Omai Mine
| Location | |
|---|---|
  Omai Gold Mines Location in Guyana | |
| Cuyuni-Mazaruni | |
| Country | Guyana |
| Coordinates | 5°25′48″N 58°45′47″W / 5.430°N 58.763°W |
| Production | |
| Products | Gold |
The Omai Gold Mine is located in Guyana on the north coast of South America near the west bank of the Essequibo River in the interior of the country. Access to Omai is by road from the capital of Georgetown on the coast, and from the town of Linden approximately 60 km away. There is an operational airstrip on site which can accommodate aircraft from Georgetown (40 min. flight). Gold mining at Omai is known from at least the 1880`s, and when it opened as a large scale mine in 1992 By Cambior, the mine was the largest gold mine in the Guiana Shield and a major source of income and employment in Guyana. During the period from 1992 – 2005, Omai produced 3.7 Moz of Gold at an average grade of 1.5 g/t Au from the Fennell and the Wenot open pits.
Omai has been known for over 130 years[1]. In the period 1886-1896 some 1890 kg (60,000 ozs.) of gold was extracted from the placer deposits of Gilt, Dunclain and L'Esperence creeks by small-scale miners ('pork-knockers") using primitive hand methods. The German Syndicate leased the concession in 1896 and carried out extensive investigation of bedrock quartz lodes by primitive drilling and tunneling and produced 1860 kg (59.800 ounces) of gold. After limited success in dredging the alluvial flats, the Syndicate surrendered the property to the pork-knockers in 1907. By 1911 most of the workable ground was considered exhausted! This was based on the gold price and mining methods of the period.
Anaconda (British Guiana Mines Ltd.) acquired to rights in 1946. |п order to 'prove-up reserves, close spaced diamond-drilling focused on the primary mineralization in the Omai Stock. Seventy-three AX-diameter boreholes were drilled for a total of 12,400 m. On the basis of surface drilling assay results, a 120 m shaft was sunk near Red Hill and 900 m of cross-cuts were made at the 62 meter level. By 1950, Anaconda had identified a primary ore body with reserves of 54.4 million tonnes grading 1.19 g/t Au. However, due to a combination of low gold prices and a favorable соррег market at the advent of the Korean War, the company decided to relinquish the property in 1951. The detailed mapping and sampling by Anaconda geologists (the best trained in the world at that time) is still of great value.
ln May 1985, Golden Star Resources Ltd., a junior Alberta-based mineral exploration company, signed an Exclusive Exploration Permit (E.E.P.) with the Guyana government over the Omai area. The Canadian engineering consultancy. S.N.C. Inc., was contracted to undertake an evaluation-feasibility study of the oxidized zone. They initiated a systematic alluvial and saprolite evaluation program, which included extensive auguring and test-pitting to depths of about 4 m. Indicated alluvial reserves of 750.000 tons grading at 3.2 g/t Au and indicated saprolite reserves of 1.2 million tonnes grading at 1.44 g/t Au were defined. On the basis of these encouraging surface results, GSR bored six deep diamond drill-holes, totalling 2125 m, to test mineralization in the Omai Intrusive Stock (Later became the Fennell pit).
Placer Dome inc. of Vancouver, British Columbia, became the senior joint-venture partner at Omai in March 1987. Over a three year period Placer carried out a comprehensive. integrated exploration program which included surface mapping, trenching, soil geochemistry and ground geophysics coupled with approximately 24,000 m of diamond drilling. The alluvial deposits in the vicinity of Gilt and Dunclain Creeks were tested using Banka and sonic drilling techniques.
In February 1989, overlapping geochemical and radiometric anomalies drew attention to the Wenot Lake area some 800 rn to the south of the Omai Stock). Subsequentlv, eight exploratory holes were drilled leading to the discovery of a second ore body overlain bv a supergene-enriched saprolite blanket. In total, 130 "SAP" and "DDH" holes totalling 10,550 m were drilled in order to accurately define the Wenot Lake ore zone. Placer estimated the combined primary, saprolite and alluvial reserves of the Omai gold deposits. Using a 0.7 g/t and 0.5 g/t cutoff value for hard rock and saprolite respectively, geological (indicated) reserves were calculated at 52.8 million tonnes grading at 1.35 g/t Au. However, Placer withdrew from the original agreement for "economic reasons" and complete control of the Omai E.E.P. reverted back to GSR.
