Essay: Formation of Magmatic-Hydrothermal Ore Deposits

Introduction:

Magmatic- hydrothermal ore deposits provide the main source for the formation of many trace elements such as Cu, Ag, Au, Sn, Mo, and W. These elements are formed in a tectonic setting, by fluid dominated magmatic intrusions in Earth’s upper crust, along convergent plate margins where volcanic arcs are created. Vapor and hypersaline liquid are the two forms of magmatic fluid important to the ore deposits. The term ‘fluid’ as it is being referred is non-silicate, aqueous liquid or vapor; Hypersaline liquid is also known as brine and is indicative of a salinity of >50wt%. The salinities in magmatic environments that can form ore deposits have a substantial range from a very low .2-.5wt% to the hypersaline >50wt%. The salinity of a fluid was thought to be one of the main contributing factors to which elements formed under certain specific conditions however, recent developments support a new theory that is discussed later. There are multiple different types of ore deposits such as skarn, epithermal (high and low sulphidation), porphyry and pluton-related veins. However, there are two different ore deposits, porphyry and epithermal, that produce the greatest abundance of trace elements around the world (Hedenquist and Lowenstern, 1994).

Porphyries, one type of ore deposit which occurs adjacent to or hosted by intrusions, typically develop in hypersaline fluid and are associated with Cu” Mo” Au, Mo, W or Sn. Another type of ore deposit which occurs either above the parent intrusion or distant from the magmatic source is known as epithermal and relates to Au-Cu, Ag-Pb, Au (Ag, Pb-Zn). The term epithermal rightfully refers to ore deposits formed at low temperatures of <300”C and at shallow depths of 1-2km (Hedenquist and Lowenstern, 1994). The epithermal ore deposits can be further separated in to two different types, the high sulfidation and the low sulfidation deposits which are shown in Figure 1. High sulfidation epithermal deposits form above the parent intrusion near the surface and from oxidized highly acidic fluids. These systems are rich in SO2- and HCl-rich vapor that gets absorbed in to the near surface waters causing argillic alteration (kaolinite, pyrophyllite, etc.). The highly acidic waters then get progressively neutralized by the host rock. Low sulfidation occurs near the surface as well but away from the source rock, as seen in Figure 1, and is dominated by meteoric waters. The fluids are reduced with a neutral pH and CO2+, H2S, and NaCl as the main fluid species. The main difference between the two epithermal fluids is how much they have equilibrated with their host rocks before ore deposition (White and Hedenquist, 1995). In addition to the two main types of ore forming deposits, there are certain environments where they are capable of occurring.

There are three important reoccurring ore-forming environments around the globe that produce these trace elements. The first of which is the deep crust where gold deposits form due to mixing and phase separation among the aquo-carbonic fluids. The second is the granite-related Sn-W veins which provide the interaction of hot magmatic vapor and hypersaline magmatic liquid with cool surface-derived meteoric water, as a widespread mechanism for ore mineral precipitation by fluid mixing in the upper crust. Third is the Porphyry-Epithermal Cu-Mo-Au systems resulting from the varying density and degree of miscibility of saline fluids between surface and magmatic conditions that propose the role of fluid phase separation in ore-metal fractionation and mineral precipitation (Heinrich et al., 2007).
Figure 1 (Hedenquist and Lowenstern, 1994)

PORPHYRY-EPITHERMAL Cu-Mo-Au

The formation of magmatic-hydrothermal ore deposits is a complicated but not so timely process that undergoes numerous phases. A general depiction can be seen in Figure 2 showing the different components involved in the system. Hydrothermal ore deposits are initiated by the ”generation of hydrous silicate magmas, followed by their crystallization, the separation of volatile-rich magmatic fluids, and finally, the precipitation of ore minerals in veins or replacement deposits.’ (Audetat, Gunther, and Heinrich, 1998). Porphyry magma chambers have been dated using individual zircon grains. Since the magma reservoirs in which porphyry deposits form occur in the upper crust, they are found to have a maximum life span of <1 Ma. The porphyry stocks struggle to remain ‘at the temperature of mineralization(>350”C) for more than even a few tens of thousands of years, even with massive heat advection by magmatic fluids.’ (Quadt et al., 2011) The hosted zircons analyzed contain significantly different ages that range over a span of millions of years indicating multiple pulses of porphyry emplacement and mineralization. Diffusive equilibrium occurs even faster than mineralization between magmatic fluids and altered rocks. Thermal constraints suggest that the porphyries and their constituent ore fluids underwent the ore-forming process in multiple spirts of as little as 100 yrs. each. The methods behind this are handled and discussed later (Quadt et al., 2011).

