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Karouni |
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Guyana |
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Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Karouni orogenic gold deposit is located ~170 km SW of the capital of Georgetown and 35 km NW of the 150 t Au Omai mine in north-central Guyana. It comprises two zones, Smarts and Hicks, that are 2 km apart.
The region surrounding Karouni is of a broad area of low topographic relief ~60 to 100 m above sea level, covered by dense tropical forest, and broken by incised river valleys. Gold discoveries, including the major Omai and Rosebel deposits has largely resulted from activity by local artisanal gold miners, exploiting placer gold in the creeks and rivers. Large-scale dredging and hydraulic mining on the Karouni area, known historically as the Kaburi district, produced up to 4.5 t of gold. The only known hard rock gold in the district was from a small shaft at a prospect known as El Dorado. Modern exploration at Karouni began in 1974 with the discovery of the Hicks zone by Cominco Ltd. Work continued sporadically through the 1990s and 2000s, but the remoteness from infrastructure and the small size of the known resource precluded its development. The Smarts zone was discovered by Azimuth Resources in 2011, adding approximately 18.5 t of Au to the Karouni project. Karouni was acquired by Troy Resources in 2013, and mine development began in 2014, with production commencing in 2015. Mining continued in 2018 in four open pits one at Smarts and three at Hicks.
Karouni is one of the few active gold mines in the Rhyacian (i.e., 2.2 to 2.05 Ga) Trans-Amazonian Province of the northern Guiana Shield. The 0.9 million km2 Guiana Shield is a Palaeo- to Neoproterozoic granite-greenstone terrane located between the Amazon and Orinoco rivers in northeastern South America. The Trans-Amazonian Province is composed of greenschist facies volcano-sedimentary rocks and associated granitoid intrusive rocks of Rhyacian age which trend along the northeast margin of the shield (Vanderhaeghe et al., 1998; Santos et al., 2000). Other than some small Archaean fragments in southern Venezuela (Imataca Complex) and in Amapá, Brazil (Cupixi terrane), no substantial Archaean terrane is known in the Guiana Shield (Gibbs and Barron, 1993; Avelar et al., 2003; Delor et al., 2003).
The Karouni deposit is hosted within 2.2 to 2.1 Ga volcano-sedimentary rocks of the Barama-Mazaruni Supergroup. This supergroup comprises a sequence of basal basalt ±ultramafic rocks grading into intermediate to felsic volcanic rocks, overlain by tuffaceous and turbiditic sedimentary rocks in packages with a total thickness of 8 to 10 km (Gibbs, 1980). The ages of these rocks are constrained by U-Pb zircon ages of 2131±10 Ma obtained from felsic volcanic rocks of the equivalent Yuruari Formation in eastern Venezuela (Day et al., 1995) and a crosscutting 2120 ±2 Ma felsic porphyry dyke at Omai (Norcross, 1997; Norcross et al., 2000).
The protracted Trans-Amazonian orogeny was the main deformation event in of the Guiana Shield, resulting from the collision of the Guiana Shield with the Archaean Leo-Man craton of West Africa (Ledru et al., 1994; Vanderhaeghe et al., 1998; Delor et al., 2003). It has been divided into:
• D1 stage, interpreted to have commenced as early as 2180 Ma and continued until ~2120 Ma with the intrusion of multiple tonalite-trondhjemite-granodiorite (TTG) style granitoids and coeval volcano-sedimentary greenstone belts in an interpreted oceanic volcanic arc (Vanderhaeghe et al., 1998). Deformation during this event are interpreted as mainly gravity driven (Delor et al., 2003).
• D2a stage, resulting from the collision of this volcanic arc with a West African cratonic nucleus, representing a shift to compressional and transpressional tectonics leading to crustal thickening, regional greenschist metamorphism and folding of the volcano-sedimentary suites into shallowly plunging, NW-SE trending synclines with a steeply dipping penetrative axial planar cleavage (Vanderhaeghe et al., 1998; Delor et al., 2003). NW-SE strike-slip shear zones, e.g., the North Guiana trough shear zone and Makapa-Kuribrong and Issano-Appaparu shear zones were developed during this event (Walrond, 1980; Elliott, 1992; Vanderhaeghe et al., 1998; Voicu et al., 2001). Activity on these shear zones is interpreted to have resulted in the development of late alluvial-fluvial filled basins mapped throughout Suriname and French Guiana (Delor et al., 2003).
• D2b stage, corresponded to a shift in the far-field stress event which led to closure and deformation of these late basins and reactivation of the NW-SE shear zones with dextral strike-slip movement (Vanderhaeghe et al., 1998; Delor et al., 2003). The D2b deformation is poorly constrained, but is interpreted to have occurred between 2070 and 2060 Ma based on syntectonic quartz-monzonite intrusions in French Guiana (Delor et al., 2003). Late in the Trans-Amazonian orogeny hydrothermal activity is interpreted to be responsible for gold mineralisation at the Rosebel and Omai deposits (Voicu et al., 2000; Daoust et al., 2011). The region is also crosscut by an extensive late set of mafic dykes related to the opening of the Atlantic in the Jurassic (Deckart et al., 1997).
The stratigraphy at Karouni comprises three sequences:
• Mafic volcanic sequence, consisting of mafic volcanic rocks and volcanic breccias,
• Lower sedimentary sequence, composed of immature volcaniclastic sandstone and conglomerate located conformably above the basalts, and
• Uppermost sedimentary sequence, composed of laminated, fine-grained sandstone and siltstone.
