VOLATILE TRANSPORT IN A CONVECTING MAGMA COLUMN - IMPLICATIONS FOR PORPHYRY MO MINERALIZATION

We propose that convection in a column of silicic magma allows transpo rt of volatile components from large (>50 km(3)) magma chambers to the sites of shallow (similar to 3 km) porphyry-type Mo deposits. Using c onstraints from the Henderson and Pine Grove systems, we show that eve n at temperatures as low as 700 degrees C, a granitic magma, with 30 v ol% phenocrysts can flow through a magma column with a 150 m radius at a rate >10 km(3)/yr-enough magma to contribute the Mo necessary to fo rm an ore deposit in a geologically reasonable time frame. This proces s requires an efficient mechanism for bubble separation at the top of the magma column to produce degassd magma that can descend down throug h the column. It also requires slow rates of crystallization in the si licic magma, consistent with experimental studies. Hydrothermal fluids released from the convecting magma column can explain many geologic f eatures of the deposits.