Strontium isotope systematics of hydrothermal minerals from epigenetic Archean gold deposits in the Yilgarn Block, Western Australia
Strontium isotope systematics of hydrothermal minerals from epigenetic Archean gold deposits in the Yilgarn Block, Western Australia
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DOI：
10.2113/gsecongeo.86.4.780
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发表时间：
1991-07
期刊：
影响因子：
5.8
通讯作者：
A. Mueller;J. D. Laeter;D. Groves
中科院分区：
文献类型：
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作者：
A. Mueller;J. D. Laeter;D. Groves
介绍了西澳大利亚Yilgarn地块1个花岗岩型和14个绿岩型太古代金矿床中热液白钨矿和电气石样品的锶同位素组成。白钨矿、电气石等贫Rb矿物的87 Sr/ 86 Sr比值与金具有密切的共生关系，通过直接测量，可准确估算太古代含金热液的局部锶同位素组成。根据脉石矿物学特征，将绿岩型金矿床划分为围岩蚀变为石榴辉石黑云母和角闪石黑云母方解石的矽卡岩型金矿床（产于绿岩带下部10 ~ 15 km）和围岩蚀变为黑云母或绢云母铁白云石钠长石的中温型金矿床。矽卡岩中流体的87 Sr/ 86 Sr比值在0.7020 ~ 0.7035之间，有石榴子石-辉石-黑云母蚀变，具有角闪岩-黑云母-方解石蚀变的矽卡岩-石英矿体中，从0.7020到0.7043;中温金矿中，从0.7014到0.7028。来自同一矿区的白钨矿的锶同位素组成的变化被解释为反映了热液流体引入的放射性锶和与该流体相互作用期间从围岩释放的非放射性锶之间的混合。所有矿床的绿岩容矿岩石主要是变质超镁铁质或镁铁质火山岩及其侵入岩，具有太古代地幔低的87 Sr/ 86 Sr初始比值（0.7007-0.7012）和低的Rb/Sr比值。另一方面，在花岗岩基，其下和边界的绿岩带的花岗闪长岩和花岗岩岩体的初始87 Sr/ 86 Sr比值变化从0.7023到0.7044。锶同位素数据表明，深部金夕卡岩主要是由花岗岩基中深源岩浆演化或与岩浆相互作用形成的。这些数据并没有清楚地限制流体在更高水平的中温热液系统中的起源。然而，他们是不一致的模式，主要涉及由脱挥发分的下部的绿岩带所产生的变质流体。
Strontium isotope compositions are presented for samples of hydrothermal scheelite and tourmaline from one granitoid-hosted and 14 greenstone-hosted Archean gold deposits in the Yilgarn block, Western Australia. The direct measurement of the 87 Sr/ 86 Sr ratios of Rb-poor minerals such as scheelite and tourmaline, which show an intimate paragenetic relationship to gold, allows an accurate estimate of the local strontium isotope composition of the Archean gold-bearing hydrothermal fluids. The population of greenstone-hosted gold deposits is sub-divided according to gangue mineralogy into gold skarns with garnet-pyroxene-biotite and amphibole-biotite-calcite wall-rock alteration, which formed in the lower parts of the green-stone belts at depths of 10 to 15 km, and medium-temperature gold deposits with biotite or sericite-ankerite-albite wall-rock alteration, which formed in the upper parts of the belts at depths of 3 to 7 km.The 87 Sr/ 86 Sr ratios of the fluids vary from 0.7020 to 0.7035 in skarns with garnet-pyroxene-biotite alteration, from 0.7020 to 0.7043 in skarn-quartz orebodies with amphibole-biotite-calcite alteration, and from 0.7014 to 0.7028 in the medium-temperature gold deposits. The variations in the strontium isotope composition of scheelite from the same mining district are interpreted to reflect mixing between radiogenic strontium, introduced by the hydrothermal fluid, and nonradiogenic strontium released from the wall rocks during interaction with this fluid. The greenstone host rocks of all deposits, mainly metamorphosed ultramafic or mafic volcanic rocks and their intrusive equivalents, are characterized by the low initial 87 Sr/ 86 Sr ratio (0.7007-0.7012) and low Rb/Sr ratio of the Archean mantle. On the other hand, the initial 87 Sr/ 86 Sr ratios of granodiorite and granite plutons in the batholiths, which underlie and border the greenstone belts, vary from 0.7023 to 0.7044. The strontium isotope data indicate that the deep-seated gold skarns formed predominantly from fluids which evolved from or interacted with anatectic magmas in the granitoid batholiths. The data do not as clearly constrain the origin of the fluids in the higher level, medium-temperature hydrothermal systems. They are, however, inconsistent with models involving predominantly metamorphic fluids generated by the devolatilization of the lower parts of the greenstone belts.