Lave flows or sills of high MgO basalt and high TiO2 dolerite are intercalated within the lower volcanic and lower sedimentary sequences, whilst intermediate to felsic volcanic rocks and possible tuffaceous felsic volcanic rocks have been noted at the base of the laminated silt and sandstone unit. All of these units have been metamorphosed to lower greenschist facies and many, especially those in proximity to the shear hosting the Smarts and Hicks zones, have undergone intense ductile deformation. The entire volcano-sedimentary package has a general NW strike and dips at 70 to 90°NE and SW. It is exposed in the regional NW-SE Karouni syncline. Whole-rock geochemical analysis indicates the sequence was formed in an oceanic island-arc environment, whilst the high MgO basalts have mantle-like characteristics. At least three phases of calc-alkaline magmatism have also affected the region, comprising quartz-monzonite plutons, granodiorite stocks and dykes, and late granite plutons.
The Smarts and Hicks zones occur along the NW-striking Smarts-Hicks shear zone, a second-order splay of the regional-scale Makapa-Kuribrong shear zone. The Smarts and Hicks orebodies are localised within dilatational bends formed at changes in strike of the Smarts-Hicks shear zone during late dextral transcurrent movement.
Four vein sets are recognised at Karouni:
i). early unmineralised V1 calc-silicate veins, divided into garnet-actinolite-epidote-calcite-quartz ±pyrite ±pyrrhotite V1a and dark grey quartz ±calcite V1b veins. Both a similarly deformed and are overprinted by D1 fabrics;
ii). gold-mineralised V2a shear and V2b extensional quartz-carbonate veins, both of which are are surrounded by semiconcentric, 1 to 4 m wide zones with distinct hydrothermal alteration assemblages. Most V2a veins are thin (1 to 2 cm) foliation-parallel stringers, but can locally occur as more massive, 1 to 2 m thick elongate pods that are up to 20 m long and pinch and swell along strike. V2b veins typically have higher grades of 5 to 100 g/t Au and fill brittle fractures in competent host rocks. They vary in thickness to up to 1 m but are more typically 2 to 10 cm wide. Rheological contrast played a dominant role in the formation of these veins with shear-hosted, NW-striking, steeply dipping quartz-carbonate-chlorite ±tourmaline-pyrite-gold V2a veins preferentially hosted in ductile deformed high MgO basalts, whereas mineralised north-south trending, quartz-carbonate-chlorite ±tourmaline-pyrite-gold V2b extensional veins are hosted within rheologically competent high TiO2 dolerite sills and granodiorite dykes. The alteration assemblage in the latter progresses towards the veins from a chlorite-calcite-rutile to an albite-dominated mineralogy. The interaction of these structures with favorable lithologies is apparently key for localising high-grade mineralisation (Tedeschi et al., 2018).
iii). late, weakly mineralised V3 quartz-carbonate veins only locally present in the Smarts open pit. They trend east-west, dip steeply to the north, and are typically 2 to 10 cm thick. They have a disseminated pyrite halo similar to that of the V2b veins. Grab samples indicate grades from 1.2 to 3.5 g/t Au. Like the V2b veins, they occur prominently within brittle, high TiO2 dolerite (Tedeschi et al., 2018);
iv). V4a late unmineralized quartz-epidote and V4b calcite veinlets ubiquitous within the Smarts and Hicks deposits. V4a are steeply dipping, north-south striking quartz-epidote veins and breccias, crosscutting all earlier structures and are generally less than a few mm in width with associated gold mineralisation. The same mineral assemblage fills and cements cataclastic breccia and gouge in the late north-south to NE-SW faults. V4b veins are late crosscutting calcite veinlets a few mm in thickness (Tedeschi et al., 2018).
The alteration assemblage at Karouni is characterised by its dominant sodic composition and distinct lack of potassic alteration (Tedeschi et al., 2018). Gold occurs as inclusions in coarse, disseminated pyrite associated with the proximal alteration zones and as coarse native gold within the quartz-carbonate veins. Minor gold is also located within telluride minerals. The high TiO2 dolerites were a favorable chemical trap due to their high magnetite content, suggesting sulphidation via redox reaction as a possible mechanism of gold deposition (Tedeschi et al., 2018). Modeling of the hydrothermal alteration indicates a wall rock-dominated system with limited addition or subtraction of major elements with the exception of C, S, and Na. Modeling has also shown strong trace element enrichment of W-Bi-Ag-Te-Mo-Pb, all of which correlate with gold. Detailed analysis indicate a geochemically and isotopically homogeneous system with only minor trace element variation due to differences in host rock, suggesting a single hydrothermal pulse correlative with the late stages of the Trans-Amazonian orogeny (Tedeschi et al., 2018).
Reserves and Resources
Published Ore Reserves and Mineral Resources at 30 June, 2018 (Troy Resources website, viewed April, 2019) were:
Proved + Probable Reserve - 1.536 Mt @ 2.43 g/t Au;
Measured + Indicated Resource - 6.031 Mt @ 2.20 g/t Au;
Inferred Resource - 7.684 Mt @ 2.10 g/t Au.
NOTE: Minerals Resources are inclusive of Ore Reserves.
The information in this summary is drawn from Tedeschi et al., 2018 parts I and II.
The most recent source geological information used to prepare this decription was dated: 2018.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Tedeschi, M., Hagemann, S.G. and Davis, J., 2018 - The Karouni Gold Deposit, Guyana, South America: Part I. Stratigraphic Setting and Structural Controls on Mineralization: in Econ. Geol. v.113, pp. 1679-1704.
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Tedeschi, M., Hagemann, S.G., Roberts, M.P. and Evans, N.J., 2018 - The Karouni Gold Deposit, Guyana, South America: Part II. Hydrothermal Alteration and Mineralization: in Econ. Geol. v.113, pp. 1705-1732.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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