The auriferous veins at Yirisen, Masumbiri, Sierra Leone, occurring mainly in the form of sericitic quartz-sulphide lodes and stringers, are hosted in metamorphosed volcano-sedimentary assemblages invaded by at least two generations of granitic intrusions. Detailed microthermometric studies of fluid inclusions from the veins coupled with laser Raman spectroscopic analysis show that the inclusions contain aqueous fluids of variable salinity (5 to 60 wt.% NaCl equivalent) and dense carbonic fluids (pure CO2: 1.08 > d > 0.88 g/cm(3)). Optical observations and analysis on opened inclusions by scanning electron microscopy (SEM) reveal that some of the aqueous inclusions contain a number of daughter minerals: halite, sylvite, Ca-, Fe-, Mg- and possibly Li-bearing chlorides, and anhydrite; nahcolite occurs also in some of the CO2 inclusions. The SEM runs also detected a small amount of electrum, suggesting that silver might be a bi-product of the mineralisation. The aqueous and carbonic fluids remained immiscible throughout the formation and evolution of the hydrothermal veins. A few mixed (H2O + CO2) inclusions apparently resulted from accidental trapping of both fluids in the same cavity. The wide range of salinities observed in the aqueous inclusions is attributed to the mixing of relatively hot, low-salinity aqueous fluids and colder, high-salinity brines. The CO2-rich and low-salinity H2O inclusions are considered to be derived from the metamorphic decarbonation/dehydration of the greenstone pile whilst the high-salinity brines are believed to be basinal in origin, pressure-temperature (P-T) conditions of entrapment, inferred from the intersection of representative isochores of the immiscible fluids, indicate that the formation of the veins started at T = 400 degrees C and P about 3 kbar, in the presence of the high-density CO2 and low-salinity H2O fluids. At about 200 degrees C, pressure fluctuations (incremental opening of the vein) correspond to the trapping of the lower-density CO2 inclusions and high-salinity brines. It is proposed that the decarbonation/dehydration processes (possibly aided by later magmatic processes) expelled and mobilised the gold from the greenstone pile and concentrated it in the COl(2)-bearing hydrothermal fluid in the form of Au-chloride complexes. High thermal gradients are believed to have caused the upward migration of this fluid from the bottom of the greenstone pile through structurally controlled conduits. We contend that phase separation of the H2O-CO2 metamorphic fluid, aided possibly by some wall-rock alteration, most probably triggered a decrease in ligand activity and thus, precipitation of the gold into lodes. Percolation of the basinal brines is thought to have remobilised some of the gold together with some silver. (C) 1999 Elsevier Science B.V. All rights reserved.
