Sulphide mineralization and wall-rock alteration in ophiolites and modern oceanic spreading centres

Massive and stockwork Fe-Cu-Zn (Cyprus type) sulphide deposits in the upper parts of ophiolite complexes represent hydrothermal mineralization at ancient accretionary plate boundaries. These deposits are probable metallogenic analogues of the polymetallic sulphide deposits recently discovered along modern oceanic spreading centres. Genetic models for these deposits suggest that mineralization results from large-scale circulation of sea-water through basaltic basement along the tectonically active axis of spreading, a zone of high heat flow. The high geothermal gradient above 1 to 2 km deep magma chambers emplaced below the ridge axis drives the convective circulation cell. Cold oxidizing sea-water penetrating the crust on the ridge flanks becomes heated and evolves into a highly reduced somewhat acidic hydrothermal solvent during interaction with basaltic wall-rock. Depending on the temperature and water/rock ratio, this fluid is capable of leaching and transporting iron, manganese, and base metals; dissolved sea-water sulphate is reduced to sulphide. At the ridge axis, the buoyant hydrothermal fluid rises through permeable wall-rocks, and fluid flow may be focussed along deep-seated fractures related to extensional tectonic processes. Metal sulphides are precipitated along channelways as the ascending fluid undergoes adiabatic expansion and then further cooling during mixing with ambient sub-sea-floor water. Vigorous fluid flow results in venting of reduced fluid at the sea-floor/sea-water interface and deposition of massive sulphide. A comparison of sulphide mineralization and wall-rock alteration in ancient and modern spreading centre environments supports this genetic concept.Massive sulphide deposits in ophiolites generally occur in clusters of closely spaced (< 1–5 km) deposits. Individual deposits are a composite of syngenetic massive sulphide and underlying epigenetic stockwork-vein mineralization. The massive sulphide occurs as concordant tabular, lenticular, or saucer-shaped bodies in pillow lavas and pillow-lava breccia; massive lava flows, hyalcoclastite, tuff, and bedded radolarian chert are less commonly associated rock types. These massive sulphide zones are as much as 700 m long, 200 m wide, and 50 m thick. The pipe-, funnel-, or keel-shaped stockwork zone may extend to a dehpth of 1 km in the sheeted-dike complex. Several deposits in Cyprus are confined to grabens or the hanging wall of premineralization normal faults.Polymetallic massive sulphide deposits and active hydrothermal vents at medium- to fast-rate spreading centres (the East Pacific Rise at lat. 21°N, the Galapagos Spreading Centre at long. 86°W, the Juan de Fuca Ridge at lat. 45°N., and the Southern Trough of Guaymas Basin, Gulf of California) have interdeposit spacings on a scale of tens or hundreds of metres, and are spatially associated with structural ridges or grabens within the narrow (< 5 km) axial valleys of the rift zones. Although the most common substrate for massive sulphide accumulations is stacked sequences of pillow basalt and sheet flows, the sea-floor underlying numerous deposits in Guaymas Basin consists of diatomaceous ooze and terrigenous clastic sediment that is intruded by diabase sills. Mound-like massive sulphide deposits, as much as 30 m wide and 5m high, occur over actively discharging vents on the East Pacific Rise, and many of these deposits serve as the base for narrow chimneys and spires of equal or greater height. Sulphides on the Juan de Fuca Ridge appear to form more widespread blanket deposits in the shallow axial-valley depression. The largest deposit found to date, along the axial ridge of the Galapagos Spreading Centre, has a tabular form and a length of 1000 m, a width of 200 m, and a height of 30 m.The sulphide assemblage in both massive and vein mineralization in Cyprus type deposits is characteristically simple: abundant pyrite or, less commonly, pyrrhotite accompanied by minor marcasite, chalcopyrite, and sphalerite. With few exceptions, the composition of massive sulphide ranges from 0.3 to 5 wt. % Cu, from 0.1 to 3 wt. % Zn, from 0.5 to 30 ppm Au, and from 1 to 50 ppm Ag. The only