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
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 TransAmazonian
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
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.