Hydrocarbons as ore fluids: An experimental study of the solubility of metals in crude oil and its analogues

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In several environments of ore-formation, there are close spatial associations between hydrocarbons and metallic mineralisation. For all of these environments, the conventional wisdom is that the ores form by deposition from aqueous fluids, generally hydrothermal liquids. Where a role is attributed to the hydrocarbons, it is to promote metallic mineral deposition by reducing the oxidation state of the fluid. This is interpreted to cause precipitation of the ore minerals by destabilising the corresponding aqueous metallic complexes and/or reducing dissolved sulphate to sulphide (the ore minerals are dominantly sulphides). An alternative explanation for the association, which few researchers have considered, is that the hydrocarbons represent the relicts of liquid petroleum that transported the metals to the sites of ore formation. According to this hypothesis, the ore metals precipitate during subsequent distillation of the petroleum, which drives off the light hydrocarbons, leaving behind a residue of bitumen. The distillation greatly increases the concentration of the metals causing them to saturate and precipitate as sulphide minerals by reacting with reduced sulphur present in the oil. This model of ore-formation provides an attractive explanation for the concentration in organic-bearing (bitumen) sedimentary rocks of metals like Ni, Pt, Pd and Hg, which are known to have a strong affinity for hydrocarbons and do not form strong aqueous complexes. The hypothesis may also apply to some deposits of metals like Zn (e.g., Mississippi Valley Type deposits), which although they are readily transported in hydrothermal fluids, are also commonly present in high concentrations in crude oils. The research proposed here will start addressing the question of whether liquid hydrocarbons can form ore fluids, by conducting experiments designed to determine the solubility of selected metals in crude oils and simpler hydrocarbon liquids, by investigating their speciation in these liquids, and by evaluating their partitioning between hydrocarbon liquids and aqueous solutions (e.g., brines) at temperatures up to 300 °C.