A magmatic-hydrothermal transition in Arkaroola (northern Flinders Ranges, South Australia): from diopside–titanite pegmatites to hematite–quartz growth

A set of Palaeozoic diopside-titanite veins are present in Mesoproterozoic metagranites and metasediments that constitute the basement (Mt Painter Inlier) of the Adelaide Fold Belt (South Australia). These massive veins (up to 1 m) of pegmatitic nature contain large crystals of diopside, LREE-Y-enriched titanite (up to 40 cm in length) and minor amounts of quartz. They can be used to trace the system's development from a high-temperature magmatic stage through to a massive hydrothermal event. The pegmatitic origin of these veins is evident from a complex fluid-melt inclusion assemblage, consisting of a highly saline inhomogeneous fluid and relicts of melt. Immiscibility of melt and heterogeneous highly saline fluids (exceeding 61 eq. mass% NaCl) is preserved in primary inclusions in diopside and secondary inclusions in titanite, indicating relatively shallow conditions of formation (510 +/- 20 degrees C and 130 +/- 10 MPa). Graphic intergrowth of diopside and albite occurs at the contact with granitic pegmatites. The system evolved into hydrothermal conditions, which can be deduced from a later population of only fluid inclusions (homogeneous and less saline, ~ 40 eq. mass% NaCl), trapped around 350 +/- 20 degrees C and 80 +/- 10 MPa. During quartz crystallization, the conditions moved across the halite liquidus resulting in a heterogeneous mixture of brine and halite crystals, which were trapped at 200 +/- 20 degrees C and 50 +/- 10 MPa. Brecciation and a palaeo-geothermal system overprinted the pegmatitic veins with an epithermal hematite-quartz assemblage and lesser amounts of bladed calcite and fluorite, in an intermittently boiling hydrothermal system of fairly pure H2O at 100-140 degrees C and 1-5 MPa. Remobilization of LREEs and Y from titanite and/or the granitic host rock is evidenced by precipitation of apatite, allanite and wakefieldite in an intermediate stage. Occasional incorporation of radioactive elements or minerals, presumably U-rich, in the fluorite is responsible for radiolysis of H2O to H2. [PUBLICATION ABSTRACT]