Summary:
Geology
Siguiri is situated in the northern part of the Siguiri Basin of Guinea, and is underlain by Lower Proterozoic rocks of the Birimian metasedimentary and volcano-sedimentary formations. Where exposed, the sediments consist of a well-bedded turbiditic sequence of greenschist facies siltstones, sandstones, greywackes and minor conglomerates, with some brecciated and possibly volcanic members. However, stratigraphic relationships in the area are poorly understood due to poor exposure and a thick lateritic duricrust covering large portions of the lease.
The Siguiri mineralisation occurs as secondary gold in alluvial or colluvial gravel in lateritic cover and primary vein-hosted mineralisation. The veins are quartz dominant and display various styles and orientations, with a sub-vertical northeast-trending conjugate quartz vein set predominating in most of the open pits, irrespective of the bedding orientation. Auriferous quartz veins show a robust lithological control and are best developed in the sandstone/greywacke units.
The primary structural and lithological trend in the current mining area of Block 1 changes from a north-south orientation in the south to a northwest-southeast orientation in the north.
The geology of Block 2 differs from Block 1 as it is mainly underlain by metavolcanic and volcanoclastics. However, mineralisation styles appear similar to those in Block 1, with Saraya appearing to be located on a north-south orientated structure.
Deposit type
Three main sedimentary packages are recognised in the Siguiri district, the Balato, Fatoya and Kintinian Formations. The basal Balato Formation is dominated by shale, siltstone, and greywacke centimetre-scale alternations. The overlying Fatoya Formation consists of metre-scale beds of greywacke fining towards the west. The Kintinian Formation is a thick package of shale and sandstone with a basal clast-supported conglomerate. The orebodies are structurally controlled, and the area has undergone at least three distinct phases of deformation, with initial north-south compression developing minor folds. The second and largest deformation event is associated with east-west to the east-northeast and west-southwest directed compression leading to north-south structural architecture. The third event was a northwest and southeast compression that refolded existing structures. A deep oxidation (weathering) profile is developed in the region, varying between 50 and 150m.
Mineralisation style
Primary gold mineralisation occurs in all three lithostratigraphic units of the Siguiri region, although most of the known mineralisation is found in the central and more competent Fatoya Formation. In some deposits, the mineralisation shows strong lithological control and is preferentially developed in coarsergrained units with higher fracture or vein densities than finegrained rocks. The mineralisation dominantly follows sub-vertical north-south thrusts, northeast to southwest dextral shear zones, and west-northwest to east-southeast sinistral faults associated with the main (D2) deformation event. The mineralised veins are remarkable for the relative consistency of their northeast orientation, despite the highly variable orientation of bedding and significant structures.
Mineralised veins are more intensely developed along major structural trends, with quartz-carbonate- sulphide veining developed along structures. Some of these structures have developed as incipient faults and are represented by discrete stockworks of mineralised quartz-carbonate veins occurring along a trend instead of clearly defined continuous structures.
Mineralisation characteristics
Two styles of primary mineralisation have been recognised at Siguiri. The first is characterised by precipitation of gold-bearing pyrite associated with proximal albite and distal carbonate alteration and opening of carbonate-pyrite veins. The second style corresponds to east-northeast to west-southwest trending native gold-bearing quartz veins with carbonate selvages which cross-cut carbonate-pyrite veins and show arsenopyrite (pyrite) halos.