peter & goodfellow 1996 brunswick belt exhalites

I Mineralogy, bulk and rare earth element geochemistry of massive sulphide-associated hydrothermal sediments of the Brunswick Horizon, Bathurst Mining Camp, New ~runswickl

I Jan M. Peter and Wayne D. Goodfellow

Abstract: Massive sulphides are spatially and temporally associated with iron formation (IF) and other hydrothermal sedimentary rocks in the vicinity of the Brunswick No. 12, Brunswick No. 6, and Austin Brook deposits, Bathurst Mining Camp. Sulphide-, carbonate-, oxide-, and silicate-predominant IF is present. Carbonate-predominant IF is best developed in and around the Brunswick No. 12 deposit, whereas hematite-bearing IF is absent here but prominent in the Austin Brook - Brunswick No. 6 area. The IF is composed dominantly of Si, CO,, Fe, Mn, and Ca. Minor constituents include Mg, P, Ti, Al, and S. Statistically significant interelement correlations between Eu, Fe, Mn, Pb, Zn, Cd, Au, Ca, Sr, Ba, P, CO,, and S indicate that these elements were precipitated from hydrothermal fluids vented onto the seafloor. Positive interelement correlations between Si, Ti, Al, Mg, K, Zr, rare earth elements (REE's) except Eu, Sc, V, Y, Yb, Co, Ni, and Cr reflect the presence of detrital clastic mafic and aluminosilicate minerals and hydrogenous sedimentary components. Felsic volcanic and pyroclastic rocks are considered to be the source for the detritus. REE patterns of IF at Brunswick No. 12 display similarities with those of modem high-temperature hydrothermal vent solutions, seawater, and host rhyolitic tuff and sedimentary rocks. These patterns are largely controlled by the relative proportions of hydrothermal and detrital components. The IF formed from reduced hydrothermal fluids vented into a stratified marine basin. The mineral precipitates were widely dispersed from the sites of venting and massive sulphide accumulation.

Rbsurnc! : Dans le camp minier de Bathurst, on observe que les sulfures massifs sont spatidement et temporellement associCs i la formation femfere et B d'autres roches skdimentaires hydrothermales de la pCriphCrie des gisements Brunswick No. 12 et No. 6 et Austin Brook. La formation femfere comporte des faciks i prkdominance de silicates, oxydes, carbonates et sulfures. Le facib de la formation ferrifire i prkdominance de carbonates est particulErement bien dCveloppC autour du gisement de Brunswick No. 12, oii le faciks de la formation ferrifere riche en hematite est absent, cependant ce dernier f a c h est dominant dans la rkgion d'Austin Brook - Brunswick No. 6. La formation ferrifere est composk principalement de Si, CO,, Fe, Mn et Ca. Les constituants mineurs incluent Mg, P, Ti, A1 et S. Les analyses statistiques permettent d'Ctablir des corrClations inter-ClCments significatives entre Eu, Fe, Mn, Pb, Zn, Cd, Au, Ca, Sr, Ba, P, CO, et S; elles indiquent que ces ClCments furent pr6cipitCs i partir des fluides tmanant de sources hydrothermales sur fond ockanique. Les corrklations inter-ClCments positives entre Si, Ti, Al, Mg, K, Zr, tltments des terres rares sauf Eu, Sc, V, Y, Yb, Co, Ni et Cr refhetent la presence dans les fragments dktritiques de minCraux aluminosilicatCs et mafiques et de composants hydrog6niques skdimentaires. Les volcanites felsiques et les roches pyroclastiques sont considCrCes cornrne la source du materiel dktritique. Les diagrammes des terres rares fournis par la formation ferrifere ?i Brunswick No. 12 sont semblables ?i ceux que procurent les solutions hydrothermales de haute temperature Cmises par les Cvents actuels, l'eau de mer, et les tufs rhyolitiques et roches skdimentaires encaissants. Les courbes de ces diagrammes dkpendent largement des proportions relatives des composants hydrothermaux et dktritiques. La formation ferrifere fut formCe par les fluides hydrothermaux rCducteurs des Cvents qui pCnCtr8rent les skdirnents stratifiks d'un bassin marin. Les prCcipitCs minCraux furent largement dispersCs en partant des Cvents et de l'aire d'accumulation des sulfures massifs. [Traduit par la rddaction]

Received January 14, 1995. Accepted November 22, 1995. Introduction

J.M. Pete9 and W.D. Goodfellow. Mineral Resources Some massive sulphide deposits are spatially and temporally Division, Geological Survey of Canada, 601 Booth Street, associated with ferruginous cherty sediments. Examples Ottawa, ON KIA OE8, Canada. include the "tetsusekiei" (red chert) at the Fukazawa Mine, ' Geological Survey of Canada Contribution 30495. a Kuroko-type deposit in Hokuroku, Japan (Kalogeropoulos

Corresponding author (e-mail: and Scott 1983), the Main Contact Tuff at the Millenbach [email protected]. gc. ca) . deposit in Noranda, Quebec (Kalogeropoulos and Scott 1989),

Can. J. Earth Sci. 33: 252-283 (1996). Printed in Canada 1 Imprim6 au Canada

Peter and Goodfellow

Fig. 1. Location and generalized geology map of the area encompassing the Brunswick Horizon. Heavy black line marks the surface trace of the Brunswick Horizon, a layered hydrothermal sediment (adapted from van Staal et al. 1992 and Brunswick Mining & Smelting Ltd.). CNE, Captain North Extension; QSR, Quebec Smelting & Refining.

Pabineau River FauR

LEGEND TETAGOUCHE GROUP

Gabbro 5 Graphitic & Mn-Fe Boucher

sedimentary rocks Brook Basalt 1 Fm.

3 Rhyolite & sedimentary rocks -Flat Landing Brook Fm. 1 QuartzAeldspar crystal tuff & sedi-

mentary rocks-Nepisiguit Falls Fm. TETAGOUCHE & MlRAMlCHl GROUP a Graphitic sedimentary rocks &

quartz wacke , , .- Fault

Thrust fault - Iron formation & exhalative horizon Massive sulphide deposit

and the Key Tuffite at the Matagami deposit, Quebec (Liaghat and MacLean 1992). Such rocks are now generally thought to represent chemical sediments that deposited from hydrothermal fluids that vented at the seafloor, and for this reason they are commonly referred to as "exhalites."

In the Bathurst area of northern New Brunswick, iron formation (IF) also occurs within and near the massive sulphide deposits. Previous petrographic and geochemical investiga- tions of IF associated with massive sulphide mineralization in the Bathurst area include Bhatia (1970), Davies (1972), Whitehead (1973), Graf (1977, 1978), Saif (1977, 1980, 1983), Saif et al. (1978), and Troop (1984). These workers described the mineralogy and bulk geochemical compositions and recognized that the IF is, at least in part, a chemical sediment, can be subdivided into sulphide-, carbonate-,

oxide-, and silicate-predominant phases, and suggested that the distribution of these was the result of varying redox conditions.

van Staal et al. (1992) have shown that in the Bathurst area there are at least three distinct stratigraphic horizons at which massive sulphides and hydrothermal sediments occur. One of these, the ~runswick Horizon (BH), encompasses the stratigraphic horizon at which the Brunswick No. 12, Bruns- wick No. 6, and Austin Brook deposits occur. Herein we report the results of systematic sampling and mineralogical and geochemical analysis of BH IF and other possible exhalative rocks. Our aims are to provide overview descriptions and mineralogy and elucidate (1) the source of the con- stituent phases, (2) the processes by which elements were distributed on the palm-seafloor during hydrothermal vent-

254 Can. J. Earth Sci. Vol. 33. 1996

Fig. 2. Generalized stratigraphic section for the Tetagouche and Mirarnichi groups in the Bathurst area (modified from van Staal et al. 1992). Shown is the position of the Brunswick Horizon.

ITETAGOUCHE GROUP^ BOUCHER Bk. Fm.

alkali basalvwacke-shale rh ythmite

FLAT LANDING Bk. Fm. rhyolite and related sedimentary rocks1 continental tholeiite

NEPlSlGUlT FALLS Fm. Brunswick Horizon (BH): iron formation &

, @ sulphideslred metallifero~s shale & chert a tuffite 6 quartz and feldspar-phyric rhyolite

PATRICK Bk. Fm. AUENIGIAN epiclastic wacke-black shale rhythmite

ing, and (3) the depositional environment in the Bathurst area.

Tectono-stratigraphic setting and distribution of the Brunswick Horizon

The rocks of the Bathurst area comprise a Cambro-Ordovician sequence of metamorphosed sedimentary and bimodal volcanic rocks that were intruded by mafic to felsic plutons and were subsequently deformed during a protracted orogeny from the Late Ordovician to the Early Devonian (van Staal and Fyffe 1991).

In the Bathurst area (Fig. I), sedimentary rocks of the Cambro-Ordovician Miramichi Group form the base of the stratigraphic section and consist of phyllite, greywacke, quartzite, and carbonaceous (graphite-rich) shales and argillites of the Knights Brook and Chain of Rocks formations (Fig. 2). These rocks are disconformably overlain by Teta- gouche Group rocks that consist of calcarenites, siltites, and conglomerates of the Valee Lourdes Formation, carbonaceous shales of the Patrick Brook Formation, and felsic volcanic and volcaniclastic rocks of the Nepisiguit Falls Formation (Fig. 2). The rocks of the Nepisiguit Falls Forma- tion are rhyolitic, porphyritic to tuffaceous, and have been variously interpreted to be of epiclastic to pyroclastic origin (e.g., Lentz and Goodfellow 1992b; McCutcheon 1990). In general, these felsic volcaniclastic rocks range in composition from rhyodacite to rhyolite (e.g., Whitehead and Good- fellow 1978). The Nepisiguit Falls Formation is overlain by aphyric and feldspar-phyric rhyolites, hyaloclastites, crackle breccias, and minor sedimentary rocks of the Flat Landing Brook Formation, which in turn is overlain by mafic volcanic

, , , , , , , , , , , , , , , ,,,,,, 3,

VALLEE LOURDES Fm. ,:::::, calcarenite & siltite, conglomerate 0

IMIRAMICH~ GROUP]

0 KNIGHTS Bk. Fm. quartzite - black shale rhythmite

@& CHAIN OF ROCKS Fm. quartzite & green shale

U-Pb zircon age date locality

a fossil locality

and sedimentary rocks of the Boucher Brook Formation of the Tetagouche Group.

The major- and trace-element geochemical characteristics of the mafic and felsic volcanic rocks and associated intru- sions of the Tetagouche and Fournier groups (e.g., van Staal 1987; van Staal et al. 1991; Fyffe 1990) indicate that felsic volcanic rocks were derived from continental crust melted during incipient rifting (Lentz and Goodfellow 1992a; van Staal 1987), whereas back-arc rifting is responsible for alkaline felsic to mafic volcanism, which progressed into a mature back-arc environment characterized by tholeiitic mafic volcanics in the eastern portion of the area.

