Magmatic Volatile Phase Research Articles

An experimental study of high temperature potassic alteration

A magmatic volatile phase (MVP) may be a supercritical fluid, low salinity vapor, or high salinity brine. The MVP causes hydrothermal alteration and can also transport metals in ore forming environments. Unfortunately, there is a lack of experimental data regarding mineral alteration by high salinity liquids (brines) in porphyry systems. This study was designed to expand the experimental data set on mineral alteration by chloride-rich subcritical, vapor-undersaturated high-salinity brines. The phase stability and equilibrium brine compositions in the K-feldspar–andalusite/sillimanite–quartz–brine system (the brine was 70wt% KCl equivalent; KCl–HCl–H2O) were determined from 600 to 750°C and 50 to 80MPa for the equilibrium:KAlSi3O8+HCl=1/2Al2SiO5+KCl+5/2SiO2+1/2H2O. The stable mineral phase was ascertained by an examination of crystal morphology, optical microscopy, and by the compositions of the run products. The mineral stability fields were plotted as a function of temperature, pressure, and molar (KCl/HCl) concentration ratios, CKClbrine/CHClbrine, of the brine. K-feldspar was observed to be stable over the entire temperature range of the experiments whereas andalusite was only stable at temperatures <750°C, sillimanite was found to be the stable Al2SiO5 polymorph at 750°C. The K-feldspar–andalusite/sillimanite–quartz–brine equilibrium boundaries at 80MPa and 600, 650, 700, and 750°C are at CKClbrine/CHClbrine of 38±6, 29.5±0.5, 22±8, and 6±2, respectively (uncertainties are half-widths of the reversal brackets). Pressure effects were examined by fixing temperature at 600°C while varying pressure to 50, 65 and 80MPa. The K-feldspar–andalusite–quartz–brine phase boundaries at 600°C and 50, 65 and 80MPa were determined to be at CKClbrine/CHClbrine of 11±3, 30±10 and 38±6, respectively. These data show that with increasing pressure and/or decreasing temperature, the equilibrium boundaries shift to higher CKClbrine/CHClbrine values in the brine. These phase boundaries are not consistent with the metastable extensions of the reaction boundaries involving low salinity fluids and illustrate that the chloride content of the MVP has an impact on the equilibrium KCl/HCl during potassic alteration. A thermodynamic model was developed from the determined equilibrium boundaries that allows for the comparison of low- and high-salinity systems. The model shows that there is a more substantial deviation from ideality in high-salinity systems when compared to low-salinity systems and that these deviations can be quantified by the calculated activity, aKClbrine/aHClbrine, and activity coefficient, γKClbrine/γHClbrine, ratios determined in this study.

Experimental evidence for the alteration of the Fe3+/ΣFe of silicate melt caused by the degassing of chlorine-bearing aqueous volatiles

The temporal evolution of the redox conditions undergone by degassing magmas has petrologic implications for magmatic phase equilibria. We present new experimental data that demonstrate that a Cl-bearing magmatic volatile phase may affect the concentration and activity of ferrous iron in silicate liquid via the preferential mass transfer of Fe 2+ into the volatile phase. The data are consistent with a model wherein the exsolution of a Cl-bearing volatile phase from the melt modifies the MgO/FeO ratio of the residual degassed melt, increasing the Fe 3+ /∑Fe ratio of the melt. Degassing-induced alteration of the melt may cryptically affect the phases that constitute the hypersolidus crystallizing assemblage. The new results indicate that a low- to moderate-salinity magmatic volatile phase may contain several weight percent iron, present as FeCl 2 . As such, passive degassing over geologically plausible, protracted periods has the potential to extract significant quantities of Fe 2+ , resulting in a residual melt depleted in FeO relative to Fe 2 O 3 . Notably, preferential mass transfer of Fe 2+ from the melt to the fluid results in self-oxidation of the melt, driving the oxygen fugacity of the melt across the phase boundary that demarcates the stability fields of sulfide and sulfate phases, stabilizing sulfate at the expense of sulfide. Such a process has significant implications for the remobilization of chalcophile metals during the evolution of magmatic-hydrothermal systems.

Gold and copper partitioning in magmatic-hydrothermal systems at 800 °C and 100 MPa

