Silicate-sulfide liquid immiscibility in modern arc basalt (Tolbachik volcano, Kamchatka): Part I. Occurrence and compositions of sulfide melts

Silicate-sulfide liquid immiscibility plays a key role in the formation of magmatic sulfide ore deposits but incipient sulfide melts are rarely preserved in natural rocks. This study presents the distribution and compositions of olivine-hosted sulfide melt globules resulting from silicate-sulfide liquid immiscibility in primitive arc basalts. Abundant sulfide droplets entrapped in olivine from primitive basalts of the 1941 eruption and pre-historic eruptive cone “Mt. 1004” of the Tolbachik volcano, Kurile-Kamchatka arc. Inclusions range from submicron to 250 μm in size, coexist with sulfur-rich glass (≤ 1.1 wt% S), and, in some cases, with magmatic anhydrite. Saturation in sulfide occurred early in the evolution of a water- and sulfur-rich magma, moderately oxidized (QFM + 1 to +1.5), which crystallized high-Mg olivine (Fo₈₆ˍ₉₂), clinopyroxene and Cr-spinel. The process developed dense “clouds” of sulfide in relatively small volumes of magma, with highly variable abundances of chalcophile metals. The low degree of sulfide supersaturation promoted diffusive equilibration of the growing droplets with the melt in Ni and Cu, resulting in high concentrations (≈ 38 mol%) of CuS and NiS in the earliest sulfide liquids. The Tolbachik samples provide a glimpse into deep arc processes not seen elsewhere, and may show how arc magmas, despite their oxidized nature, saturate in sulfide.This study was supported by the Russian Science Foundation grant # 16-17-10145. This is CRPG contribution #253

Ab initio molecular dynamics simulation and free energy exploration of copper(I) complexation by chloride and bisulfide in hydrothermal fluids

Abstract not availableYuan Mei, David M. Sherman, Weihua Liu, Joël Brugge

The effect of C02 on the speciation of bromine in low-temperature geological solutions: an XANES study

CO2-rich solutions are common in geological environments. An XANES (X-ray absorption near-edge structure) study of Br in CO2-bearing synthetic fluid inclusions has revealed that Br exhibits a strong pre-edge feature at temperatures from 298 to 423 K. Br in CO2-free solutions does not show such a feature. The feature becomes smaller and disappears as temperature increases, but reappears when temperature is reduced. The size of the feature increases with increasing X(CO2) in the fluid inclusion, where X(CO2) is the mole fraction of CO2 in the solution [n CO2 /(n CO2 + n H2 O + n RbBr); n indicates the number of moles]. The pre-edge feature is similar to that shown by covalently bonded Br, but observed and calculated concentrations of plausible Br-bearing covalent compounds (Br2, CH3Br and HBr) are vanishingly small. An alternative possibility is that CO2 affects the hydration of Br sufficiently that the charge density changes to favour the 1s-p level transitions that are thought to cause the pre-edge peak. The distance between the first two post-edge maxima in the XANES also decreases with increasing X(CO2). This is attributed to a CO2-related decrease in the polarity of the solvent. The proposed causes of the observed features are not integrated into existing geochemical models; thus CO2-bearing solutions could be predicted poorly by such models, with significant consequences for models of geological processes such as ore-formation and metamorphism

Fluid properties and origins of the Lannigou Carlin-type gold deposit, SW China: Evidence from SHRIMP oxygen isotopes and LA-ICP-MS trace element compositions of hydrothermal quartz

