"A consideration on the alteration of dacite below the Fukazawa ore deposits, Fukazawa Mine, Akita Prefecture"

Abstract

Recently many studies regarding alteration of acidic volcanic rocks below the black ore deposits were performed. For example, DATE and TANIMURA (1973) showed that Na₂O was subtracted from, and K₂O and MgO were added to the acidic volcanic rock during mineralization. Furthermore, ISHIKAWA et al. (1976) indicated that CaO was also subtracted from dacite.
This time, the authors restudied the chemical data of Na₂O, K₂O, CaO and MgO from dacite below the Fukazawa ore deposits that ISHIKAWA et al. (1976) used, and the followings were revealed 1) Below the black ore deposits, Na₂O-poor dacite develops in the extent of 3.0km (NW-SE direction)×1.5km(NE-SW). 2) The fbrmation of Na₂O-poor dacite depends onmineralization of black ore deposits. And in the area of Na₂O poor dacite, parts relatively rich in Na₂O formed by alteration differentiation can be seen. 3) AlteredIndex(K₂O+MgO/Na₂O+K₂O+MgO+CaO)that wasproposedbyISHIKAWA et al. (1976) relys definitely on its Na₂O content. Thus it is required to find Na₂O poor dacite for exploration of black oredeposits. 4) It is not always effective to use independendentlv subtractional trend of CaO and additional trend of K₂O and MgO in dacite as indications for exploration, but they may beuseful as an auxiliary instrument.