proterozoic contraction extension tectonics of the southern sao francisco region minas gerais brazil the southern sao francisco region comprises the southern portion of the sao francisco craton which is underlain by archean basement and its fringing proterozoic orogenic belts included in this region are three geologic provinces the quadrilatero ferrifero qf the sao francisco basin and the cordilheira do espinhaco we synthesize preliminary results of ongoing structural analysis reconnaissance field studies and published reports to provide a concise review of the geology of the region and to propose a working tectonic model for its proterozoic evolution interpretation of superimposed folds faults and foliations leads to the conclusion that the southern sao francisco region records the effects of four principal tectonic events that occurred subsequent to deposition of the lower proterozoic minas supergroup the first event resulted in formation of a northwest verging fold thrust belt the second event led to development of high angle reverse faults open folds and basement uplift the third event is manifested in the qf by the formation of normal faults and by the intrusion of mafic dikes these extensional structures may be associated with formation of the sedimentary basins east and south of the sao francisco craton the final event resulted in formation of a west verging fold thrust belt that affected the qf the cordilheira espinhaco and the sao francisco basin the events that we describe can be tentatively associated with regionally recognized orogenic periods trans amazonian uruacuano brasiliano

isotopic constraints on the structural relationships between the lesser himalayan series and the high himalayan crystalline series garhwal himalaya nd and sr isotope systematics may provide important constraints on the location of major thrust systems that separate lithologically similar sedimentary sequences the potential of the technique is illustrated by this isotopic study of the main central thrust system of the himalaya nd isotope data from the garhwal himalaya indicate that metasedimentary rocks from the vaikrita group nd 14 to 19 correlate closely with those from the high himalayan crystalline series which constitutes the hanging wall lithologies of the main central thrust in contrast metasedimentary rocks from the munsiari group nd 23 to 28 show marked similarities to the lesser himalayan series in the footwall of the main central thrust sr isotopes support the correlations in that the vaikrita group shows partial reequilibration at 500 ma whereas the munsiari group has not undergone sr isotope homogenization since 1800 ma thus the vaikrita thrust that juxtaposes these two formations is recognized as the main central thrust in garhwal himalaya the thrust coincides approximately with the location of the kyanite isograd confirming that inverted metamorphism is characteristic of both hanging wall and footwall of the main central thrust along the tons thrust known locally as the srinagar thrust 50 km south of the main central thrust low grade quartzarenites with nd sr isotope and trace element characteristics typical of lesser himalayan formations have been emplaced on phyllites and siltstones with geochemical characteristics of the high himalayan crystalline series the field relationships most probably result from out of sequence thrusting in which lesser himalayan series rocks to the north were emplaced over low grade equivalents of the high himalayan crystalline series preserved in the external part of the orogen this study establishes the value of isotope data for lithostratigraphic correlations within orogenic belts

behavior of gold in the lateritic equatorial environment weathering and surface dispersion of residual gold particles at dondo mobi gabon abstract a supergene gold dispersion halo has developed at dondo mobi southern gabon from the weathering of an auriferous lisvenite under equatorial humid rain forest conditions morphological and chemical studies were performed on gold particles extracted from both fresh and weathered rocks from different parts of the dispersion halo the results demonstrate that the gold particles are residual and are subjected to increasing weathering upward through the profile from the protore to the surface in the central part of the halo and at the surface both laterally upslope and downslope weathering continues downward into the formerly barren saprolites which were derived from proterozoic schist and archean gneiss rim of the halo thus the au content progressively decreases as does the ag content of gold particles with distance from the center of the ore body gold particles subjected to increased weathering are smaller with more rounded shapes and surfaces pitted by dissolution the dispersion pattern is controlled by au dissolution and translocation of residual gold particles these processes involve an autochthonous evolution in the central part of the halo and a para autochthonous evolution at the rim of the halo at dondo mobi chloride and humic substances are the only ligands present in significant concentrations that are capable of complexing au thermodynamic calculations demonstrate that under such acidic conditions ph ranges from 4 to 6 gold stability increases with increasing silver content the dominant au complexes formed have been calculated to be an aurous hydroxichloride complex and an organometallic complex both of which are stable at surficial conditions thus both chemical and translocation processes control the mobility of au and gold particles in equatorial lateritic weathering profiles

chemical and physical transfers in an ultramafic rock weathering profile part 1 supergene dissolution of pt bearing chromite chemical weathering and supergene dissolution processes of pt bearing chromite have been studied in a lateritic weathering profile developed on ultramafic rocks in new caledonia southwest pacific the chemical distributions of alkaline earth transition metals and precious metals including pt and pd were determined in a weathering profile varying from bedrock at the base upward through coarse and fine saprolites and capped by a mottled zone and a lateritic colluvial nodular horizon chemi cal analyses and mass balance calculations suggest that progressive weathering of the parent rock is characterized by an enrichment of fe co and mn a segregation of ni at the boundary between the bedrock and the coarse sapolite and in the lower part of the fine saprolite and a depletion of mg ca si al and cr the higher concentration of transition metals at the interface between the coarse and fine saprolite is due to vertical transfer and precipitation at the base of the weathering profile in such a lateritic environment the pt bearing chromite grains are progressively dissolved and the pt group minerals pgm are released in the weathering mantle with a preferential depletion of pd with regard to pt

isotope studies reveal a complete himalayan section in the nanga parbat syntaxis many models of orogenesis invoke simple anatomies for mountain belts comprising a small number of major tectonic provinces separated by major faults in the himalayan arc three main tectonic provinces the lesser himalayan high himalayan and tethyan series have been recognized over 2500 km along strike from bhutan to kashmir however their extension westward to the nanga parbat syntaxis remains equivocal we have supplemented detailed field work in the area with isotopic analysis aimed at revealing distinct signatures for each of the three main provinces using sr isotopic data to refine previous nd based discrimination we demonstrate that the three main tectonic provinces of the central himalaya also occur in the western syntaxis of northern pakistan these three units are thus continuous along the entire orogenic arc however their metamorphic grade is generally higher in the syntaxis than in the central himalaya challenging the validity of distinctions commonly drawn on this basis elsewhere in the mountain belt the corollary is that these high grade units probably continue beyond the syntaxis into northwestern pakistan which suggests that although the precollisional materials may be identical the western syntaxis marks a change in tectonic style from the main orogen this conclusion in turn requires that the burial and exhumation history in the western himalaya be radically different from that in the central himalaya and thus necessitates a reexamination of models for india asia collision

correlation of lithotectonic units across the eastern himalaya bhutan clastic sediments deposited in foreland basins and offshore fans allow the evolution of an orogen to be reconstructed provided their source regions are properly characterized isotopic data from the lithotectonic units of the eastern himalaya bhutan indicate clear isotopic differences directly comparable with the equivalent units from the central himalaya zircons from metaquartzites of the high himalayan series that range in age from 980 to 1820 ma and an orthogneiss intruded at 825 9 ma bracket deposition to between 816 and 980 ma model ages derived from nd isotopes of associated metapelites range from 1700 to 2200 ma in contrast zircons from a metaquartzite from the lesser himalayan series range from 1850 to 2550 ma a paleoproterozoic deposition age ca 1750 ma is inferred from the age of a metarhyolite associated with the sediments model nd ages of metapelites from the lesser himalayan series range from 2500 to 2600 ma the bhutanese himalayan units can be correlated more than 1000 km westward along strike from catchments of the brahmaputra to the headwaters of the sutlej on the basis of their structural position provenance and isotopes because current discharge from the brahmaputra carries the dominant sedimentary flux into the bengal fan results from this work validate the interpretation of isotopic variations observed in offshore deposits in terms of the unroofing history of the component lithologies of the himalaya

