u pb detrital zircon and 39ar 40ar muscovite ages from the eastern parts of the karagwe ankole belt tracking paleoproterozoic basin formation and mesoproterozoic crustal amalgamation along the western margin of the tanzania craton abstract the karagwe ankole belt forms part of a complex system of orogenic belts in central east africa that record the amalgamation of archaean and or paleoproterozoic continental kernels in the proterozoic the actual timing of tectonic processes including initial basin formation and subsequent deformation is only poorly constrained so that the geodynamic significance of the karagwe ankole belt within the broader system of proterozoic orogens in central east africa is not well understood this paper presents u pb ages of detrital zircons constraining the age of sediments of the eastern parts of the karagwe ankole fold belt overlying the western tanzania craton and 39ar 40ar muscovite ages from mylonites and phyllonites associated with the main phase of subsequent shortening and fold and thrust tectonics d2 39ar 40ar muscovite ages from the main detachment between archaean basement gneisses and imbricated cover rocks of the kagera supergroup in the eastern frontal parts of the karagwe ankole belt constrain the timing of the main fabric forming event and thrusting d2 to 1326 10 ma this age is at the young end of the age range for the main phase of granite plutonism and mafic dyke emplacement that have previously been related to crustal extension and associated intracontinental rifting if correct and representative for the main phase of deformation in the rest of the karagwe ankole belt the ages may rather point to a collisional event between the congo and tanzania cratons west of the karagwe ankole belt in mesoproterozoic times indicating a closer correlation with rocks of the kibara belt sensu stricto to the immediate south u pb detrital zircon ages for the older muyaga and overlying bukoba group of the kagera supergroup confirm the paleoproterozoic timing of sedimentation from ca 1780 ma but before ca 1568 ma the presence of a distinct group of ca 1750 1790 ma zircons in the bukoba group indicate uplift erosion and subsequent reworking of sediments and volcanic units contained in the lower muyaga group to the west the ages also suggest a correlation between the bwezigoro group of sw uganda and the bukoba group indicating the presence of a far more extensive paleoproterozoic and or early mesoproterozoic foreland basin overlying the western margin of the archaean tanzania craton and paleoproterozoic uganda block

power spectral analysis of auroral occurrence frequency power spectral analyses have been carried out on a data set of swedish visual auroras for the period 1721 to 1943 besides the anticipated 11 year annual and semiannual cycles an unexpected peak near 1 4 years is found the amplitude of this latter peak while smaller than that of the 6 month and 12 month peaks is statistically significant at between the 5 and 1 level and is found consistently in a sequence of spectra covering consecutive overlapping approximately 22 year data segments the semiannual variation becomes extremely weak during the prolonged solar activity minimum at the beginning of the nineteenth century and also shows some lessening during the prolonged solar activity minimum near the beginning of the twentieth century the 1 4 year peak shows a strong modulation of about 65 to 68 years and an additional long term trend of the order of centuries the origin of this peak is unknown

long term north south asymmetry in solar wind speed inferred from geomagnetic activity a new type of century scale solar oscillation a significant and very similar annual variation in solar wind speed and in geomagnetic activity was recently found around all the four solar cycle minima covered by direct sw observations since mid 1960 s we have shown that the phase of this annual variation reverses with the sun s polarity reversal depicting a new form of 22 year periodicity the annual variation results from a small north south asymmetry in sw speed distribution where the minimum speed region is shifted toward the northern magnetic hemisphere here we study the very long term evolution of the annual variation using early registrations of geomagnetic activity we find a significant annual variation during the high activity solar cycles in mid 19th century and since 1930 s most interestingly the sw speed asymmetry in mid 19th century was opposite to the present asymmetry i e the minimum speed region was then shifted toward the southern magnetic hemisphere this change of asymmetry suggests for a possible new form of century scale oscillation in the north south asymmetry of the sun we explain the asymmetry in terms of a relic magnetic field dislocated slightly in the north south direction from the heliographic equator the change in the asymmetry would result from the century scale north south oscillation of the location of the relic field across the ecliptic

kibaran a type granitoids and mafic rocks generated by two mantle sources in a late orogenic setting burundi in the mesoproterozoic northeastern kibaran belt of burundi central africa two distinct late kibaran magmatic suites coexist both including a type granitoids they are located along the boundary zone between the kibaran mobile belt western internal domain and the archaean tanzanian craton overlain by mesoproterozoic foreland deposits eastern external domain intense deformation high temperature metamorphism and intrusion of abundant peraluminous anatectic crustal granites occur only in the former domain whereas the mesoproterozoic sedimentary cover of the latter is much less or even nearly undeformed nor metamorphosed the first late kibaran magmatism 350 km long kabanga musongati with an emplacement age of 1275 ma u pb on zircon is mainly composed of mafic and ultramafic layered rocks with subordinate a type acidic differentiates moderately enriched in incompatible elements initial isotopic ratios srlr 0 708 nd 8 indicate an old continental lithospheric mantle origin the emplacement of these late kibaran magmatic rocks was controlled by late lateral shear possibly contemporaneous with the latest intrusions of the kibaran peraluminous synkinematic granites of the western internal domain 1330 1260 ma the second late kibaran magmatism 40 km long gitega makebuko and bukirasazi alignment with an emplacement age of 1249 s ma u pb on zircon is limited in volume it is mainly granitic in composition a type can be strongly enriched in incompatible elements and comprises both syenites and mafic rocks initial isotopic ratios srlr 0 702 na 4 5 to 1 4 point to an oib type asthenospheric lower continental lithospheric mantle origin with only slight contamination by the lower crust during differentiation this group was also intruded during the late lateral shear in both groups liquid lines of descent can be reconstructed although some of the rocks have been strongly albitized this indicates that the granites are produced by differentiation of less evolved magmas and not by crustal anatexis upwelling of the asthenosphere along the tanzanian craton can generate by adiabatic pressure release the oibtype basic melts and provide the heat necessary to melt the continental lithospheric mantle sources this mechanism assigns a major role to a lithosphere scale late kibaran shear event occurring at the end of the regional compressive deformation between two rheologically contrasted domains ascent of the asthenosphere continental lithospheric mantle delamination and late orogenic extensional collapse of the western internal domain are suggested as a possible geodynamic model for the entire northeastern kibaran belt additional work is however necessary to test this model finally our results indicate that in the northeastern kibaran belt the kibaran orogeny ended at 1250 ma despite various reactivation events occurring later e g at 1137 ma

filling the bushveld complex magma chamber lateral expansion roof and floor interaction magmatic unconformities and the formation of giant chromitite pge and ti v magnetitite deposits his mind was like a soup dish wide and shallow irving stone on william jennings bryan a compilation of the sr isotopic stratigraphy of the bushveld complex shows that the evolution of the magma chamber occurred in two major stages during the lower open system integration stage lower critical and lower main zone there were numerous influxes of magma of contrasting isotopic composition with concomitant mixing crystallisation and deposition of cumulates larger influxes correspond to the boundaries of the zones and sub zones and are marked by sustained isotopic shifts major changes in mineral assemblages and development of unconformities during the upper closed system differentiation stage upper main zone and upper zone there were no major magma additions other than that which initiated the upper zone and the thick magma layers evolved by fractional crystallisation the lower and lower critical zones are restricted to a belt that runs from steelpoort and burgersfort in the northeast to rustenburg and northam in the west and an outlier of the lower and lower critical zone up to the lg4 chromitite layer in the far western extension north of zeerust it is only in these areas that thick harzburgite and pyroxenite layers are developed and where chromitites of the lower critical zone occur these chromitites include the economically important c 1 m thick lg6 and mg1 layers exposed around both the eastern and western lobes of the bushveld complex the upper critical zone has a greater lateral extent than the lower critical zone and overlies but also onlaps the floor rocks to the south of the steelpoort area the source of the magmas also appears to have been towards the south as the mg chromitite layers degrade and thin northward whereas the lg layers are very well represented in the north and degrade southward sr and os isotope data indicate that the major chromitite layers including the lg6 mg1 and ug2 originated in a similar way extremely abrupt and stratigraphically restricted increases in the sr isotope ratio imply that there was massive contamination of intruding melt which hit the roof of the chamber and incorporated floating granophyric liquid which forced the precipitation of chromite kruger 1999 kinnaird et al 2002 therefore each chromitite layer represents the point at which the magma chamber expanded and eroded and deformed its floor nevertheless this was achieved by in situ contamination by roof rock melt of the intruding critical zone liquids that had an orthopyroxenitic to noritic lineage the main zone is present in the eastern and western lobes of the bushveld complex where it overlies the critical zone and onlaps the floor rocks to the south and the north where it is also the basal zone in the northern lobe the new magma first intruded the northern lobe north of the thabazimbi murchison lineament interacted with the floor rocks incorporated sulphur and precipitated the platreef along the floor rock contact before flowing south into the main chamber this exceptionally large influx of new magma then eroded an unconformity on the critical zone cumulate pile and initiated the main zone in the main chamber by precipitating the merensky reef on the unconformity the upper zone magma flowed into the chamber from the southern bethal lobe as well as the tml this gigantic influx eroded the main zone rocks and caused very large scale unconformable relationships clearly evident as the gap areas in the western bushveld complex the base of this influx which is also coincident with the pyroxenite marker and a troctolitic layer in the northern lobe is the petrological and stratigraphic base of the upper zone sr isotope data show that all the pge rich ores including chromitites are related to influxes of magma and are thus related to the expansion and filling of the magma chamber dominantly by lateral expansion with associated transgressive disconformities onto the floor rocks coincident with major zone changes these positions in the stratigraphy are marked by abrupt changes in lithology and erosional features over which succeeding lithologies are draped the outcrop patterns and the concordance of geochemical isotopic and mineralogical stratigraphy indicate that during crystallisation the bushveld complex was a wide and shallow lobate sill like sheet and the rock strata and mineral deposits are quasi continuous over the whole intrusion