Figure 2. Illustration of ore-forming magmatic-hydrothermal system, emphasizing scale and transient nature of hybrid magma with variable mantle (black) and crustal (gray) components. Interacting processes operate at different time scales, depending on rate of melt generation in mantle, variable rate of heat loss controlled by ambient temperature gradients, and exsolution of hydrothermal ‘uids and their focused ‘ ow through vein network, where Cu, Au, or Mo are enriched 100 fold to 1000 fold compared to magmas and crustal rocks (combining Dilles, 1987; Hedenquist and Lowenstern, 1994; Hill et al., 2002; Richards, 2003). (Quadt et al., 2011)

Chemical and temperature gradients are important due to the selective dissolution and re-precipitation of minerals in rare elements to also form ore deposits. Most ore deposits form in the upper crust due to the advection of magma and hot fluids into cooler rocks, creating the rather steep temperature gradients. Temporary steep gradients in pressure, density, and miscibility in response to the brittle deformation of rocks to form vertical vein networks proves physical properties of miscible fluids to be of equal importance. H2O-CO2”-NaCl controls the composition of crustal fluids causing variations in the physical properties and in turn affecting the chemical stability of dissolved species. (Heinrich 2007)

Evidence from fluid inclusions suggests the interaction of multiple fluids in volcanic arcs through fluid mixing as well as fluid phase separation. These fluid inclusions can provide insight in to the substantial role the geothermal gradient plays in the formation of these ore deposits and why they only occur under certain environmental conditions. Salinity was thought to be a main contributing factor (primary control) to which elements were precipitated but it is now debated that vapor and sulfur play a key role, especially in terms of Cu-Au deposits. Supporting evidence suggests the likelihood of one bearing greater significance than the other so both are discussed and compared. The addition of sulfur causes Cu and Au to prefer the vapor phase. Figure 3 shows the vapor/liquid concentrations surpass 1 and allow the elements to more easily shift in to the vapor phase where they can then be transported.

Figure 3 (Left). Experimental data for the partitioning of a range of elements between NaCl-H2O-dominated vapor and hypersaline liquid, plotted as a function of the density ratio of the two phases coexisting at variable pressures (modified from Pokrovski et al. 2005; see also Liebscher 2007, Figs. 13, 14). As required by theory, the fractionation constant of all elements approaches 1 as the two phases become identical at the critical point for all conditions and bulk fluid compositions. Chloride-complexed elements, including Na, Fe, Zn but also Cu and Ag are enriched to similar degrees in the saline liquid, according to these experiments in S-free fluid systems. Hydroxy-complexed elements including As, Si, Sb, Sb and Au reach relatively higher concentrations in the vapor phase, but never exceed their concentration in the liquid (mvapor/mliquid < 1). Preliminary data by Pokrovski et al. (2006a,b) and Nagaseki and Hayashi (2006) show that the addition of sulfur as an additional complexing ligand increases the concentration ratios for Cu and Au in favor of the vapor (arrows); in near-neutral pH systems (short arrows) the increase is minor, but in acid and sulfur-rich fluids (long arrows) the fractionation constant reaches ~ 1 or more, explaining the fractionation of Cu and Au into the vapor phase as observed in natural fluid inclusions. (Heinrich 2007)

VAPOR AND HYPERSALINE LIQUIDS

The solubility of ore minerals increases as water vapor density increases with the transient pressure rise along the liquid-vapor equilibrium curve. The nature of this occurrence suggests ‘that increasing hydration of aqueous volatile species is a key chemical factor determining vapor transport of metals and other solute compounds.’ (Heinrich et al., 2007) The high salinity in hypersaline fluid systems allows for the vapor and liquid to coexist beyond waters’ super critical point. The increasing water vapor density accompanied by an increase in temperature leads to higher metal concentrations as an inherent result of increased solubility of the minerals in vapor. ‘[Observed] metal transport in volcanic fumaroles’ and even higher ore-metal concentrations in vapor inclusions from magmatic-hydrothermal ore deposits’ (Heinrich et al., 2007) has led to research in order to quantify vapor transportation. Fractionation is of key importance because all elements behave differently when it occurs to the coexisting vapor and the hypersaline liquid. Certain elements such as ”Cu, Au, As, and B partition into the low-density vapor phase while other ore metals including Fe, Zn, and Pb preferentially enter the hypersaline liquid.’ (Heinrich et al.,2007). This basically means that vapor is now known to contain higher concentrations of ore metals than any other known geological fluid.

