High-level Magma Chamber Research Articles

Electrical image of magmatic system beneath the Xiangshan volcanogenic uranium deposit, southeast China: Linking magmatic evolution and uranium metallogenesis

Abstract The uranium deposit of the Early Cretaceous Xiangshan caldera (southeast China) represents the world’s third-largest volcanogenic uranium deposit. While the hydrothermal system defines uranium mineralization in shallow volcanic-intrusive complexes, we argue that the underlying magmatic system, identified in magnetotelluric data from the Xiangshan deposit, is equally crucial in controlling the origin and migration of ore-forming fluids and driving uranium element transport. As imaged by a three-dimensional resistivity model, a volcanic conduit containing ~4% saline fluid underlies the Xiangshan peak, interpreted to control the formation of alkali fluids that aid the early alkaline uranium mineralization. A deep-seated intrusion also imaged beneath the northwestern Xiangshan caldera likely facilitates the formation of later acidic uranium mineralization and provides the requisite heat flux. Its enhanced conductivity is attributed to graphite liberalization along grain boundaries caused by CO2-bearing volatiles upwelling or igneous carbon injection. The merged zone of the volcanic conduit and intrusion, which is located at a depth of ~10–15 km, functions as a high-level magma chamber. It is fed episodically by low partial melting of lower-crustal rocks and may contain significant uranium concentrations. Essentially, the transport and enrichment of uranium elements are enabled by such a vertical extensive trans-crustal magmatic system, resulting in the emplacement of the shallow volcanogenic uranium deposit. This work is a compelling example regarding the inherited structural control of the magmatic system on volcanic-related uranium metallogenesis.

Geochemical Signatures of Felsic Volcanic Rocks in Modern Oceanic Settings and Implications for Archean Greenstone Belts

Abstract Felsic volcanic rocks are abundant in ancient greenstone belts and important host rocks for volcanogenic massive sulfide (VMS) deposits. About half of all VMS deposits are hosted by dacite or rhyolite, an association that reflects anomalous heat flow during rifting, partial melting of basaltic crust, and fractional crystallization in high-level magma chambers. For over 30 years, geochemical signatures of these rocks (e.g., F classification of Archean rhyolites) have been widely used to identify possible hosts for VMS deposits in ancient greenstone belts. However, comparisons with modern oceanic settings have been limited, owing to a lack of samples of felsic volcanic rocks from the sea floor. This is changing with increasing exploration of the oceans. In this study, we have compiled high-quality geochemical analyses of more than 2,200 unique samples of submarine felsic volcanic rocks (>60 wt % SiO2) from a wide range of settings, including mid-ocean ridges, ridge-hot-spot intersections, intraoceanic arc and back-arc spreading centers, and ocean islands. The compiled data show significant geochemical diversity spanning the full range of compositions of rhyolites found in ancient greenstone belts. This diversity is interpreted to reflect variations in crustal thickness, the presence or absence of slab-derived fluids (dry melting versus wet melting), and mantle anomalies. Highly variable melting conditions are thought to be related to short-lived microplate domains, such as those caused by diffuse spreading and multiple overlapping spreading centers. Systematic differences in the compositions of felsic volcanic rocks in the modern oceanic settings are revealed by a combination of principal components analysis, unsupervised hierarchical clustering, and supervised random forest classification of the compiled data. Dacites and rhyolites from midocean ridge settings have moderately depleted mantle signatures, whereas rocks from ridge-hot-spot intersections and ocean islands reflect enriched mantle sources. Felsic volcanic rocks from arc-back-arc systems have strongly depleted mantle signatures and well-known subduction-related chemistry (strong large ion lithophile element enrichment in combination with strong negative Nb-Ta anomalies and low heavy rare earth elements [HREEs]). This contrasts with felsic volcanic rocks in Archean greenstone belts, which show high field strength element and HREE enrichment (so-called FIIIb-type) due to a less depleted mantle, a lack of wet melting, and variable crustal contamination. The differences between modern and ancient volcanic rocks are interpreted to reflect the lower mantle temperatures, thinner crust, and subduction-related processes in present-day settings. We suggest that the abundance of FIIIb-type felsic volcanic rocks in Archean greenstone belts is related to buoyant microplate domains with thickened oceanic crust that were better preserved on emerging Archean cratons, whereas in post-Archean tectonic settings most of these rocks are subducted.

Chapter 2.2b Palmer Land and Graham Land volcanic groups (Antarctic Peninsula): petrology

Abstract Large-volume rhyolitic volcanism along the proto-Pacific margin of Gondwana consists of three major episodes of magmatism or ‘flare-ups’. The initial episode (V1) overlaps with the Karoo–Ferrar large igneous provinces at c. 183 Ma. A second (V2) episode was erupted in the interval 171–167 Ma, and a third episode (V3) was emplaced in the interval 157–153 Ma. The magmatic events of the V1 and V2 episodes of the Antarctic Peninsula are reviewed here describing major and trace elements, and isotopic (Sr, Nd, O) data from rhyolitic volcanic rocks and more minor basaltic magmatism. An isotopically uniform intermediate magma developed as a result of anatexis of hydrous mafic lower crust, which can be linked to earlier, arc-related underplating. The subsequent lower-crust partial melts mixed with fractionated mafic underplate, followed by mid-crust storage and homogenization. Early Jurassic (V1) volcanic rocks of the southern Antarctic Peninsula are derived from the isotopically uniform magma, but they have mixed with melts of upper-crustal paragneiss in high-level magma chambers. The V2 rhyolites from the northern Antarctic Peninsula are the result of assimilation and fractional crystallization of the isotopically uniform magma. This process took place in upper-crust magma reservoirs involving crustal assimilants with an isotopic composition akin to that of the magma. A continental margin-arc setting was critical in allowing the development of an hydrous, fusible lower crust. Lower-crustal anatexis was in response to mafic underplating associated with the mantle plume thought to be responsible for the contemporaneous Karoo magmatic province and rifting associated with the initial break-up of Gondwana.

