u pb evidence for abitibi gold mineralization postdating greenstone magmatism and metamorphism two widely held theories for the origin of archaean lode gold deposits the magmatic1 and metamorphic replacement2 models predict that gold emplacement should have occurred near the end of late syn kinematic greenstone magmatism here we report that at the camflo mine in the abitibi subprovince of canada u pb ages for rutile and titanite in auriferous alteration 2 625 7 myr are much younger than the age of the host quartz syenite 2 680 4 myr and ages for regional magmatism 2 670 myr 3 and metamorphism 2 680 myr 4 mineral associations and pb pb systematics indicate that the hydrothermal minerals record crystallization ages that are probably coeval with the gold mineralization these ages and others from the abitibi subprovince4 6 rule out a direct connection between gold and magmatism or metamorphism associated with regional folding of the greenstone belt they are broadly contemporaneous with metamorphism at deep crustal levels7 and may record late devolatilization of the lower crust

geochemistry of precambrian mafic magmatic rocks of the western himalaya india petrogenetic and tectonic implications abstract precambrian sequences of the higher himalayan crystallines vaikrita group and the lesser himalaya chail jutogh and jaunsar groups in the garhwal and himachal regions of the western himalaya include abundant metamorphosed mafic lavas and dykes amphibolites a gabbroic body within the chail group has been dated at 1907 91 ma initial 87 sr 86 sr ratio of 0 7022 0 0008 by the whole rock rb sr method this age is consistent with several age data 1800 2000 ma whole rock rb sr method assigned to the granitoids and gneisses of this region these amphibolites and gabbros exhibit low ti tholeiite characteristics the lesser himalayan samples are enriched in light rare earth elements lree and large ion lithophile element lile with distinct negative nb sr p and ti anomalies conversely the vaikrita group samples are characterized by less enriched lree lile and absence of the above anomalies but have distinct positive sr anomalies the chemical characteristics of the lesser himalayan samples are remarkably similar to the basal aravalli volcanics of the nw indian shield inferred to reflect significant components from enriched mantle sources and crustal contamination samples of the vaikrita group appear to have been influenced by an asthenospheric mantle and are less contaminated as indicated by the absence of negative nb p and ti anomalies these distinct geochemical characteristics are used to demarcate the main central thrust along the vaikrita thrust the chail and jutogh groups are affiliated with the lesser himalaya and the chail group gabbro body may represent portion of one among the numerous magma chambers that fed a large precambrian magmatic province in the south of the main central thrust the similar emplacement ages 1800 2000 ma for the mafic and felsic magmatic rocks suggest that this region experienced a major episode of crustal generation and evolution in a rift environment

40ar 39ar laser dating of a single grain of magnetite abstract we have determined an 40 ar 39 ar age spectrum for a single grain of magnetite using laser step heating a 300 m magnetite grain from a grenville front gneiss was heated in 14 steps to 1600 c excess 40 ar resulted in a saddle shaped age spectrum with an integrated age of 1962 39 ma however the minimum of the saddle gave an age of 1144 107 ma consistent with typical grenvillian cooling ages of 1100 900 ma we have also dated a single grain of biotite from the same hand sample as the magnetite the spectrum yielded an integrated age of 1261 3 6 ma the thermal release patterns of 40 ar and of ca k 37 ar 39 ar were very different for the magnetite and biotite thus 40 ar can be retained in measurable quantities in magnetite our results are a first step toward the attempted direct dating of different components of nrm in paleomagnetism

