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Information on Japanese mineral deposits

Base and Rare Metal Mineralizations of Japan

Japan's base and rare metal metal mineralizations were formed during the Late Cretaceous to Paleogene age at the margin of the Asian continent, and from Neogene to Pleistocene age after formation of the island arc. Distributions of rare metals are regulated by petrographic characteristics of genetically related granitoids, deposit type, occurrence mode of ore, and crystal chemistry of host minerals. Advanced microanalyses and trace element geochemistry are assisting base and rare metal geology by clarifying the behavior and origin of trace elements.

Radiometric ages of hydrothermal products from Japan's deposits exhibit bimodal distribution, with Late Cretaceous and Miocene peaks (Fig. 1). Mineral deposits older than Cretaceous period account for a small fraction of total abundance and mostly occur in the accretionary complexes.

The Cretaceous peak represents mineral deposits associated with felsic magmatism at the margin of the Asian continent prior to development of the Japanese island arc. Most of the deposits occur in the Inner Zone of SW Japan, where Mo, Pb and Zn deposits are distributed north of the zone, while Sn, W and Cu occur south (Fig. 2). This difference in distribution is attributed to variations in petrographic character of granitoids: the northern zone is characterised by magnetite-series granitoids and the southern zone by the ilmenite-series. Magnetite-series (magnetite bearing) granitoids commonly show higher oxidation states than those of ilmenite-series (magnetite free) granitoids. It has been suggested that these two series represent magma types with intrinsic differences in oxidation state.

A Miocene peak in mineralization mainly corresponds to kuroko, veins in the Green Tuff Region, and polymetallic veins in the Outer Zone of SW Japan (Fig. 3). Most of them were emplaced after the formation of the Japanese island arc. Kuroko deposits were developed within deep marine environments, while veins in the Green Tuff Region were formed later than kuroko in uplifted terraines. Polymetallic veins of the Outer Zone are associated with ilmenite series granites which cooled very rapidly.

Rare metal distribution in these deposits has been examined by various analytical methods. Our investigations have found that rare metal distribution is controlled not only by petrochemistry of granitoids but also by deposit type, occurrence mode of ore, and crystal chemistry of constituent minerals in each ore. For example,

  1. Co is detected only in skarns.
  2. Sb is higher in Kuroko and Au-Ag veins than in other deposit types.
  3. In is high in tin-polymetallic veins (Fig. 4), slightly high in skarn and in kuroko deposits.
  4. Bi is high in Pb concentrate of skarn, and tin-polymetallic veins.
  5. Ga and Ge are high in sphalerite, Tl in pyrite, and Sr in barite, within kuroko deposits; Ga is especially high in kuroko sphalerite found in intensely altered host rock.
  6. Cd is strongly partitioned into sphalerite regardless of deposit type.
In addition to the beneficiation, we are contributing to clarify trace element behavior in magma-hydrothermal systems, to confirm/refine deposit models, and speculate on the sources of elements. PIXE microprobe analyses of rock-forming minerals from granitoids in a mining district revealed that incompatible elements show positive correlation with differentiation index (DI) of host rocks. Zn, Pb, Ga and Sn which do not form ore minerals in the region exhibit trends similar to incompatible elements, but Cu and As show negative correlation with DI. This fact implies that Cu and As can be partioned directly into mineralizing fluid from magma, while Zn, Pb, Ga, and Sn require extraction from minerals through water/rock interaction to enter hydrothermal fluid.

Through the study of tin-polymetallic ore, we noticed that the rare-metal rich portion of an orebody has a larger magmatic contribution than other portions of the deposit, and speculated that Bi and In are magmatic elements. We have determined that coupled substitutions are important in development of high grade In-bearing ore, e.g., In3+ + Cu+= 2(Zn2+ + Fe2+). Thus sphalerite can contain high amounts of In, up to 21%, but chalcopyrite can only incorporate 800 - 1000 ppm In.

In the case of the kuroko deposits, ore-forming process induced from geology, ore texture (Fig. 5), and trace element concentrations of ore strongly support our recent genetic model (Fig. 6).

Base and rare metal geological studies greatly assist exploration efforts through integration with advanced microanalytical and trace element geochemical studies.


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