Modern exploration began in 1965 when the area was examined by the Mt Lyell Mining & Railway Co Ltd. Exploration in the liscenced areas has largely focused on locating tin, tungsten, and fluorite mineralisation associated with the younger Devonian (Dolcoath) Granite. Importantly, exploration for base metals finished before the discovery of the Cadia porphyry deposit and it’s exploration characteristics.

• 1960’s Mt Lyell Mining Company
• 1973 Freeport
• 1970’s Comalco
• Early 1980’s Shell
• Mid 1980’s CRA
• Late 1980’s RGC
• Rio Tinto Exploration 1996-1997
• Other work

1960’s Mt Lyell Mining Company

In 1966, the Mt Lyell Mining Company conducted a regional airborne magnetometer over the Staverton area (EL8/65), including the Five Mile Rise Goldfield and Forth River Valley. The Cethana anomaly (then named Anomaly 24) was noted at the time as being the highest amplitude anomaly within the region.

The aeromagnetic anomalism was interpreted as being an expression of ‘basic’ Cambrian lithologies. In contrast, gold mineralisation at Campbells Reward was thought to be Devonian and unrelated to the aeromagnetic anomaly. Subsequent work showed the anomaly to be hosted within felsic rocks. However, the assumption that the two features were unrelated persisted until the mid 1980’s.

By 1967, the Mt Lyell Mining Company had completed reconnaissance -80# (mesh) stream sediment sampling and geological mapping programs over EL8/65. The Cethana anomaly sits on a promontory into the main Forth River and is drained mainly by two very small seasonal streams, neither of which were sampled by the Mt Lyell company.

Mt Lyell Mining Company noted only that the small stream flowing into the Forth River at the western end of the Cethana anomaly was anomalous in zinc (150ppm) and possibly anomalous in copper (22ppm). By contrast the exposed mineralisation on the Five Mile Rise Goldfield yielded strong gold anomalism. Several areas were recommended for follow up stream sediments including the possibly anomalous copper in stream sediments (34ppm Cu) draining from the magnetic anomaly 12 (later aeromagnetic anomaly C) above the Powerful mine.

Mt Lyell Mining Company identified two locations where disseminated chalcopyrite and pyrrhotite occurred in ‘granite associated porphyry’ upstream from the Devon Mine, they also located minor disseminated chalcopyrite in the Cambrian rocks on the southeast slopes of the Five Mile Rise.

In 1967-8, the Cethana anomaly was surveyed using a ground magnetometer. Peak magnetism was associated with north dipping sheared quartz-magnetite-chlorite schist on the southern side of the main Cethana aeromagnetic anomaly. The results of rock chip samples collected from the schist were not reported, but did not reveal any ’significant economic concentrations of elements’. In contrast, a small number of soil samples collected on the southern flank of the anomaly revealed cobalt anomalism of 380ppm, as well as lesser Zn and Cu anomalism (c100ppm).

Mt Lyell geologists were uncertain as to whether Co anomalism was due to Tertiary basalt. However, the geochemistry of 16 Tertiary basalts from the region (provided courtesy of John Everard, Mineral Resources Tasmania) show an average Co content of 50ppm, with an SD of only 7ppm. If the residual soil value of 400ppm is derived from the basalt then it is highly unusual.

As a result of their investigations, the Mt Lyell geologists recommended more detailed soil sampling (including Au) and two drill holes. However, subsequent years saw exploration focused on other areas due to the nearer to surface expression of mineralisation. This, coupled with a disastrous loss of base camp due to flooding of the Iris River meant work on the Cethana anomaly was never completed and the ground was finally relinquished.

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1973 Freeport

Freeport’s main target was outcropping porphyry copper mineralisation, but considered the chances of finding gold, tin and tungsten or strata bound lead-zinc-copper. They employed consultants whose activities consisted of mapping, rock chip sampling and stream sediment sampling. They concluded it was unlikely that a Cu-porphyry ore body existed in the Dove River area. Freeport’s Licence was approximately the same area as the current Dove River licence. This type of exploration has not precluded a buried porphyry system. Freeport also erroneously thought the Dove granite to be younger (Devonian) in age even though Mt Lyell had indicated otherwise.

