Magmatic Component Research Articles

Petrogenesis of the St Peter Suite, southern Australia: Arc magmatism and Proterozoic crustal growth of the South Australian Craton

The Palaeoproterozoic ca. 1620–1608 Ma St Peter Suite, located in the central Gawler Craton, South Australia comprises mafic to felsic magmatic rocks. The intermediate to felsic calc-alkaline magmatic component, preserves geochemical characteristics reminiscent of subduction-related arc magmatism. The calc-alkaline magmas are enriched in large ion lithophile elements (LILE), depleted in Nb and Ti, and have moderately evolved to relatively juvenile initial Neodymium isotopic signatures ( ɛ Nd(1620): −0.8 to +3.7). These geochemical and isotopic characteristics are comparable to other documented Proterozoic subduction-related magmatic rock suites (e.g. those in Laurentia, Baltica, central and western Australia). We propose that the St Peter Suite preserves evidence of arc magmatism and relatively juvenile crustal growth on the southern margin of Australia. At ca. 1608 Ma, subduction ceased and migrated to the present-day north as a result of collision of the Archaean Gawler nuclei with the St Peter Suite magmatic arc terrane. This interpreted collision formed part of a broader margin incorporating the development of ca. 1590–1550 Ma arc rocks in the Musgrave Province in central southern Australia. This interpreted setting situates the voluminous ca. 1595–1575 Ma Gawler Range Volcanics and Hiltaba Suite in a back-arc environment, consistent with other Proterozoic granite–rhyolite terranes of the Laurentian Margin and Baltic Shield, which are interpreted to represent an inboard expression of subduction-related convergent margin processes.

Carbonate-replacement Pb–Zn–Ag ± Au mineralization in the Kamariza area, Lavrion, Greece: Mineralogy and thermochemical conditions of formation

Carbonate-replacement Pb–Zn–Ag ± Au deposits in the Kamariza area, Lavrion district, Attica, Greece, are genetically related to the emplacement of Miocene andesitic dikes within a rapidly extending continental back-arc basin, which formed during exhumation of the Attic-Cycladic Crystalline Belt. Replacement veins as well as chimneys and mantos of massive sulfides are the major orebody types with mantos grading into chimneys and veins. Ore minerals are similar among the various types of orebodies in the Kamariza area and consist of sulfides and sulfarsenides (pyrite, arsenopyrite, chalcopyrite, galena, sphalerite, gersdorffite, marcasite), native metals (Au and Bi), Sn-bearing phases (petrukite), sulfosalts and sulfbismuthites of Ag, Bi, Cu, Pb, As, Sb (tetrahedrite-group minerals, bournonite, boulangerite, stephanite, pyrargyrite, semseyite, enargite, bismuthinite, lillianite homologues, Cu-matildite, aikinite, Ag-aikinite, mummeite, emplectite, wittichenite). The elemental association of Bi, Au, and Ag is common. The assemblages gersdorffite-bismuthinite-native gold and native gold-native bismuth are evidence for a contribution of magmatic components to the hydrothermal system. A fluctuation in the sulfidation states of the ore fluid during the evolution of the Kamariza system is evident from the deposition of early arsenopyrite, as well as of enargite-luzonite and both low-Fe and Fe-rich sphalerite in the same samples. Microthermometry of fluid inclusion assemblages show that carbonate replacement mineralization was deposited from a warm to hot (100°C to 400°C), low to moderately saline (1.8 to 17.3 wt% NaCl equiv) fluid. Eutectic temperatures of fluid inclusions as low as −55°C suggest the presence of CaCl2 in addition to NaCl, in the ore fluid. The Kamariza deposit occurs distal to the Plaka granodiorite intrusion and the associated porphyry-Mo mineralization, but is likely to be genetically related to a granitoid buried at depth.

Looking into a volcanic area: An overview on the 350 m scientific drilling at Colli Albani (Rome, Italy)

