The melt rocks of the Vredefort impact structure - Vredefort Granophyre and pseudotachylitic breccias: Implications for impact cratering and the evolution of the Witwatersrand Basin

The unique combination of its large size (250-300 km diameter), deep levels of erosion (>7 km), and widespread regional mining activity make the Vredefort impact structure in South Africa an exceptional laboratory for the study of impact-related deformation phenomena in the rocks beneath giant, complex impact craters. Two types of impact-generated melt rock occur in the Vredefort Structure: the Vredefort Granophyre - impact melt rock - and pseudotachylitic breccias. Along the margins of the structure, mining and exploration drilling in the Witwatersrand goldfields has revealed widespread fault-related pseudotachylitic breccias linked to the impact event. There, volumetrically limited melt breccia occurs in close association with cataclasite or mylonitic zones associated with bedding-parallel normal dip-slip faults that formed during inward slumping of the crater walls, and in rare subvertical faults oriented radially to the center of the structure. This association is consistent with formation of pseudotachylites by frictional melting. On the other hand, rocks in the Vredefort Dome - the central uplift of the impact structure - contain ubiquitous melt breccias that range in size from sub-millimeter pods and veinlets to dikes up to tens of meters wide and hundreds of meters long. Like fault-related pseudotachylites in the goldfields and elsewhere in the world, they display a close geochemical relationship to their wallrocks, indicating local derivation. However, although mm/cm- to, rarely, dm-scale offsets are commonly found along their margins, they do not appear to be associated with broader fault zones, are commonly considerably more voluminous than most known fault-related pseudotachylites, and show no consistent relationship between melt volumes and slip magnitude. Recent petrographic observations indicate that at least some of these melt breccias formed by shock melting, with or without frictional melting. Consequently, the non-genetic term “pseudotachylitic breccia” has been adopted for these Vredefort occurrences. These breccias formed during the impact in rocks at temperatures ranging from greenschist to granulite facies, and were subsequently annealed to varying degrees during cooling of the central uplift.In addition to the pseudotachylitic breccias, nine clast-laden impact melt dikes (Vredefort Granophyre), each up to several kilometers long, occur in vertical radial and tangential fractures in the Vredefort Dome. Unlike the pseudotachylitic breccias, they display a remarkably uniform bulk composition and clast populations that are largerly independent of their wallrocks, and they contain geochemical traces of the impactor. They represent intrusive offshoots of the homogenized impact melt body that originally lay within the crater. U-Pb single zircon and Ar-Ar dating indicates that the Vredefort Granophyre and pseudotachylitic breccias, and the Witwatersrand pseudotachylites all formed at 2020±5 Ma - the age of the impact event, making the breccias a convenient time marker in the evolution of the structurally complex Witwatersrand basin with its unique gold deposits.     

Quantifying saline groundwater seepage to surface waters in the Athabasca oil sands region

Western Canadian oil sands contain over 170 billion barrels of proven unconventional petroleum reserves currently extracted at 1.8 million barrels per day by either surface mining, or by in situ techniques that require subsurface injection of steam and hydrocarbon solvents. Natural high-salinity springs are known to add water and entrained inorganic and organic constituents to the Athabasca River and its tributaries in the region of ongoing bitumen production. However, the magnitude and synoptic distribution of these saline inputs has remained unquantified. Here, a chloride mass balance is used to estimate saline groundwater discharge to the Athabasca River from 1987 to 2010. Results show that the highest saline water discharge rate to the Athabasca River occurs between Ft. McMurray and the Peace-Athabasca Delta, supported by subcrop exposure of lower Cretaceous- and Devonian-aged formations bearing saline waters. Further, the input of saline groundwater is found to be an important control on the chemistry of the lower Athabasca River, despite comprising 10⁻¹ to 3% of the Athabasca River’s discharge. The flux of natural saline groundwater entering the Athabasca does not appear to have increased or decreased from 1987 to 2010. The origin of seep salinity is interpreted as relict subglacial meltwater that has dissolved Devonian-aged evaporites, supported by saline Na-Cl type waters with low ¹⁸O/¹⁶O and ²H/¹H ratios relative to modern precipitation. The magnitude of groundwater discharge and its impact on the Athabasca River’s chemistry in the area of ongoing bitumen development warrants the incorporation of natural groundwater seepages into surface water quality monitoring networks.     

