MICROSTRUCTURAL EVIDENCE OF CYCLIC HYDRAULIC AND CATACLASTIC BRECCIATION IN A DILATIONAL FAULT ZONE, JERVOIS RANGE REGION, CENTRAL AUSTRALIA

Hydraulic and cataclastic (fault) brecciation are important processes influencing fault zone strength and mineralization. However, outcrop and microstructural studies of single-event breccia zones have yet to establish microstructural criteria distinguishing between these two brecciation processes, which is important in assessing whether both occurred to form one breccia zone.

The Jervois Range region of central Australia has experienced Proterozoic mineralization involving hydrothermal remobilization, making it an ideal area to study the structural setting of brecciation and possibly associated mineralization. NW-SE striking fault breccia zones originated during Neoproterozoic intracontinental basin formation and currently dextrally offset Paleozoic basin strata. Slickenlines plunge gently NW and SE. Oblique quartz dikes adjacent to, and truncating against, the breccia zones with tension gash geometry indicate dextral shear. The fault zones contain hematite-quartz breccia, with no clasts of host rocks observed. The breccia zones are characterized by fine-grained, purple-red (hematite rich) domains, cm- to dm-scale clasts, and cm-scale discontinuous quartz veins. Locally, across gradients perpendicular to the zone boundaries, the percentage of fine-grained material increases as the average clast size decreases.

Representative breccia samples have been used to catalog microstructures indicative of cataclasis or hydraulic brecciation and cross-cutting relationships between them. Microstructural relationships within the clasts include (1) white quartz matrix containing clasts of fine-grained hematite-rich domains and (2) fine-grained hematite-rich matrix containing white quartz clasts. The hematite-rich domains are overall finer grained and contain smaller quartz clasts, a texture similar to fault zone gouge. Discontinuous veins observed in the field are dilational antitaxial quartz veins. These microstructures are tentatively interpreted to represent (1) hydraulic brecciation, (2) cataclastic brecciation, and (3) static crystallization in veins. Since clasts include each of these relationships, at cm- and m-scale, this in addition to field relationships are all interpreted to represent cycles of both brecciation processes in a dilational shear zone.