hybrid seismic modelling a technique to combine physical and computer methods for vertical wave incidence a hybrid seismic modelling technique has been developed to investigate complex geological phenomena those parts of a geological structure which are too complicated to be treated theoretically are studied by two dimensional physical models other sections of the structure which can be treated theoretically i e inhomogeneities in the vertical direction are modelled by computer methods a feedback process is used to combine the results of both physical and computer modelling horizontally layered coal seam models are presented to test the hybrid modelling technique for normal incidence a comparison of the hybrid seismograms with pure synthetic seismograms shows an acceptable conformity for normal incidence a hybrid zero offset section is shown to investigate a complex geological structure in the ruhr coalfield in germany

applications of uthpb isotope systematics to the problems of radioactive waste disposal abstract concentrations of u th and pb and the isotopic composition of pb for whole rock samples of granitoids show 1 that open system behavior is nearly universal in the surface and near surface environment and 2 that elemental mobility is possible to depths of several hundred meters several identified or at least postulated factors that control u and or pb mobility include 1 the mineralogical sites for u and its daughter products 2 access of groundwater to these sites 3 the volume of circulating water and 4 the chemistry of the groundwater studies of granitic samples from peralkaline complexes in the arabian shield have shown that most samples lost less than 20 of their u during recent exposure to the near surface environment most of the u in these samples appears to be firmly bound in zircons in contrast most surface and shallow drill core samples of the granite of lankin dome granite mountains wyoming have lost 70 of their u most of the u in these samples is weakly bound in biotite and epidote family minerals the granite recovered during the illinois deep drill hole project stephenson county illinois is mineralogically similar to the granite of lankin dome but this granite lost radiogenic pb rather than u probably as a result of exposure to groundwater that had a markedly different chemistry from that in the granite mountains studies of the sherman granite wyoming and the gotemar granite southeastern sweden have shown that u and or pb mobility is greatest in and near fractured rock the greater mobility is interpreted to be the result of both a larger water rock ratio in the fractured rock and exposure to water over an increased surface area and consequently a greater number of uranium sites several types of geochemical and mineralogic data can be used to identify rock water interaction in granites however if rock samples have favorable radiogenic to common pb ratios both the amount and approximate timing of u or pb mobility can be obtained through the use of isotopic studies such information can be extremely important in the search for favorable hosts for containment of radioactive waste rocks such as the gotemar granite have undergone considerable rock water interaction most of which occurred 400 myr ago and little in recent times thus a search for zones that have experienced only a little interaction with water may provide a misleading prediction as to the ability of such zones to shield radioactive wastes from the modern biosphere from an isotopic point of view an ideal candidate for evaluation as a host rock for radioactive wastes would have the following characteristics 1 a high ratio 2 of radiogenic to common pb in order to optimize precision of the results 2 a simple two stage geologic history so that results could be interpreted without multiple working hypotheses and 3 an originally high percentage 50 of labile u so that the results would be highly sensitive to even small amount of rock water interaction these characteristics should produce rocks with marked radioactive disequilibrium in surface samples the disequilibrium should grade to radioactive equilibrium with increasing depth until zones in which water has not circulated are found extensive regions of such zones must exist because u th pb systematics of most analyzed granitoids demonstrate closed system behavior for almost all of their history except for their recent history in the near surface environment

introduction to the illinois deep hole project commonwealth edison recently drilled 3 engineering test holes in n w illinois the 3 holes uph 1 2 and 3 were 0 6 1 6 and 1 6 km deep respectively and the latter 2 penetrated nearly one kilometer of precambrian granitic basement in 1980 continuous core from all 3 holes and access to uph 3 were made available for scientific investigations through the illinois deep hole project the principal study of these holes was an integrated investigation of surface and in hole experiments plus detailed petrologic geochemical and physical property studies of the core samples 50 references

