The Dimako–Mboscorro lode gold deposit has two styles of mineralization: quartz veins (4.5–54 ppm Au) and altered wallrock (up to 2.6 ppm Au). The veins occur in steeply dipping brittle–ductile shear zones in a foliated biotite metagranite. The alteration envelop around the gold-bearing quartz veins shows lateral zoning comprising a silicified/ ferruginized and mylonitic zone (SFZ) at the centre, through a transitional sericite zone (SZ), to an outer, slightly sheared, but mineralogically less altered, though bleached, metagranite zone (BZ) at the shear zone boundaries. Hydrothermal alteration in these zones is characterized by silicification (SFZ, SZ), hematite formation (SFZ), alkali metasomatism with muscovite/sericite formation (SFZ, SZ, to a much lesser extent BZ), and pyritization (SFZ, SZ). The auriferous quartz veins are concordant to the host structure but discordant to the foliation in the metagranite. The main ore minerals in the veins and in the SFZ and SZ zones are gold, hydrothermal hematite, sphalerite, galena, pyrite, chalcopyrite, arsenopyrite, chalcocite, covellite and colloidal supergene hematite. Gold occurs as inclusions in sphalerite (15.7±0.4 wt% Ag; 83.6±0.2 wt% Au), as coarse grains associated with euhedral hematite pseudomorphs (18.5±1.0 wt% Ag; 82.0±1.1 wt% Au), as stringers within fractures in quartz (13.3±0.1 wt% Ag; 87.1±0.5 wt% Au), as well as fine to coarse grains related to limonite, goethite and colloidal hematite (8.1±1.8 wt% Ag; 92.1±2.2 wt% Au). A hydrothermal metal association of Ag, Au, As, Ba, Pb, V, W, and Zn is identified in the wallrock and quartz veins. These metals, together with K-enrichment, are potential pathfinder elements for wider lithogeochemical exploration in this region, especially when combined with the lateral wallrock zonation pattern. Preliminary fluid inclusion data from the auriferous quartz vein samples indicate that gold was probably deposited from low salinity ( 2 O–CO 2 fluids that have total homogenization temperatures of 263–335°C.

Geology and geochemical aspects of lode gold mineralization at Dimako–Mboscorro, SE Cameroon

Three types of tourmaline occurrence have been identified in the area of Bin Yauri, Nigeria, mesothermal lode-gold mineralization. These are: (1) stratabound tourmalinites in pelitic metasediments, (2) tourmaline in a hydrothermal alteration assemblage within hornfelsed wall rocks, (3) tourmaline in auriferous quartz and quartz-carbonate veins. Although the tourmaline occurrences are all within or close to the contact aureole of a granodiorite intrusion, geochemical characteristics of the tourmalines are broadly similar and reflect a common metasedimentary source. Two stages of tourmalinization are envisaged. The earlier (ca. 1100 Ma) involved syngenetic-diagenetic formation of tourmalinites, while the later (ca. 500 Ma) involved epigenetic (hydrothermal) tourmaline-gold mineralization, possibly derived by dehydration and devolatilization of metasedimentary sequences containing tourmaline-rich rocks or tourmalinites. Electron microprobe analyses indicate that the tourmalines are intermediate members of the schorl-dravite solid solution series. Plots involving FeO, MgO, and/or Al2O3 from these analyses are used to constrain the sources and processes of tourmalinization. Two metallogenic implications are derived from this study. One is that, although the tourmalinites are barren of gold and base metal mineralization, their occurrences nevertheless encourages exploration for syngenetic-exhalative massive sulphide deposits in the region. The other implication applies to the potential use of tourmaline in deciphering the physico-chemical conditions of gold-mineralizing fluids in the Bin Yauri area.

Tourmalinization related to Late Proterozoic Early Palaeozoic lode gold mineralization in the Bin Yauri area, Nigeria

Petrochemical characterization of Neoproterozoic Colomine granitoids, SE Cameroon: Implications for gold mineralization

