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Thursday, February 17, 2011
Gold in Greenstone Belts
Gold is often associated with greenstone belts that according to geologists are a group of variably metamorphosed mafic to ultra mafic volcanic sequences that are often associated with sedimentary rocks that can be found in Archean and Proterozoic terrenes where they are found nested in between bodies of gneiss and granite.
An openpit gold mine operating in a greenstone belt in Australia
The name greenstone comes from the greenish color that is present in many of the metamorphic materials that are found in the rocks composing a greenstone belt. This color is often imparted by actinolite, chlorite and other green amphibole minerals that are typically found associated with these green colored belts of rock.
Many geologists feel that the gold found in greenstone belts is leached from the greenstone by supercritical water to be deposited in the surrounding rocks in quartz veins or as disseminated gold.
Pillow lava typical of that found in a greenstone belt.
Greenstone belts are typical of continental cratons that can range from as little as several dozen to several thousand kilometers in length. Most greenstones contain a great number of individual rock units that is considered a stratigraphic grouping in its own right when they are found on continental sized scales. They are considered by geologists as basically metamorphosed volcanic belts that are associated with ancient oceanic spreading centers or is even island arcs.
Although greenstone belts are usually composed of volcanic rocks with the most prevailing being basalt there are also small amounts of sedimentary rocks that are interwoven, or interleaved with the various volcanic rocks. One of the most common rocks associated with greenstone belts is conglomerate that is often gold bearing as it is in the Abitibi Greenstone Belt in Canada.
The following list of greenstone belts shows how pervasive they are worldwide: