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Saturday, 9 May 2015

THE RUBY HILL GARNET DEPOSIT NEAR BINGARA, NEW ENGLAND NSW AUSTRALIA



Google Earth photo - the signpost coming from the north
I’ve passed by this place several times without going in. As there are many references to it as a fossicking area, I suppose it must be open to the public, though whether this is public or private land I’m unable to say. It is well signposted, more visibly so when travelling south from Bingara, being on the right hand side of the road linking Bingara to Barraba (commonly called the Fossickers Way). The most precise distance from Bingara I’ve read is 18.6km, though from what point in the town this was measured is not stated. There are no rubies there – they proved to be garnets – but there have been several dubious claims of diamonds being found there. More of this later.
The earliest reference to the place appears to be DA Porter, “Notes on Some Minerals and Mineral Locations in the Northern Districts of NSW”, published in the Journal of the Royal Society of NSW in 1894. I read this many years ago and there is no certainty that Porter is referring to the present day Ruby Hill. The next reference is by JM Curran, in the same Journal in 1896. JF Lovering quotes Curran as saying that a red, pyrope-rich garnet was found in a coarsely-crystallising basic rock within the basalt at Ruby Hill. The easiest reference to read from the early days is in “The Mineral Resources of NSW”, by EF Pittman, 1901. The DIGS reference is R0005113. Read pages 392-395.
Photo from Pittman, 1901
Pittman wrote as follows: “A small hill, about eighty feet in height, which is intersected by several basalt dykes, was pegged out some years ago by a miner named Butt, who was attracted by some red crystals in the basalt. Under the impression that these crystals were rubies, Butt drove a tunnel, forty feet in length, in one of the hard basalt dykes, and only desisted on being informed that the red stones were garnets and of no value.” Pittman went on to tell of the search for diamonds on the same hill. He gives the date 1889 for the formation of a diamond prospecting syndicate. The upshot of this was that “Ten diamonds, weighing in the aggregate four and one-eighth carats, were obtained from it. The diamonds were remarkable in that they each had a distinct depression or pitting on one of the faces.
The first edition of “Gemstones”, Mineral Industry series 18, page 66, (1960) (DIGS reference R00050829) throws doubt on the validity of this discovery; however it also states that “in 1922 ER Dickens treated alluvial material in the vicinity of Ruby Hill, and it is reported that some hundreds of small diamonds were obtained but were not sold”. I wonder what became of them? There are other Ruby Hills in NSW so perhaps these diamonds came from elsewhere. The second edition (1980, DIGS reference R00050830) mentions only the earlier find.
Map from Pittman, 1901
What emerges from these readings is that the garnets were found both in the breccia, which makes up much of the hill, and in the dykes which intrude it. They are apparently derived from fragments of eclogite found in the breccia (along with many other types of rock, which is typical of brecciated volcanic pipes) and in the later basalt, which they could have entered from the breccia when it was intruded by the basaltic magma. They might also have been carried up from a considerable depth from the same source rock as that penetrated by the breccia.
Lovering addressed the nature of the eclogite fragments in an article in the Journal of the Royal Society of NSW in 1964 (Volume 97, pages 73-79). He stated “Eclogites occurring as inclusions in igneous pipes are particularly important in that they may well represent direct samples of the upper mantle caught up in the magma of the pipe during its passage to the earth’s surface”. He prefers to call the inclusions “garnet granulite” and adds that individual xenocrysts of garnet are also found in the breccia and dykes, no doubt derived from the breakdown of the granulite.
Because of the (apparently) superficial resemblance of the occurrence to South African diamond bearing kimberlite pipes, there has been quite a lot of interest in this locality as a potential source of diamonds.  One such report, written by GL Rolfe for Anaconda Australia Inc. in 1979 (DIGS reference R00015604), concluded that the rocks there were unlikely to be diamantiferous. Commenting on the garnets, the report states “The most common xenocryst in both the breccia and basalt is garnet. The garnets are generally orange pink in colour, size varies from 1 to 5mm. Reaction of the garnet with the magma during ascent to the surface has resulted in kelyphitic rims one to two mm wide. The keliphytic rims consist of clay mineral and zeolites. The rims are wider in the basalt than the breccia, indicating a longer period of disequilibrium between the garnet and magma”.
Reports from fossickers of outings to Ruby Hill are few. Some say they found little or nothing, others that there were numerous small garnets in the soil. Obviously, the soil derived from the dykes is going to be the most productive, so these places need to be identified from the map, rather than by digging at random. It seems that nearly all the gems will be found in a small sized mesh sieve, a coarser sieve being used to remove larger pieces of rock etc. There will probably be no water on site, so dry sieving to concentrate the gems followed by a dip in water you will need to take with you is the most likely approach. There may be water in nearby Hall’s Creek, but this will almost certainly be on private property.
Some other useful references: Geological Tour of the Barraba-Bingara District - Bob and Nancy’s Geological Tours Site (here) and a thread from Prospecting Australia’s website (here). There are some interesting photographs in this thread, but as there is no mention of sapphires being found at Ruby Hill in any other reference, the bluish stones may have come from somewhere else. Some object for scale (such as a coin) in the photos would have helped.
My gem hunting/mining You Tube playlist is here. I have three other playlists - the Blue Mountains, Glen Innes and New Zealand.

1 comment:

  1. Was a lot of garnets there about 25 years ago. My Grandfather used to cut gemstones and he used to take some of us grandkids out to ruby hill after it had rained and you could see the garnets shining on top of the ground literally everywhere (he was sorting out the fossickers from the frolickers for upcoming trips to the Hart Ranges i now believe) but i do not remember if he ever found any garnets that were of any significant quality or size.

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