Warroo Copper-Gold Project
- Open pit gold deposit mined in the 1980s
- Poorly understood geology and mineralisation
- Very little modern exploration, but some high-grade intercepts
- Never explored for Copper, despite historical copper mines
The Warroo gold mine and the associated Ti-Tree, Commodore and Glenrosa prospects show the strongest mineralisation known within the project, but the Angus Valley-Ashton copper workings, to the north-west, attest to mineralisation being relatively more widespread. The area is radically unexplored and poorly understood.
The Warroo project consists of EPM 26178 which covers an area of 35 sub-blocks amounting to 115 km2 and EPM 27725 being 51 sub-blocks and 167 km2.
The Warroo Project area is located 50 km west of Stanthorpe and 35 km north-east of Texas
Peter Gregory, a Principal Geologist with the GeoDiscovery consulting group, based in Brisbane, completed a study of the EPM in 2016. The work involved a brief review of historical exploration combined with a GIS study of aeromagnetic, radiometric, satellite and gravity imagery to give a better understanding of the exploration potential of the tenement. This work has been under-utilised in defining prospective targets.
The sedimentary rocks exposed in the general tenement area are part of the New England Fold Belt. They are dominantly the Late Devonian-Early Carboniferous Texas Beds comprising volcaniclastic turbidites, basalt, chert, intraformational conglomerate and limestone olistoliths (Figure 22) and are preserved as an imbricate stack of thrust slices (Flood and Aitchison, 1992). The rocks are part of a subduction complex developed near a convergent plate margin and related to a west dipping subduction zone.
The sequences are intensely deformed with dips usually steep and beds overturned. Major dextral transform faulting in the later Carboniferous combined with oroclinal bending has further reoriented the sequences.
In the early Permian deep marine basins developed over part of the area and remnants are preserved as small inliers for example near Texas (Silver Spur Beds), the Pikedale Silver Mine (Pikedale Beds) and near the Warroo and Ashton prospects (Terrica Beds) within the tenure.
Several bodies of S-type granite at Mount Bullaganang northwest of Warroo Mine and a smaller body southeast of Ashton prospect (granite to monzodiorite) are known. Several intermediate dykes are recorded from drill holes and Warroo pit. The Mount Bullaganang Granite would appear to be in the axial zone of the orocline.
The early Jurassic Marburg Sandstone unconformably overlies all the older units in the north and west.
Published literature describes the Warroo-Commodore, Glenrosa, Ashton and Angus Valley occurrences as granitoid-related mesothermal vein deposits emplaced in metasedimentary rocks. Denaro (1992) describes the deposits as veins in shear zones in pyritic shale/siltstone and tuff of the Texas Beds and in Permian sediment outliers. The. Ashton Copper Mine was discovered in 1897 and worked until about 1900. A small supergene resource was exploited with assays of to 30% Cu and 5g/t Ag recorded (Olgers et al., 1974).
The Warroo Mine produced 21 tonnes of copper, 283ozs of Au and 901 ozs of Ag in the period 1910-1911. A further 923 ozs of Au were produced from 1912-1913 when the mine was worked by tributers (Denaro, 1989). When the mine was reopened in 1931, eight tonnes of ore produced 5ozs of Au. The more recent mining by Valdora in the period 1990-1992 saw approximately 214,000 tonnes at 1.94g/t mined for a heap leach operation, but the final production figures are not known (Catherall and Hockings, 1992). The mineralisation dominantly occurs as laminated sulphidic quartz veins in shear zones (main zone to 1.2m wide) within zones of silicification, pyrite alteration and thrusting in shales/siltstones of the Texas Beds.
The Commodore copper mine was found in 1905 and is located northwest of Warroo. Total production recorded in the period 1906-1913 is 206 tonnes of Cu (grade 6-25% Cu). Gold grades in the supergene zone were to 25g/t, but there is no record of total production (Denaro, 1989). The mineralisation was exploited to a depth of 35m and laterally for 85m (Olgers et al., 1974).
