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

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.

Spatial and Metallogenic Relationships between Different Hydrothermal Vein Systems in the Southern Arburèse District (SW Sardinia)

“…Overall, the orientation of the veins suggests crosscutting relationships between the two systems, although the bad outcrop preservation does not allow the direct recognition of their interaction. Such a situation is similar to what is reported in other mineralized sites along the Southern Arburèse district (e.g., Sa Menga and Acqua is Prunas mines [8]), where greisentype orebodies and five-element vein swarms are contiguous but not well exposed, thereby questioning the succession of mineralizing events.…”

“…In recent studies focused on the genetic aspects of this class of deposits, abrupt redox variations and intake of hydrocarbons in fluids in tectonically active environments have been identified as the main controlling factors [4,5]. This general frame, albeit with differences related to regional and/or local geological conditions, has been confirmed by further studies on European deposits in Germany, Switzerland, Italian Alps and Sardinia [6][7][8]. The SW Sardinia (Italy) is characterized by multiple metallogenic events that originated different types of ore deposits ranging in age from Cambrian to post-Variscan times [9].…”

“…Our study is set in the old Pira Inferida mine, in the central section of the Southern Arburèse district, a key area due to the simultaneous occurrence of large Ni-Co-As-Bi-Ag ± Au five-element veins and a wide granite-related quartz-wolframite vein swarm ("Togoro" veins in Figure 1b,c), both hosted in Ordovician very low-grade metasandstones-metasiltstones. The mine operated during the 1930's and only the Ni-Co arsenide and Pb-Zn sulfide ores have been systematically exploited; the five-element veins of Pira Inferida were included and briefly described in a recent study of the Southern Arburèse district by [8]. Conversely, very little data are available from the poorly studied wolframite-bearing veins.…”

New data on gersdorffite and associated minerals from the Peloritani Mountains (Sicily, Italy)

“…Gersdorffite from Contrada Zillì represents the first crystalchemically well-characterized Italian occurrence of this nickel sulfarsenide, reported since the end of the 19th century and then found in several localities (e.g., Lovisato, 1894;Artini, 1903;Dessau, 1936;Triscari, 1985;Benvenuti, 1991;Carrozzini et al, 1991;Garuti et al, 2001;Moroni et al, 2019). However, only in the paper by Triscari (1985) is a quite full set of chemical and crystallographic data of gersdorffite given, along with some data on associated tetrahedrite.…”

Isselite, Cu6(SO4)(OH)10(H2O)4⋅H2O, a new mineral species from Eastern Liguria, Italy

“…This peculiar mineral assemblage occurring in the Monte Ramazzo–Lagoscuro ore deposit is quite different from other hydrothermal sulfide mineralisation described so far in ophiolites from the Eastern Liguria, where (Ni,Co)-enrichment occurs mostly in pyrite and/or accessory minerals like millerite, siegenite and pentlandite (e.g. Cortesogno et al ., 1977; Schwarzenbach et al ., 2012; Moroni et al ., 2019).…”