Porphyry-type ore deposits sometimes contain fluid inclusion compositions consistent with the partitioning of copper and gold into vapor relative to coexisting brine at the depositional stage. However, this has not been reproduced experimentally at magmatic conditions. In an attempt to determine the conditions under which copper and gold may partition preferentially into vapor relative to brine at temperatures above the solidus of granitic magmas, we performed experiments at 800 °C, 100 MPa, oxygen fugacity ( f O 2 sys ) buffered by Ni–NiO, and a S 2 sys fixed at either 3.5 × 10 −2 by using intermediate solid solution–pyrrhotite, or 1.2 × 10 −4 by using intermediate solid solution–pyrrhotite–bornite. The coexisting vapor (∼3 wt.% NaCl eq.) and brine (∼68 wt.% NaCl eq.) were composed initially of NaCl + KCl + HCl + H 2O, with starting HCl set to <1000 μg/g in the aqueous mixture. Synthetic vapor and brine fluid inclusions were trapped at run conditions and subsequently analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Our experiments demonstrate that copper and gold partitioned strongly into the magmatic volatile phase(s) (MVP) (i.e., vapor or brine) relative to a silicate melt over the entire imposed range of a S 2 sys . Nernst style partition coefficients between coexisting brine (b) and melt (m), D b/m (±1 σ), range from 3.6(±2.2) × 10 1 to 4(±2) × 10 2 for copper and from 1.2(±0.6) × 10 2 to 2.4(±2.4) × 10 3 for gold. Partition coefficients between coexisting vapor (v) and melt, D v/m range from 2.1 ± 0.7 to 18 ± 5 and 7(±3) × 10 1 to 1.6(±1.6) × 10 2 for copper and gold, respectively. Partition coefficients for all experiments between coexisting brine and vapor, D b/v (±1 σ), range from 7(±2) to 1.0(±0.4) × 10 2 and 1.7(±0.2) to 15(±2) for copper and gold, respectively. Observed average D b/v at an a S 2 sys of 1.2 × 10 −4 were elevated, 95(±5) and 15 ± 1 for copper and gold, respectively, relative to those at the higher a S 2 sys of 3.5 × 10 −2 where D b/v were 10(±5) for copper and 7(±6) for gold. Thus, there is an inverse relationship between the a S 2 sys and the D b/v for both copper and gold with increasing a S 2 sys resulting in a decrease in the D b/v signifying increased importance of the vapor phase for copper and gold transport. This suggests that copper and gold may complex with volatile S-species as well as Cl-species at magmatic conditions, however, none of the experiments of our study at 800 °C and 100 MPa had a D b/v ⩽ 1. We did not directly determine speciation, but infer the existence of some metal–sulfur complexes based on the reported data. We suggest that copper and gold partition preferentially into the brine in most instances at or above the wet solidus. However, in most systems, the mass of vapor is greater than the mass of brine, and vapor transport of copper and gold may become more important in the magmatic environment at higher a S 2 sys , lower f O 2 sys , or near the critical point in a salt-water system. A D b/v ⩽ 1 at subsolidus hydrothermal conditions may also occur in response to changes in temperature, f O 2 sys , a S 2 sys , and/or acidity. Additionally, both copper and gold were observed to partition into intermediate solid solution and bornite much more strongly than into vapor, brine or silicate melt. This suggests that, although vapor and brine are both efficient at removing copper and gold from a silicate melt, the presence of Cu–Fe sulfides can sequester a substantial portion of the copper and gold contained within a silicate melt if the Cu–Fe sulfides are abundant.

Alkali metals control the release of gold from volatile-rich magmas

Magmatic-hydrothermal ore deposits are a major resource of gold. Volatiles exsolving from magmas rising through the Earth's crust are the key agent for enrichment of gold in these deposits. We report the first experimental determination of gold solubilities in the magmatic volatile phases at conditions typical of most magmas associated with gold deposits (T = 1000 °C, P = 150 MPa). We show that gold hydrosulfide complexes supersede gold chloride complexes, and more importantly, that the stability of gold hydrosulfide complexes is greatly increased by the presence of minute concentrations of KCl or NaCl (0.1–0.5 mol/kg H 2O). The amplifying effect of alkali chlorides on the solubility of gold in H 2S bearing volatiles may explain the preferential association of many giant hydrothermal gold deposits with high-potassium alkaline mafic to intermediate igneous rocks, which exsolve volatiles that simultaneously contain both H 2S and alkali chlorides in significant concentrations.

Deposition of a high-sulfidation Au assemblage from a magmatic volatile phase, Volcán Popocatépetl, Mexico

Volcán Popocatépetl (Popo) is a Quaternary stratovolcano located 70 km SE of Mexico City in the Trans-Mexican Volcanic Belt. The summit crater of Popo is the site of recent ash eruptions, degassing bursts, and dacite-dome growth. Fresh pumice samples collected on the flanks of Popo contain sparse, very fine-grained, base- and precious-metal phases that crystallized from magmatic volatiles trapped in vesicles. The assemblages consist of pyrite, sphalerite, chalcopyrite, tennantite, galena, enargite, magnetite, chromite, and barite; some samples also contain calaverite, stannite, AuCu telluride, Ag sulfide, AgCu sulfide, AgFe sulfide, Ag bromide, Ag chloride, Bi chloride, halite, kaolinite, and alunite. These assemblages are similar to those found in high-sulfidation epithermal Au deposits, and provide evidence consistent with a direct magmatic source for metals in those deposits. Bulk-sample concentrations of base and precious metals are not anomalously high and can be accounted for mainly by the presence of ore minerals in vesicles; this suggests that a high proportion of the metal content of the original magma was partitioned into a volatile phase which subsequently was trapped and condensed.