chalcophile and platinum group element pge concentrations in the sulfide minerals from the mccreedy east deposit sudbury canada and the origin of pge in pyrite magmatic sulfide deposits consist of pyrrhotite pentlandite chalcopyrite pyrite and platinum group minerals pgm understanding the distribution of the chalcophile and platinum group element pge concentrations among the base metal sulfide phases and pgm is important both for the petrogenetic models of the ores and for the efficient extraction of the pge typically pyrrhotite and pentlandite host much of the pge except pt which forms pt minerals chalcopyrite does not host pge and the role of pyrite has not been closely investigated the ni cu pge ores from the south range of sudbury are unusual in that sulfarsenide pgm rather than pyrrhotite and pentlandite are the main carrier of pge probably as the result of arsenic contribution to the sulfide liquid by the as bearing metasedimentary footwall rocks in comparison the north range deposits of sudbury such as the mccreedy east deposit have as poor granites in the footwall and the ores commonly contain pyrite our results show that in the pyrrhotite rich ores of the mccreedy east deposit os ir ru rh ipge and re are concentrated in pyrrhotite pentlandite and surprisingly in pyrite this indicates that sulfarsenides which are not present in the ores were not important in concentrating pge in the north range of sudbury palladium is present in pentlandite and together with pt form pgm such as ptpd tebi 2 platinum is also found in pyrite two generations of pyrite are present one pyrite is primary and locally exsolved from monosulfide solid solution mss in small amounts 2 wt together with pyrrhotite and pentlandite this pyrite is unexpectedly enriched in ipge as pt and the concentrations of these elements are oscillatory zoned the other pyrite is secondary and formed by alteration of the mss cumulates by late magmatic hydrothermal fluids this pyrite is unzoned and has inherited the low concentrations of ipge and re from the pyrrhotite and pentlandite that it has replaced

can biological toxicity drive the contrasting behavior of platinum and gold in surface environments abstract in spite of the similar chemical properties of gold au and platinum pt au is highly mobile in surface environments whereas pt appears to be far less so in this study we assess if geomicrobial processes are likely to cause these differences as the mobility of pt and au should differ little based on thermodynamic solubility alone to achieve an accurate comparison it is important that both metals occur in the same environment mineral and groundwater samples were obtained from the fifield pt au field in new south wales australia where pt and au nuggets occur in a series of tertiary eluvial and alluvial paleo placers in particular we studied the m scale dispersion of au and pt within an extraordinary 10 mm sized fragment of ferruginous paleochannel material which contained abundant native au and isoferroplatinum grains gold grains displayed complex secondary morphologies indicative of biogeochemical transformations whereas isoferroplatinum grains appeared smooth and well rounded and showed no signs of supergene transformation gold grains were surrounded by a dusting of highly pure metallic au particles 10 m in diameter whereas no metallic pt particles were detected a search for ionic pt was also unsuccessful a series of biotic and abiotic incubation experiments was conducted to investigate the hypothesis that these differences in mobility are driven by interactions with microbiota biofilms consisting of metallophilic bacteria formed on ultraflat au surfaces and caused significant surface transformations in contrast only subtle changes were observed on pt surfaces incubated under similar conditions minimal inhibitory concentrations for au complexes are more than an order of magnitude lower than those measured for pt complexes in cupriavidus metallidurans cells this higher cell toxicity of mobile au compared to pt complexes can lead to toxic levels of mobile au in the vicinity of au grain surfaces the elevated toxicity drives the formation of au detoxifying biofilms that catalyze the biomineralization of spheroidal nano particulate and bacteriomorphic au in contrast this does not occur on isoferroplatinum grains due to the lower toxicity of pt in conclusion these results are consistent with microbial adaptation to element toxicity driving the cycling of precious metals in surface environments

sunspot periods in the late precambrian glacial climate and solar planetary relations cycles conspicuous in late precambrian 680 myr glacial varves in australia reveal strong climatic periods of about 11 22 145 and 290 yr and a weaker period near 90 yr the 11 22 and 90 yr cycles equate with sunspot periods and the longer cycles with solar and climatic periods as indicated by tree ring studies the varve cycles provide clear evidence of 11 and 22 yr climatic periods in the geological record and may carry important implications for solar physics and planetary science

geochronology of the central tanzania craton and its southern and eastern orogenic margins geological mapping and zircon u pb hf isotope data from 35 samples from the central tanzania craton and surrounding orogenic belts to the south and east allow a revised model of precambrian crustal evolution of this part of east africa the geochronology of two studied segments of the craton shows them to be essentially the same suggesting that they form a contiguous crustal section dominated by granitoid plutons the oldest orthogneisses are dated at ca 2820 ma dodoma suite and the youngest alkaline syenite plutons at ca 2610 ma singida suite plutonism was interrupted by a period of deposition of volcano sedimentary rocks metamorphosed to greenschist facies directly dated by a pyroclastic metavolcanic rock which gave an age of ca 2725 ma this is supported by detrital zircons from psammitic metasedimentary rocks which indicate a maximum depositional age of ca 2740 ma with additional detrital sources 2820 and 2940 ma thus 200 ma of episodic magmatism in this part of the tanzania craton was punctuated by a period of uplift exhumation erosion and clastic sedimentation volcanism followed by burial and renewed granitic to syenitic magmatism in eastern tanzania handeni block in the heart of the east african orogen all the dated orthogneisses and charnockites apart from those of the overthrust neoproterozoic granulite nappes have neoarchaean protolith ages within a narrow range between 2710 and 2630 ma identical to but more restricted than the ages of the singida suite they show evidence of ediacaran pan african isotopic disturbance but this is poorly defined in contrast granulite samples from the wami complex nappe were dated at ca 605 and ca 675 ma coeval with previous dates of the eastern granulites of eastern tanzania and granulite nappes of adjacent ne mozambique to the south of the tanzania craton samples of orthogneiss from the northern part of the lupa area were dated at ca 2730 ma and clearly belong to the tanzania craton however granitoid samples from the southern part of the lupa block have palaeoproterozoic ubendian intrusive ages of ca 1920 ma outcrops further south at the northern tip of lake malawi mark the se continuation of the ubendian belt albeit with slightly younger ages of igneous rocks ca 1870 1900 ma which provide a link with the ponte messuli complex along strike to the se in northern mozambique in sw tanzania rocks from the mgazini area gave ubendian protolith ages of ca 1980 1800 ma but these rocks underwent late mesoproterozoic high grade metamorphism between 1015 and 1040 ma one granitoid gave a crystallisation age of ca 1080 ma correlating with known mesoproterozoic crust to the east in se tanzania and ne mozambique however while the crust in the mgazini area was clearly one of original ubendian age reworked and intruded by granitoids at ca 1 ga the crust of se tanzania is a mixed mesoproterozoic terrane and a continuation from ne mozambique hence the mgazini area lies at the edge of the ubendian belt which was re worked during the mesoproterozoic orogen south irumide belt providing a further constraint on the distribution of ca 1 ga crust in se africa hf data from near concordant analyses of detrital zircons from a sample from the tanzania craton lie along a pb loss trajectory lu hf 0 extending back to 3 9 ga this probably represents the initial depleted mantle extraction event of the cratonic core furthermore the hf data from all igneous samples regardless of age from the entire study area including the neoproterozoic granulite nappes show a shallow evolution trend lu hf 0 028 extending back to the same mantle extraction age this implies the entire tanzanian crust sampled in this study represents over 3 5 billion years of crustal reworking from a single crustal reservoir and that the innermost core of the tanzanian craton that was subsequently reworked was composed of a very depleted mafic source with a very high lu hf ratio our study helps to define the architecture of the tanzanian craton and its evolution from a single age source in the early eoarchaean

late orogenic gold mineralization in the western domain of the karagwe ankole belt central africa auriferous quartz veins from the byumba deposit rwanda abstract gold mineralization in the kab karagwe ankole belt of central africa dominantly occurs as quartz lode and ferruginous breccia systems primary gold mineralization in the kab is possibly linked to a deformational event in the early neoproterozoic which is accompanied by shear movements and post compressional magmatism the byumba deposit is chosen due to its position in the kab mesoproterozoic metasedimentary stratigraphy akanyaru supergroup interesting structural trend nw trending shear zone and its closeness to magmatic intrusions explorative drillings from the byumba deposit in rwanda were logged and sampled for further petrographic investigation in situ rb sr geochronology laser ablation inductively coupled plasma triple quadrupole mass spectrometry and micro x ray fluorescence xrf major and trace element mapping distinct phases of folding and shear deformation are present at byumba and three main phases of quartz veining are identified pre syn post folding the first quartz vein generation is layer parallel and folded along the bedding indicative of a pre folding origin v1 generation a second generation of quartz veins v2a is concentrated in fold hinges and related to the primary folding of the layers cleavage parallel and often boudinaged v2b quartz veins are emplaced during late stage folding post folding v3 massive or sigmoidal quartz veins crosscut the folds and cleavage element mapping of the gold by using xrf shows that chlorite rich quartz veins v3 host the primary gold mineralization in the form of sub micron gold and of small disseminated blebs cores additionally recorded a supergene enrichment phase of gold at reduction zone boundaries in situ rb sr geochronology on white mica of the crosscutting post folding quartz veins v3 point towards an early neoproterozoic age of 900 ma the gold mineralization at byumba is interpreted to be related to the regional early neoproterozoic compressional deformation event based on 1 the post folding timing of the auriferous veins 2 the observation of shear related textures both in the host rock and the veins and 3 the location of the byumba deposit within a regional shear structure this association with compressional shearing points towards an orogenic origin for gold mineralization however a magmatic hydrothermal influence from contemporaneous granitic magmatism cannot be excluded

