Ore shoots in folded and fractured rocks – Insights from 3D modelling of the Fosterville gold deposit (Victoria, Australia)

Abstract

At the Fosterville gold deposit, high concentrations of visible and refractory gold are hosted in arrays of quartz and quartz-carbonate veins. They are distributed in elongate zones to form south plunging ore shoots within a folded and fractured turbidite sequence. Here we distinguish the influence of faults versus folds at Fosterville, by assessing the deposit-scale structural framework and hydrothermal fluid flow using a combination of 3D implicit modelling, structural analysis and 3D fault slip/dilation tendency analyses. Gold assay and bedding trend data, from a large drillhole dataset, were spatially interpolated and used to analyse and measure the orientation of ore bodies and folded host rocks. Field measurements of fold axis orientations were compared to bedding measurements in oriented drillcore, Bingham axial analysis of bedding trends, and orientation data from a 3D implicit model. Both field and drill core structural data show that ore shoots at Fosterville do not parallel the first order, gently (≤10°) south-plunging folds. Instead the orientation of the ore shoots is coincident with an intersection lineation between steeply west-dipping bedding-parallel faults and moderately dipping linkage faults that obliquely cut across the main structural grain, resulting in ore shoot plunges larger than 20°. Slip and dilation tendencies suggest that linkage faults had a higher likelihood of activity in the inferred far-field stress regimes at the time of mineralisation, relative to other structures at Fosterville. These linkage faults are therefore interpreted to have been preferential fluid flux conduits in the system despite appearing to be subordinate structures at the scale of the deposit. This new structural model for Fosterville highlights that fluid flow and ore shoot formation are strongly controlled by obliquely oriented structures that are unlike the main structural grain of first order folds and faults.