MAGM-3: Técnicas analíticas en geociencias

Gold-associated Cobalt mineralization in El Toqui Mine deposit, Aysén, Chile.

Héctor Suazo1, John Mortimer2, Olga Veloso3, Mauricio Belmar1. (1) Centro de Mineralogía Avanzada, SGS Minerals (2) Laguna Gold Limited, Australia (3) Laguna Gold, Chile

Introduction El Toqui is a Lower Cretaceous age stratiform deposit with massive Zn-Pb sulphides mineralization with some Au and Ag bearing minerals in a skarn type mineralogical association hosted in mainly late Jurassic submarine volcanic and sedimentary rock (Wellmer et al, 1983). Besides Zn and Pb high grades zones and silver and gold rich ones, there are some high values of As, Bi and Co grades. Cobalt particularly stand out due to last times rise of its price because of a higher demand for the electric car industry. A complete mineralogical characterization was carried out in 8 samples with comparative high cobalt grades (3000- 13000 ppm approximately) along with 75 hand specimens and/or drillcore complementary samples from different deposit´s zones. The major goal of this study was to identify main ore and related gangue alteration minerals, aiming to identify cobalt and gold mineralization and trying to characterize major ore and gangue minerals associated with Au and Co that could be used as tracer ones of cobalt and gold bearing minerals. Methodology For all samples, a petrochalcographical analysis on a thin-polished section was carried out, identifying completely all the alteration and ore minerals. Some complex minerals or those difficult to identify under the microscope were checked by x-ray diffraction for major ones, mainly in case of alteration gangue minerals and with TIMA-X automated mineralogy analysis for minerals in lower amounts, like sulphides or native metals. Gold and some silver minerals were identified mainly by polished section analysis by reflected light microscopy by its optical properties and checked with TIMA-X bright phase search analysis complemented with specific point measurements. Differences in the optical properties and in some cases in their BSE levels were key tools for the correct identification within very narrow composition range minerals. Results and discussion Alteration Minerals Main alteration minerals identified include as major ones, calcic pyroxenes like diopside, hedenbergite with minor johannsenitic (Mn) component and garnets of grossular and andradite type with some pyralspitic component (Mn-Mg) as well. K-Feldspar, quartz and calcite use to be commonly associated to pyroxenes and garnets. Calcic amphiboles like actinolite, ferrohornblende and ferropargasite and epidote-zoisite-clinozoiste are founded as an overprint to pyroxene and garnet alteration respectively. Other important calchosilicates founded were ilvaite, vesuvianite, prehnite, scapolite, wollastonite and locally manganese minerals like bustamite and rhodonite. Carbonates like ankerite, siderite, dolomite and kutnohorite were also identified, as late products usually as veinlets infill. Other common late minerals recognized are magnesium and ferric chlorite, sericite-muscovite, chlorite-smectite, clays, biotite, limonites and hematite. Ore Mineralization Major Zn-Pb-Fe sulphides deposit´s mineralization consists in sphalerite, pyrrhotite, galena associated with lesser amounts of pyrite, arsenopyrite, chalcopyrite and marcasite. Copper rich zones (chalcopyrite) are commonly associated with pyrrhotite rich zones and locally with more magnetite contents. On the other hand, arsenic richer zones are narrowly related with more contents of bismuth, cobalt and gold which is observed as a wide variety of bismuth tellurides, bismuth-lead sulfosalts and tellurides, glaucodot and cobaltite as main cobalt bearing minerals, while lead richer zones with higher contents of galena have a good correlation with higher silver contents, mainly as a silver/gold tellurides like hessite-petzite. The mineralogical association between ore and gangue alteration minerals can be summarized as follows: Massive sphalerite and pyrrhotite mineralization is closely related with main and massive garnet-pyroxene alteration, while arsenopyrite rich zones more related with gold bearing minerals and also with higher cobalt contents are associated with a retrograde lower temperature association of amphibole-epidote-carbonate-chlorite with variable amounts of ilvaite and prehnite. Cobalt mineralization Cobalt higher-grade zones are characterized by a mineralogical association with cobaltite and glaucodot closely related with arsenopyrite and its cobaltiferous variety “danaite” in zones with massive sulphide ore mineralization also containing massive pyrrhotite and sphalerite and commonly also chalcopyrite and minor pyrite and marcasite. In these mineralogical associations, main gangue alterations minerals are pyroxene with overprinting of retrograde alteration with amphibole, epidote, and minor chlorite and ilvaite. In zones with more amphibole-chlorite overprinting there is a higher amount of marcasite instead of pyrite and cobalt mineral associated it´s alloclasite rather than glaucodot. Alloclasite it´s a glaucodot dimorph but it´s possible to recognize it because its lower anisotropy and characteristic prismatic or radial habit. Finally, in more copper rich zones and with higher ratios of pyrrhotite/sphalerite and higher amounts of pyrite, cobalt is also present in cobaltiferous and very anisotropic pyrite and scarcely cobalt isotropic minerals like carrollite and linnaeite related with chalcopyrite. In these samples, main alteration minerals are pyroxene-calcite and k-feldspar. Gold and Silver mineralization Main gold/silver bearing minerals identified in analysed samples are mainly as a tellurides or native metals and are closely related with bismuth higher contents and usually also with higher cobalt values due to occurrence of associated glaucodot (Figure 1). Main gold minerals identified were native gold, electrum, petzite (Au-Ag telluride), maldonite, hunchunite and possibly jonassonite, with these last three ones commonly occurring in symplectitic intergrowths with native gold and bismuth, as long as for silver bearing minerals are hessite, stutzite and very scarce volynskite. All the gold and in lesser extent silver mineralization are closely related