SULFUR CONTRIBUTION TO VAPOR

‘Sulfur is a major component in volcanic fluids and magmatic-hydrothermal ores including porphyry-copper, skarn, and polymetallic vein deposits, where it is enriched to a greater degree than any of the ore metals themselves’ (Seo, Guillong, and Heinrich, 2009). Sulfur is necessary for the precipitation of sulfide minerals such as pyrite and anhydrite. Sulfide is an essential ligand in metal-transporting fluids to increase the solubility of Cu and Au. Introducing sulfur to Cu and Au in vapor phase can also cause them to be relatively volatile. Sulfur however changed the conditions in which Cu and Au enter the vapor phase, as seen in Figure 3, and shines light on why it’s possible for Cu and Au to partition in to low density magmatic vapor (Heinrich et al., 2007). Sulfur basically makes it easier for Cu and Au in particular to enter in to the vapor phase where they can then be more easily transported making sulfur a key to the high concentrations of ore in the vapor phase.

Methods and Results:

ZIRCON DATING using LA-ICP-MS and ID-TIMS Figure 4(Above, Left). Rock slab from Bajo de la Alumbrera, showing early andesite porphyry (P2, left part of picture and xenolith in lower right corner) that solidi’ed before becoming intensely veined and pervasively mineralized by hydrothermal magnetite + quartz with disseminated chalcopyrite and gold. After this ‘rst pulse of hydrothermal mineralization, a dacite porphyry intruded along an irregular subvertical contact (EP3, right part of picture), before both rocks were cut by second generation of quartz veins (diagonal toward lower right). (Quadt et al., 2011) Figure 5(Above, Right). A: Concordia diagram with isotope dilution’thermal ionization mass spectrometry (ID-TIMS) results from the ‘rst (red ellipses, P2) and second (blue ellipses, EP3) Cu-Au mineralizing porphyry of Bajo de la Alumbrera. B, C: For comparison, published laser ablation’inductively coupled plasma’mass spectrometry (LA-ICP-MS) analyses and their interpreted mean ages and uncertainties on the same age scale (replotted from Harris et al., 2004, 2008; LP3 is petrographically indistinguishable from EP3, but cuts also second phase of ore veins). All errors are ” 2”. MSWD’mean square of weighted deviates. (Quadt et al., 2011) ‘Porphyry Cu ”Mo ” Au deposits form by hydrothermal metal enrichment from fluids that immediately follow the emplacement of porphyritic stocks and dikes at 2-8km depth’ (Quadt et al., 2011) Samples were taken from two porphyry Cu-Au deposits, the first samples taken from Bajo de la Alumbrera, a volcanic complex located in northwestern Argentina. Uranium-Lead LA-ICP-MS, laser ablation- inductively coupled plasma- mass spectrometry, and ID-TIMS, isotope dilution- thermal ionization mass spectrometry, analyses were performed on zircons from the samples to conclude concordant ages of single crystals between two mineralizing porphyry intrusions. The LA-ICP-MS data was taken previously and is represented by Figure 5, B and C. ID-TMS analyzed samples from the two intrusions. One sample known as BLA-P2 is quartz-magnetite(-K-feldspar-biotite) altered P2 porphyry while the other sample, taken 5m from the EP3 contact to exclude contamination, is known as BLA-EP3. BLA-EP3 ”truncates the first generation of hydrothermal quartz-magnetite veinlets associated with P2, and is in turn cut by a second generation of quartz veins’ (Quadt 2011). The results were compared with the previously existing data and the P2 porphyry grain ages are shown to range from 7.772 ”0.135 Ma to 7.212 ”0.027 Ma. The maximum age for subvolcanic intrusion, solidification, and first hydrothermal veining of P2 is as late as 7.216 ”0.018 Ma (P2-11 is the most precise of the young group) when the zircons crystallized from the parent magma. ‘The EP3 porphyry truncated these veins and provided concordant single-grain ages with a range from 7.126 ”0.016 Ma to 7.164 ”0.057 Ma. It is ultimately concluded that the two intrusions are separated in age by .090 ”0.034Ma. With this data, it can be said that the two porphyries intruded within a period of 0.124 m.y. from each other. Figure 6. Concordia diagrams with isotopic dilution’thermal ionization mass spectrometry (ID-TIMS) results from three porphyries (A: KM10, KM2, 5091-400; B: KM5; C: D310) bracketing two main pulses of Cu-Au mineralization at Bingham Canyon (Utah, USA); Re-Os (molybdenite) data are from Chesley and Ruiz (1997). (Quadt et al., 2011) The second samples were taken from Bingham Canyon in Utah,USA and were found in pre-ore, syn-ore, and post-ore porphyry intrusions. All three of the porphyry intrusions were dated using ID-TIMS analysis and yielded the results seen in figure 6. It was found that two Cu-Au mineralization pulses occurred. The first is associated with a quartz monzonite porphyry which existed prior to the mineralization of the Cu-Au in the porphyry. A second pulse of Cu-Au is known to occur because it cuts through the latite porphyry and truncates the first veins. Thirty-one concordant ages were taken collectively from the three intrusions and the most precisely dated of the grains concluded all the porphyries overlap in an age range of 38.10 ‘ 37.78 Ma. A single outlying grain of younger age is present in the oldest intrusion and is thought to be attributed to residual Pb loss. Upon interpretation of the three porphyries and the two Cu-Au pulses, a window of .32Ma is the time it took for their occurrence. In all three of the intrusions there are significantly older concordant grains dated as far back as 40.5Ma which hosts a minimum life time of the magmatic reservoir to be .80 ‘ 2 million years in age. (Quadt et al., 2011) Errors in the analyzed zircon grains can be minimalized if crystals that have undergone Pb loss are avoided or have been removed by chemical abrasion. The lifetime of the mineralization of a single porphyry is important for alternative physical models of magmatic-hydrothermal ore deposits which are expected to be constrained to a lifetime of less than 100k.y. Comparison of the porphyry intrusions in both sites provided substantial evidence of the relatively short lifespan of their formation. In both sites, the two consecutive pulses occur >1M.y. apart, .09M.y. and .32M.y. respectively.

