Linking uplift and mineralisation at the Mount Novit Zn-Pb-Ag Deposit, Northern Australia: Evidence from geology, U–Pb geochronology and sphalerite geochemistry

The subeconomic Mount Novit Zn-Pb-Ag deposit is located approximately 20 km south of Mount Isa, Queensland. In contrast to the nearby Mount Isa, Hilton and George Fisher Zn-Pb-Ag deposits, mineralisation at Mount Novit is situated to the west of the regional-scale Mount Isa Fault and is hosted in the Moondarra Siltstone as opposed to the Urquhart Shale. Lower-grade (<4 wt.% Zn + Pb) Zn-Pb-Ag mineralisation primarily replaces pre-existing carbonate alteration and veining and consists of pyrrhotite, pyrite and sphalerite with lesser galena. Higher-grade (>10 wt.% Zn + Pb) mineralisation occurs as a matrix supported breccia dominated by sphalerite and pyrrhotite with galena, pyrite, and magnetite. In-situ U–Pb geochronology was completed on apatite and two textural varieties of monazite. Fine-grained (<50 µm) subhedral to anhedral monazite is located within highly foliated biotite alteration directly adjacent Zn-Pb-Ag mineralisation and yields a mean weighted ²⁰⁷Pb/²⁰⁶Pb age of 1527 ± 18 Ma (MSWD = 1.06). This age is consistent with the formation of highly foliated biotite alteration during D₃ deformation of the Isan Orogeny. Apatite from the same fabric yields a lower intercept age of 1443 ± 29 Ma (MSWD = 1.30). Consistent with previous studies, this age is interpreted to represent the age of a major thrusting event along the Mount Isa Fault that resulted in the cooling of the Mount Novit area below ∼375 °C. Coarse-grained monazite is coeval with Zn-Pb-Ag mineralisation and yields a mean weighted ²⁰⁷Pb/²⁰⁶Pb age of 1457 ± 11 Ma (MSWD = 0.28). Sphalerite from Mount Novit has low concentrations (<1 ppm) of Ge and Ga and a relatively high concentration of In (5 to >10 ppm), possibly reflecting the leaching of the metals from an underlying basement unit. The GGIMFis geothermometer (Frenzel et al., 2016) produced a mean formation temperature of 345 ± 52 °C. The timing and temperature of Zn-Pb-Ag mineralisation is consistent with the age and cooling temperature of apatite presented in this study. Based on these correlations, we suggest that Zn-Pb-Ag mineralisation at Mount Novit was emplaced during an episode of major thrusting along the Mount Isa Fault, with the precipitation of Zn-Pb-Ag mineralisation driven by the cooling of the Mount Novit area below ∼375 °C. A key implication of this study is a new model for synorogenic Zn-Pb-Ag mineralisation to the south of Mount Isa, which contrasts with the widely accepted regional-scale syngenetic metallogenic model.