The BH, which includes massive sulphides, is hosted by metamorphosed greenish-grey mudstones at, or near, the contact between the Nepisiguit Falls and Flat Landing Brook formations of the Tetagouche Group (Figs. 1, 2). Here, quartz- and feldspar-phyric and aphyric felsic volcanic, volcaniclastic, and sedimentary rocks of the Nepisiguit Falls Formation are the predominant stratigraphic footwall lithologies to the massive sulphide deposits and IF'S. The distribution of the outcropping and subcropping BH is depicted by a solid black line in Fig. 1. IF horizons to the west, along which the Quebec Smelting & Refining (QSR), Headway, Esker, Flat Landing Brook, and Captain North Extension (CNE) deposits are situated, have not been conclusively cor- related with the BH. In the area of the Brunswick No. 12 deposit (Fig. 3), Boucher Brook Formation alkali basalts and related sediments overlie the Flat Landing Brook Formation and are interbedded with red argillites.

van Staal(1987) has identified six generations of folding, three of which control the distribution and morphology of the


Fig. 3. Geology map of the Brunswick No. 12 deposit and environs (after Brunswick Mining & Smelting Ltd.). Shown are collars and traces of surface and underground diamond drill holes sampled (see Appendix 1 for sample descriptions).

LEGEND 147027' I ~ ? 8 ' 147%'

Nepisiguit Falls Frn. Patrick Brook Fm. I*,tl PORPHYRY DYKE IRON FORMATION AND QUARTZ WACKES -N7- GRAPHITIC SEMMENTARY ROCKS -

GABBRO MASSIVE SULPHIDE -- CONTACT; DEFINED. APPROXIMATE

BH deposits. Greenschist facies metamorphism coincided with regional deformation (van Staal 1985). Blueschist facies assemblages generated by subduction have been identified in the northern part of the belt (van Staal 1987).

Sampling and analytical methods

Representative samples of IF and other rocks of possible hydrothermal sedimentary origin were collected from out- crops, underground exposures, and drill core along the entire strike length of the BH (Fig. 1); sampling focussed particularly on the Brunswick No. 12 (Fig. 3), Brunswick No. 6, and Austin Brook (Fig. 4) massive sulphide deposits and immediate environs. Samples were collected along strike about 3 km to the south and 2 km to the north of the Bruns- wick No. 12 deposit (Peter 1991) (Fig. 3); along-strike distances in the Brunswick No. 6 - Austin Brook area are more difficult to ascertain due to the complex nature of the folding of the BH, particularly to the east of the deposits (Fig. 4); north of Brunswick No. 6, samples were collected along more than 3 km of strike extent to the FAB (named by Fred A. Boylen) sulphide occurrence (Fig. I), and samples were collected along an estimated 13 km or more of strike to the east and south (Figs. 1, 4).

Bulk compositions were determined at the Geological Survey of Canada (GSC), Ottawa, and by X-Ray Assay Laboratories (XRAL), Don Mills, Ontario. At the GSC, major and selected trace elements (SO2, Ti02, N2o3, Fe203, MnO, MgO, CaO, Na20, K20, P2O5, Ba, Co, Cr, Cu, Zn, Ni, V, Y, As, Bi, Mo, Sb, Se, Rb, Sr, Sc, Zr, Y, Nb) were analyzed by inductively coupled plasma emission spectroscopic methods (ICP-ES). Rare earth elements (REE's) were also analyzed by ICP-ES following preconcentration by multiacid dissolution and ion exchange. Ag and Pb were

determined by atomic absorption (AA) methods, and FeO, H20, C02, C, S, and loss on ignition were measured by chemical methods. All Au, Cd, and Hg analyses were by XRAL using the following techniques: Au by fire assay - direct coupled plasma (FADCP); Cd by flame atomic absorption (FAA); and Hg by wet chemical techniques. Some of the samples were analyzed at XRAL in the following manner: major and selected trace elements (Si02, A1203, CaO, MgO, Na20, K20, Fe203, MnO, Ti02, P205, Cr, Rb, Sr, Y, Zr, Nb, Ba) by standard whole-rock techniques; FeO, Hg, and H20 by wet chemical methods; Be, Na, Mg, Al, P, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Sr, Y, Zr, Mo, Ag, Cd, Sn, Sb, Ba, La, Ta, W, Pb, and Bi by ICP; Cd by AA; C and S by Leco@ furnace; and C02 by coulomb methods. Analyses were monitored using certified IF reference standards (Abbey et al. 1983). Chondrite-normalizing values for REE plots are those recom- mended by Rock (1987). North American shale composite (NASC) normalizing values are the mean of three analyses in Gromet et al. (1984) (analyses 5 -7 in their Table 2), as suggested by these authors. For statistical manipulation, analyses reported below analytical detection limits were replaced by a value equal to one half of the lower detection limit for data plotting and statistical calculations.

Description and mineralogy of the Brunswick Horizon

IF samples We use the term IF to include all banded ferruginous and fine- grained, quartzose sedimentary rocks, which are largely, or partly, of hydrothermal origin. Samples were classified according to the gross predominance of one or more of the following minerals: magnetite, hematite, siderite, chlorite,

Can. J. Earth Sci. Vol. 33, 1996

Fig. 4. Geology map of the Brunswick No. 6 and Austin Brook deposits and environs (modified from Luff et al. 1993). Shown are collars and traces of sampled surface and underground diamond drill holes.

Fig. 5. Graphical geological log of underground diamond drill hole 12-1277 from the Brunswick No. 12 deposit showing complex primary relationships between sulphide- and silicate-predominant , and mixed silicate - carbonate IF.

f l s l bndlng Brook Fm. Knights Brwk Fm.

aaphyrlc to feldspar phyrrt: ~hycl~te, tuff. 0 Ih1"'y bedded graphltlcshale and ayslal tuRRe 5- 169 quartz wacke

Na~$abuit FmII. Fm. r- Drrlihole collar and trace

and sulphide (Fig. 5). Fine-grained quartz, or recrystallized chert, is generally present.

For the most part, IF samples are fine to very fine grained and well laminated to bedded (e.g., Figs. 6A, 6B, 6E, 6F), except where deformation has obscured such primary features. For the purposes of presentation and discussion, BH IF samples were classified into the following general groups: sulphide-bearing IF, carbonate-predominant or -bearing IF, oxide-predominant or -bearing IF, and silicate- predominant IF.

Sulphide-bearing IF Sulphide-bearing IF is represented by massive sulphide as well as IF with an appreciable sulphide component (greater than about 10 wt. % S). Massive sulphide mineralization can

hanging-wall rhyolite tuff

=transition: tuff to irqn formation chlorite iron formatron magnetite-siderite iron formation

-siderite iron formation \chlorite-siderite iron formation

chlorite iron formation =chlorite-siderite iron formation \chlorite Iron formation

chlorlte-s~der~te Iron formation - -

------.7------------

- - - - - - - - - - - - - - - - - . chlorite iron formation

chlorite-(sulphide) iron formation

massive sulphide (pyrite-sphalerite- galena-chalcopyrite)

massive sulphide (pyrite- magnetite)

massive sulphide (sphalerite- galena-pyrite-calcite)

be massive, recrystallized with coarse, euhedral sulphide crystals, or brecciated, with fragments of pyrite in a pyrrho- titic matrix. Sulphide and gangue minerals include pyrite, pyrrhotite, sphalerite, galena, chalcopyrite, quartz, chlorite, magnetite, and calcite; lesser minerals that may be present include arsenopyrite, bornite, bournonite, covellite, marca- site, and tetrahedrite (e.g., Jambor 1979; Lea and Rancourt 1958; Stanton 1959), barite (McNulty 1981; W.M. Luff, personal communication, 1992), and edingtonite, a hydrated barium aluminosilicate (Grice et al. 1984).

Carbonate-predominant IF Carbonate-predominant (siderite) IF is most prominent in and around the Brunswick No. 12 deposit; in the Austin Brook - Brunswick No. 12 area, carbonate occurs as a minor to trace component of some IF samples. It is tan to grey, and consists of alternating laminae of very fine grained quartz (chert) and siderite (Fig. 6F). Predominant minerals include siderite, calcite, ankerite, dolomite, kutnahorite (Ca(Mn,Mg,Fe)(C03),), and quartz (after chert); lesser

Peter and Goodfellow 257

Fig. 6. Macroscopic features of iron formation samples. (A) Fine-scale layering of magnetite, siderite, chert, and chlorite. (B) Graded beds of upward-fining silt to mud-sized detrital(?) quartz grains in chlorite IF; (C) Rip-up clasts consisting of clasts of chert and (or) siderite (and hematitic chert at Brunswick No. 6 deposit consisting of red chert containing very fine hematite inclusions) within chlorite IF. (D) Erosional contact (scour). (E) Pebble dents. (F) Pseudo-ooliths in siderite-predominant IF.

minerals that may be present include stilpnomelane, chlorite, apatite, sericite, potassium feldspar, albite, spessartine garnet, magnetite, pyrite, pyrrhotite, sphalerite, and galena. Samples that also contain appreciable oxide or silicate component can contain laminae of magnetite and (or) chlorite.

Siderite, ankerite, and calcite occur as discrete anhedral grains and masses of intergrown grains along carbonate-rich layers with chlorite, stilpnomelane, and magnetite and also as ooliths (Fig. 6D). van Staal(1985) reported that calcite in the BH is Mn rich, and ankerite porphyroblasts typically have more Fe-rich rims. Scanning electron microscopy (SEM) studies by van Staal(1985) identified complex growth zoning in calcite defined by varying Fe, Mn, or Mg contents. Very fine grained to fine-grained quartz occurs as complexly intergrown, ragged, anhedral, interlocking grains in quartz- rich layers and was likely deposited as chert. In places, secondary, euhedral pyrite porphyroblasts occur within the siderite laminae (Fig. 6F).

Oxide-predominant IF Previous workers have recognized both magnetite-rich and hematite-rich oxide IF in the vicinity of the Brunswick No. 6 and Austin Brook deposits (Boyle and Davies 1964; Davies 1972; Rodgers 1965). Magnetite-rich oxide IF com- prises banded magnetite, hematite, chlorite, chertlquartz, and siderite. Accessory minerals include sericite, stilpnomelane, apatite, calcite, pyrite, pyrrhotite, chalcopyrite, galena, and sphalerite. Samples that are transitional to silicate- or carbonate-predominant IF locally also contain beds or laminae of chlorite and (or) siderite, respectively.

At the Brunswick No. 12 deposit, magnetite IF is dark

grey colored, with alternating laminae or beds ( < 0.1 mm to -2 cm) of magnetite and chert and, in places, pyrrhotite (Fig. 6A). At the Austin Brook and Brunswick No. 6 deposits, the magnetite IF, which is best developed south of Austin Brook, grades into hematite oxide IF, which is composed of laminated hematite, magnetite, and quartz after chert. Here, hematite occurs within hematite-magnetite IF as 0.2- 1.5 mm wide, red, aphanitic, hematitic chertlquartz laminae that for the most part are devoid of any other minerals. These layers, which are commonly disrupted and brecciated, and subsequently deformed into foliated fragments, are interlayered with quartzlchert, siderite and magnetite laminae. In places, hematitic, quartzose fragments and beds contain 5 - 10% fine-grained pyrite. Specular hematite occurs rarely and is likely the product of subsequent metamorphism. In the Brunswick No. 12 area, however, hematite- bearing IF is conspicuously absent.

Magnetite typically occurs as small octahedra, occurring along generally monomineralic layers, and as disseminated grains. Where recrystallization has been significant, euhedral magnetite is common (Fig. 7A). Where hematite is present, it is subordinate to magnetite, and chlorite and siderite are minor. Hematite occurs as very fine plates and anhedral grains with magnetite, coarse-grained, interlocking and isolated plates, and as very fine grains homogeneously distributed within quartzlchert layers. Quartz typically occurs as complexly intergrown, ragged, anhedral, interlocking grains in quartzose layers after chert; however, where it has been recrystallized, larger quartz crystals are developed. Chlorite and stilpnomelane form thin layers within magnetite and hematite (Fig. 7B) as felted masses along certain layers.

