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Geological Society, London, Special Publications doi: 10.1144/SP402.2 p121-145. 2014, v.402;Geological Society, London, Special Publications Evandro L. Klein deposition sources, and mechanisms of Au transport and Brazil: a review of the physico-chemical properties, Ore fluids of orogenic gold deposits of the Gurupi Belt, service Email alerting new articles cite this article to receive free e-mail alerts whenhereclick request Permission part of this article to seek permission to re-use all orhereclick Subscribe Collection London, Special Publications or the Lyell to subscribe to Geological Society,hereclick Notes © The Geological Society of London 2014 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://www.lyellcollection.org/cgi/alerts http://www.geolsoc.org.uk/permissions http://www.lyellcollection.org/site/subscriptions http://sp.lyellcollection.org/ http://sp.lyellcollection.org/ Ore fluids of orogenic gold deposits of the Gurupi Belt, Brazil: a review of the physico-chemical properties, sources, and mechanisms of Au transport and deposition EVANDRO L. KLEIN CPRM/Geological Survey of Brazil. Av. Dr. Freitas, 3645, Belém, State of Pará, Brazil, CEP: 66095-110 (e-mail: evandro.klein@cprm.gov.br) Abstract: The Neoproterozoic Gurupi Belt in northern Brazil developed at the southwestern margin of the Palaeoproterozoic São Luı́s-West Africa Craton. Orogenic gold deposits of this belt are hosted in Palaeoproterozoic (2160–2147 Ma) metavolcano-sedimentary and calc-alkaline granitoid rocks formed in arc and/or back-arc settings during a protracted Rhyacian orogeny (2240–2080 Ma). These host rock assemblages were tectonically and isotopically reworked during the Neoproterozoic and represent the reworked margin of the craton, that is, the external domain of the Neoproterozoic (Brasiliano-Pan African) orogen. The location of the gold deposits is controlled by the Tentugal shear zone, which represents the tectonic boundary between craton and the Gurupi Belt, and its subsidiary structures. Gold occurs in veins and in association with pyrite, and subordinately arsenopyrite and chalcopyrite, in strongly altered and variable deformed host rocks. Geological characteristics, petrographic, fluid inclusion, and isotopic evidence indicate near-neutral, reduced aqueous-carbonic metamorphic fluids, with local contributions from host rocks at the deposit site. Ore deposition occurred at about 300–370 8C and up to 3 kbars in response to fluid immiscibility and fluid-rock reactions (sulphidation, desulphidation, carbonatiza- tion, CO2 removal) and local fluid mixing and oxidation. The Gurupi Belt is a Neoproterozoic mobile belt developed at the southwestern margin of the São Luı́s-West African Craton (Fig. 1) during the Brasiliano-Pan African cycle of orogenesis (see Klein & Moura 2008 for a review). Gold is the main known mineral resource in this belt, and the metal has intermittently been mined by artisanal miners since the 17th century. There is no official historical record on the artisanal production, but unofficial data indicate over 16 t of gold has been extracted from Chega Tudo, Serrinha, and Cacho- eira. To date, more than 120 gold occurrences are reported, in addition to more developed deposits (Klein & Lopes 2011). The known gold resources of four deposits are equal to about 120 t Au (Table 1), which will be mined in open pit. The opening of the first organized, industrial operation is planned to occur at Cipoeiro. In the last decade efforts have been made to improve the understanding of the geological evol- ution of the Gurupi Belt (Ribeiro 2002; Klein et al. 2005b; Teixeira et al. 2007; Klein & Lopes 2009, 2011; Palheta et al. 2009) and of individual deposits (Torresini 2000; Ribeiro 2002; Yamaguti & Villas 2003; Klein et al. 2005a, 2006, 2007, 2008b). Results provided constraints on: (1) the nature and age of the host rocks; (2) the nature of the hosting structures; (3) the composition of the ore fluids; and (4) the T–P conditions of ore depo- sition. An integration of these studies is presented in this review, which attempts to link geological processes documented at the provincial scale with those documented in individual deposits. Special attention will be given to the circulation of fluids that have led to the formation of the deposits, espe- cially the evaluation of the mechanisms of gold transport and deposition. Other parameters that are critical for the development of genetic models have not yet been determined. Such parameters are mostly dependent on the absolute age of gold depo- sition, and also include the tectonic setting of ore formation. A brief discussion of these issues is addressed throughout the review. Geological overview São Luı́s cratonic fragment Considering that a significant part of the Gurupi Belt is made of reworked units of the São Luı́s cratonic fragment (Fig. 1), the geology and evol- ution of this cratonic fragment is briefly outlined here (based on Klein et al. 2008a, 2009, 2012; Palheta et al. 2009). Three main stratigraphic units crop out in the cratonic area (Fig. 1a). The oldest rocks known in the São Luı́s cratonic fragment belong to the Aurizona Group, which is an island arc-related metavolcano-sedimentary sequence of 2240 + 5 Ma that was intruded by shallow grano- phyric rocks at 2214 + 3 Ma and by the juvenile, From: Garofalo, P. S. & Ridley, J. R. (eds) 2014. Gold-Transporting Hydrothermal Fluids in the Earth’s Crust. Geological Society, London, Special Publications, 402, 121–145. First published online March 19, 2014, http://dx.doi.org/10.1144/SP402.2 # The Geological Society of London 2014. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ Fig. 1. (a) Location of the São Luı́s cratonic fragment and Gurupi Belt (hatched). For the sake of scale, only the major rock units of the São Luı́s cratonic fragment are considered here. (b) Simplified geological map of the Gurupi Belt showing the main gold deposits and showings. E. L. KLEIN122 by guest on August 7, 2014http://sp.lyellcollection.org/Downloaded from http://sp.lyellcollection.org/ subduction-related, metaluminous to slightly pera- luminous calc-alkaline granitoids developed in an island arc between 2168 and 2147 Ma. These gran- itoids form batholiths and stocks that are included in the Tromaı́ Intrusive Suite, which comprise mainly tonalites and granodiorites and subordi- nated quartz-diorite, monzogranite, and syenogra- nite. Minor units, which do not appear in the scale used in Figure 1a, include andesites, dacites, felsic tuffs, and subordinately basic volcanic rocks depos- ited over the Aurizona Group and the Tromaı́ Suite at 2164–2160 Ma. These volcanic associations have metaluminous to peraluminous, high-K calc- alkaline to tholeiitic signature and have been inter- preted to have formed in mature or transitional arc with minor back-arc component and active continental margin, respectively. Peraluminous, collision-type granites of c. 2100 Ma belong to the Tracuateua Intrusive Suite, crop out in the western portion of the cratonic area and are covered by Pha- nerozoic sequences. Late- to post-orogenic small plutons of highly evolved/shoshonitic granite of 2056–2076 Ma and are the youngest rocks known so far in the São Luı́s cratonic fragment. Based on rock association, geochronology, and on geochemical and Nd isotope signatures, these Palaeoproterozoic associations of the São Luı́s cra- tonic fragment are interpreted as a section of the Rhyacian orogen, which records an accretionary stage at 2240–2150 Ma and a collisional stage at c. 2100 Ma (Klein et al. 2008a, 2009, 2012). This scenario correlates with what is described for similar successions of the Eburnean-Birimian ter- ranes of the West-African Craton (Klein & Moura 2008 and references
