precious metals in magmas of east greenland factors important to the mineralization in the skaergaard intrusion a survey of precious metals ni and cu levels and their mutual relationships in several magma suites of the east greenland volcanic passive margin shows that the magma type which was parental to the skaergaard intrusion was markedly enriched in pt pd and especially au and relatively depleted in cu compared to the preceding voluminous plateau basalt volcanism it is likely that elevated levels of platinum group minerals pgm and au were critical to the generation of ore grade precious metals in the intrusion the discovery of a major pcm and au deposit within the intrusion combined with these results suggests a set of criteria which might be critical to the formation of skaergaard type pcm deposits elsewhere in the province and worldwide the skaergaard mineralization appears to be a hitherto undescribed class of pgm and au mineralizations

new global marine gravity model from cryosat 2 and jason 1 reveals buried tectonic structure gravity models are powerful tools for mapping tectonic structures especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment we combined new radar altimeter measurements from satellites cryosat 2 and jason 1 with existing data to construct a global marine gravity model that is two times more accurate than previous models we found an extinct spreading ridge in the gulf of mexico a major propagating rift in the south atlantic ocean abyssal hill fabric on slow spreading ridges and thousands of previously uncharted seamounts these discoveries allow us to understand regional tectonic processes and highlight the importance of satellite derived gravity models as one of the primary tools for the investigation of remote ocean basins

investigations of the stillwater complex part v apatites as indicators of evolving fluid composition variations in the f cl and oh contents of apatite are not constrained by crystal chemical factors in contrast to micas and amphiboles and thus changes in the abundance of these components provide an indicator of halogen fugacity variations and insights into the degassing history of igneous rocks microprobe analysis of intercumulus apatites from the stillwater complex reveal that cl rich apatites typically containing 6 0 wt cl occur throughout the lower 1 3 of the complex excluding the basal series a change from cl rich to more f rich apatite occurs within olivine bearing zone i ob i of the banded series the host zone of the platiniferous j m reef although apatite compositions are somewhat variable above the j m reef more f rich apatites predominante and typically contain 1 2 wt f and 3 0 wt cl the most f rich apatites occur in the uppermost exposed cumulates pristine apatites from coeval sills and dikes from below the complex and from the basal series are similarly f rich in all apatites the cl and f contents are lower in rocks affected by later metamorphic fluids rare earth element ree concentrations in chlorapatites show a marked peak in the olivine rich rocks of the j m reef and contain up to 2 wt ce2o3 la2o3 the trend of first increasing then decreasing cl f ratios with stratigraphic height is modeled by a vapor driven zone refining process occurring within the cumulate pile causing cl enrichment in the interstitial melt accompanied by degassing at the top of the magma chamber causing overall loss of cl from the magma as crystallization proceeded the abrupt change from cl rich to more f rich apatites within ob i is interpreted as the result of a breakdown of the cl rich zone refining front and mixing with cl poor supernatant melt any high temperature fluids that exsolved and circulated through the lower 1 3 of the complex must have been enriched in cl and could have transported ree and trace metals

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

slab flattening and the rise of the eastern cordillera colombia abstract the topographic growth of a mountain belt is commonly attributed to isostatic balance in response to crustal and lithospheric thickening however deeper mantle processes may also influence the topography of the earth here we discuss the role of these processes in the eastern cordillera ec of colombia the ec is an active double vergent fold and thrust belt that formed during the cenozoic by the inversion of a mesozoic rift and topography there has risen up to 5 000 m cocuy sierra the belt is located 500 km away from the trench where two separate portions of the nazca plate subduct below the south american plate north of 5 n the ec rises above a flat slab subduction region volcanic arc migration implies slab shallowing by 10 ma and flattening up to the present day configuration at 6 ma the occurrence of a high vp vs anomaly and clustered seismicity below the belt at 160 km depth delineates the slab geometry and has been related to dehydration of the slab suggesting the presence of a hydrated mantle wedge we compiled thermochronologic data and inverted for the exhumation history of the chain over the last 20 ma using the age elevation relationship and the different closure temperatures of multiple thermochronologic systems results indicate that exhumation rates increased during the plio pleistocene at different wavelengths and amplitudes the small wavelength and large amplitude signals could be related to shallow crustal deformation whereas the source of the long wavelength and moderate amplitude signal has yet to be identified pulses of fast exhumation are found to be concomitant with the uplift that occurred from 7 ma to the present day previous studies suggested that the high topography of the chain cannot be achieved solely through isostatic adjustment the highest residual topography is centered on the highest elevations of the ec whereas the lowest residual topography corresponds to the magdalena valley following the regional slab geometry we propose that the recent uplift and exhumation events were triggered by the transition from regular to flat slab subduction along with the hydration of the mantle wedge above the slab we test the dynamic feasibility of our hypothesis with a series of numerical models for the present day state predicting the correct trends in elevation requires a flat slab geometry and a weak and buoyant mantle wedge

