Red Bed Basins Research Articles

Origins and chemical evolution of hydrothermal fluids in the Shazhou volcanic-related type U deposit at the Xiangshan district, South China: New insights from LA-ICP-MS analysis of fluid inclusions

The volcanic-related type U mineralization in South China has been associated with the crustal extension occurred since the late Cretaceous. Uranium deposits of this type are spatially related to the Cretaceous red-bed basins and synchronous mafic magmatism. However, the contribution of basinal brines or mantle-derived fluids to the U mineralization is not well understood. This study examines the role of different hydrothermal fluids involved in the U mineralization process and traces their sources by using LA-ICP-MS analysis of fluid inclusions from the Shazhou deposit in the Xiangshan district, which is one of the largest volcanic-related type U ore-fields in the world. Two types of fluids were involved in the U mineralization process. One fluid was of higher salinity, enriched in U, Mo, and Cl, such that having low Br/Cl (0.3–1.0 × 10-3) similar to the basinal brines sourced from meteoric water. The other fluid was of lower salinity, enriched in Th and Sr, yet depleted in U and Mo, with Br/Cl ratios of 1.5–2.5 × 10-3, overlapping the range of mantle-derived fluids associated with the mafic dikes intruded into the felsic wall rocks at the U mineralization age. The high-U fluid and the low-U fluid also show distinct Rb/Na, K/Na, and Rb/Cs, indicating their respective origins of basinal brines and mantle-related fluids. The chemical variation of ore-forming fluids at the Shazhou deposit was comparable to the scenario at the Zoujiashan deposit, as reported in previous studies, suggesting similar fluid evolution history related to U mineralization in the west and the north of the Xiangshan U orefield. Our study suggests in situ analysis of Br/Cl of fluid inclusions can be a useful proxy to discriminate multiple hydrothermal fluids associated with volcanic-related type U mineralization in South China and to individually trace their origins.

Apatite as a Probe into the Nature and Origin of Hydrothermal Fluids Responsible for U Leaching in the Lujing Granite-Related U Deposits, South China

Abstract Hydrothermal fluids have been suggested to be capable of leaching U and other elements (e.g., rare earth elements; REEs) from U-fertile granites to form granite-related U deposits. However, the nature and origin of the hydrothermal fluid responsible for transporting these elements are poorly constrained. Apatite accommodates both U and REEs, and its composition can be modified by hydrothermal fluids with certain compositions. This study investigated in situ chemical and Sr-Nd isotope compositions of primary apatite in altered wall granites from the Lujing U deposits in the Zhuguang batholiths. Large-scale alterations of apatite occurred during the hematitization stage. Uranium was extensively leached out of the apatite, and its total REE concentrations were decreased from as high as 12,667 ppm to a few hundred ppm during the alteration, whereas REE-bearing mineral inclusions were absent in altered apatites. The release of U and REEs was associated with decrease of Na, Mn, and Fe and increase of Ca, Cl, and Eu anomalies in altered regions of apatite. According to apatite compositional variations, the fluid that induced hematitization was oxidizing, rich in Ca and Cl, but poor in Na. The elevated Cl in the fluid is crucial for mobilizing both U and REEs, whereas PO43− can also be a major ligand for U6+ transport given the massive dissolution of apatite during the leaching process. Altered regions of apatite contain high radiogenic strontium, implying assimilation of the fluid by clastic sediments in the adjacent red-bed basins sourced from the Precambrian crystalline basement rocks. The oxidizing, Ca- and Cl-rich, but Na- and F-poor fluid that induces hematitization shows significant potential in leaching U and REEs from the wall granite and providing ore-forming materials for U mineralization.

