Intermediate-density Fluid Research Articles

Unravelling the effects of magmatic fractionation, fluid phase separation and dilution on the composition of magmatic-hydrothermal fluids of the Cornubian Batholith (SW England)

The compositional evolution of magmatic-hydrothermal fluids from the SnW mineralized Cornubian batholith was investigated via in situ fluid inclusion LA-ICP-MS microanalysis and reveals a large degree of variation between intrusive stages and at the sample scale, with complexities due to superposition of several mechanisms affecting fluids chemistry during the protracted evolution of the batholith. Despite large ranges of salinities (from <1 wt% to 48 wt% NaCleq.), the effects of individual processes such as magmatic differentiation, fluid saturation and boiling, fluid mixing and dilution can all be discerned on the basis of fluid inclusion trace element geochemistry combined with detailed petrography and microthermometry.In all samples, different fluid types (aqueous liquid-rich, vapor-rich and brine inclusions) were identified in variable proportions and microthermometry revealed complex temporal trends. Granite-related samples from the G5 intrusive stage are characterized by intermediate density fluids undergoing magmatic fractionation under pressure conditions above 1.5 kbar, indicated by successively decreasing fluid salinities with concomitant increases in Li, B, Rb and Cs concentrations. The latter two elements later strongly partitioned in the brine phase upon boiling, together with most of the other Cl-complexing elements (e.g., Fe, Mn, Pb, Zn). Those brines subsequently underwent progressive dilution with meteoric waters that caused a decrease in the concentration of all elements (besides B) and homogenization temperatures. In G3 granite from Dartmoor, on the other hand, brine inclusions represent the earliest fluid type while intermediate density fluids are absent, and a large salinity range in the fluid inclusion record associated with a decrease in homogenization temperatures represents their progressive dilution with meteoric fluid. The lack of intermediate density fluids at Dartmoor indicates fluid exsolution at relatively low pressures resulting in immediate phase separation into brine and vapor.All studied samples associated with ore mineralization display only the trend of dilution of high-salinity magmatic fluids. Their transition metal contents (e.g. Fe, Mn, Pb and Zn) show the expected positive correlation with salinity of the fluids. Conversely, the compositional trends observed for Sn and particularly W are more enigmatic, as they do not appear to significantly fractionate during most of the physicochemical processes outlined above and appear to be unrelated to fluid salinity, opposite of what would be expected for Sn as it is dominantly transported in Cl-complexes.This study highlights the key advantages of high-resolution geochemical fluid inclusion studies in discerning a variety of magmatic and post-magmatic processes in fluids in comparison to bulk fluid inclusion techniques or more simplistic fluid inclusions studies which might overlook important aspects of the typically complex history of fluids evolution in magmatic-hydrothermal systems.

Combined Focused Ion Beam – Scanning Electron Microscope and Synchrotron X-ray Fluorescence analysis of multi-solid and melt inclusions from the super-giant Grasberg Cu[sbnd]Au deposit, Indonesia

Previous studies of the mineralised quartz veins from the Grasberg CuAu deposit in Indonesia have shown that they contain multi-solid inclusions and silicate melt inclusions. The multi-solid inclusions exhibit salinities up to 75 wt% NaCl equiv. and ultrahigh homogenisation (Vapour + Liquid + Solid → Liquid) temperatures, up to 1281 °C, but the processes leading to their formation are poorly understood. To determine the composition of these inclusions they were analysed by Focused Ion Beam – Scanning Electron Microscope (FIB-SEM) and Synchrotron X-ray Fluorescence (SXRF) techniques. FIB-SEM analysis of two representative multi-solid inclusions (MSI) showed that they contain a large halite and a smaller sylvite crystal and mixed KFe chloride crystals. K-feldspar crystals occurring inside and outside the multi-solid inclusions indicate that they were trapped from a melt phase. The ore-mineral chalcopyrite was also identified in one inclusion. Opening the multi-solid inclusions under vacuum resulted in a coating enriched in Na, K, Fe and Cl that precipitated on the crystals and walls of the inclusions. FIB-SEM analysis of a coexisting silicate melt inclusion showed that it consisted mainly of a devitrified glass with approximately K-feldspar composition that also contained small crystals of iron oxide. Further milling of the melt inclusion exposed a CuFe-rich silicate mineral. SXRF analysis of another multi-solid inclusion indicated that Br and Zn occur in the aqueous phase whereas K, Ca, Fe, Pb and Mn are hosted by solid phases within the inclusion.Modelling of mineralisation processes demonstrate that a supercritical, intermediate density fluid (with ~5 wt% NaCl) underwent an initial pressure drop causing phase separation and the formation of a vapour-rich fluid and a small fraction of brine, which led to the formation of salt hydrates, salt melts or both within the MSI. The coexistence of silicate melt inclusions and the occurrence of hematite and, K-feldspar in MSI suggest that a coeval silicate melt is also associated with mineralisation at the Grasberg deposit. The high salinity, multi-solid inclusions and melt inclusions form early in the vein paragenesis and record the origin and initial processes of ore deposition. Furthermore, the high metal-bearing capacity of salt melts suggests that the observation of coexisting silicate and salt melts may indicate the presence of a fertile mineral system.

Hydrothermal fluid evolution in the Escondida porphyry copper deposit, northern Chile: evidence from SEM-CL imaging of quartz veins and LA-ICP-MS of fluid inclusions

The origin of hypogene alteration and mineralization features in the Escondida porphyry Cu deposit resulted from intense overprinting related to three main hydrothermal stages. The beginning of each stage is recorded by deposition of bright quartz crystals on vein walls that precipitated from high-temperature fluids. In the deepest zones of the deposit, the earliest stage started with exsolution of intermediate-density fluids, which transported high concentrations of Cu, whereas Mo was not detected. However, in the shallow zones, the early stage began with depressurization and unmixing of intermediate-density fluids, which generated both a hypersaline and vapour-rich fluid phase, producing an important geochemical segregation between both phases. The transitional stage also started with circulation of intermediate-density fluids but never experienced unmixing. These fluids transported the highest Mo concentrations in the deposit; however, Cu displays lower concentrations relative to intermediate-density fluids from the early stage. The beginning of the late stage was also associated with intermediate-density fluids; however, Cu and Mo were below the detection limits in most of the analysed fluids. During the evolution of the three stages, the fluids experienced gradual cooling, which promoted the precipitation of euhedral and zoned quartz crystals that overgrew the early high-luminescence quartz generations. Sometimes, sulfide minerals display euhedral crystal boundaries with zoned quartz, suggesting that mineralization started during these phases. The latest events detected in each main stage are linked to cooler and low-salinity fluids, from which dark quartz and hypogene sulfides precipitated along microfractures and interstitial spaces developed in the earlier quartz generations.

