Using alteration halos to determine the mass of volatiles expelled and the rate of expulsion in a magmatic porphyry system with application to the porphyry copper system at Butte, Montana

Abstract

The rate of volatile expulsion, the duration of venting, the total volume of volatiles expelled, and the size of intrusion required to supply them are key parameters in the formation of any magmatic-hydrothermal ore deposit that can be estimated from the morphology of the hydrothermally altered rock. The radius of pervasively altered rock around the fluid source and the taper of alteration halo with distance from the pervasively altered zone constrain the duration and rate of volatile expulsion. The volume of altered rock records the total mass of volatiles expelled. An analysis of the Pittsmont Dome at the porphyry copper system at Butte, Montana provides an example. The moles of hydrogen ion consumed during alteration are calculated from the mineralogy of unaltered and altered rock. Published composition estimates are used to determine the concentration of reactable hydrogen ion in the magmatic fluid. Diffusion from the vein to a reaction front at the edge of the halo surrounding it constrains the rate of hydrogen ion loss from the vein. Semi-analytic and finite difference simulations of alteration halo formation show how the radius of the pervasively altered zone and the steepness of the taper beyond that zone depend on the rate of volatile expulsion; the faster the fluid velocity, the less steep the taper. Data from the Pittsmont Dome suggest 23 to 30 billion tons of magmatic fluid was expelled over a period of less than 20 years. Assuming 5 wt% magmatic water in the porphyry intrusion, a spherical intrusion ~7 km in diameter is needed to supply the volatiles for just this part of the Butte mineralization system.