Cathodoluminescence study of gold-bearing conglomerate of the Witwatersrand Basin, South Africa.

Abstract

The gold ore of the Harmony mine is hosted by conglomerate of the Basal Reef, Central Rand Group in the Witwatersrand Basin, the world's largest gold-producing province of South Africa. The conglomerate contains detrital rock fragments such as quartzite, shale, with minor greenstone. It accompanies mineral fragments of quartz, feldspar, zircon, chromite, pyrite, uraninite and minor gold. Quartz grains in the studied samples typically occur as rounded pebbles or granules. Under a cathodoluminescence (CL) microscope, quartz shows various colors, light blue, bluish purple, reddish purple, orange and brown. Preservation of several colors suggests that the metamorphic grade of conglomerate is very low. Grains of quartz were probably supplied from different source rocks because luminescence colors of quartz pebble are variable even in neighboring pairs. Quartz grain associated with uraninite shows zonal texture with inner part and thin rim under the CL microscope. A thin layer (∼ 50 μm) with reddish luminescence typically surrounds the inner part of a quartz grain which shows bluish to brownish luminescence. Such texture of quartz suggests that the inner parts of quartz grains represent original CL color, and that the outer parts were caused by radiation damage. Some quartz shows the thin layer of reddish CL color in its crack. This texture shows the relation of the hydrothermal solution containing uranium and precipitation of microsize uraninite. Chemical composition of gold grains are 82.0-85.4 atm.% Au, and Hg content in gold is very low. Low Hg concentration in gold is consistent with the weakness of post-depositional hydrothermal modification of gold ore.