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Monday, 9 March 2015

SAPPHIRE DEPOSITS AT BACK PLAIN NEAR GLEN INNES NSW AUSTRALIA



Before you read this blog, I recommend that you read two others. They are my blog entry “The Red Rock Sapphire Deposit” (here) and Jewellery Pirate’s blog “Back Plains Creek” (here). They deal 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.


My gemstone/mining You Tube playlist may be found here. I have three other playlists - the Blue Mountains, Glen Innes and New Zealand.



2 comments:

  1. Thanks, Jerry. You'd be amazed at how many times I typed in "Back Pain" as I prepared this blog. Symptomatic, I guess.

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