A Regional Survey of the Extent and Magnitude of Eutrophication in Mediterranean Estuaries of Southern California,USA

The magnitude and extent of eutrophication was assessed at 27 segments in 23 estuaries in the Southern California Bight (SCB) between October 2008 and 2009. We applied thresholds from the existing assessment frameworks from both the European Union and the U.S. National Eutrophication Assessment to measurements of three indicators [macroalgae biomass and cover, phytoplankton biomass, and dissolved oxygen (DO) concentration] to categorize eutrophic condition in each estuary. Based on these frameworks, a large fraction of segments had moderate or worse eutrophic condition—78 % based on macroalgae, 39 % for phytoplankton, and 63 % for DO. Macroalgal biomass exceeding 70 g dw m^?2 and 25 % cover was found at 52 % of sites during any sampling event and in 33 % of segments for 8 weeks or longer, a duration found to negatively impact benthic infauna. Duration of hypoxic events (DO?<?4 mg L^?1) was typically short, with most events less than one day; although 53 % of segments had at least one event longer than 24 h. Assessment frameworks of eutrophic condition are likely to evolve over time as the body of literature on eutrophication grows, including aspects such as the applicability of indicators in specific habitat types, indicator thresholds, and how event frequency and duration are incorporated. This paper informs this debate by discussing how eutrophic conditions in SCB estuaries are categorized using different indicators and thresholds. To this end, categorization of estuarine eutrophic condition was found to be very sensitive to the type of threshold, how data are integrated to represent duration or spatial extent, and how indicators are used as multiple lines of evidence.     

Coronal Cavity Survey: Morphological Clues to Eruptive Magnetic Topologies

We present a survey on coronal prominence cavities conducted using 19 months of data from the Atmospheric Imaging Assembly (AIA) instrument aboard the Solar Dynamics Observatory (SDO) satellite. Coronal cavities are elliptical regions of rarefied density lying above and around prominences. They can be long-lived (weeks to months) but are often observed to eventually erupt as part of a coronal mass ejection (CME). We determine morphological properties of the cavities both by qualitatively assessing their shape, and quantitatively fitting them with ellipses. We demonstrate consistency between these two approaches, and find that fitted ellipses are taller than they are wide for almost all cavities studied, in agreement with an earlier analysis of white-light cavities. We examine correlations between cavity shape, aspect ratio, and propensity for eruption. We find that cavities with a teardrop-shaped morphology are more likely to erupt, and we discuss the implications of this morphology for magnetic topologies associated with CME models. We provide the full details of the survey for broad scientific use as supplemental material.     

Geology and wall rock alteration at the Hercynian Draa Sfar Zn–Pb–Cu massive sulphide deposit, Morocco