The 1963–1964 eruption of Agung volcano (Bali, Indonesia)

The February 1963 to January 1964 eruption of Gunung Agung, Indonesia’s largest and most devastating eruption of the twentieth century, was a multi-phase explosive and effusive event that produced both basaltic andesite tephra and andesite lava. A rather unusual eruption sequence with an early lava flow followed by two explosive phases, and the presence of two related but distinctly different magma types, is best explained by successive magma injections and mixing in the conduit or high level magma chamber. The 7.5-km-long blocky-surfaced andesite lava flow of ∼0.1 km3 volume was emplaced in the first 26 days of activity beginning on 19 February. On 17 March 1963, a major moderate intensity (∼4 × 107 kg s−1) explosive phase occurred with an ∼3.5-h-long climax. This phase produced an eruption column estimated to have reached heights of 19 to 26 km above sea level and deposited a scoria lapilli to fine ash fall unit up to ∼0.2 km3 (dense rock equivalent—DRE) in volume, with Plinian dispersal characteristics, and small but devastating scoria-and-ash flow deposits. On 16 May, a second intense 4-h-long explosive phase (2.3 × 107 kg s−1) occurred that produced an ∼20-km-high eruption column and deposited up to ∼0.1 km3 (DRE) volume of similar ash fall and pyroclastic flow deposits, the latter of which were more widespread than in the March phase. The two magma types, porphyritic basaltic andesite and andesite, are found as distinct juvenile scoria populations. This indicates magma mixing prior to the onset of the 1963 eruption, and successive injections of the more mafic magma may have modulated the pulsatory style of the eruption sequence. Even though a total of only ∼0.4 km3 (DRE volume) of lava, scoria and ash fall, and scoria-and-ash pyroclastic flow deposits were produced by the 1963 eruption, there was considerable local damage caused mainly by a combination of pyroclastic flows and lahars that formed from the flow deposits in the saturated drainages around Agung. Minor explosive activity and lahar generation by rainfall persisted into early 1964. The climactic events of 17 March and 16 May 1963 managed to inject ash and sulfur-rich gases into the tropical stratosphere.

Geochronology and geochemistry of the Nantianwan mafic–ultramafic complex, Emeishan large igneous province: metallogenesis of magmatic Ni–Cu sulphide deposits and geodynamic setting

The Nantianwan mafic–ultramafic complex is situated in the northwest part of the Panxi district, southwest China. It consists predominantly of gabbros, gabbronorites, and lherzolites. LA–ICP–MS U–Pb zircon dating of the gabbronorites yields an age of 259.7 ± 0.6 million years, consistent with the ages of other mafic–ultramafic intrusions in the Emeishan large igneous province (ELIP). Gabbronorites and lherzolites host Cu–Ni sulphide ores. Cumulus texture is pronounced in these rocks, containing magnesium-rich olivine (up to 81.4% forsterite). SiO2 contents of the lherzolites range from 42.93 to 44.18 wt.%, whereas those of the gabbronorites vary between 44.89 and 52.76 wt.%. Analysed samples have low rare earth element (REE) contents (23.22–30.16 ppm for lherzolites and 25.21–61.05 ppm for gabbronorites). Both lherzolites and gabbronorites have similar chondrite-normalized REE patterns, suggesting that they are comagmatic. All samples are slightly enriched in large ion lithophile elements (LILEs, e.g. Rb, Ba, and Sr) relative to high field strength elements (HFSEs, e.g. Nb, Ta, and Ti), very similar to those of ocean island basalts (OIBs). The presence of cumulus textures and geochemical signatures indicates that fractional crystallization played an important role in the petrogenesis of these rocks. Initial (87Sr/86Sr) t (t = 260 Ma) ratios and ϵNd(t) values of the mafic–ultramafic suite vary from 0.70542 to 0.70763, and −0.4 to 1.7, respectively. Compared to the Cu–Ni-bearing Baimazhai and Limahe intrusions in the ELIP, which were considerably contaminated by variable crustal materials, the Nantianwan complex exhibits much lower (87Sr/86Sr) t . Their ϵNd(t) versus (Th/Nb)PM ratios also indicate that the ore-bearing magmas did not undergo significant crustal contamination. In combination with (Tb/Yb)PM versus (Yb/Sm)PM modelling, we infer that the magmas originated from an incompatible elements-enriched spinel-facies lherzolite that itself formed by interaction between the Emeishan plume and the lithospheric mantle. Most plots of NiO versus Fo contents of olivine suggest that sulphides are separated from the parental magma by liquid immiscibility, which is also supported by bulk-rock Cu/Zr ratios of the lherzolites (7.04–102.67) and gabbronorites (0.88–5.56). We suggest that the gabbronorites and lherzolites experienced undersaturation to oversaturation of sulphur; the latter may be due to fractional crystallization in a high-level magma chamber, accounting for the sulphide segregation.

