THE SPECTRUM OF ORE-FORMING FLUIDS IN THE VMS ENVIRONMENT: MODERN AND ANCIENT

A comparison of modern black smoker fluids with published fluid inclusion data from ancient VMS reveals many similarities but also some important differences. The majority of modern black smokers discharge fluids of modified seawater composition, with in situ pH of 3.5-5.5, salinities of <1-8 eq. wt% NaCl (av. 3.5), mCO₂ <5-285 mmol (av. 40), mH₂S <0.1-41 mmol (av. 7.3), mH₂ <0.1-1.8 mmol (av. 0.1-0.2), and total metal concentrations of <10-1300 ppm (av. 290) including <1-10 ppm Cu (av. 1.6). Major element concentrations are controlled by high-temperature seawater-rock reactions and vary systematically with volcanic and tectonic setting. However, variations in pH, salinity (1/10 to 2x seawater), and gas concentrations mainly reflect boiling, supercritical phase separation, and magmatic contributions. A similar spectrum of fluids has been inferred for ancient VMS from ore mineralogy, alteration, stable isotopes, and fluid inclusions. In situ pH, H₂S, and H₂ concentrations were similar to modern vents, based on the dominant PPM buffer assemblage. However, bulk fluid compositions deviated significantly from contemporaneous seawater. Fluid inclusion salinities range from 0.5 to 38 eq. wt% NaCl (typically 5-10 wt%), and CO₂ concentrations are 50-1000 mmol, with some examples of liquid CO₂-bearing inclusions. Coexisting low-salinity and high-salinity fluids are common, similar to Cl-depleted and Cl-enriched seafloor vents, but there is also a high proportion of high-salinity (>10 eq. wt% NaCl) and hypersaline (>30 wt% NaCl) inclusions. These compositions have not been sampled at modern black smokers. This may indicate that low-salinity fluids are preferentially vented to the seafloor, while "brines" are trapped within deposits and in subseafloor stockworks. Hypersaline liquids resulting from phase separation at depth may be physically separated from seawater-dominated fluids and released only intermittently from a "brine reservoir". CO₂-rich hypersaline liquids may best represent undiluted magmatic brines periodically exsolved from deep subvolcanic plutons. These fluids may explain some unusual deep semiconformable alteration (e.g., Cl-rich amphiboles) and some anomalous Cu-rich ores in both modern and ancient deposits.