Characterization of Secondary Native Selenium in the Yutangba Se Deposit,Western Hubei,China

Secondary native Se minerals that are appreciably coarser grained and vary morphologically than primary native Se minerals were observed in the abandoned dumps of the No. 3 and No. 4 orebodies of the Yutangba Se deposit, western Hubei Province, China. The secondary native Se minerals exhibit three crystal forms, namely prismatic, tabular, and granular. Based on geochemical and mineralogical data, the secondary native Se minerals are interpreted to have resulted from natural oxidation of selenide minerals in the primary Se ores, or from vaporization of Se caused by burning of “stone coal” (a highly carbonaceous mudstone) during the production of lime. The occurrence of secondary native Se minerals in the deposit would be helpful not only to further study on mineralogy, ore geochemistry and environmental geochemistry of Se, but also to improve the technology of reclaiming Se from the black rock series.     

U–Pb Ages for Zircon Grains from Nsanaragati Alluvial Gem Placers: Its Correlation to the Source Rocks

The Mamfe Basin is located in the SW of Cameroon and is an extension of the much bigger Benue Trough in the SE of Nigeria. Along the Asenem River and its tributaries in the western part of the Mamfe Basin (close to the border with Nigeria) gem placers yielding big zircon grains were found in recent river sediments close to Nsanaragati. In order to determine the source area and to establish a possible correlation between the zircons found in the Nsanaragati placers and rocks surrounding the Mamfe Basin 56 detrital zircon grains were analysed regarding their U–Pb ages and selected trace element contents by LA‐ICP–MS techniques. Possible source areas are rocks from the Benue Trough in the West and from the Cameroon Volcanic Line (CVL) in the SE of the study area. Based on microscopic analyses it was possible to distinguish two groups of zircons: reddish and non‐reddish ones, where the latter group comprises color variations from brown to orange, yellow to even colorless. In general, the detrital zircons show high hafnium values (4576 to 12565 ppm) and very variable thorium (7.8 to 1565 ppm) and uranium values (13.4 to 687 ppm). The Th/U ratios vary from 0.4 to 2.3, allowing correlations for some zircon grains to kimberlitic, granitic or syenitic affinities. It was also possible to distinguish zircon grains crystallised in mafic mantle‐derived melts from those crystallised in felsic melts, e.g. from continental rift‐related magmatic systems. In general, the U–Pb zircon ages obtained range from 11.7 to 1949 Ma. All zircons of the reddish group yielded almost similar ages resulting in a Concordia age of 12.4 Ma (Serravallian), an age unknown from the Mamfe Basin so far. The group of non‐reddish zircons showed various ages ranging from Serravallian to Orosirian. It was possible to correlate the youngest ages with rocks known from intrusions along the CVL, dated with K/Ar or Ar/Ar methods. The most probable sources were Mount Bamenda and Mount Bambouto in the east of the Mamfe Basin. Cretaceous ages are interpreted as re‐recycled clastic sediments whose original source had been rocks in the south and the north of the Benue Trough and who had been eroded and deposited within the catchment area of the Asenem river system in the Mamfe Basin in post‐Cretaceous times. The oldest ages are assumed to represent the pan‐African and pre‐pan‐African basement of the Mamfe basin.     

Untapped Economic Resource Potential of the Neoproterozoic to Early Paleozoic Volta Basin,Ghana: A Review

Natural Resources Research - The ~?115,000 km2 Volta Basin of Ghana is one of the most studied geological terrains. However, unlike the Birimian and Tarkwaian which have been...     

Two Sides of the Same Coin: Formal and Informal Institutional Synergy in a Case Study of Wildlife Governance in Ghana

Abstract

There has been the call to forge a synergistic relationship between local ecological knowledge and formal institutions in the governance of natural resources. How do informal institutions complement the efforts of formal state regulation of natural resources? How does this complementation foster a regularized human–wildlife interaction? Adopting an ethnographic design, this study assesses the role of institutional complementation in natural resource governance using the case of Boabeng–Fiema Monkey Sanctuary (BFMS) in Ghana, West Africa. We purposively selected 33 informants relevant to the BFMS governance process. The study observes that the synergy between formal and informal institutions strengthens wildlife protection in BFMS and the surrounding villages. The usefulness of informal rules is enhanced if appropriately complemented with a formal institutional arrangement. Over time, it becomes necessary for informal rules to grow in dynamism to depict the principles of collaboration, inclusivity, and benefit arrangements.     

