Structural controls on gold mineralisation and the nature of related fluids of the Paiol gold deposit, Almas Greenstone Belt, Brazil

https://doi.org/10.1016/j.oregeorev.2003.05.001Get rights and content

Abstract

Chemical analyses suggest that the metavolcanic rocks of the Almas Greenstone Belt (AGB), Tocantins State, Brazil have a continental affinity, possibly related to a continental rift environment. They were metamorphosed to amphibolite facies during a regional tectono-metamorphic event (Dn), retrogressed to greenschist facies assemblages and then hydrothermally altered within dextral strike–slip shear zones (Dn+1). Fracture sets related to Dn+2 intersect Sn+1.

The Paiol Gold Mine is one of several mineralised zones within metabasic and meta-intermediate rocks of the AGB. It exploits shoots of sulphide–Au–quartz mineralisation that occupy dilational zones approximately perpendicular to an elongation lineation (Ln+1) within mylonitic foliation Sn+1 (Sn+1=S within the S–C fabric). The dilational zones probably formed due to dextral displacement on sinistrally en echelon C surfaces. Minor amounts of gold may have been introduced or remobilised during Dn+2.

Coexisting primary and pseudosecondary fluid inclusions in mineralised quartz veins from ore shoots comprise a high-salinity three-phase type (Type II) and a lower salinity two-phase type (Type I). Homogenisation temperatures for Type II inclusions range from 200 to 410 °C and Type I from 90 to 320 °C. The inclusions and their temperature ranges are believed to reflect heat exchange and some mixing between the two fluid types under relatively constant ambient temperatures, but variable (though broadly declining) fluid temperatures. This took place late in Dn+1 in conjunction with greenschist facies retrogression and localised hydrothermally induced metasomatism.

Introduction

Greenstone belts are sites of important gold-bearing ore deposits Groves and Bennett, 1993, Groves and Barley, 1994, and the Archaean and Palaeoproterozoic greenstone belts of Brazil (Schobbenhaus et al., 1984) are no exception to this. The genesis of gold mineralisations within one of the Brazilian greenstone belts, the Archaean Almas Greenstone Belt (AGB; Costa, 1984), is the concern of the present study.

The AGB is one of several greenstone belts located in the southern part of the Tocantins Province within a tectonically complex tract (the Goias Median Massif) that lies between the Brasilia-Uruaçu fold belt to the east and the Paraguai-Araguaia fold belt to the west (Fig. 1). It comprises a sequence of metavolcanic rocks (approximately 15 km long and 2 km thick—the Córrego Paiol Formation), overlain by a metasedimentary unit (approximately 10 km long and 300 m thick—the Morro do Carneiro Formation) that is cut by late kinematic granitic intrusions.

The metabasic and meta-intermediate (metadacite and meta-andesite) volcanic rocks of the Córrego Paiol Formation have experienced amphibolite facies metamorphism. The metabasic rocks now consist of fine-grained amphibolites and metadiabase, in which hornblende–plagioclase assemblages locally preserve sub-ophitic texture. Where affected by shear zones, the amphibolite facies rocks have been retrograded and hydrothermally altered to albite–carbonate–chlorite schists that are zonally distributed about centres of quartz–pyrite–gold mineralisation (Silva et al., 1990). Such centres include the Paiol Gold Mine and other small gold deposits (Fig. 2). The metasedimentary Morro do Carneiro Formation now consists of sericite phyllite, carbonaceous, banded iron formation, quartzite, felsic metavolcanic rock, metachert and tourmalinite that were metamorphosed to greenschist facies and later affected by hydrothermal alteration.

The Paiol Gold Mine is located approximately 35 km south of Almas (Fig. 2). It was discovered in 1980 and constitutes the largest known gold deposit within the region. Although records are fragmentary, it has been worked intermittently since 1996 and was active until 2001. Open-cut workings cover an area of 300×800 m and extend 80 m below surface (Fig. 3). Production to date is in the order of 4 t Au from 10.5 Mt of ore.

To better understand factors influencing emplacement of the gold mineralisation at the Paiol Gold Mine and other localities within the Córrego Paiol Formation (Fig. 2), structural and fluid inclusion studies were undertaken at the Paiol Gold Mine, and the metavolcanic host rocks of the Córrego Paiol Formation were chemically analysed.

Section snippets

Regional setting

The southeastern region of the Tocantins Province contains gneiss–migmatite terranes and Archaean granite–greenstone belts. They are part of the Goias Median Massif that lies between the Brasilia-Uruaçu and the Paraguai-Araguaia fold belts (Fig. 1; Costa, 1984, Costa et al., 1984). According to Borges et al. (1991), transtensional deformation of the oldest gneiss–migmatite basement in the region resulted in the formation of basins that were infilled by a sequence of greenstone and supracrustal

Host rocks: petrogenetic, geochemical and isotopic aspects

The rocks that host the Paiol Gold Mine and most of the other small deposits in the Almas region of the AGB are the altered metavolcanic rocks of the Córrego Paiol Formation (Fig. 2). Notable exceptions to this are the granite-hosted Vira Saia II mineralisation and a deposit within the Morro do Carneiro Formation (Fig. 2).