platinum mineralization in the kapalagulu intrusion western tanzania low grade copper and nickel mineralization was found near the eastern shore of lake tanganyika at kungwe bay in the early part of the twentieth century the mineralization occurs in harzburgite at the base of a layered gabbro complex known as the kapalagulu intrusion emplaced between the paleoproterozoic ubendian basement and overlying neoproterozoic itiaso group metasediments several mining and exploration companies continued the geophysical and drilling exploration for base metals throughout the last century culminating in the discovery of high grade platinum group element pge mineralization associated with chromitite and sulfide bearing harzburgite within the southeastern extension of the kapalagulu intrusion known as the lubalisi zone that is covered by a layer of nickel rich laterite regolith the poorly layered southeastern harzburgite forms part of the 1500 m thick lower ultramafic sequence and resembles a dike like body that flares upwards into a succession of well layered gabbroic rocks of the upper mafic sequence no pge mineralization has been found in the layered gabbro all the mineralization is associated with the chromite and sulfide rich harzburgite of the lower ultramafic sequence and the laterite regolith overlying the mineralized harzburgite the lubalisi zone harzburgite is underlain by basal dunite and overlain by an interval of layered harzburgite and troctolite and this ultramafic sequence is folded into a syncline that plunges towards the northwest that has been modified by major dolerite filled faults orientated subparallel to the fold axial surface extensive deep drilling in the lubalisi zone of the kapalagulu intrusion shows that the folded harzburgite can be subdivided into a lower feldspathic harzburgite a harzburgite containing chromitite seams and intervals of sulfide and chromite mineralization known as the main chromite sulfide succession mcss an overlying sulfide rich harzburgite and an upper feldspathic harzburgite impersistent stratiform pge mineralized horizons occur within the mcss harzburgite from which drill core samples were taken for platinum group mineral pgm characterization from two drill holes where the pge reefs reach the surface there is residual pge mineralization within the laterite regolith from which drill core samples were taken from various laterite lithological units for pgm characterization as the harzburgite pge reefs contain significant concentrations of both sulfide and chromite including chromitite seams they resemble the pge rich chromitite seams of the bushveld complex rather than the pge bearing main sulfide zone of the great dyke and main sulfide layer of the munni munni complex the dominant pd pgm in three pge reef samples varies ranging n 164 relative wt from bismuthides 63 bismuthtellurides 19 and tellurides 6 to tellurides 39 bismuthtellurides 24 stannides 14 and alloys 13 and to antimon arsenides 33 stannides 21 bismuthides 17 tellurides 13 and alloys 10 from 13 5 to 21 0 of the total pd occurs as a solid solution in pentlandite the three samples have similar pt pgm modal distributions n 172 relative wt the dominant pt mineral is sperrylite 79 58 and 47 followed by tellurides 15 17 21 alloys 2 1 1 and sulfides 2 1 0 comparison of pd pt ratios from assays to those calculated from minerals show that the data for the pt and pd pgm are very robust confirming the concentration methodology and characterization study of samples from a shallow drill hole penetrating the laterite regolith shows that the primary pd mineralization has not survived oxidation is mainly dispersed but some was reconstituted to form secondary minerals cabriite unnamed tellurides a selenide a pd te hg mineral alloys and pd bearing secondary sulfides millerite and heazlewoodite the primary pt minerals are more resistant to oxidation and dissolution especially sperrylite and isoferroplatinum but it is likely that other pt alloys tetraferroplatinum and tulameenite are of secondary origin after dissolution of pt tellurides

weathering of pge sulfides and pt fe alloys in the freetown layered complex sierra leone fresh and weathered rocks and saprolite from horizon b of the freetown layered complex contain platinum group minerals pgm the pgm in the fresh rocks are 1 7 m across including cooperite pts isoferroplatinum pt3fe minor tetraferroplatinum ptfe tulameenite pt2fecu os bearing laurite rus2 and other base metal sulfide bms bearing pgm the weathered rocks contain fewer of those pgm but a high proportion of disordered cu pd bearing pt fe alloys the saprolite hosts scarce smaller 1 3 m ordered ptfe and disordered ptfe3 the pt fe alloys became increasingly fe rich as weathering proceeded pt fe oxides appeared during weathering copper sulfides associated with the primary pgm and cooperite with 3 pd were destroyed to provide the minor cu and pd found in some of the disordered pt fe alloys platinum and pd bearing saprolites have retained the original rock fabric and to a depth of about 2 m surround residual rocks that show progressive weathering corestones ground water passing through the saprolite has transported pt and pd and probably au in solution down slope into saprolite over unmineralized rocks transport is marked by changes in the pt pd ratio indicating that the metals have moved independently palladium is present in marginally higher concentrations in the deeper saprolite than in the corestones suggesting some retention of pd in the deeper saprolite platinum and pd are less concentrated in the upper saprolite than the deeper saprolite indicating surface leaching alteration occurred over a long period in an organic and microbial rich environment that may have contributed to the leaching and transport of pge

textural mineralogical and stable isotope studies of hydrothermal alteration in the main sulfide zone of the great dyke zimbabwe and the precious metals zone of the sonju lake intrusion minnesota usa stratigraphic offsets in the peak concentrations of platinum group elements pge and base metal sulfides in the main sulfide zone of the great dyke and the precious metals zone of the sonju lake intrusion have in part been attributed to the interaction between magmatic pge bearing base metal sulfide assemblages and hydrothermal fluids in this paper we provide mineralogical and textural evidence that indicates alteration of base metal sulfides and mobilization of metals and s during hydrothermal alteration in both mineralized intrusions stable isotopic data suggest that the fluids involved in the alteration were of magmatic origin in the great dyke but that a meteoric water component was involved in the alteration of the sonju lake intrusion the strong spatial association of platinum group minerals principally pt and pd sulfides arsenides and tellurides with base metal sulfide assemblages in the main sulfide zone of the great dyke is consistent with residual enrichment of pt and pd during hydrothermal alteration however such an interpretation is more tenuous for the precious metals zone of the sonju lake intrusion where important pt and pd arsenides and antimonides occur as inclusions within individual plagioclase crystals and within alteration assemblages that are free of base metal sulfides our observations suggest that pt and pd tellurides antimonides and arsenides may form during both magmatic crystallization and subsolidus hydrothermal alteration experimental studies of magmatic crystallization and hydrothermal transport deposition in systems involving arsenides tellurides antimonides and base metal sulfides are needed to better understand the relative importance of magmatic and hydrothermal processes in controlling the distribution of pge in mineralized layered intrusions of this type

evidence of deposition from individual tides and of tidal cycles from the francis creek shale host rock to the mazon creek biota westphalian d pennsylvanian northeastern illinois abstract a silt clayrock sequence in the francis creek shale pennsylvanian westphalian d of northeastern illinois is characterized by 1 the presence of numerous clayrock paired bands and 2 cyclic variations in thickness of silt between and within these clayrock pairs the sequence is interpreted to result from tidal influence on inner estuarine sediments the clayrocks representing deposition from suspension at times of tidal slack the interbedded silts and fine sands representing deposition from bottom traction during ebb and flood tidal flow the cyclic variations in thickness is interpreted to represent lunar neap spring forcing of tidal range in a dominantly diurnal tidal system the association of tidal characteristics provides a means whereby the rate of sedimentation can be quantified at about 1 0 m yr of compacted sediment during times of active deposition such a high rate of sedimentation would account for the rapid burial and preservation of soft bodied organisms as indicated by the associated mazon creek biota similar rates in similar settings i e tidal estuaries have recently been reported although the depositional setting of many westphalian sequences was favorable to the preservation of tidal features few examples have been recorded it is suggested that evidence of westphalian tides may have been misinterpreted or overlooked in field studies elsewhere

constraints on the formation of platinum group element deposits in igneous rocks mechanisms of concentration of the platinmn group and other chalcophile elements iu several platinum group element deposits in layered intrusions are evaluated in light of recently published data on element partitioning between immiscible sulfide and silicate melts the similarity of the partition coefficieut d super sulfide melt silicate melt of ir to that of pd indicates that the observed fractionation of these elements in nature resulted from a process other than or in addition to sulfide liquid immiscibility in experiments designed to test the possibility that this fractionation was due to the presence of os ir alloy sulfide and silicate melts were equilibrated with ir metal at 1 450 degrees c 8 kbars total pressure and under sulfur and oxygen fugacity conditions appropriate for nature the ir content of the sulfide liquid was found to be or 3 7 wt percent and that of the coexisting silicate melt inferred to be or 1 000 ppb these concentrations are far in excess of the ir contents of natural sulfides or rocks and indicate that os ir alloys are not stable in the presence of sulfide or silicate melts in nature using our preferred values of d super sulfide melt silicate melt for the chalcophile elements and their estimated concentrations in the mantle limits are determined for the possible ranges of concentrations and relative abundances of these elements in immiscible sulfide liquid formed in magma derived by partial melting of the mantle the ratios of highly chalcophile to moderately chalcophile elements and of metal to sulfur of several deposits including the j m reef stillwater complex the ug 2 bushveld complex and the lac des isles complex are inconsistent with concentration solely by segregation of sulfide liquid in contrast the merensky reef fits the geochemical criteria for formation by accumulation of magmatic sulfide stratigraphic variations in chalcophile element concentrations through the ug 2 and the main sulfide zone of the great dyke also indicate that metals must have been redistributed on at least the meter scale our analysis implies either different relative mobilities of the chalcophile elements relative to one another in a process postdating initial sulfide liquid deposition or concentration by a different mechanism