Petrochemistry of subvolcanic dike swarms associated with the Golden Revenue Au–Cu and the Stoddart Mo–Cu ± W mineralizations (Dawson Range, Yukon Territory, Canada) and implications for ore genesis

Petrologic and geochemical features enabled us to constrain the petrogenesis of dikes spatially associated with the Golden Revenue (GR) Au–Cu and the Stoddart Mo–Cu ± W mineralization, in the Mountain Freegold, Yukon Territory, Canada. Both dike sets are aplitic and porphyritic and characterized by broken and embayed feldspar and quartz phenocrysts/phenoclasts set in a fine-grained groundmass. In both GR and Stoddart mineralized zones, aplitic dikes are typically related to the lower T magma relative to the porphyritic dikes, as indicated by zircon and apatite saturation temperatures. GR dikes are essentially dacitic to rhyolitic in compositions, whereas dikes from Stoddart display a wide compositional range spanning from basaltic–andesitic to rhyolitic compositions. GR and Stoddart dike suites are classified as high-K calc–alkaline units and exhibit adakitic affinities with their depletion in HREE + Y relative to LREE and subsequently high Sr/Y and La/Yb ratios. Dikes associated with the Stoddart Mo–Cu ± W mineral occurrence are oxidized (as suggested by the elevated X Mg of biotite and hornblende, and magnetic susceptibility data) relative to dikes associated with Au–Cu mineralization, and the former were fed by a high-level magma chamber, whereas the latter are associated with a deep-seated magma chamber. 87Sr/ 86Sr i values from GR and Stoddart dikes are near equal (0.7052–0.7083 and 0.7054–0.7076, respectively), and the εNd i of both dike suites is equal as well (− 0.1). While Pb isotope compositions of GR dikes are heterogeneous and span a wide composition range ( 206Pb/ 204Pb i: 18.823–19.577; 207Pb/ 204Pb i: 15.593–15.681, and 208Pb/ 204Pb i: 38.539–39.017), by contrast dikes from Stoddart yielded relatively homogeneous Pb isotope compositions ( 206Pb/ 204Pb i: 19.103–19.389; 207Pb/ 204Pb i: 15.606–15.667, and 208Pb/ 204Pb i: 38.672 to 38.99). The difference in fO 2 and in isotope compositions, as well as in the depth of magma emplacement between the two dike sets indicates magma derived from different sources, and it is suggested that those differences further controlled the variation in styles of mineralization. The preservation of petrographic features, such as miarolitic cavities, quartz phenocrysts and phenoclasts, and the presence of intermineral porphyry dikes suggest a close temporal relationship between dike emplacement and the hydrothermal activity, thus consistent with the magmatic–hydrothermal transition.

Petrogenesis of volcanic and intrusive rocks of the Zhuanqiao stage, Luzong Basin, Yangtze metallogenic belt, east China: implications for ore deposition

The Mesozoic Luzong volcanic basin is located in the Lower Yangtze River fault-depression, along the northern margin of the Yangtze Block. Volcanic and plutonic rocks are widely distributed in the basin and are spatially related to Cu, Fe, Au, Pb, and Zn ore deposits. The magmatic rocks are dominated by an alkali-rich shoshonitic suite of volcanics and intrusives, including both a trachybasalt-basaltic trachyandesite-trachyte series and a diorite-monzonite-syenite-alkali feldspar granite series. Volcanic sequences within the Luzong Basin are subdivided into four stages, namely the Longmenyuan, Zhuanqiao, Shuangmiao, and Fushan stages with magmatism occurring between 136 and 124 Ma. Most mineral deposits were formed during the Zhuanqiao stage from about 134 to 131 Ma, during a longer (140–130 Ma) period of transition from compression to extension within the Luzong Basin. Major element concentrations in the Zhuanqiao volcanic rocks range from 53.97 to 62.59 wt.% SiO2, 0.57–0.94 wt.% TiO2, 3.52–6.07 wt.% Na2O, and 3.60–8.66 wt.% K2O. Combined K2O + Na2O totals lie between 7.87 and 12.49 wt.%, and K2O + Na2O/Al2O3 ranges between 0.47 and 0.71. In comparison, the Zhuanqiao intrusions have SiO2 concentrations between 52.19 and 67.70 wt.%, TiO2 between 0.26 and 1.13 wt.%, Na2O between 1.33 and 6.55 wt.%, and K2O between 2.15 and 8.25 wt.%, with K2O + Na2O values ranging from 5.92 to 12.39 wt.% and K2O + Na2O/Al2O3 ratios between 0.37 and 0.75. The Zhuanqiao volcanic rocks have an average ∑REE (rare earth element) content of 223.17 ppm, whereas the average ∑REE of the Zhuanqiao intrusives is 250.06 ppm. Weak negative Eu anomalies (Eu/Eu* between 0.69 and 1.00) characterize the volcanics, whereas a larger range with more significant negative Eu anomalies (Eu/Eu* values of 0.35–0.94) is present in the plutonics. The Zhuanqiao volcanic rocks and plutons have (Nb/Th)PM ratios of 0.05–0.14 and 0.02–0.14 and (La/Sm)PM of 3.33–4.95 and 3.36–7.29, respectively. Initial 87Sr/86Sr and 143Nd/144Nd ratios and ϵSr(t) and ϵNd(t) of the Zhuanqiao volcanic rocks are 0.7059–0.7067, 0.51194–0.51218, 22.0–33.3, and −10.2 to −5.5, respectively; those of the intrusives are 0.7060–0.7082 and 0.51203–0.51214 and 23.3–54.7 and −8.6 to −6.4, respectively. The geochemistry presented here suggests that both volcanic and intrusive Zhuanqiao stage magmatic rocks were cogenetic and were probably derived from an enriched mantle (EM) source, most probably metasomatized mantle relating to the EM type-I (EM-I). The Zhuanqiao stage magmatism apparently was controlled by differentiation and assimilation of crustal material in a high-level magma chamber. Formation of the Zhuanqiao stage magmatic rocks coincided with the formation of much of the Cu and Fe mineralization within the Luzong Basin. This coeval magmatism and mineralization during the Zhuanqiao stage can be subdivided into three differing mineral deposit affinities: magmatism associated with magmato-hydrothermal vein Cu (e.g. the Jingbian, Shimenan, and Chuanshandong deposits), porphyry-hosted Fe (e.g. the Luohe and Nihe deposits), and skarn Fe (e.g. the Longqiao deposit). Combining geological characteristics with variations in SiO2, K2O + Na2O, and MgO enabled the division of the Zhuanqiao stage magmatism in the Luzong Basin into geochemically distinct units associated with these differing styles of mineralization.

