Magmatic-hydrothermal Systems Research Articles

Earthquake-induced variations in the composition of the water in the geothermal reservoir at Vulcano Island, Italy

During 1979–1989, variations were observed in the oxygen composition of the water contained in the geothermal reservoir at Vulcano Island, Italy. The reservoir water, that has a magmatic origin, showed an oxygen composition of +1.0±0.5‰ δ 18O during periods without local tectonic earthquakes, and an oxygen composition of +3.4±0.5‰ δ 18O after the highest-energy seismic activity that occurred recently near the island. A slight increase of the δ 18O value in the reservoir water was also observed after a low-energy sequence of tectonic earthquakes that occurred at very shallow depth just beneath Vulcano Island. These 18O variations in the reservoir water are consistent with earthquake-induced increases in the contribution from high-temperature δ 18O-rich magmatic condensate to the geothermal reservoir, and with subsequent decreases in the δ 18O value due to 18O exchanges at the temporarily increased reservoir temperature during reactions between the highly reactive magmatic condensate and the local rocks. Only minor changes in the deuterium composition of the reservoir water occurred with time, as the δD value in the magmatic condensate released from the magma after major local earthquakes quickly approached the δD value of the water contained in the geothermal reservoir. Also the chloride concentration in the reservoir water appears to be linked to the contribution from the magmatic fluid. This chloride content seems not to have undergone major changes with time, as it may be buffered by temporary increases in the reservoir temperature up to values >300°C induced by major local earthquakes. This mechanism may possibly occur also in other magmatic–hydrothermal systems.

Boron in the Bolivian tin belt

Tourmaline alteration and high boron contents are typical features of the magmatic-hydrothermal systems of the Bolivian tin province. The average boron content in melt inclusions of quartz phenocrysts from tin porphyry systems is 225 ppm (1σ-variation range: 110–420 ppm; n=12) and suggests a magmatic boron input to the hydrothermal tin systems, and not shallow post-magmatic leaching of boron from pelitic country rocks. Boron data from melt inclusions correlate positively with cesium, rubidium and arsenic, and negatively with lithium, titanium and zirconium, and define magmatic fractionation trends. The generally high B, As, Cs and Li contents in melt inclusions suggest involvement of pelitic source lithologies undepleted in these fluid-mobile components, i.e. first-cycle metamorphic rocks. Magmatic fractionation modified the trace-element contents within a one-log-unit range. Bulk-rock Nd isotope data (ɛNd−5 to −10) are in agreement with the dominantly intracrustal geochemical signature of the Bolivian tin porphyry systems, but also imply a variable but minor mantle input. The metallogeny of the tin belt is likely a consequence of intracrustal melting of Lower Paleozoic pelitic and slightly carbonaceous source material, combined with an extended magmatic evolution. The long-lived thermal preparation of the root zones of the silicic systems is provided by mafic magma which also leaves a chemical imprint in the form of the hybrid dacitic bulk composition of the tin porphyry systems.

Advances in fluid inclusion analysis using the nuclear microprobe

The steady development, since 1987, of nuclear microprobe methods for in situ, non-destructive fluid inclusion analysis has resulted in techniques for standardless, quantitative analysis of individual fluid inclusions in minerals. These techniques offer detection sensitivities of around 20–50 ppm in the fluid using PIXE, in a short analysis time, and provide simultaneous multi-element data. PIXE is complemented by γ-ray analysis using PIGE, which provides a useful, simultaneous light element (Li, B, F, Na, Mg, Al) detection capability, and by H recoil analysis using ERDA. These techniques have been used to tackle problems in a range of geological environments, and have yielded unique insight into ore forming processes in magmatic-hydrothermal systems.

