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1 Geological Survey of Japan, 1-1-3 Higashi, Tsukuba, 305, Japan
2 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025
We propose that convection in a column of silicic magma allows transport of volatile components from large (>50 km3) magma chambers to the sites of shallow (
3 km) porphyry-type Mo deposits. Using constraints from the Henderson and Pine Grove systems, we show that even at temperatures as low as 700 °C, a granitic magma with 30 vol% phenocrysts can flow through a magma column with a 150 m radius at a rate >10 km3/yr—enough magma to contribute the Mo necessary to form an ore deposit in a geologically reasonable time frame. This process requires an efficient mechanism for bubble separation at the top of the magma column to produce degassed magma that can descend down through the column. It also requires slow rates of crystallization in the silicic magma, consistent with experimental studies. Hydrothermal fluids released from the convecting magma column can explain many geologic features of the deposits.
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