Geochemistry, mineralogy and genesis of the Ayazmant Fe–Cu skarn deposit in Ayvalik, (Balikesir), Turkey

The Ayazmant Fe–Cu skarn deposit is located approximately 20 km SE of Ayvalık or 140 km N of Izmir in western Turkey. The skarn occurs at the contact between metapelites and the metabasites of the Early Triassic Kınık Formation and the porphyritic hypabyssal intrusive rocks of the Late Oligocene Kozak Intrusive Complex. The major, trace, and rare earth-element geochemical analysis of the igneous rocks indicate that they are I-type, subalkaline, calc-alkaline, metaluminous, I-type products of a high-level magma chamber, generated in a continental arc setting. The 40Ar– 39Ar isochron age obtained from biotite of hornfels is 20.3 ± 0.1 Ma, probably reflecting the age of metamorphic–bimetasomatic alteration which commenced shortly after intrusion into impure carbonates. Three stages of skarn formation and ore development are recognized: (1) Early skarn stage (Stage I) consisting mainly of garnet with grossular-rich (Gr 75–79) cores and andradite-rich (Gr 36–38) rims, diopside (Di 94–97), scapolite and magnetite; (2) sulfide-rich skarn (Stage II), dominated by chalcopyrite with magnetite, andraditic garnet (Ad 84– 89), diopside (Di 65– 75) and actinolite; and (3) retrograde alteration (Stage III) dominated by actinolite, epidote, orthoclase, phlogopite and chlorite in which sulfides are the main ore phases. 40Ar– 39Ar age data indicate that potassic alteration, synchronous or postdating magnetite–pyroxene–amphibole skarn, occurred at 20.0 ± 0.1 Ma. The high pyroxene/garnet ratio, plus the presence of scapolite in calc-silicate and associated ore paragenesis characterized by magnetite (± hematite), chalcopyrite and bornite, suggests that the bulk of the Ayazmant skarns were formed under oxidized conditions. Oxygen isotope compositions of pyroxene, magnetite and garnet of prograde skarn alteration indicate a magmatic fluid with δ 18O values between 5.4 and 9.5‰. On the basis of oxygen isotope data from mineral pairs, the early stage of prograde skarn formation is characterized by pyroxene (Di 94–97)-magnetite assemblage formed at an upper temperature limit of 576 °C. The lower temperature limit for magnetite precipitation is estimated below 300 °C, on the basis of magnetite–calcite pairs either as fracture-fillings or massive ore in recrystallized limestone-marble. The sulfide assemblage is dominated by chalcopyrite with subordinate molybdenite, pyrite, cubanite, bornite, pyrrhotite, galena, sphalerite and idaite. Gold–copper mineralization formed adjacent to andradite-dominated skarn which occurs in close proximity to the intrusion contacts. Native gold and electrum are most abundant in sulfides, as fine-grained inclusions; grain size with varying from 5 to 20 µm. Sulfur isotope compositions obtained from pyrrhotite, pyrite, chalcopyrite, sphalerite and galena form a narrow range between − 4.8 and 1.6‰, suggesting the sulfur was probably mantle-derived or leached from magmatic rocks. Geochemical data from Ayazmant shows that Cu is strongly associated with Au, Bi, Te, Se, Cd, Zn, Pb, Ni and Co. The Ayazmant mineralizing system possesses all the ingredients of a skarn system either cogenetic with, or formed prior to a porphyry Cu(Au–Mo) system. The results of this study indicate that the Aegean Region of Turkey has considerable exploration potential for both porphyry-related skarns and porphyry Cu and Au mineralization.

Petrology of the Paleocene Picrites and Flood Basalts on Disko and Nuussuaq, West Greenland

The c. 62^60 Ma rift-related Paleocene volcanic succession in West Greenland comprises voluminous picrites (the Vaigat Formation) overlain by flood basalts (the Maliga“ t Formation). A detailed stratigraphy for the c. 3 km combined succession on Disko and Nuussuaq has been established that provides evidence of the evolution of the volcanic systems, processes and sources through time. Picrites constitute about one-third of the total erupted volume. The primary magmas for the whole succession were highly magnesian; the weighted average of 16� 6 wt % MgO for the uncontaminated Vaigat Formation is a lower limit for the primary magmas. During the emplacement of the Vaigat Formation, the magmas fractionated olivine in the feeder channels before eruption but generally did not stop in magma chambers. Only minor magma batches stopped in high-level magma chambers, where many became crustally contaminated. At the transition to the Maliga“ t Formation magma chambers developed, presumably in the lower crust. From then on, the main eruption products were basalts with compositions buffered by refilling^fractionation^tapping processes. Large-scale cyclicity can be seen, with more and less fractionated steady-state compositions. The upper part of the Maliga“ t Formation is contaminated throughout, except for the three youngest flows. Repeated pulses of new picritic magmas can be seen in both formations. The asthenospheric mantle source was heterogeneous and the dominant component was depleted, mid-ocean ridge basalt (MORB)-like mantle with Nb/ La51. An additional, less depleted mantle component akin to the Iceland source is evident in the upper Vaigat Formation but disappeared again at the transition to the Maliga“ t Formation. A third mantle component similar to the source for the E-type lavas on Baffin Island may be present in the earliest rocks. Melting took place in the garnet peridotite facies because the melting column was curtailed by the 100 km thick lithosphere. The picrites show coupled isotopic and elemental variations even within single units, indicating that the accumulated primary melts were not homogenized when they left the melting column. An incompatible-element-rich lithospheric mantle component is seen in the ManaŒ tdlat Member of alkaline picrites and basalts. Single scattered tholeiitic lavas somewhat enriched in the same trace elements are considered to be slightly contaminated with this component.The volume of alkaline and enriched rocks is very small. The crustal contaminants are the sediments in the Nuussuaq Basin. The contamination processes included bulk assimilation, inmixing of partial melts of the sediments, assimilation^fractional crystallization processes, and exchange reactions. When carbon-rich shales were assimilated severe reduction led to formation of graphite or metallic iron. All rocks more evolved than basalt arose by contamination processes. The units of contaminated rocks constitute 5^9% of the Vaigat Formation and around 30% of the Maliga“ t Formation.