Oxygen isotope and trace elements characteristics of ore-related quartz from Lannigou gold deposit, a typical fault-control second largest Carlin-type Au deposit in SW China, provide valuable information about the prop- erties and origins of the ore-forming fluids. Previous works has focused on vein quartz instead of ore-related jasperoid-like quartz grain due to hard separation. Based upon different cathodoluminescence (CL) textural patterns and intensities observed within the quartz, four generations of quartz from this deposit were identified: Quartz I is sedimentary detrital quartz; Quartz II is syn-mineralization jasperoid-like quartz intergrown with and/or containing fine (< 10 μm) As-bearing pyrite and arsenopyrite; Quartz III is late-mineralization veinlet quartz with a small of medium-coarse-grained (about 50–100 μm) As-bearing pyrite; and Quartz IV is the post- mineralization drusy thick-veined quartz. Analyses by sensitive high-resolution ion microprobe (SHRIMP) and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) were used to determine O isotope and trace elements compositions of quartz formed in different stages. The trace elemental (e.g., Al, Li, Ge, Ti) variation of quartz from Quartz II to IV reflect the effects of fluid saturation in CO2 as the result of decarbonization reactions, followed by a progressive fall in CO2, whereas the fluid pH and intensity of argillic alteration increase, pro- gressively. The range of δ18OVSMOW values in Quartz II (12.1–24.8‰) is wide but narrows in Quartz III (24.1–27.8‰) and IV (24.3–26.9‰). The estimated fluid δ18O values of early- or main-stage (calculated from Quartz II) is 3.2–15.9‰, the late-stage (calculated from Quartz III) is 12.5–16.2‰, and the post-stage (calcu- lated from Quartz IV) is 9.0–11.5‰. It is suggested that the ore-forming fluids of Lannigou gold deposit reflect a mixed source: a low O isotope end-member that was magmatic- or basement related, whereas the high O isotope end-member may have been fluid interacted with wall-rock, whose influence strengthened during the miner- alization episode. The narrow δ18OVSMOW ranges of Quartz III and IV also suggest that any meteoric water influence on the late hydrothermal system was weak.This research was jointly funded by the key project of National Natural Science Foundation of China (41830432), National Natural Science Foundation of China for Youth Scientist (41903044) and the China Postdoctoral Science Foundation (2018M643531

Redistribution of trace elements during prograde metamorphism from lawsonite blueschist to eclogite facies; implications for deep subduction-zone processes

The transfer of fluid and elements from subducting\ud crust to the overlying mantle wedge is a fundamental\ud process affecting arc magmatism and the chemical differentiation of the Earth. While the production\ud of fluid by breakdown of hydrous minerals is well understood, the liberation of trace elements remains\ud generally unconstrained. In this paper, we evaluate the\ud behaviour of trace elements during prograde metamorphism\ud and dehydration using samples of high-pressure,\ud low-temperature metamorphic rocks from New Caledonia.\ud Samples examined include mafic and pelitic rocktypes\ud that range in grade from lawsonite blueschist to eclogite facies, and represent typical lithologies of subducting\ud crust. Under lawsonite blueschist facies conditions,\ud the low temperatures of metamorphism inhibit equilibrium partitioning between metamorphic minerals and allow for the persistence of igneous and detrital minerals. Despite this, the most important hosts for trace-elements include lawsonite, (REE, Pb, Sr), titanite (REE, Nb, Ta), allanite (LREE, U, Th), phengite (LILE) and zircon (Zr, Hf). At epidote blueschist to eclogite facies conditions, trace-element equilibrium may be attained and epidote (REE, Sr, Th, U, Pb), garnet (HREE), rutile (Nb, Ta), phengite (LILE) and zircon (Zr, Hf) are the major trace-element hosts. Chlorite, albite, amphibole and omphacite contain very low concentrations of the investigated trace elements. The\ud comparison of mineral trace-element data and bulk-rock\ud data at different metamorphic grades indicates that trace\ud elements are not liberated in significant quantities by\ud prograde metamorphism up to eclogite facies. Combining\ud our mineral trace-element data with established phase equilibria, we show that the trace elements considered\ud are retained by newly-formed major and accessory minerals during mineral breakdown reactions to depths of up to 150 km. In contrast, significant volumes of fluid are released by dehydration reactions. Therefore, there is a decoupling of fluid release and trace element release in subducting slabs. We suggest that the flux of trace elements from the slab is not simply linked to mineral breakdown, but results from complex fluid-rock interactions and fluid-assisted partial melting in the slab

Sources of auriferous fluids associated with a Neoarchean BIF-hosted orogenic gold deposit revealed by the multiple sulfur isotopic compositions of zoned pyrites

