The Greenstone Belts
THE Barama-Mazaruni Supergroup is composed of a group of greenstone belts (supracrustals) that were formed in an ancient oceanic environment. The Acasta Gneiss in the Northwest Territory of Canada contains the oldest well preserved supracrustals, dated at 4.03-3.58 billion years ago.
The Guyana greenstone belts were formed in the lower Proterozoic some 2.25 billion years ago. Some settlements resting on, or adjacent to, greenstone belts in Guyana are Matthews Ridge, Arakaka, Baramita, and Port Kaituma in the Barima-Waini Region.
Others include Aurora, Ekereku and Arau in the Cuyuni sub-region of Region Seven (Cuyuni/Mazaruni); Enachu, Imbaimadai, Kurupung, Issano and Tamakay in the Mazaruni sub-region of Region Seven; and Mahdia in Region Eight (Potaro/Siparuni), to name a few.
The belts are named physiographic provinces because most of the rocks are greenish in colour, and the outcrops occur in wide linear or curving linear structures.
Greenstone belts are extremely common, and are found on almost every continent. Generally speaking, greenstone belts are composed of volcanic rocks that also have small amounts of sedimentary rocks interwoven within them.
The Guyana greenstone belts are thought to have been formed at oceanic spreading centres in the lower Proterozoic, when northern Guyana was the floor of a continental ocean.
One theory on their formation suggests that divergent Tectonic Plate activity, ie, the movement of Earth’s crustal plates away from each other over millions of years, led to the stretching and cracking (or rifting) of this ocean floor.
Hot liquid rock from the earth’s interior (magma) forced itself upwards through these cracks in the ocean floor, depositing lava on the surface. The heavy lava sank into the oceanic crust, forming shallow depressions. The lava belt deepened with time, and narrowed as it sank into the ductile crust. Finally, convergence of the lava pile as it sank halted the volcanism, and sediments filled the basin that formed as it sank into the crust.
Another theory suggests that greenstone belts may represent old oceanic crust between proto-continents near subduction zones, where one tectonic plate slid under another. When the proto-continents collided, they collapsed, and the oceans filled with basalt (a volcanic rock) and graywacke (a sedimentary rock) to form greenstone belts.
Generally, greenstone belts host economical deposits of many metals, including gold, manganese, uranium and copper. Gold is found in Archean age greenstone belts in Australia, southern Africa and Canada.
While greenstone belts host a variety of minerals, gold is most commonly found along the edges of those belts, within quartz veins cutting greenstone belts, and sometimes in some specific types of granitoid rocks intruding into rocks of greenstone belts.
This explains the intense concentration of gold mining activities in the Barima /Waini/North West District (Arakaka, Baramita), the Cuyuni /Mazaruni (Isseneru), to name a few of the more prolific gold producing areas in Guyana.
At Omai, Region Seven(Cuyuni/Mazaruni), the heart of the Mazaruni Greenstone Belt, Omai Gold Mines Limited extracted some 3.8 million ounces of gold between 1993 and 2005 in a quartz diorite intrusion in the Fennel Pit, and in rhyolitic dykes in the Wenot Pit.
Sandspring Resources Ltd reported Toroparu gold-copper deposit, with Measured and Indicated resources of 6 million ounces of gold and inferred resources of 4 million ounces of gold in the Puruni River, a tributary of the Mazaruni.
In the Cuyuni, Guyana Goldfields Inc. recently announced that Measured and Indicated resources at Aurora alone amounted to 5.8 million ounces of gold, and was growing as exploration continued.
Apart from gold, greenstone belts are also a treasure house of other mineral resources, such as manganese, copper, and some other metals, such as nickel, zinc and lead, which have not yet been discovered in Guyana.
Last February, the Canadian mining company, Reunion Gold reported finding significant manganese mineralization in four target hills of the Matthews Ridge Project in the Barama greenstone belt.
The Reunion Gold drilling programme also discovered two new manganese occurrences with strike lengths of 300 to 400 metres about two kilometres north of the former Matthews Ridge manganese mine area, and initial trenching at the westernmost discovery indicated multiple manganese beds, confirming the potential of the area.
The Canadian company, U308 (also known in Guyana as Prometheus Resources), has been exploring for uranium near the Kurupung Batholith at the foot of the Roraima Basin since 2006, and has been reporting encouraging results, indicating the viability of uranium mining.
Minerals can be found in other areas of Guyana, but are more generally distributed and not as concentrated as in the greenstone belts.

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