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Wednesday, 19 June 2013

QUARTZ AT “DOWN THE HILL” YARROW CREEK NSW AUSTRALIA



Any fossicker visiting Yarrow Creek soon notices that at least 90% of the gravel he or she digs up is made of quartz, with the balance usually consisting of granite and odd bits and pieces which are probably xenoliths derived from the local granite. So what, you are probably thinking. Well, the fact is that the size of most of the quartz pieces is considerably greater than the quartz which is a normal constituent of the granite. The sand, so plentiful in the creek is, of course, derived from the granite.

So, where does the quartz gravel come from? The obvious answer is, from veins and/or pegmatite dykes in the granite. However, no granite outcrop that I’ve ever seen there has any veins or dykes in it. So if these quartz veins do exist, they must be in places where there are no outcrops.

Down the Hill” is one such place. There is an extensive alluvial flat immediately upstream of a rocky area where lots of gravel can be found. 



What makes this gravel different is the presence of lots of chunks of white quartz (up to 1 kg in weight) which is different from the small pieces of quartz seen upstream. It would be reasonable to assume, from its rough appearance, that the quartz hasn’t travelled very far
 
Not only that, but there are other pieces of quartz which are distinctly different from the white ones. Each of these is a single, waterworn, smoky crystal; the largest of which I’ve seen weighs just under 5 kg. Most of these are not particularly clear, but certainly worth finding. 


So, apparently, there must be at least two quartz veins or dykes not far upstream from “Down the Hill” which are shedding this material. 


I guess we will never really know.






You will find the NSW Geological Survey booklet “Silica” by DK Griffin (1972) a useful source of information. The DIGS reference is Industry 34.




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