Shale basins,sulfur-deficient ore brines and the formation of exhalative base metal deposits |
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| Authors: | Fernando Tornos Christoph A. Heinrich |
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| Affiliation: | 1. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada;2. Department of Earth Sciences, Laurentian University, Sudbury, ON, Canada;1. Département de Géologie et de Génie Géologique, Université Laval, Quebec, QC G1V0A6, Canada;2. Département de Génie Chimique, Université Laval, Quebec, QC G1V0A6, Canada;3. Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, ON, N2L3G1, Canada;4. Geological Survey of Canada, Ottawa, ON K1A0E8, Canada;5. Agnico Eagle Mines Limited, Val d''Or, Qc, Canada;1. Institute of Mineralogy, Uralian Branch of RAS, Miass 456000, Russia;2. South Urals State University, Department of Geology, October Ave. 16,Miass 456318, Russia;3. John de Laeter Centre for Isotope Research, Curtin University, Kent St., Bentley 6102, WA, Australia;4. Kazan Federal University, 18 Kremlyovskaya St., Kazan, 420008, Russia;5. Irish Centre for Research in Applied Geosciences (iCRAG), University College Dublin, Belfield, Dublin 4, Ireland;6. University of Tasmania, CODES, Hobart 7000, Tasmania, Australia |
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| Abstract: | The massive sulfide deposits of the Iberian Pyrite Belt are interbedded with felsic volcanic rocks and shale, and underlain by several thousand meters of siliciclastic sedimentary rocks known as the PQ Group. Isotope geochemistry and regional geology are both consistent with equilibration of the ore-forming fluids with the PQ Group, prior to ore deposition near the former seafloor. The average Cu:Zn:Pb ratio of the PQ Group rocks (ca. 26:55:19) is similar to the weighted average of all the massive sulfide orebodies combined (ca. 25:52:23).The genetic relationship between massive sulfide deposits and a siliciclastic sedimentary metal source is explained here by a thermodynamic model, proposing that mildly reducing redox conditions imposed by equilibration with the sedimentary rocks are most critical for the formation of an effective ore-forming fluid. Relatively metal-rich but organic-poor pyrite-bearing shale undergoing dewatering of saline pore fluids is an effective source for the generation of sulfur-deficient but relatively iron and base metal-rich brines. Thus, we propose that the giant deposits of the Iberian Pyrite Belt owe their existence not to exceptionally metal-enriched (e.g., magmatic) fluids, but to the existence of a fairly ordinary but large metal source in reactive siliciclastic sediments, combined with an underlying igneous heat source and a particularly efficient mechanism of sulfide precipitation by mixing with H2S-rich fluids at or near the seafloor.Essentially similar mineral equilibria are imposed when saline fluids are buffered by typical continental basement rocks. Leaching of retrograde minerals and possibly residual salts from their magmatic or metamorphic prehistory is expected to generate similar, variably metal-rich but relatively sulfide-deficient fluids. Thus, the existence of mildly reducing rocks can be the dominant chemical control in the source of fluids generating many volcanogenic, Irish-type or sedex deposits, many of which are known to precipitate their metal load in response to biogenic sulfide addition at the ore deposition site. |
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