Are Vapor-Like Fluids Viable Ore Fluids for Cu-Au-Mo Porphyry Ore Formation?

Abstract Ore formation in porphyry Cu-Au-(Mo) systems involves the exsolution of metal-bearing fluids from magmas and the transport of the metals in magmatic-hydrothermal plumes that are subject to pressure fluctuations. Deposition of ore minerals occurs as a result of cooling and decompression of the hydrothermal fluids in partly overlapping ore shells. In this study, we address the role of vapor-like fluids in porphyry ore formation through numerical simulations of metal transport using the Gibbs energy minimization software, GEM-Selektor. The thermodynamic properties of the hydrated gaseous metallic species necessary for modeling metal solubility in fluids of moderate density (100–300 kg/m3) were derived from the results of experiments that investigated the solubility of metals in aqueous HCl- and H2S-bearing vapors. Metal transport and precipitation were simulated numerically as a function of temperature, pressure, and fluid composition (S, Cl, and redox). The simulated metal concentrations and ratios are compared to those observed in vapor-like and intermediate-density fluid inclusions from porphyry ore deposits, as well as gas condensates from active volcanoes. The thermodynamically predicted solubility of Cu, Au, Ag, and Mo decreases during isothermal decompression. At elevated pressure, the simulated metal solubility is similar to the metal content measured in vapor-like and intermediate-density fluid inclusions from porphyry deposits (at ~200–1,800 bar). At ambient pressure, the metal solubility approaches the metal content measured in gas condensates from active volcanoes (at ~1 bar), which is several orders of magnitude lower than that in the high-pressure environment. During isochoric cooling, the simulated solubility of Cu, Ag, and Mo decreases, whereas that of Au reaches a maximum between 35 ppb and 2.6 ppm depending on fluid density and composition. Similar observations are made from a compilation of vapor-like and intermediate-density fluid inclusion data showing that Cu, Ag, and Mo contents decrease with decreasing pressure and temperature. Increasing the Cl concentration of the simulated fluid promotes the solubility of Cu, Ag, and Au chloride species. Molybdenum solubility is highest under oxidizing conditions and low S content, and gold solubility is elevated at intermediate redox conditions and elevated S content. The S content of the vapor-like fluid strongly affects metal ratios. Thus, there is a decrease in the Cu/Au ratio as the S content increases from 0.1 to 1 wt %, whereas the opposite is the case for the Mo/Ag ratio; at S contents of &amp;gt;1 wt %, the Mo/Ag ratio also decreases. In summary, thermodynamic calculations based on experiments involving gaseous metallic species predict that vapor-like fluids may transport and efficiently precipitate metals in concentrations sufficient to form porphyry ore deposits. Finally, the fluid composition and pressure-temperature evolution paths of vapor-like and intermediate-density fluids have a strong effect on metal solubility in porphyry systems and potentially exert an important control on their metal ratios and zoning.

Fluid inclusion evidence for hydrothermal evolution of the Sadaigoumen porphyry Mo deposit on the northern margin of the north China Craton

The Sadaigoumen porphyry Mo deposit is located on the northern margin of the North China Craton (NCC). In this study, in situ fluid-inclusion analyses including microthermometry, laser Raman spectroscopy, and LA–ICP–MS microanalysis, as well as Quartz H–O isotopes are present to precisely constrain the hydrothermal evolution of the Sadaigoumen deposit. Five stages of hydrothermal activity are identified, with Stage 1 characterized by the development of magnetite with rare sulfide minerals; Stage 2 by K-feldspar as the typical vein mineral; Stage 3 by molybdenite mainly in veins; Stage 4 by the development of fluorite–quartz and quartz–pyrite veins; and Stage 5 by barren carbonate–quartz veins. Fluid inclusion types were distinguished based on petrographic observations and microthermometric analysis, and fluid compositions were detected by LA–ICP–MS. Microthermometric analyses indicate decreasing homogenization temperatures in primary fluid inclusions from Stage 1 to Stage 3. Intermediate-density fluids of stages 1 and 2 are characterized by high homogenization temperatures (420 °C–470 °C), moderate salinities (6.3–11.2 wt%), and high Mo contents (52–541 ppm). Hydrothermal evolution during Mo mineralization in Stage 3 is characterized by immiscible inclusion assemblages that formed through phase separation from intermediate-density fluid. Fluid inclusions of Stage 3 were trapped at pressures fluctuating around the two-phase fluid surface, leading to at least two fluid immiscible phases (373 °C–414 °C and 311 °C–341 °C). The Mo contents of fluid inclusions vary greatly in Stage 3, with Mo tending to be enriched in brine. In stages 4 and 5, fluids display characteristics that indicate mixing with meteoric water, with homogenization temperatures (<299 °C) and salinities (<5.8 wt%) decreasing together with Mo content. The Cs/(Na + K + Mn + Fe) ratios of fluid inclusions are stable from stages 1 to 4, indicating a constant supply of ore-forming fluids from an underlying, geochemically uniform magma chamber. The fluid inclusion data suggest that Mo and Cu in the initial exsolved fluids of the Sadaigoumen porphyry Mo deposit display different migration and precipitation progresses, which is of great significance to the study of the delicate ore-forming progress of porphyry Mo (–Cu) deposits in the northern NCC.

Evolution of the Magmatic-Hydrothermal System at the Santa Rita Porphyry Cu Deposit, New Mexico, USA: Importance of Intermediate-Density Fluids in Ore Formation

