Journal of Petrology | Volume 38 | Number 12 | Pages 1619-1633 | 1997
© Oxford University Press 1997
A Review of Shallow, Ore-related Granites: Textures, Volatiles, and Ore Metals
Laboratory for Mineral Deposits Research, Department of Geology, University of Maryland at College Park College Park, MD 20742–4211, USA
Received January 1, 1997; Revised typescript accepted August 1, 1997
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ABSTRACT |
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 TOP ABSTRACT IntroductionGeneral Narrative on the...Water Concentrations, and...Timing of Volatile Saturation...Granite TexturesGeneral Narrative on the...References |
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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.
KEY WORDS: granites; ores; textures; volatiles
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Introduction |
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TOPABSTRACTIntroductionGeneral Narrative on the...Water Concentrations, and...Timing of Volatile Saturation...Granite TexturesGeneral Narrative on the...References |
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Granite-related ore deposits include porphyry and skarn type ores, and other hydrothermal deposits that are spatially and temporally associated with granitic (sensu lato) rocks. In this paper, I emphasize characteristics found in porphyry style (Cu–Mo–Au) deposits, and some related systems; however, my treatment here is not meant to cover the full range of granitic rocks and mineralization that make up the granite-related ore systems. The formation of granite-related ore deposits by orthomagmatic–hydrothermal processes is dependent upon many physical and chemical magmatic factors, including (but not limited to) the composition and evolution of the associated magmas, the rate of magma ascent, the depth and geometry of magma emplacement, and the local temperature and stress fields at the site of magma emplacement. Detailed studies of the igneous rocks associated with ore deposits can therefore yield critical information regarding the source of metals and ligands in the ore-generative hydrothermal solutions that are operative in the formation of granite-related ores. In this review, I will present a series of narratives, consistent, in my view, with prevailing field and experimental data on the orthomagmatic–hydrothermal theory for the genesis of some granite-related deposits.
Some high-level granites (i.e. those that are emplaced at depths of 
8 km or less) represent the crystallized remains of magmas that were associated with subvolcanic, ore-generative, hydrothermal systems. Therefore, studies of shallow granites, aimed at understanding magmatic evolution and volatile phase exsolution at shallow levels, are necessary if we are to understand ore genesis and the deep inputs to present-day geothermal systems as well as evaluate hazards associated with the volcanic manifestations of these systems. Further, many studies (e.g. Dilles & Einaudi, 1992
) suggest that magmas are not the sole source of ore metals in some porphyry, skarn or high-temperature vein type deposits, so the precise role played by orthomagmatic processes in the genesis of some deposit types still remains to be elucidated. In this paper I will review select results from my past research, clarify what I perceive to be some problems related to volatiles in magmas, and discuss some of the problems that remain with regard to magmatic volatiles and ore metals.
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General Narrative on the Development of Granite-Related Ore Systems At the Magmatic Stage |
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TOPABSTRACTIntroductionGeneral Narrative on the...Water Concentrations, and...Timing of Volatile Saturation...Granite TexturesGeneral Narrative on the...References |
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The formation of granite-related ore deposits is a by-product of the irreversible magmatic transfer of heat and mass from the Earth's interior to its surface. Commonly, the rise of magma (aluminosilicate liquid±oxide/silicate crystals ± sulfide crystals or liquid ± hydrosaline liquid±vapor bubbles) at epizonal levels leads to either magmatic volatile phase (MVP) saturation if the system is initially volatile phase undersaturated, or the further exsolution of volatiles if the magma is already saturated with respect to one or more volatile phases. The MVP is thought to be a critical agent in ore formation because of the affinity displayed by many ore metals for a volatile phase relative to rock-forming minerals (see Candela & Piccoli, 1995
), and because of the high fluidity and buoyancy of the MVP. These physical attributes allow the volatile phase, in some circumstances, to accumulate near and above suitable apical structures where it may precipitate a significant proportion of its sparingly soluble load. It should be noted that in the Earth's epizone, volatile saturation may occur early relative to crystallization progress because of low prevailing load pressure (approximated by the vertical component of stress). Hence, volatile saturation at epizonal levels, commonly, is neither late, nor near-solidus; that is, a large amount of crystallization may occur under water-saturated conditions (if water is the dominant volatile constituent in the magma). A magma is said to be in a state of volatile saturation when bubbles of vapor grow by mass transfer from the melt, or are present in equilibrium with the magma. The nucleation and growth of bubbles of gas within the body of a liquid is referred to as boiling. Boiling occurs when the sum of the vapor pressures of the components of a liquid, e.g. 
is equal to the load pressure; boiling can result from a decrease in the load pressure or an increase in . A decrease in load pressure for a given vapor pressure of the dissolved constituents (at constant temperature and melt composition), results in what is referred to as first boiling. First boiling is fairly common in magmatic systems, and can occur as a result of magma ascent, or release of magmatic pressure by mechanical failure of the magma chamber. As many melts are known to possess concentrations of water on the order of 3–6 wt %, some with CO2 concentrations of up to a few thousand ppm (Lowenstern, 1994), partial pressures of magmatic gases can sum to a few hundred MPa. For a melt with 4 wt % water and 800 ppm CO2, 
(PH2O+PCO2)200 MPa (Lowenstern, 1994). If the magma ascends from a depth of 12 km, it will saturate with respect to a volatile phase (first boiling) at 8 km. Further, as water in a melt is an important source of the entropy of mixing that stabilizes felsic melts at temperatures below 950°C, the decreasing equilibrium concentration of water that results from first boiling (but not from isobaric second boiling) can result in irreversible crystallization. We will return to the question of melt crystallization upon rise in the section of this paper devoted to textures.
Volatile exsolution can also be brought about isobarically: if a liquid mixture of volatile and relatively non-volatile components is cooled below its liquidus and begin to crystallize phases that are dominantly anhydrous, the mole fraction of the volatile components in the mixture, and, concomitantly, , will increase. In this case, volatile saturation occurs upon cooling even though tends to decrease with decreasing temperature. This second mode of boiling, which is always accompanied by crystallization of non-volatile components and is isobaric in the limiting idealized case, is called second boiling. As an example, let us consider the crystallization of a vapor-undersaturated melt with 2 wt % water and 400 ppm CO2 (corresponding to 1000 mmol H2O/kg of melt, and 10 mmol CO2/kg of melt, respectively) at a constant pressure of 200 MPa. After 50% crystallization, the concentrations of water and carbon dioxide have doubled, yielding, as before, 200 MPa at 800–900°C. Under these conditions, the partial pressures of both water and carbon dioxide are on the order of 100 MPa (Lowenstern, 1994), and therefore the mole fractions of water and carbon dioxide in the vapor phase are sub-equal. If no carbon dioxide were present, the melt would not have saturated with respect to a volatile phase until the water concentration had risen to 6 wt % (at 67% crystallization of the melt), showing that volatile saturation occurred much earlier from any given aliquot of melt because of an initial concentration of carbon dioxide of only 400 ppm. However, the amount of vapor that can be evolved from a melt with only 800 ppm CO2 is severely limited, as <0.1 wt % CO2 is present in the melt. It should be noted that the solidus temperature does not increase during second boiling. In fact, in a melt with a few hundred ppm carbon dioxide, the activity of water increases with progressive exsolution of a CO2–H2O MVP during second boiling, and the solidus temperature of the magma actually drops during volatile exsolution. In the first boiling case, the melt would saturate with an H2O–CO2-rich volatile phase at 200 MPa vs 100 MPa for the CO2-free system.
