Emplacement of the huge Zn-Pb-Ag vein system of Montevecchio, Arburese (SW Sardinia): geological, mineralogical, geothermometric and isotopic inputs towards a new metallogenic model

The Montevecchio-Ingurtosu district (SW Sardinia) was one of the largest Pb-Zn producers of Italy between 1878 and 1968, with important yields of Ag and other byproducts (Cd, Ge, In, Ni, Co). Nonetheless, detailed studies of the Montevecchio orebodies and mineral assemblages are surprisingly scarce and old. Mineralization is hosted in a NE-SW-trending, 10 km-long set of steep veins emplaced parallel and radial to the contact of the Variscan Arburese granitoid complex. Host rocks are Paleozoic low-grade metapelites and metavolcanics variably affected by contact metamorphism. Recently the ore system was systematically sampled and analyzed for major to trace elements, microthermometry and stable isotopes (on carbonates). Different ore types were observed, not necessarily belonging to the same ore system. The Montevecchio ore consists of galena and sphalerite with minor Ag-Cd tetrahedrite, chalcopyrite, accessory Co-Ni sulfarsenides, minor pyrite and rare Au in quartz, Fe-Mn-Zn carbonate or barite gangue. Other ore types found contain galena with Cdrich sphalerite plus scheelite, or Fe-rich sphalerite, pyrrhotite, argentite and rutile in calcite-adularia-fluorite gangue. The new studies reveal some intriguing aspects of the Montevecchio assemblages. Major and trace elements redistributed among sulfides and sulfosalts according to complex patterns. Ghost microtextures in quartz gangue and rhythmic Fe and Zn-rich growth zones in primary carbonates suggest multistage deposition. Galena reveals ubiquitous, extreme enrichments in sub-μ mono/polyphase sulfosalt blebs. In zoned sphalerite partitioning of minor and trace elements (Fe, Cd, Mn, Ag, Co, Ni, Cu, Pb, Ge, In, Sn, Se, Pd and Rh) may vary in different veins. Cd, Ag and Ge also partition into tetrahedrite during polyphase deposition, depending on galena abundance. Microthermometry on fluid inclusions in sphalerite and ore-related quartz and based on trace elements in sphalerite (Frenzel et al., 2016) suggest deposition by high-salinity fluids between 160° and and 220°C for the Montevecchio system (in agreement with stable isotope signatures of siderite), while sphalerite in the other ore types records higher temperatures (360°-440°C). Different ore assemblages marked by distinct thermal conditions and ghost textures, coupled with widely variable Pb isotope signatures for Montevecchio ore suggest the existence of an early, minor, probably granite-related “ghost” ore phase overprinted by the huge flood of metal-loaded hypersaline fluids of unknown origin and moving across regional-scale tectonic corridors. The same crustal-scale tectonic pattern that controlled magma ascent and emplacement in the Arbus igneous complex at 304 Ma was probably critical in focusing these late mineralizing fluids. Frenzel, M., Hirsch, T. & Gutzmer, J. (2016): Gallium, germanium, indium, and other trace and minor elements in sphalerite as a function of deposit type. A meta-analysis. Ore Geol. Rev., 76, 52-78.