This post is a review and summary of an excellent paper: Bradley Cave, Richard Lilly, Alexander Simpson, Lucy McGee, A revised model for the George Fisher and Hilton Zn-Pb-Ag deposits, NW Queensland: Insights from the geology, age and alteration of the local dolerite dykes, Ore Geology Reviews 154 (2023) 105311. https://doi.org/10.1016/j.oregeorev.2023.105311
- The George Fisher and Hilton Zn-Pb-Ag deposits are located approximately 20 km north of Mount Isa.
- A dolerite dyke has been discovered at the George Fisher Zn-Pb-Ag deposit and the dolerite dyke have a large spike in TiO2 and V values.
- In drill core, the dolerite dykes occur as a light grey to grey-brown coloured rock that is commonly overprinted along its margins by Zn-Pb-Ag mineralisation.
- In-situ U-Pb geochronology performed on igneous apatite produce a lower intercept age of 1611 +/- 21 Ma and 1619 +/- 22 Ma for the dolerite dykes at the George Fisher and Hilton deposits, respectively.
- The dolerite dykes have experienced multiple stages of post-emplacement hydrothermal alteration/veining.
- Monazite from a quartz-albite-K-feldspar vein in the Hilton dyke produces a lower intercept age of 1513 +/- 16 Ma.
- To assess the timing of alteration in the adjacent George Fisher Zn-Pb-Ag deposit, in-situ Lu-Hf geochronology was performed on pre-mineralisation calcite from a section of stratabound Zn-Pb-Ag mineralisation, and a paragenetically late cross-cutting sphalerite-calcite vein.
- Calcite from the pre-mineralization alteration assemblage produces a Lu-Hf age of 1501 +/- 32 Ma.
- Calcite from a cross-cutting vein that post-dates Zn-Pb-Ag deposition produced a Lu-Hf age of 1289 +/- 26 Ma. The 1289 Ma age is associated with late faulting and movement along the adjacent Mount Isa Fault (B. Cave pers comms 2023).
- The paragenetic equivalents of the hydrothermal alteration/veining in the dolerite dykes are found in the adjacent Zn-Pb-Ag orebodies.
- The maximum age of alteration within the dolerite dykes is constrained by the monazite age of 1513 +/- 16 Ma, and the maximum age of stratabound Zn-Pb-Ag mineralisation is constrained by the Lu-Hf age of 1501 +/- 32 Ma.
- The dolerite dykes intruded during the early Isan Orogeny at ca. 1620 Ma, and experienced subsequent hydrothermal alteration during D3 of the Isan Orogeny coeval with Zn-Pb-Ag mineralization. Post mineralization faulting occurred during D2 of the Isan Orogeny, at ca. 1290 Ma.
The change of metallogenic model for the George Fisher and Hilton Zn-Pb-Ag deposits has significant implications to local and regional-scale exploration. Previously the emphasis had been on rocks of a highly carbonaceous nature adjacent to a major crustal structure. Brad Cave et. al. would suggest that structure is more important than host lithology, however I would disagree. The dominance of major metal deposits within the Urquhart Shale suggests that the reductive nature of these highly carbonaceous rocks played an important role in the evolution of these mineral deposits. If you combine a major crustal structure for fluid egress and a reactive lithology you must enhance the opportunity for creation of a major metal deposit – we sure see it elsewhere. Is it possible that the reason that the the Mt Isa-George Fisher deposits have no associated magnetitie, hematite or related magnetic anomalies is that the fluids were buffered by the carbonaceous shales?
Brad Cave (pers comms, 2023) commented, “Magnetite is not reported in these deposits often, but there is a fair amount there. Adjacent the hanging wall at Hilton there is a lot of magnetite associated with high Cu and Pb grades. At Isa, there are large zones associated with the presence of magnetite and biotite. Similarly at Mount Novit, there is also magnetite adjacent to Zn-Pb-Ag ore. However, we haven’t seen any at the George Fisher deposit yet”.
In the Mt Isa region (see the image below) the Upper Soldier Cap Group to the east of the Mt Margaret Fault consists of strongly carbonaceous sediments and mafic volcanics and intrusives of the Toole Creek Volcanics. These rocks were deposited in a deep water environment and are the same age as the Urquhart Shale.
The Continental Copper acreage, shown below, covers 50km of this geology and is coincident with:
- A pronounced magnetotelluric anomaly which has been modelled as a pipe-line body extending into the mid-crust. This appears to be coincident with the Gidyea Suture, the Proterozoic collision zone between the North Australian Craton to the west and the Laurentia Craton to the east,
- Very high Cu-Pb and Zn geochemistry within a regional Eh anomaly along the eastern side of the Mt Margaret Fault,
- Lead isotope geochemistry indicating a Proterozoic lead-source and,
- A strong untested EM conductor.
Over time there has been considerable controversy regarding the timing of mineralization at Mt Isa. The earliest geological investigators concluded that the mineralization was generated during deformation and subsequent to deposition of the host Urquhart Shale. Then, despite the overwhelming geological and observational evidence, the syngeneticists arrived. Modern geochemistry, geochronology and geology has conclusively supported the earliest geologist’s contention. Bradley Cave and colleagues have published conclusive data on the large George Fisher and Hilton deposits confirming similar timing of Zn-Pb-Ag mineralization to that at Mt Isa.
The Hilton (12.2 Mt @ 6.4 % Zn, 4.6 % Pb and 102 g/t Ag) and George Fisher (51 Mt @ 7.4 % Zn, 3.4 % Pb and 55 g/t Ag) Zn-Pb-Ag deposits are located in the Western Fold Belt of the Mount Isa Inlier, 20 km north of the Mount Isa Cu-Zn-Pb-Ag deposit.
The Syn-sedimentary Model
Earlier works proposed a syn-sedimentary metallogenic model for Zn-Pb-Ag mineralisation, with mineralisation occurring during, or slightly after the deposition of the host Urquhart Shale at ca. 1655 Ma. If a syn-sedimentary metallogenic model is used as a basis for exploration, reduced Proterozoic shales of the Isa Superbasin that are located adjacent to major syn-sedimentary normal or strike-slip faults are deemed the most suitable host for economic Zn-Pb-Ag mineralisation.
The Syn-deformational Model
In contrast, a syn-deformation metallogenic model has also been proposed for these deposits, which favors formation of Zn-Pb-Ag mineralisation during late-stage deformation (ca. 1520 Ma) of the Isan Orogeny. If a syn-deformational metallogenic model is favored, sites of increased structural heterogeneity with localised dilation and fracturing that were active, or formed during the late stages of the (1620–1500 Ma) Isan Orogeny are deemed the most suitable locations to host economic Zn-Pb-Ag mineralisation.
At the Hilton Zn-Pb-Ag deposit, a N–S trending dolerite dyke has previously been described and occurs along the deposit-scale Dyke Trace Fault. A previous investigation into the dolerite dyke concluded that it intruded during D2 deformation of the Isan Orogeny, and was interpreted to post-date the formation of Zn-Pb-Ag mineralisation. Cave et al provide the geological, geochemical and isotope data to support this contention.