Given this background, we can consider the following general scenario, which sets up the balance of the paper. A vapor-undersaturated magma, rising in the crust, saturates with respect to a volatile phase by first boiling. The magma may of course reach the surface and erupt; on the other hand, the magma ascent rate may be slowed by external far-field stresses or by an increase in viscosity because of crystallization or bubble production [however, see Dingwell, (1997)]. The change in viscosity of a magma upon rise, as a result of crystallization, will be equal to the rate of change in the viscosity of the magma as it crystallizes, times the ratio of the rate of crystallization relative to the rate at which the melt is rising. As magma ascent slows, some crystallization will occur because of irreversible heat loss to the magma's cold epizonal surroundings, and volatile exsolution will be driven, increasingly, by crystallization (second boiling) rather than by decompression (first boiling); we note, however, that the adiabatic crystallization that occurs upon ascent, driven by first boiling, will also further promote volatile exsolution. Ponding of the increasingly viscous magma may be aided by tectonic factors. In many compressional settings, local dilational pull-aparts provide space for magmatic intrusion. It is not clear whether internal magmatic factors (e.g. the balance of buoyancy vs viscous drag forces) or the rates of brittle strain of the crust, are the limiting controls on the rate of magma ascent; most likely, one or the other factor will be dominant. In cases where the crustal strain rate is limiting, then magma may rise only as rapidly as brittle deformation occurs, and magma ascent and emplacement will be punctuated by periods of complete crystallization of melt. Such a pluton would be assembled over a period of time, and the volatile phase saturation scenario outlined here would be a quasi-periodic process (Candela & Blevin, 1995a), with magmas sometimes emplaced in concentric fashion, and at other times emplaced adjacent to previous magma pulses (Vigneresse & Bouchez, 1997). Hence, whether a magmatic–hydrothermal system is produced as a result of a single pulse or multiple pulses of magma, and whether the multiple pulses of magma produce a single pluton or multiple intrusive phases, will be a complex function of the rate of magma generation, upper-crustal strain rate, and the time-integrated physical properties of the magma (Hanson & Glazner, 1995). Depending upon the mode and rate of magma emplacement, a pluton may represent a single batch of magma, or it may represent a time-integrated chamber, with the magma present at any time corresponding to only a small portion of the total inferred pluton volume. The geometry of volatile egress, therefore, in relation to the present-day geometry of a given pluton, will depend upon the rate of strain at the site of emplacement, and the rate of magma supply.
In summary, we might expect a magma, after some differentiation at depth, to experience volatile exsolution, and further crystallization, upon ascent. As the magma is emplaced, near-isobaric volatile exsolution will commence. During the phase changes occurring at each stage, ore metals and other magmatic constituents will partition among the melt, mineral and volatile phases, in accord with the appropriate partitioning and solubility equilibria (as modified by kinetic constraints when irreversibility of the magmatic processes is considered). Of course, melts richer in volatile components can be expected to reach MVP saturation earlier (early relative to crystallization progress) than melts poorer in volatiles; further, earlier volatile phase saturation is also expected for hotter melts transported to shallower levels. As we shall see, the timing of volatile saturation is critical in our understanding of ore genesis according to the orthomagmatic–hydrothermal model. In the next few sections of this paper, I will discuss a number of factors related to the magmatic stage of development of granite–ore systems, including crystal–melt equilibria for ore metals, water concentrations in the melt, and pressure of emplacement. I will then proceed to discuss textures in high-level granites and the processes of MVP egress, followed by a narrative on the hydrothermal aspects of the MVP exsolution process.
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Water Concentrations, and Pressure of Emplacement |
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TOPABSTRACTIntroductionGeneral Narrative on the...Water Concentrations, and...Timing of Volatile Saturation...Granite TexturesGeneral Narrative on the...References |
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Initial water concentrations with special reference to the role of hornblende
The initial water concentration of a melt can only be defined for a given initial melt composition. In many cases, this is taken to be the composition of either the granitic rock in question or some primitive member of the suite. It should be noted also that the concentration of water in the melt phase is different from the water concentration in the magma. A magma with 2 wt % water and 50% crystals, and which is volatile phase undersaturated, contains a melt phase with 4 wt % water. The composition of the bulk magma may correspond to the composition of a true partial melt, a mixed melt, melt plus restite, or a fractionated melt; postulating the water content and bulk composition of inferred parental magmas is, therefore, a difficult task.
Hornblende is a hydrous mineral that is present in many tonalitic to granodioritic I-type igneous rocks. It is not common in granites (sensu stricto) and the very highly evolved and 'specialized’ granites associated with many lithophile metal deposits do not contain hornblende (however, it does occur as a phenocryst phase in some rhyolites). By definition, hornblende does not occur in S-type granites (White & Chappell, 1977). For these systems, studies of melt inclusions in phenocrysts (Thomas, 1994) or water concentrations in cordierite (Carrington & Harley, 1996) are yielding promising results with regard to the estimation of water concentrations at the time of trapping of the inclusions, and at the ‘water closure temperature’ of cordierite, respectively. However, how these water concentrations can be related to a particular stage of crystallization and melt evolution still must be addressed.
The use of hornblende as an indicator of water concentrations in melts, and as a geobarometer, has not been discussed in a magmatic–hydrothermal context. Given that both new phase equilibrium data on hornblende stability and new formulations of the amphibole geobarometer have recently been published, the balance of this section will be devoted to a discussion of hornblende. The presence of hornblende can yield information on the minimum water concentration in melts with which it is in equilibrium; below a certain fugacity of water (for a given bulk magma composition and at a given temperature and pressure), hornblende will decompose. The limiting dehydration reaction (shown schematically) for supersolidus hornblende stability, according to Rutherford, (1993), is:
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According to experiments on the Mount St Helens dacite (62–64 wt % SiO2) by Rutherford & Devine, (1988), the minimum water concentration in the melt needed to stabilize hornblende at T=750°C, partial pressure of water equal to total pressure, and f(O2)=NNO (nickel–nickel oxide) + 1, is on the order of 4–4.5 wt %, or water vapor pressures of 100–140 MPa. This yields a minimum fugacity of water for hornblende stability, in a magmatic bulk composition corresponding to Mount St Helens dacite, of 95 MPa (i.e. partial pressure of water of 120 MPa and fugacity coefficient of 0.8) at 750°C.
At about 900°C and a total pressure of 220 MPa, Rutherford & Devine, (1988) reported that hornblende is destabilized in the presence of a CO2–H2O vapor at mole fractions of water in the vapor less than 0.6. That is, hornblende was destabilized at partial pressures of water below 220 x 0.6
130 MPa, or f(H2O)120 MPa, given a fugacity coefficient for pure water 0.9, and assuming ideal mixing in the vapor at these temperatures and pressures.
In summary, hornblende stability is limited to fugacities of water greater than 95–120 MPa at temperatures of 750–900°C, at least for dacitic magmas at total pressures on the order of 100–200 MPa, and for the oxygen fugacities that prevail in many arc and I-type granite magmas.
We can consider a hypothetical example to see how these data may be used to estimate the initial magmatic water concentration. The estimate of water concentrations in granitic rocks is fraught with many difficulties, and this example is probably close to a best case scenario. If a magma series, believed to be related by fractional crystallization, has a porphyritic granite of roughly dacitic composition (to conform to the experiments of Rutherford and coworkers) with 25% phenocrysts, including a few percent hornblende, then one might infer that hornblende crystallized during the first 25% of the crystallization history of the system. This suggests that the melt had a minimum initial concentration of water equal to 3–3.5 wt % near its liquidus (yielding on the order of 4–4.6 wt % at 25% crystallization). As the temperature of hornblende crystallization in this case would be on the order of 900°C, the higher figures for these water concentrations would probably be more accurate; i.e. the best estimate for a minimum initial water concentration would be 4±0.5 wt %. The presence of hornblende, and an estimate of when it began to crystallize, is probably the best method, currently, for the estimation of initial water concentrations in granitic systems.
