Granite-hosted gold mineralization in the Midlands greenstone belt: a new type of low-grade gold deposit in Zimbabwe

Abstract

In 1992, the Ford gold deposit was rediscovered during field work in the Kwekwe district near the Indarama mine, approximately 200 km southwest of Harare, Zimbabwe. Based on diamond drilling and open pit operations, estimated ore reserves are at least 3 Mt with an average gold content of 2.5 g/t. The gold deposit is located within a porphyritic granite dike with a thickness of 20–50 m, striking 800 m NNW-SSE. It dips 60–70° to the NE and intrudes a volcano-sedimentary sequence of tholeiitic basalts, acid volcanics, and banded iron formations of the Bulawayan Group (2900–2700 Ma). The intrusion of the dike occurred at 2541 ± 17 Ma (Pb/Pb step leaching technique) within a second order structure and is related to displacement along transcrustal deformation zones such as the Sherwood- and Taba-Mali deformation zones. Gold mineralization is confined to the s-shaped part of the dike intrusion. At the present stage of mining, the deposit is characterized by the absence of major veins, the occurrence of disseminated pyrite throughout the orebody, and a distinct alteration pattern comparable to that of porphyry copper deposits. The central zone of the dike shows a typical K-feldspar-albite-sericite-pyrite (±biotite?) alteration, followed by a narrow external propylitic zone. Native gold with an average Ag content of 5 wt.% and a grain size of 5–100 μm is rare and occurs within pyrite and secondary K-feldspar. Sulphide mineral separates of pyrite and minor arsenopyrite probably contain invisible gold (up to 120 ppm) amenable to cyanidation. Anomalously high gold values of ∼7 ppm have been found in the transition between the K-feldspar-albite-sericite-pyrite alteration and the propylitic zone, indicating that the mineralizing fluids have experienced major physico-chemical changes in the transition zone. The regional tectonic position of the orebody suggests that the emplacement of the granite and the gold mineralization are structurally controlled. The Pb isotope composition of several leachates of pyrite indicate isotope disequilibrium with magmatic minerals and point to a contamination of the mineralizing fluid by Pb from older (sedimentary?) sources. Stable isotope geochemistry of sulphides and carbonates as well as the metallogeny of the deposit compare to shear-zone hosted gold mineralization in the Kwekwe district, for which a deep crustal origin has been discussed. Although this study documents contrasting evidence for a porphyry-gold versus a shear-zone type of mineralization, it is suggested that gold-bearing fluids were syntectonically introduced into a ductile shear zone within the granite dike either during cooling of the intrusion or later in Archaean or early Proterozoic times.