Cobalt-Te-Au occurrence, genesis, and metallogenic potential of the Jiama skarn orebody, Tibet, China

Abstract

Located in the Eastern Gangdese belt of Tibet, the Jiama skarn orebody is a Cu-polymetallic orebody with significant Pb, Zn, Au, and Ag by-products. The genesis of this orebody is revealed through an analysis of Co, Au, Ag, and Te occurrences. Based on textural and mineralogical relationships, ore formation can be divided into three paragenetic stages. Stage I, the prograde skarn phase, resulted in the formation of andradite, diopside, and wollastonite. This was succeeded by a retrograde period encompassing two sulfide stages—Stage II (Cu-Fe sulfides) and Stage III (Pb-Zn sulfides)—both of which are characterized by the deposition of various sulfides with quartz and calcite. Cobalt was identified in two forms: as the discrete minerals cobaltite (CoAsS) and carrollite ((Ni,Cu)Co₂S₄), and as an isomorphous substitution enriching sulfides. Tellurium is present within melonite, and Au occurs as native gold. Comparison between ore concentrate powder and ore shows loss of Co and enrichment of Au and Ag during beneficiation. Mantle-derived magma and/or juvenile lower crust involvement was likely during Jiama mineralization based on Co-Te-Au mineralization and other isotope characteristics. Three ore-forming fluids were revealed by the mineral assemblages of Co-Te-Au-Ag-Bi. The first ore-forming fluid was accompanied by Co-Cu mineralization and occurred at ΔNNO of 0.4 to 3.9; the second ore-forming fluid was accompanied by Co-Pb-Zn mineralization, while the last phase was dominated by Pb-Zn-Ag-Au-Bi-Te mineralization and occurred at logƒS₂ of −12.8 to −8.3 and logƒTe₂ of −10.5.