Late Miocene Magmatic‐Hydrothermal Systems in the Jozankei‐Zenibako District, Southwest Hokkaido, Japan

Abstract: Hydrothermal systems related to magmatic intrusions in the Jozankei‐Zenibako district, southwest Hokkaido are examined, based on field observations, K‐Ar ages, and alteration mineral assemblages. The study reveals five major magmat–ic–hydrothermal systems of Late Miocene in age, comprising Ogawa (9. 7 Ma), Jozankei (9. 5–9. 0 Ma), Otarunaigawa (8. 7 Ma), Asarigawa (8. 8 and 6. 7 Ma) and Hariusu (6. 7 Ma). The Ogawa system is related to granodiorite, and the Jozankei, Otarunaigawa and Asarigawa systems are related to quartz porphyry.The Ogawa system includes potassic, sericitic, propylitic and advanced argillic alteration as well as base‐metal mineralization, represented by the Toyotomi deposit. The Jozankei and Otarunaigawa systems lack significant potassic alteration, and are accompanied by sericitic and propylitic alteration. The Otarunaigawa system is associated with base‐metal mineralization at Toyohiro and Inatoyo. The Asarigawa and Hariusu systems include advanced argillic and argillic alteration, as well as iron sulfide deposits. The presence of potassic alteration only in the Ogawa system is ascribed to deeper emplacement (˜3 km from the surface) of the intrusive magma. These systems formed in terrestrial environments that existed from ca. 11 Ma to 8. 5 Ma and after 7. 5 Ma in the district.Age–data compilation shows that the major advanced argillic alteration events in southwest Hokkaido, including those in the Jozankei‐Zenibako district, formed during the periods from 9. 7–6. 5 Ma and 3. 5–1. 5 Ma. These periods correspond to the timing of normal subduction of the Pacific plate beneath the Northeast Japan arc. Normal, in contrast to oblique, plate subduction is characterized by andesitic, polygenetic volcanism and associated advanced argillic alteration.

Metal fractionation between magmatic brine and vapor, determined by microanalysis of fluid inclusions

The major and trace element compositions of individual fluid inclusions from a range of magmatic-hydrothermal ore deposits were analyzed by laser-ablation inductively coupled plasma-mass spectrometry, to explore the behavior of ore-forming components during fluid phase separation (“boiling”) in high-temperature saline fluid systems. Data from 13 samples showing unambiguous evidence for coeval trapping of a liquid brine and a coexisting vapor phase identify two groups of elements with drastically different geochemical behavior. Na, K, Fe, Mn, Zn, Rb, Cs, Ag, Sn, Pb, and Tl preferentially partition into the brine (probably as Cl complexes), whereas Cu, As, Au (probably as HS complexes), and B selectively partition into the vapor. Fluid phase separation is probably a major, previously underestimated process in the chemical differentiation that contributes to the extreme range of selective element enrichments in magmatic-hydrothermal systems, from deep plutons through porphyry-style and greisen deposits to epithermal mineralization and volcanic fumaroles.

Presence of native gold and tellurium in the active high-sulfidation hydrothermal system of the La Fossa volcano (Vulcano, Italy)

Grains of native gold and tellurium were found in siliceous hydrothermally altered rocks in the high-temperature (170–540°C) fumarolic field of the La Fossa volcano (Island of Vulcano). In addition to Au and Te, Pb–Bi sulfides (cannizzarite) and Tl-bromide chloride were found as sublimates in the hottest fumarolic vents of the crater rim. The chemical composition of altered rocks associated with sublimate deposition indicate the presence of a significant concentration of Te (up to 75 ppm), while gold concentrations are very low (<9 ppb). Pb, Bi and Tl are strongly enriched in the hottest and less oxidized fumarolic vents, reaching concentrations of 2186, 146 and 282 ppm, respectively. These elements are transported (generally as chloride complexes) to the surface by volcanic gases, and several of these (Bi, Te, Tl) are originated from magma degassing. The silicic alteration is produced by the flow of fluids with pH<2. High acidity results from introduction of magmatic gases such as SO 2, HCl and HF released by the shallow magmatic reservoir of La Fossa volcano. The silicic alteration found at Vulcano may represent an early stage of the `vuggy silica' facies which characterizes the high-sulfidation epithermal ore deposits, confirming the analogies existing between this type of ore deposit and magmatic-hydrothermal systems associated with island-arc volcanoes.