Solid inclusions of magmatic halite and sylvite in felsic granitoids, Sierra Norte, Córdoba, Argentina

This study provides evidence for the existence of halite and sylvite solid inclusions in igneous quartz and feldspars, the first to be reported in intrusive rocks, and to partially constrain the physicochemical environment that lets halides crystallize under magmatic conditions. Halite and sylvite solid inclusions were found included in quartz and feldspars from a micrographic–granophyric assemblage in a miarolitic aplite and, rarer, in alkali-feldspar from a miarolitic monzogranite. Monzogranite and aplite represent I-type, K-enriched postcollisional rocks of the Late Cambrian–Early Ordovician Sierra Norte–Ambargasta batholith in the Eastern Sierras Pampeanas. Both granitoids fall among the most evolved felsic rocks of the batholith, with aplite approaching haplogranitic compositions. Halite is far more common than sylvite and the presence and distribution of one or both halides are erratic within the felsic intrusive bodies. Halides occur as small skeletal grains, commonly in cross-shaped aggregates of less than 50 μm. No K or Na was found at the detection limits of EDS in either halite or sylvite respectively. Textural relationships suggest that the alkali-chlorides separated from the melt near the minima along the quartz–feldspar cotectics of P H 2O > 160 < 200 MPa in a silica-, and potassium-rich magmatic system at approximately 750–700 °C, prior to the H 2O-vapor saturated miarole-forming stage. Computed ratios for the magmatic volatile phase (MVP) coexisting with melt at the early stage of aplite crystallization are: NaCl/HCl = 0.11–0.97 and KCl/HCl = 0.24–1.62, being the highest range of values (0.79–0.97 and 1.45–1.62, respectively) found in those alkali-chloride-bearing samples. Maximum HCl/ΣCl (MVP) (0.28 to 0.31) indicates higher total Cl concentration in the MVP of alkali-chloride-bearing aplites, which is much higher in the halite-free aplite samples (HCl/ΣCl (MVP) = 0.59 to 0.74). One miarolitic monzogranite sample, where halite solid inclusions are present, also yielded the highest ratios for NaCl/HCl (MVP) (0.91) and KCl/HCl (MVP) (1.46), and the HCl/ΣCl (MVP) is 0.30. A high HCl concentration in the fluid phase is suggested by the log f(HF)/ f(H 2O) = − 4.75 to − 4.95, log f(HCl)/ f(H 2O) = − 3.73 to − 3.86, and log f(HF)/ f(HCl) = − 0.88 to − 1.22, computed at 750 °C after biotite composition. The Cl concentrations at 800 °C, computed with a D v/l Cl = 0.84 + 26.6 P ( P at 200 MPa), yielded values within the range of ∼ 70 to 700 ppm Cl in the melt and ∼ 4000 to 40 000 ppm Cl in the coexisting MVP. The preferential partitioning of Cl in the vapor phase is controlled by the D v/l Cl; however, the low concentration of Cl in the melt suggests that high concentrations of Cl are not necessary to saturate the melt in NaCl or KCl. Cl-saturation of the melt and coexisting MVP might have been produced by a drop in Cl solubility due to the near-haplogranitic composition of the granitoids after extreme fractionation, probably enhanced by fluctuating reductions of the emplacement pressure in the brittle monzogranite host. Liquid immiscibility, based in the differential viscosity and density among alkali-chloride saturated hydrosaline melt, aluminosilicate felsic melt, and H 2O-rich volatiles is likely to have crystallized halite and sylvite from exsolved hydrosaline melt. High degrees of undercooling might have been important at the time of alkali-chloride exsolution. The effectiveness of alkali-chloride separation from the melt at magmatic temperatures is in line with the interpretation of “halite subtraction” as a necessary process to understand the origin of the “halite trend” in highly saline fluid inclusions from porphyry copper and other hydrothermal mineralizations, despite the absence of the latter in the Cerro Baritina aplites, where this process preceded the exsolution of halite-undersaturated fluids. Pervasive alteration of the monzogranite country rock as alkali-metasomatic mineral assemblages, the mineral chemistry of some species, and the association of weak molybdenite mineralization are compatible with the activity of alkaline hypersaline fluids, most likely exsolved during the earliest stages of aplite consolidation.

Copper partitioning in a melt–vapor–brine–magnetite–pyrrhotite assemblage

The effect of sulfur on the partitioning of Cu in a melt–vapor–brine ± magnetite ± pyrrhotite assemblage has been quantified at 800 °C, 140 MPa, f O 2 = nickel–nickel oxide (NNO), log f S 2 = - 3.0 (i.e., on the magnetite–pyrrhotite curve at NNO), log f H 2 S = - 1.3 and log f SO 2 = - 1 . All experiments were vapor + brine saturated. Vapor and brine fluid inclusions were trapped in silicate glass and self-healed quartz fractures. Vapor and brine are dominated by NaCl, KCl and HCl in the S-free runs and NaCl, KCl and FeCl 2 in S-bearing runs. Pyrrhotite served as the source of sulfur in S-bearing experiments. The composition of fluid inclusions, glass and crystals were quantified by laser-ablation inductively coupled plasma mass spectrometry. Major element, chlorine and sulfur concentrations in glass were quantified by using electron probe microanalysis. Calculated Nernst-type partition coefficients (±2 σ) for Cu between melt–vapor, melt–brine and vapor–brine are D Cu v / m = 63 ± 31 , D Cu b / m = 240 ± 80 , and D Cu v / b = 0.27 ± 0.10 , respectively, in the S-free system. The partition coefficients (±2 σ) for Cu between melt–vapor, melt–brine and vapor–brine are D Cu v / m = 316 ± 22 , D Cu b / m = 443 ± 68 , and D Cu v / b = 0.69 ± 0.16 , respectively, in the S-bearing system. Apparent equilibrium constants (±1 σ) describing Cu and Na exchange between vapor and melt and brine and melt were also calculated. The values of K Cu,Na v / m are 34 ± 21 and 128 ± 29 in the S-free and S-bearing runs, respectively. The values of K Cu,Na b/m are 33 ± 22 and60 ± 5 in the S-free and S-bearing runs, respectively. The data presented here indicate that the presence of sulfur increases the mass transfer of Cu into vapor from silicate melt. Further, the nearly threefold increase in D Cu v / b suggests that Cu may be transported as both a chloride and sulfide complex in magmatic vapor, in agreement with hypotheses based on data from natural systems. Most significantly, the data demonstrate that the presence of sulfur enhances the partitioning of Cu from melt into magmatic volatile phases.

Melt inclusions in veins: linking magmas and porphyry Cu deposits.

At a porphyry copper-gold deposit in Bajo de la Alumbrera, Argentina, silicate-melt inclusions coexist with hypersaline liquid- and vapor-rich inclusions in the earliest magmatic-hydrothermal quartz veins. Copper concentrations of the hypersaline liquid and vapor inclusions reached maxima of 10.0 weight % (wt %) and 4.5 wt %, respectively. These unusually copper-rich inclusions are considered to be the most primitive ore fluid found thus far. Their preservation with coexisting melt allows for the direct quantification of important oreforming processes, including determination of bulk partition coefficients of metals from magma into ore-forming magmatic volatile phases.

Alkali exchange equilibria between a silicate melt and coexisting magmatic volatile phase: an experimental study at 800°C and 100 MPa

Many experimental studies have been performed to evaluate the composition of coexisting silicate melts and magmatic volatile phases (MVP). However, few studies have attempted to define the relationship between melt chemistry and the acidity of a chloride-bearing fluid. Here we report data on melt composition as a function of the HCl concentration of coexisting brines. We performed 35 experimental runs with a NaCl-KCl-HCl-H 2O brine (70 wt% NaCl [equivalent])-silicate melt (starting composition of Qtz 0.38Ab 0.33Or 0.29, anhydrous) assemblage at 800°C and 100 MPa. We determined an apparent equilibrium constant K ′ meas (K, Na) =( C Na m × C KCl b )/( C NaCl b × C K m ) for the equilibrium NaCl b +Σ K m =Σ Na m + KCl b , (where CKClb, CNaClb, CKm, and CNam are total concentrations of potassium and sodium chloride in the brine, and potassium and sodium in the melt, respectively) as a function of the HCl concentration in the brine (C HCl b). Although K′ meas (K, Na) was not affected by variations in KCl/NaCl of the brine, it did vary inversely with C HCl b. The relationship is given by K ′ meas (K, Na) = K ′ ex (K, Na) + a C HCl b [where C HCl b is in wt% and a = 0.03; K′ ex (K, Na) = 0.40 ± 0.03 (1σ) and represents the exchange of model sodium and potassium between chloride components in the brine and the aluminate components (NaAlO 2 and KAlO 2) in the melt. This empirical result will be discussed in light of a structural hypothesis; however, validation of the model awaits determinations based on spectroscopy or transport properties–thermodynamic relations alone cannot be used as evidence of structure. The form of this equation is consistent with a model wherein sodium is present in the melt as both sodium aluminate and sodium hydroxide components, and HCl reacts with the NaOH component in the melt to produce NaCl and H 2O. The correlation between fugacity of H 2O ( fH 2 Osys), model NaOH m/ΣNa m, aluminum saturation index (ASI), and the ratio (HCl/NaCl) b of an exsolving MVP is complex. fH 2 Osys and the ASI are the main controls on model NaOH m/ΣNa m in the system, with model NaOH m/ΣNa m increasing with increasing fH 2 Osys. This relationship can be used to estimate the C HCl b in subaluminous systems, an improvement over previous models. Data for metal partitioning between a volatile phase and melt are commonly presented in the literature as metal–sodium exchange equilibria (i.e., K Cu,Na for the exchange of copper and sodium). However, the variation in K′ meas (K, Na) observed in this study implies that the treatment of metal partitioning between a volatile phase and melt as metal–alkali exchange equilibria is complex because alkali partitioning is not constant and suggests that experimental partitioning studies need to carefully control the HCl/NaCl in experimental vapors and brines. This effect may explain discrepancies in metal–alkali exchange equilibria presented in the literature. Therefore, metal–alkali exchange cannot be described fully by a single metal–alkali equilibrium but must be examined by multiple equilibria.

Geology and Host-Rock Alteration of the Henty and Mount Julia Gold Deposits, Western Tasmania

