Pyrrhotite and associated sulphides and their relationship to acid rock drainage in the Halifax Formation, Meguma Group, Nova Scotia

The physical disruption of sulphide-bearing metasedimentary rocks of the Halifax Formation leads to oxidation of iron-sulphide minerals and the generation of acid rock drainage (ARD). Although pyrrhotite occurs in many places throughout the Halifax Formation, previous ARD studies have not considered in detail the mineral chemistry, texture, and distribution of this mineral nor how these factors may potentially influence the development of ARD. For this study, pyrrhotite-bearing samples of the Halifax Formation were collected in the field and from drill core at four locations in southwestern Nova Scotia. Samples were taken from different geological settings, such as proximal and distal to granitic intrusions and from different stratigraphic positions, to obtain a variety of mineral assemblages. Petrographic, microprobe and X-ray diffraction work indicate that the pyrrhotite in all samples is mainly monoclinic Fe7Sg, and its composition is relatively homogeneous regardless of geological environment. Inclusions of chalcopyrite and detectable quantities of As, Co and Ni are common. In regionally metamorphosed, grecnschist-facies areas, pyrrhotite is preferentially aligned along cleavage planes and thus is easily accessible to oxidizing air and fluids. Because pyrrhotite is regionally developed, contains potentially toxic trace elements, and occurs along cleavage planes, it is considered to play a significant role in ARD development in the Halifax Formation. Also, pyrrhotite oxidizes substantially faster than many other sulphide minerals and may be especially significant in the early stages of ARD. RÉSUMÉ La dislocation physique des roches métasédimentaires sulfuriféres de la Formation d'Halifax mène à une oxydation des minéraux renfermant du sulfure de fer ainsi qu'à la production d'exhaures de roches acide (ERA). Même si on relève la présence de pyrrhotine en de nombreux endroits partout à l'intérieur de la Formation d'Halifax, les études antérieures des ERA ne se sont pas attardées de fa÷on approfondie sur la nature chimique minérale, la texture et la répartition de ce minéral ni sur la manière dont ces facteurs peuvent éventuellemcnt influer sur l'apparition des ERA. Les chercheurs ont, aux fins de cette étude, prélevé sur le terrain ainsi que sur des carottes de sondage à quatre emplacements dans le sud-ouest de la Nouvelle-Écosse, des échantillons de la Formation d'Halifax renfermant de la pyrrhotine. On a prélevé les échantillons de différents cadres géologiques stratigraphiques, comme des intrusions proximales et distalcs à granitiques et différentes positions stratigraphiques, afin d'obtenir toute une variété d’associations minérialogiques. Des travaux à la microsonde, de diffraction aux rayons X et pétrographiques révèlent que la pyrrhotine de tous les échantillons est essentiellement du F7Sg monoclinique et qu'elle est d'une composition relativement homogene, peu importe I'environnement géologique. Les inclusions de chalcopyrite et de quantités détectablcs d'As, de Co et de Ni sont courantes. Dans les secteurs des faciès des schistes verts régionalement métamorphisés, la pyrrhotine est principalement alignée le long de plans de clivage et elle est ainsi facilement accessible aux fluides et à l'air oxydants. Vu la présence régionale de la pyrrhotine, vu qu'elle renferme des éléments traces pouvant être toxiques et vu qu'elle sc trouve le long de plans de clivage, on considère qu'elle joue un rôle prépondérant dans la production des ERA à l'intérieur de la Formation d'Halifax. La pyrrhotine s'oxyde par ailleurs bcaucoup plus rapidement que de nombreux autres minéraux sulfurls et ce facteur peut être particulièrement déterminant dans les premiers stades de la production des ERA. [Traduit par la rédaction

Paleo-Hydrothermal Predecessor to Perennial Spring Activity in Thick Permafrost in the Canadian High Arctic, and Its Relation to Deep Salt Structures: Expedition Fiord, Axel Heiberg Island, Nunavut