rb sr dating of sphalerites from tennessee and the genesis of mississippi valley type ore deposits mississippi valley type mvt ore deposits commonly consist of some combination of lead zinc and iron sulphides accompanied by barite fluorite dolomite and calcite they are thought to form by fluid expulsion from sedimentary successions1 3 but their exact origin has remained controversial because of the scarcity of reliable geochronological data in a recent and widely quoted model the mvt deposits of the central and eastern united states formed as a result of large scale lateral fluid flow driven by the tectonic squeezing of sedimentary successions during the late palaeozoic alleghenian orogeny 330 250 myr 4 here we report the first direct rb sr dating of sphalerites from an mvt deposit and propose this as a useful geochronological technique for sulphide mineralization with poor age control rb sr data for sphalerites from coy mine east tennessee define an isochron age of 377 29 myr ruling out any genetic connection with the alleghenian orogeny instead mvt mineralization in the eastern united states was apparently generated from fluids expelled during thrusting associated with the older acadian orogeny 380 350 myr

botryoidal hematite from the schwarzwald germany heterogeneous uranium distributions and their bearing on the helium dating method abstract neutron induced fission tracks of small grain mounts of two co genetic botryoidal hematites eisenbach hematite manganese mineralization district mittelschwarzwald germany reveal substantial heterogeneous 235 u distributions 235 u fission tracks in the three shells of one hematite yield total uranium concentrations of 26 48 and 160 ppm 2 errors of approximately20 respectively the other hematite consists of a coherent mineralogical unit devoid of shell type features in which four localities have been probed with a range in total uranium between 26 and 600 ppm induced fission tracks from a larger section of the single unit hematite show a subparallel arrangement of five well defined 235 u zones with densities ranging from about 10 6 tracks cm 2 in zone 5 to approximately 25 10 6 tracks cm 2 in zone 2 additionally in zones 3 5 gradual internal track density variations of over 100 can be recognized a traverse shows that there is an overall decrease in radioactivity from zone 1 to zone 5 an autoradiograph confirms the existence of five zones with different activities and allows correlation of the high activity zone 2 with a 0 5 mm thin internal hematite micro layer analyses by mass spectrometer of the triple shell hematite resulted in the following concentrations 26 45 and 166 ppm uranium 2 errors of1 and 0 55 3 25 l g 4 he 2 errors of3 the four probed localities in the single unit hematite show smaller variations at 40 73 ppm uranium 2 errors of0 5 and 0 75 1 37 l g 4 he 2 errors of3 spectrometry of thorium yields values in the 1 5 ppm range for both hematites the uranium and 4 he concentration differences measured allow construction of two internal u th he isochrons yielding identical ages 2 error of 156 1 ma and 155 6 ma concordance of both isochrons in our opinion is strong evidence for a geologically significant age we interpret the data as emplacement ages further we suggest that botryoidal hematite has the potential to be a prolific chronometer for the helium method on mesozoic hematite bearing mineralizations

palaeomagnetic estimates of crustal shortening in the himalayan thrusts and zangbo suture this article reports new palaeomagnetic results from precisely dated late palaeocene limestones from the former northern margin of the indian subcontinent now in tibet between the major himalayan thrusts and the india tibet suture zone convergence in the suture and intracontinental thrusting can be estimated separately and are respectively found to be of the order of 550 km 650 km and 400 km 400 km comparison with kinematic models derived from magnetic anomalies constrains the age of the collision at about 50 myr

effective stress response of a plane sea bed under wave loading this note describes the determination for a wide class of wave pressure forms of the response to waveloading of an infinite half space consisting of a porous elastic soil skeleton under such loading no volumetric strains occur in the soil mass and there is no coupling between the pore water and the skeleton also an analytical relationship has been established between stresses and pore pressures so that the computation of the pore pressures is sufficient to solve the complete effective stress problem the paper gives an integral formulation for the pore pressure as a function of the wave pressure

