Paragenesis of gold mineralization at the Kiyuk Lake Project, Kivalliq Region, Nunavut, Canada

ABSTRACT Exploration for gold in Nunavut has been primarily focused on Archean greenstone belts in the north and coastal regions of the territory, resulting in large areas of underexplored terrain in the south. The Kiyuk Lake property is located in the underexplored southwest corner of the Kivalliq Region of Nunavut within the Hearne domain of the ∼1.9 Ga western Churchill Province. The property is hosted by Proterozoic calc-silicate and clastic sedimentary units of the Hurwitz Group (<2.4–1.9 Ga) and the unconformably overlying Kiyuk Group (1.9–1.83 Ga). Gold mineralization in Proterozoic sedimentary rocks is rare in the Canadian Shield, so the Rusty Zone at Kiyuk Lake presents a unique opportunity to study the enigmatic gold mineralization hosted in such sedimentary rocks. Mineralization at the Rusty Zone is hosted by an immature lithic wacke cut by thin intermediate dikes that are associated with hydrothermal breccias composed of Fe-carbonate, calcite, calcic-amphibole, Fe-sulfide, Fe-oxide minerals, and gold. Textural and timing relationships suggest that the gold mineralization is post-sedimentary and syn- to post-intrusion of intermediate dikes. Stable isotope thermometry suggests that mineralization took place between 450 and 600 °C, and geochronological studies indicate that the intrusion and mineralization occurred before or about 1.83 Ga. Using basement breaching thrusts faults as conduits to the surface, over-pressurization along a later normal fault is thought to be the primary cause for the localized breccia pipe that controls gold mineralization. The hydrothermal fluids are postulated to be volatile-rich aqueous solutions exsolved from a source of cooling magmas at depth. Although sub-economic at present, the occurrence of high-grade gold in a Paleoproterozoic basin such as Kiyuk Lake could signal a new opportunity for exploration for gold in the Canadian Shield.

Amp-TB2: An Updated Model for Calcic Amphibole Thermobarometry

Amphibole (Amp) plays a crucial role in the study of several earth and planetary processes. One of its most common applications is in thermobarometry, especially for volcanic-magmatic systems. However, many thermobarometers require the input of melt composition, which is not always available in volcanic products (e.g., partially crystallized melts or devitrified glasses), or show rather high errors for characterizing the depth of magma chambers. In this work, a new version of amphibole thermobarometry based on the selection of recently published high-quality experimental data is reported. It is valid for Mg-rich calcic amphiboles in magmatic equilibrium with calc-alkaline or alkaline melts across a wide P-T range (up to 2200 MPa and 1130 °C) and presents the advantage of being a single-phase model with relatively low errors (P ±12%, T ±22 °C, logfO2 ±0.3, H2O in the melt ±14%). A user-friendly spreadsheet (Amp-TB2.xlsx) for calculating the physico-chemical parameters from the composition of natural amphiboles is also reported. It gives warnings whenever the input composition is incorrect or diverges from that of the calibration data and includes diagrams for an easy graphical representation of the results.

Analysis of Baseline and Perimeter Air Monitoring Data from the Calaveras Dam Replacement Project (CDRP), Fremont, California

<p>This paper presents the results of 810 pre-project baseline samples collected over four years (2010-2011), and 7,210 offsite (ambient) and 14,314 perimeter samples collected over 7 years (2012-2018) during the CDRP project. The principal asbestos particles were chrysotile from serpentinite, and glaucophane-winchite amphibole from blueschist. The baseline data showed that asbestos concentrations measured at each station are not representative of a regional average background, rather, they reflect contributions from several variables such as: location on or near NOA-containing units, wind direction, intensity of localized soil disturbance, and time of year. The data shows that baseline sampling prior to a project cannot be used as a measure of “background” during the project. The analysis of amphibole composition in air and rock/soil samples was applied to differentiate local source impacts from the primary CDRP asbestos emissions. Of particular value was the application of the calcic-amphibole to total amphibole ratio (Ca index) measured during ABS sampling and comparison with the ratios measured in the samples. This analysis delineated three primary amphibole sources: 1) alluvium in the Sunol Valley with a high Ca index, 2) imported road surfacing material with a moderate Ca index, and 3) blueschist with a low Ca index. When the data was sorted by wind direction, the analysis showed that the contribution of CDRP-generated asbestos to monitoring stations was significant near the point of disturbance only, and did not significantly impact offsite stations that were located at or near sensitive receptors. The asbestos measured at the offsite stations were correlated with local geologic units. The analysis verified that the CDRP emissions were well below the project-specific risk-based thresholds established for the CDRP project, documenting that the offsite receptors were not exposed to an adverse risk by CDRP activities.</p>