Hornblende barometry
Methods to determine the pressure of crystallization of hornblende have recently been summarized and discussed by Anderson, (1996) and Ague, (1997). The essence of the technique relies on the fact that the aluminum concentration in amphibole increases with increasing pressure in the presence of a low-variance granitic phase assemblage. The aluminum-bearing exchange components responsible for the variation in the aluminum concentration in amphibole appear to be the edenite (NaAlSi–1) and Tschermak (Al2Mg–1Si–1) exchanges, which operate on the additive tremolite component (the pargasite component comprises equal proportions of these three components). Various calibrations for the Al-in-hornblende geobarometer are available, but the interested reader is referred to the papers by Anderson and Ague, and references therein. In all but Ague's method, the barometer depends upon the presence of a large number of mineral and fluid phases: quartz, plagioclase, K-feldspar, biotite, hornblende, sphene, Fe–Ti oxides + melt + vapor. However, Ague has identified an equilibrium among the mineral phase components:
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, but with the added benefit of not requiring the presence of as many phases. However, before the amphibole barometer can be used, some issues regarding the stability of hornblende need to be clarified. The preceding discussion on the stability of hornblende as a function of water fugacity illustrates some of the problems related to the lower pressure limit of hornblende stability, and the following discussion considers, qualitatively, the limited range of temperatures over which biotite and hornblende may coexist.
As N. L. Bowen noted in his now famous ‘reaction series’ early in this century, hornblende can react with melt to yield biotite. According to Abbott, (1981) and Vythnal et al., (1991), ferromagnesian mineral-bearing assemblages above the solidus evolve, with decreasing temperature, from (shown schematically): melt + hbl, to melt + hbl + bt (an even reaction involving direct precipitation of biotite and hornblende), to the odd reaction (melt + hbl 
bt), and finally to melt + bt. I express the equilibrium along the odd reaction curve as
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); the activity of Al2O3 increases from zero as the aluminum saturation index (ASI) of the melt increases beyond unity. The crystallization of hornblende, at least at low pressures, causes the ASI of the melt to rise because low-pressure hornblende is low in aluminum. Generally, the assemblage hornblende + biotite occurs over only a portion of the temperature range between the temperature of appearance of hornblende and the solidus. Temperatures and pressures estimated from hornblende geothermometers and geobarometers may reflect the limited temperatures of coexistence of the pertinent phases rather than the temperature of the vapor-saturated solidus. Many granites (sensu lato) probably do not have hornblende + biotite coexisting stably at the water-saturated solidus, except for melts with low ratios of normative or to an, such as diorites. In the next section, the significance of the estimated water concentrations and pressures of emplacement will be related to the efficiency with which ore metals can be removed into the magmatic volatile phase during volatile exsolution.
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Timing of Volatile Saturation Versus Crystallization Progress |
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TOPABSTRACTIntroductionGeneral Narrative on the...Water Concentrations, and...Timing of Volatile Saturation...Granite TexturesGeneral Narrative on the...References |
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The competition for ore metals (and other elements) between the MVP and the crystallizing magmatic mineral assemblage during crystallization is a first-order control on the efficiency with which metals can be removed from magma into an evolving MVP, or ‘ore fluid’ (Candela & Holland, 1986
). The values of the pertinent crystal–melt and MVP–melt partition coefficients, together with the timing of crystallization relative to saturation with respect to a water-rich MVP, is a major factor in determining whether the MVP or the crystallizing phases take up a higher proportion of a given ore metal. My coworkers and I have presented detailed accounts of ore metal partitioning in felsic melt–crystal systems elsewhere (see Candela, 1992; Jugo, 1997), and the interested reader is referred to those and other papers for values of the pertinent partition coefficients. Here, I will discuss briefly the qualitative effects of oxygen fugacity and crystallization on the behavior of ore metals in granitic systems.
Based on field associations of mineral deposits associated with I- versus S-type granites, Burnham & Ohmoto, (1980) suggested that oxygen fugacity may affect the type of mineralization associated with granites. They suggested that copper and molybdenum deposits were associated with oxidized granites, and tin and tungsten deposits were associated with more reduced systems. Similar conclusions had been reached by Ishihara, (1977). Candela, (1992) summarized experimental data for the crystal–melt partitioning of copper, molybdenum and tungsten, and showed that the experimental results are consistent with the effects of magmatic oxygen fugacity. Those results suggest that f(O2)-dependent crystal–melt equilibria play a significant role in producing the observed relationships. Similarly, Lehmann, (1990) showed, based on field data, that tin behaves as a compatible element at low oxygen fugacities. Of the elements copper (Lynton et al., 1993), gold (Jugo, 1997), molybdenum and tungsten (Candela & Bouton, 1990), and tin (Lehmann, 1990), only tin and tungsten do not experience enhanced partitioning in favor of the melt phase with increasing oxygen fugacity, either as a direct effect on the partition coefficient (e.g. as in the case of molybdenum), or because of destabilization of the host phase (as in the case of copper or gold).
Molybdenum and tungsten can be partitioned into Ti-bearing accessory phases, such as ilmenite, magnetite, sphene and biotite. Both gold and copper are partitioned strongly into certain accessory magmatic sulfides [which are stable at f(O2)<NNO + 1; Carroll & Rutherford, 1985] and protracted crystallization of magmatic sulfide minerals, or exsolution of an immiscible sulfide melt at higher temperatures, can result in the strong depletion of melts with respect to copper and gold. If a significant amount of copper- and gold-bearing sulfide is removed from the magmatic system before saturation of the melt with respect to a volatile phase, the probability of formation of a copper- or gold-rich ore or protore will be significantly reduced. In I-type systems with oxygen fugacities between quartz–fayalite–magnetite (QFM) and NNO, a small amount of pyrrhotite crystallization might remove a significant amount of copper, but not gold. For some metals, such as copper and gold, that possess high MVP–melt partition coefficients (see Candela et al., 1996), the role of f(O2) is obviated in cases where hot, primitive melts with high initial water concentrations are emplaced in the uppermost levels of the Earth's crust. Under these conditions, volatile exsolution may occur before significant fractionation of sulfides, regardless of the oxygen fugacity. However, copper mineralization is usually restricted to relatively oxidized magmas at shallow levels, as even small amounts of pyrrhotite can remove significant amounts of copper from a crystallizing melt (Jugo, 1997). Candela & Holland, (1986) suggested that magmatic sulfides might, in some cases, react out of the hypersolidus mineral assemblage upon saturation of a magma with a volatile phase, as sulfur is removed from the melt along with other exsolving volatiles. Further, Keith et al., (1997) argued convincingly that metals sequestered in sulfides can be repartitioned into the MVP after volatile phase saturation, if magmatic sulfides are crystallized at, or brought to, the level of emplacement where volatile exsolution occurs. This process depends on the proportion of ore metals in a magmatic system that is partitioned into these phases during in situ crystallization at the level of emplacement versus the proportion of ore metals that is partitioned into crystallizing phases at depths below the level of emplacement (Candela & Piccoli, 1995) and remains at sites distal to MVP exsolution. In the latter case, ore metals are certainly lost from the system.
In general, early saturation with a water-rich volatile phase, as has been suggested for porphyry copper deposits (Candela & Holland, 1986; Candela, 1989) allows the evolving magmatic volatile phase maximum access to crystal-, or sulfide liquid-compatible ore metals, such as copper or gold, while they are still dissolved in the melt. In a relatively dry magma that is crystallizing at a relatively deep level, exsolution of a water-rich magmatic volatile phase will occur late in magmatic crystallization progress. Given that copper generally behaves as a compatible element, and molybdenum and tungsten are, for the most part, more incompatible than copper, early magmatic crystallization of phases capable of sequestering large proportions of magmatic copper can lead to low efficiencies of removal of copper into ore fluids relative to molybdenum and tungsten (Candela & Holland, 1986; Candela, 1989). The general zonation of shallow Cu deposits and deeper Mo and W deposits in some terranes supports this idea (Candela, 1992).
