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THE ARENOPLIS-MARA ROSA GOLD-COPPER BELT,NEOPROTEROZOIC GOIS MAGMATIC ARC

CLAUDINEI GOUVEIA DE OLIVEIRA1, CLAUDIA LIMA DE QUEIROZ1 AND MRCIO MARTINS PIMENTEL1

ABSTRACT The Gois Magmatic Arc hosts several gold and copper-gold deposits, which were originated during several stages of the tectonic,magmatic and metamorphic evolution of this Neoproterozoic Arc, between ca. 0.9 and 0.6 Ga. The nature of these deposits, their spatialdistribution, and timing of mineralization agree with the model involving the continuous evolution of a collisional belt. They also display someof the characteristics of the orogenic gold deposits. The Zacarias Au-Ag-Ba Deposit was originated by volcano-exhalative processes in an intra-oceanic subduction setting, whereas the Chapada Cu-Au Deposit (porphyry copper-type mineralization) is connected with plutonic rocks alsoassociated with an intra-oceanic arc. The Posse Au Deposit was formed during the syn-collisional phase, shortly following the metamorphic peakof the Brasiliano Orogeny (ca. 630 Ma). Finally, the genesis of the Mundinho Au-Cu-Bi Mineralization is related with the emplacement ofgranite intrusions associated with post-orogenic extension.

Keywords: Gois magmatic arc, gold and copper-gold mineralization, orogenic evolution.

INTRODUCTION Recently, Groves et al. (1998) introduced theterm orogenic gold deposits, as a substitute for the so-calledmesothermal gold deposits. According to the original definition ofLidgreen (1933 in Groves et al. 1998), the term mesothermal refersonly to ore deposits formed at depths between 1.2 to 3.6 km. Suchdepth range, however, does not include the majority of gold depositsclassified as mesothermal, in which pressure varies between 1 and 3kbar and temperature ranges from 300 to 400C.

Orogenic gold deposits are classified according to their depths offormation as: (i) epizonal (< 6 km, 150-300C), (ii) mesozonal (6-12km, 300-475C) and (iii) hypozonal (> 12 km, > 475C). The term"orogenic deposits" also carries a tectonic significance, since most ofthe deposits are formed in compressive and transpressiveenvironments, within convergent plate margins during collisional andaccretionary orogeny (Groves et al. 1998).

In the central part of Brazil, the Neoproterozoic Gois MagmaticArc, a large segment of 900-650 Ma juvenile crust, hosts several goldand copper-gold deposits, which are herein included in the so-calledArenoplis-Mara Rosa gold-copper belt. The aim of this paper is toreview the main features of these deposits and to discuss models fortheir origin and evolution. Some of them are characterized as typicalorogenic gold deposits.

THE GOIS MAGMATIC ARC The Tocantins StructuralProvince corresponds to a large Neoproterozoic (Brasiliano/Pan-African) orogenic zone developed between two major continentalblocks: The Amazon Craton in the west; and the Sao Francisco Cratonin the east (Fig. 1). The eastern part of the Province is occupied by theBrasilia Belt, which includes mainly a thick metasedimentary sequenceand a large area where juvenile Neoproterozoic arc rocks are exposed(The Gois Magmatic Arc). Within the Tocantins Structural Province,the most pervasive group of structures shows mass transport towardsthe Sao Francisco Craton. This pattern is interpreted as the mostconspicuous features of the Neoproterozoic deformation, during theBrasiliano Cycle (Fucket al.1994).

The Arenoplis-Mara Rosa gold-copper belt is located in the Gois

Magmatic Arc, which extends for more than 400 km the western andnorthern parts of the Gois State (Fig. 1). Two main areas of Neoproterozoic juvenile crust have been identified. These are hereafterreferred to as the Arenoplis and Mara Rosa Arcs, respectively in thesouthern and northern ends of the arc (Pimentel et al. 1991,1997). TheMara Rosa Arc is the focus of this paper because it contains themajority of deposits.