Neogene Metasomatism in the Subcontinental Lithosphere beneath SE Asia—Evidence from Modal and Cryptic Phosphorus Enrichment in Peridotites and Pyroxenites from Southern Laos

Metasomatism is the prime process to create compositional heterogeneity of the upper mantle. Mineralogical and mineral chemical changes of the mantle triggered by metasomatism can be used to deduce the nature of the metasomatic agent(s) and to constrain the timing of metasomatism. This information is vital for an understanding of the secular evolution of a given mantle segment and the magmatic processes occurring therein. For this study spinel-lherzolites and -websterites were collected from ∼16 Myr old alkali-basaltic lava flows that were extruded on the Bolaven Plateau in south–central Laos. These xenoliths are fragments of the shallow continental lithosphere of the SE Asian peninsula and originate from a mantle segment that acted as source for Cenozoic basaltic volcanism in the wake of the India–Asia collision. In both rock types modal metasomatism formed apatite ± whitlockite ± phlogopite ± calcic amphibole ± calcite ± orthopyroxene. The principal metasomatic phase is apatite, which appears in three varieties. Type-I apatite is ±inclusion-free and associated with phlogopite, calcic amphibole, calcite and lamellar orthopyroxene. It is high in Na and low in P and shows low analytical totals indicating a type-B carbonate–apatite component. Type-I apatite presumably precipitated from a P-alkali-rich mixed H2O–CO2 fluid with low large ion lithophile element (LILE)–light rare earth element (LREE) contents. Type-II apatite shows a spongy texture and has lower Na and higher P contents with higher analytical totals. Crosscutting discontinuous zones of type-II characteristics within type-I apatites indicate type-II formation through an exchange Na+ + CO32– = PO43– + Ca2+ by a later fluid with lower aCO2. REE-rich type-III apatite is the youngest type and formed by infiltration of basaltic melts as part of spongy rims around clinopyroxene. One lherzolite contains whitlockite in addition to apatite. Whitlockite formation is ascribed to a short-lived metasomatic event involving a fluid with extremely low aH2O. Disequilibrium between whitlockite and the bulk assemblage is indicated by hydrous silicates in the immediate vicinity of whitlockite and by substantial H2O contents of 250–370 µg g–1 in clinopyroxenes and 170–190 µg g–1 in orthopyroxenes. High-density (1·15–≥1·17 g m–3) CO2–fluid inclusions in the whitlockite-bearing sample provide evidence for the presence of low-aH2O fluids at mantle depths. The spinel-herzolites may also show cryptic metasomatism evidenced by P zoning in olivine, which is characterized by P-poor (<20–130 µg g–1) cores and P-rich (170–507 µg g–1) rims, the latter in part with oscillatory zoning on a µm scale. Element correlations indicate [4]Si4+ + [6](Mg, Fe)2+ = [4]P5+ + [6]Li+, 2 [4]Si4+ + 4 [6](Mg, Fe)2+ = 2 [4]P5+ + 3 [6](Mg, Fe)2+ + [6]vac and/or 5 [4]Si4+ = 4 [4]P5+ + [4]vac as major P incorporation mechanisms. High P–T experiments conducted at 2 GPa and 950–1050 °C yield apatite-saturated P contents of olivine in the range ∼360–470 µg g–1. Most P concentrations in olivines from the xenoliths including those in the P-rich rims, however, are significantly lower than the apatite-saturated values, which indicates disequilibrium uptake of P during growth of the P-rich rims by dissolution–reprecipitation. Diffusion modeling indicates that the P zoning must have formed within decades prior to the eruption of the host basalts. This is consistent with the preservation of Li disequilibrium partitioning between olivine and pyroxenes in some of the xenoliths. All metasomatic phenomena were assigned to two metasomatic events, both of which were in close temporal relation with the eruption of the xenolith host basalts: an older event-1 formed type-I apatite, hydrous silicates, calcite and orthopyroxene and caused the modification of type-I apatite composition towards that of type-II. It is also likely to be responsible for whitlockite formation and P zoning in olivine. A younger event-2 comprises all paragenetic, textural and compositional modifications of the xenolith assemblages associated with the infiltration of basaltic melts.

