Evolution and paragenetic context of low δD hydrothermal fluids from the Panasqueira W-Sn deposit, Portugal: new evidence from microthermometric, stable isotope, noble gas and halogen analyses of primary fluid inclusions

Abstract

The evolution, paragenetic context and origin of remarkably low δD hydrothermal fluids from the Hercynian Panasqueira W-Sn-Cu(Ag) deposit have been investigated through a combined microthermometric, stable isotope, halogen and noble gas fluid inclusion study. Large variations in δD between −60 ‰ and −134‰ have been observed in primary fluid inclusions from growth zones in a quartz crystal (Pa66) paragenetically constrained to the main sulfide stage (MSS) of ore formation. The same fluids exhibited relatively constant fluid inclusion homogenisation temperatures (254 to 260°C), salinities (7.4 to 8.7 wt.% NaCl equivalent) and calculated fluid δ 18O (3.8 to 4.4‰). All the fluids exhibited 40Ar excesses. Mean molar Br/Cl and I/Cl ratios varied from 2.3 × 10 −3–4.2 × 10 −3 and 8 × 10 −4–10 × 10 −4 respectively, with the more Br-rich fluids being associated with the more deuterium-depleted fluids. The low palaeolatitude of Panasqueira throughout the main stages of ore formation and the overwhelming predominance of meteoric fluids in the main, late and post-ore mineralising fluids with δD of −40 to −65‰ lends little support for any models involving low δD palaeometeoric water. The limited range of 40Ar∗/Cl ratios (1.1 × 10 −5–1.6 × 10 −5) also precludes boiling during the MSS or mixing of meteoric water with highly fractionated deuterium-depleted magmatic waters as viable mechanisms for producing the low δD fluids. The high Br/Cl and I/Cl of the MSS fluids indicate significant fluid interactions with organic rich sediments or metasediments, in particular regionally abundant Carboniferous coals or coaly sediments that are inferred to be depleted in deuterium by around 85‰ compared to palaeometeoric water. The large range of δD observed in the MSS fluids may be explained by isotopic exchange of palaeometeoric water with these coals at varying water/rock (wt./wt.) ratios between 0.02 and 0.002 with fluid oxygen isotopic compositions controlled by exchange with the Beira Schists. Such a model is consistent with the requirements that any satisfactory model must take into account viz. (i) the relative constancy of temperature, salinity, 40Ar∗ and fluid δ 18O during excursions to low δD; (ii) the high I/Cl and Br/Cl ratios observed; and (iii) the coincidence of these low δD fluids with the onset of precipitation of polymetallic sulfides and phosphates under reducing conditions. The model indicates that (meta)sedimentary sources of ore forming components became increasingly important during the evolution of the Panasqueira Sn-W deposit and accounts in part for the observed progression from oxide-dominated to base-metal sulfide-dominated parageneses.