Volcanic facies, geochemistry and setting of VMS deposits in the Ambler Range, Alaska, and at Parys Mountain, Wales.

Barrett, T.J., 1997. GAC-MAC Annual Meeting, Ottawa, Ontario. May 1997, p. A-8.
In the Ambler district of northwestern Alaska, mid-Paleozoic volcanic sequences are host to the Smucker, Arctic and Sun deposits (each with resources of some 15-30 Mt). In the Dead Creek area (8 km from the Arctic deposit), several sulfide horizons occur within a bimodal but folded volcanic sequence. In this area, quiet accumulation of carbonaceous dacitic volcaniclastic material was followed by a sulfide-barite mineralization event and eruption of low-Zr rhyolite B, then eruptions of dacite. There appears to be a second sulfide horizon associated with high-Zr rhyolite B. These various eruptions took place mainly as pyroclastic flows, which halted further sulfide accumulation in the marine basin. Only when they ceased, and carbonaceous sediments began to slowly accumulate, could further sulfides be deposited. Rhyolites and dacites are of medium-K, transitional to calc-alkaline magmatic affinity, and are interpreted to have been erupted on continental margin, rifting of which allowed emplacement of rather unfractionated, mantle-derived tholeiitic basalts. The nearby Arctic deposit is subjacent to a separate felsic volcanic centre. The sulfide lenses at Arctic are underlain by carbonaceous volcaniclastic sediments, which are commonly chloritized. Eruptions of rhyolite A (and some rhyolite B) eventually buried and terminated the sulfide-depositing system, although hydrothermal alteration affected the hangingwall rhyolites.

At Parys Mountain in Anglesey, Wales, massive sulfide lenses are hosted by a lower Paleozoic volcanic sequence. One horizon occurs at the base of a thick (>200m) sequence of felsic pyroclastic flows and lesser massive rhyolites, at or near the contact with underlying mudstones and volcaniclastic beds. Compositionally, the felsic volcanic rocks are mostly tholeiitic rhyolite A, with a smaller group of transitional rhyolite B. Alteration of rhyolite flows has produced common moderate silica- and K-enrichment, and strong Na-Ca depletion. Some zones of extreme silica addition also occur. Mafic units in part of the footwall are of transitional-to-alkaline E-MORB affinity. Only relatively low-Zr rhyolites (<300 ppm Zr) were sampled in the present study. They lack a subduction signature. Data from other studies indicate that high-Zr (500-1200 ppm Zr) peralkaline rhyolites are also present at Parys Mountain and elsewhere in the Welsh Basin. Based on the general stratigraphic setting and chemical comparisons with modern peralkaline rhyolites and enriched basalts, the Parys Mountain deposits are interpreted to have formed in a marine, ensialic marginal basin in which rifting, foundering and volcanism occurred in response to asthenopheric upwelling.

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