age and sources of gold mineralization in the marmato mining

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Ore Geology Reviews 3

Age and sources of gold mineralization in the Marmato miningdistrict, NW Colombia: A Miocene–Pliocene epizonal gold deposit

Colombo Celso Gaeta Tassinari a,⁎, Fabio Diaz Pinzon a, Juaquin Buena Ventura b

a Centro de Pesquisas Geocronológicas, Instituto de Geociências, Universidade de São Paulo, Rua do Lago 562,Cidade Universitária, São Paulo, CEP 05508-080, Brazil

b INGEOMINAS-Servicio Geológico y Minero, Diagonal 53, no 34-53, Bogota, Colombia

Received 4 May 2005; accepted 10 March 2007Available online 24 March 2007

Abstract

Epigenetic gold mineralization occurs in the Marmato mining district, within the Calima Terrain of the Setentrional Andes,Colombia. Regional rocks associated with this mineralization include: graphite- and chlorite-schists of the Arquia Complex;metamorphosed during the Cretaceous, Miocene sandstones, shales and conglomerates of the Amagá Formation; as well aspyroclastic rocks (clasts of basalt, andesites and mafic lavas) and subvolcanic andesitic/dacitic bodies of the Combia Formation (9to 6 Ma). The subvolcanic Marmato stock hosts mesothermal and epithermal low-sulfidation Au–Ag ores in the form ofdistensional veins, stockwork, and quartz veinlets within brecciated zones. Ore minerals are pyrite, sphalerite and galena withsubordinate chalcopyrite, arsenopyrite, pyrrhotite, argentite and native gold/electrum.

Sericitized plagioclase from a porphyry dacite yielded a K–Ar age of 5.6±0.6 Ma, interpreted as the age of ore deposition. This is inclose agreement with the age of reactivation of the Cauca–Romeral Fault System (5.6±0.4 Ma), which bounds the Calima Terrain. Aporphyry andesite–dacite (6.7±0.1 Ma), hosting the Au–Ag veins, shows a measured 87Sr/86Sr between 0.70440 and 0.70460, εNdbetween+2.2 and +3.2 and 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of 18.964 to 19.028; 15.561 to 15.570; and 38.640 to 38.745,respectively. The 87Sr/86Sr and εNd values of rocks from the Arquia Group range from 0.70431 to 0.73511 and −12.91 to +10.0,respectively, whereas the corresponding Pb isotopic ratios (206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb) range from 18.948 to 19.652;15.564 to 15.702; and 38.640 to 38.885, respectively. 87Sr/86Sr and εNd values obtained on sulfides from the gold quartz veins, whichoccur at shallow and intermediate levels, range from 0.70500 to 0.71210 and from −1.11 to +2.40. In the deepest veins, εNd values liebetween +1.25 and +3.28 and the 87Sr/86Sr of calcite and pyrite fall between 0.70444 and 0.70930. The 206Pb/204Pb, 207Pb/204Pb and208Pb/204Pb ratios of all mineralization are in the ranges 18.970 to 19.258; 15.605 to 15.726 and 38.813 to 39.208, respectively.Carbonates have an average 87Sr/86Sr ratio of 0.70445, which is within the range of values measured in the host dacite. The Sr isotopicdata indicate that carbonic fluids have a restricted hydrothermal circulationwithin the host igneous body, while the Sr, Pb andNd isotopiccompositions of the sulfides suggest that the fluids not only circulated within the Marmato stock, but also throughout the ArquiaComplex, inferring that these rocks offer a potential target for mineral exploration. Based on geological and geochronological evidence,the epizonal Marmato gold ores formed during the Miocene to Pliocene, as a result of cooling of the Marmato stock and reactivationalong a crustal-scale fault zone related to thermal processes in an accretionary oceanic–continental plate orogen.© 2007 Elsevier B.V. All rights reserved.

Keywords: Marmato district; Colombia; Geochronology; Gold; Andes

⁎ Corresponding author. Fax: +55 11 30913993.E-mail address: [email protected] (C.C.G. Tassinari).

0169-1368/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.oregeorev.2007.03.002

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506 C.C.G. Tassinari et al. / Ore Geology Reviews 33 (2008) 505–518

1. Introduction

The Marmato gold mining district is located in theSetentrional Andes of Colombia, in the State of Caldas,50 km NNW of Manizales (Fig. 1). Most of the area ismountainous with steep morphological features andpeaks of up to 1850 m. The climate is tropical, with rainyseasons from April to June, as well as the months of

Fig. 1. Geological map of the Colombian A

October and November. The district has been continu-ously producing gold since the middle of the 15thcentury. Today, mining activities are centered aroundseveral small mines, predominately located on the upperpart of Marmato Mountain, including the operations ofPlata Fría, LaMaria, Aguaceral, La Negra, La Felicia andLa Palma. However, the largest gold ore deposit, locatedon the lower part of Marmato Mountain, is Marmato

ndes (modified from Toussaint, 1993).

507C.C.G. Tassinari et al. / Ore Geology Reviews 33 (2008) 505–518

Bajo with ore grades of approximately 12 g/t Au(Rodriguez and Warden, 1993). The district containsreserves of up to 15.9 million. Oz Au gold, with anaverage grade in excess of 2.2 g/t (F. Ortiz, pers. comm.);ca. 500 tonnes of ore is estimated to be mined each day.Despite the current levels of exploitation, the Marmatogold mining district has only begun to be properlyexplored for its metallogenic potential. As a conse-quence, it remains one of the last frontiers for modern-day gold exploration and development in SouthAmerica.

