Mineral Pairs Research Articles

Carbon and oxygen isotopic composition of fluid inclusion CO2 and CH4 within the Archean Junction gold deposit, Kambalda, Western Australia

ABSTRACT The Junction orogenic gold deposit in the St Ives district of Western Australia is characterized by a syn-gold mineral assemblage dominated by quartz-calcite-albite-biotite-chlorite-pyrrhotite. The deposit had a light hydrocarbon,CO2, and O2 soil-gas signature above known mineralization prior to mining and it has been proposed that the source of the light hydrocarbon gases in the soil was gas-rich fluid inclusions trapped in gold-related alteration minerals, particularly calcite, albite, quartz, and pyrrhotite. Linking the soil gases with those in the deposit is extremely difficult. However, establishing that the gases in the soil are indeed present within the deposit and that those gases are related to the syn-Au alteration minerals is achievable through stable-isotope studies. Carbon and O stable-isotope compositions of pre-gold, syn-gold, and post-gold quartz veins; syn-gold and post-gold calcite; and CO2 and CH4 in the fluid inclusions that each of these minerals host were investigated to establish if the various mineral and fluid-gas species in the deposit are in isotopic equilibrium with each other, an important first step to relate syn-ore minerals with the relevant gases. Pre-ore Mo-type quartz veins contain CO2 (δ13Cgas = –1‰) and CH4 (δ13Cgas = ca. –33‰) in fluid inclusions at a ratio of ca. 93:7. The paucity of Mo-type quartz veins in the deposit suggests that these veins were not the main source of the soil-gas signature. Syn-gold alteration post-dates the Mo-type quartz veins. Quartz and co-existing calcite in the Au-bearing Junction shear zone have δ18Omineral values around 12.0 and 10.5‰, respectively. Multiple co-existing quartz-calcite pairs indicate that gold deposition occurred at ∼400 °C. This temperature agrees with mineral equilibria temperature estimates, the entrapment temperatures of fluid inclusions, and temperature modelling of solid-solution mineral phases. The temperature dictates that the quartz and calcite are in isotopic equilibrium with each other. The calcite in the Junction shear zone has δ13Cmineral values from –7.4 to –2.5‰, indicating that the CO2-rich ore fluid had a δ13Cfluid value of –3.7 ± 0.9‰. CO2 and CH4 released from quartz-hosted fluid inclusions have δ13Cgas values from –4.3 to +3.5‰ (mean = –1.5 ± 1.9‰) and –50.5 to –35.2‰, respectively. The isotopic composition of the fluid inclusion CO2 is in disequilibrium with co-existing CH4 that was co-released from the same quartz vein and the calculated δ13Cfluid value from co-existing calcite. Isotopic mass balance calculations using the two co-released gases show that the CO2 was initially in equilibrium with the syn-ore calcite but has since re-equilibrated with CH4 at temperatures below 200 °C. The abundance of CH4 in some quartz veins suggests that the syn-gold vein assemblage could be the source for the soil-gas anomaly. Post-gold veins contain quartz and calcite that have δ18Omineral values of ca. 11.0 and 10.0‰, respectively. Individual mineral pairs indicate precipitation at ∼320 °C from a fluid with a δ18Ofluid value of 4.7 ± 0.9‰, distinct from that which formed the syn-gold quartz veins. The post-gold calcite has δ13Ccalcite values from –7.5 to –5.4‰, indicative of formation from a CO2-bearing fluid having a δ13Cfluid value of –4.6 ± 0.9‰. The δ13Cfluid values are indistinguishable from fluid inclusion CO2 values of –3.6 ± 0.9‰, indicating no post entrapment re-equilibration, which suggests that CH4 was at trace volumes or absent in the post-gold quartz veins. These data lead to the conclusion that post-entrapment reequilibration between fluid inclusion CO2 and CH4 has occurred, but that the two gases were likely in equilibrium at the time of entrapment. This has implications for the interpretation of C isotope studies that focus on fluid inclusion CO2 measured from other gold and base-metal deposits, especially when the isotopic value of that CO2 is assumed to represent a specific source for the ore-forming fluids. The data also lead to a model that proposes that the syn-gold alteration assemblage could have produced the soil-gas anomalies observed above the mineralization.

Late Permian medium-pressure metamorphism in the eastern Songnen Massif, eastern Central Asian Orogenic Belt (NE China): Implications for the final closure of the Paleo-Asian Ocean

The nature of late Paleozoic metamorphism in the eastern Central Asian Orogenic Belt (CAOB) is controversial. This paper reports mineral chemistry, petrological, and zircon and titanite U–Pb age data for representative rocks in the Dongfengshan Group from the eastern margin of the Songnen Massif. We use these data to determine the rock associations, P–T conditions, and timing of metamorphism. The P–T conditions were estimated from 54 mineral pairs in representative rocks, including garnet–biotite schist, garnet–two-mica schist, garnet–biotite–plagioclase gneiss, and garnet amphibolite, and indicate the Dongfengshan Group experienced metamorphism at P = 5.5–7.1 kbar and T = 592–630 °C (i.e., a geothermal gradient ranging from 25 to 30 °C/km) under medium-pressure lower amphibolite-facies conditions. The biotite–plagioclase gneiss and garnet amphibolite yielded titanite U–Pb ages of 251 and 260 Ma, respectively, which are interpreted to represent the regional metamorphic age (260–251 Ma). The consistency of these dates and ages of metamorphic rims in zircons from a biotite plagiogneiss (252 Ma) confirm that medium-pressure metamorphism occurred in the late Permian rather than the Neoproterozoic, as assumed previously. Based on metamorphic, magmatic, stratigraphic, and paleontological constraints, as well as the regional tectonic history, we propose that late Permian medium-pressure metamorphism in the Dongfengshan Group occurred during the collision and amalgamation of the Siberian and North China cratons following the final closure of the eastern Paleo-Asian Ocean.