The Mount Julia-Henty gold deposits are a series of small tonnage ( 20 Mt) of quartz-sericite altered volcanics. The alteration system is hosted in Cambrian submarine volcaniclastic and hyaloclastic dacites associated with interbedded carbonates and calcareous volcaniclastic sandstones located at the Mount Read Volcanics Tyndall Group-Central Volcanic Complex boundary. The alteration zone forms a subvertically dipping tabular sheet over 3 km in length and between 10 and 100 m in width. Although this zone is broadly strata bound in the north, it crosscuts stratigraphy and is hosted 50 to 300 m deeper to the south, well below the Central Volcanic Complex-Tyndall Group contact. The alteration zone is bound updip by the South Henty fault and downdip by abruptly decreasing alteration intensity. Mineralization is confined to lenses within the subvertically dipping alteration zone. The alteration zone is strongly deformed due to reactivation of the South Henty fault. The sericitic alteration assemblage is strongly foliated and mylonitized and the orebody is now located on the steeply west dipping, overturned limb of a broad, shallowly south plunging syncline. Alteration is distinctly asymmetric and can be subdivided into three categories: (1) footwall alteration, (2) alteration associated with mineralization (A zone), and (3) hanging-wall alteration. Alteration in the southern area (Mt. Julia) is similar to the north (Zone 96) but has a much lower sulfide content. Footwall alteration consists of intensely sericite ± pyrite ± carbonate altered schistose rhyolitic and dacitic volcanics. The main mineralized zone (A zone) is also zoned from intensely leached, massive quartz alteration, to quartz-sericite alteration, to an outer quartz-sericite-pyrite-chlorite alteration. Minor massive pyrite and massive sulfide lenses are located at the top of the A zone. Hanging-wall alteration consists of chlorite-albite-quartz alteration in andesitic volcaniclastic rocks and albite-quartz alteration of rhyolitic volcaniclastic rocks and lavas. Bedded carbonates and calcareous volcaniclastic rocks are mainly associated with the upper parts of the A zone alteration zone but are also found toward the footwall and well up into the overlying sequences. The footwall and A zone alteration zones are intensely Na2O depleted and K2O enriched due to feldspar destruction and sericitization. The hanging-wall alteration is strongly Na2O enriched due to intense albitization. Aluminum and K2O have been strongly depleted from the massive quartz alteration within the A zone, suggesting leaching by highly acidic fluids. Mineralization consists of pyrite and chalcopyrite with lesser galena and sphalerite and with minor gold, electrum, galenobismuth, and native bismuth. Gold, copper, and bismuth are mainly confined to the massive quartz and quartz-sericite alteration zones, whereas the outer quartz-sericite-chlorite alteration halo is dominated by pyrite. Metal zonation extends from a gold-silver-rich core associated with copper, lead, and bismuth, to a proximal halo of copper, lead, and bismuth, and then to a distal halo of zinc. Modeling of oxygen and carbon stable isotopes suggests that the carbonates formed from an early phase of basin-wide magmatic CO2 devolatilization that commenced early in the hydrothermal event and continued for a long time period. The carbonates precipitated due to mixing of small amounts of magmatic CO2-rich fluid with seawater at and below the seawater interface. An alternative model suggests that the observed carbon and oxygen stable isotope signature results from fluid-rock interaction between a preexisting carbonate and a later hydrothermal fluid. The Au-Cu-Bi-Ag association and alteration zonation suggests that the Henty-Mount Julia system formed from an evolving submarine Cambrian hydrothermal system, with some low-sulfidation epithermal characteristics. Phase separation of magmatic volatiles from metalliferous magmatic brines formed the alteration halo first, followed by the mineralizing event. Deposition occurred through mixing of the magmatic fluid with bicarbonate/H2S-rich seawater circulating through the unconsolidated Lynchford Member and Central Volcanic Complex volcanics. Mineral deposition was less efficient at Mount Julia where the magmatic volatile phase was dominant and the late, reduced seawater influx less focused, resulting in discontinuous, lower grade mineralization and extensive areas of barren massive quartz and quartz-sericite alteration. The metal zonation is a result of both sulfur availability and decreasing temperature. The Henty-Mount Julia alteration system possibly represents a gold-rich end member of a large, regional, submarine magmatic copper-gold hydrothermal event. This mineralizing event is probably responsible for most of the deposits found on the southeastern side of the Henty fault, including the Mount Lyell deposits.

Carbon dioxide in magmas and implications for hydrothermal systems

This review focuses on the solubility, origin, abundance, and degassing of carbon dioxide (CO2) in magma–hydrothermal systems, with applications for those workers interested in intrusion-related deposits of gold and other metals. The solubility of CO2 increases with pressure and magma alkalinity. Its solubility is low relative to that of H2O, so that fluids exsolved deep in the crust tend to have high CO2/H2O compared with fluids evolved closer to the surface. Similarly, CO2/H2O will typically decrease during progressive decompression- or crystallization-induced degassing. The temperature dependence of solubility is a function of the speciation of CO2, which dissolves in molecular form in rhyolites (retrograde temperature solubility), but exists as dissolved carbonate groups in basalts (prograde). Magnesite and dolomite are stable under a relatively wide range of mantle conditions, but melt just above the solidus, thereby contributing CO2 to mantle magmas. Graphite, diamond, and a free CO2-bearing fluid may be the primary carbon-bearing phases in other mantle source regions. Growing evidence suggests that most CO2 is contributed to arc magmas via recycling of subducted oceanic crust and its overlying sediment blanket. Additional carbon can be added to magmas during magma–wallrock interactions in the crust. Studies of fluid and melt inclusions from intrusive and extrusive igneous rocks yield ample evidence that many magmas are vapor saturated as deep as the mid crust (10–15 km) and that CO2 is an appreciable part of the exsolved vapor. Such is the case in both basaltic and some silicic magmas. Under most conditions, the presence of a CO2-bearing vapor does not hinder, and in fact may promote, the ascent and eruption of the host magma. Carbonic fluids are poorly miscible with aqueous fluids, particularly at high temperature and low pressure, so that the presence of CO2 can induce immiscibility both within the magmatic volatile phase and in hydrothermal systems. Because some metals, including gold, can be more volatile in vapor phases than coexisting liquids, the presence of CO2 may indirectly aid the process of metallogenesis by inducing phase separation.

