Factors Controlling Hydrothermal Nickel and Cobalt Mineralization-Some Suggestions from Historical Ore Deposits in Italy

We compare three poorly known, historical Ni-Co-bearing hydrothermal deposits in different geological settings in Italy: The Ni-Co-As-Sb-Au-bearing Arburese vein system (SW Sardinia), the Co-Ni-As-rich Usseglio vein system (Piedmont), and the small Cu-Ag-Co-Ni-Pb-Te-Se stockwork at Piazza (Liguria). These deposits share various (mineralogical, chemical, thermal, and stable isotopic) similarities to the Five Element Vein-type ores but only the first two were economic for Co-Ni. The Sardinian Ni-rich veins occur in Paleozoic basement near two Variscan plutons. Like the Co-rich Usseglio vein system, the uneconomic Piazza deposit is hosted in an ophiolite setting anomalous for Co. The Sardinian and Usseglio deposits share a polyphasic assemblage with Ni-Co-As-Sb-Bi followed by Ag-base metal sulfides, in siderite-rich gangue, whereas Piazza shows As-free, Ag-Pb-Te-Se-bearing Co-Ni-Cu sulfides, in prehnite-chlorite gangue. Fluid inclusions indicated Co-Ni arsenide precipitation at approximate to 170 degrees C for Usseglio, whereas for the Sardinian system late sulfide deposition occurred within the 52-126 degrees C range. Ore fluids in both systems are NaCl-CaCl2-bearing basinal brines. The chlorite geothermometer at Piazza provides the range of 200-280 degrees C for ore deposition from CO2-poor fluids. Enrichments in Se and negative delta C-13 in carbonates suggest interaction with carbonaceous shales. These deposits involve issues about source rocks, controls on Co/Ni and possible role of arsenic and carbonate components towards economic mineralization.