Epithermal Gold
-Alteration of Deposits

Erosion has a very important part to play in the occurrence of epithermal gold deposits. Very few epithermal deposits can be related directly to deep seated intrusive bodies, but this relationship can be seen where deep erosion has occurred. Most deposits have no visible relationship with plutonic rocks. As these ore deposits are found at or near the surface and in tectonically active areas, they are susceptible to immediate erosion. Subsequently this type of deposit is dominantly found in areas of Tertiary volcanism, and is almost unknown in pre Cenozoic lithologies.

The alteration in country rocks around hydrothermal veins is usually extensive. High porosity and permeability values in wall rocks allow deep migration of the mineralization fluids. The difference in temperature between the cool country rocks and the hot invading fluids causes reactions which results in a new assemblage of minerals. The main products of this mineralization are chlorite, alunite, sericite, zeolites, clays, adularia, silica and pyrite. Propylitization is the main zone of alteration in which chlorite is the main mineral. Silica, chlorite, sericite and pyrite of the alteration halos are usually fine grained. Near the surface it is not uncommon to find a broad argillic zone of alteration often containing alunite. Some deposits are found to have an aluminous advanc ed argillic alteration zone in which kaolinite, sericite, pyrophyllite and andalusite with minor corundum, topaz and diaspore is found. Carbonate minerals such as calcite, dolomite, ankerite and rhodochrosite are also products of alteration. Montmorillonite and kaolinite clays may be abundant forming zones parallel to the walls of veins.

Often supergene gossans are found above epithermal ore deposits. This is a weathering process in which an iron oxide cap is formed above the deposit. Also pyrite within the altered wall rock is oxidized to limonite, goethite and hematite. What often occurs in copper deposits when supergene enrichment takes place is very minimal in an epithermal deposit, only minor enrichment of gold and silver takes place.