Exploration was focused on the flanks and cusps of the granite due to identifying alteration in the granite at the Powerful prospect associated with minor disseminated pyrite-chalcopyrite. Freeport located significant copper anomalism in both stream sediments and rock chip (to 0.16% in fractures in the Dove River near the Devon Mine and several regional 600ppm samples from fractured granite and quartzite and Molybdenum values to 160ppm in quartzite in the East near the Powerful Granite) but did not follow this up. Freeport also identified haematite alteration in the “Lorinna Greywacke” and concluded that this was caused by the late stages of the Dove Granite with chlorite veins containing minor gold and base metals.

The consultants prefaced their investigation with the assertion that the Five Mile Rise gold-sulphide deposits were related to granite intrusion and the Devon Mine was described as being near or at the Pre-Cambrian contact with the Dove Granite. This has since been identified by Pluton geologists as a granite-volcanic rock contact. The EL was relinquished in 1973.

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1970’s Comalco

In 1974, the Cethana anomaly was included within exploration licence 7/74, held by Comalco. Like EL8/65, EL7/74 included large tracts of land and included deposits located north of the outcropping Dolcoath granite. Comalco’s exploration was primarily focused on locating extensions to fluorite mineralisation previously found at Moina. The fluorite was to be used in Comalco’s aluminum smelters. Most reports up until 1980 deal almost exclusively with exploration in the Moina area.

Like the Mt Lyell company, Comalco embarked on a regional stream sediment program in the mid 70’s, however, unlike Mt Lyell, Comalco used -20# (mesh) in the mistaken belief that -80# would not yield enough fine material in the steep terrane.

Ironically, Freeport had already run tests a year or so earlier and had shown that sampling using -40# underestimated results using -80# by 60% to 85%. Comalco did ultimately realise their mistake when areas of known mineralisation failed to show up in -20# data. They switched to using more conventional -80#.

Streams north and south of the Cethana anomaly were sampled using only -20#. Like the Mt Lyell company, Comalco did not include the small seasonal streams draining the Cethana anomaly. Not surprisingly, Comalco’s -20# results show only very weak zinc anomalism (c85ppm) and moderate F anomalism (500-1300ppm) in the vicinity of the Cethana anomaly.

Comalco went on to explore the Cethana anomaly further. Unlike the Mt Lyell company, Comalco assumed that magnetite associated with the Cethana anomaly was of Devonian age. This exploration model appealed because a Devonian deposit was more likely to yield a fluorine-rich ‘wrigglite’, similar to that hosted by Ordovician rocks at Moina. A ground geophysical and geological program included gridding, ground magnetic and soil sampling(Pb, Zn, Cu, Co) was undertaken over the Cethana anomaly.

This assumption that mineralisation was Devonian was pivotal. What must have been disappointing to Comalco geologists was the almost complete lack of metal anomalism in Ordovician rocks. Samples of quartz-veined scree, typical of the Ordovician sandstone, were also barren. The geologists did note Cu and Pb anomalism in Cambrian rocks on the flanks of the Cethana anomaly but went on to conclude that this reflected nothing more than ‘ elevated background’. This, despite values in soils up to 32x that already calculated as background for the Cambrian volcanics.

Comalco suggested that gold and lead at Five Mile Rise may be remobilised from volcanogenic base metal deposits and recommended an EM survey. Comalco mapped the Dove Granite adjoining the Moina Sandstone at the Golden Cliff, this work was never completed. This area was later relinquished by Shell in 1983.

Given proximity to Campbells Reward, it was again recommended that the Cethana area be sampled for gold. Again, this work was not completed.

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Early 1980’s Shell

EL7/74 was transferred to the Commonwealth Aluminum Corporation in early 1979 and then in joint venture to the Shell company of Australia in early 1980 (TCR82-1728). Like Comalco, Shell considered its focus to be Devonian mineralisation associated with the main wrigglite/pyrrhotite/sphalerite skarns in the Shepherd & Murphy Mine (Moina), and a possible low-grade Sn-Au zone in the Tin Spur area.

Shell reviewed Comalco’s exploration of the Cethana anomaly (now renamed Lorinna North). They conducted their own regional aeromagnetic survey and noted that measured magnetic susceptibilities in surface rocks at Cethana did not account for the intensity of the aeromagnetic anomaly at Cethana.