Academic Reminiscences and Thermodynamics-Kinetics of Thermo-Barometry-Chronology

This article has three major components that include, in addition to the technical aspects, reminiscences of my academic upbringing, my move to the USA from India, and my professional career. I have recounted many stories that I hope convey some sense of time, especially in these two countries with vastly different cultures, my personal journey with its ups and downs and how I made the transition to an academic career path in USA even though that was not in my future plan as a young man. The development of the field of thermobarometry and its integration with diffusion and crystal kinetic modelling of compositional zoning (or lack thereof) and cation ordering in minerals have led to important quantitative constraints on the pressure-temperature-time evolution of terrestrial rocks and meteorites. I review the historical developments in these areas and a segment of my own research spanning the period of 1964-2021. The foundational works of the thermometry of metamorphic rocks and palaeothermometry were laid at the University of Chicago around 1950. Subsequently, the synergetic growth of thermodynamics and experimental studies in petrology in the 1960s and 1970s, along with the introduction of electron microprobe as a nondestructive analytical tool with micron scale resolution, gave a major boost to the field of thermobarometry. There were also significant new developments in the field of thermodynamics of solid solutions in the petrology community and demonstration from observational data, countering strong scepticism, that the principles of classical thermodynamics were applicable to “complex natural systems”. The section on thermodynamic basis of thermobarometry concludes with a discussion of the thermodynamics of trace element and single mineral thermometry. I further deal with the experimental protocols, along with selected examples, for phase equilibrium studies that provide the bedrock foundation for the field of thermobarometry based on elemental compositions of coexisting minerals in a rock. It is followed by an account of the controversies and international meetings relating to the aluminum silicate and peridotite phase diagrams that play crucial roles in the thermobarometry of metamorphic rocks and mantle xenoliths, respectively. The construction of quantitative petrogenetic grids to display stability relations of minerals in multicomponent–multiphase systems came into play in the field of metamorphic petrology in the mid-1960s and early 1970s. Augmented by experimental data, these petrogenetic grids led to important discoveries about the P-T-<em>f</em>(O<sub>2</sub>) and bulk compositional controls on the stability of certain “index” minerals that are used to define metamorphic isograds and different types of regional metamorphism; one such grid also opened up a new field that came to be known as ultra-high temperature metamorphism. The construction of petrogenetic grids has now evolved to computer based calculations of complex equilibrium P-T phase diagrams, commonly referred to as “pseudosections”, by minimisation of Gibbs free energy of a system with fixed bulk composition. I discuss these historical developments and modern advancements. Subsequently I highlight some aspects of thermobarometry and diffusion kinetic modelling of selected natural samples along with their broader implications and present a critical discussion of different protocols for thermobarometry of natural assemblages. Following up on the introductory historical perspective of development of palaeothermometry, I discuss the modern advancements using density functional theory (DFT). Examples of DFT based calculations have been shown for hydrogen isotope fractionation in mineral-water/hydrogen systems and “clumped isotope” thermometry. The hydrogen isotope fractionation data led the development of new low temperature palaeothermometers using serpentine-talc/brucite mineral pairs. These results enable simultaneous solutions of both temperature and source of fluid in the serpentinisation process of rocks. The final section is devoted to high temperature thermochronology dealing with the problems of closure temperature of decay systems in minerals and the use of bulk and spatial resetting of mineral age according to a specific decay system to determine cooling rates of the host rocks. Complications arise in the interpretation of mineral ages determined by such decay systems as <sup>176</sup>Lu-<sup>176</sup>Hf or the short- lived system <sup>53</sup>Mn-<sup>53</sup>Cr in which the parent nuclide has a much lower closure temperature than the corresponding daughter product. Numerical simulations help explain the discrepancy between the <sup>176</sup>Lu-<sup>176</sup>Hf and <sup>147</sup>Sm-<sup>143</sup>Nd ages of garnets in metamorphic rocks and enable construction of the entire T-t cycle from the discrepant ages and some additional constraints.

Mineralogical and geochemical characterization and implications for provenance of Roman granite columns in ancient Tripolis (Denizli, Turkey)

Roman granite columns of the monumental fountain (Nymphaeum) in Tripolis ancient city (Denizli, Western Turkey) have porphyro-phaneritic texture and are characterized by the presence of a K-feldspar megacrysts. The columns mainly consist of K-feldspar (microcline and orthoclase), quartz, plagioclase, mafic minerals (biotite and amphibole), and minor amount of titanite, apatite, zircon, and opaque minerals. Electron probe microanalysis (EPMA) reveals that plagioclase is oligoclase (An16–23), trioctahedral mica is ferro-biotite and siderophyllite (Mg#: 0.20–0.24), and amphibole is hastingsite (Mg#: 0.19–0.24) in composition. Calculated temperature and pressure values from amphibole–plagioclase mineral pairs are 933 to 1000 °C (average 971 ± 13.5 °C) and are 3.5 to 6.9 kbar (average 4.8 ± 0.7 kbar), respectively. Roman granite columns are mildly alkaline and characterized by metaluminous nature. In Primitive Mantle (PM), Oceanic Ridge Granite (ORG), and Continental Crust (CC) multi-element normalization diagrams, the columns show LILE enrichment relative to HFSE. Furthermore, they bear A-type parental magma characteristics that occurred in a late/post-orogenic tectonic setting. Detailed mineralogical–petrographic, mineral chemistry, geochemical, and some archeological data support the assumption that Roman granite columns have similar features to Red Aswan Granite in Egypt and were imported to Tripolis.

