Evolution of a Carboniferous carbonate-hosted sphalerite breccia deposit,Isle of Man

A newly discovered, extensive sphalerite-bearing breccia (~7.5 wt.% Zn) is hosted in dolomitised Carboniferous limestones overlying Ordovician–Silurian metasedimentary rocks on the Isle of Man. Although base metal sulphide deposits have been mined historically on the island, they are nearly all quartz vein deposits in the metamorphic basement. This study investigates the origin of the unusual sphalerite breccia and its relationship to basement-hosted deposits, through a combination of petrographic, cathodoluminescence, fluid inclusion, stable isotope and hydrogeologic modelling techniques. Breccia mineralisation comprises four stages, marked by episodes of structural deformation and abrupt changes in fluid temperature and chemistry. In stage I, high-temperature (T _h > 300°C), high-salinity (20–45 wt.% equiv. NaCl) fluid of likely basement origin deposited a discontinuous quartz vein. This vein was subsequently dismembered during a major brecciation event. Stages II–IV are dominated by open-space filling sphalerite, quartz and dolomite, respectively. Fluid inclusions in these minerals record temperatures of ~105–180°C and salinities of ~15–20 wt.% equiv. NaCl. The δ³⁴S values of sphalerite (6.5–6.9‰ Vienna-Canyon Diablo troilite) are nearly identical to those of ore sulphides from mines in the Lower Palaeozoic metamorphic rocks. The δ¹⁸O values for quartz and dolomite indicate two main fluid sources in the breccia’s hydrothermal system, local Carboniferous-hosted brines (~0.5–6.0‰ Vienna standard mean ocean water) and basement-involved fluids (~5.5–11.5‰). Ore sulphide deposition in the breccia is compatible with the introduction and cooling of a hot, basement-derived fluid that interacted with local sedimentary brines.     

The geological history and geodynamics of the Earth

The Earth formed through a hot accretion process. Almost simultaneously, the core and the mantle were separated from each other. At the final stages of the accretion process, the outer layer approximately 2000 km thick was molten, thus representing a magma ocean. This magma ocean produced the primary crust of the Earth. Surface waters were precipitated from the atmosphere and released from the crystallizing magma ocean. The plate tectonic processes started at around 4.3 to 4 Ga BP. In the Archean, the overall tectonic mechanism was quite specific, due to substantially higher mantle temperature and thicker oceanic crust. The normal plate tectonics acted during the Proterozoic and Phanerozoic with the periodic assembly of continents, which are known as supercontinent cycles.     

Redox problems in the “metallogenic specialization” of magmatic rocks and the genesis of hydrothermal ore mineralization

Considering the history and current state of the problem of the so-called metallogenic specialization of magmatic rocks, the paper places emphasis onto various aspects of the genesis of ore mineralization depending on the redox state of magmas (as a logical continuation of S. Ishihara’s works), fluids, and host rocks. These problems were inadequately poorly explored and discussed by researchers dealing with ore deposits. Various possible variants of ore-forming redox processes for different types of mineral deposits, with ore mineralization affiliated to granites (Ta, Sn, W, Mo, and Be) and mafic magmas (Au, Ag, U, Cu, Zn, Pb, As, Sb, and Hg) and, accordingly, to crustal and mantle origin, are discussed. On the basis of analyzed geological data, including those published over the past three decades, it is shown that the redox state of ore-producing magmas commonly significantly impacted not only the ore potential of magmatic complexes but also the genetic type of the ore mineralization. The redox state of the fluids predetermined the transport and precipitation speciation of metals. Influence mechanisms of hydrocarbons from sedimentary country rocks and gaseous products of their pyrolysis on ore deposition of various metals are considered. Understanding these mechanisms can be helpful for predicting the possible precipitation sites of ore mineralization of noble, radioactive, and chalcophile metals.     

A Leaky-Conduit Model of Transient Flow in Karstic Aquifers

Karst Flow Model (KFM) simulates transient flow in an unconfined karstic aquifer having a well-developed conduit system. KFM treats the springshed as a two-dimensional porous matrix containing a triangulated irregular network of leaky conduits. The number and location of conduits can be specified arbitrarily, perhaps using field information as a guide, or generated automatically. Conduit networks can be tree-like or braided. Rainwater that has infiltrated down from the surface leaks into the conduits from the adjacent porous matrix at a rate dictated by Darcy’s law, then flows turbulently to the spring via the conduits. KFM is calibrated using the known steady state; geometry and recharge determine the steady fluxes in the conduits, and the head distribution determines conduit gradients and sizes. Spring flow can vary with time due to spatially and temporally variable recharge and due to prescribed variations in the elevation of the spring. KFM is illustrated by four examples run on a test aquifer consisting of 27 nodes, 42 elements, and 26 conduits. Three examples (drought, uniform rainstorm, storm-water input to one element) are simulations, while the fourth uses data from a spring-basin flooding event. The qualitative fit between the predicted and observed spring discharge in the fourth example provides support of the hypothesis that the dynamic behavior of a karst conduit system is an emergent property of a self-organized system, largely independent of the locations and properties of individual conduits.     

