The physical-chemical properties of Au-bearing hydrothermal fluids from orogenic and intrusion-related deposits based on a record of multi-technique, fluid inclusion analytical data

From our work towards constructing a database on the properties of Au-transporting hydrothermal fluids in the Earth’s Crust, we present preliminary microthermometric determinations, Raman spectra, volume fraction determinations of fluid phases via spindle stage measurements, and Laser Ablation-ICP-Mass Spectrometric analyses for a set of 25 fluid inclusion assemblages. This record has been collected for a number of well-documented orogenic (Sigma and Beaufor, Canada; Wattle Gully, Australia; Berezovskoe, Russia; and Pestarena, Italy) and intrusion-related (Fairbanks district, Alaska; Teremkin and Talatui, Transbaykalia, Russia) Au deposits from various tectonic settings and age. Where possible, data have been collected for assemblages petrographically associated with Au nuggets, in order to better constrain stages of ore fluid entrapment. Data show that the ore fluid of orogenic deposits can be represented by a two- to three-phase (aqueous liquid, carbonic liquid, vapour) H2O-CO2-NaCl model fluid, with a relatively low bulk salinity (< 7.1 eq. wt% NaCl), a φ=13-50 vol% vapour fraction, and a relatively uniform vapour composition (CO2±CH4-N2). A subordinate H2O-NaCl fluid is also present. Th(total) range from values as low as 200 °C (Wattle Gully) to 400 °C (Sigma), and homogenisation occurs by bubble and liquid disappearance. LA-ICP-MS data show that the most abundant analyte in this fluid is Na (4000-28000 μg/g), with subordinate K (300-4500 μg/g) and B (200-2800 μg/g), and minor to trace amounts of (in the most commonly recognised order of decreasing abundance) As, Cu, Sr, Rb, Mg, Mn, Li, Ba, Cs, Sb, and Pb (range: 1-450 μg/g). These trace components are not systematically determined in all the studied deposits or samples. Au has been determined only in few assemblages from the Sigma deposits to be in the 0.5-5 μg/g range. Intrusion-related deposits contain two-phase aqueous (liquid, vapour) fluid inclusions, but multi-phase aqueous inclusions containing 1 or more solids (halite and opaque phases), and an aquo-carbonic fluid, are also common in some deposits, corresponding to a much broader range of bulk salinities (0.4-56 eq. wt% NaCl). The volume fraction of the vapour phase is similar to that of orogenic deposits, and its composition is dominated by CO2 (±CH4). The Th(total) documented in quartz associated with early- and main-stage Au range from about 300 °C to 600 °C, and homogenisation occurs by bubble and liquid disappearance. LA-ICP-MS data show that the most abundant analyte in this fluid is Na (3500-55000 μg/g), with subordinate K (850-9000 μg/g) and B (400-1500 μg/g). With the exception of Fe, Zn, Pb, and Mn, which have a high bulk concentration in the halite-saturated fluid (e.g., 28000 μg/g of Fe), the components As, Cu, Sr, Rb, Mn, Li, Ba, Cs, and Pb have been determined in the liquid-vapour and vapour-rich inclusions to be in the 2-300 μg/g concentration range. All these data show remarkable compositional similarities between the fluid types, which will be discussed in light of possible ore fluid source and evolution