re os isotopic constraints on the source of platinum group minerals pgms from the vestřev pyrope rich garnet placer deposit bohemian massif abstract mineralogical studies of the heavy fraction from a holocene pyrope rich garnet placer deposit at vest ev krkonose piedmont basin bohemian massif have identified the presence of very rare grains of platinum group minerals pgm pt fe alloy grains are accompanied by os ir ru minerals native osmium iridium and ruthenium with inclusions of pt fe alloy and hongshiite ptcu this mineral assemblage is typical for several mantle settings including ophiolites the chemistry of the os ir ru minerals shows an enrichment of the pgm in ru which is typical of ophiolites the grain morphology of pgm and pyrope rich garnet mostly rounded with numerous euhedral subhedral grains does not exclude a common source in situ laser ablation mc icp ms was used to measure the re os isotopic compositions of single os rich grains which show heterogeneous subchondritic os isotopic compositions 187 os 188 os 0 12082 0 12505 0 00003 this precludes their low temperature origin and indicates derivation of platinum group elements pges essentially from mantle derived rocks without a significant contribution of crustal os the mantle model age t ma and re depletion model age t rd model ages range from 0 4 to 1 0 ga and most likely reflect a long history of melt depletion that affected the mantle sources of pgm

platinum group element distribution in base metal sulfides of the merensky reef from the eastern and western bushveld complex south africa base metal sulfides in magmatic ni cu pge deposits are important carriers of platinum group elements pge the distribution and concentrations of pge in pentlandite pyrrhotite chalcopyrite and pyrite were determined in samples from the mineralized portion of four merensky reef intersections from the eastern and western bushveld complex electron microprobe analysis was used for major elements and in situ laser ablation inductively coupled plasma mass spectrometry la icp ms for trace elements pge ag and au whole rock trace element analyses were performed on representative samples to obtain mineralogical balances in merensky reef samples from the western bushveld both pt and pd are mainly concentrated in the upper chromitite stringer and its immediate vicinity samples from the eastern bushveld reveal more complex distribution patterns in situ la icp ms analyses of pge in sulfides reveal that pentlandite carries distinctly elevated pge contents whereas pyrrhotite and chalcopyrite only contain very low pge concentrations pentlandite is the principal host of pd and rh in the ores palladium and rh concentrations in pentlandite reach up to 700 and 130 ppm respectively in the samples from the eastern bushveld and up to 1 750 ppm pd and up to 1 000 ppm rh in samples from the western bushveld only traces of pt are present in the base metal sulfides bms pyrrhotite contains significant though generally low amounts of ru os and ir but hardly any pd or rh chalcopyrite contains most of the ag but carries only extremely low pge concentrations mass balance calculations performed on the merensky reef samples reveal that in general pentlandite in the feldspathic pyroxenite and the pegmatoidal feldspathic pyroxenite hosts up to 100 of the pd and rh and smaller amounts 10 40 of the os ir and ru chalcopyrite and pyrrhotite usually contain less than 10 of the whole rock pge the remaining pge concentrations and especially most of the pt up to 100 are present in the form of discrete platinum group minerals such as cooperite braggite sperrylite moncheite and isoferroplatinum distribution patterns of whole rock cu ni and s versus whole rock pd and pt show commonly distinct offsets the general sequence of offset patterns of pge and bms maxima in the order from bottom to top is pd in pentlandite pd in whole rock cu ni and s the relationship is not that straightforward in general some of the reef sequences studied only partially show similar trends or are more complex in general however the highest pd concentrations in pentlandite appear to be related to the earliest volumetrically rather small sulfide liquids at the base of the merensky reef sequence a possible explanation for the offset patterns may be rayleigh fractionation

the roles of magmatic and hydrothermal processes in pge mineralization ferguson lake deposit nunavut canada the ferguson lake ni cu co platinum group element pge deposit in nunavut canada occurs near the structural hanging wall of a metamorphosed gabbroic sill that is concordant with the enclosing country rock gneisses and amphibolites massive to semi massive sulfide occurs toward the structural hanging wall of the metagabbro and a low sulfide high pge style of mineralization sulfide veins and disseminations locally occurs 30 50 m below the main massive sulfide water rock interaction in the ferguson lake ni cu co pge deposit is manifested mostly as widespread post metamorphic epidote chlorite calcite veins and replacement assemblages that contain variable amounts of sulfides and platinum group minerals pgm pgm occur as inclusions in magmatic pyrrhotite and chalcopyrite in both the massive sulfide and high pge zones at the contact between sulfides and hornblende or magnetite inclusions in the massive sulfide in undeformed sulfide veins and adjacent chlorite and or epidote halos in hornblende adjacent to hydrothermal veins and in plagioclase chlorite aggregates replacing garnet cemented by sulfide the pgm are mostly represented by the kotulskite pdte sobolevskite pdbi solid solution but also include michenerite pdbite froodite pdbi2 merenskyite pdte2 mertieite ii pd8 sb as 3 and sperrylite ptas2 and occur in variety of textural settings those that occur in massive and interstitial sulfides interpreted to be of magmatic origin and formed through exsolution from base metal sulfides at temperatures 600 c are dominantly bi rich i e te bearing sobolevskite whereas those that occur in late stage hydrothermal sulfide silicate veins and their epidote chlorite alteration halos tend to be more te rich i e bi bearing kotulskite the chemistry and textural setting of the various pgm supports a genetic model that links the magmatic and hydrothermal end members of the sulfide pgm mineralization the association of pgm with magmatic sulfides in the massive sulfide and high pge zones has been interpreted to indicate that pge mineralization was initially formed through exsolution from base metal sulfides which formed by magmatic sulfide liquid segregation and crystallization however the occurrence of pgm in undeformed sulfide bearing veins and in their chlorite epidote halos and differences in pgm chemistry indicate that hydrothermal fluids were responsible for post metamorphic redistribution and dispersion of pge

surface transformations of platinum grains from fifield new south wales australia a growing literature is demonstrating that platinum pt is transformed under surface conditions yet bio geochemical processes at the nugget soil solution interface are incompletely understood the reactivity of pt exposed to earth surface weathering conditions highlighted by this study may improve our ability to track its movement in natural systems e g focusing on nanoparticles as a strategy for searching for new undiscovered sources of this precious metal to study dissolution re precipitation processes of pt and associated elements grains of pt fe alloy were collected from a soil placer deposit at the fifield pt field australia optical and electron microscopy revealed morphologies indicative of physical transport as well as chemical weathering dissolution pits cavities striations colloidal nano particles and aggregates of secondary pt platelets as well as acicular iron fe hydroxide coatings were observed fib sem ebsd combined with s m xrf of a sectioned grain showed a fine layer of up to 5 mm thick composed of refined aggregates of 0 2 to 2 mm sized crystalline secondary pt overlying more coarsely crystalline pt fe alloy of primary magmatic origin these results confirm that pt is affected by geochemical transformations in supergene environments structural and chemical signatures of grains surfaces rims and cores are linked to the grains primary and secondary trans formational histories and pt mobility can occur under earth surface conditions intuitively this nanophase pt can disperse much further from primary sources of ore than previously thought this considerable mineral reactivity demonstrates that the formation and or release of pt nanoparticles needs to be measured and incorporated into exploration geochemistry programs

a new wind vector algorithm for c band sar ocean wind speed and wind direction are estimated simultaneously using the normalized radar cross sections spl sigma sup 0 corresponding to two neighboring 25 km blocks within a given synthetic aperture radar sar image having slightly different incidence angles this method is motivated by the methodology used for scatterometer data the wind direction ambiguity is removed by using the direction closest to that given by a buoy or some other source of information we demonstrate this method with 11 envisat advanced sar sensor images of the gulf of mexico and coastal waters of the north atlantic estimated wind vectors are compared with wind measurements from buoys and scatterometer data we show that this method can surpass other methods in some cases even those with insufficient visible wind induced streaks in the sar images to extract wind vectors

evidence for 18 6 year lunar nodal drought in western north america during the past millennium analysis of 38 tree ring chronologies yields evidence for enhanced drought conditions every 18 6 years in the north american interior for the past millennium cross spectra between a drought area index and 26 temperature records confirm the analysis of currie 1979 1981d as regards a lunar nodal mn term in these data discussion of the mn tidal constituent which appreciably modulates amplitude and phase of four main constituents m2 k1 o1 mf is presented and implications for agriculture are surveyed

