1 bridget suite - geoscience australia · 2020. 7. 8. · mosquito creek formation. the formation...

1 BRIDGET SUITE

1.1 Timing 1730 Ma

1.2 IndividualAges

Primary Ages:

1. Par nell Quartz Monzonite 1731 ± 14 Ma, Rb-Sr

Source: Collins et al. 1988.

1.3 RegionalSetting

The Bridget Suite includes small stocks of granite ranging in composition fromhornblende monzogranite to quartz monzonite, and associated hornblende porphyrydykes. The suite forms a north-northwest trending belt in the Bamboo Creek area of the Archaean Pilbara Block of Western Australia. It intrudes the Mosquito CreekFormation, Mount Bruce Supergroup, Warrawoona and Gorge Creek Groups. It is theonly known Proterozoic granite suite in the Pilbara Block.

The Bridget Suite is named after the Bridget Adamellite, and includes the ParnellQuartz Monzonite and several other unnamed plutons.

1.4 Summary The suite is Sr-undepleted, Y-depleted (which is rare for Australian Proterozoicgranites), fractionated, and shows minor alteration. It has a well-developed contactaureole, and dykes of aplite, granophyre and pegmatite are common. It intrudesArchaean sediments and mafic to intermediate volcanics, some of which are potentialhosts to mineralisation.

1.5 Potential Geochemcially the suite appears to have some potential, but it must be noted that this isbased on a small (possibly unrepresentative) sample set. The suite also intrudes hostrocks with some potential for being reactive to an oxidised fluid. There are numerousquartz-vein gold deposits within the vicinity of the suite, but these appear to be syn-deformational (and earlier than the Bridget granites) and are also more widespreadthan exposures of the granite, suggesting that these granites are not the cause of themineralisation. Further work is required on both the granites and the nearby golddeposits; the low potential assigned to the suite is based on inadequate data.

Cu: LowAu: LowPb/Zn: LowSn: LowMo/W: LowCon fi dence Level: 111

1.6 DescriptiveData

Location: Regional mapping in the Pilbara Block (Hickman and Lipple 1975; Hickman 1983)outlined a north-northwest trending belt of Proterozoic hornblende monzogranite and quartzmonzonite. This belt occurs between Bamboo Creek and Windywindina Creek on theNULLAGINE and YARRIE 1:250 000 sheet areas.

Dimensions and area: This suite occurs over a strike length of between 140 and 160 km, andhas a width of about 25 km. The individual plutons have a small outcrop area, the largest beingthe Bridget Adamellite which is 7 x 2.5 km.

1.7 Intrusives Component plutons: Bridget Adamellite (Hickman 1978), Parnell Quartz Monzonite (Collinset al. 1988), several other unnamed intrusives marked as Pgh and Pph on NULLAGINE(Hickman 1978) and YARRIE (Hickman et al. 1983).

© Geoscience Australia 2001 Proterozoic Pilbara Block 1.1

Form: Most of the bodies are elongate, suggesting preferential intrusion along a pre-existingfault. The Parnell Quartz Monzonite shows primary ‘magmatic’ foliation (Collins et al. 1988).

Metamorphism and Deformation: In places throughout the suite, large irregular plagioclasecrystals have sericitised cores, and biotite is replaced by chlorite.

Dominant intrusive rock types: Hornblende monzogranite, quartz monzonite.

Colour: Pinkish-grey, greyish, brick red.

Veins, Pegmatites, Aplites, Greisens: Felsic microgranite dykes crop out sporadicallythroughout the Parnell Quartz Monzonite; they rarely exceed 1 m in width and several tens ofmetres in length. These dykes are typically saccharoidal in texture and contain distinct evenlydispersed hornblende crystals <5 mm in length. Aplitic dykes radiate from the northern andsouthern ends of the pluton. Like the microgranite dykes, they are approximately 1 m wide, butare continuous over several hundred metres. They also contain granophyric phases which grade into pegmatite containing stellate clusters of muscovite. These dykes also occur elsewherethroughout the rest of the suite.

Distinctive mineralogical characteristics: Plagioclase is the dominant mineral, followed by K-feldspar, quartz, hornblende and biotite. Accessories include apatite, titanite and magnetite,with rare zircon and allanite. Trace amounts of fluorite are present within the inner core of theParnell Quartz Monzonite.

