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Optical image and microchemical analysis of gold grains from a weathered profile of the Minvoul greenstone belt, northern Gabon

Published online by Cambridge University Press:  30 August 2019

Mario Iglesias-Martínez Open the ORCID record for Mario Iglesias-Martínez [Opens in a new window] ,
Berta Ordóñez-Casado Open the ORCID record for Berta Ordóñez-Casado [Opens in a new window]  and
Edgar Berrezueta Open the ORCID record for Edgar Berrezueta [Opens in a new window]
Mario Iglesias-Martínez*
Affiliation:
Escuela Técnica Superior de Ingenieros de Minas, Universidad Politécnica de Madrid, c/Ríos Rosas, 21, 28003 Madrid, Spain
Berta Ordóñez-Casado
Affiliation:
Instituto Geológico y Minero de España, c/Matemático Pedrayes, 25, 33005 Oviedo, Spain
Edgar Berrezueta
Affiliation:
Instituto Geológico y Minero de España, c/Matemático Pedrayes, 25, 33005 Oviedo, Spain
*
Author for correspondence: Mario Iglesias-Martínez, Email: [email protected]
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Abstract

Morphological characterization and quantification of gold particles by optical image analysis (OIA) and by compositional analysis of microprobes using scanning electron microscope and electron microprobe analysis techniques were carried out on gold grains from the Minvoul area (Archean greenstone belt in Gabon). Large grains of almost pure gold were found throughout a weathering profile, which consisted of saprolite, mottled clay zone, iron duricrust, pisolitic gravels and yellow latosol. In the deeper horizons, gold was dissolved as shown by corrosion features on the surface of particles with average sizes of 2.6Φ and 2.35Φ in the saprolite and mottled clay zones, respectively. The occurrence of secondary gold in the duricrust was indicated by the larger size of the nuggets (average, 1.8Φ), the high fineness (> 995 in average) and the close textural relationship between gold and neoformed iron oxyhydroxides. The uppermost horizons composed of yellow latosol and pisolitic gravels were interpreted as transported materials based on their size distribution (average 0.85Φ and 1.34Φ), sorting and shape parameters. The best morphological parameter to describe the whole weathering profile was found to be the perimeter/area ratio. The highest ratios were recorded in the saprolite (average 0.192 μm−1), and decreased towards the surface (average 0.057 μm−1). The combination of the OIA technique and the microchemical analysis of gold grains allowed us to define specific morphological and compositional characteristics of the gold particles for each horizon. Both methods proved to be of great utility to understand gold concentration, dissolution and dispersion processes in supergene environments.

Keywords

goldlateriteweatheringoptical image analysisGabon
Type
Original Article
Information
Geological Magazine , Volume 157 , Issue 2 , February 2020 , pp. 307 - 320
Copyright
© Cambridge University Press 2019 

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