Draa Sfar is a siliciclastic–felsic, volcanogenic massive sulphide (VMS) Zn–Pb–Cu deposit located 15 km north of Marrakesh within the Jebilet massif of the western Moroccan Meseta. The Draa Sfar deposit occurs within the Sarhlef series, a volcano-sedimentary succession that hosts other massive sulphide deposits (e.g., Hajar, Kettara) within the dominantly siliciclastic sedimentary succession of the lower Central Jebilet. At Draa Sfar, the footwall lithofacies are dominated by grey to black argillite, carbonaceous argillite and intercalated siltstone with localized rhyodacitic flows and domes, associated in situ and transported autoclastic deposits, and lesser dykes of aphanitic basalt and gabbro. Thin- to thick-bedded, black carbonaceous argillite, minor intercalated siltstone, and a large gabbro sill dominate the hanging wall lithofacies. The main lithologies strike NNE–SSW, parallel to a pronounced S1 foliation, and have a low-grade, chlorite–muscovite–quartz–albite–oligoclase metamorphic assemblage. The Draa Sfar deposit consists of two stratabound sulphide orebodies, Tazakourt to the south and Sidi M'Barek to the north. Both orebodies are hosted by argillite in the upper part of the lower volcano-sedimentary unit. The Tazakourt and Sidi M'Barek orebodies are highly deformed, sheet-like bodies of massive pyrrhotite (up to 95% pyrrhotite) with lesser sphalerite, galena, chalcopyrite, and pyrite. The Draa Sfar deposit formed within a restricted, sediment-starved, fault-controlled, anoxic, volcano-sedimentary rift basin. The deposit formed at and below the seafloor within anoxic, pelagic muds.The argillaceous sedimentary rocks that surround the Draa Sfar orebodies are characterized by a pronounced zonation of alteration assemblages and geochemical patterns. In the more proximal volcanic area to the south, the abundance of medium to dark green chlorite progressively increases within the argillite toward the base of the Tazakourt orebody. Chlorite alteration is manifested by the replacement of feldspar and a decrease in muscovite abundance related to a net addition of Fe and Mg and a loss of K and Na. In the volcanically distal and northern Sidi M'Barek orebody alteration within the footwall argillite is characterized by a modal increase of sericite relative to chlorite. A calcite–quartz–muscovite assemblage and a pronounced decrease in chlorite characterize argillite within the immediate hanging wall to the entire Draa Sfar deposit. The sympathetic lateral change from predominantly sericite to chlorite alteration within the footwall argillite with increasing volcanic proximity suggests that the higher temperature part of the hydrothermal system is coincident with a volcanic vent defined by localized rhyodacitic flow/domes within the footwall succession.     

Composition-volume changes during hydrothermal alteration of andesite at Buttercup Hill, Noranda District, Quebec

Hydrothermally altered andesites in the upper member of the Amulet formation at Buttercup Hill, Noranda, Quebec represent part of the aquifer and cap of a self-sealing geothermal system that focussed the discharge of hydrothermal fluids during the formation of massive Cu-Zn sulfide deposits. Five alteration facies are recognized

1. 1) pervasive greenschist faciés regional metamorphism (least-altered andesite)

2. 2) epidotization-silicification

3. 3) albitization-silicification

4. 4) chloritization

5. 5) sericitization-silicification. Alteration is localized on permeable zones such as amygdules, fractures, flow tops, discordant breccia dikes, and conformable breccia horizons.

Epidotized-silicified andesite is enriched in Ca-Sr-Eu and depleted in Mg and first transition series metals (FTSM) relative to least-altered andesite. Albitized-silicified andesite is significantly enriched in Na and depleted in most FTSM relative to least-altered andesite. The abundances and inter-element ratios of the rare-earth elements (REE) and most high field-strength elements (HFS: Y, Zr, Th, U, Hf, Ta) are similar in least-altered, epidotized-silicified and albitized-silicified andesites. The most silicified andesites are strongly enriched in Na-Si, strongly depleted in Mg and divalent FTSM and slightly but systematically depleted in REE and most HFS elements. Serialized andesites were previously silicified; they are very strongly enriched in K-Rb-Cs-Ba, very strongly depleted in Na-Ca-Sr-Eu and slightly depleted in light REE relative to silicified andesite. Chloritized andesitic rocks exhibit heavy REE and HFS element ratios similar to those of leastaltered andesite, but are relatively strongly enriched in Mg and divalent FTSM, strongly depleted in Si and large ion lithophile (LIL) elements and slightly depleted in light REE.

The coupled behavior of the heavy REE and most HFS elements during epidotization, albitization, silicification, chloritization and serialization suggests that they were inert during hydrothermal modification of the andesite. Mass balance calculations suggest that volume was conserved during epidotization-silicification and albitization-silicification, but that intense silicification was accompanied by volume increases up to 30 percent.