Geology, tectonic settings and iron ore metallogenesis associated with submarine volcanism in China: An overview

Abstract

Submarine volcanogenic iron oxide (SVIO) deposits are one of the most important sources of high-grade iron ores in China. The spatial distribution of the deposits is controlled by the tectonic settings including arc, back-arc and rift environments, with the SVIO deposits mostly concentrated in the western part of China namely, the southwestern Yangtze Craton, Western and Eastern Tianshan, and Altay orogens and the Kaladawan iron ore district in the eastern part of the Altyn Tagh region. The Chinese SVIO deposits range in age from Paleoproterozoic to Mesozoic, and were formed during two main metallogenic epochs in the Proterozoic and Paleozoic. More than 70% of the SVIO deposits formed in the Paleozoic, with three important SVIO-metallogenic provinces recognized, in the Altay, Eastern and Western Tianshan orogens. These SVIO deposits are hosted in lithofacies that are related to submarine magmatism, such as lavas and associated pyroclastic and volcaniclastic-sedimentary rocks. The iron orebodies are hosted in different volcanic lithofacies with different features. Moreover, the different volcanic lithofacies in which the Fe ores are hosted also provide information as to their spatial relationship, ranging from distal to proximal to the eruption center or vent. Many of these deposits are characterized by well developed skarns, and could be interpreted either by a distal position of the ore system in question and/or exposed igneous rocks or active magma chamber, or a relationship to early metamorphism and continuous alteration at relatively high temperature followed by retrograde alteration as temperatures decline. Geological and geochemical evidence suggests that these deposits were formed as a result of submarine magmatic activity, including subaqueous volcanic eruptions, associated volcano-sedimentary lithofacies, and related post-magmatic hydrothermal activity. Iron oxide ore probably formed the hydrothermal fluids which generated the skarns could be a mixture of evolved magma-derived water and convecting sea water driven by the heat from the shallow active magma chamber, whereas volcano-sedimentary deposits could be formed by the fallout of the ore-bearing materials to the sea floor emanating from submarine eruption columns, or fractional precipitation of iron which had been introduced locally into the bottom water by volcanic-origin hydrothermal solutions and by leaching from the relatively iron-rich volcanic rocks. The formation of these various styles of Fe ore deposits is controlled by several key factors, such as magma differentiation, lithofacies of host rocks, temperature and chemical compositions of hydrothermal fluids, as well as the depth of sea water. In combination with their geological characteristics, geodynamic mechanisms and metallogenesis, we propose a genetic model in which the origin of these deposits can be related to the space–time evolution of the submarine volcanism, and their relationship to volcanic lithofacies variation, such as central, proximal and distal environments of ore formation.