Characterisation of chemically related asbestos amphiboles of actinolite: proposal for a specific differentiation in the diagram (Si apfu versus Mg/Mg+Fe2+)

Aggregates and rocks from quarries located in metropolitan France and New Caledonia, all likely to contain asbestiform amphiboles, were analysed by a routine laboratory (AD-LAB). Morphological observations were made using transmission electron microscopy and chemical analyses were obtained with energy dispersive X-ray spectroscopy. The chemical analyses obtained from amphiboles were treated in such a way that they could be plotted in a diagram (Si apfu versus Mg/Mg+Fe2+). The points corresponding to analysed particles, classified as asbestos, define a broader compositional domain than that corresponding to the compositional areas of actinolite and tremolite. The creation of two new domains is proposed. Samples of basic metavolcanics and amphibolites collected by the Geological and Mining Research Bureau (BRGM) in different quarries of the Armorican Massif and the Massif Central containing calcic amphibole fibres have been the subject of polarized light microscope and electron microprobe analyses. The representative points of the spot chemical analyses performed on the very fine and ultrafine fibres are contained in the range defined previously. The diagram that has been determined from chemical analyses coupled with morphological and dimensional observations can help the “routine laboratories” to better characterise asbestiform calcic amphiboles, but it also allows comparisons with geological observations.

Water transport by subduction: Clues from garnet of Erzgebirge UHP eclogite

A key question concerning the water budget of Earth’s mantle is how much water is actually recycled into the mantle by the subduction of eclogitized oceanic crust. Hydrous phases are stable only in quartz eclogite not coesite eclogite so that water transport to greater depths is mainly governed by structural water in omphacite and garnet. Here we explore if garnet can be used as a proxy to assess the amount of this water. Available data on the water contents of garnet in coesite eclogite vary over orders of magnitude, from a few up to ca. 2000 ppm. By implication, the maximum bulk-rock water contents are unrealistically high (wt% level). New data from the Erzgebirge indicate moderate amounts of structural H2O stored in garnet (43–84 ppm), omphacite (400–820 ppm), and in the bulk coesite eclogite (ca. 280–460 ppm). Higher garnet water contents occur, but these are not primary features. They are related to molecular water in fluid inclusions that can be attributed to eclogite-facies fluid influx postdating the metamorphic peak. Fluid influx also caused the uptake of additional structural water in garnet domains close to fluid inclusions. Such secondary H2O incorporation is only possible in the case of primary water-deficiency indicating that garnet hosted less water than it was able to store. This is insofar astonishing as comparably high H2O amounts are liberated by the breakdown of prograde eclogite-facies hydrous minerals as a result of ultrahigh-pressure (UHP) metamorphism. Judging from Erzgebirge quartz eclogite, dehydration of 5–10% hydrous minerals (±equal portions of zoisite+calcic amphibole) produces 1500–3000 ppm water. We infer that the largest part of the liberated water escaped, probably due to kinetic reasons, and hydrated exhuming UHP slices in the hanging-wall. Depending on the physical conditions, water influx in eclogite during exhumation (1) produces fluid inclusions and simultaneously enhances the structural water content of nominally anhydrous minerals—as in the Erzgebirge—and/or (2) it may give rise to retrograde hydrous minerals. We conclude that eclogite transports moderate quantities of water (several hundred parts per million) to mantle depths beyond 100 km, an amount equivalent to that in ca. 1% calcic amphibole.