A 350 m deep borehole was drilled in the Colli Albani volcanic district (Central Italy) in order to: understand the shallow crust structure beneath the volcanic complex; characterize the rock physical properties especially through in-situ measurements and, afterward, laboratory experiments; assess the local present-day stress field; install a broad-band seismometer at depth. The borehole is located adjacent to the western rim of the Tuscolano–Artemisio caldera, where several phenomena of unrest recently occurred. In 1989–90 a seismic swarm affected this area and a related uplift was recognized. In addition, high gas concentrations (mainly CO 2 and H 2S), in aquifers and soils, caused illnesses and casualties among inhabitants and animals in the past. We describe the investigations carried out at the drill site and the results achieved from data analysis. Wire-line drilling produced a complete stratigraphic record of the Quaternary volcanic units down to the Plio–Pleistocene sedimentary sequence and geophysical logs allowed a characterization of the rock physical properties. From a tectonic point of view, data provided by Dipmeter and Borehole Televiewer were used for investigations on the recent and present-day stress field and the results are compared to those available in the literature. In the volcanic units we recognized two main fracture systems, SW and NW dipping. Several faults intersecting the borehole show planes with oblique striae, indicating a prevalent strike–slip component of the movement. Finally, borehole breakout analysis defined an active stress field with a ∼ E–W oriented minimum horizontal component. At the end of the drilling, a blow-out occurred, due to pressurized fluids trapped into the sandy unit drilled in the last few meters of the hole. Sampling these fluids gave an additional value to the borehole, providing information about the deep volcanic circulation and its possible connection to a deep-seated magma chamber. The main results show water with a Na–HCO 3 chemistry and the highest salinity ever recognised in the area (Electrical Conductivity = 10.12 mS/cm). Stable O and H isotopes reveal a meteoric origin of water and the absence of tritium points out a long residence time in the aquifer. Emitted gas is CO 2-dominated, with N 2 as second most important component. Helium isotopic composition of the gas allows us to estimate a magmatic component ranging in the interval 40–50%, one of the highest in the Colli Albani. Carbon isotopes of CO 2 (− 0.53‰ vs. PDB) suggest that it could derive partly from a magmatic source and partly by the thermal decarbonation of the carbonatic basement.

Magmatic He distribution around Unzen volcano inferred from intensive investigation of helium isotopes in groundwater

The ascending magmatic component around Unzen volcano was investigated by intensive sampling and measurements of helium concentration and isotopic ratios in groundwaters including thermal waters (and gases). In the eastern part of the Unzen graben, even shallow groundwaters contain large amount of dissolved magmatic helium with high 3He/ 4He ratio, about 7 times higher than the atmospheric value. This value is almost identical as those in thermal water of this region (Shimabara Spa). In contrast, no significant amount of magmatic He was incorporated into the shallow groundwaters in other regions. The 3He/ 4He distribution among thermal waters inside the graben does not follow the trend observed in many active volcanoes of decreasing the ratio with increasing the distance from a conduit. The distribution of 3He/ 4He ratios around Unzen volcano reflects the difference in residence time in thermal water reservoir and/or difference in the fluxes of magmatic gas components in each region. The results of model calculation, which considered the evolution of 4He concentration and 3He/ 4He ratio in the reservoir, indicated that the residence times of Obama Spa (western side of the graben) and Unzen Spa (central part) were about 28,000 and 11,000 years, respectively, which are significantly higher than those of Shimabara Spa (eastern part). These values are highly variable according to several parameters, such as crustal He flux, aquifer thickness and porosity etc, and should be taken as a first order approximation. Observed 3He/ 4He ratios, however, reflect the qualitative difference of thermal water residence time as well as the magnitude of magmatic He fluxes among three regions. The magmatic 3He flux out from the thermal water reservoir of Shimabara Spa is about four times higher than those of Obama and Unzen Spas. All of these results are not inconsistent with the concept that magmatic gas ascends through the densely existing EW faults in the eastern part of the Unzen graben as well as a conduit.

Helium deficit in high‐3He/4He parent magmas: Predegassing fractionation, not a “helium paradox”

We evaluated He‐Ne‐Ar elemental compositions of mid‐ocean ridge basalt (MORB) glasses and basalt glasses from the Hawaiian hot spot (Loihi Seamount, Kilauea) to determine the (3He/22Ne)solar, 4He*/21Ne*, and 4He*/40Ar* MORB‐type and “plume”‐type end‐member compositions within a binary mixing model. Although the 3He/22Ne ratios of “plume” magma and MORB magma marginally agree with current estimates of solar compositions, the calculated 4He*/21Ne* ratios are lower than the theoretical production ratio and indicate that the “plume” magma component experienced a predegassing fractionation event. This fractionation event resulted in a deficit in both radiogenic 4He* and primordial 3He and accounts for the differences in 3He/22Ne, 4He*/21Ne*, and 4He*/40Ar* ratios and 3He concentrations for both mantle domains. This resolves an apparent contradiction previously termed “mantle He paradox,” though the derived 3He concentrations are also at odds with a primitive mantle concept that requires still higher concentrations. We propose that the primary fractionation of He/Ne and He/Ar ratios is related to a more compatible behavior of He relative to Ne, Ar during partial melting. One model is presented: at depths below a shallow MORB source region a rising mantle plume causes formation of low‐degree partial melts, which are deficient in He. During melt ascent, these “plume”‐derived melts mix with melts of the shallow (MORB‐type) mantle source that experienced a higher degree of melting and hence show no He deficit. Depending on the relative proportion of mixing the resulting oceanic basalts will encompass the whole observed range from Loihi‐type and MORB‐type noble gas isotope compositions.

