The present study gives new insight on the formation conditions of dolomite and magnesite in an early–middle Miocene succession related to a half-graben rift-sag basin on the western margin of the Red Sea. The studied Miocene succession comprises two units of siliciclastic–carbonate rocks separated by a magnesite bed. The succession is enriched with epigenetic–supergenetic polymetallic minerals, dominated by zinc-bearing ferromanganese oxides. These represent oxidized Mississippi Valley-type deposits (MVT) formed during uplifting in late Miocene–Pliocene time. Multistage dolomitization (four dolomite types: D1–D4) and magnesite authigenesis, enhanced by tectonic uplifting and faulting related to the Red Sea rifting, have been recorded. The first dolomite phase (D1) is pervasive early diagenetic dolomicrite (replacement type), which is dominant in the lower unit. Magnesite occurs as microcrystalline aggregates exclusive to the lower unit, where its authigenesis was after D1 and before D2. Occurrence of magnesite was mostly related to a restricted environment in a sag fault-bounded basin with shallow evaporative hypersaline conditions in coastal areas. D2 dolomite occurs in the lower and upper units as replacement and/or cement type of medium- to coarse-crystalline dolomite crystals. The three magnesium-rich carbonates (D1, magnesite and D2) are related to successive events of sea-level fall and rise in mesohaline and hypersaline conditions. Enrichment of magnesite and D2 dolomite with Na (up to 2.16 wt.%) and Sr (up to 1483 ppm) supports their formation under more saline evaporative conditions if compared with D1 dolomite which was formed in near-normal sea water or mesohaline fluids. The third and fourth dolomite phases (D3 and D4) are late diagenetic pore-filling coarsely crystalline and zoned, and restricted mainly to faulted areas associated with the polymetallic ore deposits. Elemental analyses of the four dolomite phases show different chemistries, i.e., non-ferroan dolomites (D1 and D2), alternation of manganiferous and non-ferroan zones (D3) and/or ferroan-type dolomite (D4). Stable- isotope values of the four dolomite types (δ18OVPDB of –7.82‰ to –5.88‰) and geochemistry suggest involvement of shallow evaporative conditions in coastal areas, enhanced either by dry and hot climate or by hydrothermal process in their formation. Nonetheless, the localized occurrence of D3 and D4 types along the faults, their concomitant occurrence with the epigenetic–supergenetic polymetallic ore deposits, and the preservation of unaltered feldspar grains ruled out the meteoric-water interaction and reinforce the fault-controlled and deep-seated hot fluid evolution for these two dolomite types. The underlying ultramafic and serpentinite rocks along with the intercalated magnesium-rich clays and/or the modified seawater most probably played a critical role in the diagenesis and/or precipitation of dolomite and magnesite. The proposed model can contribute to better understanding the genetic mechanisms of magnesite and dolomite hosted by mixed siliciclastic–carbonate deposits and their relations with MVT mineralization conditions in rift basins.

中文翻译：

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红海西缘裂谷盆地白云石-菱镁矿形成和多金属矿化：古环境、热液和构造意义

