Age constraints on gold mineralization and Paleoproterozoic crustal evolution in the Ashanti belt of southern Ghana

Abstract

U Pb isotopic dating in the region of the Ashanti belt in Ghana has been carried out to improve understanding of crustal evolution and gold mineralization in this Paleoproterozoic terrane. A U Pb zircon age of ca 2174 ± 2 Ma for a belt-type granitoid from Sekondi accords with previous ages on belt-type plutons in Ghana, but metamorphic titanite within this granitoid gives an age of 2092 ± 3 Ma, coeval with intrusion of the late basin-type granitoids. The within-belt granitoid at Banso village has an age close to 2097 ± 3 Ma, which falls into the age range typical for basin-type granitoids (2116-2088 Ma). These results indicate that widespread plutonism in the Birimian sedimentary basins also produced magmatic and metamorphic effects within the volcanic belts. Most dated detrital zircons from Birimian metasediment samples at the Ashanti gold mine agree in age at 2155 ± 2 Ma, with others that are slightly older. This gives a maximum estimate for the depositional age of the sediments and for gold mineralization at the Ashanti gold mine. Hydrothermally altered and auriferous granitoid plutons along the western flank of the Ashanti belt, at Anyankyerim, Nhyiaso, Yaomensakrom and Esuajah (Ayanfuri), all have intrusion ages within error of 2105 ± 2 Ma. This is also the age of metamorphic titanite in an unmineralized basin-type granitoid at Kwapia. Rutiles from the hydrothermally altered Yaomensakrom and the mineralized Esuajah (Ayanfuri) granitoid were dated at 2098 ± 7 and 2086 ± 4 Ma, respectively, and probably provide minimum ages on gold mineralization. These data indicate a close temporal association among auriferous granitoid plutons near the western margin of the Ashanti belt at 2105 Ma. Gold mineralization may be only slightly younger than magmatic emplacement of the plutons. Granitic plutonism in the sedimentary basins, metamorphism, and hydrothermal gold mineralization in southern Ghana are all part of a major crust formation event, the Eburnean orogeny, that can be constrained to the time range 2120-2080 Ma.