Through the 4.6 Gyr of Earth’s history, ore deposit has been formed via close linkage with the evolution of this planet. Ore deposit is an abnormally enriched material of targeted elements and its enrichment mechanism macroscopically controlled by “Geology” and microscopically controlled by “Geochemistry.” “Front line of economic geology” is the first collaborative special issue by Chikyukagaku (Geochemistry) and Shigen-Chishitsu, aiming to review the metallogenesis, recent progresses and unsolved questions in each type of ore deposit for future researches on economic geology. This collaborative special issue is followed by issue 2 in the next year.

Skarns are rocks consisting mainly of calc-silicate minerals (skarn minerals) such as clinopyroxene and garnet, formed by hydrothermal metasomatism along the contact zone typically between limestone and granitic rock. They are sometimes associated with economical Pb–Zn, Mo, Cu, Fe, Sn, and W mineralization (skarn deposits). Skarns consist mainly of one or two skarn minerals, and often show a zonation. In case of skarns (endoskarns) derived from Al-rich rocks such as granitic rocks and sedimentary rocks, in addition to temperature and pressure, the supply of Ca from limestone controls the zonation. On the other hand, in case of skarns (exoskarns) derived from limestone, the supply of Si, Al, Fe and Mg from related granitic rock and other rocks, degassing of CO₂, and redox state govern the zonation. The mineralized metal species in skarn deposits are determined by the granitic rock related to the skarn formation. Isotope analyses revealed that magmatic water supplied from the related granitic rock is greatly involved in the early stage of skarn formation, and influence of meteoric water tends to increase in the later stage.

Since the discovery of rare-earth elements and yttrium (REY)-rich mud in the Pacific Ocean, a variety of research has been conducted to understand its spatial distribution and genesis. In this paper, we review the latest research outcomes on REY-rich mud, including the discovery of “extremely REY-rich mud” containing >5,000 ppm of total REY, a promising deep-sea mineral resource in Japan’s exclusive economic zone surrounding Minamitorishima Island. Then, we introduce a new statistical approach based on independent component analysis (ICA) to clarify the origin of REY-rich mud in the Pacific and Indian oceans, with a theoretical background and a protocol of ICA application on geochemical data. Independent components extracted from a multi-elemental dataset of ～4,000 samples demonstrate distinctive geochemical features, and their spatiotemporal distributions indicate that the sedimentation rate is an underlying key factor for REY-enrichment. We also refer to an important link between the genesis of REY-rich mud and Earth system dynamics. Finally, we focus on some challenges to be overcome. One of the most significant questions concerns the formation mechanism of the extremely REY-rich mud. An important key to this question is the depositional age of the extraordinary sediment layer.

Ferromanganese crusts have been used as archives for monitoring long-term evolution of seawater chemistry throughout the Cenozoic. Because of their high metal concentrations, ferromanganese crusts are logical targets for analyzing both radiogenic and stable metal isotopes. An extensive dataset of metal isotopes is now available from ferromanganese crusts. However, further work is needed to validate age models of ferromanganese crusts and to develop frameworks to interpret the isotope signatures. This paper is intended to provide an overview of paleoceanographic study using ferromanganese crusts with special attention to their isotope records. We begin by reviewing age-dating methods of ferromanganese crusts using beryllium and osmium isotopes. Subsequent section summarizes strontium, neodymium, lead, and stable metal (such as molybdenum and thallium) isotope records of ferromanganese crusts which have been used to understand the timing of phosphatization, changes in ocean circulation patterns, and evolution of oceanic elemental cycles. Finally, we will discuss possible directions for future paleoceanographic studies using isotopic compositions of ferromanganese crusts.

Fluid inclusions are commonly found in minerals comprising ore deposits, and record the information of chemical compositions of fluids associated with coupled phenomena of mass transfer and chemical reaction during the ore deposit formation. Various analytical methods have been applied to determine chemical compositions of fluid inclusions. In-situ analysis of individual fluid inclusion can track the detail of chemical evolution of fluids. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has become the most powerful technique for the quantitative multi-element and isotope analysis of the individual fluid inclusion, owing to the recent progress in the LA system and the development of high-sensitivity ICP-MS. Here, I summarize the key aspects of LA-ICP-MS analytical protocol of individual fluid inclusion. Some applications are reviewed, which emphasize the chemical composition of individual fluid inclusions by LA-ICP-MS analysis to constrain the metal mobility, metal source, and the origin of fluid. Individual fluid inclusion analysis by LA-ICP-MS provides accurate chemical composition of fluid and contributes to understanding the characteristics of fluid related to the formation of ore deposit and various geological processes.