cenozoic anatexis and exhumation of tethyan sequence rocks in the xiao gurla range southwest tibet article i nfo in order to advance our understanding of the suturing process between continental landmasses a geologic and geochronologic investigation was undertaken just south of the india asia suture in southwestern tibet the focus of this study the xiao gurla range is located near the southeastern terminus of the active right lateral strike slip karakoram fault in southwestern tibet the range exposes metasandstone phyllite schist locally of sillimanite facies calc gneiss and marble paragneiss pyroxene quartzite metagranite and variably deformed leucogranite these metamorphic rocks are exposed in the footwall of a domal top to the west low angle normal detachment fault structurally beneath neogene quaternary basin fill and serpentinized ultramafic rocks of the kiogar jungbwa ophiolite the detachment is interpreted to be the northeastern continuation of the gurla mandhata detachment fault system that bounds metamorphic rocks of the gurla mandhata range 60 km to the southwest u pb geochronology on five detrital zircon samples of schist phyllite and quartzite yielded maximum depositional ages that range from 644 363 ma and age probability distributions that are more similar to tethyan sequence rocks than lesser himalayan sequence rocks a felsic gneiss yielded a metamorphic zircon age of 35 3 0 8 ma with a significant population of early paleozoic xenocrystic core ages the gneiss is interpreted to be the metamorphosed equivalent of the cambro ordovician gneiss that is exposed near the top of the greater himalayan sequence leucogranitic bodies intruding metasedimentary footwall rocks yielded two distinct u pb zircon ages of 23 ma and 15 ma locally rocks exposed in the hanging wall of this fault and of the southward dipping northward verging great counter thrust to the north consist of serpentinite bearing melange and conglomerate of inferred paleogene age dominated by carbonate clasts the melange is intruded by a 44 ma granite and the stratigraphically highest conglomerate unit yielded detrital zircon u pb ages similar to tethyan sequence rocks we attribute the middle eocene magmatism south of the suture to break off of the neo tethyan oceanic slab in addition our observations are consistent with the late eocene shortening and crustal thickening within the tethyan himalayan sequence early middle miocene leucogranite emplacement being related to underthrusting and melting of the greater and possibly lesser himalayan sequences and late miocene arc parallel extension in the hinterland of the southward propagating himalayan thrust belt

gold a tracer of the dynamics of tropical laterites laterites have developed essentially from long term exposure of cratonal rocks to the atmosphere and hydrosphere the properties of gold density and relative insolubility make residual gold particles tracers of surficial processes causing mineralized laterites gold particles are useful in distinguishing between in situ chemical weathering and physical transport in african lateritic systems developed on slowly eroding stable landforms in an equatorial rainforest setting chemical dissolution and translocation of fine particles are the dominant processes episodic alluvial and or colluvial transport appears to contribute to laterite formation in the subsahelian steppe the use of gold particles coupled with the examination of in situ 10 be from auriferous quartz indicates the applicability of such methods for examination of lateritic weathering dynamics of the tropical transcontinental lithosphere

neogene foreland basin deposits erosional unroofing and the kinematic history of the himalayan fold thrust belt western nepal sedimentological and provenance data from the lower miocene pliocene dumri formation and siwalik group in western nepal provide new information about the timing of thrust faulting and the links between erosional unroofing of the himalaya and the cenozoic 87 sr 86 sr record of the ocean in western nepal the dumri formation is an 750 1300 m thick fluvial sandstone and overbank mudstone unit the siwalik group is 4200 m thick and consists of a lower member 850 m of 2 12 m thick fluvial channel sandstones and oxidized calcareous paleosols a middle member 2400 m of very thick 20 m channel sandstones and mainly organic rich histosols and an upper member 1000 m composed of gravelly braided river deposits paleocurrent data indicate that middle miocene pliocene rivers in western nepal flowed southward transverse to the thrust belt throughout deposition of the siwalik group no evidence was found for an axial fluvial trunk system i e the paleo ganges river in siwalik group sandstones a major increase in fluvial channel size is recorded by the transition from lower to middle siwalik members at 10 8 ma probably in response to an increase in seasonal discharge modal petrographic data from sandstones in the dumri formation and the siwalik group manifest an upsection enrichment in potassium feldspar carbonate lithic fragments and high grade metamorphic minerals modal petrographic analyses of modern river sands provide some control on potential source terranes for the miocene pliocene sandstones the dumri formation was most likely derived from erosion of sedimentary and low grade metasedimentary rocks in the tibetan tethyan himalayan zone during early miocene emplacement of the main central thrust the presence in dumri sandstones of plagioclase grains suggests exposure of crystalline rocks of the greater himalayan zone perhaps in response to tectonic unroofing by extensional detachment faults of the south tibetan detachment system during deposition of the lower siwalik group 15 11 ma emplacement of the dadeldhura thrust sheet one of the synformal crystalline thrust sheets of the southern himalaya on top of the dumri formation supplied abundant metasedimentary lithic fragments to the foreland basin a steady supply of plagioclase grains and high grade minerals was maintained by deeper erosion into the main central thrust sheet from 11 ma to the present k feldspar sand increased steadily suggesting that granitic source rocks became widely exposed during deposition of the upper part of the lower siwalik group this provenance change was caused by erosion of passively uplifted granites and granitic orthogneisses in the dadeldhura thrust sheet above a large duplex in the lesser himalayan rocks since the onset of deposition of the conglomeratic upper siwalik group 4 5 ma fault slip in this duplex has been fed updip and southward into the main boundary and main frontal thrust systems we obtained 113 u pb ages on detrital zircons from modern rivers and siwalik group sandstones that cluster at 460 530 ma 850 1200 ma 1 8 2 0 ga and 2 5 ga an abundance of cambrian ordovician grains in the siwalik group suggests sources of siwalik detritus in the granites of the dadeldhura thrust sheet and possibly the greater himalayan orthogneisses the older ages are consistent with sources in the greater and lesser himalayan zones an overall upsection increase in zircons older than 1 7 ga suggests increasing aerial exposure of lesser himalayan rocks none of the detrital zircons even in the modern river samples yielded a cenozoic age that might suggest derivation from the cenozoic greater himalayan leucogranites but this may be attributable to the inheritance problems that characterize the u pb geochronology of the leucogranites when compared with recent studies of the 87 sr 86 sr composition of paleosol carbonate nodules and detrital carbonate in paleosols from the siwalik group the provenance data suggest that erosion and weathering of metamorphosed carbonate rocks in the lesser himalayan zone and cambrian ordovician granitic rocks of the crystalline thrust sheets in central and eastern nepal may have played a significant role in elevating the 87 sr 86 sr ratio of middle miocene synorogenic sediments in the indo gangetic foreland basin and the bengal fan as well as global seawater

platinum enrichment at low temperatures and related microstructures with examples of hongshiite ptcu and empirical pt2hgse3 from itabira minas gerais brazil low temperature pt enrichment is illustrated with reference to two hypogene platiniferous minerals i a pd cu bearing pt2hgse3 an unnamed phase and ii hongshiite ptcu with up to 0 7 wt hg the former has patches oxidized to a pt o species with curvilinear microcracks originated by desiccation the latter displays parallel sets of cu depleted pt hg enriched patches along which microfractures occur such microstructures are interpreted to arise from two processes i oxidation and formation of a hydrated pt o compound which tends to succumb to a pt rich alloy by deoxygenation and dehydration leaving behind curvilinear desiccation cracks ii metal leaching conducive to native pt by removal of cu from hongshiite via micropores that coalesce along crystallographic planes to result in reticulated microcracks platinum enrichment either by oxidation or metal leaching ideally produces characteristic microcracks which may be indicative of the origin i e detrital vs supergene of alluvial and eluvial pt rich alloys

brittle microstructures in gold nuggets a descriptive study using backscattered electron imaging abstract gold nuggets from specular hematite rich sulphide free auriferous veins jacutinga at gongo soco an iron ore deposit in the quadrilatero ferrifero of minas gerais brazil have inclusions of palladium arseno antimonides isomertieite and mertieite ii and hematite both isomertieite and mertieite ii are commonly fractured and their fragments are displaced in relation to each other without any optically recognisable brittle failure of the host gold more rarely isomertieite has sub parallel s shaped intragranular microfractures terminating in angular edges hematite shows microfracturing and displacement as well as microgouge formation and slumping it occasionally exhibits deformation twins microcracking also occurs in gold itself goethite seals microcracks in gold with or without angular fragments of goethite crack sealing in nugget gold by hg pd enriched gold hematite kaolinite infill indicates recurrent fracturing and mineralisation although the microstructures are enigmatic two hypotheses are suggested for their formation i fluid pressure increase and its sudden seismogenic release could fracture erstwhile ductile gold ii the ductile gold transferred tectonic stresses to the harder mineral inclusions which deformed in a brittle manner

