1978 Volume 28 Issue 151 Pages 325-335
The Fukazawa Zn-Pb-Cu-Ag deposit is a typical Kuroko deposit in the Hokuroku district. The felsic volcanic rocks around the deposit have suffered hydrothermal alteration. The footwall of the stratiform massive ores is characterized by the Mg-chlorite-sericite assemblage. FeMg-chlorite, sericite and feldspar are present in the outer zone. The regional diagenetic alteration is also recognized in upper part of the volcanic pile.
The chemical variations involved in alteration of the footwall volcanic rocks of felsic composition are the depletion of Na2O and CaO and the increase of MgO, S and As. These features are recognized in the case of the Iwami deposit of the Kuroko type. CI in weakly altered volcanic rocks are 300-500 ppm in the Iwami area and 20-40 ppm in the Fukazawa area. The low Cl content of volcanic rocks in the Fukazawa area might be due to leachine by thermal waters related to base metal mineralization of younger age.
Trends of chemical variation of altered rocks can be represented by AL2O 3-MgO-(CaO+Na2O+K2O) mole proportional diagram. Alteration associated with Kuroko mineralization reveals the extensive Mg enrichment in the felsic volcanic rocks. The Mg enrichment is found in the case of Archean massive sulfide deposit in Cananda. The Mg enrichment associated with Na depletion can be considered as a unique characteristic of hydrothermal alteration involved sea water.