Internal textures, multiple sulfur isotopic compositions, and contents of gold, selenium, and molybdenum of ore-related pyrites from a Neoarchean carbonate-facies BIF-hosted gold deposit in Quadrilátero Ferrífero were investigated to elucidate the source(s) of sulfur. Sodium hypochlorite etching and BSE imaging revealed pervasive zoning in pyrites. Five different growth zones (Py1a, Py1b, Py2, Py3, and Py4) and six types of zoning (Type A–Type F) were identified. Two pyrite generations were distinguished in ores: G1 (Py1a and Py1b) and G2 (Py2, Py3, Py4). Both G1 and G2 have positive Δ S, but the magnitudes of G1 are higher. The G1-relevant fluids can be shallow-sourced, whereas the G2-related auriferous fluids are most likely deep-sourced metamorphic fluids derived from devolatilization of the lower succession of the Nova Lima Group (metavolcanics and metasedimentary rocks) during metamorphism, with fluid–rock interactions during fluid ascent and at the depositional site. The negative δ S, higher selenium contents, and carbonates inclusions of Py1a contrast with the positive δ S, lower selenium contents, and carbonaceous material inclusions of Py2, Py3, and Py4. The Lamego system possibly started with shallow-sourced oxidized and low-gold fluids (Py1a), closely followed by mixing in of deep-seated reduced auriferous fluids (Py2, Py3, Py4) initiated by tectonic activities. The oscillatory zoning of Py2 with the highest gold contents consists of alternating gold-rich and gold-poor laminae, indicating that fault-valve activity is a trigger of gold deposition. The sulfidation of siderite in BIF (desulfidation of auriferous fluids) related to the formation of ore-related pyrites also contributed to gold precipitation. 33 34 34This study was supported by the Australian Research Council DP140103393 to TR

Geology and intrusion-related affinity of the Morila Gold Mine, Southeast Mali

The ∼8 Moz Morila gold mine, hosted within Paleoproterozoic Birimian volcano-sedimentary rocks of southeast Mali, is spatially and temporally associated with prolonged (2098-2065 Ma) arc magmatism during the late stages of the Eburnean orogeny. Visible gold at Morila is associated with variably deformed polymineralic veins containing native bismuth, maldonite, aurostibite, rare tellurobismuthite, and löllingite, suggesting a proximal intrusion-related source for this period of gold mineralization. This early formed mineralization is contained within a zone of hornblende hornfels contact metamorphism and is spatially associated with syn- to post-D emplacement of 2098 to 2091 Ma quartz-diorite, granodiorite, and leucogranite magmas. The occurrence of immiscible Au-Sb-Bi-Te blebs within sills or dikes associated with gold mineralization at the Morila deposit explicitly links granitic magmatism with gold mineralization This early intrusion-related gold system was overprinted by a younger post-D stage of hydrothermal alteration recorded by sulfidation along a north-northeast-trending zone characterized by disseminated idioblastic arsenopyrite porphyroblasts that contain polygonal gold blebs. Silicate alteration during this stage includes albitization of plagioclase and the growth of randomly distributed biotite and titanite, the latter typically surrounding ilmenite. Uranium-Pb dating of this generation of titanite yields a preliminary age for late-stage sulfidation of 2074 ± 14 Ma, which brackets mineralization to the interval 2098 ± 4 to 2074 ± 14 Ma. The geochemistry and isotope systematics of syn- to post-tectonic intermediate intrusions at the Morila deposit point to their derivation in a suprasubduction zone setting and emplacement into tectonically thickened crust. Based on these observations, it is suggested that the Morila gold deposit formed during late-stage collisional orogenesis involving the accretion of juvenile volcanic arc terranes against the Archean Man (Liberian) cratonic nucleus. This setting is analogous to younger Phanerozoic active continental margin settings which host the best-described examples of intrusion-related gold systems

Petrogenesis of the Greenhills Complex, Southland, New Zealand: magmatic differentiation and cumulate formation\ud at the roots of a Permian island-arc volcano

A Permian (265 Ma) intrusive complex which formed as a magmatic feeder reservoir to an immature island-arc volcano is fortuitously exposed in southern New Zealand. Known as the Greenhills Complex, this intrusion was emplaced at shallow crustal levels and consists of two layered bodies which were later intruded by a variety of dykes. Cumulates, which include dunite, olivine clinopyroxenite, olivine gabbro, and hornblende gabbro-norite, are related products of parent-magma fractionation. Both primary (magmatic)\ud and secondary platinum-group minerals occur within\ud dunite at one locality. Using the composition of cumulus\ud minerals, mafic dykes and melt inclusions, we have determined that the parent magmas of the complex were\ud hydrous, low-K island-arc tholeiites of ankaramitic affinities. Progressive magmatic differentiation of this\ud parent magma generated fractionated melt of high-alumina\ud basalt composition which is now preserved only as dykes which cut the Complex. Field evidence and cumulus mineral profiles reveal that the magma chambers experienced turbulent magmatic conditions during cumulate-rock formation. Recharge of the chambers by primitive magma is likely to have coincided with eruption of residual melt at the surface. Similar processes are inferred to account for volcanic-rock compositions in other parts of this arc terrane and in modern island-arc systems