Barrett, T.J., 1995b. Stratigraphic, Lithogeochemical and Petrographic Relations of some Volcanic Rocks at the Parys Mountain Massive Sulfide Deposit, Wales. Unpublished Report for Anglesey Mining plc, Wales. pp. 120. The Parys Mountain deposit is located in a sequence of Ordovician volcanic rocks and shales in Anglesey, Wales. Although historic Cu production from 1760-1904 was from Cu-rich veins, exploration since 1961 has concentrated on synvolcanic massive sulfides. These appear to lie mainly at the base of a thick (>300m) felsic sequence, at or near the contact with underlying mudstones. In this study, 3 drill holes that intersected volcanic stratigraphy, mudstones and mineralization were examined, described and sampled. The felsic volcanic rocks are mainly pyroclastic flows, with some massive rhyolites. Many primary volcanic textures remain, including glassy fiamme and shards, spherulites and small amygdules. No definite phenocrysts were seen in any felsic samples. Altered glass is common. Fiamme in pyroclastic flows are up to several centimetres long, and are strongly altered to sericite+chlorite; the matrix consists of quartzo-feldspathic 'silty' grains with sericite and minor chlorite. In some pyroclastic flows, felsic lithic fragments are also present (up to a few centimetres across). Massive rhyolites are flow-banded with local zones of flow-breccia rubble. Mudstones contains some graded felsic tuff beds. The felsic volcanic rocks fall into two compositional subgroups termed rhyolites A and B, with most samples belonging to the former group. Based on REE patterns and Zr/Y ratios, rhyolite A is of tholeiitic affinity, whereas rhyolite B is of transitional affinity. Rhyolites A and B are effectively separated, even where altered, using a Nb-Zr plot. Rhyolite A (precursor) has a relatively high Nb content, and is probably not subduction-related. Two mafic samples from the footwall are enriched in Ti, Zr, Nb and P; their overall chemistry suggests that they are intra-plate mafic rocks of transitional-to-alkaline affinity. Two mudstones have REE patterns with strong enrichments in the light to heavy REE, indicating derivation from a different source area (peralkaline or alkaline). The felsic rocks are commonly moderately silica- and K-enriched, and strongly Na-Ca depleted. This is consistent with the lack of observed feldspar. Some rhyolites contain notable Fe + Mg enrichments (as chlorite or ankerite). Because of the strong downhole variations in alteration of rhyolites A and B, it is necessary to use immobile element ratios such as Al2O3/Zr and Zr/Nb to recognize original volcanic units. Mass changes can also be plotted downhole in order to search for hydrothermal alteration effects. In the present study, only low-Zr rhyolites were sampled (<300 ppm Zr). However, data in Southwood (1982) indicate that high-Zr peralkaline rhyolites containing 500-800 ppm Zr, and locally up to 1200 ppm, are also present at Parys Mountain. Peralkaline rhyolites are in fact common in both the Welsh Basin and in southeast Ireland; they are typified by high Zr contents (500-1200 ppm) and about 3 to 4% each of Na2O and K2O (Leat et al., 1986; McConnell et al., 1991). These authors interpreted the peralkaline rhyolites as products of ensialic magmatism related to tensional faulting of a continental margin. The rhyolites examined in the present study appear to have been derived from a proximal vent which erupted subalkaline, pyroclastic, fiamme-bearing units and lesser massive flows mostly of one initial composition (rhyolite A). The vent may have been a seafloor synvolcanic fault zone, now represented by the Penymynedd Fault, which also may have been the conduit for some of the VMS-mineralizing fluids. The zones of White Rock, which are locally mineralized, are dominated by hydrothermal silica which may represent a seafloor sinter, or discordant fracture infilling of altered host rocks; some white rock also may have been brecciated by seafloor faulting. It is speculated that the Parys Mountain peralkaline felsics (Southwood's 1982 data) were derived from a different source area relative to rhyolites A and B, possibly from the Snowdon volcanic region, where peralkaline rhyolites are common. Based on comparisons with modern peralkaline rhyolites, the overall setting for the Parys Mountain massive sulfides is interpreted to be a relatively shallow marine, faulted and foundering ensialic basin behind a volcanic arc. VMS deposits hosted by chemically similar rhyolites have recently been found in the Yukon, where they occur in a tectonic setting similar to Parys Mountain.
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