Scheelite skarn mineralization associated with the Tinos pluton, Cyclades

Abstract

Scheelite (CaW 04) mineralization has developed as disseminations, veinlets and mainly pods within garnet-pyroxene skarn and homfelses in two locations within the contact aureole of a syntectonic calc-alkaline granodiorite to leucogranite pluton on the island of Tinos, Hellas. The pluton was emplaced during a transition from compression (granodiorite) to extension (leu­ cogranite). Opening of conduits assisted by carbofracturing allowed fluid circulation during extension, which resulted in a change of character of contact metamorphism from thermal-isochemical to infiltration metasomatism. During infiltration metasomatism ion exchange involved the plutonic and the country rocks and was intensified by carbofracturing resulting from the pyrometaso- matic loss of C 0 2 from the marbles. Skarn formation may have been initiated due to thermal effects of the pluton with local mass transfer in the host schists and marbles. The Tinos tungsten skarn is an oxidized skarn that retains relics of an earlier less oxidized stage probably mirroring a change in the redox conditions of the leucogranite. Skarn pyroxene is diopside, however, hedenbergite has been preserved locally. Cores of garnet megacrysts from an early skarn formation are rich in grossular component and exhibit sector-zoning, characteristic of growth under contact metamorphic conditions at log f02 that ranged between -19.8 and -22.9. Gar­ net overgrowths from a later stage are predominantly andradite and display euhedral crystal boundaries, oscillatory zoning, zonal anisotropy and growth as open space fillings, which are features typical of hydrothermal garnet. Scheelite crystals are enclosed within and display equilibrium boundaries to hydrothermal garnet. They have primary fluid inclusions with an average homogeni­ zation temperature Th of 375 °C and salinity of -14 wt% NaCl equivalent. Th of primary fluid inclusions in the coexisting hydro- thermal garnet ranges from 310 to 340 °C, of primary inclusions in quartz from quartz-calcite veinlets formed by carbofracturing from - 400 to 375 °C and salinity -14 wt% NaCl equivalent. These quartz-calcite veinlets cut mainly the early garnet cores, rich in the grossular component, but rarely the hydrothermal andraditic garnet overgrowths, indicating that scheelite mineralization is associated with the late leucogranitic phase of the Tinos pluton.