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

The evolution, paragenetic context and origin of remarkably low delta D hyd rothermal fluids from the Hercynian Panasqueira W-Sn-Cu(Ag) deposit have be en investigated through a combined microthermometric, stable isotope, halog en and noble gas fluid inclusion study. Large variations in delta D between -60 parts per thousand and -134 parts per thousand have been observed in p rimary fluid inclusions from growth zones in a quartz crystal (Pa66) parage netically constrained to the main sulfide stage (MSS) of ore formation. The same fluids exhibited relatively constant fluid inclusion homogenisation t emperatures (254 to 260 degrees C), salinities (7.4 to 8.7 wt.% NaCl equiva lent) and calculated fluid delta(18)O (3.8 to 4.4 parts per thousand). All the fluids exhibited Ar-40 excesses. Mean molar Br/Cl and I/Cl ratios varie d from 2.3 x 10(-3)-4.2 x 10(-3) and 8 x 10(-4)-10 x 10(-4) respectively, w ith the more Br-rich fluids being associated with the more deuterium-deplet ed fluids. The low palaeolatitude of Panasqueira throughout the main stages of ore for mation and the overwhelming predominance of meteoric fluids in the main, la te and post-ore mineralising fluids with delta D of -40 to -65 parts per th ousand lends little support for any models involving low delta D palaeomete oric water. The limited range of Ar-40*/Cl ratios (1.1 x 10(-5)-1.6 x 10(-5 )) also precludes boiling during the MSS or mixing of meteoric water with h ighly fractionated deuterium-depleted magmatic waters as viable mechanisms for producing the low delta D fluids. The high Br/Cl and I/Cl of the MSS fluids indicate significant fluid intera ctions with organic rich sediments or metasediments, in particular regional ly abundant Carboniferous coals or coaly sediments that are inferred to be depleted in deuterium by around 85 parts per thousand compared to palaeomet eoric water. The large range of delta D observed in the MSS fluids may be e xplained by isotopic exchange of palaeometeoric water with these coals at v arying 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 mus t take into account viz. (i) the relative constancy of temperature, salinit y, Ar-40* and fluid delta(18)O during excursions to low delta D; (ii) the h igh I/Cl and Br/Cl ratios observed; and (iii) the coincidence of these low delta D fluids with the onset of precipitation of polymetallic sulfides and phosphates under reducing conditions. The model indicates that (meta)sedim entary sources of ore forming components became increasingly important duri ng the evolution of the Panasqueira Sn-W deposit and accounts in part for t he observed progression from oxide-dominated to base-metal sulfide-dominate d parageneses. Copyright (C) 2000 Elsevier Science Ltd.