Conflicting evidence on the timing of mesothermal and paleoplacer gold mineralisation in early Proterozoic rocks from Southwest Ghana, West Africa

Abstract

Rocks of early Proterozoic age (ca. 2100 Ma) host the major gold deposits in Ghana. The deposits are either located in mesothermal quartz vein systems or hosted in a quartz pebble conglomerate that represents a paleoplacer. Both types of mineralisation are largely confined to the Ashanti Belt, one of four parallel northeasttrending volcanic belts. While the stratigraphy and structure of the belts are similar, the Ashanti belt is characterised by a more tectonised northwest margin where most of the epigenetic gold deposits are located. In these deposits, gold mineralisation is located in faults that parallel the regional trend of the belts and were active late in the deformation history of the terrane. The auriferous quartz pebble conglomerate is part of a elastic sequence that is largely derived from the adjacent volcanic and plutonic rocks with the gold widely regarded as having originated from eroded vein deposits. Structural data, however, show that both the volcanic rocks and elastic sequence were deformed jointly prior to epigenetic gold mineralisation. Thus, the quartz vein deposits could not have been the source of the paleoplacer mineralisation. The paleoplacer gold could have originated from one of several possible sources but none has been unequivocally identified.

Geology and Geochemical Aspects of Ore Formation at the Prestea Mesothermal Vein Gold Deposit in the Birimian System of Ghana

“…According to a review of the geology of the Prestea mine by Adadey (1983), Appiah et al (1991), andEisenlohr (1992), the gold deposits occur in the upper Birimian close to the boundary with the Tarkwaian (Fig. 2).…”

The application of P–T–X(CO2) modelling in constraining metamorphism and hydrothermal alteration at the Damang gold deposit, Ghana

“…Fluid inclusion and mineral thermometry techniques suggest a spread of formation temperatures and pressures for Ghanaian orogenic gold deposits, typically in the range 300–450 °C and 2–5 kbar, with a dominantly low‐salinity fluid (Mumin et al ., ; Schmidt Mumm et al ., ; Yao et al ., ; Wille & Klemd, ). In addition to the orogenic deposits, the Tarkwaian System sediments host widespread palaeoplacer‐type deposits hosted in quartz‐pebble conglomerate horizons (Milési et al ., ; Eisenlohr, ; Pigois et al ., ).…”

Diferenças Entre os Depósitos Auríferos do Grupo Aguapei (Estado de Mato Grosso) e os Clássicos Exemplos de Ouro em Conglomerados

“…Os primeiros trabalhos de exploração de ouro, em Ghana, datam de 1453 com a chegada dos portugueses naquele país (Kesse 1984). Os maiores depósitos estão associados a veios de quartzo mesotermais e a conglomerados quartzosos (paleoplacer), ambos confinados a Ashant Belt (Eisenlohr 1992). Esta faixa é constituída por rochas supracrustais do Paleoproterozóico (ca.…”

“…Os litótipos do depósito de Tarkwa foram metamorfisados na fácies xisto verde a anfibolito. Segundo Eisenlohr (1992), a ocorrência de ouro está restrita aos metaconglomerados polimíticos do Grupo Kawere, constituídos por clastos de rochas vulcânicas máficas e félsicas, silexito, folhelhos e quartzo, derivados do Supergrupo Birimian, bem como seixos do Granito Dixcove. O ouro está concentrado em lentes de conglomerados com 0,5 a 2 m de espessura, 50 -150 m de largura e 200 -600 m de comprimento.…”

“…Na maioria dos depósitos clássicos em conglomerados, como os de Jacobina, Witwatersrand e Mporokoso, a origem do ouro é controversa (Barnicoat et al1997;Mascarenhas et al1998). Quanto aos depósitos em rochas metassedimentares de Ghana (Supergrupo Tarkwaian), apesar de haverem defensores da origem paleoplacer modificado (Milési et al 1992), a singenética é aceita pela maior parte dos pesquisadores (Eisenlohr 1992).…”