258 Can. J. Earth Sci. Vol. 33, 1996

Fig. 7. Microscopic features of nonsulphide phases in iron formation samples. (A) Euhedral magnetite grains in oxide-predominant IF. (B) Fibrous intrgrowth of stilpnomelane. (C) Pseudo-ooliths of siderite and calcite overgrowths. (D) Pseudo-ooliths of siderite.

Pyrite occurs along discrete layers (up to about 30 cm thick), and occurs as disseminated euhedral cubes and aggregates of cubes in both the oxide- and silicate- predominant IF (Fig. 8A). Pyrrhotite is generally interstitial to euhedral pyrite (Fig. 8B). Sphalerite and galena occur as irregular, discrete grains interstitial to pyrite and as cores within pyrite (Fig. 8C). These may be replacement minerals or may themselves replace pyrite. Arsenopyrite occurs as euhedral rhombohedral crystals, which are commonly concentrated along discrete layers (Fig. 8D). Apatite occurs as minor disseminated anhedral to euhedral grains within quartzlchert-rich layers and, less commonly, as apatite-rich layers. Accessory rutile, zircon, epidote, and ilmenite are also generally present.

Silicate-predominant IF Previous workers have recognized that chlorite-rich rocks occur abundantly in and around the massive sulphide deposits (e.g., Boyle and Davies 1964; Davies 1972; Rodgers 1965). These rocks are generally banded or schistose and predominantly composed of dark green to black, felted to massive fine-grained chlorite; pyrite and (or) pyrrhotite are locally

important. Other common minerals are chertlquartz, quartz, ~til~nomelane, sericite, albite, potassium feldspar, biotite, zircon, ilmenite, rutile, epidote, apatite, magnetite, spessartine garnet, chalcopyrite, galena, sphalerite, arsenopyrite, and bornite. ~uartzbccurs as complexly intergrown, ragged, anhedral, interlocking grains in minor green-, pink-, or grey- colored laminae after chert, Mixed silicate-carbonate or silicate-oxide IF also contains siderite or magnetite laminae.

Chlorite in chlorite-rich layers occurs as coarse, intergrown fibers, felted mats, and coarse aggregates. Silicate- predominant IF grades into chlorite schist, with the Fe content of chlorite in the former being higher than chlorite in the schist, based on anomalous Berlin-blue birefringence, which is characteristic of iron-rich chlorites. In this study we have not characterized the chlorite by electron microprobe or SEM methods. Davies (1972) analyzed chlorite from the Austin Brook IF and noted the of high-iron ripidolite and brunsvigite, with the latter predominating in silicate- predominant IF.

Stilpnomelane, along with magnetite and hematite, forms thin layers of felted grains in silicate IF. van Staal (1985) conducted SEM analyses of several stilpnomelane crystals in


Fig. 8. Microscopic features of sulphide phases in iron formation samples. (A) Euhedral cubes of pyrite. (B) Euhedral arsenopyrite laths and pyrite with interstitial pyrrhotite and quartz. (C) Inclusion of galena within pyrite. (D) Euhedral arsenopyrite rhombohedron with pyrrhotite inclusion.

IF and found ferri-stilpnomelane, ferro-stilpnomelane, and parsettensite (Mn-rich stilpnomelane). He concluded that stilpnomelane formed during deformation and metamorphism, likely from precursor minerals.

Spessartine garnet is confined to the Brunswick No. 6 - Austin Brook area and occurs in the silicate IF as rounded, 1 - 3 mm diameter, subhedral grains, commonly concentrated along discrete layers. It has not been found in the vicinity of Brunswick No. 12. X-ray diffraction and SEM analyses by van Staal(1985) indicate that Mn is the dominant cation. On the basis of microstructures, garnet is thought to have formed post-F2 and pre-F3 (van Staal 1985). Grunerite occurs as porphyroblasts that crosscut layering in places and is commonly associated with garnet and stilpnomelane along certain layers. Pre-D2 porphyroblastic grunerite occurs at Brunswick No. 6, Austin Brook, and the IF exposed to the east of these deposits (Fig. 4) as radiating bundles of acicular crystals.

Sericite occurs as fine-grained, felted books commonly containing intergrown chlorite. Biotite occurs as fine laths

intergrown with chlorite and sericite. In some places, biotite flakes are aligned along metamorphic cleavages, where they replace chlorite or sericite. van Staal (1985) also reports the presence of albite, barium muscovite, and barium feldspars (celsian and hyalophane) in IF. W. Luff (personal communication in van Staal 1985) reports the presence of trace barite in the IF. Euhedral magnetite occurs as disseminated grains and aggregates of grains commonly displaying pressure shadows of quartz or chlorite.

Rare angular to rounded quartz grains have been interpreted by van Staal(1985) to be relict quartz phenocrysts and fragments. On the basis of mineralogy and bulk geochemistry, most chlorite, as well as sericite (muscovite or phengite) and rutile, represent tuffaceous - sedimentary detritus. In the Japanese tetsusekiei and Canadian Main Contact Tuff at Noranda, these minerals are also thought to be largely detrital in origin (Kalogeropoulos and Scott 1983, 1989).

Pyrite forms discrete layers up to about 30 cm thick, and as disseminated cubes and aggregates thereof (Fig. 8A). Pyrrhotite, sphalerite, and galena also form layers containing

260

Table 1. Bulk geochemical data for Brunswick Horizon and host-rock samples.


-

Sample: POAM6 POA060 POA063 POA064 POA065 POA071 POA093 POAlOl POA114 POA125 POA148 Type: sid IF chl IF mt -hem mt IF mt - hem chl-sid chl-sulf sid IF mt -sid fw sed chl-sid

IF IF IF IF IF IF

SiO, (wt. %) A1203 TiO, FeO F%O, MnO MgO CaO Na20 K2O P2Q5 co2 C S H20- Total

Fe(t)

Be @pm) Sc v Cr Co Ni

Cu Pb Zn Cd As Sb Ag Bi Sn Mo

Ba Rb Sr Y Zr Ta Nb

Hg (PP~) Au

La @pm) Ce Nd Sm Eu Gd DY Ho Er Tm Yb

Notes: -, not analyzed; chl, chlorite; fw sed, footwall sediment; hem, hematite; hw sed, hanging-wall sediment; IF, iron formation; mt, magnetite; po, pyrrhotite; sid, siderite; sulf, sulmde.


POA158 POA187 POA208 POA222 POA223 POA254 POA334 POA400 POA423 POA451 POA455 POA474 POA557 mt IF mt-sid mt-chl chl IF mt -chl chl -sid- chl-sid- chl-po hw sed hem IF hem IF mt -sid- mt -sid-

IF IF IF mt IF mt IF IF hem IF hem IF


pyrite blebs and less common porphyroblasts. Galena forms very fine, discrete grains and cores within pyrite (Fig. 8C). Euhedral arsenopyrite forms isolated grains and aggregates, which are commonly concentrated along discrete layers (Fig. 8D).

Sedimentary structures The most prominent feature of the BH IF is ubiquitous fine- scale mineralogical banding (Fig. 6A). This banding can be regular, wavy, or lensoid, and is defined by alternating monomineralic bands of chlorite, siderite, magnetite, hematite, and fine-grained quartz after chert. Individual bands vary in width from 0.5 to 20 mm. Although most bands reflect deformational overprinting effects, in some bands there is a change in the grain size of minerals and this likely reflects primary layering; most of these layers have sharply defined bases and are interpreted to represent original bedding.

Silt layers in chlorite IF fine upward to mud-sized detrital(?) quartz grains (Fig. 6B). Rip-up quartz (chert) - siderite clasts at Brunswick No. 12 and hematitic quartz (chert) clasts within chlorite IF at Brunswick No. 6 have been observed. In a few places, erosional contacts (scours) between two IF groups (Fig. 6C) and pebble dents are present. Scours are indicative of resedimentation from shal- lower to deeper depths and deposition from turbidity currents. However, the abundance of nongraded laminations and bedding indicates that turbidity currents were only infrequent. Pebbles associated with pebble dents are rarely angular and commonly tapered, likely as a result of deformation.

The excellent preservation of fine laminations and the lack of bioturbation is evidence against metazoan activity (Tyler and Twenhofel 1952a, 1952b; Walter and Hofmann 1983), and the conspicuous absence of benthic fossils indicates deposition in deeper water below wave base in a reducing basin. Cross-bedding in IF is not necessarily indicative of deposition in a shallow-water environment above wave base, as this feature is found in the modern environment (e.g., at 2400 m water depth at Middle Valley, northern Juan de Fuca Ridge; Rigsby et al. 1994). Brachiopod and pelecypod fossils in the host shaly sedimentary rocks of the Nepisiguit Falls Formation (Bolton 1968; Fyffe 1982; Gummer et al. 1978; Neuman 1984) are also not necessarily indicative of shallow- water conditions, as such fauna are known to thrive in deep-water, inhospitable conditions such as near seafloor hydrothermal vents (e.g . , Jones 1985).

Pseudo-ooliths in carbonate-predominant IF are present in a few places (Figs. 6D, 7C, 7D); these structures resemble ooliths in appearance and size. They contain cores of sphene, carbonate, and other extremely fine-grained, unidentified minerals with successive overgrowths of carbonate. Such features have also been noted by McMillan (1969) in the Heath Steele Belt, by van Staal(1987) along the BH, and by Saif (1983) in "spotted IF" near Nepisiguit Falls. These structures were interpreted by these authors to have formed in shallow water. However, examination by optical microscopy shows that the cores of many of these structures contain grains with planar crystal faces, indicating that they likely formed by successive overgrowths of carbonate on subhedral nucleus minerals during the subsequent metamorphic events. In summary, sedimentary and pseudo-sedimentary features provide equivocal indicators of water depth.

It was not possible to correlate IF intervals of differing mineralogies between adjacent drill holes. In some places, there is extreme variability in the IF groups between adjacent drill holes. Additionally, many drill holes failed to intersect IF even in the immediate vicinity of the Brunswick No. 12 deposit.

Bulk composition

Appendix 1 gives locations and descriptions of representative samples analyzed; bulk compositional data for these samples are presented in Table 1. The BH IF samples are composed principally of quartz and iron oxides. C02, Fe, Mn, and Ca are major to minor components, and Mg, P, and S are present in minor amounts.

Interelement correlations and recognition of hydrothermal, detrital, and hydrogenous components within IF

For each element, histograms and probability plots were created for the different sample populations (not shown) to determine whether the distribution is normal, bimodal, or polymodal. Log-transformed data were used for populations that were not normally distributed in the construction of a matrix of Pearson correlation coefficients for major and trace elements in BH samples (not shown). Significant positive interelement correlations exist between two groups of elements:

(1) Eu, Fe, Mn, Pb, Zn, Cd, Au, Ca, Sr, Ba, P, C02, and S. Figure 9 is a mosaic of bivariate scatter plots for Fq03 , FeO, Fe(t), CaO, MnO, P2O5, C02, Sr, Pb, Zn, and Hg that show significant positive interelement correlations. These elements are present in iron oxides, carbonate, and sulphide minerals that are attributed to hydrothermal sources. These minerals were likely precipitated from hydrothermal fluids that vented onto the paleoseafloor (Bostrom 1973a; Fleet 1983).