a gold and platinum mineralized layer in gabbros of the kap edvard holm complex field petrologic and geochemical relations the lower layered series a cumulates of the kap edvard holm complex contain a stratiform layer 1 to 20 m thick and 6 km long that is mineralized with gold and platinum group elements pges the lower layered series a cumulates 650 m thick are dominated by modally layered gabbro with lesser volumes of granular anorthosite and troctolite that together form 2 to 30 m thick layers that are most abundant above the mineralized layer minor volumes of poikilitic wehrlite form a few 1 to 2 m thick layers throughout the section field and petrographic relations as well as cryptic variations in cumulus mineral compositions indicate that the lower layered series a formed in an open system magma chamber from repeated magma injections within the mineralized layer average concentrations over 3 m thickness are 250 ppb pt 40 ppb pd and 50 ppb au with individual samples containing up to 5 ppm pt and 6 ppm au the concentrations of ir os ru and rh are uniformly low 20 ppb stratigraphic zoning of pt pd and au is consistent along strike with peak au concentrations overlying pt and pd peaks platinum minerals are primary pt fe alloy sperrylite and moncheite with secondary pt sulfide overgrowths gold occurs as alloys with ag and cu these minerals are found in contact with primary and secondary silicates and oxides and with intercumulus base metal sulfides that consist of the assemblage bornite chalcopyrite digenite deposition of the mineralized layer is associated in space and time with injections of plagioclase saturated primitive magma that mixed with more evolved resident magma these events caused efficient partitioning of au and pge into sulfides derived from a large volume of magma deposition of these sulfides to form the mineralized layer and subsequent deposition of the overlying anorthosite layers the wehrlite layers however were formed by postcumulus reactions between gabbro cumulates and hydrous intercumulus melts probably derived from the compacting cumulus pile subsolidus reactions between primary minerals and meteoric hydrothermal solutions caused extensive alteration of the mineralized gabbro oxidation and desulfurization of cu fe sulfides and local recrystallization of pge minerals

the formation of low volume high tenor magmatic pge au sulfide mineralization in closed systems evidence from precious and base metal geochemistry of the platinova reef skaergaard intrusion east greenland the majority of magmatic platinum group element pge deposits in layered mafic ultramafic intrusions are characterized by visible cu ni sulfide mineralization often located in the lower parts of a magmatic stratigraphy that show evidence of multiple magma injections and crustal contamination the platinova reef hosted by the 55 ma skaergaard intrusion east greenland is an example of a rarer type of pge deposit typified by a pd au cu dominant ni pt poor sulfide assemblage present in the upper parts of the host intrusion such deposits are generally considered to form through prolonged fractional crystallization of the magma with magnetite crystallization playing a role in late stage s saturation in the case of the skargaard intrusion the crystallization of the cumulate rocks and the generation of the mineralization hosted within them occurred within a closed system from a single homogenized batch of magma the platinova reef is characterized by offsets in the stratigraphic position of individual metal peaks an anomalously low sulfide content 1 ppm the intermediate zone pd 0 1 1 ppm the au zone au 1 ppm and the upper cu zone cu 200 ppm with precious metals 3 to 10 4 ppm low cu pd ratios and a remarkable upward decrease in pd au ratio from 40 to 1 whereas the sulfides in the cu zone are lower tenor 10 3 ppm pd higher volume and have high cu pd ratios and pd au to explain these features we present a multistage model of preconcentration dissolution upgrading and further metal enrichment this involves initial s saturation of the skaergaard magma at a late stage in the crystallization history the sulfide liquid produced scavenged metals from the entire volume of the chamber as it sank but was then dissolved as it came into contact with hotter fe rich and s undersaturated magma at the bottom of the chamber producing a highly metal rich magma layer at the bottom of the chamber this layer then became s saturated as a result of continued magnetite crystallization with the tiny sufide droplets that segregated into this precious metal enriched magma layer becoming highly enriched in pge themselves these were then trapped in situ by the crystallizing cumulate pile forming the subzone pd zone and intermediate zone sulfides formed in the rest of the chamber which was pge depleted at this stage would have been of low tenor these sulfide droplets grew and settled concentrating any remaining au when they encounter the floor to form the au zone and formed the low tenor sulfides of the cu zone above this we propose that the platinova reef exhibits an unusual example of a style of magmatic precious metal mineralization formed through sulfide dissolution upgrading in the latter stage of crystallization of a closed system the offsets in individual metal peaks mineralization demonstrate the relative partitioning of pd pt au cu into a sulfide liquid from a silicate magma and demonstrates the importance of partitioning behavior in low volume magmatic sulfide melts this feature can lead to the characteristic stratigraphic offsets in peak metal abundances the role of magnetite crystallization appears to be an important control on s saturation in such systems and thus may be an important indicator of the location of mineralized horizons in addition in such deposits indicators of crustal contamination by external s and multiple injections of magma common features of more ni rich pge reefs are less likely to be present