late precambrian tidal rhythmites in south australia and the history of the earth s rotation sedimentary rhythmites of siltstone and fine sandstone from late precambrian c 650 800 ma glaciogenic formations in south australia are interpreted as distal ebb tidal deposits that record variability in the velocity and range of palaeo ebb tides variations in lamina thickness encode a full spectrum of palaeotidal cycles including semidiurnal diurnal fortnightly and monthly tidal cycles as well as the lunar apsides perigee and nodal cycles a half yearly oscillation is attributable largely to a beat between the fortnightly tidal cycles of luni solar conjunction and lunar declination the lunar nodal cycle is discernible as an amplitude modulation of this beat oscillation the data allow determination of the earth9s palaeorotation and the past dynamics of the earth moon system with an accuracy previously unattainable for the precambrian the late precambrian c 650 ma year contained 13 1 0 5 lunar months and c 400 20 days and the late precambrian lunar month c 30 5 1 5 days these values suggest an average equivalent phase lag near 3 since late precambrian time rather than the present value of 6 the period of 19 5 0 5 years determined for the lunar nodal cycle c 650 ma ago indicates a lunar distance 96 9 1 7 of the present distance the low rate of lunar recession since late precambrian time revealed by the rhythmite data militates against a close approach of the moon during the proterozoic precambrian sedimentary rhythmites may hold a key to the early history of the earth9s rotation

geochemistry and mineralogy of platinum group elements at hartley platinum mine zimbabwe the main sulfide zone msz is the most prominent layer of the great dyke layered intrusion carrying economic platinum group element pge mineralization at the hartley platinum mine the msz is up to some meters thick and is composed of a basal pge rich subzone which overlaps slightly with a base metal sulfide rich subzone on top based on the degree of sulfide mineralization and pge ratios the subzones can be further divided into geochemically distinct layers stratigraphically upwards the msz displays metal profiles characterized by increasing cu ni pt pd and ppge ipge ratios accompanied by a general element decoupling in the order pd pt base metals more specifically ipge pd pt ni cu co s sulfides au te bi the fine structure of the msz is regarded to reflect primary magmatic features of consecutive batches of sulfide accumulation concomitant scavenging of pge and fractionation sulfide deposition was followed by a second limited subsolidus stage of pge redistribution mineralogically most of the pd and rh is hosted in pentlandite whereas pt is dominantly present in the form of discrete platinum group minerals pgm within the msz sequence sperrylite is present throughout the pge subzone of the msz cooperite braggite occur mainly in its basal part and the pt pd bismuthotellurides concentrate at the top these findings indicate that a large proportion of the pge primarily concentrated in sulfide under magmatic conditions was redistributed in the subsolidus stage and formed discrete pgm with available reactant partners chemical gradients and magmatic hydrothermal fluids probably led to small scale redistribution of pge within the msz

annual variation in near earth solar wind speed evidence for persistent north south asymmetry related to solar magnetic polarity we study the annual variation in solar wind speed at earth s orbit and in geomagnetic activity since mid 1960 s the two parameters depict a very similar annual variation during the whole period annual variation has maximum amplitude around sunspot minima the phase of annual variation reverses soon after solar maxima following the sun s polarity reversal and indicating a new type of 22 year periodicity stronger solar wind is found at or close to the earth s highest northern southern resp heliographic latitudes during solar minima with a negative positive magnetic polarity this implies an asymmetric sw speed distribution across heliographic equator such that the minimum speed region during solar minimum times is displaced away from heliographic equator towards the northern magnetic hemisphere this may result e g from a systematically larger extension of polar coronal holes from the sun s magnetic south pole toward solar equator we exclude the earlier explanations proposed for annual variation such as accumulation of small comets within 1 au or internal solar variation

contrasting mineralogical and processing potential of two mineralization types in the platinum group element and ni bearing kapalagulu intrusion western tanzania abstract the kapalagulu layered ultramafic and mafic intrusion is emplaced between the paleoproterozoic ubendian basement and overlying neoproterozoic itiaso group metasedimentary rocks located near the western shore of lake tanganyika high grade platinum group element pge mineralization 1 6 g t pt pd au is associated with chromitite and sulfide bearing harzburgite within the southeastern extension of the intrusion known as the lubalisi zone which is covered by a layer of nickel rich 0 2 2 ni laterite regolith that contains linear areas of pge mineralization in the lubalisi zone the mineralization may be divided into several significant geometallurgical domains a high grade pge mineralization 1 6 g t pt pd au associated with stratiform pge reefs and chromitite seams within a harzburgite unit b high grade pge mineralization up to 12 g t pt pd au associated with small bodies and veins of nickel massive sulfide within harzburgite below pge bearing reefs and chromitite seams c low grade pge mineralization 0 1 0 5 g t pt pd au associated with a sulfide mineralized harzburgite unit above the pge bearing reefs d laterite style residual pge mineralization 0 2 4 g t pt pd au associated with chromite concentrations in the saprolite and overlying red clay horizons of the laterite regolith and e supergene ni associated with the saprock and overlying saprolite clay mineralogical study of three samples from the pge reef consisting of high grade pge chromitite and harzburgite indicate that this mineralization will give a good metallurgical response to conventional grinding and floatation due to the relatively coarse grained nature of the pgm p80 from 37 to 52 m association with base metal sulfides and unaltered gangue minerals wilhelmij and cabri 2016 in contrast mineralogical and metallurgical study of the ni and pge mineralized laterite indicate that it cannot be processed using conventional mineral processing techniques but that a hydrometallurgical route should be used to recover the base and precious metals because any process is very much deposit controlled significant metallurgical and geometallurgical testing of mineralized samples as well as pilot plant testing will be required to arrive at feasibility studies

horizontal and vertical variations in noble metal distribution in the great dyke of zimbabwe a model for the origin of the pge mineralization by fractional segregation of sulfide abstract a study based largely on diamond drilling undertaken specifically for research purposes has shown that in the darwendale subchamber of the great dyke of zimbabwe there are three principal sulfide zones that are located in subunits 1a and 1b of cyclic unit 1 sulfide zone 1 the main sulfide zone msz is subdivided into three subzones the base of the msz occurs at the top of the main bronzitite layer of cyclic unit 1 extending into the overlying websterite the noble metals are concentrated at the bases of each sulfide zone and subzone and decrease in concentration rapidly upwards whereas variations in the concentrations of base metals such as cu correlate well with variations in the amounts of sulfide across each zone when calculated in terms of their concentration in 100 sulfides it is found that pd decreases upwards most rapidly followed by pt and then au with the cu concentration remaining essentially constant the decrease of the noble metals is interpreted in terms of simple rayleigh fractionation as the consequence of the removal of sulfide the observed correlation between the tops and bottoms of sulfide zones with small variations in the mg number of bronzite is taken to indicate that in zones 2 and 3 sulfide segregation started as a result of crystallization increasing the s content of the resident magma in the chamber until it reached that of sulfide saturation coupled with fresh inputs of sulfide unsaturated more primitive magma causing the resident magma to become unsaturated again this recharge by primitive magma also raised the pge content of the sulfides segregating from the trapped intercumulus liquid indicating that the resident magma was becoming recharged in pge in this way subunits i and ii of sulfide zone 1 are thought to contain too much sulfide to be explicable in this way and it is suggested that mixing with a plagioclase saturated magma possibly generated around the flanks of the intrusion was responsible for these subunits the sulfides of each of the sulfide zones are more concentrated at the margin than at the axis of the great dyke but have a lower tenor of noble metals it is proposed that the axially located sulfides were the consequence of magma sulfide equilibration at a higher r factor magma sulfide ratio than those at the margin the magma sulfide partition coefficients required to explain the percentage difference in the tenors of the different noble metals between axis and margin in this way are the same as those required to explain the vertical variations in noble metal tenor in terms of the fractional segregation model

platinum group element distribution in the oxidized main sulfide zone great dyke zimbabwe in the great dyke mafic ultramafic layered intrusion of zimbabwe economic concentrations of platinum group elements pge are restricted to sulfide disseminations in pyroxenites of the main sulfide zone msz oxidized ores near the surface constitute a resource of ca 400 mt mining of this ore type has so far been hampered due to insufficient recovery rates during the oxidation weathering of the pristine ores most notably s and pd are depleted whereas cu and au are enriched the concentrations of most other elements including the other pge remain quite constant in the oxidized msz pge occur in different modes 1 as relict primary pgm mainly sperrylite cooperite and braggite 2 in solid solution in relict sulfides dominantly pd in pentlandite up to 6 500 ppm pd and 450 ppm pt 3 as secondary pgm neoformations i e pt fe alloy and zvyagintsevite 4 as pge oxides hydroxides that replace primary pgm as the result of oxidation 5 hosted in weathering products i e iron oxides hydroxides up to 3 600 ppm pt and 3 100 ppm pd manganese oxides hydroxides up to 1 6 wt pt and 1 150 ppm pd and in secondary phyllosilicates up to a few hundred ppm pt and pd in the oxidized msz most of the pt and pd are hosted by relict primary and secondary pgm subordinate amounts are found in iron and manganese oxides hydroxides the amount of pge hosted in solid solution in sulfides is negligible considerable local variations in the distribution of pge in the oxidized ores complicate a mineralogical balance experiments to evaluate the pge recovery from oxidized msz ore show that using physical concentration techniques i e electric pulse disaggregation hydroseparation and magnetic separation the pge are preferentially concentrated into smaller grain size fractions by a factor of 2 highest pge concentrations occur in the volumetrically insignificant magnetic fraction this indicates that a physical preconcentration of pge is not feasible and that chemical bulk leaching methods need to be developed in order to successfully recover pge from oxidized msz ore

climate record in varved sediments of the eocene green river formation three time series of varves from the eocene green river formation in colorado consisting of 1469 1869 and 4158 annual thickness measurements were analyzed by a variety of statistical techniques to characterize climate variations the analyses reveal that the time series are lognormal after adjustment for a trend moreover the analyses suggest that the time series are generally nonstationary and are noiselike in structure fourier spectra and maximum entropy spectra of the time series are reddish indicating the importance of nonoscillatory changes in varve thickness at time scales ranging from tens to hundreds of years there are no statistically significant peaks in fourier spectra for the entire time series however one segment of one of the three time series displays statistically significant peaks corresponding to periods of 10 8 and 5 4 years overall the time series are suggestive of a nonperiodic and weakly predictive climate during the time periods corresponding to deposition of the varves