The Woumbou–Colomine–Kette district is located in the East Metallogenic Province of the Republic of Cameroon. Northeast- to E-striking splays of the regional Pan-African crustal-scale Sanaga Shear Zone control widespread gold mineralization. Gold occurs within local shear zones developed along granite-gneiss contacts, as well as within the margins of the Pan-African intrusions. Field investigations coupled with mineralogical and textural studies reveal two stages of gold mineralization. Stage 1 led to pyrite, chalcopyrite, sphalerite, pyrrhotite, bismuthinite, tellurobismuthite, galenobismuthite, and native gold deposition with quartz, sericite/muscovite, chlorite, calcite, and ankerite as gangue minerals. Stage 2 included the precipitation of pyrite, hematite, tellurobismuthite, and native gold. A SEM–CL study distinguished the two stages of quartz, as well as an early remnant barren quartz depositional stage. Two fluid inclusion assemblages (FIAs) with four types of fluid inclusions can be observed. The FIA 1 is aqueous-carbonic (±N2±CH4), with carbonate daughter minerals, which is related to stage 1 of the gold mineralization and composed of low salinity (~7.5 wt% NaCl equiv) H2O-CO2-NaCl (type 1), CO2-rich (type 2), and H2O-rich (type 3a) fluid inclusions. The inclusions provide evidence of episodic phase separation, with unmixing at P~2 kbar, T~300 °C, and at a paleodepth of ~7 km. The FIA 2 consists of aqueous (type 3b) fluid inclusions with a salinity of 0.2 to 11.7 wt% NaCl equiv and trapping temperatures between 205 and 245 °C. The δ18Oquartz (+11.0 to +11.7‰), δDfluid inclusions (−46.2 to −40.9‰) and δ13Cfluid inclusions (−4.9 to −4.1‰) for gold-bearing quartz veins suggest a metamorphic source for the ore-forming fluids, although some contribution from the mantle and/or magmatic source(s) cannot be ruled out. The δ34S (+6.5 to +7.0‰) for gold-bearing pyrites suggests a metamorphic source for the sulfur that transported gold and was involved in the precipitation of sulfide minerals. These data are consistent with a mesozonal orogenic gold deposit model for the Woumbou–Colomine–Kette district, which is an important part of the emerging greenfields exploration region for Pan-African orogenic gold and with the more significant targets defined by granite-gneiss contacts along NE- and E-striking shear zones.

Gold occurrences of the Woumbou–Colomine–Kette district, eastern Cameroon: ore-forming constraints from petrography, SEM–CL imagery, fluid inclusions, and C–O–H–S isotopes

Tectonic evolution of Neoproterozoic rocks, eastern Cameroon: Implication for gold mineralization in the Bétaré Oya and Woumbou–Colomine–Kette districts

The geochemical and structural controls on gold mineralization in the Colomines area, Pan-African belt of eastern Cameroon

Ophiolites are the remnants of ancient oceanic crust that can serve as tools to reconstruct the evolutionary history of fossil oceans. At Boumnyebel, the western part of the Yaoundé group, the sole of the lower unit (magnesian mica-schists and gneiss) of the Pan-African nappes is made up of ophiolitic rocks (talc schists, chlorite schists, serpentinites, amphibolites, pyroxenites, hornblendites). Structurally, the lower unit has a flat-lying schistosity with some recumbent folds while the underlying TTG gneisses are marked by a high-dipping foliation at Mapan. The contact between the Pan-African mica schists and the basement formations (Congo craton) is therefore discordant. In ophiolitic rocks, amphibole is actinolite, tremolite or magnesio-riebeckite. Chlorite compositions correspond to clinochlore. Talc shows notable substitution of Mg by Fe whereas substitution of Mg by Fe and Al prevails in serpentine. Spinel compositions are either Cr-rich with high FeO/Fe2O3 values (peridotitic spinels) or Cr-poor with low FeO/Fe2O3 values (melt-derived or magmatic spinels). Main mineral features indicate a metamorphic retromorphosis of the original lherzolites, harzburgites, dunites and pyroxenites to hornblendites and serpentinites. This is followed by hydrothermal alteration (at various temperatures: 250–570 °C and low pressure: < 2 kbar) that yielded chlorite schists and talc schists. Chemically, talc schists are made up of SiO2, MgO and minor FeO and Al2O3, while chlorite schists and hornblendites contain notable amounts of Al2O3 and CaO, respectively. All the ultramafic rocks display the same range of high Ni, Cr, Co contents with various LREE enrichments but flat HREE patterns. Multi-element patterns show that most talc schist are depleted in almost all elements. All the ultramafic rocks are marked by deep Nb, Ta, K, Pb, Sr, Ti, Zr and Hf troughs suggesting that the ultramafic rocks possibly derived from heterogeneous mantle sources varying from fertile to refractory peridotites. Amphibolites are generated from N-MORB, IAT or CA geotectonic settings. The compositions of spinel and ultramafic whole rocks reveal an arc setting that has evolved from divergent plate setting (oceanisation, N-MORB) to convergent plate (IAT, CA, SSZ D-MORB) settings. In summary, ophiolites emplaced in various geotectonic settings and were amalgamated during the Pan-African collision. 

Petrology of ophiolites of Memel, Nsimè-Kellé and Mapan (Yaoundé group): Evidence of the geodynamic evolution of the Pan-African orogeny in South Cameroon

Calistus Gentry Fuh

Papers

The geochemical and structural controls on gold mineralization in the Colomines area, Pan-African belt of eastern Cameroon

Petrology of ophiolites of Memel, Nsimè-Kellé and Mapan (Yaoundé group): Evidence of the geodynamic evolution of the Pan-African orogeny in South Cameroon