common gangue minerals — quartz, chlorite, calcite, and gypsum generally make up less than 10 percent of the massive zone.Sulphide assemblages in massive sulphide samples recovered from the Juan de Fuca Ridge (abundant sphalerite, wurtzite, and pyrite; minor marcasite, chalcopyrite, and galena), East Pacific Rise (abundant sphalerite, pyrite, and chalcopyrite; minor wurtzite, marcasite, and pyrrhotite), and Guaymas Basin (abundant pyrrhotite and sphalerite; minor chalcopyrite) contrast with ophiolitic deposits. Bulk analyses of two zinc-rich sulphide samples from the Juan de Fuca Ridge yield the following average values: Zn, 56.6 wt. %; Cu, 0.2 wt. %; Pb, 0.15 wt. %; Fe, 4.9 wt. %; Ag, 260 ppm; and Cd, 775 ppm. Other minerals precipitated with sulphides at hydrothermal-vent sites include anhydrite, barite, gypsum, Mg-hydroxysulphate-hydrate, talc, sulphur, and amorphous silica.Massive sulphide lenses in some Cyprus-type deposits are underlain by a silica-rich zone consisting of massive quartz, opaline silica, red jasper, or chert mixed with disseminated and veinlet Fe-Cu-Zn sulphides. Some deposits are overlain by ochre, a gossanous Mn-poor Fe-rich bedded deposit composed of goethite, maghemite, quartz, and finely disseminated sulphide. In the Solomon Islands, ochre is overlain by siliceous sinter containing anhydrite, barite, and sulphide; the sinter contains anomalous Ag, Au, Cu, Zn, and Hg, and grades upward into Fe-rich chert and manganiferous wad. Amorphous Fe-Mn deposits (umber) and Mn-bearing chert enriched in Ba, Co, Cu, Ni, Cr, Pb, and Zn are common features near the top of ophiolite sequences. Although their genetic relation to sulphide mineralization is uncertian, they probably formed during off-axis hydrothermal discharge.At modern, medium-rate spreading centres, thin blankets of unconsolidated hydrothermal sediment have been observed near hydrothermal sulphide deposits. Basalt fragments recovered with massive sulphide from the Juan de Fuca Ridge have surfaces coated with smectite, magnetite, hematite, opaline silica, and Fe---Mn-oxyhydroxides. Sediment mounds composed largely of nontronitic clay and hydrated Fe and Mn oxides, and more distal metalliferous (Fe, Mn, Cu, Ni, Pb, Zn) sediment on the flanks of ceanridges, are also products of off-axis hydrothermal processes.Pillow lavas, diabase dikes, and gabbro in ophiolite sequences, and deeper, layer 2 basalt and diabase recovered from oceanic ridges, are altered to greenschist-facies assemblages (albite + chlorite + actinolite ± sphene ± quartz ± pyrite) during high-temperature sub-sea-floor hydro-thermal metamorphism near the axis of spreading. Chemical changes in the wall-rock during this large-scale sea-water/rock interactive episode depend on the water/rock ratio and temperature but generally include gains in Mg, Na and H₂O and losses of Ca. Subsequent low temperature sea-water/rock interaction away from the axis of spreading results in fracture-controlled zeolitefacies alteration, characterized by smectite, caledonite, zeolite, calcite, prehnite, hematite, marcasite, and pyrite. This retrograde alteration involves increases in total Fe, K, and H₂O and decreases in Mg and Si in the wallrock; Ca may be lost or gained.Wall-rock alteration in Cyprus type stockwork zones is more striking, in that the basalt and diabase between veins of Fe---Cu-Zn sulphides, quartz, and chlorite have undergone partial to complete conversion to fine-grained aggregates of quartz + chlorite + illite + pyrite; kaolinite and palygorskite may be present in minor amounts. Calcium and Na are strongly depleted; K, Al, Ti, Mn, and Ni are leached to a lesser extent; and Fe, S, Cu, Zn, and Co are strongly enriched in the wall-rock underlying massive sulphide. Mafic rocks at depth in the volcanic pile may be enriched in K, Rb, and Li, and depleted in Cu, Co, and Zn. Lavas lateral to and overlying massive sulphide mineralization may have low concentrations of Cu and high concentrations of Zn and Co relative to background levels.Mutual consideration of hydrothermal sulphide deposits and associated wall-rock alteration in ophiolites and at modern oceanic spreading centres can provide useful criteria for the development of regional exploration models for ophiolitic terrains.     

