Meghan Lewis

Covellite (CuS) has traditionally been assumed to form as a result of supergene oxidation processes and it is difficult to distinguish ore mineral assemblages containing hypogene covellite from those of supergene origin. The mineralogical relationships surrounding covellite in the Cu-Fe-S system are poorly constrained with respect to temperature, particularly above 250^oC.

This thesis was a test to determine, among other things, whether hypogene and supergene covellite can be distinguished based on a given set of criteria. It was determined that supergene and hypogene covellite differ both texturally (in habit and mineral assemblages), and compositionally with respect to minor constituents (specifically iron).

Covellite is a prominent mineral in the supergene zones of the Chuquicamata porphyry copper deposit, Chile, one of the largest concentrations of copper in the world. It has been determined through this study, however, that a significant amount of the covellite at Chuquicamata is of hypogene origin. With respect to textural occurrences, supergene covellite in Chuquicamata is generally massive or very finely granular, while hypogene covellite most commonly occurs in acicular or feathery exsolutions from high-temperature digenite or as a network of lamellae replacing other copper-sulphides. The replacement assemblage covellite + chalcopyrite, has long been presumed to be a result of supergene covellite replacing hypogene chalcopyrite. However, grain morphology suggests that the covellite is in fact hypogene. The observation of primary anhydrite as part of this assemblage is further evidence of hypogene covellite.

The presence of a second (metastable) assemblage covellite + chalcopyrite is clearly not a replacement assemblage. The coexistence of these two minerals may be due to the breakdown of high-temperature idaite (Cu_5.5FeS_6.5) below 223^oC, when pyrite fails to nucleate. Further evidence of hypogene covellite resulting from the breakdown of a higher-temperature idaite + covellite assemblage comes from the significant Fe content of primary covellites in this system. Of the more than 100 samples studied in detail, most contain hypogene covellite; fully half of the covellite analyses containe, which varies systematically from 0 to <5 weight %, and appears to indicate crystallisation along the covellite-idaite tie line. The Fe content of primary covellites has not been described before and is a real phenomenon. Supergene covellite rarely deviates from the stoichiometric composition CuS.

The hypogene sulphide assemblages were studied in the context of experimentally-determined sulphide phase equilibria to constrain temperatures of mineralisation. Assemblages from the potassic zone of alteration at Chuquicamata can be constrained to a temperature range of approximately 400 to 600^oC (based on idaite assemblages), although they may have been as low as 300^oC, while those from the quartz-sericite zone have temperatures between 300 and 435^oC (based on covellite-digenite assemblages). Late-stage hydrothermal polymetallic veins formed above 275-320^oC.

Sulphur isotopic compositions of coexisting sulphides and sulphates from Chuquicamata are temperature-dependent and were intended as a complement to the petrologic study. Coexisting sulphur-bearing minerals give temperatures of mineralisation between 300 and 375^oC for samples from the potassic zone; however, the coarse anhydrite veins that were sampled probably resulted from the later quartz-sericite stage. Isotopic temperatures are therefore in agreement with temperatures of mineralisation inferred from sulphide phase equilibria.

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Pages: 235
Supervisor: Marcos Zentilli