Publication: The carbonate-hosted Zn-Pb deposits of NW Spain: stratabound and discordant deposits relates to the variscan deformation
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1996
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Society of Economic Geologists, Inc
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Abstract
The carbonate Vegadeo Fm (Lower-Middle Cambrian, NW Spain) hosts abundant Zn-Pb mineralization. Minor stratiform ore showings occur in mineralogically and geochemically undisturbed limestones of the Lower Member. They are interpreted as related to diagenetic processes, with sulfur derived from the abiogenic reduction of sulfates interbedded in the carbonates and lead leached from the nearby detrital rocks. The major mineralization, stratabound deposits in the Upper Vegadeo Fm and discordant replacements (Rubiales-type), occurs inrelationship with pervasive epigenetic silicification and minor hydrothermal breccias. They are interpreted as of Late Variscan age. Here, the mineralization seems to be formed by reaction of the hosting carbonates with low-saline (<7 wt% NaCI eq.) water-rich fluids that circulated along major lithologic contacts or extensional faults. Isotopic composition of the hydrotherrnal fluids falls within the field of low-grade metamorphic or basinal waters ( δ180=-l.0 to +6.3 ‰; δD=-43 to -3l ‰). The ore-fonning process occurred at temperatures between 150 and 250ºC and at low fluid pressures (<500 b). Lead isotopes suggest tha_t most of the base metals were derived from the hydrothermal remobilization of the diagenetic ores in a "lead-frozen" system. Sorne of the sulfur appears to have had the same source, but there was also a significant input from biogenically -reduced sulfur from the endosing sbales. These Variscan ores have many of the descriptive features of carbonate-hosted deposits, but the fluid compositions, P-T conditions of formation, and tectonic setting were fundamentally different. They can be interpreted as equivalent to MVT deposits, but formed in the intemal zones of orogenic belts.