The Gounkoto Au deposit, West Africa: Constraints on ore genesis and volatile sources from petrological, fluid inclusion and stable isotope data

Lambert-Smith, J. S., Lawrence, D. M., Vargas, C. A., Boyce, A. J. , Treloar, P. J. and Herbert, S. (2016) The Gounkoto Au deposit, West Africa: Constraints on ore genesis and volatile sources from petrological, fluid inclusion and stable isotope data. Ore Geology Reviews, 78, pp. 606-622. (doi: 10.1016/j.oregeorev.2015.10.025)

Abstract

The Loulo–Gounkoto complex in the Kédougou–Kéniéba Inlier hosts three multi-million ounce orogenic gold deposits, situated along the Senegal–Mali Shear Zone. This west Malian gold belt represents the largest West African orogenic gold district outside Ghana. The Gounkoto deposit is hosted to the south of the Gara and Yalea gold mines in the Kofi Series metasedimentary rocks. The ore body is structurally controlled and is characterised by sodic and phyllic alteration, As- and Fe-rich ore assemblages, with abundant magnetite, and overall enrichment in Fe–As–Cu–Au–Ag–W–Ni–Co–REE + minor Te–Pb–Se–Cd. Fluid inclusion analysis indicates that the deposit formed at P–T conditions of approximately 1.4 kbar and 340 °C and that two end member fluids were involved in mineralisation: (1) a moderate temperature (315–340 °C), low salinity (< 10 wt.% NaCl equiv.), low density (≤ 1 g·cm− 3), H2O–CO2–NaCl–H2S ± N2–CH4 fluid; (2) a high temperature (up to 445 °C), hypersaline (~ 40 wt.% NaCl equiv.), high density (~ 1.3 g·cm− 3), H2O–CO2–NaCl ± FeCl2 fluid. Partial mixing of these fluids within the Jog Zone at Gounkoto enhanced phase separation in the aqueo-carbonic fluid and acted as a precipitation mechanism for Au. These findings demonstrate the widespread, if heterogeneously distributed, nature of fluid mixing as an ore forming process in the Loulo–Gounkoto complex, operating over at least a 30 km strike length of the shear zone. Stable isotope analyses of ore components at Gounkoto indicate a dominant metamorphic source for H2O, H2S and CO2, and by extension Au. It thus can be reasoned that both the aqueo-carbonic and the hypersaline fluid at Gounkoto are of metamorphic origin and that the high levels of salinity in the brine are likely derived from evaporite dissolution.

• Item Type: Articles
• Additional Information: Isotope analyses were carried out through a NERC Facilities Grant (IP-1252- 0511) to P. J. Treloar. A. J. Boyce is funded by NERC support to the Isotope Community Support Facility at SUERC.
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Boyce, Professor Adrian
• Authors: Lambert-Smith, J. S., Lawrence, D. M., Vargas, C. A., Boyce, A. J., Treloar, P. J., and Herbert, S.
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre
• Journal Name: Ore Geology Reviews
• Publisher: Elsevier B.V.
• ISSN: 0169-1368
• ISSN (Online): 1872-7360
• Published Online: 28 October 2015