Into the depth of the Arburèse vein system: arsenide-sulfide evolution in the Ni-Co ores

The Arburese region (SW Sardinia) is well-known for a large swarm of mineralized veins hosted in low-grade metamorphic rocks, extending for about 20 km around the Late Variscan (304±1 Ma) Arbus pluton, made up of a core of cordierite-bearing biotite leucogranites and an external shell of granodiorites with subordinate mafic rocks (Cuccuru et al., 2015). The largest veins occur in the northern and western sectors (past Montevecchio district), with a primary mineral association of Pb-Zn (Cu, Ag) sulfides in quartz and siderite. Recent works on the southern branch of the vein system (Naitza et al., 2015) evidenced a more complex metallogeny, with Ni-Co-As-Sb-Bi-Pb-Zn-Cu-Ag ores. New microscopy, SEM-EDS and EPMA data allow to highlight the depositional history of these ores, defining two stages of mineralization. In the Arsenide stage, early precipitation of quartz with abundant Ni monoarsenide (nickeline) was followed by growth of Ni antimonide (breithauptite) on nickeline, by Ni, Ni-Co, Co, Fe diarsenides, triarsenides, sulfoarsenides (rammelsbergite, skutterudite, safflorite/löllingite, cobaltite, gersdorffite/ullmannite) and Bi minerals (bismuthinite and native Bi), replacing nickeline and forming rims and inclusions on previously crystallized phases. The Sulfide stage was preceded by cataclasis of the ores; initial abundant precipitation of quartz, siderite and sphalerite, cementing the fragments of the arsenide ores, was followed by Pb-Cu-Ag sulfides (galena, chalcopyrite, tetrahedrite/freibergite, bournonite, proustite/pyrargirite, stephanite), and, at last, by ankerite, calcite and pyrite. These sequences of mineralization suggest a relationship with distinct hydrothermal fluids: 1) an earlier fluid, carrying Ni, Co, As and Sb, deposited as arsenides and sulfoarsenides into structurally-controlled traps in response to variations in physicochemical conditions; 2) a late, carbonate and Pb-Zn-Cu-Ag -rich fluid, permeating fractures re-opened after a tectonic phase that involved the previous mineralization, and originating a “Montevecchio-type” mineral association. Depositional histories similar to the Arsenide stage have been reported in other hydrothermal Ni-Co deposits worldwide, as in the Bou Azzer district, Morocco (Ahmed et al., 2009). The geochemistry of these ores requires the presence in the geological sequence of a Ni-Co-rich source (e.g. mafic rocks) from which these elements could be leached by acidic and moderately oxidizing hot fluids. Analogous conditions may have been attained in the Arbus pluton, which includes an olivine-bearing end member (Secchi et al., 1991), possible Ni-Co source for fluids of magmatic origin. During the emplacement of the igneous complex, widespread uralitization of pyroxenes in granodiorites point to high fluid/rock interactions; moreover, quartz- Fe-cordierite graphic intergrowths in leucogranites support a late dehydration of magmas. References Ahmed A.H., Shoji A., Ikenne M. (2009). Mineralogy and Paragenesis of the Co-Ni Arsenide Ores of Bou Azzer, Anti-Atlas, Morocco. Econ. Geol. 104, 249-266 Cuccuru S., Naitza S., Secchi F., Puccini A., Casini L., Pavanetto P., Linnemann U., Hofmann M., Oggiano G. (2015): Structural and metallogenic map of late Variscan Arbus Pluton (SW Sardinia, Italy), Journal of Maps, DOI: 10.1080/17445647.2015.1091750. Naitza S., Cuccuru S., Oggiano G., Secchi F. (2015). New observations on the Ni-Co ores of the southern Arburese Variscan district (SW Sardinia, Italy). Geoph. Res. Abs. 17, EGU2015-12659. Secchi F. A., Brotzu P., Callegari E. (1991). The Arburèse igneous body (SW Sardinia, Italy) - An example of dominant igneous fractionation leading to peraluminous cordierite-bearing leucogranites as residual melts. Chem. Geol., 92, 213-249.