direct dating of sulfides by rbsr a critical test using the polaris mississippi valley type znpb deposit abstract the rb sr dating of sphalerites is a powerful method for directly determining the age of base metal deposits and testing models for large scale fluid flow however the uncertainty over the exact host phases for the trace amounts of rb and sr and the causes of variability in rb sr have caused concern over the reliability of the method here we show that the polaris mvt deposit with a geologically well constrained age of formation confirmed by paleomagnetic measurements and hosted in significantly older sedimentary rocks yields a consistent rb sr age of 366 15 ma providing the first unequivocal vindication of the reliability of the method

amplified erosion above waterfalls and oversteepened bedrock reaches erosion rate at the upstream end of the flow acceleration zone above a waterfall fr is the froude number at this setting and n ranges between 0 5 1 7 this amplification expression suggests that erosion at the lip could be as much as 2 5 times higher relative to erosion at a normal setting with identical hydraulic geometry utilizing this erosion amplification expression in numerical simulations we demonstrate its impact on reach scale morphology above waterfalls amplified erosion at the lip of a waterfall can trigger the formation of an oversteepened reach whose length is longer than the flow acceleration zone provided incision wave velocity vi at the upstream edge of the flow acceleration zone is higher than the retreat velocity of the waterfall face such an oversteepened reach is expected to be more pronounced when vi increases with increasing slope the simulations also suggest that oversteepening can eventually lead to steady state gradients adjacent to a waterfall lip provided vi decreases with increasing slope flow acceleration above waterfalls can thus account at least partially for prevalent oversteepened bedrock reaches above waterfalls using the cosmogenic isotope cl 36 we demonstrate that incision wave velocity upstream of a waterfall at the dead sea western escarpment is probably high enough for freefall induced oversteepening to be feasible

lead isotopic patterns of galena in the piedmont and blue ridge ore deposits southern appalachians massive sulfide deposits of the southern appalachians form two distinct groups of lead isotope ratios which correlate with their geologic settings piedmont massive sulfides are associated with volcanic rocks of predominantly cambrian age they have model lead ages which are too young an effect of u pb enrichment of source rocks lead isotope ratios of piedmont massive sulfide define a trend of decreasingly radiogenic values from the mineral district virginia to the stone hill deposit alabama which probably reflects decreasingly ensialic volcanism from northeast to southwest a regression through the data has a slope which corresponds to a secondary isochron age of about 3 7 b y the line is proposed to represent mixing between variable amounts of upper crustal lead decreasing to the southwest with lead from a source relatively depleted in u pb which has however experienced either continuous or episodic enrichment of uranium relative to lead the occurrence of a similar trend in galenas from kings mountain belt ores is further evidence for such a source massive sulfides of the blue ridge geologic province occur in upper precambrian metasedimentary sequences and are typically low in lead content lead isotope ratios of galenas from ducktown and the gossan lead are highly enriched in 206 pb compared to amounts expected for deposits of that age the anomalously radiogenic component is probably the result of lead derived from the predominantly clastic sediments of the host sequences the radiogenic component of the 206 pb coupled with the narrow range of 207 pb 204 pb at ducktown indicates that much older continental crustal rocks were not involved in the ore forming process galenas from a vein in the hamme tungsten district north carolina form a tight grouping which is similar to lead in k feldspars from postorogenic approximately 300 m y old granitic plutons the mineralized veins of the virginia gold pyrite belt have galenas with a wide spread of radiogenic lead isotope ratios and do not appear to be related to massive sulfide deposits of the central virginia piedmont

duration of the early cambrian u pb ages of volcanic ashes from avalon and gondwana volcanic zircons from three ashes give a u pb date of 511 1 ma on trilobite bearing upper lower cambrian upper branchian series strata of southern new brunswick that correlate into the siberian middle botomian toyonian stage interval this very young age on the late but not latest early cambrian of avalon is consistent both with a 519 1 ma age on the older caerfai bay shales of south wales that are tentatively correlated into strata with the oldest avalonian trilobites lower branchian and with a 517 1 5 ma age on the antatlasia gutta pluviae zone trilobites of morocco determination of a 522 2 ma zircon age on the moroccan subtrilobitic lower cambrian lie de vin formation is consistent with an earlier reported 521 7 ma age from the lie de vin but suggests that a 526 4 ma age on australian trilobite bearing lower cambrian rocks may be too old a 33 ma duration of the avalonian early cambrian and an 8 ma length of the avalonian trilobite bearing lower cambrian support proposals that