Abstract The Santa Rita porphyry Cu deposit in New Mexico, USA, is characterized by a stockwork of three vein types that differ in morphology, mineralogy, and associated alteration assemblages. Early quartz veins associated with potassic alteration are composed of recrystallized granular quartz grains that host ubiquitous hypersaline liquid and vapor-rich fluid inclusions. The early quartz likely formed at high (≳500°C) temperatures and lithostatic pressures. Hypogene Cu mineralization at Santa Rita is in sulfide veins that reopened or crosscut the early quartz veins. The sulfide veins are surrounded by alteration halos containing chlorite and K-feldspar. Rare quartz crystals are present in some of these chalcopyrite and pyrite veins. The cores of the quartz crystals contain hypersaline liquid and vapor-rich fluid inclusions, whereas the rims mostly contain hypersaline liquid inclusions. The quartz crystals are interpreted to have formed close to the ductile-brittle transition as a result of the pressure drop from lithostatic to hydrostatic conditions. Formation of the quartz crystals was postdated by the deposition of Cu sulfides. Grain boundaries between the quartz and the sulfide minerals are irregular in shape, with sulfide crosscutting growth zones in the quartz. The Cu sulfides are interpreted to have formed from intermediate-density fluids that form secondary fluid inclusion assemblages in all earlier-formed quartz types. Microthermometric investigations showed that these fluid inclusion assemblages homogenize at ~385° to 435°C by critical or near-critical behavior and have salinities of &amp;lt;10 wt % NaCl equiv. The precipitation of Cu sulfides occurred as a result of cooling of these fluids following their escape from the lithostatic into the hydrostatic realm. Retrograde quartz solubility caused the corrosion of earlier-formed quartz during Cu sulfide deposition. The youngest veins at Santa Rita are composed of quartz and pyrite. These veins are associated with intense sericite alteration that overprinted all earlier alteration assemblages. The late quartz hosts primary and secondary liquid-rich fluid inclusions, but no intermediate-density fluid inclusions were identified. This quartz vein type formed at temperatures &amp;lt;360°C and hydrostatic pressures. The paragenetic relationships show that hypogene Cu mineralization at Santa Rita postdated potassic alteration of the host rocks. The Cu mineralization was formed by cooling intermediate-density fluids with critical or near-critical densities as they escaped from lithostatic to hydrostatic conditions.

Understanding the evolution of magmatic-hydrothermal systems based on microtextural relationships, fluid inclusion petrography, and quartz solubility constraints: insights into the formation of the Yulong Cu-Mo porphyry deposit, eastern Tibetan Plateau, China

The evolution of the magmatic-hydrothermal system that formed the Eocene Yulong Cu-Mo porphyry deposit of the eastern Tibetan Plateau, China, is reconstructed based on the study of the sequence of stockwork vein sets showing systematic crosscutting relationships in hand specimen. Early A veins associated with potassic alteration of the porphyritic host are composed of vitreous quartz that is extensively recrystallized and has a dark blue cathodoluminescence. Fluid inclusions contained in the quartz have been affected by post-entrapment modification. Some of the A veins were reopened and contain ribbons of microscopic molybdenite flakes. Formation of the molybdenite was associated with recrystallization of the earlier formed quartz and concomitant with the development of a dark purple to brownish cathodoluminescence signature. Reopening of the early veins also resulted in the formation of AB veins containing euhedral quartz crystals that contain encapsulated molybdenite flakes and are intergrown with coarse laths of molybdenite. These euhedral quartz crystals showing a dark purple to brownish cathodoluminescence host vapor-rich, intermediate-density fluid inclusions as well as hypersaline liquid and vapor-rich fluid inclusions, indicating that fluid immiscibility at hydrostatic pressure conditions occurred during crystal growth. Later, C veins that crosscut and commonly reopen the earlier A and AB veins lack quartz as a gangue mineral and consist entirely of chalcopyrite and pyrite with minor molybdenite. Sulfide formation was accompanied by chlorite or sericite alteration of the wall rocks. Late D veins containing quartz and pyrite with associated sericitic alteration of the porphyritic wall rocks crosscut all earlier vein types. Only minor amounts of molybdenite and chalcopyrite are present. Euhedral quartz crystals in the D veins contain liquid-rich fluid inclusions that have not been affected by post-entrapment modification. The results of this study indicate that Mo and Cu mineralization at Yulong was introduced by single-phase, intermediate-density magmatic-hydrothermal fluids as the porphyry system was transitioning from ductile to brittle conditions. The bulk of the Mo endowment predated Cu mineralization and formed during the change from lithostatic to hydrostatic conditions and associated immiscibility of the magmatic-hydrothermal fluids. Chalcopyrite precipitation followed as the single-phase, intermediate-density magmatic-hydrothermal fluids was escaping into the hydrostatic-pressured regime. Cooling of the magmatic-hydrothermal fluids appears to have been the principal control on metal precipitation.

Molybdenum isotopic fractionation in the Tibetan Yulong Cu-Mo deposit and its implications for mechanisms of molybdenite precipitation in porphyry ore systems

Large variations in Mo isotopes (up to ~2‰ δ98Mo) of molybdenite from many porphyry ore deposits have been reported. However, the mechanisms of Mo isotopic fractionation and their implications for mineralization processes remain poorly understood. Here we report contrasting Mo isotopes of molybdenite formed in two discrete, successive mineralizing stages of the giant Yulong porphyry Cu-Mo deposit in eastern Tibet which, when combined with available fluid inclusion data and high-precision molybdenite Re-Os ages, provide a better understanding in the mechanisms of Mo isotopic fractionation and molybdenite precipitation. The initial ore-forming fluids of both mineralizing stages at Yulong exsolved from magmas as a single-phase, intermediate-density fluid, which subsequently separated into coexisting brine and vapor phases upon ascent and decompression. The relatively uniform Mo isotopes (δ98Mo = 0.03–0.18‰) of the early Cu-Mo stage is consistent with the precipitation of molybdenite from relatively stagnant, high-density brines at shallow levels after phase separation. In contrast, the large spread of Mo isotopes (δ98Mo = −0.16–0.60‰) of the transitional Mo stage requires a process similar to instantaneous Rayleigh fractionation and suggests molybdenite precipitation from rapidly ascending single-phase fluids before phase separation in deep parts of the deposit . These arguments imply that the amount of Mo precipitation from the single-phase fluids likely determines the degree of Mo isotopic fractionation. Further quantitative modeling results suggest that the relatively large Mo isotope variation of molybdenite in many porphyry ore deposits may be simply explained by the precipitation of significant amount of Mo from the initial single-phase fluids. Therefore, apart from the precipitation of molybdenite with economic importance from metal-enriched brines that condensed from the single-phase fluids, the direct precipitation of molybdenite from the single-phase fluids may represent another significant mechanism of molybdenite precipitation in porphyry ore systems.