Precise initial abundance of Niobium-92 in the Solar System and implications for p-process nucleosynthesis

The niobium-92-zirconium-92 (92Nb-92Zr) decay system with a half-life of 37 Ma has great potential to date the evolution of planetary materials in the early Solar System. Moreover, the initial abundance of the p-process isotope 92Nb in the Solar System is important for quantifying the contribution of p-process nucleosynthesis in astrophysical models. Current estimates of the initial 92Nb/93Nb ratios have large uncertainties compromising the use of the 92Nb-92Zr cosmochronometer and leaving nucleosynthetic models poorly constrained. Here, the initial 92Nb abundance is determined to high precision by combining the 92Nb-92Zr systematics of cogenetic rutiles and zircons from mesosiderites with U-Pb dating of the same zircons. The mineral pair indicates that the 92Nb/93Nb ratio of the Solar System started with (1.66 ± 0.10) × 10-5, and their 92Zr/90Zr ratios can be explained by a three-stage Nb-Zr evolution on the mesosiderite parent body. Because of the improvement by a factor of 6 of the precision of the initial Solar System 92Nb/93Nb, we can show that the presence of 92Nb in the early Solar System provides further evidence that both type Ia supernovae and core-collapse supernovae contributed to the light p-process nuclei.

Reduction of oxidized sulfur in the formation of the Grasberg porphyry copper-gold deposit, Papua, Indonesia

The reduction of oxidized sulfur is essential in porphyry copper deposits whose mineralization predominantly comprises copper sulfides, whereas their source magmas are oxidized with most of their sulfur as SO42− and with exsolved fluids having SO2>>H2S. To estimate the redox state of sulfur, we examined drill cores that intersect potassic and unaltered intrusive rocks in the deeper levels of the Grasberg porphyry copper-gold deposit. Magmatic oxybarometers such as the amphibole-titanite-magnetite-quartz assemblage, anhydrite, and amphibole consistently show that Grasberg ore-forming magmas were oxidized with fO2 > FMQ+3. Initial hydrothermal events formed sulfide-free, anhydrite-rich K-feldspar, and biotite alteration, followed by successive vein stages of (1) magnetite, (2) biotite, (3) quartz, (4) anhydrite-chalcopyrite, (5) chalcopyrite ± sericite selvages, and (6) pyrite-chalcopyrite-quartz + sericite selvages. The δ34S values of sulfide-sulfate mineral pairs indicate SO2-derived SO42− and H2S in SO42−/H2S molar proportions of ~ 4:1 to ~ 3:1 at > 550 °C. The hydrothermal fluid then likely followed a rock-buffered trajectory and became more reduced at < 550 °C. Hydrothermal biotite that replaces igneous amphibole and biotite has a phlogopitic composition, suggesting that Fe2+ was liberated from igneous mafic minerals and oxidized by reaction with SO42− to form magnetite, resulting in sulfide formation by the simplified reaction 12FeO + SO42− + 2H+ → 4Fe3O4 + H2S.

First-principles calculations of equilibrium nitrogen isotope fractionations among aqueous ammonium, silicate minerals and salts

Nitrogen isotopes are a robust tool to study geological nitrogen cycle between Earth’s reservoirs. However, the application of nitrogen isotope system to understanding geological processes has been limited by the lack of constraints on equilibrium isotope fractionation factors between mineral and fluid and between mineral pairs. Here, we use first-principles methods to calculate the nitrogen isotope fractionations among aqueous ammonium, ammonium- and/or nitrate-bearing salts, and ammonium-bearing silicate minerals that commonly occur in Earth’s lithosphere. Our results show a first-order, large nitrogen isotope fractionations between the nitrate group and the ammonium group, with 15N being more enriched in the nitrate group. In detail, the nitrogen isotope fractionations among nitrate group minerals (NaNO3, KNO3, Ba(NO3)2, NH4NO3) are very small. The nitrogen isotope fractionations are also small among the ammonium group (e.g., aqueous ammonium, ammonium salts, phyllosilicate, and tectosilicate minerals) except inosilicate minerals (e.g., diopside and jadeite), which are however significantly more enriched in 15N than the other ammonium-bearing minerals. These results suggest that nitrogen isotopes may serve as a robust geothermometer only when the rock contains coexisting ammonium and nitrate minerals, or contain diopside and/or jadeite together with other ammonium-bearing silicate minerals. Given that common silicate minerals in crustal rocks (clays, micas, feldspars) do not significantly discriminate nitrogen isotopes, nitrogen isotopes can thus be used as a sensitive tool to trace material source and infer geochemical processes that may cause isotope disequilibrium, such as metamorphic devolatilization, hydrothermal alteration, crust-mantle interaction. Our results also demonstrate that authigenic clay minerals can inherit the nitrogen isotopic signature of organic matter and aqueous ammonium and thus can serve as an environmental proxy. However, it is crucial to distinguish detrital minerals from authigenic minerals in sediments when reconstruct marine and lacustrine environments in the deep time (Archean and Proterozoic).