u pb monazite ages of the kabanga mafic ultramafic intrusions and contact aureoles central africa geochronological and tectonic implications mafic ultramafic rocks of the kabanga musongati alignment in the east african nickel belt occur as bushveld type layered intrusions emplaced in metasedimentary sequences the age of the mafic ultramafic intrusions remains poorly constrained though they are regarded to be part of ca 1375 ma bimodal magmatism dominated by voluminous s type granites in this study we investigated igneous monazite and zircon from a differentiated layered intrusion and metamorphic monazite from the contact aureole the monazite shows contrasting crystal morphology chemical composition and u pb ages monazite that formed by contact metamorphism in response to emplacement of mafic ultramafic melts is characterized by extremely high th and u and yielded a weighted mean 207pb 206pb age of 1402 9 ma which is in agreement with dates from the igneous monazite and zircon the ages indicate that the intrusion of ultramafic melts was substantially earlier by 25 m y 95 confidence than the prevailing s type granites calling for a reappraisal of the previously suggested model of coeval bimodal magmatism monazite in the metapelitic rocks also records two younger growth events at ca 1375 ma and ca 990 ma coeval with metamorphism during emplacement of s type granites and tin bearing granites respectively in conjunction with available geologic evidence we propose that the kabanga musongati mafic ultramafic intrusions likely heralded a structurally controlled thermal anomaly related to nuna breakup which culminated during the ca 1375 ma kibaran event manifested as extensive intracrustal melting in the adjoining karagwe ankole belt producing voluminous s type granites the grenvillian aged ca 990 ma tin bearing granite and related sn mineralization appear to be the far field record of tectonothermal events associated with collision along the irumide belt during rodinia assembly since monazite is a ubiquitous trace phase in pelitic sedimentary rocks in contact aureoles of mafic ultramafic intrusions and in regional metamorphic belts our study highlights the potential of using metamorphic monazite to determine ages of mafic ultramafic intrusions and to reconstruct postemplacement metamorphic history of the host terranes georef subject absolute age africa east africa gabbros igneous rocks intrusions mesoproterozoic metamorphic rocks metamorphism metasedimentary rocks monazite neoproterozoic nesosilicates norite orthosilicates phosphates plutonic rocks precambrian proterozoic silicates tanzania u pb ultramafics upper precambrian zircon zircon group

structure of the surface wind field from the seasat sar an intensive analysis of the vector wind field of one seasat data set pass 1339 is described wind speed and direction signatures are found in seasat sar synthetic aperture radar images and the resulting estimates are compared with the seasat a scatterometer system sass and simultaneous noaa p 3 aircraft measurements a power law is presented to relate sar measured backscatter to sass estimated wind speed and the sar estimates are shown to agree with the sass estimates to within a standard error of 0 7 m s over a range of wind speeds from 3 m s to 13 m s the surface expressions of atmospheric roll vortices are apparent in several of the sar images and may be responsible for the wind direction signature in these cases wind field estimates averaged over regions of variable sizes are possible because of the high resolution of sar imagery a method for extracting low wave number directionality and its variability from sar spectra is described and sar direction estimates obtained from spectra of 6 4 km square images are shown to have a precision of approximately 10 although a comparison data set that could validate these higher resolution estimates is lacking averages over 40 km square regions are in good agreement with the other wind field information the sar wind direction estimates yield a more complete interpretation in a region with a turning wind field near a front that is ambiguous with only sass observations in this region the flow patterns of the high resolution estimates appear consistent with our knowledge of the overall circulation considering all the observations however in another region the small scale variability is too large and random to represent real wind variability although averages derived from these estimates are still reliable

layering and precious metals mineralization in the rincon del tigre complex eastern bolivia geologic investigations supporting recent technically successful platinum exploration of the rincon del tigre complex a ca 4 6 km thick layered sill have provided significant new understandings of the layering development and precious metals mineralization of one of south america s largest mafic ultramafic intrusions three macrocyclic units are recognized from the base up basal unit unit 1 and unit 2 each most likely the product of a major replenishment with mafic magma of one compositional type the ultramafic rocks of all three macrocyclic units largely comprise repeated minor cyclic units of poikilitic harzburgite granular harzburgite olivine bronzitite each a few meters thick with occasional development of bronzitite at the top mafic rocks absent from unit 1 and present only locally in the basal unit form a well developed norite gabbro magnetite gabbro sequence in the upper half of unit 2 associated with the base of the magnetite gabbro is the precious metals zone a persistent zone of low grade subeconomic sulfide and precious metals mineralization 80 to 185 m thick the precious metals zone comprises an upper cu sulfide rich portion its base the sulfide phase boundary about 23 to 70 m above the magnetite phase boundary and a lower precious metals rich portion the lower part straddling the magnetite phase boundary the individual precious metals are concentrated in separate sulfide poor subzones each up to many tens of meters thick in the stratigraphic order rh pt pd au with peak offsets at the same scale modal layering of silicates and magnetite is well developed in the mafic rocks as is modal and cryptic precious metals layering of the sulfides in the precious metals zone all at three different superimposed scales from about 3 m to several tens of meters a possible interpretive framework for the cryptic layering and perhaps for much of the other layering including the meter scale layering of the ultramafic rocks is provided by the likely compositional stratification of the resident magma initiated by the fluid dynamics of the replenishment and mixing processes and in the magnetite gabbro by repeated convective overturns caused by the onset of magnetite precipitation the characteristics of the precious metals zone fit an orthomagmatic model the composition stratigraphic relations and modal correlation with magnetite of the sulfides all imply that s saturation was induced by loss of feo to magnetite precipitation this took place after prolonged closed system fractionation following the last major magma injection at the base of unit 2 during which crystallization of olivine pyroxene plagioclase and finally magnetite precipitation led to overall s cu and precious metals enrichment and ni and finally fe depletion of the magma the development of the mineralization and its complex layering was controlled by sulfide segregation rayleigh fractionation the different but generally very high partition coefficients dsilicate sulfide for each precious metal and repeated convective overturns at three different superimposed scales the decoupled metals distribution is analogous and similar in origin to that of the main sulfide zone of the great dyke zimbabwe and of the main sulfide layer offset portion of the munni munni complex western australia but the stratigraphic order of metal enrichment peaks is different pt pd au vs pd pt au this suggests a reversal in the relative magnitudes of the partition coefficients dpt and dpd that was possibly brought about by changes in the temperature and o s and fe activities of the magmas between the middle and late stages of crystallization constancy of d values during fractionation of mafic magma should not be assumed the precious metals zone is one of several skaergaard type strata bound precious metals zones associated with magnetite gabbros in the upper levels of many layered intrusions and the origin of the skaergaard mineralized zone itself including the potentially economic platinova reefs can be explained in terms of the precious metals zone model presented here the very low grade great thickness and wide separation of individual metal subzones in the precious metals zone are similar to those of the subeconomic axial facies of the main sulfide zone in the wider parts of the great dyke cf the economic more compact marginal facies on the basis of the lateral and vertical thermal model of the great dyke magma chamber the subeconomic status of the precious metals zone may be in large part a function of the specific fluid dynamics of the rincon del tigre magma chamber

direct evidence for archean crust in the western domain of the karagwe ankole belt rwanda implications for neoarchean to paleoproterozoic crustal evolution abstract neoarchean orthogneiss dated at 2629 9 ma by sims zircon u pb has been discovered in the rusizian terrane of the western domain of the karagwe ankole belt rwanda redefining the nw margin of the tanzania craton metamorphism of the orthogneiss at 1959 4 ma was associated with paleoproterozoic granitic arc magmatism in the rusizian terrane over the period ca 2000 1840 ma the major and trace element composition of the peraluminous calc alkalic orthogneiss is consistent with magmatism in an arc setting at an active continental margin its nd depleted mantle model age of ca 2800 ma is consistent with derivation by partial melting and reworking of older crust at the nw margin of the tanzania craton the paleoproterozoic thermal event was related to the accretional collision between the congo and tanzania cratons during the assembly of the nuna supercontinent future tectonic modelling of the karagwe ankole belt must take into account the presence of archean basement beneath the western domain

estimation of sea surface wind vector using radarsat data abstract synthetic aperture radars sar have higher spatial resolution than scatterometers and we can obtain more detailed wind vector information from space borne sar data this type of high spatial resolution wind information can be very useful particularly in coastal regions where the scatterometer wind information can be altered by the coastal effects because the larger footprint spatial averaging of backscattered energy is unavoidable this paper tested radarsat data with cmod 4 and cmod ifr2 algorithms for extracting wind information over selected coastal test areas around the korean peninsula because the cmod algorithms were originally developed for the c band vv polarized scatterometer data we applied currently available polarization ratio models e g kirchhoff elfouhaily and thompson models for radarsat data processing three test areas the west sea and east sea on both sides of the korea peninsula and the open sea off jeju island were tested with the radarsat fine standard and scansar beam mode data the correlation of the wind vector results with the buoy and meteorological station data agrees well with each other but with some variations the rms error for the sar derived wind direction is somewhat great and the wind speed rms error using cmod 4 algorithm is about 1 7 m s kirchhoff 1 84 m s elfouhaily and 2 4 m s thompson polarization ratio model respectively

