2 Ore geology – an introduction
Ore geology is an economical term used to describe valuable mineralization. This involves certain metallic rocks and also gems. There are two types of ore, syngenetic and epigenetic. The syngenetic ores are formed at the same time as the host rock and the epigenetic are formed after the host rock.
Ore mineralization can occur as either hypogene or supergene. Hypogene is when the miner-alization occurs as the hydrothermal solution ascends. The Supergene mineralization occurs when there is a descending solution. This process occurs because of weathering and that can be responsible for the in situ enrichment of ore minerals (Robb, 2005).
Orogenic gold associations occur during regional tectonic zones and deposits occur in epi-zonal, mesozonal, and hypozonal formations. An example of how this can occur is through a contact between two different rocktypes in a shear zone (fig 3) (Bark et al 2005).
Hydrothermal deposits can occur in different environments as epithermal, mesothermal and hypothermal. The epithermal deposits are formed at depths less than 1500 metres and tem-perature of about 50 to 200oC. The mesothermal deposits are formed at depths of abour 1500 to 4500 metres and temperatures of about 200 to 400oC. The hypothermal deposits are formed in the deepest of the three facies at depths greater than 4500 metres and at temperature of about 400 to 600oC (Robb, 2005).
Tectonic regions where ore can form are convergent and divergent margins. The convergent margins have low geothermal gradients and can reach temperatures for hydrous metamorphic minerals to be unstable. The rise of batholiths becomes a main process for the hydrothermal process. In divergent margins magmas are extruded and seawater penetrates the fractures. The hot water reacts with the basalt and dissolves minerals such as calcium and epidiot, increasing the acidic. This acidic water then dissolves element such as copper and zinc creating sulfides (Wenk & Bulakh, 2004). These become VMS (volcanogenic massive sulfide) and examples of these are the Kukorko ores outside of Japan that is a Zn-Pb-Cu VMS.
A picture on some of the things i have been doing the past weeks
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