EVIDENCE FOR OPEN-SYSTEM BEHAVIOR IN IMMISCIBLE Fe S O LIQUIDS IN SILICATE MAGMAS: IMPLICATIONS FOR CONTRIBUTIONS OF METALS AND SULFUR TO ORE-FORMING FLUIDS

Magmatic sulfides are generally accepted as forming by segregation of an immiscible sulfide liquid from a host silicate melt. Immiscible sulfides have been observed in many types of igneous rocks; however, some types of plutonic and volcanic rocks lack sulfides. We have examined a suite of samples from Mount Pinatubo (Philippines), Volcan Popocatepetl (Mexico), Satsuma–Iwojima (Japan) and Mount St. Helens, Bingham Canyon, Tintic District, and Clear Lake (U.S.A.). The samples reflect a range of crystallization histories and compositions; they range from rhyolite to basalt to trachyandesite, with f (O2) at the time of eruption ranging from below the fayalite – magnetite + quartz (FMQ) buffer to well above the nickel – nickel oxide (NNO) buffer. Textural and chemical evidence from our suite of samples indicate that sulfides initially were present, but were modified prior to complete cooling of the parent melt, giving rise to Fe-oxide globules. The globules formed through: (1) segregation of an immiscible Fe–S–O melt, and possibly, further separation of immiscible Fe–S and Fe–O liquids, and (2) undersaturation with respect to sulfide, causing removal of S from the immiscible sulfide melt. Sulfide undersaturation may have been caused by magma degassing (passively or during eruption), or magma mixing. The recognition of modified magmatic sulfides is important because, with extensive degassing, base and precious metals ( e.g. , Cu, Au) could be stripped from a melt by a S-rich magmatic volatile phase and entrained into a magmatic-hydrothermal fluid, ultimately giving rise to porphyry-type or related mineralization. For a melt containing 0.01 modal % magmatic sulfides, efficient degassing of only 10 km3 of magma could yield enough Cu to form a giant deposit.

Estimation of aqueous HCl and Cl concentrations in felsic systems

Two models are presented which can be used to make order of magnitude estimates of the concentration of HCl and Cl in the magmatic volatile phase (MVP). One model is based primarily upon aplite chemistry, and the other is based primarily on apatite chemistry; the models are not entirely independent, and additional information can be gained by using the models in conjunction with one another. Combining both methods allows for the calculation of a model bulk Cl concentration of magmatic vapor. Both models are applied to a natural system: the Billy Lake granite and Rush Creek quartz–monzodiorite in the Sierra Nevada Batholith of eastern California. The bulk composition of aplite and other granitic rocks of (vapor-saturated) near-minimum melt compositions can be used to estimate maximum HCl/total Cl for the magmatic vapor phase. The composition of the aplites in the Billy Lake–Rush Creek (BLRC) system has been used to determine model HCl/KCl and HCl/NaCl for the vapor in equilibrium with the aplite melts; both ratios are on the order of unity (1.1). These values suggest that the maximum HCl/Cl∼1/3, corresponding to a concentration of HCl in the magmatic vapor of ∼0.02 molal if the magmatic system was saturated with a brine phase. Apatite compositions can be used to estimate the HCl concentration in the magmatic vapor as a function of temperature. Based on the P 2O 5 and SiO 2 concentrations in the aplites, and the solubility of apatite in silicate melts, apatite began to crystallize at 820°C in the aplites, which also yields an estimate of 0.02 molal for the concentration of HCl for the BLRC system by the apatite method. Within error, both models produce the some result.

Experimental investigation of H 2 O and Cl ? solubilities in F-enriched silicate liquids; implications for volatile saturation of topaz rhyolite magmas

To interpret the degassing of F-bearing felsic magmas, the solubilities of H2O, NaCl, and KCl in topaz rhyolite liquids have been investigated experimentally at 2000, 500, and ≈1 bar and 700° to 975 °C. Chloride solubility in these liquids increases with decreasing H2O activity, increasing pressure, increasing F content of the liquid from 0.2 to 1.2 wt% F, and increasing the molar ratio of ((Al + Na + Ca + Mg)/Si). Small quantities of Cl− exert a strong influence on the exsolution of magmatic volatile phases (MVPs) from F-bearing topaz rhyolite melts at shallow crustal pressures. Water- and chloride-bearing volatile phases, such as vapor, brine, or fluid, exsolve from F-enriched silicate liquids containing as little as 1 wt% H2O and 0.2 to 0.6 wt% Cl at 2000 bar compared with 5 to 6 wt% H2O required for volatile phase exsolution in chloride-free liquids. The maximum solubility of Cl− in H2O-poor silicate liquids at 500 and 2000 bar is not related to the maximum solubility of H2O in chloride-poor liquids by simple linear and negative relationships; there are strong positive deviations from ideality in the activities of each volatile in both the silicate liquid and the MVP(s). Plots of H2O versus Cl− in rhyolite liquids, for experiments conducted at 500 bar and 910°–930 °C, show a distinct 90° break-in-slope pattern that is indicative of coexisting vapor and brine under closed-system conditions. The presence of two MVPs buffers the H2O and Cl− concentrations of the silicate liquids. Comparison of these experimentally-determined volatile solubilities with the pre-eruptive H2O and Cl− concentrations of five North American topaz and tin rhyolite melts, determined from melt inclusion compositions, provides evidence for the exsolution of MVPs from felsic magmas. One of these, the Cerro el Lobo magma, appears to have exsolved alkali chloride-bearing vapor plus brine or a single supercritical fluid phase prior to entrapment of the melt inclusions and prior to eruption.