active lower crustal deformation and himalayan seismic hazard revealed by stream channels and regional geology abstract thick tibetan crust stores energy driving large earthquakes along its margins at its southern margin it is hypothesized that its lower crust is extruded between two shear zones such that strain in the ductile lower crust is fed into the higher seismogenic portions of the crust predicting rock uplift between these structures the upper shear zone is regionally referred to as the south tibetan detachment std and the lower shear zone as the main central thrust mct we use field mapping and ksn channel steepness analysis to test this hypothesis in northwestern nepal within a notable gap in historic seismicity here we show an 170 km 40 km swath of rapid rock uplift at the southern margin of the tibetan plateau that overlaps a regional anticline suggesting active folding the area of rapid uplift coincides with peak interseismic strain determined by gps and overlies the inferred locked to creeping transition along the india asia plate boundary the fold is cored by thick ductiley deformed crust and defined by a shear zone interpreted as the std because the std is folded it cannot facilitate active extrusion of a mid crustal channel a regional cross section across the fold shows that while the uppercrust has little post miocene shortening the lower middle crust is anomalously thick we explain this by duplexing whereby thrust sense shear zones stack ductile lower middle crust beneath the std north downdip of the locked to creeping transition the plate boundary here last ruptured during the mw 8 2 ad 1505 earthquake whose inferred rupture area encompasses the region of rapid rock uplift shown here predicting a high slip potential integrating the structural geology and ksn analysis with microseismicity patterns and the 1505 surface rupture suggests that in western nepal the locked to creeping transition is 40 km wide and lies 100 120 km north of the himalayan thrust front

platinum group elements in impactites of the icdp chicxulub drill core yaxcopoil 1 are there traces of the projectile this study presents results of platinum group element pge analyses of impactites from the yaxcopoil 1 yax 1 and yucatan 6 drill cores of the 180 km diameter chicxulub crater these are the main elements used for projectile identification they were determined by nickel sulfide fire assay combined with inductively coupled plasma mass spectrometry the concentration of pge in the samples are low the concentration patterns of the suevite samples resemble the pattern of the continental crust we conclude that any meteoritic fraction in these samples is below 0 05 a syn and post impact modification of the pge pattern from meteoritic toward a continental crust pattern is very unlikely the globally distributed fallout at the cretaceous tertiary k t boundary however has high pge concentrations therefore the lack of a significant meteoritic pge signature in the crater is not an argument for a pge poor impactor taking the results of three dimensional numerical simulations of the chicxulub event into account the following conclusions are drawn 1 the main fraction of the impactor was ejected into and beyond the stratosphere distributed globally and deposited in the k t boundary clay and 2 the low amount of projectile contamination in the yax 1 lithologies may reflect an oblique impact however the role of volatiles in the mixing process between projectile and target is not well understood and may also have played a fundamental role

early weathering of palladium gold under lateritic conditions maquine mine minas gerais brazil since the 80 s studies have shown that au is mobile in supergene lateritic surficial conditions they are based either on petrological thermodynamic studies or experimental works in contrast few studies have been done on the mobility of the pt group elements pge moreover at the present time no study has addressed the diaerential mobility of au ag and pd from natural alloys in the supergene environment the aim of this study is to understand the supergene behavior in lateritic conditions of au ag pd alloys of the au ore locally called jacutinga at the maquinemine iron quadrangle minas gerais state brazil the field work shows that the host rock is a lake superior type banded iron formation bif and that the au mineralization originates from sulfide barren hydrothermal processes primary ag pd bearing au has developed as xenomorphous particles between hematite and quartz grains the petrological study indicates that the most weathered primary au particles with rounded shapes and pitted surfaces were found under the duricrust within the upper friable saprolite this layer however is not the most weathered part of the lateritic mantle but it is where the quartz dissolution resulting porosity is the most developed the distribution of au contents in the weathered rocks are controlled by the initial hydrothermal primary pattern no physical dispersion has been found most of the particles are residual and very weakly weathered this characterizes early stages of au particle weathering in agreement with the relatively low weathering gradient of the host itabiritic formations that leads essentially to the development of isostructural saprolite lateritic mantle limited dissolution of primary au particles issued from the friable saprolite induces pd ag depleted rims compared to primary au particle pd ag contents in addition limited very short distance in situ dissolution reprecipitation processes have been found at depth within the primary mineralization as illustrated by tiny supergene almost pure au particles the supergene mobility order pd ag au as reflecting early weathering stages of au ag pd alloys under lateritic conditions is proposed 1999 published by elsevier science ltd all rights reserved

stratigraphy structure and tectonic evolution of the himalayan fold thrust belt in western nepal regional mapping stratigraphic study and 40ar 39ar geochronology provide the basis for an incremental restoration of the himalayan fold thrust belt in western nepal tectonostratigraphic zonation developed in other regions of the himalaya is applicable with minor modifications in western nepal from south to north the major structural features are 1 the main frontal thrust system comprising the main frontal thrust and two to three thrust sheets of neogene foreland basin deposits 2 the main boundary thrust sheet which consists of proterozoic to early miocene lesser himalayan metasedimentary rocks 3 the ramgarh thrust sheet composed of paleoproterozoic low grade metasedimentary rocks 4 the dadeldhura thrust sheet which consists of medium grade metamorphic rocks cambrian ordovician granite and granitic mylonite and early paleozoic tethyan rocks 5 the lesser himalayan duplex which is a large composite antiformal stack and hinterland dipping duplex and 6 the main central thrust zone a broad ductile shear zone the major structures formed in a general southward progression beginning with the main central thrust in late early miocene time eocene oligocene thrusting in the tibetan himalaya north of the study area is inferred from the detrital unroofing record on the basis of 40ar 39ar cooling ages and provenance data from synorogenic sediments emplacement of the dadeldhura thrust sheet took place in early miocene time the ramgarh thrust sheet was emplaced between 15 and 10 ma the lesser himalayan duplex began to grow by 10 ma simultaneously folding the north limb of the dadeldhura synform the main boundary thrust became active in latest miocene pliocene time transport of its hanging wall rocks over an 8 km high footwall ramp folded the south limb of the dadeldhura synform thrusts in the subhimalayan zone became active in pliocene time the minimum total shortening in this portion of the himalayan fold thrust belt since early miocene time excluding the tibetan zone is 418 493 km the variation depending on the actual amounts of shortening accommodated by the main central and dadeldhura thrusts the rate of shortening ranges between 19 and 22 mm yr for this period of time when previous estimates of shortening in the tibetan himalaya are included the minimum total amount of shortening in the foldthrust belt amounts to 628 667 km this estimate neglects shortening accommodated by small scale structures and internal strain and is therefore likely to fall significantly below the actual amount of total shortening

transtensional deformation in the central himalaya and its role in accommodating growth of the himalayan orogen 1 field mapping structural analysis and geochronologic data from northwestern nepal reveal major normal right slip motion along a previously unrecognized west northwest striking system of shear zones that we term the gurla mandhata humla fault system gmh the gmh obliquely cuts across the greater himalayan crystalline sequence and into the lesser himalayan imbricate thrust belt via two right step over structures the average slip direction on the gmh parallels the strike of the himalayan orogen motion along this fault system has resulted in an apparent left separation of the south tibet detachment main central thrust zone and lesser himalayan imbricate thrust belt along a north striking segment of the fault system we estimate a minimum of 21 km of net slip on the southern branch of the gmh by restoring the trace of the main central thrust zone parallel to the average slip direction on the fault taking into account slip estimates from the northern branch of the gmh yields a minimum net slip estimate of 24 4 to 32 4 km for the gmh the 232th 208pb ion microprobe monazite ages from leucogranite bodies indicate that motion on the gmh occurred after 15 ma its initiation immediately followed crustal thickening between the main central thrust zone and indus yalu suture zone motion on the gmh is contemporaneous with arc normal contraction in the southernmost himalayan orogen these observations can be explained by a model that involves foreland propagating structural systems facilitating arc normal contraction in the foreland and arc parallel extension in the hinterland that work together to maintain the arcuate shape of the himalayan orogen

dissolution features of gold particles in a lateritic profile at dondo mobi gabon abstract this paper describes the morphological evolution of gold particles in a lateritic weathering profile under rain forest inherited from primary quartz veins associated with amphibolite the gold particles can be recognized from the weathering front up through the saprolite and overlying regolith to the land surface in the central part of the supergene gold dispersion halo gold particles undergo some changes in the uppermost 10 m voids and corrosion pits appear on particle surfaces their numbers and sizes increase toward the land surface edges of particles become progressively more blunt from the bottom to the top of the section the morphological changes are attributed to dissolution of gold over time possible causes for such dissolution are discussed occurrence of gold particles in the soil could be helpful for gold explorations in the humid tropics

