Abstract
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 ≈170 °C for Usseglio, whereas for the Sardinian system late sulfide deposition occurred within the 52–126 °C range. Ore fluids in both systems are NaCl-CaCl2-bearing basinal brines. The chlorite geothermometer at Piazza provides the range of 200–280 °C for ore deposition from CO2-poor fluids. Enrichments in Se and negative δ13C 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.
Highlights
Nickel-and cobalt-rich hydrothermal deposits are interesting both for the economic importance of their metals and for aspects of their genesis still not completely understood
LV and LVS inclusions are characterized by very low temperature of initial melting: the appearance of the “orange peel” texture, which is typically observed 5–10 degrees below the eutectic temperature [62] generally occurs between −55 and −45 ◦ C, and in the biggest inclusions initial melting is observed at ca. −52 ◦ C
The Ni–Co-rich vein systems of SW Sardinia and Usseglio (Western Alps) show strong similarities: in spite of the different tectonic setting (SW Sardinia: Paleozoic basement and late Variscan plutonic complexes, Usseglio: metaophiolite unit), these vein systems show comparable mineral assemblages, characterized by a Ni- or Co-dominated, arsenide-rich stage followed by the deposition of base metal sulfides + tetrahedrite
Summary
Nickel-and cobalt-rich hydrothermal deposits are interesting both for the economic importance of their metals (especially highly sought-after cobalt) and for aspects of their genesis still not completely understood. The detailed studies of the impressive, early works on these deposits, dating back to the 1980s and summarized in [1], outlined several important features including the low-temperature characters of this type of mineralization and its tendency to regional-scale development. In this work we try to contribute to the discussion by presenting some preliminary geological, mineralogical and geochemical data about three Ni- and Co-bearing ore deposits located in various parts of Italy and poorly studied so far. We are considering two actual historical mining districts rich in Co and Ni and analogous to “five element vein type” deposits, and comparing them with a peculiar hydrothermal, Cu–Ag mineralization carrying only accessory Co–Ni enrichments but located in a geological context favorable for Co–Ni mineralization at a regional scale. The third deposit is the poorly known, gabbro-hosted Piazza Cu–Ag-rich orebody located in one of the low-metamorphic ophiolite complexes outcropping in the Northern
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