FLUID INCLUSIONS: Sn-W VEINS

Mineral deposits of Sn-W are commonly formed by the mixing of magmatic fluids with external fluids along the contact zones of granitic intrusions (Heinrich, 2007). Tin precipitation was proven to be driven by the mixing of hot magmatic brine with cooler meteoric water by using LA-ICP-MS to measure fluid inclusions taken before, during, and after the deposition of Cassiterite(Sn02). (Audetat, Gunther, and Heinrich 1998). The fluid inclusions that formed in minerals during the time of the ore formation recorded temperatures between 500-900”C at several kilometers depth. The average size range of the inclusions is between 5 and 50 micrometers. In order to prove the importance of fluid-fluid interaction in the formation of magmatic-hydrothermal ore deposits, the Yankee Lode was analyzed. The Yankee Lode is a magmatic-hydrothermal vein deposit located in eastern Australia and is a part of the Mole Granite intrusion. This vein consists of primarily quartz and cassiterite that’s well preserved in open cavities. Two quartz were analyzed, their crystals have the same pattern of hydrothermal growth and precipitation represented by successive zones of inclusions as seen in Figure 7.

Fig. 7 (A) Longitudinal section through a quartz crystal from the Yankee Lode Sn deposit, showing numerous trails of pseudosecondary fluid inclusions and three growth zones recording the precipitation of ilmenite, cassiterite, and muscovite onto former crystal surfaces. The fluid inclusions shown in the right part of the figure represent four different stages in the evolution from a magmatic fluid toward a meteoric water-dominated system. Thtot corresponds to the final homogenization temperature. (Audetat, Gunther, and Heinrich, 1998)

