Duck Creek Anticline
The West Project tenements include Exploration Permits for Minerals (EPMs) Mining Leases (MLs), covering a combined area of 703kmÇ centred approximately 36km south‐west of Cloncurry. The project area can be accessed from Cloncurry by sealed roads and the Townsville‐Mt Isa railway line runs through the project area with some 14.5km of the line within the EPMs.
The Duck Creek tenements are characterised by many narrow, typically east‐west striking, crush/shear zone hosted, predominately quartz‐malachite style of copper orebodies. Output from the very early historical copper mines in the area whilst modest in tonnage was typically high‐grade (~20% Cu +/‐), due in most part to the need for selective mining driven by the logistics of getting the ore to market.
The area is also host to one of only three historical cobalt producers in Queensland, producing an estimated 333 tonnes @ 13.48% Co (and 36.84% Cu) between 1927‐1931, of which only 25.25 tonnes @ 21.70% Co was recorded as being sold.
There are also sporadic cases of modest tonnage, yet high‐grade historical gold (+/‐copper) ore production.
In the Duck Creek area, oxide weathering profiles continue generally to around 30m depth from surface and combined with the narrow nature of mineral enrichment, has contributed to historically small‐scale oxide‐dominated mining activity, that very rarely reached depths of 50m or so via narrow, selective underground methods. Exceptions include at the Horseshoe Prospect (JORC 2004 resource 0.9Mt @ 1.47% Cu), which has limited confirmed strike extent to date, but includes deeper intersections from RC drilling of 35m @ 2.73% Cu (from 198m down‐hole depth) and 37m @ 1.72% Cu (from 186m down‐hole depth).
There are hundreds of surface copper workings and mineralised occurrences in the Duck Creek tenement area, yet very little deeper exploration has been undertaken to date. Geophysics for the area is limited and is a focus for Transition.
The largest identified resource on Transition’s Duck Creek tenements is at the Horseshoe Mine (ML2778) where previous production included ~90,000 tonnes of oxide ore of unknown grade. A remaining resource of 0.9Mt @ 1.49% Cu (predominately sulphides) has been defined (reported to JORC2012 guidelines).
Transition’s West Project tenements are largely located on the Duck Creek Anticline, which is interpreted as a ramp anticline on the Mitakoodi Culmination (MacCready et al. 1998). This regional open fold structure and associated west‐vergent thrust system formed during the Isan Orogeny (MacCready et al., 1998; O’Dea et al., 2006; Potma and Betts, 2006).
The core of the Mitakoodi Culmination comprises a suite of felsic volcanics that were previously identified as belonging to the Argylla Formation. However, dating anomalies indicate these Mitakoodi “Argylla Fm” volcanics represent a different suite of lithologies produced by a younger period of magmatism with an alkaline affinity in an intra‐plate setting, possibly a back‐arc basin. This contrasts to the geochemical signatures accepted Argylla Fm. volcanics that indicate subduction zone magmas.
The “Argylla Fm” lithologies in the Mitakoodi Culmination are conformably overlain by units of Marraba Volcanic, a suite of interbedded mafic volcanics and fine clastic units. Some mafic units are quite massive and most likely represent high‐level ultramafic to mafic plutonic bodies. The volcanic supracrustal package is overlain by a thick (up to 2000 m) succession of quartzo‐feldspathic arenite termed the Mitakoodi Quartzite. The arenite contains minor interbedded quartzite units, as well as lenses of siltstone and metabasalt with a unit of thinly bedded tuff near the top of the quartzite. The top of the quartzite also contains a unit of porphyritic rhyolite which crops out for a strike length of >3 km. As rhyolite is a high viscosity magma that forms lava domes, not extensive flows, the rhyolite sequence more is interpreted to be a pyroclastic ignimbrite sheet or tuff.
Zircons from the “rhyolite” yield a SHRIMP U‐Pb age of 1755 ± 4 Ma (Page et al. 1997), which is within error of the age of the felsic alkaline volcanic unit dated by Neumann et al., (2009) from the core of the Mitakoodi Culmination.