Heat production and moho temperatures in cratonic crust: evidence from lower crustal xenoliths from the slave craton

Ambient Moho temperatures and lower crustal heat production are surprisingly poorly constrained in cratons. Here we address these problems using 15 lower crustal xenoliths from the Diavik A-154 kimberlite, Slave craton, Canada. Iron‑magnesium exchange geothermometry on small biotite and amphibole inclusions in garnet indicates that the Slave craton lower crust was at a temperature of ≤500 °C at the time of kimberlite eruption (~55 Ma). The ambient lower crustal temperature was likely lower than 500 °C because the thermometers record the closure temperature of diffusional Fe2+-Mg exchange between touching mineral pairs. New measurements of K, U and Th concentrations in the constituent minerals, together with xenolith modes, allow reconstruction of the heat-producing element (HPE) K, U, and Th budget of the Slave craton lower crust. Metasedimentary granulites have an average heat production of 0.29 ± 0.01 μW/m3 (n = 3) whereas mafic granulites have an average heat production of 0.13 ± 0.03 μW/m3 (n = 12). Our new data clearly show that plagioclase abundance in both lithologies has a major influence on overall lower crustal heat production, being an important reservoir of all three HPE. Combining the heat production of mafic and metasedimentary granulites in their observed 80:20 proportions results in an average heat production value for the Slave craton lower crust of 0.16 ± 0.03 μW/m3.Using these heat production estimates, modeled Moho temperatures beneath Diavik of ~450–470 °C are broadly consistent with maximum lower crustal temperatures indicated by geothermometry. The low HPE contents predicted for cratonic lower crust must result in lower temperatures in the deep crust and mantle lithosphere, and in turn higher estimates for the thickness of mantle lithosphere. This effect becomes larger as the thickness of the low-HPE lower crustal layer increases. In the specific case of the central Slave craton, we find that model estimates of the diamond potential of the mantle lithosphere, as judged by the proportion of lithospheric mantle in the diamond stability field, are not strongly affected by small variations in lower crustal heat production and Moho temperature.

Applications and Limitations of Elastic Thermobarometry: Insights From Elastic Modeling of Inclusion‐Host Pairs and Example Case Studies

AbstractElastic thermobarometry can be used to constrain the pressure and temperature conditions of mineral crystallization by exploiting the difference in the elastic evolution of a mineral inclusion and its host during cooling and decompression. In this work we examine the pressure‐temperature sensitivity of &gt;5,000 untested inclusion‐host pairs. Hosts such as diamond and zircon are ideal host minerals because their low compressibility makes them rigid containment vessels. Highly compressible inclusions such as albite, graphite, and quartz serve as the most reliable barometers. We provide three case studies of inclusion‐host pairs from different geologic settings to demonstrate the advantages and challenges associated with these mineral pairs. Apatite inclusions in olivine from Yellowstone caldera mostly record negative residual pressures (tension) and suggest magmatic crystallization at ~0.4 GPa. Rutile inclusions in garnet from Verpeneset eclogites record near ambient conditions and do not recover reasonable metamorphic conditions of rutile entrapment. These results suggest that stiff inclusions may have a tensile strain limit, a possible limitation of elastic thermobarometry. Albite inclusions in epidote from a blueschist (Syros, Greece) record geologically reasonable entrapment pressures, but a large range of residual pressures that may be caused by the complex anisotropy of both phases. Our theoretical and applied results indicate that elastic thermobarometry has the potential to be used to understand petrologic processes in diverse geologic environments, including mantle, metamorphic, and magmatic settings but that each elastic thermobarometer requires careful evaluation.

Sr–Nd–O isotopic evidence of variable sources of mantle metasomatism in the subcratonic lithospheric mantle beneath the Grib kimberlite, northwestern Russia

To provide new insights into the type and extent of mantle metasomatism in the subcratonic lithospheric mantle, we examined the Sr–Nd–O isotopic compositions of orthopyroxene, clinopyroxene, garnet, ilmenite and phlogopite from sheared garnet lherzolite, granular garnet harzburgites and lherzolites and clinopyroxene-phlogopite rocks from the Grib kimberlite in the Arkhangelsk diamond province in northwestern Russia.Clinopyroxene and orthopyroxene from sheared garnet lherzolite initially have a close value of 87Sr/86Sr(t) (~0.7034) and close weak positive εNd. Orthopyroxene and clinopyroxene are in oxygen isotope equilibrium with coexisting olivine. Clinopyroxene from a garnet harzburgite has a low 87Sr/86Sr(t) isotope ratio of 0.70266. Clinopyroxene from granular garnet lherzolites has a relatively narrow variation in 87Sr/86Sr(t) (0.70456–0.70582) and considerably larger variations in εNd (−4.3 − +1.0) isotope ratios. Garnet displays elevated initial 87Sr/86Sr(t) values (0.70540–0.70633). Ilmenite shows a narrow range in 87Sr/86Sr(t) (0.70497–0.70522) coupled with εNd values of +0.4 and +3.5. These isotopic data suggest granular garnet lherzolite of mantle metasomatism took place during the interaction of kimberlite melts with SCLM that contained mica-amphibole-rutile-ilmenite-diopside (MARID)-type metasomes. Clinopyroxenes from clinopyroxene-phlogopite (phlogopite wehrlite) xenoliths display a broader range in 87Sr/86Sr(t) (0.70486–0.70813) that is significantly higher than the kimberlite values and a circa-chondritic εNd (−0.1 − +1.3) with a restricted δ18O range (5.11‰–5.33‰). More radiogenic Sr isotopic composition decoupled from Nd isotopes could have been induced by metasomatic melt/fluid related to a subducted material.The isotopic compositions of mantle minerals preserve Sr–Nd isotopic evidence of pre-kimberlite metasomatic events that were probably due to incomplete reequilibration with ultramafic carbonated melt. Based on mineral pairs Rb–Sr isochrons and a clinopyroxene-based Sm–Nd errochron, these mantle metasomatic events correspond to ~550–600 Ma and ~1200 Ma episodes of magmatic–thermal activity.

