Summary:
The Namdini gold deposit is a large, structurally controlled, orogenic gold deposit within the Nangodi Greenstone Belt, with numerous features similar to deposits found elsewhere in late Proterozoic Birimian terranes of West Africa. To date the Namdini gold deposit has been delineated over a strike length of 1.15 km, up to 300 m wide and 700 m deep.
The Namdini Gold Project appears to be a typical Birimian gold deposit and is hosted in a mixture of altered meta-volcanic sediments, diorite and tonalite. It is associated with quartz-carbonate veins and disseminated pyrite and arsenopyrite in both veins and wall rocks. The mineralization is strongly structurally controlled and the deposit appears to be located in an oblique, sinistral structure in a regionally extensive deformation zone.
Currently, the structural overprinting, geometries, and kinematics are thought to be consistent with deposition of mineralization in both D1 and D2 foliation-forming events. It is likely that this represents two stages of gold deposition punctuated by a deformation hiatus between D1 and D2. The emplacement of successive mineralization-associated vein stages does not represent a single event. The formation of intense D2 structures, in particular the penetrative S1-S2 composite foliation, have strongly modified or destroyed D1 overprinting and geometric relationships.
Typical mineralised zones, with characteristic light green alteration within metavolcanics.
Intense silica-chlorite-sericite-pyrite, +/arsenopyrite alteration with a strong D1 planar fabric deformed by D2 folding.
Syn-D1 pyrite clusters show strong rotational deformation with quartz strain shadows that have been further deformed during D2.
Syn-D2 sulphides are developed in the fold hinges of D2 folds.
Ubiquitous finely disseminated sulphides (pyrite strongly dominating over arsenopyrite) occur throughout the rock mass.
Host rocks
Namdini gold mineralization is located in the Nangodi Greenstone Belt within a host sequence of metavolcaniclastics, granitoids (tonalite), and diorites. The deposit is bounded on the hangingwall and footwall sides by metasediments.
Geological description of the Birimian units intersected during exploration drilling include:
- Meta-sedimentary rocks are fine-grained chlorite-muscovite schists.
- Meta-volcaniclastic rocks are very fine-grained, chlorite-muscovite phyllites.
- Granitoid samples are classified as altered, sheared, sulphide-bearing tonalite. A tectonic foliation is developed in the intrusive rocks, but is not pervasive in the granitoid.
- Diorite rocks are assumed to be late stage intermediate diorite stock and dykes. They occur as altered (shearing, silicification, chlorite, and sericite) or unaltered diorite, as well as quartz diorite speckled with quartz and feldspar.
Weathering profile
In the mineralized area, the tropical weathering profile extends to a maximum depth of around 30 m, comprising the following material types:
> Strongly oxidized: total oxidation of all primary minerals with little or no primary rock texture. This zone ranges from 1.0 to 7.5 m in thickness.
> Moderately oxidized: some primary rock texture, total oxidation of feldspar to clay, and total oxidation of sulphides. This zone ranges from 0.5 to 13.0 m in thickness.
> Transition: strong primary rock textures with partial oxidation of feldspars and sulphides. The Transition zone ranges from 2.0 to 14.5 m in thickness.
Mineralization
Drilling has outlined mineralization with three-dimensional continuity, with a size of approximately 1,500 m long, 550 m wide, and 450 m in depth.
In all rock types, the mineralization is accompanied by visible disseminated sulphides of pyrite and arsenopyrite in both the veins and wall rocks. In diamond drill core the mineralized zones are visually distinctive due to the presence of millimetre to centimetre wide quartz-carbonate veins that are commonly folded and possess yellowbrown sericite-carbonate selvedges.
Visible gold was identified by Cardinal, Orefind, and RPA. Visible gold’ s instances occurred in strongly altered granite and were associated with silica-sericite shears that had sub-millimetre widths, as well as in the diorite.
Gold at Namdini is dominated by electrum (80% of measured grains) with the remainder as native gold.
Gold grains are generally fine; QEMScan analysis indicates 89% <10 microns.
Visible gold is rare and usually occurs as a vein selvage with chlorite in late-stage remobilised veins.
Gold dominantly occurs as inclusions and fractures in:
- Pyrite (75%);
- Arsenopyrite (8%);
- Quartz and other silicates (6%);
- Chalcopyrite (2.5%).
Circa 5% of gold grains are free gold.
Sulphide mineralisation
Sulphides in decreasing order of abundance are:
- Pyrite, arsenopyrite, chalcopyrite, galena;
- Trace sphalerite, molybdenite, pyrrhotite, gersdorffite and tennantite;
- Rutile, ilmenite and magnetite are alsob associated with gold mineralised zones;
- Pyrite mostly exhibits grain sizes in the 20 to 140 micron range.
Rutile, ilmenite and magnetite are also associated with gold mineralised zones.
Pyrite mostly exhibits grain sizes in the 20 to 140 micron range.
Zoned pyrites are common.
The mean total sulphur level for Namdini is 1%. Low sulphur levels will allow for the production of a high-grade gold-sulphide flotation concentrate on site for CIL treatment.