The Characteristics and Mechanism of Island-Arc Volcanism on the Central and Southern Fildes Peninsula, King George Island, Antarctica

The volcanic rock series on the Fildes Peninsula is the product of the later subduction of the Pacific plate beneath the Antarctic plate. It consists mainly of basalt, basaltic andesite and andesite with minor dacite. Its isotopic ages range from 64.6 ± 1 to 43 ± 2 Ma, belonging to Palaeocene to Eocene. Volcanism in the area may be divided into two phases. The contents of major oxides, rare earth elements (REE) and trace elements in volcanic rocks formed in different phases show regular changes, which are mainly related to the rock associations of these phases. Isotope geochemical studies indicate that the primitive magma in the area originating by partial melting in the upper mantle underwent fractional crystallization and ascended to the high-level (shallow) magma chamber. Before eruption the primitive basalt-andesitic magma was subjected to differentiation in the high-level magma chamber, forming zones of derivative magmas of different compositions. In various phases magma-conducting faults experienced periodic extension and cut through various derivative magma zones in different parts of the peninsula, leading to the eruption of magmas of different compositions on the surface and the formation of volcanic rock associations of corresponding compositions.

Major element, trace element, and Sr, Nd and Pb isotope studies of Cenozoic basalts from the South China Sea

The whole rock K-Ar ages of basalts from the South China Sea basin vary from 3.8 to 7.9 Ma, which suggest that intra-plate volcanism after the cessation of spreading of the South China Sea (SCS) is comparable to that in adjacent regions around the SCS, i.e., Leiqiong Peninsula, northern margin of the SCS, Indochina block, and so on. Based on detailed petrographic studies, we selected many fresh basaltic rocks and measured their major element, trace element, and Sr-Nd-Pb isotope compositions. Geochemical characteristics of major element and trace element show that these basaltic rocks belong to alkali basalt magma series, and are similar to OIB-type basalt. The extent of partial melting of mantle rock in source region is very low, and magma may experience crystallization differentiation and cumulation during the ascent to or storing in the high-level magma chamber. Sr-Nd-Pb isotopic data of these basaltic rocks imply an inhomogeneous mantle below the South China Sea. The nature of magma origin has a two end-member mixing model, one is EM2 (Enriched Mantle 2) which may be originated from mantle plume, the other is DMM (Depleted MORB Mantle). Pb isotopic characteristics show the Dupal anomaly in the South China Sea, and combined with newly found Dupal anomaly at Gakkel ridge in Arctic Ocean, this implies that Dupal anomaly is not only limited to South Hemisphere. In variation diagrams among Sr, Nd and Pb, the origin nature of mantle below the SCS is similar to those below Leiqiong peninsula, northern margin of the SCS and Indochina peninsula, and is different from those below north and northeast China. This study provides geochemical constraints on Hainan mantle plume.

Petrology and geochemistry of Tertiary volcanic rocks from the İkizce (Ordu) area, NE Turkey: Implications for the evolution of the eastern Pontide paleo-magmatic arc

Tertiary volcanism in the İkizce region at the western edge of the eastern Pontides paleo-magmatic arc is represented by basaltic and andesitic rocks associated with sediments deposited in a shallow basin environment. The basaltic rocks contain plagioclase (An 58–80), olivine (Fo 82–84), clinopyroxene (Wo 44–48En 35–42Fs 7–17), hornblende (Mg# = 0.68–0.76) phenocrysts, and magnetite microcrysts, whereas the andesitic rocks include plagioclase (An 25–61), clinopyroxene (Wo 46–49En 38–43Fs 11–13), hornblende (Mg# = 0.48–0.81), biotite (Mg# = 0.48–0.60) phenocrysts, titanomagnetite, apatite, and zircon microcrysts. Geochemical data indicate magmatic evolution from tholeiitic-alkaline transitional to calc-alkaline characteristics with medium-K contents. The geochemical variation in the rocks can be explained by fractionation of common mineral phases such as clinopyroxene, olivine, hornblende, plagioclase, magnetite, and apatite. The trace elements’ distributions of the volcanic rocks show similarities to those of E-Type MORB, have a shape that is typical of rocks from subduction-related tectonic setting with enrichment in LILE and to a lesser extent in LREE, but depletion in HFSE. The rocks evolved from a parental magma derived from an enriched source formed by subduction induced metasomatism of basaltic rocks, the latter formed through clinopyroxene ± olivine controlled fractionation in a high level magma chamber. The andesitic rocks developed through hornblende ± plagioclase controlled fractionation in shallow level magma chamber(s).

