Basin-Scale Numerical Modeling to Test the Role of Buoyancy-Driven Fluid Flow and Heat Transfer in the Formation of Stratiform Zn-Pb-Ag Deposits in the Northern Mount Isa Basin

Abstract

Numerical fluid-flow and heat-transport modeling was undertaken on a well-constrained geologic section through the northern Mount Isa basin in order to test the possibility of forming stratiform Zn-Pb-Ag deposits by buoyancy-driven free convection of marine fluids. The major two-dimensional geologic section used for the modeling was based on recent geologic mapping and sequence stratigraphic interpretation, combined with geophysical interpretation from regional seismic, aeromagnetic, and gravity data sets. The basin fill, termed the Leichhardt, Calvert, and Isa superbasins, forms a south-dipping wedge of sedimentary and minor volcanic rocks up to 25 km thick. A number of major subvertical synsedimentary normal faults cut through this fill and are rooted in the basement. Two potential aquifer sequences have been identified: sandstones and volcanic rocks of the Big supersequence at the base of the Calvert superbasin, and sandstones and conglomerates comprising the Mount Guide Quartzite at the base of the older Leichhardt superbasin. There are three stratiform Zn-Pb-Ag deposits in the younger Isa superbasin, close to the section-line selected for modeling. Century is a world-class and high-grade stratiform zinc deposit which contains over 14 Mt of Zn metal hosted by 1595 Ma black shales and siltstones. Pb-Pb isotope studies suggest the deposit formed about 1575 Ma. Walford Creek and Blue Bush are large but low-grade, pyrite-rich, and zinc-poor stratiform deposits hosted in 1640 Ma carbonaceous shales and siltstones. Numerical modeling of fluid flow at about 1575 Ma shows that buoyancy-driven convection is controlled by the relationship between the faults and aquifers. The synsedimentary faults, given high permeabilities in the model because they were active during the mineralizing event, act as either recharge or discharge zones for fluid flow. Marine fluids commonly recharge the basin via the minor faults and flow through the sandstone and volcaniclastic aquifer sequences at depths of 5 to 10 km. These fluids have the potential to leach zinc and lead from the clastic material comprising aquifer and adjacent volcanic rock layers. The heated metalliferous fluids discharge to the surface where the aquifers intersect the major faults. Hydrothermal discharge temperatures from the Termite Range fault were computed to be in the range 100 degrees to 180 degrees, with fluid velocities of 1 to 8 m per year. These conditions are suitable for the formation of a Century-sized Zn deposit at the discharge point adjacent to the Termite Range fault over a period of 0.65 m.y., provided a suitable chemical trap environment is present. Several numerical simulations were run with different aquifer and fault properties designed to understand the hydrological constraints for the formation of major Zn deposits. Aquifer permeability, fault permeability, and fault penetration depth were shown to be the major factors controlling the fluid-flow and temperature regime. Significantly higher discharge fluid temperatures and velocities were obtained when the depth of penetration of the Termite Range fault was increased from 15 to 30 km. The results of the numerical modeling are significant in understanding geologic and hydrological controls on the formation and location of major stratiform Zn deposits. The permeability and thickness of potential aquifers, their depth in the basin, the presence of stacked aquifer sequences, and the relationship between aquifers and synsedimentary faults all have important exploration implications. In particular, the Termite Range, Fish River, and Elizabeth Creek faults are interpreted to be major deep faults that have controlled basin-wide convective fluid flow, and therefore the location of major base metal deposits.