This analysis suggests that magmas possessing early-formed low Al-amphibole (high initial water content melts emplaced at shallow levels) should have a high probability of forming high (Cu + Au)/(Mo + W) ores. Conversely, magmas with late, high Al-hornblende might be associated with a lower potential for high (Cu + Au)/(Mo + W) ore formation.
In summary, there are two prime effects on the efficiency of ore metal removal from magmas: namely, the sequestering of ore metals into crystallizing magmatic phases, which is a function of both the oxygen fugacity and the mineralogy of the fractionating assemblage (Candela, 1992); and the timing of volatile saturation relative to crystallization (Candela & Holland, 1986), with early volatile exsolution allowing ore metals, especially those that are crystal compatible, to be available for partitioning into the MVP.
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Granite Textures |
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TOPABSTRACTIntroductionGeneral Narrative on the...Water Concentrations, and...Timing of Volatile Saturation...Granite TexturesGeneral Narrative on the...References |
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It was recognized rather early in the study of granite-related ores that the granitic rocks associated with mineralization can be texturally complex. Variations in texture are observed commonly within a given pluton, or among the plutons of a suite. The textural variants present may include porphyritic, aplitic, and pegmatitic domains, miarolitic cavities, graphic and micrographic texture, and dendritic, acicular and/or skeletal crystals, in addition to normal (i.e. hypidiomorphic granular) granitic texture. Variations in texture and morphology in igneous systems are functions of the mineral in question, the composition of the phase and the associated melt, the physical history of the melt, the presence of pre-existing phase boundaries (e.g. nuclei), ripening, post-crystallization deformation, and the magnitude of undercooling. For a given phase and a given melt composition, the magnitude of undercooling exerts a major influence on the crystallization kinetics and crystal morphology in the crystallizing system (Swanson, 1977
). Undercooling (
T) is defined as the difference between the temperature at which the melt saturates with respect to a mineral, and the temperature at which the mineral actually nucleates and grows. The undercooling is the driving force for crystal nucleation and growth; i.e. the free energy of crystallization is a function of T. Further, crystallization textures evolve with increasing undercooling (e.g. a given mafic melt may crystallize yielding a gabbroic texture at low undercooling, a diabasic texture at intermediate undercooling and a basaltic texture at high undercooling). Much has been made in the petrological literature about the necessity of pre-existing nuclei (heterogeneous nucleation) in the crystallization of igneous systems; however, the variations found in crystal number density in shallow granites, and as a function of the degree of undercooling in experimental studies, show that this concept is not a prime factor in understanding the textures of shallow granites. Granitic rocks that are emplaced at shallow levels, where undercooling is expected to vary significantly, show the greatest variety of textures. Magmas at deeper crustal levels, or magmas in regions of high heat flow, will cool more closely and acquire an equilibrium texture. Therefore, most undeformed granitic rocks fall on a continuum from near-equilibrium textures, which tell us very little about magmatic conditions, to texturally rich rocks (far from equilibrium), which possess a historical record.
Porphyritic texture is reported commonly in shallow, ore-associated granites. When the same minerals are found among both the phenocrysts and the groundmass, it is safe to ascribe this texture to two-stage cooling, with the fine-grained groundmass produced by a sudden increase in undercooling. For a given saturation temperature for the phases in question, the undercooling may be increased by lowering the actual temperature at which the crystals grow. The movement of an MVP-undersaturated magma into colder country rock, presumably by magma ascent to shallower levels, increases the temperature difference between the country rock and the magma. The increasing rate of heat flow out of the magma, in turn, results in a more rapid decrease in the temperature of the melt before crystals grow (i.e. higher T) and therefore a finer grain size. On the other hand, ascent of an MVP-saturated magma results in the lowering of the equilibrium water concentration in the melt. As the water concentration in the melt decreases (first boiling), the undercooling increases because of the rise in temperature required for equilibrium between, for example, quartz + feldspar and the melt at the new, lower water concentration in the melt. In principle, whether or not the melt rises volatile undersaturated may be discerned from the texture of magmatic quartz. The presence of rounded cores in quartz (Ratajeski, 1995) could indicate that at least some MVP-undersaturated ascent has occurred, because quartz solubility increases with decreasing pressure in water-undersaturated melts. Volatile saturation following volatile phase undersaturated rise should induce disequilibrium growth textures (e.g. skeletal quartz growth) over the corroded quartz cores. If the magma rises further, increasing undercooling may result, and skeletal quartz may be overgrown by more dendritic forms (see Fig. 1), either in the groundmass, or as decorations around the pre-existing phenocrysts (Ratajeski, 1995). Further, undercooling can produce, in the course of crystallization of felsic melts: the generation of quartz eyes [produced because of the ease of quartz nucleation relative to feldspar, yielding quartz phenocrysts (Piccoli et al., 1996)]; pegmatite (London, 1992); graphic or micrographic intergrowths (Fenn, 1986); granophyric (skeletal quartz) texture (Swanson & Fenn, 1986); aplite; variants of, or intermediates between these textures; miarolitic cavities, isolated, or interconnected (Candela & Blevin, 1995b).
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Piccoli et al., (1996)
suggested tentatively that increasing undercooling in dikes of the Tuolumne Intrusive Suite produced a sequence of dike textures ranging from normal granitic texture, to finer, more allotriomorphic variants, to pegmatite–aplite. Piccoli et al., (1996) also found that graphic texture in pegmatites and the abundance of pegmatite also increased with undercooling. Graphic or micrographic texture is defined as a cellular quartz–feldspar intergrowth (see Fig. 2). Micrographic texture occurs commonly as a component of the texture described otherwise as granophyric, fine grained, and/or aplitic. Apparently, during growth of feldspar, a planar feldspar interface degrades into a cellular micrographic quartz–feldspar intergrowth, which, in turn, can pass into dendritic quartz in a feldspar matrix (see Fig. 3). The term granophyric is usually used to described the less regular quartz–feldspar intergrowths found in aplites and fine-grained groundmass; these textures usually comprise, in part, skeletal quartz, quartz dendrites, irregular micrographic patches or feathery feldspar. The formation of skeletal quartz–quartz dendrites–granophyre, and graphic or micrographic textures also require degrees of undercooling beyond the range that obtains during the production of normal granite texture.
