Mineral Resources Potential - Geothermal Resources

34 Mined Resources Potential or Ethiopia
of metallic mineral deposits (gold, base metals, rare metals, Ni-Cr-
Co and others), for potential investors to enjoy such rewarding
business opportunities.
3.1 Gold deposit
Occurrence
Due to its relative chemical inertness, gold is usually found as the
native metal or alloy. Occasionally large accumulations of native
gold (also known as nuggets) occur but usually gold occurs as
minute grains. These grains occur between mineral grain
boundaries or as inclusions within minerals. Common gold
associations are quartz often as veins and sulfide minerals. The
most common sulfide associations are pyrite, chalcopyrite, galena,
sphalerite, arsenopyrite, stibnite and pyrrhotite. Rarer mineral
associations are petzite, calaverite, sylvanite, muthrnannite,
nagyagi te and krennerlte.
Gold is widely distributed in the Earth's crust at a
background level of 0.03 g11,000 kg (0.03 ppm by weight) (Boyle,
1987). Hydrothermal ore deposits of gold occur in metamorphic
rocks and igneous rocks -alluvial placer deposits originate from
these sources. The primary source of gold is usually igneous rocks
or surface concentrations. A deposit usually needs some form of
secondary enrichment to form an economically viable ore deposit:
either chemical or physical ,pro'cesses like erosion or solution or
more generally metamorphism, which concentrates the gold in
sulfide minerals or quartz. There are several primary deposit types;
common ones are termed reef or vein. Primary deposits can be
weathered and eroded, with most of the gold being transported into
stream beds where it congregates with other heavy minerals to
form placer deposits. In all these deposits the gold is in its native
form. Another important ore type is in sedimentary black shale and
limestone deposits containing finely disseminated gold and other
platinum group metals. Gold occurs in sea water at 0.1 to 2 mg/t