Minerama Book 8: SAPPHIRE

This is the final Minerama book. I would have liked to have done more but the opportunity had passed.

 

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Shoalhaven District Geology.

SAPPHIRE DEPOSITS AT BACK PLAIN NEAR GLEN INNES NSW AUSTRALIA

SAPPHIRE DEPOSITS AT BACK PLAIN 

NEAR GLEN INNES NSW AUSTRALIA

Before you read this blog, I recommend that you read my blog entry “The Red Rock Sapphire Deposit” (here). It deals with gemstone deposits in the same general area as I am covering here.

A good starting point in understanding the sapphire deposits at Back Plain (not Back Plains, by the way) is the map at the back of “Records of the Geological Survey of NSW, 14 Part 1 (1971)”, DIGS reference R00050764. If you haven’t downloaded this document yet, you should do so. The map shows only that Wellingrove Creek (left) and the Wellingrove Fossicking Area (on the letters Pgw)) are sources of sapphire. Both are marked in green. The reddish area near the centre of the map (marked Ts) is apparently an area of Tertiary sediments associated with the basalt.

The next map is extracted from the “Quarterly Notes of the Geological Survey of NSW 103, April 1997” (page 13). The DIGS Reference is R00040935. My extract from the map shows the following sapphire bearing areas of interest to us here: Back Plain Creek in the centre, with a tributary to the west of it and the creek flowing down from the Wellingrove Fossicking Area into Wellingrove Creek.

The third map has been extracted from the Grafton-Maclean Metallogenic Map (DIGS reference R00056102).  To help you align it with the other maps, the Wellingrove Fossicking Area is approximately in the centre, shown as 1835. 1836 is identified in the accompanying notes as Pablos Mine and 1837 as the Back Plain Creek deposit, elsewhere called the Truro Mine. We can distinguish two types of sapphire deposits on this map. Firstly, the close cross hatched areas (Wellingrove Creek, the stream draining the Wellingrove Fossicking Area and Back Plain Creek, not named on the map) which are regular alluvial deposits, and secondly, the broad hatched areas (one around Pablos Mine and the other near the top of the map adjoining an area of Tertiary basalt labelled Tb. These are locations where spinel, sapphire and zircon are to be found in the soil, possibly derived from tuffaceous sediments intercalated with the earliest basalt flows. The Red Rock deposit (see above) is like that and the Wellingrove Fossicking Area deposit could be a contemporaneous alluvial concentration derived from such sediments.

There isn’t much information available about these deposits, except for one (Truro) and even that isn’t clear

Ruwenzorie Road turnoff (on right)

cut. Starting from the north, we have a site which has at least the potential to produce some gems. This is the area shown in broad hatching along Ruwenzorie Road, which branches off to the right (north) from the Strathbogie Road. This is before you come to the back road from Wellingrove (Polhill Road). This road crosses Back Plain Creek after 2.5km at a place where there are some rocky outcrops. I only went there once and found some colour easily enough (without getting wet or covered with leaches). It’s certainly worth a try, if it’s still accessible. Then there is the possibility of gems in the soil on the road side. You could try dry sieving and then take the concentrate to Back Plain Creek to see what’s in it.

Pablos Mine location (from Mindat)

Pablos Mine appears to be on a tributary of Back Plain Creek south of the Wellingrove Fossicking Area. All I can find from the metallogenic notes is that it was worked by Pablos Mining (1970-73) by shallow pits or scrapings. It’s described as a “modern placer (fluvial)”.

The problem starts with the Back Plain Mine. The metallogenic notes contain quite a bit of information – worked by SC Burgis (1971), Pablos Mining (1970-73) and Nunan Pty Ltd circa 1990. “This deposit comprises high grade sapphires and abundant zircon. The headwaters of the creek drain from a red soil 

Back Plain Mine location (from Mindat)

plateau where the deposits were quite rich. The abundant zircon contrasts strongly with Reddestone Creek immediately to the east where zircon is relatively minor. Alluvial placer”. The difficulty arises when you look at the Mindat information for the Back Plain Mine (here). It is clearly identified as the Back Plain Mine (Truro). This would be fine, except that we have a lot of information about the Truro Mine from the NSW Department of Minerals and Energy publication Minfo 27 (1990) which deals with the opening of the mine belonging to TJ and PV Nunan Pty Ltd. You can download the document from DIGS, reference number R00040997. The included map shows that the mine was on the right hand 

Truro Mine location (from Minfo 27)

side of the Strathbogie Road, past the Wellingrove turnoff. This is several kilometres from the indicated site of the Back Plain Mine. Perhaps the wash from the Back Plain Mine was trucked to Truro Mine for treatment. That there was a mine near Truro homestead is beyond doubt. You could see the activity from the Strathbogie Road, but when it closed down I can’t say.

My thanks go to the NSW Geological Survey, Mindat and Google Maps for information and illustrations used in this blog. Without these it would be difficult to make sense of this interesting area.

To explore the satellite images better, I suggest a Google search for "Mindat Pablos Mine NSW" and "Mindat Back Plain Mine NSW". You can then follow the "Maps" link, "Maps Pages" and select "satellite"to get to the images shown above.

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Shoalhaven District Geology.

FRENCHIES SWAMP CREEK NEAR GLEN INNES NSW AUSTRALIA

FRENCHIES SWAMP CREEK 

NEAR GLEN INNES NSW AUSTRALIA

The Yarrow Creek junction 2013

Frenchies Swamp Creek is one of only two tributaries of Yarrow Creek over the entire length of the two properties we have regularly fossicked. The other one is a mere trickle and hardly noticeable. Just who Frenchie was I have no idea, but the swamp bearing his name covers a swathe of country between Yarrow Creek and the Mt Slow road near the Henry River bridge.

You might think that swamps are unlikely places to find gemstones, but it has been the swampiest ground at Pretty Valley that has been the most productive, so it doesn’t pay to jump to conclusions. In the case of Frenchies Swamp, the creek flows through some rocky areas along its course and gemstones have been found in all these places. I have followed the main creek up to the limit of the last swamp (about 4km from Yarrow Creek)

3 carat zircon

and gemstones are to be found in the gullies running into the swamp there. There are more small tributaries running into the swamp around its margin but I never had the opportunity to check these out.

The junction with Yarrow Creek is not obvious except when water levels are high. It can be followed up to the first swamp (about 400m) through rocky ground with quite a bit of typical quartz gravel in places. How this got there through kilometres of swamp is one of those little mysteries. The junction is at “Sue’s Favourite Spot”, just below the "Glory Hole" section.

                                                                                 The gemstones are identical to those in Yarrow, with perhaps a lower 

Corundum fragment 1998

garnet percentage. The fact that there is any garnet at all is significant, because it implies that the source of the mineral must be in the catchments of both creeks. There is another possibility, that the swampy ground represents a former course of the Yarrow and that there are in fact large gemstone deposits present under the swampy areas.

It’s been quite a while since I checked out anything but the lowest part of the creek. In fact, it’s been 12 years since I went much further than that. Access is complicated by the fact that we never had a 4 wheel drive vehicle and relied on others on the few times we went there. Walking from The Crossing is the best bet, allowing 20 minutes to the point where the track crosses the creek.

I’m looking forward to hearing of good finds there one day. The landowner certainly believes that Frenchies Swamp Creek has more potential than Yarrow Creek. I hope he proves to be correct.

Check out the videos and blogs of Sue's Favourite Spot and the Glory Hole for more information and ideas. 

Photographs are unfortunately rather few. Sorry about that. The only one of a day's best finds was done on a scanner. Be assured that there were many good finds made over the years.

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Song Studies. Bible studies based on hymns and songs

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Shoalhaven District Geology.

Best finds from an outing in 2006

MISCELLANEOUS MINERALS FROM YARROW CREEK NEAR GLEN INNES NSW

MISCELLANEOUS MINERALS FROM YARROW CREEK 

NEAR GLEN INNES NSW

All gem hunters in Yarrow Creek soon become familiar with the most abundant minerals to be found there. Quartz, as always, leads all the rest in quantity. Mostly it's waterworn, but the occasional near pristine crystal turns up. 

Along with the quartz, orthoclase felspar is sometimes seen. No doubt, both the quartz and felspar are derived from veins in the granite, which outcrops in most places fossickers go. 

Black spinel (pleonaste, blackjack), corundum (a high proportion of which is gem quality sapphire), zircon and pyrope garnet are found everywhere. Sometimes the vagaries of the currents during floods cause the zircon and garnet to be concentrated separately, though always with spinel and corundum.

The less common minerals,but still common enough to be found every visit or so, are enstatite and schorl tourmaline. Topaz is found once in a while, perhaps one or two pieces in a year of regular weekly outings. Once a blue specimen was found. Just once a pale specimen of ruby  turned up in my sieve, and this has already been illustrated in the blog entry on pink sapphires.

Finally, there are the miscellaneous things that turn up, which, while not all one-offs, will bother most 

Ilmenite

fossickers trying to identify them. Ilmenite is reasonably common but not normally noticed. It occurs as cylindrical, metallic looking lumps, though why it is in this form I do not know .

carnelian 3cm across

Silicified wood 11cm long

I've seen the occasional piece of agate, though I failed to photograph one. The photo of the specimen of orange chalcedony (carnelian) is a one-off, as are the silicified wood and rutile. The chalcedony would be related to the silicification of wood trapped beneath the nearby basalt flows, though I would have expected them to be more abundant. 

Rutile 2cm long

The specimen of rutile is an unexpected find, mainly because I have never seen another. Such a vast amount of rutile has accumulated in heavy mineral deposits along our coasts that I would have expected more to turn up in creeks like Yarrow which drain in that direction.

Lastly, here is a specimen about which I know nothing. It was found amongst the sieve concentrates, so it's reasonably dense. It doesn't look like the rest of the corundum, which I thought it was initially. It remains a mystery mineral.

Unknown mineral

Check my gem hunting videos from around Glen Innes here . 

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Limestone Caves of NSW

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Song Studies. Bible studies based on hymns and songs

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

Shoalhaven District Geology.

The mineral we all want to find: 16.8 carat blue sapphire