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Perkoa |
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Burkina Faso |
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Zn Ag Ba Pb
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Super Porphyry Cu and Au
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The Perkoa massive sulphide Zn-Ag-Ba deposit is located ~120 km west of Ouagadougou, in western Burkina Faso, west Africa (#Location: 12°22' 31"N, 2°36' 0"W).
The Perkoa deposit is hosted in a felsic to intermediary series of volcanic and volcanoclastic rocks, within the Lower Palaeozoic Birimian Supergroup of West Africa. It is located in the central part of the Boromo greenstone belt, which has been metamorphosed to lower greenschist facies. At least three phases of deformation have affected the Boromo belt, into which mafic to felsic dykes and granitic bodies were emplaced in several intrusive phases (Jensen et al., 2018; Schwartz and Melcher, 2003).
In the deposit area, the host sedimentary rocks, lavas and pyroclastics strike NE-SW and generally dip steeply to the NW. Several units of andesitic lavas with subordinate andesitic tuffs, separated by sequences of tuffs interlayered with fine grained clastic sedimentary rocks, make up the host lithological package. A number of syn- and post-tectonic intrusive bodies have been emplaced into the meta-sedimentary rocks, ranging from large plutons of granitic and dioritic composition to smaller ultramafic to rhyolitic intrusions (Jensen et al., 2018; Schwartz and Melcher, 2003).
The mineralisation at Perkoa occurs as a series of stacked, NE-SW striking tabular lenses hosted, and separated by, andesitic tuffaceous material that has been overturned with an average dip of ~70°NW. Six mineralised lenses have now been identified, hosted within the tuffs, comprising the main lens known as the 'Footwall Lens' which is split into normal and high grade domains due to plutonic activity which locally remobilised zinc; one minor thin <2 m thick lens occurs in the footwall, but is sometimes in contact with the main orebody, and four minor satellite lenses in the stratigraphy above, known as the 'Hanging Wall Lenses'. The main lens/orebody, which has sharp hanging wall and footwall contacts, contains >90 percent of the resource. It is semitabular with dimensions of 400 x 450 m, and an average thickness of 10 m, varying up to 25 to 30 m. The satellite orebodies which have an average thickness of 4 m make up the remainder of the resource. The deposit is characterised by its high concentrations of zinc and barium mineralisation, and relatively low levels of lead and copper (Jensen et al., 2018; Schwartz and Melcher, 2003).
The immediate host andesitic tuffaceous rocks are intruded by a 2175±1 Ma quartz diorite to granodiorite in the structural footwall (stratigraphic hanging wall), although it locally intrudes into the main orebody, creating enriched remobilised areas. The volcanics in contact with the granodiorite are locally metasomatised to a silica-magnetite-garnet skarn. The main orebody is subparallel to, and is on average 15 m from, the steeply dipping intrusive contact. The satellite orebodies are discontinuous but also show a preferred orientation subparallel to the intrusive contact, on average 75 m stratigraphically below the intrusion. The host sequence is structurally overlain by andesite, cumulate rich basic rocks and carbonaceous schist (Jensen et al., 2018; Schwartz and Melcher, 2003).
Dykes crosscut the deposit, most notably one with a granitic composition that is 5 m to 20 m thick, strikes ENE-WSW and dips at an average of 50°, which has replaced areas of mineralisation. Smaller sets of andesitic and basaltic dykes also crosscut the deposits and are typically sub-horizontal and sub-vertical respectively. The deposit is also cut by several felsic to intermediate intrusive rocks, interpreted as the feeders of lavas and intrusions found above and
below the mineralised sequence (Jensen et al., 2018).
A major reverse fault, which is bifurcated into several minor to major structures, cross cuts the entire orebody from its southern tip in the open pit to the northern point of the deepest part of the mine, and seems to play an important role on the geological continuity of the orebody and other structural elements (Jensen et al., 2018).
