The current interpretation is that Montagne d’Or is a deformed volcanogenic massive sulphide deposit (Ross 2014).

The Montagne d’Or prospect consists of a family of tabular mineralized bodies that form closelyspaced sub-parallel east-northeast (084°) striking and steeply (72°) south-dipping mineralized zones. Mineralization has been encountered over a strike length of more than 2,500 m and to a vertical depth of at least 200 m. Only a small portion of the gold mineralization has been subjected to upper level oxidation. The significant fine-grained gold mineralization is principally affiliated with sulphide veins and masses within fresh country rock that begins at shallow depths.

Two significant styles of gold mineralization have been recognized although they show a gradational relationship between each other:

-SMS with gold mineralization
-Sulphides in disseminated stringers with gold mineralization

SMS was a term coined by previous operators and was used to support a “VMS” type model for the
mineralization. It is characterized by a high sulphide content (>20%) and occurs over intervals ranging from tens of centimetres to up to 4 m. This mineralization was later interpreted to represent zones of thicker, deformed and transposed sulphide ± quartz-rich veins and a denser distribution of disseminated sulphide as compared to that of the disseminated type.

The SMS also includes sulphide-rich breccia dykes, which host rolled and milled clasts of host rock within a ductily deformed pyrite-chalcopyrite-pyrrhotite matrix. In addition, bornite is present, and minor amounts of arsenopyrite have been identified petrographically. There is a clear correlation between sulphide veinlets and sulphide-rich breccia zones and high gold grades.

Mineralization is hosted by felsic, mafic and intercalated mafic/felsic rocks to varying degrees. However, approximately 80% of the gold mineralization in the deposit occurs within the more felsic units, mainly the Felsic tuff unit.

The mineralization appears as elongated lenses of higher grade material within broader zones of low grade but anomalous mineralization (0.25 g/t Au to 0.4 g/t Au). Several distinct anomalous mineralized domains are recognized, separated by barren intercalated mafic and felsic rocks.

The Montagne d’Or deposit is now thought to be part of a stratiform/stratabound deposit type. Mineralization consists of pyrite, pyrrhotite and chalcopyrite with minor sphalerite, magnetite and
arsenopyrite. Arsenopyrite, although observed, does not appear to have an obvious relationship with either gold or copper mineralization. Distinct phases are reported as stratiform disseminated sulphides, stockwork sulphide veinlets and layers of semi-massive sulphides that are tectonically transposed. The latter facies is considered as syn-volcanic in origin and as the most favourable occurrence for gold mineralization.

The disseminated sulphide veins could be related to feeder zones and/or remobilized on fold hinges and shear zones. In addition, evidence is found for tectonic remobilization with sulphides concentrated within fold hinges and pressure shadows, and cross-cutting sulphide-bearing veins.

Visible gold occurs in chlorite-rich zones or is spatially related to sulphide mineralization (after Giraud, Tremblay, Jébrak and Lefrançois, 2014). This particular 1 m interval ran 80.75 g/t Au. There is generally an increase in gold grades as sulphide (excluding pyrrhotite) content increases. Microscopic studies indicate that gold occurs as very fine grains in the host rock groundmass and at the junctions of quartz crystals. Gold has only very rarely been seen as inclusions within sulphide minerals.

This Feasibility study assumed that open pit mining methods will use front-end loaders (FELs) and hydraulic excavators to load haul trucks for waste and ore haulage. Mining activities will include site clearing, removal of growth medium (topsoil), free-digging, drilling, blasting, loading, hauling and mining support activities. Material within the pit will be generally blasted on a 5 m high bench. Most of the saprolite material (approximately 18% of the total material to be mined) can be loaded directly with hydraulic excavators without the need for blasting. Most ore will be sent directly to the primary crusher. The stripped waste material will be placed in dumps to the north of the pit, and lower-grade ore placed in a stockpile, near the primary crusher location.

The treatment plant design incorporates the following unit process operations:

-Primary jaw crushing to produce a coarse crushed product;
-A crushed ore surge bin with bin overflow conveyed to a dead stockpile. Ore will be reclaimed from the dead stockpile to feed the milling circuit when the crushing circuit is offline;
-A single stage semi-autogenous grinding circuit with recycle crushing (SS SAC) circuit with a 14 MW SAG mill in closed circuit with a pebble crusher and hydrocyclones to produce an 80% passing 75 micron grind size;
-Gravity concentration and removal of coarse gold from the milling circuit recirculating load and treatment of gravity concentrate by intensive cyanidation and electrowinning to recover gold to doré;
-Pre-leach thickening to increase the slurry density feeding the (CIL) circuit to minimize CIL tankage, smooth out fluctuations in the milling circuit, improve slurry mixing characteristics and reduce overall reagent consumption;
-Leach/CIL circuit incorporating a leach tank and six CIL tanks with carbon for gold adsorption providing a total of 31 hours leach time;
-A 10 tonne split AARL (Anglo American Research Laboratories) elution circuit treating loaded carbon, electrowinning and gold smelting to produce doré;
-Tails wash thickener to reduce the weak acid dissociable cyanide (CNWAD) contained in the CIL tails prior to cyanide destruction and to recover free cyanide (CNFREE) in the process water and recycle to the milling circuit.