Mineralogical evolution of the Las Cruces gossan cap (Iberian Pyrite Belt): From subaerial to underground conditions |
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| Institution: | 1. Department of Earth Sciences, University of Huelva, Avenida de las Fuerzas Armadas, S/N, 21071 Huelva, Spain;2. Geological Area, Mining Drepartment of Cobre Las Cruces S.A., Ctra. SE-3410, Km 41,100, 41860 Gerena, Seville, Spain;3. Cobre Las Cruces S.A., Ctra. SE-3410, Km 41,100, 41860 Gerena, Seville, Spain;1. Earth Science Institute, Slovak Academy of Sciences, 840 05 Bratislava, Slovakia;2. Clermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans, BP 10448, F-63000 Clermont-Ferrand, France;3. CNRS, UMR6524, LMV, F-63038 Clermont-Ferrand, France;4. IRD, R 163, LMV, Clermont-Ferrand, France;5. Department of Mineralogy and Petrology, Comenius University, 842 15 Bratislava, Slovakia;6. Institute of Geological Sciences, Masaryk University, 611 37 Brno, Czech Republic;7. Matúškova 11, 831 01 Bratislava, Slovakia;8. Geoafrica Prospecting Services cc, PO Box 24218, Windhoek, Namibia;9. Earth Science Institute, Slovak Academy of Sciences, 917 04 Banská Bystrica, Slovakia;1. Department of Earth and Environmental Sciences, University of Windsor, Windsor, ON N9B3P4, Canada;2. Section of Earth and Environmental Systems, University of Toyama, 3190 Gofuku, Toyama-shi 930-8555, Toyama, Japan;3. Centro de Astrobiologia, CSIC-INTA, Crta. Ajalvir Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain;4. Department de Mineralogía y Petrología, Universidad del País Vasco, Apdo. 644, 48080 Bilbao, Spain;5. Instituto Geológico y Minero de España, Rios Rosas 23, 28003 Madrid, Spain;1. Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China;2. Department of Earth Sciences, Durham University, Durham DH1 3LE, UK;3. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;4. 321 Geological Team, Bureau of Geology and Mineral Exploration of Anhui province, Tongling 244033, China;5. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China |
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| Abstract: | The Las Cruces VMS deposit is located at the eastern corner of the Iberian Pyrite Belt (SW Spain) and is overlain by the Neogene–Quaternary sediments of the Guadalquivir foreland Basin. The deposit is currently exploited from an open pit by Cobre Las Cruces S.A., being the supergene Cu-enriched zone the present mined resource. The Las Cruces orebody is composed of a polymetallic massive sulfide orebody, a Cu-rich stockwork and an overlying supergene profile that includes a Cu-rich secondary ore (initial reserves of 17.6 Mt @ 6.2% Cu) and a gossan cap (initial reserves of 3.6 Mt @ 3.3% Pb, 2.5 g/t Au, and 56.3 g/t Ag).The mineralogy of the Las Cruces weathering profile has been studied in this work. Textural relationships, mineral chemistry, deposition order of the minerals and genesis of the Las Cruces gossan are described and discussed in detail. A complex mineral assemblage composed by the following minerals has been determined: carbonates such as siderite, calcite and cerussite; Fe-sulfides including pyrite, marcasite, greigite and pyrrhotite; Pb–Sb sulfides and sulfosalts like galena, stibnite, fulöppite, plagionite, boulangerite, plumosite, and the jordanite–geocronite series, Ag–Hg–Sb sulfides and sulfosalts including miargyrite, pyrargyrite, sternbergite, acanthite, freibergite, cinnabar, Ag–Au–Hg amalgams; and Bi–Pb–Bi sulfides and sulfosalts such as bismuthinite, galenobismutite, others unidentified Bi–Pb-sulfosalts, native Bi and unidentified Fe–Pb–Sb-sulfosalts. Remains of the former oxidized assemblage appear as relicts comprised of hematite and goethite.Combining paragenetic information, textures and mineral chemistry it has been possible to derive a sequence of events for the Las Cruces gossan generation and subsequent evolution. In that sense, the small amount of Fe-oxyhydroxides and their relict textures replaced by carbonates and sulfides suggest that the gossan was generated under changing physico-chemical conditions. It is proposed that the Las Cruces current gossan represents the modified residue of a former gossan mineralization where prolonged weathering led to dissolution and leaching out of highly mobile elements and oxidation of the primary sulfides. Later, the gossan was subject to seawater-gossan interaction and then buried beneath a carbonated-rich cover. The basinal fluids-gossan interaction and the equilibration of fluids with the carbonated sediments brought to the carbonatization and sulfidation of the gossan, and thus to the generation of Fe-carbonates and Pb–Sb-sulfides.The Las Cruces mineral system likely represents a new category within the weathering class of ore deposits. |
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