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

thermochronologic constraints on the late cenozoic exhumation history of the gurla mandhata metamorphic core complex southwestern tibet how the tibetan plateau is geodynamically linked to the himalayas is a topic receiving considerable attention the karakoram fault plays key roles in describing the structural relationship between southern tibet and the himalayas in particular considerable debate exists at the southeastern end of the karakoram fault where its role is interpreted in two different ways one interpretation states that slip along the dextral karakoram fault extends eastward along the indus yalu suture zone bypassing the himalayas the other interprets that fault slip is fed southward into the himalayan thrust belt along the gurla mandhata detachment gmd to evaluate these competing models the late miocene history of the gmd was reconstructed from thermokinematic modeling of zircon u th he data three east west transects reveal rapid cooling of the gmd footwall from 8 0 1 3 ma to 2 6 0 7 ma model simulations show a southward decrease in slip magnitude and rate along the gmd in the north initiation of the gmd range between 14 and 11 ma with a mean fault slip rate of 5 0 0 9 mm yr the central transect shows an initiation age from 14 to 11 ma with a mean fault slip rate of 3 3 0 6 mm yr in the south initiation began between 15 and 8 ma with a mean fault slip rate of 3 2 1 6 mm yr the initiation ages and slip rates match the karakoram fault across several timescales supporting the idea that the two are kinematically linked specifically the data are consistent with the gmd acting as an extensional stepover with slip transferred southward into the himalayas of western nepal

supergene neoformation of pt ir fe ni alloys multistage grains explain nugget formation in ni laterites ni laterites from the dominican republic host rare but extremely platinum group element pge rich chromitites up to 17 5 ppm without economic significance these chromitites occur either included in saprolite beneath the mg discontinuity or as floating chromitites within limonite above the mg discontinuity both chromitite types have similar iridium group pge ipge enriched chondrite normalized patterns however chromitites included in limonite show a pronounced positive pt anomaly investigation of heavy mineral concentrates obtained via hydroseparation techniques led to the discovery of multistage pge grains i os ru fe ir grains of porous appearance are overgrown by ii ni fe ir and ir fe ni pt phases which are overgrown by iii pt ir fe ni mineral phases whereas ir dominated overgrowths prevail in chromitites from the saprolite pt dominated overgrowths are observed within floating chromitites the following formation model for multistage pge grains is discussed i hypogene platinum group minerals pgm e g laurite are transformed to secondary pgm by desulphurization during serpentinization ii at the stages of serpentinization and or at the early stages of lateritization ir is mobilized and recrystallizes on porous surfaces of secondary pgm serving as a natural catalyst and iii at the late stages of lateritization biogenic mediated neoformation and accumulation of pt ir fe ni nanoparticles occurs the evidence presented in this work demonstrates that in situ growth of pt ir fe ni alloy nuggets of isometric symmetry is possible within ni laterites from the dominican republic

gold mass balance in lateritic profiles from savanna and rain forest zones abstract the gold mass balance was studied in two areas gold mineralization one in a savanna zone in south mali the other in a rain forest zone in french guiana in the ferricrete profile of the savanna zone the gold mass balance is strongly negative within the ferricrete itself where almost all the primary gold stock is leached beneath the ferricrete an absolute accumulation of fine secondary gold is found in the mottled zone the gold cycle is controlled by the migration of fe and associated with the neogenesis of iron oxides particularly hematite the lateritic profile in the rain forest zone is truncated and a soil is developing from the underlying saprolite in equilibrium with the equatorial conditions an absolute accumulation of gold is found in the soil with gold migration and reprecipitation being controlled by the evolution of the macroporosity the global mass balance of gold in the guiana deposit shows a leaching rate of 40 in the weathered part of the mineralization this process occurs mostly at the weathering front in the two examples gold shows opposite behaviors due to different pedogenic and paleoclimatic conditions this might influence the interpretation of geochemical soil surveys in various intertropical environments

evolution of north himalayan gneiss domes structural and metamorphic studies in mabja dome southern tibet field structural and metamorphic petrology investigations of mabja gneiss dome southern tibet suggest that contractional extensional and diapiric processes contributed to the structural evolution and formation of the domal geometry the dome is cored by migmatites overlain by sillimanite zone metasedimentary rocks and orthogneiss metamorphic grade diminishes upsection and is defined by a series of concentric isograds evidence for three major deformational events two older penetrative contractional and extensional events and a younger doming event is preserved metamorphism migmitization and emplacement of a leucocratic dike swarm were syntectonic with the extensional event at mid crustal levels metamorphic temperatures and pressures range from 500 8c and 150 450 mpa in chloritoid zone rocks to 705 65 8c and 820 100 mpa in sillimanite zone rocks we suggest that adiabatic decompression during extensional collapse contributed to development of migmatites diapiric rise of low density migmatites was the driving force at least in part for the development of the domal geometry the structural and metamorphic histories documented in mabja dome are similar to kangmar dome suggesting widespread occurrence of these events throughout southern tibet q 2004 elsevier ltd all rights reserved

isotopic constraints on the age and provenance of the lesser and greater himalayan sequences nepalese himalaya the main central thrust of the central himalaya juxtaposes the lesser himalayan sequence footwall with the greater himalayan sequence hanging wall both of which are regarded as components of the northern passive margin of india considerable uncertainty surrounds their respective ages and sedimentary derivation u pb and sm nd isotopic studies of samples from the langtang area central nepal demonstrate important distinctions between the two rock packages the greater himalayan sequence had a sedimentary provenance that included a major source of 0 8 1 0 ga zircons implying a late proterozoic age the source of the precambrian part of the lesser himalayan sequence contained 1 87 2 60 ga zircons and its depositional age is probably middle proterozoic nd isotopic characteristics of the two sequences are different at 21 ma the greater himalayan rocks had end values between 14 6 and 18 5 while their lesser himalayan counterparts had values between 21 4 and 25 9 giving rise to major differences in their model ages the lesser and greater himalayan sequences at langtang are interpreted as correlative to the lower and upper parts of the vindhyan supergroup currently exposed south of the gangetic plain in northern india on the basis of available age data the exposed hanging wall of the main central thrust in central and eastern nepal contains no archean early proterozoic basement if the fault does root into the basement at depth it must do so farther north than recently published cross sections indicate this implies a total displacement on the fault that is substantially greater than the 140 210 km required by tectonic overlap in the eastern himalayas

isotopic characteristics of the gurla mandhata metamorphic core complex implications for the architecture of the himalayan orogen isotopic data from the gurla mandhata metamorphic core complex provide insight on the character of rocks exposed within it and permit possible correlations of these rocks to those exposed in the himalayan thrust belt whole rock sr and nd isotopic analyses show that rock units in the metamorphic core have isotopic signatures that correlate to those of greater and lesser himalayan rocks exposed in the foreland these data are the first to document lesser himalayan rocks north of the higher himalayan physiographic zone when combined with previously published structural reconstructions of the himalayan orogen these data reveal a crustal architecture that requires significant uplift of lesser himalayan rocks in the hinterland with respect to their position in the foreland a greater amount of underthrusting of the lesser himalayan rocks and more shortening in the hinterland within the tethyan himalaya physiographic zone than currently estimated

seismotectonics of the nepal himalaya from a local seismic network the national seismological network of nepal consists of 17 short period seismic stations operated since 1994 it provides an exceptional view of the microseismic activity over nearly one third of the himalayan arc including the only segment between longitudes 78 e and 85 e that has not produced any m 8 earthquakes over the last century it shows a belt of seismicity that follows approximately the front of the higher himalaya with most of the seismic moment being released at depths between 10 and 20 km this belt of seismicity is interpreted to reflect interseismic stress accumulation in the upper crust associated with creep in the lower crust beneath the higher himalaya the seismic activity is more intense around 82 e in far western nepal and around 87 e in eastern nepal western nepal between 82 5 and 85 e is characterized by a particularly low level of seismic activity we propose that these lateral variations are related to segmentation of the main himalayan thrust fault the major junctions between the different segments would thus lie at about 87 e and 82 e with possibly an intermediate one at about 85 e these junctions seem to coincide with some of the active normal faults in southern tibet lateral variation of seismic activity is also found to correlate with lateral variations of geological structures suggesting that segmentation is a long lived feature we infer four 250 400 km long segments that could produce earthquakes comparable to the m 8 4 bihar nepal earthquake that struck eastern nepal in 1934 assuming the model of the characteristic earthquake the recurrence interval between two such earthquakes on a given segment is between 130 and 260 years

