Sr Isotopic Composition of Hydrothermal Epidote and Carbonate in the Sudbury Structure Determined by LA-MC-ICP-MS Analysis: Implications for Fluid Sources and Architecture
The 1.85 Ga Sudbury impact structure comprises the 2.5 km thick Sudbury Igneous Complex (SIC), overlain by breccias of the Onaping Fm (OF), carbonates of the Vermilion Fm (VF) and a sequence of post-impact sediments (Onwatin and Chelmsford formations). Circulation of deep formational brines, Proterozoic seawater, and orthomagmatic fluids have been postulated at various stratigraphic levels of the structure. Epidote is commonly found in miarolitic cavities (Ep-Kfs-Qtz-Chl-Hbl) throughout the SIC. These cavities are considered to represent late-stage magmatic fluids derived from the SIC. Mineralogically similar veins are also present, and represent the pathways through which the orthomagmatic fluids migrated away from their source. Epidote is also found in the OF as a replacement phase, and filling amygdules in syn-depositional aphanitic dykes and peperite bodies. In the OF, calcite occurs in a basin-wide semi-conformable alteration zone, which is considered to have precipitated from a fluid mixture dominated by Proterozoic seawater, but with a component of magmatic fluids derived from the SIC. Previous Sr isotopic values of the OF carbonate alteration (0.7140), were interpreted to reflect interaction of this fluid with basement lithic fragments in the OF. The overlying VF was considered to have precipitated from similar fluids. Calcite is also found in amygdules (Qtz- Chl-Cal-Ep-Sulph) in syn-depositional aphanitic dikes in the OF, and as discordant aggregates and replacement phases in the VF. We have determined the initial 87Sr/86Sr ratios of various textural types of epidote and carbonate, using femtosecond laser ablation MC-ICP-MS, providing in-situ analysis of individual crystals. Bulk mineral separates of selected samples were also analyzed by isotope dilution mass spectrometry. 87Sr/86Sr ratios of SIC-hosted cavities and vein epidote lie mainly between 0.7070 and 0.7112, with some significantly higher values between 0.7125 and 0.7177. OF epidote also displays variable 87Sr/86Sr ratios, similar to those of the SIC epidote. Notably, the granophyre-hosted miarolitic epidote has a narrow range (0.7071 to 0.7076). Published whole rock 87Sr/86Sr ratios for the SIC range from 0.7064 to 0.7073, which are similar to the granophyre-hosted miarolitic epidote, but lower than most of the other SIC-hosted epidote. We propose that the granophyre-hosted miarolitic epidote represents late-stage orthomagmatic fluids derived from the SIC, and that the more radiogenic values reflect mixing with fluids that had interacted with Archean and Huronian basement rocks (deep formation waters). Our data are also consistent with a model in which the VF carbonate (0.7070 to 0.7081) precipitated from Proterozoic seawater (or modified seawater), as they are isotopically similar to carbonate concretions in the Chelmsford Fm (0.7057 to 0.7081), and to Proterozoic seawater (∼0.7055). In contrast, the higher values observed in the OF carbonates (0.7089 to 0.7150) suggest involvement of more radiogenic fluids, isotopically similar to those that circulated through the SIC. Some carbonates associated with base metal sulfide mineralization in the OF and VF may have also precipitated from similar fluids. In total, these data indicate a fluid connection between the foot-wall and hanging-wall of the SIC.
