Magmatic vapor plumes and ground-water interaction in porphyry copper emplacement

Abstract

Porphyry copper deposits, all showing similar geological characteristics, occur in Tertiary and older orogenic-volcanic belts around the w•orld. Recent isotope and fluidinclusion studies have shown that in a number of deposits the development of the characteristic ore alteration pattern, at some stage, involved the interaction of meteoric ground waters with saline fluids evolved from a magma. A fluid dynamic model is proposed for porphyry copper emplacement which focuses on the interaction of a buoyant low-salinity magmatic vapor plume with surrounding ground water. As the magmatic vapor rises and cools, high-salinity liquid condenses in a two-phase plume core, drains under gravity, and is diverted to vertical lower salinity stream lines tangential to the two-phase core boundary. Cool ground water is entrained into the rising fluid, giving rise to a buoyant dispersion plume. The potassic core and inner part of the phyllic alteration envelope of the porphyry copper system is regarded, in compliance with isotopic data, as the remnant imprint of the plume on the ground-water regime.Although the model may be modified to a ground-water source for the "magmatic fluid," the authors favor an orthomaglnatic hypothesis by which w, ater and essential ore components are derived from a cooling magma column convecting lighter, more volatile components from a deeper level. The temperature profile of the steady-state plume is calculated using empirical data for permeability and heat input from the active Wairakei and Broadlands geothermal systems. Chemical implications of the physical model are in accord with the observed alteration-mineralization patterns and available high-temperature solubility data. Metals enter the system as hydroxyl or chloride complexes in the low-salinity magmatic gas precipitating in response to ground-water entrainment, temperature, and wall-rock induced pH and ]:o2 variations. Some transport analogies are tentatively drawn with the observed chemistry of volcanic gases.The plume model also provides an interpretation of the characteristics of the deep portion of active geothermal systems and may be extended to other ore-forming systems such as epithermal veins and massive sulfides. In the majority of such hydrothermal systems, if ore formation occurred below around 350øC, the magmatic input may be marked by the then predominant entrained ground-water component.

The solubility and speciation of molybdenum in water vapour at elevated temperatures and pressures: Implications for ore genesis

“…It is also commonly assumed that if a vapour phase exsolves at ail, it is a low density fluid, but at typical porphyry conditions (i.e., 600-700°C and 500-1000 bars), vapour has a relatively high density. An additional misconception is that the mass of the exsolving vapour is much smaller than that of the co-evolved liquid phase (Burnham, 1979), and is thus of little importance in metal transport, but a model proposed by Henley and McNabb (1978) demonstrated that the mass of an exsolving magmatic plume is vapour-dominated. Thus, several of the underlying assumptions for liquid-dominated metal transport are unfounded, and so the vapour phase should be more seriously considered as a transport agent in oreforming processes.…”

“…The high Mo solubility at low temperature observed in the present study indicates that the vapour phase may play a much more important role in the transport of Mo in natural systems than previously thought. For example, as the vapour phase is thought to dominate porphyry-forming systems (Henley and McNabb, 1978), vapour transport may be an important process in the formation of economic Mo porphyry deposits.…”

“…Although conventional models for the formation of porphyry Cu-Mo deposits assume that metals are transported by an aqueous liquid (e.g., Burnham, 1979;Reynolds and Beane, 1985), a small number of researchers have proposed that the vapour phase may also play an important role in metal transport, as vapour commonly dominates the hydrothermal system in porphyry settings (e.g., Henley and McNabb, 1978;Eastoe, 1982). This latter idea has recently gained credibility as a result of studies reporting elevated concentrations of metals in vapour-rich fluid inclusions (Heinrich et al, 1992Ulrich et al, 1999 (Symonds et al, 1987).…”

Electrical resistivity image of the upper crust within the Taupo Volcanic Zone, New Zealand

“…At these depths the saline geothermal fluids are distributed over the base of the whole convection system. Henley and McNabb [1978] argue that a phase separation occurs in brines at high temperatures, resulting in the formation of a boundary layer of highly concentrated brine between the convective plumes and hightemperature rock. Thus the interface region will be characterized by low resistivity.…”

Controle polygenetique sur la localisation des gisements proximaux et distaux dans le champ polymetallique-stannifere a Xinlu Guangxi Chine meridionale

“…One apparent effect of the convecting ground water on the venting fluid is the dispersion of heat and salinity by fluid mixing at the edges of the venting plume (Henley and McNabb, 1978). This effect may be stronger in the central part of a concave intrusion environment, where the flow direction of the convecting ground water is just opposite to that of the venting fluid.…”