The aqueous geochemistry of the rare-earth elements and yttrium 4. Monazite solubility and REE mobility in exhalative massive sulfide-depositing environments

Although there is considerable evidence that rare-earth elements (REE), and particularly the light REE (LREE), have been mobile in the alteration zones below many massive sulfide deposits, there is some disagreement over the cause of this mobility. Most researchers have argued or implied that the REE were mobilized by analogues of modern seafloor hydrothermal vent fluids. However, the few measurements that have been made of the REE concentrations of these modern fluids suggest that their ability to mobilize REE is negligible. In order to shed further light on this problem we have carried out calculations of monazite solubility (monazite is thought to be the principal host for the LREE) in a model vent fluid at temperatures from 200° to 300°C. These calculations show that at 300°C the model fluid will contain 0.08 ppb Ce, 0.07 ppb La and 0.03 ppb Nd. At 200°C the concentrations increase to 0.54 ppb, 0.37 ppb and 0.15 ppb, respectively. A decrease in pH of one unit at 300°C also increases REE solubility significantly (7.4 ppb Ce, 5.7 ppb La and 1.1 ppb Nd). These solubilities are similar to the concentrations of REE measured in seafloor hydrothermal vent fluids. However, they are considerably lower than required to account for the scale of REE mobility in alteration zones associated with massive sulfide deposits. If the mass ratio of typical hydrothermal vent fluid to rock reached 1000, concentrations of REE in the altered rock the predicted to be in the range < 1 ppm to several ppm, whereas REE depletions up to several tens of ppm and enrichments in excess of 1000 ppm have been reported. Our calculations thus suggest that ancient analogues of modern seafloor hydrothermal vent fluids could not have transported REE in the quantities required to explain the extent of REE mobility documented for alteration zones associated with massive sulfide deposits. This finding requires that either ancient massive sulfide-forming hydrothermal systems were different in some critical respect from modern seafloor hydrothermal systems or that REE enrichment/depletion occurred during later metamorphism or alteration of massive sulfide deposits.