Abstract
Two methods for estimating modal composition of alteration minerals in host rocks of the Hosokura Pb-Zn vein deposits are developed.
(i) First one is to calculate Altered Rock Normative Mineral (A.R.N.M.) based on bulk chemistry data. This enables us to estimate normative composition of alteration minerals; that is, quartz, plagioclase (albite+arnorthite), K-feldspar, chlorite (Fe- and Mg-chlorite), montmorillonite, and calcite (Table 1).
Calculated results coincide very well with observed relative amounts of constituent minerals which were determined by combining X-ray powder diffraction and microscopic observation. Furthermore, normative compositions of plagioclase (albite and anorthite) and chlorite (Fe- and Mg-end members) show good agreement with actual chemical composition of these solid solutions. Therefore, the proposed A.R.N.M. method appears to be useful for the present purpose.
(ii) Second method is to estimate mineral mode on the basis of X-ray diffraction data. At first, we measure the intensity (area) of a characteristic peak of each alteration mineral. Then normalized X-ray diffraction intensity of the mineral was calculated as a ratio of the area to the total areas of characteristic peaks of quartz, K-feldspar, plagioclase, sericite, chlorite, montmorillonite, and calcite. The normalized intensity shows strong positive correlation with normative percent of the mineral. This proportional relation was used to construct calibration curve to compute weight percent of the mineral from the X-ray peak intensity. Estimated chemical composition of altered rocks based on this method coincides very well with actual analytical results. Therefore, it is concluded that the second method is also valid for the purpose.
Furthermore, these two methods are also applicable not only to the intermediate type alteration (UTADA, 1977) but also to the acidic or alkaline alteration after partial modification.
