The Juomasuo Au–Co deposit, currently classified as an orogenic gold deposit with atypical metal association, is located in the Paleoproterozoic Kuusamo belt in northeastern Finland. The volcano-sedimentary sequence that hosts the deposit was intensely altered, deformed, and metamorphosed to greenschist facies during the 1.93–1.76 Ga Svecofennian orogeny. In this study, we investigate the temporal relationship between Co and Au deposition and the relationship of metal enrichment with protolith composition and alteration mineralogy by utilizing lithogeochemical data and petrographic observations. We also investigate the nature of fluids involved in deposit formation based on sulfide trace element and sulfur isotope LA-ICP-MS data together with tourmaline mineral chemistry and boron isotopes. Classification of original protoliths was made on the basis of geochemically immobile elements; recognized lithologies are metasedimentary rocks, mafic, intermediate-composition, and felsic metavolcanic rocks, and an ultramafic sill. The composition of the host rocks does not control the type or intensity of mineralization. Sulfur isotope values (δ34S − 2.6 to + 7.1‰) and trace element data obtained for pyrite, chalcopyrite, and pyrrhotite indicate that the two geochemically distinct Au–Co and Co ore types formed from fluids of different compositions and origins. A reduced, metamorphic fluid was responsible for deposition of the pyrrhotite-dominant, Co-rich ore, whereas a relatively oxidized fluid deposited the pyrite-dominant Au–Co ore. The main alteration and mineralization stages at Juomasuo are as follows: (1) widespread albitization that predates both types of mineralization; (2) stage 1, Co-rich mineralization associated with chlorite (± biotite ± amphibole) alteration; (3) stage 2, Au–Co mineralization related to sericitization. Crystal-chemical compositions for tourmaline suggest the involvement of evaporite-related fluids in formation of the deposit; boron isotope data also allow for this conclusion. Results of our research indicate that the metal association in the Juomasuo Au–Co deposit was formed by spatially coincident and multiple hydrothermal processes.

中文翻译：

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芬兰东北部 Kuusamo 带 Juomasuo Au-Co 矿床成矿事件的地球化学特征

Juomasuo Au-Co 矿床位于芬兰东北部古元古代库萨莫带，目前被归类为非典型金属组合的造山型金矿床。在 1.93-1.76 Ga Svecofennian 造山运动期间，拥有该矿床的火山-沉积层序发生了强烈的改变、变形和变质为绿片岩相。在这项研究中，我们利用岩石地球化学数据和岩石学观察研究了 Co 和 Au 沉积之间的时间关系以及金属富集与原岩组成和蚀变矿物学的关系。我们还基于硫化物微量元素和硫同位素 LA-ICP-MS 数据以及电气石矿物化学和硼同位素研究了与沉积物形成有关的流体的性质。原始原岩的分类是根据地球化学上的固定元素进行的；公认的岩性是变质沉积岩、镁铁质、中间成分和长英质变质火山岩，以及超镁铁质基石。主岩的成分不控制矿化的类型或强度。从黄铁矿、黄铜矿和磁黄铁矿中获得的硫同位素值 (δ34S − 2.6 至 + 7.1‰) 和微量元素数据表明，两种地球化学上不同的 Au-Co 和 Co 矿石类型由不同成分和来源的流体形成。还原变质流体负责沉积以磁黄铁矿为主的富钴矿石，而相对氧化的流体则沉积了以黄铁矿为主的 Au-Co 矿石。觉马索主要蚀变成矿阶段如下：(1) 早于两种矿化类型的广泛白化；(2) 第一阶段，与绿泥石（±黑云母±角闪石）蚀变相关的富钴矿化；(3) 阶段 2，与绢云母化相关的 Au-Co 矿化。电气石的晶体化学成分表明与蒸发岩相关的流体参与了矿床的形成；硼同位素数据也支持这一结论。我们的研究结果表明，Juomasuo Au-Co 矿床中的金属组合是由空间重合和多次热液过程形成的。电气石的晶体化学成分表明与蒸发岩相关的流体参与了矿床的形成；硼同位素数据也支持这一结论。我们的研究结果表明，Juomasuo Au-Co 矿床中的金属组合是由空间重合和多次热液过程形成的。电气石的晶体化学成分表明与蒸发岩相关的流体参与了矿床的形成；硼同位素数据也支持这一结论。我们的研究结果表明，Juomasuo Au-Co 矿床中的金属组合是由空间重合和多次热液过程形成的。