Taking into account that the gold metallogenicevolution of the Marmato area is not fully understood,the main purpose of the research reported here was tocharacterize the age and sources of the Marmato golddeposits, the role of the Miocene subvolcanic intrusionsin the formation of the ore deposits and the isotopiccharacter of the gold mineralization. Application ofradiogenic isotopes in ore deposit studies and mineralexploration is useful to determine genetic models. Thismodelling involves identifying the nature and age of thesources, the timing of fluid migration, the type andtiming of geological control, the timing and temperatureof ore formation and tectonic relations, characterizationof hydrothermal fluid sources, and fluid–rock interac-tion process (Gulson, 1986; Kerrich, 1991).

2. Geologic background

2.1. Tectonic setting

In Colombia, the Andes are comprised of three N–Strending ridges, referred to as the Eastern, Central andWestern Cordilleras (Fig. 1). The Colombian Andes isdivided, by the N–S trending Cauca Romeral ShearZone, into two domains. The Eastern and CentralCordilleras (oriental domain) are composed of Precam-brian and Paleozoic continental basement, which areintruded by several Mesozoic and Cenozoic graniticplutons and covered by sedimentary sequences. TheWestern Cordillera (occidental domain) was formed bysuccessive accretions of allochthonous terranes withoceanic affinities (Toussaint and Restrepo, 1989). Thesedomains were intruded by several subvolcanic intru-sions, which constitute the Combia Formation. One ofthese intrusions is the Miocene Marmato stock, whichhosts the Marmato Au–Ag mineralization.

The Marmato district is located within the CalimaTerrain (Toussaint and Restrepo, 1991), and is boundedby the Cauca and Romeral Faults (Fig. 1). The Au–Agmineralization occurs as dilational veins hosted in theporphyritic dacite and andesites of the Miocene Combia

Formation, which outcrops out in the northern part ofthe Cauca–Romeral Fault System (CRFS; MacDonald,1980). The CRFS is a regional structure that trends N–Sfrom Ecuador to the Colombian Caribbean coast. Thismega-shear originated during the Mesozoic, but hasbeen intermittently reactivated since that time. Duringthe Cretaceous, it had a right-lateral component insouthwestern Colombia (Paris and Romero, 1994), butwhen reactivated during the Miocene (Vinasco, 2001), itdipped east and exhibited left-lateral and reverse compo-nents in northwestern Colombia (Paris and Romero,1994; Ego and Sebrier, 1995; Taboada et al., 2000).Recent activity is demonstrated by the 1999 Armeniaearthquake along the CRFS (Taboada et al., 2000). Fromeast to west, the CRFS is composed of a series ofsubparallel faults including the San Jerónimo–Romeral(SJRF), Silvia–Pijao (SPF), Cauca–Almaguer (CAF)and Calí–Patia (CPF) faults (González and Maya, 1995)(Fig. 1).

The Eastern Cordillera (EC) and the Central Cordil-lera (CC) are located to the east of the SJRF (Fig. 1). TheCC is composed of a polymetamorphic Precambrian andPaleozoic basement that was affected by Mesozoic andCenozoic magmatism. During the Jurassic, largeextensive batholiths and widespread volcanoclasticsequences were formed. Overlying sedimentary rocksof Cretaceous and Cenozoic ages are also widespread inthe CC. The EC is characterized by Cretaceous marinesequences and locally by clastic Cenozoic sedimentaryrocks. Paleozoic sedimentary rocks and Triassic volca-nic rocks are also present. During the Cretaceous, theserocks were affected by normal faults while during theMiocene they were subsequently deformed by thrustingand folding, which produced uplift and shortening of theEC (Taboada et al., 2000). The EC formed by successiveaccretion of terranes until the Lower Paleozoic, or alter-natively since the upper Paleozoic to Upper Cretaceous(Toussaint and Restrepo, 1989), and was subsequentlyaffected by rifting during the Permian or Triassic.

The Andean segment, located to west of the CPF, iscomposed of the Western Cordillera (WC) and the“Serranía del Baudo” (SB) domains. The SB domain hasan oceanic affinity and is composed of basalts andmarine sedimentary rocks of Cretaceous age. It wasaffected by Cenozoic volcanism and plutonism andcovered by Cenozoic sedimentary rocks. It was made upby the accretion of allochtonous terranes, which ac-cording to Etayo et al. (1986), are the Cañas Gordas,Dagua, Atrato–San Juan–Tumaco and Baudo terranes.Restrepo and Toussaint (1988), however, consider themas the Calima and Cuna terranes. The SB comprisesbasaltic plateaus and oceanic island arc sequences of


Cretaceous age, which were accreted during the Creta-ceous to Miocene (Aspden et al., 1987; Restrepo andToussaint, 1988; Duque-Caro, 1990; Ordonez, 2001;Kerr et al., 2002).

Accretion of the Panamá–Chocó block onto theWestern Cordillera during the Miocene caused reacti-vation of the CRFS with a left-lateral component (Egoand Sebrier, 1995) and crustal thinning, allowing theemplacement of volcanic rocks and several subvolcanicbodies, including the Marmato stock and others of theCombia Formation (Duque-Caro, 1990).

Etayo et al. (1986) defined the Cauca–Romeralterrain as a crustal segment bounded by the SJRF andCAF, and proposed a Cretaceous age for its accretion.They also termed the Miocene–Pliocene volcanogenicrocks, which covered the Colombian Andes, as “Supraya-cente Terciario”. Restrepo and Toussaint (1988) definedthe Calima terrain as a block bounded by the SJRF and theSB (Cuna terrain), which developed during the Creta-ceous. This terrain is the so-called “Andean block”.Therefore, the Marmato stock, included in the CombiaFormation, is part of the “Suprayacente Terciario” andintruded the Cauca–Romeral terrain, or alternatively, ispart of the Tertiary volcanism in the Andean block thatintruded the Calima terrain.

The low sulfidation epithermal gold mineralization isspatially associated with subvolcanic andesite and daciteintrusions, which formed during accretion of the Panamá–ChocoArc along thewesternmargin of theAndeanCentralCordillera. The Au–Ag mineralization thus occurs in anorogenic tectonic environment.