The Glenrosa prospect has not been exploited and was defined by Duval Mining as a geochemical find, but displays similar characteristics to Warroo, but with little or no Au.
The Ti-Tree prospect 800m south of Warroo is a relatively recent discovery of an epithermal gold vein and siliceous alteration related to a northwest structure.
Duval Mining (Australia) Ltd targeted sediment-hosted, disseminated replacement Carlin-type gold deposits and stockworks in the Texas Beds (Seed, 1984, 1985, Hextall, 1986). Exploration initially consisted of regional and detailed geochemical sampling and mapping which yielded the Glenrosa prospect and the confirmed the detection of the Warroo prospect. Follow-up work comprised the drilling of 26 percussion drillholes and 1 diamond hole at Warroo which defined a new zone of gold mineralisation and yielded a best result of 8m at 13.46g/t Au. This mineral zone had strike length of over 1km with uncertain extensions under soil cover to the northwest, but also to the southeast. The Glenrosa holes were disappointing. The further exploration involved an IP–resistivity survey over the Warroo Grid and defined several highly prospective targets in an en echelon pattern which were tested by 30 angled RC holes for 1,943m of drilling. This showed that pyrite in veinlets or disseminations to 5% accounted for the chargeability anomalism. While gold was associated with shear zones these were too sparse or irregular and low grade to constitute a potential resource.
Valdora Minerals Ltd carried out regional mapping, heavy mineral sampling with various sieve sizes and neutron activation analysis. This program generated several new areas of interest but no follow up works seems to have been completed. Drilling on the property was conducted at Commodore and Warroo. An open cut resource of 160, 000 tonnes at 2.86g/t Au at cut-off grade of 0.5g/t Au was defined (14, 700 ozs) at Warroo in the supergene zone, but the small Cu resource defined at Commodore was not of economic interest. Other drilling outside the main zones did not produce encouraging results. Significant work was done by outside consultants regarding a potential mining operation and environmental aspects (Eeson and Gould, 1988a, 1988b, Eeson, 1989, Catherall, 1991).
Budmead Pty Ltd and Silklan Pty Ltd reported on the mining at Warroo and the early problems of grade control and dilution (Catherall, 1992, Catherall and Hockings, 1992, Gunter, 1993). Comment is made that while most of the historical gold mined came from the Central Shear at Warroo, most of the gold mined in 1992 came from the supergene zone where a low angle thrust (Flat make Shear) controls gold deposition and has sub-vertical feeders streaming off the hanging wall and acting as channelways for both primary sulphide deposition and supergene enrichment (Catherall and Hockings, 1992)
Hillcrest Resources Ltd conducted a review of previous work, commissioned a Landsat interpretation and a review of the new Government Texas aeromagnetic survey and a new review of the known prospects. No significant follow up work was completed.
Qld Tantalite Pty Ltd/Qld Gold and Minerals NL realised the potential for Intrusion related gold mineralisation and believed that the Warroo Cu-Au prospect had all the characteristics of a porphyry/stockwork Cu-Au system and that Commodore was part of the same system. This belief was based on the recognition of complex fracture patterns and the small veins and shoots of gossanous mineralisation previously reported. Potential for a sizeable low grade disseminated gold-base metal resource was suggested. A substantial review of newly acquired government data was undertaken with strong emphasis on geophysical interpretation. Various features were identified giving rise to a re-evaluation of the exploration prospectivity of the area. Follow up mapping and surface geochemical sampling in the Ashton–Angus Valley corridor defined a zone of pyritic siliceous breccia, copper zonation and a new Permian outlier. Rock sampling was more definitive than soil sampling.
