Trace element signatures in scheelite associated with various deposit types: A tool for mineral targeting

IF 3.4 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
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引用次数: 0

Abstract

Scheelite is a widespread mineral in several geological settings and its trace element composition provides valuable information about the source and composition of the hydrothermal fluids. In this study, scheelite from 22 magmatic-hydrothermal deposits and 2 orogenic Au deposits (Hangar Flats and Corcoesto) were analyzed by EPMA and LA-ICP-MS. Magmatic-hydrothermal scheelite, together with literature data are investigated using partial least square-discriminant analysis (PLS-DA) and Random Forest (RF) classifier, to evaluate the use of scheelite as a robust indicator mineral for W-bearing deposit targeting. Cathodoluminescence images show that scheelite is texturally homogeneous in reduced intrusion-related gold systems (RIRGS) and varies from homogeneous to heterogeneous in other magmatic-hydrothermal and orogenic Au deposits. Scheelite displays six REE chondrite-normalized patterns, which are a function of the source and composition (mainly salinity) of the mineralizing fluids and partitioning with co-genetic minerals (e.g., garnet, clinopyroxene). The PLS-DA highlights that scheelite trace element composition from magmatic-hydrothermal deposits varies following different deposit types (e.g., oxidized and reduced skarns, porphyry WMo, RIRGS, quartz-vein/greisen SnW), and that such compositional variation reflects mainly the difference of fO2 and composition of mineralizing fluids. Additionally, scheelite from magmatic-hydrothermal deposits are chemically distinct to those formed dominantly by metamorphic fluids in orogenic settings as shown by their higher Mo, Nb and Mn, and lower Sr contents and predominantly negative Eu anomalies. Metamorphic scheelite can be discriminated from that of orogenic Au deposits by their lower Pb, As and REE contents and LREE/HREE ratios, which are related to local host rock composition and metamorphic grade. Using Na, Mg, Mn, As, Sr, Y, Nb, Mo, Pb, ΣREE concentrations and Eu anomaly as predictors, the RF model yields an overall prediction accuracy of 97 % for test data as function of deposit types (89.2 % for RIRGS, 100 % for porphyry WMo, 97.8 % for quartz-vein/greisen SnW, 96.9 % for oxidized skarn, 98.1 % for reduced skarn and 99.3 % for orogenic Au deposits). Application of RF classifier to scheelite composition from orogenic Au and skarn- and greisen-type W deposits from literature yields an overall prediction of ∼79 % (91 % for oxidized skarn, 71.4 % for quartz-vein/greisen SnW and 74.2 % for orogenic Au deposits) showing that scheelite is an efficient indicator mineral for Au and W deposits targeting. Metamorphic scheelite is predicted mostly as orogenic Au scheelite (83 %), reflecting the genesis of metamorphic fluids and similar geological setting, suggesting that RF classifier can be also used to predict the fluid sources.

与各种矿床类型相关的白钨矿中的微量元素特征:矿物定位工具
白钨矿是一种广泛存在于多种地质环境中的矿物,其微量元素组成为了解热液的来源和组成提供了宝贵的信息。本研究利用 EPMA 和 LA-ICP-MS 分析了 22 个岩浆热液矿床和 2 个造山金矿(Hangar Flats 和 Corcoesto)中的白钨矿。利用偏最小平方判别分析(PLS-DA)和随机森林分类器(RF)对岩浆热液型白钨矿和文献数据进行了研究,以评估白钨矿作为含 W 矿床目标的可靠指示矿物的用途。阴极发光图像显示,白钨矿在还原侵入相关金系统(RIRGS)中质地均匀,而在其他岩浆-热液和造山金矿床中则从均匀到异质不等。白钨矿显示了六种REE软玉归一化模式,这与成矿流体的来源和成分(主要是盐度)以及与共生矿物(如石榴石、倩辉石)的分区有关。PLS-DA突出表明,岩浆-热液矿床中的白钨矿微量元素组成随矿床类型(如氧化和还原矽卡岩、斑岩WMo、RIRGS、石英脉/砾岩SnW)的不同而变化,这种组成变化主要反映了成矿流体的fO2和组成的不同。此外,岩浆-热液矿床中的白钨矿与造山环境中主要由变质流体形成的白钨矿在化学性质上截然不同,这表现在它们具有较高的钼、铌和锰含量,较低的锶含量,以及主要为负的Eu异常。变质白钨矿的铅、砷和 REE 含量较低,LREE/HREE 比值也较低,这与当地的主岩成分和变质品位有关,因此可以将它们与成岩金矿床的白钨矿区分开来。使用 Na、Mg、Mn、As、Sr、Y、Nb、Mo、Pb、ΣREE 浓度和 Eu 异常作为预测因子,RF 模型对测试数据的矿床类型函数的总体预测准确率达到 97%(RIRGS 为 89.2%,斑岩型 WMo 为 100%,石英脉/砾岩型 SnW 为 97.8%,氧化矽卡岩为 96.9%,还原矽卡岩为 98.1%,成因金矿床为 99.3%)。将射频分类器应用于文献中的成因金矿床和矽卡岩及格瑞森型W矿床的白钨矿成分,得出的总体预测结果为79%(氧化矽卡岩为91%,石英脉/格瑞森SnW为71.4%,成因金矿床为74.2%),表明白钨矿是金矿床和W矿床目标的有效指示矿物。变质白钨矿主要被预测为成因金白钨矿(83%),反映了变质流体的成因和相似的地质环境,表明射频分类器也可用于预测流体来源。
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来源期刊
Journal of Geochemical Exploration
Journal of Geochemical Exploration 地学-地球化学与地球物理
CiteScore
7.40
自引率
7.70%
发文量
148
审稿时长
8.1 months
期刊介绍: Journal of Geochemical Exploration is mostly dedicated to publication of original studies in exploration and environmental geochemistry and related topics. Contributions considered of prevalent interest for the journal include researches based on the application of innovative methods to: define the genesis and the evolution of mineral deposits including transfer of elements in large-scale mineralized areas. analyze complex systems at the boundaries between bio-geochemistry, metal transport and mineral accumulation. evaluate effects of historical mining activities on the surface environment. trace pollutant sources and define their fate and transport models in the near-surface and surface environments involving solid, fluid and aerial matrices. assess and quantify natural and technogenic radioactivity in the environment. determine geochemical anomalies and set baseline reference values using compositional data analysis, multivariate statistics and geo-spatial analysis. assess the impacts of anthropogenic contamination on ecosystems and human health at local and regional scale to prioritize and classify risks through deterministic and stochastic approaches. Papers dedicated to the presentation of newly developed methods in analytical geochemistry to be applied in the field or in laboratory are also within the topics of interest for the journal.
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