Greenockite and zincian greenockite in epithermal polymetallic Ag-Au-Te mineralization, Tinos Island, Hellas: Description and conditions of formation

摘要

Greenockite-CdS and zincian greenockite-(Cd_(0.65)Zn_(0.35))S were identified in low sulfidation epithermal Au-Ag-Te mineralization at Panormos Bay, Tinos Island, Hellas. Both minerals occur in stage VII of nine ore paragenetic stages as minute moderately reflective grains, up to 15 mu m in length. The zincian greenockite displays a gray-bluish color, with a slight gray to gray-yellow anisotropy and yellowish to brownish-red internal reflections. It: is intergrown with greenockite that coexists with stage VII - wurtzite, siderite, smithsonite and magnetite. Based on silicate, sulfide, and carbonate stabilities and fluid inclusion data, the approximate conditions of formation of stage VII and of zincian greenockite and greenockite are estimated to have been near 215 deg C, at a hydrostatic pressure of 115 to 125 bars, fS_2 = 10~(-14.1) to 10~(-14.5), fO_2 = 10~(-44.2) to 10~(-44.6), pH = 6.1 and aH_2S_(aq) = 10~(-3.6). For this temperature of formation and from previously published experimental data it is suggested that the Panormos Bay zincian greenockite has a wurtzite structure. The zinc and cadmium ion contenfof stage VII solutions is estimated as 0.09 and 0.34 ppm, at 115 and 125 bars respectively. It is suggested that substitution of cadmium by zinc is a rather complicated process depending on precipitation mechanism, pH, fO_2, aH_2S and possibly a_(Cl~-) of the fluid, associated with local mineral-fluid equilibrium.