Inter- and Intracrystalline Isotopic Disequilibria: Techniques and Applications

In the last ~20 years, we have seen a significant expansion of techniques related to geochemical and isotopic microsampling of materials in the earth sciences. From constraining pre-eruptive histories of flood basalt magmas to identifying the natal rivers of origin of anadromous fishes, these techniques have had significant impacts in a wide variety of scientific fields. Nowhere has the impact been greater than in identifying the sources, processes, and timing of processes involved in igneous magmatic systems. Both technique refinements and the development of new technologies have aided in advancing microsampling applications, thus allowing for a better understanding of the sources and mechanisms responsible for changing geochemical and isotopic signatures in natural systems. In this chapter, we focus on the techniques and technologies associated with radiogenic isotope microsampling and review applications of these techniques as utilized in scientific investigations. Isotope microsamping is a logical extension of earlier studies that evaluated individual components of magmas and magma systems, including melts and minerals. From the use of petrographic microscopes and the later introduction of the electron microprobe, the focus on internal chemical variations in melts and minerals is critical to assessing the petrogenetic histories of igneous rocks. Even today, these technologies are used to ensure that further trace element and isotopic analyses are undertaken in a textural and major element context. For trace elements and isotopes, early studies (e.g., Cortini and van Calsteren 1985) confirmed variations in the melt and mineral components of many igneous rocks but focused on mineral or glass separates. Potential information associated with isotopic variations retained by individual crystals or internal variations within individual crystals was lost. Later studies such as Geist et al. (1988) focused on isotope variations within single megacrysts to constrain mixing scenarios between basaltic, andesitic, and rhyolitic magmas, and Davidson et al. (1990) used …

The Miocene igneous rocks in the Basal Unit of Lavrion (SE Attica, Greece): petrology and geodynamic implications

AbstractThe Miocene igneous rocks in the Basal Unit of the Lavrion area form part of the granitoid province of the central Aegean. Undeformed, subvertical dykes of quartz-syenite to granodiorite and granite porphyries, and a little deformed but variably altered granodiorite stock intrude metamorphic rocks of the Basal Unit. A 9.4 ± 0.3 Ma K–Ar age on feldspar for a dyke rock provides a minimum age for the igneous activity in the Basal Unit. East–west orientation of porphyry dykes is indicative of a regional extensional stress field with roughly north–south direction. Substantial extension in the Basal Unit after granodiorite emplacement is evident from widespread quartz veining associated with hydrothermal alteration of the granodiorite and the occurrence of mineralized tension gashes cutting the hydrothermally altered hornfelses. Final emplacement of the Blueschist Unit over the Basal Unit by extensional detachment post-dates contact metamorphism of the rocks surrounding the granodiorite. Geochemical diagrams show a continuous range of compositions from the dykes to the granodiorite. Radiogenic isotope compositions are compatible with a common magmatic source for the two lithologies. Elemental variations, as well as the considerable geochemical similarity of the dyke rocks to the Hercynian paragneiss of the central Cyclades, indicate that crustal melts were significant components during the evolution of the igneous rocks with fractional crystallization as an important process during later stages of evolution. The granodiorite displays geochemical signatures indicative of a significant mafic mantle-derived magma component.

Noble gas and halogen constraints on regionally extensive mid-crustal Na–Ca metasomatism, the Proterozoic Eastern Mount Isa Block, Australia

Fluid inclusions in late-Isan quartz veins associated with regional Na–Ca alteration (albitisation), in the Mary Kathleen Fold Belt and the Cloncurry District of the Eastern Mt Isa Block, have been analysed for naturally occurring and neutron produced isotopes of Ar, Kr and Xe. The noble gases have been extracted using a thermal decrepitation procedure that enables partial deconvolution of the different fluid inclusion types, including variably saline aqueous, liquid carbon-dioxide and mixed aqueous-carbonic varieties. The variably saline (<5–65 wt%) aqueous fluid inclusions dominate and have 40Ar/ 36Ar values of less than 2700 in most of the samples from across the region. These fluid inclusions have extremely variable molar Br/Cl values of 0.3–4 × 10 −3 and I/Cl values of 0.2–35 × 10 −6, and the fluids are interpreted to represent sedimentary formation waters derived from the upper crust that have dissolved variable quantities of halite (or scapolite) to achieve their ultra-high salinity. Fluid inclusions in a sample from the Snake Creek anticline in the Cloncurry District have the highest 40Ar/ 36Ar value of ∼25,000 demonstrating a deep magmatic or metamorphic fluid origin in this case. A magmatic origin is favoured because this fluid is very similar to a fluid involved in Iron-Oxide-Copper-Gold (IOCG) mineralization at Ernest Henry that was also interpreted to have a magmatic origin, and because the aqueous fluid inclusions have Br/Cl of ∼1–2 × 10 −3 and I/Cl of ∼10 × 10 −6 that are similar to mantle-derived igneous rocks. Carbon-dioxide fluid inclusions dominate samples from the Knobby Quarry in the Mary Kathleen Fold Belt and have a maximum 40Ar/ 36Ar value of 6000–7000. These fluid inclusions are estimated to have a 36Ar concentration of 1–4 ppb, that is similar to the range determined for aqueous fluid inclusions in all the other samples. In addition, aqueous and carbonic fluid inclusions in all the samples have similar 129Xe/ 36Ar plus 84Kr/ 36Ar values that are unfractionated, and close to the air—Air Saturated Water (ASW) range. These data are interpreted to indicate an independent and dominantly metamorphic origin for CO 2, and the presence of mixed aqueous-carbonic fluid inclusions are attributed to fluid mixing and/or mingling, rather than fluid unmixing. However, the data do not preclude the presence of a minor magmatic CO 2 component in the samples from the Knobby Quarry. Fluid inclusions in most of the samples from the Mary Kathleen Fold Belt have higher Br/Cl and I/Cl values, higher 36Ar concentrations and lower maximum 40Ar/ 36Ar values than fluid inclusions in samples from the Cloncurry District. This suggests Na–Ca alteration in these different parts of the Eastern Mt Isa Block occurred independently. However, fluid inclusions associated with Na–Ca alteration in the Cloncurry District have a very similar composition to fluid inclusions in IOCG mineralization-stage quartz veins from Ernest Henry. These data are therefore compatible with a genetic relationship between regional Na–Ca alteration and IOCG mineralization in the Cloncurry District. In both cases the ultra-saline hydrothermal fluids had a dominant origin from sedimentary formation water, but are interpreted to contain a magmatic component sourced from the late-Isan Williams-Naraku Batholiths which may have driven fluid convection.

