1988 Volume 38 Issue 211 Pages 407-418
Intense Sn-As-Zn-Ag vein mineralization was found at the 26th Level of the Besshi mine, of which deposit is a conformable massive sulfide type (Besshi-type). The mineralization resulted in formation of such rare Sn minerals as rhodostannite, hocartite and a franckeite-like mineral, as well as common stannite and cassiterite. Pyrite, arsenopyrite, sphalerite, tetrahedrite, manganese carbonates, quartz, tourmaline and so on occur in association with these Sn minerals. The prominently polymetallic ores are composed of minerals that were formed at several stages during the mineralization sequence.
The present microprobe analyses, combined with previous chemical data, indicate that the substitution of Ag for Cu is extensive in rhodostannite, whereas that of Zn for Fe is very limited. The substitution relationship of these elements in stannite and hocartite is just the opposite to that found in rhodostannite. Divalent Sn may substitute for Pb in the franckeite-like mineral, which is considered to be mainly responsible for the extensive solid solution observed in this mineral.
The mineralization may have taken place nearly simultaneously with contact metamorphism, which converted the massive pyrite ores and surrounding pelitic schists to massive pyrrhotite ores and biotite hornfels, respec-tively. A granitic intrusion, which is supposed to be hidden in the deeper part of the Besshi mining district, probably caused the vein mineralization and contact metamorphism. The geological situation and mineralogical characteristics of the vein mineralization are quite similar to those observed in the Sn deposits adjacent to Miocene granitic intrusives in the Outer Zone of Southwest Japan at Kyushu. The Miocene Sn metallogenic province in the Outer Zone of Southwest Japan at Kyushu should be extended eastward to the Besshi mining district in Shikoku.