the effect of temperature on metal mobility in subseafloor hydrothermal systems constraints from basalt alteration experiments abstract crystalline basalt was reacted with a na ca k cl fluid of seawater chlorinity at 400 350 and 300 c 400 bar and a fluid rock mass ratio of 1 to assess the effect of temperature on metal mobility in ridge crest hydrothermal systems basalt alteration at 400 c yielded a fluid chemistry characterized by high zn cu fe mn sio 2 h 2 s and h 2 concentrations lowering temperature to 350 and then to 300 c caused dissolved metal concentrations to decrease drastically black smoker vent fluids generally have measured temperatures 355 c the dissolved concentrations of zn cu fe and mn observed in these fluids however could only be achieved or exceeded during the 400 c phase of the experiment indicating that hot spring fluids have lost heat and possibly dissolved fe and base metals on ascent to the seafloor from a high temperature reaction zone the relative rates at which zn cu fe and mn achieve new steady state concentrations in response to temperature reduction during basalt alteration are cu fe zn mn thus dissolved zn and especially mn concentrations are more likely to record accurately temperatures characterizing subseafloor reaction zones while fe and cu which respond rapidly to changes in temperature are likely to precipitate along fluid flow paths especially in near seafloor locations this trend is confirmed by the relative abundance of dissolved metals in hot spring fluids variably influenced by heat loss processes calculations suggest that hot spring fluids may lose heat by conduction through sulfide mounds and chimney walls at flow rates observed for many vent systems since heat loss processes influence the chemistry especially dissolved metals as well as measured temperatures of axial hot springs fluids these effects must be taken into account to model accurately phase relations and fluid mineral equilibria in deeply seated subseafloor reaction zones

age and duration of weathering by 40k 40ar and 40ar 39ar analysis of potassium manganese oxides supergene cryptomelane k1 2 mn3 mn4 8o16 h2o samples from deeply weathered pegmatites in southeastern brazil subjected to 40k 40ar and 40ar 39ar analysis yielded 40k 40ar dates ranging from 10 1 0 5 to 5 6 0 2 ma million years ago laser probe 40ar 39ar step heating of the two most disparate samples yielded plateau dates of 9 94 0 05 and 5 59 0 10 ma corresponding within 2 to the 40k 40ar dates the results imply that deep weathering profiles along the eastern brazilian margin do not reflect present climatic conditions but are the result of a long term process that was already advanced by the late miocene weathering ages predate pulses of continental sedimentation along the eastern brazilian margin and suggest that there was a time lag between weathering and erosion processes and sedimentation processes

4he diffusion in 40ar retentive minerals abstract 4he vacuum diffusion experiments were carried out by stepwise degassing minerals normally used in k ar chronometry such as hornblende nepheline and others the study aims at 4he diffusion parameters for uppermost crustal temperatures and thus at he retentivities of the minerals the results indicate long time near quantitative 4he retention in hornblende pyroxene langbeinite and perhaps nepheline for 0 5 mm grains at least at temperatures below about 70 c sanidine and muscovite lose 4he over geologic periods at lower temperatures only the minerals of the first group are possible u th he chronometers or geothermometers provided that other dating problems such as alpha loss and u migration can be settled closure temperatures for rapidly cooling 2 mm hornblende crystals are found to be in the range of 190 to 170 c tentative u he indices indicate that diffusion parameters of the same order of magnitude as determined for the he retentive minerals may be used to select minerals to be further examined for their u th he datability e g ore minerals

geochemistry and petrogenesis of late archaean aravalli volcanics basement enclaves and granitoids rajasthan abstract major trace and rare earth element data are presented for the basal volcanics of the aravalli supracrustal sequence for the mafic enclaves within the banded gneiss complex bgc adjacent to the supracrustals and for granitoids within the bgc and their petrogenesis discussed with regard to the problematic relationship between basement and cover in terms of major elements the aravalli lavas range from magnesian 22 mgo komatiites or picrites to fe rich tholeftes but differ from a typical archaean greenstone volcanic sequence in that their trace element patterns are relatively enriched in incompatible and light rare earth elements moreover the range of patterns observed requires a number of distinct mantle sources as well as different degrees of melting there are two distinct types of mafic enclave within the bgc the larger lenses correspond quite well with the range of variation seen in the aravalli sequence and could be feeders the other type close to the bgc aravalli junction has more alkalic characteristics high nb zr and could represent rift volcanics or tectonically accreted fragments of ocean islands the data suggest that the aravalli sequence is not an accreted oceanic terrane but are more consistent with interaction of a deep mantle plume with rather mature sub continental lithosphere the early archaean age of parts of the bgc the chemistry of the granitoids and the alkaline character of the enclaves are also broadly compatible with this model the character of the aravalli sequence is intermediate between that of an archaean greenstone belt and a phanerozoic flood basalt province