(2) Si, Ti, Al, Mg, K, Zr, REE except Eu, Sc, V, Y, Yb, Co, Ni, and Cr. Figure 10 is a mosaic of bivariate scatter plots for Al2O3, Ti02, MgO, Zr, Sc, Fe, Ni, Co, and Cr that show significant interelement correlations; all element pairs except those involving Fe(t) display positive correlations. All of these elements (except Fe) are characteristically present in detrital aluminosilicate minerals, and it is likely that the presence of these minerals in the samples accounts for positive interelement correlations.

Fe and Mn in marine sediments are derived mainly from hydrothermal sources and are therefore a measure of the hydrothermal contribution to the sediments, whereas the A1 is of detrital origin and is added to the sediment by clay minerals (Bostrom and Peterson 1969; Bostrom 1973b). The highly positive correlation of A1203 with Ti02 for the BH samples (Fig. 10G) indicates that Al was originally deposited in clay minerals. Figures 1 1 A, an A1 - Fe - Mn ternary plot, shows that most BH samples fall in the hydrothermal field. Samples plotting outside of this field and in the non- hydrothermal field are predominantly chlorite IF comprised largely of detrital minerals.

The relative contributions of terrigenous clastic and hydrothermal components are illustrated in Fig. 11B where elements of clastic origin are represented by A1 and Ti and

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Fig. 11. (A) A1 - Fe - Mn ternary plot for iron formation samples from the Brunswick Horizon (n = 249). Shown are fields for non-hydrothermal and hydrothermal sediments (Bostrom 1973b), metalliferous sediments from the East Pacific Rise (EPR) (Bostrom 1973a) and Deep Sea Drilling Project (DSDP) Leg 31 (Bonatti et al. 1979), umbers associated with Cyprus massive sulphides (Robertson and Hudson 1973), and oceanic sediments (Dymond et al. 1973; Toth 1980). (B) Plot of FelTi versus AlI(A1 + Fe + Mn) for iron formation samples from the Brunswick Horizon (n = 249) (after Bostrijm 1973~). This plot is used to estimate the hydrothermal contribution to the sediments. Shown are Brunswick Horizon samples, average pelagic Pacific Ocean sediment, terrigenous matter, East Pacific Rise metalliferous sediment, and hanging-wall and footwall sediments to the Brunswick Horizon. Also shown are fields for various basalts in the Brunswick area.

LEGEND Al

( A ) LEGEND

0 magnetite-siderite IF A magnetite IF

0 siderite IF o ch1orite:sideritemagnetite IF

+ chlorite-siderite IF magnetite-siderite- hemat~te IF

0 chlorite IF V hematite IF

x sulphide IF footwall sediment

A magnetite-chlorite IF 0 hanging-wall sediment

magnetite-hematite IF uth Padlc red clays

DSDP Leg 31 metalliferou

East P8cific Rise Metalliferous Sediment

Pacific Ocean .- Modern Sediment

Belt Tuff, Hanging-. Wall & Footwall

Brook Tholeiite

Brook Tholeiite


hydrothermal elements are represented by Fe and Mn. This plot is also useful in discriminating between possible detrital sources. The BH samples define a continuous mixing curve between relatively pure metalliferous sediment and tuffaceous sedimentary rocks of the Flat Landing Brook and Nepisiguit Falls formations, with most samples plotting near the end-member composition for metalliferous sediments.

The compositions of different mafic volcanic rocks that overlie massive sulphide mineralization, with the exception of the Forty-Mile Brook tholeiite, are well removed from the trend for the BH exhalites. Fe and Ti are unlikely to have been fractionated into the detrital component by sedimentary processes, and the basalts can be ruled out as possible sources for the clastic component within these samples. The Forty-Mile Brook tholeiites are mid-ocean-ridge basalts (MORB) and are not subaerial; furthermore, they are con- temporaneous with the BH IF and are unlikely to have been uplifted and eroded during the time of IF deposition. The composition of the Miramichi Group sediments suggests that these sediments are a possible source of detritus in the IF samples; however, several IF samples plot to the right of these rocks, indicating that the host sediments are a more likely source.

Adsorption of certain elements from seawater (e.g., Ni, Co, U, Th) onto hydrothermal precipitates and inorganic sediment particles and organic particle surfaces that have a surface charge can also play a significant role in determining the composition of the metalliferous sediment. However, the BH samples have low abundances of these elements and, therefore, do not have a significant hydrogenous component (Fig. 12A).

Element distributions in the Brunswick Horizon

Metalliferous sediments composed of Fe - Si - Mn oxyhydroxides are found at oceanic ridge crests on the modern ocean floor (e.g., Barrett 1987; Dymond 1981; Pottorf and Barnes 1983). In modern metalliferous sediments, Fe hydroxides and goethite are prevalent (e.g., Germain- Fournier 1986; Kastner et al. 1986). The discovery of modern submarine hydrothermal plumes that vent fine- grained sulphide (pyrrhotite, sphalerite, pyrite, chalcopyrite), amorphous silica, barite, anhydrite, and various silicate minerals at mid-ocean-ridge and back-arc spreading centres (e.g., Klinkhammer and Hudson 1986; Mottl and McConachy 1990; Rudnicki and Elderfield 1993; Trocine and Trefry 1988) has provided a modern analogue for ancient exhalites. Studies of modern hydrothermal plumes have greatly advanced our knowledge of metal dispersion under oxygenated conditions. Some particles settle out of the buoyant plume and constitute "fallout" (Feely et al. 1987). Walker and Baker (1988) and McConachy (1988) have shown from particle size analysis that most of the mass of this material will settle in the proximity of the vent field. Thus, modern metalliferous sediments and hydrothermal plume particulates serve as a valid analogue for the BH.

The Si content of modern seafloor hydrothermal solutions is generally controlled by equilibrium with respect to quartz or chalcedony at the temperature and pressure of reaction (e.g., Von Damm 1990), and the large variations in silica

contents of these fluids arise because of the various cooling and mixing pathways possible for a fluid as it vents into seawater. Silica contents in BH IF range from 5.96 to 79.00 wt. % (mean = 39.67 wt. %; n = 249), much of which resides in detrital aluminosilicates and silicates. For this reason, the bulk compositional data do not indicate a hydrothermal origin for silica. Within samples considered to contain the largest hydrothermal component, there is also considerable variation in Si02 contents; hematite IF ranges between 64.8 and 9 1 wt. % (mean = 75.24 wt. %), whereas magnetite IF ranges between 15.6 and 64.4 wt. % (mean = 34.56 wt. %). Remains of siliceous organisms were not noted in any of the BH samples observed; however, recrystallization during regional dynamic metamorphism and deformation may have destroyed them.

Modern metalliferous sediments have anomalously high Fe and Mn (e.g., Bostrom 1973a; Lyle et al. 1986). BH samples are also rich in these elements. Magnetite- and siderite- bearing IF samples average between 20 and 37 wt. % Fe(t), whereas chlorite-bearing IF samples contain on average 16.25 wt. % Fe(t). MnO contents average 5.31 wt. % for siderite IF, with siderite-bearing IF having slightly lower mean values; chlorite IF averages 1.47 wt. % , hematite IF averages 1.01 wt. %; magnetite IF averages 3.34 wt. %; manganiferous sediment samples range up to 6.67 wt. % (mean = 3.09 wt.%). The overall negative correlations between Fe(t) and Ti02 and Zr (Figs. 10C, 10J) indicate that most Fe is hydrothermal and not detrital in origin. The positive correlations between Fe203 (and Fe(t)) and Pb, P2O5, and Zn, illustrated in Figs. 9A, 9C, 9F, and 9K, likely reflect the coprecipitation of apatite and sulphides from hydrothermal solutions. Positive correlations between MnO and C02 and MnO and FeO (Figs. 9D, 9E) reflect the presence of Mn and Fe in carbonate minerals.

Ba contents average 648 pprn for chlorite IF, 690 pprn for hematite IF, 895 pprn for magnetite IF, 891 pprn for siderite IF, 766 pprn for magnetite - siderite IF, 1 120 pprn for manganiferous sediments, and host rock sedimentary samples average 1065 ppm; chlorite-bearing IF samples generally have somewhat lower Ba contents, averaging between 200 and 350 ppm. Barium, like Sr, can be of hydrothermal origin (Arrhenius and Bonatti 1965; Meylan et al. 1981). Barium concentrations up to 4 wt. % occur in the very Fe and Mn rich sediments in the Bauer Deep (Bostrom and Peterson 1966; Hanor 1968) and Ba contents up to 2 wt. % have been noted in metalliferous sediments along the East Pacific Rise (EPR) (Bostrom and Peterson 1966). Craig (1969) noted extreme enrichments in Ba of the hot brines of the Red Sea (over 25 000 times seawater). Electron microprobe data for BH IF samples indicate that carbonate, sericite, biotite, and stilpnomelane in the IF contain trace to minor Ba (J. Peter, unpublished data).

P2O5 contents average 0.67 wt.% for chlorite IF, 1.65 wt.% for magnetite IF, 1.21 wt.% for siderite IF, 1.72 wt. % for magnetite -siderite IF, 1.22 wt. % for magnetite - chlorite IF, 0.16 wt. % for manganiferous sediment, 1.5 wt. % for sulphide IF, 0.13 wt. % for hematite IF, with hematite- bearing IF also having low concentrations. Modern seafloor hydrothermal vent fluids, such as those at Loihi Seamount, can be enriched in P relative to seawater (Karl et al. 1988). However, biological phosphorus can also be important in the.


Fig. 12. (A) (Ni + Co + Cu) x 10 - Fe - Mn ternary plot for BH IF samples (n = 249). Also shown are the general fields for hydrothermal sediments (Bonatti et al. 1972) and hydrogenous nodules (Bonatti et al. 1972), Fe-Mn crusts (Toth 1980), for Bauer Deep sediments (Sayles and Bischoff 1973), East Pacific Rise deposits (axial zone, crest flanks, and deeper ridge flanks) (Germain-Fournier 1986), South Pacific biogenous oozes and red clays, siliceous clays from the Central East Pacific nodule belt, and Clarion-Clipperton zone associated Mn nodules (Karpoff et al. 1988). (B) Bivariate plot of P205 versus Y for BH iron formation samples (n = 249). Shown is the trend for deep sea sediments from Marchig et al. (1982). Note that the BH samples do not fall on this trend and display a distinct lack of correlation between the two elements; this is characteristic of modem metalliferous sediments. (C) Cu-Pb-Zn ternary plot for Brunswick Horizon iron formation samples. Also shown are fields for chemical and clastic end members of Tetsusekiei (iron quartz) samples associated with Kuroko deposits of the Hokuroku district (Kalogeropoulos and Seott 1983), umbers associated with massive sulphide deposits of Cyprus (Robertson and Hudson 1973), and exhalites associated with the Windy Craggy massive sulphide deposit (Peter 1992). Note the similarity of most Brunswick Horizon samples to the Tetsusekiei samples.