basement controls on sediment hosted zn pb deposits a pb isotope study of carboniferous mineralization in central ireland lead isotope ratios of galenas from carbonate hosted zn pb or ba or cd or ag deposits in central ireland are homogeneous within individual deposits but vary between deposits the variation correlates directly with geographic position defining isoplumbs and not with style of mineralization age of host rock or structural or physico chemical factors the least radiogenic pb occurs in the northwest and the most radiogenic in the southeast this pattern elegantly reflects irish pre caledonian basement geology lewisian gneisses in the northwest and late proterozoic upper crustal rocks in the southeast that the northwestern end member of the irish pb mixing trend is ultimately lewisian is demonstrated by the concordance of the irish pb isotope trend with the trends defined in proterozoic to tertiary rocks and ores of scotland which contain pb of demonstrably lewisian derivation although geologic relations and pb isotope ratios unambiguously implicate basement in the ore forming process we are unable to discriminate between lewisian gneisses themselves and detritus derived from lewisian rocks in the caledonian metamorphic complex as the direct source of pb comparison of the irish deposits with mcarthur river and mississippi valley deposits leads us to conclude that style of mineralization syngenetic syndiagenetic epigenetic is not a fundamental aspect of sediment hosted zn pb deposits but is a reflection of local stress regimes and physico chemical conditions the fundamental link between the types of sediment hosted zn pb deposits may be the onset of a condition such as extension or compression that provides a mechanism for driving fluids in extensional regimes seismic pumping processes may channel fluids toward the surface or into fractures or favorable pore spaces resulting in a wide variety of styles of mineralization the lack of dilatency in sediments in compressional regimes however may inhibit surface exhalation and favors epigenetic styles of mineralization

factors underlying piping in the basilicata region southern italy abstract piping tunnelling erosion is a widely spread process in the plio pleistocene marine clays of the basilicata region in southern italy the pipes are often closely concentrated along the surface drainage networks at different depths with a tunnel length and a diameter varying from just a few centimetres to some metres the formation and evolution of pipes in the badlands can be explained through 1 geo structural characteristics of the clay 2 material properties such as a high exchangeable sodium percentage 3 favourable climatic conditions and 4 hydraulic gradient along the path controlled by the gully or ravine bottom that is acting as a local drain the exposure and the extension of joints at the surface and in the bedrock along the slope are the main factors required for development of the pipes the absence of the piping erosion on the slopes where a reasonably thick layer of the soil material covers the open joints shows that although certain material properties and climatic conditions play an important role they are not sufficient to develop extensive piping erosion in the area

soil erosion assessment tools from point to regional scales the role of geomorphologists in land management research and implementation geomorphological research has played an important role in the development and implementation of soil erosion assessment tools because policy and management approaches include the use of soil erosion assessment tools soil erosion research directly affects the public in terms of providing information on natural hazards and human impacts and also as the basis for regulatory policy on land management for example soil loss calculations and geomorphological expertise are used to support soil conservation planning both through agricultural legislation that defines maximum tolerable soil loss rates and through federal and local legislation that requires soil erosion controls on many construction sites to be useful for decision makers soil erosion models must have simple data requirements must consider spatial and temporal variability in hydrological and soil erosion processes and must be applicable to a variety of regions with minimum calibration the growing use of erosion models and geographic information systems gis in local to regional scale soil and water conservation raises concerns about how models are used this has prompted interest in methods to assess how models function at management scales and with the types of data that are commonly available to users a case study of a gis based soil erosion assessment tool using the process based water erosion prediction project wepp shows that using commonly available data rather than research grade data can have predictably a significant impact on model results if model results are then used in management decisions it is critical to assess whether the scale and direction of variation in results will affect management and policy decisions geomorphologists provide unique perspectives on soil erosion and can continue to affect policy through soil erosion research this research should focus on fundamental processes but equally important is continued development and evaluation of models that are matched to real world data availability geomorphic settings and information needs d 2002 elsevier science b v all rights reserved

