Re–Os dating and sulfur isotope composition of molybdenite from tungsten deposits in western Namaqualand, South Africa: implications for ore genesis and the timing of metamorphism
Re–Os dating and sulfur isotope composition of molybdenite from tungsten deposits in western Namaqualand, South Africa: implications for ore genesis and the timing of metamorphism
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DOI：
10.1007/s001260050276
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发表时间：
2000-11
影响因子：
4.8
通讯作者：
H. Stein;J. Raith
中科院分区：
文献类型：
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作者：
H. Stein;J. Raith
层状/层控和花岗岩相关的模式已被用来解释钨（钼）矿床的成因在奥基普铜矿区西部纳马夸兰，南非。显然，层控矿化（富铁黑钨矿和副黑钨岩）出现在Wolfram组的高级（> 750 °C，5-6 kbar）变岩中的平行于叶理的石英脉中，很少出现在硅化浅色花岗岩和伟晶岩的小岩体中。Nababeep Tungsten Far West、Kliphoog、Narrap、Tweedam等4个矿床的辉长岩Re-Os年龄介于1000 ± 4 ~ 1026 ± 5 Ma之间。这些辉长岩定义了一个约束良好的187 Re-187 Os等时线，年龄为1019 ± 6 Ma，可作为W（Mo）成矿年龄。这个年龄是显着年轻于元古宙原岩年龄的表壳岩和侵位年龄的主要侵入套，但地质证据需要重叠的高级变质作用的时期。我们认为W（Mo）成矿作用与1020 Ma的壳内岩浆作用有关，从而排除了1060 Ma的Concordia花岗岩作为成矿流体的来源。5个钨（钼）矿山的8个辉长花岗岩的δ 34 S正同位素组成范围较窄（+3.6 ~+4.5‰），与富SO2流体和与花岗岩有关的成因模式一致。峰后变质变形和变质作用的钨（钼）矿石是最有可能有关的纳马夸造山带的退变质阶段，其中重叠侵位的晚造山，演化花岗岩和伟晶岩，以及钨（钼）矿床的形成在西部纳马夸。因此，退变质Namaquan的影响至少延伸到1020年马，或者，这些W（Mo）静脉的影响，约束较差的后期事件（如早期泛非）。
Both stratiform/stratabound and granite-related models have been used to explain the genesis of W(Mo) deposits in the Okiep copper district in western Namaqualand, South Africa. Apparently, stratabound mineralization (Fe-rich wolframite with accessory molybdenite) occurs in foliation-parallel quartz veins in high-grade (∼750 °C, 5–6 kbar) metapelites of the Wolfram Formation, and less commonly in small bodies of silicified leucogranites and pegmatites. Six Re–Os ages for molybdenites from four deposits (Nababeep Tungsten Far West, Kliphoog, Narrap, Tweedam) range between 1000 ± 4 and 1026 ± 5 Ma. These molybdenites define a well-constrained187Re–187Os isochron with an age of 1019 ± 6 Ma, which is interpreted as the age of W(Mo) mineralization. This age is significantly younger than Proterozoic protolith ages for supracrustal rocks and the emplacement ages for the main intrusive suites, but geologic evidence requires overlap with a period of high-grade metamorphism. We suggest that W(Mo) mineralization is genetically linked to intra-crustal magmatic processes at ∼1020 Ma, thereby precluding the ∼1060 Ma Concordia granite as the source for mineralizing fluids. A narrow range of positive δ34S compositions (+3.6 to +4.5‰) for eight molybdenites from five W(Mo) mines is consistent with a SO2-rich fluid and a granite-related genetic model. Post-peak metamorphic deformation and metamorphism of W(Mo) ores is most likely related to the retrograde stage of the Namaquan orogeny, which overlaps emplacement of late-orogenic, evolved granites and pegmatites, and the formation of W(Mo) deposits in western Namaqualand. Therefore, the effects of retrograde Namaquan metamorphism extend at least to ∼1020 Ma or, alternatively, these W(Mo) veins were affected by a poorly constrained later event (e.g. early Pan-African).