CHLORINE AS AN EXPLORATION GUIDE FOR THE PLATINUM-GROUP ELEMENTS IN LAYERED INTRUSIONS

Recent theoretical and experimental work has suggested that Cl may be important for the transport of the platinum-group elements (PGE). A survey of the halogen geochemistry of apatite in layered intrusions, including new data from four intrusions from the Yilgarn block of Western Australia (the Jimberlana Intrusion, the Windimurra complex and the Mount Thirsty and Ora Banda Sills) and the Duluth Complex in Minnesota, shows that apatite from most intrusions is characteristically fluorian, with endmember chlorapatite components typically less than 20 mole percent. In contrast, the cumulate sections below both the J-M reef of the Stillwater Complex and the Merensky Reef of the Bushveld Complex contain Cl-rich apatite approaching endmember chlorapatite. Although the Still-water and Bushveld intrusions are not the only intrusions that contain zones of PGE enrichment, the chlorapatite association suggests that intrusions with Cl-rich assemblages are more likely than not to contain economic PGE deposits. Furthermore, this association is consistent with theoretical considerations supporting chlorine as important for the transport of the PGE. It is suggested that the halogen geochemistry, as recorded in primary halogen-bearing minerals, may be a useful exploration guide for layered intrusions that may contain above-average grade deposits of these elements.