The hydrothermal system of Central Jebilet (Variscan Belt,Morocco): A genetic association between bimodal plutonism and massive sulphide deposits?

Central Jebilet (Moroccan Variscan Belt) hosts several Cu and Pb–Zn massive sulphide deposits that are associated with a suite of gabbroic and microgranitic, tholeiitic to alkaline, intrusions emplaced 330 Ma ago. The intrusions and ore bodies form structural lineaments within marine Visean shales that are affected by very low to low-grade post-Visean metamorphism and contemporaneous shortening accompanied by the development of conjugate ductile to brittle shear zones. The ductile shear zones are localised in thermally softened aureoles around magmatic intrusions, while brittle deformation is common far from the intrusions. The intrusions have induced a contact metamorphism that reaches the hornblende hornfels facies, and their emplacement was accompanied by hydrothermal activity that leached base metals from the felsic intrusions.The massive sulphide deposits consist of steeply dipping elongate lenses that are located in shear zones 1–1.5 km away from the intrusions. They are dominated by pyrrhotite (up to 90%), sphalerite, galena, chalcopyrite, pyrite and arsenopyrite forming a mylonitic texture. Their wall rocks are altered to syntectonic mineral assemblages similar to those found in the alteration zones associated with the magmatic intrusions. The massive sulphide deposits located near the felsic intrusions are rich in lead and zinc compared to those located near the mafic intrusions, which are copper deposits. These relationships indicate that the whole Central Jebilet hydrothermal system could be described in terms of a lateral secretion of base metals from source zones (i.e. bimodal intrusions) to discharge zones (i.e. the Jebilet sulphide deposits). The metapelites in the contact metamorphic zone around felsic intrusions contain zincian ilmenite that was probably related to interaction of the host rocks with chlorine-rich fluid carrying zinc and other metals leached from the microgranites.     

块状硫化物矿石中硫化物的压溶和增生及成矿意义——以加拿大西部矿床为例

加拿大西部块状硫化物矿石普遍地发生过硫化物的压溶和增生。增生作用根据增生体的成分可以分为同质增生和异质增生，根据动力环境可以分为静态增生和动态增生。三晶嵌接结构可以是静态增生的产物。压溶和增生是块状硫化物矿床成岩和变质过程中的重要作用。脉石矿物的压溶可使原生矿石就地加富，硫化物的压溶可使成矿物质发生再活化。增生可促进矿质沉淀。富含硫化物的地层之所以能成为地球化学障而有利于后期热液叠加和层控矿床的形成，硫化物晶芽的增生是一种重要机制。     

The formation and detection of metal dispersion halos in organic lake sediments

The application of organic-rich lake sediment (gyttja) to exploration geochemistry is discussed. Gyttja was collected from some 42 lake sites in the Red Lake-Uchi Lake volcanic-sedimentary sequences.The existance of free sulphide ion in most gyttja is confirmed and theoretical calculations are used to define the approximate concentrations of sulphide ion which determine whether the accumulation of Cu⁺, Cu²⁺, Zn²⁺ and Fe²⁺ is by metal sulphide precipitation, organic complexing or both. These calculations show that in the sediments collected in this study, the accumulation of copper will probably be by metal sulphide formation, whereas for zinc, organic complexing and possibly sulphide precipitation are likely mechanisms.It is shown that the background concentrations of copper and zinc on a dry matter basis increase with the content of organic matter in the sediment and it is further shown that the quantity of interstitial water in the sediment increases linearly with the organic content. Also it is demonstrated that the organic matter in the sediment is dispersed in the interstitial water in approximately the same concentration irrespective of the sediment composition.On the assumption that copper sulphide colloids are dispersed in the interstitial water of the sediment, it is suggested that the copper content of gyttja should be expressed as μg/g interstitial water for meaningful interpretation. Similarly, the zinc content should be expressed relative to the organic matter content.The ability of reagents to selectively extract only chemically dispersed metals and not silicate lattice metals from gyttja is discussed.Both the analytical procedures and the interpretations are applied to the samples collected.     

