A carbon, oxygen, hydrogen and sulfur isotopic study of the gold mineralization at Loulo, Mali

Abstract The isotopic compositions of oxygen, hydrogen, carbon and sulfur have been determined on silicate, carbonate and sulfide samples from auriferous tourmalinized sandstone, its footwall and hanging wall, of Birimian age (2.1 Ga), at Loulo, in Mali. The sandstones have undergone two phases of deformation. The samples were taken from three different orebodies — Loulo 0 , Loulo 3 and P 64 . For Loulo 0 , δ 18 O-values for vein quartz are consistently between + 16.1 and + 16.6‰ (SMOW) for oxygen, with one exception at + 11.6‰. The δ 18 O-values for detrital quartz grains from the tourmalinized sandstone are also 16.1‰. Tourmaline has a high δ 18 O of + 13.9‰ . The δ 18 O-values for total rocks are also relatively homogeneous at + 13.2 to + 15.7‰, the lowest values (+ 13.2 and + 14‰) being those for the footwall samples. Dolomites from Loulo 0 and P 64 have δ 18 O-values between 15.1 and + 19.5‰ (SMOW) for oxygen and δ 13 C-values of − 13.5 to − 2.0‰ (PDB), with the exception of one sample from the hanging wall, which is of sedimentary origin ( δ 13 C + 0.5‰ , δ 18 O + 22.2‰ ). The sampled dolomites are in veinlets or veins or disseminated in rocks and were deposited by hydrothermal fluids. The δ 34 S-values of samples from the three orebodies range between + 6.0 and + 12.9‰ (CDT). They fall into different groups according to location, but pre- and post-schistosity sulfides within each group are indistinguishable. The results are interpreted as indicating the existence of two hydrothermal systems enriched in 18 O at different times. One of these gave rise to tourmalinization, silicification and dolomitization of the sandstone and the deposition of sulfides. The high δD-values of the tourmalines, in agreement with petrological data, indicate that the fluid of this system was of marine origin. The two phases of folding intervened between the activity of this hydrothermal system and the initiation of the second system, which gave rise to the stockwork. This fluid, rich in 18 O, was possibly of metamorphic origin, with a low water/rock ratio. It remobilized the pre-schistosity sulfides, concentrating them into post-schistosity veins. The (partly remobilized?) silica and carbonates were deposited under temperature conditions such that their isotopic compositions were similar to those observed for the tourmalinized sandstone. The vertical extent of this hydrothermal system is as yet poorly known. It is very extensive horizontally and strongly pervasive in the tourmalinized sandstone, but in the vertical sense appears to be confined to fractures in the footwall.