aspects of the proterozoic geochronology of northern sweden abstract it has been proved that u pb zircon dating provides reliable formation ages for magmatic rocks affected by medium or lower grade metamorphism the timing of the main volcanic plutonic and metamorphic episodes of the early to mid proterozoic evolution of northern sweden has therefore been verified a preliminary and schematic chronology of these events is presented archaean ortho and para gneisses 2 70 ga supracrustal rocks affected by the svecokarelian orogeny mafic volcanic phase 2 20 1 93 ga felsic volcanic phase 1 91 1 86 ga svecokarelian orogeny 1 88 1 80 ga post orogenic migmatization 1 80 ga

geochemistry and petrogenesis of garhwal volcanics implications for evolution of the north indian lithosphere abstract middle proterozoic magmatism in the garhwal himalayas consists of an extensive sequence of mafic volcanics plus associated mafic dyke sheets geochemical data reveal that the volcanic suite is tholeiitic with the dyke sheets showing considerably more fe enrichment than the lava flows all the rocks are distinctly enriched in incompatible elements and light ree relative to primordial mantle abundances but have an equally distinct continental signature reflected in marked negative nb sr p and ti anomalies in their mantle normalised spidergrams although such geochemical characteristics are commonly thought to indicate contamination of the magmas with a continental crust component the evidence is strongly in favour of the compositional characteristics being inherited from their mantle source other proterozoic volcanics in northern india have closely similar geochemical features as have other classic occurrences of younger continental flood basalt suites in adjacent areas of gondwanaland these geochemical characters are probably imposed upon the subcontinental lithosphere at the time of continent formation but in the case of younger cfb suites this may have been modified to a greater or lesser extent by subsequent additions of plume mantle material

high field strength element geochemistry of mafic intrusive rocks from the bhagirathi and yamuna valleys garhwal himalaya india abstract incompatible high field strength element geochemistry divides mafic intrusive rocks of garhwal himalaya exposed in the bhagirathi and yamuna valleys into two distinct types viz older garhwal mafic intrusive rocks gmir1 and younger garhwal mafic intrusive rocks gmir2 gmir1 is mainly associated with the central crystallines whereas gmir2 belongs to the garhwal group they outcrop close to the main central thrust mct zone and extend in a nw se direction following the strike direction both types are metamorphosed to amphibolite facies and classified as sub alkaline high fe tholeiitic basalts they are enriched in large ion lithophile and high field strength elements relative to a primitive mantle source gmir2 has higher concentrations of incompatible high field strength elements than the gmir1 further on multi element spidergrams gmir1 samples do not show any elemental anomaly but gmir2 samples show distinct negative nb and sr anomalies gmir1 multi element and rare earth element trends are similar to n morb whereas gmir2 follows trends observed in the within plate basalts continental flood basalts type discrimination diagrams based on incompatible trace elements corroborate the n morb nature of gmir1 and cfb wpb nature of gmir2 geochemical modeling indicates that these mafic intrusive rocks were derived by close system fractional crystallization of depleted for gmir1 to variably enriched parental basalts for gmir2 probably these two contrasting mafic rock types have been juxtaposed during the himalayan orogeny proterozoic mafic rocks of similar geochemical characteristics have been reported from different parts of the northern indian lithosphere observed composition suggests its genetic association with the sub continental lithosphere rather than effect of crustal contamination

