Structural configuration of the Central African Copperbelt: roles of evaporites in structural evolution, basin hydrology, and ore location
Structural configuration of the Central African Copperbelt: roles of evaporites in structural evolution, basin hydrology, and ore location
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中非铜矿带的构造配置：蒸发岩在构造演化、盆地水文和矿石位置中的作用
DOI：
10.5382/sp.21.07
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发表时间：
2018
期刊：
影响因子：
--
通讯作者：
D. Broughton
中科院分区：
文献类型：
--
作者：
D. Selley;R. Scott;P. Emsbo;L. Koziy;M. Hitzman;S. Bull;M. Duffett;S. Sabagenzi;J. Halpin;D. Broughton
中非铜带是世界上主要的沉积物承载铜矿省。它包含在Katangan盆地，一个克拉通内裂谷，记录开始生长在10840 Ma和反转在10535 Ma。在铜矿带地区，盆地具有大致对称的形式，中央沉积中心最大，包含位于刚果民主共和国和赞比亚边界北方一侧的11 km地层，边缘浓缩序列。已知从盆地的边缘和过渡到U-Ni-Co和Pb-Zn-Cu矿石向沉积中心的最大。大多数矿石类型位于前近盆地宽的盐席或相关的盐动力学结构的500米晕内，位于盆地边缘位置的矿石除外，在那里原生盐没有沉积。层状Cu ± Co矿石产于盐内（刚果型）、盐下（赞比亚型）和盐缘（Kamoa型）部位。从这些存款类型的前两个散装破碎-浸出流体包裹体数据揭示了一个主要的残余钾长石卤水协会。一个可能的签名的矿流体，盐水是在沉积过程中产生的盆地范围内的盐层和占用大量的子和盐内含水层从~800马。相关的强烈Mg ± K交代作用仅限于这些水平，表明在盆地历史的长时间内，封盖和封闭盐保持不可渗透，将深层含水层与盆地填充物的上部隔离开来。765 ~ 740 Ma，盆地刚果部分的盐片发生了盐动力学变化。盐被横向抽出，供给底辟，最终导致局部焊接或破坏以前的水文密封。在这些点，更深层次的残留卤水被吸入盐内地层，与还原元素相互作用，形成层状矿石。根据盐覆盖层堆积过程中原始盐厚度、沉积物供给速率和体积以及构造作用的相互作用，可能盐熔作用贯穿盆地的北方和中部。水文结构的这种根本性变化的可变时间限制在造山运动的最早阶段的盐动开始期间，但是，矿石的几何形状和相关的蚀变模式要求矿化之前的寄主地层的特征复杂的碎片。因此，虽然早期的造山时间是允许的，矿化在伸展盆地发展的后期阶段更有可能。原位还原元素的主机赞比亚型层状铜±钴矿石是在连续的水文沟通与盐下含水层，这样的矿石形成可能已经开始从10800马卤水引入事件。在这个地区的盐的nonhalokinetic字符允许完整的密封保持超流体静力孔隙压力，促进流体循环，直到盆地生长的后期阶段，并可能早期造山作用。位于沉积中心最大和盆地南部的矿石的渗滤液数据，经历了相对较高的变质记录残留和岩盐溶解相关的盐水混合。盐溶解可能是由底辟的出现或热和/或机械引起的岩盐渗透性增加而引发的。虽然岩盐溶解发生在造山作用期间和之后是肯定的，但在沉积中心最大值的伸展过程中，可能局部存在有利于盐溶解的条件。盐的渗透性增加到一定程度，成为主要的含水层。盐作为隔水层的性质丧失，原来深层的残余盐水与蒸发岩溶解相关的盐水的新相混合，在盆地填充物的中间水平产生矿石。在成矿的最后阶段，记录在沉积中心的最大的造山后Pb-Zn-Cu矿化，流体的盐度主要来自溶解的残余体的盐。
The Central African Copperbelt is the world’s premier sediment-hosted Cu province. It is contained in the Katangan basin, an intracratonic rift that records onset of growth at ∼840 Ma and inversion at ∼535 Ma. In the Copperbelt region, the basin has a crudely symmetrical form, with a central depocenter maximum containing ∼11 km of strata positioned on the northern side of the border of the Democratic Republic of Congo and Zambia, and marginal condensed sequences The largest Cu ± Co ores, both stratiform and vein-controlled, are known from the periphery of the basin and transition to U-Ni-Co and Pb-Zn-Cu ores toward the depocenter maximum. Most ore types are positioned within a ∼500-m halo to former near-basin-wide salt sheets or associated halokinetic structures, the exception being that located in extreme basin marginal positions, where primary salt was not deposited. Stratiform Cu ± Co ores occur at intrasalt (Congolese-type), subsalt (Zambian-type), and salt-marginal (Kamoa-type) positions. Bulk crush-leach fluid inclusion data from the first two of these deposit types reveal a principal association with residual evaporitic brines. A likely signature of the ore fluids, the brines were generated during deposition of the basin-wide salt-sheets and occupied voluminous sub and intrasalt aquifers from ~800 Ma. Associated intense Mg ± K metasomatism was restricted to these levels, indicating that capping and enclosing salt remained impermeable for prolonged periods of the basin’s history, isolating the deep-seated aquifers from the upper part of the basin fill. From ∼765 to 740 Ma, the salt sheets in the Congolese part of the basin were halokinetically modified. Salt was withdrawn laterally to feed diapirs, ultimately leading to localized welding or breaching of the former hydrological seal. At these points, deeper-level residual brines were drawn into the intrasalt stratigraphy to interact with reducing elements and form the stratiform ores. It is probable that salt welding occurred diachronously across the northern and central parts of the basin, depending upon the interplay of original salt thickness, rates and volumes of sediment supply during accumulation of salt overburden, and tectonism. The variable timing of this fundamental change in hydrologic architecture is poorly constrained to the period of halokinetic onset to the earliest stages of orogenesis; however, the geometry of the ores and associated alteration patterns demands that mineralization preceded the characteristically complex fragmentation of the host strata. Thus, while an early orogenic timing is permissible, mineralization during the later stages of extensional basin development was more likely. In situ reducing elements that host Zambian-type stratiform Cu ± Co ores were in continuous hydrological communication with subsalt aquifers, such that ore formation could have commenced from the ∼800 Ma brine introduction event. The nonhalokinetic character of the salt in this region allowed the intact seal to have maintained suprahydrostatic pore pressures, facilitating fluid circulation until late stages of basin growth and possibly early stage orogenesis. Leachate data from ores positioned in the depocenter maximum and southern parts of the basin that underwent relatively high grade metamorphism record mixing of residual and halite dissolution-related brines. Salt dissolution was likely triggered by emergence of diapirs or thermally and/or mechanically induced increased permeability of halite. While it is certain that halite dissolution occurred during and after orogenesis, conditions favorable for salt dissolution may have existed locally during extension in the depocenter maximum. The permeability of salt increased to a point where it became the principal aquifer. The salt’s properties as an aquiclude lost, originally deep-seated residual brine mixed with new phases of evaporite dissolution-related brine to produce ores at middle levels of the basin fill. During the final stages of ore formation, recorded by postorogenic Pb-Zn-Cu mineralization in the depocenter maximum, the salinity of fluids was dominantly derived from the dissolution of remnant bodies of salt.