FLUID INCLUSION CL-BR-NA RATIOS IMPLICATE RESIDUAL EVAPORATIVE BRINES IN THE FORMATION OF SUPERGIANT SEDEX ZINC DEPOSITS

The supposition that sedex (“sedimentary-exhalative”) deposits formed from saline basinal brines derived from their host basin was a critical advance in understanding sedex deposit genesis. While direct study of sedex ore fluids has been limited, a few robust fluid inclusion studies have verified that sedex deposit form from 100 to 200° C brines with salinities between 10 - 30 wt %TDS. Despite the importance to sedex genesis, the source(s) of dissolved salt (and water) in ore-forming brines is not well understood.

New fluid inclusion solute analyses of six sedex deposits (four categorized as “super giants”) by a one-of-a-kind dual ion chromatography (IC) system constrain the primary salinity source of ore-forming brines to evaporatively concentrated seawater. Particularly diagnostic are Cl/Br and Na/Br ratios that overlap values of residual brines evolved during seawater evaporation and salt deposition. Cl/Br molar ratios are below the value for modern seawater of 660 and range down to 220, indicating brines reached halite saturation. Moreover, the Cl/Br and Na/Br data plot in the center of a relatively small compositional field previously established for MVT brines; confirming previous hypotheses that both deposit types formed from fluids of similar origins. Neither deposit type provides evidence for halite–dissolution as a primary source for the parent fluid, therefore residual brines are considered fundamental to the genesis of basin-hosted Zn-Pb-Ag deposits.

These interpretations are supported by high-resolution stratigraphic, biostratigraphy and geochronology analysis of the sedex-hosting basins that reveal mineralizing episodes corresponding with discrete periods of intense evaporation of seawater on the basin platform margin. Moreover previous paleolatitude reconstructions have shown that sedex deposits formed at latitudes conducive to high evaporation rates.

This new fluid inclusion solute data forms a cornerstone of a new USGS model for sedex deposits. In combination with numerical fluid flow modeling and detailed geologic analysis of sedex basins, this new data suggest that the evaporation of seawater on the margin of sedex basins is critical to ore formation. Thus, evidence for seawater evaporation is at the top of a hierarchy of quantifiable geologic criteria for sedex assessment.