Oxiana Exploration Pty Ltd in joint venture with Qld Gold and Minerals NL completed extensive soil and rock sampling at Ashton-Angus Valley, Glenrosa and Warroo-Warroo Southwest. This identified an epithermal style quartz breccia vein with gold mineralisation at the Ti Tree prospect 800m south of Warroo. Anomalism associated with follow-up IP geophysics for the Ti Tree-Warroo area resulted in the drilling of 5 RC holes testing for a low-grade bulk tonnage sheeted veins system. Subsequent work by Qld Gold and Minerals suggested an alternative geological interpretation such that the Oxiana drilling strategy may have been sub-optimal. Qld Gold and Minerals also completed the drilling of 14 RC holes to 40m depth to test the heap leach potential of the existing stacks at Warroo Mine. However, gold grades greater than 1g/t were very restricted and would not support an operation at that time (Day, 2008).
Aeromagnetic, radiometric and satellite imagery are very helpful in defining structure and hydrothermal alteration, outcrop limits of intrusions and even possible buried intrusions. These are useful components in assessing the geological understanding and exploration potential of an area.
The regional aeromagnetic data shows the Warroo EPM to be located east of a large oval magnetic feature that appears to be at moderate depths (Figure 23). Strong northwest structural trends occur close to the north eastern edge of this buried body and within the Mount Bullaganang Granite and extending southeast to the Warroo area. Dyke-like intrusives extend from the buried body at depth into the Texas Beds. The orocline folded structure can be seen to the north within the Texas Beds.
A ternary radiometric image of the same area shows strong potassium levels associated with the Mount Bullaganang Granite and extending north and northwest in an arc controlled by northwest and east west structures (Figure 24). In addition, there are other smaller areas of potassic alteration south of the tenure and southwest of the tenure in the Mount Gunyan area with a major southwest structure running southwest through Warroo to Mount Gunyan and possibly defining the edge of buried intrusions in part. Specific ovoid patches in and on the edge of the granite may indicate specific altered zones or compositionally different intrusive plugs. Chert units help to define ovoid structures possibly due to doming and contact metamorphism by buried intrusives.
(with interpreted faults (black) and buried intrusives (white polygons))
Satellite imagery shows that Glenrosa sits in this structural corridor but that west-northwest structures run through the prospect area and extend several kilometres to the west to intersect the granite (Figure 25). Despite the northeast structural corridor, the northwest structures through Warroo extend into the granite and also southeast of Warroo which suggests that there may be further potential in exploring the extension to the northwest-southeast corridor and particularly its undercover intersection with the northeast corridor.
More mafic intrusive plugs are suggested for potassium low ovoid zones southeast of Ashton where these are aligned along a northeast trend and north of a defined east west silicification zone defined by Valdora. The possibility of intrusives underlying Ti-Tree and northeast of Warroo is shown based on oval patterns on the satellite and radiometric imagery.
The previous figures indicate how more recent geoscientific data can be used to re-evaluate the geology of an area and with the application of either new or alternative exploration models a fresh view to selection of prospective areas can be applied.
Historical exploration to date has focused on the Warroo/Ti-Tree area as this has the best surface and drilled gold and silver results. The deposit can be shown to be associated with major northwest structures east of their intersection with a major northeast structural corridor that extends southwest to Mt Gunyan northeast of Texas, QLD. Buried granites are suggested to underlie part of this area and gold and silver occurrences in that region area suggested to be related to such intrusives.
Good potential exists to define mesozonal to epizonal intrusive related gold mineralisation related to these structural corridors and certainly on extensions to the Warroo and Ti Tree structures. Other structures within the potassic alteration arc in part surrounding Mt Bullaganang and extending to Ashton also have exploration potential. The extensive west-northwest structural corridor that appears to extend from Glenrosa and onto the northern edge of the granite has had no exploration apart from some focussed exploration on the observed alteration and mineralisation at Glenrosa proper where results for gold are low. This is one example of where a consistent exploration approach to identified structures/structural corridors is needed to identify new targets.
Smaller interpreted satellite intrusions may have gold potential as sheeted veins to breccia bodies and stockworks as may specific potassic altered zones within the Mt Bullaganang Granite.
A better understanding of the structural controls on known mineralisation including the shallow thrust emphasis of previous explorers is needed. However, other styles of mineralisation with different controls may be present and an open-minded approach is needed for success.