Conference program

November 23, 2011
Session 3
10:40 a.m.

Composition of fluid inclusion volatiles: Significance and use in gold exploration in metamorphic environments
Damien Gaboury (UQAC)

Several recent studies have determined the P-T conditions involved in the formation of gold deposits, as well as the source of the fluids and gold, and the conditions of solubility-precipitation. Fluid inclusions provide the only direct evidence of the fluids involved in metallogenesis. We therefore maintain that the gas composition of such fluids should reveal fundamental processes involved in forming ore deposits. A unique apparatus, developed by the author, can be used to analyze the gas composition of families of fluid inclusions by mass spectrometry. The apparatus generates a spectrum of fluid compositions (CH₄, H₂O, N₂, H₂S, SO₂, CO₂, C₂H₆, Ar, H₂, He) as a function of the decrepitation temperature (Td: 100 to 500°C). To test the validity of the approach, samples of quartz veins from Archean (Abitibi) and Paleoproterozoic (Birimian) deposits were analyzed using the method presented in Gaboury et al. (Economic Geology, 2008). Samples from Casa Berardi (n=15) and Joanna (n=13) were taken from mineralization occurring along major faults in a sedimentary context; samples from Beaufor (n=10) were hosted in granodiorite; and those from Mana (Burkina Faso: n=4) and Liberi (Niger: n=4) in deformed volcano-sedimentary rocks.

All the fluids are rich in CO₂ and contain H₂S, in accordance with their respective roles as a pH buffer for maximum gold solubility and as a gold ligand. The barren samples are collectively depleted in CO₂ and H₂S, or have Td temperatures below the gold solubility range of 250-450°C. The mineralized samples collectively show a Td offset for H₂O-CO₂, indicating phase separation, a fundamental mechanism in gold precipitation. Hydrocarbons (CH₄ and C₂H₆) are omnipresent and provide evidence of a contribution from rocks rich in organic matter. Such rocks (shales) are known to be the best source rocks for generating metamorphic fluids and liberating gold from sedimentary pyrites during their conversion to pyrrhotite. Also commonly present was N₂, which is considered a likely indicator of the contribution from shales. Helium, present at Casa Berardi and Joanna, may indicate the presence of a major fault, which is considered an important metallotect. The significance of other gases remains to be determined.

Gas composition thus sheds light on important processes involved in the genesis of orebodies. In exploration, quartz fragments found in tills or at surface in lateritic environments could potentially serve as exploration vectors based on their gas contents.