Differentiating Detrital and Metamorphic Monazite in Greenschist-Facies Sandstones From the Witwatersrand Supergroup
Monazite is a robust and reliable geochronometer of low-temperature metamorphic and hydrothermal events. It is a widespread accessory phase in sedimentary rocks metamorphosed at prehnite-pumpellyite to lower greenschist facies grade, and also in a range of hydrothermal ore deposits. Its ability to date multiple fluid-flow events in low-grade metasedimentary belts has been largely neglected, possibly because of a misconception that it is rare in these rocks and possibly because of misidentification of metamorphic monazite grains as detrital. Both detrital and metamorphic monazites are present in sandstone and conglomerate from the Witwatersrand Supergroup but can be distinguished by their occurrence, chemistry and age. Detrital grains were unstable during regional greenschist-facies metamorphism, and show evidence for a number of destructive reactions dependent on bulk rock composition and the original composition of the monazite. In quartz sandstone and conglomerate, detrital grains were present in heavy mineral bands with pyrite, zircon and chromite. The monazite grains have been pseudomorphed by intergrowths of apatite, florencite and Th-silicate, as well as matrix muscovite and chlorite. In some samples, Th-silicate forms only minute specks but in others it forms larger prismatic crystals that comprise up to 2% of some pseudomorphs. These variations may reflect differences in the original compositions of the detrital grains. In other samples detrital monazite cores, dated at 2.8-3.0 Ga, are enclosed within 2.04 Ga metamorphic rims. These composite grains formed by dissolution and reprecipitation of monazite during metamorphism. The cores and rims have distinctly different compositions, and the metamorphic rims show pronounced zoning of REE. In more calcic sandstone monazite occurs in heavy mineral bands with chromite, zircon, rutile, pyrite, apatite, Th-silicate, allanite and baddeleyite. These sandstones are notably rich in Ca-bearing minerals such as epidote, calcite and titanate, and monazite has been partially replaced by fine intergrowths of allanite, apatite and Th-silicate. Careful characterisation of monazite in low-grade metasedimentary rocks can distinguish detrital grains from metamorphic, and open the way for precise geochronology of low-temperature events.
Mineralogical and Geochemical Study of Titanite Associated With Copper Mineralization in the Hopper Property, Yukon Territory, Canada
Copper mineralization in central Yukon is well known, but the metallogeny of the Ruby Range batholith, west of the copper belt, is poorly understood. The Hopper property, situated in the south western part of the Yukon in the Yukon-Tanana terrane, contains copper mineralization hosted by granodiorite and quartz feldspar porphyry of cal-alkaline affinity. These rock units, interpreted to be part of the Ruby Range batholith, intruded metasediments of the Ashihik Metamorphic Suite rocks. Mafic dykes cross cut the intrusion followed by aplite dykes. Small occurrences of skarn also occur in the area and some of these contain copper mineralization. The copper mineralization at the Hopper property appears to have a porphyry-type affinity. However, it is associated with a shear zone and propylitic alteration unlike other typical copper porphyry-type deposits. This raises the question whether or not the mineralization is orthomagmatic in origin, i.e., whether or not this is a true porphyry system. The main zone of mineralization is 1 kilometer long and 0.5 kilometer wide. It is characterized by disseminated chalcopyrite and pyrite, which also occur along fractures. Molybdenite mineralization was found to be associated with slickensides. Alteration minerals associated with the copper mineralization are chlorite, epidote-clinozoisite, carbonate and titanite. Chlorite and epidote-clinozoisite are concentrated in the mineralized zone, whereas an earlier potassic alteration shows a weaker spatial correlation with the mineralization. The association of the mineralization with propylitic alteration leads us to believe the mineralization is shear related, although a deeper porphyritic system may be present at depth. Two populations of titanite at the Hopper property are recognized based on their shape, size and association with other minerals. The first population, defined by a length of 100 micrometers to 1 centimeter, euhedral boundaries, and planar contacts with other magmatic phases, is interpreted to be magmatic in origin. The second population is 10 to 500 micrometers long, anhedral and shows a close association with chlorite and chalcopyrite. This type of titanite is hydrothermal in origin. Preliminary electron microprobe analyses of titanite show the magmatic titanite grains have higher concentrations of Al, Fe, Nb, Ce, Zr and Mn, and lower concentrations of Ti and Ca compared to hydrothermal titanite grains. This corresponds with substitutions of Al, Fe, Nb, and Ce to Ti and substitutions of Ce, Zr to Ca. The association of titanite with propylitic alteration and its susceptibility to trace element substitutions make this an ideal test case to evaluate magmatic versus hydrothermal titanite.