oxygen isotope study of precambrian granites from the illinois deep hole project seventeen whole rock samples and 27 mineral separates from the precambrian granites in drill cores uph 1 2 and 3 have been analyzed the 180 values for quartz remain exceedingly uniform 7 6 8 6 throughout the entire length of the three cores and are indicative of their primary igneous values the 180 for the feldspars are also uniform for samples located 70 m or deeper below the unconformity 6 0 6 9 however in the upper 70 m of the granite body a gradual upward increase of 180 in the feldspars is observed to 8 8 similarly 18 quartz feldspar show typical plutonic values 1 4 2 1 in the main part of the precambrian sections except in the upper 70 m where q f become progressively smaller upward until a reversed fractionation negative q f is observed in samples several meters below the unconformity the increase of 180 in the feldspars is interpreted as a result of hydrothermal alteration of the granites at low temperatures probably between 110 260 c the source of the hydrothermal fluid responsible for the alteration most likely derived from the formation waters in the upper cambrian ordovician sedimentary rocks overlying the unconformity the alteration occurred at least 135 200 m y ago when there was also a lead mobilization event the relative low primary 18o 16o ratios in the granites suggest that the granites may have derived from partial melting of low 180 metamorphic rocks in the lower crust

zinc copper and lead in mid ocean ridge basalts and the source rock control on zn pb in ocean ridge hydrothermal deposits the contents of zn cu and pb in mid ocean ridge basalts morb and the morb source rock control on zn pb in ocean ridge hydrothermal deposits are examined the values of zn cu and pb for submarine mid ocean ridge basalts morb are respectively in ppm average morb 75 75 and 0 7 west valley juan de fuca ridge jfr 87 64 and 0 5 southern jfr 120 and 0 5 and 21 n east pacific rise epr 73 78 and 0 5 values of zn pb range from about 100 240 and cu pb from 100 156 in this study zn is found to correlate positively with tio2 feo mean square of weighted deviates mswd of 1 6 for jfr basalt and inversely with mg number mswd of 3 5 therefore contrary to statements in the literature that zn should be compatible in morb zn is a mildly incompatible element and must be enriched in the glass phase relative to olivine as zn does not fit into the other major phenocryst phase plagioclase in the source of morb zn likely is most enriched in oxides spinel magnetite and titanomagnetite copper generally does not correlate well with other elements in most morb data examined when differentiation is dominated by olivine cu has a tendency to behave incompatibly e g at mg numbers 70 but overall cu shows some tendency towards being a compatible element particularly along the mid atlantic ridge a behavior presumably due to separation of sulfides in which cu but not zn is markedly enriched copper thus may be in dispersed sulfides in the source of morb ocean ridges provide important data on source rock controls for sulfide deposits because in sediment starved ridges much is known about the possible source rocks and mineralization is presently occurring in contrast to zn pb 5 in continental hot cl rich brines zn pb in the hottest sediment starved ridge black smoker hydrothermal fluids at 21 n epr is about 110 similar to local morb 145 but cu pb is closer to 30 possibly due to subsurface deposition of cu at the jfr the best value of zn pb in the hydrothermal fluids is about 175 again similar to local morb 240 but cu is very low in the fluids that are at temperatures less than 300 c the large morb like zn pb in the hottest black smoker fluids suggests a source rock control for the metals that prohibits significant galena in the black smoker deposits of sediment starved ridges in contrast exhalative deposits on sediment swamped ridges have significant galena its presence is suggestive of pb derivation from sediments an origin supported by pb isotope studies of lehuray and colleagues in 1988

