New Cryogenian, Neoproterozoic, and middle Paleozoic U–Pb zircon ages from the Caledonia terrane, southern New Brunswick, Canada: better constrained but more complex volcanic stratigraphy

New U–Pb zircon ages from volcanic, plutonic, and sedimentary units in the Avalonian Caledonia terrane of southern New Brunswick provide better timing constraints in this geologically complex area. Previous ca. 620 Ma ages from the Broad River Group are now corroborated by additional dates from felsic tuff in the Gordon Falls Formation and rhyolite in the former Fairfield (now East Branch Black River) Formation of 620 ± 5 Ma and 622 ± 1.9 Ma, respectively. Combined with ages ranging from ca. 625 Ma to 615 Ma from crosscutting plutons, the data suggest that the minimum age of the Broad River Group is about 615 Ma. A quartzfeldspar porphyry dyke in mafic volcanic rocks of the previously undated Long Beach Formation yielded an igneous crystallization age of 685 ± 10 Ma, the oldest unit yet dated in the Caledonia terrane but similar in age to porphyry in the Stirling belt in the Avalonian Mira terrane of Nova Scotia. The age of the Coldbrook Group was constrained previously by U–Pb (zircon) ages of volcanic rocks between 560 and 550 Ma as well as by similar ages from comagmatic plutons. Five additional samples from both volcanic and plutonic units lie in the same range of 560–550 Ma, including errors, demonstrating that the Coldbrook Group and related plutons formed in less than 10 million years. Such a large volume of mainly felsic magma erupted and emplaced in a short time span suggests a “supereruption/supervolcano” environment such as the late Cenozoic southwestern USA but not yet recognized at ca. 560–550 Ma elsewhere in Avalonia. Two units yielded Paleozoic ages: felsite of the Bloomsbury Mountain Formation with a zircon population at 427 ± 9 Ma, indicating a Silurian maximum emplacement age, and dacite of the Grassy Lake Formation with several zircon grains at 382.8 ± 8.3 Ma, indicating a maximum age of middle Devonian, the first rocks of this age to be identified in the Caledonia terrane

Determination of the Mg/Mn ratio in foraminiferal coatings: An approach to correct Mg/Ca temperatures for Mn-rich contaminant phases

The occurrence of manganese-rich coatings on foraminifera can have a significant effect on their bulk Mg/Ca ratios thereby biasing seawater temperature reconstructions. The removal of this Mn phase requires a reductive cleaning step, but this has been suggested to preferentially dissolve Mg-rich biogenic carbonate, potentially introducing an analytical bias in paleotemperature estimates. In this study, the geochemical composition of foraminifera tests from Mn-rich sediments from the Antarctic Southern Ocean (ODP Site 1094) was investigated using solution-based and laser ablation ICP-MS in order to determine the amount of Mg incorporated into the coatings. The analysis of planktonic and benthic foraminifera revealed a nearly constant Mg/Mn ratio in the Mn coating of ∼0.2 mol/mol. Consequently, the coating Mg/Mn ratio can be used to correct for the Mg incorporated into the Mn phase by using the down core Mn/Ca values of samples that have not been reductively cleaned. The consistency of the coating Mg/Mn ratio obtained in this study, as well as that found in samples from the Panama Basin, suggests that spatial variation of Mg/Mn in foraminiferal Mn overgrowths may be smaller than expected from Mn nodules and Mn–Ca carbonates. However, a site-specific assessment of the Mg/Mn ratio in foraminiferal coatings is recommended to improve the accuracy of the correction.We acknowledge the financial support provided by ETH Research Grant ETH-04 11-1 (A.P.H.), and the Swiss National Science Foundation grants PZ00P2_141424 (A.M.-G.) and PP00P2_144811 (S.L.J.). This work was also funded (in part) by The European Research Council (ERC grant 2010-NEWLOG ADG-267931 HE)

Quantifying the effect of solid phase composition and structure on solid-liquid partitioning of siderophile and chalcophile elements in the iron-sulfur system

