ALBERT

Description: Strain localization and fluid flow patterns adjacent to the massive quartz-hosted reefs of the Bendigo gold-field have been investigated using new structural observations, three-dimensional geometry of fold-fault relationships, and gold distribution. Quartz reef and vein formation in the Bendigo goldfield is associated with an initial E-W compression event that produced a penetrative upright cleavage and N-S–trending fold hinges. In the immediate hanging wall of quartz reefs with high gold grades, the S 1 cleavage is locally deformed by an E-W–trending S 2 cleavage that is attributed to a low-strain N-S–oriented compressive event. Further evidence of this low-strain event is provided by gently S plunging quartz-carbonate slickenline lineations on S 1 cleavage surfaces and the reef margin surfaces and visible gold within the outer portions of the reef. The reefs are commonly situated on or immediately to the south of a left-flex (sinistral) change in strike of the axial surface of the folds, and the flexure occurs close to an anticlinal culmination defined by the double-plunging N-S–striking folds. The folds become deformed with meter to decimeter wavelengths, which generally affect the stiff units, in a disharmonic style. Accompanying this is a reactivation of reef margins by oblique slip and the formation of dilational sites and tensional vein arrays that provide the conduits for fluid flow and gold deposition. Numerical models that best simulate the observed distribution of strain, based upon the location of quartz reefs, involve NW-SE–oriented contraction with a significant sinistral strike-slip component. This contraction event has an overall N-S–sense orientation and is therefore consistent with deformation features in the immediate hanging wall of quartz reefs. The models also demonstrate a geometrical dependence on the localization of strain and, thus, the potential localization of fluid flow. Volume and shear strain increase with the flex angle, and variations in the magnitude of strain are also associated with variations in the strike angle of the flexure. This analysis highlights that the modification of preexisting fold structures and quartz veins by a low-strain event may enhance the localization of dilation sites within individual quartz reefs where there are subtle changes in the orientation of the reef margins. Such numerical modeling can also improve ranking of exploration targets based on fold geometry and provide a better understanding of fluid flow in hydrothermal mineral systems.

Description: Gold mineralization at the Ballarat East deposit, central Victoria, Australia, is hosted in lodes that are historically known as "leather jackets." These are quartz-dominant vein arrays related to low-displacement, W-dipping faults (≤45°) that transect the core and/or eastern limb of tight, asymmetric N-S-trending anticlines. The leather jacket lodes typically have dip extents from 5 to 65 m, widths of ≤20 m, and strike lengths up to hundreds of meters, but their along-strike continuity is disrupted by oblique, low-displacement listric faults known as "cross courses." The gold lodes are characterized by distinct phases of sulfide paragenesis with minor gold + arsenopyrite + pyrite defining the early sulfide stage. Late-stage coarse gold was precipitated with galena + sphalerite ± pyrrhotite ± chalcopyrite (late pyrite also occurs). The gold mineralization events are linked to low-strain mineralized fracture networks, which are closely related to the final deformation stages and the amplification of the major folds enclosing the lodes. This amplification produced domal fold culminations, with plunges ≤30°, and localized minor parasitic folds with shallower plunges (≤10°). A network of dilation sites, on the W-dipping faults, preferentially developed in the cores of anticlines, particularly in zones where there are changes in strike of bedding or fault bifurcation and refraction through contrasting sandstone and interbedded packages of sandstone and shale. Numerical three-dimensional simulations were undertaken to test our geologic observations and replicate conditions controlling the emplacement of the leather jacket lodes. Two different scenarios were investigated: first, to determine how changes in the local stress field orientation influences dilation and fluid infiltration; secondly, to test variations in fault geometry during the last stages of deformation—that is, within the final 2% of shortening, when most of the mineralized sites were created. Results show that strain and fluid flow localized along refracted sections of faults and around changes in dip, specifically on the shallower dipping sections within subvertical sandstone units. This is consistent with the observation that high-grade gold-bearing quartz is associated with localized changes in fault dip in thicker sandstone and sandstone-shale packages. There was also a component of strike-slip motion and near-field NW-to-SE or N-to-S stress fields, which can be attributed to the development of a component of out-of-plane motion during the development of fold culminations. The preferred model for the distribution of the high-grade auriferous vein arrays defining the leather jacket lodes is one of fold amplification and extension parallel to the fold axes, which produced an increasing out-of-plane relaxation. The main fluid conduits responsible for the leather jacket style of mineralization involve infiltration along steep bedding-parallel faults and veins that link up with the arrays of low-displacement W-dipping faults.