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Miarolitic cavities and pegmatite
Miarolitic cavities (see Fig. 4) comprise crystals of supersolidus minerals that terminate in, and project into, a quasi-spherical void (or mass of hydrothermally precipitated minerals). Further, miarolitic cavities and pegmatites share a common textural characteristic: external nucleation. External nucleation (London, 1992
) is defined as nucleation on a bounding surface of a system (as in a typical, crustified, hydrothermal vein), rather than at a point that is internal to the system (as in the crystallization of an aplite or a normal-textured granitic rock). One can readily identify crystals that have nucleated internally versus externally; however, whether internal nucleation has occurred heterogeneously or homogeneously is almost never apparent, and therefore is not a very useful concept when examining real rocks. External nucleation is the main characteristic of pegmatites. Because of the close textural similarity between pegmatitic and miarolitic texture, a short digression on the pegmatite texture is in order. Pegmatites are recognized by many geologists by the fact that the crystals form encrustations along the bounding surfaces of the body (again, as in a well-crustified hydrothermal vein) rather than by grain size; hence the term ‘micropegmatite’, which I think is an apt term for the externally nucleated texture found in some small dikes, or other minor bodies of externally nucleated igneous minerals, where the crystals are <1 cm in length. London et al., (1989) and London, (1992) demonstrated experimentally that pegmatitic textures do not require water saturation for their formation, and also pointed out that other experimental studies (e.g. Swanson, 1977) in addition to their own have shown that feldspar growth rates are lower at water saturation than under water-undersaturated conditions. The average size of crystals in a rock is inversely proportional to the number of crystals in a given volume, and is proportional to the ratio of the growth rate to the nucleation rate. The variation in the ratio of growth to nucleation rates can result from a multiplicity of causes, including the magnitude of undercooling, and the build-up of the concentration of non-quartzofeldspathic components in the melt, especially water, fluorine and phosphorus (London et al., 1989; London, 1992). In my opinion, based on the aforementioned experimental studies, my observations in the field, and observations and interpretations reported by others (e.g. Swanson & Fenn, 1986; London, 1992) pegmatite and aplite both form, at least in part, as a result of undercooling. Subtle variations in melt chemistry, temperature, and undercooling affect the relationship between growth and nucleation rates of the constituent minerals, thereby determining whether pegmatite (dominated by external nucleation of at least feldspar) or aplite (dominated by internal nucleation) forms; however, aplite seems to dominate with increasing undercooling. Generally, there can be multiple kinetic pathways to achieving external nucleation, and pegmatitic texture may be generated in different environments for different reasons. Miarolitic cavities share with pegmatite the characteristic of external nucleation; however, this does not suggest that miarolitic cavities, along with the many modes of occurrence of pegmatite, share a common genesis. Indeed, unless a pegmatite is miarolitic, there is no compelling reason to suppose the melt was water saturated during its crystallization (although many pegmatite magmas may well have crystallized at or near water saturation). Some pegmatites probably crystallize from near-solidus melts that passively flow into sub-horizontal voids created by the foundering of layers or mattes of crystals. The space created in this way may be infilled with nuclei-poor, relatively cool (i.e. near-solidus) melt, wherein external nucleation of crystals may occur more rapidly than internal nucleation. Similarly, pluton borders may contain pegmatite (stockscheider when it is composed primarily of quartz, feldspar and mica), not because of intrusion of ‘late, water-saturated, magmas along the contact’, but because of intermediate undercooling, or because of dilational pumping of residual melt away from preexisting crystals that normally would serve as nuclei at lower, near-solidus temperatures.
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Miarolitic cavities are generally taken as the best evidence, found in plutonic rocks, for volatile phase saturation in magmas. The crystals project into what is now a void, and what was occupied by the magmatic volatile phase. The paucity of internally nucleated crystals in miarolitic cavities reflects, in part, the fact that silicate melt did not exist in that volume, and only crystals nucleating near the melt–MVP interface could grow into the cavity. Miarolitic cavities are generally restricted, though not entirely so, to epizonal granites, particularly those that are finer grained. Miarolitic cavities are not found in most granites, even though the overwhelming majority of most granites have seen volatile saturation at some stage in their history; that is, the absence of miarolitic cavities from most granites tells us nothing about whether those granites were saturated with a volatile phase or not. These factors suggest the hypothesis that the formation of miarolitic cavities requires bubble growth during magma ascent and decompression, and (at least partial) quenching of melt during that process. Vapor bubbles that form during isobaric crystallization of a water-saturated melt are probably small compared with the size of most of the mineral grains in a normal, granite-textured rock. Therefore, the overwhelming majority of bubbles that form during the crystallization of a granite are not preserved as miarolitic cavities. The probability of forming miarolitic cavities increases with decreasing crystal size because bubble size and crystal size approach each other. Although bubbles can grow by coalescence and by addition of volatile constituents from the devolatilizing melt, those processes can occur in all crystallizing, devolatilizing granitic melts. However, in an ascending, volatile-saturated melt, bubbles increase in size by decompression. Together with the concomitant increase in solidus temperature, the simultaneity of bubble growth and crystallization driven by increasing undercooling results in the simultaneous development of miarolitic cavities, micropegmatite, and micrographic and/or dendrite-bearing (‘granophyric’) aplite (Candela & Blevin, 1995b
). The combination of larger bubbles and smaller crystals allows for some small portion of the volume of the magmatic volatile phase to be preserved as miarolitic cavities. However, this textural preservation is selective, and the volume of miarolitic cavities is unrelated to the actual proportion of the MVP vs melt.
The absence of miarolitic cavities from most granites indicates that decompression to pressures below the vapor pressure of the melt did not occur to such an extent that significant crystallization accompanied decompression and bubble growth. When a relatively low crystallinity magma decompresses and saturates with respect to a volatile phase, and crystallizes significantly, then miarolitic cavity formation can occur. For example, if a melt with 4 wt % water ascends to within about 4 km of the Earth's surface, then vapor bubbles begin to form as the 100 MPa isobar is crossed, and miarolitic cavity generation may begin thereafter during crystallization and progressive rise of the melt to higher levels (as long as the rate of ascent is sufficiently slow to allow crystallization). As higher initial water contents obtain, miarolitic cavity formation may occur at deeper levels in the crust. If we accept the data from melt inclusions in volcanic rocks, felsic melts can have vapor pressures as high as a few hundreds of MPa. Therefore, formation of miarolitic cavities by bubble growth and decompressive crystallization will be limited, generally, to the uppermost crust. As a generalization, the deeper the interval of crystallization is in the crust, the lower the probability of forming miarolitic cavities; miarolitic cavities should become relatively uncommon at pressures of crystallization of 300 MPa or greater. There is, however, no maximum pressure for the formation of miarolitic cavities.
Many of the textures that have been ascribed to water saturation or volatile exsolution, such as pegmatite–aplite, granophyre, or graphic or micrographic textures, have been produced experimentally at volatile-undersaturated conditions (Fenn, 1986; Swanson & Fenn, 1986; London, 1992). Rather, as indicated herein and elsewhere, these textures indicate that the magma experienced varying degrees of undercooling, which may be related to the temperature of the country rock and/or the exsolution of a volatile phase upon ascent to higher levels in the crust. For magmas of a given composition, water content and crystallinity, emplaced at a given level in the epizone, higher undercoolings will be produced in regions of lower heat flow. This may be why many porphyry-Mo related plutons possess highly variable textures, complete with unidirectional solidification textures (some components of which are magmatic, and possess dendritic, skeletal, or pegmatitic components).
In summary, most of the textural variability found in high-level and/or ore-related granites is probably generated by various degrees of undercooling. This undercooling may result in part from first boiling, so that many of the observed textures are indirectly related to volatile exsolution. However, many of these textures have been produced experimentally in the absence of volatile saturation, so saturation is not a necessary prerequisite for the formation of pegmatite, aplite, ‘granophyre’ or graphic textures. Miarolitic cavities, however, appear to be an exception to the rule. Further, the generation of undercooling by volatile phase exsolution is restricted to first boiling. Granite textures are moot with regard to volatile phase exsolution produced by second boiling, as this process occurs without generating any substantive undercooling. Fine-grained rocks that crystallized during second boiling owe their texture to undercooling produced by conductive heat loss at shallow levels or coincidental decompression that may have accompanied second boiling. Further research, building upon the seminal works of Shannon et al., (1982), Kirkham & Sinclair, (1988), and Lowenstern & Sinclair, (1996), is required to determine how these many granite textures relate to the genesis of specific mineral deposits, and, more importantly, how they differ among deposit types.
