Bulk compositions of intimate intergrowths of chalcopyrite and sphalerite and their genetic implications

To determine the bulk chemical compositions of chalcopyrite containing starlike sphalerite and sphalerite including dotlike chalcopyrite, specimens from various types of ore deposits in Japan were used for modal and electron microprobe analyses. According to the analytical results, most of the measured zinc contents in chalcopyrite containing starlike sphalerite are less than 0.8 at%, corresponding to the maximum solubility of zinc in chalcopyrite as determined experimentally at 400°C. However, specimens from the Maruyama deposit in the Tsumo mine contain 1.2–1.4 at% Zn, which are within the solubility limit of an intermediate solid solution (ISS) above 400°C. It is therefore concluded that starlike sphalerite in chalcopyrite are exsolution products derived from primary chalcopyrite solid solution and/or zincic ISS. Measured copper contents in sphalerite including dotlike chalcopyrite yield considerably higher values, i.e., 1.5–6.0 at%, which exceed the solubility limits of copper in sphalerite solid solution as determined experimentally. This result suggests that not all the chalcopyrite dots were exsolved from sphalerite, but that most of them are the product of some other mechanisms.