Yulong Deposit, Eastern Tibet: A High-Sulfidation Cu-Au Porphyry Copper Deposit in the Eastern Indo-Asian Collision Zone

The Yulong ore body is the largest Cu deposit (6.22 million metric tons [Mt] at 0.99% Cu) in the 300 km long Himalayan porphyry copper belt, and is controlled by major Cenozoic strike-slip faults in the eastern Indo-Asian collision zone. It is associated with a steeply dipping, pipe-like multiphase (42-35 Ma) monzogranitic stock. The host rocks are potassic calc-alkaline or shoshonitic, and show geochemical affinities with adakites. They appear to have been derived from a thickened lower crustal source in East Tibet. The Yulong deposit consists of a ring-shaped, high-grade Cu-Au zone overlying and/or surrounding a porphyry-type Cu-Mo ore body. Cu-Mo mineralization produced a steeply dipping, pipe-like, veinlet-disseminated ore body within the stock. Associated hydrothermal alteration produced K-silicate and quartz-sericite assemblages within the stock, and contemporaneous propylitic alteration in the Upper Triassic sandy-slate wall rock. Fluid inclusion and δ18O-δD data indicate that the ore-forming fluid was supercritical, and exsolved from a high-level magma chamber at >620°C; it then separated into a hypersaline aqueous liquid and a coexisting low-salinity vapor at 340°-600°C. The high-grade Cu-Au zone (3 Mt at 4.74% Cu, and 4.5 g/t Au) is dominated by a supergene chalcocite-malachite blanket resting on an underlying supergene/hypogene sulfide transition unit and a hypogene pyrite-chalcopyrite sulfide unit. The Cu-Au zone was controlled by a subhorizontal or gently outward dipping breccia horizon developed along the marginal fracture zone near the roof of the stock, produced by hydrothermal brecciation due to regional uplift and/or fluid boiling. Alteration associated with the hypogene Cu-Au mineralization was texture-destructive advanced argillic alteration, characterized by associations of quartz, kaolinite, dickite, endellite, montmorillonite, hydromica, and minor alunite. It mainly developed within the breccia horizons, and partially over-printed the early-formed K-silicate zone and the quartz-sericite zone. Associated mineralization was of the high-sulfidation epithermal-type, characterized by chalcocite, tennantite, covellite, bornite, and minor pyrite, which formed the main ore body in the high-grade Cu-Au zone. Epithermal fluids also caused the dissolution of early-formed sulfides and remobilization of Cu-Mo, the latter transported into the intense advanced argillic alteration halo within the mineralized stock. This late-stage alteration and mineralization is attributed to a CO2-rich, low-temperature (<350°C), low-salinity (<12 wt% NaCl equiv.) meteoric fluid, involving input of magmatic fluid. Based on alteration, mineralization, fluid inclusion and stable isotopic data, a two-stage genetic history has been reconstructed for the Yulong deposit. It spans (1) a magmatic hydrothermal environment reflecting the emplacement of the monzogranite stock and Cu-Mo introduction through (2) hydrothermal fluid infiltration of breccia zones to epithermal overprinting.

DURATION OF MAGMATIC, HYDROTHERMAL, AND SUPERGENE ACTIVITY AT CERRO RICO DE POTOSI, BOLIVIA

New high precision 40Ar/39Ar dating of sanidine and biotite from two rhyolitic domes and an ignimbrite, combined with existing fission-track data and a hydrothermal sericite age, suggests that the world-class Ag deposit at Cerro Rico was emplaced during a protracted period of magma-related hydrothermal activity beginning at 13.77 ± 0.03 Ma and continuing for at least 0.2 m.y. This may have been sustained by a large single injection or repeated injections of fractionated Ag-enriched magma into a high-level magma chamber. K-Ar dating of alunite indicates that supergene oxidation had begun by about 13.5 Ma, soon after dome emplacement, and progressed semicontinuously for at least 7.5 m.y. This oxidation, while not leading to significant enrichment, has significantly enhanced the economic viability of the disseminated part of the orebody.

Siderophile and Chalcophile Metal Variations in Flood Basalts from the Siberian Trap, Noril'sk Region: Implications for the Origin of the Ni-Cu-PGE Sulfide Ores

The Talnakh, Kharaelakh, and Noril’sk I intrusions of the Noril’sk region are coeval with the Permo-Triassic Siberian trap flood basalt and contain one of the largest known resources of Ni-, Cu-, and platinum group element (PGE)-enriched sulfide mineralization. The ~3.5-km-thick stratigraphy of the basalts consists of a Lower sequence of high Ti alkalic, subalkalic, and picritic basalts, and an Upper sequence of low Ti basalts. The stratigraphy of the Upper sequence consists of a group of picritic and tholeiitic basalts assigned to the Tuklonsky Formation, overlain by the Nadezhdinsky Formation which is represented by a sequence of contaminated tholeiites that show an upward decline in the degree of contamination into the overlying Morongovsky and Mokulaevasky Formations. The Nadezhdinsky Formation is depleted in Ni, Cu, and PGE, with the most depleted basalts having concentrations below the analytical determination limits of 0.06 ppb for Pd and 0.1 ppb for Pt and a factor of at least 100 less than the Pd and Pt concentrations of the rest of the Upper sequence. The greatest PGE depletion occurs a short distance above the base of the Nadezhdinsky, and these flows are overlain by basalts that exhibit a gradual recovery in PGE concentration toward values typical of the Mokulaevsky Formation and the overlying basalts (~6.8–12.1 ppb Pt and ~7.2–16.5 ppb Pd). Within the Nadezhdinsky Formation, there is a strong correlation between Pd/Zr and La/Sm ratios that indicates that the magmas with the largest contribution from crustal material are also the most PGE depleted. Moreover, the contribution of crust to the magma and the degree of metal depletion decreased through time. The PGE-undepleted Upper sequence tholeiites have Pd/Pt of 0.45 to 2.0, and the ore-forming sulfides (Pd/Pt ~3–4) were probably formed from magmas with Pd/Pt ratios at the upper limit of this range; the metal-depleted basalts have lower values (Pd/Pt ~0.3). The high Pd/Pt ratio of the ores and the low Pd/Pt ratio of the basalts indicate that the processes of ore formation and flood basalt magmatism were important because they indicate that the basalts have the Pd/Pt ratio of a magma that has segregated sulfide with the same Pd/Pt ratio as the Noril’sk ores. Moreover, the ore deposits are located in the region where the metal-depleted Nadezhdinsky Formation is ~500 m thick and forms a >5,000-km3 volcanic center. The continuous changes in basalt chemistry through the stratigraphy of the Upper sequence are consistent with processes that took place in a very large staging chamber rather than within multiple ~5- to 10-km3 discrete high-level magma chambers on the scale of the mineralized intrusions. In the staging chamber, Tuklonksy-type magmas interacted with crust to produce the contaminated basalts of the Nadezhdinsky Formation. The staging chamber containing the crustally contaminated magma was then replenished with PGE-undepleted magma. Initially, the replenishing magma became sulfide saturated, thereby producing the ore-forming sulfides, and the magma in the staging chamber remained sulfide-saturated. As further injections of PGE-undepleted magma entered the staging chamber, the melt became sulfide undersaturated and magmas tapped from it were progressively less PGE depleted. The deposits of the Noril’sk, Talnakh, and Kharaelakh intrusions were formed by injection of olivine-bearing melts containing immiscible sulfide that were produced in the staging chamber. The ores inherited a heavy sulfide isotope composition possibly by reaction with evaporite-laden country rocks. The unradiogenic composition of the Os in the ores is consistent with interaction between the crustally contaminated magma (which had become depleted in the PGE and radiogenic Os) in the staging chamber with later pulses of uncontaminated mantle-derived magma. The high PGE contents of PGE-undepleted Siberian trap are due to interaction of plume-generated picritic magmas with lithosphere that had been depleted in sulfide by previous melt extraction.