In situ elemental and Sr-Nd isotopic compositions of hydrothermal apatite from the Shazhou U deposit in the Xiangshan complex: Implications for the origins of ore-forming fluids of volcanic related U deposits in South China

• In situ Sr-Nd isotopes of apatite are effective in tracing origins of ore forming fluids in the Shazhou U deposit. • The reducing U-poor fluid was derived from mantle-derived magma which formed the mafic dikes in wall rocks. • The oxidizing U-rich fluid originated from percolation of meteoric water in the adjacent red-bed basins. • The Late Cretaceous mafic magmatism and red-bed basins are two key factors controlling U mineralization in South China. The origin of ore-forming fluids of volcanic related U deposits remains controversial. This study reports in situ elemental and Sr-Nd isotopic compositions of apatite associated with U mineralization from the Shazhou volcanic related U deposit in South China, which provides a new perspective to constrain the origin and evolution of its ore-forming fluids. Hydrothermal apatites from the hematized rocks and U-mineralized rocks show distinct textures and chemical features, indicating two types of fluids involved in the U mineralization process. The hematitization fluid was oxidizing and enriched in U and Cl. A reducing fluid with lower Cl/F and U was introduced and mixed with the oxidizing hematitization fluid to cause U precipitation. Apatite from the hematized rocks has a lower 87 Sr/ 86 Sr (0.709246–0.714383) than that of apatite in U ore veins (0.710383–0.719318). The εNd(t) of apatite in the hematized rocks is from −7.6 to −9.4, whereas apatite in U ore veins has higher εNd(t) ranging from −1.2 to −8.4, close to that of mafic dikes intruded into the volcanic rocks. In situ Sr-Nd isotopes of apatite indicate the oxidizing fluid originated from the red-bed basins, whilst the origin of the reducing fluid was related to the mantle-derived mafic dikes. The reducing fluid has experienced assimilation by the metamorphic basement rocks to gain higher radiogenic strontium. This study demonstrates that in situ elemental and Sr-Nd isotopic compositions of apatite are effective in identifying different ore-forming hydrothermal fluids and constraining their origins and chemical evolution.

Application of the Wide Field Electromagnetic Method for Oil and Gas Exploration in a Red-bed Basin of South China

Red-bed basins with rich hydrocarbon source rocks are widely distributed in south China, such as Banshi Basin in southern Jiangxi, which may have very good prospects for oil and gas exploration. However, due to poor ground conditions, and complex geological structures, seismic exploration and conventional electromagnetic methods cannot provide useful information for hydrocarbon evaluation. This study uses the wide field electromagnetic (WFEM) method to investigate the distribution and geoelectric characteristics of the target stratum of a red-bed basin in Jiangxi province, China. The inversion results demonstrate that the WFEM method could quickly delineate the favorable area and determine the location of the parametric well, confirming that the WFEM method is an effective geophysical exploration method for evaluating hydrocarbon resources in red-bed basins.

An Early Cretaceous record of trachyte from the red bed basin of the southwest Dabie Orogen, Central China: Implications for tectonic evolution and geodynamics

Zircon U–Pb ages of approximately 130 Ma were obtained from trachytes in Xiaochang County, at the southwestern margins of the Dabie Mountains, in Central China. The SiO2 and K2O contents of these trachytes range between 56.78–63.77% and 3.76–5.07%, respectively, while Rittmann indexes (δ) range between 3.7 and 6.8. These characteristics indicate that the trachytes belong to the alkaline series. The trachytes are enriched in large‐ion lithophile elements (LILEs) but are depleted in high‐field‐strength elements (HFSEs). The initial 87Sr/86Sr values of the trachytes range from 0.7060 to 0.7072, with negative εNd(t) values varying from −10.43 to −12.88. The weighted averages of the zircon εHf(t) and TDM2 values are −11.7 ± 0.5 and 1921 ± 28 Ma, respectively. The presence of Early Cretaceous K‐rich volcanic rocks at the southwestern margins of the Dabie Mountains has been confirmed for the first time. This finding fills a gap regarding the presence of Mesozoic volcanic rocks in this region and limits the earliest possible formation age of the red bed basin at the southwestern margins of the Dabie Mountains to the Early Cretaceous. The trachytes originate from partial melting of enriched mantle. They are closely related to major regional faults and represent the eastern extension of the Qinling–Daba alkaline rock belt.