the kinematic evolution of the nepalese himalaya interpreted from nd isotopes neodymium nd isotopes from the himalayan fold thrust belt and its associated foreland basin deposits are useful for distinguishing between himalayan tectonostratigraphic zones and revealing the erosional unroofing history as controlled by the kinematic development of the orogen neodymium isotopic data from the himalayan fold thrust belt in nepal n 35 reveal that the lesser himalayan zone consistently has a more negative nd 0 value than the greater and tibetan himalayan zones our data show the average nd 0 value in the lesser himalayan zone is 21 5 whereas the greater and tibetan himalayan zones have an average nd 0 value of 16 these consistently distinct values throughout nepal enable the use of nd isotopes as a technique for distinguishing between lesser himalayan zone and greater himalayan zone rock the less negative nd 0 values of the greater himalayan rocks support the idea that the greater himalayan zone is not indian basement but rather a terrane that accreted onto india during early paleozoic time neodymium isotopic data from eocene through pliocene foreland basin deposits n 34 show that sediment provenance has been dominated by greater and tibetan himalayan detritus across nepal the nd t values in the synorogenic rocks in western and central nepal generally show an up section shift toward more negative values and record the progressive unroofing of the different tectonostratigraphic zones at 10 ma in khutia khola and 11 ma in surai khola a shift in nd t values from 16 to 18 marks the erosional breaching of a large duplex in the northern part of the lesser himalayan zone this shift is not seen in eastern nepal where the nd t values remain close to 16 throughout miocene time because there has been less erosional unroofing in this region

very high purity gold form lateritic weathering profiles of nilambur southern india laterite profiles in the nilambur gold prospect of southern india contain dust grains and nuggets of supergene gold whereas chemical rounding and corrosion cavities characterize the gold grains in upper zones of intense weathering the lower moderately to slightly weathered horizons contain grains with jagged contours and less common etch pits scanning electron microscopy reveals a variety of incipient growth patterns of secondary gold including spongy filamental petalitic or dendritic patterns spectacular examples of painted gold with a bright luster that precipitated over dull primary grains are documented electron microprobe analyses of secondary gold grains show up to 99 93 au and exceedingly high fineness values i e 1000 au au ag ranging from 991 to 999 compared to the normal fineness of 950 reported for supergene gold from other terrains particle induced x ray emission analyses indicate that fe is the commonly associated trace metal thus linking supergene gold genesis in nilambur to a model involving ferrolysis whereby mobility of gold in weathering profiles was controlled by pyrite oxidation the nature of occurrence grain morphology textural parameters and unusual fineness of gold grains in nilambur laterites testify to distinct low temperature processes attending the chemical dissolution migration and reprecipitation of gold during tropical weathering

first field identification of the cuonadong dome in southern tibet implications for ew extension of the north himalayan gneiss dome the cuonadong dome exposes in east southern margin of the north himalayan gneiss domes nhgd which is reported first time in this study the cuonadong dome is located at the southern part of the zhaxikang ore concentration area which is divided into three tectono lithostratigraphic units by two curved faults around the dome geometry from upper to lower or from outer to inner the upper unit middle unit and lower unit and the outer fault is nading fault while the inner fault is jisong fault the cuonadong dome is a magmatic orthogneiss and leucogranite mantled by orthogneiss and metasedimentary rocks which in turn are overlain by jurassic metasedimentary and sedimentary rocks the grades of metamorphism and structural deformation increase towards the core which is correspondence with the ridang formation low metamorphic schist tourmaline granitic biotite gneiss garnet mica gneiss and mylonitic quartz mica gneiss the cuonadong dome preserves evidences for four major deformational events firstly top to s thrust d1 early approximately n s extensional deformation d2 main approximately e w extensional deformation d3 and late collapse structural deformation d4 around the core of the cuonadong dome which are consistent to three groups lineation approximately n s trending lineation including l1 and l2 e w trending l3 and l4 with plunging towards outside of the dome respectively the formation of the cuonadong dome was probably resulted from the main e w extensional deformation which is a result of eastward flow of middle or lower crust from beneath tibet accommodated by northward oblique underthrusting of indian crust beneath tibet the establishment of the cuonadong dome enhanced the e w extension of the nhgd which is further divided into two structural dome zones according to the different extensional directions approximately n s extensional north himalayan gneiss domes ns nhgd and e w extensional north himalayan gneiss domes ew nhgd the ns nhgd developed by a dominantly n s contraction and locally extensional regime and keep a close relationship to the south tibetan detachment system whereas the ew nhgd formed by an e w extensional deformation along the north south trending rifts

gold a useful tracer in sub sahelian laterites sequential and connected lateritic sequences from the tertiary to present day can be observed in sub sahelian conditions in northern burkina faso michel 1973 grandin 1 975 here complex iron auriferous crust systems have developed on the stable west african craton from proterozoic auriferous greenstone belts and volcanic tuff in order to distinguish between chemical and physical processes which have led to the development of the lateritic systems we propose to use visible gold particles as pathfinders in this paper we focused on the lowland lateritic system profile of pit p4 located downslope to the middle land lateritic system where gold is associated with hydrothermally altered quartz veins and pebbles fig 1

interpretation of a ca 1600 1580 ma metamorphic core complex in the northern gawler craton australia abstract the mount woods domain in the gawler craton south australia records a complex tectonic evolution spanning the palaeoproterozoic and mesoproterozoic the regional structural architecture is interpreted to represent a partially preserved metamorphic core complex that developed during the 1600 1580 ma hiltaba event making this one of the oldest known core complexes on earth the lower plate is preserved in the central mount woods domain which comprises the mount woods metamorphics these rocks yield a detrital zircon maximum depositional age of 1860 ma and were polydeformed and metamorphosed to upper amphibolite to granulite facies during the 1740 1690 ma kimban orogeny the upper plate comprises a younger succession the skylark metasediments deposited at 1750 ma within the upper plate sedimentary and volcanic successions of the gawler range volcanics were deposited into half graben that evolved during brittle normal faulting the skylark shear zone represents the basal detachment fault separating the upper and lower plate of the core complex the geometry of normal faults in the upper plate is consistent with ne sw extension both the upper and lower plates are intruded by 1795 1575 ma hiltaba suite granitic and mafic plutons the core complex was extensively modified during the 1570 1540 ma kararan orogeny exhumation of the western and eastern mount woods domain is indicated by new 40ar 39ar biotite cooling ages that show that rock packages in the central mount woods domain cooled past 300 c 50 c at 1560 ma which was 20 million years before equivalent cooling in the western and eastern mount woods domain exhumation was associated with activity along major syn kararan orogeny faults

isotopic and structural constraints on the location of the main central thrust in the annapurna range central nepal himalaya five isotope enhanced geologic transects in the southern annapurna range of central nepal elucidate structural geometries near the main central thrust whole rock nd isotopes and u pb ages of detrital zircons unambiguously distinguish greater himalayan hanging wall and lesser himalayan footwall metasedimentary rocks nd 0 values for lower lesser himalayan rocks typically range from 20 to 26 whereas greater himalayan rocks usually have nd 0 values of 19 to 12 lower lesser himalayan rocks yield detrital zircons with an age peak at ca 1880 ma and no detrital zircons younger than ca 1550 ma in contrast greater himalayan rocks yield detrital zircons with a prominent broad peak of ages at ca 1050 ma and no detrital zircons younger than ca 600 ma the protolith boundary between greater and lesser himalayan rocks is up to 1 km farther south than usually mapped on the basis of lithology field and microstructural observations indicate the presence of a top to the south ductile shear zone superimposed on this boundary confirming this shear zone as the main central thrust no evidence exists for large scale structural mixing of greater and lesser himalayan rocks along the main central thrust in the annapurna range

