Composition-volume changes during hydrothermal alteration of andesite at Buttercup Hill, Noranda District, Quebec

Hydrothermally altered andesites in the upper member of the Amulet formation at Buttercup Hill, Noranda, Quebec represent part of the aquifer and cap of a self-sealing geothermal system that focussed the discharge of hydrothermal fluids during the formation of massive Cu-Zn sulfide deposits. Five alteration facies are recognized

1. 1) pervasive greenschist faciés regional metamorphism (least-altered andesite)

2. 2) epidotization-silicification

3. 3) albitization-silicification

4. 4) chloritization

5. 5) sericitization-silicification. Alteration is localized on permeable zones such as amygdules, fractures, flow tops, discordant breccia dikes, and conformable breccia horizons.

Epidotized-silicified andesite is enriched in Ca-Sr-Eu and depleted in Mg and first transition series metals (FTSM) relative to least-altered andesite. Albitized-silicified andesite is significantly enriched in Na and depleted in most FTSM relative to least-altered andesite. The abundances and inter-element ratios of the rare-earth elements (REE) and most high field-strength elements (HFS: Y, Zr, Th, U, Hf, Ta) are similar in least-altered, epidotized-silicified and albitized-silicified andesites. The most silicified andesites are strongly enriched in Na-Si, strongly depleted in Mg and divalent FTSM and slightly but systematically depleted in REE and most HFS elements. Serialized andesites were previously silicified; they are very strongly enriched in K-Rb-Cs-Ba, very strongly depleted in Na-Ca-Sr-Eu and slightly depleted in light REE relative to silicified andesite. Chloritized andesitic rocks exhibit heavy REE and HFS element ratios similar to those of leastaltered andesite, but are relatively strongly enriched in Mg and divalent FTSM, strongly depleted in Si and large ion lithophile (LIL) elements and slightly depleted in light REE.

The coupled behavior of the heavy REE and most HFS elements during epidotization, albitization, silicification, chloritization and serialization suggests that they were inert during hydrothermal modification of the andesite. Mass balance calculations suggest that volume was conserved during epidotization-silicification and albitization-silicification, but that intense silicification was accompanied by volume increases up to 30 percent.     

Cultural sediment analyses and transitions in early fishing activity at Langenesværet,Vesterålen,Northern Norway

The development of specialized and commercial fishing activity in the island archipelago of Lofoten and Vesterålen in northern Norway is a critical foundation from which to understand the subsequent spread of commercial fishing across the north Atlantic region during the medieval and early modern period. One little understood aspect of this development is the relationship between medieval commercial fishing stations (fiskevaer) and earlier fishing activity. In this article, cultural sediment deposits at Langenesværet, Vesterålen, Northern Norway provide an opportunity to examine this relationship and its implications for current historical models of fishing development in northern Norway and the north Atlantic region. Conventional and AMS radiocarbon dating techniques are used to establish a chronology for the deposits, while activities associated with the sediments are characterized using thin‐section micromorphology supported by proton induced X‐ray emission spectrometry (PIXE). The results suggest that the site commenced formation as early as ca. 3000 B.C. and that the site was first used for specialized fishing activity from the early centuries A.D. The medieval commercial fiskevaer settlement at Langenesværet was introduced to an area that had a longstanding tradition of specialised fishing activity. © 2000 John Wiley & Sons, Inc.     

A Fissure-Controlled Acidic Volcanic Centre (Ordovician) at Yr Arddu,North Wales

The Yr Arddu Tuffs (Ordovician) are a sequence of predominantly welded acid ash-flow tuffs in the outlier centred on Yr Arddu (North Wales). The tuffs accumulated above a NNE-trending fracture, the Yr Arddu Fracture, which had previously influenced sedimentation. The heterogeneity of the tuffs and their restricted development suggest that they represent proximal accumulations from small or suppressed (boiling-over) eruption columns. The eruptions and emplacement developed, at least in the initial stages, in a submarine environment. Lithological, geochemical and palaeontological evidence indicates that the tuffs represent the earliest eruptive phase of the Lower Rhyolitic Tuff Formation. They were later intruded by comagmatic rhyolite domes, whose alignment reflects the continued influence of the early fracture.