MVP egress and interconnected miarolitic texture
Whereas magmatic vapor±brine may remove ore metals from the apical regions of an intrusion into the overlying hydrothermal system, magmatic transport of ore metals to the apical regions of the magma chamber is essential, as the volume of the apical magma, by itself, is rarely sufficient to provide the full complement of ore metals to an ore zone. How then does the magmatic volatile phase gain egress from a magma? To a large extent, this depends upon: whether the magma is saturated with respect to a volatile phase upon ascent, and whether a vapor, or vapor plus brine are present; the pressure during volatile exsolution, and the volume proportion of crystals:silicate liquid:brine:vapor; the rate of magma rise after volatile saturation; the rate of cooling of the magma at a given pressure; and many other factors. If the magma is already saturated with a vapor phase upon emplacement, and the crystallinity of the magma is low, then some bubble rise and/or bubble-laden plume rise is possible (Candela, 1991). Indeed, ascent of the magma may be considered as the rise of a bubble-laden plume, with ascent driven in part by the buoyancy imparted to the magma by bubbles of vapor. Buoyancy imparted by brine is lower in magnitude, but magmatic brines formed at near-solidus temperatures and at 100 MPa have densities on the order of 1000 kg/m3, significantly lower than the bulk density of magmas.
Rise of individual bubbles certainly occurs in magmas during some stages of evolution after volatile saturation. However, as the proportion of crystals increases, the pathway for bubble rise becomes tortuous. Further, bubble rise hypotheses suffer from the fact that volatile exsolution, resulting from rapid crystallization or rapid pressure reduction, probably produces many small bubbles, rather than a few large bubbles. Smaller bubbles have higher ratios of drag forces to buoyancy forces, and therefore rise more slowly. Both of these factors are likely to limit models that depend upon bubble rise in shallow ore environments, except for large bubbles that form during ascent in crystal-poor magma. In addition to the rise of bubble-laden magma plumes, Candela, (1991) suggested that the MVP may advect through interconnected regions of the volatile phase, allowing for transport of hydrothermal ‘ore fluids’ through the magma. Shinohara & Kazahaya, (1995) have criticized this hypothesis, suggesting that the model relies on ‘channels’ between bubbles that are unstable with respect to coalescence, and the large channel itself would rise because of its buoyancy, relegating this mechanism to a transient phenomenon. However, the model was proposed as a phenomenological ‘fractal percolation cluster’ model, not as a detailed physical model. Indeed, coalescence of volumes of the MVP is not an unreasonable mechanism for connectivity. Whether the percolation cluster of volatile phase volumes rises as discrete packets, or simply remains open, with the volatile phase flowing through it, depends upon the rate of rise of the MVP relative to the rate at which it is produced by exsolution. Whether MVP expulsion is steady state, or occurs by pulse-like slug flow, is beyond what is currently testable. Although researchers may perform simplified calculations meant to model the complex process of MVP exsolution, the suggestion of specific textural geometries, which can be tested in the field by ‘postdiction’, provides a definitive test of process-related hypotheses. Whereas the percolation model proposes ‘interconnected regions of MVP’, and because miarolitic cavities represent the best evidence for the existence of a separate MVP in plutonic environs, interconnected miarolitic cavities are wholly consistent with the supersolidus MVP percolation hypothesis. In Candela & Blevin, (1995b), we have described, illustrated, and documented, both macroscopically and microscopically, the occurrence of interconnected miarolitic cavities in shallow ore-related granites (see Fig. 5a–c). These textures have been described in the Ruby Creek phase of the Stanthorpe Granite (New England Orogen, Eastern Australia), where they provide strong field evidence for the supersolidus MVP percolation hypothesis. The MVP percolation model also suggests that supersolidus percolation will occur only under certain circumstances (relatively low pressures and high water concentrations in the ‘initial’ magma); that is, the process is critical, and does not ‘work’ in every case, consistent with the observation that mineralization is less probable at greater depths (a few kilobars). Igneous structures supporting this model have also been found in the Capitan Pluton, New Mexico (Dunbar et al., 1996). Candela, (1991) and Candela & Blevin, (1995b) considered this process to be a general one for shallow (pressures 200 MPa and less) wet magmas (initial water concentrations close to saturation). We suggested that this process operates during second boiling under the appropriate conditions, and is sometimes ‘frozen in’ and preserved when plumes of magma, containing interconnected channels, become upwardly mobile because of buoyancy. High-level granites exhibiting this texture should be considered ‘prospective’ from an exploration viewpoint.
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General Narrative on the Development of Granite-Related Ore Systems At the Hydrothermal Stage |
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During crystallization of an MVP-saturated melt, pressure may increase above the total load pressure because of the exsolution of the low-density vapor phase. Burnham, (1979)
suggested that this excess pressure may cause failure of the country rock leading to flow of fluids (melt, magma or MVP), or even to volcanic eruption. However, the preferred orientations found in many mineralized systems suggested that these near-field stresses are coupled strongly to the far-field stress regime, producing a complex geometry of fractures that may vary from regionally controlled to those that are related to the geometry of the pluton. Venting at more than one locus within a large chamber creating apophyses, veins, or vein dikes (or even subvolcanic necks) will lower the prevailing total pressure of the chamber to lithostatic or lower (if the boundary region between the lithostatically and hydrostatically pressured systems is breached), possibly triggering volatile exsolution by first boiling and groundmass crystallization. If the decrease in pressure is large (tens of MPa) chamber failure can provide a large percolation network (by increasing the molar volume of a vapor phase). Under these conditions melt flow may also occur, with crystals left behind. This hydrodynamic dispersion of melt and crystals may, for example, produce non-miarolitic, crowded and overcrowded porphyries of monzodioritic composition, on one hand, and more highly fractionated quartz-rich, and possibly miarolitic rocks elsewhere down pressure in the system.
During first or second boiling at low (<150 MPa) pressures, a magma may saturate with respect to a vapor, a brine (Webster, 1997), or both, depending upon the ratio of chlorine to water in the magma, the initial water concentration, and the pressure of crystallization (Metrich & Rutherford, 1992; Webster, 1992; Shinohara, 1994; Candela & Piccoli, 1995). When two volatile phases exsolve they may be transported through the magma toward the top of the chamber by a variety of mechanisms (Candela, 1991; Candela & Blevin, 1995a, b). However, the dynamics of vapor movement through a magma is likely to be wholly different from that of a brine. MVP percolation will probably be dominated by vapor. The vapor, brine and melt will tend to stratify gravitationally, producing regions within the upper reaches of the magma chamber and the superjacent subsolidus hydrothermal system enriched in the lower-density aqueous vapor phase. The higher-density brine may be trapped within the crystallizing melt below. Because of its high density relative to both the vapor and the surrounding hydrological regime, the brine phase would tend to resist migration away from the immediate vicinity of the magma chamber (Henley & McNabb, 1978). Brine may probably remain dispersed. In fact, early, deep generation of brine within the pluton may not be optimum for ore formation. If both brine and vapor form simultaneously, the vapor from different portions of the crystallization interval may mix, and the only meaningful vapor compositions would be averaged compositions. The vapor volumes would form percolation clusters (under the appropriate conditions of pressure and initial water concentration of the melt) with vapor streaming toward the apical regions of the pluton. Brine bubbles may not form percolation clusters (their mass fraction is too low, and their density is too high); most probably, they sit where they form. They would be buoyant in the magma, rising small distances tortuously; a large proportion of brine probably becomes trapped in the pluton, only to be removed by circulating exogeneous fluids. If, on the other hand, the brine forms late (e.g. near the solidus), a cupola near the maximum pressure at which brine can form at near-solidus temperatures (120 MPa) would be the locus of brine formation and storage; otherwise a brine dispersed through the crystallized pluton could become just another phase of plutonic dispersal of chloride-complexed ore metals. It should be noted that in the magma, brine is at a pressure of ‘lithostatic’ or greater, and upon catastrophic failure of the roof, the brine may be propelled into the lower reaches of the overlying hydrologic system (see Rye, 1993). The brine may indeed re-equilibrate with the subsolidus pluton at temperatures of 400–700°C, profoundly altering its character (Frank, 1996). The region between the carapace and the brittle geothermal system above is a quasiplastic region where meteoric water is scarce, and fractures heal rapidly. As Fournier, (1987) has pointed out, and as Rye and later Hedenquist & Lowenstern, (1994) have emphasized, the continual accession of vapor plumes from the deep magmatic zone into the overlying geothermal system may be punctuated by catastrophic periodic pulses of more saline brine that are driven upward from the upper reaches of the magmatic system by the large pressure gradients that are produced when the brittle–ductile transition is breached. Hence, vapor or supercritical gas on one hand, and brine on the other hand, may be transported through the magmatic chamber and into the overlying hydrothermal system in very different ways. Further, it is worth emphasizing that chlorinity, and hence the metal-carrying capacity of vapor and brine, can vary strongly over rather small regions of P–T–X space, especially for bulk MVP compositions that are near-critical at the pressures and temperatures close to the water-saturated granite solidus, and the proportion of ore substance transported in gas-like or liquid-like form can vary appreciably from one system to another.