In general, the Córrego Paiol, Morro do Carneiro Formations and TTG rocks variably record a sequence of metamorphic, structural and metasomatic (including sulphide–Au

Structural geology of Paiol Gold Mine

The main structures at the Paiol Gold Mine were generated in a transcurrent shear zone, 80–400 m wide and up to 1800 m long, as delineated by Kwitko et al. (1995). Structures related to the Dn+1 event (deformation terminology from Kwitko et al., 1995) predominate, whereas the Dn structures are rarely observed, particularly as the foliations (Sn and Sn+1, corresponding to the two deformations) are almost coplanar. The final Dn+2 event produced an overprinting fracture system comprising synthetic

Gold, metasomatism and structure

Iron-rich metabasic rocks host the gold at Paiol Gold Mine. During the Dn+1 event, the original basic rocks were retrograded, transformed to schists within transcurrent shear zones and partly metasomatised and mineralised. The schists show hydrothermal zoning; actinolite and chlorite schists occur in the outer zone, albite–sericite–carbonate schists in the intermediate zone and carbonate–quartz schists (Fig. 10B) in the central zone. Quartz veins and quartz-rich portions of the central zone

Fluid inclusion types

The samples studied Fig. 2, Fig. 4, Fig. 81 consisted of sulphide–Au–quartz veins intersected in drill core and collected directly from mine workings. The abundant fluid inclusions were classified as primary, pseudosecondary and secondary, based on the criteria of Roedder (1984). They were then subdivided into three types depending upon the phases present at room temperature.

Type I inclusions are two-phase with a

Discussion and conclusions

The Paiol Gold Mine in the Almas Greenstone Belt is situated in a sequence of volcanic rocks that have been metamorphosed to amphibolite facies, retrogressed to greenschist facies and then variably affected by hydrothermal alteration. Gold was deposited in hydrothermal zones where the rocks are now quartz–sericite schists and quartz–carbonate schists. Chemical data from unaltered metavolcanic rocks show that they have continental affinities, probably related to a continental rift environment.

Acknowledgments

The authors are grateful to FAPESP (São Paulo Research Foundation), Process 96/06260-7 and 97/10885-5, for financial support for this research project. We thank CVRD (Cia. Vale do Rio Doce) for help during fieldwork and sampling at the mine site. Detailed reviews and suggestions by Paul Duuring, David Groves and Brian Marshall contributed substantially to the final paper, but the views and interpretations presented are the responsibility of the authors.

References (58)

  • S.H. White et al.

    On mylonite in ductile shear zones

    Journal of Structural Geology

    (1980)
  • R.J. Bodnar et al.

    MacFlinCor: a computer program for fluid inclusion data reduction and manipulation

  • M.S. Borges et al.

    Organização lito-estrutural do pre-Cambriano da região de Almas-Dianópolis, sudeste do Estado de Tocantins

  • M. Boryta et al.

    Geochemistry and origin of the Archaean Beit Bridge complex, Limpopo Belt, South Africa

    Journal of the Geological Society of London

    (1990)
  • P.E. Brown et al.

    MacFlinCor: a computer program for fluid inclusion data reduction and manipulation

  • A.C. Cattell et al.

    Archaean basic magmas

  • M. Choussidon et al.

    Sulphur isotope composition of orogenic spinel lherzolite massifs from Ariege (N.E. Pyrenees, France): an ion microprobe study

    Geochimica et Cosmochimica Acta

    (1990)
  • K.C. Condie

    Geochemical characteristics of Precambrian basaltic greenstones. In Early Precambrian basic magmatism

  • Costa, J.B.S., 1984. Aspectos litoestruturais e evolução crustal da região centro-norte de Goiás.. Centro de...
  • J.B.S. Costa et al.

    Geologia da região de Porto Nacional, GO

    Revista Brasileira de Geociências

    (1984)
  • Cruz, E.L.C.C., 1993. Geologia e Mineralizações aurı́feras de terreno granitóide-greenstone de Almas-Dianópolis,...
  • E.L.C.C. Cruz et al.

    Geology and tectonic evolution of the Tocantins granite-greenstone terrane: Almas-Dianópolis region, Tocantins State, Central Brazil

    Revista Brasileira de Geociências

    (1998)
  • E.L.C.C. Cruz et al.

    Mineralizações aurı́feras filoneanas do Terreno granito-greenstone do Tocantins

    Revista Brasileira de Geociências

    (1999)
  • J.C.M. Danni et al.

    Archean and lower Proterozoic units in central Brazil

    Geologische Rundaschau

    (1982)
  • G.H. Davis

    Structural Geology of rocks and regions

    (1984)
  • B. de Vivo et al.

    Fluid Inclusions in Minerals: Methods and Applications. Short Course of the Working Group (IMA).Inclusions and Minerals, Pontignanc, Siena, Italy

    (1994)
  • M. Dubois et al.

    The H2O–NaCl–MgCl2 ternary phase diagram with special application to fluid inclusions studies

    Economic Geology

    (1997)
  • M.A.D. Ferrari et al.

    Variações quı́micas nas rochas vulcânicas do Greenstone Belt de Almas—TO

  • M.A.D. Ferrari et al.

    Tectônica e a mineralização aurı́fera da Mina Córrego Paiol Almas—TO

  • Cited by (0)

    View full text