Summary:
Deposit Type
The Omai Property hosts mesothermal orogenic gold deposits. The Wenot and Gilt Creek Deposits represent similar mesothermal gold mineralized systems emplaced in different host rocks, specifically in sheared volcanic and sedimentary rocks and a quartz diorite intrusion, respectively. Mesothermal gold deposits are generally considered to form as a result of hydrothermal fluid activity during the final stages of tectonism in the orogen (i.e., the deposits are syn- or late-tectonic). They are almost always proximal to crustal-scale fault zones within the low metamorphic grade portion of the orogen. The orogenic gold deposits themselves consist of quartz-carbonate vein systems and carbonate-sericite alteration zones, generally with a relatively low proportion of sulphides. The immediate host rock units tend to exhibit more brittle deformation than the surrounding units. The sediment host rocks and diorite dykes exhibit more ductile deformation.
Orogenic gold deposits occur intermittently through 3 Ga of geologic time and are perhaps best known in the Archean greenstone belts of the Superior Craton (Canada) and the Yilgarn Craton (Western Australia). The host rocks and structural setting of the Wenot and Gilt Creek Deposits are strikingly similar to the well-known Lamaque and Sigma Gold Mine Deposits in Val-d’Or, Québec (Canada). Both deposits there are similarly hosted by a regional-scale shear zone and an adjacent intermediate intrusion.
Mineralization
The Wenot and Fennel Gold Deposits were historically subject to open pit mining. The Wenot Gold Deposit is hosted mainly in tabular quartz-feldspar porphyry dykes and strongly silicified rhyolite dykes, and subordinately in andesites and metapelites within the Wenot Shear Zone. The Gilt Creek Deposit, 400 m north of Wenot, under Fennel Pit, is hosted mainly in the epizonal Omai Stock, a quartz diorite intrusion, and to a minor extent, the surrounding tholeiitic basalts and calc-alkaline andesites. The geological features and geochronological data for the Wenot and Gilt Creek Gold Deposits suggest that they are genetically related and represent a contemporaneous metallogenic event related to the latest brittle-ductile phases of the TransAmazonian Orogeny at ~2.0 Ga.
Primary Gold Mineralization
Two types of gold-bearing veins are present in the Wenot Pit (Voicu, 1999a):
1) Vein Sets or Stockworks. These are found within the more competent, brittle units on the Property, such as the sub-vertical dykes and sills of silicified rhyolite and quartz-feldspar porphyry in the Wenot Pit area. These veins are typically in the mm to cm thickness range. The veins pinch out on entering the more ductile surrounding units; however, they can continue into these units for as much as 10 m. The veins are surrounded by carbonate-sericite-silica-chlorite alteration halos and, where the veining is densest, the halos overlap to form completely altered host rock; and
2) Lode Veins. These veins are present in all units (except late diabase dykes and gabbro sills); however, are most common within the brittle sub-vertical felsic units. They are generally nearly flat-lying with dip of <30° (northwest or southeast).
In the Wenot Deposit, the sub-horizontal gold veins display random strikes and dips, which results in a typical stockwork environment (Voicu et al., 1999b). The sub-vertical veins are not confined to particular rock types and cut across all stratigraphic contacts. These veins are less common than the sub-horizontal veins.
The geometrical and textural relationships of the two Omai vein sets suggest that they are broadly contemporaneous. The vein systems are classified as crack and seal, laminated, breccias, and open-space filling veins. Most veins formed in two filling stages and a late fracture-filling stage during protracted hydrothermal fluid activity. The hydrothermal fluid temperature was in the range of 200º to 400°C (Elliott, 1992). Some features of Omai vein textures are comparable to those described in Archean orogenic gold deposits, whereas others resemble the vein textures described in the circum-Pacific Tertiary epithermal deposits.
The metallic minerals represent <1% of the vein volume and consist of various sulphides together with tungstates, native elements, tellurides, and sulphosalts. The main metals of the mineralization are Au, Ag, Te, W, Bi, Pb, Zn, Cu, Hg and Mo. The major gangue minerals in the veins are quartz and carbonates (ankerite and calcite), albite, sericite, chlorite, tourmaline, rutile and epidote (Voicu, 1999).
The gold mineralization occurs primarily as native gold and as tellurides, such as petzite and calaverite, in the quartz-carbonate veins (Voicu, 1999c). Refractory gold is present as inclusions within pyrite and pyrrhotite. Pyrite and pyrrhotite are the main sulphide phases, whereas sphalerite and chalcopyrite are minor. Galena is associated with visible gold (Elliott, 1992). Scheelite is commonly observed in the quartz and quartz-ankerite veins. The associated rock alteration consists mainly of carbonates-quartz-sericite-albite-tourmaline-rutile and epidote.
Secondary Gold Mineralization
Coarse gold also occurs in laterite zones near the Wenot and Gilt Creek (Fennel) Pits, and as alluvial placers on the Omai Property. Prospective gold targets occur along the strike of the Wenot Shear Zone, as revealed by auger drilling of laterite and saprolite, and limited core drilling (AMEC, 2012a). For example, the laterite area overlying the eastern strike extension of the Wenot Shear Zone, known locally as the East Wenot Extension-Boneyard area, has been extensively worked by artisanal miners.
A second laterite zone, on the upper parts of Broccoli Hill, is located 200 m east of the Gilt Creek (Fennel) Pit. Broccoli Hill has a long history of artisanal mining on the hill flanks and in creek beds. Significant historical alluvial workings on Broccoli Hill date back to the 1890s on the southern flank and, more recently, on the western and northeastern flanks. In the early 1990s, Golden Star Resources surficial and auger sampling surveys generated encouraging, broad gold-in-soil anomalies over a 750 m x 500 m area. An 8 m deep auger sample returned 12.4 g/t Au. Historically, Broccoli Hill had never been diamond-drilled.
Alluvial placer gold appears to be present within the Property area. Mahdia Gold press releases in 2013 and 2014 announced production from the “Roraima” paleochannel, by Mahdia Gold and a joint-venture partner. Mahdia reported production of 59 ounces of gold in March 2014 and 118.5 ounces of gold in June 2014 from this operation (Douchane, 2014; Gordon and Bending, 2014).
From the limited historical Cambior-OGML drilling, the primary mineralization styles are known to continue beyond the bounds of the Wenot Pit along strike to the west and east within the Wenot Shear Zone, and to occur to the north at the Gilt Creek (Fennel) Pit.