The Candelaria silver deposit, Nevada - preliminary sulphur, oxygen and hydrogen isotope geochemistry

Thomson, B., Fallick, A.E., Boyce, A.J. and Rice, C. (1994) The Candelaria silver deposit, Nevada - preliminary sulphur, oxygen and hydrogen isotope geochemistry. Mineralium Deposita, 29(4), pp. 318-329. (doi: 10.1007/BF00191038)

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The pre-Cenozoic geology at Candelaria, Nevada comprises four main lithologic units: the basement consists of Ordovician cherts of the Palmetto complex; this is overlain unconformably by Permo-Triassic marine clastic sediments (Diablo and Candelaria Formations); these are structurally overlain by a serpentinitehosted tectonic mélange (Pickhandle/Golconda allochthon); all these units are cut by three Mesozoic felsic dike systems. Bulk-mineable silver-base metal ores occur as stratabound sheets of vein stockwork/disseminated sulphide mineralisation within structurally favourable zones along the base of the Pickhandle allochthon (i.e. Pickhandle thrust and overlying ultramafics/mafics) and within the fissile, calcareous and phosphatic black shales at the base of the Candelaria Formation (lower Candelaria ‘shear’). The most prominent felsic dike system — a suite of Early Jurassic granodiorite porphyries — exhibits close spatial, alteration and geochemical associations with the silver mineralisation. Disseminated pyrites from the bulk-mineable ores exhibit a δ 34S range from — 0.3‰ to + 12.1‰ (mean δ 34S = +6.4 ± 3.5‰, 1σ, n = 17) and two sphalerites have δ 34S of + 5.9‰ and + 8.7‰ 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 contribution of sulphur from diagenetic pyrite in the host Candelaria sediments (mean δ 34S = — 14.0‰) cannot be ruled out. Sulphur in late, localised barite veins (δ 34S = + 17.3‰ and + 17.7‰) probably originated from a sedimentary/seawater source, in the form of bedded barite within the Palmetto basement (δ 34S = + 18.9‰). Quartz veins from the ores have mean δ 18O = + 15.9 ± 0.8‰ (1σ, n = 10), which is consistent, over the best estimate temperature range of the mineralisation (360°–460°C), with deposition from 18O-enriched magmatic-hydrothermal fluids (calculated δ 18O fluid = + 9.4‰ to + 13.9‰). Such enrichment probably occurred through isotopic exchange with the basement cherts during fluid ascent from a source pluton. Whole rock data for a propylitised porphyry (δ 18O = + 14.2‰, δD = — 65‰) support a magmatic fluid source. However, δD results for fluid inclusions from several vein samples (mean = — 108 ± 14‰, 1σ, n = 6) and for other dike and sediment whole rocks (mean = — 110 ± 13‰, 1σ, n = 5) reveal the influence of meteoric waters. The timing of meteoric fluid incursion is unresolved, but possibilities include late-mineralisation groundwater flooding during cooling of the Early Jurassic progenitor porphyry system and/or meteoric fluid circulation driven by Late Cretaceous plutonism.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Boyce, Professor Adrian and Fallick, Professor Anthony
Authors: Thomson, B., Fallick, A.E., Boyce, A.J., and Rice, C.
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Mineralium Deposita
ISSN (Online):1432-1866

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