Melt and fluid evolution in an upper-crustal magma reservoir, preserved by inclusions in juvenile clasts from the Kos Plateau Tuff, Aegean Arc, Greece

Pre- to syn-eruptive fluids released by the magmatic system forming the non-welded rhyolitic Kos Plateau Tuff ignimbrite, Greece, were characterized using fluid inclusions entrapped in quartz from granitic clasts entrained from the roof or walls of the feeding magma reservoir. This fluid was initially of intermediate density and relatively low salinity (3–11 wt% NaClequivalent). Following significant pressure drop(s), it separated into immiscible brine and CO2-bearing vapor occurring as texturally less mature inclusions. The transition from single- to two-phase-fluid occurred at magmatic conditions, as supported by high fluid inclusion homogenization temperatures (ca. 700 °C) and simultaneous trapping of fluid inclusions with silicate melt inclusions. The large recorded pressure drop together with texturally immature fluid inclusion shapes suggests a possible direct relation between the fluid phase change and the incipient 161 ka caldera-forming eruption. All inclusion types occur predominantly along (pseudo-) secondary trails in the granitic clasts, indicating efficient entrapment in microfractures formed at high crystallinity, as attested to by elevated concentrations of incompatible elements in the silicate melt inclusions.Fast quenching of the inclusions due to eruption prevented significant post-entrapment chemical modification, providing pristine samples of magmatic fluid compositions. Element concentrations in co-existing fluids and silicate melt were analyzed using laser ablation inductively-coupled plasma mass spectrometry and compared in view of element transporting capabilities and magmatic-hydrothermal ore formation. Economically important metals such as Cu, Zn, Mo, and W were extracted from the silicate melt by the fluid. In particular, Cu concentrations in the single-phase fluid in the range of 100–500 ppm agree well with previous experimental and modeling studies and, therefore, represent values expected for primary magmatic fluids. Most elements got further enriched in the brine upon phase separation, except Li, B, and As, which may have contributed significantly also to the vapor phase. Considering the similarity between vapor and intermediate-density fluid in terms of salinity, however, the metal-enriched brine phase only comprised a minor fraction of the bulk fluid.

Cyclic Dilution of Magmatic Metal-Rich Hypersaline Fluids by Magmatic Low-Salinity Fluid: A Major Process Generating the Giant Epithermal Polymetallic Deposit of Cerro de Pasco, Peru

The giant mid-Miocene Cerro de Pasco Cordilleran polymetallic (Zn-Pb-Ag-Cu-Bi) deposit in central Peru formed during three successive mineralization stages resulting in low- to high-sulfidation mineral associations emplaced at a paleodepth from <500 to 1,500 m: (1) pyrrhotite pipes grading outward to sphalerite and galena replacement bodies (stage A), (2) deep quartz-pyrite veins (stage B1) and a funnel-shaped massive replacement body of pyrite-quartz (stage B2) with quartz-sericite ± kaolinite alteration, and (3) well-zoned Zn-Pb-(Bi-Ag- Cu) carbonate-replacement orebodies (stage C1) and E-W–trending Cu-Ag-(Au-Zn-Pb) enargite-pyrite veins (stage C2); stages C1 and C2 are accompanied by advanced argillic alteration. Field evidence indicates that the epithermal polymetallic mineralization has formed in the shallow part of a porphyry system. A detailed microthermometric and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS) study of fluid inclusions hosted in gangue and, using near-infrared microscopy, opaque ore minerals from the different mineralization stages as well as available bulk and in situ secondary ion mass spectrometry (SIMS) stable isotopic data allow reconstruction of the evolution and tracing of the source(s) of the mineralizing fluids. Fluid inclusions are two phase (L + V), homogenize to liquid at moderate temperatures (~220°–280°C in stages A, B1, B2, and C2 and down to 150°C in stage C1), and show a wide salinity range (1.2–19 wt % NaCl equiv) with no evidence of phase separation. Fluids during mineralization stages A, B1, B2, and C1 are the result of mixing between a moderate-salinity metal-rich magmatic fluid and a low-salinity fluid at the (shallow) site of deposition. The moderate-salinity metal-rich magmatic fluid cannot be interpreted as an evolved intermediate-density fluid or its boiling product due to its salinity (up to 19 wt % NaCl equiv), its high base metal concentration (>1 wt % Mn, Fe, Zn, and Pb), and its high Li, B, As, and Sb contents (up to several thousands of ppm). The obtained results suggest that the moderate-salinity metal-rich magmatic fluid results from mixing at depth between metal-rich hypersaline fluids and low-salinity magmatic fluids exsolved late in the lifetime of the magmatic-hydrothermal system. The moderate-salinity metal-rich magmatic fluid resulting from this deep mixing rose to the epithermal environment, where it in turn mixed with low-salinity fluids that were stored below the paleowater table and had similar temperatures to the moderate-salinity fluid. In contrast, enargite-pyrite veins of stage C2 were formed by the ascent of CO2-bearing, contracted vapor-like fluids that subsequently mixed with cold meteoric water. No interaction with the moderate-salinity, metal-rich magmatic fluids has been registered in stage C2. The similarity between fluid compositions and evolution during stages A, B1, B2, and C1 contrasts with their significantly different mineral assemblages that are rather controlled by changing fO2, pH, fS2, and temperature. Trace element LA-ICP-MS analyses of sphalerite, pyrite, and enargite also reveal important compositional differences. The trace elements in the measured minerals reside to a significant extent in micro- to nanoscale solid sulfide and/or sulfosalt inclusions. A direct correlation between fluid composition recorded in the studied fluid inclusion assemblages and the measured trace element composition of sphalerite, pyrite, and enargite has not been found in most cases. The cyclic rise of metal-rich, moderately saline fluids that issued from dilution of hypersaline fluids stored at depth by low-salinity magmatic fluids is a major process in the formation of stages A, B1, B2, and C1 of the giant epithermal polymetallic deposit of Cerro de Pasco. Such a mechanism may also explain moderate-salinity fluids (≈20 wt % NaCl equiv) recorded in other magmatic-related polymetallic epithermal deposits worldwide.

Nature and evolution of fluid inclusions in the Cenozoic Beiya gold deposit, SW China