We report experimentally determined partition coefficients between solid and liquid phases for bulk compositions on either side of the Fe–FeS eutectic for a suite of siderophile, chalcophile, and lithophile elements. Experiments were performed at conditions of 1.5 and 2 GPa in pressure (P), 1323 K in temperature (T), and virtually identical eutectic sulfide liquid compositions in equilibrium with either solid face centered cubic Fe or solid FeS. This enabled isolation of the effect of solid phase composition and structure from pressure–temperature–melt composition effects. Solid phase–liquid metal partition coefficients (D values) for Ge, Re, Ni, Co, Cr, Mn, V, Sn, Pb, Re and W differ significantly if partitioning occurs between identical metallic liquids but different solid phases, whereas Zn, Cu and Mo are virtually unaffected. For all elements except Ge and Sn, measured solid Fe–liquid sulfide partition coefficients at 1.5 and 2 GPa are inconsistent with model predictions based on atmospheric pressure experiments, indicating that such models may not be appropriate for modeling core crystallization processes at non-ambient pressure. The framework of a lattice strain-based model of solid–liquid metal partitioning (Stewart et al., 2009) enables us to quantify the effect of the solid phase. Changing the solid phase from Fe to FeS leads to systematic increases in r0 (from 1.54 to 1.65 Å) and apparent Young's modulus E (from 112 to 178 GPa). These systematic changes can be used to predict element partitioning in eutectic solid FeS-bearing systems from measurements in eutectic solid Fe-bearing systems. Although changing the solid phase from face centered cubic Fe to FeS is an end member example, our data suggest that changes with pressure in the structure (e.g., to hexagonally close packed at high pressure) and composition (e.g. to higher S content at high pressure) of solid iron could affect the partitioning of elements between Fe and liquid metal during the solidification of planetary cores

Fluid Inclusion Studies in Opaque Ore Minerals: I. Trace Element Content and Physical Properties of Ore Minerals Controlling Textural Features in Transmitted Near-Infrared Light Microscopy

Physical properties and compositions of ore-forming fluids in magmatic-hydrothermal systems have been mostly investigated by conventional fluid inclusion studies in transparent gangue minerals that are assumed to be “co-genetic” with the mineralization. However, ore precipitating fluids can be directly studied by analyzing fluid inclusions in opaque ore minerals, such as wolframite, stibnite, pyrite and enargite, by using near-infrared (NIR) petrography and microthermometry in combination with laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of individual inclusions. Although results of NIR fluid inclusion studies of ore minerals were first published in 1984, the technique is still not commonly used in fluid inclusion research due to a number of limitations related to the analytical equipment used, sample preparation and NIR mineral transmittance. In this contribution we present new data on the applicability of NIR fluid inclusion studies in ore minerals (pyrite, stibnite, enargite, and wolframite) from a series of magmatic-hydrothermal systems, according to their chemical composition and physical behavior during microthermometry. Our results reveal strong correlation between NIR transmittance and high trace element (Co, Ni, Cu, As) content in pyrite, enargite (Fe, Bi), and wolframite (Sc, V, Fe). Despite this, the restricted distribution of these elements in oscillatory and sector zoning has allowed observation of NIR mineral features and fluid inclusions. Energy absorption of opaque minerals, either as light energy during microscopy or as thermal conductive energy during fluid inclusion microthermometry, presents a second limitation for NIR fluid inclusion studies. Our results confirm the relevance of fluid inclusion studies in ore minerals by combining NIR microscopy and microthermometry. Despite some limitations due to trace element composition of the host mineral and its physical behavior at high temperature, a successful NIR fluid inclusion study can be performed on some ore minerals that are opaque in the visible light range, allowing the direct study of hydrothermal ore-forming fluids

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

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

Sulfide replacement processes revealed by textural and LA-ICP-MS trace element analyses: Example from the early mineralization stages at Cerro de Pasco, Peru