Controls on boron isotopic fractionation of tourmaline in magmatic and submarine hydrothermal ore-forming systems

Controls on boron isotopic fractionation of tourmaline in magmatic and submarine hydrothermal ore-forming systems

Hydrogen-alkali exchange between silicate melts and two-phase aqueous mixtures: an experimental investigation

Experiments were performed in the three-phase system high-silica rhyolite melt + low-salinity aqueous vapor + hydrosaline brine, to investigate the exchange equilibria for hydrogen, potassium, and sodium in magmatic-hydrothermal systems at 800 °C and 100 MPa, and 850 °C and 50 MPa. The K aqm/melt H,Na and K aqm/melt H,K for hydrogen-sodium exchange between a vapor + brine mixture and a silicate melt are inversely proportional to the total chloride concentration (ΣCl) in the vapor + brine mixture indicating that HCl/NaCl and HCl/KCl are higher in the low-salinity aqueous vapor relative to high-salinity brine. The equilibrium constants for vapor/melt and brine/melt exchange were extracted from regressions of K a q m / m e l t H , N a and K a q m / m e l t H , K versus the proportion of aqueous vapor relative to brine in the aqueous mixture (Faqv) at P and T, expressed as a function of ΣCl. No significant pressure effect on the empirically determined exchange constants was observed for the range of pressures investigated. Model equilibrium constants are: K aqv/melt H,Na(vapor/melt)=26(±1.3) at 100 MPa (800 °C), and 19( ± 7.0) at 50 MPa (850 °C); K aqv/melt H,K=14(±1.1) at 100 MPa (800 °C), and 24(±12) at 50 MPa (850 °C); K aqb/melt H,b(brine/melt)= 1.6(±0.7) at 100 MPa (800 °C), and 3.9(±2.3) at 50 MPa (850 °C); and K aqb/melt H,K=2.7(±1.2) at 100 MPa (800 °C) and 3.8(±2.3) at 50 MPa (850 °C). Values for K aqv/melt H,K and K aqb/melt H,K were used to calculate KCl/HCl in the aqueous vapor and brine as a function of melt aluminum saturation index (ASI: molar Al2O3/(K2O+Na2O+CaO) and pressure. The model log KCl/HCl values show that a change in melt ASI from peraluminous (ASI = 1.04) to moderately metaluminous (ASI = 1.01) shifts the cooling pathway (in temperature-log KCl/HCl space) of the aqueous vapor toward the andalusite+muscovite+K-feldspar reaction point.

Crater Lake heat losses estimated by remote sensing

Volcanic crater lakes represent the uppermost parts of magmatic‐hydrothermal systems, and can be regarded as calorimeters in order to estimate subsurface heat fluxes. Here, thermal infrared images recorded by the Landsat Thematic Mapper (TM) are employed to map surface temperatures across Poás (Costa Rica), Ruapehu (New Zealand), and Kawah Ijen and Kelut (Indonesia) volcano lakes. First, lake areas are constrained by inspection of the nonthermal bands. These estimates are used to define “lake” pixels in the thermal band, and the temperature of each flagged pixel is calculated. Alternatively, the “hottest” pixel can be assumed to represent the temperature of the whole lake surface, mitigating the effects of mixed land/water pixels. Taking appropriate values for air temperature, humidity and windspeed, surface heat flux densities and power outputs are then derived from bulk aerodynamic or other formulae. Improved capabilities for monitoring crater lakes will be provided by forthcoming satellite missions, notably the Enhanced Thematic Mapper (ETM+) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER).