Published versionIt is surprising to encounter active saline spring activity at a constant 6° C temperature year-round not far away from the North Pole, at latitude 79° 24′N, where the permafrost is ca. 600 m thick and average annual temperature is -15° C. These perennial springs in Expedition Fiord, Queen Elizabeth Islands, Canadian Arctic Archipelago, had previously been explained as a recent, periglacial process. However, the discovery near White Glacier (79° 26.66′N; 90° 42.20′W; 350 m.a.s.l.) of a network of veins of hydrothermal origin with a similar mineralogy to travertine precipitates formed by the springs suggests that their fluids have much deeper circulation and are related to evaporite structures (salt diapirs) that underlie the area. The relatively high minimum trapping temperature of the fluid inclusions (avg. ~200 ± 45° C, 1σ) in carbonate and quartz in the vein array, and in quartz veins west of the site, explains a local thermal anomaly detected through low-temperature thermochronology. This paper reviews and updates descriptive features of the perennial springs in Expedition Fiord and compares their mineralogy, geochemistry, and geology to the vein array by White Glacier, which is interpreted as a hydrothermal predecessor of the springs. The perennial springs in Axel Heiberg Island are known for half a century and have been extensively described in the literature. Discharging spring waters are hypersaline (1-4 molal NaCl; ~5 to 19 wt% NaCl) and precipitate Fe-sulfides, sulfates, carbonates, and halides with acicular and banded textures representing discharge pulsations. At several sites, waters and sediments by spring outlets host microbial communities that are supported by carbon- and energy-rich reduced substrates including sulfur and methane. They have been studied as possible analogs for life-supporting environments in Mars. The vein array at White Glacier consists of steep to subhorizontal veins, mineralized fractures, and breccias within a gossan area of ca. 350 × 50 m. The host rock is altered diabase and a chaotic matrix-supported breccia composed of limestone, sandstone, and anhydrite-gypsum. Mineralization consists of brown calcite (pseudomorph after aragonite) in radial aggregates as linings of fractures and cavities, with transparent, sparry calcite and quartz at the centre of larger cavities. Abundant sulfides pyrite and marcasite and minor chalcopyrite, sphalerite, and galena occur in masses and veins, much like in base metal deposits known as Mississippi Valley Type; their weathering is responsible for brown Fe oxides forming a gossan. Epidote and chlorite rim veins where the host rock is Fe- and Mg-rich diabase. The banded carbonate textures with organic matter and sulfides are reminiscent of textures observed in mineral precipitates forming in the active springs at Colour Peak Diapir. Very small fluid inclusions (5-10 μm) in two generations of vein calcite (hexagonal, early brown calcite we denominate “cal1” lining vein walls; white-orange sparry calcite “cal2” infilling veins) have bulk salinities that transition between an early, high-salinity end-member brine (up to ~20 wt% NaCl equivalent) to a later, low-salinity end-member fluid (nearly pure water) and show large fluctuations in salinity with time. Inclusions that occupy secondary planes and also growth zones in the later calcite infilling (deemed primary) have Th ranging from 100° C to 300° C (n = 120, average~200° C; independent of salinity), 2 orders of magnitude higher than average discharging water temperatures of 6° C at Colour Peak Diapir. Carbon isotope composition (δ13CVPDB) of the White Glacier vein array carbonates ranges from approximately -20 to -30‰, like carbonates formed by the degradation of petroleum, whereas carbonates at Colour Peak Diapir springs have a value of -10‰. Oxygen isotope composition (δ18OVSMOW) of vein carbonates ranges from -0.3‰ to +3.5‰, compatible with a coeval fluid at 250° C with a composition from -3.5‰ to -7.0‰. These data are consistent with carbonates having precipitated from mixtures of heated formational waters and high-latitude meteoric waters. In contrast, the δ18OVSMOW value for carbonates at Colour Peak Diapir springs is +10‰, derived from high-latitude meteoric waters at 6° C. The sulfur isotope (δ34SVCDT) composition of Fe-sulfides at the perennial springs is +19.2‰, similar to the δ34SVCDT of Carboniferous-age sulfate of the diapirs and consistent with lowtemperature microbial reduction of finite (closed-system) sulfate. The δ34SVCDT values of Fe-sulfides in the vein array range from -2.7‰ to +16.4‰, possibly reflecting higher formation temperatures involving reduction of sulfate by organics. We suggest that the similar setting, mineralogical compositions, and textures between the hydrothermal vein array and the active Colour Peak Diapir springs imply a kinship. We suggest that overpressured basinal fluids expelled from the sedimentary package and deforming salt bodies at depth during regional compressional tectonic deformation ca. 50 million years ago (Eocene) during what is known as the Eurekan Orogeny created (by hydrofracturing) the vein array at White Glacier (and probably other similar ones), and the network of conduits created continued to be a pathway around salt bodies for deeply circulating fluids to this day. Fluid inclusion data suggest that the ancient conduit system was at one point too hot to support life but may have been since colonized by microorganisms as the system cooled. Thermochronology data suggest that the hydrologic system cooled to temperatures possibly sustaining life about 10 million years ago, making it since then a viable analogue environment for the establishment of microbial life in similar situations on other planets