A Review of Shallow, Ore-related Granites: Textures, Volatiles, and Ore Metals

Shallow, texturally complex granites can be associated, spatially and temporally, with hydrothermal alternation and mineralization. The formation of a granite-related ore deposit represents the confluence of many chemical and physical geological factors, including: magma composition (e.g. concentrations of ore metals, chlorine , and water); magma oxidation state; the relative timing of crystallization, magma ascent, magmatic volatile phase exsolution; and the depth of emplacement. Early magmatic crystallization of ore–metal-rich minerals coupled with the exsolution of the magmatic volatile phase is promoted by deeper emplacement levels and lower magmatic water concentrations, and leads to low efficiencies of removal of compatible elements from melts into the magmatic volatile phase(s). Water concentrations in the melt and pressures of crystallization can be estimated from phase equilibria and textural relationships of hornblende in some felsic systems. Physical factors affect the dynamics of fluid (magma, melt and volatile phase) movement, which in turn affect the size of zones of alternation and mineralization; granite textures can record the partial history of some dynamic processes. Miarolitic cavities are good evidence for magmatic volatile phase exsolution; both miarolitic cavities and pegmatitic texture exhibit evidence for external nucleation of crystals. Nucleation-controlled phenomena, expressed as variations in granite textures, can result from a multiplicity of causes including undercooling of the melt; textural elements such as pegmatite, aplite, skeletal crystals, and graphic textures can form as a result of undercooling. Interconnected volumes of the magmatic volatile phase allow for advection of hydrothermal ore fluids through the magma. In shallow granitic systems, miarolitic cavities can be interconnected, consistent with this hypothesis.

A Review of Shallow, Ore-related Granites: Textures, Volatiles, and Ore Metals

Shallow, texturally complex granites can be associated, spatially and temporally, with hydrothermal alternation and mineralization. The formation of a granite-related ore deposit represents the confluence of many chemical and physical geological factors, including: magma composition (e.g. concentrations of ore metals, chlorine , and water); magma oxidation state; the relative timing of crystallization, magma ascent, magmatic volatile phase exsolution; and the depth of emplacement. Early magmatic crystallization of ore–metal-rich minerals coupled with the exsolution of the magmatic volatile phase is promoted by deeper emplacement levels and lower magmatic water concentrations, and leads to low efficiencies of removal of compatible elements from melts into the magmatic volatile phase(s). Water concentrations in the melt and pressures of crystallization can be estimated from phase equilibria and textural relationships of hornblende in some felsic systems. Physical factors affect the dynamics of fluid (magma, melt and volatile phase) movement, which in turn affect the size of zones of alternation and mineralization; granite textures can record the partial history of some dynamic processes. Miarolitic cavities are good evidence for magmatic volatile phase exsolution; both miarolitic cavities and pegmatitic texture exhibit evidence for external nucleation of crystals. Nucleation-controlled phenomena, expressed as variations in granite textures, can result from a multiplicity of causes including undercooling of the melt; textural elements such as pegmatite, aplite, skeletal crystals, and graphic textures can form as a result of undercooling. Interconnected volumes of the magmatic volatile phase allow for advection of hydrothermal ore fluids through the magma. In shallow granitic systems, miarolitic cavities can be interconnected, consistent with this hypothesis.

Exsolution of magmatic volatile phases from Cl-enriched mineralizing granitic magmas and implications for ore metal transport

