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 ~350ĄC.
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 (~850ĄC ¨
~500ĄC) 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 ŇinstantaneousÓ cooling of the supercritical
vapor from ~500ĄC to ~370ĄC. 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 IITM
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 400ĄC isotherm. The diagram
was constructed from tabulated and derived fluid physical properties.