A mineralogical investigation into the formation of ore-barren endoskarn: An example from the Tonglushan porphyry system, eastern China

Abstract

The formation of ore-barren endoskarns in association with exoskarn Cu-Fe deposits, rather than extensive porphyry-style alteration and mineralization, is investigated in the Tonglushan quartz monzodiorite porphyry system of eastern China. The Tonglushan endoskarn underwent: 1) muscovite-dominated alteration; 2) prograde and 3) retrograde skarnification; 4) potassic and 5) sodic alteration; and 6) carbonatization, and the quartz monzodiorites distal to the endoskarn minor potassic alteration and weak Fe-Cu sulphide mineralization, and then sodic alteration. The stage 1 muscovite-dominated alteration was caused by hot and saline magmatic aqueous fluids which then mixed with Ca- (±CO2)-bearing fluids from exoskarnification of surrounding carbonates, to cause prograde (stage 2) and then retrograde (stage 3) endoskarnification of the variously altered quartz monzodiorite. A second pulse of moderately hot, weakly acidic to neutral magmatic fluids produced potassic alteration in the endoskarn (stage 4), and similar fluids caused porphyry-style potassic alteration and weak Cu-Fe mineralization in the quartz monzodiorite distal to the endoskarn and extensive sulphide precipitation in the exoskarns. Subsequent sodic alteration (stage 5) in both environments was due to ingress of Na-(CO2)-rich fluids from surrounding carbonate rocks.Primary Cu mineralization is absent in the endoskarns and instead concentrated in the exoskarns which had a more reducing and alkaline environment and into which fluid flow was focused. The latter was due to decarbonation of wall rock marbles and related upwards migration of CO2 to produce a self-sustaining chimney effect which further drew in fluids towards the marbles to form, alter and mineralize the exoskarns. Porphyry-style mineralization in the quartz monzodiorites was limited as magmatic-hydrothermal fluids ascending within the magmatic system were CO2-rich, due to magmatic assimilation of carbonate, and therefore incapable of carrying elevated concentrations of Cu. This contribution offers new insights into why certain porphyry systems host ‘barren’ endoskarns and mineralized exoskarns rather than porphyry-style Cu deposits.