Marginal facies and compositional equivalents of Bushveld parental sills from the Molopo Farms Complex layered intrusion, Botswana: Petrogenetic and mineralization implications
Marginal facies and compositional equivalents of Bushveld parental sills from the Molopo Farms Complex layered intrusion, Botswana: Petrogenetic and mineralization implications
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DOI：
10.1016/j.oregeorev.2017.12.001
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发表时间：
2018
影响因子：
3.3
通讯作者：
J. Kaavera;H. M. Rajesh;T. Tsunogae;G. Belyanin
中科院分区：
文献类型：
--
作者：
J. Kaavera;H. M. Rajesh;T. Tsunogae;G. Belyanin
岩浆的组成被认为是层状超基性或基性侵入岩的母岩，通常用与周围岩石接触的岩石来研究。博茨瓦纳Molopo Farms杂岩的基性到超基性岩石，其大小约为~ 2.05 Ga Bushveld杂岩层状侵入物的1/3，被认为是由形成Bushveld岩石的相同岩浆形成的。在可用的不同岩心中，MF38的底部与乡村岩石相交。另外两个岩心MF9和MF11也被记录下来进行比较。钻芯MF38在底部保留了活化的乡村岩石，随后是基性（黑岩）到超基性（橄榄岩-辉石岩）岩石。黑云母具有淬火织构，具有细长、骨架状、成分分带的正辉石晶体，并含有绿云母。值得注意的是，正辉石矿物化学成分与报道的Bushveld杂岩冷缘相似。结合岩心MF38底部活化的乡村岩石，解释为一个边缘相单元。边缘相黑岩的全岩地球化学特征与Bushveld杂岩的寒缘和B1组成相似，认为它代表了已知最早的Molopo Farms杂岩母岩浆组成。在MF38岩心地层较高位置发现的中粒含辉云母辉石岩，其成分与Bushveld杂岩中的B1UM相当。此外，MF9岩心辉长岩的全岩地球化学特征与B3相当，被认为是Bushveld杂岩的亲本-分化端元。在Molopo Farms复合体岩石中未观察到与Bushveld复合体相当的B2成分。考虑到Molopo Farms杂岩比Bushveld杂岩的厚度小，并且它保留了与最早的岩浆（B1， B1UM和冷缘）相似的基性和超基性岩石，以及Bushveld岩浆（B3）中分化程度最高的岩石，认为Molopo Farms杂岩演化得更快。因此，它很可能不会形成布什维尔德杂岩的岩石，而布什维尔德杂岩含有含铬铁矿层，以PGE矿化而闻名。相反，有人认为Molopo农场复合体是铜镍硫化物成矿的更好目标。岩浆磁黄铁矿-辉黄铁矿-黄铜矿组合在研究岩石中显著存在，边缘相黑岩中与硫化物有关的微量PGM，以及与硫化物有关的蚀变特征，提出了一种可能的硫化物成因模式。
The composition of magmas considered to be parental to layered ultramafic to mafic intrusions are commonly studied using rocks occurring along their marginal portion, in contact with the surrounding country rock. With a size, approximately 1/3rd of the ∼2.05 Ga Bushveld Complex layered intrusion, mafic to ultramafic rocks of the Molopo Farms Complex in Botswana are suggested to have formed from the same magma that formed the Bushveld rocks. Of the different drill cores available, the base of MF38 intersects the country rock. Two other drill cores, MF9 and MF11, were also logged for comparison. Drill core MF38 preserves remobilized country rock at the base, followed up by mafic (norite) to ultramafic (peridotite-pyroxenite) rocks. The norite is quench textured with elongate, skeletal, compositionally zoned orthopyroxene crystals, and contain phlogopite. Significantly, the orthopyroxene mineral chemical composition is similar to that reported for chill margin from the Bushveld Complex. Together with the remobilized country rock beneath, norite at the base of drill core MF38 is interpreted to represent a marginal facies unit. The whole-rock geochemical characteristics of the marginal facies norite are similar to those of chill margin and B1 compositions from the Bushveld Complex, arguing for it to represent the earliest known parental magma composition to the Molopo Farms Complex. A medium-grained phlogopite-bearing pyroxenite occurring higher up the stratigraphy in drill core MF38 is considered compositional equivalent to B1UM from the Bushveld Complex. Further, the whole-rock geochemical characteristics of gabbronorites from drill core MF9 are comparable to B3, considered parental to differentiated end members of the Bushveld Complex. No compositional equivalent to B2 from the Bushveld Complex was observed in the Molopo Farms Complex rocks. Considering the fact that the Molopo Farms Complex is less thick than the Bushveld Complex, and that it preserves mafic and ultramafic rocks showing similarity to the earliest magma (B1, B1UM and chill margins) as well as the most differentiated of the Bushveld magma (B3), it is argued that the Molopo Farms Complex evolved faster. Thus it likely did not form rocks of the Bushveld Complex, which hosts the chromite-bearing layers, known for PGE mineralization. Instead, it is argued that the Molopo Farms Complex is a better target for Cu-Ni sulphide mineralization. A possible sulphide genesis model is presented in relation to the prominent occurrence of magmatic pyrrhotite-pentlandite-chalcopyrite assemblage in the studied rocks, trace amounts of PGM associated with sulphides in the marginal facies norite, and the alteration features associated with sulphides.