Land degradation and management of red beds in China: Two case studies

Red beds cover approximately 9.5% of China, and are home to approximately 144 million people. In total, 83% of these lands are distributed in humid regions making it an important part of research on red bed soil erosion in China in these areas. This paper presents the main types of land degradation in red bed landscapes and the status of current soil erosion in a typical red bed basin, the Nanxiong Basin located in the north of Guangdong Province, China, and establishes the connection between management strategies and regional economic development in humid red bed regions of China. The soil erosive modulus was calculated in the Nanxiong Basin by using RUSLE (The Revised Universal Soil Loss Equation). The results of overlapping analyses demonstrated that appropriate measures, such as the Return Farmland to Forests initiative, should be taken at the junction of central red bed areas and mountainous areas in order to mitigate current soil erosion. Two examples are presented to demonstrate this: the tourism development in Mt. Danxiashan, a noted scenic mountainous area near Nanxiong Basin, and the land degradation mitigation in the Nanxiong Basin. Both examples promote local economic growth while simultaneously protecting the environment. A ‘stakeholder’ strategy is pursued at Mt. Danxiashan, which can help residents to understand their positive effects on the environment as well as increase their income. The second example, in Nanxiong City, showcases how local farmers became stakeholders by implementing contract responsibility and self-support systems for economic forests and terraced land in the 1980s.

Plate subduction, oxygen fugacity, and mineralization

Plate subduction is the largest natural factory that processes elements, which controls recycling and mineralization of a variety of elements. There are three major ore deposit belts in the world: the circum-Pacific, the central Asian, and the Tethys belts. All the three belts are closely associated with plate subductions, the mechanism remains obscure. We approached this problem from systematic studies on the behaviours of elements during geologic processes. This contribution summaries the recent progress of our research group. Our results suggest that porphyry Cu deposits form through partial melting of subducted young oceanic crust under oxygen fugacities higher than ΔFMQ ∼+1.5, which is promoted after the elevation of atmospheric oxygen at ca. 550 Ma. Tin deposits are associated with reducing magmatic rocks formed as a consequence of slab rollback. The Neo-Tethys tectonic regime hosts more than 60% of the world’s total Sn reserves. This is due to the reducing environment formed during the subduction of organic rich sediments. For the same reason, porphyry Cu deposits formed in the late stages during the closure of the Neo-Tethys Ocean. Tungsten deposits are also controlled by slab rollback, but is not so sensitive to oxygen fugacity. Subduction related W/Sn deposits are mostly accompanied by abundant accessory fluorites due to the breakdown of phengite and apatite. Decomposition of phengite is also significant for hard rock lithium deposits, whereas orogenic belt resulted from plate subduction promote the formation of Li brine deposits. Cretaceous red bed basins near the Nanling region are favorable for Li brines. Both Mo and Re are enriched in the oxidation-reduction cycle during surface processes, and may get further enriched once Mo-, Re-enriched sediments are subducted and involved in magmatism. During plate subduction, Mo and Re fractionate from each other. Molybdenum is mainly hosted in porphyry Mo deposits and to a less extent, porphyry Cu-Mo deposits, whereas Re is predominantly hosted in porphyry Cu-Mo deposits and sedimentary sulfide deposits.

The Dynamic Change of Vegetation Cover and Associated Driving Forces in Nanxiong Basin, China

Natural climate change and human activities are the main driving forces associated with vegetation coverage change. Nanxiong Basin is a key ecosystem-service area at the national level with a dense population and highly representative of red-bed basins, which are considered as fragile ecological units in humid regions. In this study, the authors aimed to determine the trends in vegetation cover change over past two decades and the associated driving forces in this study area. The Normalized Difference Vegetation Index (NDVI) of 2000–2015, derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) remote sensing dataset along with the application of statistical methods and GIS (geographic information system) techniques were used to quantify vegetation cover change. The results show that human-induced factors can explain most variations at sites with significant cover change. That is to say that human activities are the main drivers of vegetation dynamics in this study area, which shows a significant reduction trend in vegetation cover during the industrialization and urbanization processes of the study period and noticeable recovery trend in 2000–2015 under the plantation and enclosed forest policy.

Th-U-Pb zircon geochronology of the Palaeoproterozoic Hartley Formation porphyry by six methods, with age uncertainty approaching 1 Ma.

