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Minor Elements in Layered Sphalerite as a Record of Fluid Origin, Mixing, and Crystallization in the Navan Zn-Pb Ore Deposit, Ireland

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Gagnevin, D.; Menuge, J. F.; Kronz, Andreas; Barrie, C.; Boyce, A. J.
Economic Geology
This study tests the utility of the minor to trace element composition of sphalerite to discriminate between possible sources of ore-forming fluids, and to constrain processes involved in ore genesis of the world-class Irish-type Navan Zn-Pb orebody, Ireland. Detailed petrography and electron microprobe microanalyses were performed on layered sphalerite previously analyzed for Zn, Fe, and S isotope compositions. Layered sphalerite displays a wide range of chemical composition at both sample and crystal scales. The color, style, and scale of layering show variations with chemical composition, but none of these correlations are consistent between samples. However, there are strong intersample correlations between chemical and S-Fe-Zn isotope compositions at the millimeter scale. Sphalerite precipitated from deep, hydrothermal fluids with 34S-enriched sulfide is enriched in Cd, Sb, Cu, and Ag, whereas Fe and As are enriched in sphalerite precipitated from shallow, bacteriogenic brines with isotopically light S. Significant chemical variations also occur at the micrometer scale in sphalerite regardless of its genetic affinity. These variations are interpreted as being due to variations in temperature, pH, and sulfur activity following the arrival at the site of deposition of pulses of hydrothermal fluids at specific stages of sphalerite growth. Therefore the chemistry of sphalerite, coupled with its texture, appears to be a powerful tool to elucidate fluid typing and ore genesis. The results support the hypothesis that layered sphalerite forms by rapid crystallization due to mixing of two fluids of contrasting physicochemical properties, a process required for the formation of Irish-type and some other hydrothermal deposits including, most notably, Mississippi Valley-type (MVT) deposits. The existence of layered sphalerite, coupled with its texture and chemical composition, may therefore provide useful insight for the mineral exploration industry in Ireland and elsewhere.
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