Vertical profiles of trace metals and acid-volatile sulphide in a dynamic sedimentary environment: Lake Ketel,The Netherlands

Fine-scale solid-phase trace metal and acid-volatile sulphide profiles are presented for sediment cores taken at four locations in Lake Ketel, a dynamic sedimentation basin of the river Rhine in The Netherlands. The purpose of this study was to relate these profiles to the dynamics of sediment transport in the lake. The internal cycling of solids in the lake affects the distribution of trace metals and acid-volatile sulphide in the sediments. Comparison of vertical profiles of trace metals and sulphide indicates significant spatial variation in the dynamics of sediment transport in Lake Ketel. The construction of a dredge spoil storage depot in 1996 seems to have significantly influenced the hydrodynamic and sediment dynamics in Lake Ketel. Consequently, deposition rates in Lake Ketel based on comparison of solid-phase trace metal concentrations in the sediments and historical levels in Rhine suspended matter can only be used as rough estimates.     

Types of massive sulphide deposits in the Urals

Massive sulphide deposits in the Urals are found within volcanic and volcanic-sedimentary sequences of Ordovician to Middle Devonian ages. Four types of economic sulphide deposits have been recognized: Cyprus, Besshi, Urals and Baimak. The Cyprus-type copper sulphide deposits are hosted by mafic volcanites that occur in the basal parts of Palaeozoic volcanic sequences. The Besshi-type copper-zinc deposits are located within clastic sedimentary rocks intercalated with basalts and andesites. Zinc-copper deposits of the Urals-type are hosted by bimodal rhyolite-basalt assemblages, which occur at a higher stratigraphic level than those of Cyprus- and Besshi-types. The Baimak-type zinc-copper-barite deposits are associated with intrusive quartz porphyries which occur in the upper parts of bimodal volcanic successions. In addition there are some sulphide deposits of zinc-lead-barite and zinc-copper composition hosted by Ordovician terrigenous sequences which occur within depressions in Precambrian blocks. These types of sulphide deposits have been formed at various stages of divergence and convergence of the Earth's crust during the orogenic history of the Urals. Received: 27 June 1997 / Accepted: 14 May 1998     

沿海表层沉积物中重金属的有效结合态

以浙江沿海表层沉积物为研究对象,系统地研究了重金属有效结合态与沉积环境、矿物组成和人为排放等环境因素之间的相互关系.结果表明:沿海表层沉积物中重金属的积累数量,呈现出潮汐河口沉积物>沿海沉积物>强潮汐河口沉积物的趋势,这反映了不同沉积类型对重金属富集作用的差异;重金属的有效结合态,总体上以铁锰氧化物结合态>碳酸盐结合态>硫化物及有机结合态>可交换态的顺序存在,但是不同的重金属或同种重金属在不同的采点,主要有效结合态的比例存在明显的差异,这不仅与重金属的地球化学性质、沉积环境和沉积物中的粘土矿物组成有关,更受到重金属污染物的人为排放量的影响.     

Geochemical investigations associated with the Wilga and Currawong massive sulphide deposits, Benambra, Victoria

The Wilga and Currawong copper-zinc massive sulphide deposits in northeastern Victoria occur within a sequence of Silurian volcanics and sediments. The Wilga deposit which was discovered in mid 1978 consists of a single lens while the Currawong deposit, discovered in early 1979, consists of at least two lenses.The first indication of the presence of base metal mineralization in the area was provided by an assessment of stream sediment geochemical data contained in open-file Exploration Licence reports at the Victorian Department of Minerals and Energy.The massive sulphide mineralization does not outcrop, but the ore horizons are weakly mineralized and give rise to stringer gossans as far as 150 m up dip from ore grade mineralization. These can be identified by their trace element chemistry (anomalous values of Bi, Fe, As, Au, Pb, Hg, Se, Co, Ag and Mn) corresponding to the trace element signature of both stringer and massive sulphides.Soils in the area are essentially skeletal and residual with some colluvial movement on the steeper slopes. The soils are highly anomalous in Cu, Pb, and Zn over the projected horizon of the Wilga mineralization and the No. 2 lens at Currawong.The stream sediment responses at both Wilga and Currawong result from a combination of chemical and elastic dispersion. Downslope from the surface expression of the Wilga mineralization a spring discharges directly into the Tambo River. The spring has a very low pH and is rich in base metals resulting in enhanced metal values in both stream water and stream sediments.Analyses of selected samples of the more prominent vegetation species have failed to show a clear relationship to the mineralization.     