Response of the U-Pb System in Zircon to Ultrahigh-Temperature Metamorphism
Zircons from igneous and metamorphic rocks often tend to yield more or less discordant U-Pb ages. This discordance may be due to real Pb-loss or is an artifact of mixing material from zones having different ages during whole or multiple grain dissolution. The latter problem can be addressed by spot analyses of individual growth zones in a single grain. Discordance due to Pb-loss may have several causes that need to be understood for a reliable interpretation of the resulting ages. In principle, Pb can be lost by diffusion or during recrystallization in response to thermal overprinting in a magmatic or metamorphic environment. To assess the behavior of the U-Pb system during ultrahigh temperature (UHT) metamorphism, we have analyzed zircons from quartzites that occur as xenoliths or come from the contact aureole of the Proterozoic Kadavur anorthosite complex, SE India. Cathodoluminescence (CL) imaging shows that the zircon grains consist of rounded cores surrounded by sub- to euhedral rims, some of them separated by a mantle. Interfaces between the zones are usually sharp. The cores show weak CL and are either structureless or have faint oscillatory, sector or patchy zoning. The rims display oscillatory zoning, but the highly luminescent mantles typically show no internal structure. All rims and mantles have similar CL characteristics indicating that they probably record the same growth event(s). U-Pb ages were obtained for single spots in individual CL zones by laser ablation ICP-MS. The cores provide a cluster of both concordant and discordant ages in the range of 3.4 to 1.9 Ga that are interpreted to be of detrital origin. Spot ages from zircon mantles and rims define two concordant age populations with weighted means of 914±22 Ma (2ó) and 815±11 Ma, respectively. Among these, the younger is interpreted to date the anorthosite intrusion, whereas the older are of ca. 914 Ma represents regional granulite facies metamorphism. The preservation of old and concordant ages in a large number of the zircon cores provides strong evidence that the UHT (>1150°C) conditions in and around the anorthosite intrusion did not cause Pb-loss in zircons. This implies that diffusion of Pb in pristine or only slightly metamict zircons cannot be a cause for the discordance of U-Pb ages in metamorphosed zircons. Consequently, discordant U-Pb ages may require loss of Pb from metamict zircons either by diffusion or recrystallization.
Thermobarometry in the Hadean: The Nuvvuagittuq Greenstone Belt
The 4.28 Ga 142Nd model age of the faux-amphibolite formation makes it the oldest assemblage of the Nuvvuagittuq Greenstone Belt (Northeastern Superior Province, Quebec, Canada) and the oldest rocks yet found on Earth. The protolith of the faux-amphibolite, however, is uncertain. The bulk chemistry suggests that it is most likely mafic and basaltic to basaltic-andesite in composition (samples have 36-63 wt% SiO2 and 3.5- 14 wt% MgO), although it has very low Ca-content compared to typical basalt. This low-Ca content is reflected in the crystallization of the amphibole cummingtonite, as opposed to hornblende, that is characteristic of the faux-amphibolite's adjacent gabbro sill. This suggests that Ca and other elements were mobile, perhaps during metamorphism. On the other hand, we do not see low-Ca in the adjacent gabbro sill suggesting either a more complex history for the faux-amphibolite, i.e. a metamorphic event before the emplacement of the gabbro sill, or Ca-depletion as the result of weathering processes. The faux-amphibolite is a heterogeneous gneiss with the mineral assemblage: cummingtonite + quartz + biotite + plagioclase ± anthophyllite ± garnet ± alkali-feldspar with the majority of the biotite replaced by retrograde chlorite. The garnets are heavily fractured, poikioblastic and, apart from the rims, are not zoned with respect to Fe and Mg. The garnets, as well as the groundmass, contain inclusions of zircon, rutile, ilmenite, monazite and other REE phosphates, and iron sulfides. Preliminary garnet-biotite geothermometry has been done that supports upper amphibolite to granulite facies metamorphism. Unzoned garnets from different parts of the faux-amphibolite record distinct Fe-Mg exchange temperatures that range from 730 to 940 °C (assuming a constant pressure of 5 kbar) suggesting the preservation of a metamorphic field gradient. Further geothermobarometry with trace element and accessory phases will be used to further describe the PT path as the resulting trace element zoning profiles may record reactions and loss of phases during the prograde PT path; phases that otherwise are not preserved. Thus, future work using the LA-ICPMS will aim to better constrain the metamorphic compositional changes to the faux-amphibolite's protolith.