The partitioning of copper between silicate melts and two-phase aqueous fluids: An experimental investigation at 1 kbar, 800° C and 0.5 kbar, 850° C

Experiments were performed in the three phase system high-silica rhyolite melt+low-salinity aqueous vapor+hydrosaline brine, to investigate the partitioning equilibria for copper in magmatic-hydrothermal systems at 800° C and 1 kbar, and 850° C and 0.5 kbar. Daqm/mlt Cu and apparent equilibrium constants, Kaqm/mlt Cu,Na, between the aqueous mixture (aqm=quenched vapor+brine) and the silicate melt (mlt) are calculated. Daqm/mlt Cu increases with increasing aqueous chloride concentration and is a function of pressure. Kaqm/mlt Cu,Na=215(±73) at 1 kbar and 800° C and Kaqm/mlt Cu,Na=11(±6) at 0.5 kbar and 850°C. Decreasing pressure from 1 to 0.5 kbar lowers Kaqm/mlt Cu,Na by a factor of approximately 20. Data revealed no difference in Kaqm/mlt Cu,Na or Daqm/mlt Cu as a function of the melt aluminium saturation index. Within the 2-phase field the Kaqm/mlt Cu,Na show no variation with total aqueous chloride, indicating that copper-sodium exchange between the vapor, brine and silicate melt is independent of the mass proportion of vapor and brine. Model copper-sodium apparent equilibrium constants for the hydrosaline brine and the silicate melt revealed a negative dependence on pressure. Model apparent equilibrium constants for copper-sodium exchange between the brine and vapor were close to unity at 1 kbar and 800° C.

Microanalysis of ore-forming fluids using the scanning proton microprobe

Much of the uncertainty in our early method of microanalysis of fluid inclusions using PIXE stemmed from the heterogeneous internal structure of each inclusion, which typically contains a vapor bubble and daughter crystals, and the non-uniform beam dose distribution. This paper reports on recent work to improve the accuracy of quantitative analysis through the use of beam-scanning techniques which provide a controlled beam-dose distribution. Improvements have been made to the PIXE yield modelling to better account for large vapor bubbles, and all fluid inclusion analysis routines have been incorporated as standard features of the GeoPIXE software package. The method has been tested using synthetic fluid inclusions in quartz (solutions of Na, K, Zn, Rb and Cs chlorides). The results demonstrated an accuracy improved to ∼10–15%. Examples show the application of the method for the analysis of ore-forming fluids in magmatic hydrothermal systems.

Active hydrothermal systems during the recent uplift of Nanga Parbat, Pakistan Himalaya

During the last 10 m.y., the Nanga Parbat Haramosh Massif in the northwestern Himalaya has been intruded by granitic magmas, has undergone high‐grade metamorphism and anatexis, and has been rapidly uplifted and denuded. As part of an ongoing project to understand the relationship between tectonism and petrologic processes, we have undertaken an isotopic study of the massif to determine the importance of hydrothermal activity during this recent metamorphism. Our studies show that both meteoric and magmatic hydrothermal systems have been active over the last 10 m.y. We suggest that the rapid uplift of the massif created a dual hydrothermal system, consisting of a near‐surface flow system dominated by meteoric water and a flow regime at deeper levels dominated by magmatic/metamorphic volatiles. Meteoric fluids derived from glaciers near the summit of Nanga Parbat were driven deep into the massif along the transpressional faults causing δ18O and δD depletions in the gneisses and marked oxygen isotopic disequilibrium between mineral pairs from the fault zones. The discharge of these meteoric fluids occurs in active hot springs that are found along the steep faults that border the massif. At deeper levels within the massif, infiltration of low δ18O magmatic fluids caused δ18O depletions in the gneisses within the migmatite zone. These low δ18O fluids were derived from the young (&lt;4 Ma), relatively low δ18O granites (∼8‰c) that are found within the core of the massif. Geochronological evidence in the form of fission track and 40Ar/39Ar cooling ages and U/Pb ages on accessory minerals from the granites and gneisses provide a constraint on the timing of fluid flow in the surface outcrops we examined. Fluid infiltration in the migmatite zone rocks located along the Tato traverse was coeval with metamorphism, granite emplacement, and rapid denudation, in the interval 0.8–3.3 Ma. Finally, we infer from the presence of active hot springs that significant flow systems continue to be active at depth within the central portion of the Nanga Parbat‐Haramosh Massif.

