Petrography and geochemistry of rocks in a prospect southwest of Prestea, Ghana to complement stream sediments and soils gold anomaly in the area

Abstract

Forty three (43) stream sediments and seven hundred and forty three (743) soil data were used to delineate gold anomalies. This was followed by thin and polished section petrography and XRF geochemistry of 17 rock samples to produce geological and gold prospectivity map for a prospect located in the Birimian Supergroup, southwest of Prestea, Ghana. The thinly foliated rocks are strongly deformed and partially to strongly altered. Mineralogical composition is plagioclase, amphibole, chlorite, sericite, epidote, quartz, pyrite, arsenopyrite, marcasite, magnetite, pyrrhotite and trace gold. The rocks are quartz chlorite schist, chlorite amphibole schist and quartz schist. Peak metamorphism is to amphibolite facies and retrograde metamorphism is to greenschist facies. Though primary textures were destroyed, geochemical variation plots classified the protoliths as rhyolite, dacite and andesite. XRF analysis of SiO2 ranges from (59.50 – 77.00 wt %), Al2O3 (14.20 – 21.10 wt %), Na2O (0.69 – 2.52 wt %), CaO (0.01 – 1.56 wt %), Total FeO (0.65 – 13.45 wt %), TiO2 (0.53 – 0.82 wt %) and MgO (0.14 – 2.44 wt %). Gold concentration is higher in soils from metamorphosed dacite and rhyolite with assay values ranging from 50 – 630 ppb than in soils from metamorphosed andesite (50 – 90 ppb). Consequently, prospectivity map shows that two gold anomalous zones occur at the contact between the metamorphosed dacite and metamorphosed rhyolite or metamorphosed andesite with stream sediments values 43 – 987 ppb and 43 – 985 ppb respectively. Therefore, exploration for gold in this area must target the contact between the metamorphosed dacite and metamorphosed rhyolite or metamorphosed andesite.