VAPOR-TRANSPORT OF ORE METALS: A REVIEW
BAO Zhiwei(Key Laboratory of Mineralization Dynamics, Guangzhou Institute of Geochemistry, Guangzhou, GD 510640, China)
The idea that vapors can transport metals in appreciable concentrations and may play an important role in the formation of ore deposits was suggested some 360 years ago, however, in the modern era it has been largely ignored. Significant metal concentrations have been reported from volcanic gases worldwide, and the possibility of concentrating directly to form potential ore deposits has been recognized. The development of new micro-analytical techniques, especially the synchrotron XRF, PIXE, LAM-ICP-MS, enables us to measure the concentrations of ore metals in fluid inclusions directly. The results show that metals such as Au, Cu, and Mo, are significantly concentrated in the gas-rich fluid inclusions. Evidences showing that vapor-transport of metals may play a very important role in the formation of hydrothermal deposits also come from the melt-inclusion investigations. During the magma degassing or boiling process, metals such as Cu and Au are preferentially concentrated in vapors rather than melts, while up to 65% of volatiles may be exsolved from the crystallizing magmas. Vapor solubiltiy experiments suggest that the solubilities of metals in the vapor phases are much more significant than were expected. Thus, vapor-transport of metals may play a very important role in the formation of a variety of ore deposit types, particularly the high temperature hydrothermal deposits associated with magmatism where vapor is a major or dominant phase in the hydrothermal system.