The origin of discordant Fe-rich wehrlite bodies and the role of metasomatism and liquid immiscibility in the evolution of the Bushveld Igneous Complex and its platinum deposits

不一致的富铁韦尔铁矿体的起源以及交代作用和液体不混溶性在布什维尔德火成岩杂岩及其铂矿床演化中的作用

• 批准号: 243884595
• 负责人: Dr. Ilya Veksler, Ph.D.
• 金额: --
• 依托单位: Sektion 3.1: Anorganische und Isotopengeochemie
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2013
• 资助国家: 德国
• 起止时间: 2012-12-31 至 2016-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/243884595?language=en
• 关键词: origin; discordant; Fe; rich; wehrlite

The Bushveld Igneous Complex in South Africa is the Earths largest natural repository of several strategic precious and industrial metals, namely the platinum-group, chromium and vanadium. The sheer scale of the complex (hundreds of kilometres across), as well as the limited surface exposure, particularly in the highly industrialised western sector, have obstructed understanding of the Complex and its fabulous mineral wealth. Despite the obstacles and because of the great scientific and economical importance, geoscientists have used Bushveld for several decades as a unique natural laboratory, where theories of magma emplacement, differentiation and mineralization have been explored and tested. One of the major unresolved problems is the relationship between magmatic crystallization and metasomatic processes in shaping the diversity of igneous rocks in the Complex and other large, strongly differentiated intrusions. This project addresses important aspects of that major problem by studying the origin of a suite of extremely Fe-rich olivine-pyroxene rocks (wehrlite). The bodies, also known under the name of iron-rich ultramafic pegmatites (IRUP), are believed to result from large-scale replacement reactions imposed on pre-existing gabbronoritic cumulate rocks by metasomatizing melts or fluids of unknown origin and composition. It has been proposed that the replacement was caused by downward percolation of dense, Fe-rich silicate melts produced by silicate liquid immiscibility in the apical part of the intrusion. However, large-scale liquid immiscibility in the Bushveld Complex and in other mafic intrusions is contentious, so the origin of the Fe-wehrlite suite remains unclear. The goal of the project is to test the possible link between metasomatizing liquids and silicate liquid immiscibility and to assess the geochemical effects of the metasomatic transformation, including re-distribution of Fe-Ni-Cu sulphides and the platinum-group elements. The proposed working programme combines targeted geochemical and petrographic studies of natural rocks with melting experiments on mixtures of natural rocks and synthetic glasses. The experimental objectives are to reproduce the replacement reactions and establish better phase equilibria constraints on the compositions of metasomatizing fluids. The origin of the replacement Fe-wehrlite and the associated reactions are not only of great theoretical interest but also of serious practical importance because the replacement disrupts the platinum-group ore bodies and cause significant problems for mining.

南非的布什维尔德火成岩杂岩是地球上最大的天然矿藏，蕴藏着几种战略贵金属和工业金属，即铂族、铬和钒。该建筑群的巨大规模(数百公里宽)，以及有限的地表曝光率，特别是在高度工业化的西部地区，阻碍了人们对该建筑群及其惊人的矿产财富的了解。尽管存在这些障碍，而且由于其巨大的科学和经济重要性，地球科学家几十年来一直将布什维尔德作为一个独特的自然实验室，在那里探索和测试岩浆侵位、分异和成矿的理论。尚未解决的主要问题之一是岩浆结晶和交代过程之间的关系，这些过程塑造了复杂和其他大型、强烈分异侵入体中火成岩的多样性。该项目通过研究一套极富铁的橄榄石-辉石岩(魏利岩)的成因，解决了这一主要问题的重要方面。这些遗体也被称为富铁超镁铁伟晶岩(IRUP)，据信是通过交代未知来源和成分的熔体或流体作用于先前存在的辉长岩堆积岩而产生的大规模置换反应的结果。认为交代作用是由侵入岩顶端硅酸盐液体不混溶产生的致密富铁硅酸盐熔体向下渗流所致。然而，布什维尔德杂岩和其他镁铁质侵入体中的大规模液体不混溶是有争议的，因此铁辉橄榄岩套的起源尚不清楚。该项目的目标是测试交代液体和硅酸盐液体不混溶之间的可能联系，并评估交代转变的地球化学影响，包括Fe-Ni-Cu硫化物和铂族元素的重新分布。拟议的工作方案结合了对天然岩石的有针对性的地球化学和岩石学研究，以及对天然岩石和合成玻璃混合物的熔化实验。实验目标是重现置换反应，并对交代流体的组成建立更好的相平衡约束。由于交代作用破坏了铂族矿体，给采矿带来了严重的困难，因此，交代铁辉橄榄石的成因及其伴生反应不仅具有重要的理论意义，而且具有重要的实际意义。

项目成果

Electrochemical Processes in a Crystal Mush: Cyclic Units in the Upper Critical Zone of the Bushveld Complex, South Africa

• DOI: 10.1093/petrology/egv036
• 发表时间: 2015-06
• 期刊: Journal of Petrology
• 影响因子: 3.9
• 作者: I. Veksler;D. Reid;P. Dulski;J. Keiding;Mathias Schannor;L. Hecht;R. Trumbull

Immiscible iron- and silica-rich liquids in the Upper Zone of the Bushveld Complex

• DOI: 10.1016/j.epsl.2016.03.016
• 发表时间: 2016-06-01
• 期刊: EARTH AND PLANETARY SCIENCE LETTERS
• 影响因子: 5.3
• 作者: Fischer, Lennart A.;Wang, Meng;Holtz, Francois
• 通讯作者: Holtz, Francois

Letter. Experimental confirmation of high-temperature silicate liquid immiscibility in multicomponent ferrobasaltic systems

• DOI: 10.2138/am-2015-5285
• 发表时间: 2015-05
• 期刊: American Mineralogist
• 影响因子: 3.1
• 作者: T. Hou;I. Veksler