Orogenic gold mineral systems of the Western Lachlan Orogen (Victoria) and the Hodgkinson Province (Queensland): Crustal metal sources and cryptic zones of regional fluid flow

• Crustal metamorphic devolatilisation is accepted for both orogenic gold provinces.
• Gold endowment of each province was limited by volume of hydrous sulphidic rocks.
• Linear metallogenic zones were controlled by cryptic deep crustal domain boundaries.
• The boundaries focused near-vertical flow of mineralising fluids.
• Major faults often controlled fluid flow indirectly and at a more local scale.

Orogenic gold mineral systems in the Western Lachlan Orogen (Victoria) and the Hodgkinson Province (Queensland) produced gold provinces characterised by vastly different scales of gold endowment and strongly uneven distribution of gold mineralisation within each province. The volume of hydrous pyrite-bearing rocks undergoing metamorphic devolatilisation during thermo-tectonic events driving orogenic gold mineral systems represents a fundamental first-order constraint on the total gold endowment and its broad spatial distribution, both between and within the provinces. Most of the largest gold deposits in both regions occur in linear, richly-endowed metallogenic zones, oblique to the dominant regional structures and related to deep crustal domain boundaries. These boundaries, with only subtle surface expressions, were the major regional structural controls which promoted focused near-vertical flow of mineralising metamorphic fluids above the outer margins of cratonic blocks in the lower crust. Recognised major faults represented only more local scale and often indirect controls on the focused fluid flow, particularly effective above the deep cratonic block boundaries overlain by relatively thick crustal source rocks.