wind vector retrieval using ers 1 synthetic aperture radar imagery an automated algorithm intended for operational use is developed and tested for estimating wind speed and direction using ers 1 sar imagery the wind direction comes from the orientation of low frequency linear signatures in the sar imagery that the authors believe are manifestations of roll vortices within the planetary boundary layer the wind direction thus has inherently a 180 spl deg ambiguity since only a single sar image is used wind speed is estimated by using a new algorithm that utilizes both the estimated wind direction and spl sigma sub 0 values to invert radar cross section models the authors show that 1 on average the direction of the roll vortices signatures is approximately 11 spl deg to the right of the surface wind direction and can be used to estimate the surface wind direction to within spl plusmn 19 spl deg and 2 utilizing these estimated wind directions from the sar imagery subsequently improves wind speed estimation generating errors of approximately spl plusmn 1 2 m s for ers 1 sar data collected during the norwegian continental shelf experiment in 1991

distribution and controls of platinum group element and base metal mineralization in the darwendale subchamber of the great dyke zimbabwe three major sulfide zones containing platinum group elements pge have been identified in the upper part of the ultramafic sequence of the great dyke the pge are at the bases of the sulfide zones the lower two have sparse sulfide mineralization over a thick interval the upper zone contains higher grade in a thinner stratigraphic layer none of the current models for the formation of stratabound sulfide deposits in layered intrusions adequately accounts for the mineralization the two lower sulfide zones are interpreted as resulting from progressive fractional segregation of sulfide from the silicate magma whereas the upper zone may have resulted from mixing of a relatively primitive magma at the base of the chamber with an evolved liquid derived from near the roof

solar cycles recorded in carboniferous glacimarine rhythmites western argentina relationships between climate and sedimentary environment abstract a carboniferous glacial episode has been widely known in the western part of argentina since ca 1910 in some sections it is associated with rhythmic sedimentation one of those rhythmite successions is interpreted as composed by varves annually deposited sedimentary cycles accumulated in a glacimarine environment it is further analyzed as a time series searching for cyclic signals several localities show varve like successions but only a few of them are not disturbed by reworking of bottom currents the rio francia section san juan argentina is well suited for varve counting as no current produced structures are recognized through the rhythmite interval and outcrops are good enough for counting directly in the field five varve types which show differences mainly in thickness grain size and dropstone frequency are recognized internally they are mostly symmetric distinguishing them from traditional normally graded varves a feature that has also been observed in holocene glacimarine varves the different varve types indicate varying positions with respect to the ice front because of disturbances or lack of well defined textural or colour contrasts only the three varve types with intermediate thicknesses were counted three units including 1028 690 and 465 varves were measured and then analyzed as time series in order to detect cyclic signals some differences in the quality of the cyclic signals can be seen in spectral analyses depending on the main type of the varve that composes the unit however a peak around 12 yrs can be seen in most of the spectra and is interpreted as the sunspot cycle record a secondary peak near 24 26 yr also may be related to this phenomenon hale cycle four major cycles recognized in the field but not in the spectra that represent major glacial advances and retreats can also be related to a solar periodicity of 2500 3000 yr the recognition of sunspot cycles in these carboniferous varves is not unusual as they are found in other ancient varve successions random processes typified by the distribution of dropstones played an important role in disturbing the climatic signal in this sedimentary environment the random process of sediment transport and deposition by ice rafting disturbs the cyclic climatic signal mainly in proximal varve successions increased water motion and distance to the source of sediment toward the open sea tend to produce more homogeneous very fine grained distal varves eventually causing the loss of varve like appearance the remaining varves located at intermediate positions with respect to the glacio related sediment source seem to be the best suited for recording climatic cyclicity as shown by spectral analyses

lateral and vertical distribution of platinum group elements and petrogenetic controls on the sulfide mineralization in the p1 pyroxenite layer of the darwendale subchamber of the great dyke zimbabwe the great dyke of zimbabwe is a major resource of platinum group elements pge and represents the world9s second largest deposit after the bushveld complex the darwendale subchamber contains more than 75 percent of all the pge mineralization in the great dyke and is the subject of this investigation the mineralization is closely associated with sulfide bearing zones located in pyroxenites toward the top of the ultramafic sequence this environment contrasts with many other mineralized layered intrusions in that substantial plagioclase crystallization did not occur prior to the formation of the pge enriched sulfide layers several sulfide layers occur within the bronzitites of the topmost cyclic unit rather than at the base suggesting that a major mixing event of residual magma with a new influx of primitive liquid was not the primary control of the mineralization the lower sulfide zones are disseminated over 10 to 20 m and contrast with the uppermost sulfide zone s1 which is highly mineralized and generally less than 1 5 m in thickness in the s1 zone values for pt are as high as 8 ppm over a narrow interval with pt pd close to 2 the maximum enrichment of pge occurs at the first appearance of sulfide as a result of the high partitioning of these elements into sulfide liquid the order of enrichment in the sulfide is ir pd pt raquo au cu ni and as the abundance of sulfide increases this phase becomes highly depleted in these elements the enrichment pattern is consistent with fractional segregation of sulfide repetition of the mineralization results from periodic disturbance of the magma system which is related to relatively small pulses of new magma and overturns within the stratified magma column the stratified magma was sustained by the repeated influxes of new magma entering as fountains a series of peaks for pt pd and the base metals occurs in the s1 zone contrasting patterns in the vertical profile for pt and pd are attributed to the differences in the effective partition coefficients of these two elements in an environment of rapidly changing sulfide proportions marked lateral variations in the degree and style of mineralization are observed from axis to margin of the subchamber whole rock concentrations of pt are highest nearest to the margin in the very narrow and highly mineralized sulfide zone s1 but these fall to low values in the axis where the zone is more disseminated in any given vertical section the maximum concentration of pge in sulfide is similar in each of the sulfide zones the total amount of pt for the entire mineralized zone and the concentration of pt in sulfide are however significantly higher in the axial facies compared to the margin there are also major lateral variations in maximum values of cu ni and ni cu ni these lateral variations are attributed to a difference in the thermal regimes arising from the flared shape of the great dyke in cross section and its influence on the fluid dynamics of the magma chamber extensive borehole drilling has permitted detailed two dimensional contour patterns to be deduced for the distribution of pt and pd and the base metals in the s1 suboutcrop over a large area of the darwendale subchamber

review of platinum group element distribution and mineralogy in chromitite ores from southern iran abstract the platinum group element pge potential of four chromite mining districts in southern and south eastern iran was investigated using geochemical and mineralogical methods a total of 20 mines and prospects were sampled in the neyriz ophiolite the abdasht and sikhuran complexes in the esfandagheh district and the faryab district chromitite and dunite were analysed for major trace element and pge geochemistry and the platinum group element mineralogy pgm in all mining districts chromites analysed from mantle tectonite probable transition zone and cumulate zone chromitites are refractory low in tio2 pge concentrations range from 5000 ppb with median values around 200 ppb in all complexes investigated the 187os 188os isotope ratios of chromitite indicate chondritic to slightly suprachondritic initial os mantle normalised pt ir ratios are generally low in the sikhuran complex pods of sulphide bearing chromitite 0 2 1 2 sulphides occur in a dunite zone interpreted as an ultramafic cumulate zone chromites in chromitite are characterised by high cr 75 81 and intermediate mg numbers 45 65 in associated dunite chromites cr 74 80 are more fe rich mg 29 50 chromitite and dunite carry up to 5 2 ppm total pge and are characterised by high normalised pt ir up to 5 pt pd large degrees of partial melting in a supra subduction zone geotectonic setting probably in a two stage melting process produced boninitic melts that crystallised refractory cr mg rich and ti poor chromite in some cases sulphide saturation was achieved during and after chromite formation saturation during chromite crystallisation is evidenced by ni cu rich sulphide droplets preserved in some chromitites sulphur saturation after chromite crystallisation governed the formation of ni rich sulphide liquids that exsolved monosulphide solid solution which later crystallised to form pge bearing pentlandite platinum and pd did not partition into pentlandite but crystallised as pt and pd rich pgm associated with the pentlandite

coastal wind field retrievals from ers synthetic aperture radar images a unique series of european remote sensing satellite ers 1 and 2 c band synthetic aperture radar sar images was obtained off the southern coast of norway during the coast watch 95 experiment in september 1995 in this paper we carry out a systematic analysis of the mesoscale coastal wind field conditions expressed in the sar images four different categories of phenomena including windrows fetch limited seas wind fronts and oceanographic fronts are examined and discussed the quantitative retrievals of the wind field are based on examination of both the sar image backscatter characteristics and the spectral properties results are compared and validated against coincident ship and buoy data providing complementary and independent observations of the oceanographic and meteorological conditions

the ages of the kabanga north and kapalagulu intrusions western tanzania a reconnaissance study the kabanga north and kapalagulu intrusions form part of a 500 km belt of intrusions within the kibaran orogenic belt of central africa some of the intrusions within the belt host important ore deposits notably ni sulfides at kabanga and ni laterites at musongati and kapalagulu despite the economic significance of the intrusions their ages have remained unclear here we present shrimp ii u pb zircon ages interpreted as magmatic ages of crystallization for the kabanga north intrusion 1403 14 ma and for the kapalagulu intrusion 1392 26 ma these data along with other geochronological and petrological data support a model of a broadly coeval and cogenetic suite of mafic ultramafic intrusions throughout the kibaran orogenic belt from northwest tanzania through burundi and possibly as far south as the democratic republic of congo

