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

Rhenium-osmium geochronology and isotopic systematics in New Zealand source rocks and oils

<p>The Re-Os radiogenic isotope system has over the past three decades been successfully applied to organic-rich sedimentary rocks and oils as a geochronometer and geochemical tracer. The Re-Os geochronometer has provided a direct way of constraining the depositional age of organic-rich sediments as well as the timing of oil generation events. Osmium isotopic compositions have further been utilised in understanding past climatic, oceanographic and geological events recorded in sediments, and in correlating oils to their source. Thus far, however, Re-Os studies of organic-rich sediments have mainly focused on marine black shales where Re and Os are primarily sourced from seawater. The work presented in this thesis seeks to investigate factors controlling Re-Os systematics and potential for geochronology in a range of fluvio-deltaic coaly rocks and terrestrial organic matter-dominated marine sediments, and associated oils from New Zealand’s Taranaki and East Coast basins. The Re-Os data presented here yield the first radiometric age for the late Paleocene Waipawa Formation (57.5 ± 3.5 Ma), a marine sedimentary unit that was formed by episodic input of large amounts of terrestrial woody plant matter resulting in high average sedimentation rates of up to ~10.6 cm/ky. This age is consistent with available biostratigraphic age determinations. The formation possesses Re (38.9 ± 17.6 ppb) and Os (526 ± 75.8 ppt) concentrations similar to those found in typical marine sediments containing amorphous organic matter deposited under much lower sedimentation rates. This indicates that organic matter type and sedimentation rate may not play a significant role in sequestration of these elements in organic-rich sediments. Unlike the Waipawa Formation, coals and coaly mudstones with varying degrees of marine influence (purely terrestrial to strongly marine-influenced) from the Rakopi, North Cape, Farewell and Mangahewa formations record low average Re (0.37 ± 0.25 ppb) and Os (24.5 ± 11.9 ppt) concentrations. These concentrations are up to two orders of magnitude lower than those of similarly marine-influenced coals from the Matewan coalbed, USA, suggesting that Re and Os enrichment in coals does not simply correlate with the level of marine influence; the timing and nature of the marine influence, as well as chelation ability of organic-rich sediments, are equally important. The initial 187Os/188Os (Osi) values for the Waipawa (~0.28) and underlying Whangai (~0.36) formations are broadly similar to those reported for coeval pelagic sediments from the central Pacific Ocean, further constraining the low-resolution marine 187Os/188Os record of the Paleocene. A broad correlation between this record and global temperature (δ18O and TEX86) and carbon isotope (δ13C) records is observed from the middle Paleocene to early Eocene, which is inferred to reflect climate-modulated changes in continental weathering patterns. Unlike the marine sediments, significant variations are noted in the Osi of the Taranaki Basin coaly rocks. These are linked to depositional and diagenetic conditions, degree of water connectivity with the open ocean, and sediment source. The large variations in Osi values combined with small ranges in 187Re/188Os ratios and relatively young ages are considered as factors that hindered development of Re-Os isochrons in these rocks. Crude oils sourced from the Taranaki coals and coaly mudstones also record low average Re (0.31 ± 0.09 ppb) and Os (14 ± 7.6 ppt) concentrations and have 187Re/188Os and 187Os/188Os ratios that do not correlate on an isochron diagram. The lack of an isochron fit for these oils is mainly attributed to a large variation in Osi values (0.47-1.14) resulting from the heterogeneous nature of their potential Rakopi and North Cape coaly source rocks and a lengthy (20 Myr) oil generation event. These Osi values, however, overlap with 187Os/188Os values for the potential source rocks at the time (ca.10 Ma) of oil generation (0.38-1.26), suggesting that Os isotopes may be utilised in tracing these oils. Crude oils that have potentially been sourced from the Waipawa and Whangai formations record much higher Re (2.86 ± 1.92 ppb) and Os (166 ± 142 ppt) concentrations than the coaly-sourced oils, and show Os isotopic compositions that either correlate with those of their potential source rocks (e.g., oil Osi = ~0.63 compared with Waipawa Formation 187Os/188Os = 0.48–0.68 at time of oil generation) or differ due to likely secondary alteration processes within the reservoir such as thermochemical sulfate reduction (TSR).</p>

Rhenium-osmium geochronology and isotopic systematics in New Zealand source rocks and oils

