Multielement Imaging Reveals the Diagenetic Features and Varied Water Redox Conditions of a Lacustrine Dolomite Nodule

The genesis of dolostone has long been puzzling for more than two centuries. Although much work has been done on investigating the process of dolomitization, little emphasis has been put on examining the diagenetic water redox condition with the wealthy geochemical information preserved in primary dolomite, which is believed to archive the aqueous environment as well as biotic and/or abiotic effects during formation. In situ interpretation with high resolution is a prerequisite in refined research of dolomite. Here, we reported the multielement imaging results of a lacustrine dolomite nodule with the host black shale from the Songliao Basin, northeast of China. Micro X-ray fluorescence (μ-XRF) with a spatial resolution down to 10 μm was used for in situ scanning. Two key parameter settings of the μ-XRF, including single-point exposure time and spatial resolution, were optimized to achieve a better result in a reasonable scanning time scale. The final imaging data graphically revealed dynamic variation of elemental distributions, including elements enriched in dolomite (e.g., Ca, Mg, Fe, and Mn), clastic quartz (Si), and clay minerals (e.g., Al and K) and redox-sensitive trace elements (e.g., Cr, Mo, V, and U). The well-preserved laminated structures inside the nodule and the features with a magnesium-rich core wrapped with an iron-concentric outer layer and a manganese-concentric shell together indicated its primary form as dolomite and a gradual transformation into ankerite as well as manganese-ankerite. The elemental variation indicates a varied bottom water redox condition, which involved from sulfidic to ferruginous and manganous zones. Here, we propose that the intermittent supplies of sulfate and Fe-/Mn-oxidized minerals interrupting the black shale deposition while favoring dolomitization might be brought by the oxidized and salted seawater. And this lacustrine dolomite is expected to be a potential fingerprint mineral in tracking the seawater intrusions to the Songliao Basin which happened 91 million years ago.

Ultrabasites and the sources of noble metals (Au, Pt, Pd) in the North Caucasus

Ультрабазиты Северного Кавказа являются древнейшими магматическими породами, возраст которых определяется как нижнепалеозойский (ордовик-силур). Эти породы играют ключевую роль в металлогении благородных металлов Кавказа. С ультрабазитами связаны гигантские запасы металлов в региональной углеродсодержащей девонской черносланцевой толще Северного Кавказа. Практическое значение имеют пермскские пропилиты (Чучкурское месторождение) и металлоносные юрские коры выветривания ультрабазитовых массивов. До настоящего времени не оценены благородные металлы лиственитов Северного Кавказа, сопровождающих ультрабазиты. Цель работы. Показать, что ультраосновные породы являются донорами благородных металлов (Au, Pt, Pd) для Северного Кавказа на протяжении всего фанерозоя. Подтвердить пространственно-временную связь ультрабазитов с благороднометалльным оруденением в черных сланцах Кавказа. Доказать на примере Беденского массива ультрабазитов наличие рудоносной коры выветривания, залегающей на размытой поверхности серпентинитов. Методы исследований. Сделан краткий исторический обзор исследований ультрабазитов с ориентацией на генетические связи благородных металлов (Au, Pt, Pd) с ультрабазитовой магмой. Проведен анализ геологических материалов по наиболее крупным на Северном Кавказе Беденскому и Малкинскому ультрабазитовым массивам - их составу, геохимическим особенностям и потенциальной рудоносности. Результаты работы. По химическому составу ультрабазиты Северного Кавказа отнесены к альпинотипному формационному типу. Установлена рудообразующая роль альпинотипных ультрабазитов для благородных металлов в осадочных и метаморфических толщах фанерозоя региона. Благородные металлы активно проявляются в металлогении фанерозойских геотектонических эпох: каледонской, герцинской, киммерийской и альпийской, имея единый первичный ультрабазитовый источник. Свежие, неизмененные серпентиниты в настоящее время не обнаруживают аномальные содержания благородных металлов, представляющих промышленный интерес. Насыщение серпентинитов благородными металлами проявляется при их экзогенной переработке в обогащенных продуктах их разрушения. Потенциально промышленными осадочными комплексами, состоящими из обогащенных продуктов разрушения серпентинитов, являются коры выветривания серпентинитов и осадочные черносланцевые толщи Северного Кавказа. Ультрабазитовая магма на Северном Кавказе послужила первичным глубинным источником благородных металлов в осадочных и метаморфогенных фанерозойских толщах Кавказа The ultrabasites of the North Caucasus are the oldest igneous rocks, whose age is defined as the Lower Paleozoic (Ordovician-Silurian). These rocks play a key role in the metallogeny of the noble metals of the Caucasus. Huge reserves of metals in the regional carbonaceous Devonian black shale stratum of the North Caucasus are associated with ultrabasites. Permian propylites (Chuchkurskoye deposit) and metal-bearing Jurassic crusts of weathering of ultrabasic massifs are of practical importance. To date, the noble metals of the listvenites of the North Caucasus accompanying ultrabasites have not been evaluated. Aim. Show that ultrabasic rocks are donors of noble metals (Au, Pt, Pd) for the North Caucasus throughout the entire Phanerozoic. Confirm the spatio-temporal relationship of ultrabasic rocks with noble metal mineralization in the black shales of the Caucasus. Prove the presence of an ore-bearing weathering crust on the eroded surface of serpentinites using the example of the Bedensky ultrabasic massif. Methods. A brief historical review of ultrabasite studies with a focus on the genetic relationships of noble metals (Au, Pt, Pd) with ultrabasite magma is made. The analysis of geological materials on the Bedene and Malkin ultrabasite massifs, the largest in the North Caucasus, is carried out - their composition, geochemical features and potential ore content. Results. According to the chemical composition, the ultrabasites of the North Caucasus are attributed to the alpinotype formation type. The ore-forming role of alpinotype ultrabasites for noble metals in sedimentary and metamorphic strata of the Phanerozoic region has been established. Noble metals are actively manifested in the metallogeny of the Phanerozoic Geotectonic epochs: Caledonian, Hercynian, Cimmerian and Alpine, having a single primary ultrabasic source. Fresh, unaltered serpentinites do not currently show anomalous contents of noble metals of industrial interest. The saturation of serpentinite with noble metals is manifested during their exogenous processing in the enriched products of their destruction. Potentially industrial sedimentary complexes consisting of enriched products of destruction of serpentinites are the weathering crusts of serpentinites and sedimentary black-shale strata of the North Caucasus. Ultrabasic magma in the North Caucasus served as the primary deep-seated source of precious metals in sedimentary and metamorphogenic Phanerozoic strata of the Caucasus