direct radiometric age determination of carbonate diagenesis using u pb in secondary calcite abstract carbonate rocks and corals from the middle devonian lucas formation ontario have 238 u 204 pb values ranging from 132 to 559 and give concordant apparent 206 pb 238 u and 207 pb 235 u ages in approximate agreement with the primary age of sedimentation a carbonate rock from one part of the section gives an anomalously young age and contains patches of secondary calcite spar this spar is predominantly pale purple under cathodoluminescence cl and contains 250 600 ppb u and 15 22 ppb pb the values range from 2 10 3 10 4 making the pb very radiogenic apparent 206 pb 238 u ages range from 208 to 249 ma and 232 th 238 u values are from 0 005 to 0 040 the younger ages are correlated with an increase in orange cl and higher apparent values the age scatter is attributed to mixing of a subordinate calcite spar which infiltrated the older spar the young calcite is bright orange under cl has 7200 ppb u of 6 2 10 4 a of 0 23 and is dated at 45 ma the age of the older spar has been determined by a sequential dissolution technique the resulting u pb data can be plotted as u pb age spectra and yield lplateau dates ranging from 231 5 1 4 to 244 9 1 6 ma and an integrated 206 pb 238 u age of 238 2 1 1 ma also the sequential dissolution increases the values progressively to a5 10 4 making these ages independent of the initial pb correction the occurrence of two young ages of diagenetic calcite in these strata is consistent with the idea of recurrent fluid activity along fracture zones in subsurface sediments in the michigan basin this study shows that it is now possible to date secondary carbonate diagenesis by u pb and by inference to determine precisely the timing of activity on ancient fracture zones in carbonate rocks

fluid inclusion evidence of basinal brines in archean basement thunder bay pb zn ba district ontario canada fluid inclusions from three quartz galena sphalerite barite calcite veins in the thunder bay district of western ontario contain liquid vapor halite and homogenize by vapor disappearance or halite dissolution at temperatures of 90 200 c cyclically frozen liquid vapor type i inclusions undergo four melting events upon gradual warming initial melting at 55 to 46 c ice disappearance at 30 2 to 25 4 c inversion of hydrohalite to halite at 8 0 to 0 7 c and halite melting at 14 0 to 56 3 c liquid vapor halite type ii inclusions behave similarly but have higher t m ice 27 2 to 21 7 c and t m halite 105 203 c scanning electron microscopy and energy dispersive analysis of fluid inclusion derived decrepitates indicate that the solutes consist of nacl cacl 2 kcl and are consistent with the low temperature phase observations in that they define two distinct populations based on cacl 2 cacl 2 nacl ratios the temperatures and compositional trends defined by the inclusion results are similar to those documented for basinal brines and from fluid inclusions in mississippi valley type ore deposits the thunder bay veins cross the basal unconformity of the middle proterozoic sibley basin and extend into archean basement granites such that the fluid inclusions results provide direct evidence that basinal waters infiltrated basement rock in western ontario the inclusion fluids and associated mineralization are thought to result either from dewatering of the sibley basin during keweenaw age rifting or from the introduction of exotic paleozoic basinal waters when the michigan basin extended over the region

u th pb ree systematics of organic rich shales from the ca 2 15 ga sengoma argillite formation botswana evidence for oxidative continental weathering during the great oxidation event article i nfo article history accepted 23 october 2008 theredox sensitive geochemicalbehaviorof uraniumpermits the useof th uratiosasa geochemicalproxy for the oxidation state of the atmosphere during deposition due to the effects of post depositional uranium mobilityonth uratiosduringeventsinvolvingoxygenated fluids directmeasurementsofth uratiosareoften misleading even for drill core samples because both of these elements radioactively decay and produce lead isotopes the pb isotope composition may reflect the depositional th u ratio although the th u ratios induced bychangesshortlyafterdeposition maynotbe distinguished fromthe true depositional th uratios in order to effectively evaluate the time integrated th u ratio a values for the initial depositional pb isotope composition must be determined or accepted from the models for the whole earth while the timing for the rise of atmospheric oxygen is reasonably well constrained now its effect on continental weathering and ocean redox state remains poorly constrained and debated the ca 2 15 ga sengoma argillite formation of botswana contains organic rich shales deposited during the great oxidation event the slope of the

