Multi-phase water-rhyolite interaction and ore fluid generation at Aljustrel, Portugal

Detailed study of the Aljustrel volcanic rocks, through petrography, mineral chemistry, whole-rock geochemistry, and previously published stable isotope data, show that the rocks experienced extreme metasomatism. The metasomatic phenomena presented are explained accurately by a sea-water convective model. The evolution of the sea-water-derived fluid as it progresses downwards through the rocks is evident, from cold, oxidising, Mg-bearing, to hot, reducing, Mg-free, and enriched in Si, Fe, Cu, Zn. Several lines of evidence (feldspar mineralogy, paragenetic relations between regional and ore-zone alteration, and stable isotope data) show that the temperature of the hydrothermal circulation increased through time, and that the water-to-rock ratio evolved from very high to low values. According to our data, about 30 ppm Cu and some 100 ppm Zn were removed from the Aljustrel Volcanics. This implies, for both metals, the same amount of leached rock to produce the 2 Mt Cu and 6 Mt Zn contained in the massive sulphide deposits of the Aljustrel area. With regard to Fe, about 1% was on average removed from the rocks, producing enough metal to account for all the Fe in the MS deposits and overlying chemical sediments. This study documents semi-quantitatively the provenance of the main metals in major massive sulphide deposits via sea water leaching of the immediate host rocks.

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Resumen (translated by E. Pascual) Un estudio detallado de las rocas volcánicas de Aljustrel, mediante petrografia, química mineral, geoquímica de roca total y datos de isótopos estables previamente publicados muestra que estas rocas han sufrido metasomatismo en grado extremo. Los fenómenos metasomáticos presentados se explican con precisión mediante un modelo convectivo con agua de mar. Es evidente la evolución del fluido derivado del agua de mar conforme progresa en profundidad a través de las rocas, desde un fluido frío, oxidante y con magnesio a otro reductor, sin magnesio y enriquecido en Si, Fe, Cu y Zn. Varias líneas de evidencia (mineralogía de los feldespatos, relaciones paragenéticas entre la alteración regional y de las zonas mineralizadas, isótopos estables) muestran que la temperatura y la circulación hidrotermal se incrementaron con el tiempo y que la relación agua-roca evolucionó de valores muy altos a bajos. De acuerdo con nuestros datos, de las rocas volcánicas de Aljustrel se han extraído unas 30 ppm de Cu y unas 100 ppm de Zn. Esto implica, para ambos metales, que la misma roca lixiviada ha producido los 2 Mt de Cu y los 6 Mt de Zn contenidos en los depósitos de sulfuros masivos del área de Aljustrel. En cuanto al Fe, una media de alrededor de 1% fue extraída de las rocas, produciendo metal suficiente para explicar todo el Fe de los depósitos de sulfuros masivos y de los sedimentos químicos suprayacentes. Este estudio documenta de forma semicuantitativa la procedencia de los principales metales en depósitos mayores de sulfuros masivos, mediante lixiviado por agua de mar de sus rocas huéspedes inmediatas.

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Received: 1 February 1997 / Accepted: 30 July 1997     

Algal uptake of U and some other metals: Implications for global geochemical cycling

Freshwater green algae Ankistrodesmus sp, cultured in dilute solutions of metals chelated with EDTA, revealed massive uptake of U. Examination of Thames River waters and selected micro-organisms of its aquatic ecosystem revealed an average dissolved U concentration of 1.46 ppb, representing about twice the global mean riverine solute concentration of 0.6 ppb U. Suspended particulates, principally algae, contain U at 10³ to 10⁵ times that of their river habitat, averaging 28 300 ppb U. One community of the filamentous algae Spirogyra sp. contained intracellular crystals tentatively identified as the Ca-oxylate weddelite. Studies of algae from a variety of environments have shown large concentration factors for a wide range of elements. Algae from an acid U mine tailings site contain up to 2.7% U (dry weight) plus very high concentrations of other metals. Micro-organisms may act as an important sink for metal solutes in the hydrosphere.     

纪念陈光远教授(英文)

仅以此文悼念陈光远教授。陈光远教授毕生献身于地质科学 ,是一位卓越的地质学家。本人与中国同行的交往始于 1 973年 ,当时我应邀参加了华北地台古老变质岩系的地质旅行。后来我又多次访问中国 ,并于 1 996年在北京成功地主持召开了第三十届国际地质大会。陈光远教授是我有幸结识的杰出的中国同行之一 ,在我访问中国期间他曾带我参观考察了他多年从事研究工作的矿区。陈光远教授毕业于西南联大 ,并在瑞典获得副博士学位 ,回到祖国后一直致力于地质学教学与研究工作。他为中国矿产资源的开发做出了不朽的贡献 ,发表的成果及著作涉及金矿、铁矿等许多领域 ,尤其突出的是他编著的胶东金矿省矿物学填图图集开拓了找矿矿物学的新篇章。陈光远教授生前曾多次获奖 ,他的成果得到国际同行的赞誉。他是挪威、瑞典、芬兰、丹麦地质学会的终身会员 ,并被推选为俄罗斯科学院外籍院士。陈光远教授去世前正在与我合作进行有关植树造林防止水土流失的中加合作项目 ,他的去世也是该项目的巨大损失。可喜的是孙岱生教授及其同事将负起重任 ,完成陈光远教授未竟的事业。     

The production of granitic melts during ultrametamorphism

Data concerning the occurrence and mineralogy of granitic rocks and migmatites indicate that granitic liquids are not generally water saturated and do not form on minimum melting curves. Experiments on the partial fusion of crustal materials containing water only in hydrated phases demonstrate that water undersaturated melts with compositional affinities to the granite family are produced at temperatures and pressures in accord with observations on the metamorphio state of migmatite terrains.     

Tectonics and Chemical Transport: The Rules of the Resource Game

The most basic solid earth resources for use by man include construction materials, energy resources, metals, ceramics, soils. With the exception of construction materials, the formation of economic mineral deposits and soils involves the thermal or gravity driven flow of various types of geofluids. The search for mineral deposits thus involves the understanding of the exact nature of fluids, their mass and flow patterns. Given the giant thermal disturbances associated with the formation and destruction of Gondwana, it must be no surprise that these regions contain very significant mineral resources on a large scale.     

Gold in “primitive” Archaean volcanics

Gold concentrations have been determined by neutron activation in sixteen samples of komatiites and related rocks from Barberton, South Africa. All the rocks give Au values in the ranges 1–1.5 ppb with no evidence for anomalously high values in such old primitive igneous types. As the rocks are extensively hydrated, it is possible that Au has been leached from the rocks during later hydration processes.     

Adularia,the characteristic mineral of felsic spilites

Sea floor rhyolites from the Iberian Pyrite Belt show strong enrichment in potassium and adularia is observed to replace plagioclase. This process is in accord with thermodynamic data for the exchange reaction with ocean water which favours adularia up to about 140° C. Archaean felsic intrusives and extrusives exhibit sodium enrichment, a contrast which we attribute to lower K levels in sea water and, or, higher ocean temperatures. All of these rocks show ¹⁸O enrichment of + 6 to + 8, in compliance with low temperature sea water exchange.     

The coordination number of ferrous ions in silicate glasses

Mössbauer and optical absorption measurements on a number of silicate glasses show that most of the ferrous ions in these glasses are distributed over two types of octahedral sites. Less than 0.5% of all ferrous ions occupy tetrahedral sites. It is shown that the Bernal liquid model of random close packing does not apply to silicate glasses. Attention is drawn to the fact that measurements on silicate glasses cannot provide reliable information about coordination numbers in silicate liquids.