<p>The Re-Os radiogenic isotope system has over the past three decades been successfully applied to organic-rich sedimentary rocks and oils as a geochronometer and geochemical tracer. The Re-Os geochronometer has provided a direct way of constraining the depositional age of organic-rich sediments as well as the timing of oil generation events. Osmium isotopic compositions have further been utilised in understanding past climatic, oceanographic and geological events recorded in sediments, and in correlating oils to their source. Thus far, however, Re-Os studies of organic-rich sediments have mainly focused on marine black shales where Re and Os are primarily sourced from seawater. The work presented in this thesis seeks to investigate factors controlling Re-Os systematics and potential for geochronology in a range of fluvio-deltaic coaly rocks and terrestrial organic matter-dominated marine sediments, and associated oils from New Zealand’s Taranaki and East Coast basins. The Re-Os data presented here yield the first radiometric age for the late Paleocene Waipawa Formation (57.5 ± 3.5 Ma), a marine sedimentary unit that was formed by episodic input of large amounts of terrestrial woody plant matter resulting in high average sedimentation rates of up to ~10.6 cm/ky. This age is consistent with available biostratigraphic age determinations. The formation possesses Re (38.9 ± 17.6 ppb) and Os (526 ± 75.8 ppt) concentrations similar to those found in typical marine sediments containing amorphous organic matter deposited under much lower sedimentation rates. This indicates that organic matter type and sedimentation rate may not play a significant role in sequestration of these elements in organic-rich sediments. Unlike the Waipawa Formation, coals and coaly mudstones with varying degrees of marine influence (purely terrestrial to strongly marine-influenced) from the Rakopi, North Cape, Farewell and Mangahewa formations record low average Re (0.37 ± 0.25 ppb) and Os (24.5 ± 11.9 ppt) concentrations. These concentrations are up to two orders of magnitude lower than those of similarly marine-influenced coals from the Matewan coalbed, USA, suggesting that Re and Os enrichment in coals does not simply correlate with the level of marine influence; the timing and nature of the marine influence, as well as chelation ability of organic-rich sediments, are equally important. The initial 187Os/188Os (Osi) values for the Waipawa (~0.28) and underlying Whangai (~0.36) formations are broadly similar to those reported for coeval pelagic sediments from the central Pacific Ocean, further constraining the low-resolution marine 187Os/188Os record of the Paleocene. A broad correlation between this record and global temperature (δ18O and TEX86) and carbon isotope (δ13C) records is observed from the middle Paleocene to early Eocene, which is inferred to reflect climate-modulated changes in continental weathering patterns. Unlike the marine sediments, significant variations are noted in the Osi of the Taranaki Basin coaly rocks. These are linked to depositional and diagenetic conditions, degree of water connectivity with the open ocean, and sediment source. The large variations in Osi values combined with small ranges in 187Re/188Os ratios and relatively young ages are considered as factors that hindered development of Re-Os isochrons in these rocks. Crude oils sourced from the Taranaki coals and coaly mudstones also record low average Re (0.31 ± 0.09 ppb) and Os (14 ± 7.6 ppt) concentrations and have 187Re/188Os and 187Os/188Os ratios that do not correlate on an isochron diagram. The lack of an isochron fit for these oils is mainly attributed to a large variation in Osi values (0.47-1.14) resulting from the heterogeneous nature of their potential Rakopi and North Cape coaly source rocks and a lengthy (20 Myr) oil generation event. These Osi values, however, overlap with 187Os/188Os values for the potential source rocks at the time (ca.10 Ma) of oil generation (0.38-1.26), suggesting that Os isotopes may be utilised in tracing these oils. Crude oils that have potentially been sourced from the Waipawa and Whangai formations record much higher Re (2.86 ± 1.92 ppb) and Os (166 ± 142 ppt) concentrations than the coaly-sourced oils, and show Os isotopic compositions that either correlate with those of their potential source rocks (e.g., oil Osi = ~0.63 compared with Waipawa Formation 187Os/188Os = 0.48–0.68 at time of oil generation) or differ due to likely secondary alteration processes within the reservoir such as thermochemical sulfate reduction (TSR).</p>

Paleoenvironment and Organic Matter Accumulation Mechanism of Marine–Continental Transitional Shales: Outcrop Characterizations of the Carboniferous–Permian Strata, Ordos Basin, North China

In the Carboniferous–Permian period, several organic-rich black shales were deposited in a marine–continental transitional environment in the Linfen area on the eastern margin of the Ordos Basin. Integrated sedimentological and organic geochemical analyses are performed on an outcrop in order to clarify the relationship between paleoenvironment and organic matter accumulation. The results of this study show that the marine–continental transitional strata of the Upper Carboniferous Benxi Formation to Lower Permian Taiyuan and Shanxi Formation exposed in the Linfen area are composed of sandstone, shale, coal, and limestone. Total organic carbon (TOC) contents of the studied samples were mainly distributed in the range of 0.59%–35.4%, with an average of 7.32%. From Benxi Formation to Shanxi formation, the humidity gradually increased, and the climate gradually changed from hot and humid to warm and humid during Carboniferous to Permian. The deposition of the Shanxi Formation ended with the climate returning to hot and humid, having an oxic-suboxic conditions and a high paleoproductivity. Paleoredox conditions and paleoproductivity are the two vital factors controlling the formation of organic matter in black shales. The transitional environment characterized by oxic-suboxic, relatively high deposition rate, and various source of organic matter, although different from the marine environment, provides a good material basis for the deposition of organic-rich shales.