Mineral Distribution in Alteration Zones of the Matsumine Ore Deposit of the Hanaoka Mine

Abstract

The Matsumine ore deposit of the Hanaoka mine, Akita Prefecture, Japan, is one of the typical Kuroko (copper-lead-zinc sulfide) deposits, which are recently considered by many geologists as submarine exhalative origin. It occurs in the Miocene marine pyroclastic rocks. Alteration zones surrounding the ore deposit consist successively of silicified zone (IV), clay zone (III), sericite-chlorite zone (II), and montmorillonite zone (I) from lower or inner to upper or outer.
Zone IV, which is altered rhyolite lava dome, is characterized by association of quartz and well-crystallized sericite. Zone III is a lenticular clay bed, enclosing the main ore deposits, and is abundant in interstratified sericite/montmorillonite, particularly at the marginal hanging wall and the end of clay bed. Mg-chlorite, gypsum, and pyrite are also common in the main part of zone III (IIIB), but at the end of clay bed (IIIA) they are rare and plagioclase is found. The main part of zone II(IIA) is compósed principally of sericite minerals (sericite and interstratified sericite/montmorillonite), chlorite, quartz, and feldspar, the last of which is lacking in the portion of this zone right above the ore deposits (IIB). Zone I, characterized by common occurrence of montmorillonite, contains mordenite in the lower portion and opal in the upper, but right over the ore deposits mordenite is almost lacking and the lowest level of opal is close to the boundary with Zone II. The boundary of zones I and II crosses stratigraphic boundaries.
These complicated zonal patterns may have both stratigraphic and concentric natures, and together with the variation of mineralogical properties of sericite minerals suggest that there were syngenetic and epigenetic alterations. The syngenetic alteration, which formed the clay bed, is inferred as reactions between the hydrothermal ore solutions and volcanic glasses in the sea bottom. The epigenetic alteration, which formed the concentric zonings after accumulation of pyroclastics above the ore deposits, may have been principally made by a hydrothermal solution which continued to ascend after ore mineralization.