本研究对红海西缘半地堑裂谷盆地早-中新世演替中白云岩和菱镁矿的形成条件提供了新的认识。所研究的中新世层序包括两个由菱镁矿床隔开的硅质碎屑碳酸盐岩单元。该层序富含表生-表生多金属矿物，以含锌铁锰氧化物为主。这些代表了在晚中新世-上新世时期隆起期间形成的氧化密西西比河谷型沉积物（MVT）。多阶段白云石化（四种白云岩类型：D1-D4）和菱镁矿自生，由与红海裂谷相关的构造抬升和断层增强，已被记录。第一个白云岩阶段（D1）是普遍的早期成岩白云岩（置换型），在较低的单元中占主导地位。菱镁矿以微晶聚集体的形式出现在下部单元中，其自生发生在 D1 之后和 D2 之前。菱镁矿的出现主要与沿海地区具有浅蒸发高盐条件的凹陷断层界盆地的受限环境有关。D2 白云石作为中到粗晶白云石晶体的替代和/或胶结类型出现在下部和上部单元中。三种富镁碳酸盐（D1、菱镁矿和 D2）与中盐和高盐条件下海平面下降和上升的连续事件有关。用 Na 富集菱镁矿和 D2 白云石（高达 2.16 wt. %）和 Sr（高达 1483 ppm）与在接近正常海水或中盐流体中形成的 D1 白云石相比，支持它们在更多盐分蒸发条件下形成。第三和第四白云岩阶段（D3和D4）为晚期成岩孔隙充填粗晶带状，主要局限于与多金属矿床相关的断层区。四种白云岩相的元素分析显示出不同的化学成分，即非铁质白云岩（D1 和 D2）、锰和非铁质区交替（D3）和/或铁质型白云岩（D4）。四种白云岩类型的稳定同位素值（δ18OVPDB 为 –7.82‰ 至 –5.88‰）和地球化学表明，沿海地区的浅层蒸发条件受干热气候或形成过程中的热液作用增强。尽管如此，D3和D4类型沿断层的局部产状，与表生-表生多金属矿床的伴生，以及未改变的长石颗粒的保存，排除了大气-水相互作用，加强了断层控制和深部这两种白云岩类型的热流体演化。下伏的超镁铁质和蛇纹岩以及夹层的富镁粘土和/或改性海水很可能在白云石和菱镁矿的成岩和/或沉淀中发挥了关键作用。所提出的模型有助于更好地理解以硅质碎屑-碳酸盐混合矿床为主体的菱镁矿和白云石的成因机制及其与裂谷盆地 MVT 成矿条件的关系。D3和D4型沿断层的局部赋存，与表生-表生多金属矿床的伴生，以及未改变的长石颗粒的保存，排除了大气-水相互作用，加强了断层控制和深部热流体这两种白云岩类型的演化。下伏的超镁铁质和蛇纹岩以及夹层的富镁粘土和/或改性海水很可能在白云石和菱镁矿的成岩和/或沉淀中发挥了关键作用。所提出的模型有助于更好地理解以硅质碎屑-碳酸盐混合矿床为主体的菱镁矿和白云石的成因机制及其与裂谷盆地 MVT 成矿条件的关系。D3和D4型沿断层的局部赋存，与表生-表生多金属矿床的伴生，以及未改变的长石颗粒的保存，排除了大气-水相互作用，加强了断层控制和深部热流体这两种白云岩类型的演化。下伏的超镁铁质和蛇纹岩以及夹层的富镁粘土和/或改性海水很可能在白云石和菱镁矿的成岩和/或沉淀中发挥了关键作用。所提出的模型有助于更好地理解以硅质碎屑-碳酸盐混合矿床为主体的菱镁矿和白云石的成因机制及其与裂谷盆地 MVT 成矿条件的关系。它们与表生-表生多金属矿床同时出现，以及未改变的长石颗粒的保存排除了大气-水相互作用，并加强了这两种白云岩类型的断层控制和深部热流体演化。下伏的超镁铁质和蛇纹岩以及夹层的富镁粘土和/或改性海水很可能在白云石和菱镁矿的成岩和/或沉淀中发挥了关键作用。所提出的模型有助于更好地理解以硅质碎屑-碳酸盐混合矿床为主体的菱镁矿和白云石的成因机制及其与裂谷盆地 MVT 成矿条件的关系。它们与表生-表生多金属矿床同时出现，以及未改变的长石颗粒的保存排除了大气-水相互作用，并加强了这两种白云岩类型的断层控制和深部热流体演化。下伏的超镁铁质和蛇纹岩以及夹层的富镁粘土和/或改性海水很可能在白云石和菱镁矿的成岩和/或沉淀中发挥了关键作用。所提出的模型有助于更好地理解以硅质碎屑-碳酸盐混合矿床为主体的菱镁矿和白云石的成因机制及其与裂谷盆地 MVT 成矿条件的关系。并且未改变的长石颗粒的保存排除了大气-水相互作用，并加强了这两种白云岩类型的断层控制和深部热流体演化。下伏的超镁铁质和蛇纹岩以及夹层的富镁粘土和/或改性海水很可能在白云石和菱镁矿的成岩和/或沉淀中发挥了关键作用。所提出的模型有助于更好地理解以硅质碎屑-碳酸盐混合矿床为主体的菱镁矿和白云石的成因机制及其与裂谷盆地 MVT 成矿条件的关系。并且未改变的长石颗粒的保存排除了大气-水相互作用，并加强了这两种白云岩类型的断层控制和深部热流体演化。下伏的超镁铁质和蛇纹岩以及夹层的富镁粘土和/或改性海水很可能在白云石和菱镁矿的成岩和/或沉淀中发挥了关键作用。所提出的模型有助于更好地理解以硅质碎屑-碳酸盐混合矿床为主体的菱镁矿和白云石的成因机制及其与裂谷盆地 MVT 成矿条件的关系。下伏的超镁铁质和蛇纹岩以及夹层的富镁粘土和/或改性海水很可能在白云石和菱镁矿的成岩和/或沉淀中发挥了关键作用。所提出的模型有助于更好地理解以硅质碎屑-碳酸盐混合矿床为主体的菱镁矿和白云石的成因机制及其与裂谷盆地 MVT 成矿条件的关系。下伏的超镁铁质和蛇纹岩以及夹层的富镁粘土和/或改性海水很可能在白云石和菱镁矿的成岩和/或沉淀中发挥了关键作用。所提出的模型有助于更好地理解以硅质碎屑-碳酸盐混合矿床为主体的菱镁矿和白云石的成因机制及其与裂谷盆地 MVT 成矿条件的关系。