carbonate hosted zinc lead deposits in the lower cambrian of hunan south china a radiogenic pb sr isotope study zinc lead deposits hosted by lower cambrian carbonate rocks near huayuan in the hunan guizhou orogenic belt southern china have been studied by means of pb and sr isotopes the deposits are characterized by two mineralization stages 1 a main economic stage comprising cockade style sphalerite galena pyrite marcasite fluorite barite and calcite and 2 a subsequent noneconomic stage with nodular rosette sphalerite saddle dolomite and bitumen both ore stages are postdated by a generation of late calcite main stage sulfides 206pb 204pb 18 162 18 225 display a steep linear trend on conventional pb pb diagrams which indicates binary mixing of lead components bearing the imprint of ancient 207pb dominated crustal rocks and reservoirs with elevated th u ratios possibly located in the lower crust nodular rosette sphalerites fall outside this line but reflect a second mixing trend between mineralizing fluids characterized by 206pb 204pb 18 3 and lead isotopically similar to specific main stage cockade sphalerites suggesting either remobilization of pb from main stage ores or contemporaneous mixing of three distinct lead components in the course of an evolving hydrothermal event the 87sr 86sr ratios of diagenetic and hydrothermal carbonate cements and barites range from 0 70905 to 0 71006 and increase systematically with the paragenetic sequence indicating progressive mixing of marine sr derived from the lower cambrian carbonate host rocks with radiogenic sr supplied by external mineralizing fluids in order to identify possible local metal sources u pb and rb sr analyses were performed on six whole rock samples from the lower cambrian sequence and their time integrated pb and sr isotope evolution was compared to the isotopic signature of the mineralization whereas shales and black shales of the cambrian balang and niutitang formations underlying the ore bearing qingxudong carbonate unit appear to be possible external sources for the radiogenic sr found in hydrothermal carbonates the u pb data do not permit these shales to be considered as primary source rocks for the ore lead both the pb and the sr isotopes indicate mineralizing fluids that had interacted with various external metal sources some of them most probably being located in sedimentary or crystalline basement rocks this refutes prevailing genetic models favoring the hydrocarbon and metal rich lower cambrian niutitang black shales as the most likely source reservoir for the zn and pb in the deposits of the huayuan district the pb and sr isotope mixing trends observed for the ore and gangue preciptates reflect small scale heterogeneous fluid systems that were not capable of producing large deposits this may indicate that the huayuan district is not a favorable target for future exploration of economic ore deposits it also limits the prospects of successful radiometric dating of the mineralization since conventional geochronologic models require initial isotopic homogeneity

mixing line isochrons a new interpretation of galena lead isotope data from southeastern british columbia ainsworth bluebell slocan and moyie camps mixing of lead from two different sources both of which follow simple growth curve lead evolution the more radiogenic source is thought to represent evolution of lead in the upper crustal continentally derived sedimentary rocks which host these deposits the two stage bluebell growth curve provides an estimate of the uranium poor lead source which may be the lower crust the upper mantle or a mixture of both mixing line isochrons drawn between these two curves at ages corresponding to the best estimates for the mineralization ages at these three mining camps closely fit the lead isotope data this supports the age estimates for all three camps and in particular substantiates a lower cambrian age for the original mineralization in the ainsworth and bluebell deposits mixing is assumed to take place at the time of mineralization and may be attributed to magmatic activity in the moyie and slocan camps applicability of the model to a large part of the omineca belt may be used to date both epigenetic and syngenetic or penecontemporaneous deposits of unknown age modified journal abstract