provenance ages and alteration histories of shales from the middle archean buhwa greenstone belt zimbabwe nd and pb isotopic evidence abstract shales of the ca 3 0 ga buhwa greenstone belt zimbabwe were derived from a compositionally diverse provenance whose ages determined by ion probe analyses of detrital zircons in interbedded sandstones range from 3 8 to 3 1 ga geochemical data for the shales were previously interpreted to indicate that sediments had been derived from an intensely weathered source ree concentrations in the shales were interpreted to suggest that the provenance was compositionally mixed with components of felsic tonalite and alkalic granitoid and mafic rocks sm nd and nd isotopic compositions of these rocks can be used to model initial nd isotopic ratios at the time of sedimentation endsed as well as model crustal formation ages tdm the former at the age of sedimentation range from 0 6 to 10 8 consistent with a range of provenance ages the latter range from 4 46 ga to 2 99 ga the oldest crustal formation ages up to 0 7 ga older than known detrital components are interpreted here to indicate that the sm nd system of the sediments experienced open system behavior the implied alteration would have included an increase in sm nd by about 20 25 percent probably in the form of preferential loss of nd with respect to sm the pb isotopic compositions of whole rock samples are quite radiogenic with a range of 206pb 204pb from 25 5 to 154 an array of ten samples lies scattered about a line with a 207pb 204pb 206pb 204pb slope age of about 2 73 ga five individual samples were sequentially leached to further test the timing and characteristics of this u th pb alteration event these arrays of a whole rock three leach steps and a residue also form linear pb pb arrays one is more scattered with ages ranging from 2260 360 ma to 2824 170 ma suggesting that all samples experienced a latest archean to earliest proterozoic enrichment in u pb this age range also may be the approximate age of sm nd enrichment for the shales all samples both whole rocks and leached samples lie grouped on a 208pb 204pb 206pb 204pb diagram around a line with 232th 238u 3 5 2 9 to 3 9 because of the lack of large differences in the th u of the samples through large ranges of u pb we interpret this consistency in th u to mean that the shales of the buhwa belt experienced pb loss rather than u and th gain circumstances that may be responsible for pb loss in a sedimentary basin include loss of saline fluids during basin dewatering such an event would likely have been related to folding associated with the thrusting and magmatic intrusion of the adjacent limpopo belt suggesting that uplift dewatering and geochemical and isotopic alteration can be genetically related

alleghenian age of the upper mississippi valley zinc lead deposit determined by rb sr dating of sphalerite mississippi valley type mvt ore deposits in north america are generally thought to have formed from warm brines which migrated from sedimentary basins to sites of deposition in sedimentary strata of the neighbouring craton long after unification of the sediments1 3 when and how these migrations occurred is not well understood but recent geological evidence suggests that continental scale brine migration can be triggered by tectonic collisions4 6 indirect evidence7 9has suggested an association of north american mvt mineralization with the late palaeozoic alleghenian ouachita orogeny recently however nakai et al 10obtained a rb sr age of 377 29myr for sphalerites from the east tennessee mvt district which they linked to the middle palaeozoic acadian orogeny here we report rb sr dates for sphalerites from the mvt type section the upper mississippi valley umv zinc lead district rb sr data from two distinct sphalerite bands from the west hayden ore body yield two independent but indistinguishable isochrons of permian age 269 6 myr and 270 4 myr consistent with a brine migration episode initiated by the alleghenian ouachita orogeny in conjunction with the previous data indicating a link to the acadian orogeny10 11 our results indicate that north american mvt deposits formed in at least two distinct tectonic episodes

