Sediment-hosted Zn-Pb-Ag (+ Cu) deposits in the Mount Isa region: An epigenetic approach to mineralisation

The Mount Isa Inlier, north-central Australia, contains numerous sediment-hosted Zn-Pb-Ag-Cu deposits. Previous research on these deposits has resulted in two contrasting metallogenic models: (1) Zn-Pb-Ag mineralisation was formed during the deposition of the host Urquhart Shale at ca. 1655 Ma and was overprinted by Cu mineralisation at ca. 1500 Ma; (2) Zn-Pb-Ag mineralisation formed during the late stages of the 1620􀍴1500 Ma Isan Orogeny and is coeval with regional-scale Cu mineralisation. This thesis focuses on the textural, geochemical, and geochronological aspects of four Zn-Pb-Ag-Cu deposits in the Mount Isa region in order to constrain a coherent metallogenic model. This work is concentrated on: (1) The Mount Isa Cu-Zn-Pb-Ag deposit; (2) The Mount Novit Zn-Pb-Ag deposit; (3) The George Fisher and Hilton Zn-Pb-Ag deposits. (1) Detailed petrography across Cu- to Zn-Pb-Ag transition zones at the Mount Isa deposit suggests that chalcopyrite, sphalerite, and galena are texturally coeval. Furthermore, the variation in trace element geochemistry of sphalerite, galena, and chalcopyrite across the Cu to Zn-Pb-Ag transition suggests that the separation of Cu and Zn-Pb-Ag mineralisation likely resulted from a small (>50ºC) temperature gradient across the orebody. Monazite inclusions within silica-dolomite alteration produce ages from 1623 ± 25 Ma to 1587 ± 43 Ma, constraining the maximum age of Cu-Zn-Pb-Ag mineralisation. Monazite from the 1100 level Cu orebody produces a range of ages from 1620􀍴1360 Ma, reflecting either continuous dissolution-reprecipitation reactions, or variable resetting from a late fluid. Monazite from illite-chlorite-rutile alteration underlying the deposit produces an age of 1376 ± 32 Ma, reflecting a late fluid event associated with uplift along the Mount Isa Fault. (2) The Mount Novit Zn-Pb-Ag deposit is located 20 km south of Mount Isa. Through a combination of petrology, sphalerite geochemistry and multi-mineral U-Pb geochronology, a metallogenic model is constructed for this deposit. The data illustrates that highly foliated biotite-alteration formed at 1527 ± 18 Ma, followed by Zn-Pb-Ag mineralisation at 1457 ± 11 Ma. Zn-Pb-Ag mineralisation occurs primarily as replacement and is coeval with regional-scale uplift along the Mount Isa Fault. (3) The Hilton and George Fisher Zn-Pb-Ag deposits are situated approximately 20 km north of Mount Isa. The trace element geochemistry of six textural varieties of sphalerite was analysed from the Hilton Zn-Pb-Ag deposit to assess its usefulness as a metallogenic tracer. Sphalerite that co-crystallised with sulphides were depleted in the elements Fe, Co, In, Sn, Sb, Ag and Tl. Sphalerite formed via chemical remobilisation were enriched in Zn, Ge, Ga and Sn, and depleted in Fe, Tl, Co and Bi relative to the parent minerals. Dolerite dykes exist within the Hilton and George Fisher Zn-Pb-Ag deposits. Apatite U-Pb geochronology constrains the emplacement age of the dolerite dykes to ca. 1620 Ma. Monazite U-Pb and calcite Lu-Hf geochronology constrains widespread pre-mineralisation hydrothermal alteration in the dolerite dykes and adjacent orebodies to ca. 1510􀍴1500 Ma, with late faulting occurring at ca. 1280 Ma. The geochronological evidence suggests that the dolerite dykes were emplaced during the earliest phase of the Isan Orogeny and experienced hydrothermal alteration and Zn-Pb-Ag mineralisation during the late stages of the Isan Orogeny. The evidence presented in this thesis favours an epigenetic syn-deformation metallogenic model for the formation of Cu and Zn-Pb-Ag mineralisation throughout the Mount Isa area.

Author dc:creator

• Cave, Bradley Wade

Advisors dc:contributor.advisor

• Hatch, Karin
• Lilly, Richard
• McGee, Lucy

• Mount Isa
• Geology
• Ore Deposit
• Epigenetic
• SEDEX
• Zn-Pb-Ag Deposits

Language dc:language.iso

en

Handle dc:identifier.uri

https://hdl.handle.net/2440/137690

OAI identifier oai:identifier

oai:digital.library.adelaide.edu.au:2440/137690