Structural And Alteration Controls On Gold Mineralization In The Rhyacian Rocks Of The Josephine Deposit, NW Ghana

Abstract

This thesis focuses on the structures and alterations controlling gold mineralization in the Josephine deposit found in NW Ghana. The lithological and structural maps were produced from field lithological and structural data in conjunction with airborne geophysical data. The lithologies observed in the deposit comprise of meta-sediments (mainly quartzite), granitoid batholiths, mafic intrusion (dolerite), and gneiss. The quartzite which hosts the mineralization have experienced greenschist metamorphism. Through mapping and structural analysis, the following sequence of deformational events; DJO0, DJO1, DJO2 and DJO3was identified. DJO0 is synonymous to the primary preserved sedimentary features observed in the rocks and mainly observed in the quarzitic rocks. Gold mineralization in the deposit is structurally controlled and mainly associated with the DJO1 NNW-trending dextral and steeply dipping sinusoidal shear zone. The gold occurs together with arsenopyrite along the foliation planes of the host rocks, quartzite. Alteration minerals assemblage observed within the deposit give indication the gold precipitated under greenschist metamorphic facies. The ore body geometry is about 15-40m wide with a strike length of 800m trending in the NNW direction. Silicification, seritization, chloritization and sulphidation (quartz + sericite + chlorite + sulphide) are the main alterations associated with the ore zone in the deposit. Sulphides observed in the deposit are arsenopyrite, pyrite, chalcopyrite and sphalerites with gold more associated with the arsenopyrite mineral. The gold occurs as free gold within the fractures of the arsenopyrite mineral. The pathfinder elements associated with gold in the deposits are mainly antimony or stibnite (Sb), Te (Tellurium), S (Sulphur), Cu (Copper), Ag (Silver), and Lead (Pb) which is characteristic of orogenic gold. Isocon analysis suggests the altered mineralized zone has gained 16.6% mass and volume increase because of addition of the fluids. This together with the zonation of the arsenopyrite gives an indication of hydrothermal process responsible for the mineralization with an unknown source