The effects of vegetative ash on infiltration capacity, sediment transport, and the generation of progressively bulked debris flows

Through the alteration of the physical characteristics of a landscape, such as the destruction of vegetation and the formation of a hydrophobic layer, a fire can dramatically amplify erosion rates. On the basis of field observations, it has been proposed that the deposition of a layer of ash on the ground surface can enhance the erosion of mountainous terrain by surface runoff and might even be a necessary condition for the generation of progressively bulked debris flows. In this study, a flume was constructed to investigate the role of ash in increasing both the volume and the transport capacity of runoff. The experiments demonstrated that the presence of ash on the soil surface reduces the ability of flowing water to infiltrate; this effect is even greater when the ash has been pre-wetted. In addition, the ability of ash slurries to infiltrate decreases with increasing ash concentration. The results also indicate that the transport capacity of runoff is enhanced by the incorporation of ash into the flow because of the increased fluid density. However, the addition of ash reduces the boundary Reynolds number such that, at high ash concentrations and with fine-grained sediment, sediment transport declines as the flow becomes hydraulically smooth. The experimental results were also used to evaluate the ability of steep flow fronts, a common characteristic of debris flows and flash floods, to increase sediment transport rates. Finally, it is proposed that ash slurries may evolve into progressively bulked debris flows through a positive feedback between fluid density, transport capacity, and erosivity.     

A 17,900-year multi-proxy lacustrine record of Lago Puyehue (Chilean Lake District): introduction

This paper introduces the background and main results of a research project aimed at unravelling the paleolimnological and paleoclimatological history of Lago Puyehue (40° S, Lake District, Chile) since the Last Glacial Maximum (LGM), based on the study of several sediment cores from the lake and on extensive fieldwork in the lake catchment. The longest record was obtained in an 11-m-long piston core. An age-depth model was established by AMS ¹⁴C dating, ²¹⁰Pb and ²³⁷Cs measurements, identification of event-deposits, and varve-counting for the past 600 years. The core extends back to 17,915 cal. yr. BP, and the seismic data indicate that an open-lake sedimentary environment already existed several thousands of years before that. The core was submitted to a multi-proxy analysis, including sedimentology, mineralogy, grain-size, major geochemistry and organic geochemistry (C/N ratio, δ¹³C), loss-on-ignition, magnetic susceptibility, diatom analysis and palynology. Along-core variations in sediment composition reveal that the area of Lago Puyehue was characterized since the LGM by a series of rapid climate fluctuations superimposed on a long-term warming trend. Identified climate fluctuations confirm a.o. the existence of a Late-Glacial cold reversal predating the northern-hemisphere Younger Dryas cold period by 500–1,000 years, as well as the existence of an early southern-hemisphere Holocene climatic optimum. Varve-thickness analyses over the past 600 years reveal periodicities similar to those associated with the El Niño Southern Oscillation and the Pacific Decadal Oscillation, as well as intervals with increased precipitation, related to an intensification of the El Niño impact during the southern-hemisphere equivalent of the Little Ice Age.     

Mineral Paragenesis and Ore‐Forming Processes of the Dongping Gold Deposit,Hebei Province,China

The Dongping gold deposit is located near the center of the northern margin of the North China Craton. It is hosted in the Shuiquangou syenite and characterized by large amounts of tellurides. Numerous studies have addressed this deposit; the mineral paragenesis and ore‐forming processes, however, are still poorly studied. In this contribution, a new mineral paragenesis has been evaluated to further understand ore formation, including sulfides (pyrite, chalcopyrite, galena, sphalerite, molybdenite, and bornite), tellurides (altaite, calaverite, hessite, muthmannite, petzite, rucklidgeite, sylvanite, tellurobismuthite, tetradymite, and volynskite), and native elements (tellurium and gold). Molybdenite, muthmannite, rucklidgeite, and volynskite are reported for the first time in this deposit. We consider the Dongping gold deposit mainly formed in the Devonian, and the ore‐forming processes and the physicochemical conditions for ore formation can be reconstructed based on our newly identified ore paragenesis, that is, iron oxides → (CO₂ effervescence) → sulfides → (fTe₂/fS₂ ratio increase) → Pb‐Bi‐tellurides → (condensation of H₂Te vapor) → Au‐Ag‐tellurides → (mixing with oxidizing water) → carbonate and microporous gold → secondary minerals → secondary minerals. The logfO₂ values increase from the early to late stages, while the fH₂S and logfS₂ values increase initially and then decrease. CO₂ effervescence is the main mechanism of sulfides precipitation; this sulfidation and condensation of H₂Te vapor lead to deposition of tellurides. The development of microporous gold indicates that the deposit might experience overprint after mineralization. The Dongping gold deposit has a close genetic relationship with the Shuiquangou syenite, and tellurium likely originated from Shuiquangou alkaline magmatic degassing.     