Trace elemental and sulfur-lead isotopic variations in metamorphosed volcanogenic massive sulfide (VMS) mineralization systems: An example from the Keketale Pb-Zn(-Ag) deposit, NW China

The effects of post-VMS (volcanogenic massive sulfide) deformation/metamorphism on sulfide trace element and sulfur-lead isotopic compositions remain unclear. The greenschist to lower amphibolite facies metamorphosed Keketale VMS Pb-Zn(-Ag) deposit, located in the Devonian Maizi volcanic-sedimentary basin in the Chinese Altay, NW China, provides an opportunity to resolve the abovementioned issue. Two mineralization stages are recognized at Keketale, including (1) primary banded, massive and disseminated ores derived from sea-floor hydrothermal mineralization and (2) ore remobilization, as represented by intensively-deformed ores and quartz-polymetallic sulfide ore veins that crosscut the primary ores. Sulfide trace elements data suggest that Stage II pyrite and pyrrhotite have higher contents of Cu, Zn, Ag, Sb and Pb, whereas contents of these elements are higher in Stage I sphalerite and galena than their Stage II counterparts. Such elemental remobilization was probably caused by the rapid intragrain diffusion and subsequent fluid-mediated liberation with trace elements re-precipitated as particles within the same grain and nearby minerals, respectively. The δ34SCDT values in both stages show a trimodal distribution (−25.9 to −22.7‰, −17.0 to −10.6‰ and 0.6 to 2.1‰). Sulfide mineral pairs formed during primary VMS mineralization are not in sulfur isotopic equilibrium, and only localized re-equilibration occurs during deformation and metamorphism. In addition, the lead isotopic values of galena from remobilized sulfide veins overlap with those of the Stage I galena and pyrite. These data indicate that regional deformation and metamorphism of the Keketale deposit can be approximated as a closed system in terms of sulfur-lead isotopes. Whilst the presence of the anomalously low δ34S values (minimum −25.9‰) and the distinct lead isotopic ratios of sulfides from the calcite-quartz veins argue for that in the late part of the metamorphic event, some exotic lead and light sulfur were sourced from outside the ore field. In situ δ34S values of Stage I sulfides vary widely (−17.0 to +2.1‰), suggesting that the sulfur was originated from bacterial sulfate reduction (BSR) of marine sulfates with probably some magmatic input. Lead isotopes of the sulfides and mineralized meta-sedimentary-volcanic rocks display a linear distribution between the mantle and upper crust line in the 206Pb/204Pb vs. 207Pb/204Pb diagram. The lead source may be derived from both felsic volcanic rocks and mafic igneous rocks (and/or magmatic volatiles), and the sulfur-lead isotopes may result from a shallow-level seawater circulating process. An additional group of lead isotopic compositions of the meta-volcanic rocks show elevated present-day 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values, probably due to decay of Th and U carried by deeply-penetrative circulation (~13 km) of CO2-F-Cl-rich hydrothermal (metamorphic) fluids. Both syn-VMS shallow (seawater) and post-VMS deep (metamorphic fluids) circulation processes are critical for the metal concentration at Keketale.

Mineralogical and Isotopic Characteristics of Sodic-Calcic Alteration in the Highland Valley Copper District, British Columbia, Canada: Implications for Fluid Sources in Porphyry Cu Systems

AbstractThe Highland Valley Copper porphyry Cu (±Mo) district is hosted in the Late Triassic Guichon Creek batholith in the Canadian Cordillera. Fracture-controlled sodic-calcic alteration is important because it forms a large footprint (34 km2) outside of the porphyry Cu centers. This alteration consists of epidote ± actinolite ± tourmaline veins with halos of K-feldspar–destructive albite (1–20 XAn) ± fine-grained white mica ± epidote. The distribution of sodic-calcic alteration is strongly influenced by near-orthogonal NE- and SE-trending fracture sets and by proximity to granodiorite stocks and porphyry dikes. Multiple stages of sodic-calcic alteration occurred in the district, which both pre- and postdate Cu mineralization at the porphyry centers.The mineral assemblages and chemical composition of alteration minerals suggest that the fluid that caused sodic-calcic alteration in the Guichon Creek batholith was Cl bearing, at near-neutral pH, and oxidized, and had high activities of Na, Ca, and Mg relative to propylitic and fresh-rock assemblages. The metasomatic exchange of K for Na, localized removal of Fe and Cu, and a paucity of secondary quartz suggest that the fluid was thermally prograding in response to magmatic heating. Calculated δ18Ofluid and δDfluid values of mineral pairs in isotopic equilibrium from the sodic-calcic veins and alteration range from 4 to 8‰ and −20 to −9‰, respectively, which contrasts with the whole-rock values for least altered magmatic host rocks (δ18O = 6.4–9.4‰ and δD = −99 to −75‰). The whole-rock values are suggested to reflect residual magma values after D loss by magma degassing, while the range of hydrothermal minerals requires a mixed-fluid origin with a contribution of magmatic water and an external water source. The O-H isotope results favor seawater as the source but could also reflect the ingress of Late Triassic meteoric water. The 87Sr/86Srinital values of strongly Na-Ca–altered rocks range from 0.703416 to 0.703508, which is only slightly higher than the values of fresh and potassic-altered rocks. Modeling of those data suggests the Sr is derived predominantly from a magmatic source, but the system may contain up to 3% seawater Sr. Supporting evidence for a seawater-derived fluid entrained in the porphyry Cu systems comes from boron isotope data. The calculated tourmaline δ11Bfluid values from the sodic-calcic domains reach 18.3‰, which is consistent with a seawater-derived fluid source. Lower tourmaline δ11Bfluid values from the other alteration facies (4–10‰) suggest mixing between magmatic and seawater-derived fluids in and around the porphyry centers. These results imply that seawater-derived fluids can infiltrate batholiths and porphyry systems at deep levels (4–5 km) in the crust. Sodic ± calcic alteration may be more common in rocks peripheral to porphyry Cu systems hosted in island-arc terranes and submarine rocks than currently recognized.