The Beiya Au deposit in the Sanjiang metallogenic belt, SW China, is a porphyry-skarn deposit that formed in an intracontinental setting. Three stages of hydrothermal veins, namely quartz veins (Stage 1), quartz-pyrite veins (Stage 2), and Au-rich sulfide-quartz veins (Stage 3) were identified on the basis of field relationship and petrography under scanning electron microscopy cathodoluminescence (SEM-CL). Intermediate density aqueous fluid inclusions (ID1, ID2), high density brine inclusions (B1, B2), and low density vapor inclusions (V1, V2) have been identified in both Stage 1 and Stage 2 veins. Only liquid-rich fluid inclusions (L3) were found in Stage 3 veins. Raman spectroscopy revealed that all the inclusions contain abundant CO2. H2S was identified in ID2 and V2 inclusions. Salinities and homogenization temperatures of the intermediate density aqueous fluid inclusions (ID1, ID2), high density brine inclusions (B1, B2), and low density vapor inclusions (V1, V2) are 7.4 ± 1.1–11.9 ± 2.2, 33.6 ± 0.5–39.2 ± 1, and 1.6 ± 0.7–5.8 ± 0.8 (wt% NaCl equiv.) and 400 ± 35–440 ± 37, 395 ± 20–414 ± 6, and 386 ± 5–416 ± 4 °C, respectively. Salinities and homogenization temperatures of the L3 inclusions range from 4.9 ± 1.6 to 9.8 ± 0.7 (wt% NaCl equiv) and from 301 ± 28 to 398 ± 4 °C, respectively. Initial intermediate density aqueous fluid inclusions contain 2000–10,000 ppm Fe, 2000–4000 ppm Cu, 200–600 ppm Zn, 200–500 ppm Pb, 17–89 ppm Mo, 1–8 ppm Ag, and 0.9–5.9 ppm Au, comparable to that of intermediate density inclusions in subduction-related porphyry deposits. Brine inclusions are rich in metals and have 9000–24600 ppm Fe, 3400–11,000 ppm Cu, 1200–5200 ppm Pb, 500–3000 ppm Zn, hundreds of ppm Mo, and tens of ppm Ag. The corresponding vapor inclusions contain lower metal concentrations: 400–6000 ppm Fe, 1700–4700 ppm Cu, 50–270 ppm Pb, 100–500 ppm Zn, and minor Mo and Ag. Low to intermediate density fluids are abundant and are inferred to have been the main metal transporting medium. A temperature decrease to approximately 350 °C during Stage 3 triggered metal precipitation.

A fluid inclusion and stable isotope study of the Pebble porphyry copper-gold-molybdenum deposit, Alaska

The hydrothermal fluid evolution in the supergiant Pebble porphyry Cu-Au-Mo deposit in southwest Alaska has been constrained using fluid inclusion and light stable isotope data. The deposit is related to ~90Ma granodiorite porphyry intrusions and mineralization occurs primarily in sodic-potassic, potassic, and advanced argillic alteration zones.Throughout the deposit vein quartz hosts intermediate-density fluid inclusion assemblages with an average salinity of 9.5wt% NaCl equivalent and homogenization temperatures of 377 to 506°C. Pressure corrections result in trapping temperatures with a maximum of 610°C. This early fluid exsolved from a crystallizing magma at depth and underwent phase separation under varying conditions during cooling and depressurization forming two distinct fluid compositions. The two fluids formed two hydrothermal alteration assemblages which have different metal signatures. In the hydrothermal core in the eastern part of the system, high-density quartz veins, potassic alteration and the most economically significant mineralization are related to ~52wt% NaCl equivalent brines that coexist with very low-density vapor. In the western part of the system peripheral to the hydrothermal core, lower quartz vein densities accompanied by sodic-potassic alteration and lower grade mineralization are associated with ~38wt% NaCl equivalent brines and low-density vapor. Sulfide mineralization precipitated late in this early alteration event at temperatures between 375 and 330°C.Fluids related to sodic-potassic alteration minerals have δ18O signatures from +6.0 to +10.5‰ VSMOW and δD values from −70 to −41‰ VSMOW, which are consistent with a magmatic fluid source. Fluids related to potassic alteration assemblages have δ18O of −1.7 to +2.3‰ VSMOW and δD of −68 to −59‰ VSMOW, which suggests that the magmatic fluids evolved to lighter oxygen isotope signatures due to quartz precipitation during alteration.Lower temperature (~280°C) sericite and illite alteration followed the high-temperature magmatic stage and formed from the rising vapor plume as it mixed with varying amounts of meteoric fluid. Sericite in quartz-sericite-pyrite alteration on the periphery of the deposit and illite from areas of illite±kaolinite alteration which overprints sodic-potassic alteration and forms quartz-illite-pyrite alteration within the deposit are related to low-salinity aqueous fluids that have a significant magmatic component (+2.1 to +4.1‰ δ18O and −76 to −67‰ δD). Illite alteration which overprints both potassic alteration and quartz-sericite-pyrite alteration formed from fluids with a large meteoric component (−9.1 to −4.8‰ δ18O and −101 to −90‰ δD). The higher vein density and therefore higher permeability in the core of the deposit facilitated ingress of larger amounts of meteoric fluid compared with the more peripheral region.Advanced argillic alteration and associated high-grade mineralization are related to a second pulse of magmatic fluid which did not intersect the two phase field during cooling and depressurization. The fluid contracted to form a 3wt% NaCl equivalent aqueous fluid that resulted in a sericite-pyrite-bornite-digenite bearing assemblage with a magmatic signature (4.6 to 6.5‰ δ18O and −108 to −102‰ δD) at 340°C. Cooling and mixing of this fluid with meteoric water resulted in a fluid that formed a pyrophyllite-quartz-sericite-chalcopyrite bearing assemblage (−8.9 to −0.6‰ δ18O and −110 to −96‰ δD) at 300°C. Compared to older potassic and sodic-potassic alteration, the advanced argillic alteration has a highly depleted δD signature which is attributed to degassing of a crystallizing intrusive fluid source.

Zoned Base Metal Mineralization in a Porphyry System: Origin and Evolution of Mineralizing Fluids in the Morococha District, Peru

