X-ray and microprobe analyses of wolframite series from the Kaneuchi mine, Kyoto Prefecture, Japan

By means of X-ray diffraction and microprobe analyses, compositions of wolframites from the wolfram-quartz veins at the Kaneuchi mine, Southwest Japan, have been examined. In view of the compositional variation within a single crystal, its shape, and coexisting minerals, wolframites have been deposited only with an oscillatory zoning of less than about 3mole% MnWO4 and their compositions change in part by later ore solutions along a peripheral part or crack. Bulk compositions of wolframites obtained by X-ray analyses range from 35.1 to 61.1mole% MnWO4, throughout the mine. However, it is worth while to mention that wolframites in the Eastern veins are higher in MnWO4 contents by about 10mole% than those in the Western veins. A positive correlation in chemical composition between wolframites and associated wall rocks strongly indicates that wall rock chemistry plays a very important role in controlling the wolframite compositions concerned. Regardless of a variety of wall rocks, such compositional differences of wolframites observed between the two vein swarms are concluded to be mainly controlled also by solution pH. Somewhat higher pH is responsible for wolframites to be more Mn-rich in the Eastern veins than in the Western veins.