U-Pb geochronology and trace element analysis of apatite and calcite from Ernest Henry.

Abstract

Ernest Henry is the largest known IOCG deposit in the Eastern Succession of the Mount Isa inlier, NW Queensland. To improve our understanding of the timing of alteration and mineralisation in the Ernest Henry deposit, we attempt to date apatite from the Interlens (a pre-mineralization structure) and the ore-stage breccia, and calcite from the Interlens, ore-stage breccia and post-mineralization alteration using the in-situ U-Pb LA-ICP-MS method. This also approach provides the opportunity to examine the trace element composition of the minerals, which can be used to identify their sources and constrain metasomatic processes.

Coarse-grained apatite from Interlens was dated at 1581 ± 16 Ma, coeval with regional peak metamorphic conditions and D2 deformation of the Isan Orogeny. Finer-grained apatite from the Interlens produced an age of 1557 ± 23 Ma, possibly representing regional D2, D2.5 or D3 deformation, coeval with retrograde metamorphic conditions. Ore-stage apatite produced an age of 1529 ± 39 Ma, coeval with the accepted age for sulphide mineralisation, D3 deformation and the formation of the nearby Mount Margaret Granite. Calcite samples were unable to be dated by this method, as the samples were dominated by common lead.

Trace element analysis indicate that apatite from the Interlens and ore-stage assemblage were sourced from magmatic/hydrothermal fluids. Furthermore, metasomatism and coupled dissolution re-precipitation reactions of apatite were induced by a Na and/or Ca rich fluid, possessing varying amounts of Cl and S. Calcite from this study displays similarities with altered granites, and greisen type deposits, likely the result of fluid diffusing through the heavily altered Mount Fort Constantine host rocks. This study also geochemically links calcite from the Ernest Henry and the nearby E1 deposit, suggesting the REE composition of calcite may be used to link hydrothermal systems from various deposits.