John's New England Minerals and General Geology Blog: May 2015

Saturday, 9 May 2015

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

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. Search Trove.

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.

Another useful reference: A thread from Prospecting Australia’s website (here). There are some interesting photographs in this thread, but as there is no mention of sapphire 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.

https://johnsbluemountainsblog.blogspot.com/2013/12/links-to-all-blog-entries-and-relevant.html All Blue Mountains blogs and videos

https://johnsnewenglandminerals.blogspot.com/2013/12/list-of-all-blog-entries-to-date.html

All New England and other Geology blogs and videos

https://www.blogger.com/blog/posts/1325599670344725659?pli=1

Limestone Caves of NSW

https://www.blogger.com/blog/post/edit/1147003616874115974/2430103125095714141

Song Studies. Bible studies based on hymns and songs

https://www.blogger.com/blog/posts/1515863000684737715

Shoalhaven District Geology.

Wednesday, 6 May 2015

GEM DEPOSITS ALONG THE SARA RIVER, NEW ENGLAND NSW AUSTRALIA

GEM DEPOSITS ALONG THE SARA RIVER

NEW ENGLAND NSW AUSTRALIA

This area is one of the most interesting gem producing regions in New South Wales. It’s also one of the most frustrating because of the difficulty in gaining access to the river and its tributaries. The map is extracted from the Records of the Geological Survey of NSW 14(1), which contains a valuable article titled “Sapphires in the New England District, New South Wales”. If you would like to download a copy, try Trove or locate DIGS.

Backwater-Kookabookra area, 40km south-east of Glen Innes

A further complication is that the river is sometimes called the Mitchell River; adding to the confusion is the fact that the nearby Mann River also has the Mitchell River as an alternative name!

There is plenty of information in issues of “Gold, Gem and Treasure” magazine, the Australian Lapidary Forum (ALF) website (here) and on DIGS (if you can locate it).

Practically every tributary of the Sara (Mitchell) river is worth fossicking. Unfortunately, they are nearly all either on private land, in a National Park or simply nowhere near a road or track. If you wish to get onto private land areas, you are going to have to approach landowners for permission. Many of them will have already had enough of trespassers, shooters, people who dig holes and don’t backfill them, fire lighters and fence cutters, so be prepared for an earful.

It isn’t at all obvious from the map, but the river rises on the upper left of the map (west of Bullwarra), passes under the Backwater road (signposted as the Mitchell River) and flows south of Mt Mitchell via the Horse Shoe Bend to Kookabookra and beyond. The creek south of it is coloured green to indicate that it carries sapphire. The fact is, so do most of the streams and gullies on the map.

As a guide for readers, I’ll say something about the three areas I have actually been to. Firstly, upstream of the Backwater road bridge. We checked this out once for a possible Minerama trip but rejected it because of our poor finds that day. This was on Bullwarra property. The area was mined for gems, probably in the 1970’s, and all I found was a few bits of sapphire on the track (evidently spilled with gravel on the way to the treatment plant) and a nugget of cassiterite (tinstone) two or three centimetres across, which I saw lying on the ground. We were unaware of the fact that Cockatoo Creek was also gem bearing, otherwise we would have checked it out too.The presence of gold and cassiterite (tin) in the wash is normal for the area. Whether any mining was ever carried on in this creek I cannot say.

The Horse Shoe Bend section of the river is accessible by 4WD vehicles along the Horse Shoe Bend fire trail, which is reached by following McGarry’s Lane and Aqua Park Road eastwards off the Backwater road, a little south of the Yarrow Creek bridge. The descent is quite spectacular, with Crown Mountain (granite outcrops) straight ahead at the beginning of the trail. From the fossicking point of view, the problem is that most of the area is now in the Warra National Park. See this article from National Parks and Wildlife (here). Although much is said about the flora, fauna, history etc nothing is said about the reason many people would want to go there – fossicking. I have been there only once and found gold and small gemstones in the river easily enough. Another DIGS document (Reference number R00046781) makes for interesting reading at this stage. It’s a summary of the prospecting done in the area by Mr W Madgwick of Glen Innes in 1974. There is private property all around and trespassers are certainly not welcome.

Sara River at Kookabookra. Photo by Wwoofa, ALF

Most of the information available concerns the third area, at Kookabookra. This is well known as a fossicking area which produces quartz crystals, black tourmaline (schorl), gold, cassiterite (tin), topaz, sapphire and zircon. Fortunately for our generation, the early miners were only after gold and tin and discarded the rest. The actual area is well described in the blogs I listed near the beginning of this blog. This land is, I believe, a crown land lease held by the adjacent landowner who has given permission for fossickers to work there. Please respect this permission and do not trespass elsewhere, camp, light fires or leave rubbish behind. There is a further accessible area at the Sara River bridge a kilometre or so downstream, on the Ward’s Mistake road. This branches off the Pinkett road on the right a short distance back towards Glen Innes.

The Mitchell River Dredge from the Pinkett history book

This stretch of the river was extensively dredged, especially in the depression years. Sue’s grandfather worked as a woodcutter for the Mt Mitchell (Mitchell River) dredge and her mother began her schooling at the nearby Kookabookra school. If you can find a copy, the little book “Pinkett, Mogg’s Swamp, Kookabookra Past and Present” (1988) makes good reading.