The Palaeoproterozoic Hartley Formation in the Olifantshoek Group was deposited in one of the rift-related Waterberg (sensu lato) red bed basins which formed on the Kaapvaal Craton after the 2.05 Ga Bushveld intrusions and coeval thermal event. The age of these basins is not well constrained due to the shortage of directly dateable rock types. The Hartley Formation contains rare quartz-porphyry lavas interbedded with the dominant basalts and these provide the means to date the formation by analyses of zircon. In this work zircon from one sample has been dated by six Th-U-Pb methods, namely Laser Ablation ICP Quadrupole Mass Spectrometry, Laser Ablation ICP High-resolution Mass Spectrometry, Laser Ablation ICP Multicollector Mass Spectrometry U-Pb (also Lu-Hf), Nordsim Ion probe U-Pb and Th-Pb; and Krogh method ID-TIMS. Our precise ages give a combined age of 1915.2 ± 1.1 Ma. Including one published ion probe date from the only other known occurrence of quartz porphyry, the results only agree if the quoted analytical errors are increased by 20%, which gives a combined result of 1915.6 ± 1.4 Ma. This is considered a reliable, precise and accurate age for the Hartley Formation and supersedes the published Kober method 207Pb/206Pb age of 1928 ± 4 Ma. The new Lu-Hf zircon data, supported by published whole rock Sm-Nd and Rb-Sr data, suggests that both the dominant basalts and the rare quartz porphyries of the Hartley Formation were derived from mafic source rocks which had been in the crustal domain from Archaean times. By contrast with the intracratonic rifts of the other Waterberg Basins, the Olifantshoek Supergroup reflects the development of a western passive margin as the Archaean Kaapvaal Craton rifted and drifted. This was followed by accretion of the Rehoboth Province along the Kalahari Line, accompanied by the development of the east-vergent Kheis Province thrust complex. This created a larger cratonic block against which the 1.2 Ga collisions of Namaqua-Natal terranes impacted. The Kheis Province now yields ~1.17 Ma cooling ages, reflecting the Namaqua collisions, but the true age of the Kheis event is still enigmatic.

Thermal origin of continental red beds in SE China: An experiment study

The origin of continental red beds in SE China is the result of high diagenetic temperatures, rather than an arid climate during their deposition. Here we present results from an experimental study where black mud was heated to demonstrate the formation of red beds. Diffuse reflectance spectroscopy (DRS) of heated samples enables determination of the relative proportion of goethite and hematite. Iron in black mud is predominantly in the form of goethite that has an initial dehydration temperature of ca.150°C. Increasing temperature or prolonged heating time is accompanied by decreasing goethite and organic content, increasing hematite and red colouration. Heat provided to subsiding red bed basins is supplied by cooling of an intracrustal granitic magma layer. The thermal model can explain vertical colour, temperature, redox and mineral zonation in red bed sequences, from red (hematite-bearing), through green-yellow (Cu, Zn, V sulphide mineralization) to grey-black (hydrocarbon, halite-bearing) sediments. The model can also be used to help prospect for hydrocarbon and halite deposits in the SE China red bed basins.

Properties and Evolution of Ore‐forming Fluid in Continental Sandstone‐type Copper Deposit: a Case Study from Liuju and Haojiahe Deposits in Chuxiong Redbed basin in Yunnan Province, China

Properties and Evolution of Ore‐forming Fluid in Continental Sandstone‐type Copper Deposit: a Case Study from Liuju and Haojiahe Deposits in Chuxiong Redbed basin in Yunnan Province, China

