CONICET | Buscador de Institutos y Recursos Humanos

The La Huerta range is part of the Western Sierras Pampeanas and consists of a Proterozoic-Lower Paleozoic medium- to high-grade crystalline basement (granulitic gneisses, amphibolites, marbles-calcsilicate rocks, coronitic gabbros and granitoid rocks) and rhyolitic extrusives and breccias of Upper Carboniferous-Lower Triassic age. The mineralized zones are mostly associated with the occurrence of metacarbonate outcrops in contact with the high-grade granulite basement. Reflected light microscopic investigation and scanning electron microscopy including energy dispersive X-ray analyses of representative mineralized samples from the Caledonia-Albion gold mines, show the presence of several ore minerals: pyrite + sphalerite + galena + chalcopyrite ± arsenopyrite ± pyrrhotite ± greenockite ± bornite ± tennantite ± tetrahedrite ± covellite ± chalcocite ± aikinite ± wittichenite ± matildite ± berryite ± miharaite ± geocronite ± hematite ± magnetite. One of the most conspicuous features of the mineralization is the occurrence of hypogene greenockite in different parageneses and textures: 1) it occurs as small, individual crystals associated with bornite, galena, chalcopyrite, pyrite, magnetite, hematite and tiny sphalerite grains along late-stage, narrow veinlets and cracks. This greenockite has a low Zn-content of < 1 mol% ZnS, but a varying Fe-content from 0.2 up to 4.5 mol% FeS. Composition of sphalerite varies between 0.4 and 1.3 mol% CdS component and 0.5 to 8 mol% FeS component, 2) greenockite occurs typically as hypogene replacement product of sphalerite. It forms narrow, discontinuous rims, often together with Cd-rich sphalerite, around corroded, primary Cd-poor sphalerite. These sphalerite-greenockite intergrowths are rimmed by a corona of tiny (< 1 µm) greenockite grains enclosed in quartz. Similar replacement textures can also be observed within cracks of the primary, Cd-poor sphalerite. Greenockite is Zn-rich with 10-25 mol% ZnS component and < 0.1 to 3 mol% FeS component. Tiny greenockite grains within quartz have similar composition. Primary sphalerite is Cd-poor with ca. 0.3 mol% CdS component, but variable Fe-content ranging from < 1 to 21 mol% FeS. Late-stage, Cd-enriched sphalerite has up to 4.5 mol% CdS, but is always low in Fe (< 3 mol% FeS component). In rare cases, greenockite occurs as euhedral crystals within galena, accompanied by various Ag-Cu-Bi(Pb)-sulphides. This greenockite has a Zn-content of 5-7 mol% ZnS component and FeS < 0.1 mol%. Apart from greenockite, several complex Ag-Cu-Bi-Pb-sulphides are reported for the first time for the Blanca Creek mineralization with typical analyses: aikinite (intergrown with galena): Pb0.99Cu0.99Fe0.06Bi0.99S2.97; berryite (with sphalerite or intergrown with galena): Pb2.98(Ag2.03Cu3.04)5.07Bi7.21S15.74; matildite (intergrown with or exsolutions within galena): Ag1.03Bi0.99S1.98; tennantite (usually intergrown with galena, with Cd- and Bi-content up to 4.2 and 26.7 wt%, respectively) and wittichenite (intergrown with greenockite or galena): Cu3.00Bi1.00S3.00. Very rare sulphide minerals, encountered in only a few grains are: geocronite (one monophase inclusion in pyrite): Pb13.97(Sb4.39As2.02)6.41S22.62; miharaite (with berryite and chalcopyrite in cracks of pyrite): Pb0.98Cu3.92Fe1.21Bi1.03S5.86; tetrahedrite (accompanied by galena): (Cu9.54Fe2.01Ag0.42)11.97Sb4.08S12.95 as well as several not yet identified Ag-Cu-Bi-(Pb)-sulphides associated with galena. The presence of complex Ag-Cu-Bi-Pb sulphides points to a late stage, low-temperature overprint of the primary sulphide parageneses, accompanied by the formation of hypogene greenockite, at least partly at the expense of sphalerite. The almost end-member composition of greenockite and late-stage sphalerite also point to a low temperature formation. This is in contrast to greenockite and sphalerite from active fumaroles, where both minerals crystallize at temperatures from 400-725°C, showing a nearly complete range of solid solutions.

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