REPEATED HYDRAULIC (AND COSEISMIC?) FRACTURING FORMING M-SCALE DILATIONAL STRUCTURES: A PILOT STUDY FROM THE JERVOIS RANGE REGION, CENTRALI AUSTRALIA

The SE Georgina Basin, central Australia, and its basement are highly prospective for shale gas and Cu mineral systems, respectively. The timing and extent of hydrothermal fluid activity, fault formation, and fault reactivation all pose exploration challenges. Prior work in the region suggests the earliest faults formed during ~NE-SW Neoproterozoic intracontinental rifting and were reactivated in the Paleozoic. NW-SE trending, m-scale quartzite dikes and quartz-hematite breccia bodies occur in basement exposures along the south-central basin’s margin. These cross-cut basement structures and are locally unconformably overlain by Neoproterozoic and early Paleozoic basin strata.

This pilot study examined samples from a NW-SE trending breccia body adjacent to a fault set that continues NW along strike underneath the Dulcie syncline, one of several possible exploration targets in the Georgina basin. The breccia zone is unconformably overlain by late Neoproterozoic Georgina strata. Slickensides on the breccia indicate oblique- to strike-slip motion, consistent with map-view offset of the overlying strata along the fault set. Microstructures including clast size, clast shape, clast and matrix material type, and cross-cutting contact type have been documented using a combination of light microscopy, scanning electron microscopy (SEM) and EDS element mapping of representative areas, and hot and cold cathodoluminescence imagery (CL). Microstructures indicating cataclasis are extremely rare. Clasts from adjacent basement rocks are not observed. 150 distinct cross-cutting relationships between 32 different materials were identified (between cm-scale domains and within clasts). The different materials are interpreted as Si ± Fe-rich fluids. The different cross-cutting relationships are interpreted as different episodes of hydraulic brecciation accommodating dilational strain. Vugs in several samples suggest dilation under low confining pressure. Intriguingly, rounded, FeO rimmed clasts are common in all samples and at all scales of microstructural observation. Strong textural similarity with those documented in dolomitic fault breccias, and the above field relationships, lead to the question: Are these textures evidence of co-seismic dilation related to Neoproterozoic extension?