The geology and geochemistry of the Omai goldfield, Guyana

The Omai goldfield consists of mesothermallode gold mineralisation and associated saprolitealluvial placer deposits, hosted within Palaeoproterozoic granite-greenstone terrain of the Guiana Shield. Total mining reserves are estimated at 44.8 million tonnes, grading at 1.43 glt Au. The goldfield lies along an ESE-trending, regional-scale structure referred to as the Issano-Appaparu shear zone. At Omai, the gold deposits are located in two discrete ore zones - the Omai Stock Zone and the Wenot Lake Zone. The bulk of primary mineralisation is centred on a high-AI, quartz diorite-tonalite boss (the Omai Stock), where wall-rock alteration is dominated by a hydrothermal sericite-carbonate assemblage. The primary ore package of Au-W-Te-S mineralisation is contained in a series of narrow (1-5cm), quartzcarbonate (ankerite) veins. Visible gold is commonly associated with galena and microscopic tellurides. Provisional fluid inclusion studies have indicated that the parent hydrothermal fluid was H20-C02 -bearing (- 5.0 mol% CO2 ), of low salinity (0-1.8 wt. % NaCI equiv.) and moderate density (0.96 g/cm3). The depositional temperature of the fluid was probably in the region of 200-400oC. Preliminary 6'80 values are consistent with a magmatic andlor metamorphic source. The Majuba Suite greenstones adjacent to the Omai Stock are also partially mineralised. These rocks are dominated by primitive, low-K, high-Fe tholeiitic (HFT) basalts and minor basic intrusives which are characterised by flattish REE patterns of around 10 times chondrite. A subordinate, calc-alkaline series (CAS), of mostly andesitic composition, is intercalated within the volcanic pile. In terms of trace element geochemistry, the Majuba Suite is chemically comparable to volcanic rocks from modern island arc settings. The REE geochemistry, which proved to be extremely diagnostic throughout, indicates that the Omai Stock is genetically related to the regional Tigri pluton. This and other Trans- Amazonian soda granitoid plutons in the area have chemical affinities akin to calc-alkaline volcanic arc granitoids of Phanerozoic orogenic belts. Several lamprophyric (appinite) pipes are spatially and temporally associated with the Omai Stock. Although the appinites are only weakly mineralised, they may have a genetic significance regarding the source of the gold. Both the Omai Stock and Majuba Suite are cut by a series of partially mineralised, ultrabasic to intermediate dykes (the Gilt Creek Suite) which intruded along shallow-dipping fracture zones. These rocks are typically Mg-rich, LREE enriched (LanlYbn = 7.84-32.6) and appear to be the product of alkaline arc magmatism. A few post-orogenic mafic dykes (POMDs), of probable Mesozoic age, are also identified in the area. These rocks are strictly nonmineralised and are chemically correlatable with tholeiitic basalts from continental rift-related settings. In the Wenot Lake Zone, lode mineralisation is hosted within thin bands of high-silica rhyolites (HSRs), also of the Majuba Suite, close to a sheared contact with phyllitic tuffs. The HSRs have a distinct chemistry characterised by elevated Si02 (> 75.0%), Na, Nb, Zr and REE contents with anomalously low concentrations of Mg, Ti, Ca, K, Rb and Sr. These rocks are further characterised by wing-shaped REE patterns containing deep negative Eu anomalies.

Grantor dc:publisher

Oxford Brookes University

Year dc:date

1992

Author dc:creator

• Elliott, Ray G.

Contributors dc:contributor

• Colley, Howard

Statement dc:rights

• All rights reserved

Language dc:language

en

DOI dc:identifier

https://doi.org/10.24384/33h3-mn63

OAI identifier oai:identifier

tle:74da8535-dba5-4554-ad8b-79f393b341a7:d6bd9758-527a-46cd-bfe2-c433766e8fca:1