Porphyry-related base metal vein and replacement mineralization (i.e., Cordilleran polymetallic mineralization) in the Morococha district, central Peru, is part of a large magmatic-hydrothermal system associated with the emplacement of several late Miocene porphyry intrusions and formation of important Cu-Mo mineralization. Zn-Pb-Ag-Cu veins overprint the giant Toromocho porphyry Cu-Mo deposit in the center of the district and display a typical concentric base metal zonation (Cu → Zn, Pb → Ag) covering an area of approximately 50km2. A detailed fluid inclusion study supports the hypothesis that base metal mineralization precipitated from cooled and evolved metal-rich, intermediate-density porphyry-type fluids. In early stages of Cordilleran base metal vein formation, fluid inclusions have low salinities of ~2 to 5 wt % NaCl equiv, CO2 contents of 3 to 10mol %, and homogenization temperatures (Th) of 380° to 340°C. They are similar to intermediate-density fluid inclusions trapped in a milky quartz vein predating Cordilleran polymetallic mineralization, with similar low salinities (3.0–3.8 wt % NaCl equiv) and low CO2 contents (6.5–8 mol %), but higher Th of ~420° to 410°C. During cooling of the intermediate-density fluids from 400° to 300°C, the lithostatic pressure regime changed to a hydrostatic one. The fluids underwent pressure drop as well as phase separation (i.e., unmixing) and lost most of their CO2. They acquired moderate salinities, in some cases intermediate (~up to 16 wt % NaCl) to brine compositions. However, the bulk of the magmatic fluid retained low salinity while it continued to cool under open-system conditions and precipitated tennantite-tetrahedrite, chalcopyrite, enargite, sphalerite, and galena. Upon cooling below 270°C, the fluids deposited abundant rhodochrosite and quartz, while following the boiling curve toward lower P-T conditions. These data record an evolution of mineral precipitation from deep (minimum depth of 2–1.5 km) to shallow environments (300–800 m). Oxygen, hydrogen, and carbon stable isotope data indicate that the hydrothermal fluids have a dominantly magmatic signature and were diluted by meteoric waters during the carbonate stage. Copper (5,000–18,000 μ g/g), sulfur (up to 12,000 μ g/g), and iron (2,100–6,000 μ g/g) concentrations in the intermediate-density fluid inclusions in the milky quartz veins are approximately 5 to 10 times higher than in intermediate-density inclusions of the early Cordilleran base metal veins. The base metals Zn, Pb, and Mn have comparable concentrations between 100 and 1,000 μ g/g for both types of fluid. These findings suggest that the fluids identified in Cordilleran polymetallic veins are compositionally similar to the porphyry-type fluids and could have derived from the latter after precipitation of Cu- and Fe-bearing sulfides in a deeper porphyry environment. The new data explain the commonly observed base and precious metal zonation patterns encountered in porphyry-centered districts (e.g., Bingham, Butte) and show that both porphyry and polymetallic mineralization can precipitate from similar magmatic-hydrothermal fluid pulses.

Melt and Fluid Inclusions in Hydrothermal Veins: The Magmatic to Hydrothermal Evolution of the Elatsite Porphyry Cu-Au Deposit, Bulgaria

The magmatic to hydrothermal transition in the Late Cretaceous Elatsite porphyry Cu-Au-(Mo-platinum group element) deposit has been studied in a suite of samples with clear timing relations between porphyry dikes, magmatic-hydrothermal veins, silicate melt inclusions in quartz veins, fluid inclusion generations, and ore minerals. Ore mineralization occurs late, at temperatures ~200° to 300°C below those of the early, multistage interplay between magmatic and hydrothermal processes at near-magmatic temperature-pressure conditions. Crosscutting relations and petrography indicate that shortly after the intrusion of the earliest, monzodioritic dikes, fluids precipitated a first generation of granular quartz veins accompanied by potassic alteration. The second vein generation with crystalline quartz textures and K-feldspar alteration halos formed at the same time as the second, granodioritic pulse of porphyry intrusions. Cathodoluminescence imaging of quartz growth textures reveals that the earliest fluid inclusions in the crystalline quartz veins are of intermediate density and ~8 wt % NaCl equiv salinity, probably trapped at near-lithostatic pressures of ~1,200 to 1,300 bars at near-magmatic temperatures (≤730°C), and implies a depth of ~4 to 5 km. Depressurization led to phase separation, indicated by a first generation of coexisting brine and vapor inclusions trapped at temperatures of ≥640°C and suprahydrostatic pressures of ≥920 bars. A second quartz generation in crystalline quartz veins precipitated during progressive depressurization and hosts assemblages of coexisting brine, vapor, and silicate melt inclusions, trapped at temperatures in excess of 600°C and suprahydrostatic pressures of 630 to 880 bars. Some open-spaced quartz veins were filled with aplite during this stage. Field relations and geochemical evidence suggest that the aplites as well as the silicate melt inclusions in hydrothermal quartz veins represent volumetrically minor residual melts that evolved directly from granodiorite porphyries at the level of deposit formation, and do not represent aliquots of metal-supplying magma at depth. Fluid inclusions coexisting with the silicate melt inclusions are metal rich, but these fluids predate sulfide precipitation and are, therefore, not the dominant fluids responsible for the Cu-Au mineralization at Elatsite. Melt-fluid-metal separation processes recorded in these co-trapped silicate melt and fluid inclusions in vein quartz are small-scale, local phenomena and do not appear to be suited for understanding and quantifying metal segregation in the deeper source. Microthermometry data of fluid inclusions, trapped during the later stages of the formation of the second quartz generation, show further depressurization to ~260 to 325 bars and cooling to ~460°C, representing a hot hydrostatic regime. Bornite, chalcopyrite, and magnetite seem to be slightly later precipitated at temperatures ≤460°C in separate veins or opened spaces of preexisting veins. Dissolution textures of the second quartz generation indicate a subsequent local redissolution of vein quartz, most likely as a result of passing through a window of retrograde quartz solubility upon further cooling below 460°C. The next, economically most important chalcopyrite-pyrite stage is largely devoid of quartz precipitation and is mostly expressed as “paint veins.” Due to the lack of fluid inclusions, the temperature-pressure conditions of formation of these veins could not be constrained. The waning phase of hydrothermal activity is represented by a quartz-carbonate-zeolite stage, formed at low temperature (~145°C), as indicated by microthermometry of fluid inclusions trapped in quartz from this stage. Laser ablation-inductively coupled plasma-mass spectrometry analyses of fluid inclusions show very high Cu contents in early intermediate-density fluid inclusions and in the first vapor inclusion generation, followed by a drastic decrease in the second generation of vapor inclusions. The decrease correlates with the appearance of anhydrite inclusions in the second quartz generation and may indicate that a lack of sulfur in these later vapor inclusions led to Cu partitioning into the brine. Alternatively, it is also possible that, unlike early vapor inclusions, the later vapor inclusions were not susceptible to postentrapment copper enrichment, in accordance with recent experiments. A progressive Cu enrichment in the brine phase correlates well with depressurization prior to mineralization. Mass balance considerations based on analyzed fluid components and fluid phase relations indicate that brine was volumetrically minor and therefore likely stagnant, but this may have prepared ore precipitation by accumulating Cu stripped from ascending vapor.

