In any given hole, two immobile element ratios can be conveniently plotted downhole in order to examine lithological variation, e.g. Al2O3/TiO2 and Zr/TiO2, as these elements are not mobile to any signifcant degree in the alteration zones of typcial VMS deposits. In the example shown, the ratios are parallel to each other and show little change in lithology over >200 metres of drilling, indicating that one rhyolite type (A) occurs down to the level of mineralization. Some of these rhyolites are strongly altered. Note that a thin basaltic unit underlies the mineralization in this hole. The geochemistry of the least altered rhyolites and basalts at Parys mountain is consistent with an origin in a rifted extensional marine basin formed in foundered continental crust, similar to the setting of the Snowdonia Volcanic Complex in northern wales (Leat et al, 1986; Howells et al., 1991).





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