GEOLOGY AND GEOCHRONOLOGY The Gois MagmaticArc is made largely of tonalitic/dioritic orthogneisses, which underlielarge areas between narrow N20-30E volcano-sedimentary sequences.In the Chapada-Mara Rosa Area, these supracrustal rocks form threeindividual NNE belts, known as the eastern, central and western belts,separated from each other by metatonalites/metadiorites. These beltsare made of metabasalts, intermediate and felsic metatuffs, fine-grainedmetagraywackes, garnet mica schists, metacherts, iron formations,

quartzites and metaultramafic rocks, metamorphosed in the Figure 1 -Regional geologic map of the Northern Sector of the TocantinsProvince, in Central Brazil (after Fuck et al. 1994).

1 Universidade de Brasilia, Institute de Geocincias, Brasflia-DF, Brazil, 70910-900. e-mail: gouveia@unb.br

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greenschist- to amphibolite-facies conditions (Arantes et al. 1991).Uranium-lead zircon data for small elongated bodies of myloniticgranites this felsic rock in the Posse Gold Mine yield a crystallizationage of 862 8 Ma. Titanites from the same sample yield ametamorphic age of 632 4 Ma (Pimentel et al. 1997).

Amphibolites of the volcano-sedimentary sequence are eithertholeiitic, rich in Mg, Ni and Cr and similar to boninites, or calc-

alkaline. According to Palermo (1998), the former could representfragments of oceanic crust and the latter are related to the arcmagmatism. In the Chapada Area, garnet and epidote amphibolites arechemically similar to modern-arc tholeiites and are interpreted as beingoriginated in a back-arc setting (Kuyumjian 1989).

Metasedimentary rocks represented by feldspathic garnet micaschists and fine-grained biotite gneisses are abundant in thesupracrustal belts, especially in the western belt. Samarium-neodymium isotopes for these rocks indicate TDM values dominantly inthe range 0.9-1.2 Ga, with only one analysis yielding a significantlyolder model age of 1.6 Ga. This indicates that they are the products oferosion of the arc rocks, with little contribution from older sources.The deposition of the original sediments must have taken place farfrom any old continental source area and probably happened in anintra-oceanic setting. Samarium-neodymium garnet-whole rockisochrons for these metasedimentary rocks indicate ages ofca. 733,765, 604 and 610 Ma. These were interpreted as resulting from twometamorphic episodes: an early one at ca. 760 Ma and a latermetamorphic event with typical Brasiliano ages (ca. 600 Ma).

In the Chapada-Mara Rosa Area, the medium- to coarse-grained,grey metaplutonic rocks of dioritic to tonalitic compositions, locallyshow well preserved plutonic textures such as enclaves and porphyritictextures. They are geochemically very primitive with SiO. contents ofless than 60%, with a calcic to calc-alkaline character, characterized bylow Rb, Nb, Y, Zr and REE. They are similar to M-type granitoids ofimmature island arcs (Pimentel et al. 1997). Uranium-lead zircon dataindicate crystallization of the protolith at 856 13 Ma. Neodymiumisotopic data indicate the very primitive nature of the original magma,with TDM values of ca. 0.9-1.0 Ga and Nd (T) of +4.6. The lastdeformational event affecting the Mara Rosa rocks at ca. 600 Ma wasimmediately followed by the intrusion of several granitic intrusions

(e.g. the Faina, Angelim, Estrela and Amador granites) as wells asgabbro-dioritic bodies. The granite bodies include mainly biotitegranites and two-mica leucogranites, with local granodioritic facies(Pimentel et al. 1997). The mafic intrusions are made of diorites and,to a lesser extent, gabbros, and very commonly display magma mixingstructures.

Similarly to the Arenpolis Area, the Precambrian geologicalevolution of the Mara Rosa arc ended, therefore, with an importantbimodal magmatic event. In both cases this is interpreted to beassociated with final uplift and collapse of the Brasiliano Orogen.