whole rock pb and sm nd isotopic constraints on the growth of southeastern laurentia during grenvillian orogenesis the conventional view that the basement of the southern and central appalachians represents juvenile mesoproterozoic crust the final stage of growth of laurentia prior to grenville collision has recently been challenged new whole rock pb and sm nd isotopic data are presented from mesoproterozoic basement in the southern and central appalachians and the granite rhyolite province as well as one new u pb zircon age from the granite rhyolite province these data combined with existing data from mesoproterozoic terranes throughout southeastern laurentia further substantiate recent suggestions that the southern and central appalachian basement is exotic with respect to laurentia sm nd isotopic compositions of most rocks from the southern and central appalachian basement are consistent with progressive growth through reworking of the adjacent granite rhyolite province however pb isotopic data including new analyses from important regions not sampled in previous studies do not correspond with pb isotopic compositions of any adjacent crust the most distinct ages and isotopic compositions in the southern and central appalachian basement come from the roan mountain area eastern tennessee western north carolina the data set indicates u pb zircon ages up to 1 8 ga for igneous rocks inherited and detrital zircon ages 2 0 ga sm nd depleted mantle model t dm ages 2 0 ga and the most elevated 207 pb 204 pb observed in southeastern laurentia the combined u pb geochronologic and sm nd and pb isotopic data preclude derivation of southern and central appalachian basement from any nearby crustal material and demonstrate that grenville age crust in southeastern laurentia is exotic and probably was transferred during collision and assembly of rodinia these new data better define the boundary between the exotic southern and central appalachian basement and adjacent laurentian granite rhyolite province

on the discretization and cost effectiveness of a finite element solution for hillslope subsurface flow abstract an investigation is carried out into the effects of the size of time and space steps on the discharge and hydraulic potential predictions of a finite element rainfall runoff model the institute of hydrology distributed model version 4 accuracy of hydrological predictions is viewed in the light of computer processing times and recommendations are given for suitable discretizations for this type of model

u th pb systematics in hydrothermally altered granites from the granite mountains wyoming abstract u th pb systematics were investigated in 15 samples representing two types of deuterically altered archean granite albitized and silicified epidotized granite from the granite mountains wyoming the loss of k feldspar during both types of deuteric alteration was accompanied by an extreme reduction of pb content from roughly 40 ppm to less than 12 ppm in the most altered samples nine of the 15 samples yield anomalously young whole rock pb pb and th pb ages compared to concordia ages for zircons and to whole rock pb pb and th pb ages for samples of unaltered granite the young ages are interpreted to be the result of radiogenic pb loss during a middle proterozoic metamorphism that disturbed several isotopic systems in the unaltered granite the loss of radiogenic pb from the whole rock systems of the deuterically altered granites is most likely due to the absence of microcline in many granitic rocks potassium feldspar tends to act as a receptor for pb that has been mobilized and this effect may account for the closed system behavior of pb in whole rock samples of the unaltered granite it may also account for the apparent gain of radiogenic pb during the proterozoic by one sample which was collected at the edge of an alteration zone the deuterically altered granites may have also lost u during the proterozoic but except for two samples the dominant u loss occurred relatively recently probably during tertiary uplift and erosion the low common pb contents and rather high u and th contents of the hydrothermally altered granites might seem to indicate that these rocks are well suited for geochronologic studies in the u th pb system because such rocks should have a high percentage of radiogenic pb unfortunately open system behavior in response to post crystallization metamorphism shows that apparent ages for these types of rocks must be interpreted with caution

isotopic composition of uranium and thorium in crystalline rocks the 238u 234u 230th 232th system has been investigated in 84 silicic crystalline rocks obtained from drill cores surface and near surface samples in california wyoming colorado and illinois results of these analyses displayed on ternary diagrams with apexes for 238u 234u and 230th indicate five predominant geochemical processes that affected uranium in the rock 1 bulk uranium leaching where 238u and 234u were removed with little or no fractionation 2 preferential 234u leaching by alpha recoil displacement 234u recoil loss with lesser 238u loss 3 234u recoil loss with little or no 238u loss 4 uranium assimilation where both 238u and 234u were added with present day 234u 238u activity ratios varying from 0 8 to 1 2 and 5 addition of 234u and 230th by daughter emplacement processes 234u 230th recoil gain evidence for the existence of 234u and 230th recoil gain in rocks is the most important finding of this investigation radioactive disequilibrium occurs in the majority of rocks analyzed where 234u recoil loss is the predominant process associated with incipient weathering u assimilation and 234u 230th recoil gain occur under conditions of substantial water penetration along fractures and into weathered zones in the rocks relatively unfractured and petrographically fresh rocks from the uph 3 drill hole in northern illinois are closest to being in radioactive equilibrium for any suite of rocks included in this study and they demonstrate that equilibrium during the last 0 5 m y can be maintained over a substantial vertical distance where there has been little or no movement of water in the basement rock the 238u 234u 230th system can be a sensitive indicator of geologically recent u mobility and rock water interaction both in petrographically fresh core samples and in sealed fracture zones

