THE IMPORTANCE OF SALINITY IN CONTROLLING ORE-FORMING FLUID MIGRATION IN SEDEX ORE SYSTEMS

It has long been recognized that high salinity (evaporitic) conditions of sedimentation are a critical factor in the development of basinal brines that form stratiform Zn-Pb-Ag (or Sedex) ore deposits. However the relationship between the evaporitic conditions and the ore-forming processes have not been fully understood. Recent numerical experiments in the McArthur Basin of northern Australia have greatly increased our knowledge of the ore-forming fluid migration in that area. In these previous studies, the buoyancy force is assumed the only driving force for fluid migration. It is well known that buoyancy force is generated from the fluid density variation due to the fluctuations in both temperature and salinity distribution; however, the previous models have only considered the thermal effect and ignored the contribution from salinity variation.

In this study, we develop a finite element algorithm to fully couple transient subsurface fluid, heat and solute transport in multi-dimensional and discretely fractured porous media. This computer package is employed to conduct numerical computation of saline fluid flow in the McArthur Basin related to the formation of stratiform Zn-Pb mineralization. We simulate hydrothermal fluid migration and solute transport, and examine the factors potentially controlling the ore-forming processes. A variety of scenarios are investigated, including those with zero salinity on the top (meteoric surface water) and high salinity on the bottom (sedimentary brine), high salinity on the top (evaporitic conditions) and uniform initial salinity of seawater throughout the domain, and other complex patterns. Our results indicate that the salinity distribution has an important impact on the hydrothermal ore-forming fluid migration, and may promote or impede fluid circulation depending on the boundary and initial conditions of the salinity distribution. These results provide more realistic models to compare and contrast the genetic theories for stratiform Zn-Pb-Ag deposits in sedimentary basins.