There are indications of boiling fluid throughout the entire history of the quartz precipitation due to the presence of both low-density vapor inclusions and high-density brine inclusions. Apparent salinities of both inclusions were taken using microthermometric measurements and ‘Pressure for each trapping stage was derived by fitting NaClequiv values and homogenization temperatures (Thtot) of each fluid pair into the NaCl-H2O model system’ (Audetat, Gunther, and Heinrich, 1998). This data basically shows that there were three pulses of extremely hot fluid injected into the system before cool water mixing and had a consecutive temporary increase in pressure. The pressure increases are noted along with some of the various fluid inclusions analyzed in Figure 7. In this system tin is the main precipitating ore-forming-element as represented in Figure 8. The initial Sn concentration of 20 wt% starts to drop drastically at the onset of cassiterite precipitation. By stage 23, represented in Figure 8C, only 5 wt% of the initial concentration of Sn remains. At this same stage in non-precipitating elements, the fluid mixture still contains 35% of the magmatic fluid indicating the chemical and cooling(thermal) effects of fluid mixing are the cause for the precipitation of cassiterite. Three pulses of magmatic fluid occurred before the formation of cassiterite was initiated in response to Sn precipitation, however the onset of cool meteroric groundwater mixing didn’t occur until the third pulse. This proves the fluid-fluid mixing is critical to the formation of trace elements (Audetat, Gunther, and Heinrich, 1998).

There is another component occurring in this system along with the precipitation of Sn, the magmatic vapor phase selectively transporting copper and boron into the liquid mixture represented by Figure 8D. Boron’s initial marked reduction occurs at stage 25 in Figure 8D, exactly where tourmaline begins to precipitate. Note that the concentration of B remained near its original magmatic value in stage 23 and 24 when simultaneously the none precipitating elements underwent substantial dilution. B also decreased in stages 26 and 27 relative to its initial value but not as much as would be expected considering the continual growing and extracting of B from the fluid to form tourmaline. Copper follows the same trend as Boron of having the same original magmatic value in stages 23 and 24, indicating an excess of these two elements. The vapor and brine inclusions in the vapor phase were found to be selectively enriched in Cu and B. This explains the excess to be condensation of magmatic vapor into the mixing liquids as Cu and B prefer to partition to the vapor phase as opposed to the saline liquid like the other elements. It has been suggested that Cu can be stabilized in a sulfur-enriched vapor phase as opposed to metals which stabilize in brine by chloro-complexes. Gold, Au, is thought to behave similarly to Cu which could explain why it is selectively coupled with Cu and As in high sulfidation epithermal deposits. (Audetat, Gunther, and Heinrich, 1998).

Fig. 8. (Left) Evolution of pressure, temperature, and chemical composition of the ore-forming fluid, plotted on a relative time scale recorded by the growing quartz crystal. (A) Variation in temperature and pressure, calculated from microthermometric data. Hot, magmatic fluid was introduced into the vein system in three distinct pulses before it started to mix with cooler meteoric groundwater. (B) Concentrations of non-precipitating major and minor elements in the liquid-dominant fluid phase, interpreted to reflect progressive groundwater dilution to extreme values. (C) A sharp drop in Sn concentration is controlled by the precipitation of cassiterite. (D) B and Cu concentrations reflect not only mineral precipitation (tourmaline) but also the selective enrichment of the brine-groundwater mixture by vapor-phase transport. (Audetat, Gunther, and Heinrich, 1998)

Fig. 9 (Right) Partitioning of 17 elements between magmatic vapor and coexisting brine, calculated from analyses of four vapor and nine brine inclusions in two ‘boiling assemblages.’ At both pressure and temperature conditions recorded in these assemblages, Cu and B strongly fractionate into the magmatic vapor phase. (Audetat, Gunther, and Heinrich, 1998)

SALT PRECIPITATION

Fluids are released from the upper crustal plutons associated with magmatic-hydrothermal systems. These fluids are usually saline and phase separation occurs into very low salinity vapors and high-salinity brines as discussed earlier. Salt precipitation can have a major impact on the permeability of a system and the ore formation along the liquid-vapor-halite curve making certain ore deposits precipitate out more than others. Halite-bearing fluid inclusions were analyzed from porphyry deposits using microthermometry to discover the inclusions can homogenize by halite dissolution. (Lecumberri-Sanchez et al., 2015).

