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 second collaborative special issue by two Japanese academic journals of 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 and geochemistry. This collaborative special issue will be followed by issue 3 in the next year.

Mine drainage from approximately 100 abandoned/closed Japanese mines has been treated by neutralization over the last forty years. Coprecipitation is a main mechanism for removal of several toxic elements from mine drainage during the neutralization. In order to develop more efficient treatment methods, it is important to understand the coprecipitation mechanism quantitatively.

In many cases, coprecipitation can incorporate more toxic elements into the hydroxide than adsorption. In coprecipitation of arsenic to ferrihydrite, surface precipitation of low crystalline ferric arsenate was formed, as initial As/Fe molar ratio was increased. This surface precipitation enabled arsenic removal by ferrihydrite with high efficiency such as 1 mol-As / mol-Fe. On the other hand, in coprecipitation of chromate to ferrihydrite, structure of ferrihydrite was expanded due to inner-sphere surface complexation of chromate into inside of ferrihydrite, as initial Cr/Fe molar ratio was increased. These phenomena of surface precipitation and inner-sphere surface complexation will occur because surface complexation of arsenate or chromate is faster than ferrihydrite formation, and additional surface complexation of Fe³⁺ion onto them can be formed.

Seafloor massive sulfide (SMS) deposit is expected to be an available metal resource; however, it might become a potential source for metal-containing drainage around mining areas to generate metal cations and acids by their oxidative dissolution. In this paper, we reviewed the metal mobilization from various hydrothermal sulfides into seawater and discuss the possibility of metal rich seawater generation during SMS-mining operation. The results from leaching experiment using natural hydrothermal sulfide in seawater showed the metal dissolution behaviors were significantly different with the initial oxidation states of the hydrothermal sulfide surfaces. The non-oxidized samples (i.e., non-exposed to atmosphere) primarily and gradually released Zn and Pb into seawater, whereas the oxidized hydrothermal sulfides rapidly released various metals (Mn, Cu, Zn, As, Cd, Pb) into oxic seawater within several minutes. The result suggests the hydrothermal sulfides in each mining process would have different metal dissolution potentials; once the sulfides are oxidized by air and oxic seawater during lifting from seafloor to the vessel, various metals and metalloids can be released rapidly if the oxidized sulfides are spilled into the ocean.

Phosphorous is one of the three major nutrients (N, K, P) required by plants and about 80 of phosphate mined in the world is used to produce chemical fertilizers. “Phosphate rock” is the term generally used in industry to describe mineral assemblages with a high concentration of phosphate minerals, consisting commonly of francolite (Ca₅(PO₄, CO₃)₃(OH, F, Cl)) - apatite (Ca₅(PO₄)₃(OH, F, Cl)) series.

Apatites contain a variety of REE concentrations from several thousands ppm to several wt.%. They are generally enriched in LREE, however, some apatites are rich in HREE: apatites in sedimentary phosphate rocks and deep sea mud, and igneous rocks. In addition, both Th and U contents in apatite are relatively low, compared with common REE minerals such as monazite and xenotime. Thus, the extraction of REE from apatite as by-products in the existing plants for phosphate fertilizer would be highly economically efficient and feasible compared with the development of new REE deposits.

This paper reviews firstly REE and actinide substitution mechanism of apatite and apatite-group minerals, and secondarily the classification and variety of apatite deposit types by focusing on REE resource (especially HREE).I finally present the studies concerning precipitation, solvent extraction and adsorption for REE in the process of phosphoric acid production.