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复合体是地球上最大的几种战略贵金属和工业金属（即铂族、铬和钒）的天然储藏库。复杂的规模（数百公里），以及有限的表面暴露，特别是在高度工业化的西部地区，阻碍了对复杂及其惊人的矿产财富的理解。尽管存在这些障碍，但由于其在科学和经济上的重要性，地球科学家们几十年来一直将布什维尔德作为一个独特的天然实验室，在那里对岩浆就位、分异和成矿理论进行了探索和测试。其中一个尚未解决的主要问题是岩浆结晶和交代过程之间的关系，在形成复杂的火成岩和其他大型，强烈分化的侵入体的多样性。该项目通过研究一套极富铁的橄榄辉石岩（韦尔岩）的起源来解决这一主要问题的重要方面。这些天体也被称为富铁超镁铁质伟晶岩（IRUP），被认为是由于交代熔体或未知来源和成分的流体对预先存在的辉长岩堆晶岩进行大规模置换反应而产生的。有人提出，置换是由于向下渗透的致密，富铁的硅酸盐熔体产生的硅酸盐液体的不溶性在顶端的侵入。然而，在布什费尔德杂岩和其他镁铁质侵入体中的大规模液体不可渗透性是有争议的，因此铁-韦氏岩套件的起源仍然不清楚。该项目的目标是测试交代作用液体与硅酸盐液体不可渗透性之间可能存在的联系，并评估交代作用转化的地球化学影响，包括铁镍铜硫化物和铂族元素的重新分布。拟议的工作方案将对天然岩石进行有针对性的地球化学和岩相学研究与对天然岩石和合成玻璃的混合物进行熔化实验相结合。实验的目的是再现置换反应，并建立更好的相平衡的交代流体的组合物的约束。交代铁-韦氏体的成因及其相关反应不仅具有重要的理论意义，而且具有重要的实际意义，因为交代作用破坏了铂族矿体，给采矿带来了重大问题。

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