High Al contents in quartz and hydrothermal alteration of the “Roseki” deposits in the Mitsuishi district, Southwest Japan

抄録

Numerous pyrophyllite (“Roseki”) deposits are found in Late Cretaceous volcanic rocks in Chugoku province, Southwest Japan. Judging from mineralogical characteristics, following five types of altered rocks can be distinguished: 1) silicified rock, 2) spotted Toseki, 3) illite-rich Roseki, 4) illite rock developed as a narrow zone and 5) pyrophyllite-bearing Roseki. In addition, clay veins are commonly observable in these alteration rocks. Concentration of various elements including trace elements such as Li, Na, Mg, Al, K, Ca, Mn, Fe, Co, Ni, Cu, Zn, Rb and Pb were measured by atomic absorption spectrophotometry. Trace elements such as ⁷Li, ²³Na, ²⁷Al, ³⁹K, ⁴⁹Ti, ⁵⁵Mn, ⁵⁷Fe, ⁶⁹Ga and ¹¹⁸Sn in quartz were measured by a laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS). Authigenic quartz in small druse in silicified rocks contain a large amount of Li, Na, K, Fe and Al. Al and Li contents in authigenic quartz vary within 0.01-1.6 wt% and 33-75 ppm, respectively. The Al content of 1.6 wt% in natural quartz is the highest values hitherto recorded in previous papers. These analytical data permitted to establish geochemical characteristics of each altered rocks and the host volcanic rocks. Examining concentration of several elements in relation with aluminum content reveals several significant facts; Li is more concentrated in quartz than that in illite. Average content of K in the altered rocks is rather low compared with that of the host rhyolitic rocks. Elements such as Fe, Mn, Na, Mg, Ca and Zn are contained only in very small amount in the altered rocks. They are lower in altered rocks than those in the host rhyolitic rocks. These observations strongly suggest that main alteration processes of the district were the results of leaching and/or dissolution of the host rhyolitic rocks during the post volcanic hydrothermal activities.