Organic Petrographic and Geochemical Evaluation of the Black Shale of the Duwi Formation, El Sebaiya, Nile Valley, Egypt

This study evaluates the palynologic, organic, inorganic, and petrographic properties of organic-rich black shale (Mahamid Mine) in the El Sebaiya area, Nile Valley, Egypt. Black shale is composed of quartz (50%), calcite (10%), kaolinite (25%) and montmorillonite (15%). Organic and inorganic analyses revealed that this shale was deposited under oxic to anoxic marine conditions during strong chemical weathering. Black shale has poor to very good organic richness, and poor to fair hydrocarbon potential. Organic petrography indicates that the kerogen is mixed types II/III and III and is immature to marginally mature (%VRo is 0.44 and 0.53). Liptinite macerals consist of alginite, cutinite, and bituminite. The hydrocarbon products to be generated at higher maturity are expected to be oil and gas.

Supplemental Material: New insights into organic matter accumulation from high-resolution geochemical analysis of a black shale: Middle and Upper Devonian Horn River Group, Canada

Geochemical data, organic petrographic analysis, and supplemental figures.

Supplemental Material: New insights into organic matter accumulation from high-resolution geochemical analysis of a black shale: Middle and Upper Devonian Horn River Group, Canada

Geochemical data, organic petrographic analysis, and supplemental figures.

Fluids as primary carriers of sulphur and copper in magmatic assimilation

Magmas readily react with their wall-rocks forming metamorphic contact aureoles. Sulphur and possibly metal mobilization within these contact aureoles is essential in the formation of economic magmatic sulphide deposits. We performed heating and partial melting experiments on a black shale sample from the Paleoproterozoic Virginia Formation, which is the main source of sulphur for the world-class Cu-Ni sulphide deposits of the 1.1 Ga Duluth Complex, Minnesota. These experiments show that an autochthonous devolatilization fluid effectively mobilizes carbon, sulphur, and copper in the black shale within subsolidus conditions (≤ 700 °C). Further mobilization occurs when the black shale melts and droplets of Cu-rich sulphide melt and pyrrhotite form at ∼1000 °C. The sulphide droplets attach to bubbles of devolatilization fluid, which promotes buoyancy-driven transportation in silicate melt. Our study shows that devolatilization fluids can supply large proportions of sulphur and copper in mafic–ultramafic layered intrusion-hosted Cu-Ni sulphide deposits.