Summary
16 polished thin sections have been prepared from core samples from four drill holes from the eastern part of the Nussir eu deposit with enrichments of Ag, Au, Pt and Pd. The main aims of the study were to describe the various copper-sulphide minerals and to identify noble metal-bearing minerals and their relationship to the copper sulphides. Some of the polished thin sections have been investigated by use of scanning electron microscope (SEM) and semi-quantitative analyses have been performed.
The host rocks of the Nussir deposit are fine-grained sandstones and siltstones that contain carbonate-rich layers. They are heterogeneously defonned, and both rocks with well-preserved primary layering and rocks that exhibit strong ductile deformation have been studied.
Bornite and chalcocite are the major copper sulphides. They mainly comprise cement for the clastic grains of the sandstone and suggest a diagenetic origin for their deposition rather than strictly epigenetic fonnation related to defonnation of the host rock. Accessory sulphide minerals include chalcopyrite, covelline, wittichenite, carrolite and cinnabar. They commonly occur as replacement of and/or exsolution from the major Cu sulphides. Alternatively they have been formed through epitactic growth. They are partly formed during later defonnation and regional metamorphism.
Gold and silver are closely associated the Cu mineralisations. Electrum (AuAg, 3-15 Ilm) is found in contact with and as inclusions and in cracks in bornite. Additionally, silver occur in amalgam (AgHg), hessite (All2Te), Ag-bearing clausthalite (PbSe), bohdanowiczite (AgBiSe,), naumannite (Ag,Se) and unidentified AgBiTe mineral phase often associated native bismuth (Bi) and wittichenite (Cu3BiS3). These minerals commonly comprise exsolutions or droplets in the major Cu sulphides and are often enriched at their rims.
Platinum most frequently occurs as microscopic grains of sperrylite (PtAs2) that form clusters of inclusions in bornite and disseminated, interstitial grains in the silicate matrix of the sandstone. Palladium is a minor constituent of some sperrylite, but more commonly comprises complex grains of isomertietite (Pd"Sb2As2) and occurs in unidentified PdHgTe and PdBiHg mineral phases which can be Ag-bcaring, both in contact with, and separate from the major Cu sulphides.