The massive sulphide ore is predominantly composed of Fe-rich sphalerite (30%), pyrite (25%), barite (10%), hexagonal pyrrhotite (5%), magnetite (5%), and white mica. Quartz is mainly present as host rock relics. Significant quantities of hydrothermal quartz were developed in the structural footwall of the deposit, where the main orebody is close to the quartz diorite (0.5 m away). Albite, (Ba, K) feldspar, Ba rich biotite, chlorite, tourmaline, andradite rich garnet, ilmenite, rutile, titanite, and galena are subordinate. Trace amounts of chalcopyrite, arsenopyrite, Ag rich tetrahedrite, and molybdenite are also found within the massive sulphide ore. A strongly altered quartz-microdiorite dyke containing disseminated zinc spinel + magnetite ± Fe-poor sphalerite occurs adjacent to the massive, Fe-rich sphalerite ore of one of the satellite orebodies (Schwartz and Melcher, 2003).
The 'Footwall Lens' is composed of roughly the same amount (20 to 25%) of coarse grained sphalerite and pyrite, a minor content of pyrrhotite,
plus localised galena associated with silver. Locally barite (up to 15%) is a common constituent of the gangue. In the upper northeastern area of the deposit, the Footwall High Grade Lens has >50% Zn near the contact between the sulphide lens and the quartz diorite/granodiorite pluton, due to the late heat driven re-modification of the sulphide mineralisation. Zinc grades in the footwall average between 9 and 15% (Jensen et al., 2018).
The smaller Hanging Wall Lenses are typically thinner than the Footwall Lens, but may be up to 15 m thick and represent massive sulphide bands intercalated within the tuffs, vein type, sulphide replacement, or disseminated mineralisation. In these lenses, the pyrite and pyrrhotite content is higher than that of the Footwall Lens, while barite content is lower and almost no lead and silver are present. Due to the interbedded and disseminated nature of the Hanging Wall Lenses, they are typically of lower average zinc grade, although, they are found to have some enriched zones in proximity to fault zones and/or intrusions (Jensen et al., 2018).
Minor high temperature alteration, ferruginous and copper minerals have been found in the Hanging Wall lenses, especially at depth or in relation to local structural features. At depth a progressive enrichment in copper is found in the hanging wall of the main lens. Although the deposit only contains trace copper, generally <10 ppm, values in the hanging wall are progressively increasing up to 0.6% (Jensen et al., 2018).
The magmatic rocks in the Perkoa area have a juvenile character according to the Nd-Sm isotopic data, like most Birimian rocks, although the Perkoa basalts show the influence of a subduction component in various discrimination diagrams. Birimian basalts from other areas have geochemical signatures of MORB or oceanic, within-plate basalts. The massive sulphide ore has been metamorphosed during a regional thermal event under predominantly isotropic stress, probably 100 to 200 m.y. after the emplacement of the quartz diorite. Sphalerite and magnetite were remobilized, whereas pyrrhotite formed at the expense of pyrite. Peak metamorphic temperatures of >460°C at pressures of >1 kbar are indicated by almandine rich garnet, the presence of andradite, and geothermometers involving arsenopyrite and pyrrhotite. Due to the metamorphic modifications of the ore and host rock, however, it is not possible to establish the age relationships between the main ore forming event and granitoid intrusions (Schwartz and Melcher, 2003).
The estimated ore reserves (Schwartz and Melcher, 2003) were 5.7 Mt @ 18.2% Zn, 10% BaSO4, 0.06% Pb, and 26 g/t Ag (50 ppm Cu).
Remaining Ore Reserves and Mineral Resources as of 31 December 2020 (Trevali Mining Reserve and Resources statement, 2021) were:
Proved + Probable reserves - 1.53 Mt @ 11.94% Zn.
Measured + Indicated resources - 3.54 Mt @ 11.98% Zn,
Inferred resources - 0.57 Mt @ 8.68% Zn.
For detail consult the reference(s) listed below.
Significant sections of this summary are drawn from: Jensen, T., Blakley, I.T., Jacquemin, T. and Patel, A.A., 2018 - Technical Report on the Perkoa Mine, Burkina Faso; An NI 43-101 Technical Report prepared by Roscoe Postle Associates Inc., for Trevali Mining Corporation; 192p.
The most recent source geological information used to prepare this decription was dated: 2018.
Record last updated: 17/4/2021
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.
Perkoa
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Selected References:
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Schwartz M O, Melcher F 2003 - The Perkoa Zinc Deposit, Burkina Faso: in Econ. Geol. v98 1463-1485
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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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