Genetic and exploration significance of the zinc ratio (100 Zn/(Zn + Pb)) in massive sulfide systems

Abstract

Comparison of the 100 Zn/(Zn + Pb) ratio (zinc ratio) of various styles of mineralization in the Mount Read Volcanics of Tasmania indicates that volcanogenic massive sulfide deposits have a restricted range of mean values (60-77) and low standard deviations (less than 15) whereas other mineralization styles have a broader but lower range of mean values (39-61) and higher standard deviations (greater than 26).Thermochemical calculations on lead and zinc transport in saturated ore solutions show that the solution zinc ratio is controlled only by temperature and salinity and is independent of pH, f (sub O 2 ) , and activity of dissolved sulfur. Zinc ratios calculated for saturated ore solutions at conditions typical of the deposition of sphalerite and galena in massive sulfide deposits (250 degrees -150 degrees C and 0.5-1.5 m NaCl) are very similar to those observed in polymetallic massive sulfide deposits, in particular those of the Rosebery, Hercules, and Hellyer deposits in western Tasmania. This similarity, along with the shape of the Pb vs. Zn plots, suggests that zinc and lead (as chloride complexes) were saturated in the metal-deficient (m (sub Sigma metal) < m (sub H 2 S) ) hydrothermal fluid that formed the Rosebery and Hellyer ores.Variations in the zinc ratio between individual massive sulfide deposits in the Mount Read Volcanics indicate that lead-zinc saturation of the ore solution rather than lead-zinc content in the footwall volcanic source rocks was the major control on the zinc ratio of the deposits. However, this is probably not the case for the Pb-poor, Cu-Zn-type massive sulfides (e.g., Cyprus deposits, Canadian Archean deposits, and present-day black smokers), where low concentrations of lead in the basalt-dominated source rocks result in complete undersaturation of lead in the hydrothermal solutions. The low mean values and high standard deviations of Cambrian vein deposits in the Mount Read Volcanics, and in Ordovician and Devonian deposits of the Dundas trough, may be explained by a lack of zinc-lead saturation in the relevant hydrothermal fluids.The characteristic zinc ratio of volcanogenic massive sulfide-style mineralization can be used to distinguish these deposits from other mineralization types in the Mount Read Volcanics. Effective use of this simple technique requires a reasonable number of anomalous samples (minimum = 15) and an unoxidized sample medium (drill core samples are preferable). In combination with geology and lead isotope studies, the zinc ratio provides an excellent early screening of prospects.