A 144m percussion hole (PD1) was drilled approximately in the centre of the anomaly but away from previously detected geochemical anomalism and distal to the Campbells Reward Mine.

PD1 passed through 58m of Tertiary cover before intersecting weak metal anomalism in Cambrian magnetite-altered volcanic. The hole was assayed every 2m through cover but only once every 10m in the mineralised volcanic. The hole suffered from water problems and sample dilution. Although only mildly anomalous in copper (the log showing up to 280ppm Cu and 290ppm Zn), the results were mistakenly under-reported as being a maximum of only 105ppm Cu.

Gold was not assayed.

Susceptibilities measured from drill chips were believed at the time to explain the anomaly as being a magnetite-altered andesite. No attempt was made to determine the cause of the alteration.

Shell also noted that the Comalco grid was not centred on the anomaly but was rather biased south of the anomaly and into areas of Ordovician and Tertiary cover. This was probably due to misregistration of the aeromagnetic anomaly noted in the 1960’s geophysical data. Instead of collecting new samples, Shell re-assayed soil samples previously collected by Comalco, but this time for Sn, W, As, and Bi. They did not explore the possibility of extensions to Pb and Cu anomalism Comalco had previously identified in Cambrian rocks along the western edges of their grid.

Like Comalco, Shell assumed a Devonian age for mineralisation, consistent with the age of mineralisation in their main areas of focus around Moina. Unlike Moina, they found no appreciable surface mineralisation associated with the Cethana anomaly.

Shell re-submitted two lines of Comalco soil samples for gold assay. PD1 had already shown that Cu-Zn anomalism extended no more than a few meters into overlying Tertiary cover. Yet, only six of 39 soil samples resubmitted for gold were from soils overlying Cambrian rocks. The balance were from areas of thick Ordovician or Tertiary cover. None of the samples were from areas previously showing Copper or Lead anomalism. None of these samples were from the vicinity of the Campbells Reward Goldmine. All the samples were up hill and/or in separate catchments to sites previously showing metal anomalism.

All samples produced <50ppb Au, or below detection. By regional standards, this level of Au is now considered anomalous.

Shell identified the “Lorinna East” bullseye anomaly adjacent to the Powerful mine as ‘possibly a plug with a south dip’. They drilled one 200m long percussion hole into the bullseye anomaly. The lithologies intersected were Tertiary basalt to 58m, Tertiary clays to 80m, Pre-Cambrian schist with minor granitic veining to 200m. Susceptibilities in the basalt were considered too low to explain the aeromagnetic anomaly. Remanent magnetism studies were meant to be done on these basalts. The conclusion was that remnant magnetism caused the anomalies even though the aeromagnetic signal ‘could not be formational’ according to the report and no evidence tendered that remnant magnetism studies were done. The drill hole contained elevated Ba, Cu, Zn, however was not assayed for gold, despite being located adjacent to the Powerful gold mine.

The exploration licence area was reduced in 1984 but no work was completed on the prospects within the current Dove River licence area retained in the 7/74 licence.

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Mid 1980’s CRA

In 1985, CRAE became managers in EL7/74 in a three-way joint venture with the Commonwealth Aluminum Corporation and Shell. CRAE embarked on another very widely spaced reconnaissance stream sediment survey. The only sample collected from the vicinity of the Cethana anomaly was upstream of the Lorinna Road, both up-stream and east of the Cethana anomaly.

In 1986/7 CRAE reprocessed and reinterpreted Shell’s aeromagnetic data. They ‘rediscovered’ the Cethana anomaly although this time it was referred to as Anomaly 36. It was again noted to be the largest anomaly within the region and, again, the association was made between the anomaly and the Campbells Reward Goldmine. CRAE disregarded that anomaly as a basic volcanic containing high magnetite. This interpretation made no reference to the earlier work already identifying the rock as variously rhyolitic, dacitic and andesitic. It also did not consider that the magnitude of the anomaly exceeded that easily explained by most basic Cambrian volcanic rocks known from the Mt Read Volcanic belt.

Exploration licence 7/74 was subsequently dropped, with the joint venture maintaining tenure over the Moina fluorite deposit via Retention Licence 10/1988.