secondary k feldspar at the precambrian paleozoic unconformity southwestern ontario the precambrian paleozoic boundary in the subsurface of southwestern ontario commonly is characterized by secondary k feldspar in the weathered and altered precambrian granitoid gneisses at the unconformity secondary k feldspar has replaced preexisting minerals and also occurs as discrete crystals of adularia overgrowths on altered minerals and microcrystalline veinlets the k feldspar is chemically pure or99 100 and has high 18o values 18 9 to 21 4 vienna standard mean ocean water features that indicate crystallization at low temperatures secondary k feldspar also occurs in cambro ordovician clastic and carbonate rocks that immediately overlie the precambrian basement k ar 453 9 to 412 8 ma and rb sr 440 50 ma dates obtained for secondary k feldspar from the precambrian host rocks suggest that its crystallization is unrelated to precambrian weathering or early diagenesis of the immediately overlying cambro ordovician strata estimated crystallization temperatures for the seconda

age and significance of precambrian basement samples from northern illinois and adjacent states geochronologic results are reported for 12 localities where deep drilling has penetrated precambrian basement in iowa illinois indiana michigan and kentucky for the northern illinois deep drill holes uph 1 and uph 3 a zircon u pb age of 1463 10 m y was obtained rb sr data for these samples indicate mild disturbance of this system but are not interpreted in terms of any specific younger event u pb ages for other localities fall in the interval 1450 1510 m y and indicate that this was a major period of igneous activity throughout the southern great lakes region rb sr results indicate that alteration was ubiquitous and that most ages in the range 1100 1400 m y probably represent disturbed systems the results obtained so far tend to support interpretations in which kentucky indiana and illinois are primarily underlain by a rhyolite and epizonal granite terrane that is 1450 1500 m y old older proterozoic rocks may be absent or more deeply buried

the possible bearing of the granite of the uph deep drill holes northern illinois on the origin of mississippi valley ore deposits the pb pb th pb and u pb systems of whole rock drill core samples of the granite of uph record at least one postcrystallization event treatment of the data on a concordia plot indicate the timing of this disturbance was probably 260 35 m y ago and probably no older than 400 m y ago the upper intercept age on concordia is 1416 20 m y and the pb pb isochron age for the four least disturbed rocks plus the four potassium feldspars is 1451 19 m y these ages are in close agreement with rb sr whole rock and u pb zircon ages determined by others and should be the age of granite formation considerable pb loss is indicated for many of the shallower samples that have a more red rather than pink color probably to a factor of two to three or more as a result of the disturbance although the granite remains pb rich today 40 80 ppm in some other studies of disturbed u th pb systems the radiogenic lead has been taken up by potassium feldspars however the pb for most samples seems to have been lost from the system for the granite of uph one sample with an unusually high pb content of about 195 ppm has the most radiogenic pb of any sample but normal values of th u this sample has obviously gained radiogenic pb by more than a factor of four and confirms that pb was indeed mobilized in parts of the system within the limits of the uncertainties equal amounts of pb were lost or gained in both the u pb and th pb systems which is compatible with mobilization being restricted to lead there is some tendency however for the more affected samples to have lower th u values and higher u contents a complex model in which u is gained pb lost and 208pb lost preferentially to 207pb and 206pb cannot be ruled out u and th have somewhat different crystallographic locations and decay energies so that 208pb might be in more accessible sites than the other pb isotopes fission track mapping of the samples indicates most of the u today is located in submicroscopic sites along edges and cleavages of biotite and iron oxides so u is readily available for remobilization lead extraction with only minor u or th removal may have been accomplished by chloride rich brines lead isotope data on the granite of uph indicates that some of the pb in the ore in the nearby upper mississippi valley lead zinc district may have been derived from these granites an even better match is made with the ores of southeastern missouri although a granite like that of uph would only be one end member most likely the radiogenic end member of a mixed source curiously similar aged and heavy metal rich granites from southeastern missouri are not a good match for theckn nearby ores basement rocks such as the granite of uph must be reconsidered as an important contributor to the ores of the mississippi valley but it is unlikely that they constitute the sole source