Fission track age determinations of rocks from north Greenland

Apatite fission track (FT) ages and length characteristics of samples obtained from Cambrian to Paleocene-aged sandstones collected along the margin of Nares Strait in Ellesmere Island in the Canadian Arctic Archipelago are dominated by a thermal history related to Paleogene relative plate movements between Greenland and Ellesmere Island. A preliminary inverse FT thermal model for a Cambrian (Archer Fiord Formation) sandstone in the hanging wall of the Rawlings Bay thrust at Cape Lawrence is consistent with Paleocene exhumational cooling, likely as a result of erosion of the thrust. This suggests that thrusting at Cape Lawrence occurred prior to the onset of Eocene compression, likely due to transpression during earlier strikeslip along the strait. Models for samples from volcaniclastic sandstones of the Late Paleocene Pavy Formation (from Cape Back and near Pavy River), and a sandstone from the Late Paleocene Mount Lawson Formation (at Split Lake, near Makinson Inlet) are also consistent with minor burial heating following known periods of basaltic volcanism in Baffin Bay and Davis Strait (c. 61-59 Ma), or related tholeiitic volcanism and intrusive activity (c. 55-54 Ma). Thermal models for samples from sea level dykes from around Smith Sound suggest a period of Late Cretaceous – Paleocene heating prior to final cooling during Paleocene time. These model results imply that Paleocene tectonic movements along Nares Strait were significant, and provide limited support for the former existence of the Wegener Fault. Apatite FT data from central Ellesmere Island suggest however, that cooling there occurred during Early Eocene time (c. 50 Ma), which was likely a result of erosion of thrusts during Eurekan compression. This diachronous cooling suggests that Eurekan deformation was partitioned at discrete intervals across Ellesmere Island, and thus it is likely that displacements along the strait were much less than the 150 km that has been previously suggested for the Wegener Fault

Weathering of Devonian monzogranites as recorded in the geochemistry of saprolites from the South Mountain Batholith, Nova Scotia, Canada

