HYDROTHERMAL HISTORY OF THE MARY-KATHLEEN FOLD BELT, MT ISA-BLOCK, QUEENSLAND

In the Mary Kathleen Fold Belt, in the centre of the Proterozoic Mt Is a Block, four phases of deformation-related hydrothermal activity are recognized that occurred over a time range from at least 1750 Ma to 11 00 Ma. Intense metasomatism occurred during all four phases due to the involvement of highly saline fluids, with the apparent NaCl source be ing the evaporitic Corella Formation rocks. In all phases, the scale o ver which the hydrothermal systems operated was greater than or equal to 1 km, with commensurate time integrated fluid fluxes of the order o f 10(4) m(3)/m(2) or more. Phase 1 involved the development of convect ion cells and fault- and shear-zone fluid pathways related to emplacem ent of intrusions in and above a major crustal subhorizontal extension al shear zone. Gold mineralization and some U-REE mobilization occurre d at this time. Intrusive rocks and contact aureoles formed discrete k ilometre-scale bodies within the Corella Formation during Phase 1. As a consequence, fluid flow during Phases 2 and 3 (regional deformation and amphibolite-facies metamorphism) was localized around the boundari es of very competent Phase 1 intrusions and contact metamorphic aureol es. Fluids infiltrating the rocks during this phase were complex NaCl- CaCl2-KCl-H2O-CO2 brines that may have undergone phase separation. Thi s fluid flow culminated in the formation of widespread calcite vein sy stems (with copper) and the Mary Kathleen U-REE vein style orebody, bu t was sufficiently localized by the pre-existing structural heterogene ity that many parts of the Corella Formation did not experience signif icant fluid how at this time. Phase 4 post-metamorphic oxidizing alter ation was localized around major cross-cutting faults. In all phases, the oxygen and carbon in the fluid is dominated by igneous or 'average -crustal' oxygen isotopic values, suggesting that devolatilization of the Corella Formation metacarbonates did not provide the bulk of the f luid. However, if the high fluid salinities were due to evaporite diss olution from the Corella Formation, this requires selective or non-equ ilibrium evaporite dissolution during the early phases of hydrothermal activity. During the later stages, when all the salt was taken up in metamorphic scapolite, the high salinities may have been derived by eq uilibration of retrograde fluid with scapolitic rocks.