soil erosion assessment using geomorphological remote sensing techniques an example from southern italy the aim of this study is to assess of the distribution and map the geomorphological effects of soil erosion at the basin scale identifying newly formed erosional landsurfaces nefels by means of an integration of landsat etm 7 remotely sensed data and field surveyed geomorphological data the study was performed on a 228 6 km2 wide area located in southern italy the study area was first characterized from a lithological pedological land use and morpho topographic point of view and thematic maps were created then the georeferenced landsat etm 7 satellite imagery was processed using the rsi envi 4 0 software the processing consisted of contrast stretching principal component analysis pca decorrelation stretching and rgb false colour compositing a field survey was conducted to characterize the features detected on the imagery particular attention was given to the nefels which were located using a global positioning system gps we then delimited the regions of interest roi on the landsat etm 7 imagery i e polygons representing the ground truth discriminating the nefels from the other features occurring in the imagery a simple statistical analysis was conducted on the digital number dn values of the pixels enclosed in the roi of the nefels with the aim to determine the spectral response pattern of such landsurfaces the nefels were then classified in the entire image using a maximum likelihood classification algorithm the results of the classification process were checked in the field finally a spatial analysis was performed by converting the detected landsurfaces into vectorial format and importing them into the esri arcviewgis 9 0 software application of these procedures together with the results of the field survey highlighted that some objects in the classified imagery even if displaying the same spectral response of nefels were not landsurfaces subject to intense soil erosion thus confirming the strategic importance of the field checking for the automatically produced data during the production of the map of the nefels which is the final result of the study these objects were eliminated by means of simple geomorphologically coherent intersection procedures in a geographic information system gis environment the overall surface of the nefels had an area of 22 9 km2 which was 10 of the total the spatial analysis showed that the highest frequency of the nefels occurred on both south facing and southwest facing slopes cut on clayey marly deposits on which fine textured and carbonate rich inceptisols were present and displaying slope angle values ranging from 12 to 20 the comparison of two satellite imageries of different periods highlighted that the nefels were most clearly evident immediately after summer tillage operations and not so evident before them suggesting that these practices could have played an important role in inducing the erosional processes copyright 2009 john wiley sons ltd

strontium isotope trends during diagenesis in ore bearing carbonate basins strontium isotope studies 120 analyses were carried out on ore and gangue minerals from five ore districts in different carbonate basins most of the samples were separated from consecutive diagenetic generations even for diverse stratigraphic positions and mineralogies very consistent trends are recognized in all cases87sr 86sr ratios increase with advancing diagenetic stage however each district shows a different behaviour in the magnitude of increase over the first diagenetic generation which ideally is close to contemporaneous seawater the increase of the strontium ratios in the subsequent diagenetic generations is explained by equilibration during crystallization with fluids evolving towards more radiogenic compositions this is interpreted as a consequence of the reaction of the fluids with rb bearing phases mainly detrital clay minerals variation ranges of87sr 86sr ratios can constitute a sensitive tool for determining the stage of basin evolution in a similar way as vitrinite reflectance or illite crystallinity this method has the disadvantage that the increase of radiogenic strontium depends not only on the evolution stage i e temperature pressure and time but also on the amount of rb bearing minerals present in the basin on the other hand this method has the important advantage that within a given basin this last parameter can be modelled each evolution stage is recorded in the cements or crystallization generations by its strontium isotope signatures which tend towards increased radiogenic values with advancing diagenesis

87 sr 86 sr ratio measurements on sulfides carbonates and fluid inclusions from pine point northwest territories canada an 87 sr 86 sr ratio increase accompanying the mineralizing process crystallization of fine grained early diagenetic dolomite and later coarse dolomite has occurred in sr isotope equilibrium with devonian sea water 87 sr 86 sr 0 7082 extraction of radiogenic sr from silicates in the metal source sediments by mineralizing brines modified journal abstract

cockade breccia product of mineralisation along dilational faults cockade breccias are fault fills in which individual clasts are completely surrounded by concentric layers of cement they occur particularly in low temperature near surface hydrothermal veins at least six mechanisms have been proposed for the formation of cockade breccia like textures but only two repeated rotation accretion and partial metasomatic replacement of clast minerals have been supported by detailed evidence a typical example of cockade breccia from the gower peninsula south wales shows clear evidence for the rotation accretion mechanism in particular overgrown breakage points in cement layers where cockades were previously touching each other and rotated geopetal infills of haematitic sediment based on the available evidence it is proposed that cockade textures result from low rates of cement growth compared to high rates of dilational fault slip seven criteria are given for the correct identification of cockade breccias

