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Title: Mineralogical characterisation and interpretation of a precious metal-bearing fossil gossan, Las Cruces, Spain
Author: Blake, Christopher
ISNI:       0000 0004 2751 5928
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2008
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The Las Cruces VMS deposit lies on the southern margins of the Iberian Pyrite Belt, Spain. The primary base metal massive sulphide is overlain by a supergene enriched zone and precious metal gossan that remains well preserved under approximately 150 metres of Tertiary marl. The mineralogy, mineral textures and associations of five boreholes containing precious metal gossan mineralisation were characterised using a combination of optical microscopy, SEM and XRD techniques. The mineralogy and geochemical profile of the gossan suggests that it was formed under near-surface weathering conditions, resulting in the development of the supergene zone and a mature gossan profile characterised by elevated levels of Au and Ag. The Au and Ag probably remobilised as chloride complexes under strongly acid, oxidising conditions, precipitating as high fineness Au and discrete Ag-bearing phases lower in the gossan profile. The original Fe-oxyhydroxkJe dominated gossan mineral assemblage has subsequently been extensively replaced by later stages of siderite, greigite, galena and high fineness Au mineralisation that reflect marked changes in the oppositional environment relative to the original gossan mineral assemblage. Fluctuating oxidising and reducing conditions, coupled with biogenic processes within the Niebla Posadas aquifer, situated directly above the present day Las Cruces gossan, provide a suitable mechanism for the formation of the extensive siderite and greigite mineralisation as well as precious metal remobilisation as a thiosulphate complex under near-neutral to alkaline conditions. Strongly negative 613C stable isotope values for the siderite are consistent with biogenic processes involving Fe3* and/or sulphate reducing bacteria as well as a significant influence from the oxidation of methane.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available