Four lithological zones within the Parnell Quartz Monzonite are observable in the field (Collinset al. 1988). From margin to core these are:

(i) A relatively fine (<1 mm) even-grained mafic margin that is pinkish-grey in colour andcontains distinctive small mafic clots and hornblende crystals. A weak to moderate, steeplydipping primary foliation defined by hornblende prisms is subparallel to the contact. Themarginal phase reaches 150 m in width at the southern end of the body, is present as a narrowstrip in the north, but is truncated elsewhere by the ‘main phase’, which intrudes the margin.

(ii) The ‘main phase’ of the pluton is an even, medium-grained, greyish rock containingabundant conspicuous hornblende crystals up to 10 mm in length. Small phenocrysts of K-feldspar are rare. This phase makes up the majority of the pluton, is elliptical, and whollycontains the two ‘core’ phases.

(iii) An ‘outer core’ phase is intrusive into, and distinguished from, the main phase by a greaterabundance of K-feldspar megacrysts, commonly 10 mm in length; quartz is also conspicuous in hand specimen. This phase occurs in two bodies, both roughly circular in plan; the southernbody is approximately 400 m in diameter and has an inner core, whereas the northern body isless than 200 m in diameter and is homogeneous.

(iv) The ‘inner core’ phase contains distinctive, relatively large (>20 mm) pink K-feldsparmegacrysts set within a brick-red matrix that contains hornblende crystals. This phase forms aprominent rocky inselberg around which contact relationships are rarely observed. A weak K-feldspar megacryst alignment forms a concentric pattern within this core.

The lithological zoning is one of increasing grain size from an even, medium to fine-grainedmargin to a coarse-grained, porphyritic core. This variation is due primarily to an increase in K-feldspar abundance and size. Hornblende abundance varies antipathetically with K-feldsparabundance, but neither the change in relative amount nor crystal size is significant. There is amajor intrusive discontinuity between the marginal and main phases, but further changes to thecore are more gradational.

Breccias: None.

Alteration in the granite: Minor sericitisation of plagioclase and chloritisation of hornblende.

1.8 Extrusives None.

1.9 CountryRock

Contact metamorphism: The observable metamorphic aureole around the Parnell QuartzMonzonite is ~50 m wide and consists of hornfels composed of foxy-red biotite, quartz,orthoclase, and minor plagioclase as a fine-grained polygonal mosaic. A more subtle aureole up to about 500 m wide is represented as a dark tonal pattern on aerial photographs. This zone has a


BRIDGET SUITE

relatively sharp outer boundary and indicates a major subsurface expression of the bodyextending approximately 10 km to the southeast (Collins et al. 1988).

Reaction with country rock: Unknown.

Units the granite intrudes: The Parnell Quartz Monzonite and Bridget Adamellite intrude theMosquito Creek Formation. The Formation consists of schistose psammitic, psammopeliticand pelitic metasediments with minor conglomerate units. The Formation is thought to be~3000 Ma old, based on Rb-Sr dating of intrusive granites (Hickman 1983).

Other plutons intrude basalt, andesite, rhyolite and dacite lavas of the Pilbara Supergroup;various basalts and volcaniclastics of the Fortescue Group; and the Bamboo Creek Porphyry.

Dominant rock types: Basalt, andesite, rhyolite, dacite, tuff with mudstone and shale, quartz-plagioclase porphyry.

Potential hosts: The basalt and tuff with mudstone and shale, are potential hosts to oxidisedfluids.

1.10 Mineralisation Many small Au (±Sb) deposits occur in pelites of the Mosquito Creek Formation (Hickman1983). Typically, the deposits are small quartz veins. At the Blue Spec mine, shearing appearsto have occurred during mineralisation (ie., mineralisation is syn-deformation) because itcontrols the shape and limits of the ore bodies and visibly deforms crystals of stibnite. The ageof mineralisation is unknown, however Pb-Pb dating of galena at the Braeside Pb field (which is also syn-deformation) gives an age of ~2700 Ma (Richards 1978; D. Huston, pers. comm.).Further, there are Archaean granites nearby which have also intruded the Mosquito CreekFormation and could be associated with the mineralisation. Therefore, it is considered unlikelythat the Bridget Suite gave rise to the mineralisation in the area, but the possiblity cannot beruled out completely. Further work needs to be done on assessing the fractionation behaviour ofthe granite, and on dating the mineralisation.

1.11 GeochemicalData

Data source: All data are from Collins et al. 1988.

