A CARBON, OXYGEN, HYDROGEN AND SULFUR ISOTOPIC STUDY OF THE GOLD MINERALIZATION AT LOULO, MALI

The isotopic compositions of oxygen, hydrogen, carbon and sulfur have been determined on silicate, carbonate and sulfide samples from aurife rous tourmalinized sandstone, its footwall and hanging wall, of Birimi an age (2.1 Ga), at Loulo, in Mali. The sandstones have undergone two phases of deformation. The samples were taken from three different ore bodies - Loulo 0, Loulo 3 and P64. For Loulo 0, deltaO-18-values for v ein quartz are consistently between + 16.1 and + 16.6 parts per thousa nd (SMOW) for oxygen, with one exception at + 11.6 parts per thousand. The deltaO-18-values for detrital quartz grains from the tourmalinize d sandstone are also 16.1 parts per thousand. Tourmaline has a high de ltaO-18 of + 13.9 parts per thousand. The deltaO-18-values for total r ocks are also relatively homogeneous at + 13.2 to + 15.7 parts per tho usand, the lowest values ( + 13.2 and + 14 parts per thousand) being t hose for the footwall samples. Dolomites from Loulo 0 and P64 have del taO-18-values between + 15.1 and + 19.5 parts per thousand (SMOW) for oxygen and deltaC-13-values of - 13.5 to - 2.0 parts per thousand (PDB ), with the exception of one sample from the hanging wall, which is of sedimentary origin (deltaC-13 + 0.5 parts per thousand, deltaO-18 + 2 2.2 parts per thousand). The sampled dolomites are in veinlets or vein s or disseminated in rocks and were deposited by hydrothermal fluids. The deltaS-34-values of samples from the three orebodies range between + 6.0 and + 12.90 parts per thousand (CDT). They fall into different groups according to location, but pre- and post-schistosity sulfides w ithin each group are indistinguishable. The results are interpreted as indicating the existence of two hydrothermal systems enriched in O-18 at different times. One of these gave rise to tourmalinization, silic ification and dolomitization of the sandstone and the deposition of su lfides. The high deltaD-values of the tourmalines, in agreement with p etrological 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, whic h gave rise to the stockwork. This fluid, rich in O-18, was possibly o f metamorphic origin, with a low water/rock ratio. It remobilized the pre-schistosity sulfide's, concentrating them into post-schistosity ve ins. The (partly remobilized?) silica and carbonates were deposited un der temperature conditions such that their isotopic compositions were similar to those observed for the tourmalinized sandstone. The vertica l extent of this hydrothermal system is as yet poorly known, It is ver y extensive horizontally and strongly pervasive in the tourmalinized s andstone, but in the vertical sense appears to be confined to fracture s in the footwall.