Isotopic Microsampling of Magmatic Rocks

Radiogenic isotope ratios can be used as a kind of petrogenetic DNA to identify the source components of magmas. Technical advances allowing us to measure isotopic compositions at the sub-crystal scale have led to the realisation that many magmatic rocks are isotopically heterogeneous. Crystals traditionally regarded as phenocrysts grown from the host magma have now been shown to be wholly or partly out of isotopic equilibrium with the glass or groundmass in which they are contained. Many of these crystals are likely to be recycled from earlier cumulates. Combining these fingerprinting techniques with the other approaches described in this issue offers an unprecedented opportunity to understand the processes and timescales through which magmas are assembled, differentiated and delivered to sites of eruption or emplacement.

Pb and other ore metals in modern seafloor tectonic environments: Evidence from melt inclusions

Many modern seafloor tectonic environments are host to hydrothermal systems and associated polymetallic sulfide deposits. Metal transport and precipitation are controlled by magmatic processes such as pre-eruptive degassing and the hydrothermal cycle. The original availability of Pb and other ore metals in a given setting is dependent on concentrations in the original magmatic source or additional enrichment processes. We have examined the Pb budget of melt inclusions from nine modern seafloor settings representing back-arcs, mid-ocean ridges and seamounts. Melt inclusions provide information on the characteristics of parental magmas, including insights into metal budgets. Trace element data in melt inclusions hosted in plagioclase, olivine and pyroxene were obtained by laser-ablation inductively-coupled mass-spectrometry. Results from back-arcs emphasize the impact of slab-subduction and dehydration processes on the chemical characteristics of generated magmas. Volatile- and fluid-mobile element-rich melt inclusions at Manus basin and Okinawa trough reflect a robust contribution of elements from the subducting slab as evidenced by relatively low Ce/Pb ratios. At Bransfield strait, on the other hand, melt inclusions are volatile poor, and fluid-mobile element ratios are similar to mid-ocean ridge values indicating little or no contribution from the slab. High Cu concentrations at Manus basin and Okinawa trough can be explained by fluxing of ferric iron from the subducting slab benefiting the production of sulfate over sulfide. Metal budgets for seamounts located on and nearby the axis of mid-ocean ridge segments appear to be independent of any input of mantle plume material. Results from the southern Explorer ridge (strong lower mantle influence, transitional- and enriched-MORBs), Pito and Axial seamounts (moderate lower mantle influence, transitional-MORBs) and a Foundation near-ridge seamount (little to no mantle influence, normal-MORB) show that, despite similar tectonic environments and varying contributions of mantle plume material, Cu, Zn and Pb values do not vary significantly between the enriched and non-enriched magma components of a given setting.

Three magmatic components in the 1973 eruption of Eldfell volcano, Iceland: Evidence from plagioclase crystal size distribution (CSD) and geochemistry

The 1973 eruption of Eldfell volcano, Iceland, appears to have been a short, simple event, but textural and geochemical evidence suggest that it may have had three different magmatic components. The first-erupted fissure magmas were chemically evolved, rich in plagioclase (∼ 18%) and had shallow, straight crystal size distribution (CSD) curves. The early lavas were less evolved chemically, had lower plagioclase contents (∼ 13%) and steeper, slightly concave up CSDs. The late lavas were chemically similar to the early lavas, but even richer in plagioclase than the initial magmas (∼ 24%) and had the steepest CSDs. There was no chemical evidence for plagioclase fractionation, but compositional diversity could be produced by clinopyroxene fractionation which must have occurred at depth. We propose that the eruption started with old, coarsened (Ostwald ripened) magma left over from a previous eruption, possibly that which produced Surtsey Island ten years earlier. The early flows may be mixtures of small amounts of this old magma with a new, low crystallinity, uncoarsened magma or a completely new magma. The late flows are another new magma from depth, chemically similar to the early flows, but which has grown plagioclase under increasing saturation (undercooling) perhaps during its ascent. All three magmatic components may have originated from the same parent, but had varying degrees of clinopyroxene fractionation, plagioclase nucleation and growth, and coarsening.