Exsolution of immiscible vapor and liquid phases from a crystallizing silicate melt: Implications for chlorine and metal transport

The compositional variation of a Cl-bearing aqueous fluid exsolving from a crystallizing silicate melt is evaluated for the system haplogranitic melt-(Na, K)C1-H 2O, with attention to the effect of vaporliquid immiscibility on the aqueous fluid. Above critical pressures for the aqueous fluid ( P > 1.5 kb at 800°C), where the fluid is single phase, the Cl concentration in the aqueous fluid ( C cl aq ) gradually decreases with crystallization of a silicate melt under Rayleigh fractionation, as demonstrated by previous studies. However, the variation of Ca is quite different below critical pressures, where vapor-liquid immiscibility occurs, in particular at low pressures ( P < 1.3 kb at 800°C). Under such conditions, a vapor phase will exsolve when the initial Cl H 2O ratio in a melt is small and Ca will increase with crystallization. A high Cl H 2O ratio at low pressure results in liquid exsolution and decreasing Ca with crystallization. Eventually both vapor and liquid phases will exsolve simultaneously regardless of the initial exsolving phase. During this simultaneous exsolution, the composition of the exsolving vapor and liquid phases, the mass ratio of these phases and the Cl concentration of the silicate melt will all remain constant. Comparison of the model and published experimental data indicates that the simultaneous exsolution of vapor and liquid may occur from a normal rhyolite crystallized at pressures below the critical pressure for the aqueous fluid. Therefore, simultaneous exsolution of the immiscible aqueous phases is likely to be a common phenomenon in magmatic hydrothermal systems at conditions for the vapor-liquid immiscibility. Model calculations at 0.6 and 1.2 kb indicate that major amounts of Cl in the system will be distributed to the liquid phase particularly at the lower pressure. This suggests that the majority of chloridecomplexed metals exsolving from a magma may be transported in the liquid phase under shallow crustal conditions.

Isotope and Fluid Inclusion Studies of Geological and Hydrothermal Processes, Northern Peru

Mineralization in the Hualgayoc district of northern Peru occurs in altered Miocene felsic intrusions and in mid-Cretaceous platform sedimentary rocks of the Goyllarisquizga, Inca, and Chulec formations. The ores occur both as stratiform and stratabound pyritiferous base-metal deposits (mantos), and as steeply dipping, sedimentary and intrusive rock-hosted base-metal veins. Igneous rocks in the district are affected by propylytic, sericitic-argillic, sericitic, potassic, and acid-sulfate alteration. K-Ar and Rb-Sr dating and geological evidence indicate multiple stages of intrusive activity and hydrothermal alteration, including close spatial emplacement of two or more separate Miocene magmatic-hydrothermal systems. K-Ar dates on sericite, hydrothermal biotite, and alunite indicate that the most important hydrothermal episodes in the district took place ≈13.24 and 12.4 Ma. Other K-Ar dates on altered rocks in the district may reflect various amounts of resetting by the emplacement of the 9.05 ± 0.2 Ma Hua...

