The Huonville Sphene Pit - Reprinted and Updated From An Article first published in the Australian And New Zealand Mineral Collector Magazine, Issue 3: September 2007.

Approximately thirty kilometres south of Broken Hill, on the flank of a small rise, is a curiosity of sorts. On this hill is a pit that was worked periodically over the last seventy years as a source of gem quality sphene – the old alternative gemstone name for the mineral titanite. The pit is reached either from the east via Huonville Station or from the west via Ascot Vale Station and the Copper Blow. The deposit is approximately two kilometres north of the Huonville to Copper Blow track and is marked on the NSW Geological Survey Metallogenic Map as RK 257.

The deposit was first found sometime around the 1930’s and was exploited by Mr H. Hore, who sent a few fragments to the Mining Museum in Sydney for identification. He worked the deposit for the gem sphenes and little is known about how much gem material was extracted and cut during this time. In the 1950’s three stones, the largest of one carat, were cut from material collected by Mr R.O. Chalmers of the Australian Museum, who had visited and studied the mineralogy at the site. He noted during his visit that the original outcrop of the deposit had been excavated to a point that there was none still in situ. For many years following it was abandoned and the location effectively lost. It was rediscovered in late 1970’s by a geologist – Graham Bradley – working for the Geological Survey who shared the discovery with other academics including Professor Ian Plimer, then working at the now closed Broken Hill Campus of the University of New South Wales. This sparked renewed interest and new study of the mineralogy and origin of the deposit.

The titanite occurs in a pipe style deposit that is suggested to have formed during retrograde metamorphism along fracture zones in the Little Broken Hill Gabbro. This gabbro is largest of several ultrabasic intrusives in the Broken Hill block post dating the high-grade metamorphic rocks that dominate in the region. The gabbro was probably fractured and sheared during the Delamarian Orogeny and the final stages of uplift, around five hundred million years ago. The outcrop of the gabbro has a distinct red-brown colour and is clearly seen on aerial photographs contrasting darker against the surrounding metamorphic rocks.

The shallow pit itself has opened up the vein that was approximately three to five metres long and up to 0.3 metres wide with a central core of both coarse calcite and smaller calcite veinlets. Within the coarse calcite are actinolite needles giving the calcite a green hue. Platy crystals of ilmenite are sometimes found cutting through the calcite. The edge of the vein has coarsely crystalline albite, up to 20mm, grading into a fine-grained albite rock then altered amphibole and fine grained chlorite rock. Chlorite also occurs coating the top of the coarser albite. The fine-grained albite rock is quite porous and it often coated along fractures by calcrete. This calcrete is thought to have formed by dissolution and redeposition of the calcite. Along the contact of the albite and the calcite are the large titanite crystals. The titanite crystals are typically wedge shaped and yellow-green in colour, this colour is probably due to an increased iron content with Fe2+ substituting for Ti4+ ions within the crystals (Plimer 1983). They range from micro-crystals scattered across the albite to 30 mm crystals sitting proud. Some excellent matrix pieces have been obtained by etching away the calcite to expose the titanite on albite, while others are already exposed but covered with residual calcrete. Within the fine-grained albite rock are micro crystals of epidote, magnetite and actinolite. While the mineral assemblage of albite – titanite – calcite – epidote is similar to world renowned localities such as Capelinha: Brazil, Gilgit: Pakistan and Imilchil: Morocco, the only other noted Australian titanite location with a similar mineralogy is the Entire Valley in the Harts Range, Northern Territory. Those found here also stand proud, have similar size ranges but are mostly brown, unlike the Huonville green titanite. The albite from these two locations differs considerably in size. While fist size single crystals are common in the Harts Ranges, the albite crystals from Huonville rarely exceed 20mm across.

Since its rediscovery in the 1970’s the dump material at the pit was methodically picked over by numerous individuals and small groups in the ensuing years. As a result the tops of the dumps have been raked several times and the surface material picked clean. A stockpile of the coarse green calcite, with blocks up to 200mm on edge once sat at the south-western side of the pit, however it was near totally removed by a few over zealous fossickers in the 1980’s. The days of specking the surface and picking up good titanite crystals have gone and only via sieving the dump material are the remaining crystals able to be found. Even through this activity there is no guarantee of finding good crystals, as the dump material has been well turned and finding a new spot to dig is difficult. The most recent visit to this site by the Broken Hill Mineral Club was in May 2009 and after much sieving found a handful of small crystals, with the largest being 20mm. While the dumps at the pit have been well turned and are becoming near exhausted, the flank of the hill down from the workings looks to be the best area to now dig and sieve and worth a revisit. The pit itself has been partially backfilled by a large pile of rubble containing rocks up to 0.5 metres in size and hence the original vein opened up in the early years cannot be seen. Those persons lucky enough to have good titanite specimens from this location should consider them rare and valuable, for it is unlikely that more will come out in the future.

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