HERMIONE, EVOLUTION OF ATe-BEARING EPITHERMAL MINERALIZATION, ARGOLIS, HELLAS

The Cu-Te-bearing pyrite deposits of Hermione, Argolis are hosted in Miocenic ophiolites. The ophiolites are overlain by a shale-sandstone formation with intercalations of limestones and manganiferous sedimentary rocks. The ore deposits form irregular lenticular or stratiform ore bodies, and veins. These ore bodies are related to volcanic activity in an arc-related rift at the margins of a palaeocontinent. Late N- to NNE-trending, sinistral, milky quartz-pyrite-calcite veins cut the host ophiolites. Alteration haloes of quartz-calcite, albite-sericitechlorite, and chalcedony-epidote-clay minerals are developed in the lavas as concentric shells, or as envelops that parallel the quartz veins. The telluriumbearing mineralization is developed in two successive stages, characterized by the assemblages: pyrite-(pyrrhotite)-magnetite-chalcopyrite-sphalerite (Stage I) and galena-sphalerite-freibergite-marcasite-chalcocite (Stage II), followed by a supergene stage. The cobaltiferous pyrite-chalcopyrite geothermometer defined two ranges of last-equilibration temperatures: 220° to 250°Cfor Stage I, and 120° to 195°Cfor Stage II. The calculated δ18 Ο and SD compositions of the mineralizing fluids, at 200° and 250°C, reflect the dominance of a magmatic component. The calculated δ SH2S fluid values reveal a magmatic source for the sulphur, with minor contribution from submarine sediments, whereas tellurium is proposed to be derived from a mafic-ultramafic source.

Hydrothermal circulation at Mount St. Helens determined by self-potential measurements

The distribution of hydrothermal circulation within active volcanoes is of importance in identifying regions of hydrothermal alteration which may in turn control explosivity, slope stability and sector collapse. Self-potential measurements, indicative of fluid circulation, were made within the crater of Mount St. Helens in 2000 and 2001. A strong dipolar anomaly in the self-potential field was detected on the north face of the 1980–86 lava dome. This anomaly reaches a value of negative one volt on the lower flanks of the dome and reverses sign toward the dome summit. The anomaly pattern is believed to result from a combination of thermoelectric, electrokinetic, and fluid disruption effects within and surrounding the dome. Heat supplied from a cooling dacite magma very likely drives a shallow hydrothermal convection cell within the dome. The temporal stability of the SP field, low surface recharge rate, and magmatic component to fumarole condensates and thermal waters suggest the hydrothermal system is maintained by water vapor exsolved from the magma and modulated on short time scales by surface recharge.

Magmatic Fluids in the Breccia-Hosted Epithermal Au-Ag Deposit of Rosia Montana, Romania