Based on the hypothesis formed from the examination of fluid inclusions that there is widespread halite saturation in magmatic-hydrothermal fluids, further data was collected and studied. Roughly 11,000 fluid inclusions from 57 different porphyry systems were used to identify halite bearing inclusions. There were about 6,000 halite-bearing inclusions in the data set. These inclusions were then subdivided in to two different methods of homogenization, by vapor bubble disappearance or by halite dissolution and found that 90%, 52 out of the 57, of the porphyry systems homogenized by halite dissolution. The pressure at homogenization was then calculated based on the PVTX, pressure-volume-temperature-composition, properties of H2O-NaCl and found the pressures at fluid inclusion homogenization exceeds 300MPa. If significant fluid-inclusion migration is expected, several millimeters, then water loss can occur and would result in salinity changes as well as density changes. This however is not the proposed idea because migration of no more than a few micrometers is common. If no migration is evident, this leads the more plausible explanation that heterogeneous entrapment of halite due to highly variable temperatures, ”100”C, occurred. This means it is thought that halite saturation occurs at the time of trapping. The coexistence of vapor inclusions with homogenized brine inclusions are a result of halite saturation along the liquid-vapor-halite curve. Trapped halite found in the surface of another growing mineral has also been observed and means that, ‘heterogeneous entrapment of solid halite inside FIs is a natural consequence of halite saturation’ (Lecumberri-Sanchez et al., 2015).

Figure 10. Left: Pressure-salinity projection of the H2O-NaCl phase diagram at 400 ”C (Driesner and Heinrich, 2007) showing a potential mechanism for copper sulfide mineralization via halite (H) saturation. Destruction of the liquid (L) phase results in partitioning H2O to the vapor (V), and Cu and Fe to the solid phase. Right side shows the same process schematically. (Lecumberri-Sanchez et al., 2015)
Halite saturation usually occurs at the eutectic where vapor, liquid, and halite are all in existence together. Since halite precipitates at shallow crustal levels, other ore minerals are able to precipitate out of liquid. The Na,Cl, Fe& Cu-rich liquid + vapor phase traverses the phase boundary to the more stable vapor + halite stage as seen in Figure 10. Once this eutectic point is reached, liquid decreases and starts precipitating out the Cu-Fe sulfides (”Au) that was in liquid. It can be concluded that salt saturation acts as a precipitation mechanism in magmatic-hydrothermal fluids. This allows for the rapidly ascending vapor phase to transport sulfur and gold upward however the mechanism is limited by the availability of reduced sulfur. The disproportionation of SO2 similarly occurs at temperatures around which halite saturation occurs which provides the needed sulfur. This indicates that salinity is not the only key component to the formation of magmatic-hydrothermal deposits, sulfur is of equal if not more importance. Lecumberri-Sanchez et al., 2015).

SULFUR in a Porphyry Cu-Au-Mo System

In order to better understand the role sulfur plays in high temperature metal segregation by fluid phase separation, two porphyry Cu-Au-Mo deposits were examined along with two granite related Sn-W veins, and barren miarolitic cavities. The fluid inclusion assemblages underwent microthemometric analysis to measure salinities. No modification after entrapment occurred and the temperature range for homogenization of the brine inclusions was between 323”8 to 492”8 ”C. This indicates heterogeneous entrapment of variable temperatures, ”100”C, signifying halite saturation at the time of fluid inclusion trapping. LA-ICP-MS was used to measure absolute element concentrations with Na as a standard. The results were coupled with the microthemometry data to estimate the P-T conditions of the brine + vapor entrapment. (Seo et al., 2009)

Sulfur quantification in fluid inclusions was done by using two different ICP-MS instruments, a sector-field MS and the quadrupole MS, on homogeneous inclusions with similar salinities (42.4” 1.2 NaCl equiv.wt%). The size of the inclusions being analyzed can inhibit the ability to detect sulfur. The results of the quantification are such that the dominant components of the coexisting brine-vapor inclusions are NaCl, KCl, FeCl2, Cu and S. The concentrations of Cu to S are very similar and follow the same trend as seen in Figure 11 when normalized to Na (the dominant cation component). Figure 11 shows the correlation of S/Na to Cu/Na with a slope of 1 and mole ratio of 2:1, S:Cu. Figure 12 represents the fractionation behavior of how some elements prefer the brine and some prefer vapor. The elements are normalized to Pb which prefers brine and shows Au, Cu, and S are clearly correlated in their partitioning in to vapor. Figure 11 and 12 also indicates the significance of the environment in which the samples had formed. The Sn-W samples show the concentrations of Cu and S to prefer vapor where as in the porphyry Cu-Mo-Au samples show a Cu and S enrichment in the vapor phase relative to the salt components but the absolute concentrations in vapor are lower than in the brine. The overall combination of the two fluid phases in the porphyry Cu-Mo-Au are much higher in S, Cu, and Au than those in the Sn-W mineralizing fluids. The importance of sulfur and chloride as complexing agents in both of the fluid phases can be represented by the exchange equilibria:

