District-scale targeting for gold in the Yilgarn Craton: Part 2 of the Yilgarn Gold Exploration Targeting Atlas

Part 2 of the Yilgarn Gold Exploration Targeting Atlas evaluates 22 targeting criteria that are considered useful for targeting prospects for more-detailed exploration within a project area. As in Part 1, the emphasis is on a conceptual approach, targeting mineralization that is either not exposed at the surface or is exposed as a 'distal footprint' that might provide a vector to buried 'proximal' mineralization. Gold in the Yilgarn Craton is found in a diverse range of host rocks, so host-rock lithology is not an efficient targeting criterion. Virtually all gold mineralization is controlled by proximity to lithological contacts, particularly contacts between units of strongly contrasting rheological properties. Targeting 'rheological contacts' within a specific range of orientations increases discovery efficiency, but the optimum contact orientations vary from district to district and may differ for small deposits versus large deposits. Analyses presented here for the Wiluna, Laverton, and Kalgoorlie – Ora Banda districts of the Eastern Goldfields Superterrane show a strong association between gold mineralization and Mafic Group intrusions. Proximity to district-scale faults is an effective targeting criterion, but is generally not as efficient as targeting rheological contacts. Fault intersection density, and proximity to fault jogs and faults bends, provide effective district-scale targeting tools. Although fault density is generally an effective targeting criterion, 50% or more of the gold endowment in some districts is within domains identified by the software as having zero fault density. These deposits are in fact controlled by faults or fractures that are too small to appear at the scale of the shape files used in the analyses. There is little advantage in targeting right-stepping over left-stepping jogs, or clockwise over anticlockwise fault bends. Fold axes are a powerful targeting criterion. Targeting anticlinal fold axes is two to three times more effective than targeting undivided fold axes. In high-temperature metamorphic settings such as the Southern Cross district, boudinage at all scales represents an important (perhaps the most important) targeting element for gold. Subjected to regional deformation, folds and boudins create stress heterogeneity, and computer-based techniques for predicting variations in minimum stress (paleostress modelling and stress transfer modelling) are other useful approaches for district-scale gold exploration in all metamorphic settings. District-scale maps of maximum downhole gold concentrations highlight the flow paths of gold-bearing ore fluids and provide a striking validation of published models relating fluid flow to heterogeneous stress distribution. Arsenic distribution is partially decoupled from gold, and anomalous arsenic may be partly controlled by proximity to thick sequences of metasedimentary (including felsic volcaniclastic) rocks. In regional shear zones, arsenic footprints are more extensive along strike than gold anomalies. Pathfinder element suites can be used to determine the extent of oxidized/alkaline (W+Mo+Bi) and reduced/acid (As+Sb) hydrothermal activity. The oxidized suite has potential for location of buried or poorly exposed intrusions. Oxidized and reduced hydrothermal cells can also be distinguished on the basis of white-mica species by using multispectral and hyperspectral mineral data or chemical parameters that identify albite-rich versus muscovite-rich hydrothermal alteration. However, the more useful spectral parameters for targeting gold mineralization are white-mica intensity and Fe-chlorite abundance. A chalcophile index (e.g. W+Mo+Bi+As+Sb) or the rare alkali index ([Rb+Cs]/Th)N are as effective or more so than using juxtaposed or overlapping oxidized and reduced pathfinder element suites. Geological complexity (expressed as fractal dimension) was not shown to be effective for targeting gold mineralization at district scale.