Parent mineralogy, paleoenvironment, and subsequent diagenetic and erosional history contribute to the nature of the paleoweathering profiles (saprolites) developed on the South Mountain Batholith. Saprolites of three different ages (pre-Carboniferous, pre-Triassic, and pre-Pleistocene) developed on Devonian monzogranite and exhibit increases in oxidation and hydration and decreases in rare earth elements with increasing weathering. In addition, changes in Ca, Ba, Rb, Zn, Pb, Co, and other elements in the oldest saprolite indicate element mobility during weathering. Re-exposure of these partially weathered profiles at surface in today's acidic and oxygen-rich environment may result in further migration of elements from these saprolites. RÉSUMÉ La minéralogie originelle, le paléoenvironnement, et les antécédents diagénétiques et d'érosion subséquents sont autant de facteurs qui déterminent les profils de paléométéorisation (saprolithes) observés dans le batholithe de South Mountain. Des saprolithes de trois époques différentes (antérieures au Carbonifère, à l'ère triasique, et au Pléistocène) se sont formées sur du monzogranite. On observe une oxydation et une hydratation accrues, une diminution de la présence des éléments du groupe des terres rares, et une météorisation plus grande. De plus, des altérations de Ca, Ba, Rb, Zn, Pb, Co et d'autres éléments dans les saprolithes plus anciens indiquent le caractère meuble des éléments pendant la météorisation. Une nouvelle exposition de ces profils en partie météorisés dans le milieu acide et riche en oxygène d'aujourd'hui pourrait entraîner une autre migration des éléments contenus dans ces saprolithes. [Traduit par la rédaction

Spatial coincidence and similar geochemistry of Late Triassic and Eocene-Oligocene magmatism in the Andes of northern Chile: evidence from the MMH porphyry type Cu-Mo deposit, Chuquicamata district

The MMH porphyry type copper-molybdenum deposit in northern Chile is the newest mine in the Chuquicamata District, one of largest copper concentrations on Earth. Mineralized Eocene-Oligocene porphyry intrusions are hosted by essentially barren Triassic granodiorites. Despite a century of exploitation, geologists still have problems in the mine distinguishing the Triassic granodiorite from the most important ore-carrying Eocene porphyries in the district. To resolve the problem, internally consistent high-quality geochemical analyses of the Triassic and Tertiary intrusives were carried out: explaining the confusion, they show that the rock units in question are nearly identical in composition and thus respond equally to hydrothermal alteration. In detail, the only difference in terms of chemical composition is that the main Eocene-Oligocene porphyries carry relatively less Fe and Ni. Unexpectedly, the mineralized Eocene-Oligocene porphyries have consistently less U and Th than other Tertiary intrusions in the district, a characteristic that may be valuable in exploration. The supergiant copper-molybdenum deposits in the Central Andes were formed within a narrow interval between 45 and 31 Ma, close to 7% of the 200 My duration of "Andean" magmatism, which resulted from subduction of oceanic lithosphere under South America since the Jurassic. Although recent work has shown that subduction was active on the margin since Paleozoic times, pre-Andean (pre-Jurassic) "Gondwanan" magmatism is often described as being very different, having involved crustal melting and the generation of massive peraluminous rhyolites and granites. This study shows that the indistinguishable Late Triassic and Eocene-Oligocene intrusions occupy the same narrow NS geographic belt in northern Chile. If it is accepted that magma character may determine the potential to generate economic Cu-Mo deposits, then Late Triassic volcano-plutonic centres in the same location in the South American margin could have contained valuable ore deposits, although their preservation will depend on the level attained by pre-mid Jurassic erosion. Both Late Triassic and Eocene-Oligocene magmatic events occurred during the waning stages of vigorous volcano-plutonic cycles, and both preceded apparent gaps in igneous activity (Rhaetian and post-Oligocene), abrupt lateral shifts of the volcanic front and radical changes in the character of the magmas generated. Both Late Triassic and Eocene-Oligocene intrusions were emplaced along the same narrow strip of crust; it is probable that they both exploited the same deep crustal structures. The Eocene-Oligocene magmatic front was controlled by an orogen-parallel shear system caused by oblique subduction; it is possible that Late Triassic magmatism along the same belt had a similar setting. The identified Rhaetian gap in subduction and magmatism may have widespread implications.CODELCO's Gerencia de Recursos Mineros y Desarrollo Distrital MZGeoscience Inc. of Halifax, Canad