Miarolitic textures in high-level granites may grudgingly yield hints as to means of egress of MVPs from crystallizing plutons, and other granite textures may, indirectly, bear on the complex relations among variables such as pressure, volatile concentrations in the melt, ascent rate, and the local thermal structure of the crust. The actual concentrations of ore metals, sulfur, HCl, etc., in the MVP that enter the deep parts of ore, protore, or geothermal systems, depend upon how magmatic evolution has affected critical parameters such as: the concentration of H2O, CO2, HCl, and total sulfur in the magma; the f(O2) of the magma; when, relative to volatile saturation, the magma saturates with sulfides or other phases capable of sequestering ore metals; and where those crystalline materials reside relative to the regions of ‘ore-fluid’ exsolution. Fortunately, experimental work on granite textures (London, 1992), ore metals (Candela, 1992; Williams et al., 1995), phase equilibria in igneous systems (Rutherford, 1993), apatite and estimates of magmatic chloride concentrations (Piccoli & Candela, 1994), granite geobarometry (see Anderson, 1996), ore metal sequestering in sphene (Piccoli et al., 1994), sulfide and oxide chemistry (see Whitney, 1985; Stimac & Hickmott, 1995; Jugo, 1997), and many other studies are beginning to yield, albeit stubbornly, information on many of the intensive parameters important in determining the chemical and physical properties of devolatilizing magmatic systems and the associated volatile phases that give rise to granite-related ore systems.
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Acknowledgements |
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This research benefited from the support of the NSF (Grant EAR9506631) and the US DOE (Grant DEFG0790ID13025). I would like to thank the many past and present members of the Laboratory for Mineral Deposits Research, University of Maryland (their papers are cited in this work) who have contributed to the overall model presented here, and especially Dr Philip Piccoli. Thanks are due to Professor Jay Ague, who kindly made a preprint of his paper available to me. I would also like to thank my colleagues in the Geology Department at the Australian National University, especially Dr Phillip Blevin and Professor Bruce Chappell, who have not only supported my many forays into Eastern Australia, but have also shared their knowledge and wisdom related to granites and mineralization, much of which inspired the material presented in the latter half of this paper. I am grateful to Professors Bernd Lehmann and Jeffrey Keith, and an anonymous reviewer, for insightful and constructive reviews of the manuscript.
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FOOTNOTES |
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www: http://www.geol.umd.edu/pages/facilities/lmdr/lmdr.html
* Telephone: 1-301-405-2783. Fax: 1-301-314-9661. e-mail: candela{at}geod.umd.edu
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References |
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|
TOPABSTRACTIntroductionGeneral Narrative on the...Water Concentrations, and...Timing of Volatile Saturation...Granite TexturesGeneral Narrative on the...References |
|---|
Abbott R. N Jr. AFM liquidus projections for granitic magmas, with special reference to hornblende, biotite, and garnet. Canadian Mineralogist (1981) 19:103–110.
Ague J. J. Thermodynamic calculation of emplacement pressures for batholithic rocks, California: implications for the Al-in-hornblende barometer. Geology (1997) 25:563–566.
Anderson J. L. Status of thermobarometry in granitic batholiths. Transactions of the Royal Society of Edinburgh: Earth Sciences (1996) 87:125–138.[Web of Science]
Burnham C. W. Magmas and hydrothermal fluids. In: Geochemistry of Hydrothermal Ore Deposits—Barnes H. L., ed. (1979) 2nd edn. New York: John Wiley. 71–136.
Burnham C.W., Ohmoto H. Late-state processes of felsic magmatism. Society of Mining Geologists of Japan (1980) 8(Special Issue):1–11.
Candela P. A. Magmas, volatiles and metallogenesis. In: Ore Deposition Associated with Magmas. Reviews in Economic Geology, Society of Economic Geologists—Naldrett A., Whitney J., eds. (1989) 4:223–233.
Candela P. A. Physics of aqueous phase exsolution in plutonic environments. American Mineralogist (1991) 76:1081–1091.[Abstract]
Candela P. A. Controls on ore metal ratios in granite-related ore systems: an experimental and computational approach. Transactions of the Royal Society of Edinburgh: Earth Sciences (1992) 83:317–326.[Web of Science]
Candela P. A., Blevin P. L. Physical and chemical magmatic controls on the size of magmatic–hydrothermal ore deposits. In: Giant Ore Deposits, II—Clark A. H., ed. (1995a) Kingston: Queens University. 2–37.
Candela P. A., Blevin P. L. Do some miarolitic granites preserve evidence of magmatic volatile phase permeability? Economic Geology (1995b) 90:2310–2316.
Candela P. A., Bouton S. L. The influence of oxygen fugacity on tungsten and molybdenum partitioning between silicate melts and ilmenite. Economic Geology (1990) 85:633–640.
Candela P. A., Holland H. D. A mass transfer model for copper and molybdenum in magmatic hydrothermal systems: the origin of porphyry-type ore deposits. Economic Geology (1986) 81:1–19.
Candela P. A., Piccoli P. M. Model ore–metal partitioning from melts into vapor and vapor/brine mixtures. In: Magmas Fluids and Ore Deposits. Mineralogical Association of Canada Short Course—Thompson J. F. H., ed. (1995) 23:101–128.
Candela P. A., Piccoli P. M., Williams T. J. Preliminary study of gold partitioning in a low-sulfur, high oxygen fugacity melt/volatile phase system. Geological Society of America, Abstracts with Programs (1996) 28:A-402.
Carrington D. P., Harley S. L. Cordierite as a monitor of fluid and melt H2O contents in the lower crust: an experimental calibration. Geology (1996) 24:577–672.
Carroll M. R., Rutherford M. J. Sulfide and sulfate saturation in hydrous silicate melts. Journal of Geophysical Research (1985) 90:C601–C612.
Dilles J. H., Einaudi M. T. Wall-rock alteration and hydrothermal flow paths about the Ann-Mason porphyry copper deposit, Nevada—a 6-km vertical reconstruction. Economic Geology (1992) 87:963–2001.[Web of Science]
Dingwell D. B. The brittle–ductile transition in high-level granitic magmas: material constraints. Journal of Petrology (1997) 38:000–000.
Dunbar N. W., Campbell A. R., Candela P. A. Physical, chemical, and mineralogical evidence for magmatic fluid migration within the Capitan pluton, southeastern New Mexico. Geological Society of America Bulletin (1996) 108:318–333.
