Hydrothermal alteration and geochemical proximity indicators to ore at the Metsämonttu Zn–Pb–Cu–Au–Ag deposit, Uusimaa belt, southern Finland

IF 3.4 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Janne Hokka , Hanna Leväniemi , Tuomas Leskelä
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引用次数: 0

Abstract

The Paleoproterozoic Metsämonttu Zn–Pb–Cu–Ag–Au deposit (1.5 Mt at 3.5 wt% Zn, 0.8 wt% Pb, 0.3 wt% Cu, 13.2 wt% S, 25 g/t Ag, and 1.4 g/t Au, production 1952–1974) is the largest past-producing mine in the Aijala–Orijärvi area (Orijärvi formation, Aijala member) within the Uusimaa belt, southern Finland. The Aijala member is characterized by 1.9–1.88 Ga felsic-dominated volcanic-sedimentary supracrustal rocks with intercalated sedimentary carbonates and iron formations. The area is underexplored, and little deposit-scale research has been carried out, despite the lateral continuum to the world-class ore district of Bergslagen in south-central Sweden. To better understand the Metsämonttu VMS-related alteration system, we reassessed the previously described metamorphic mineral assemblages and their protolith rock compositions by using mobile and immobile element geochemistry. This resulted in the definition of four metamorphosed alteration mineral assemblages and eight chemostratigraphic rock units. The chemostratigraphic results suggest that the lithological and/or structural setting of the Metsämonttu succession is more complex than previously considered. The stratigraphic footwall is mainly characterized by an extensive cordierite + anthophyllite ± biotite ± phlogopite + pyrite ± pyrrhotite (Mg–Fe–S) assemblage dominated by mafic rocks, designated Mafic B1 and Mafic B2, and Andesite A1. The main sulfide mineralization is hosted by tremolite + diopside ± biotite ± phlogopite ± chlorite skarn (Ca–Mg–K). A sericite-bearing muscovite + quartz ± biotite ± phlogopite + pyrite (K–Si–S) assemblage is composed of felsic to mafic protoliths (Mafic B1–B2, Andesite A1, Dacite A1, Dacite B1, Dacite C1) and extends several tens of meters into the stratigraphic hanging-wall. A quartz + pyrite ± muscovite (Si–S) assemblage represents the immediate ore-proximal alteration and is derived from rocks with a rhyolitic composition (Rhyolite A1).

Limited drill core samples near Metsämonttu mineralization, along with restricted surface alteration, pose challenges in studying geochemical variations from distal to ore proximal areas and limits the ability to model the shape and size of the alteration zone. Large mass changes suggest that the alteration was hydrothermal, and due to several protolith compositions, the alteration is interpreted to be predominantly discordant to stratigraphy. A 60-m-wide alteration halo surrounds the Metsämonttu deposit. Major and trace elements, namely MgO, Na2O, K2O, SiO2, Fe2O3, Cu, Zn, Pb, S, Ag, Tl, Hg, Se, Te, Sn, Sb, Rb, and Sr, and the indices modified alteration index (MAI), Ishikawa alteration index (AI), chlorite‑carbonate-pyrite index (CCPI), S/Na2O can be used for chemical vectoring, which can assist regional or near-mine exploration. Elements MnO, Ba, Cd, Bi, As, Ni, Co, W, Ga, Mo, and indices Hashigushi index and advance argillic alteration index (AAAI) were less useful as geochemical exploration indicators.

Abstract Image

芬兰南部乌西马带 Metsämonttu Zn-Pb-Cu-Au-Ag 矿床的热液蚀变和地球化学近矿指标
古新生代的梅萨蒙图(Metsämonttu)锌铅铜金矿床(1.5 百万吨,含锌 3.5 wt%、铅 0.8 wt%、铜 0.3 wt%、硒 13.2 wt%、银 25 g/t 和金 1.4 g/t,1952-1974 年产量)是芬兰南部乌西马(Uusimaa)带艾雅拉-奥里耶尔维(Orijärvi)地区过去最大的矿山。艾雅拉岩层的特征是以 1.9-1.88 Ga 长英岩为主的火山-沉积超基性岩,夹有沉积碳酸盐岩和铁质地层。尽管该地区与瑞典中南部的世界级矿区伯格斯拉根(Bergslagen)具有横向连续性,但该地区勘探不足,几乎没有开展过矿床规模的研究。为了更好地了解与 Metsämonttu VMS 相关的蚀变系统,我们利用移动和非移动元素地球化学方法重新评估了之前描述的变质矿物组合及其原岩成分。最终确定了四个变质蚀变矿物组合和八个化学地层岩石单元。化学地层学结果表明,梅萨芒图岩系的岩性和/或构造背景比之前认为的更为复杂。地层底壁的主要特征是以黑云母岩(黑云母岩 B1 和黑云母岩 B2 以及安山岩 A1)为主的广泛的堇青石 + 直闪石 ± 黑云母 ± 辉绿岩 + 黄铁矿 ± 黄铁矿(Mg-Fe-S)组合。主要硫化物矿化由透闪石+透辉石±斜长石±辉绿岩±绿泥石矽卡岩(Ca-Mg-K)包裹。绢云母+石英±斜长石±辉绿岩+黄铁矿(K-Si-S)集合体由长英岩至黑云母原岩(黑云母 B1-B2、安山岩 A1、黑云母 A1、黑云母 B1、黑云母 C1)组成,延伸至地层悬壁数十米处。Metsämonttu 矿化附近的钻孔岩芯样本有限,地表蚀变也很有限,这给研究从远端到近端矿石区域的地球化学变化带来了挑战,并限制了对蚀变带的形状和大小进行建模的能力。巨大的质量变化表明,蚀变是热液蚀变,由于多种原岩成分,蚀变被解释为主要与地层不和谐。Metsämonttu矿床周围有一个60米宽的蚀变晕。主要元素和微量元素,即氧化镁(MgO)、氧化钠(Na2O)、氧化钾(K2O)、二氧化硅(SiO2)、氧化铁(Fe2O3)、铜(Cu)、锌(Zn)、铅(Pb)、硒(Si)、银(Ag)、碲(Tl)、汞(Hg)、硒(Se)、碲(Te)、锡(Sn)、锑(Sb)、铷(Rb)和锶(Sr),以及修正蚀变指数(MAI)、石川蚀变指数(AI)、绿泥石-碳酸盐-黄铁矿指数(CCPI)、S/Na2O指数可用于化学矢量分析,这有助于区域或近矿勘探。元素 MnO、Ba、Cd、Bi、As、Ni、Co、W、Ga、Mo 以及指数 Hashigushi 指数和 advance argillic alteration index (AAAI) 作为地球化学勘探指标的作用较小。
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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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