中国丹霞世界自然遗产地丹霞地貌发育机制探讨

中国丹霞世界自然遗产地丹霞地貌发育的红层盆地全部形成于中生代，尤其是白垩纪盆地内部形成了巨厚的红色碎屑岩系，为丹霞地貌的发育提供了物质基础，随着地壳的隆升，内力是矛盾的主要方面。此外，研究地的丹霞地貌主要分布在红层盆地的边缘地带，在大断裂线的控制下，丹霞地貌空间展布以北东方向为主，构造隆升叠加在上述水平分异之上，局部导致丹霞地貌垂直分异，造成同一地带丹霞地貌的发育不均衡，使丹霞地貌空间分异更加复杂。随着构造隆升的丹霞岩体，在外力的不断侵蚀下，发育成各具形态的丹霞地貌类型。这样，在内力“主导”和外力“雕塑”的共同作用下，发育了绚丽多姿的丹霞地貌。 Danxia landforms which interacted by the internal and external forces are integrations with time, space, and substances. From the time category, all red bed basins of red inland continental facies clast formation which were formed under hot-dry climate environments in the Mesozoic era provided material bases for the development of Danxia landforms. As the crustal monement, the internal force is the main factor of contradictions at that time. From the space category, Danxia landforms are mainly distributed in the verges of red bed basins. The spatial extension of the study areas is mainly along NE direction under the control of main fault line, the directions of sub-faults and sub-joints which are NW, NNE, or EW superimpose the main fault line of NE direction and bring complicities to the Danxia landforms horizontal differentiation. The development of Danxia landforms are acted by the internal and external forces then. From the perspective of substance category, tectonic uplifts which superimpose those horizontal differentiations which bring vertical differentiation of Danxia landforms. Caves and grooves were developed as the lithologic differences eroded by external forces and developed next stages. So the colorful various Danxia landforms were developed under the main action of the internal forces and sculpture action of the external forces.

APPLICATION OF REMOTE SENSING TO 1:50000 AREAL GEOLOGICAL MAPPING IN CONTINENTAL RED BED BASIN

APPLICATION OF REMOTE SENSING TO 1:50000 AREAL GEOLOGICAL MAPPING IN CONTINENTAL RED BED BASIN

Geological evolution of Longhushan World Geopark in relation to global tectonics

The South China fold belt has experienced a complex series of tectonic events that span 1.0 billion years of earth history. Longhushan (龙虎山) World Geopark is located on the Proterozoic suture between the Yangtze craton and Cathyasia block and highlights the long history of this belt. Collision of the Cathyasia and Yangtze cratons 1.0 billion years ago was associated with the formation of the Rodinian supercontinent where most of the planet’s landmasses were amalgamated into one block. Jurassic through Early Cretaceous magmatism was associated with the inland migration of the continental margin arc associated with the penetration of a flat slab after subduction of the Kula-Farallon ridge. Slab roll-back in the Early to Middle Cretaceous opened many extensional basins across the South China fold belt, including the Xinjiang (信江) basin in which Longhushan is located, and these were filled largely with continental red beds deposited by fluvial systems in the hot torrid climate. The beds are richly fossiliferous, including remains of many dinosaurs and dinosaur eggs. Subduction of the Kula-Pacific plate in the Middle Cretaceous caused a short magmatic pulse, and then, the basins subsided slowly through the remainder of the Cretaceous. Cenozoic uplift of the red bed basins was initiated by the India-Asia collision. The uplift was associated with the formation of many faults, joints, and brittle structures that dissected the red bed deposits. Fluvial erosion of the red beds was enhanced along the brittle structures, and different locations have developed very distinctive and structurally controlled geomorphological features including mesas, kopjies, and isolated stone peaks that are known in China as Danxia (丹霞) land-forms. Together, these features form Danxia landscapes, and Longhushan World Geopark exhibits a complete range of the Danxia landscapes from juvenile, to mature and to old stages of development. The United Nations Educational, Scientific, and Cultural Organization (UNESCO) World Geopark program ensures that these features and geologic history will be preserved in their pristine state and be available for study and appreciation for generations to come.

Age and genesis of the Danxia landform on Jianglang Mountain, Zhejiang Province

Jianglang Mountain is situated at the transitional zone of South China fold-system, Jiangshan-Shaoxing deep fracture zone and Baoan-Xiakou-Zhangcun fracture zone. The forming of the Xiakou basin was attributed to the pull-apart fault depression by the above fractures in earlier Cretaceous, afterward, series deposits such as Guantou formation (K1g), Chaochuan formation (K1c) and Fangyan formation (K1f) which belong to Yongkang group, the lower Cretaceous layer accumulated in the Xiakou basin. In late Cretaceous, the above fractures occurred to extrude and the basin began to uplift, meanwhile, amounts of tension fissures and joints were produced since Cenozoic, which accelerated water-dicing into bed-rock. Consequently, landform-building processing: weathering, eroding and collapsing etc. were prevalent as finally to develop the so-called Danxia landform. The Jianglang Mountain landscape zone of the Danxia landform to apply for world natural relics are relying on unique and unparalleled peak, sky-split valley with vivid stones and reviving of platform. What is more, there is significance of study at lithology, stratigraphy and paleo-biology. According to dating for specimen of ophitic vein through-crossing the Yongkang group of Yafeng Peak by K-Ar method, this article revealed the uplift age of red-bed basin to be 77.89±2.6 MaBP (K2) i.e. late Cretaceous, and it is the first chronological datum of Danxia landform research in China.