2.2. Geology of the Marmato district

The oldest exposed rocks in the Marmato miningdistrict are metasedimentary and metavolcanic rocks ofthe Arquia Complex (González and Maya, 1995). Thisunit is composed of discontinuously alternating quartz–sericitic schist, amphibolite–schist and amphibolite(Calle, 1984), as well as graphite schist and quartz–biotitic schist (Lopez-Rendon, 1991) (Fig. 2), and wasmetamorphosed to the upper greenschist–lower am-phibolite facies during the Cretaceous. Small bodies ofTriassic gabbro and ultramafic rocks constitute a sepa-rate lithostratigraphic unit in the area (Vinasco, 2001).The ultramafic rocks occur as isolated bodies with aN–S strike, and comprise serpentinites and serpenti-nized peridotites.

The Cretaceous Támesis stock is composed of grano-diorite to diorite–gabbroid rocks with hypidomorphic andlocally pegmatitic textures and yields a K–Ar hornblendeage of 124±6 Ma (Calle, 1984). The Quebradagrande

Complex is a lower Cretaceous volcanosedimentarymarine unit (González and Maya, 1995) that outcrops asbasalts in the lower part of the Marmato Hill.

The Amagá Formation is a sedimentary sequenceformed during the Oligocene and Miocene (Van DerHammen, 1960), and subsequently affected by exten-sional deformation and episodic volcanic events (Sierraet al., 2000). The formation is divided into three mem-bers; the lower member consists of layers (5 m thick) ofconglomerate, grey-green and light sandstone and clays-tone. The middle member consists of shale, light and greysandstone and minor conglomerates and coal layers up to3 m thick. The upper member, exposed in the Marmatoarea, is dominated by cemented sandstones and laminatedclaystones, with thin conglomerates and coal layers presentlocally (Calle et al., 1984).

The Combia Formation is a thick volcanosedimen-tary unit, which is divided into two members (González,1980). The upper member comprises sedimentary rocksand volcanic ash. The lower member is characterized bypyroclastic rocks, which include tuffs, lapilli tuffs,aglomerates and breccias with clasts of basalt andandesites. Mafic lavas with aphanitic to porphyritictextures (augite phenocrystals), and subordinate tuffa-ceous intercalations also occur. Andesitic and daciticsubvolcanic bodies, with K–Ar ages ranging from 9 Mato 6 Ma (Restrepo et al., 1981), are a part of the CombiaFormation.

TheMarmato stock of the Combia Formation intrudedgraphitic schists of the Arquia Complex and sedimen-tary rocks of the Amagá Formation (Fig. 2). It iscomposed of porphyritic volcanic rocks with daciticand andesitic compositions. The porphyritic andesitesoutcrop along the border of the stock, whereas theporphyritic dacites appear in the central part of the body,exhibiting gradational contacts between these compo-sitional types.

2.3. Structural features

The main structure in the Marmato area is the Cauca–Almaguer fault, which regionally is east-dipping andshows a left-lateral displacement with reverse components.Within the Marmato Mining District the structural featuresare related to sub-vertical faults with N45W–N70W trend(Marmato Fault) and extensional sulfide-bearing quartzveins containing gold (Fig. 2).

Rossetti and Colombo (1999) characterized threedeformation events in the area: (1) a ductile event thatpreceded emplacement of the Marmato Stock and whichonly affected the schists of the Arquía Complex; (2) aductile-brittle event that formed simultaneously with the

Fig. 2. Geological map of the Marmato mining district (after Mora and Cuellar, 1982).

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emplacement of the Marmato Stock producing catacla-sites along the border of the subvolcanic body and foldsand discontinuities within the Amagá Formation; and(3) a brittle event, after the stock emplacement, asso-ciated with the reactivation of the CPRFS. This eventformed mineralized faults that trend N70W and barrenthat trend N50E.

Within the Marmato mining district, the structuralfeatures related to gold mineralization are sub-verticalfaults, as exemplified by the N25E-trending ObispoFault, as well as extensional veins with N, NE and NWtrends (Fig. 2).

3. Gold mineralization

The mineralization at Marmato Mountain occurs atthree different topographic levels: the near-surface Echan-dia Zone (1450 to 1600 m above sea level [a.s.l.]), whichcorresponds to the upper part of Marmato Mountain, theintermediary Cien Pesos Zone (1300 to 1450 m a.s.l.)and at depth in the Marmato Bajo Zone (1160 to1260 m a.s.l.), part of the lower Marmato Mountain.Mineralization occurs in quartz veins, distentional veins,stockwork structures, and in narrow brecciated zoneswith quartz veinlets that are associated with the regionalCauca–Romeral Shear zone. In general the ore mineral-ogy is variable, comprising pyrite, sphalerite, galena,chalcopyrite, arsenopyrite, Ag-sulfosalts, native gold andelectrum. The gangue minerals are mainly calcite andquartz. Sericitization, propylitization, argillitization andsilicification are the main types of wall-rock alteration.The average gold grades are between 2 and 8 g/t. Based onthese characteristics the mineralization is considered aslow-sulfidation epithermal type, but, at lowest exposedlevels in the system, geological evidence is consistentwith a transition to a higher temperature mesothermalenvironment (Rodriguez and Warden, 1993).

Gold mineralization is spatially associated withMiocene andesite and dacite porphyries of the CombiaFormation. The andesite porphyry of the Marmatostock occurs in the near-surface Echandia zone (PlataFria mine) and is composed of plagioclase phenocrysts(andesine) hydrothermally altered (propylitized) tosericite and carbonate, hornblende and biotite (alteredto chlorite), calcite and Fe-oxides within a fine-grained matrix composed of plagioclase, chlorite,and minor magnetite, apatite, epidote and zircon. Thedacite porphyry, which hosts most of the auriferousquartz veins, is composed of plagioclase phenocrysts,which are propylitically altered to sericite, carbonateand epidote, quartz, hornblende and biotite (altered tochlorite and calcite). All these phenocrysts occur in a

fine-grained matrix of plagioclase, quartz, sericite andcarbonates.

