The Role of Low Salinity Fluids in Metal Enrichment

Abstract:

In recent decades the importance of brines for ore-formation has been demonstrated for a wide range of deposit types and geological settings. Chloride can reach high concentrations in fluids of diverse origins and it complexes many metals in solution. Nevertheless, there are elements which are preferentially complexed by other ligands and are transported and concentrated in low-Cl fluids. Magmatic fluids normally have Cl as the dominant cation and may be very saline. Transition metal levels in magmatic brines from arc settings in particular are often very high, reflecting strong chloride complexing and associated mineralization is often dominated by metals such as Fe which reach high concentrations in brines. Not all magmatic hydrothermal fluids are brines however, and where Cl is less abundant the composition of the dissolved load can be very different. Because of “salting out” effects, highly saline fluids are relatively low in dissolved gases, whereas low salinity fluids can carry significant loads of CO₂, H₂S and other volatile species, including B, As, Sb and Hg. Of the volatile species with the potential to complex specific metals only H₂S has been investigated in any detail. Arsenic is particularly abundant in some low-salinity fluids, however the possible role of As-complexing in transport of Au or other metals is not known. There is little evidence for enhancement of metal solubility by CO₂ under upper crustal conditions, except through lowering of pH, but there is some evidence that CO₂-saturated brines may mobilize Ni more effectively than brines alone. Sulfate is an important anion in a range of magmatic and hydrothermal fluids and may be primary or arise from oxidation of magmatic H₂S in geothermal systems. Notably, sulfate preferentially forms aqueous complexes with the HREE, and may play a role in HREE-enrichment. Fluids in which Cl does not dominate the dissolved load have potential to cause enrichment in a diverse range of elements, but this is most likely to be realised in systems where Ca is relatively low, since it would otherwise remove many of the ligands of interest from solution.