Lithology, kinematics and geochronology related to Late Mesozoic basin-mountain evolution in the Nanxiong-Zhuguang area, South China

The Nanxiong red-bed basin and its adjacent Zhuguang granite form a distinctive basin-mountain landform in the Nanling region, South China. Research results suggest that the Zhuguang granite is a polyphase composite pluton developed on the metamorphic basement of the paleo-Tethys-paleo-Asian tectonic regime and possesses geometrical and kinematic features of hot-doming extensional tectonics at the middle-upper crustal level, which is considered as a magmatic complex that resulted from the collision-orogeny during the Indosinian Period, the subduction-consuming during the Early Yanshanian Period and the intra-continental basaltic underplating and deep-seated geodynamics during the Late Yanshanian Period. The Nanxiong basin is a Late Cretaceous-Paleogene asymmetric faulted basin that is characterized by a fault boundary on the northern side and an uncomformable boundary on the southern side, its deposit center was migrated gradually from south to north. Structural kinematic results on the basin-mountain coupled zone demonstrate that the ductile and the brittle rheological layers show a quite coincident sense of shear, implying that it is a continuous process from the ductile extensional deformation followed by locally sinistral strike-slip shear at a middle-crustal level to the brittle tensional deformation at a upper-crustal level during formation of granitic doming extensional tectonics. The Zhuguang granite and the Nanxiong faulted basin constructed a semi-graben tectonic system. Lithological and geochemical results suggest that the Late Triassic to Jurassic granitic bodies in the Zhuguang have some similar features: high SiO2, Al2O3, K2O contents, alkalinity index > 2.8, ANKC value > 1.1, LREE-enriched pattern with high REE contents, marked negative Eu anomalies, enrichment in Rb and Th, depletion in Ba and Nb, showing a K-rich and Al-rich calc-alkaline affinity, which suggest a continuous magmatic evolution from Late Triassic to Jurassic. Formation of Nanxiong basin and evolution of basin-mountain system were controlled both by the Zhuguang granitic-doming and the regional extensional tectonics. Development of the olivine basalt in the basin suggests that tension action was very strong during lava eruption. The magma-type zircon grains of basalt from the Nanxiong basin yielded the SHRIMP age of 96±1Ma, providing reliable geochronological constraint on the tectono-thermal event and basin-mountain evolution in the Nanling region, South China.

Metal Transport and Deposition in Hydrothermal Veins Revealed by 213nm UV Laser Ablation Microanalysis of Single Fluid Inclusions