PIXE microanalysis of fluid inclusions and its application to study ore metal segregation between magmatic brine and vapor

The composition of fluid trapped as inclusions during the growth of minerals, or during later healing of fluid-filled cracks, provides information on the processes that form rocks and ore deposits. A method has been developed for the quantitative PIXE microanalysis of intact fluid inclusions. The method addresses the complex 3D geometry of the inclusion, and its orientation, internal structure and relation to X-ray take-off angle. The effects of inclusion geometry are significant for low energy X-rays due to the increased absorption caused by the increased path length through the mineral that is not accounted for in a planar yield model. Consideration of these effects yields a more reliable analysis method for lighter elements, such as Cl. The method was applied to study low-salinity vapor and high-salinity brine inclusions in quartz from a granite-hosted quartz-cassiterite vein in the Mole Granite (New England plateau, Australia). Cation ratios in the brine compared with the source granite closely match experimental data on equilibrium metal distribution between chloride-bearing aqueous fluids and silicate melts, confirming an earlier interpretation that these inclusions represent samples of magmatic fluid. The analysis of the vapor inclusions showed strong partitioning of Cu into the vapor phase. This surprising result indicates that liquid-vapor separation and preferential transport of Cu (and by inference Au) may be a major mechanism of base and precious metal segregation and ore formation within magmatic hydrothermal systems.

Sediment-hosted gold deposits: Distal products of magmatic-hydrothermal systems

Sediment-hosted or Carlin-type gold deposits are currently thought to have been generated at shallow levels in geothermal systems, the gold having been scavenged from host sedimentary sequences by meteoric hydrothermal fluids. In contrast, we propose that gold was contributed by magmatic hydrothermal fluids and was deposited on the peripheries of base and precious-metal districts, up to several kilometres from progenitor intrusions. Support for our model is provided by relations at Bau in eastern Malaysia, Bingham in Utah, and elsewhere, combined with geologic, chronologic, and metallogenic observations from the three principal alignments of gold-bearing deposits in Nevada. The proposed model has important exploration implications, and can be tested.

Disseminated tin mineralization in the roof of the Bushveld granite pluton at the Zaaiplaats Mine, with implications for the genesis of magmatic hydrothermal tin systems

The Bobbejaankop Granite is a hydrothermally altered facies equivalent of the typical Bushveld (Nebo) granite. On a regional scale, the Bobbejaankop usually occurs in the upper part of the sheetlike Nebo pluton, but it is particularly well developed in a roof cupola at the Zaaiplaats tin mine. Several styles of endogranitic tin mineralization occur at the Zaaiplaats mine, but disseminated deposits hosted by the Bobbejaankop Granite form the main producing ores.Cassiterite, commonly associated with scheelite and sulfides, is interstitial to the major silicate minerals in the disseminated ores. The ores are confined to a favorable zone that can be defined on geochemical-statistical grounds as a tabular body of varying thickness (55-66 m) with an upper contact that is parallel to, and some 66 to 83 m below, the pluton roof contact. Geochemical halos around the ore-bearing zone are absent or poorly developed and, except for the anomalous tin contents in this zone, there are few obvious differences between mineralized and barren Bobbejaankop Granite.The Zaaiplaats rocks are more heavily altered than equivalent granites in the southeastern Bushveld Complex, and granite in the disseminated tin zone is more altered than barren Bobbejaankop Granite. A complex history of sequential alteration can be recognized, beginning with high-temperature feldspathic (albitization and microclinization) alterations and followed by chloritization, sericitization, and lastly, silicification. These events induced many chemical changes. Compared with Bobbejaankop variants in the southeastern Bushveld, the entire Zaaiplaats suite is enriched in K, Ca, Rb, Cu, and Sn, and depleted in Na. Within the mine area, granite from the mineralized zone contains higher Na, Sn, W, As, S, and F, but lower K and Rb contents than the barren granite. These differences reflect increasing degrees of fluid-controlled precipitation and alteration.For the most part, the Bushveld granites are not metallogenically specialized in the manner of other tin mineralized plutons. Ore genesis cannot be explained by geochemical heredity, or contamination during or after emplacement. Fractional crystallization cannot directly produce significant tin concentrations; the Bushveld magma contained relatively low (