Thermodynamic Constraints on REE Mineral Paragenesis in the Bayan Obo REE-Nb-Fe Deposit, China

Hydrothermal processes have played a significant role in rare earth element (REE) precipitation in the Bayan Obo REE-Nb-Fe deposit. The poor preservation of primary fluid inclusions and superposition or modification by multiphase hydrothermal activities have made identification of physico-chemical conditions of ore-forming fluids extremely difficult. Fortunately, with more and more reliable thermodynamic properties of aqueous REE species and REE minerals reported in recent years, a series of thermodynamic calculations are conducted in this study to provide constraints on REE precipitation in hydrothermal solutions, and provide an explanation of typical paragenesis of REE and gangue minerals at Bayan Obo. During the competition between fluocerite and monazite for LREE in the modelled solution (0.1 M HCl, 0.1 M HF and 0.1 M trichloride of light rare earth elements (LREE) from La to Sm), all LREE would eventually be hosted by monazite at a temperature over 300 °C, with continuous introduction of H3PO4. Additionally, monazite of heavier LREE would precipitate earlier, indicating that the Ce- and La-enriched monazite at Bayan Obo was crystallized from Ce and La pre-enriched hydrothermal fluids. The fractionation among LREE occurred before the ore-forming fluids infiltrating ore-hosting dolomite. When CO2 (aq) was introduced to the aqueous system (model 1), bastnaesite would eventually and completely replace monazite-(Ce). Cooling of hot hydrothermal fluids (&gt;400 °C) would significantly promote this replacement, with only about one third the cost of CO2 for the entire replacement when temperature dropped from 430 °C to 400 °C. Sole dolomite addition (model 2) would make bastnaesite replace monazite and then be replaced by parisite. The monazite-(Ce) replaced by associated bastnaesite and apatite is an indicator of very hot hydrothermal fluids (&gt;400 °C) and specific dolomite/fluid ratios (e.g., initial dolomite at 1 kbar: 0.049–0.068 M and 0.083–0.105 M at 400 °C and 430 °C). In hot solution (&gt;430 °C) that continuously interacts with dolomite, apatite precipitates predating the bastnaesite, but it behaves oppositely at &lt;400 °C. The former paragenesis is in accord with petrography observed in this study. Some mineral pairs, such as monazite-(Ce)-fluorite and monazite-(Ce)-parisite would never co-precipitate at any calculated temperature or pressure. Therefore, their association implies multiphase hydrothermal activities. Pressure variation would have rather limited influence on the paragenesis of REE minerals. However, temperature and fluid composition variation (e.g., CO2 (aq), dolomite, H3PO4) would cause significantly different associations between REE and gangue minerals.

A Mineral X-ray Linear Attenuation Coefficient Tool (MXLAC) to Assess Mineralogical Differentiation for X-ray Computed Tomography Scanning

X-ray computed tomography (XCT) is becoming one of the most important techniques in the geosciences. The technique relies on linear attenuation coefficient differences in order to reveal the internal structure of the rocks. In this work, we present a new excel macro tool, called MXLAC, which is a data bank with an excel interface that uses density, X-ray energy and the mineral chemical formula to allow users to calculate mineral linear attenuation coefficients that can then be used to determine discrimination between mineral pairs. Elements within a mineral and specified by the chemical formula, determine how the X-ray beam is attenuated. Analysis of a variety of scanned mineral pairs with similar densities and attenuation coefficients indicates that an attenuation coefficient difference of greater than or equal to 6% at 45.5 keV effective X-ray energy is required for effective discrimination between two minerals using XCT with single energy scanning. This means that mineral pairs, such as quartz and pyrophyllite cannot be discriminated using the current XCT instruments due to the fact that the attenuation coefficient difference is less than 1.9% at 45.5 keV effective X-ray energy. Garnets and a variety of other minerals were used as examples to illustrate the importance of knowing the actual chemical formula of the mineral to demonstrate whether they can be partially or fully discriminated from each other.

Influence of calcium nanoparticles (CaNPs) on nutritional qualities, radical scavenging attributes of Moringa oleifera and risk assessments on human health

This study investigated stimulatory potentials of zero-charged calcium nanoparticles (CaNPs) on nutritional contents and free radical scavenging abilities of Moringa oleifera. M. oleifera seeds were planted on soil wetted with water (control: group A), 100 mgL−1 Ca(NO3)2, 100, 75 and 50 mgL−1 CaNPs as groups B, C, D and E respectively. Elemental compositions were determined with inductively coupled plasma optical emission spectrometer (ICP-OES). CaNPs beneficially improved crude fibre, ash, K and Ca by 124.82, 152.5, 10.66 and 28.91% respectively in M. oleifera. There were significant declines in concentrations of Na, Ni, Pb, Cd and Cr in M. oleifera planted on CaNPs amended soil. CaNPs significantly improved free radical scavenging ability from 43.85 to 74.06%, phenolic contents from 248.82 to 555.88 mg quercetin/g and flavonoid contents from 120.75 to 148.08 mg quercetin/g. Reduction in oxidative stress biomarker (MDA) from 1.83 to 1.54 mmolg−1 in M. oleifera planted on CaNPs amended soil was also recorded. Strong correlations among essential mineral pairs indicate M. oleifera is a source of multiple mineral nutrients and a poor source of toxic heavy metals. Results of estimated daily intake of mineral nutrients and toxic hazard quotients of heavy metals suggest M. oleifera planted on CaNPs as the safest for consumption with respect to heavy metal toxicity. Lifetime risk assessment reveals a 50% reduction in cancer risk from consumption of M. oleifera planted on CaNPs soil. Thus, it is evidently clear from results of this study that CaNPs enhanced nutritional quality of M. oleifera.