fluid inclusion and carbon oxygen and strontium isotope study of the polaris mississippi valley type zn pb deposit canadian arctic archipelago implications for ore genesis the polaris deposit is one of the largest mississippi valley type deposits in the world with 22 million tonnes of ore at 14 zn and 4 pb contained in a single compact orebody surrounded by dolomitized host rocks using detailed sampling of carbonates in the orebody and the dolostone halo this paper aims to characterize the temporal and spatial evolution of the mineralizing system and to understand the mechanisms that controlled the accumulation of this large compact zn pb deposit five types of dolomite have been distinguished including three replacement rd and two pore filling dolomites pd the paragenetic order is rd1 rd2 rd3 pd1 and pd2 pore filling calcite pc postdates all other minerals in most cases sulfides and dolomite did not co precipitate but sphalerite and galena largely overlap with rd3 and pd1 various dolomites are dissolved or replaced by sulfide precipitating fluids sulfides in turn can be overgrown by dolomites colloform texture in sphalerite is widespread fluid inclusions were studied in rd3 pd1 pd2 sphalerite and pc the overall ranges of homogenization temperatures t h and last ice melting temperatures t m ice for fluid inclusions in dolomites and sphalerite are from 67 to 141 c and from 46 7 to 27 0 c respectively consistent with warm basinal brines with high salinities and ca na ratios gas chromatographic analysis of these fluid inclusions indicates low concentrations of hydrocarbons 0 06 mol c o and sr isotopes were analyzed for all dolomites and pc as well as for the fine grained host limestone and early diagenetic calcite sc rc the isotopic values of rd2 rd3 pd1 and pd2 cluster tightly and form largely overlapping domains with respect to the host limestone they are depleted in 18o similar in 13c and slightly enriched in 87sr there are no regular spatial variations for fluid inclusion and isotope data indicating an overall geochemical homogeneity in the hydrothermal system however certain samples close to the fracture zones in the orebody with slightly elevated t h and 87sr 86sr values and depleted 18o values suggest that the fracture zone was the conduit for the hot brines based on the geological and geochemical characteristics of the deposit we propose that sulfide precipitation at polaris was caused by mixing of a reduced metal rich sulfur poor fluid with a reduced metal poor sulfur rich fluid at the site of mineralization the metal carrying fluid ascended along fractures from below the deposit and was hotter than the host rocks whereas the reduced sulfur carrying fluid was delivered to the site of mineralization laterally and was in thermal equilibrium with the host rocks this model can readily explain the dissolution of dolomite during sulfide precipitation and the abundance of colloform sphalerite as well as the low concentrations of hydrocarbons in fluid inclusions

the impact of the farming practice of remodelling hillslope topography on badland morphology and soil erosion processes abstract badland landforms created in plio pleistocene clay landscapes of the basilicata region of southern italy form marginal features in a semi arid landscape dominated by the widespread cultivation of subsidised cereals these badland features are high relative relief forms exhibiting a high drainage density and steep slopes with slope angles typically in excess of 35 economic incentives to increase agricultural productivity have resulted in the practice of remodelling these marginal areas using heavy earth moving equipment remodelling the badland features creates longer slopes at lower angles which can be cultivated using conventional farm machinery these changes in hillslope morphology have altered the degree to which soil erosion processes operating in these areas are spatially coupled in the badland areas erosion and deposition are strongly localised with minimal sediment delivery to ephemeral or perennial channel systems the economically driven change in land use from visually striking badland areas to newly remodelled fields for agricultural use results in an increase in the coupling of drainage networks and a net increase in soil erosion

