- Possible copper VMS-style deposits
- No modern exploration
- No gold assays in the previous drilling
- Numerous small-scale historic mines and known mineral occurrences.
Silverwood Project is located 15km South of Warwick in Queensland and has 234km2 of tenure. Situated in the Calliope Island Arc which hosts the Mount Morgan gold mine (+8Moz), Silverwood comprises numerous small-scale historic mines and numerous known mineral occurrences.
(source GSQ report by Siemon & Huber 1979)
The oldest rocks in the area are the Silurian to Early Devonian Silverwood Group comprising the basal Riadon Stud Formation, the Connolly Volcanics and the Rosenthal Creek Formation (Figure 16). Mineralisation is confined to the upper units. Unconformably overlying the Silverwood Group are the Early Permian Rhyolite Range Beds and the Early Jurassic Marburg Formation. Intruding the sequence to the west is the Herries Adamellite and to the southeast the Stanthorpe Adamellite both of Early Triassic age. Dykes of andesitic, dacitic and rhyolitic composition intrude the sequence, which has undergone low grade metamorphism with little deformation.
The Connolly Volcanics is a succession of andesitic and dacitic flows and tuffs. sediments and biohermal limestone. Detritus in the sediments consists of reworked volcanics and sediments, limestone of Ordovician and Devonian age and granite. Sediments including limestone, and volcanics of the Rosenthal Creek Formation conformably overlie the Connolly Volcanics. The succession is typical of an island arc environment with waning volcanism.
The Silverwood copper mine workings were first developed in 1917 and in 1923, ore was sent to Mount Morgan for processing. In 1966, the Stanthorpe Mining Company began mining the lodes and the mine was active until 1970 (Siemon and Huber 1979). Production is thought to have been about 42.5 tonnes of copper.
Sulphide mineralisation in the southern Silverwood Valley is restricted to the Connolly Volcanics, and the Rosenthal Creek Formation where the principal occurrences are at Grieves Quarry and the Silverwood Copper mine.
Prospecting of a copper occurrence near Silverwood siding commenced about 1970. The Silverwood copper mine (Figure 16) includes the Day Dawn and Wilson’s Lodes with copper mineralisation occurring as chalcopyrite mainly in a highly siliceous brecciated zone, 0.3m to 1m wide. The workings are located along the contact between the andesitic pyroclastics (Connolly Volcanics) and the sediments of the Rosenthal Creek Formation. Barren dyke intrusions appear to have remobilised earlier formed sulphides into favourable structural sites, with the dykes themselves containing only minor pyrite. Historical Ore Reserves were estimated at some 10, 000 tons, but H&SC is unable to report this as it is well before the establishment of the JORC Code. A treatment plant to produce copper concentrates was erected 5.6 km south of Warwick in 1969 and was later used for crushing limestone from Grieves Quarry which began in 1971. Limestone is reported to have been produced from the Silverwood area between 1962 and 1965, but the sites of production are unknown.
The Myrtle Copper deposit is a small prospect of scattered pits and small dumps, to the south of Silverwood Copper Mine, and occurs in the same stratigraphic position.
The Grieves Quarry deposit is a stratabound copper-lead-zinc-silver deposit hosted in the Early Devonian sequence i.e. the Connolly Volcanics, comprising of interbedded tuffs and sediments with massive reef limestone, overlain by siltstone, mudstone and minor tuff. The tuffs and sediments were deposited in an off-reef environment in an island arc system with a cratonic hinterland of granitic, volcanic and sedimentary rocks. Crosscutting the sequence are at least two phases of dykes of the Devonian to Triassic age. All rock types have undergone low-grade metamorphism
The first real modern exploration in the Silverwood Valley was carried out by the Geological Survey of Queensland (“GSQ”) between 1967 and 1971 where two diamond drilling programmes were undertaken at the Grieves Quarry for a total of 17 diamond holes with 1,505 metres drilled. The GSQ diamond drilling intercepted numerous significant intersections of massive sulphide mineralisation at the Grieves Quarry and the best intercepts are shown in Table 5. The GSQ did not assay for gold, focussing on base metals copper, zinc, lead and silver.
Amoco Minerals Australia Pty Limited (1974) and Penzoil of Australia Ltd, (1978) were the earliest to explore for massive base-metal sulphides. Programs included geological mapping, airborne electromagnetic surveys, ground induced polarization, magnetic and geochemical sampling surveys, but no drilling.
In the 1980s, Ashton Mining Limited (1980-1982), CRA Exploration Pty Limited (1986-1987), Australian Gold Resources Ltd (1988) and Stevenson Enterprises Pty Ltd (1989) examined the area with a gold-in-massive sulphide target in mind; there was limited fieldwork and no drilling.
The most recent work is recorded by Metallica Minerals Limited (“Metallica”) (2000-2002), Icon Resources Ltd (2008-2009) and Invictus Gold Ltd, (2013), all of which carried out reviews but limited fieldwork. Limestone quarrying was considered by Metallica.
Grieves Quarry Base Metal Deposit
At Grieves Quarry, base metal mineralisation has been located in three zones over a length of 150m and to a depth of at least 130m. The stratabound mineralisation is hosted in both a lens of bioclastic reef limestone and in more widely distributed tuffaceous sediments (Figure 17). There is some suggestion of a transgressive nature to the mineralisation at the limestone/tuffaceous sediment contact. The exposed host units have a relatively steep dip to the east before shallowing to a 20-30o dip east some 50 to 80m below surface.
Pyrite is ubiquitous in the area and, in parts of the deposit, has been converted to pyrrhotite. In massive sulphide zones associated with the upper parts of the limestone lens, sphalerite, chalcopyrite and galena are subordinate to pyrite and pyrrhotite in a matrix of calcite and chlorite.
Stockworks that form the lateral parts of the massive sulphide zones contain sphalerite, chalcopyrite and galena with minor iron sulphides with chlorite and calcite as gangue. Similar stockworks with only minor galena overlie the massive sulphide mineralisation but are separated from it by a barren tuffaceous horizon. Chalcopyrite is usually present as discrete grains associated with pyrrhotite.
The dominance of chlorite in the sulphide zones can be explained by an intermediate to basic volcanic source rather than an acidic volcanic source. The presence of calcite is attributed to recrystallisation of limestone material which in turn may suggest dissolution and brecciation associated with hydrothermal fluids. There is no mention of any high temperature calcsilicate minerals suggesting that there is no whole scale metasomatic replacement and the mineralisation may well be related to exhalite processes or even Mississippi Valley-type (MVT) mineralisation.