The dacite porphyry of the Marmato stock containsabundant low-sulfidation ore veins, composed mainly ofquartz, carbonate, pyrite and sphalerite. The minerali-zation occurs as extensional veins, stockwork structuresas well as narrow brecciated zones with quartz veinlets.In general the ore veins exceed 250 m in length and areb2 m wide. These veins are laterally zoned with pyriteand sphalerite dominating the margins and the core ofthe vein, respectively. The relative proportion of sulfidestends to increase with depth.

At shallow levels, within the Echandia zone, the oreconsists of variable amounts of pyrite, sphalerite andgalena. Accessory and trace minerals include chalcopy-rite, arsenopyrite, pyrrhotite, argentite and native gold/electrum. Native gold in this zone occurs as inclusionsin pyrite and sphalerite, and also as intergrowths withpyrrhotite. At intermediate depth, within the Cien Pesoszone, the ore minerals are pyrite, sphalerite, gold, chal-copyrite and pyrrhotite. Native gold is mainly associatedwith pyrite and sphalerite and occurs as inclusions andintergrowths. At greater depth, within the Marmato Bajozone, ore minerals are pyrite, pyrrhotite, sphalerite, arse-nopyrite, chalcopyrite, native gold, galena and marcasite.Native gold is associated with pyrite, Fe-rich sphalerite,chalcopyrite, arsenopyrite, galena and pyrrhotite, as inclu-sions and filling micro-fractures (Ortiz, 1992; Bedoya,1998). According to Rodriguez andWarden (1993), about80% of the gold occurs as grains 20 to 30 μm in sizewithin the sulfides.

The gangue minerals in all zones are carbonates(mainly calcite) and quartz. In general, the sulfides fromthe three zones are fractured, and in places are filled byclays, carbonates and quartz. The Au/Ag ratio in pyreticore is variable across theMarmatomining district, rangingfrom1:1 to 4:1; in galena ore, the Au/Ag ratio is about 1:1.The Au/Ag ratios were calculated using the Au and Agconcentrations measured within each mineral type.Rodriguez and Warden (1993) reported ratios of Au/Agfrom 1:1 to 1:250, averaging 1:3.7 for the Marmato Bajoore.

The ore mineral assemblage represents three differentstages of formation. The first stage is dominated bypyrite, the second stage by sphalerite, and the third stageby pyrite and calcite. Within the Marmato Bajo and CienPesos zones the gold is exclusively associated with thefirst and second stages, respectively. In the Echandiazone, however, gold was deposited both during the firstand final stages (Lopez-Rendon and Bedoya, 1989).

Although disseminated Au–sulfide mineralizationextends up to 1 m into the wall-rocks, hydrothermal


alteration is confined to a narrow, metre-scale sheathcharacterized by silicification, argillic alteration and kaolini-sation around the veins. Sericite alteration is pervasiveadjacent to the gold–quartz veins. Plagioclase in thedacite porphyry is replaced by fine sericite andsubordinate carbonate (Fig. 3A). Locally, pyrite ispresent within plagioclase crystals (Fig. 3B). Carbonateand chlorite also result from the alteration of the biotiteand hornblende. Silicification, argillization and albiti-zation also occur locally.

Propylitic alteration has affected the porphyry daciteand andesite, and is not restricted to the mineralizedquartz veins. Phenocrysts of plagioclase feldspar arereplaced by sericite and carbonate, whereas hornblendeand biotite were replaced by chlorite and carbonate andlocally by epidote. According to Rossetti and Colombo(1999) the propylitization event is related to the for-mation of the Marmato stock and preceded the goldmineralizing event.

Fig. 3. (A) Transmitted light photomicrograph of strongly sericitizedplagioclase. (B) Transmitted light photomicrograph of pyrite within asericitized plagioclase crystal.

Fluid inclusion microthermometric results (Bedoya,1998; Rossetti and Colombo, 1999) indicate homoge-nization temperatures for primary fluid inclusions inquartz between 250 and 300 °C, with low to moderatesalinities (1.6 to 8.1 equiv. wt.% NaCl). In a broad sense,the Marmato Au–Ag mineralization can be classified asadularia–sericite type, similar to several other epither-mal deposits classified by Sillitoe (1988).

4. Analytical procedures and sampling

TheRb/Sr, Pb–Pb, Sm–NdandK–Ar isotopic analyseswere carried out onwhole rock, ore and gangueminerals atthe Geochronological Research Center, University of SãoPaulo. Samples used for isotopic analyses wereselected on the basis of petrographic and mineralog-ical characteristics derived using thin and polishedsections and back-scattered scanning electron micro-scope imaging.

The samples investigated in this study include:(a) mineralized host rocks, porphyry dacite, porphyryandesite and associated rocks with ages ranging from 9to 6Ma (samplesC J1; 66A; IGM6921; 8763); (b) countryrocks, that include graphite schist (sample FHD 6),biotite schist (sample CJ 03), amphibolite (samples CJ04;06) with Cretaceous metamorphic age, basalt(sample IGM 119089), pyroxenite and dunite (samplesCJ02; IGM 6912), with Triassic ages; and (c) ore andgangue minerals from the gold quartz veins of theEchandia (samples FHD 1; 2), Cien Pesos (samplesFHD-14; 16) and Marmato Bajo zones (samples FHD21; 25; 19; 27; 23; 24).