The bimodal nature of this magmatism suggests that the input ofheat required to promote large-scale melting of the crust was most probably provided by the emplacement and/or underplating of maficmagma into the continental crust. This abundant post-Brasilianogranitic magmatism is spatially confined to the regions underlain bythe juvenile arc rocks. This suggests that these were preferable sites ofuplift and melting of the mantle and continental crust or, alternatively,that the juvenile crust be'haved as a more "fertile" material for granite production. Field and geochronological evidence suggest that theemplacement of these bodies occurred under extension, whichaccompanied rapid uplift and unroofing of the region. Amphibolitefacies metavolcanic rocks with U-Pb titanite ages of ca. 590 Ma areintruded by ca.560 Ma sub-volcanic microgranite dikes.

NEOPROTEROZOIC DEFORMATION IN CENTRALBRAZIL The Brasiliano structures can be grouped into twodeformational phases, Dn and Dn+1. During Dn, southeast-verging,tangential structures were generated. These encompass tangential shearzones and thrust faults, always accompanied by tight to isoclinal folds(NE to NW axis), locally a-folds, and a conspicuous NW stretchinglineation. These structural features were rotated during Dn+1 mainly

into NW- and NE-directed structures. It also formed open to gentlefolds (NS axes), a pair of synchronous crenulation cleavages (NS andEW axes) and two types of directional shear zones, of first and thirdorders. The first-order shear zones are oriented about N50W, have asinistral character, and may be accompanied by N70E dextral shear

zones, whereas the third-order shear zones are mainly NS-striking anddextral

In the Chapada-Mara Rosa Area, the most important structuralfeature is the Rio dos Bois Fault System (Fig. 1), which has beeninterpreted as a single fault, originated during the Brasiliano Cycle,with an oblique reverse dextral sense. However, geophysical imagesshow that the NE trace of the Rio dos Bois System does not belong to

the same structure of the north concave trace. The north concave tra-ce separates the Archaean greenstone belts of Crixas Region from thePalaeoproterozoic Santa Terezinha Sequence, showing a generalnorthward transport. The northeast trace separates the Santa TerezinhaSequence from the Chapada-Mara Rosa Neoproterozoic Sequence,and extends into the greenstone terrains, crosscutting them in anoblique dextral sense. Thus, the concave-to-the-north trace isinterpreted as an ancient structure, due to the TransAmazonian Cycle(Palaeoproterozoic), whereas the northeast trace is interpreted as a Neoproterozoic structure. Also important in the Chapada-Mara RosaArea are the NS-dextral shear zones, which may control somemineralizations.

GOLD AND GOLD-COPPER DEPOSITS The ArenoplisMara Rosa Region contains some important Au (Posse, Zacarias,

Fazenda Nova) and Cu-Au (Chapada, Bom Jardim, Mundinho)deposits (Fig. 1). Mining companies have investigated the region sincethe beginning of the 1970's. During this period, investments inexploration were discontinued several times, due to gold pricefluctuations in the international market.

Gold and gold-copper deposits of the Mara Rosa Belt occur in fourmain associations: i) Au-Ag-Ba (e.g. Zacarias Deposit), which isinterpreted as a stratiform volcanogenic-type deposit (Poll 1994); ii)Cu-Au (e.g. Chapada Deposit), which has been interpreted either asvolcanogenic- (Kuyumjian 1989), or as a porphyry-type deposit(Richardson 1988); Hi) Au-only deposit (e.g. Posse Deposit), whichhas been interpreted as epigenetic disseminated deposit controlled bya mesozonal shear zone (Palermo 1998); and iv) Au-Cu-Bi (e.g.Mundinho deposit), which are considered as vein-type depositscontrolled by magnetite-rich diorites.

