Characterisation of a recently discovered zone of intense hydrothermal alteration, deformation and unusual Au mineralization at Anglogold Ashanti's Kopanang gold mine

• Main Author: Cloete, Louis Michiel
• Published: 2012
• Subjects: Hydrothermal alteration; Gold ores geology
• Online Access: http://hdl.handle.net/10210/4327

M.Sc. === A prominent zone of deformation, identified as a possible hydrothermal fluid pathway, has recently been discovered by the author in the 64 SW4 21Line workings at Kopanang Gold Mine. The zone is between 3m and 16m wide, has a NE SW trend and dips steeply (65°-70۫°) towards the SE. Very little to no displacement seems to occur along this zone of alteration that is marked by intensive albite veining; the surrounding host rocks are also intensively altered (albitized). Albitization resulted from sodium metasomatism, during which Na was exchanged for K, and was accompanied by quartz dissolution which resulted in vug formation. This alteration event is also characterized by REE enrichment, which was found to be most intense in carbonate (ankerite) rich albitized vein fill material. A study of fluid inclusions indicates that the main hydrothermal fluid was of relatively low salinity (14.46 to 19.84 wt% NaCl ), having homogenization temperatures between 59.6 and 117°C. From these measurements it was concluded that a likely emplacement temperature for the studied feature would be in the order of 200°C-300°C at a pressure of roughly 0.5 to 1Kbar. A marked increase of Au grades is observed where the Vaal Reef, the most important economical reef horizon exploited at the mine, is affected by deformation, veining, and associated alteration. In light of the above observations it appears only reasonable to expect that this zone of deformation and hydrothermal alteration could either be the direct cause of, or at least have a prominent influence on the high gold values occurring in the area. A causal link between high Au grades, deformation and hydrothermal fluid flow would have obvious economic implications. The study therefore strives to characterise the extent and style of deformation, constrain the effects of hydrothermal alteration and nature of hydrothermal fluids associated with the structure, and assess its importance for Au mineralization