Zinc and sulfur isotope variation in sphalerite from carbonate-hosted zinc deposits, Cantabria, Spain

摘要

We studied zinc and sulfur isotopes and the chemical composition of sphalerite samples from Picos de Europa (Aliva mine) and sphalerite and hydrozincite samples from La Florida mine, two carbonate-hosted Mississippi Valley-type (MVT) deposits located in northern Spain; despite being close, they are hosted in carbonatic rocks of different ages, Lower Carboniferous and Lower Cretaceous, respectively. The two generations of sphalerite at Picos de Europa show different δ~(66)Zn values (stage 1 sphalerite +0.24 per mil and stage 2 sphalerite from -0.75 to +0.08 per mil). Both generations also differ in the sulfur isotope composition (stage 1 has δ~(34)S=+6.6 and stage 2 has δ~(34)S=-0.9 to +2.9 per mil) and the chemical composition (stage 1 sphalerite, compared to stage 2 sphalerite, is significantly enriched in Pb, As, Mn, Sb, slightly enriched in Ag, Ni, and Cu and depleted in Co, Ga, Tl, Te, Ge, and Sn). We suggest that Zn isotope fractionation was controlled predominantly by pH and T changes. High Zn isotope values reflect rapid precipitation of sphalerite from higher-temperature acidic fluids that carried Zn mostly as chloride species after interaction with carbonate rocks while lower Zn isotope values most likely resulted from a longer precipitation process from fluid at higher pH and decreasing T that carried dominantly Zn sulfide species. At La Florida, sphalerite samples show light ~(66)Zn-depleted signatures with δ~(66)Zn values from -0.80 to -0.01 per mil (mostly between -0.80 and -0.24 per mil) and δ~(34)S values from +10.7 to +15.7 per mil without any relationship between the ~(66)Zn and δ~(34)S values. Here, the variation in Zn isotope values is interpreted as related to mixing of fluids from two reservoirs. The Zn was carried by a single deep-seated and higher T (-250-320 °C) fluid, and precipitation took place after mixing with a connate S-rich fluid in a system with mH_2S>mZn~(2+) as a result of change in pH, T, and Zn predominant species. The light δ~(66)Zn accompanied by heavy δ~(34)S values resulted from fractionation of Zn aqueous sulfides at near-neutral pH and decreasing T. Hydrozincite samples show much heavier δ~(66)Zn values (+ 0.21 to +0.33 per mil), consistent with fractionation during supergene processes.