Nicholas Mortimer, Dave Craw, Doug MacKenzie, Jim Mortensen, Uwe Ring, Iain Pitcairn

This paper provides a regional-scale background for understanding gold-mineralising processes in the Otago Schist during the Cretaceous. At this time the schist belt was in the latter stages of formation as an accretionary complex with 2000 km strike length on the Pacific margin of Gondwana. The Otago Schist is interpreted as an exhumed accretionary wedge of structurally stacked clastic metasedimentary rocks with minor metabasic rocks. Metamorphic grade reached upper greenschist facies. Gold and other related elements were mobilised from the metasedimentary rocks during metamorphism, and these elements contributed to the high levels of orogenic gold endowment (>18 million ounces in the schist belt. Mesozoic Gold deposits were emplaced in two distinct pulses, one at the beginning of the Early Cretaceous (~140-135 Ma) and the other at the end of the Early Cretaceous (~112-100 Ma). These mineralising pulses were driven by regional tectonic events that may have involved episodic underplating of subducting material, and/or subduction of spreading ridges. The earlier event was most closely associated with metamorphic processes that mobilised gold from the accreted metasediments during convergent tectonics, and appears to have been the more economically significant. The later event took place as accretionary processes ceased and the Zealandia portion of Gondwana began to undergo intracontinental rifting.