Physical quality and mineral composition of new Mesoamerican bean lines developed for cultivation in Brazil

The present study was undertaken to evaluate whether new common bean lines present differences in their physical quality and mineral composition traits; investigate correlations between those traits; and to select common bean lines for use in human nutrition and in breeding programs. Four experiments were carried out to examine 17 genotypes (cultivars and lines) of carioca beans (beige seed coat with brown streaks) and black beans, which represent the technological advances of six breeding programs in Brazil. A significant genotype × environment interaction was observed for all evaluated traits, except for potassium concentration. Most physical quality traits were correlated with each other (r ≥ 0.848), and almost all evaluated pairs of minerals exhibited a positive correlation with each other (r ≥ 0.609). Indirect selection based on the highest values for normal grains and uptake is efficient for identifying fast-cooking common bean lines, and it is possible to select mineral-biofortified common bean lines. The LP 09-33 and LEC 01-16 carioca bean lines and the TB 03-11 and CHP 01-182-48 black bean lines present high physical quality and better mineral composition, and their use in human nutrition and in breeding programs is therefore recommended.

Calibrating equilibrium Fe isotope fractionation factors between magnetite, garnet, amphibole, and biotite

Equilibrium isotope fractionation factors are the basis for application of stable isotopes in geological studies. Experimental calibration and theoretical calculations have been employed to determine Fe isotope fractionation factors for a variety of minerals, however, these methods have their limitations. An alternative approach to calibrating inter-mineral Fe isotope fractionation factors is to use well-characterized geological samples; this approach has unique advantages over the other methods including attainment of equilibrium at relatively low temperatures. In this study, we investigated the Fe isotope composition of magnetite (Mt), garnet (Grt), amphibole (Amp) and biotite (Bt) from the metamorphosed Yingshan iron formation in South China. Two independent geothermometers, quartz–magnetite oxygen isotope and amphibole–garnet–biotite Fe–Mg exchange geothermometers, give a consistent metamorphic temperature of 538 ± 39 °C. The Fe isotope composition of the different Fe-bearing minerals is highly variable in different ironstone samples, with δ56Fe values (relative to IRMM-014) ranging from −0.23 to +0.37‰ in magnetite, −0.44 to +0.09‰ in amphibole, −0.78 to +0.02‰ in garnet, and −0.61 to +0.04‰ in biotite. Despite that, the offsets of δ56Fe values for mineral pairs are consistent, implying attainment of equilibrium isotope fraction between these minerals. Inter-mineral Fe isotopic fractionations (±2 standard deviation) measured from the multiple mineral pairs are Δ56FeMt–Grt = +0.55 ± 0.08‰, Δ56FeMt–Amp = +0.25 ± 0.06‰, Δ56FeMt–Bt = +0.42 ± 0.11‰, Δ56FeAmp–Grt = +0.34 ± 0.15‰, Δ56FeAmp–Bt = +0.26 ± 0.04‰, and Δ56FeBt–Grt = +0.13 ± 0.08‰. Based on the well-defined metamorphic temperature (538 ± 39 °C) and internally consistent inter-mineral fractionation factors, the temperature-dependent functions for equilibrium Fe isotope fractionation between the following mineral pairs are derived:103lnαMt–Grt = 0.36(±0.05) × 106/T2, 103lnαMt–Bt = 0.28(±0.07) × 106/T2,103lnαMt–Amp = 0.16(±0.04) × 106/T2, 103lnαAmp–Grt = 0.22(±0.10) × 106/T2,103lnαAmp–Bt = 0.17(±0.03) × 106/T2, and 103lnαBt–Grt = 0.09(±0.05) × 106/T2,where Fe3+/ΣFe ratio is 0.05 ± 0.02 in garnet, 0.29 ± 0.04 in biotite, and 0.24 ± 0.06 in amphibole for the above functions. The equilibrium Fe isotope fractionation factors derived in this study enable estimation of the metamorphic temperature of rocks that contain these mineral pairs, and identification of secondary processes that may have induced disequilibrium Fe isotope distribution in rocks, such as retrograde metamorphism and hydrothermal alteration.

Metasomatic Vein Formation by Stationary Fluids in Carbonate Xenoliths at the Eastern Margin of the Bergell Intrusion, Val Sissone, Italy