An experimental study of the solubility of MoO3 in aqueous vapour and low to intermediate density supercritical fluids

The solubility of MoO3 in HCl-bearing water vapour and vapour-like aqueous fluids having a density between 0.005 and 0.343gcm−3 was determined experimentally for temperatures between 300 and 500°C. Molybdenum concentration in the quenched experimental condensates ranged from 3 to 481ppm. The corresponding molybdenum fugacity increased exponentially with increasing water fugacity, indicating that the dissolution of MoO3 proceeded via the reaction:MoO3,s+yH2Og=MoO3(H2O)yKs1,yand that the hydration number (y) increases with increasing water fugacity. A set of equilibrium constants (Ks1,y) was calculated from the experimental results and fitted to a linear function involving the reciprocal of temperature for values of y between two and eight, and to a polynomial function for a hydration number of one. Modelling of an H–Cl–S–O-bearing intermediate density supercritical fluid shows that molybdenite solubility is increasingly sensitive with decreasing temperature to changes in oxygen fugacity due to fluid–rock interaction. In a H2S–SO2 gas-buffered, intermediate density fluid containing 1wt% S, the molybdenum concentration decreases with decreasing temperature and pressure from 89 to 0.07ppm. Values of MoO3 solubility calculated using the thermodynamic data reported in this study for temperatures and pressures above the range considered in our experiments agree well with those determined experimentally by other researchers. Moreover, the predicted Mo concentrations are similar to those measured in vapour and intermediate density fluid inclusions from Cu–Mo and Mo porphyry deposits.

Separation of Molybdenum and Copper in Porphyry Deposits: The Roles of Sulfur, Redox, and pH in Ore Mineral Deposition at Bingham Canyon

The giant Bingham Canyon porphyry Cu-Mo-Au deposit (Utah) is associated with Eocene subvolcanic intrusions. It shows a distinct metal zonation above a barren core, with dominantly shallow Cu-Au mineralization (Cu stage) following the early quartz monzonite porphyry (QMP) intrusion, and spatially deeper Mo mineralization (Mo stage) occurring in a separate vein set exclusively after a late quartz latite porphyry (QLP) intrusion that truncates earlier Cu-Au veins. To understand this metal separation and the geochemical process of molybdenite mineralization, we investigated fluid inclusions by microthermometry, Raman spectroscopy, and laser ablation inductively couple plasma mass spectrometry (LA-ICP-MS) microanalysis in low- and high-grade quartz veins of both mineralization stages. In deep, low-grade quartz veins interpreted to represent the root zone of the Cu stage we found high concentrations of Cu, S, and Mo in the fluid inclusions, whereas in low-grade Mo-stage veins, we found lower Cu, but similar concentrations of S and Mo, compared to the inferred input fluids to the Cu stage. Sulfur and copper concentrations were similar in intermediate-density-type fluid inclusions in deep low-grade Cu-stage samples, whereas intermediate-density-type inclusions in low-grade Mo-stage veins have S contents that exceed their Cu contents. In high-grade Mo-stage vein, we found large variations of Mo concentrations in coexisting brine and vapor inclusions. Compared to the P-T conditions of the Cu precipitation stage (90–260 bars and 320°–430°C), the Mo-precipitating fluids were trapped at higher pressures and temperatures of 140 to 710 bars and 360° to 580°C. Mass-balance calculation based on the compositions of intermediate-density inclusions and brine + vapor assemblages, interpreted to be derived by phase separation during decompression of the ascending single-phase intermediate-density fluid, indicate that the mass of vapor phase exceeded that of brine by about 9:1 in both mineralization stages. Combining this mass balance with the analyzed vapor/brine partitioning data indicates that more than 70% of Mo and S (by mass) in the deposit were deposited from the vapor phase. Earlier Cu-Au deposition was similarly dominated by vapor, but recently published data about postentrapment Cu diffusion in and out of fluid inclusions cast doubt on previous quantifications, suggesting that almost none of the copper was deposited by brine. Mo is less likely to be modified by selective diffusion, and high Mo contents (max 0.0054 Mo/Na in intermediate density; 380 μ g/g Mo in brine) in the hydrothermal fluids were maintained from the early Cu stage to the late Mo stage. This indicates that Mo concentration was not the decisive factor for separate precipitation of late Mo ore at Bingham Canyon. Instead, the metal separation may be explained by a reduction in redox potential and an increase in acidity in the evolving source region of the fluids, i.e., a large subvolcanic magma reservoir. This is indicated by the stoichiometry of chalcopyrite and molybdenite precipitation reactions, a tentative difference in the Fe/Mn ratio in fluids of both veining stages, incipient muscovite alteration along high-temperature molybdenite veins, and an increasing tendency for Mo to fractionate from brine to vapor. We suggest that the early Cu-stage fluids were slightly more oxidized and neutral, allowing Cu-Fe sulfides to saturate first, while molybdenite saturation was suppressed and Mo was lost from the early ore stage. By contrast during the later Mo stage, the fluids were more reduced and acidic, thereby allowing selective saturation of molybdenite as the first precipitating sulfide in the cooling and expanding two-phase fluid, consistent with textural observations. This interpretation may imply more generally that small differences in redox potential and acid/base balance of the magmatic source of porphyry-mineralizing systems may be decisive in the temporal and spatial separation of the two metals.

The Bingham Canyon Porphyry Cu-Mo-Au Deposit. III. Zoned Copper-Gold Ore Deposition by Magmatic Vapor Expansion

Fluid inclusion microthermometry and laser-ablation ICPMS microanalysis are combined with geological and textural observations to reconstruct the spatial and temporal evolution of magmatic fluids that formed the subvolcanic porphyry Cu-Au(-Mo) ore deposit at Bingham Canyon, Utah. The Bingham Canyon orebody is exposed over ~1.6 km vertically and has the shape of an inverted cup with distinct metal zoning. Fluid inclusions in the barren but highly veined and potassically altered deep center of the system have intermediate density (~0.6 g cm−3) and a salinity of ~7 wt percent NaCl equiv. They have subequal concentrations of Na, K, Fe, and Cu and contain minor CO2. The intermediate-density fluids were trapped as a single phase, mostly at >500°C and >800 bars. The Au-Cu-rich center near the top of the orebody contains low-density vapor inclusions (~0.2 g cm−3) coexisting with brine inclusions containing ~45 wt percent NaCl equiv. The vertical transition of different inclusion types indicates phase separation of the single-phase input fluid upon volume expansion associated with a pressure drop to 200 ± 100 bars. Mass-balance calculation based on all analyzed inclusion components indicates that the mass of the vapor phase exceeded that of the brine by ~9/1. The vapor contained Cu as its dominant cation (~1.5 wt %) and contributed about 95 percent of the total amount of copper transported to the base of the orebody. Bornite, chalcopyrite, and native gold were precipitated in a narrow temperature interval from 430° to 350°C, into secondary pore space created by local redissolution of vein quartz as a result of retrograde quartz solubility in the vapor-dominated fluid system. Intermediate-density fluid inclusions in the deepest parts of the peripheral copper ore zone have identical density and composition, including similar gold contents, as those in the deep center. Microthermometry and statistical estimation of phase proportions in the inclusions show that the vapor in the peripheral Cu-rich but Au-poor ore zone remained denser, and the separating brine was less saline (~36 wt % NaCl equiv), compared to vapor and brine in the central Au-Cu ore zone. This indicates that the peripheral fluids experienced a lower degree of phase separation, due to slightly higher fluid pressure at equivalent temperature, compared to more strongly expanding fluids in the center of the system. The systematic zoning of Au/Cu within the ore shell, despite compositionally similar input fluids, is interpreted to have resulted from slightly different pressure-temperature-density evolution paths of magmatic fluids. Copper was selectively precipitated in the peripheral ore zone, in contrast to complete coprecipitation of Au and Cu in the central upflow zone of the vapor plume. The formation of particularly rich Cu-Au ore in the center of the upward-expanding fluid plume is consistent with published experimental data, showing that the solubility of metals in hydrous vapor decreases sharply with falling pressure, due to destabilization of the hydration shell around metal complexes in expanding vapor. This interpretation supports the classic vapor plume model for porphyry copper ore formation but additionally emphasizes the role of sulfur-bearing complexes as a key chemical control on magmatic-hydrothermal metal transport and the deposition of Cu and Au in porphyry ores. Our interpretation of selective Cu ± Au precipitation as a function of vapor density can explain the more general observation that most gold-rich porphyry copper deposits are formed in shallow sub-volcanic environments, whereas deeper seated porphyry Cu-(Mo) deposits are generally gold poor.