(Ni + Co + Cu)x 10

( A )

Clarbn-Cllppertan

Central E. Pacific nodule belt days

S. pacific V / 1 \

Fe EPR deposits ~ ~ ~ ' d e ~ o s i t s -EPR deposits (axial zone) (crest flanks) (deeper ridge flanks)

LEGEND

o magnetite-siderite IF

siderite IF

+ chlorite-siderite IF

IJ chlorite IF

x sulphide IF

A magnetite-chlorite IF

magnetite-hematite IF

A magnetite IF

o chlorite-sideritemagnetite IF

magnetite-siderite-hematite IF

'I hematite IF

fwtwall sediment

hanging-wall sediment

marine environment (Skinner 1993). Most of the phosphorus in deep-sea sediments occurs as skeletal biogenous remnants (apatite) with typical enrichment of Sc, Y, and rare earth elements (REE's) (e.g., Marchig 1978), which results in a positive correlation between P and Y, Sc, or La in deep-sea sediments (Marchig et al. 1982). However, metalliferous sediments or exhalites do not have correspondingly high Y values and P values because P is mobilized by hydrothermal leaching and coprecipitated with iron hydroxides without

:\ ,Cyprus umbers

Kuroko Tetsusehe~ chemleel end member

incorporating Sc, Y, and La. BH IF, which contains greater than detection limit amounts of Y displays a distinct lack of correlation between Y and P2O5 contents (Fig. 12B), characteristic of modern metalliferous sediments of hydrothermal origin. Thus, apatite, a ubiquitous accessory mineral in some BH samples, is likely of hydrothermal origin.

Figure 9G illustrates the strong correlation between CaO and P2O5 in the BH samples indicating that these elements are present in fine-grained apatite. The linear trend in


Fig. 9G lies along the apatite line, and some samples contain excess CaO, which is present as carbonate. The high positive correlation between F q 0 3 and P2O5 in the BH samples (Fig. 9F) is likely due to the coprecipitation of apatite and carbonate. Significant positive correlations between P2O5 and CaO and P2O5 and Pb (Figs. 9G, 9H) result from the local substitution of Pb for Ca in apatite.

Figure 9J illustrates the correlation between Sr and CaO indicative of substitution of Sr for Ca in the CaC03 lattice (Lyle et al. 1984). Sr is present in modem C1-enriched hydrothermal fluids of the EPR at 13 ON (Michard et al. 1984) and in fluids that react with sediment, such as those of the Guaymas Basin (Von Damm et al. 1985). Strontium isotope values for siderite from BH IF are intermediate between those for continental crust and Ordovician seawater and indicate Sr likely was contributed from seawater modified by subsurface interaction with crustal rocks or derivative sediments during hydrothermal circulation (J.M. Peter and W.D. Goodfellow, unpublished data). Figure 9D illustrates a high positive correlation between C02 and MnO, indicating that Mn is present mainly in carbonate.

Base metal contents of the BH IF samples vary widely (Cu, 1 - 11 10 ppm, mean = 52 ppm; Pb, 1 -5900 ppm, mean = 1408 ppm; Zn, 8-32 000 ppm, mean = 625 ppm; n = 249), and generally relate to the abundance of sulphide minerals. Figure 12C is a ternary plot of weight percent values of Cu -Pb - Zn for BH IF samples. Also shown for reference are umbers associated with Cyprus massive sulphide deposits, chemical and clastic tetsusekiei or iron quartz samples associated with the Japanese Kuroko deposits, and exhalites associated with the giant Windy Craggy Cu-Co Besshi-type massive sulphide deposit in northwestern British Columbia. A salient feature is the Pb-rich (and Zn-rich for many samples) and Cu-poor nature of the BH samples. The BH samples are compositionally most similar to chemical tetsusekiei hosted by rhyolites and dacites, although the former are even more Cu poor. The base metal tenor of the BH samples is very different from the Cyprus umbers and Windy Craggy exhalites, both of which are hosted by mafic rather than felsic volcanic rocks. This difference probably relates to the chemistry of the rocks that the hydrothermal fluids have interacted with. On the basis of metal tenor, the BH and Kuroko deposits have derived their contained metals from sialic crustal rocks or derivative intrusive or volcanic rocks.

Within the BH samples, significant positive correlations exist between Zn and Hg and Zn and Fe(t) as shown in Figs. 9B and 9K, respectively. Positive correlations also exist between Pb and Fe203, Fe(t), and P2O5, shown in Figs. 9A, and 9H respectively. These correlations likely indicate coprecipitation of Zn and Pb sulphides with carbonate and apatite. Davies (1972) also noted a strong positive correlation between Zn and Mn in the oxide-predominant IF at Austin Brook.

BH IF contains from 0.5 to 5490 pprn As, mean = 119 ppm, and 0.2 to 250 pprn Sb, mean = 8 ppm. Most hematite- and magnetite-bearing BH IF samples contain low As (less than about 10 pprn); however, chlorite-, siderite-, and sulphide-predominant or -bearing and mixed IF contains higher As. The elevated As contents of these latter samples are in contrast to the generally low average As contents of

the hanging-wall and footwall sedimentary rocks (mean = 19 ppm; n = 22) and clastic sedimentary rocks in general (1 - 13 ppm; Onishi 1978). Arsenic in the BH IF resides in arsenopyrite (Fig. 8D) and is likely of hydrothermal origin.

Arsenic contents average 5 pprn in modern deep-sea sediment (Onishi and Sandell 1955); however, As is enriched in deep-sea sediments proximal to active spreading centres (up to 230 ppm) (Bostrom and Valdes 1969; Bostrom and Peter- son 1969; Marchig et al. 1982). These As enrichments are attributed to hydrothermal sources, followed by its fractionation into amorphous iron phases. More recent work has shown that As is adsorbed onto freshly precipitated Fe - Mn oxyhydroxides and is associated with hydrothermal plume particulates and ridge-crest sediments (e.g., Birolleua et al. 1988; German et al. 1991). As is also enriched in other rocks of hydrothermal sedimentary origin associated with massive sulphide mineralization in exhalites, tourmaline, and garnet- bearing metasediments in the vicinity of the Broken Hill Pb - Zn - Ag orebodies, Australia (Lottermoser 1989), and in the umbers associated with the Troodos Ophiolite, Cyprus (Elderfield et al. 1972).

Hg contents of the BH IF vary between 2.5 and 600 ppb, mean = 52 ppb (n = 249). The positive correlation of Hg with Zn in the BH IF samples (Fig. 9B) indicates both elements are of hydrothermal origin. Direct evidence of hydrothermal Hg has been found on the Mid-Atlantic Ridge, where anomalously high Hg contents occur in the bottom water (Carr et al. 1975). Bignell et al. (1976) noted the presence of Hg (as well as Mn, Cu, and Zn) anomalies in metalliferous sediment up to 10 km from the Atlantis 11 Deep, Red Sea deposits.

The Au content of the BH IF varies from less than the lower limit of detection to 310 ppb, mean = 8.8 ppb (n = 249). Au displays significant positive correlations with Ag (r = 0.488; n = 249), As (r = 0.347; n = 249), Zn (r = 0.293; n = 249), Hg (r = 0.259; n = 249), and S (r = 0.288; n = 249); this may indicate that these elements occur at least partly in solid solution either within native gold or in other sulphides. Mercer (1975) noted positive correlations between Au and Pb and Zn in massive sulphides from the Brunswick No. 12 deposit, and further work by Chryssoulis and Agha (1990) found that Au is present as electrum, native gold, or invisible gold in arsenopyrite and pyrite. The BH IF with the highest Au content also has the highest Fe content and the second highest Mn content. This suggests that Au is of hydrothermal origin. Such a correlation has also been established for modern metalliferous sediments by Bostrom (1973b). Other studies have also noted anomalous Au contents in metalliferous sediments (Crocket et al. 1974; Piper and Graef 1974; Hendricks et al. 1969), whereas non- hydrothermal, normal marine sediments are not anomalous in Au (e.g., Piper 1974; Anoshin et al. 1969).

A1203 contents are highest in chlorite IF and average 10.64 wt.%; in chlorite-bearing IF, d 2 O 3 contents vary from 5.40 to 7.30 wt. %, with averages of 6.60 wt. % in hematite IF, 5.80 wt. % in magnetite IF, 3.31 wt. % in magnetite-hematite IF, 4.41 wt. % in siderite IF, 1.70 wt. % in magnetite - siderite - hematite IF, and 7.03 wt. % for manganiferous sediment. TiOz contents are highest in chlorite IF and average 0.52 wt. %; in chlorite-bearing IF, Ti02 contents vary from 0.29 to 0.36 wt.%, with averages of


0.32 wt. % in hematite IF, 0.32 wt. % in magnetite IF, 0.16 wt. % in magnetite -hematite IF, 0.17 wt. % in siderite IF, 0.11 wt. % in magnetite -siderite -hematite IF, and 0.34 wt . % for manganiferous sediment.

The strong positive correlations between A1203 and Ti02, Zr, Sc, and MgO, shown in Figs. 10G, 10H, 101, and 10K, respectively, and between TiOz and Sc, Zr, MgO, and Al2O3, illustrated in Figs. 10A, 10B, 10E, and 10G, respectively, indicate that these elements are of detrital origin and likely reside in chlorite and clay-sized detritus. Al is also present in other silicates such as chamosite, ripidolite, Al-greenalite, greenalite, and stilpnomelane. The high positive correlation of Zr with MgO, shown in Fig. 10F, indicates its association with detrital ferromagnesian minerals. The high positive correlations of Sc with Ti02, Zr, and Al2O3 (Figs. 10A, 10D, and 101, respectively) indicate that it resides within the detrital aluminosilicate fraction. The negative correlations between Ti02 and Fe(t) (Fig. 10C) and between Zr and Fe(t) (Fig. 10J) likely arise from dilution effects of hydrothermal components.

Significant positive correlations exist between Co and Cr (r = 0.534; n = 249), V (r = 0.470; n = 249), and Ti02 (r = 0.422; n = 249); between Ni and Cr (r = 0.482; n = 249), V (r = 0.408; n = 249); and between V and Ni (r = 0.408; n = 249), Co (r = 0.470; n = 249) and Cr (r = 0.588; n = 249). Figures 10L and 10 M display the strong correlations between Ni and Co and Cr, respectively. Deep-sea ferromanganese nodules of hydrogenous origin are generally enriched in heavy metals such as Ni, Co, Cu, and Zn scavenged from seawater and adsorbed onto Mn hydroxides (Fig. 12A) (e.g, Elderfield et al. 1981; Nohara 1978). It has been suggested by Dymond et al. (1973) that Ni is partly of hydrothermal derivation in modern metalliferous sediments. However, in the Brunswick samples, Ni displays no positive correlations with elements of hydrothermal origin, and on this basis such an origin is ruled out. Rather, Co, Ni, and Cr in the BH samples are likely present in detrital mafic minerals.

Rare earth element geochemistry

The behavior of REE's in natural systems is governed by the systematic contraction of trivalent ionic radii with increasing atomic number and by redox chemistry of Ce and Eu. Thus, the REE's are effective tracers of physical and chemical processes in the solid and fluid Earth. Herein, the REE's are used to measure the contribution of hydrothermal fluids, seawater, and detrital sediment to the BH samples, and to con- strain the redox conditions under which they were deposited. REE compositions of BH IF are presented in Table 1. Since the fine-scale layering in the samples results from multiple precipitation events, the REE analyses represent an averaged record of many events, as no effort was made to collect microsamples of individual laminae.