The K–Ar age determination was carried out usingtechniques described by Amaral et al. (1966). Potassiumwas determined by flame photometry with a MicronalB-262 machine using a lithium internal standard. The Arextraction was made in a high vacuum system withpressure usually b10−7 mm/Hg. Isotopic analyses of thepurified argon were determined with an MS-1 automaticnuclide-type mass spectrometer. All ages have beencalculated with the decay constants recommended bySteiger and Jaeger (1977) and are given with standarderror (1σ) estimates. The constants used in the calcu-lations are:

λβ 4.962×10−10 year−1

λε 0.581×10−10 year−1

(40Ar/36Ar) atm 295.540K 0.01167% K total

The Rb–Sr and Sm–Nd analyses were preparedusing the procedures of Kawashita (1972) and Sato et al.

Table 1Sm–Nd analytical data

No. Rock Material Sm (ppm) Nd (ppm) 147Sm/143Nd 144Nd/143Nd Error εNd

FH 2 Au–Qz vein Pyrite 0.047 0.231 0.1241 0.512581 (28) −1.1FH 14 Au–Qz vein Pyrite 0.056 0.258 0.1316 0.512512 (38) −2.4FH 16 Au–Qz vein Pyrite 0.033 0.151 0.1315 0.512764 (17) +2.5FH 21 Au–Qz vein Pyrite 0.654 3.059 0.1292 0.512784 (14) +2.8FH 25 Au–Qz vein Pyrite 0.025 0.107 0.1389 0.512738 (17) +1.9FH 19 Au–Qz vein Pyrite 0.12 0.49 0.1483 0.512806 (89) +3.3FH 23 Au–Qz vein Pyrite 0.018 0.076 0.1402 0.512781 (14) +2.8CJ 01 Dacite porphyry WR 2.961 14.345 0.1248 0.512802 (16) +3.2CJ 66 Dacite porphyry WR 2.978 14.948 0.1204 0.512790 (09) +3.0IGM69 Dacite porphyry WR 3.545 17.787 0.1207 0.512752 (11) +2.2IGM87 Hornblende diorite WR 28.853 111.04 0.1571 0.512833 (11) +3.8FHD 6 Graphite schist WR 2.082 9.294 0.1354 0.512061 (21) −11.3CJ 03 Biotite schist WR 11.219 57.289 0.1184 0.511976 (09) −12.9CJ 04 Mafic metavolcanic WR 2.957 7.961 0.2248 0.513182 (14) +10CJ 08 Mafic metavolcanic WR 5.068 19.402 0.1580 0.512654 (12) +0.3IGM11 Basalt WR 2.948 9.794 0.1820 0.512878 (11) +4.9CJ 02 Pyroxenite WR 0.056 0.145 0.2348 0.513167 (24) +10

Abbreviations: Au–Qz vein=gold–quartz vein; WR=whole rock. Errors are expressed as 2σ.


(1995), which involve HF-HNO3 dissolution plus HClcation exchange separation. No visible solid residueswere observed after dissolution. The Sr isotopic ratioswere normalized to 86Sr/88Sr=0.1194; replicate analy-ses of 87Sr/86Sr for the NBS987 standard give a meanvalue of 0.71028±0.00006 (2σ), with measured Srblanks of 5 ng. Nd isotopic ratios were normalized to

Table 2Sr isotopic composition of gangue and ore minerals and country rocks

Sample no. Lithology Material

FHD1 Gangue vein CalciteFHD16 Au–Qz vein CalciteFHD19 Au–Qz vein CalciteFHD27 Hydrotermal breccia CalciteFHD16 Au–Qz vein PyriteFHD21 Au–Qz vein PyriteFHD25 Au–Qz vein PyriteFHD19 Au–Qz vein PyriteFHD19 Au–Qz vein Sphalerite-L1FHD19 Au–Qz vein Sphalerite-L2CJ 01 Dacite porphyry WRCJ 66 Dacite porphyry WRIGM69 Diodite porphyry WRIGM87 Hornblende diorite WRFHD 6 Graphite schist WRCJ 03 Biotite schist WRCJ 04 Mafic metavolcanic WRCJ 06 Mafic metavolcanic WRIGM11 Basalt WRIGM69 Dunite WRCJ 02 Pyroxenite WR

Abbreviations: Au–Qz vein=gold–quartz vein; L1=first-leached; L2=seco

146Nd/144Nd=0.72190. The averages of 143Nd/144Ndfor La Jolla and BCR-1 standards were 0.511847±0.00005 (2σ) and 0.512662±0.00005 (2σ), respective-ly. The blanks measured b0.03 ng Nd. Isotopic analysesof Sr and Nd were carried out on a multicollector VG354 Micromass and Finnigan-MAT 262 mass spectro-meters, respectively.

Rb (ppm) Sr (ppm) 87Sr/86Sr Error

0.704721 0.0000854.5 306.6 0.704660 0.0000351.2 384.7 0.704512 0.00004911.2 103.1 0.704442 0.000451

0.707160 0.0003300.705030 0.0007000.709370 0.0003000.706790 0.0005000.705150 0.0001100.705920 0.000230

39 1172.1 0.704447 0.00005650.2 1064.4 0.704599 0.00008554.6 964 0.704477 0.0000497.3 906.4 0.703876 0.000125

0.735108 0.000125186.7 247.2 0.714914 0.00009331 109.5 0.704306 0.00005672.1 133.5 0.706200 0.00007837.9 502 0.703957 0.0000630.7 1.6 0.704750 0.0002822.1 8.2 0.706841 0.000057

nd-leached; WR=whole rock. Errors are expressed as 2σ.

