Petrogenesis of the Bakoudou TTGs and high-K biotite granites and associated gold mineralization, southern Gabon: An Archean example of a porphyry–epithermal transitional gold system

Abstract

The Bakoudou gold deposit, southern Gabon, is hosted in the Archean basement granitoids of the Chaillu Massif. These granitoids of the Bakoudou deposit are subdivided into two main groups: TTGs and porphyritic biotite granites. The TTGs show geochemical characteristics similar to those of other TTGs worldwide, such as high Al2O3 (13.49–18.93 wt%) and Na2O (3.27–4.66 wt%) and low K2O (1.01–1.98 wt%), with low K/Na (0.15–0.32) ratios. Their Mg# (Mg/(Mg + Fe)*100) values vary from 34.81 to 50.76, with low to moderate Nb/Ta and Zr/Sm ratios (7.03–23.43 and 19.13–75.25, respectively). They are characterized by moderate (La/Yb)N (6.70–41.71) and Sr/Y (20.80–72.13) ratios similar to those of adakites, negative Nb, Ta and Ti anomalies and LREE-enriched and HREE-depleted patterns, without significant Eu anomalies (Eu/Eu* = 0.93–1.11, average = 1.00). These geochemical characteristics suggest that the TTG rocks were formed by partial melting of hydrous basaltic crust, such as garnet-rich amphibolite. The porphyritic biotite granites show high-K calc-alkaline affinity. They are characterized by high SiO2 (71.01–74.86 wt%) and K2O (3.29–4.92 wt%) but relatively low MgO (0.38–0.92 wt%), with Mg# values varying between 31.78 and 40.27. The rocks show low to moderate Sr/Y (13.75–26.66) ratios and LREE-enriched and HREE-depleted patterns, with negative Eu anomalies (Eu/Eu* = 0.36–0.58, average = 0.47). These geochemical features indicate that the porphyritic biotite granites originated from partial melting of TTG lithologies.The orebody of the Bakoudou gold deposit occurs as quartz veins hosted in the TTG shear zones. Quartz is the main gangue mineral, with minor sulfides such as pyrite and chalcopyrite. Gold is present in solid solution (Au+1) and as nanoparticles of native gold (Au0) in pyrite and mostly as free gold. Pyrite samples analysed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) show high Ni (3.24–3600 ppm) and Co (4.68–4140 ppm) and low As (<34.50 ppm) and Au concentrations (<2.90 ppm) compared to Cu, which has rather high concentration of <3570 ppm. Moreover, the Au–As relationships of pyrites show a porphyry–epithermal transitional signature. Therefore, the Bakoudou gold deposit can be considered an example of Archean porphyry–epithermal transitional gold system, formed from magmatic fluids originating from a hidden porphyry system at depth during cooling and emplacement of the potassic granites. The disseminated free gold grains in the quartz veins may have originated from boiling of the mineralizing fluid (phase separation) and/or from remobilization of invisible gold in pyrite by later tectonic events and surficial weathering.