GSR entered into a joint-venture agreement with Cambior Inc in May, 1990. Cambior completed 8,500 m of definition drilling to confirm grade, continuity and geological boundaries in the mineralized zones. Following the completion of a successful feasibility study at the end of 1990, Cambior Inc and GSR signed a deal with the government of Guyana to develop Omai under a subsidiary, Omai Gold Mines Ltd. Construction started and the first gold was poured at the end of 1992, a very rapid startup. The published mining reserves were 44.8 million tonnes, grading at 1.43 g/t Au, and planned annual production of 8.2 tonnes (255,000 ozs.) over the first three years (Mining Magazine, Sept., 1991).
In 2019, Avalon Gold Exploration acquired a PL license from the Government of Guyana and will conduct exploration at Omai for new gold resources.
Geology
Geologically the Omai deposit has an orogenic style of gold mineralisation and is a slightly younger (Paleoproterozoic) analogue for similar Archean-aged deposits found in other cratonic areas worldwide such as the Canadian Shield. The deposit is split into two related zones: the Wenot zone hosted by saprolitised felsic units that run parallel to a regional tectonic structure, and the Fennell zone which is found within a nearby carrot-shaped granitoid plug. The free gold is contained within narrow horizontal quartz-scheelite veins.
The twin Wenot and Fennell deposits were mined from 1993 to 2006, concurrently from surface in open pits. Additional gold ore is likely to remain beneath the Fennell pit and along strike from the Wenot zone. Local prospectors, known as pork-knockers have historically worked the saprolite in the mine area for nuggets of gold.
Environmental Incident
On 19 August 1995 a tailings dam broke and leaked tailings into the Essequibo River. 4.2 million cubic metres of cyanide-containing slurry escaped. Eighty kilometres of the Essequibo River were declared an environmental disaster zone. Large numbers of fish were killed.[2] Production at the mine was halted for several months while the spill was investigated. The principal mine owners were Cambior Inc., based in Canada; Golden Star Resources Inc, based in Colorado, USA; and the government of Guyana. Cambior owned 65%, and Golden Star 30%, of the mine. Attempts were made in Guyana and in Canada to sue the companies. The Guyana case sought $2 billion in damages. These cases were dismissed in Canada in 1998, and in Guyana in 2002 and 2006.[3]
The environmental alert on the river was lifted after one week, but Indigenous villagers on the river were still using alternative water sources, at considerable inconvenience, seven years after the spill.[4]
According to a 2002 article in Geotechnical News, the dilute contaminant could not have caused all of the alleged environmental effects.[5]
See also
References
- ^ Elliott, Roy Graham (1992). The geology and geochemistry of the Omai goldfield, Guyana (Ph.D. thesis). Oxford Brookes University.
- ^ Mineral Policy Institute, "Chronology of Major Tailings Dam Failures" http://www.mpi.org.au/2014/08/chronology-of-major-tailings-dam-failures/ mpi.org] Accessed June 2018.
- ^ Environmental Justice Atlas https://ejatlas.org/conflict/omai-gold-mine-tailings-dam-guyana
- ^ McGill Research Group Investigating Canadian Mining in Latin America, "Omai, Guyana"
- ^ "This event caused debatable environmental damage with reports of downstream devastation far outstripping the ability of the dilute contamination to ever accomplish." -- Michael P Davies, "Tailings Impoundment Failures: Are Geotechnical Engineers Listening?" Geotechnical News, September 2002. http://www.pebblescience.org/pdfs/Dam_failuresDavies2002.pdf
External links
- S. Vick, R Squires and others, "Preliminary Report on Technical Causation Omai Tailings Dam Failure" (1995). http://wise-uranium.org/mdgr.html Accessed June 2018.