The Zacarias orebody, characterized by the gold-silver-barite

association, is concordant with the host amphibolites of probablevolcanoclastic origin. The ore zone consists of concordant lenses ofquartz, barite, barium-muscovite (oellacherite), pyrite, and minorsphalerite, galena, chalcopyrite, zincian spinel, magnetite, electrum,freibergite and boulangerite, with subordinate tetrahedrite/bournonite.Molybdenite and covellite are also present in very small amounts (Poll1994). Gold content is proportional to barite content. Goldconcentration in barite quartzites varies from 3.0 to 15.0 g/t, whileoellacherite quartzites contain generally less than 3.0 g Au/t (Poll1994).

The Chapada copper-gold deposit consists of a disseminated pyrite-chalcopyrite-magnetite mineralization, hosted by feldspathic quartzitesand biotite-rich schists of probable tuffaceous origin. Bornite,chalcosite, sphalerite, galena, pyrrothite and molybdenite aresubordinate. Sulfides are deformed, indicating a pre-metamorphicorigin of the deposit. Gold occurs in chalcopyrite or between sulfidegrains. The geological resource stands at 30 Mt of oxidized ore,grading 0,6 g/t gold and ca. 200 Mt sulfide ore zone grading 0.4 g Au/t and 0,43 % Cu.

The Posse Gold Deposit is hosted by microcline gneisses derivedfrom felsic rocks. Basic metavolcanic rocks occur at the footwallsequence and locally host mineralization. Hydrothermal alteration isintense and affects both the hangingwall and the footwall sequences.An outer alteration halo is composed by a propylitic assemblage,composed of epidote-pyrite-sericite. The inner zone is represented bystrong silicification with abundant associated sulfide (pyrite). Goldoccurs mainly as native grains, but also associated with tellurides(frohbergite and calaverite). Gold mineralization is controlled by aseries of parallel lenses trending N20-30E/50 NE and plunging 35-45 SW. The deposit has a maximum length of 1 km and is 300-meterswide in the southern portion and 400 meters wide in its northern

portion. The gold reserve is 1.7 Mt grading 2.24 g/t (down to 60meters) (Arantes et al. 1991).

The occurrences controlled by the NS-transcurrent system are veinsor quartz segregations containing quartz, sulfides (pyrite, chalcopyrite,bismutinite), magnetite and ilmenite. The veins are hosted in granites

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(e.g. Mundinho deposit ) and magnetite-rich diorites (Surucaoccurrence), which include an association of wall rocks represented bymagnetite quartzites and magnetite-pyrite-muscovite-quartz schists.

GOLD MINERALIZATION AND HYDROTHERMALALTERATION Hydrothermal mineral parageneses have beendescribed in several of the gold-copper mineralizations of Chapada-

Mara Rosa Region. The necessity of working with metamorphosed anddeformed products, with no evidence of a neat zoning of the halos ofhydrothermal alteration, led Richardson (1986) and Kuyumjian (1989)to distinct propositions about the genetic model involved in thegeneration of the hydrothermal associations in the Chapada deposit.

Kuyumjian (1989, 1991) gives special attention to the alterationzones that occur adjacent to the mineralization in the Chapada deposit.The author suggests that epidote-rich and epidosite zones result fromintensive, high-temperature interaction of sea water with basaltic rocksprior to the tectonic event responsible for the penetrative schistosity inthe Chapada rocks. The epidosites may represent the feeder systemthrough which hydrothermal solutions moved to form the ore deposits.The magnetite pyritic quartz sericite schists, also very common in theChapada-Mara Rosa Region, could represent a metamorphosed phyllicalteration zone formed as a result of the metasomatism imposed byhydrothermal solutions. The author also emphasizes that the presenceof a staurolite-gedrite rock closely associated with the mineralization

in the Chapada deposit may represent the metamorphosed chloridealteration zone, while the presence of microcline-bearing schists, richin K2O, suggest that potassic alteration occurred during the formationof the copper-gold mineralization in Chapada.

