Search This Blog

Tuesday, 9 July 2013

THE DUNDEE GRAVEL BEDS




I first noticed quartz gravel in a road cutting on the Ranger’s Valley road near Dundee over 20 years ago. The location is 4.1km from the New England Highway, immediately before the road crosses the abandoned Wallangarra railway line.


For several hundred metres the road passes through low cuttings of poorly compacted sandstone containing layers of gravel. So far as
I have seen, all the pebbles are quartz and all are quite waterworn. Some are clear or smoky, but these make up only a few percent of the whole.


The obvious question is, where have they come from? The local rock is the Dundee Rhyodacite (locally called the blue granite).
It is a quartz deficient rock and I have not seen evidence of quartz veins in it anywhere. It is unlikely to be the source of the quartz pebbles.

The nearest major stream is the Severn River, which runs a km or so to the south. I haven’t seen gravel like this is the river but it is still possible that this outcrop represents an old course of the river. 


I believe that the most likely source of the gravel is the eastern end of the Mole Tableland in the vicinity of Stannum, about 30km to the north. Observations at Stannum reveal that a former stream bed vanishes beneath a younger flow of basalt. This is the classic “deep lead” situation and the Stannum Deep Lead has been traced beneath the basalt at least as far south as the Deepwater-Emmaville road. There are in fact up to 10 separate flows, which would have followed down the old valley, completely burying it and its alluvium.


Tin (cassiterite) has been mined from this gravel near Stannum, but the wash became less economic as it was traced southwards beneath the basalt. This is to be expected, as the cassiterite naturally became finer as it was carried further from its source. Not only that, but it becomes more difficult to locate the centre of the old lead beneath the basalt and most of the boreholes which were sunk missed the payable ground which might exist in the centre of the old valley.


The gravel at Dundee has no basalt cap, and the nearest outcrop of the Stannum basalt is a few kilometres to the north, but it is quite possible that it is part of the alluvium present in that ancient valley.


Reports on the Stannum Deep Lead mention zircon and spinel among the denser minerals found with the tin, but not topaz. I found no topaz when I carried about 20kg of gravel down to the Severn River and washed it. There was a residue of dense black sand which no doubt included cassiterite. Others have reported finding topaz here but this might have been clear quartz which is easily mistaken for it.


The easiest way to get specimens of clear and smoky quartz at this spot is to gouge out pebbles that look dark. Otherwise, if conditions allow, dry sieve the loose material and then wash it in a convenient puddle or take it down to the river. (The Severn River Road branches off the Ranger’s Valley road before you reach the cutting.)





A good reference is “The Stannum Alluvial Tin Deposits” by PL Rasmus, 1968 (DIGS GS1968/221).
 



Why not check out my You Tube channel here where you can view gem hunting, mining and general geology videos. New subscribers are always welcome.

No comments:

Post a Comment