sulfide and platinum mineralization in the merensky reef evidence from hydrous silicates and fluid inclusions the base metal sulfides of the merensky reef are associated with hydrous silicates and intense deuteric hydrous alteration of cumulus and postcumulus silicates biotite and phlogopite crystallized in the vicinity of sulfides from a volatile enriched highly fractionated intercumulus melt amphibole chlorite and talc are later alteration phases of cumulus pyroxene and intercumulus plagioclase biotite is often accompanied by zircon rutile and quartz accessory quartz hosts a complex suite of h2o nacl cacl2 co2 ch4 fluid inclusions which have thus far not been described from the merensky reef the earliest fluid inclusion compositions are nacl h2o with less than 10 vol water co2 coexisting with a halite daughter crystal and brine and polyphase inclusions with up to six daughter and accidental phases and high contents of divalent cations the maximum trapping temperature is around 730 c at 4 to 5 kb pressure later inclusion generations are h2o nacl co2 h2o and pure co2 and ch4 the presence of cl rich fluids during the intercumulus stage of the crystallizing merensky reef is directly related to the mode of sulfide precipitation prior to sulfide unmixing in a hydrous magma sulfur is likely to be present as h2s sulfur saturation causes reaction of h2s with oxides of the silicate melt to form a sulfide melt plus water during reaction the magma is enriched in water until a separate fluid unmixes it carries all compounds with high fluid melt partition coefficients as well as metals capable of forming oh and cl complexes precious metals are assumed to have fractionated into the cl rich fluid as cl complexes rather than being dissolved in the sulfide melt during the cooling evolution of the fluid the precious elements precipitate around the periphery of sulfide melt droplets the model proposed explains the distribution pattern of platinum group minerals in the merensky reef better than any orthomagmatic mineralization concept offered so far

platinum group element oxides from the pirogues ophiolitic mineralization new caledonia origin and significance a number of platinum group element pge oxides and hydroxides have been discovered in a pge mineralization in the new caledonia ophiolite complex they include pt fe oxides containing variable amounts of oxygen 5 50 at ir fe rh oxides metal oxygen approximately 3 fe rh pt oxides and pt ir fe rh oxides metal oxygen 1 and an ru mn fe phase with a very low metal oxygen ratio interpreted as hydroxide these minerals have been found either as inclusions in chromite crystals from chromitite or as discrete grains from alluvial concentrates pt fe oxides are characterized by unusual textures including zoning and symplectite optical and scanning electron microscope descriptions of all the pge oxides are given together with electron microprobe analyses including the quantitative determination of oxygen the pge oxides may result from the alteration of magmatic pgm alloys and from primary crystallization of oxides in lateritic conditions their presence indicates the existence of mechanisms of transport and crystallization of pge as oxide in surface conditions mechanisms contributing to the redistribution and enrichment of pge in laterite

direct dating of ultramafic sills and mafic intrusions associated with ni sulfide mineralization in the thompson nickel belt manitoba canada the paleoproterozoic thompson nickel belt a segment of the northwest margin of the archean superior craton in manitoba is one of the largest nickel producing regions in the world and the second largest ni cu pge platinum group elements mining camp in canada after sudbury the nickel sulfide ores are hosted in or associated with ultramafic sills of komatiitic affinity that were emplaced into sulfur rich metasedimentary rocks of the ospwagan group and subsequently strongly deformed and metamorphosed amphibolite to granulite facies during the trans hudson orogeny a u pb zircon age of 1880 2 1 4 ma for a metaperidotite olivine orthopyroxenite from the upper part of a boudinaged ultramafic sill at the pipe 2 deposit is interpreted as the age of crystallization of the intrusion the first age from an ultramafic sill containing mined nickel sulfide in the thompson nickel belt this age confirms a direct petrogenetic relationship between ni cu pge mineralization and widespread mafic ultramafic magmatic activity molson igneous events at ca 1 88 ga along the superior province craton margin e g thompson nickel belt fox river belt cape smith belt new quebec orogen at the south pit of the thompson mine a u pb zircon age of 1860 7 0 7 ma for a composite mafic intrusion garnet amphibolite to layered metagabbro identifies a new age of mafic magmatic activity in the thompson nickel belt that is unrelated to nickel sulfide mineralization the thermal effects of the trans hudson orogeny are recorded by a range of zircon dates and ca 1760 ma metamorphic titanite from the south pit intrusion and are consistent with interpretations that regional metamorphism occurred as a single progressive event the results of this study open up the possibility of using the high precision u pb geochronology of mineralized ultramafic rocks to precisely constrain the duration of mafic ultramafic magmatism in the thompson nickel belt and assess potential age differences between ultramafic sills across and along strike of this world class nickel metallotect

lacustrine varve formation through time abstract studies using sediment traps in lakes reveal that the seasonal flux of sediment regulates both the composition and timing of deposition of materials that reach the bottoms of lakes if the bottom waters of a lake are partly or totally anoxic the seasonally deposited materials are preserved as annual groupings of laminae varves common components that form individual laminae consist of allochthonous clastic material derived from the drainage basin precipitated carbonate minerals diatom frustules iron rich and manganese rich flocs autochthonous organic detritus and autochthonous and allochthonous materials resuspended from the bottom the style of varving has changed over geologic time reflecting changes in biologic evolution and types of materials available precipitated iron rich laminations were common in the middle precambrian graded sets of clastic organic laminations persisted through the precambrian prior to the evolution of bioturbating benthic organisms glaciolacustrine varves appear to have retained their distinctive character through time carbonate rich varves occurred sporadically in the precambrian and phanerozoic with the exception of diatoms major components of modern lacustrine varves were present through the paleozoic and mesozoic and yet varves are rare in strata of these ages and may have accumulated in marine to brackish water environments diatoms were introduced into lacustrine systems in early tertiary time and are common components of varves from then on diatom laminae combined with a greater chance for geologic preservation of younger lake deposits have increased the number of geologically young occurrences of varved sediments however seasonal associations of modern varve components and the processes they represent are present in ancient deposits and provide clues to the interpretation of ancient environments

can near inertial internal waves in the east sea be observed by synthetic aperture radar 1 near inertial 18 hours internal waves were observed in the mid western part of the east sea on may 18 and 19 2004 using c band synthetic aperture radar sar images current and temperature measurements obtained during the field experiment from 10 may to 9 june 2004 iwxes2004 internal wave experiment in the east sea were analyzed to investigate the processes by which near inertial internal waves can be seen in sar images and how they progress the spatial distributions of horizontal wavelength and phase speed estimated from two successively acquired sar images are consistent with those inferred from water temperature and current measurements carried out during the field experiment based on these two independent observations sar and iwxes2004 we report a strong possibility that the observed wave patterns in the sar images during iwxes2004 are near inertial internal waves propagating westward off the east coast of the korean peninsula with a phase speed of about 0 3 m s

an overview of chalcophile element contents of pyrrhotite pentlandite chalcopyrite and pyrite from magmatic ni cu pge sulfide deposits we have compiled the trace element concentrations in pyrrhotite pentlandite chalcopyrite and pyrite from magmatic ni cu pge ore deposits with the aim of understanding their petrogenesis and whether these minerals can be used as indicator minerals among the samples there are some of the most studied world class ni cu aguablanca duluth jinchuan noril sk talnakh kharaelakh sudbury voisey s bay and others and pge dominated bushveld lac des iles stillwater great dyke and penikat deposits crustal assimilation may be constrained using as se and sb se ratios in pentlandite the degree of interaction between the silicate and sulfide liquids r factor can be estimated by the content of highly chalcophile elements dsulf liq sil liq above 1000 in sulfide minerals the fractional crystallization of the sulfide liquid can be traced using se te ratios of pentlandite pyrite formed by exsolution from mss has higher rh ru ir and os than co existing pyrrhotite whereas pyrite formed by hydrothermal alteration of pyrrhotite inherits the rh ru ir and os contents of the pyrrhotite it replaced sulfide minerals are preserved in transported glacial cover and their trace element chemistry can be used to discriminate their source pentlandite from ni cu deposits has much lower rh and pd concentrations than those from pge dominated deposits pyrite from magmatic deposits has higher co sb and se as ratios relative to pyrite from hydrothermal deposits and chalcopyrite from magmatic deposits has much higher ni and lower cd concentrations than those from hydrothermal deposits

alteration of platinum group minerals and dispersion of platinum group elements during progressive weathering of the aguablanca ni cu deposit sw spain the distribution mineralogy and mobility of the platinum group elements pge in the surface environment are poorly understood this study of the lower less altered and upper more altered gossan overlying the aguablanca ni cu pge magmatic deposit spain has shown that the platinum group minerals pgm are progressively oxidised and dispersed into iron oxides that form the gossan a combination of the characterization of pge in host pgm using a scanning electron microscope and measurement of pge at lower concentrations in host iron oxides using laser ablation inductively coupled plasma mass spectrometry la icp ms has for the first time allowed the total distribution of the pge within a gossan to be documented this study has revealed a complete in situ alteration and dispersion sequence of the pgm including 1 breakdown of both the more stable pt arsenides pt pd tellurides and the less stable bismuthotellurides 2 formation of partially oxidised pgm 3 development of a wide range of oxidised pt and pd bearing phases 4 subsequent formation of fe pge oxides and pge hydroxides 5 incorporation of pge into ferruginous supergene products and lastly 6 concentration of pge at the edges of veins and iron oxides dispersion of pd is greater than for the other pge with pd being widely distributed throughout the iron oxides this oxidising environment produced pge oxides rather than pge alloys also commonly found in the surface environment especially in placers these results provide critical evidence for the stages of mineralogical change from pge host mineralogy in magmatic ores to surface weathering producing pge oxides

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