Evolution of Magmatic Vapor to Gold-Rich Epithermal Liquid: The Porphyry to Epithermal Transition at Nevados de Famatina, Northwest Argentina

Fluid inclusions in the subeconomic porphyry Cu-Mo-Au system at Nevados de Famatina and closely associated high-sulfidation epithermal Cu-Au-(Ag-As-Sb-Te) veins at La Mejicana, northwest Argentina, were studied to reconstruct the evolution of hydrothermal fluids from their deep magmatic source to the shallow epithermal environment. Field geology, vein petrography, fluid inclusion microthermometry, and single-inclusion microanalysis by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) were combined to determine the evolution of pressure, temperature, and ore metal concentrations in the fluids. Cathodoluminescence imaging complements transmitted light petrography to constrain the successive stages of quartz formation and the entrapment sequence of the fluid inclusion populations. Aqueous liquid inclusions of ~5 wt percent NaCl equiv salinity trapped in quartz-sericite-pyrite (QSP) veins between 360° and 325°C contain unusually high concentrations of Cu (400–5,000 ppm), As (~200 ppm), Sb and Te (both up to 100 ppm), and Au (several ppm) along with other ore-forming elements. These veins are paragenetically transitional between subeconomic porphyry copper mineralization exposed in the valley floor, and high-sulfidation epithermal veins with high Au grades, which are preserved along a high ridge adjacent to the porphyry stock. Low-density vapor and hypersaline liquid (i.e., brine) inclusions were trapped in early-formed quartz of the porphyry stockwork veins, between 450° and >600°C. The brine contains lower Cu and Au concentrations than coexisting vapor inclusions and texturally even earlier inclusions of intermediate density that are trapped in phenocrysts and some stockwork veins; the latter may be equivalent to a parental fluid. The low- and intermediate-density fluids compositionally overlap in salinity with the aqueous liquids recorded by the later transitional quartz-sericite-pyrite veins. Fluid inclusions and geologic time relations indicate that the transitional quartz-sericite-pyrite veins were the channelways for metal-rich aqueous liquids that generated the high-sulfidation epithermal Cu-Au deposit. These mineralizing liquids are interpreted to have evolved by continuous density increase (“contraction”) from a low-salinity, S-rich magmatic fluid of low to intermediate density. This vaporlike fluid was produced by exsolution from a magma that existed at greater depth during the late stages of cooling of the magmatic-hydrothermal complex, and probably underwent minor brine separation prior to reaching the present level of exposure. Epithermal mineral precipitation occurred upon dilution of the low-salinity magmatic fluid with meteoric water, which entered the hydrothermal system as it was cooled and successively eroded during continued magmatic fluid ascent.

The dynamics of steady, partial‐depth intrusive gravity currents

Experiments of intrusive gravity currents generated by lock exchange offer insights into atmospheric and oceanic flows. However, whereas many previous investigations have considered the full‐depth’ lock exchange problem, in which the intermediate density fluid initially spans the entire channel depth, less is known about ‘partial‐depth’ releases, which represent a more appropriate analogue to environmental flows where the inceptive, localized interfacial mixing is relatively weak and/or the upper and lower ambient layers are of significant vertical expanse. Here, we consider this circumstance using a combination of experimental, (two‐dimensional) numerical and analytical techniques with a particular focus on equilibrium flow for which there is no auxiliary concentration or dilution of the active scalar, and the interface ahead of the intrusion remains approximately horizontal. In this case, the initial (steady) speed of propagation, U, can be well‐predicted by adapting a shallow‐water model for gravity currents that employs as its front condition the relationship of Benjamin (1968). When, in the initial state, the upper and lower halves of the density field are (stretched) mirror images of one another, the front travels at constant speed beyond 10 lock lengths, as was noted in the case of full‐depth lock releases by Sutherland et al. (2004) and Sutherland and Nault (2007). However, when this symmetry is broken either by altering the relative depth of either ambient layer or by changing the intrusion density, the flow begins to decelerate after travelling as few as three lock lengths.

Evolution and mixing of asymmetric Holmboe instabilities

When a stably stratified density interface is embedded in a region of strong velocity shear, hydrodynamic instabilities result. Here we generalize the stratified shear layer to allow an offset between the centre of the shear layer and the density interface. By including this asymmetry, and keeping the density interface thin with respect to the shear layer, the asymmetric Holmboe (AH) instability emerges. This study examines the evolution and mixing behaviour of AH instabilities, and compares the results to the well-known Kelvin–Helmholtz (KH) and Holmboe instabilities. This is done by performing a series of direct numerical simulations (DNS). The simulation results show that there are two different mixing mechanisms present. The first is a feature of KH instabilities and leads to the mixing and production of intermediate density fluid. The second mixing mechanism is found in AH and Holmboe instabilities and consists of regions of mixing and turbulence production that are located on one or both sides of the density interface. Since the Holmboe-type instabilities do not generate a large-scale overturning of the central isopycnal, the density interface is able to retain its identity throughout the mixing event. The amount of mixing that takes place is found to be strongly dependent on the degree of asymmetry in the flow.