Fenn P. M. On the origin of graphic granite. American Mineralogist (1986) 71:325–330.[Abstract]
Fournier R. O. Conceptual models of brine evolution. US Geological Survey Professional Paper (1987) 1350(2):45–61.
Frank M. R., Candela P. A., Piccolo P. M. Thermodynamics of alteration equilibria in a high salinity quartz-saturated portion of the system Al2O3–SiO2–H2O–KCl–KH–1. Geological Society of America, Abstracts with Programs (1996) 28:A-153.
Hanson R. B., Glazner A. F. Thermal requirements for extensional emplacement of granitoids. Geology (1995) 23:213–216.
Hedenquist J. W., Lowenstern J. B. The role of magmas in the formation of hydrothermal ore deposits. Nature (1994) 370:519–527.
Henley R. W., McNabb A. Magmatic vapor plumes and ground-water interaction in porphyry copper emplacement. Economic Geology (1978) 73:1–20.
Ishihara S. The magnetite-series and ilmenite-series granitic rocks. Mining Geology (1977) 27:293–305.
Jugo P. Experimental study of the behaviour of copper and gold in the system haplogranite-CuFeS2–FeS–Au–H2O–HCl–O2–S2 at 850°C and 100 MPa. (1997) College Park: University of Maryland. M.Sc. Thesis.
Keith J. D., Whitney J. A., Hattori K., Ballantyne G. H., Christiansen E. H., Barr D. L., Cannan T. M., Hook C. J. The role of magmatic sulphides and mafic alkaline magmas in the Bingham and Tintic mining districts, Utah. Journal of Petrology (1997) 38:1679–1690.
Kirkham R. V., Sinclair W. D. Comb quartz layers in felsic intrusions and their relationship to porphyry deposits. In: Recent Advances in the Geology of Granite-related Mineral Deposits. Canadian Institute of Mining—Taylor R. P., Strong D. F., eds. (1988) 31:50–71.
Lehmann B. Metallogeny of Tin (1990) Berlin: Springer-Verlag.
London D. The application of experimental petrology to the genesis and crystallization of granitic pegmatites. Canadian Mineralogist (1992) 30:499–540.[Web of Science]
London D., Morgan VI G. B., Hervig R. L. Vapor-undersaturated experiments with Macusani glass + H2O at 200 MPa, and the internal differentiation of granitic pegmatites. Contributions to Mineralogy and Petrology (1989) 102:1–17.[Web of Science]
Lowenstern J. B. Dissolved volatile concentrations in an ore-forming magma. Geology (1994) 22:893–896.
Lowenstern J. B., Sinclair W. D. Exsolved magmatic fluid and its role in the formation of combined layered quartz at the Cretaceous Logtung W-Ma deposit, Yukon Territory, Canada. Transactions of the Royal Society of Edinburgh: Earth Sciences (1996) 87:291–304.[Web of Science]
Lynton S. A., Candela P. A., Piccoli P. M. Experimental study of the partitioning of copper between pyrrhotite and a high-silica rhyolitic melt. Economic Geology (1993) 88:901–915.
Metrich N., Rutherford M. J. Experimental study of chlorine behavior in hydrous silicic melts. Geochimica et Cosmochimica Acta (1992) 56:607–616.[Web of Science]
Piccoli P. M., Candela P. A. Apatite in felsic rocks: a model for the estimation on initial halogen concentrations in the Bishop Tuff (Long Valley) and Tuolumne Intrusive Suite (Sierra Nevada Batholith) magmas. American Journal of Science (1994) 294:92–135.
Piccoli P. M., Candela P. A., Jugo P. J., Frank M. R. Contrasting syn–late magmatic intrusive behavior of aplite dikes in the Tuolumne Intrusive Suite, California: implications for magma rheology. Cordilleran Section of the Geological Society of America (a symposium in honor of Paul Bateman) (1996) 28:101.
Piccoli P., Candela P., Rivers M. Titanite microchemistry and the interpretation of magmatic and hydrothermal processes in granitic systems. Geological Society of America, Abstracts with Programs (1994) 26:499.
Ratajeski K. Estimation of initial and saturation water concentrations for three granitic plutons in the North–Central Great Basin, Nevada. (1995) College Park: University of Maryland. M.Sc. Thesis.
Rutherford M. J. Experimental petrology applied to volcanic processes. EOS Transactions, American Geophysical Union (1993) 74:49–55.
Rutherford M. J., Devine J. D. The May 18, 1990 eruption of Mt. St. Helens: 3, Stability and chemistry of amphibole in the magma chamber. Journal of Geophysical Research (1988) 93:949–959.
Rye R. O. The evolution of magmatic fluids in the epithermal environment: the stable isotope perspective. Economic Geology (1993) 88:733–753.
Shannon J. R., Walker B. M., Carten R. B., Geraghty E. P. Unidirectional solidification textures and their significance in determining relative ages of intrusions at the Henderson Mine, Colorado. Geology (1982) 10:293–297.
Shinohara H. Exsolution of immiscible vapor and liquid phases from a crystallizing silicate melt: implications for chlorine and metal transport. Geochimica et Cosmochimica Acta (1994) 58:5215–5221.[Web of Science]
Shinohara H., Kazahaya K. Degassing processes related to magma chamber crystallization. In: Magmas Fluids and Ore Deposits. Mineralogical Association of Canada Short Course Volume—Thompson J. F. H., ed. (1995) 23:47–70.
Stimic J., Hickmott D. Ore metal partitioning in intermediate to silicic magmas: PIXE results on natural mineral/melt assemblages. In: Giant Ore Deposits, II—Clark A. H., ed. (1995) Kingston: Queens University. 182–220.
Swanson S. E. Relation of nucleation and crystal growth rate to the development of granitic textures. American Mineralogist (1977) 62:966–978.[Abstract]
Swanson S. E., Fenn P. M. Quartz crystallization in igneous rocks. American Mineralogist (1986) 71:331–342.[Abstract]
Thomas R. Estimation of the viscosity and water content of silicate melts from melt inclusion data. European Journal of Mineralogy (1994) 6:513–536.
Vigneresse J. L., Bouchez J. L. Successive granitic magma batches during pluton emplacement: the case of Cabeza de Araya (Spain). Journal of Petrology (1997) 38:1767–1776.
Vythnal C. R., McSween H. Y. Jr., Speer J. A. Hornblende chemistry in southern Appalachian granitoids: implications for aluminum hornblende thermobarometry and magmatic apatite. American Mineralogist (1991) 76:176–188.[Abstract]
Webster J. D. Fluid–melt interactions involving Cl-rich granites: experimental study from 2 to 8 bar. Geochimica et Cosmochimica Acta (1992) 56:679–687.[Web of Science]
Webster J. D. Chloride solubility in felsic melts and the role of chloride in magmatic degassing. Journal of Petrology (1997) 38:1793–1807.
White A. J. R., Chappell B. W. Ultrametamorphism and granitoid genesis. Tectonophysics (1977) 43:7–22.[Web of Science]
Whitney J. A. Fugacities of sulfurous gases in pyrrhotite-bearing silicic magmas. American Mineralogist (1985) 69:69–78.[Web of Science]
Williams T. J., Candela P. A., Piccoli P. M. The partitioning of copper between silicate melts and two-phase aqueous fluids: an experimental investigation at 1 kilobar, 800°C and 0.5 kilobar, 850°C. Contributions to Mineralogy and Petrology (1995) 121:388–399.[Web of Science]
Williams T. J., Candela P. A., Piccoli P. M. Hydrogen–alkali exchange between silicate melts and two-phase aqueous fluids: an experimental investigation. Contributions to Mineralogy and Petrology (1997) 128:114–126.[Web of Science]
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