On the other hand, Richardson (1986) combines geologic andgeochemical hydrothermal alteration evidences rather thanmineralogical to suggest that the Chapada deposit represents theremains of a porphyry copper deposit of island-arc setting, in whichmost of the mineralization was hosted by wall rock. The evidencecomprise: an intensely disseminated distribution of the copper andgold and the absence of massive sulfide lens; the grade, tonnage andgold values, perfectly compatible with those of island-arc porphyrycopper deposits; the sulfide and oxide minerals at Chapada, as well astheir zonations, including the peripheral pyritic shell and the centralmagnetite-rich zone, are the same as those commonly seen in porphyrycopper deposits; the magmatic affiliation deduced from the S isotope

composition (zero per mil); the chemistry of the alteration around thedeposits is also similar to that seen in porphyry copper deposits, withenrichment in K2O and depletion in Na2O and CaO.

The studies in the Posse Deposit are also controversial, not onlyregarding the genesis of the mineralization but also the nature of thewallrock. For Arantes et al. (1991), the microcline gneisses that hostthe deposit represent metamorphic products of acid volcanic rockspreviously altered by the circulation of aqueous solutions. The genesisof the mineralization might have been connected with this volcano-exhalative interaction. On the other hand, Palermo (1998) interpretsthe microcline gneisses as original alkaline granitic intrusions, withevidence of hydrothermal alteration (serialization, albitization), whichresembles those which follow the process of greisenization. The mainevent of gold mineralization, however, is not would not be connectedwith this late-magmatic episode, but with a later event after the peak ofmetamorphism and deformation event. Gold mineralization iscontrolled by halos of propilitization, albitization, sericitization,

silicification, carbonation and deposition of oxides (magnetite,ilmenite), sulfides (pyritechalcopyritepyrrhotite) and tellurides(Au, Ag, Bi, Pb, Fe).

Ideas concerning the genesis of the Zacarias deposit converge to aconsensus. This deposit, with lenticular geometry and hosted by basic

metavolcanic rocks and chemical metasedimentary rocks, is constitutedby different kinds of sulfide- and barite-rich rocks generated in a proximal volcano-exhalative environment. Hydrothermal alteration products are represented by aluminous, quartz-biotite schists, probablya product of metamorphism of clay-sericite-rich debris derived fromhydrothermal vents. Other minor mineral alteration products includetalc, chlorite, epidote, sericite, quartz, K-feldspar and calcite (Poll1994).

Similarly to the gold deposits hosted by the hydrothermally alteredgranitic mylonites (Mundinho deposit), there is no direct relation between the hydrothermally altered wallrock (epidote-rich rocks toepidosites and magnetite-pyrite-quartz-muscovite schists) and themineralized rocks. In this case, the halos of potassic alteration whichaccompanied the mineralization were controlled by transcurrent-NSdeformation, which clearly post-date the alterations which previouslyaffected the wallrock.

DISCUSSION AND CONCLUSIONS The gold and coppergold deposits, located within the Gois Magmatic Arc, can betemporally and spatially related to the magmatic evolution model of acollisional belt proposed by Harris et al. (1986), as well as to theerogenic gold deposits model postulated by Groves et al. (1998).These models are based on a continuous evolution of collisional plates,which can be divided into four stages with distinct magmatic

characteristics: i) subduction stage; ii) syn-tectonic collisionalmagmatism stage; Hi) post-tectonic collisional magmatism stage; andiv)post-orogenic extension stage.

In the Tocantins Province the first stage, intra-oceanic subduction,probably occurred between 0.8 and 0.9 Ga and is represented bytholeiitic volcanic rocks and calc-alkaline plutonics (M- and I-typegranitoids) of island arcs and back-arc basins. During this stage thedeposits of Zacarias (Au-Ag-Ba), Chapada (Cu-Au) and Bom Jardim(Cu-Au) were formed. The Zacarias deposit was generated due toproximal volcano-exhalative brines (Poll 1994), whereas the Chapadaand Bom Jardim deposits are probably related to porphyritic copper-gold genesis.