lead isotope geochemistry of mississippi valley type deposits of the southern appalachians lead isotope compositions of sphalerite fluorite barite and sparry dolomite in mississippi valley type deposits of the southern appalachians fall into four different isotopic groups most sphalerite forms two small clusters whereas barite fluorite and sparry dolomite compositions are more scattered these lead isotope compositions reflect the distribution of the deposits which are hosted by two main palcoaquifers lower cambrian clastic and carbonate rocks host the austinville zn pb cartersville ba embreeville zn and doughtery zn deposits all of which fall in a cluster of lead isotope compositions near 207 pb 204 pb 15 73 and 206 pb 204 pb 19 45 lower ordovician carbonate rocks of the knox group host the copper ridge zn eve mills zn greeneville fall branch ba f mascot jefferson city zn powell river zn pb russell tazewell ba f and sweetwater ba zn f deposits which have more complex compositions sphalerite from mascot jefferson city eve mills sweetwater and russell tazewell plots in a cluster around 207 pb 204 pb 15 67 and 206 pb 204 pb 19 0 whereas that from copper ridge plots in both clusters fluorite from sweetwater falls on a long array in 207 pb 204 pb 206 pb 204 pb space that has a two stage age of 1 55 to 1 9 ga for t 2 360 ma the previously published 87 sr 86 sr isochron age obtained for the mascot jefferson city district barite and sparry dolomite compositions are relatively similar and have lower 207 pb 204 pb and 208 pb 204 pb ratios than most sphalerite and fluorite the spatial distribution of lead isotope compositions indicates that there were at least two mineralizing solutions represented by the two lead clusters the upper cluster solution dominated the lower ordovician palcoaquifer whereas the lower cluster dominated the lower cambrian palcoaquifer and that the two solutions mixed in most areas except the mascot jefferson city district where zinc mineralization is strongest although the specific sources of these leads cannot be determined from available data on possible source rocks the two lead clusters probably came from relatively young well mixed sedimentary basins and the lead in the fluorite must have come from a source with an unusally high u th ratio such as phosphoritc this study confirms that the isotopic composition of lead in southern appalachian mississippi valley type deposits differs significantly from that found in mid continent deposits

origin of a giant orebody at navan ireland the navan base metal deposit in ireland formed 360 myr ago1 and is one of the world s largest orebodies with 70 million tonnes of 12 zn pb much of the mineralization was sedimentary exhalative other well known examples of this class of the so called sedex deposits are mount isa in australia sullivan in canada and meggen in germany several models for the formation of such deposits have been suggested here we report the first systematic stratigraphic pb isotopic study of galenas in any giant exhalative zn pb orebody we document evidence that the sources of pb in the navan orebody evolved to produce increased dispersion and less radiogenic averages of lead isotopic compositions this is as expected if the orebody formed from a hydrothermal convection cell which expanded to scavenge metals from progressively deeper parts of the continental crust

strontium isotopic evidence for the origin of barites and sulfides from the mississippi valley type ore deposits in southeast missouri several barites from the lead barite district contain between 1 170 and 1 633 ppm sr their 87 sr 86 sr ratios range from 0 7098 to 0 7118 in contrast to the values of 0 7082 to 0 7089 for the host carbonate rocks four galenas from the viburnum mine in southeast missouri provide a rb sr age of 392 or 21 m y and an initial isotopic composition of 0 7104 or 0 0004 indicating the period of mineralization to be early devonian modified journal abstract

isotopic footprints of the giant precambrian caixiashan zn pb mineralization system abstract carbonate replacement zn pb deposits are formed through fluid circulation within carbonate rocks and are an important global source of zn and pb the caixiashan carbonate replacement deposit is one of the largest zn pb systems in china the mesoproterozoic syn sedimentary stage i of the caixiashan deposit is characterized by sulfides with negative 34s values 25 3 to 8 6 generated by organic sulfate reduction which co precipitated with the host rocks the neoproterozoic main sulfide precipitation stage ii was caused by thermo sulfide reduction resulting in positive 34s values 7 9 12 3 carbonates from stage i have 13cpdb of 1 2 to 2 8 and 18osmow of 17 1 24 0 similar to those of marine carbonate rocks dolomite and calcite from stage ii have 13cpdb ranging from 5 1 to 0 2 and 18osmow of 14 1 19 7 consistent with dissolution of carbonate rock in contrast dolomite and calcite from stage iii have mantle c o signatures with the 13cpdb ranging from 7 5 to 0 4 and 18osmow from 5 8 to 13 2 c o isotopic signatures suggest that the original carbonates were dissolved by the acid generated from sulfide precipitation consistent with the sulfide precipitation process subsequent precipitation of carbonates created large volumes of dolomite alteration that coated the sulfides and possibly contributed to the preservation of the ore as dolomite would dissolve more slowly than calcite carboniferous magmatism stage iii resulted in a magmatic overprint on the stage ii sulfide the metal sources of these sulfide ore packets are mainly derived from the mesoproterozoic carbonate rocks as indicated by the lead isotopes of sulfides and also c o isotopes of carbonates the deposit is characterized by a complex paragenesis with multiple overprinting sources of hydrothermal fluids and sulfide over 700 m y the syn sedimentary stage 1019 ma and main mineralization stage 859 837 ma at caixiashan are chronologically and tectonically associated with the assembly and break up of the rodinia supercontinent whereas the magmatic hydrothermal modification stage 353 ma is associated with the closure of the paleo asian ocean during the early carboniferous