Figures 13A- 131 shows chondrite-normalized REE distribution patterns of BH IF. Chondrite-normalized patterns for magnetite IF (Fig. 13A), magnetite - hematite IF (Fig. 13B), magnetite - siderite IF (Fig. 13C), siderite IF (Fig. 13D), chlorite -siderite IF (Fig. 13E), and magnetite - chlorite IF (Fig. 13F) display strong enrichment in the LREE, nil to strong negative Ce anomalies, and moderate to

strong positive Eu anomalies (discussed below). The similarity of REE patterns of oxide- and carbonate-predominant IF (Figs. 13A- 13F) indicates that the effects of the crystallographic control, in the sense of Morgan and Wandless (1980), on the REE distribution are insignificant. Hydrother- mal sulphides are unable to accommodate significant REE contents (e.g., Campbell et al. 1981; Graf 1977; Robertson and Fleet 1976). Hydrothermal carbonates are known to contain significant REE (e.g., Kontak and Jackson 1995; Moller et al. 1984, 1979). Therefore, the residence sites of REE's in the BH samples must be in carbonate minerals and (or) in other nonsulphide phases that are present in minor to trace amounts (e.g . , monazite, sphene, apatite, allanite, chlorite, etc.).

Samples with the most chlorite (magnetite-chlorite IF, Fig. 13F; chlorite IF, Figs. 13H, 131) commonly do not have a negative Ce anomaly. Indeed, only one sample of chlorite IF displays a positive Eu anomaly and the others have negative Eu anomalies, or no anomaly. The REE patterns for quartz -pyrite stockwork (sample POA-056), sulphide - chlorite IF (sample POA-093), and recrystallized, silicified pyrite- "chert" IF (sample POA-037) are shown in Fig. 13J. The patterns for these are topologically similar, except for the slightly greater light rare earth element (LREE) enrichment of the former and the negative Ce anomaly of the latter. The REE patterns for volcanic, volcaniclastic, and sedirnen- tary host rocks (Figs. 13K, 13L) show LREE enrichment, no appreciable Ce anomalies, and moderate to strong negative Eu anomalies. For comparison, the REE patterns for North American shale composite (NASC) and post-Archean Aus- tralian shale (PAAS), average shale compositions from the two continents, are shown in Fig. 13M. PAAS is an average of 35 shale analyses from mainly cratonic sequences of Aus- tralia (Nance and Taylor 1976) and is similar to NASC. Most IF samples (except chlorite IF) have lower REE contents than NASC and PAAS and the host sedimentary rocks.

Positive Eu anomaly Eu occurs in solution in either the 2 + or 3 + valence state, depending on the redox conditions. The Eu anomaly (EuIEu*) is defined here using the expression of McLennan (1989):

Eu - -- Eun Eu* (Smn x Gdn)0.5

where n is the NASC normalizing value, a mean of composite shales from North America (Gromet et al. 1984). EuIEu* for BH IF samples varies between 0.29 and 11.80.

REE patterns for some of the magnetite IF (Fig. 13A), the magnetite - hematite IF (Fig. 13B), and two highly silicified quartz-pyrite samples collected from what is interpreted to be the feeder zone to the overlying massive sulphides of the Brunswick No. 12 deposit (Luff et al. 1992) display the least LREE enrichment of the BH samples (Fig. 13J). These samples are topologically very similar to modem submarine hydrothermal vent fluids from basalt- and sediment-domi- nated spreading centres (Figs. 14A, 14B) in that they have positive Eu anomalies, although they have higher overall REE abundances. Fouquet et al. (1993) report REE patterns for end-member hydrothermal fluids from the Vai Lili site in

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Fig. 13. Chondrite-normalized rare earth element plots of Brunswick Horizon and host rocks samples. (A) Magnetite IF. (B) Magnetite - hematite IF. (C) Magnetite - siderite IF. (D) Siderite IF. (E) Chlorite - siderite IF. (F) Magnetite - chlorite IF. (G) Chlorite-siderite-magnetite IF. (H, I) Chlorite IF. (J) Quartz-pyrite stockwork material from the Brunswick No. 12 deposit and sulphide-chlorite IF. (K, L) Host volcanic (rhyolite), volcaniclastic (rhyolite tuff, crystal tuff), and hanging-wall and footwall sedimentary rocks. (M) North American shale composite (NASC) (Gromet et al. 1984) and post-Archean Australian shale (PAAS) (Nance and Taylor 1976). (N) Mixing calculations between POA-125, a typical footwall sedimentary rock sample, and POA-056, Brunswick No. 12 quartz-pyrite stockwork, believed to best represent the end-member hydrothermal fluid.

the Lau Basin, situated in a back-arc rift floored by volcanic rocks of intermediate to felsic composition. They do not report actual analytical concentrations; however, their patterns are similar to those reported for hydrothermal fluids from MORB settings. Given the differences in the REE composition of typical MORB's, such as those that underlie the 21 ON EPR hydrothermal site, and felsic volcanic rocks that underlie the Brunswick No. 12 deposit (e.g., Lentz and Goodfellow 1992b), the similarity of REE patterns for Brunswick No. 12 IF's to modern hydrothermal vent fluids suggests that the REE patterns of hydrothermal vent fluid are not controlled by the composition of the host rocks. A significant difference between the REE patterns of the BH IF and modern vent fluids is that the former have Ce depletions with respect to the adjacent REE that are absent in modem vent fluids but are characteristic of seawater (Fig. 14C); the significance of this is discussed below.

In an earlier REE study, Graf (1977) found positive Eu anomalies in IF samples from the Brunswick No. 6 and Austin Brook areas. He argued that these anomalies origi- nated from hydrothermal solutions that had reacted with underlying volcanic rocks. He also suggested that during such reactions, solutions became reduced and enriched in Eu because of preferential alteration of feldspar, which is anomalously enriched in Eu. These conclusions were con- firmed by recent work on hydrothermal solutions that have reacted with volcanic rocks at high temperature (Campbell et al. 1988; Michard et al. 1983). The Eu3+/Eu2+ redox potential is strongly dependent on temperature, is slightly dependent on pH, and is almost unaffected by pressure; thus, at high temperatures, the reacted fluids become reduced, and this favors the stability of Eu2+, thus preserving the Eu anomalies (Sverjensky 1984). Even at oxygen fugacities close to the hematite-magnetite buffer, temperatures of more than 25°C are necessary to stabilize considerable amounts of divalent Eu (Bau 1991), which is a prerequisite for the fractionation of Eu from the trivalent REE's (i.e., formation of a positive Eu anomaly). In summary, positive Eu anomalies are most likely to develop during fluid-rock interaction at high temperatures under reducing, slightly acid conditions.

Positive Eu anomalies are absent in deep-sea sediments from modem spreading ridges and Deep Sea Drilling Project sites in the eastern Pacific (Figs. 14D, 14E), in exhalites associated with the Kuroko deposits of Japan and the coticules (Mn-garnet-bearing sediments) of west Georgia (Fig. 14F), the exhalites of the giant Broken Hill Pb-Zn deposit in Australia (Fig. 14G), and the ochres and umbers associated with the Cyprus copper deposits (Fig. 14H). Clearly, this anomaly is not preserved in modern and ancient oxidized oceans because of the rapid oxidation of hydrother-

mal Eu2+ to Eu3+ in the water column. Any precipitates formed also strongly adsorb seawater REE's that lack a positive Eu anomaly.

However, in some environments positive Eu anomalies are preserved. Positive Eu anomalies have been found in modem seafloor massive sulphides at 21°N, EPR (Bence 1983), and REE patterns similar to the Brunswick IF's have been reported by Barrett et al. (1990) for recent massive sulphide - sulphate - silica samples from the Southem Explorer Ridge. Silica- barite-rich crusts, precipitated at about 100°C and having REE patterns similar to those of some of the BH IF samples analyzed in this study, were interpreted by Alt (1988) as products of a 13"N East Pacific Rise-type fluid, which had already deposited sulphides beneath the seafloor. Furthermore, metalliferous muds from the Red Sea deeps also show positive Eu anomalies and variably developed negative Ce anomalies (Calvez et al. 1988; Courtois and Treuil 1977) (Fig. 141). Most Archean IF's are also characterized by the enrichment in Eu relative to other REE's (e.g., Alibert and McCulloch 1993; Bau and Moller 1993; Derry and Jacobsen 1990). REE patterns for several Precambrian IF are shown in Fig. 14J.

Figures 15A - 15F are plots of EuIEu* versus various elements and element ratios for BH IF. Figure 15A clearly illustrates the strong positive correlation with the FeITi ratio, an indicator of hydrothermal input (Bostrom 1973a) (Fig. 11B). Furthermore, there are also good correlations between EuIEu* and P205, Sr, Pb, Zn, and Sb (Figs. l5B- l5F). These correlations indicate scavenging of Eu by precipitated apatite.

Negative Ce anomaly The REE patterns shown in Fig. 13 illustrate the moderate to strong negative Ce anomalies in certain IF samples. The degree of Ce enrichment or depletion with respect to adjacent REE, or the Ce anomaly (CetCe*), has been expressed by Toyoda and Masuda (1991) as

Ce - -- 5Ce,

Ce* (4Lan + Sm,)

where n is the NASC normalizing value, a mean of composite shales from North America (Gromet et al. 1984). CeICe* ranges between 0.48 and 0.96 for the BH IF. Nega- tive Ce anomalies have CeICe* less than 0.9; a value between 0.9 and 1.1 is not considered to be a negative Ce anomaly.

Ce3+ is oxidized in the oceans to Ce4+ (Elderfield and Greaves 1982; Goldberg 1961; Goldberg et al. 1963). Under oxidizing conditions, Ce4+ is highly insoluble at near- neutral to high pH and is preferentially incorporated or


0.00001 1 end-member hydrothermal vent fliiid

1

Broken Hill exhalite

10°S, East Pacifii Rise rn 4 6 , East Pacific Rise

Kuroko deposit "tetsusekiei" 100 west Georgia coticules

', Cyprus ochres and umbers

Adarns Mine, Midipicoten, & Hamersley IF

1 ° i 1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Trn Yb Lu

R l l l r ~ r ~ l r l l l l l ~ l l

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Trn Yb LU


Fig. 14. Chondrite-normalized rare earth element plots for (A, B) end-member hydrothermal vent fluids from several sites in the modem oceans (13"N and 21°N, East Pacific Rise, Trans-Atlantic Geotraverse site, Mid-Atlantic Ridge, and Guaymas Basin, Gulf of California (Klinkharnmer et al. 1994; Michard and Albarede 1986)); (C) surface and deep modem seawater from the Atlantic and northwest Pacific oceans (De Baar et al. 1985; Hogdahl et al. 1968); (D, E) deep-sea sediments from the East Pacific Rise at 21°S, 13"S, 10°S, 4"S, and 2"N, Galapagos Rift, Carlsberg Ridge, Central Indian Ocean, Lau Basin, and the Red Sea (Kunzendorf et al. 1988); (F) "tetsusekiei" or fermginous chert associated with Japanese Kuroko deposits (Kalogeropoulos and Scott 1983) and coticules (Mn gamet-bearing sedimentary rocks) from west Georgia (Wonder et al. 1988); (G) exhalites associated with the giant Broken Hill Pb-Zn deposit in Australia (Lottermoser 1989); (H) ochres and umbers (exhalites) from Cyprus-type massive sulphide deposits, Cyprus (Robertson and Fleet 1976); (I) metalliferous muds from the Red Sea (Courtois and Treuil 1977); and (J) iron formation from the Adams mine and Michipicoten area in Ontario and the Hamersley basin, Australia (Graf 1977, 1978).

Fig. 15. Bivariate plot of EuIEu* versus selected elements and element ratios for Brunswick Horizon iron formation samples from the Brunswick No. 12 deposit area: (A) FeITi; (B) P205; (C) Sr; (D) Pb; (E) Zn; and (F) Sb.