The entire package of lithologies that crop out in the Mitakoodi Culmination represent a previously unrecognized intraplate suite, that was possibly deposited in a back‐arc rift environment.
The Mitakoodi Domain was subsequently intruded by phases of the Wimberu Granite, the aqges of which are within error of the timing of crystallization of xenotime (1534±20 Ma) and monazite (1469±35 Ma) from Transition’s heavy REE bearing Highway Breccia discovery.
Wimberu Granite is considered to be a member of the Eureka Supersuite of plutons within the Williams Batholith that are believed to be associated with IOCG mineralisation (Hackenberg, 2018). In this regard, the felsic alkaline core of the Wimberu Granite, the youngest phase of the intrusion, is interpreted to be associated with a Cu‐rich hematite breccia system.
The heat source, that melted the mantle wedge and lithosphere to produce the Williams Batholith was most likely associated with plume generated ultramafic and mafic magmatism.
In fact, plume magmatism during the geotectonic evolution of the EFB was suggested by Betts et al., (2007 and 2009) based on geodynamic arguments.
Previously identified plume magmatic events include:
(1) the emplacement of the Lunch Creek Gabbro and Burstall Granite at 1739 ± 3 Ma and 1738 ± 2 Ma (Neumann et al., 2009);
(2) the emplacement of alkaline lithologies associated with the Williams Batholith between 1550 and 1500 Ma (Duncan et al., 2011). This later event was also accociated with emplacement of ultramafic lamprophyres and phoscorites in the Mary Kathleen Belt (KDC unpublished data).
The discovery of ~1755 Ma intraplate volcanics in the Mitakoodi Domain that are pre‐Burstall granite and Lunch Creek Gabbro in age, represent an earlier and previously unrecognized period of intraplate plume magmatism.
Thus, through improved understanding of the crustal evolution and geochronology of the Mitakoodi Domain, it is apparent that the region preserves evidence for three periods of intraplate (plume) magmatism: i.e., ~1755 Ma, ~1738 Ma and ~1500 Ma.
This has significant metallogenic implications because plume magmas are associated with mantle upwellings that rise from the core‐mantle boundary a region of the Earth that is very enriched in precious metals. This effectively explains the source for the polymetallic suite of elements (Cu‐Ni‐Co‐W‐Au‐Y‐HREE‐Pd‐Pt) seen in the Mitakoodi mineral system. The continental crust is not the source these elements.
The West Project area is dominated by an area historically referred to as the Duck Creek group of copper workings, within which is a densely mineralised, historically heavily worked area, extending some 20km north‐south and 5km east‐west.
Mineralisation is typically characterised by narrow oxide‐copper‐bearing quartz‐rich structures that whilst modest in surface expression, are of a style of mineralisation that facilitate localised enrichment and selective historical mining of high‐grade lenses.
The majority of historical mining activity appears to have occurred between 1879‐1930, when ore grades for shipment by camel or bullock train were generally required to be above 20% Cu. Sporadic mining activity also occurred in the 1940s, though was impacted by World War 2, and from 1960‐1970 cupiferous siliceous flux was mined to supply the Mt Isa copper smelter.
A blast furnace was constructed on Mt Start (at the intersection of the Cone and Duck Creeks) and was important for the region as it meant lower grade (+5% Cu) ore could finally be mined profitably. However, its operation was problematic (lack of water and fuel), and it subsequently became a short‐lived facility. Visual inspection of dumps at the location indicate very modest throughput.
It appears that ore in the 5‐10% range (too low for shipping to the coast), stockpiled at many prospects to be fired at the furnace, were never processed and, to this day, remain strewn throughout the Duck Creek tenements.
The Duck Creek Workings remain one of the most densely mineralised and heavily worked areas in the Cloncurry‐Mt Isa region and due to its history and small‐scale of the deposits, has possibly been overlooked by modern explorers.
Modern exploration has been sporadic, selective and somewhat limited, resulting in a poor understanding of the geology of the area, and potentially the economic importance of the significant number of historical workings.