A Review of Fluid Inclusion Temperatures and Salinities in Mississippi Valley-type Zn-Pb Deposits: Identifying Thresholds for Metal Transport

Abstract

More than 2800 published fluid inclusion data (primary inclusions) from eighteen typical Mississippi Valley-type (MVT) Zn-Pb deposits and districts were examined to study possible relationships between MVT fluid properties and metal tonnages and ore grades. The mean Th and salinity values for fifteen MVT deposits and districts are 122°C ± 21°C and 20.7 ± 2.6 wt% CaCl 2 equivalent, respectively. In general, temperatures increase from pre-ore to ore stages, and salinities decrease from ore to post-ore stages. From post-ore to late calcite stages, salinity and temperature decrease with various trends within specific districts. Several conclusions are drawn: first, results show that Hanor’s (1996) salinity threshold for metal-rich basinal fluids applies to typical MVT ore fluids (~16 wt% CaCl 2 equivalent). Second, the lower limit of mean Th values of measured MVT deposits and districts (n = 15) of ~75°C is similar to Hanor’s (1996) observed temperature threshold for metal-rich basinal brines of ~60°C (n = 224). Third, there is no statistically significant relationship between fluid inclusion homogenization temperatures (Th) and metal tonnages or ore grades. Fourth, there seems to be a tendency for large tonnage districts/deposits to be associated with mean fluid salinities that are relatively low (~16 to 21 wt% CaCl 2 equivalent) within the overall range of salinity means (~16 to 26 wt% CaCl 2 equivalent). There is also a correlation between higher ore grades and higher salinities. Finally, comparison with other sedimentary rock-hosted Zn-Pb deposit types shows a trend of salinity decrease and temperature increase from MVT, sandstone-hosted and diapir-related Pb-Zn deposits, to vein-type and Irish-type Pb-Zn deposits, and then to sedex deposits. However, more data are needed for sedex deposits. The salinity threshold for significant Zn-Pb transport decreases to ≥~10 wt% CaCl 2 equivalent at ~250°C. This study suggests that fluid inclusion analysis during exploration may be useful in assessing ore-forming potential (threshold salinity) and possible ore grades (salinity).