Fig. 16. Bivariate plot of CeICe* versus selected elements and EuIEu* for Brunswick Horizon iron formation samples from the Brunswick No. 12 deposit area: (A) Fe(t); (B) CaO; (C) P205; (D) A1203; (E) Co; and (F) EulEu*.

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0

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adsorbed in octahedral sites of precipitates or as Ce02 on the surfaces of grains in bottom sediments (Koeppenkastrop and De Carlo 1992). It also precipitates as a colloidal hydroxide unlike trivalent REE and is rapidly incorporated in Mn nodules relative to the other REE's in the 3 + state (see discussions by Liu et al. 1988; McLennan 1989; and Elder- field et al. 1981). Mn nodules and Fe -Mn crusts are, therefore, significantly enriched in Ce relative to the trivalent REE (Elderfield and Greaves 1981). Other REE's are in the 3 + state and are lost from solution without discernible fractionation of other individual REE's. This results in depletion of normal seawater in Ce relative to the other REE's (Hogdahl et al. 1968; Masuda and Ikeuchi 1979) (Fig. 14C).

Numerous studies have

I

shown that authigenic sediments accurately reflect the relative Ce and Eu ahndances of the waters from which they formed (e.g . , Piper 1974; Shimizu and Masuda 1977), and negative Ce anomalies such as those in seawater are also found in modern pelagic clays (Elder- field and Greaves 1981; Shaw and Wasserburg 1985) and in modem deep-sea and metalliferous sediments (Dymond et al. 1973; Piper and Graef 1974) (e.g., Figs. 14D, 14E, 141).

Stockwork material from the Brunswick No. 12 deposit lacks a negative Ce anomaly (Fig. 13J) and in this respect is similar to reduced modern seafloor hydrothermal vent fluids (Michard et al. 1983). On mixing with oxidized seawater, modern hydrothermal fluid precipitates Fe -Mn oxides and


hydroxides and apatite. These minerals are shown to be highly efficient scavengers of REE from the ambient seawater without any preferential scavenging of Ce4+ (Ruhlin and Owen 1986). Olivarez and Owen (1991) have shown that the rate of REE scavenging from seawater by the hydrothermal precipitates is proportional to the REE concentration of the vent fluid. Thus, the original REE pattern of hydrothermal precipitates will be modified, depending on the degree of mixing with seawater and length of time of exposure to seawater.

The strong negative correlations between CeICe* and Fe(t) (Fig. 16A), CaO (Fig. 16B), and P2O5 (Fig. 16C) reflect varying degrees of mixing between detritus lacking a Ce anomaly and hydrothermal and hydrogenous components. This interpretation is supported by the high positive correlation between CeICe* and A1203 (Fig. 16D). A positive correlation between CeICe* and Co (Fig. 16E) indicates that Co cannot have been scavenged from seawater; rather, it likely is present in detrital mafic minerals. The process of Ce scavenging from the ambient water column by hydrothermal carbonate and apatite cannot be ruled out, however. A correlation between the Ce anomaly and phosphate content has been observed in modern Pacific sediment by Toyoda et al. (1990).

Elderfield and Greaves (1982) found that the large Ce deficiency in seawater is not uniform throughout the water column but it is highest in bottom waters and decreases to the surface. They suggested that either Ce is increasingly depleted by sedimentation on the ocean margins or less Ce is available at the bottom. More recent work by De Baar et al. (1988) has shown that deep anoxic ocean waters in the Cariaco Trench have normalized Ce contents similar to the neighboring LREE's, apparently due to dissolution of Ce under reducing conditions. Shimizu and Masuda (1977) noted that a significant negative Ce anomaly is characteristic of radiolarians from the open-ocean environment of the Pacific, but this anomaly is absent in radiolarians from the continental shelf and marginal basins. Based on the presence of Ce depletions in deep-sea cherts, Shirnizu and Masuda (1977) suggested that Ce is depleted in the waters of open ocean but not those of shallow seas. Fleet et al. (1980) specu- lated along similar lines about glauconites they had analyzed. These authors concluded that Ce depletions in the waters of open oceans and the lack thereof in shallow water is due primarily to the redox conditions of the waters.

The presence of the low CeICe* values in the BH IF can only result from the intrinsic redox behavior of Ce and must be derived from the precipitation of some insoluble Ce(1V) phase elsewhere in the ocean basin in which the BH was deposited. The presence and magnitude of the CeICe* values for BH IF suggest they interacted with somewhat oxidizing seawater some time in their history. Magnetite IF has the largest negative Ce anomalies (mean CeICe* = 0.51), whereas siderite IF has smaller such anomalies (mean CeICe* = 0.60), indicating more reducing conditions for the latter samples. Precambrian carbonate IF from Isua, West Greenland, has REE patterns with pronounced negative Ce anomalies and positive Eu anomalies (Dymek and Klein 1988), as do Proterozoic siderite IF from the Transvaal, South Africa (Klein and Beukes 1989). In this respect, these patterns are similar to BH siderite-predominant IF, and the

authors also attributed these patterns to mixing with ambient seawater.

Detrital signature in the REE patterns LREE enrichments are typical of sediments and continental crustal material. Negative Eu anomalies in the tuffaceous sedimentary rocks at Brunswick No. 12 (Figs. 13K, 13L) are the result of the absence or removal of plagioclase that is high in Eu. Compared with NASC, the sedimentary host rocks of the BH have slightly to moderately pronounced negative Eu anomalies. The REE pattern of a particular BH IF sample is extremely dependent on the amount of clastic contamination, since REE abundances may be more than two orders of magnitude higher in clastic detritus than in seawater precipitates (e.g., Parekh et al. 1977).

From the FeITi versus Al/(Al + Fe + Mn) plot (Fig. 1 lB), it is apparent that there is a variable component of detrital sediments present in most of the IF samples. Strong evidence for a major detrital component is also apparent in REE patterns of some of the samples. Some IF samples, particularly chlorite-predominant samples, display LREE enrichment (Figs. 13H, 131) characteristic of continental detritus (e.g., NASC; Fig. 13M). Furthermore, most chlorite IF samples display moderate to strongly negative Eu anomalies, which would indicate little incorporation of hydrothermal precipitates. Most of the IF samples display moderate to strong negative Ce anomalies, and a few of the chlorite-predominant IF samples also show negative Ce anomalies, whereas some BH samples display no Ce negative anomalies, and (or) nil to a negative Eu anomaly (e.g., chlorite IF; Figs. 13H, 131).

To evaluate the influence of detrital sediment incorporated in the IF samples, a simple mixing calculation was per- formed. Figure 13N shows linear mixing between sample POA-125, typical footwall sediment, and POA-056, quartzose and pyritic stockwork from the Brunswick No. 12 deposit that is believed to best represent the REE pattern of endmember hydrothermal solutions. Because of the strong negative Eu anomalies in the host sediments, masking of a positive Eu anomaly in the IF samples is possible by the incorporation of detritus. Inspection of Fig. 13N reveals that greater than about 30% sediment would completely mask the hydrothermal positive Eu anomaly. The presence of the negative Ce anomaly in the BH IF REE patterns requires the interaction of ambient seawater. The Brunswick IF REE patterns therefore reflect contributions from hydrothermal fluid (and precipitates), detrital sediment, and ambient seawater.

In one transect of samples collected stratigraphically upwards through the IF horizon at the Brunswick No. 12 deposit, there is a general systematic increase in the CeICe* ratio from about 0.48 at the base to about 0.9 near or at the top (Fig. 17A) and a systematic decrease in the EuIEu* ratio from about 5 at the base to about 0.7 near or at the top (Fig. 17B). Furthermore, there is an inverse correlation between the magnitude of CeICe* and EuIEu* (Fig. 17C). Such a concomitant upward decrease in EuIEu* and increase in CeICe* cannot be explained by changing redox conditions. Such variations are readily explained, however, by the steadily upward increasing proportion of detritus lacking negative CeICe* and positive EuIEu* within the IF. This is supported by the upward increases of A1203 and TiOz (not


Fig. 17. Geochemical profile through the Brunswick Horizon iron formation associated with the Brunswick No. 12 deposit. Diamond drill hole 12-2590, East Ore Zone, Section 22N. (A) CeICe* versus depth. (B) EulEu* versus depth. (C) CelCe* versus EuIEu* for these samples.

and adjacent sedimentary rocks are strong evidence for closed-system behavior of the REE, because REE mobility during diagenesis or metamorphism would have resulted in homogenization of the REE patterns.

Hanging wall

I-

68 - Hanging wall

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54 - Stratigraphic tops

52 . . . . ,

shown). In effect, these variations record the decay of hydrothermal activity against relatively constant sedimentation, assuming sedimentation rates remained constant during IF deposition.

Preservation of the primary rare earth element signature Dehydration, decarbonatization, and metasomatism may have altered the original chemical composition of the BH samples. However, the different REE patterns (CeICe* and EuIEu* anomalies) for samples from the same horizon and for BH IF

Model for element dispersal in the Brunswick Horizon

The BH IF was deposited predominantly by chemical sedimentation, either from a buoyant hydrothermal plume or a dense brine. Relative base metal abundances in the BH IF indicate that these metals were likely leached from the underlying host rocks. Many drill holes in the immediate vicinity of the Brunswick No. 12 deposit failed to intersect IF; this may be a primary depositional feature resulting from the presence of multiple hydrothermal vent sites that deposited mineralogically distinct IF over relatively limited lateral distances, or it may be due to complications arising from folding and faulting that occurred during structural deformation. Boundaries between IF and footwall rhyolite tuffs and hanging- wall sedimentary rocks are typically gradational and only rarely sharp, and suggest that detrital sediments were deposited during chemical sedimentation. The abundance of nongraded laminations and bedding in the IF indicates that turbidity currents were infrequent.

On the modem ocean floor, hydrothermal vents, a commonly used analogue for ancient massive sulphide deposits, give rise to predominantly buoyant plumes of warm water, which form as the hydrothermal solutions are vented into ocean-bottom waters. A direct result of the mixing of the hot solutions rich in dissolved metals and H2S with cold, alkaline seawater is the precipitation of a cloud of fine-grained polymetallic sulphide minerals, which has the appearance of black smoke. The particles eventually settle to the ocean floor either as unaltered minerals or oxides. Under the oxidizing conditions of the modern oceans, these particles are largely oxidized, and no significant accumulation of hydrothermal precipitates builds up. Based on sulphur isotope compositions of Brunswick No. 12 massive sulphides (Goodfellow and Peter 1996) and trace sulphides in carbonaceous sedimentary rocks beneath the deposit (W.D. Good- fellow and J.M. Peter, unpublished data), as well as the presence of abundant siderite-predominant IF, the sulphide deposits and IF of the BH were formed from minerals precipitated from reduced hydrothermal fluids vented into a stratified marine basin.

Attempts were made to interpret the BH in the context of the model of James (1954). In a few places the stratigraphy is broadly similar (i.e., massive sulphide -, siderite +

magnetite + chlorite). However, because of the complex interlamination of monomineralic bands of magnetite, carbonate, and chlorite (e.g . , Fig. 7) and the absence of one or several of these intervals, the divisions of James (1954) are of limited use. Furthermore, the influence of depth as the controlling factor on the different facies of IF, as proposed by James (1954), has been discounted by many workers. Complex mineralogic relationships probably result from a number of factors, including fluctuating physicochemical conditions of mineral precipitation as governed by paleo- redox changes in the local basin of sulphide deposition and more regionally; overprinting of primary minerals by metamorphic assemblages; and structural deformation. Varying


paleo-redox conditions under which the IF was deposited are indicated by the general disposition of abundant carbonate (siderite) IF and the absence of hematite IF at the Brunswick No. 12 deposit (i.e., reducing), versus the preponderance of magnetite-predominant IF, which grades into hematite- bearing IF at the Austin Brook and Brunswick No. 6 deposits (less reducing).