213nm UV laser ablation provides a powerful and cost-effective new method for the chemical microanalysis of individual fluid inclusions. The instrument offers significantly improved energy absorption in many geological materials compared to 266nm UV lasers resulting in improved ablation characteristics. The laser is not strongly absorbed by either air or commonly used optical material, permitting relatively easy delivery and control of the beam. Ablation of multielement standard solutions in pure quartz glass capillaries was found to be an effective method for external calibration of instrument sensitivity. Internal calibration for calculation of absolute element concentrations in inclusion fluids was carried out using chloride concentration estimated from microthermometric data. Using this approach, analyses of groups of cogenetic primary and secondary inclusions using a 30 element analyte menu gave typical relative standard deviations for absolute element concentrations of around 20 percent for alkali and alkali earth elements and 20 to 50 percent for metals, comparable with those obtained by 193nm Excimer laser ablation. Results for Ca are consistent with independent estimates made from microthermometric measurements and a range of other elements are in reasonable agreement with previously published bulk chemical analyses. Overall, the data give good charge balance, within 5 percent. The technique was used to test the hypothesis that the metal content of hydrothermal ore deposits is primarily controlled by the metal content of the mineralizing solutions rather than selective precipitation mechanisms. We studied Pb(-Zn) mineralized ‘crosscourse’ veins from Southwest England that are known to have formed from low temperature brines expelled from Permo-Triassic red-bed basins adjacent to the Cornubian peninsula, probably during the Triassic. These fluids are comparable with the solutions that formed Mississippi Valley-type Pb-Zn deposits. Analyses of primary fluid inclusions across growth-zoned quartz crystals from one of these veins indicate the orefluids at an early stage of vein growth are relatively Pb-rich (up to 295 ppm Pb, up to 86 ppm Zn; Pb/Zn = 1.3-16.1) and display marked temporal variations in chemistry. Gradual increases in K, Ca, Sr, Ba and Zn are observed across a significant (∼10 mm) interval of quartz growth. These increases are interpreted to reflect mineral dissolution-precipitation reactions occurring within the fracture system exploited by the fluids after being expelled from the basin source area. Superimposed on this trend are comparatively short time-scale (∼1 mm of quartz growth) chemical excursions characterized by decreases in the concentrations of Cl, K, Ca, Ba, Sr, Pb and Zn of up to two orders of magnitude. These excursions are believed to be due to local, volumetrically significant, mineral precipitation events and indicate the operation of processes that result in the simultaneous supersaturation of a number of phases. It is suggested that initial galena precipitation (>98% Pb loss from solution) was due to reduction as oxidized fluids encountered the first strongly reducing lithology on their flow path out of the basin source region. Several later dilution events were caused by fluid mixing and are inferred to have caused the co-precipitation of sulfides (galena, sphalerite; up to 98% loss of metals from solution) with sulfates (anhydrite, barite; up to 91% Ca and 97% Ba loss from solution) and/or silicates (K-mica; up to 96% loss of K from solution). However, at only one stage is galena (±anhydrite, K-mica) observed in the sample studied suggesting that fluid inclusions may commonly trap depleted fluids after mineral precipitation has already occurred some distance upstream, rather than reflecting precipitation events in their immediate vicinity. The primary ore-forming fluid in this case study contained elevated Pb and Zn and, at least initially, displayed a higher Pb/Zn ratio than predicted for fluids saturated with respect to both sphalerite and galena. Therefore, we suggest that the Pb-rich tenor of the vein deposit is primarily controlled by the metal content of the ore fluids, which is ultimately a function of the geochemistry of the basin source. In this example, the source area is dominated by oxidized terrestrial clastic rocks. Reactions within the aquifer and selective precipitation mechanisms at the site of ore formation also appear to have modified fluid geochemistry but played a second order role in controlling the metal budget of the mineral deposit formed.

Platinum-group element occurrences in Britain: magmatic, hydrothermal and supergene

