Rare metal-bearing and barren granites, Eastern Desert of Egypt: geochemical characterization and metallogenetic aspects

F.H. Mohamed
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引用次数: 34

Abstract

Three “younger granite” plutons from the Eastern Desert of Egypt are studied: petrographic and geochemical characteristics of the barren pink granites at Wadi Sikait and Wadi Nugrus are similar, of alkaline, mildly peraluminous nature and are enriched in LIL-elements and LREE with moderate negative Eu anomalies. In contrast, the Sn-Ta-W-bearing albite granite of Abu Dabbab is alkaline, peraluminous muscovite granite; its chemical specialization is manifested by the pronounced enrichment in Ta, Sn, W, F, Rb and Li coupled with marked depletion in Ca, Ti, Mg, Sr and Ba. Elemental ratios (e.g., K/Rb, Rb/Sr, Ba/Rb) discriminate the albite granite and the pink granites into “mineralized and barren granites”, respectively.

The albite granite is derived from Na-rich magma of within-plate characteristics. Fluorine was an important complexing anion during magmatic evolution history. The albite granite is emplaced at shallow depth (<100 MPa) and at the intersection of structural weaknesses. The pink granites might have a crustal and/or LIL-element enriched mantle sources, in which the subduction-related fingerprints are partly obliterated. For both types, reactivation of regional structures played a significant role in magma generation.

Acid metasomatism is mainly manifested by the development of thin greisen veins along fracture systems in the albite granite. The chemistry of greisenization using mass balance approach reveals that the process is accompanied by dramatic increase in SiO2, Fe2O3, MnO, F, Sn and Li as well as significant loss in Na2O, K2O, Ba, Nb and Zn. The process causes a significant increase in volume (30%). Changes in chemical components are consistent with the observed mineralogical changes. Microprobe results reveal that the wolframite crystals are typically huebnerite with Fe-rich cores and Mn-rich rims. Compositional variations in wolframite crystals are attributed to the physicochemical conditions (pH, T, etc.) and chemistry of the ore-bearing fluids.

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埃及东部沙漠含稀有金属和贫瘠花岗岩:地球化学特征和成矿方面
研究了来自埃及东部沙漠的3块“年轻花岗岩”岩体:Wadi Sikait和Wadi Nugrus的贫瘠粉红色花岗岩的岩石学和地球化学特征相似,均为碱性、轻度过铝质,富集li -元素和轻稀土元素,具有中度负Eu异常。Abu Dabbab含sn - ta - w钠长花岗岩为碱性过铝质白云母花岗岩;其化学专一性表现为Ta、Sn、W、F、Rb和Li富集,Ca、Ti、Mg、Sr和Ba富集。元素比值(如K/Rb、Rb/Sr、Ba/Rb)分别将钠长石花岗岩和粉红色花岗岩区分为“矿化花岗岩”和“贫瘠花岗岩”。钠长石花岗岩来源于富钠岩浆,具有板内特征。氟是岩浆演化过程中重要的络合阴离子。钠长石花岗岩位于浅层深度(< 100mpa)和构造弱点的交汇处。粉红色花岗岩可能具有富含地壳和/或li元素的地幔源,其中与俯冲有关的指纹部分被湮没。对于这两种类型,区域构造的再活化在岩浆生成中起着重要作用。酸交代作用主要表现在钠长石花岗岩中沿断裂系统发育的细灰白色脉体。采用质量平衡法对灰化过程进行化学分析,结果表明,灰化过程中SiO2、Fe2O3、MnO、F、Sn和Li含量显著增加,Na2O、K2O、Ba、Nb和Zn含量显著减少。该过程导致体积显著增加(30%)。化学成分的变化与观察到的矿物学变化一致。微探针结果表明,黑钨矿晶体具有典型的富铁核和富锰边缘的辉白晶石结构。黑钨矿晶体组成的变化与含矿流体的物理化学条件(pH、T等)和化学性质有关。
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