909 MAGM-3: Técnicas analíticas en geociencias

with Bi/Pb tellurides like tellurobismuthite, hedleyite, joseite-B, pilsenite, kochkarite, alekseite, tetradymite and altaite as the main ones and with Bi/Pb sulfosalts like cosalite, galenobismuthite and aschamalmite. All these sulfosalts are also in a fine symplectitic intergrowth among them. (see Figure 1). Conclusion Although before this study, some occurrence type for gold and silver mineral were known, there was few detailed information about mineralogical associations for different ore types. This study points out that that the main sphalerite-pyrrhotite ore is related with the dominant pyroxene-garnet alteration, with pyrrhotite more related to pyroxene massive alteration and commonly also with k-feldspar. Chalcopyrite is also related with that kind of alteration and associated with pyrite and local magnetite. Other chalcosilicate minerals like ilvaite, vesuvianite, scapolite and prehnite are more related with marginal mineralization zones, although ilvaite was also commonly identified in zones with gold-silver-bismuth tellurides with arsenopyrite-glaucodot occurrence. Arsenic rich zones dominated by arsenopyrite occurs as a later product overprinted to sphalerite-pyrrhotite association and is commonly associated with occurrence of marcasite and impure pyrite (optically anisotropic) consisting in arsenical and/or cobaltiferous pyrite. Gold and Silver minerals are associated with arsenopyrite rich zones, but more exactly with glaucodot occurrence, cobaltiferous arsenopyrite or cobaltite. Last one occurs in gold richer zones. Furthermore, these gold-silver minerals are related with a lower temperature alteration overprinted to pyroxene-garnet one and consist of calcic amphibole, chlorite and phengite-muscovite. Cobalt occurs mainly as a glaucodot, cobaltite, cobaltiferous arsenopyrite (var. “danaite”) and cobaltiferous pyrite and alloclasite instead of glaucodot in marcasite richer zones. Carrollite and linnaeite are very scarce and are associated only in chalcopyrite richest zones. Silver minerals are more related with galena richer zones and with occurrence of Pb-Bi minerals like sulfosalts and tellurides and native bismuth also. Gold, electrum and other gold/ silver bearing minerals like petzite, hessite, maldonite and volynskite are strongly related with the occurrence of a variety of Bi-Pb tellurides and sulfosalts like tellurobismuthite, hedleyite and cosalite as the main ones. Finally, the occurrence of Au together with Bi-tellurides, makes bismuth an important gold tracer or pathfinder and suggest the possibility that gold could be scavenged from Bi-melts as have been suggested in former studies in other gold deposits (Ciobanu et al, 2005) but would be necessary make further studies like fluid inclusions. Acknowledgements I would like to thank to all Laguna Gold and SGS Minerals people who collaborated and enabled the development of this work. References Ciobanu, L; Cook, N; Pring, A. (2005) ‘Bismuth tellurides as gold scavengers’, Society for geology Applied to mineral deposits; Min. deposit research: Meet the global challenge, Springer, Berlin, 1383-1386. Wellmer, F; Reeve, E; Wentzlau, E; Westenberger, H (1983) ´Geology and ore deposits of the Toqui district, Aysen, Chile´, Economic Geology 78, 1119-1143.

Figure 1: Gold mineralization with native gold (Au) related with cobalt mineralization with cobaltiferous arsenopyrite (Arpy) and glaucodot (Gcdt), sphalerite (Sph) and bismuth tellurides and sulfosalts cosalite (Cos), pilsenite (Pls), kochkarite (Kchk), joseite-B (Jos), hedleyite (Hdy) and tellurobismuthite (Tbm).

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