AbstractFluid flow is an important mechanism associated with heat and mass transport within the Earth’s crust. The study of veins, which represent channelling of fluids, can thus be key in understanding these fluid movements, unravelling fluid composition and origin, paleo stress regimes, and the history of the host-rock. New stable isotope data on carbonates and silicates have been combined with phase petrology, mass balance, and field observations to evaluate the formation mechanism of metasomatic reaction veins in dolomitic xenoliths in the Bergell tonalites (Val Sissone, Italy). Multiple generations of extensional veins can be followed from the contact zone between the dolomites and the intrusion to a few meters within either the tonalites (with the epidote–quartz veins), or within the dolomites, where they terminate. Each type of vein contains a central zone, which is formed by open fracture crystallization. This central fracture is framed by relatively symmetric replacement zones, where the original dolomite reacted to form either forsterite, diopside, tremolite or talc, all accompanied by calcite in either a succession of reaction zones or in simpler bi-mineral (silicate + calcite) veins. The δ18O and δ13C values across the veins allow temperatures to be estimated from different mineral pairs (silicate + calcite), and which confirm vein formation along a retrograde cooling path of the intrusion. At least four different fluid infiltration events are required, the first one around 555 °C to form the forsterite–calcite veins, followed by the epidote–quartz veins at temperatures around 430 °C, then the tremolite–calcite veins at around 390 °C, and finally the talc–calcite veins at around 140 °C. The shape of the δ18O and δ13C profiles, which are flat across the central part and the replacement zones of the veins (buffered by the intrusion), change substantially over short distances. Both of these isotope profiles overlap with the equally sharp mineralogical front between the veins and the unreacted dolomites. These profiles are interpreted to be the result of an isotopic exchange mechanism driven by dissolution and re-precipitation reactions. All veins are oriented perpendicular to the contact with the intrusive body, except for the late talc veins. Elevated fluid pressures, above the confining pressures caused by the regional and intrusion emplacement stress field, are suggested to be responsible for the initial fracturing of the carbonates and intrusive rocks. The contact zones between the tonalites and carbonates likely served as fluid conduits, where fluids accumulated and the pressure built up until hydrofracturing occurred. We propose that the veins formed through episodic pulses of highly reactive fluids that remained stationary during reaction, rather than a system where fluids flushed through the veins. Based on the XCO2-constrained mass balance, the formation of the veins would only require a relatively small amount of fluid, which could potentially originate from the intrusive rocks in vicinity of the xenoliths. Veining is not ubiquitous around the Bergell intrusion, suggesting that it only may have been a localized event and thus there is no need to involve a larger convective hydrothermal system for their formation.

Boron isotope composition of coexisting tourmaline and hambergite in alkaline and granitic pegmatites

The boron isotopic composition of tourmaline and hambergite (Be2BO3[OH,F]) from peraluminous (n = 12), peralkaline (n = 1), and peralkaline nepheline syenite (n = 16) pegmatites has been measured by secondary ion mass spectrometry, for which a new hambergite reference material was developed. The focus of this study is on nepheline syenite pegmatites from the Larvik Plutonic Complex (Norway) and one peralkaline pegmatite related to the nearby Eikeren-Skrim Complex (Norway), where we investigate the source of boron as being from magmatic vs. external fluids. Tourmaline-hambergite mineral pairs were also analysed from peraluminous pegmatite localities (Russia, Tajikistan, and Pakistan) to test for systematic B-isotope fractionation between these two minerals.Tourmaline and hambergite from peraluminous granitic pegmatites have light boron ratios (δ11B = −12.9to −1.0‰) associated with S-type granites, whereas peralkaline granitic and nepheline syenite pegmatites have boron ratios (δ11B = −1.7 to 11.8‰), which we interpret is a result of heavy‑boron enrichment from external fluids. Our data show that hambergite tracks isotope variations of its geochemical setting and could therefore be used as a proxy mineral in place of tourmaline when geochemical stability favours hambergite.The results suggest a slight but consistent partitioning of B-isotopes between tourmaline and hambergite, with Δ11B = δ11Btourmaline−δ11Bhambergite in the range of approximately −3‰ to −5‰. Boron is in trigonal coordination with oxygen in both of these mineral phases as verified by NMR. Single crystal XRD analyses of tourmaline and hambergite reveal consistent longer <B-O > distances of tourmaline relative to hambergite. We attribute the boron isotopic fractionation to the longer <B-O > bond-lengths in tourmaline compared with hambergite.

Calibration of oxygen isotope fractionation and calcite‐corundum thermometry in emery at Naxos, Greece

AbstractCorundum (Crn), including sapphire, occurs in emery pods surrounded by marble on the island of Naxos, Greece. The emery formed from bauxite deposited in karst that was metamorphosed to 400–700°C at 20–15 Ma. Many of these rocks initially appeared well suited for refractory accessory mineral (RAM) thermometry, which uses oxygen isotope fractionation between a RAM – corundum – and a modally dominant phase with faster diffusion of oxygen – calcite (Cc) – to determine peak metamorphic temperatures. However, previous attempts at oxygen isotope thermometry were confounded by highly variable fractionations (Δ18O) measured at mm‐scale and the uncertain calibration of Δ18O(Cc‐Crn) versus temperature. Secondary ion mass spectrometry (SIMS) permits in situ analysis of δ18O in corundum and calcite at the 10‐μm scale in adjacent grains where textures suggest peak metamorphic equilibrium was attained. SIMS analyses of adjacent mineral pairs in eight rocks yield values of Δ(Cc‐Crn) that systematically decrease from 7.2 to 2.9‰ at higher metamorphic grade. Pairing these data with independent temperature estimates from mineral isograds yields an empirical calibration of 1,000 lnα(Cc‐Crn) = 2.72 ± 0.3 × 106/T2 (T in K). The new fractionations (2.7‰ at 1,000 K) are significantly smaller than those calculated from the modified increment method (6.5‰ at 1,000 K; Zheng, Geochimica et Cosmochimica Acta, 1991, 55:2299–2307; Zheng, Mineral Mag, 1994, 58A:1000–1001), which yield unreasonably high temperatures of 630 to 1,140°C when applied to the new Naxos data. The new calibration of Δ(Cc‐Crn) can be combined with published fractionations to calculate A‐factors for corundum versus a range of 14 other minerals. These new fractionation factors can be used for thermometry or to constrain the genesis of corundum. A compilation of gem corundum δ18O values shows that many igneous sapphires, including important deposits of basalt‐associated sapphire, are mildly elevated in δ18O relative to the calculated range in equilibrium with mantle values (4.4–5.7‰) and formed from evolved magmas.