Mistral Ruby Mine (Hillrise Prospect), Spriggs Creek Bore (Spriggs Bore), Ambalindum Station, Harts Range (Harts Ranges; Hartz Range; Hartz Ranges), Central Desert Region, Northern Territory, Australiai

Regional Level Types
Mistral Ruby Mine (Hillrise Prospect)	Mine
Spriggs Creek Bore (Spriggs Bore)	- not defined -
Ambalindum Station	- not defined -
Harts Range (Harts Ranges; Hartz Range; Hartz Ranges)	Mountain Range
Central Desert Region	Region
Northern Territory	Territory
Australia	Country

"During the latter part of 1978, it was observed that corundum crystals excavated from subsurface outcrops in the vicinity of Mount Brady were pinker than any previously recovered. Exploratory bulldozer costeaning was carried out on various areas. Several hundred kilograms of red corundum were recovered from these initial excavations, which are nowhere deeper than 5 metres. This discovery is the first ruby deposit recorded in Australia having a commercial significance. It is located six kilometres southeast of Mount Brady in the eastern Harts Range.
The ruby is found in a distinctive layer of greenish gneiss of unusual composition within the Riddock Amphibolite. The best ruby crystals occur mainly in lenticular somewhat dislocated leucocratic bands in which the proportion of anorthite may exceed 90%. The larger ovoid ruby masses occur towards the base of the gneiss lens as irregular segregations."
Mistral Mines NL originally held the leases of this area.

Rubies were discovered here by John Bruce and John Vitosky in 1978, prospecting for Hillside Properties Pty Ltd. A lot of dis-organised digging followed with their D6 bulldozer, uncovering pale pink to red rubies from shallow excavations. A geologist was employed, but detailed work was not carried out until Mistral Mining NL took over led by colourful mining entrepreneur Joseph Gutnick.

Michael Katz from the N.S.W University was invited to look over the deposit, October 1980, in the company of Mistral geologists. Katz would later write a letter to the company imploring them to release specimens to museums, universities, and the collectors market. Mistral produced some rubies for its jewellery stores in southern Australia during 1981. After they abandoned the mine, it was the focus for fossickers. Barfuss Corporation Pty Ltd who took over the lease seven years later, complaining about the material taken. Specimens may come from any of these sources.

In 1982, thirty five third year geology students from the Adelaide University descended on the mine site for a one week mapping exercise. After a Federal government grant, the following year five Adelaide University geology staff, four post graduate students, and three honours students spent six months studying the site. One, R.W. Lawrence, completed his Ph.D thesis on the deposit.

At least one person in the Northern Territory Mines Department was unimpressed in the focus on geological study, rather than progressing to mining. When the company's report (Lawrence, R.W., Adams, P.J.,Exploration Licence 1801/1956. Report to the Northern Territory Mines Department. Annual Report 1982-1983), stated exploration at the deposit had been 'highly successful', the department added a hand written note to the report:
'In which way? Presumably for the university'.

The 1987 stock market crash, poor quality of the rubies found, difficulties finding a market, the limited resource, and the hard rock requiring explosives which damaged the already fractured rubies, all led to Mistral abandoning the property. Ignoring the prior Mistral work, Barfuss Corporation Pty Ltd came to the same conclusion by 2003 after seven years of investigations.

The mine is 25 kilometres south of the Plenty Highway. To reach, head along a track south, leaving the Plenty Highway about one kilometre before the Entire Creek crossing. After 2 kilometres the Entire Bore will be reached, continue south. Eventually a track will come in from the east. Alternatively take the track south immediately after the Entire Creek crossing on the Plenty Highway. Eventually a track will come in sharply from the north-west. Take this track, cross the Entire Creek, and you will end up at the same intersection mentioned above. Go south-south-west, the track turning west. A dam is on the south side of the track here. The track will wend generally west through hilly country to the mine site. The eastern side of the mine contains a mining camp, the shallow discontinuous workings extending south approximately 2 kilometres across hilly country. The mine could be under an active lease at any time, and see main Ambalindum Station heading for access requirements otherwise.

The basal unit at the mine site is Bruna Granitic Gneiss, being a sheet like body, mylonitic in the upper portion. Above this is the Irindina Supracrustal Assemblage of semi pelitic to pelitic biotite-garnet gneisses containing sillimanite, distinct units of marble and quartzite with diopside-plagioclase-quartz calc-silicate boudins in marble, and three amphibolite units. Partly above this is the Entire Anorthosite, containing some leucocratic gneiss, separated from the main body by a 2 metre garnet-biotite schist.

Rubies are found in two forms. The first is irregularly shaped ruby corundum in matted dark green amphibolite, and secondly a very hard white anorthosite. In this specimens show opaque pink to red hexagonal crystals to one inch in diameter. Pale pink tend to be from the upper levels, however a number of factors effect the level of chromium which influence its colour. Some ruby corundum has altered partly or wholly to pale bright green diaspore. The amphibolite may contain masses of ruby with massive schorl. Also found is fine-grained clear dark green to black epidote in white calcareous sediments. Vermiculite and garnets are found nearby, and where possible will be written separately.

There has been several theories over the years on how the material was formed. The following is the most plausible. High temperature, pressure and water caused the ultramafics to be replaced with amphibolite. This required silica from the adjacent rocks. This changed the composition of the leucogneiss which had infolded into the ultramafics, resulting in an excess of alumina, which could no longer be incorporated into the silica. Ruby corundum distribution along the linear line is erratic, based over how advanced the above process locally reached. Olivine requires more silica to form amphibolite than pyroxene rich rocks. Factors such as the amount of leucogneiss infolding to the volume of silica also influenced subsequent formation of ruby corundum. The second generation of ruby corundum was crystallised in a stress fold as hexagonal crystals as thin plates. The southern workings show deformation in the form of warped plates. Ruby corundum is found generally on a north-south trend at the mine site, due to a large asymmetrical Z shaped fold, trending north-east.

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