alteration lateritique du gite aurifere d ovala gabon et formation d une anomalie superficielle de dispersion les sols de la region d ovala sud gabon presentent une anomalie superficielle en or a l aplomb d une mineralisation de la roche mere lateritisee sur 50 m d epaisseur de bas en haut le profil d alteration comprend trois horizons principaux saprolitique nodulaire et sablo argileux cette alteration est developpee in situ a partir de gneiss archeens et de roches metamorphiques proterozoiques du facies schiste vert dans la roche fraiche l or est associe avec du quartz de la pyrite de la tourmaline et de la muscovite lorsque l alteration progresse les particules auriferes sont liberees et chimiquement arrondies depuis la mineralisation primaire jusqu a la surface les mecanismes de dissolution amenuisent de plus les particules d or et les appauvrissent en argent une dispersion mecanique laterale vers l aval des versants se superpose a l evolution verticale ce mecanisme fournit les particules de plus petite taille dans l horizon superficiel des mecanismes a la fois chimiques et physiques sont a l origine de la formation du halo supergene

transfer and deposition of gold in the congo watershed abstract gold has been measured in water and suspended particulate matter samples coming from the oubangui congo system gold anomalies up to 800 pico m l were measured at two different locations these are interpreted as the result of gold transport by dissolved organic complexes everywhere else the gold content is in the range of 10 11 m l and gold is present as an aurous hydroxichloride complex the knowledge of both discharge rates and gold content allows a quantification of the gold input into the atlantic ocean respective values for dissolved and particulate gold input are equal to 2 86 t a and 2 95 t a both chemical denudation and physical weathering rates were computed from this new set of data

the platiniferous au pd belt of minas gerais brazil and genesis of its botryoidal pt pd aggregates we delineate an approximately 240 km long north trending au pd pt belt in minas gerais brazil which hosts hydrothermal pd pt bearing gold mineralization in its southern part quadrilatero ferrifero and itabira district and alluvial pd pt au mineralization in its northern part southern serra do espinhaco the hydrothermal pd pt bearing gold mineralization locally known as jacutinga occurs as late orogenic quartz hematite talc kaolinite veins which truncate the brasiliano regional tectonic foliation of the itabiritic host rocks of the paleoproterozoic itabira iron formation the alluvial pd pt au mineralization occurs as botryoidal pd pt nuggets and delicate palladiferous gold aggregates in clastic and organic rich valley infill on quartzite and hematitic rocks of the paleo to mesoproterozoic espinhaco supergroup the au pd pt belt follows the regional trend of major thrust faults related to the 0 6 ga brasiliano orogeny and the hydrothermal jacutinga mineralization could be regarded as an oxidized variant of the orogenic gold deposit spectrum secondary botryoidal and arborescent pt pd aggegates and primary detrital au pd grains are peculiar to the alluvial platiniferous deposits of the northern part of the belt in the serra do espinhaco we suggest that their source was jacutinga style vein mineralization as deduced from abundant detrital hematite and rutile and the discovery of detrital hongshiite a pt cu alloy that forms bonanza pockets within jacutinga at itabira reconnaissance laser ablation inductively coupled plasma mass spectrometry analyses for trace metals in the botryoidal pt pd grains show that they have elevated tl and se contents of 0 9 to 354 and 54 to 2 700 ppm respectively these elements correlate positively with a tl se ratio of about 0 08 a value that contrasts with tl se 1 for hongshiite from the hydrothermal jacutinga system at itabira the low and roughly constant tl se ratios of the alluvial samples are close to the average value of 0 12 for fluvial waters we suggest that the botryoidal pt pd aggregates grew in situ by electrochemical metal accretion from dilute solutions carrying pt and pd from the dissolution of detrital pt pd minerals from jacutinga the connection between jacutinga and platiniferous alluvia indicates that there is a prospective trend of about 240 km for pd pt bearing gold mineralization reaching from ouro preto to diamantina the very high oxidation state required for efficient low temperature aqueous mobilization of pt pd and au was apparently controlled by the quartzitic and hematitic rocks in the serra do espinhaco but not in the southern belt where greenstone rocks buffered fluids to moderate ph and lower oxidation states

structural evolution and sequence of thrusting in the tethyan fold thrust belt and indus yalu suture zone southwest tibet regional mapping of a north south traverse from the india nepal china border junction to mount kailas in southwest tibet combined with previously published geochronologic and stratigraphic data is the basis for an incremental restoration of the tethyan fold thrust belt and deformation along the indus yalu suture zone from north to south the major structural features are 1 the indus yalu suture zone composed of five south dipping thrust faults involving rocks interpreted to represent parts of the former indian passive margin and asian active margin 2 the tethyan fold thrust belt composed of a dominantly north dipping system of imbricate thrust faults involving precambrian through upper cretaceous strata and 3 the kiogar jungbwa thrust sheet a linelength cross section reconstruction indicates a minimum of 176 km of north south horizontal shortening partitioned by the tethyan fold thrust belt 112 km and indusyalu suture zone 64 km sequential restoration of the cross section shows that the locus of shortening prior to the late oligocene occurred significantly south possibly 60 km of the indus yalu suture zone within the tethyan fold thrust belt and that a significant amount of unsubducted oceanic lithosphere was present south of the suture in southwest tibet until that time an implication of this result is that postcollision oligocene miocene high k calcalkaline magmatism may be explained by melting due to active subduction of oceanic crust beneath the kailas magmatic complex until the late oligocene a regional profile across the tibetan himalayan orogen from

palladium bearing gold deposit hosted by proterozoic lake superior type iron formation at the caue iron mine itabira district southern sao francisco craton brazil geologic and structural controls the caue iron mine is located in the itabira district in the southern sao francisco craton three geologic units metamorphosed to amphibolite facies have been identified 1 an archean volcano sedimentary sequence 2 a proterozoic iron formation unit and 3 a proterozoic quartzite unit the iron formation unit is cut by amphibolite dikes and tectonically imbricated with talc schists and amphibolites of the volcano sedimentary sequence all the rocks were affected by three phases of folding and associated thrusting and boudinage in the iron formation unit the d 1 and d 2 structures have been generated by a progressive simple shear regime with a transport direction consistently east over west d 3 structures are characterized by open folds with an associated crenulation cleavage the caue palladium bearing deposit comprises five orebodies hosted by jacutinga in the iron formation unit jacutinga is a highly sheared metamorphosed hydrothermally altered lake superior type iron formation composed of hematite quartz talc phlogopite with minor amounts of tourmaline apatite and monazite in the corpo y orebody the highest gold concentrations occur in quartz veins whereas in the corpo x central aba leste and aba notre orebodies the highest gold grades occur in hematite veins with local concentrations of quartz the gold rich veins are parallel to s 1 foliation in all orebodies except aba norte in the aba norte the gold rich boudinaged hematite vein is parallel to s 2 the ore minerals are palladian gold gold palladium palladium oxide and palladseite the main mineralizating event was synchronous with the peak of thermal metamorphism t 600 degrees c and with d 1 shearing and thrusting the hydrothermal alteration associated with this event is characterized by the formation of tale phlogopite and tourmaline and hematite and quartz veins generating the jacutinga rock type weathering of the jacutinga resulted in the alteration of silicates to kaolin and oxides to goethite

quantitative approach to physical and chemical gold mobility in equatorial rainforest lateritic environment mobility of gold throughout the global weathering system of dondo mobi south gabon has been quantatively addressed by calculating the gold mass transfer during lateritic weathering of auriferous lisvenite archean gneiss and proterozoic schist and by proposing a thermodynamic approach to gold solubility under equatorial rainforest acidic conditions both chemical and short distance translocation processes govern the mobility of gold and gold particles during the long term evolution of collapsed open weathering systems which result from the combined interaction of rock meteoric water and biological agents free gold particles are translocated from the lisvenite derived soil towards formerly barren archean gneiss and porous proterozoic formations derived from schist to form a 200 m wide supergene gold dispersion halo which took shape over about 0 86 1 26 million years simultaneously with physical translocation of gold dissolution of gold makes the total balance of gold transfer through the whole weathering system strongly negative which is the converse of what is generally believed to occur in lateritic systems taking into account hydrolysis reactions three main gold complexes are proposed as a function of corresponding inorganic and organic ligand concentrations these are au oh h o aucioh and au oh fa these gold complexes leave the dondo mobi weathering mantle and move towards the river system in the course of both physical and chemical processes involved in the weathering of the formation

