Summary:
The Sanbrado deposit is associated with the Lower Proterozoic system of the Birimian Supergroup (2150 – 2100 Ma) comprising metavolcanic (arc) and metasedimentary (basin) rocks. The Birimian Supergroup has been intruded by two distinctive granitoid types. The larger basin-type granitoids (Eburnean Events) can be subdivided into the initial Eburnean event corresponding to a major phase of crustal thickening as a result of shortening, folding and granitoid emplacement, followed by regional-scale north to northeast trending transcurrent faulting. Large scale fluid migration along these major, deep-seated structures is inherent to most orogenies. Hydrothermal gold-bearing fluids follow secondary and tertiary fault systems, adjacent to the main structures at shallower crustal levels.
The Mankarga prospect area (M1, M3 and M5) is characterised by a sedimentary pile which is mostly composed of undifferentiated pelitic and psammitic metasediments as well as volcanosedimentary units. This pile has been intruded by a variably porphyritic granodiorite, overprinted by shearing and mylonites in places, and is generally parallel to sub-parallel with the main shear orientation. In a more regional context, the sedimentary pile appears “wedged” between regional granites and granodiorites. The alteration mineralogy varies from chloritic to siliceous, albitic, calcitic and sericite-muscovite. Gold mineralisation in the project area is mesothermal orogenic in origin and structurally controlled. The project area is interpreted to host shear zone type quartz-vein gold mineralisation. Observed gold mineralisation at the Mankarga prospects appears associated with quartz vein and veinlet arrays, silica, sulphide and carbonate-albite, tourmaline-biotite alteration. Gold is free and is mainly associated with pyrrhotite, pyrite, minor chalcopyrite and arsenopyrite disseminations and stringers.
The M3 and M5 gold deposits sit within discrete high strain zones which occur along the margins of major granitoids. These high strain zones can range from meters to tens of meters wide and sit within the belts which are themselves characterised by moderate to high strain.
The main rock types are variably strained clastic metasediments and mafic to intermediate intrusives. Regional metamorphic grade has reached greenschist facies with prograde biotite contributing to foliation development. Most rocks have undergone some degree of retrograde metamorphism resulting in chlorite, sericite, epidote, albite, leucoxene and calcite rich rocks.
Metasediments comprise a mixture of black shale, laminated metasiltstone and lithic greywacke, and are intruded by both mafic and intermediate (diorite and granodiorite) intrusive with xenoliths of sediment common in the intrusive phases.
Most of the belt rocks, including within belt intrusive, are moderately to strongly foliated. The granitoid terranes that bound the belts are strongly foliated along their margins but less foliated towards their interiors. Foliation has formed in response to co-axial strain with the highest amount of simple shear occurring within the high strain corridors which form along the margins of the major granitoids. The best mineralisation at both M5 and M1 South is typically within or close to zones of strong deformation.
Gold mineralisation is associated with the main hydrothermal event which produced strong silicification of the surrounding rock during reactivation of the pre-existing structures and fabrics.
This interpretation places gold mineralisation at post peak metamorphism after the bulk of the deformation, during late D2 (regional Birimian deformation) within a roughly WNW-ESE (to NW-SE) stress field. Deformation and shearing along the high strain corridors has resulted in a pressure shadow, south of the main northern granitoid as the M1 and M5 high strain zones peel away (trending SE and SW respectively) from the same granitoid body. Conjugate movement along these two corridors, sinistral along M1 and dextral along M5, is consistent with the late D2 stress field and has resulted in dilational opening and high grade steeply plunging ore shoots - along left-hand flexures at M1 and right-hand flexures at M5.
Late D3 deformation is at a high angle to D2 and reactivated D2 structures with an opposite sense of shear.
The kinematics during mineralisation were strike-slip; however, the bulk of the deformation was most likely related to thrusting, with strike slip movement with gold mineralisation occurring towards the end of the orogeny.
Dimensions
Known mineralisation at M1 extends along strike for approximately 1 km, is up to 50 m wide and up to 1,000 m in depth.
The M5 mineralisation extends along strike for approximately 3 km, is up to 100 m wide and 450 m in depth.
MV3 mineralisation extends along strike for 850 m, is up to 20 m wide and 250 m in depth.
Mineralisation at all deposits remains open at depth.