Metal Transport and Deposition in Hydrothermal Veins Revealed by 213nm UV Laser Ablation Microanalysis of Single Fluid Inclusions

Abstract

213nm UV laser ablation provides a powerful and cost-effective new method for the chemical microanalysis of individual fluid inclusions. The instrument offers significantly improved energy absorption in many geological materials compared to 266nm UV lasers resulting in improved ablation characteristics. The laser is not strongly absorbed by either air or commonly used optical material, permitting relatively easy delivery and control of the beam. Ablation of multielement standard solutions in pure quartz glass capillaries was found to be an effective method for external calibration of instrument sensitivity. Internal calibration for calculation of absolute element concentrations in inclusion fluids was carried out using chloride concentration estimated from microthermometric data. Using this approach, analyses of groups of cogenetic primary and secondary inclusions using a 30 element analyte menu gave typical relative standard deviations for absolute element concentrations of around 20 percent for alkali and alkali earth elements and 20 to 50 percent for metals, comparable with those obtained by 193nm Excimer laser ablation. Results for Ca are consistent with independent estimates made from microthermometric measurements and a range of other elements are in reasonable agreement with previously published bulk chemical analyses. Overall, the data give good charge balance, within 5 percent. The technique was used to test the hypothesis that the metal content of hydrothermal ore deposits is primarily controlled by the metal content of the mineralizing solutions rather than selective precipitation mechanisms. We studied Pb(-Zn) mineralized ‘crosscourse’ veins from Southwest England that are known to have formed from low temperature brines expelled from Permo-Triassic red-bed basins adjacent to the Cornubian peninsula, probably during the Triassic. These fluids are comparable with the solutions that formed Mississippi Valley-type Pb-Zn deposits. Analyses of primary fluid inclusions across growth-zoned quartz crystals from one of these veins indicate the orefluids at an early stage of vein growth are relatively Pb-rich (up to 295 ppm Pb, up to 86 ppm Zn; Pb/Zn = 1.3-16.1) and display marked temporal variations in chemistry. Gradual increases in K, Ca, Sr, Ba and Zn are observed across a significant (∼10 mm) interval of quartz growth. These increases are interpreted to reflect mineral dissolution-precipitation reactions occurring within the fracture system exploited by the fluids after being expelled from the basin source area. Superimposed on this trend are comparatively short time-scale (∼1 mm of quartz growth) chemical excursions characterized by decreases in the concentrations of Cl, K, Ca, Ba, Sr, Pb and Zn of up to two orders of magnitude. These excursions are believed to be due to local, volumetrically significant, mineral precipitation events and indicate the operation of processes that result in the simultaneous supersaturation of a number of phases. It is suggested that initial galena precipitation (>98% Pb loss from solution) was due to reduction as oxidized fluids encountered the first strongly reducing lithology on their flow path out of the basin source region. Several later dilution events were caused by fluid mixing and are inferred to have caused the co-precipitation of sulfides (galena, sphalerite; up to 98% loss of metals from solution) with sulfates (anhydrite, barite; up to 91% Ca and 97% Ba loss from solution) and/or silicates (K-mica; up to 96% loss of K from solution). However, at only one stage is galena (±anhydrite, K-mica) observed in the sample studied suggesting that fluid inclusions may commonly trap depleted fluids after mineral precipitation has already occurred some distance upstream, rather than reflecting precipitation events in their immediate vicinity. The primary ore-forming fluid in this case study contained elevated Pb and Zn and, at least initially, displayed a higher Pb/Zn ratio than predicted for fluids saturated with respect to both sphalerite and galena. Therefore, we suggest that the Pb-rich tenor of the vein deposit is primarily controlled by the metal content of the ore fluids, which is ultimately a function of the geochemistry of the basin source. In this example, the source area is dominated by oxidized terrestrial clastic rocks. Reactions within the aquifer and selective precipitation mechanisms at the site of ore formation also appear to have modified fluid geochemistry but played a second order role in controlling the metal budget of the mineral deposit formed.