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