Granite Tor Projects Area
The Granite Tor Project is held 100% by Corona as an exploration licence of about 166 km² (EL25/2010). It is centred about 12km due east of the former mining township of Tullah on the Murchison Highway. It is about 60km NNE of Queenstown. There are no roads or vehicle tracks in the EL – the nearest road being at the south end of the Hydro Tasmania Lake Mackintosh storage. It has always been regarded as a remote, inaccessible area which has hindered mineral exploration and development. The country is deeply dissected and rugged with thick bush clad valleys and rocky, button grass clad higher ground.
EL 25/2010 encompasses a Devonian age granitoid pluton – the Granite Tor Granite, which is analogous with several other plutons in the west coast district of Tasmania that have associated economic tin deposits, in particular, and lesser tungsten plus accompanying base metal mineralisation. These include:
- the Meredith Granite, 30km WNW, where the recently discovered Mt Lindsay contact skarn tin and tungsten deposits of Venture Minerals Ltd have indicated and inferred resources of 43Mt @ 0.4% Sn equivalent grade;
- the Pine Hill Granite, 20km WSW, near to where the historic Renison Bell Mine of Metals X Ltd has previously mined and have current resources totalling 24.5Mt @ 1.4% Sn;
- the Heemskirk Granite, 40km SW, where there are contact skarn tin deposits plus distal sulphidic deposits being drilled by Stellar Resources Ltd and with current resources of 4.4Mt @ 1.1% Sn + Pb, Zn & Ag.
The Granite Tor granitoid, which intrudes Mesoproterozoic metasedimentary rocks, has only had limited prospecting and production since the first recorded discovery of alluvial tin in 1910 near Bluff River in the far east of the EL. This prospect was briefly visited by Government geologists in 1944 and in 1966 who indicated that there was no further chance of finding significant alluvial terrace deposits on the steep hillside. They did, however, point to the exposure of a bedrock tin bearing vein system exposed in the previously sluiced area.
Apart from limited Geological Survey traversing in the district, there appears to have been no modern exploration until Alcoa of Australia Ltd took up an EL in 1978 and conducted airborne magnetic, stream sediment sampling, ground reconnaissance and limited soil sampling surveys up until 1981 whereupon they terminated exploration for tin in the EL without delineating significant mineralisation. Their stream sediment sampling indicated widespread anomalous tin and lower order tungsten values in two main areas – Bluff River in the far SE and Mt Swallow in the NW. They carried out soil sampling surveys on wide spacing at these two areas and obtained significant tin values but did not follow up on these before abandoning work on the granite area in favour of the Mt Read Volcanics area to the north.
Geology
The Granite Tor pluton, which measures about 11km E-W and 7km N-S, has been dated at 359 Ma, ie late Devonian and synchronous with the Tabberabberan Orogeny. It is categorised as an S-Type granite and is of biotite-muscovite granite to adamellite composition. It has not been ground mapped in detail but is reasonably well delineated by airborne magnetic and radiometric survey. It has significantly high tin, fluorine and lithium content.
The pluton intrudes Mesoproterozoic age metasediments of the Tyennan complex which have been observed as dominantly quartzite and quartz mica schist lithologies but Alcoa submitted some samples of calc-silicate skarn rocks, indicative of carbonate bearing original lithologies, from the Bluff River area.
The pluton is interpreted to dip shallowly under the intruded Tyennan rocks on all sides but in the Bluff River area there is an apparent NNW-SSE trending graben structure with a roof pendant of metamorphics infaulted into the pluton. There is a strong NNW-SSE lineation apparent over much of the pluton.
Mineralisation
Alluvial tin mineralisation has likely been prospected for in the area since the late 19th century and is alluded to in a 1908 government report but the rugged, dissected nature of the country has precluded significant alluvium volumes in any of the drainages.
Bluff River ProspectDiscovery of alluvial tin was made here in 1910 with minor sporadic attempts at operation over the next 20 years or so but no records of production are available. The two Geological Survey brief reports of 1944 & 1966 covering the Bluff River prospect mention the presence of a small area of sluicings on a high level terrace on an otherwise steep hillside with no remaining volume. The exposed bedrock in the sluiced area was of granite with thin veinlets of quartz-tourmaline and greisen, trending generally north, showing cassiterite and lesser wolframite. The earlier report mentions mineralised veins extending across the contact into metamorphics upslope to the north.
The Alcoa stream sediment sampling produced anomalous values in most of the small creeks and several soil sampled grid lines in the vicinity of the old workings indicated an anomalous area of 500m x 1000m. There is another soil anomaly of 1000m x 1500m about 3km to the NW with no geochemical sampling in between. They apparently recognized an area of sluicing but did not report on the bedrock geology nor follow up on the soil anomaly. They also discovered skarn altered rocks in the graben hosted roof pendant immediately to the west but the several samples assayed did not carry significant tin values. There is, however, strong magnetic anomalism in several localities in this structure that might be caused by mineralised skarns.
Mt Swallow AreaThis is on the northern flank of the Granite Tor pluton where Alcoa obtained a number of high tin stream sediment values and much lower order tungsten. A limited grid was cut and soil sampled but only low order tin values were obtained. The airborne magnetic survey by Alcoa and later by Mines Department Tasmania has indicated several highs in the metamorphics to the north of the pluton. These have not been effectively investigated to date and possibly represent skarn targets.
Targets
he Bluff River area is possibly the principal target area at this stage and mapping is required of bedrock at the old Bluff River workings and throughout the previously defined soil anomaly areas to determine the density, extent and mineralisation concentration of the greisen vein systems to determine if bulk mineable deposits exist. Streamline geological mapping and panning for heavy mineral concentrates in all creeks is required here, in the Mt Swallow area and other areas in the core of the pluton where stream sediment anomalies have been noted.
Magnetic anomalies that have been observed in the peripheral metamorphic rocks and suggestive of skarn alteration also require detailed ground checking, and especially in the graben immediately west of Bluff River prospect where there is stronger magnetic anomalism and previous recognition of skarn alteration of original carbonate bearing sedimentary rocks.
The lack of vehicular access means that programs will be helicopter supported and precludes trenching which would otherwise be desirable. A light core drill will need to be used at an early stage to provide meaningful information on mineralisation scope.
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