An experimental method for gold partitioning between two immiscible fluids: Brine and n-dodecane

Abstract

Organic matter can be found in many different types of ore deposits, but its role in ore-forming processes is not yet fully understood. Here, we present an experimental method that can be used to determine the partition coefficient (DAuorg/aq: partition coefficient of Au between an hydrocarbon and an aqueous fluid) of gold between two immiscible liquids, and thus whether liquid hydrocarbon fluids such as petroleum can act as ore fluids and transport gold or other metals of interest. To investigate liquid hydrocarbons in the presence of an aqueous liquid doped with gold, we modified the HFS-340Z hydrothermal flow system (Coretest Systems, Inc.) to enable sampling at hydrothermal P-T conditions (=150 °C and ~5 bar) of each of two density-stratified immiscible liquids. A saline aqueous solution (10 wt% NaCl) was doped with gold and heated with n-dodecane (CH3(CH2)10CH3) to 105 °C and 150 °C. Each brine sample was directly followed by an organic sample until three samples of each liquid were taken. Aqua regia was added to the brine samples to stabilize the gold before ICP-MS analyses. Each organic sample was digested chemically with a mixture of ultra-pure nitric acid and hydrogen peroxide to generate carbon free solutions prior to ICP-MS analysis. This procedure generates reproducible partition coefficients for gold, or presumably any other metal, between hydrocarbon and aqueous liquids, if passivation procedures of the HFS-340Z hydrothermal flow system are strictly followed and error sources are monitored rigorously. The preferred DAuorg/aq between n-dodecane and the brine is 0.05 ± 0.04.