The breccia-hosted epithermal Au-Ag deposit of Rosia Montana is located 7 km northeast of Abrud, in the northern part of the South Apuseni Mountains, Romania. Estimated total reserves of 214.91 million metric tons (Mt) of ore at 1.46 g/t Au and 6.9 g/t Ag (10.1 Moz of Au and 47.6 Moz of Ag) make Rosia Montana one of the largest gold deposits in Europe. At this location, Miocene calc-alkaline magmatic and hydrothermal activity was associated with local extensional tectonics within a strike-slip regime related to the indentation of the Adriatic microplate into the European plate during the Carpathian orogenesis. The host rocks of the magmatic complex consist of pre-Mesozoic metamorphosed continental crust covered by Cretaceous turbiditic sediment (flysch). Magmatic activity at Rosia Montana and its surroundings occurred in several pulses and lasted about 7 m.y. Rosia Montana is a breccia-hosted epithermal system related to strong phreatomagmatic activity due to the shallow emplacement of the Montana dacite. The Montana dacite intruded Miocene volcaniclastic material (volcaniclastic breccias) and crops out at Cetate and Cârnic Hills. Current mining is focused primarily on the Cetate open pit, which was mapped in detail, leading to the recognition of three distinct breccia bodies: the dacite breccia with a dominantly hydrothermal matrix, the gray polymict breccia with a greater proportion of sand-sized matrix support, and the black polymict breccia, which reached to the surface, contains carbonized tree trunks and has a dominantly barren clastic matrix. The hydrothermal alteration is pervasive. Adularia alteration with a phyllic overprint is ubiquitous; silicification and argillic alteration occur locally. Mineralization consists of quartz, adularia, carbonates (commonly Mn-rich), pyrite, Fe-poor sphalerite, galena, chalcopyrite, tetrahedrite, and native gold and occurs as disseminations, as well as in veins and filling vugs within the Montana dacite and the different breccias. The age of mineralization (12.85 ± 0.07 Ma) was determined by 40 Ar- 39 Ar dating on hydrothermal adularia crystals from vugs in the dacite breccia in the Cetate open pit. Microthermometric measurements of fluid inclusions in quartz phenocrysts from the Montana dacite revealed two fluid types that are absent from the hydrothermal breccia and must have been trapped at depth prior to dacite dome emplacement: brine inclusions (32–55 wt % NaCl equiv, homogenizing at T h > 460°C) and intermediate density fluids (4.9–15.6 wt % NaCl equiv, T h between 345°–430°C). Secondary aqueous fluid inclusion assemblages in the phenocrysts have salinities of 0.2 to 2.2 wt percent NaCl equiv and T h of 200° to 280°C. Fluid inclusion assemblages in hydrothermal quartz from breccias and veins have salinities of 0.2 to 3.4 wt percent NaCl equiv and T h from 200° to 270°C. The oxygen isotope composition of several zones of an ore-related epithermal quartz crystal indicate a very constant δ 18 O of 4.5 to 5.0 per mil for the mineralizing fluid, despite significant salinity and temperature variation over time. Following microthermometry, selected fluid inclusion assemblages were analyzed by laser ablation-inductively coupled-plasma mass spectrometry (LA-ICPMS). Despite systematic differences in salinity between phenocryst-hosted fluids trapped at depth and fluids from quartz in the epithermal breccias, all fluids have overlapping major and trace cation ratios, including identical Na/K/Rb/Sr/Cs/Ba. Consistent with the constant near-magmatic oxygen isotope composition of the hydrothermal fluids, these data strongly indicate a common magmatic component of these chemically conservative solutes in all fluids. Cu, Pb, Zn, and Mn show variations in concentration relative to the relatively non-reactive alkalis, reflecting the precipitation of sulfide minerals together with Au in the epithermal breccia, and possibly of Cu in an inferred subjacent porphyry environment. The magmatic-hydrothermal processes responsible for epithermal Au-Ag mineralization at Rosia Montana are, however, not directly related to the formation of the spatially associated porphyry Cu-Au deposit of Rosia Poieni, which occurred about 3 m.y. later.

The Camelback Zn-Pb-Cu Deposit: A Recent Discovery in the Bathurst Mining Camp, New Brunswick, Canada

Camelback is a small (≤200 000 tonnes), moderate grade (5%–7% Zn+Pb), volcanogenic massive sulfide deposit, which occurs within the Nepisiguit Falls Formation of the Ordovician Tetagouche Group. The host rocks are tuffaceous sedimentary rocks (Little Falls member) that overlie quartz-feldspar porphyritic tufflavas (Grand Falls member). The hanging-wall sequence comprises rhyolite of the Flat Landing Brook Formation, overlain by the Forty Mile Brook tholeiitic basalt. A dike of unaltered andesite was intersected beneath the massive sulfides, but was not found in the hanging-wall sequence in the vicinity of the deposit. The stratiform part of the deposit is made up of two, steeply south dipping, subparallel massive lenses that average approximately 4 m in thickness. Each lens is zoned, with an upper part consisting mainly of a pyrite and a basal part containing pyrite, sphalerite, and galena, with Ag values. The Au content in the massive sulfides is low (average = 52 ppb for 271 samples), but tends to be enriched in the massive pyrite near the top of each lens. Oxide facies iron formation is spatially associated with the massive sulfides, and has been traced up to 1800 m along strike. Footwall hydrothermal alteration is typical of VMS systems, and is characterized by depletion in K 2 O and Na 2 O, and enrichment in MgO and Fe 2 O 3 T with proximity to massive sulfides. The massive lenses are underlain by intensely chloritic, fine-grained, tuffaceous sedimentary rocks containing locally significant sulfide (chalcopyrite > pyrite > pyrrhotite) veins, which are interpreted to represent feeder zone mineralization. Unlike many VMS deposits, there is no evidence of silicification beneath the massive sulfides at the top of the feeder zone. The oxygen isotopic compositions (δ 18 O = −1.5‰ to −0.3‰) of hydrothermal chlorite coupled with sulfur isotopic compositions (δ 34 S ≈ 12‰) of the massive sulfides suggest that seawater was the dominant fluid in the hydrothermal system. Normalized rare earth element diagrams show flat Ce profiles and positive Eu anomalies, indicating that modified seawater was involved in ore formation; i.e., high-temperature (≥350°C), acidic, and reduced prior to entrainment into the hydrothermal cell. Although, seawater was the dominant fluid in the sulfide-forming system, δ 34 S values of stringer zone sulfides (8.5‰) coupled with elevated Sn (400 ppm) in one massive sulfide sample indicate that there was at least some magmatic component in the hydrothermal cell.

