LA-ICPMS and EPMA studies of pyrite, arsenopyrite and loellingite from the Bhukia-Jagpura gold prospect, southern Rajasthan, India: Implications for ore genesis and gold remobilization

The Bhukia-Jagpura gold prospect is hosted by a Paleoproterozoic metasedimentary rock sequence that constitutes the lowermost part of the Aravalli Supergroup, lying in close proximity to the basement-cover contact. A combined ore petrographic, LA-ICPMS and EPMA studies of iron sulfides and sulfarsenides from the prospect has revealed four varieties of pyrite and two varieties of arsenopyrite with different trace element compositions. The data indicates a sedimentary-diagenetic origin for the earliest Pyrites (Py) I and II, which are devoid of any gold, in contrast to hydrothermal Py III and IV with a minor, but significant content of invisible gold (0.3–1.6 ppm). Associated loellingite (35–51 ppm) and Arsenopyrite (Aspy) I (0.28–10 ppm) contain the maximum concentration of invisible gold, while it is substantially lower in Aspy II (0.06–1.5 ppm). The compositions of the gold-bearing pyrites (low Ni:Co ratios), arsenopyrites and loellingite (rich in Co), and the presence of Au–Bi–Te mineral phases within arsenopyrite, support a magmatic hydrothermal model for the origin of gold–sulfide mineralization, which appears to have taken place during the peak of regional metamorphism, or at an earlier stage. The different stages of deformation and metamorphism released and remobilized gold in invisible and discrete forms from the sulfarsenides. During prograde metamorphism of Aspy I, gold was partially incorporated and relatively enriched within the structure of loellingite. Later, during retrograde metamorphism gold was exsolved as visible grains at the interface of loellingite and Aspy II and/or within loellingite.