Zur Kenntnis der präkambrischen,schichtgebundenen Molybdänitvorkommen in Südnorwegen

The southern Norwegian molybdenum deposits have hitherto been looked upon as typical examples of deposits of pneumatolytic — catathermal origin formed by emanations from intrusive granites. It is now demonstrated that all these deposits are to be found stratabound in a series of grey gneisses. They are accompanied by varying amounts of graphite and apatite of biogenetic origin and all of them show the same sulphide and oxide ore mineral paragenesis. It is concluded that the concentration and precipitation of metals took place in marine environments. Both the ores and their host rocks later underwent the same highmetamorphic processes.     

The stiff upper LIP: investigating the High Arctic Large Igneous Province

The Canadian Arctic Islands expose a complex network of dykes and sills that belong to the High Arctic Large Igneous Province (HALIP), which intruded volatile‐rich sedimentary rocks of the Sverdrup Basin (shale, limestone, sandstone and evaporite) some 130 to 120 million years ago. There is thus great potential in studying the HALIP to learn how volatile‐rich sedimentary rocks respond to magmatic heating events during LIP emplacement. The HALIP remains, however, one of the least well known LIPs on the planet due to its remote location, short field season, and harsh climate. A Canadian–Swedish team of geologists set out in summer 2015 to further explore HALIP sills and their sedimentary host rocks, including the sampling of igneous and meta‐sedimentary rocks for subsequent geochemical analysis, and high pressure‐temperature petrological experiments to help define the actual processes and time‐scales of magma–sediment interaction. The research results will advance our understanding of how climate‐active volatiles such as CO_2, SO₂ and CH₄ are mobilised during the magma–sediment interaction related to LIP events, a process which is hypothesised to have drastically affected Earth's carbon and sulphur cycles. In addition, assimilation of sulphate evaporites, for example, is anticipated to trigger sulphide immiscibility in the magma bodies and in so doing could promote the formation of Ni‐PGE ore bodies. Here we document the joys and challenges of ‘frontier arctic fieldwork’ and discuss some of our initial observations from the High Arctic Large Igneous Province.     

Geochemistry of hydrothermal Mn-oxide deposits from the S.W. Pacific island arc

Hydrothermal Mn-oxide crusts have been removed from the Tonga-Kermadec Ridge, the first such hydrothermal deposits to be reported in the S.W. Pacific island arc. In several respects the deposits are similar to hydrothermal Mn-crusts from oceanic spreading centre settings. They are limited in areal extent, comprise well-crystalline birnessite and generally display extreme fractionation of Mn from Fe. They are strongly depleted in many elements compared to hydrogenous Mn deposits but are comparatively enriched in Li, Zn, Mo and Cd. The Group IA and Group IIA metals show strong intercorrelations and the behaviour of Mg in the purest samples may indicate the extent to which normal seawater has influenced the composition of the deposits.Certain aspects of the deposits are not typical of hydrothermal Mn deposits. In particular at least some of the crusts have developed on a sediment or unconsolidated talus substrate. Some crusts, or specific layers within some crusts, display a chemical composition which suggests a significant input from normal seawater.     

Studies on mineralization and composition of DC ore field,Guangxi, China

DC, a multi-metal ore field, is an important and well known producer of tin in China. Besides tin, also mined are zinc, lead, antimony, copper, arsenic, tungsten, mercury, silver and many other metals. Most of the deposits in this region occur in Devonian limestones and siliceous shales. The ore deposits in this field are predominated by cassiterite sulphide type. As to the origin of those deposits, they seem closely related to biotite granite. The ore-forming material is mainly derived from magmatic rocks, and part of it may come from the host rocks. The appearance of horizontal zoning of ore deposits in this field is extremely distinct, with LXG biotite granite as the core. Found in the magmatic rocks is molybdenum mineralization. From the biotite granite outwards there displays such an occurring sequence as the following: copper (zinc) skarn ore deposits, scheelite ore deposits, wolframite ore deposits, cassiterite sulphide ore deposits, lead-zinc (antimony) ore deposits and mercury or arsenic ore deposits. In this region the mineralizing-temperature zoning is also distinguishable, basically in agreement with the horizontal zoning of ore deposits. In other words, the formation temperature of ore deposits occurring around the granite is higher than that farther away from the granite. The following mineralization periods and stages can be divided: the first period, including skarn sub-stage, cassiterite-quartz sub-stage, cassiterite-sulphide sub-stage, cassiterite-calcite sub-stage; the second period, including ferrosphalerite (galena)-franckeite sub-stage, jamesonite-boulangerite sub-stage; and the third period, including pyrite-calcite sub-stage. The characters of material composition are as follows: ore minerals are characterized by great variety, perfect crystallinity, large grain-size and abundant sulfosalts. More than 90 species of minerals have been identified. What is more interesting is that lead and antimony in some of the cassiterite-sulfide deposits are derived from jamesonite rather than galena and stibnite, which provides an example rarely encountered in metallic deposits throughout the world.     