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Late 1980’s RGC

By 1988, the Cethana anomaly was included in EL8/88, held by RGC. RGC again undertook reconnaissance stream sediment sampling, collecting both -200# and panned concentrate samples from 84 locations. In contrast to CRAE, RGC collected 2 samples from the streams north of and below the Cethana anomaly. One -200# sample proved weakly anomalous in Au (15ppb) relative to surrounding areas (<5ppb), whereas the panned concentrate returned a significantly higher 135ppb Au.

Importantly, CRAE’s sample from above the Lorinna Road returned only 0.1ppb Au. The anomalous samples from RGC, therefore, could only have originated from the Cethana anomaly.

RGC also embarked on two re-interpretations of regional geophysical data previously collected by Shell and a more recent survey over the area by the Mines Department.

RGC did not conduct any further work in the vicinity of the Cethana anomaly. Rather, they concentrated subsequent exploration efforts in Ordovician rocks in the Five Mile Rise and Round Hill areas. Ironically, soil results for Cu in the Round Hill area were significantly less than that that had already been identified at Cethana by Comalco.

A 20.5km grid was established at Five Mile Rise with an east-west base line and 15 north-south lines spaced every 200m easting. Mapping was completed from scarce outcrop and C-horizon soil samples taken every 25m in a north-south direction. Soil sampling was aided by a well developed soil profile allowing representative samples to be taken. The grid was largely located over the Ordovician Moina Sandstone. Results were not reported until 1990.

A program of stream sediment and rock chip sampling was undertaken by consultant W.Herrmann for RGC in 1989. The stream sediment survey of approximately 160 sample sites was a regional program covering three other licences and EL8/1988 which overlaps with the current Dove River tenement. Fifty gram panned concentrate (from <0.5mm and >200 mesh fraction) and -200 mesh wet sieved fractions were collected at 20 locations within the current licence and assayed by Neutron Activation Analysis.

Very high gold values were obtained from the sediments in the Five Mile Rise Goldfield. Moderate anomalism was obtained from the Powerful prospect area and moderate-high anomalism was recorded in a south draining creek, north of the Devon Mine. This anomalous sample was not repeated in the creek immediately to the east suggesting a local source

Gradient array Induced Polarisation was conducted and the survey and identified five chargeability anomalies and 6 drill targets. The geophysical contractor also made the mistake of referring to the Dove Granite as Devonian.

RGC relinquished most of its interest in the Moina and Cethana areas in 1990 without drilling any of the identified geophysical targets.

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Rio Tinto Exploration 1996-1997

Rio Tinto acquired a new Exploration Licence – EL30/1996 covering 242 km2 which is approximately eight times larger than the current Dove River Exploration License. The target style was sediment hosted fine grained sulphide poor Carlin or Sepon style gold. They were focused on the Ordovician Gordon Limestone and the Moina Sandstone where they are intruded by Devonian Granite

Rio Tinto took 12 -80# stream sediment samples and 12 panned concentrate samples. They found the Five Mile Rise area to be anomalous for gold and lead. Rio Tinto then reviewed RGC’s soil and ground geophysical data for Five Mile Rise and decided that existing small workings were not a viable target of this type. The main reason for their withdrawal was the lack of conductors in the RGC survey. The licence was surrendered November 1997.

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Other work

In 1999, the area was remapped by Mineral Resources Tasmania and a number of samples collected for petrological examination. The geologists mapping the area were unaware of the Cethana anomaly but collected samples containing up to 20% galena from areas previously shown to be anomalous in Lead. The samples were not assayed.

Western Tasmanian Regional Minerals program aeromagnetic, radiometric and electromagnetic data for the area was subsequently released in 2000/2001. In 2003, these data were used to help assess the potential of the area to yield granite-related mineralisation. The Cethana anomaly was recommended for field checking. This was not done due to a lack of funds.

In 2003, a review of the stratigraphy of the Cambrian Mt Read Volcanics in the area again mentions the aeromagnetic anomaly east of Lake Cethana (UR2003-17). This report again concludes that the anomaly is likely to be a Devonian skarn, despite work already concluding this is not the case.

Pluton acquired the Dove River licence and presented it in the 2006 Prospectus to the stock exchange, subsequently Pluton entered into a Joint Venture over the Cethana Licences in 2007 with a right to earn a 60% managing interest.

For more information on results from Pluton’s exploration see the drilling page.

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