Multi-stage growth and invisible gold distribution in pyrite from the Kundarkocha sediment-hosted gold deposit, eastern India

Abstract

Gold mineralization at Kundarkocha, India, is hosted in sheared gray quartz veins that were emplaced in carbonaceous pyritic phyllite. Gold occurs as enclosed grains within sulfides and free grains in quartz. Based on characteristic textural and chemical features, documented by X-ray element imaging, electron probe microanalysis and laser-ablation inductively-coupled plasma mass spectrometry analyses, four pyrite types were identified in carbonaceous phyllites and auriferous veins. Rock-hosted fine-grained syn-sedimentary to early diagenetic pyrite framboids (PyI) have lower contents of Co and As but consistently high gold values. Pyrite of the next generation (PyII) has numerous silicate and rare sulfide inclusions; lower contents of Co and Ni, moderate As values; the highest mean value of invisible gold and maximum concentrations of trace elements such as Li, Ti, Zn, Rb, Sr, Y, Zr, Nb, La, Ce, Ta, Th, U and Cr. Pyrite of the third generation (PyIII) shows evidence of overgrowth over PyII, contains both silicate and sulfide inclusions, and are characterized by moderate contents of Co, high Ni and low Au values and higher concentrations of large ion lithophile elements, but lesser amount of high field strength elements. Pyrites of the latest type (PyIV) occur as polycrystalline aggregates that contain inclusions of gold, sulfides and rare silicates, show oscillatory zoning of Co and As and the lowest concentrations of all other trace elements. Successive decrease in contents of majority of trace elements from PyII to PyIV is attributed to fluid-assisted recrystallization during diagenesis and low grade metamorphism.Later generation pyrites (PyII through PyIV) exhibit higher Au contents regardless of their As values, indicating occurrence of invisible gold mostly as nanoparticles, at times reaching up to 500ppm. Unlike the majority of trace elements that underwent large-scale remobilizations, gold was somehow locked up in pyrite resulting in a rather lean deposit at Kundarkocha.