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1.1 Regional Geology of the Bougouni Basin
The Kalana exploitation permit lies in close proximity to the western edge of the Bougouni basin, a large sedimentary-rock-dominated domain in southwestern Mali
The Bougouni basin is dominated by clastic sedimentary rocks that were deposited in a broad sedimentary trough, outboard of an island arc domain located further to the east (Boundiali belt), in Birimian time (circa 2.2-1.8Ga). These clastic sedimentary rocks consist of greywackes, siltstones and turbiditic mudstone and are classified on the published regional geology map as: 1) undifferentiated sedimentary rocks (Bf6a); 2) detrital sedimentary rocks (Bf6c); and 3) pelitic sedimentary rocks (Bf6d). These sedimentary rocks dominate the eastern, western and southern parts of the Bougouni basin (fig. 2).
Mafic volcanic rocks (primarily basalt) are found in small domains in the eastern and western part of the basin, associated with faults that may have brought lower stratigraphy to the present erosional level.
A large felsic intrusion, the Bougouni batholiths, occupies the central part of the basin and similar smaller intrusions cut the sedimentary rocks. Small intermediate intrusions (diorite, granodiorite) are found primarily in the eastern part of the basin but are also found in the Kalana area.
Neoproterozoic sedimentary rocks bound the basin to the east and north and lie uncomfortably on the Birimian stratigraphy.
Regional-scale dolerite dykes cut all the above lithologies and are dominated by E, NE, and NNW trends.
From an exploration viewpoint, the basin-marginal-parallel faults appear to be a primary control on the sitting of gold mineralization in southern Mali. These structures are generally oriented with NW to N trends.
The eastern Bougouni basin margin, in the Sikasso area, is controlled by northerly and northwesterly striking faults and is host to the multi-million ounce gold deposits at Morila and Syama. A large endowment of gold is located along the eastern margin of the Bougouni basin, within a definable NW-trending corridor, which has been identified as the “Morila-Syama Gold Belt”.
1.2 Geology of the Kalana District and Exploitation Permit
The geology of the Kalana district is not well known due to extensive alluvial and lateritic cover. However, mapping coverage since the 1960s has established limited bedrock exposures such that a general picture of the bedrock geology has been delineated. As well, a detailed airborne magnetic survey over the Kalanaexploration permit and a multi-element soil geochemistry programme conducted during the Or-Bagoé survey have helped interpret the underlying geology where not exposed.
The Kalana district lies along the western margin of the Bougouni basin and is underlain by sedimentary rocks of the lower part of the Upper Birimian Group (fig. 2), which have been metamorphosed to greenschist facies and intruded by syntectonic granite and later dioritic dykes and stocks.
The Kalana exploitation permit area lies on the western limb of a regional-scale syncline with steeply plunging parasitic folds. The published geology map for the Kalana district (DNGM, 1997; scale 1: 1,500,000) shows that the western third of the exploitation permit is underlain by a large granitoid body of probable batholithic dimensions. The eastern two-thirds of the property is underlain by a sedimentary rock series that are represented by sandstones, siltstones and turbiditic mudstones (fig. 3).
The regional geology map indicates the presence of NE-trending doleritic dykes in the northern part of the exploitation permit, as well as further north, which together describe a NE oriented dyke swarm marking a regional-scale crystal weakness. A second set of dolerite dykes, with a NNW trend appears on the regional geology map within the granitoid body underlying the western part of the Kalana exploitation permit. Interpretation of the Ashanti airborne data (analytical signal) and the Or-Bagoé geochemical data also indicates northerly, north-easterly, and northwesterly trending doleritic dyke sets within and marginal to, the exploitation permit.
Of particular interest to the SOMIKA exploration programme is a NW-trending linear zone or corridor of relatively small dioritic intrusions (coloured red in figure 3), which appear on the regional geology map cutting through the northern and eastern parts of the exploitation permit. These dioritic intrusions also mark a zone ofcrystal weakness or faulting which has controlled their emplacement.
Source: Final Prospectus May 27,2005 |
