Fossil Foraminifera Replaced by Sphalerite in the Shakanai Kuroko Deposits, Hokuroku District

Abstract

A fine-grained, compact kuroko ore regarded as one of the most primitive kuroko deposits, occurs in the Shakanai Mine of Hokuroku district, Japan. From this type of ore, fossil foraminifera replaced by sphalerite have been found in an uncovered, doubly-polished thin section. Because of observation only in sectional forms of the foraminifera, it is difficult to decide the species names for most of the specimens, but some well-preserved ones can be identified with regard to the genus level. They include the genera of planktonic and calcareous foraminifera such as Globorotalia, Globoquadrina and Globigerina (or Globigerinoides). Among them an important Miocene index fossil, Globorotalia cf. peripheroronda, has been recognized.
The fossil foraminifera replaced by sphalerite exist in fine-grained matrix of yellow ore mainly composed of chalcopyrite. In the fossils, two types of sphalerite can be recognized under a microscope; one is transparent, colorless sphalerite replacing the test part of the fossils, and another is opaque one filling their chambers. Electron probe X-ray microanalyses have revealed that the opaque nature of the chamber sphalerite can be attributed to small inclusions of chalcopyrite of some six weight percent.
The fossil-bearing kuroko deposit is stratiform and intercalated between altered dacite pyroclastics and mudstones. It shows many features characteristic of slumping and sliding, such as fragmentation of ores and intraformational soft-rock-deformation. It is assumed that the fossil foraminifera which were first replaced by sphalerite after their settling down onto the bottom of the ore-depositing basin, would have been physically mixed with the chalcopyrite-rich ore in an unconsolidated condition by slumping or sliding.