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Formation and Deformation of Pyrite and Implications for Gold Mineralization in the El Callao District, Venezuela

2013; Volume: 109; Issue: 2 Linguagem: Inglês

10.2113/econgeo.109.2.457

1554-0774

German Velásquez, Didier Béziat, Stefano Salvi, Luc Siebenaller, Anastassia Y. Borisova, Gleb S. Pokrovski, Philippe de Parseval,

Geochemistry and Geologic Mapping

Research Article| March 01, 2014 Formation and Deformation of Pyrite and Implications for Gold Mineralization in the El Callao District, Venezuela German Velásquez; German Velásquez 1Universidad Central de Venezuela, Instituto de Ciencias de la Tierra, Avenida Los Ilustres, Los Chaguaramos, Caracas 3895, Venezuela2Université de Toulouse, CNRS, Géosciences Environnement Toulouse, Institut de Recherche pour le Développement, Observatoire Midi-Pyrénées, 14 Av. Edouard Belin, F-31400 Toulouse, France Search for other works by this author on: GSW Google Scholar Didier Béziat; Didier Béziat 2Université de Toulouse, CNRS, Géosciences Environnement Toulouse, Institut de Recherche pour le Développement, Observatoire Midi-Pyrénées, 14 Av. Edouard Belin, F-31400 Toulouse, France Search for other works by this author on: GSW Google Scholar Stefano Salvi; Stefano Salvi † 2Université de Toulouse, CNRS, Géosciences Environnement Toulouse, Institut de Recherche pour le Développement, Observatoire Midi-Pyrénées, 14 Av. Edouard Belin, F-31400 Toulouse, France †Corresponding author: e-mail, stefano.salvi@get.obs-mip.fr Search for other works by this author on: GSW Google Scholar Luc Siebenaller; Luc Siebenaller 2Université de Toulouse, CNRS, Géosciences Environnement Toulouse, Institut de Recherche pour le Développement, Observatoire Midi-Pyrénées, 14 Av. Edouard Belin, F-31400 Toulouse, France Search for other works by this author on: GSW Google Scholar Anastassia Y. Borisova; Anastassia Y. Borisova 2Université de Toulouse, CNRS, Géosciences Environnement Toulouse, Institut de Recherche pour le Développement, Observatoire Midi-Pyrénées, 14 Av. Edouard Belin, F-31400 Toulouse, France3Geological Department, Moscow State University, Vorobevu Gory, 119899, Moscow, Russia Search for other works by this author on: GSW Google Scholar Gleb S. Pokrovski; Gleb S. Pokrovski 2Université de Toulouse, CNRS, Géosciences Environnement Toulouse, Institut de Recherche pour le Développement, Observatoire Midi-Pyrénées, 14 Av. Edouard Belin, F-31400 Toulouse, France Search for other works by this author on: GSW Google Scholar Philippe De Parseval Philippe De Parseval 2Université de Toulouse, CNRS, Géosciences Environnement Toulouse, Institut de Recherche pour le Développement, Observatoire Midi-Pyrénées, 14 Av. Edouard Belin, F-31400 Toulouse, France Search for other works by this author on: GSW Google Scholar Economic Geology (2014) 109 (2): 457–486. https://doi.org/10.2113/econgeo.109.2.457 Article history received: 23 Feb 2013 accepted: 17 May 2013 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation German Velásquez, Didier Béziat, Stefano Salvi, Luc Siebenaller, Anastassia Y. Borisova, Gleb S. Pokrovski, Philippe De Parseval; Formation and Deformation of Pyrite and Implications for Gold Mineralization in the El Callao District, Venezuela. Economic Geology 2014;; 109 (2): 457–486. doi: https://doi.org/10.2113/econgeo.109.2.457 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract The El Callao mining district is the most important gold-producing region in Venezuela. It is hosted in the Paleoproterozoic Guasipati-El Callao greenstone belt, which forms part of the Guayana craton, the Venezuelan extension of the Guiana Shield of South America. It consists of volcanic and volcanosedimentary sequences that were affected by several deformation events, particularly localized shear zones. The Colombia mine, the largest active mine in the district, produces 4 tonnes (t) (128,600 oz) of gold annually with reserves estimated at 740 t (24 Moz) and grades of up to 60 g/t. Gold mineralization is concentrated within a vein network in the Colombia corridor, a shear fracture-hosted mesh of interconnected quartz-ankerite-albite veins enclosing fragments of altered metabasaltic host rocks. Gold occurs mostly in the metabasaltic fragments and is spatially associated with pyrite, in which it occurs as invisible gold, micron-sized native gold inclusions, and filling fractures.Based on optical and scanning electron microscopy-backscattered electron observations, two types of pyrite are recognized: a simple-zoned pyrite and a less common oscillatory-zoned pyrite. Both types consist of a mineral inclusion-rich core and a clearer rim; however, in oscillatory-zoned pyrite, the latter is composed of complex rhythmic overgrowths of alternating As-rich and As-poor bands. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis and elemental mapping reveal the presence of invisible gold in all generations of pyrite. The highest concentrations (5–23 ppm Au) are found in oscillatory-zoned pyrite rims, which correlate with the highest As concentrations (16,000–23,000 ppm). In As-poor bands, Au (up to 1.5 ppm) and As (300–6,000 ppm) concentrations decrease by about an order of magnitude. Copper, Bi, Te, Sb, Pb, and Ag always occur with invisible gold, particularly in pyrite cores, suggesting that at least part of the gold occurs in sulfosalt nanoparticles of these metals and metalloids. Visible native gold grains occur as small inclusions throughout core and rim of both pyrite types, as well as in fractures within it. In both occurrences, chalcopyrite, sphalerite, tellurobismuthite, ankerite, albite, and chlorite accompany native gold, and gold fineness ranges between 900 and 930.At an early stage of vein mesh formation, pyrite formed in the metabasaltic fragments at the expense of ankerite, which, in turn, resulted from alteration of Fe-Ti oxides. Gold, together with other chalcophile elements, was incorporated within the structure of pyrite, most likely by destabilization of metal sulfide complexes during ankerite replacement. Subsequent cyclic reactivations of the shear zone caused development of pressure shadows around pyrite, generating local and repeated decreases in pressure, which triggered local boiling of the hydrothermal fluid, as evidenced by the presence of primary fluid inclusions containing immiscible liquid-rich and vapor-rich aqueous-carbonic fluids. This process was responsible for a number of physical-chemical changes in the liquid, all of which contributed to the formation of the As- and Au-rich overgrowths in pyrite: (1) removal of H2O into the vapor phase, inducing saturation of dissolved metals in the remaining liquid; (2) an increase in pH due to partition of H2S and CO2 into the vapor, thus decreasing the solubility of sulfide minerals; and (3) an adiabatic decrease in temperature, lowering the solubility of As and Au in the liquid. Waning of this process restored precipitation of As-poor pyrite, until the onset of a new cycle. Because pressure drops are more significant adjacent to open spaces, oscillatory-zoned pyrite probably crystallized near newly formed veins whereas simple-zoned pyrite formed away from them. Previously formed pyrite underwent fracturing during reactivation of the deformation, especially through the brittle deformation events that postdated shearing, resulting in local pulverization of pyrite. This newly created porosity facilitated fluid circulation and remobilization of structurally bound gold, as well as of other chalcophile elements (Ag, Cu, Bi, Te, Pb, and Sb), which reprecipitated together with pyrite in the form of native gold, sulfides, and tellurides, either as small inclusions or as larger grains within fractures. This remobilization process facilitates the exceptionally high gold tenor found in the deposit, where the Colombia corridor is intersected by the Santa Maria fault. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.