Food Web Structure and Habitat Connectivity in Fish Estuarine Nurseries—Impact of River Flow

River flow variability is known to influence estuarine production, yet knowledge on its effect upon estuarine food webs dynamics is still scarce. Stable carbon and nitrogen isotopes were used to assess the effect of river flow in the connectivity and food web interactions between the two main fish nursery areas of the Tagus estuary. The aims of the present work were to investigate the seasonal variation in food web structure and the exchange rate of individuals of marine juvenile fish among estuarine nurseries, to compare the spring of a rainy year (2001) with that of an average year (2000), and to investigate the impact of the winter floods of 2001. A low level of connectivity was observed for the fish species that use these areas as nurseries. In low river flow conditions, two isotopically distinct food webs were established in each nursery area. These food webs were very sensitive to small variations in the freshwater input. Winter floods seem to disrupt the localized food webs that are established in low river flow periods, leading to the re-establishment of a wider food web. While in rainy years this wide food web is maintained until spring, in average years the food web undergoes fragmentation into two localized and isotopically distinctive food webs. The increase in frequency of droughts due to climate change should lower the connectivity of the estuarine fish nurseries food webs, causing habitat fragmentation and consequent loss in complexity and resilience.     

Paleoclimatic inference of the mid-Holocene record of monk seal (Monachus monachus) in the Cantabrian Coast

The mid-Holocene has been widely used to test the performance of the numerical models that are commonly employed to estimate the future evolution of world climate. This period, as the pollen record shows, was characterized by higher temperatures than present in northern and Central Europe, while cooler conditions occurred in the south of the continent. This pattern is challenging numeric algorithms that widely fail to replicate the paleoclimate data of southern Europe. Here we report the discovery of a fragmented bone of a temperate water phocid (Monachus monachus) dated to 5540 ± 40 BP that was hunted and consumed on the Cantabrian Coast during that period. This find implies a hitherto unnoted phase of warm conditions associated with strengthened advention of subtropical waters to the region. As a consequence, the possibility that the oceanographic regime from that time in the Bay of Biscay was similar to the current one is reinforced, a fact that could modify our view of mid-Holocene climate in the Iberian Peninsula and have important implications in climate change studies.     

On Geodynamic Evolution of Simao Region (Southwestern Yunnan, China) during Late Paleozoic and Triassic

ＩＮＴＲＯＤＵＣＴＩＯＮＴｈｅＳｉｍａｏｒｅｇｉｏｎ (ＳＷＹｕｎｎａｎ ,Ｃｈｉｎａ) ,ｓｉｔｕａｔｅｄｂｅ ｔｗｅｅｎｔｈｅＡｉｌａｏｓｈａｎｂｅｌｔｉｎｔｈｅｅａｓｔａｎｄｔｈｅＬａｎｃａｎｇｊｉａｎｇｚｏｎｅｉｎｔｈｅｗｅｓｔ (Ｆｉｇ .1) ,ｉｓａｋｅｙａｒｅａｔｏｃｈｒｏｎｏｌｏｇｉｃａｌｌｙｃｏｎｓｔｒａｉｎｌａｔｅＰａｌｅｏｚｏｉｃｃｏｍｐｒｅｓｓｉｏｎａｌｄｅｆｏｒｍａｔｉｏｎｓａｎｄｔｈｕｓｏｆｐｒｉｍｅｉｎ ｔｅｒｅｓｔｗｈｅｎｄｉｓｃｕｓｓｉｎｇｔｈｅｐｌａｔｅ ｔｅｃｔｏｎｉｃｈｉ…     

Equivalent Permeability and Simulation of Two-Phase Flow in Heterogeneous Porous Media

The problem of calculating equivalent grid block permeability tensors for heterogeneous porous media is addressed. The homogenization method used involves solving Darcy's equation subject to linear boundary conditions with flux conservation in subregions of the reservoir and can be readily applied to unstructured grids. The resulting equivalent permeability tensor is stable as defined relative to G-convergence. It is proposed to use both conforming and mixed finite elements to solve the local problems and compute approximations from above and below of the equivalent permeability, respectively. Comparisons with results obtained using periodic, pressure and no-flux boundary conditions and the renormalization method are presented. A series of numerical examples demonstrates the effectiveness of the methodology for two-phase flow in heterogeneous reservoirs.     

Hydrography and circulation in the northwestern Bay of Bengal during the retreat of southwest monsoon

The distribution of temperature and salinity in the upper 500 m of the northwestern Bay of Bengal, adjoining the east coast of India, during the retreat of southwest monsoon (September) of 1983 is presented. This study reveals coastal upwelling (limited to the upper 40 m) induced by the local winds. Waters of higher surface salinity near the coast characterize the upwelling. The freshwater influx near the head of the Bay diluted the surface salinity to as low as 26.0 × 10⁻³. The surface circulation was weak and led to a net transport of 2.0 × 10⁶m³.s⁻¹ directed towards northeast.