岩石圈含水辉石控制岩浆硫化物矿床的定位和镍禀赋

IF 4.4 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS Mineralium Deposita Pub Date : 2024-01-04 DOI:10.1007/s00126-023-01238-z
Isra S. Ezad, Daryl E. Blanks, Stephen F. Foley, David A. Holwell, Jason Bennett, Marco L. Fiorentini
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引用次数: 0

摘要

岩浆硫化镍矿床通常与玄武岩至柯麦饭石火成岩有关,这些火成岩起源于地幔的部分熔融,地幔通常被模拟为均匀的四相橄榄岩。现有模型认为,地幔熔体的主要金属成分是橄榄石(镍)和硫化物(铜、铂族元素和少量镍)。然而,地幔熔融通常是从体积较小的地幔组合开始的,如在橄榄岩地幔中以脉石形式出现的水合辉石,其中富含水合矿物辉石、闪石和磷灰石。过去,人们可能低估或忽视了含水辉石对地幔熔体金属禀赋的贡献,部分原因是随着熔化加剧,涉及橄榄岩时,含水辉石的贡献证据被部分抹去。在这里,我们汇编了来自实验和天然岩石的新结果,证明辉石、闪石和磷灰石等含水矿物对镍的分配系数都很高(3-20),可能是火成岩地幔源中镍的重要储存库。这意味着,寄存在水相辉长岩等变质地幔岩性中的含水矿物可能是某些岩浆硫化镍矿系统的重要成分。水相辉石中含有大量的水相矿物,其模态丰度高达 30-40 Vol%,此外还有烊辉石和少量 Vol%的氧化物相,如金红石和钛铁矿。这些地幔岩性通常与板块和大陆地区有关,在这些地区,低温、低度富挥发性熔体通常会改变岩石圈橄榄岩地幔,沉积出可变的含水辉石。水合辉绿岩岩性中水合矿物的熔融温度较低,这也意味着岩浆矿床的生成可能不需要羽流等重大热扰动,因为水合辉绿岩的熔融温度比干橄榄岩低约 300-350 ℃。水合辉绿岩的部分熔体在低温下比橄榄岩熔体的体积更大。此外,下文将论证它们含有类似甚至更高浓度的镍。因此,预测性勘探模型应考虑岩石圈地幔中水合辉长岩可能局部集中的区域,因为在板块漫长的地质演化过程中,水合辉长岩可能偶尔被熔化,产生富含镍的熔体,这些熔体可能存在于地壳各层不同大小和几何形状的导管中。
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Lithospheric hydrous pyroxenites control localisation and Ni endowment of magmatic sulfide deposits

Magmatic Ni–sulfide ore deposits are generally associated with basaltic to komatiitic igneous rocks that originate by partial melting of the mantle, which is usually modelled as a uniform four-phase peridotite. Existing models accept that the key metal contributors to mantle melts are olivine (Ni) and sulfide (Cu, platinum group elements (PGEs) and minor Ni). However, melting in the mantle commonly begins in volumetrically minor mantle assemblages such as hydrous pyroxenites that occur as veins in the peridotite mantle, which are rich in the hydrous minerals phlogopite, amphibole and apatite. The contribution of hydrous pyroxenites to the metal endowment of mantle melts may have been underestimated or overlooked in the past, partly because evidence of their input is partially erased as melting intensifies to involve peridotite.

Here, we compile new results from experiments and natural rocks which demonstrate that the hydrous minerals such as phlogopite, amphiboles and apatite all have high partition coefficients for Ni (3–20) and may be important repositories for Ni in mantle sources of igneous rocks. This implies that hydrous minerals hosted in metasomatic mantle lithologies such as hydrous pyroxenites may be important contributors to some magmatic Ni–sulfide ore systems. Hydrous pyroxenites contain hydrous minerals in large modal abundances up to 30–40 vol% in addition to clinopyroxene and a few vol% of oxide phases, such as rutile and ilmenite. These mantle lithologies are commonly associated with cratonic and continental regions, where low-temperature, low-degree volatile-rich melts commonly modify lithospheric peridotite mantle, depositing variable hydrous pyroxenites.

The lower melting temperatures of hydrous minerals in hydrous pyroxenite lithologies also means that the generation of magmatic ore deposits may not require a major thermal perturbation such as a plume, as the melting temperatures of hydrous pyroxenites lie around 300–350 °C lower than dry peridotites. Partial melts of hydrous pyroxenite are more voluminous at low temperatures than melts of peridotite would be. Furthermore, it is argued in the following that they would contain similar or even higher concentrations of Ni. Thus, predictive exploration models should consider domains of the lithospheric mantle where hydrous pyroxenites may be localised and concentrated, as they may have been episodically melted throughout the long-lived geological evolution of cratonic blocks, yielding Ni-rich melts that may be hosted in conduits of varying size and geometry at various crustal levels.

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来源期刊
Mineralium Deposita
Mineralium Deposita 地学-地球化学与地球物理
CiteScore
11.00
自引率
6.20%
发文量
61
审稿时长
6 months
期刊介绍: The journal Mineralium Deposita introduces new observations, principles, and interpretations from the field of economic geology, including nonmetallic mineral deposits, experimental and applied geochemistry, with emphasis on mineral deposits. It offers short and comprehensive articles, review papers, brief original papers, scientific discussions and news, as well as reports on meetings of importance to mineral research. The emphasis is on high-quality content and form for all articles and on international coverage of subject matter.
期刊最新文献
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