The large Cerro de Pasco Cordilleran base metal deposit in central Peru is the result of three successive mineralizing stages comprising both low- and high-sulfidation mineral associations: (A) several pyrrhotite pipes grading outward to sphalerite and galena replacement bodies, (B) a massive, funnel-shaped pyrite-quartz replacement orebody, and (C) E-W–trending Cu-Ag-(Au-Zn-Pb) enargite-pyrite veins and well-zoned Zn-Pb-(Bi-Ag-Cu) carbonate-replacement orebodies. This superposition of hydrothermal events leads to complex replacement textures and crosscutting relationships. A detailed study of the textures and mineral composition of the up to 15-m-wide replacement front existing between the pyrrhotite pipes and the pyrite-quartz body allows for clarification of the relative chronology of the hydrothermal events. The results show that, in contrast to previous interpretations, the emplacement of the pyrrhotite pipes and their Zn-Pb mineralized rims precedes that of the pyrite-quartz body. The replacement textures affecting pyrrhotite and arsenopyrite and the nature of the newly formed minerals have been used as a qualitative way to track the evolution of fS2, fO2, and pH of the mineralizing fluids. Two steps of pyrrhotite replacement have been recorded. The first one takes place under moderate acidity and relatively reduced to moderately oxidized conditions and is marked by replacement of pyrrhotite by euhedral nonporous pyrite. The second step occurs under more acidic and oxidized conditions and is characterized by replacement of pyrrhotite by porous marcasite and replacement of arsenopyrite by pyrite. Subsequently, marcasite is partly replaced by fine-grained euhedral nonporous pyrite. LA-ICP-MS trace element analyses of the replaced pyrrhotite and arsenopyrite and of the newly formed marcasite and pyrite support dissolution-reprecipitation as the main mechanism for replacement. Positive correlations between some of the elements (e.g., Pb-Sb, Pb-Ag) are indicative of the possible presence of nanoscale solid inclusions as main carriers for those elements; however, coupled substitutions and incorporation of some of the elements at a ppm level into the pyrite and marcasite structures cannot be excluded. The obtained As, Sb, Pb, and Bi values in pyrite are systematically higher than published data of pyrite in epithermal and porphyry systems. Nature and trace element content of the newly formed minerals yield information on the physicochemical conditions during their precipitation, the initial trace element content of replaced minerals, and the subsequently dissolved neighboring phases. The results show that the metal concentration of the fluid is locally influenced by the composition of the dissolved minerals. This study leads to a simpler interpretation of the fluid evolution than previously proposed, with a progressive increase of fS2, fO2, and pH as a result of decreasing wall-rock buffering during the three successive mineralizing stages at Cerro de Pasco

From a long-lived upper-crustal magma chamber to rapid porphyry copper emplacement: Reading the geochemistry of zircon crystals at Bajo de la Alumbrera (NW Argentina)

The formation of world class porphyry copper deposits reflect magmatic processes that take place in a deeper and much larger underlying magmatic system, which provides the source of porphyry magmas, as well as metal and sulphur-charged mineralising fluids. Reading the geochemical record of this large magmatic source region, as well as constraining the time-scales for creating a much smaller porphyry copper deposit, are critical in order to fully understand and quantify the processes that lead to metal concentration within these valuable mineral deposits. This study focuses on the Bajo de la Alumbrera porphyry copper deposit in Northwest Argentina. The deposit is centred on a dacitic porphyry intrusive stock that was mineralised by several pulses of porphyry magma emplacement and hydrothermal fluid injections. To constrain the duration of ore formation, we dated zircons from four porphyry intrusions, including pre-, syn- and post-mineralisation porphyries based on intersection relations between successive intrusion and vein generations, using high precision CA-ID-TIMS. Based on the youngest assemblages of zircon grains, which overlap within analytical error, all four intrusions were emplaced within 29 ka, which places an upper limit on the total duration of hydrothermal mineralisation. Re/Os dating of hydrothermal molybdenite fully overlaps with this high-precision age bracket. However, all four porphyries contain zircon antecrysts which record protracted zircon crystallisation during the ∼200 ka preceding the emplacement of the porphyries. Zircon trace element variations, Ti-in-zircon temperatures, and Hf isotopic compositions indicate that the four porphyry magmas record a common geochemical and thermal history, and that the four intrusions were derived from the same upper-crustal magma chamber. Trace element zoning within single zircon crystals confirms a fractional crystallisation trend dominated by titanite and apatite crystallisation. However, zircon cathodoluminescence imaging reveals the presence of intermediate low luminescent (dark) growth zones in many crystals from all intrusions, characterised by anomalously high Th, U and REE concentrations and transient excursions in trace element ratios. A return to the same fractionation trend after this excursion excludes external compositional forcing such as magma mixing. Instead we interpret the “dark-zones” to record zircon crystallisation during a transient event of rapid growth that resulted from mafic magma injection into the base of the magma chamber, releasing a CO2-rich vapour phase into the dacitic crystal mush. We propose that this vapour phase then migrated upwards to the apical part of the magma chamber from where it was expelled, together with successive batches of magma, to form the porphyry copper deposit within a short time-span of less than a few 10,000 years. The short duration of host rock emplacement, hydrothermal alteration and mineralisation presented in this study provides critical constraints on fluid storage in magma chambers and the genesis of large porphyry copper deposits