Earthquake rupturing as a mineralizing agent in hydrothermal systems

Much fault-hosted epithermal mineralization is localized in dilational jogs between en echelon fault segments, as fissure veins or as hydrothermally cemented, high-dilation wall-rock breccias. Jog widths may range from millimetres to kilometres; vein textures record histories of incremental development. Perturbation or arrest of earthquake ruptures at dilational jogs has been observed and is believed to involve extensional fracturing at the rupture tip, locally reducing fluid pressure and inducing suctions opposing rapid slip transfer across the jog. This forced fissuring leads to brecciation by hydraulic implosion and to a concentrated fluid influx, allowing delayed slip transfer accompanied by aftershock activity. Within the southern San Andreas fault system, major dilational jogs extend throughout the seismogenic regime and form loci for magmatic-hydrothermal systems; they act as vertical pipelike conduits for enhanced fluid flow. Rupture termination at these structures has sometimes been followed by hydrothermal eruptions, suggesting that high-level boiling events are triggered by the arrest mechanism. It thus seems probable that episodic mineral deposition in the top 1–2 km of such jogs is induced by the dynamic effects of rupturing on the flanking strike-slip faults.

A mass transfer model for copper and molybdenum in magmatic hydrothermal systems; the origin of porphyry-type ore deposits

Equations have been derived which model element partitioning between silicate melts, aqueous fluids, and crystalline phases during crystallization. These equations can be used to calculate the efficiency of removal of elements from magmas into aqueous fluids as a function of (1) the bulk solid-liquid partition coefficient of these elements; (2) the initial and saturation water concentrations in the melt (which together determine the amount of melt crystallized before water saturation); and (3) the chlorine concentration of the melt (in the case of chlorine-complexed cations).The efficiency with which copper and molybdenum can be removed from silicate melts, E(Cu) and E(Mo), respectively, has been calculated. Based on geologic data, copper is modeled as a compatible element and molybdenum is modeled as an incompatible element. Under these conditions the ratio E(Mo)/E(Cu) increases as the initial water concentration of the melt decreases for a given depth of vapor evolution and a given Cl/H 2 O ratio and increases as the depth of vapor evolution increases for a given Cl/H 2 O ratio and a given initial water concentration of the melt.Cu is concentrated so efficiently into a moderately to highly saline aqueous phase that liquid-vapor extraction seems to be a reasonable process to account for the concentration of Cu in porphyry Cu deposits. Efficient extraction of Cu results when aqueous fluids are evolved early in the crystallization of the intrusion. The value of D(Mo) is small relative to D(Cu) at moderate to high chloride concentrations, and the extraction of Mo from the melts into aqueous fluids therefore tends to be less efficient. However, vapor-liquid partitioning can extract the requisite quantities of Mo from granitic melts of batholithic size if Mo acts as an incompatible element and if the water content of the magma at water saturation is on the order of several weight percent.

Copper mineralization and magmatic hydrothermal brines in the Rio Pisco section of the Peruvian coastal batholith

Cu-Fe-Mo mineralization in the Rio Pisco section of the Peruvian Coastal batholith is spatially associated with the Linga superunit, a suite of monzonitic rocks intruded into Albian volcanics. Petrochemical studies of this superunit indicate emplacement of a differentiation series at a subvolcanic level in the crust. The Cu-Fe-Mo mineralization is located principally in the Albian volcanic envelope and is essentially a low-grade porphyry copper type, although the grade is enhanced where structural controls on the movement of the ore-bearing fluids produced more sulfide-rich vein- and manto-type deposits. Alteration patterns associated with both the mineralization and the Linga superunit suggest a close, predominantly magmatic control on the nature of the hydrothermal fluids. Fluid inclusion studies of quartz from the Linga superunit support this and indicate that emplacement of magmas and mineralization took place at a depth of approximately 3 km. The characteristics of the Linga porphyry copper are compared to those of other such deposits and used to suggest a possible telescoping of geometry of the Andean model of Lowell and Guilbert (1970). Thus, in magmatic hydrothermal systems like the Linga, the deeper parts of the model are effectively brought nearer the surface.