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挪威块状硫化物矿床中单斜与六方磁黄铁矿的交生及其成因

顾连兴 尹琳 F.M.Vokes   

  1. 南京大学地球科学系
  • 收稿日期:1995-03-20 修回日期:1995-03-20 出版日期:1995-09-20 发布日期:1995-09-20

INTERGROWTHS OF HEXAGONAL AND MONOCLINIC PYRRHOTITES AND THEIR GENESIS FROM MASSIVE SULPHIDE DEPOSITS OF NORWAY

Gu Lianxing1, Yin Lin1, F. M. Vokei2   

  1. 1. Department of Earth Science, Nanjin university, 210008; 2. Department of Geology, University of Trondheim, N-7034, Trondheim, Norway
  • Received:1995-03-20 Revised:1995-03-20 Online:1995-09-20 Published:1995-09-20

摘要: 矿石的镜下结构研究进一步确证,挪威加里东造山带块状硫化物矿床中的磁黄铁矿绝大部分是沉积-成岩作用的直接产物,并在其形成之后经历了多期次的变形和退火,还有一部分磁失矿是在退变质阶段形成的,或者是由变质热液形成的。这次研究中未发现由黄铁矿变质而成的磁黄铁矿。 磁性胶体法研究揭示,无论是原生的、退变质的还是变质热液的磁黄铁矿,都可能含有六方与单斜变体的紧密交生。交生形式可以分为两种:由结晶方位控制的叶片状交生以及受裂隙和颗粒边界控制的不规则状交生,其成因分别为六方固溶体的出溶和富硫热液的交代。尽管如此,大部分矿石的碰黄铁矿中缺乏单斜出溶叶片,其原因可能有三个方面 1)矿石形成于海底贫硫环境; 2)矿石形成时的温度低于254℃;3)原有的单斜变体在矿石变质、重结晶和退火过程中受到了均匀化。

Abstract: Microscopic investigations have provided further evidence that pyrrhotite in the massive sulphide ores of the Norwegian Caledonides was mainly formed during sedimentation and diagenesis and has undergone polyphase metamorphism, deformation, recrystaliization and annealing. A minor portion of the pyrrhotite was formed during diaphthoretic and metahydrothermal events. No pyrrhotite was found in the present study that appeared to have been derived from pyrite by progressive metamorphism. Observations using a magnetic colloid revealed that intimate intergrowths of the hexagonal and monocilnic varieties can be exhibited by either sedimentary-diagenetic, retrogressive or metahydrothermal pyrrhotite. The intergrowths are classified morphologically in to crystallographically controlled lamellar patterns and fissure- or grain boundary-controlled irregular ones. Those two Patterns were form ed either by exsoiution from the hexagonal solid solution or by replacement of the hexagonal variety by sulphur-rich fluids. Recent research indicates that volcanogenic massive sulphide deposits can form at temperature higher than 300°~350℃, and even above 450℃. Such temperatures may be attained particularly in the case of proximal deposits and are favourable for the crystallization of hexagonal pyrrhotite (hpo), which will be able to exsolve moncolinic pyrrhotite (mpo) during cooling if the bulk composition plots in the hpo+mpo region. Monoclinic pyrrhotite will he absent in the case of ores that were formed under sulphur-poor sea-floor conditions or at temperatures be low 254℃. Metamorphism at temperatures exceeding that of the mpo-hpo transition will erase earlier exsolved monoclnic pyrrhotite by homogenization during reocrystallization and annealing. Such a proof of homogenization will account for the fact that the monoclinic variety is normally absent in recrystallized and annealed pyrrhotite. Well-preserved moneclinic lamellae in retrogressive and metahydrotharmal pyrrhotites may indicate that these pyrrhotites have fiever been affected by post-crystallization and high-temperature events. Most of the volcanogenic massive sulphide deposits in old orogenic belts have, to various extents, been subjected to metamorphism, recrystallization and annealing. This could be considered as one of the most important reasons for the absence of monoclinic pyrrhotite in their ores.