Honours Abstract (2005) Simon BRUZZONE

The Mt Gibraltar pluton is a small (~2200 m by ~800 m), syenitic laccolith that was emplaced within the Sydney Basin during the Early Jurassic (~182 Ma) as a direct result of the break-up of Gondwana.

The Mt Gibraltar magma intruded into the base of the Hawkesbury Sandstone causing the overlying Triassic strata to arch upwards. Eventually, continued magma influx caused these strata to undergo brittle fracture, resulting in a near-vertical fault on Mt Gibraltar’s eastern side directly above the pluton feeder channel. Zirconium concentrations indicate this magma was between 865-905ºC at the time of emplacement and estimates of cover thickness suggest a pressure of ~40 MPa. The Mt Gibraltar melt did not breach the surface, but became trapped within the sedimentary layers and formed a relatively uniform, fine-grained microsyenite composed of anorthoclase and albite feldspar, augite-aegirineaugite-aegirine, and rare arfvedsonite. Although initially volatile undersaturated, the Mt Gibraltar melt attained saturation through fractional crystallization, creating a separate volatile fluid phase that crystallized as pegmatites along contraction fractures after the microsyenite was solidified.

Three separate fluid phases can be attributed to pegmatite development at Mt Gibraltar. The first of these produced the primary mineralization of anorthoclase, augite-aegirineaugite-aegirine, zircon, and arfvedsonite-riebeckite. The second caused the alteration of primary minerals, and deposited chalcedony, quartz, stilpnomelane, fluorite, and calcite within interspaces and cavities that remained after the primary mineralization. Finally, the last fluid phase moved within, and around pegmatite veins depositing secondary calcite within pegmatite cavities and wallrock vesicles, and hydrothermally replacing pyroxene within the wallrock with iron oxides. Stable isotopic values (d18O and d13C) for calcite from Mt Gibraltar indicate that the first two phases of fluid movement were entirely magmatic, but the last was influenced by the introduction of external waters (likely to have a meteoric, connate, or groundwater source). These isotopic data suggest that the fluid temperature during the last two phases were between 195-207ºC and 118-128ºC respectively.