THE CANDELARIA SILVER DEPOSIT, NEVADA - PRELIMINARY SULFUR, OXYGEN AND HYDROGEN ISOTOPE GEOCHEMISTRY

The pre-Cenozoic geology at Candelaria, Nevada comprises four main lit hologic units: the basement consists of Ordovician cherts of the Palme tto complex; this is overlain unconformably by Permo-Triassic marine e lastic sediments (Diablo and Candelaria Formations); these are structu rally overlain by a serpentinite-hosted tectonic melange (Pickhandle/G olconda allochthon); all these units are cut by three Mesozoic felsic dike systems. Bulk-mineable silver-base metal ores occur as strataboun d sheets of vein stockwork/disseminated sulphide mineralisation within structurally favourable zones along the base of the Pickhandle alloch thon (i.e. Pickhandle thrust and overlying ultramafics/mafics) and wit hin the fissile, calcareous and phosphatic black shales at the base of the Candelaria Formation (lower Candelaria 'shear'). The most promine nt felsic dike system - a suite of Early Jurassic granodiorite porphyr ies - exhibits close spatial, alteration and geochemical associations with the silver mineralisation. Disseminated pyrites from the bulk-min eable ores exhibit a delta(34)S range from - 0.3 parts per thousand to + 12.1 parts per thousand (mean delta(34)S = + 6.4 +/- 3.5 parts per thousand, 1 sigma, n = 17) and two sphalerites have delta(34)S Of + 5. 9 parts per thousand and + 8.7 parts per thousand These data support a felsic magmatic source for sulphur in the ores, consistent with their proximal position in relation to the porphyries. However, a minor con tribution of sulphur from diagenetic pyrite in the host Candelaria sed iments (mean delta(34)S = - 14.0 parts per thousand) cannot be ruled o ut. Sulphur in late, localised barite veins (delta(34)S = + 17.3 parts per thousand and + 17.7 parts per thousand) probably originated from a sedimentary/seawater source, in the form of bedded barite within the Palmetto basement (delta(34)S = + 18.9 parts per thousand). Quartz ve ins from the ores have mean delta(18)O = + 15.9 +/- 0.8 parts per thou sand (1 sigma, n = 10), which is consistent, over the best estimate te mperature range of the mineralisation (360 degrees-460 degrees C), wit h deposition from O-18-enriched magmatic-hydrothermal fluids (calculat ed delta(18)O fluid = + 9.4 parts per thousand to + 13.9 parts per tho usand). Such enrichment probably occurred through isotopic exchange wi th the basement cherts during fluid ascent from a source pluton. Whole rock data for a propylitised porphyry (delta(18)O = + 14.2 parts per thousand, delta D = - 65 parts per thousand) support a magmatic fluid source. However, delta D results for fluid inclusions from several vei n samples (mean = - 108 +/- 14 parts per thousand, 1 sigma, n = 6) and for other dike and sediment whole rocks (mean = - 110 +/- 13 parts pe r thousand, 1 sigma, n = 5) reveal the influence of meteoric waters. T he timing of meteoric fluid incursion is unresolved, but possibilities include late-mineralisation groundwater flooding during cooling of th e Early Jurassic progenitor porphyry system and/or meteoric fluid circ ulation driven by Late Cretaceous plutonism.