The Massawa gold deposit, Eastern Senegal, West Africa: an orogenic gold deposit sourced from magmatically derived fluids?
P. J. Treloar, D. M. Lawrence, D. Senghor, A. Boyce, P. Harbidge, 2015. "The Massawa gold deposit, Eastern Senegal, West Africa: an orogenic gold deposit sourced from magmatically derived fluids?", Ore Deposits in an Evolving Earth, G. R. T. Jenkin, P. A. J. Lusty, I. Mcdonald, M. P. Smith, A. J. Boyce, J. J. Wilkinson
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The Massawa gold project is situated on the Senegalese side of the highly prospective/productive Palaeo-Proterozoic (Birimian) Kédougou–Kéniéba inlier, which hosts several world-class orogenic gold deposits/districts in western Mali (e.g. Loulo and Sadiola). The Massawa ore body has a strike length of at least 4 km and a current resource of 3.61 Moz at a grade of 2.8 g t−1. The ore body is structurally controlled and located within a package of low-grade regionally metamorphosed volcaniclastic sediments (agglomerates, tuffs and ash-tuffs), quartz–feldspar and lithic wackes, carbonaceous shales, hydrothermal breccias, and gabbro and porphyry sills. These rocks have undergone pervasive silica alteration followed by a sericite–ankerite–chlorite alteration event related to mineralization. Two major styles of mineralization are recognized at Massawa from field and laboratory studies. The first stage of sulphide–Au mineralization is associated with disseminated arsenopyrite–pyrite, which follows shear zones in the sedimentary and volcano-sedimentary host rocks. The second stage consists of quartz–stibnite±tetrahedrite veining distinguished by coarse visible gold and represents a late stage overprint on the primary mineralization. The two stages of gold mineralization are separated by a phase of quartz–molybdenite veining. A distinctive base metal trace assemblage is linked to stibnite formation including multiple Sb phases such as chalcostibite, zinkenite, roshchinite, aurostibite, jamesonite and robinsonite. Secondary ion mass spectroscopy-based gold deportment data indicate that up to 90% of stage 1 gold is held as a solid solution within either arsenopyrite or arsenian pyrite. Stable isotope data yield δ34Ssulphide values of between 0 and 4.1‰ and δ18OH2O values of 5.5–10.9‰ for all stages of mineralization, suggesting a magmatic fluid influence. This is consistent with field data that suggest that mineralization is synchronous with emplacement of a sequence of concordant felsic sheets. That mineralization occurred at shallow (<6 km) depths is suggested both by the presence of stibnite and by fluid inclusion studies. Low-temperature (homogenization temperatures between 150 °C and 230 °C) H2O–NaCl fluids (<6 wt% NaCl equiv.) and coeval CO2–CH4 inclusions, observed in both phases of mineralization, indicate trapping conditions of 220–315 °C at 1–1.65 kbar. A combination of phase petrology, fluid inclusion and stable isotope data suggests deposition of gold from low-salinity, magmatic fluids, most probably released from felsic rocks similar to those emplaced into the Massawa sequence during mineralization.
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Ore deposits form by a variety of natural processes that concentrate elements into a volume that can be economically mined. Their type, character and abundance reflect the environment in which they formed and thus they preserve key evidence for the evolution of magmatic and tectonic processes, the state of the atmosphere and hydrosphere, and the evolution of life over geological time. This volume presents 13 papers on topical subjects in ore deposit research viewed in the context of Earth evolution. These diverse, yet interlinked, papers cover topics including: controls on the temporal and spatial distribution of ore deposits; the sources of fluid, gold and other components of orogenic gold deposits; the degree of oxygenation in the Neoproterozoic ocean; bacterial immobilization of gold in the semi-arid near-surface environment; and mineral resources for the future, including issues of resource estimation, sustainability of supply and the criticality of certain elements to society.