Importance of mantle derived fluids during granite associated hydrothermal circulation: He and Ar isotopes of ore minerals from Panasqueira

Helium trapped in arsenopyrite and wolframite from the Panasqueira hydrothermal deposit in central Portugal is overwhelmingly—≈ 75%—derived from the mantle. This is surprising as the W-Cu(Ag)-Sn vein deposits are spatially and temporally associated with an S-type granite. It is difficult to reconcile ³He/⁴He ratios > 5Ra (where Ra is the atmospheric ³He/⁴He ratio) with a granite produced by crustal anatexis, therefore it seems likely that mantle derived fluids unrelated to the granite were the source of the magmatic component in the ore fluids.The mantle appears to transport ³He and heat to the surface in relatively constant proportions (³He/Q = 0.1-1 x10⁻¹² cm³ STP J⁻¹). However, the ³He/heat ratios in the Panasqueira fluids (1-10 x10⁻¹² cm³ STP J⁻¹, calculated using the ³He/³⁶Ar ratio) are considerably higher. Therefore, it does not seem likely that gradual cooling of the pluton supplied heat to the hydrothermal system (as this would result in low ³He/Q ratios). Therefore, both He and heat in this collision related, granite-associated hydrothermal deposit were supplied from the mantle.