GEOCHEMICAL AND MINERALOGICAL CONSTRAINTS ON THE GENESIS OF THE OTJOSONDU FERROMANGANESE DEPOSIT, NAMIBIA: HYDROTHERMAL EXHALATIVE VERSUS HYDROGENETIC (INCLUDING SNOWBALL-EARTH) ORIGINS
GEOCHEMICAL AND MINERALOGICAL CONSTRAINTS ON THE GENESIS OF THE OTJOSONDU FERROMANGANESE DEPOSIT, NAMIBIA: HYDROTHERMAL EXHALATIVE VERSUS HYDROGENETIC (INCLUDING SNOWBALL-EARTH) ORIGINS
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纳米比亚 OTJOSONDU 铁锰矿床成因的地球化学和矿物学限制：热液喷流成因与水成因（包括雪球地球）成因
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发表时间：
2011
期刊：
影响因子：
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通讯作者：
C. Sattler
中科院分区：
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作者：
A. Cabral;J. Moore;B. Mapani;M. Koubová;C. Sattler
泛非达马拉造山带中部新元古代达马拉超群Otjosondu锰铁矿床的地层位置尚不确定。与中南区区域剖面相比较，表明其为超群致密地层剖面Nosib群Etusis组（富铁长石-石英岩），而非先前提出的Swakop群Chuos组（二辉岩、浊积岩和薄铁层）。Otjosondu矿石由锰铁-硅酸盐-氧化物组合组成，赋存于长石-石英岩层序中，缺乏典型的Chuos组混合相。含少量透明石、闪辉石和重晶石的赤铁矿地层和含透明石的闪辉石-石英岩石与锰硅酸盐氧化物岩石在空间上呈紧密结合。重晶石是锰-硅-氧化物矿石中广泛存在且局部富集的矿物。整体岩石化学分析表明，在锰-硅-氧化物和辉石-石英岩石中存在Ag-As-Cu-Co-Bi-B-P与Mn和Sr-Mo-W与Ba的微量元素组合。这些元素的关联可以通过在含氧碱性海洋环境中的吸附/取代过程来解释。锰硅酸盐氧化矿石中重晶石硫同位素组成变化范围为+10.1 ~ +16.7‰，δ34S值范围与岩石整体铁含量呈负相关，解释为同位素较轻的热液S与较重的海水S的混合，锰铁岩石的原岩为喷发热液与含氧的含硫酸盐海水混合形成的盆地边缘沉淀。以及不同程度的碎屑添加。这些结论与形成与冰川相关的rapitan型铁和锰地层所需的深水、缺氧、硫酸盐匮乏的海洋环境相矛盾。因此，Otjosondu锰铁矿床的形成与任何Chuos/Sturtian全球冰川事件无关。由于Otjosondu矿石的贱金属和贫磷而富钡的性质，主要排除了氢成因矿床。Otjosondu的重晶石锰铁、赤铁矿石英和辉绿石石英岩可以与南非Gamsberg（破碎山型）贱金属矿床的氧化重晶石部分进行比较，并可能代表沉积物为主的喷出型贱金属矿化的远端或浅水氧化当量，如准同生的Tsongoari和Rosh Pinah矿床。
The stratigraphic position of the Otjosondu ferromanganese deposit in the Neoproterozoic Damara Supergroup in the Central Zone of the Pan-African Damara orogenic belt is uncertain. Comparison of the enclosing host rocks with regional profiles in the southern Central Zone would indicate that it is hosted by the Etusis Formation (Fe-rich feldspar-quartz rocks) of the Nosib Group, in a condensed stratigraphic section of the supergroup, and not the Chuos Formation (diamictite, turbidite and thin iron-formation layers) of the Swakop Group, as previously proposed. The Otjosondu ore comprises ferromanganese silicate-oxide assemblages that occur in a feldspathic-quartzite sequence lacking the mixtite facies typical of the Chuos Formation. Hematitic iron formation containing minor hyalophane, spessartine and barite, and hyalophane-bearing spessartine-quartz rock are present in close spatial association with the Mn silicate-oxide rocks. Barite is a widespread and locally abundant mineral in the Mn silicate-oxide ore. Bulk-rock chemical analyses indicate trace-element associations of Ag-As-Cu-Co-Bi-B-P with Mn and Sr-Mo-W with Ba in the Mn silicate-oxide and spessartine-quartz rocks. These element associations are explained by adsorption/substitution processes in an oxygenated alkaline marine setting. Sulfur-isotope compositions of barite from the Mn silicate-oxide ore vary from +10.1 to +16.7 ‰, a range of δ34S values that shows negative correlation with bulk-rock Fe content and is interpreted as mixing of isotopically light hydrothermal S with heavier sea-water S. The protoliths of the ferromanganese rocks were precipitates deposited in a basin-margin setting as a result of mixing of an exhalative hydrothermal fluid with oxygenated, sulfate-bearing sea water, and varying degrees of detrital addition. These conclusions are in contradiction to the deep-water, anoxic, sulfate-depleted oceanic settings required for formation of glacially associated Rapitan-type iron and manganese formations. The formation of the Otjosondu ferromanganese deposit is therefore unrelated to any Chuos/Sturtian global glacial event. Origins as hydrogenetic deposits are mostly excluded due to the base-metal- and P-poor, but Ba-rich nature of the Otjosondu ores. The baritic ferromanganese, hematite-quartz and spessartine-quartz rocks at Otjosondu may be compared to oxidised baritic portions of the Gamsberg (Broken Hill-type) base-metal deposit in South Africa and may represent the distal or shallow-water oxidised equivalents of sediment-hosted exhalative base-metal mineralisation such as the penecontemporaneous Tsongoari and Rosh Pinah deposits.