Geochemistry and Alteration Mapping of the Paboase Deposit at Chirano Gold Mines, Ghana

Abstract

The Paboase deposit located in the Chirano gold district is hosted in Paleoproterozoic rocks within the Sefwi-Bibiani volcanic belt of the Birimian, which has been regionally metamorphosed to greenschist facies. Gold mineralization at Paboase is largely hosted by albite-pyrite-carbonate altered mafic and porphyry intrusive rocks rather than Birimian sedimentary rocks and graphitic shear zones which are well known style of mineralization in southwest Ghana. Multi-element geochemistry shows a quite weak fractionation trend among the mafic rocks, despite the doleritic and gabbroic textures. One different exotic mafic dyke signature can be recognised. Amongst the porphyries, two different compositional groups were recognized. There is a small scale intermixing of felsic dykes within mafic volcanics.The dominant alteration signature is intense albitisation along with ferroan dolomite and pyrite. There is about 150 m wide zone where the Analytical Spectral Device (ASD) maps strong carbonate alteration. The proximal alteration is carbobonate-albite-pyrite. This is surrounded by a distal halo of sericite-chlorite. Within the porphyries, the mineralogy is sericite-rich. Sericite-chlorite alteration is very widespread in the mafic rocks. Paboase has a Tungsten-Molybdenum-Bismuth-Terrarium (W-Mo-Bi-Te) signature. The Mo, Bi and Te have a strongly skewed distribution, and they are highly correlated with gold, so their usefulness as pathfinders is limited. The tungsten is remarkably uniformly distributed in the albite-dolomite rocks, and has a halo extending far beyond the footprint of the gold. Tungsten is therefore the best pathfinder for gold at Paboase. The host rocks for the anomalous W are essentially just albite-pyrite-dolomite rocks. Arsenic is clearly depleted in the proximal parts of the system compared to the pelitic sedimentary hosted graphitic shear zones of the Ashanti and Bogoso deposits in Ghana. It is suggested that the depletion of arsenic is as a result of flushing the rocks with an oxidized fluid early in the history of the system.