Late Pleistocene granodiorite source for recycled zircon and phenocrysts in rhyodacite lava at Crater Lake, Oregon

Rhyodacite tephra and three lavas erupted ∼27 ka, interpreted to be early leaks from the climactic magma chamber of Mount Mazama, contain ubiquitous resorbed crystals (antecrysts) that were recycled from young granodiorite and related plutonic rocks of the same magmatic system. The shallow composite pluton is represented by blocks ejected in the 7.7-ka climactic eruption that formed Crater Lake caldera. Plagioclase crystals in both rhyodacite and granodiorites commonly have cores with crystallographically oriented Fe-oxide needles exsolved at subsolidus conditions. At least 80% of plagioclase crystals in the rhyodacite are antecrysts derived from plutonic rocks. Other crystals in the rhyodacite, notably zircon, also were recycled. SIMS 238U– 230Th dating indicates that zircons in 4 granodiorite blocks crystallized at various times between ∼20 ka and ≥ 300 ka with concentrations of analyses near 50–70, ∼110, and ∼200 ka that correspond to periods of dacitic volcanism dated by K–Ar. U–Th ages of zircon from a rhyodacite sample yield similar results. No analyzed zircons from the granodiorite or rhyodacite are pre-Quaternary. Zircon minimum ages in blocks from different locations around the caldera reflect ages of nearby volcanic vents and may map the distribution of intrusions within a composite pluton. Survival of zircon in zircon-undersaturated hydrous magma and of Fe-oxide needles in plagioclase suggests that little time elapsed from entrainment of antecrysts to the ∼27-ka eruption of the rhyodacite. The ∼27-ka rhyodacite is an example of young silicic magma that preserved unstable antecrysts from a known source early during growth of a large high-level magma chamber. In contrast, the voluminous 7.7-ka climactic rhyodacite pumice is virtually lacking in zircon, indicating dissolution of any granodioritic debris in the intervening period. Mineralogical evidence of assimilation may be destroyed in hot, vigorously growing silicic magma bodies such as ultimately produced the climactic eruption of Mount Mazama.

Reactivation of a rhyolitic magma body by new rhyolitic intrusion before the 15.8 ka Rotorua eruptive episode: implications for magma storage in the Okataina Volcanic Centre, New Zealand

The 15.8 ka Rotorua rhyolite eruptive episode from the Okataina Volcanic Centre comprises a plinian pumice fall deposit followed by the extrusion of two rhyolitic lava domes or flows and their associated block-and-ash flows, with a total volume &gt;1 km 3 (dense rock equivalent). Variations in mineralogy, whole-rock, glass and mineral chemistry, and calculated magmatic properties suggest that two distinct rhyolitic magmas were sequentially tapped during the eruption. The first magma erupted (T1) is characterized by low SiO 2 ( c . 76.5 wt% in glass), calcic feldspars (An 44 ), magnesian hornblendes (MgO c . 14.45 wt%), clinopyroxene, and high temperatures ( c . 835 °C) and f O 2 (1.8–2.1 ΔFMQ (where FMQ is the fayalite–magnetite–quartz buffer)). The second magma (T2) was more evolved and is characterized by higher SiO 2 ( c . 77.4 wt% in glass), Na-rich feldspars (An 24 ), less magnesian hornblendes (MgO c . 11.8 wt%), biotite, and low temperatures ( c . 750 °C) and f O 2 (0.65–1.1 ΔFMQ). Both magmas are homogeneous, but evidence for some magma mingling indicates that they were in contact during eruption. However, there was only a minor degree of hybridization, perhaps reflecting the contrasting temperatures and viscosities of the two magmas. The crystal-rich, poorly vesicular nature of the T2 ejecta indicates that it originated from a cooling, high-level magma chamber that was reactivated by intrusion of hotter, volatile-rich T1 magma. The ponding of rhyolite magmas at shallow depth and their subsequent reactivation by later rhyolitic intrusion may be an important process in the compositional evolution and eruption dynamics of many Okataina Volcanic Centre rhyolite magma bodies.