lead isotope geochemistry of mississippi valley type mineralization in the central appalachians lead isotope compositions of sulfide minerals from mississippi valley type deposits in cambrian ordovician and silurian rocks in the central appalachians plot in distinct groups one for each major district data for deposits in the nittany arch and shawangunk districts form circular clusters whereas data for the timberville and friedensville districts form more elongated fields comparison of these data to isotopic compositions of possible source rocks suggests that lead in the nittany arch shawangunk and timberville clusters came largely from paleozoic igneous rocks or sediment derived from these rocks whereas lead in the friedensville deposits came from grenville age basement rocks additional lead from the timberville district appears to have come from triassic diabase suggesting that as least some mineralization there is unusually young these results contrast strongly with those for the southern appalachians where different mississippi valley type districts have similar isotopic compositions over large areas only lead from the ordovician hosted timberville deposits in the central appalachians has an isotopic composition similar to that of any of the southern appalachian deposits and its composition is similar to that of deposits in the lower cambrian paleoaquifer of the southern appalachians rather than the ordovician paleoaquifer these relations indicate that although mississippi valley type brines in the central appalachians participated in significant cross formational flow they were not as regionally extensive as those in the southern appalachians a factor that might account for the relative scarcity of large mississippi valley type deposits in the central appalachians area

strontium isotopic geochemistry of mississippi valley type deposits east tennesse implications for age and source of mineralizing brines strontium isotopic ratios of wall rock ore and gangue minerals from the mascot jefferson city copper ridge and sweetwater mississippi valley type mvt districts and from the lost creek barite deposit in east tennessee were measured in an effort to determine the age of this mineralization and its relation to the tectonic evolution of the appalachian orogen the lower ordovician knox group which hosts the mvt mineralization contains limestones and dolomite karst breccias with 87sr 86sr ratios of 0 70900 to 0 70916 that are similar to those of early ordovician sea water primary diagenetic knox dolomite as well as recrystal line dolomite which formed by later alteration of limestone exhibits 87sr 86sr ratios of 0 70930 to 0 71024 reflecting varying degrees of contamination by solutions with more radiogenic strontium 87sr 86sr isotopic ratios of mineralization in the districts increase in the order lost creek sweetwater copper ridge mascot jefferson city and extend to values significantly higher than those of the enclosing rocks of the three possible source basins for the mvt brines the late proterozoic ocoee cambrian luttrell and ordovician sevier basins the sevier basin appears to be the only one that could have supplied brines of the appropriate composition at geologically reasonable times estimates of the change with time in the isotopic composition of sevier basin brines indicate that the maximum ages of the lost creek sweetwater copper ridge and mascot jefferson city mineralization are 520 460 405 and 395 m y respectively comparison of fluid inclusion temperatures for the sweetwater and mascot jefferson city minerlization with the thermal evolution curve for the sevier basin supports these estimates these ages suggest that the lost creek barite minerlization formed as an exhaltive deposit in the sevier basin and that the sweetwater fluorite barite mineralization formed during the later part of taconic orogeny the copper ridge and mascot jefferson city minerlization could have formed either prior to or during the alleghanian orogeny but prior to deformation of the enclosing rocks depending on the rate of istopic evolution of the sevier basin brines