The characteristics of the particular occurrence of rare-metals associated with base-metal deposits in the northern Tohoku district, Japan

Abstract

We examined the distribution of the 17 rare metals accompanied by various types of base-metal deposits in order to delineate the relationship between the concentrations, and the geologic and tectonic developments of the northern Honshu arc in the late Cenozoic age. We selected 54 probably representative samples among previously collected from 56 mines on the basis of the descriptions in the operating period, and investigated them by means of statistical studies of the analytical results. The correlation coefficients among the 17 rare metals together with 4 base metals (Cu, Pb, Zn and Fe) and 2 precious metals (Au, Ag) are as follows: Sr-Ba(0.87), In-Sn(0.83), Sn-Bi(0.74), Se-Te(0.74), In-Bi(0.66), V-Ag(0.58), Fe-Bi(0.58), Tl-Ge(0.53), Fe-In(0.52), V-Ga(0.51), Zn-Ga(0.48), Pb-Se(0.47), Fe-Sn(0.46), Ge-Te(0.41), Ge-Te(0.41) in descending order. The normative compositions of ore minerals, which were calculated from the base metal grades including Ba, have correlation coefficients against rare metals as follows: the norm barite with Sr, 0.87; Tl, 0.37; Ag, 0.32, and Sb, 0.27; norm galena with Se, 0.47; Te, 0.28; Ag, 0.24, and Ge, 0.21; norm sphalerite with Ga, 0.48; Ge, 0.40; Cd, 0.39, and Mo, 0.21; norm chalcopyrite with Bi, 0.17, and norm pyrite with Bi, 0.58; Sn, 0.48; In, 0.48, and Co, 0.27. The other silicate gangues are correlated with Ti, Au and Be with correlation coefficients of 0.54, 0.27 and 0.26, respectively.
We classified the ore deposits type into the following 6 categories on the basis of the previous studies (MITI, 2000; 2001) and recently-compiled data-base (Tsuchiya, 2008);
1) typical Kuroko deposits (T-type), 2) massive sulfide ore regarded as Kuroko analogue (A-type), 3) stringer ore similar to the Kuroko footwall mineralization (S-type), 4) stockwork to veinlets deposits with sulfate minerals (N-type), 5) fissure-filling veins (V-type) and 6) xenothermal deposits around granitoid intrusions (X-type). The variation of rare metal contents, equivalent to the sulfide+sulfate=100% basis, indicates that the concentration of the In-Sn-Co-Bi segment associated with norm pyrite is gradually increasing from the above 1) to 6), that of the Te-Se segment associated with norm galena is remarkably concentrated in 4) (N-type), and that of the Tl-Sb segment associated with norm barite is gradually decreasing from 1) to 6).
The tectonic settings in the northern Honshu arc are divided from west to east into a) Hachimori rise, b) Babame rift, c) Ani rise, d) Kuroko rift and e) fore-arc region based on our previous studies (Yoshida et al, 2005; Yamada and Yoshida, 2005). The In-Sn-Co-Bi segment has high concentration in the Ani rise (zone c) and the fore-arc region (zone e). On the contrary, the Tl-Sb segment is rich in the Kuroko rift (zone d). Te-Se is remarkably concentrated within the fore-arc region.
These statistical analyses suggest that many rare metals are strongly associated with both deposit types and tectonic settings. Indium, economically most significant, is rich in xenothermal and polymetallic deposits of the types N and V, and it exists mainly in the pyrite-rich ore with Sn, Co and Bi in the intra-rift rise and the fore-arc region.