genesis of itabirite hosted au pd pt bearing hematite quartz veins quadrilatero ferrifero minas gerais brazil constraints from fluid inclusion infrared microthermometry bulk crush leach analysis and u pb systematics fluid inclusions hosted in quartz and specular hematite from auriferous jacutinga and barren veins in the quadrilatero ferrifero qf have been studied using conventional and near infrared microscopy respectively the mineralization consists of veins that cross cut metamorphosed iron formation itabirite of the paleoproterozoic itabira group the sample suite comprises hematite from veins from the low strain domain in the w and sw of the study area as well as hematite samples from the eastern high strain domain in the central and ne parts of the qf halogen ratios of fluid inclusions in quartz and hematite from all studied deposits are consistent with a fluid evolved from dissolving and reprecipitating halite that was subsequently diluted fluid inclusions hosted in quartz and hematite are characterized by consistent na k ratios and considerable so4 contents and suggest similar formation conditions and perhaps fluid origin from a common source na k and na li fluid mineral geothermometers indicate water rock interaction at approximately 340 40 c hematites from the high strain domain contain fluid inclusion assemblages of high temperature aqueous carbonic and multiphase high salinity high temperature aqueous inclusions probably due to fluid immiscibility in the system h2o nacl co2 fluid inclusions hosted in hematite from barren veins in the low strain domain as well as in hematite from jacutinga type mineralization from the central part of the qf only host multiphase aqueous fluid inclusions all showing narrow ranges of salinity 7 2 11 7 wt nacl equiv and homogenization temperatures 148 to 229 c lower homogenization temperatures and the absence of co2 rich inclusions in specular hematite from these occurrences are attributed to carbonate precipitation and or co2 escape due to cooling during fluid migration from the high to the low strain domain pb pb and u pb systematics of gold hematite and hematite hosted fluid inclusions in combination with geochemical evidence indicate distinct sources for pd au and pb the formation of specular hematite veins may be related to retrograde metamorphic fluids being released during the brazilian orogenic cycle 600 700 ma the pb isotopic characteristics of all samples are readily reconciled in a simple model that involves two different paleoproterozoic or archean source lithologies for lead and reflects contrasting depths of fluid percolation during the brasiliano orogeny

lithostratigraphic correlations in the western himalaya an isotopic approach we present the results of a whole rock nd isotopic study of two contrasting regions of the western himalaya using the neodymium model age approach on the scale of a single orogen high grade metasedimentary rocks from zanskar yield model ages t dm that are similar to those of the high himalayan crystalline series t dm 1 2 2 0 ga nd 6 to 16 and distinct from values from the lesser himalaya t dm 2 3 3 4 ga nd 18 to 27 hence these two lithological sequences can be recognized for 2000 km along the strike of the orogen data for the basement of the nanga parbat massif at the western extremity of the himalaya t dm 2 3 2 8 ga nd 18 to 30 suggest that these rocks are not equivalent to the high himalaya as previously supposed but have affinities with the lesser himalaya a thin metasedimentary cover sequence on the margins of the nanga parbat massif is isotopically indistinguishable from the high himalaya t dm 1 6 1 8 ga nd 10 to 14 the prior misidentification of the provenance of the massif stems from its high metamorphic grade characteristic of the high himalaya but in this case related to the unique neogene history of the nanga parbat massif which has exhumed a higher grade equivalent of the lesser himalaya that is not seen elsewhere

on the stability of gold silver alloys in the weathering environment thermodynamic analysis of the stability conditions for gold silver solid solutions reveals that the silver component is unstable with respect to chlorargyrite and silver sulfate in most weathering environments as a consequence supergene secondary gold and gold found in modern river placers detrital and authigenic tends to be pure impure detrital placer gold is subject to natural purification processes as evidenced by silver depleted spongey grain margins in contrast under hydrothermal conditions the silver content of primary gold can be controlled by equilibrium reactions with argentite

weathering mantles and their significance for geomorphological evolution of central and northern europe since the mesozoic abstract weathering mantles saprolites are very widespread in central western and northern europe where they form thick near surface residual deposits often in excess of 50 m they vary in the type of weathering the bedrock lithology involved the topographic setting and the age and stratigraphic context this review paper provides information about the distribution and major characteristics of weathering mantles from a number of key areas including the british isles the central european belt of mid mountains and uplands the scandinavian mountains and the fennoscandian shield it also examines the evidence for weathering etched origin of various landforms including boulders tors inselbergs basins and some regional landforms assemblages major types of regionally important weathering mantles include ferrallitic lateritic saprolites kaolinite rich saprolites sandy to grus mantles and various residuals left after partial dissolution of carbonate rocks although dating of weathering mantles is often circumstantial it appears sufficiently precise to argue for various ages of deep weathering mesozoic pre late cretaceous mantles are particularly widespread but there are also early tertiary and miocene saprolites widely present whilst most grus weathering is likely to be of miocene to pleistocene age individual upland areas have been subjected to multiple episodes of weathering whose legacy is the present day co existence of different weathering products continental and shallow marine deposits around the uplands particularly in the north sea and intra continental graben structures are often dominated by quartz and kaolinite pointing to deeply weathered land surfaces as the source areas and extending the record of weathering on the geological time scale they demonstrate that weathering mantles repeatedly formed and were subjected to erosion and new cycles of weathering were initiated we suggest that deep unroofing of basement structures in many parts of europe has been mainly accomplished by deep weathering and stripping of pre weathered rock the phenomenon of deep weathering has been very important for geomorphology etched landforms vary in size from small residual boulders up to regional characteristic landscapes etchsurfaces such as joint valley landscapes of southern sweden or undulating multi convex and multi concave landscapes of many uplands in central europe deep weathering has played a major part in the geomorphological evolution of europe since at least the late triassic and the concept involving long term etching and stripping provides an appropriate framework to describe and explain both erosional landscapes and sediment features

isotopic study of the manaslu granite himalaya nepal inferences on the age and source of himalayan leucogranites a detailed isotopic study of the manaslu leucogranite was carried out a u pb age of 25 ma and a whole rock rb sr age isochron of 18 ma were obtained suggesting that the magmatic activity lasted at least 7 ma initial sr isotopic ratios are very high 0 740 to 0 760 and initial nd isotopic ratios are low nd in 13 to 16 and they show the existence of large isotopic variations even at the metre scale these are not the result of perturbations by fluids but rather they reflect the initial isotopic heterogeneity of the source material which has not been obliterated by magmatic processes e g fusion mixing by convection these results also support the crustal origin of this leucogranite the tibetan slab paragneisses whose sr and nd isotopic ratios are very similar to those of the granite at an age of 20 ma are the most probable parental material nd model ages for both the leucogranite and the gneisses are in the range 1 5 2 ga a model of formation of the manaslu granite by coalescence of different batches of magma is in agreement with the present data

platinum group element and gold enrichment in soils monitored by chromium stable isotopes during weathering of ultramafic rocks abstract weathering of ultramafic rocks can lead to the formation of soil profiles with high contents of fe ni co platinum group elements pge and gold traditionally these metal rich soils are known as laterites and are formed under tropical climates and stable tectonic conditions however little is known about their possible development in cold humid regions and the factors governing pge and gold mobility and enrichment under these weathering conditions are poorly constrained in this study five soil profiles developed on serpentinized chromite bearing ultramafic rocks at la cabana located in the coastal range of south central chile 38 s were studied by combining major and trace element geochemistry with chromium stable isotope data the results show that the soils developed at la cabana have higher pge and au contents than the parent serpentinite rock with pge and au reaching up to 160 ppb and 29 ppb in a limonitic soil horizon and clay saprolite respectively most soil samples have slightly negative 53 52crsrm979 values within a range of 0 089 0 012 to 0 320 0 013 average of 0 178 and are in agreement with previous data reported for modern soils a noteworthy relation between 53 52cr data and pge au contents is observed in the studied soil horizons where isotopically lighter values of 53 52cr match the higher contents of pge and gold these results show that pedogenetic processes operating at the cold and humid la cabana area are capable of increasing the total pge and au contents of certain soil horizons such processes are complex and multivariate but are primarily modulated by chromite dissolution and the formation of secondary phases such as clay minerals and oxy hydroxide phases in the soil these findings provide evidence that important weathering and pge au supergene accumulation are not only restricted to tropical latitudes and that the chromium isotope system is a useful proxy to track surface redox process and noble metal enrichment during pedogenesis

