Mineralogical siting and distribution of gold in quartz veins and sulfide ores of the Ashanti mine and other deposits in the Ashanti belt of Ghana: genetic implications |
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Authors: | T Oberthür T Weiser J A Amanor S L Chryssoulis |
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Institution: | (1) Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, D-30655 Hannover, Germany, DE;(2) Ashanti Goldfields Company Ltd., Obuasi, Ghana, GH;(3) Advanced Mineral Technology Laboratory (amtel), University of Western Ontario, 100 Collip Circle, London, Ontario N6A 5B7, Canada, CA |
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Abstract: | The Ashanti belt of Ghana constitutes a gold province which has produced a total of about 1500 t of gold historically. Gold
mineralization is found in steep, NNE-SSW to NE-SW trending shear zones predominantly transecting metasediments of the Palaeoproterozoic
Birimian Supergroup (2.2–2.1 Ga), disseminated in ca. 2.1 Ga granitoids, in paleo-conglomerates of the Tarkwaian Group (<
2135 Ma), and in recent placers. The distribution of gold, its chemistry, paragenesis and mineralogical siting in the mesothermal
ores of the major mines in the Ashanti belt, namely Konongo, Ashanti, Bogosu and Prestea mine, are the subject of this study.
At the localities studied, gold is present in two main types of ores: 1. Quartz veins with free-milling gold. The gold is
relatively silver-rich (true fineness values from 730 to 954) and is accompanied by a distinct suite of Cu, Pb, Sb sulfides.
2. Sulfide ores, consisting of arsenopyrite, pyrite and rarer pyrrhotite and marcasite, with refractory gold. The ores have
apparent fineness values larger than 910. Arsenopyrite and locally (at Bogosu) pyrite were identified as the hosts of submicroscopic
gold. Mean concentrations of gold in arsenopyrite in various samples from the different mines, obtained by secondary ion mass
spectrometry (SIMS), range from 67 to 314 ppm Au. Gold concentration mapping in individual arsenopyrite crystals from the
different deposits revealed similar patterns of gold distribution: the grains possess a gold-poor core, and elevated gold
contents are present along distinct crystal growth zones towards their rims. The outermost crystal layer is usually gold-poor.
The well-preserved distribution patterns also indicate that remobilization of gold from the sulfides played an insignificant
role in the ores of the Ashanti belt. Multiple quartz veining and growth zoning of the sulfides are interpreted as manifestations
of multiple episodes of fluid infiltration, fluid flow and mineral deposition. The bimodal occurrence of gold in spatially
closely associated quartz vein and sulfide ores indicates a genetic link between these ore types. A model implying a grossly
coeval formation of the ores from mesothermal fluids is proposed.
Received: 28 September 1995 / Accepted: 11 June 1996 |
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