Table 3Pb isotopic compositions of sulphides and country rocks

Sample no. Lithology Material 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb

FHD 2 Au–Qz veins Pyrite 19.058 (0.10) 15.706 (0.125) 39.209 (0.14)FHD 2 Au–Qz veins Sphalerite 19.100 (0.076) 15.715 (0.071) 39.170 (0.083)FHD 2 Au–Qz veins Galena 19.003 (0.026) 15.605 (0.023) 38.813 (0.023)FHD 14 Au–Qz veins Pyrite 19.067 (0.046) 15.726 (0.047) 39.208 (0.048)FHD 16 Au–Qz veins Pyrite 19.258 (0.176) 15.910 (0.175) 39.751 (0.183)FHD 21 Au–Qz veins Pyrite 18.999 (0.020) 15.615 (0.020) 38.827 (0.020)FHD 25 Au–Qz veins Pyrite 18.970 (0.057) 15.613 (0.059) 38.862 (0.060)FHD 19 Au–Qz veins Pyrite 19.065 (0.049) 15.699 (0.048) 39.115 (0.049)FHD 19 Au–Qz veins Sphalerite 19.010 (0.035) 15.619 (0.036) 38.842 (0.037)FHD 23 Au–Qz veins Pyrite 19.047 (0.051) 15.680 (0.051) 39.041 (0.052)CJ 01 Dacite porphyry WR 18.964 (0.017) 15.564 (0.018) 38.640 (0.019)CJ 66A Dacite porphyry WR 18.971 (0.027) 15.570 (0.028) 38.673 (0.028)IGM 6921 Diorite porphyry WR 18.982 (0.008) 15.569 (0.008) 38.688 (0.008)IGM 8763 Hornblende diorite WR 19.028 (0.031) 15.561 (0.032) 38.745 (0.033)FHD 6 Graphite schist WR 18.948 (0.026) 15.617 (0.029) 38.885 (0.030)CJ 03 Biotite schist WR 19.652 (0.057) 15.702 (0.056) 39.118 (0.062)CJ 06 Mafic metavolcanic WR 19.131 (0.026) 15.609 (0.026) 38.807 (0.027)

Abbreviations: Au–Qz vein=gold–quartz vein; WR=whole rock. Errors are expressed as 2σ.


The Pb isotopic analyses included washing sampleswith HCl+HNO3 and dissolution with HCl in Parr typebombs. The Pb was separated using HCl and HBr in anAG1-X8 (200–400#) column. The procedural blank forPb was 70 pg. Pb isotope ratios were corrected relative tothe values of the NBS981 standard by 1.0024 for206Pb/204Pb, 1.0038 for 207Pb/204Pb, and 1.0051 for208Pb/204Pb. The analytical errors for these isotopic ratioswere 0.15 to 0.48%, 0.13 to 1.07% and 0.104 to 0.45%,respectively. Lead isotopic analyses were determined on amulticollectorVG354Micromassmass spectrometer. For235U and 238U the decay constants used were 9.8485×10−10 year−1 and 1.55125×10−10 year−1, respectively.

Fig. 4. 87Sr/86Sr initial ratios vs. εNd valu

5. Isotopic results

Strontium, Nd and Pb isotopic compositions in 54samples of ore, gangue minerals, regional rocks and hostrocks, are reported in Tables 1, 2 and 3, respectively. Inaddition one sample of intensely sericitized plagioclasewas dated using K–Ar techniques.

5.1. K–Ar data

Analysis of a sample (FHD-24) of sericitizedplagioclase from the porphyry dacite of the MarmatoBajo zone (Fig. 3A), that hosts gold quartz veins, yields

es for sulfides and country rocks.


a K–Ar age of 5.6±0.6 Ma, which is interpreted asthe time of hydrothermal sericitization processes andof ore deposition. The analytical data are reported asfollows:

% of K

Fig. 5. (A)calculated

Error(%)

ComparisoεNd values

40Ar Rad ccSTP/g(⁎10−6)

n of measured 87Sr/86

for sulfides and coun

40ArAtm (%)

Sr ratios fortry rocks.

Age(Ma)

sulfides,

Error(Ma)

1.7661
0. 6266 0.38 65.30 5.6 0.6
The K–Ar age (5.6±0.6 Ma) is in excellent agree-ment with inferred movement along the Cauca–Romeralfault system, dated at 5.6±0.4 Ma by Vinasco (2001)with Ar–Ar step-heating techniques on biotite from adeformed amphibolite. The age is, however, slightlyyounger than the 6.7±0.06 Ma, Ar–Ar age on biotitefrom the Marmato stock reported by the same author.

country rock

5.2. Rb–Sr and Sm–Nd data

Rb–Sr and Sm–Nd isotopic compositions were mea-sured in pyrite and sphalerite to represent the oreminerals. The range of Sr and Nd isotopes in the hostporphyry dacite and andesite, regional metasedimen-tary and metavolcanic rocks, as well as Sr isotopiccompositions in calcite from mineralized quartz veinsand hydrothermal breccia, are shown for comparison inTables 1 and 2. The 87Sr/86Sr ratios and εNd are reportedas present day values, because the measured values arevery close to calculated values at the time ofmineralization.

The Sm and Nd contents of the Marmato stock varyfrom 2.96 to 28.85 ppm and 14.34 to 111.04 ppm,respectively. The 147Sm/143Nd ratio increases from 0.12in the dacite porphyry to 0.15 in the diorite. Present day

s and calcite associated with hydrothermal breccia. (B) Comparison of


εNd values vary between +2.2 and +3.2 (Table 1). Thehost porphyry andesitic–dacitic rocks have 87Sr/86Srratios between 0.70387 and 0.70445. The 87Rb/86Srratios decrease from 0.8959 in dacite porphyry to 0.005in diorite (Table 2). These isotopic compositions areconsistent with magmatic differentiation.

