THERMAL ALTERATION HISTORY OF NEOPROTEROZOIC BASALT XENOLITHS IN THE PATAWARTA AND WITCHELINA DIAPIRS, FLINDERS AND WILLOURAN RANGES, SOUTH AUSTRALIA

Patawarta Diapir is located in the Central Flinders Ranges of South Australia and contains xenoliths of basalt ranging in size from 10’s of meters to a maximum of 800 meters in diameter. The origin of these clasts is thought to be from early Neoproterozoic magmatism related to the rifting event associated with the breakup of Rodina. According to petrographic analysis, thermal alteration of the basalt xenoliths appears to be present, suggesting that the alteration resulted from the basalt being deposited as sills within the depositional evaporite sequence. A regional orogenic event known as the Delamerian Orogeny took place as early as the late Neoproterozoic through the early Ordovician could have caused additional thermal alteration of the basalt xenoliths. The age and chemical composition of the phenocrysts in the basalt xenoliths from Patawarta Diapir will be compared to those documented in the Willouran Ranges approximately 200km to the northwest in the Witchelina Diapir. Petrographical observations indicate the basalt xenoliths in the Witchelina Diapir are less thermally altered and coarser-grained than those in Patawarta Diapir.

Understanding the mineralization associated with basalt xenoliths in Patawarta diapir is significant to the mineral exploration industry because they are synchronous with copper mineralization and deposition. It is proposed that the fluids in the diapir thermally altered and leached copper ions from the basalt and carried them to the adjacent carbonate xenoliths seen in outcrop as malachite and azurite associated with calcite and quartz fractures that cross cut the carbonate stratigraphy.

Basalt and carbonate xenoliths variously termed “chips, rafts, stringers, sutures, and encased minibasins” in the petroleum industry pose serious drilling hazards because of unknown fluid presence (tar) and pressure, disrupt homogenous salt velocity models, and potentially contain large quantities of hydrocarbons. Offshore petroleum exploration in deepwater salt basins is particularly risky due to the current difficulty in imaging clasts within salt diapirs. If we can provide density information and geology models to explain their origin, it may aid in early recognition and detection of the xenoliths.