Chrysoberyl, beryl and zincian spinel mineralization in granulite-facies Archaean rocks at Dowerin,Western Australia
Chrysoberyl, beryl and zincian spinel mineralization in granulite-facies Archaean rocks at Dowerin,Western Australia
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西澳大利亚州陶尔林麻粒岩相太古宙岩石中的金绿宝石、绿柱石和锌尖晶石矿化
作者：
P. Downes;A. Bevan
摘要在西澳大利亚太古代伊尔加恩克拉通西南部的Dowerin附近，发现了包括翠绿宝石在内的金绿宝石(BeAl2O4)矿床。该矿床位于Yilgarn克拉通西南部地壳成分格雷斯湖地体的北部，麻粒岩相片麻岩(2640-2649 Ma；T=700°C，P<；6Kbar)，毗邻凯勒伯林岩基(2587±25 Ma)的边缘。Dowerin的铍矿化赋存于斜长-黑云母-石榴石片麻岩和横切电气石-斜长石脉中，位于阳起石-花岗石-金云母片岩透镜体附近。金绿宝石晶体(0.15-1.74wt.%Cr2O3；2.25-3.23wt.%FeO；微量-0.13wt.%ZnO；SiO_2<；0.05wt.%)嵌于铁铝榴石或斜长石中，并与寄主岩片麻岩中的黑云母和/或锌铝尖晶石紧密共生。黄绿宝石中的微量铬和铁可能来源于伴生的锌铁铝尖晶石和/或铁铝榴石。微量绿柱石(0.04-0.20wt.%Cr2O3；0.54-0.71wt.%FeO；微量-0.22wt.%Na2O；0.1-0.71wt.%MgO)以无面体间隙颗粒的形式存在于金绿宝石、斜长石和黑云母晶体之间，并以稀有包裹体的形式存在于金绿宝石中。结构和矿物化学证据表明，金绿宝石和锌尖晶石(含2-8wt%氧化锌的铬铁矿-铁铝尖晶石)形成于麻粒岩相区域变质作用期间，可能早于同一变质事件中变质电气石-斜长石脉的形成。形成金绿宝石、绿柱石、电气石和锌尖晶石所需的Be、B和Zn可能是在前进麻粒岩相变质过程中通过围岩变质反应释放出来的。
Abstract A deposit of chrysoberyl (BeAl2O4), including the variety alexandrite, occurs near Dowerin, in the southwestern region of the Archaean Yilgarn Craton, Western Australia. The deposit is situated in the northern part of the Lake Grace Terrain, a crustal component of the southwestern Yilgarn Craton, in granulite-facies gneisses (2640-2649 Ma; T = 700°C, P <6 kbar) adjacent to the margin of the Kellerberrin Batholith (2587±25 Ma). Beryllium mineralization at Dowerin occurs in plagioclasequartz- biotite-garnet gneiss and cross-cutting tourmaline-plagioclase veins situated adjacent to lenses of actinolite-cummingtonite-phlogopite schist. Crystals of chrysoberyl (0.15-1.74 wt.% Cr2O3; 2.25-3.23 wt.% FeO; trace-0.13 wt.% ZnO; SiO2 <0.05 wt.%) are found embedded in almandine or plagioclase, and closely intergrown with biotite and/or zincian hercynite in the host-rock gneiss. Minor Cr and Fe in the alexandrite variety of chrysoberyl were possibly derived from associated zincian hercynite and/or almandine. Trace beryl (0.04-0.20 wt.% Cr2O3; 0.54-0.71 wt.% FeO; trace-0.22 wt.% Na2O; 0.1-0.71 wt.% MgO) occurs as anhedral interstital grains between crystals of chrysoberyl, plagioclase and biotite, and as rare inclusions in chrysoberyl. Textural and mineral chemical evidence suggests that chrysoberyl and zincian spinels (chromite to hercynite containing from 2-8 wt.% ZnO) formed during granulite-facies regional metamorphism and probably pre-dated the formation of metamorphic tourmaline-plagioclase veins during the same metamorphic episode. The Be, B and Zn required to form chrysoberyl, beryl, tourmaline and zincian spinels may have been released by metamorphic reactions in host-rock metapelites during prograde granulite-facies metamorphism.