The 87Sr/86Sr ratios and εNd values of the metase-dimentary and metavolcanic rocks of the Arquia Groupare variable and range from 0.70431 to 0.73511 andfrom −12.91 to +10, respectively. The least radiogenicSr isotopic compositions and positive εNd values arerelated to amphibolites, whereas the more radiogenic Srisotopic compositions and the strong negative εNd val-ues are related to quartz–biotite and graphitic schists.The 87Sr/86Sr ratios (0.70396 to 0.7068) and εNd values(+4.45 to +10.32) of the mafic and ultramafic rocks areclose to Sr and Nd isotopic compositions obtained forthe amphibolitic schists (Fig. 4).

The 87Sr/86Sr and the εNd values obtained on sulfidesfrom gold–quartz veins range from 0.70503 to 0.70937,and from −2.46 to +3.28, respectively. Gold mineral-ization in the Marmato Bajo zone (the deepest) exhibitsthe more positive εNd values (+1.95 and +3.28) and thelowest 87Sr/86Sr ratios (0.70503), whereas the negativeεNd values and the highest average

87Sr/86Sr ratios occurin the shallower Echandia and Cien Pesos zones. TheεNd values and

87Sr/86Sr ratios of sulfides are believed toreflect the isotopic compositions of the hydrothermalfluids at the time of sulfide mineralization deposition.

The 87Sr/86Sr ratios of calcites associated with hy-drothermal breccias related to the third stage of gold-sulfide mineralization in the Echandia and Cien Pesoszones show a very narrow range and low radiogenic

Fig. 6. Uranogenic diagram, with plumbotectonic model curves from Zartmancountry rocks.

values between 0.70440 and 0.70472. These valuesoverlap with the Sr isotopic compositions measured onsamples from the host porphyritic dacite (Fig. 5A).

5.3. Lead isotope data

Pb–Pb isotopic compositions of ore minerals weremeasured in pyrite and sphalerite. The range of Pbisotopes in the host porphyry dacite and andesite, re-gional metasedimentary and metavolcanic rocks areshown for comparison (Table 3). The lead isotopic com-positions of sulfides from theMarmato mining district areradiogenic and show a slightly scattered distribution.The 206Pb/204Pb ratios range from 18.970 to 19.258,207Pb/204Pb from 15.605 to 15.726, and 208Pb/204Pbfrom 38.813 to 39.208. Isotopic compositions clusternear the orogenic and upper crust curves on the diagramof Zartman and Doe (1981) (Fig. 6). The dacite andandesite porphyries of the Marmato Stock display Pbisotopic compositions for 206Pb/204Pb, 207Pb/204Pb and208Pb/204Pb, of 18.964 to 19.028, 15.561 to 15.570 and38.640 to 38.745, respectively. The lead isotopic signa-tures of the metasedimentary rocks of the Arquia Grouprange from 18.948 to 19.652, from 15.564 to 15.702, andfrom 38.640 to 38.885 for 206Pb/204Pb, 207Pb/204Pb and208Pb/204Pb, respectively.

The average 207Pb/204Pb and 208Pb/204Pb ratios ofsulfides are more radiogenic than those for the MarmatoStock, whereas the 206Pb/204Pb ratio is less radiogenicthan that of quartz–biotite schist and amphibolite. InFig. 6, the Pb isotope data for the sulfides define twodistinct compositional fields with both sets of dataplotting above the orogenic growth curve. The Pb

and Doe (1981), showing the Pb isotopic compositions of sulfides and


isotopic composition of some samples lies betweenthose of the Marmato stock and graphitic schists. Theother cluster is located above the upper crust growthcurve, showing radiogenic compositions compatiblewith mixed source rocks, involving the Marmato stockand the amphibolite with high U/Pb ratios. The Pbisotopic composition of the sulfides (Fig. 6), suggests amixing of Pb derived from different rock sources andreflects a contribution of lead from both the Marmatostock and regional crustal rocks.

6. Discussion

6.1. Age and sources of mineralization

The age of the hydrothermal sericitization processes of5.6±0.6 Ma is interpreted as the time of ore deposition,because this wall-rock hydrothermal alteration type isconfined to a narrow zone around the gold quartz veins.Based on our K–Ar age and previous Ar–Ar geochro-nological data available for the Marmato stock (Vinasco,2001), it is possible to suggest that the gold mineraliza-tion formed during cooling of the Marmato pluton about1 Ma after crystallization, as a result of movement alongthe Cauca–Romeral shear zone that was related to pull-apart basin formation within the Cauca–Patia depression.

Previous discussions on the genesis of the Marmatogold deposit, mostly based on field, structural and pet-rographic evidence, have emphasized the epithermalnature at the shallow levels of mineralization, and meso-thermal features at the lowest levels of the Marmatomining district. At the top of the system a stockwork ofquartz veins is hosted in breccias and pyroclastic rocks.The base of the breccia zone comprises bonanza-typegold mineralization, which suggests strong hydrothermalboiling, while inferred degassing with carbonate deposi-tion suggests the presence of CO2-enriched fluids. Atlowest levels of the Marmato hill (Marmato Bajo zone) atransition towards a mesothermal environment appearslikely related to the upper part of a porphyry system(Rodriguez and Warden, 1993).

Our isotopic results confirm this interpretation, andsuggest circulation of at least two hydrothermal fluidtypes (meteoric and magmatic). Indeed, a comparison ofthe Sr (Fig. 5A) and Nd (Fig. 5B) isotopic compositionsof the sulfides from gold mineralization with those fromthe country rocks suggests that shallow mineralizationfrom the Echandia and Cien Pesos zones is characterizedby more radiogenic fluids than the Marmato Bajo zone,with significant Sr and Nd crustal contributions derivedfrom metasedimentary rocks of the Arquia Complex.Porphyritic rocks are more important sources of ore-

forming components for the deeper Marmato Bajo zone,with hydrothermal fluids likely derived from theMarmatoStock (porphyry system) and migrating through themetasedimentary sequences. The low 87Sr/86Sr values,together with the positive εNd values obtained for theMarmato stock, suggest that subduction-related mantlemagmas produced during the subduction of the NazcaPlate beneath the South American continent are the mainsource for these rocks.

