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Segilola |
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Nigeria |
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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 Segilola orogenic gold deposit is located in Osun State, Nigeria, ~120 km northeast of Lagos.
The deposit is located within the Ilesha Schist Belt which is developed in the crystalline Basement Complex rocks of the Benino-Nigerian Shield in southwestern Nigeria. This is one of a series of such Pan-African Neoproterozoic schist belts that were developed within generally NNE-SSW trending domains of Neoproterozoic meta-sedimentary, meta-volcanic sequences and intrusions. These schist belts are oriented parallel to the boundary between the West African Craton and the Pan African Province. They are deeply infolded into an Eburnian migmatite-gneiss-granite basement of Archaean to Palaeoproterozoic age and have been extensively intruded by granitoids of the ~600 Ma Pan African orogenic suite. These rocks were continuous with the Borborema Province of NE Brazil prior to the Late Mesozoic opening of the Atlantic Ocean.
The Ilesha Schist Belt is exposed over a strike length of ~200 km with a maximum width of 60 km in the south. It is followed for much of its length by the regional dextral strike-slip Ifewara Shear Zone which is interpreted to have been intermittently active from the Neoproterozoic to the Mesozoic. The Ifewara Shear Zone defines a marked structural contrast between rocks to the east, where the Segilola deposit is located, and the rocks to the west of that structure.
The host rocks at Segilola comprise an overturned sequence of strongly foliated, high-grade amphibolite-facies metasedimentary rocks that include quartz schists and a gneissic sequence intruded by a large, possibly differentiated, granodiorite sill-like body. The quartz schists are composed of quartzite, quartz-mica schist and meta-sedimentary rocks such as garnet-biotite schist. The gneissic sequence is divided into biotite paragneiss, granitic and pegmatoidal orthogneiss and undifferentiated gneiss. The granodiorite is up to 120 m thick and is massive to foliated and concordantly intrudes the sequence between the quartzites and basement biotite gneisses. From west to east, it gradually transitions from massive to weakly foliated, and then to strongly foliated as it approaches a sharp transition with the mineralised Lode Sequence. The stratigraphic sequence trends NNE and generally dips steeply towards the west.
The lodes are found in the intensely foliated and sheared rocks of the Lode Sequence which occurs at the boundary between the structural Hanging Wall Sequence basement biotite gneiss and granodiorite, and the Footwall Sequence calc silicate and mylonitic biotite-garnet schists. Pegmatitic veins, which are mostly conformable to schistosity, permeate the quartzite and footwall rocks. Gold mineralisation is associated with these ‘pegmatitic’ veins, which are late stage and weakly foliated to undeformed, and is restricted to the quartzite unit. The depth of weathering varies from 1 to 2 m in the west to 5 to 10 m in the mineralised shear zone.
Gold mineralisation occurs in fractured pale to dark grey smoky quartz veins and in sheared pegmatite veins, with silica/chlorite/carbonate alteration. It is dominantly hosted in steeply dipping vein sets or lodes that form an elongate mineralised zone striking at 10° and dipping at 60 to 70°W within a shear zone, primarily composed of biotite gneiss. The known mineralised zone (in 2021) had a strike length of ~2000 m, persisted to a depth of between 70 and 200 m, whilst the mineralised zone has a width of from 2 to 18 m in true thickness. Within this mineralised interval there are three steeply dipping vein sets or lodes:
• the Hanging Wall Lodes 100 and 300. These lodes are associated with dark grey, quartz-biotite schist with veining parallel to the gneissic fabric. Lode 100 is 2 to 4 m thick containing ~7 g/t Au, whilst Lode 300 is 5 to 8 m thick with ~2.5 g/t Au. Lode 100 is relatively discrete with sharp upper and lower contacts. Lode 300 is ~20 to 30 m west of, and parallel to, Lode 100 and is best developed in the southern part of the deposit, where it is characterised by some of the highest gold grades with finely disseminated visible gold particles in vein material;
• Other minor Lodes 400, 500 and 600, which have thicknesses and grades respectively of 1 to 3 m @ ~7 g/t Au; 1 to 3 m @ ~2 g/t Au; and 2 to 3 m @ ~1 g/t Au; These lodes are generally discontinuously developed;
• Footwall Lode 200, which is 4 to 5 m, but locally up to 15 m thick, containing ~3 g/t Au. It is hosted within a pale grey-green, strongly silicified zone of biotite schists and gneisses. Lode 200 is characterised by a wider, more diffuse, and lower-grade mineralisation developed around high-grade veins.
Higher gold grades and greater thicknesses are developed adjacent to a 5 to 20 m thick zone of intense quartz-carbonate flooding located at the eastern margin of the Hanging Wall Sequence.
The mineralogy of the deposit is characteristically simple and consistent. Gold is entirely non-refractory and commonly occurs as visible particles within either the quartz or pegmatitic quartz-feldspar veins or foliated biotitic selvedges to the veins. There are no significant direct trace element associations with gold, such as silver, although silver is present. However, metallurgical assaying indicates slightly elevated 250 to 300 ppm Cu and mineralogical studies suggest a gold-tellurium association.
The lodes have minor associated sulphides, typically pyrite, occurring as grains and blebs, often aligned parallel to foliation, commonly following either biotite-rich laminae or near pegmatite boundaries. There is also another common generation of pyrite found along fractures or as quartz-pyrite tension gashes, that are strongly discordant to the foliation, considered to represent either a late episode of mineralisation, or remobilisation. Native gold is visible in both altered wall rock and in quartz-feldspar veins. It occurs with the silver-gold telluride petzite, within pyrite, and in quartz veins. The typical size of native gold blebs is ~10 µm. Gold, either as native grains, flakes and blebs occurs together with gold-on-pyrite in alteration zones, along tension gashes, hair-like fractures, joints and minor faults.
Published Ore Reserves and Mineral Resources include (Thor Exploration Limited website viewed April 2022, consistent with Taylor and Burger, 2021):
Open Pit:
Probable Ore Reserves - 4.007 Mt @ 4.04 g/t Au containing 16 t of gold;
Indicated Mineral Resources - 3.700 Mt @ 4.5 g/t Au containing 16.55 t of gold;
Inferred Mineral Resources - 0.032 Mt @ 2.5 g/t Au containing 0.08 t of gold;
Underground:
Indicated Mineral Resources - 0.386 Mt @ 6.1 g/t Au containing 2.35 t of gold;
Inferred Mineral Resources - 0.411 Mt @ 5.0 g/t Au containing 2 t of gold.
NOTE: Resources are inclusive of Reserves.
This summary is largely drawn from Taylor, I. and Burger, A.M., 2021 - Mineral Resource and Ore Reserve Estimate, Segilola Gold Deposit, Osun Province Nigeria; an independent technical report prepared for Segilola Resources Operating Ltd, by Mining Associates Pty Ltd; 184p.
The most recent source geological information used to prepare this decription was dated: 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.
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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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