| Article ID | Journal | Published Year | Pages | File Type |
|---|---|---|---|---|
| 4730611 | Journal of Asian Earth Sciences | 2015 | 14 Pages |
•Gold evolved as lattice-bound → substrates → lode gold with Au substituting Fe.•Three generations of gold bearing arsenopyrite/loellingite and pyrite are noted.•Remobilization, redistribution and concentration of refractory gold resulted the free milling gold.•We report native silver, gersdorffite and arsenian-ullmannite for the first time.
Gold mineralization in Kundarkocha gold deposit occurs in the eastern Indian Craton that is hosted by sheared quartz–carbonate–sulfide veins emplaced within the graphitic schist, carbonaceous phyllite and talc–chlorite–serpentine schist belongs to Gorumahisani–Badampahar schist belt of Iron Ore Group. Gold mineralization exhibits both lithological and structural controls in the study area, albeit the stratigraphic control is more ubiquitously observed. Detailed mineralogical characterization coupled with electron probe microanalysis of the sulfide phases reveal the occurrences of gold in three distinct forms (i) as lattice-bound form within sulfides especially enriched in arsenopyrite, loellingite, pyrite, pyrrhotite and chalcopyrite in decreasing order of abundance; (ii) as micro inclusions or nano-scale gold inclusions within pyrite and arsenopyrite especially along the growth zones and micro-fractures as substrates and (iii) as free milling nugget gold grains either along the grain boundaries of sulfides or within the host rocks. Three generations of pyrite (Py-I, Py-II and Py-III) and arsenopyrite (Asp-I, Asp-II, Asp-III) have been identified based on textural, morphological characteristics and mineral chemistry. The lattice-bound gold content in pyrite and arsenopyrite varies from 600 to 2700 ppm and 900 to 3600 ppm respectively and increase in concentration of such refractory gold is seen in the order of chalcopyrite > pyrrhotite > pyrite > loellingite/arsenopyrite.The evolutionary stages of different forms of gold include remobilization of the lattice-bound grains in pyrite and arsenopyrite (Py-I and Asp-I) and re-concentration along the zoned-pyrite and arsenopyrite (Py-II and Asp-II) and ultimately as native gold/nuggets surrounding the sulfides as well as within the main mineralized zone. Lattice-bound gold distribution could have resulted due to metamorphic devolatilization reactions which are further aided by the influx of hydrothermal fluids. These reactions along with additional input of hydrothermal fluid paved the way for expulsion of lattice-bound gold from sulfides to concentrate as nuggets/main lode within shear fractures channelizing the fluid flow. Thermometry results of the arsenopyrite–pyrite–pyrrhotite assemblage yielded a temperature range from 375 to 390 °C which is the ideal condition for gold precipitation. Native silver, gersdorffite and arsenian-ullmannite are being reported for the first time from this deposit indicating the complexity and wide variety of mineral phases associated with gold implying magmatic-hydrothermal input of the source fluid.
