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Omai - Wernot Pit

Guyana

Main commodities: Au
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Deposit Description

The Omai gold deposit is one of the largest in South America.   It is located within Palaeoproterozoic rocks of the Guiana Shield in Guyana, 160 km SSW of Georgetown.

The deposit is hosted by the Palaeoproterozoic Barama-Mazaruni Supergroup (2.25 to 19.0 Ga) in the greenstone belt of the same name.   The supergroup is composed of alternating felsic, mafic and ultramafic volcanic flows interlayered with thick sedimentary units.   The base is dominated by tholeiitic basalts with assocaited mafic and ultramafic bodies and sills.   These are overlain by intermediate to felsic volcanics interlayered with immature clastic sediments.   There are also extensive syn- to late-tectonic calc-alkaline felsic to intermediate intrusions, ranging from granite to granodiorite, diorite, tonalite and adamellite.   One of these granites has been dated at 1945±45 Ma.

In the mine area the main hosts are mafic (massive to pillowed tholeiitic basalts) and intermediate (calc-alkaline andesites) volcanics, underlain by polygenic conglomerates (2 km thick sequence of poorly sorted cobble conglomerates with thin interlayered basalt flows) and overlain by finer grained sediments (siltstones, sandstones and greywackes).   The volcanics are cut by 2120±2 Ma quartz-feldspar porphyry and rhyolite dykes and by the 2094±6 Ma Omai granitoid (quartz-monzodiotire, granodiorite and diorite) stock, which is in turn cut by a variety of 2001±4 Ma mafic dykes and sills.

Both ductile and brittle shear zones are recognised and are broadly contemporaneous.   The former contain no mineralised veins, which are all developed within the competent rock of the mine area (eg. the Omai Stock, the porphyry and rhyolite dykes), although they may persist up to 10 m beyond the contact into the volcanics.

Six undeformed sub-vertical and sub-horizontal gold bearing vein sets have been delineated in two ore zones.   These represent a series of broadly contemporaneous crack and seal, laminated, brecciated and open space filling veins.   Veins range from a few mm's to 2 m in thickness and up to a maximum of 600 m in length, although the shallow veins may only be 10 m or less in extent.

The gangue is mainly quartz with subordinate carbonates, albite, sericite, chlorite, tourmaline, rutile and epidote, with a metallic paragenesis of Au-Ag-Te-W-Bi-Pb-Zn-Cu-Hg-Mo.   Wall rocks are altered to an assemblage including carbonates, sericite, silica, chlorite, albite, epidote, argillic minerals, pyrite and pyrrhotite.

The total mineable reserve at Omai (production+reserves) is approximately 130 tonnes of gold.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1999.    
This description is a summary from published sources, the chief of which are listed below.
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  References & Additional Information
 References to this deposit in the PGC Literature Service Collection: Want any of these papers ? Click Here
Voicu G, Bardoux M, Jebrak M, Crepeau R  1999 - Structural, mineralogical and geochemical studies of the Paleoproterozoic Omai gold deposit, Guyana: in    Econ. Geol.   v94 pp 1277-1304
Voicu G, Bardoux M, Stevenson R, Jebrak M  2000 - Nd and Sr isotope study of hydrothermal scheelite and host rocks at Omai, Guiana Shield: implications for ore fluid source and flow path during the formation of orogenic gold deposits: in    Mineralium Deposita   v35 pp 302-314


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