Exchange equilibria (1) shows the preferred equilibria shift is towards Cu-S complexes in vapor and (2-4) shows stabilizing K, Na, and Fe as chloride complexes in brine. The main significance is Cu prefers stabilization in vapor with the addition of S. (Seo et al., 2009)
This means that salinity is not the main contributing factor to the formation of Cu deposits. S is now known to be important since the ”efficiency of copper extraction from the magma is determined by the sulfur concentration in the exsolving fluids’ (Seo et al., 2009). Magmatic sulfide melt inclusions have been observed and may have formed at the time of fluid saturation in the magma. Copper is precipitated out of brine and vapor as chalcopyrite (CuFeS2) and/or bornite (Cu5FeS4) once cooled. The Cu and S enriched vapor phase has the greatest contribution. (Seo et al., 2009)

Fig. 10(Next Page) Concentrations of sulfur and copper in natural magmatic’hydrothermal ‘uid inclusions. Co-genetic pairs of vapor+brine inclusions (‘boiling assemblages’) in high temperature hydrothermal veins from porphyry Cu’Au’Mo deposits (orange to red symbols), granite related Sn’W deposits (blue’green), and a barren granitoid (black’ gray)are shown. All vapor(a) and brine inclusions(b) have sulfur concentrations equal to copper or contain an excess of sulfur (the S: Cu=1: 1 line approximates a 2: 1 molar ratio). Element ratios (c), which are not influenced by uncertainties introduced by analytical calibration (Heinrich et al.,2003), show an even tighter correlation along and to the right of the molar 2:1 line, with Cu/Na as well as S/Na systematically higher in the vapor inclusions (open symbols) than in the brine inclusions (full symbols). Averagesof3’14single ‘uid inclusions in each assemblage from single healed fractures are plotted, with error bars of one standard deviation. Scale bars in the inclusion micrographs represent 50 ”m.(Seo et al., 2009)

Fig. 11(Above). Partitioning of elements between co-genetic vapor and brine inclusions. Fluid analyses including sulfur and gold are normalized to Pb, which is most strongly enriched in the saline brine (Seward, 1984). S, Cu, Au, As and sometimes Mo preferentially fractionate into the vapor relative to the main chloride salts of Pb, Fe, Cs, K and Na. A close correlation between the degrees of vapor fractionation of S,Cu and generally also Au indicates preferential sulfur complexation of these metals in the vapor. The two boxes distinguish assemblages in which absolute concentrations of Cu and S are higher or lower in vapor compared with brine. This grouping correlates with geological environment, i.e, the redox state and pH of the source magmas and the exsolving ‘uids. (Seo et al., 2009)

Conclusion:

Throughout the many years of research multiple types of analysis have been performed such as LA-ICP-MS, sector-field MS, microthermometry, quadrapole MS, and ID-TIMS. Zircon crystals were dated to provide ages of the magmatic system in which the ore deposits formed as well as help recognize multiple pulses can occur within the same system >1M.y. apart. Fluid inclusions have been examined in great detail to bring further insight in to the magmatic pulses. These pulses are critical to fluid-fluid mixing which in turn effects the precipitation of Sn, forming cassiterite, in Sn-W veins. There are however multiple different environments for deposits to form. Porphyry-epithermal Cu-Au-Mo deposits precipitate different elements. Vapor-liquid fractionation in the porphyry-epithermal system between coexisting brine and vapor is due to the increased transport of Cu and Au in sulfur-enriched acidic magmatic-hydrothermal vapors (Pokrovski et al., 2007).

The formation of magmatic-hydrothermal ore deposits was once thought to be mainly dependent on the salinity of the fluid, hypersaline or vapor. Salinity can be used to recognize an elements referential fluid. For example, Cu and Au prefer low salinity vapors as opposed to coexisting hypersaline fluid and elements such as Pb and Fe prefer hypersaline conditions (Williams-Jones and Heinrich, 2005). Salinity can also serve as a precipitation mechanism for Cu and Au into vapor phase however it has been discovered that reduced sulfur must be present. Fluid phase separation is critical for Cu and Au to partition in to the vapor phase which is aided by sulfur-enriched acidic magmatic-hydrothermal vapors. Sulfur is in turn essential for metal transport in fluids and increasing the solubility of Cu and Au. The low salinity Cu-Au-Mo rich vapor phase is greatest contributor to Cu-Au deposits. (Pokrovski et al., 2007)

References:

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