The relationship of mineralization to petrology at Parys Mountain, Anglesey

The Lower Palaeozoic succession at Parys Mountain overlies a Precambrian basement (the Iona Series). This succession consists of Ordovician slates, overlain by, and in part interbedded with, Ordovician dacitic and rhyolitic volcanics, which in turn are unconformably overlain by Silurian slates. Both basement and Palaeozoic rocks have been deformed during Caledonian and Variscan orogenies. The resultant structure of Parys Mountain is interpreted as an east-north-easterly trending, single syncline overturned to the north. Many primary extrusive characters are retained by the volcanic rocks, despite the high degree of deformation. The lithologies and textures allow subdivision and interpretation of these rocks as dacite, lithic tuff, siliceous sinter, rhyolitic tuff, rhyolitic ignimbrite, rhyolitic tuff-lava, and rhyolitic lava. The results of 61 bulk chemical analyses are interpreted to show that the volcanism was of the orogenic calc-alkaline type from a continental margin/island arc environment. The magmas probably result from either partial melting of the crustal part of the oceanic lithosphere on a Benioff zone, or partial melting of the mantle, above a Benioff zone, under high load pressures and high water pressures. The mineral deposits are largely confined within the volcanic succession though some occur in the Ordovician and Silurian slates near to their contacts with the volcanics. The majority of the deposits form conformable lenses and tabular bodies, with subordinate deposits as veins and stockworks. The ore mineral assemblages are of chalcopyrite, galena, sphalerite, and pyrite. The general paragenetic sequence (73 sections) is pyrite--chalcopyrite--galena-sphalerite. The main mineralization episode is interpreted to be syngenetic, genetically related to the volcanism. The veins and stockworks probably result from Caledonian and Variscan remobilization of the primary mineralization. Trace element analyses (Cu, Zn, Pb, Ni, Co, Cd, Cr, Hg, Ba, Sr), on 350 specimens, detected anomalous concentrations of these elements around the mineralized zones, though some occur where no mineralization was found. The analyses also indicate a close relationship between the mineralization and the volcanic horizons, especially the siliceous sinter

A comparative study of jadeite, omphacite and kosmochlor jades from Myanmar, and suggestions for a practical nomenclature

Jadeitite boulders from north-central Myanmar show a wide variability in texture and mineral content. This study gives an overview of the petrography of these rocks, and classifies them into five different types: (1) jadeitites with kosmochlor and clinoamphibole, (2) jadeitites with clinoamphibole, (3) albite-bearing jadeitites, (4) almost pure jadeitites and (5) omphacitites. Their textures indicate that some of the assemblages formed syn-tectonically while those samples with decussate textures show no indication of a tectonic overprint. Backscattered electron images and electron microprobe analyses highlight the variable mineral chemistry of the samples. Their extensive chemical and textural inhomogeneity renders a classification by common gemmological methods rather difficult. Although a definitive classification of such rocks is only possible using thin-section analysis, we demonstrate that a fast and non-destructive identification as jadeite jade, kosmochlor jade or omphacite jade is possible using Raman and infrared spectroscopy, which gave results that were in accord with the microprobe analyses. Furthermore, current classification schemes for jadeitites are reviewed