A Hypothesis on the Alternative between Thermal Conduction Aureole and Juvenile Brine Aureole in the Vicinity of Magma Intrusives

Abstract

Silicic magma-wall rock interactions are poorly understood compared with shallow-depth geothermal phenomena. Knowledge on the surroundings of old intrusive bodies exposed at the surface suggests that such interactions may occur in two types: contact metamorphism or high temperature hydrothermal alteration related to a porphyry-type deposit. Their features are in contrast with each other and indicate that contact metamorphism may result from the thermal conduction from the magma, while hydrothermal alteration of the porphyry-type deposit may result from the juvenile brine released from the magma. Factors controlling these alternatives are considered and two hypotheses are suggested. One is the effect of the permeability of wall rocks, and the other is that of the two-phase region of juvenile magmatic fluids.The latter is probably more essential but the former is also not negligible. Both hypotheses suggest that, if the top of the magma is shallower than ca. 2.5 kilometers, hydrothermal alteration of the porphyry-type deposit occurs, whereas, if it is deeper than ca.2.5 kilometers, contact metamorphism occurs.