Trace element diffusion and incorporation in quartz during heating experiments

Abstract Heating of quartz crystals in order to study melt and high-temperature fluid inclusions is a common practice to constrain major physical and chemical parameters of magmatic and hydrothermal processes. Diffusion and modification of trace element content in quartz and its hosted melt inclusions have been investigated through step-heating experiments of both matrix-free quartz crystals and quartz crystals associated with sulfides and other minerals using a Linkam TS1500 stage. Magmatic and hydrothermal quartz were successively analyzed after each heating step for Cu, Al, and Ti using electron probe micro-analyzer. After the last heating step, quartz crystals and their hosted melt inclusions were analyzed by laser ablation inductively coupled plasma mass spectrometry and compared to unheated samples. Heated samples reveal modification of Cu, Li, Na, and B contents in quartz and modification of Cu, Li, Ag, and K concentrations in melt inclusions. Our results show that different mechanisms of Cu, Li, and Na incorporation occur in magmatic and hydrothermal quartz. Heated magmatic quartz records only small, up to a few ppm, enrichment in Cu and Na, mostly substituting for Li. By contrast, heated hydrothermal quartz shows enrichment up to several hundreds of ppm in Cu, Li, and Na, which substitute for originally present H. This study reveals that the composition of both quartz and its hosted melt inclusions may be significantly modified upon heating experiments, leading to erroneous quantification of elemental concentrations. In addition, each quartz crystal also becomes significantly enriched in Cu in the sub-surface layer during heating. We propose that sub-surface Cu enrichment is a direct indication of Cu diffusion in quartz externally sourced from both the surrounding sulfides as well as the copper pins belonging to the heating device. Our study shows that the chemical compositions of both heated quartz and its hosted inclusions must be interpreted with great caution to avoid misleading geological interpretations

Fluid Inclusion Studies in Opaque Ore Minerals: II. A Comparative Study of Syngenetic Synthetic Fluid Inclusions Hosted in Quartz and Opaque Minerals

Fluid inclusion studies give unique insights into physical conditions and composition of fluids involved in geological processes. Most studies to date are performed on transparent minerals. Near-infrared (NIR) microscopy allows also microthermometry to be performed on minerals that are opaque to the visible light such as pyrite, hematite, wolframite, enargite and stibnite. The main drawback of this technique is the underestimation of the recorded phase-change temperatures with increasing light intensity, up to several hundred percent in the case of ice-melting temperatures. Although this issue has been known for a decade, it is poorly understood. We address this problem based on a systematic study of synthetic fluid inclusions in a variety of opaque minerals. For the first time, fluid inclusions have been co-synthetized with success in quartz and opaque minerals. Fracturing the host minerals by in-situ quenching allowed for fluid–mineral equilibration prior to fluid inclusion formation. In this study, we assess the impact of mineral intrinsic parameters, mainly absorption and thermal-conductivity, and experimental settings (light source operative power, diaphragms aperture, and the use of filters) on recorded phase-change temperatures. We show that these are underestimated due to local overheating of the sample caused by radiative heating from the light source. It affects all minerals and the extent of the temperature shift of the observed phase-changes depends on sample thickness, mineral characteristics, and the amount of light reaching the sample. Thus, any calibration of the temperature shift as a function of the amount of light is complicated and in most cases impracticable. However, we demonstrate, based on co-generated inclusions in opaque minerals and quartz that yield similar values during NIR-microthermometry that for any mineral, there is a range of light power and microscope settings for which no shift is noticeable within the thermal stage accuracy. These are defined as "ideal measuring conditions" ensuring reliability of acquired microthermometry data