Anatomy of ancient passive margin slope systems: Aptian gravity-driven deposition on the Vocontian palaeomargin, western Alps, south-east France

The Aptian succession on the Vocontian palaeomargin (south-east France) consists of marl and marly calcareous pelagic slope facies together with a range of gravity-driven deposits (slumps, debris-flow deposits, turbidite packages and massive sandstones). The massive sandstones were emplaced by high-density turbidity currents and are associated with extensive clastic sills and dykes. The sedimentology is constrained by a high-resolution bio- and lithostratigraphic framework and permits a detailed analysis of the slope succession including: (1) a sequence stratigraphical analysis of the slope deposits; and (2) lateral tracing of individual sedimentary packages downslope. The resulting model for the Vocontian slope represents an alternative to the ‘classic’ Exxon delta-fed, mud-rich model. Key elements of the Vocontian model are: (1) an emphasis on lowstand slope erosion and complex slope morphology controlled by contemporary tectonism and salt diapirism; (2) slope deposition in confined erosional and structurally controlled conduits rather than the buildout of slope fans/channel-levee complexes; (3) a dominance of large-volume muddy slump and transitional debris-flow deposits, with subordinate sandy turbidites, including significant massive sandstone facies; (4) common sand injections (sills and dykes) associated with the massive sandstone facies; and (5) minimal downslope evolution of the flows, with the nature of the source sediment being the over-riding factor determining flow behaviour and deposit character. The Vocontian system is a rare instance in which large sections of a ‘fossil’ passive margin slope are preserved in the geological record. The slope deposits differ from the classic models of turbidite systems that have mainly been built from peripheral foreland basins, and the new insight makes it possible to compare ancient and present-day passive margin slope systems.     

Supergene sulphides and related minerals in the supergene profiles of VHMS deposits from the South Urals

The mineralogy and structure of the supergene profile in recently-exploited volcaniс hosted massive sulphide (VHMS) deposits of Cyprus, Uralian and Kuroko type in the South Urals, Russia, have been studied. Specific subzones enriched in secondary sulphides and associated minerals have been distinguished in residual pyrite and quartz–pyrite sands at the Gayskoye, Zapadno-Ozernoye, Dzhusinskoye and Alexandrinskoye deposits. Besides minerals which are common to the cementation subzones (covellite, chalcocite and acanthite), non-stoichiometric colloform and framboidal pyrite, pyrite–dzharkenite, pyrrhotite-like and jordanite-like minerals, metacinnabar, sphalerite, selenium-enriched tetrahedrite and unidentified As-, Sb sulphosalts of Pb or Hg and Ag, sulphur-bearing clausthalite, naumannite and tiemannite were also found. Secondary sulphide minerals in VHMS deposits of the South Urals region are characterized by light sulphur isotope compositions (− 8.1 to − 17.2‰). Superposition of the advanced oxidation of colloform pyrite, an enrichment in impurities (sphalerite, galena, and tennantite) from the primary ores, stagnant water conditions, an elevation of the water table during oxidation, and bacterial activity led to supergene concentrations of the base metals as sulphide, selenides or sulphosalts.     

