Summary:
The principal deposit at Diba and Diba NW is analogous to the Sadiola Gold Mine, located 20km north of the Diba & Lakanfla project. However, the Sadiola deposit occurs in rocks assigned to the Kofi Formation, whereas the Diba discovery occurs in rocks assigned to the Kéniébandi Formation. Until the discovery at Diba, no significant gold deposits had been recognized in the Kéniébandi Formation of western Mali.
Common features of the orogenic gold deposits in the Kéniéba-Kédougou inlier are their shear-hosted character, the intimate association of the gold mineralization with permeable lithologies and fractured host rocks, producing stratabound mineralized bodies, and the link between gold mineralization and carbonatisation and Fe-rich alteration.
The source of orogenic gold has been strongly debated. In particular, the composition and source of hydrothermal fluids that sourced the gold mineralizationin Mali had been poorly studied until very recently. Simon and Pizarro (Simon, 2013)reviewed a detailed geological study at Loulo, which included fluid-inclusion investigations, and determined that low-salinity, reduced, CO2-rich metamorphic fluids were associated with gold mineralization at Loulo, which is consistent with many orogenic gold fluid studies worldwide. The geological study considered the hyper-saline fluid as being related to a magmatic source, and its presence points to the potential role of multi-fluid sources in the formation of orogenic gold deposits. Gold precipitation may be, therefore, linked to fluid mixing between metamorphic and magmatic fluids, thus increasing metal fertility and leading to the formation of world-class orogenic gold deposits
Diba
Gold mineralisation modelled at Diba extends over an area measuring 700m x 700m. Anomalous gold at theDiba / Korali Sud property extends over 2.5km north –south (Woodman, 2007), and is defined as auger sample values >0.1g/t Au.
No sulphide or gold mineralisation has been intersected in the Lower Calcareous Sequence (LCS). Gold mineralisation is strata-bound and constrained to the Upper Calcareous Sequence (UCS). The sulphide content of the mineralised lenses is typically less than 10% by volume, and commonly as little as 1%. Disseminated sulphides are fine-to very-fine grained, and consist dominantly of pyrite, with a minor amount of arsenopyrite, chalcopyrite, tellurides and native gold.
Calcite is a ubiquitous constituent of the rocks at Diba, typically 5 -20% of the overall mineralogical composition. The mineralized dark-grey siltstone is strongly reactive to hydrochloric acid. Calcite also often occurs in veinlets and irregular pockets. The distribution of pyrite relative to calcite is inconsistent, but sometimes pyrite develops marginal to calcite crystals. Associated veinlets consist of millimetre-thick bedding-parallel veinlets of pyrite with subordinate amounts of calcite-quartz-biotite.
According to the report by Simon and Pizarro (2013), gold mineralisation in the Property is represented by native gold and calaverite within graphite, biotite and quartz-bearing silicates, and by native gold as inclusions in carbonates. Grain sizes vary between 3 -100 µm, and all are believed to have formed during hydrothermal processes. The absence of silver in calaverite would suggest moderate to high temperature of crystallization.
Simon and Pizarro (2013)also reported native gold in biotite-altered domains that contain aggregates of fine-grained titanite and rutile, as a rim on epidote, as minute inclusions in recrystallized feldspars of a sericite+biotite-altered siltstone. The strong association of gold with biotite, sericite and carbonate suggests its mobilisation and concentration during hydrothermal processes. No visual guides for determining the presence of mineralisation in the drill holes have been identified. The depositis characterized by an Au+Ag+As+Te±Sb geochemical signature.
Based on the continuity of mineralised zones >0.3g/t Au, and the interpretation of the mineralisation as sediment-hosted, disseminated epigenetic deposit, Mining Plus modelled the zones of the deposit as a series of 8 stacked lenses. These were given restricted continuity along strike and at depth.
The mineralised units extend horizontally over a 700m x 700m area, and the mineralised bodies are usually shallow-dipping (30 degrees east –ESE) and generally 20 –40m thick. The spatial distribution of the gold grades dipping approx. 30oESE supports the interpretation of the stacked lenses modelled with the grade shells.
Lakanfla
Most of the Lakanfla property is underlain by a tightly-folded sequence of (a) calcareous metapelites, meta-arenites and meta-greywacke and (b) impure carbonate rocks (marble) interbedded with carbonaceous meta-argillite and minor metasiltstone/wacke. A 2 km2, composite granite- granodiorite intrusion, elongated NE-SW, lies roughly at the centre of the explored area.
The general fabric of the sedimentary rocks (S0/S2) is steep and strikes in a north- to NNE direction; attitude varies locally however, particularly near the granite contact. Minor intrusive rocks observed at Lakanfla include pre-mineral metadiorite dykes associated with gold bearing structures such as Lakanfla West Prospect, and post-mineral microdiorite and fine-grained “volcanic” or aplite dykes that crosscut Lakanfla Central. Dolerite dykes striking 015°are the youngest rocks on the project. The southwestern edge of the extensive Taoudenni Basin, comprising Neo-Proterozoic to Phanerozoic sedimentary rocks, lies 6 km east of the Lakanfla licence boundary.
Lakanfla Central is hosted exclusively in massive, medium- to coarse-grained granite and granodiorite. Mineralisation occurs in two styles; firstly, in a wide, NE striking, steeply-dipping body of disseminated mineralisation. This orderly, disseminated orebody has a strike length of approximately 650 m and an approximate width of 140 m, with consistent lower grade typically between 0.5 g/t Au to 1 g/t Au. Mineralisation is thought to be controlled by subvertical, NE- striking brittle-ductile faults and barren dykes. In some instances, the faults are mineralised (with quartz-sulphide veinlets) and are characterized by higher gold grades, typically above 3g/t Au over 1-2 metres.