The Project is contained within seven current exploration permits that were granted to Ampella Mining Côte d’Ivoire and Ampella Mining Exploration Côte d’Ivoire, which are both 100% owned Ivoirian subsidiaries of Centamin.
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Summary:
The Doropo Project currently includes thirteen distinct mineralised bodies that host the actual resource plus numerous prospects and geochemical surface anomalies yet to be tested, fitting an area of about 170 km2 , or within a circular area of 28 km radius.
The mineral occurrences tested to date include two model types, a ‘classic’ orogenic shear-hosted gold deposit model and a quartz vein hosted gold deposit model. Both these models are coherent in nature with the majority of the other West African deposits, except on the issue of the host lithology (the granitic domain).
The granitic complex displays lozenge-shaped arrays of anastomosing shear zones. The shears have a broad south-southwest to north-northeast orientation, dipping shallowly towards the northwest.
The quartz veins mainly occur along the NW-SE orientation, and are sub-vertical or steeply dipping towards the SW. These veins show significant gold grades and often visible gold but have a limited width.
Structure and Mineralisation
Economically interesting mineralisation is associated with discrete structures of intense silica-sericite alteration, focused within and along the margins of narrow (5-10 m wide to locally 20-25 m wide) dextral shear zones. Outside of the mineralised zones, the granodiorite is fairly undeformed.
The planar zones of mineralisation define a great circle on the stereonet with a plunge of 30->295 (excluding Kilosegui). This direction appears to be coincident with the linear shoot directions within the planar zones of mineralisation and can be used to further explore the deposits (e.g. Nokpa).
Even though Kilosegui appears to be in a completely different strike orientation to the planar zones of gold mineralisation at Doropo, the trend could well be related and may have been formed under the same stress conditions. If Kilosegui is completely unrelated to Doropo, the poles of the planar grade continuity would not lie close to the great circle defined by the poles of the Doropo zones. With the inclusion of Kilosegui, the average pole to the great circle that fits through the local planar orientations is 25->266. Further drilling will support or negate this hypothesis.
The gold mineralisation is laterally terminated by NE and NW striking late-stage fractures, some of which are intruded by younger dykes. Most of the prospects are not along these trends but are more NNE-SSW striking. Some observed patterns are as follows:
- The eastern extent of CHG North prospect is terminated by a NW-striking transverse fracture;
- Chegue (CHG) South prospect is terminated to the south by NW-striking fracture;
- Han (HAN) is terminated to the SW by a NW-striking fracture;
- Attire (ATI) is terminated to the NW by a NE-striking transverse fracture.
Also, some prospects are parallel to the transverse fractures:
- Vako (VAK) appears to run along a NE-striking fracture;
- Kilosegu (KLG) and ATI are both running parallel to a NW-striking transverse fracture.
These patterns of gold mineralisation are common in Archean belts where the gold is locally continuous along the tectonic grain but are terminated by fractures and faults that clearly post-date and crosscut the tectonic grain. This means that the gold mineralisation itself is very late in the orogenic process. If gold is found to be laterally non-continuous at Doropo, it is highly likely that they are terminating against these late fractures. By recognising this relationship, it would save in drilling costs by drilling wider spaced drill fences on the ‘barren’ side of these fractures.
Alteration and Mineralisation
The alteration assemblages seen at Doropo from distant to proximal are epidote ± chlorite, chlorite ± magnetite, haematite-silica, silica-sericite-leucoxene then silica flooding ± pyrite ± gold.
In distant regions from mineralised zones, the mineral assemblage is dominated by epidote-chlorite and haematite. Haematite alteration is widespread, ranging from weak to medium intensity. Close to doleritic dykes, haematite alteration intensifies, making it challenging to determine mineralization direction, notably at the Nokpa prospect.
Proximal mineral assemblages include strong silica-sericite alteration that often overprints earlier haematite and silica alteration. The sulphides, mostly pyrite, are abundant throughout the core of the shear zone; they host part of the gold mineralisation. The other portion of the gold mineralisation occurs as native gold in quartz veins and selvages. Chlorite-magnetite alteration precedes the main pyrite-associated silica-sericite alteration phase.
Weathering Profiles
The weathering profiles encountered across the gold deposits in the Doropo Project area includes a surficial layer or soil profile, which has some degree of transported material but is dominated by sandy granitederived soils. The surficial layer rests on a mottled or unmottled saprolite layer and a saprock layer which then transitions to fresh rock. The weathered, in situ bedrock is divided into two reasonably distinct material types for the purposes of drill hole logging and modelling of the weathered zone, which overlies fresh granodiorite. The transition from saprock to fresh rock is generally a sharp contact zone.
The characteristics of the constituent parts of the weathered bedrock are as follows:
- Surficial material (soil profile): In situ and transported sandy soils, colluvium and alluvium in drainage lines and limited areas of hard laterite cuirasse on stable interfluvial areas. The soil profiles vary in thickness from 0 m to 5 m with an average thickness (outside the drainage lines) of 2 m. The BOT code (base of transported/soil profile) is used for geological modelling;
- Saprolite: Highly weathered insitu rock (mainly granodiorite), which is reduced to an orangebrown clayey sand. The original granitic texture is barely recognisable but some of the constituent minerals are present, mainly a skeletal framework of quartz grains with some remnant feldspars and clay minerals. The saprolite layer varies in thickness from 2 m to 40 m with an average thickness of 18 m. The BOS code (base of saprolite) is used for the geological modelling;
- Saprock (transition material): Weathered granodiorite which retains it’s original rock texture. Most of the constituent minerals are recognizable, including quartz, milky, partially weathered feldspars and any mica is only partially altered. The saprock layer varies in thickness from 10 m to 30 m with an average thickness of 16 m;
- Fresh rock: None of the mineral components are altered. The TPFR code (top of fresh rock) is used for geological modelling.