Stratigraphic architecture and infill history of a deglaciated bedrock valley based on georadar, seismic profiling and drilling

The sediment fill of a silled bedrock valley in Western Norway has been investigated with respect to stratigraphy and infill history using a combination of mapping, georadar, seismic profiling and drilling. A small outlet glacier occupies the head of the valley that displays a stepwise down-valley profile and terminates in a lake at 29 m above sea-level. The valley is surrounded by high, steep bedrock slopes and is characterized by a series of filled basins each limited by sills of bedrock or moraine accumulations. Till, glacial outwash and/or rockslide deposits fill in the lower half of the two larger basins. (Fan) delta deposits fringed by the deposits of alluvial fans and colluvial cones dominate the upper fill of most basins. (Fan) delta deposits interfinger downstream with lake sediments in the larger basins and fluvial deposits comprise the top fill. The overall infill pattern was controlled by deglaciation as well as basin size and shape. An overall decreasing sediment supply following deglaciation is shown in the fill of a larger basin down-valley, whereas a recently increasing sediment supply during glacier growth is reflected primarily in an upstream basin. Only the lowermost basin was exposed to a sea-level drop from 75 m above sea-level to the present lake level associated with incision and river migration. This observation is in contrast to the basins above marine influence where incision has been limited due to fixed downstream sills resulting in insignificant erosion except for some fan-head entrenchment. It follows that the fills of these small valley basins display progradational and aggradational trends of deposition and paraglacial reworking has been limited. Additionally, the study demonstrates that georadar profiling, combined with other methods, is very useful for comprehensive investigation of valley basins.     

Evolution of a Holocene delta driven by episodic sediment delivery and coseismic deformation, Puget Sound, Washington, USA

Episodic, large‐volume pulses of volcaniclastic sediment and coseismic subsidence of the coast have influenced the development of a late Holocene delta at southern Puget Sound. Multibeam bathymetry, ground‐penetrating radar (GPR) and vibracores were used to investigate the morphologic and stratigraphic evolution of the Nisqually River delta. Two fluvial–deltaic facies are recognized on the basis of GPR data and sedimentary characteristics in cores, which suggest partial emplacement from sediment‐rich floods that originated on Mount Rainier. Facies S consists of stacked, sheet‐like deposits of andesitic sand up to 4 m thick that are continuous across the entire width of the delta. Flat‐lying, highly reflective surfaces separate the sand sheets and comprise important facies boundaries. Beds of massive, pumice‐ and charcoal‐rich sand overlie one of the buried surfaces. Organic‐rich material from that surface, beneath the massive sand, yielded a radiocarbon age that is time‐correlative with a series of known eruptive events that generated lahars in the upper Nisqually River valley. Facies CF consists of linear sandbodies or palaeochannels incised into facies S on the lower delta plain. Radiocarbon ages of wood fragments in the sandy channel‐fill deposits also correlate in time to lahar deposits in upstream areas. Intrusive, sand‐filled dikes and sills indicate liquefaction caused by post‐depositional ground shaking related to earthquakes. Continued progradation of the delta into Puget Sound is currently balanced by tidal‐current reworking, which redistributes sediment into large fields of ebb‐ and flood‐oriented bedforms.     

Sources, distribution and storage of heavy metals in the Río Pilcomayo, Bolivia

The Río Pilcomayo rises on the Cerro Rico de Potosí precious metal-polymetallic tin deposits of Bolivia, and flows in a southeasterly direction for ca. 600 km to Bolivia's southern border with Argentina. Mining of the Potosí deposits has occurred continuously since 1545, generating large quantities of waste materials in the headwater of the basin. In addition, a tailings dam breach at the Porco mine in 1996 released an estimated 235 000 m³ of tailings and fluid into the upper reaches of the Río Pilaya, the largest tributary to the Pilcomayo.Concentrations of As, Sb, Cd, Cu, Pb, Hg, Ag, Tl and Zn in contemporary channel sediments upstream of the Pilaya confluence are significantly elevated above background values. Elevated levels appear to be associated with pyrite- and other sulphide mineral-bearing tailings materials transported more than 200 km downstream of the Potosí mines. Significant downstream declines in elemental concentrations occur within 15 km, and again between 150 and 200 km, from the mines. The initial decrease in concentrations is due to the rapid dilution of nearly pure tailings effluent released to the river from milling facilities near Potosí. The latter decrease results from a combination of geomorphic processes including the storage of sediment-borne metals within the channel bed and the influx of ‘clean’ sediment from several large tributaries. Downstream of the Pilaya confluence, concentrations of Cu, Pb, Hg and Zn are only slightly elevated above background values, and Ag, Cd, Sb and Tl cannot be distinguished from background levels. These data suggest that while the Porco tailings spill may have had a significant short-term impact on sediment and water quality along the lower reaches of the Río Pilcomayo, its longer-term impacts were limited. Metals stored and eroded from alluvial deposits of historical age in upstream reaches appear to be an important source of metals to the river today. An additional, and perhaps more significant source, is the release of tailings effluent to the river from modern milling operations. The transport of these contaminants downstream of Icla (203 km from Potosí) appears to be restricted by aggradational processes occurring in the vicinity of Puente Sucre. In addition, downstream of the confluence of the Río Pilaya, inputs of large amounts of ‘clean’ sediment have caused dilution of the metal contaminants. Data from other studies where similar geomorphic processes have occurred suggest that the metals in the upper Pilcomayo may eventually be moved downvalley as the aggradational processes are reversed and channel stabilisation occurs. Thus, the most significant impacts of metal contamination may not be realised in downstream areas for decades.     