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

geochemical investigations of the origin of the manaslu leucogranite himalaya nepal the himalayan orogeny is characterized by an intraplate subduction zone the thrusting of the tibetan slab over the foreland is probably responsible for the origin of the manaslu type leucogranites this geochemical study tries to examine the crustal origin of the granite to verify whether the paragneisses of the tibetan slab are the most probable parent material and to determine the subsequent petrogenetic processes the rb sr systematics suggest that 1 1 the scatter of data points in the 87sr86srvs 87rb86sr diagram may be due to the heterogeneity of the initial 87sr86sr ratios 2 2 the initial isotopic ratios are very high 0 730 0 760 and are in agreement with those of the gneisses of the tibetan slab at the time of the melting pb isotopic ratios are homogeneous they are also consistent with a crustal prehistory the ree and th contents are unexpectedly low in comparison with the much higher contents of the parent material monazite left in the residue of melting or more probably extracted in an early stage of the magmatic history may explain the observed ree and th abundances alternatively field observations show that the emplacement of the granite has been accompanied by a very strong hydrothermal activity in consequence the low ree and th contents of the granite could also be due to the loss by solution during prolonged fluid circulation throughout the granitic melt and the sr and pb isotope distribution to the effect of late to post magmatic fluids the isotopic character of which was determined by the underlying tibetan slab

oligocene initiation of the south tibetan detachment system constraints from syn tectonic leucogranites in the kampa dome northern himalaya abstract himalayan leucogranites provide a robust record of the thermal and tectonic processes that occurred during convergence of the eurasian and indian tectonic plates we present structural petrological mineralogical whole rock geochemical sr nd isotope and geochronological results for cenozoic leucogranites from the kampa dome northern himalaya the kampa leucogranites are typical syn tectonic granites and occur as a series of sills dikes and puddings in the kampa shear zone which is considered to be the exhumed portion of the south tibetan detachment system stds in the northern himalaya the kampa syn tectonic leucogranites provide information about the timing and nature of regional kinematics and record thermal conditions and the initiation of the stds whole rock geochemistry and sr nd isotope analyses show that the kampa leucogranites are characterized by the following 1 they are peraluminous and have high sio2 al2o3 and a cnk low tio2 mgo p2o5 and tfe2o3 and variable cao na2o k2o and total alkalis 2 they are enriched in large ion lithophile elements e g k and rb and depleted in high field strength elements e g zr and ti with negative eu anomalies and 3 they display variable initial 87sr 86sr compositions 0 7507580 826065 and their end i values range between 7 6 and 12 2 anatexis of graywackes and metapelites in the great himalayan crystalline series played an important role in the formation of the kampa leucogranites anatexis was controlled by fluid absent partial melting of muscovite mineralogical and petrological analyses show that zircons belong to an early crystallization phase whereas monazites are remnants of the final crystallization phase of the granitic melt zircon u pb dating reveals that anatexis in the deep himalayan crust occurred at ca 32 23 ma during the oligocene monazites yield homogeneous 208pb 232th ages of ca 23 5 ma and record rapid emplacement in the kampa shear zone these results indicate that movement on the stds was initiated during the early oligocene and became well developed during the late oligocene in the kampa region the oligocene was a unique period of counterbalance between continued plate convergence and intense extension in the himalayan orogen

tectonic evolution of the himalayan thrust belt in western nepal implications for channel flow models we present a new geologic map of western nepal and three balanced regional cross sections in the himalayan thrust belt the minimum shortening between the south tibetan detachment and the main frontal thrust is 485 743 km and suggests that total himalayan shortening may exceed 900 km all rocks involved in the thrust belt are of upper crustal affinity implying that a comparable length of indian lower crust and mantle lithosphere was subducted beneath tibet major structural features are the subhimalayan thrust system lesser himalayan imbricate zone dadeldhura thrust sheet lesser himalayan duplex ramgarh thrust sheet main central thrust sheet and a north dipping normal sense shear zone possibly related to the south tibetan detachment these structures are continuous along the entire nepalese portion of the himalayan thrust belt new 40 ar 39 ar ages from the ramgarh thrust zone greater himalayan rocks and the lower part of the tethyan sequence support a kinematic model in which major thrust systems in nepal propagated southward from early miocene time onward the geometry and kinematic history of the thrust belt in western nepal are not compatible with recent models for southward ductile extrusion of greater himalayan rocks in a mid crustal channel instead the thrust belt in western nepal behaved like a typical forward propagating thrust system involving unmetamorphosed brittlely deformed rocks in its frontal part and ductilely deformed higher grade metamorphic rocks in its hinterland region although our results do not support published versions of the channel flow model they provide additional geological and geo chronological data that will assist future attempts to develop geodynamic models for the himalayan tibetan orogenic system

diverse magma sources for the himalayan leucogranites evidence from b sr nd isotopes abstract the himalayan orogen is featured by widespread s type leucogranites ca 45 to 9 ma formed primarily from the partial melting of metapelites 800 480 ma and minor from the partial melting of amphibolite with subordinate metapelites the ama drime gneiss and mabja leucogranite pluton are both located at the footwall of the ns trending xainza dinggye normal fault in the central part of the himalayan orogen with published end t values of 21 0 to 19 6 and 19 3 to 18 3 and 87sr 86sr t values of 0 90954 0 92574 and 0 84853 0 85474 respectively in this study we presented new shrimp zircon u pb ages and laser ablation multi collector inductively coupled plasma mass spectrometry la mc icp ms tourmaline boron isotope data on the metamorphic rocks and leucogranites of the himalayan orogen the weighted mean 207pb 206pb age of the zircon cores of the ama drime gneiss t0446 2 3 is 1854 4 ma the tourmaline 11b values of the ama drime gneiss t0446 1 6 are 17 6 to 14 3 similar to those of the mabja leucogranite t0436 4 18 9 to 17 4 in contrast the tourmaline 11b values of the quedang metapelite t0389 18 and its partial melting product malashan leucogranite t0659 12a 3 are substantially higher i e 15 3 to 12 5 and 16 2 to 8 0 respectively the tourmaline 11b values of the yadoi leucogranite t0321 4 derived from partial melting of amphibolite range from 8 4 to 5 4 therefore the tourmaline b isotopes and sr nd isotopes results are consistent and may have reflected an e w extension along the southern tibetan rift system indicating a late miocene anatexis in the north himalaya region the melts derived from the partial melting of metapelites and mature crustal materials e g paleoproterozoic ama drime gneiss ascended along the n s trending xainza dinggye normal fault and were subsequently emplaced in the tethyan himalaya sequence

mobility of gold and silver in lateritic weathering profiles some observations from western australia experimental evidence suggests that very acid chloride solutions generated in lateritic profiles by ferrolysis oxidation and hydrolysis of iron are responsible for the dissolution of gold and silver au is subsequently reprecipitated by reduction of the aucl 4 super ion with fe super 2 reduction of the agcl complex does not similarly occur because of the relative redox potentials of fe super 2 fe super 3 and ag agcl thus chemical refinement of gold occurs during lateritization the development of low ph and the redistribution of gold and silver in lateritic weathering profiles appear to be more common over granitic and gneissic basement and may be inhibited by the presence of carbonate in the weathering zone of basic rock sequences modified journal abstract

chemical and physical transfers in an ultramafic rock weathering profile part 2 dissolution vs accumulation of platinum group minerals the chemical weathering of ultramafic rocks has resulted in eluvial concentration of pt group minerals pgm in lateritic weathering profiles of southern new caledonia the pt mineralization interpreted as being primary consists of pt group minerals included within chromite crystals the oc currence of pgm as free particles in the weathering profile results from the supergene dissolution of pt bearing chromite traore et al 2008 following their release in the profile supergene dissolution processes variably affect the pgm particles the behavior of pt group elements in the weathering profile is characterized by significant loss of pd and relative enrichment of pt indicating that pd is more mobile than pt in the exogenous cycle unstable pt fe cu pd alloys and pge oxides undergo chemical and mineralogical changes to acquire the chemical configuration of isoferroplatinum pt3fe which is the most stable pt phase in a lateritic environment the isoferroplatinum phase may also be dispersed throughout the weathering mantle and or accumulated in the lower parts of profiles ac cording to a translocation mechanism of residual pt rich fine particles driven by percolation of water through the connected pore spaces