Morphology and analysis of gold grains associated with auriferous quartz veins, Gopur, Keonjhar district, Orissa

Abstract

At Gopur, Orissa, auriferous quartz veins with stringers and rich disseminations of sulphides occur in the laterites derived by tropical weathering of metabasaltic rocks. Minute to coarse irregular gold grains were hand picked from the panned heavies obtained by washing the excavated wall rock materials of the quartz veins which are composed of powdery laterite +/- kaolinite +/- quartz. The gold grains display pitted, fractured, striated, wavy surface and occasional smooth contour pattern. The gold content of the quartz veins and lateritic wall rocks varies widely from < I ppm to 9 ppm. SEM-EDS and microprobe analytical results of four gold grains reveal contrasting chemical characters. One gold grain exhibits rimward increase in fineness value indicating that silver has been preferentially leached out by supergene fluid. This grain possibly represents the residually concentrated hypogene gold. The other gold grains do not show chemical gradients within the domain of individual grain and formed by precipitation from supergene fluids.