Reconstructed Palaeohydrology of a Volcano-centred Copper- and Gold-ore-forming Magmatic-Hydrothermal System

 

B.D. Rohrlach, R.R.Loucks and J.M.Palin

 

Reconstructions of the palaeohydrology of ore-forming magmatic-hydrothermal systems helps to understand the physico-chemical transport and deposition conditions for ore metals associated with porphyry Cu and high-sulphidation epithermal Cu-Au deposits in volcanic arcs. Hydrological systems associated with porphyry and epithermal ores commonly span depths of 4 to 8 kilometres in the upper crust and may extend laterally for 10 to 20 km. We have investigated the entire vertical and lateral extent of such a fossil hydrothermal system at Tampakan in the southern Philippines. The study looks at three hydrological regimes: 1) the physical properties and hydrological transport mechanics of supercritical fluids from the magmatic exsolution site, through the lithostatically-pressured rock column, to the meteoric- and magmatic-fluid mixing interface at the base of the deposit; 2) identification of the properties of the fluid end-members and the geometry of mixing and dispersion paths within the deposit in the deep hydrostatically-pressured domain; 3) the mixing proportions and geometry of hybrid magmatic and meteoric waters as they were entrained and radially dispersed by topographically forced flow down the western flank of the volcanic complex. The palaeohydrologic study utilised district-scale whole-rock oxygen isotope mapping; deposit-scale d¹⁸O and dD data obtained from hydrothermal alteration minerals; fluid inclusion data; detailed maps of regional- and deposit-scale alteration zonation patterns and assemblages constructed from PIMA II (portable infra-red spectrometer) data; the spatial variations of high-sulphidation-stage white mica chemical compositions which were calibrated with potassic white mica infra-red spectral signatures; isotopic constraints on the enthalpy of magmatic fluids; and geochronological constraints.

 

Pressure and enthalpy constraints calculated for the high-sulphidation-stage magmatic fluid at the end points of its ascent path from the intrusive stock to the deposit in pressure-enthalpy space (Figure 1) reveal that the magmatic fluid was a two-phase fluid. This fluid was dominated by a high-density vapor that ascended along a nearly isochoric decompression path through the lithostatic column, from the site of exsolution to the site of fluid mixing. The density of the vapor increased slightly from ~0.2 g/cc to ~0.3 g/cc over a vertical ascent distance of ~1.2 km. During transit, the vapor cooled conductively by ~350¡C. The slight volumetric contraction of the vapor during ascent sustained its metal carrying capacity to the fluid-mixing site in the deposit. The near-isochoric ascent of vapor through the lithostatically pressured, ductile rock column requires transport along fine-scale, transient hydrofractures, with intimate contact between the vapor and the ductile wall rocks during vapor ascent. This ensured substantial conductive cooling (~850¡C ¨ ~500¡C) along the ascent path and that thermal contraction of the vapor balanced the tendency to expand with decompression. Instantaneous isoenthalpic decompression of the magmatic-vapor-charged mobile hydrofractures at the lithostatic-hydrostatic interface (brittle/ductile transition) near the base of the deposit, was associated with ÒinstantaneousÓ cooling of the supercritical vapor from ~500¡C to ~370¡C. This pressure-temperature quenching efficiently condensed magmatic vapor to a modestly saline (5 wt.% NaCl) condensate that subsequently mixed with ambient meteoric water within a palaeo-aquifer along an erosional unconformity at ~2 km depth at the base of the hydrostatic regime. An important finding from a metal transport perspective is that, at the P-T conditions of two-phase vapor + brine exsolution from the melt, the mass fraction of high-pressure vapor was >99% and accounted for ~87% of the exsolved chlorine budget; hyper-saline brine was a minor component of the mixture. During ascent of the vapor component, the bulk of this chlorine (81-85 % of the original exsolved chlorine budget) remained within the high-pressure vapor until it was quenched at the hydrostatic interface. These hydrodynamic conditions of nearly isochoric vapor transport, vapor-dominated two-phase fluid flow and >4 wt.% chloride within the high-pressure vapor throughout its ascent path enabled efficient transport of copper-chloride complexes from the magmatic interface to the lithostatic-hydrostatic interface. The low degree of condensation during ascent through the lithostatic rock column allowed maximum transfer of enthalpy to the base of the hydrostatic zone that promoted vigorous meteoric-hydrothermal circulation in the overlying hydrostatic domain.

 

Oxygen-isotopic material balance and enthalpy balance indicates that sericite in the deep portions of the deposit and pyrophyllite at higher and peripheral regions precipitated from a hybrid fluid comprised of magmatic condensate and meteoric water. The hot, hybrid fluids formed a thermally buoyant plume that ascended and were entrained into a stratabound aquifer system on the west slope of the volcano. A substantial hydraulic head in the aquifer is implied by down-stratigraphic-slope deflections in the time-integrated proxy fluid isotherms identified and calculated from calibrations of PIMA II^TM infrared spectral parameters with the chemical composition of potassic white mica. These calibrations reveal chemical trends in the composition of potassic white micas that comprise a central, and deep-seated, high-temperature zone of nearly stoichiometric muscovite coincident with the locus of the inferred magmatic vapour plume. This zone is transitional to shallow peripheral regions where there is an increasing expression of a hydropyrophyllite molecular component in the potassic white mica crystal structure, and decreasing Cu and Au grades. These trends reflect a central, deep-level zone of high fluid temperatures, with cooling paths deflected down-palaeo-slope at shallower levels in the volcanic edifice. The calibration of infra-red spectral data from the Tampakan hydrothermal system reveal that the deposit is a superb example of hydrothermal plume-groundwater interaction and downslope dispersion. Infrared spectral parameters of white mica are shown to reflect both time-integrated palaeo-temperature gradients and relative element activity gradients for Si, Na, K, ferric Fe and Ti.

 

At the district-scale, isotopic modelling of the fluid mixing proportions and mixing geometry was undertaken using whole-rock d¹⁸O values and accounting for the effects of rock chemical variations on the bulk rock isotope fractionation factors along the fluid outflow path. The model reveals that the plume of thermally heated meteoric water and admixed magmatic condensate in the hydrostatic environment was centred within the Tampakan deposit. Substantial magmatic fluid also ascended into the hydrostatic regime along a 5 km by 1.5 km wide NNE-trending fault zone that partly controlled mineralisation. The deposit is located where gradients in the hybrid fluid temperature, and in the proportion of magmatic fluid, were greatest. Lateral outflow of the hybrid magmatic-meteoric fluids was strongly controlled by regional dilational faults that transect the volcanic centre. A two-tiered plume model is applicable to the Tampakan high-sulphidation epithermal system. Mineralisation was localised in the zone of steep temperature and pressure gradients associated with the interface between a deep lithostatic-pressured plume and a shallow hydrostatic-pressured plume.

 

 

 

 

Figure 1: Pressure versus enthalpy plot showing the one-, two- and three-phase fields for the H₂O-NaCl system with 5 wt.% NaCl, and showing vapor and liquid isotherms and isochores. The trajectory of the Tampakan high-sulphidation magmatic fluids (yellow arrows and black boxes) are shown from the point of exsolution (A) from the parental melt to the site of isoenthalpic decompression (B-C) at the brittle-ductile transition, and for the path of subsequent condensation and mixing in the deposit (D*-D**-E). The brittle-ductile transition is assigned to the 400¡C isotherm. The diagram was constructed from tabulated and derived fluid physical properties.