Structural Constraints and Numerical Simulation of Strain Localization in the Bendigo Goldfield, Victoria, Australia

Abstract

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 S1 cleavage is locally deformed by an E-W–trending S2 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 S1 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.