Data quality: Unknown.

Are the data representative? Probably – all mapped phases of the Parnell Quartz Monzonite are represented by at least one geochemical analysis, although it is not known how representativethis pluton is of the suite as a whole.

Are the data adequate? Depends on how representative the Parnell Quartz Monzonite is of thesuite as a whole.

SiO2 range (Fig. 1.1): 63 - 75 wt%.

Alteration (Figs. 1.1 & 1.2):

• SiO2: No alteration evident.• K2O/Na2O: No alteration evident.• Th/U: The samples of the main phase and outer core have slightly elevated ratios,

indicating loss of uranium. The other samples are normal.


BRIDGET SUITE

Fig ure 1.1: His to gram of SiO2 val ues.

• Fe2O3/(FeO+Fe2O3): All phases of the monzonite are slightly reduced, the dyke is morereduced. There is no evidence of alteration.

Fractionation Plots (Fig. 1.3): The geochemical data do not show any fractionation. It must bepointed out that the plots used are sensitive to K-feldspar fractionation; Collins et al. (1988)showed that crystal fractionation of plagioclase and hornblende were dominant in the coolinghistory of the Parnell Quartz Monzonite. Other evidence such as a well-developed contactaureole, zoning of the pluton, and the presence of felsic microgranite dykes, aplites andpegmatites suggest that fractionation did take place.

• Rb: Values are moderately low.• U: All values are low to very low.• Y: All values are low.• P2O5: Values range from moderate to low, decreasing with increasing SiO2.• Th: All values are low.• K/Rb: All values are moderate.• Rb-Ba-Sr: Most of the samples plot in or near the monzogranite field.• Sr: Values for the monzonite range from moderate to very high; the dyke has a moderately

low value.• Rb/Sr: All values are very low because of the high Sr content.• Ba: Values range from moderately low to low.• F: No data available.

Metals (Fig. 1.4):

• Cu: Values are low.• Pb: Values are low.• Zn: Values are moderate to low.• Sn: No data available.

High field strength elements (Fig. 1.5):

• Zr: Values are low and decrease with increasing SiO2.• Nb: Values are low and decrease with increasing SiO2.• Ce: Values are low and decrease with increasing SiO2.

Classification (Fig. 1.6):

• The CaO/Na2O/K2O plot of White, quoted in Sheraton and Simons (1992): Mostsamples plot in the granodiorite field, one in the monzogranite field and one in the granitefield.

• Zr/Y vs Sr/Sr*: Insufficient data available.• Spidergram: The suite is Sr-undepleted, Y-depleted, which is rare for Australian

Proterozoic granites.• Oxidation plot of Champion and Heinemann (1994): All samples are oxidised.• ASI: All samples are metaluminous.• A-type plot of Eby (1990): Most of the samples plot in the Palaeozoic fractionated granite

field; one plots in the ordinary granite field.

Granite type (Chappell and White 1974; Chappell and Stephens 1988): I-granodiorite.

Australian Proterozoic granite type: Sally Downs.

1.12 GeophysicalSignature

Radiometrics (Fig 1.7): The zoning of the Parnell Quartz Monzonite pluton should be evidenton a radiometric image: Th and U increase from the inner core outwards. The inner core, mainphase and outer core would be all be various shades of dark red, the dyke sampled would beyellowish, and the marginal phase would be pink-white.

Gravity: Not sufficiently detailed.

Magnetics: Not sufficiently detailed.


BRIDGET SUITE

1.13 References Collins, W.J., Gray, C.M. and Goode, A.D.T. 1988. The Parnell Quartz Monzonite: AProterozoic zoned pluton in the Archaean Pilbara Block, Western Australia, AustralianJournal of Earth Sciences, 35, 535-547.

Hickman, A.H. 1978. Nullagine , Western Australia, 1:250 000 Geological Series, GeologicalSurvey of Western Australia, Explanatory Notes, SF/51-05, 22pp.

Hickman, A.H. 1983. Geology of the Pilbara Block and its environs, Geological Survey ofWestern Australia, Bulletin 127.

Hickman, A.H. and Lipple, S.L. 1975. Explanatory notes on the Marble Bar 1:250 000 sheet,Western Australia, Geological Survey of Western Australia, Record 1974/20 (unpublished).

Hickman, A.H., Chin, R.J. and Gibson, D.L. 1983. Yarrie, Western Australia, 1:250 000Geological Series, Geological Survey of Western Australia, Explantory Notes, SF/51-01, 33pp.