All sulfides of the first and second stages of miner-alization show Sr isotopic compositions that are moreradiogenic than those of the carbonates from the thirdstage of mineralization (Fig. 5A). This suggests that thesulfide-bearing fluids have circulated not only within theMarmato Stock but also within the graphitic schist and/orquartz–sericitic schist of the Arquia Complex. Hydro-thermal fluids related to the third stage of mineralization,in equilibrium with calcite at this stage appear to havecirculated exclusively within the Marmato Stock.

Our Sr, Pb and Nd isotopic data are consistent with amixing of meteoric and magmatic fluids, with the mag-matic contribution to the ore being from the MarmatoStock. It is likely that the Cauca–Romeral shear zoneallowed circulation of hydrothermal fluids related to thefirst and second stages of ore formation and for inter-action of the ore fluids with metasedimentary rocks,most likely the graphitic schists and amphibolite of theArquia Complex.

6.2. Mineralization and geodynamics

The geodynamic setting of the Marmato mining dis-trict involves subduction-related magmatism that oc-curred in response to plate convergence in the Miocene,within a combined back arc and pull-apart basin envi-ronment. The low 87Sr/86Sr initial ratio of 0.7044 andthe positive εNd values of +2.2 for the Marmato stocksuggest a juvenile, mantle-derived magma as the mainsource of the calc–alkaline to tholeiitic magmatism,although a minor magmatic contribution resulting fromthe partial melting of the lower continental crust cannotbe discarted. The ascension of the magma into the uppercrust was controlled by fractures related to the Cauca–Romeral shear zone.

Based on previous geochronological data (Restrepoet al., 1981; Sierra et al., 2000; Ordonez, 2001; Vinasco,2001), and on isotopic results reported here, thegeological and metallogenetic evolution of the Marmatomining district area can be summarized as follows:

12 Ma The Panamá–Choco Arc collided with the SouthAmerican Continent producing reactivation of the


Cauca–Romeral Shear Zone (CRSZ) and devel-opment of pull-apart basins in the Cauca–Patiadepression.

11 Ma Calc–alkaline and tholeiitic magmas of theCombia Formation were emplaced.

6.3 Ma The Marmato Stock crystallized and was as-sociated with the reactivation of the CRSZ andfracturing of the Marmato Stock. Hydrothermalactivity commenced.

5.6 Ma Reactivation of the CRSZ, in association withwaningmagmatic activity of theMarmato Stock.Deposition of associated gold mineralization.

3.7–3.1 Ma Closure of the Pacific Ocean and CaribbeanSea through the collision of the Panamá–Choco Arc with South America.

7. Summary and conclusions

The host dacitic subvolcanic Marmato stock istypical of active continental margins, as supported bythe Pb, Sr and Nd isotope data. The body was producedduring subduction of the Nazca Plate beneath SouthAmerica from a magma derived from partial melting ofoceanic basalt and metamorphic basement of the CalimaBlock.

The main characteristics of the Marmato gold depos-its are: (1) variable amounts of pyrite, sphalerite andgalena with accessory and trace quantities of chalcopy-rite, arsenopyrite, pyrrhotite, argentite and gold native/electrum; (2) two major styles of alteration are present(sericitization and propylitization), although silicifica-tion, argillization, albitization and potassic alteration arelocally present; carbonate and chlorite also developedfrom the alteration of mafic minerals; and (3) the miner-alization is controlled by the movement along Cauca–Romeral Fault System, which served as a pathway forthe gold-bearing fluids.

Three major stages of mineralization have been dis-tinguished. The first and second stages are associatedwithwidespread hydrothermal circulationmainly throughthe Cauca–Romeral shear zone and at least two differentsource rocks. The third stage of mineralization formedfrom small hydrothermal cells that circulated exclusivelywithin the Marmato stock.

The main geological features of the shallow levelmineralization from Echandia and Cien Pesos zones,reported by Rossetti and Colombo (1999), such as theassociation with subvolcanic rocks, the abundance ofsericite and adularia, and the occurrence of open space-filling vein textures, together with our radiogenic Sr, Pband Nd isotopic compositions, are indicative of an epi-thermal environment. On the other hand the geological

characteristics of the mineralization from the lowestlevel of the Marmato hill (Marmato Bajo zone), like low87Sr/86Sr initial ratios and the positive εNd values, thepresence of potassic alteration (Mora and Cuellar, 1982)and propylitic alteration over large areas indicate that thedeeper zone of the Marmato mining district could berelated to a porphyry type hydrothermal system.

The likely age of gold mineralization (5.6 Ma) iscoeval with movement along the Cauca–Romeral FaultSystem and is slightly younger than the cooling age of6.7±0.06 Ma for the Marmato stock. The gold quartzveins are epigenetic and controlled by movement alongshear zones during the cooling of the pluton. Based onthe classification schema of Groves et al. (1998) theMio-Pliocene Marmato gold mineralization is epizonal.

The results of our study suggest that metasedimentarysequences of the Arquia Group and elsewhere within theMarmato stock may be considered as prospective targetsfor further gold mineralization.

Acknowledgements

We thank FAPESP-Fundação de Apoio a Pesquisa doEstado de São Paulo (Project number 99/11841-7) forfinancially supporting FP with a scholarship, as well asthe CNPq for a research grant to CCGT. We also thankINGEOMINAS geologists for their cooperation with thefield work and sampling, as well as the staff members ofthe Geochronological Research Center of University ofSão Paulo for their help with isotopic analyses. We areespecially grateful to David John and Paul Spry for theirconstructive comments and corrections on drafts of thispaper, Jason Kirk for final revision, and Ore GeologyReviews Editor Nigel Cook for his help during finalediting.

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age and sources of gold mineralization in the marmato mining

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