Mode of occurrence of silver in the Nakatatsu skarn type deposits has carefully been studied in terms of microscopic observation and electron microprove analysis. The results reveal that silver is concentrated in galena and some other sulfides of the system Pb-Bi-Ag. The silver-bearing minerals are classified as follows on the basis of their grain size and silver contents.
Galena A: fine-grained galena with high silver contents (0.93%Ag in average) included in aggregates of sphalerite and pyrrhotite
Galena B: coarse-grained galena with low silver contents (0.23%Ag in average) occurring in general as massive aggregates
Pb-Bi-Ag sulfides: fine-grained crystals with high silver contents (3.95%Ag in average) in aggregates of sphaler-ite and pyrrhotite.
On the other hand, from the economic viewpoint, lead-zinc ores of the mine are conventionally grouped into three types as follows.
Low Ag/Pb type ores (Ag/Pb: 0-59)
Intermediate Ag/Pb type ores (Ag/Pb: 60-239)
High Ag/Pb type ores (Ag/Pb: ≥240)
The low Ag/Pb type ores are characterized by the presence of the galena B, the intermediate Ag/Pb type by the galena A and the high Ag/Pb type by the Pb-Bi-Ag sulfides.
In the deposits are well developed the zonal distribution of the above ore types, i.e., high Ag/Pb type→intermediate Ag/Pb type→low Ag/Pb type, from the inner skarn zone near the quartz porphyry mass towards the outer limestone side. The regularity of this zonal distribution has been employed not only as a practical indicator to increase the recovery of silver during milling processes but also as a promising guide for prospecting works to acquire economically favorable ores.