Nanoscale resetting of the Th/Pb system in an isotopically-closed monazite grain: A combined atom probe and transmission electron microscopy study

Understanding the mechanisms of parent-daughter isotopic mobility at the nanoscale is key to rigorous interpretation of Ue The Pb data and associated dating. Until now, all nanoscale geochronological studies on geological samples have relied on either Transmission Electron Microscope(TEM) or Atom Probe Microscopy(APM) characterizations alone, thus suffering from the respective weaknesses of each technique. Here we focus on monazite crystals from a ~1 Ga, ultrahigh temperature granulite from Rogaland(Norway). This sample has recorded concordant UeP b dates(measured by LA-ICP-MS) that range over 100 My, with the three domains yielding distinct isotopic Ue Pb ages of 1034 ± 6 Ma(D1; Srich core), 1005 ± 7 Ma(D2), and 935 ± 7 Ma(D3), respectively. Combined APM and TEM characterization of these monazite crystals reveal phase separation that led to the isolation of two different radiogenic Pb(Pb*) reservoirs at the nanoscale. The S-rich core of these monazite crystals contains Cae Srich clusters, 5 -10 nm in size, homogenously distributed within the monazite matrix with a mean interparticle distance of 40 -60 nm. The clusters acted as a sink for radiogenic Pb(Pb*) produced in the monazite matrix, which was reset at the nanoscale via Pb diffusion while the grain remained closed at the micro-scale. Compared to the concordant ages given by conventional micro-scale dating of the grain,the apparent nano-scale age of the monazite matrix in between clusters is about 100 Myr younger, which compares remarkably well to the duration of the metamorphic event. This study highlights the capabilities of combined APM-TEM nano-structural and nano-isotopic characterizations in dating and timing of geological events, allowing the detection of processes untraceable with conventional dating methods.     

Giant submarine collapse of a carbonate platform at the Turonian–Coniacian transition: The Ayabacas Formation, southern Peru

The Ayabacas Formation of southern Peru is an impressive unit formed by the giant submarine collapse of the mid‐Cretaceous carbonate platform of the western Peru back‐arc basin (WPBAB), near the Turonian–Coniacian transition (～90–89 Ma). It extends along the southwestern edge of the Cordillera Oriental and throughout the Altiplano and Cordillera Occidental over >80 000 km² in map view, and represents a volume of displaced sediments of >10 000 km³. The collapse occurred down the basin slope, i.e. toward the SW. Six zones are characterised on the basis of deformational facies, and a seventh corresponds to the northeastern ‘stable’ area (Zone 0). Zones 1–3 display increasing fragmentation from NE to SW, and are composed of limestone rafts and sheets embedded in a matrix of mainly red, partly calcareous and locally sandy, mudstones to siltstones. In contrast, in Zones 4 and 5 the unit consists only of displaced and stacked limestone masses forming a ‘sedimentary thrust and fold system’, with sizes increasing to the southwest. In Zone 6, the upper part of the limestone succession consists of rafts and sheets stacked over the regularly bedded lower part. The triggering of this extremely large mass wasting clearly ensued from slope creation, oversteepening and seismicity produced by extensional tectonic activity, as demonstrated by the observation of synsedimentary normal faults and related thickness variations. Other factors, such as pore pressure increases or lithification contrasts probably facilitated sliding. The key role of tectonics is strengthened by the specific relationships between the basin and collapse histories and two major fault systems that cross the study area. The Ayabacas collapse occurred at a turning point in the Central Andean evolution. Before the event, the back‐arc basin had been essentially marine and deepened to the west, with little volcanic activity taking place at the arc. After the event, the back‐arc was occupied by continental to near‐continental environments, and was bounded to the southwest by a massive volcanic arc shedding debris and tuffs into the basin.     

Upscaling of CO2 vertical migration through a periodic layered porous medium: The capillary-free and capillary-dominant cases

We present an upscaled model for the vertical migration of a CO₂ plume through a vertical column filled with a periodic layered porous medium. This model may describe the vertical migration of a CO₂ plume in a perfectly layered horizontal aquifer. Capillarity and buoyancy are taken into account and semi-explicit upscaled flux functions are proposed in the two following cases: (i) capillarity is the main driving force and (ii) buoyancy is the only driving force. In both cases, we show that the upscaled buoyant flux is a bell-shaped function of the saturation, as in the case of a homogeneous porous medium. In the capillary-dominant case, we show that the upscaled buoyant flux is the harmonic mean of the buoyant fluxes in each layer. The upscaled saturation is governed by the continuity of the capillary pressure at the interface between layers. In the capillary-free case, the upscaled buoyant flux and upscaled saturation are determined by the flux continuity condition at the interface. As the flux is not continuous over the entire range of saturation, the upscaled saturation is only defined where continuity is verified, i.e. in two saturation domains. As a consequence, the upscaled buoyant flux is described by a piecewise continuous function. Two analytical approximations of this flux are proposed and this capillary-free upscaled model is validated for two cases of heterogeneity. Upscaled and cell averaged saturations are in good agreement. Furthermore, the proposed analytical upscaled fluxes provide satisfactory approximations as long as the saturation set at the inlet of the column is in a range where analytical and numerical upscaled fluxes are close.