PH dependant isotope variations in arc-type geothermal waters: New insights into their origins

In arc-type geothermal systems along the circum-Pacific rim, low pH well discharges in the geothermal systems of Japan and the Philippines are more enriched in oxygen-18 or deuterium or both as compared to neutral pH waters found in the same systems. Isotope enrichment of both oxygen-18 and deuterium means higher percentage of magmatic vapour contribution to the well discharges due to the fact that un-neutralized magmatic water carries much abundant hydrogen ions to keep the water at a lower pH. This is consistent with the observation that acid geothermal waters usually occur near the volcanic heat source of a geothermal system, where the magmatic component tends to reach a higher percentage.

Interplinian explosive activity of Santorini volcano (Greece) during the past 150,000 years

The eruptive history of Santorini was dominated by twelve major Plinian eruptions. Six of these (Middle Pumice, Vourvoulos, Upper Scoriae 1, Upper Scoriae 2, Cape Riva and Minoan) occurred during the past ∼150 ky. This study focuses on the stratigraphy and geochemistry of the interplinian pyroclastic deposits between 145 ka (Middle Pumice) and 3.6 ka (Minoan). Five interplinian eruptive intervals were identified between these major eruptions and are numbered M8-M12. The Upper Scoriae 1/Upper Scoriae 2 (M10) and Upper Scoriae 2/Cape Riva (M11) intervals are divided into sub-intervals by voluminous interplinian lava formations, the Skaros and Andesites of Oia and Therasia lavas, respectively. Each interval lasted between ∼17 and ∼45 ky and contains pyroclastic deposits of numerous eruptions indicating considerable volcanic activity between the major Plinian events. Most of these interplinian eruptions produced scoria-fall and ash-fall deposits predominantly from subplinian-type eruptions. Surge deposits, resulting from phreatomagmatic activity, are predominant in the Upper Scoriae 1/Skaros sub-interval (M10a). The Andesites of Oia/Cape Riva sub-interval (M11b) is entirely composed of subplinian pumice-fall and ash deposits. Other subplinian pumice-fall deposits occur in the intervals Vourvoulos/Upper Scoriae 1 (M9) and Upper Scoriae 1/Upper Scoriae 2 (M10). The Upper Scoriae 1/Skaros sub-interval (M10a) belongs to one of the longest interplinian intervals (M10) that is characterized by the thickest eruptive units and the largest number of eruptions. A further prominent interplinian pyroclastic formation in northern Thera is the Megalo Vouno cinder cone deposit, which occurred in a time span between the Vourvoulos eruption and the formation of the Skaros lava shield. The interplinian deposits vary from 50 to 70 wt.% SiO 2 and are classified as basalt, basaltic andesite, andesite and dacite with a typical calc-alkaline affinity in accordance with the general characteristics of Santorini. They are dominated by a mafic magma component, exemplified by extensive scoria-fall deposits, which is generally absent or only a minor constituent of the silicic Plinian eruptions. Major and trace element compositional trends indicate a significant role of fractional crystallization in the evolution of the interplinian Santorini magmas, where repeated replenishment of primitive magma appears to have prevented magmatic differentiation towards highly silicic compositions that dominate the Plinian eruptions. Pauses in igneous activity, immediately prior to the Plinian and often caldera-forming eruptions, are inferred to be crucial for the generation of more evolved magma compositions erupted during Plinian events.

Spectroscopic Mapping of the White Horse Alunite Deposit, Marysvale Volcanic Field, Utah: Evidence of a Magmatic Component

Previous studies have demonstrated that the replacement alunite deposits just north of the town of Marysvale, Utah, USA, were formed primarily by low-temperature (100°–170° C), steam-heated processes near the early Miocene paleoground surface, immediately above convecting hydrothermal plumes. Pyrite-bearing propylitically altered rocks occur mainly beneath the steam-heated alunite and represent the sulfidized feeder zone of the H 2 S-dominated hydrothermal fluids, the oxidation of which at higher levels led to the formation of the alunite. Maps of surface mineralogy at the White Horse deposit generated from Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data were used in conjunction with X-ray diffraction studies of field samples to test the accuracy and precision of AVIRIS-based mineral mapping of altered rocks and demonstrate the utility of spectroscopic mapping for ore deposit characterization. The mineral maps identified multiple core zones of alunite that grade laterally outward to kaolinite. Surrounding the core zones are dominantly propylitically altered rocks containing illite, montmorillonite, and chlorite, with minor pyrite, kaolinite, gypsum, and remnant potassium feldspar from the parent rhyodacitic ash-flow tuff. The AVIRIS mapping also identified fracture zones expressed by ridge-forming selvages of quartz + dickite + kaolinite that form a crude ring around the advanced argillic core zones. Laboratory analyses identified the aluminum phosphate-sulfate (APS) minerals woodhouseite and svanbergite in one sample from these dickite-bearing argillic selvages. Reflectance spectroscopy determined that the outer edges of the selvages contain more dickite than do the medial regions. The quartz + dickite ± kaolinite ± APS-mineral selvages demonstrate that fracture control of replacement processes is more prevalent away from the advanced argillic core zones. Although not exposed at the White Horse deposit, pyrophyllite ± ordered illite was identified using AVIRIS in localized, superimposed conduits within propylitically altered rocks in nearby alteration systems of similar age and genesis that have been eroded to deeper levels. The fracture zones bearing pyrophyllite, illite, dickite, natroalunite, and/or APS minerals indicate a magmatic component in the dominantly steam-heated system.