To understand Cl dissolution in aluminosilicate liquids and the exsolution of Cl-rich magmatic volatile phases, experiments were conducted to determine the solubility of NaCl, KCl, and H 2O in felsic liquids at 0.5 and 2 kbar. The Cl content of H 2O-poor, NaCl-saturated, and KCl-saturated silicate liquids is low (i.e., ≤1.3 wt%) to very low (i.e., ≈0.2 wt%) and varies with changes in pressure and composition; the Cl concentration increases with the F concentration and the molar (Al + Na + Ca + Mg/Si) ratio of the liquid and decreases with increasing activity of H 2O in the system. Exsolution of a volatile phase depends on the partial pressures of all dissolved volatiles, and low Cl solubilities in NaCl-saturated and KCl-saturated silicate liquids imply that exsolution of a Cl-bearing volatile phase will occur “early” in Cl-bearing granitic magmas, i.e., prior to extensive melt crystallization and/or at comparatively low water fugacities. The solubility behavior of H 2O and Cl is very similar to that of CO 2 and H 2O in felsic liquids. Small quantities of CO 2 are known to facilitate volatile phase exsolution (Holloway, 1976), and in a similar manner volatile phases may exsolve “early” in Cl-enriched granite magmas. Whereas 5 to 6 wt% dissolved H 2O is necessary for volatile phase exsolution from a CO 2-free and Cl-free haplogranite liquid at 2 kbar and 800°C, a Cl-rich brine will exsolve if the liquid contains only 1 wt% H 2O and 0.26 wt% Cl at the same conditions. These new solubility data are interpreted in light of H 2O, F, and Cl concentrations in felsic liquids, determined from silicate melt inclusions, to constrain the exsolution of Cl-bearing, magmatic volatile phases from mineralizing granitic magmas. Felsic magmas genetically associated with Cu-porphyry and Mo-porphyry mineralization contain sufficient H 2O and Cl to become saturated with respect to a hypersaline brine without strong pressure reduction, boiling of the volatile phase (i.e., exsolution of immiscible vapor and brine), or strong volatile enrichment resulting from extensive crystal fractionation. Experiments were also conducted to investigate the solubility of Mo in highly saline volatile phases coexisting with granitic liquids at 2 and 0.5 kbar. The apparent partition coefficient for Mo in the volatile phase (s) relative to silicate liquid, ( D Mo ∗ ), is defined as [the computed concentration of Mo in a volatile phase or phases/the measured concentration of Mo in granite glass]. D Mo ∗ ranges from 8 to 80 as the NaCl and KCl content of the volatile phase(s) increases from 15 to 90 wt%. Because Mo does not complex with Cl in aqueous fluids, it appears that Mo solubility may be a strong function of the activity of Na and K in alkali chloride-rich volatile phases.

Physical, chemical, and mineralogical evidence for magmatic fluid migration within the Capitan pluton, southeastern New Mexico

The ≈300 km 3 , 28.8 Ma granitic Capitan pluton, located in southeastern New Mexico, was the source of high-temperature (up to 600°C), high-salinity (up to 80 wt%) fluids that formed small REE-bearing zones of mineralization in the pluton carapace. Erosional dissection of the pluton reveals zones in the granite that are characterized by bubble-like void spaces and fissures. These could be interpreted, on one hand, as original magmatic porosity resulting from the interconnection and/or coalescence of bubbles of magmatic volatile phase, or on the other hand, as void spaces resulting from postcrystallization dissolution. These porous zones grade upward into the brecciated portions of the pluton carapace that hosts mineralization. The zones are planar features, traceable across valleys, and even between adjacent valleys on the pluton9s flanks. The orientation of the zones is roughly concentric with the exposed shape of the pluton. Chemical analyses of samples collected across a porous zone reveal that some elements, including Fe, Al, Rb, and Sr are depleted in the entire area surrounding the porous zone, whereas Si, Na, Ba, and Ta are enriched. Other elements, including K, Sb, Zn, Zr, a number of rare earth elements (REE), Pb, and Th, exhibit a slight to strong depletion surrounding the porous zone and a strong enrichment roughly centered on the zone itself. Abundance of high-salinity fluid inclusions in igneous quartz is high within the entire 2.5-m-wide zone but is significantly higher in the central porous zone. The chemical and fluid inclusion abundance patterns could be produced by a two-step process with the wide depletion caused first, by fractionation of elements from the melt into a volatile phase at the time that volatile exsolution first occurred. This could be followed by the superimposed enrichment pattern for certain elements caused by deposition of these elements from trace-element-rich fluids, which evolved in a more central, deeper part of the pluton and were transported along the same zone (although now more crystallized) of fluid migration established earlier, depositing minerals as it cooled. Textural, petrographic, and geochemical evidence suggest that volatiles may have exsolved during crystallization, coalescing into bubble-rich zones that enhance the efficiency of volatile transport.

Do some miarolitic granites preserve evidence of magmatic volatile phase permeability?

Do some miarolitic granites preserve evidence of magmatic volatile phase permeability?

Further constraints on the temporal evolution of the Oslo Rift from precise U-Pb zircon dating in the Siljan-Skrim area

Precise U-Pb ages have been obtained from zircon and baddeleyite in plutonic rocks from the Siljan-Skrim area in the Vestfold Graben Segment in the south-western part of the Permian Oslo Rift. In addition, common lead data from feldspars in the same rocks are presented. The measured common Pb compositions of the feldspars are used to correct for initial common Pb in the zircons. The common Pb data indicate that U has been present in the feldspars since the time of crystallisation, probably in a late magmatic volatile phase in fluid inclusions. The Siljan-Skrim area comprises two intrusive complexes. The northern Skrim-Mykle complex consists of two large, composite intrusive bodies, the Skrim larvikite and the Mykle ekerite. The southern Siljan-Hvarnes complex consists of a series of monzonitic to syenitic intrusions, which are nested in a systematic way, indicating decreasing emplacement age from east to west. U-Pb zircon ages range from 281.2 ± 0.6 Ma (2σ) to 279.8 ± 0.7 Ma for three samples from the northern part of the Siljan-Skrim area and from 278.5 ± 0.8 Ma to 277.3 ± 0.8 Ma for four samples from the southern part of the area. The geochronological data suggest that large amounts of magma were emplaced within very short time spans, i.e. less than 2 Ma for each of the two complexes. The high precision of the U-Pb age results allows discrimination between intrusive activity in the northern and southern parts of the Siljan-Skrim area and suggests that emplacement of magmas was episodic.