the study of molybdenites through the187re 187os chronometer we determined the re and os concentrations in 11 molybdenites of various ages 30 2700 m y and from various areas no common osmium has been detected and os was found to be purely radiogenic therefore by comparing the concentrations we were able to determine 187 re 187 os ages for most samples these ages are in good agreement with those measured in surrounding rocks and probably correspond to the age of mineralization however two or three re os ages seem unreliable being probably related to some rhenium leaching through later metamorphic and or hydrothermal events this shows that the petrographic characteristics of ores and the type of crystallization of mos 2 are important in choosing samples suitable for dating re and os determinations combined with careful petrographic investigations may give important clues on the genesis and evolution of molybdenum ores

cassiterite potential for direct dating of mineral deposits and a precise age for the bushveld complex granites u pb and pb isotopic measurements of cassiterite from the 200 ma tin deposits of belitung indonesia and 2100 ma zaaiplaats deposit in south africa demonstrate that cassiterite can be used to directly date mineral deposits rather than associated minerals such as zircon and rutile a precise u pb age of 2099 3 ma has been determined for cassiterites hosted in granites from the bushveld complex on conventional 207 pb 204 pb 206 pb 204 pb plots the data for the indonesian deposits give an age apparently older than that obtained from u pb plots perhaps indicative of inheritance of pb from an older source

excess40ar in metamorphic rocks from broken hill new south wales implications for40ar 39ar age spectra and the thermal history of the region 40ar 39ar age spectrum analyses of samples from broken hill new south wales indicate that the region has experienced a complex thermal history following high grade metamorphism 1660 ma ago the terrain cooled slowly 3 c ma 1 until about 1570 ma ago when the temperature fell below about 500 c following granitoid emplacement 1500 ma ago the region remained relatively cold until affected by a thermal pulse 520 40ma ago causing temperatures to rise to 350 c in some places during this event accumulated40ar was released from minerals causing a significant ar partial pressure to develop laboratory ar solubility data combined with the40ar 39ar age spectra gives a local estimate of this partial pressure of 10 4atm the region finally cooled below 100 c about 280 ma ago 40ar 39ar age spectrum analyses of hornblende plagioclase and clinopyroxene containing excess40ar are characterized by saddle shaped age spectra detailed analysis of plagioclase samples reveals a complex diffusion behaviour which is controlled by exsolution structures this effect in conjunction with the presumed different lattice occupancy of excess40ar with respect to radiogenic40ar appears to be responsible for the saddle shaped age spectra

u pb dating of columbites a geochronologic tool to date magmatism and ore deposits we have developed techniques for the u pb analysis of the mineral columbite fe mn ta nb 2o6 and report u pb ages obtained for three early proterozoic columbites from the baltic shield of northern sweden the u pb ages of these columbites agree with other available geochronological data u pb dating of columbite is therefore a potentially powerful tool in establishing reliable ages of pegmatites alkaline and carbonatitic intrusions and ore deposits of sn w and rees all of which often contain columbite furthermore columbite can be used to date peraluminous granites as it often occurs within li p ree pegmatites associated with such granites

40ar 39ar ages of precambrian manganese ore minerals from sweden india and morocco in order to test their chronometric potential 40ar 39ar stepheating and 4he analyses have been carried out on five manganese ore minerals of the hollandite cryptomelane series from three precambrian manganese deposits ultevis sweden sitapar india bachkoun morocco samples from the metamorphic occurrences ultevis and sitapar yielded ar ages of 1 8 ga and 0 95 ga interpreted as the age of postmetamorphic cooling hollandites ultevis and of an early k introducing alteration process subsequent to amphibolite facies metamorphism cryptomelanes sitapar both data are consistent with known chronologies of the svecokarelian and satpura orogenic cycles a date of 670 ma obtained for a hollandite from a volcanogenic vein deposit bachkoun however contrasts with published extrusion ages of 580 560 ma for the volcanic host rocks ouarzazate series probably due to incorporation of excess argon the use of the 40ar 39ar technique together with multiple isotope systematics made it nevertheless possible to establish a reasonable estimate of a mineralization age close to 580 ma measurement of fractional ar losses during vacuum step heating 500 1600 c although indicating good ar retentivities failed to define model diffusion parameters because of non linear arrhenius arrays helium diffusion results 200 1200 c indicated retention of radiogenic 4he by the samples corroborated by u he mineral dates between 0 96 and 0 31 ga potassium bearing manganese oxides are therefore able to retain argon possibly also 4he through geological times and may thus provide ages of ore forming processes and perhaps later cooling and alteration stages

