Ore Mineralogy and Mineral Chemistry of the Ashanti Gold Deposit at Obuasi, Ghana

Abstract

Abstract:The gold deposit at Ashanti occurs in the Proterozoic Birimian formation of Ghana. Two main ore types mined from the deposit are gold-bearing quartz veins, and gold-sulfide disseminations in metasediments and metavolcanics. The main sulfide minerals in the gold-sulfide disseminated ores are arsenopyrite, pyrite and pyrrhotite, and to a very minor extent, sphalerite and tetrahedrite. Carbonate alteration and sericitization are prominent in the metavolcanics and the metasediments, respectively. In the quartz veins, pyrite and arsenopyrite commonly occur in small amounts, but gold mostly occurs in contact with tetrahedrite, chalcopyrite, galena, aurostibite, and sphalerite. Pyrrhotite is absent in the quartz veins.Microprobe studies indicate that As content of homogeneous arsenopyrite grains ranges from 27.0 to 31.7 atm%, and gives mineralization temperatures from 170 to 430ºC, although mostly from 300 to 400ºC. Chlorite geothermometry using temperature dependence of substitution of Al for Si in the tetrahedral site gives formation temeratures of 330 to 400ºC, comparable to the arsenopyrite temperatures. Applying sphalerite-pyrite-pyrrhotite geobarometry to sphalerite with FeS contents from 13.6 to 12.5 mol%, the pressure was estimated to be in a range from 5.9 to 7.0 kb at the stage of elevated temperatures.Mineralogical observations, especially absence of pyrrhotite in the quartz veins, together with microprobe data for gold and associated minerals suggest that the fluids having ascended through fissures in the Ashanti deposit were reduced by the reaction with carbonaceous materials in the metasediments during the declining stage of the regional metamorphism.

Soil Heavy Metal(loid) Pollution and Phytoremediation Potential of Native Plants on a Former Gold Mine in Ghana

“…For instance, in Obuasi, Antwi-Agyei et al (2009) reported 1711 mg kg −1 while Bempah et al (2013) found a concentration of 1752 mg kg −1 , which are among the highest in the world. This is not surprising, as only low amounts of As containing Birimian volcanics occur at the Damang deposit, while the Obuasi gold ores are rich in arsenopyrite containing sulphide minerals (Osae, Kase, & Yamamoto, 1995).…”

Gold metallogeny in the Birimian craton of Burkina Faso (West Africa)

“…After the classic contributions of Junner (1932Junner ( , 1935Junner ( , 1940 on the deposits of Ghana, numerous studies have been published since 1980, characterizing gold deposits in Paleoproterozoic formations of Ghana (Kesse, 1985;Milési et al, 1989Milési et al, , 1992Leube et al, 1990;Eisenlohr and Hirdes, 1992;Blenkinsop et al, 1994;Davis et al, 1994;Hirdes and Nunoo, 1994;Höhndorf et al, 1994;Hü nken et al, 1994;Mü cke and Dzigbodi-Adjimah, 1994;Oberthür et al, 1994Oberthür et al, , 1996Oberthür et al, , 1997Oberthür et al, , 1998Hammond and Tabata, 1997;Klemd and Hirdes, 1997;Osae et al, 1999;SchmidtMumm et al, 1997;Barritt and Kuma, 1998;Yao and Robb, 2000;Allibone et al, 2002Allibone et al, , 2004Feybesse et al, 2006), Mali (Dommanget et al, 1985(Dommanget et al, , 1993Milési et al, 1989Milési et al, , 1992, Sénégal (Sylla and Ngom, 1997), and Burkina Faso (Huot et al, 1987;Sanogo and Prost, 1993;Bourges et al, 1994Bourges et al, , 1998Bamba et al, 1997;Klemd and Ott, 1997;Béziat et al, 1998…”

Surface Water Quality at Boete Slimes Dam, AngloGold Ashanti Ltd Obuasi

“…The slightly acidic pH values at certain points could be due to run-off from the natural soils in the area which are known to be acidic [1],The Kwabrafo stream, which is close to the BSD mining area, recorded the highest pH value of 7.90. Ores mined at Obuasi contain approximately 1% sulphides and 2% carbonates [5].The carbonate content is enough to neutralize acids generated during oxidation of sulphide minerals during storage. In addition lime was added during the gold recovery processes at PTP and these aids in the neutralization process.…”