广西DC矿田成矿作用和物质成分研究

广西DC矿田是我国著名的锡矿产地,除锡以外,还有锌、铅、铜等多金属矿床,以及锑、砷、硫、汞、银、分散元素等可以综合利用。     

The link between hydrothermal epigenetic copper mineralization and the Caçapava Granite of the Brasiliano Cycle in southern Brazil

Base-metal deposits in the Caçapava do Sul Copper Province are hosted by both volcanosedimentary rocks of the Bom Jardim Group and by metamorphic rocks of the Passo Feio Formation, and show a spatial relationship to the Caçapava Granite. These associations have led to much controversy about the genesis of the base-metal deposits, which has been at least partly resolved by precise dating using SHRIMP (Sensitive High Resolution Ion Microprobe) U/Pb zircon studies combined with S, Pb, and Sr isotope trace studies.The Passo Feio Formation is Neoproterozoic in age and was derived from a complex continental source, as shown by the presence of xenocryst zircons of Archaean, Paleoproterozoic, and Neoproterozoic ages. It was metamorphosed at ca. 700 Ma. The syntectonic Caçapava Granite that intruded the supracrustal rocks of the Passo Feio Formation at 562 Ma was derived from an old sialic basement.Lead-isotope data are consistent with a 562 Ma age for the base-metal sulphide deposits sited in the Passo Feio Formation. The least-radiogenic compositions lie between the field of the isotopic compositions of the Caçapava Granite and rocks of the Passo Feio Formation, suggesting that Pb in the sulphide deposits may have been derived from both sources. The Pb, like that in the Caçapava Granite and Passo Feio Formation, was derived from a primitive crustal source. Sulphur isotope data from the base-metal sulphide deposits in the Passo Feio Formation are compatible with a mixed sedimentary and magmatic source.The most logical model for ore genesis, based on the isotopic data and spatial relationships, is that magmatic metal-bearing fluids from Caçapava Granite leached metals from the Passo Feio Formation and that the deposited sulphides therefore show mixed isotopic signatures. However, there is also some isotopic evidence from the Caçapava Granite itself that suggests assimilation of S-bearing rocks of the Passo Feio Formation during emplacement. Thus, isotopic signatures could have been inherited from assimilated metal sulphides at this stage, and deposition could have been entirely from Caçapava Granite-derived magmatic fluids.Importantly, the inferred 562±8 Ma age for the deposits in the Passo Feio Formation is younger than the well-constrained age of 594±5 Ma for the Camaquã/Santa Maria deposits. Thus, the epigenetic sulphides in the Passo Feio Formation cannot be the source of these deposits as previously suggested. Other isotopic data also argue against such a model.     

Images of concentrations of dissolved sulphide in the sediment of a lake and implications for internal sulphur cycling

The high-resolution mapping of dissolved sulphide in modern sediment of a permanently stratified (meromictic) lake is possible using a novel sulphide imaging technique (called a 'sulphur print'). The technique is simple, easy to use and can map a relatively large area (up to several dm²). In situ sulphur prints from anoxic fine-grained sediments are able to reveal internal structures in apparently homogeneous materials. Sulphur prints have been used to examine the formation and distribution of dissolved sulphide in the sediment and water column of the meromictic Lake Cadagno (southern Switzerland). The prints show clearly that a previously unknown laminar convective flow of sulphide-free porewater occurs across the sediment–water interface. Such convective flow out of the sediment must be accompanied by convective flow of sulphide- and sulphate-rich lake water into the sediment, and may be an important mechanism for the accumulation of sulphur in the sediment.