pb isotope evidence on the origin of the west shropsh orefield england pb isotope data on ore galenas from the west shropshire orefield show a significant spread of 20 pb 204 pb values with minor variation in 207pb 204 pb and 208pb 204 pb ratios these indicate that lead was derived from multiple sources some of them uranium enriched and incompletely mixed prior to ore deposition four possible mineralizing agents are considered circulating sea water metamorphic waters basinal brines and convecting formation waters pb isotope data exclude a circulating sea water origin for the mineralization and best support a convecting formation water mineralizing agent a model involving a single fluid tapping multiple lead sources is proposed to explain the observed pb isotope variation

widespread buried precambrian layered sequences in the u s mid continent evidence for large proterozoic depositional basins large regions of the north american mid continent are underlain by precambrian layered rocks buried beneath phanerozoic sedimentary strata south of the wichita mountains published seismic reflection profiles show a precambrian layered assemblage extending for at least 40 km in both the north south and east west directions and industry data show that it may continue 150 km to the southeast seismic reflection data in the illinois region show a precambrian layered assemblage extending 320 km in an east west direction and 200 km in a north south direction in both cases the layered rocks are as much as 12 km thick apparent sequence boundaries onlap downlap within these assemblages suggest they are parts of large depositional basins with diffractions and dipping strat due to faulting the layered sequences correlate with regions of relatively long wavelength and low amplitude magnetic anomalies the extent of this magnetic signature suggests that about 200 000 km 2 of illinois indiana and western ohio about 50 000 km 2 of southernmost oklahoma and north central texas and about 32 000 km 2 of southern missouri and northern arkansas may be underlain by similar precambrian strata drill holes indicate that the top of the mid continent precambrian basement is composed largely of silicic igneous rocks such material may comprise a large part of the layered sequences alternatively these igneous rocks could be intermixed with or underlain by nonvolcanic meta sedimentary strata the strong reflectivity of some layers suggest that minor mafic flows and or sills may also be present analysis of u pb and nd sm isotopes within the granites and rhyolites imply that the layered sequences postdate crustal formation at 1 7 2 0 ga and predate or are contemporaneous with the 1 3 1 5 ga crystallization ages of the granites and rhyolites though these layered rocks have a spatial association with igneous rocks and thus have likely been metamorphosed the possibility tha they contain precambrian hydrocarbons that escaped heating is as yet untested

copper and sulfur isotope ratios in paleozoic hosted mississippi valley type mineralization in wisconsin usa abstract we provide the first cu isotope data for chalcopyrite in paleozoic carbonate hosted mississippi valley type mvt mineralization along with corresponding sulfur isotopes for eastern wisconsin usa chalcopyrite in this region occurs mainly as disseminated crystals and is associated with regionally extensive dolomite hosted subeconomic mvt mineralization on the western edge of the ancestral michigan basin a narrow range in 65cu values 0 0 5 for chalcopyrite is observed along a large region 125 km of the western margin of the basin that is consistent with a homogeneous cu source in the crust with a 65cu composition near 0 along with limited redox modification during regional flow or hydrothermal precipitation processes cu and s isotope values in conjunction with existing lead strontium and sulfur isotope data from other sulfide minerals in the region are consistent with a precambrian source similar to parts of the midcontinent rift system but distinct from some other precambrian deposits in the region