Richards, J.R. 1978. Lead isotopes and ages of galenas from the Pilbara region, WesternAustralia, Journalof the Geological Society of Australia, 24, 465-473.


BRIDGET SUITE


BRIDGET SUITE

Legend

1.2A: Na2O vs K2O

1.2B: Th/U vs SiO2

1.2C: Fe2O3/(FeO+Fe2O3)


BRIDGET SUITE

1.3A: Rb vs SiO2

1.3B: U vs SiO2

1.3C: Y vs SiO2

Legend


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1.3D: P2O5 vs SiO2

1.3E: Th vs SiO2

1.3F: K/Rb vs SiO2

Legend


BRIDGET SUITE

1.3G: Rb- Ba- Sr

Stronglydif fer en ti atedgran ite

Gran ite

To nal iteMon zo gran ite

Anoma lousgran ite

1.3H: Sr vs SiO2

1.3I: Rb/Sr vs SiO2

Legend


BRIDGET SUITE

1.3J: Ba vs SiO2

1.4A: Cu vs SiO2

NO FLUORINE DATA AVAILABLE

Legend


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1.4B: Pb vs SiO2

1.4C: Zn vs SiO2

Legend

NO TIN DATA AVAILABLE


BRIDGET SUITE

1.5A: Zr vs SiO2

1.5B: Nb vs SiO2

1.5C: Ce vs SiO2

Legend


BRIDGET SUITE

1.6A: CaO- Na2O-K2O

To nal ite

Gra no dio rite

Mon zo gran ite

Trondh jemite Gran ite

1.6B: Zr/Y vs Sr/Sr*

1.6C: Spider gramAll data plotted

Legend

INSUFFICIENT TRACE ELEMENT DATAAVAILABLE FOR Zr/Y vs Sr/Sr* PLOT


BRIDGET SUITE

1.6D: Re dox plot

Strongly oxi dised

Oxi dised

Re duced

Strongly Re duced

1.6E: ASI vs SiO2

1.6F: Ga/Al vs HFSE (Eby 1990)

Legend


BRIDGET SUITE

1.7A: K2O%Box- whisker

1.7B: Th ppmBox- whisker

1.7C: U ppmBox- whisker

Pro tero zoic me dian


Legend


Inner core, main phase, outer core, dyke phase, marginal phase of Parnell Quartz Monzonite

MEANS AND STANDARD DEVIATIONS

Element Mean Median Standard Minimum Maximum Number ofDeviation Items

SiO2 66.04 64.81 4.32 63.37 74.74 6TiO2 0.44 0.49 0.14 0.15 0.54 6Al2O3 13.85 14.01 0.48 13.07 14.41 6Fe2O3 2.02 2.26 0.83 0.37 2.55 6FeO 2.42 2.7 0.83 0.75 2.91 6MnO 0.09 0.1 0.04 0.01 0.11 6MgO 2.07 2.31 0.77 0.54 2.7 6CaO 3.09 3.37 0.94 1.31 3.95 6Na2O 3.88 3.89 0.13 3.66 4.02 6K2O 4.28 4.34 0.39 3.79 4.66 6P2O5 0.28 0.31 0.11 0.07 0.37 6H2O+ 0.99 1.07 0.33 0.48 1.4 6H2O- 0.2 0.18 0.06 0.13 0.29 6CO2 0.29 0.23 0.25 0.1 0.78 6Ba 713.17 828 255.8 232 894 6Rb 172.67 171 32.59 131 209 6Sr 643.5 725.5 220.1 251 838 6Pb 18.33 17.5 4.18 15 26 6Th 18 17 6.57 9 27 6U 2.33 2 1.51 1 5 6Zr 162.67 176 40.57 81 192 6Nb 6.33 7 2.25 2 8 6Y 14 15.5 4.47 5 17 6La 35.83 39.5 11.89 13 46 6Ce 57.17 62 12.91 32 68 6V 98.5 113 38.68 20 120 6Cr 60.33 62 26.03 12 88 6Ni 32.83 33.5 8.28 19 43 6Cu 19 17 7.59 11 31 6Zn 54.17 61.5 23.09 8 71 6Ga 17.33 18 1.21 15 18 6


BRIDGET SUITE

1 bridget suite - geoscience australia · 2020. 7. 8. · mosquito creek formation. the formation...

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