Analysis of a Telescoped Orogenic Gold System: Insights from the Fosterville Deposit

The Fosterville gold (Au) deposit is hosted in the Bendigo zone within the western Lachlan orogen, southeast Australia, and contains three distinct mineralization styles: (1) refractory Au in fine-grained arsenopyrite and arsenian pyrite disseminated throughout metasedimentary rocks near brittle faults, (2) visible Au hosted in fault-controlled quartz-carbonate veins associated with stibnite mineralization, and (3) vein-hosted visible Au with little or no associated stibnite. Refractory Au mineralization is found throughout the deposit, whereas visible Au ± stibnite occurs deeper in the system (>800-m depth from surface). Thus, Fosterville provides a unique opportunity to study a telescoped orogenic Au system that changes mineralization style as a function of depth. Microscopy, neutron tomography, nanoscale secondary ion mass spectrometry, and field observations have been conducted to investigate mineralogical and structural controls on the various styles of Au mineralization. These observations are used as the foundation for reactive mass transport geochemical modeling using HCh software. Results are considered in the context of an evolving mineral system over the formation history of the deposit, and relative timing of mineralization is inferred. Two alternatives for the genesis of such a system include the following: (1) metal deposition was controlled by ongoing physicochemical changes at a very shallow level in the crust in one evolving mineralization stage or (2) two or three deposits formed in the same location, with each different style of mineralization representing a separate period of fluid infiltration, each potentially tens of millions of years apart. Based on careful observations, microanalysis, and thermodynamic modeling, we suggest that the latter is more likely. Therefore, we suggest that Fosterville is to be recognized as a telescoped orogenic Au system, where relatively high temperature mineralization and alteration assemblages were overprinted vertically by later, lower-temperature assemblages.