Mineral Resources Potential - Geothermal Resources

4 MindResoums Potential of Ethiopia ;hcl
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Alteration associated with gold mineralization also invdlvk?s :'
;srflph.ildbtion (sulphide halos are a characteristic aitmtibs
phgnbmenol~ of most orogenic mesothermal vein gold depasi@)
.:ad potassium metasomatism (potassium is usually enriched in the
alteration halo around the veins). These halos overprint preexisting
alteration assemblages in the host rock. Any rock type can
1. L - host these vein systems, but, at best, they are developed in d c
'fo;cks such as badts, greenstones, gabbros and turbiditic shaky
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sedimentary rocks; this is attributable to the chemical contrasts
between host rock and ore fluids. The ore fluids are silica-rich with
Garbon dioxide and potassium; hence they react best with mafic
rwcks, which-& not contain free silica but which have calciumiron-Magnesium
silicates that can react with carbon dioxide to
- form carbonate alteration minds. Gold abundance are characteristically Iow in most
geological rnakrids, The average crustal abundance of gold is in
, the order of three parts per billion, and generallyno single rock
type is preferentially enriched in gold As a result of the low
background contents of gold, a large mount of. rock must be
affected by the hydrothernld fluids in brder for sufficient deposits
of dissolved gold to be formed. The general model for these
deposits suggests that the associated regional faults have deep
roots that extend down to the lower crust, Hydrothermal fluids,
which contain gold dissolved from a wide region, are formed, and
these are focused up dong the M t s to higher levels in the crust,
where they react with country rock to form lode gold ores. In
temporal terms, orogenic rnesothd vein gold deposits
apparently have ken restricted to specific intervals in the Earth's
history, including the Late Archean, Early Pmterozoic. Early
Paleozoic and Early Mesozoic periods. They are best deveIoped in
Archean greenstone belts within Archean cratonic areas, such as in
northern regions af Ontario and Quebec, Western Australia and
Southern Afiica.