In-situ platinum-group element-bearing mineralization of magmatic, hydrothermal and near-surface origin occurs widely in Britain. In addition, grains of platinum-group minerals and other phases that contain platinum-group elements (PGE) have been found in overburden and stream sediments without any identified bedrock source.The Caledonian mafic-ultramafic intrusions of northeast Scotland have been a major target of investigations by the British Geological Survey. Magmatic copper-nickel sulphide deposits, with up to 500 ppb Pt + Pd, have been identified in the Knock intrusion and at Arthrath, near Ellon, in contaminated and xenolithic gabbroic rocks. A magmatic origin, subsequently modified by hydrothermal reworking at high temperatures in shear zones, is proposed for the mineralization in these areas. PGE enrichments, up to 280 ppb Pt + Pd, also occur in clinopyroxene-bearing ultramafic rocks in deformed mafic-ultramafic intrusions along a major shear zone in the upper Deveron valley near Huntly.At Loch Borralan, on the northwestern margin of the Scottish Caledonides, PGE mineralization, up to 878 ppb Pt + Pd, occurs over a 2-km strike length. Two phases of mineralization are evident: a late magmatic phase comprising base-metal sulphides and rare PGE sulphides; and a later, low-temperature phase with PGE tellurides and antimonides associated with shearing, brittle fracturing and veining. A hydrothermal origin that involved the introduction of As and Sb and the redistribution of early formed sulphides is proposed for this mineralization.High PGE concentrations occur at several stratigraphic levels in the Unst ophiolite, Shetland. Pt and Pd both exceed 100 ppm close to the basal thrust of the ophiolite in a zone of talc-carbonate alteration. Lowergrade Pt-Pd-dominant mineralization has also been reported in the cumulate sequence of the complex. Elsewhere on Unst PGE mineralization more typical of ophiolites, dominated by Os, Ir and Ru, occurs in association with chromitites.Recent studies have identified platinum-group minerals (PGM) in layered ultramafic rocks in the Tertiary igneous centres on Rum, Mull and Skye in northwest Scotland. In each case abundant and diverse PGM assemblages have been recorded and a combined orthomagmatic-hydromagmatic origin has been proposed.In several parts of Britain a gold-PGE association has been identified at the boundary between Permian red-bed basins and underlying reduced strata. Complex Au-Pt-Pd grains and discrete PGM are widespread in drainage sediments and overburden derived from the Lower Devonian and Permo-Triassic rocks in the South Hams area and the Crediton Trough in Devon, and the Mauchline and Thornhill basins in southern Scotland. The genesis of the precious-metal grains is attributed to metal transportation in saline, oxidizing fluids with deposition along faults and in favourable host rocks due to mixing with reduced rocks or fluids.

The configuration of Asia prior to the collision of India: Cretaceous paleomagnetic constraints

Paleomagnetic data from Central Asia show that 1700±610 km of shortening of southern Asia since Cretaceous time have been absorbed by distributed deformation between southern Tibet and the Siberia craton. This result is based on a compilation of Cretaceous poles from the Junggar, Tarim, Tibet, Indochina, South China, North China, and Mongolia blocks, complementing the recent compilation of Enkin et al. (1992a). We propose a paleogeographic reconstruction of Asia in the Cretaceous, in which the position of Siberia is derived from the synthetic apparent polar wander path of Besse and Courtillot (1991). The resulting map, which likely represents Asia as it remained throughout the Cretaceous until the collision with India began, features an “unbent” Tibet, with an east‐west trending Andean margin at tropical latitudes and a rather continuous belt of continental red bed basins extending from Sichuan to Tarim through Tibet. The map allows one to estimate continental shortening and rotations between the blocks, which are attributed to the collision. Despite large uncertainties, these have amounts and senses which are in all cases compatible with some recent kinematic models such as that of Avouac (1991).Appendix is available with entire paper on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W.,Washington, DC 20009. Document B93‐007; $2.50. Payment must accompany order.

Origin and provenance of some exotic blocks in lower Mesozoic red-bed basin deposits, southern Arizona

In southern Arizona, a profound change in tectonic regime from quiescence to major instability took place near the beginning of Mesozoic time. One manifestation of this instability was the free gliding of large blocks of Paleozoic sedimentary rocks into fault troughs containing volcanic material and continental sediments. In the northwestern Canelo Hills, a succession of exotic blocks composed of Permian sedimentary rocks is intercalated with the Canelo Hills Volcanics of Triassic(?)-Jurassic age. The blocks are as large as 400 m thick and 2 km in strike length. Bedding faults, slickensides, and open-space breccias formed near the base of each Permian block during epidermal gliding, whereas the underlying sediments of the Canelo Hills Volcanics deformed by penecontemporaneous flow. Slickenside and fold-axis orientations place northeast-southwest constraints on the direction of block emplacements. The sense of movement is interpreted to have been northeast to southwest because paleocurrent directions for red beds within the Canelo Hills Volcanics are dominantly southwest and the stratigraphy of the glide blocks is most similar to Permian rocks to the northeast. Palinspastic reconstruction of the blocks to a provenance northeast of the Canelo Hills fits present models of distribution of Permian formations in southern Arizona. Slide block emplacement was synchronous with Triassic(?)-Jurassic vertical movements along northwest-striking fault zones in southern Arizona. One of these fault zones, the Sawmill Canyon, marks the approximate northeast boundary of the Canelo Hills and in early Mesozoic time furnished a scarp from which the slide blocks were derived.