Geochemistry and origin of the ore-forming fluids in hydrothermal-magmatic systems in tectonically active zones

Data on fluid inclusions and stable isotope compositions (O, H, C, and S) in minerals have been summarized for large-and middle-scale mesothermal gold deposits (Nezhdaninsk, Berezovsk, Kochkar’, Svetlinsk, Darasun, and Maisk), cassiterite-silicate-sulfide deposits of Sikhote Alin (Solnechnoe, Arsen’evsk, and Vysokogorsk), vein silver-base metal deposits in the Southern Verkhoyansk region (Prognoz and Kupol’noe), and epithermal copper-bismuth-silver-base metal deposits of the Karamazar district in Tajikistan (Kanimansur, Tary Ekan, and Zambarak). It is shown that ores precipitated from fluids with salinity varying from brines (up to 60 wt % NaCl equiv) to dilute fluids (1–3 wt % NaCl equiv). As a rule, fluids of different compositions entered the hydrothermal-magmatic system. A fluid mixture of H2O-CO2-NaCl±CH4±N2 predominated in the orogenic (mesothermal) gold-bearing hydrothermal systems, with deposition of the final-stage gold-bearing sulfosalts from aqueous-salt fluid. Brines played a significant role in the formation of cassiterite-silicate-sulfide and vein silver-base metal deposits. The brines often coexisted with a low-density vapor-rich fluid at the ore deposition site. The obtained data suggest a predominant magmatic component in the hydrothermal-magmatic systems, with a significant contribution of meteoric waters.

2: Hydrothermal ore deposits related to post-orogenic extensional magmatism and core complex formation: The Rhodope Massif of Bulgaria and Greece

The Rhodope Massif in southern Bulgaria and northern Greece hosts a range of Pb–Zn–Ag, Cu–Mo and Au–Ag deposits in high-grade metamorphic, continental sedimentary and igneous rocks. Following a protracted thrusting history as part of the Alpine–Himalayan collision, major late orogenic extension led to the formation of metamorphic core complexes, block faulting, sedimentary basin formation, acid to basic magmatism and hydrothermal activity within a relatively short period of time during the Early Tertiary. Large vein and carbonate replacement Pb–Zn deposits hosted by high-grade metamorphic rocks in the Central Rhodopean Dome (e.g., the Madan ore field) are spatially associated with low-angle detachment faults as well as local silicic dyke swarms and/or ignimbrites. Ore formation is essentially synchronous with post-extensional dome uplift and magmatism, which has a dominant crustal magma component according to Pb and Sr isotope data. Intermediate- and high-sulphidation Pb–Zn–Ag–Au deposits and minor porphyry Cu–Mo mineralization in the Eastern Rhodopes are predominantly hosted by veins in shoshonitic to high-K calc-alkaline volcanic rocks of closely similar age. Base-metal-poor, high-grade gold deposits of low sulphidation character occurring in continental sedimentary rocks of synextensional basins (e.g., Ada Tepe) show a close spatial and temporal relation to detachment faulting prior and during metamorphic core complex formation. Their formation predates local magmatism but may involve fluids from deep mantle magmas. The change in geochemical signatures of Palaeogene magmatic rocks, from predominantly silicic types in the Central Rhodopes to strongly fractionated shoshonitic (Bulgaria) to calc-alkaline and high-K calc-alkaline (Greece) magmas in the Eastern Rhodopes, coincides with the enrichment in Cu and Au relative to Pb and Zn of the associated ore deposits. This trend also correlates with a decrease in the radiogenic Pb and Sr isotope components of the magmatic rocks from west to east, reflecting a reduced crustal contamination of mantle magmas, which in turn correlates with a decreasing crustal thickness that can be observed today. Hydrogen and oxygen isotopic compositions of the related hydrothermal systems show a concomitant increase of magmatic relative to meteoric fluids, from the Pb–Zn–Ag deposits of the Central Rhodopes to the magmatic rock-hosted polymetallic gold deposits of the Eastern Rhodopes.