TRACE ELEMENT GEOCHEMISTRY AND PETROGENESIS OF FELSIC VOLCANIC ROCKS ASSOCIATED WITH VOLCANOGENIC MASSIVE Cu-Zn-Pb SULFIDE DEPOSITS

Volcanogenic massive Cu-Zn-(Pb) sulfide (VMS) deposits occur primarily in subaqueous rift-related environments (e.g., oceanic, fore-arc, arc, back-arc, continental margin, or continental), are hosted primarily by bimodal, mafic-felsic volcanic successions, and are typically associated with felsic volcanic rocks with specific geochemical characteristics. FI alkalic dacites and rhyodacites, despite being abundant in the rock record, are typically barren. Some FII calc-alkalic rhyodacites and rhyolites host VMS deposits, but most are barren. FIII tholeiitic and FIV depleted rhyolites and high silica rhyolites are much less abundant in the rock record but commonly host VMS deposits, regardless of age, and FIII rhyolites appear to host the largest deposits. Most petrogenetic models proposed for the formation of FII and FIII-FIV felsic volcanic rocks link felsic magma genesis to fractionation processes in high-level magma chambers now represented by associated subvolcanic intrusions, where the magma is also interpreted to have supplied the heat and/or metals required to generate and sustain the VMS-forming convective hydrothermal system. However, the relatively constant compositions of FII and FIII-FIV felsic volcanic rocks within individual areas, the high eruptive temperatures (at or above liquidus) of FIII rhyolites, and the bimodality of VMS-hosting volcanic successions indicate that fractional crystallization within subvolcanic intrusions could not have generated or significantly modified the compositions of FII and FIII-FIV magmas. This, coupled with detailed geological, geochemical, and geochronological studies indicates that many of these subvolcanic intrusions were emplaced in multiple phases and that the later, most voluminous phases often cut ore-associated, hydrothermally altered rocks. A reassessment of the physical conditions responsible for producing the geochemistry of ore-associated FII and FIII-FIV felsic volcanic rocks and a review of the compositions of felsic volcanic rocks associated with VMS deposits that range in age from Mesoarchean to Cenozoic provide important constraints on models for VMS-associated felsic volcanic rocks and their relationship to mineralization. The compositions of felsic volcanic rocks may be explained by low to moderate degrees of partial melting of mafic sources at a range of depths within rift environments where the mineralogy and composition of the source regions, modes, and degrees of partial melting, pressure and temperature of melting, and, to a lesser extent, subsequent fractionation processes, account for the compositional variations from FI through FII to FIII-FIV. Long-lived, enhanced heat flow and structural permeability of rift environments that allows partial melting to form some FII rhyolites at midcrustal levels (10–15 km) and FIII-FIV rhyolites at shallow crustal levels (<10 km), both within the zone of brittle fracture permeability, are essential to sustain the high-temperature convective hydrothermal systems that are required to form large VMS deposits and camps. Rift environments contain long-lived, thermal, magmatic, and structural corridors that focus magma ascent, heat flow, high-temperature convective hydrothermal systems, and emplacement of subvolcanic intrusions that are favorable environments for the formation of VMS deposits and FII and FIII-FIV felsic volcanic rocks.

Recharge flux to ocean-ridge black smoker systems: a geochemical estimate from ODP Hole 504B

We use the Sr-isotopic composition of rock and anhydrite, as a monitor of fluid composition, in DSDP/ODP Hole 504B to calculate the recharge flux to the axial high-temperature hydrothermal circulation. The fluid and rock Sr-isotope profiles are well fit by a tracer transport mass-balance model that approximates fluid–solid exchange by linear kinetics. The calculated time-integrated flux of 1.7±0.2×106 kg m−2 is significantly less than the 5×106 kg m−2 calculated from thermal models that assume all magma is intruded into a high-level magma chamber at the base of the sheeted dykes. Our low flux is consistent with the observed thermal structure as recorded by secondary alteration minerals in Hole 504B and the intrusion of magma as lenses distributed through the lower oceanic crust. It leaves open the questions as to how the lower oceanic crust cools and how seawater geochemical budgets balance.

Effects of repetitive emplacement of basaltic intrusions on thermal evolution and melt generation in the crust

A one-dimensional thermal conduction model simulates the repetitive intrusion of basalt sills into the deeper parts of the crust. The model assumes geothermal gradients of 10–30°C km−1, and intrusion depths at 20 and 30 km. A range of intrusion rates from 50 m of intruded basalt every 1000, 10 000 and 100 000 years cover a range of geodynamic situations. There is an initial incubation period in which the basalt intrusions solidify. Generation of silicic melts initiates when the solidus temperatures of either the basalt magma or surrounding crust is reached. At an intrusion rate of 50 m per 10 000 years incubation periods in the range 105–106 years are estimated, consistent with geochronological and stratigraphic data on many volcanic systems where there is commonly an evolution from mafic to silicic volcanism. Melt generation involves simultaneous cooling and crystallization of intruding basalt and partial melting of both new basaltic crust and pre-existing old crust. The proportion of these components depends on the fertility of the crust, in particular the abundance of hydrous minerals, and the temperature and water content of the basalt magma. For a wet (2% H2O) and cool (1100°C) basalt, melt generation can be dominated by residual liquids from basalt crystallization. For a dry (0.3% H2O) and hot (1300°C) basalt emplaced into fertile crustal rocks, such as pelite, melt generation can be dominated by partial melting of old crust. Melt proportions and temperature vary greatly across such a deep crustal intrusion zone, resulting in geochemical diversity in magmas. Segregated melts, if mixed together during ascent or in a high-level magma chamber, will be geochemical hybrids with mantle and crustal components. Intrusion rates of 50 m per 100 000 years or less are too low for large-scale melt generation in the crust. Periods of magmatic intrusion create reverse geothermal gradients and thermal anomalies in the crust which will take several million years to decay. Such anomalous zones are predisposed to remelt if a subsequent magmatic episode initiates.