The apparent enigma of the presence of negative Ce anomalies (implying oxidizing conditions) in siderite- predominant IF (siderite is indicative of deposition under reducing conditions) has not, to our knowledge, been addressed in the literature. Garrels and Christ (1965) established the stability relations in the system Fe -0 -H20 - S-C02 at 25°C. Siderite has a very large stability field at lower temperatures, but this decreases markedly at higher temperatures (Frost 1979), where siderite has a very limited stability in Eh-pH space and can only occur in unusual environments at high pco2, low dissolved sulphur, and reducing conditions. The stability field for siderite is increased as pcq increases (Nordstrom and Munoz 1986). Precipitation conditions for the BH IF in the vicinity of the Brunswick No. 12 deposit must have been such that siderite was precipitated rather than pyrite, magnetite, or hematite under redox conditions, which allowed for the formation of a negative Ce anomaly. Unfortunately, we have no evidence to estimate what pcq conditions were; furthermore, there are no available data to evaluate the aqueous stability of Ce, although Ce3+ is thought to be favored with increasing temperature and Ce4+ with increasing pressure (Wood 1990). Based on alteration mineral assemblages, the fluids that formed the Brunswick No. 12 massive sulphide deposit are thought to have been acidic (Lentz and Goodfellow 1993). With waning hydrothermal activity, fluids may have become cooler, more alkaline, and possibly more C02 rich, such that the precipitation of minerals such as siderite were favoured. Mn is fixed predominantly in carbonate minerals within the BH IF, particularly near the Brunswick No. 12 deposit, and also in garnet near the Austin Brook and Bruns- wick No. 6 deposits. Mn was also likely precipitated far from the massive sulphides in sediments at the margins of the basin under the oxidizing and more alkaline conditions.

Geothermometric estimates based on sulphur isotope compositions of coexisting mineral pairs and the arsenopyrite composition of massive sulphides in the Brunswick area give temperatures between 350 and 425°C; geobarometric studies using the chemical composition of coexisting sulphide phases give pressures between 4.5 and 5.5 kbar (1 kbar = 100 MPa) based on the iron content of ore sphalerite (e.g., van Staal 1985). These temperatures and pressures were attained during dynamic metamorphism (van Staal 1985) and have resulted in recrystallization of existing minerals and formation of new metamorphic assemblages within the BH IF. Considerable recrystallization of magnetite has taken place (e.g . , Fig. 7A), together with diffusion of iron, to produce the clustered occurrence of many magnetite euhedra. How- ever, this has taken place over only very small distances, likely on the order of a few millimetres or less. Furthermore, the occurrence of sharp, alternating, relatively monomineralic layers of siderite and magnetite (e.g . , Fig. 6E) indicate that magnetite likely did not form by oxidation of Fe-bearing carbonates (e.g., LaBerge 1964; Lougheed 1983) from secondary fluids flowing across layers. Rather, the iron

oxides were either directly precipitated as proposed by many authors (e.g., Gole and Klein 1981; Kimberley 1989), or were formed during diagenetic transformation of precipitated ferric hydroxide (e.g., Morris 1985, 1987).

Conclusions The BH IF contains sulphide-, carbonate-, oxide- and silicate- predominant intervals and is composed predominantly of Si, C02, Fe, Mn, and Ca with lesser Mg, P, Ti, Al, and S. The bulk compositions are influenced by hydrothermal elements such as Eu, Fe, Mn, Pb, Zn, Cd, Au, Ca, Sr, Ba, P, C02, and S, which were precipitated from hydrothermal fluids vented onto the seafloor, elements originating largely in detrital clastic mafic and aluminosilicate minerals (e.g., Si, Ti, Al, Mg, K, Zr, REE's except Eu, Sc, V, Y, Yb, Co, Ni, Cr), and hydrogenous Ce. Felsic volcanic and pyroclastic rocks are considered to be the source for the detritus. REE patterns of IF at Brunswick No. 12 display similarities with those of modem hydrothermal vent fluids, seawater, and host rhyolitic tuff and sedimentary rocks; REE patterns are controlled predominantly by the relative proportions of detrital and hydrothermal and hydrogenous components. Systematic variation in the REE patterns of samples collected from one vertical transect through the BH IF shows an upward increase in clastic detritus and indicates the cessation of hydrothermal activity.

The morphologies and textures of the various sulphide deposits, carbon-rich nature of sedimentary host rocks beneath some of the deposits, sulphur isotope composition of the massive sulphides as well as trace sulphides in the sedimentary host rocks, and mineralogy of the IF indicate that the massive sulphides and IF were precipitated from a hydrothermal fluid that vented into a stratified marine basin with a relatively reduced water column. The widespread extent of the BH IF indicates that mineral precipitates were dispersed far from the sites of venting and massive sulphide accumulation.

Acknowledgments

This study was conducted under the Canada - New Bruns- wick Cooperation Agreement on Mineral Development 11, 1990- 1995. Additional funding also came from Brunswick Mining and Smelting Ltd. (BM&S) and the Geological Survey of Canada under an Industrial Partners Program project. We thank BM&S, in particular Bill Luff, for provid- ing logistical support necessary for the successful completion of this project. The New Brunswick Geological Surveys Branch provided logistical support. We thank Cees van Staal, Elizabeth Anderson, Gordon Gross, David Lentz, and Steven McCutcheon for discussions and assistance. Peter Belanger and Gwendy Hall facilitated bulk analyses. Gilles St. Jean provided stable isotope analyses. We thank David Lentz and Cees van Staal for reviewing preliminary versions of this manuscript. Comments by journal reviewers Scott Swinden and Robert Whitehead helped us to clarify our argu- ments and be more concise.

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Appendix

Table Al. Descriptions of bulk geochemistry samples.


Whitehead, R.E.S., and Goodfellow, W.D. 1978. Geochemistry of volcanic rocks from the Tetagouche Group, Bathurst, New Brunswick, Canada. Canadian Journal of Earth Sciences, 15: 207-219.

Wonder, J.D., Spry, P.G., and Windom, K.E. 1988. Geochemistry and origin of manganese-rich rock related to iron formation and sulfide deposits, western Georgia. Economic Geology, 83: 1070- 1081.

Wood, S.A. 1990. The aqueous chemistry of the rare-earth elements and yttrium. 2. Theoretical predictions of speciation in hydrothermal solutions to 350°C at saturation water vapor pressure. Chemical Geology, 88: 99- 126.

Sample No. DDH m Location Description

POA-I01

WA-114

POA- 125

POA-148

POA-158

12-2590

12-1277

Float

Outcrop

Outcrop

12-1277

12-1277

A-247

A-246

A-246

A-250

A-257

Underground drill hole, Brunswick No. 12, east ore zone, 22N, 475 level; 5262457.79N 282235.24E

Underground drill hole, Brunswick No. 12; 5262016.72N 281735.58E

Float from Brunswick No. 6 road; 5253430.00N 286900.00E

Austin Brook iron mine outcrop locality; 5253062.84N 28672 1.78E

Austin Brook iron mine outcrop locality; 5254434.83N 287064.26E

Underground drill hole, Brunswick No. 12, 5262016.72N 281735.58E


Surface drill hole, Brunswick No. 12; 5263031.51N 281682.23E


Surface drill hole, Brunswick No. 12; 526322 1.48N 282482.47E



Laminated siderite-chert iron formation; predominantly interlaminated siderite and chert; laminations range from sub-mm to - 1 cm; with very thin, < 1 mm dark grey-green chlorite foliae between individual siderite and chert laminae; contains trace 1 - 2 mm diameter cubic pyrite porphyroblasts

Transition zone from rhyolite tuff to chlorite iron formation; very fine grained, deep green, with < 20% lighter colored siliceous laminae

Very fine grained, laminated magnetite - hematite - chert iron formation; composed of finely laminated magnetite and 5 - 10 mm wide hematite-stained chert laminae

Magnetite iron formation

Laminated magnetite-hematite iron formation; Canmet reference standard FeR-2 from GSC paper 83-19 (Abbey et al. 1983)

Folded, fine- to medium-grained chlorite - sierite iron formation; magnetite absent

70 cm wide intersection of soft, foliated, deep green, chloritic tuff; contains sphalerite -galena- pyrite; both contacts are gradational

Well-laminated to -bedded siderite - chert iron formation; contains minor pyrrhotite as fine-grained disseminations along select laminae in some areas

Finely laminated magnetite - siderite iron formation

Fine-grained, grey-green, finely laminated to bedded footwall sediment with minor finely disseminated 1-2 mm diameter, porphyroblastic pyrite cubes

Laminated chlorite - siderite iron formation; transition zone from underlying siderite iron formation and overlying chlorite iron formation

Well-laminated, dark grey to black, very fine grained magnetite iron formation; contains abundant chert, siderite, and minor pyrite and pyrrhotite


Table A1 (concluded).

Sample No. DDH m Location Description

POA- 187

POA-208

POA-222

POA-223

POA-254

POA-334

POA-400

POA-423

POA-45 1

POA-455

POA-474

POA-557

A-266-1

B-248

B-246

B-246

6- 175

C-005

229-2

B-72

Outcrop

Outcrop

B-075

6- 169



Surface drill hole, Brunswick No. 6; 5254825.07N 28691 l.6E



Surface drill hole, Austin Brook; 5254209.3 1N 28883 1.46E

Surface drill hole, FAB Zone; 5258457.76N 286946.84E

Surface drill hole, Austin Brook; 5253022.79N 286832.66E

Reids Brook, near confluence with Nine Mile Brook; 5253950.00N 288650.00E

Nine Mile Brook, near confluence with Neipisiguit River; 5252900.00N 288700.00E

Surface drill hole, Austin Brook; 5253332.N 28.6803.77E


Well-laminated magnetite-siderite iron formation; magnetite far more abundant than chert and siderite; some sand-sized grains and several - 1 cm wide foliated fragments of predominantly siderite

Magnetite-chlorite iron formation with minor siderite foliated veins ( < 3 %); sharp contact with hanging-wall rhyolite

Finely laminated chlorite iron formation; contains trace 1-2 mm siderite porphyroblasts

Finely laminated magnetite-chlorite iron formation

Finely laminated, intensely folded chlorite - siderite - magnetite chert iron formation

Finely laminated chlorite - siderite -magnetite iron formation, with abundant magnetite

Finely laminated chlorite-pyrrhotite iron formation; trace to 1 % pyrite and pyrrhotite along select laminae or foliae

Fine-grained, foliated hanging-wall tuff

Red, hematitic siltstone-mudstone with minor magnetite in places; Caribou Horizon

Hematitic chert or jasperite; Caribou Horizon

Finely laminated magnetite - siderite - hematite iron formation; magnetite predominant, with lesser siderite, hematite, and chert

Magnetite - siderite - hematite - chert iron formation; composed of finely interlaminated, relatively monomineralic layers; contains specularite

Notes: UTM northings and eastings are given for drill hole collars only; DDH, diamond drill hole; m, downhole meterage.

peter & goodfellow 1996 brunswick belt exhalites

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