Subsequent to this stage was syn-tectonic collisional stage, leadingto a maximum crustal thickening in the area, which is related to ametamorphic peak ofca. 630 Ma at the end of the Brasiliano orogeny.During this period, the northeast arm of the Rio dos Bois Fault System

evolved and controlled many gold occurrences in the region, includingthe Posse deposit (Au). The latter represents a mesozonal shear zone-controlled gold deposit (Palermo 1998), or the so-called erogenic golddeposit. The fluid migration in these shear zones might have started at760 Ma. This age is interpreted as another metamorphic peak, relatedto a poorly known event that occurred during an accretionary stagebetween oceanic and continental plates.

The post-tectonic collisional magmatism, and the post-orogenicextension, succeeded the maximum crustal thickening peak. This ismarked by the intrusion of alkaline granitic bodies (mainly biotitegranites and leucogranites), and magnetite bearing gabbro-dioritic bodies between 590 and 560 Ma. The Au-Cu-Bi Mineralization(Mundinho deposit), controlled by the magnetite bearing gabbro-dioritic intrusions, occurs as disseminated ore associated to theintrusions and/or veins. The veins are related to NS transcurrent shearzones (DN+1 Phase) that crosscut the Rio dos Bois System.

The evolution of the Gois Magmatic Arc provided many distinct

geotectonic environments, such as oceanic arc, back arc basin, andaccreted terrains. These environments are favorable to gold andcopper-gold deposit formation because of their depth, magmaticassociation and structural framework. The deposits here described andother small occurrences confirm this interpretation.

Arantes D., Buck P.S., Osborne G.A., Porto C.G. 1991. A Sequencia Vulcano-Sedimentarde Mara Rosa e Mineraliza9oes Auriferas Associadas.Boletim Informativo da SBG,

Niicleo Centro-Oeste,27-40.Fuck R. A. 1994. A Faixa Brasilia e a Compartimenta9ao Tectonica da Provincia Tocantins.

In: SBG. Simposio de Geologiado Centro-Oeste, 4. Brasilia.Anais, 1:184-187.Groves D.I., Goldfarb R.J., Gebre-Mariam M., Hagemann S.G., Robert F. 1998. Orogenic

gold deposit: A proposed classification in the context of their crustal distribuition andrelationship to other gold deposit types. Ore Geology Reviews, 13(1 -5):7-27.

Harris N.B.W., Pearce J.A., Tindle A.G. 1986. Geochemical characteristics of collision-zonemagmatism. In: M.P. Coward, A.C. Ries (eds). Collision Tectonics, London,Geological Society Special Publication, 19:67-81.

Kuyumjian R.M.I 989. The geochemistry and tectonic significance of amphibolites fromthe Chapada sequence, Central Brazil, Imperial College, England, PhD. Thesis, 289P-

Palermo N. 1998.Le gisement aurifere prcambrien de Posse (Golds, Brasil) dans sonscadre geologique. ENSMP, Paris, These de doctoral. 175 p.

Pimentel M.M., Heaman L., Fuck R.A. 1991. U-Pb zircon and sphene geochronology of lateProterozoic volcanic arc rock units from southwestern Gois, central Brazil. Journalof South American Earth Sciences,4:329-339.

Pimentel M.M., Whitehouse M.J., Viana M.G., Fuck R.A., Machado N. 1997. The MaraRosa arc in the Tocantins Province: further evidence for Neoproterozoic crustalaccretion in central Brazil.Precambrian Research, 81:299-310.

Poll N.J.I 994. The geology of the Zacarias gold-silver-harite deposit, Gois State, Brazil.Colorado School of Mines, USA, Master thesis, 124p.

Richardson C.J., Kesler S.E., Essene, E.J. 1986. Origin and geochemistry of the chapadaCu-Au deposit, Gois, Brazil: a metamorphosed wall-rock porphyry copper deposit.Economic Geology, 81:1884-1898.

Contribution IGC-007Received January 21, 2000

Accepted for publication May 8, 2000

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