the source and significance of argon isotopes in fluid inclusions from areas of mineralization argon isotopes in fluid inclusions in quartz veins associated with granite hosted tungsten mineralization in the southwest and north of england have been investigated in detail by the40ar 39ar technique the natural argon is present as a number of discrete components which can be identified through correlations with39ar 38ar and 37ar induced by neutron bombardment of potassium chlorine and calcium the potassium correlated component arises principally from in situ decay of potassium in solid phases in the inclusions in the case of the hemerdon tungsten deposit of southwest england the phases responsible are small 25 m captive authigenic micas which are shown to have been deposited from a fluid 268 20 ma ago shortly after the emplacement of the host granite the chlorine correlated component is present in the brines which constitute the fluid phase of the inclusions the argon in these hydrothermal fluids is made up in part of parentless or excess 40ar leached from surrounding crustal rocks and in part of dissolved ancient atmospheric argon absolute concentrations of both atmospheric and excess components in the brine can be estimated from 40arcl ratios and independent determinations of the salinity of the inclusions the absolute concentrations of the atmospheric argon are close to those found in modern meteoric water while those of the excess component can be interpreted in terms of the degree of interaction betwen the circulating fluids and country rock a calcium correlated component with a much higher ratio of excess to atmospheric argon than that in the brine was found to be a dominant phase in one sample from the hemerdon deposit indicating the presence of a solid phase probably a caso4 daughter mineral inclusions of this composition represent fluids which have had a more prolonged interaction with crustal rocks the results obtained from this study provide a systematization and a framework for future multi component argon studies of fluid inclusions together with an indication of the wide range of information which can be inferred

geochemistry and petrogenesis of early proterozoic amphibolites west central colorado u s a abstract early proterozoic supracrustal rocks exposed in west central colorado include amphibolites intimately associated with felsic volcanics and associated volcaniclastic sediments three successions are recognized the dubois succession 1770 1760 ma is comprised chiefly of amphibolites and felsic volcanics the amphibolites exhibit low concentrations of light ree and hfse and are broadly similar in composition to tholeiites from immature island arcs and in some respects to nmorb the cochetopa succession 1745 1730 ma includes large amounts of felsic volcanics and volcaniclastic sediments together with amphibolites that exhibit variable but slight incompatible element enrichment similar to basalts from evolved island arc systems and associated back arc basins the black canyon succession is coeval with the cochetopa succession and is comprised chiefly of volcaniclastic sediments with minor amphibolite these amphibolites exhibit significant incompatible element enrichment similar to basalts from continental margin arc systems geochemical modeling indicates that the amphibolites evolved by closed system fractional crystallization of depleted for the dubois succession to variably enriched parental basalts for the cochetopa and black canyon successions chiefly at shallow depths incompatible element distributions in the amphibolites indicate that mantle sources changed with time from relatively depleted 1770 1760 ma to variable but slightly enriched 1745 1730 ma a provisional model calls upon the dubois succession forming in an island arc system followed by collision with the archean wyoming province at 1750 ma the cochetopa succession was deposited in an intra arc basin developed within a new continental margin arc and the black canyon succession formed in a back arc basin associated with this arc