Different textural occurrences of Greenockite (CdS)

MOGESSIE, ABERRA; GALLIEN, FLORIAN; BERNHARD, FRANZ; BAUER, CHRISTOPH; CASTRO DE MACHUCA, BRÍGIDA; BJERG, ERNESTO

Oslo

Congreso; 33° International Geological Congress (33IGC); 2008

International Union of Geological Sciences (IUGS)

Reflected light microscopic investigations and scanning electron microscopic studies including energy dispersive X-ray analyses of representative mineralized samples of the La Huerta range, San Juan province, Argentina have been made These show the different textural occurrences of Greenockite associated with several ore minerals in varying parageneses: Pyrite + Sphalerite + Galena + Chalcopyrite ± Geocronite ± Arsenopyrite ± Pyrrhotite ± Bornite ± Tennantite ± Tetrahedrite ± Covellite ± Chalcocite ± Aikinite ± Wittichenite ± Matildite ± Berryite ± Miharaite ± Hematite ± Magnetite. The mineralized zones are mostly associated with the occurrence of metacarbonate outcrops in contact with the high grade granulite basement and the associated gabbroic intrusion. Greenockite occurs in different parageneses and textures: 1) as hypogene replacement product of sphalerite. It forms narrow, discontinuous rims, often together with Cd-rich sphalerite, around corroded, primary Cd-poor sphalerite. This sphalerite-greenockite intergrowths are rimmed by a corona of tiny 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 a similar composition. 2) 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. 3) In rare cases, greenockite occurs as euhedral crystals within galena, accompanied by various Ag-Cu-Bi(Pb)-sulfides. This greenockite has a Zn-content of 5-7 mol% ZnS component and FeS < 0.1 mol%. Very rare sulfide minerals, encountered in only a few grains are: Geocronite; Miharaite and Tetrahedrite as well as several not yet identified Ag-Cu-Bi-(Pb)-sulfides associated with galena. It is assumed that the presence of complex Ag-Cu-Bi-Pb sulfides may point to a late stage, low temperature overprint of the primary sulfide 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 may indicate 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.