rhythmic layering formed by deposition of plagioclase phenocrysts from influxes of porphyritic magma in the cuillin centre isle of skye the outer bytownite gabbros of the cuillin centre on the isle of skye include a 175 m rhythmically layered sequence consisting of alternating layers of coarse grained massive gabbro and finer grained laminated gabbro we infer that the massive layers formed from repeated influxes of plagioclase phyric magmas which flowed into an existing magma chamber and deposited their entrained phenocrysts on the chamber floor the phenocrysts accumulated rapidly trapping large amounts of pore liquid in the cumulus pile during the intervals between influxes laminated gabbro layers formed by crystallization of the resident magma these layers accumulated more slowly by crystal settling or in situ growth allowing sufficient time for their pore liquids to be expelled by compaction or adcumulus crystallization the resulting fractions of trapped pore liquid were510 in laminated gabbros and 20 40 in massive gabbros high porosity in the massive layers is attributable to rapid accumulation of a lowdensity cumulus assemblage with trapping of pore liquids facilitated by the confining effects of impermeable laminated cumulates above and below among layers whole rock sr and nd isotope ratios reveal variable degrees of contamination by the archean crust beneath skye in the lower part of the section influxes of plagioclase phyric magma are associated with a shift to more primitive mineral compositions lower sr sr and higher nd nd indicating that the replenishing magmas were initially more primitive and less contaminated than those already residing in the chamber in accordance with this interpretation massive layers have lower sr sr than laminated layers consistent with derivation of the plagioclase phenocrysts from a less contaminated source in the upper part of the section the shift toward more primitive and less contaminated compositions is reversed suggesting that over time the replenishing magmas underwent increasing degrees of fractionation and contamination within the magmatic plumbing system that fed the intrusion these observations and inferences coupled with published evidence for other isotopic reversals support a model for growth of the cuillin centre by frequent replenishment of a small volume magma chamber with pulses of magma that underwent complex histories of fractionation and contamination during their transit through the archean crust periods dominated by assimilation and fractional crystallization within the feeder system alternated with periods of vigorous recharge by relatively primitive and less contaminated magmas these cycles were superimposed on an overall trend toward less contamination through time consistent with increasing dominance of the system by uncontaminated tholeiitic magmas as the hebridean magmatic province matured

post laxfordian magnetic imprint in the lewisian metamorphic complex and strike slip motion in the minches nw scotland the palaeomagnetic imprint in the lewisian metamorphic basement was acquired during early post laxfordian times and resides in magnetite filled microfractures an early magnetizing event dated c 1820 1750 ma was largely overprinted by a second event between 1700 and 1500 ma which was probably linked to retrogressive amphibolite fades metamorphism it reaches its greatest intensity in the south of the outcrop while the relict remanence survives most prominently in scourian terranes populations of magnetic components are offset by c 95 km of presumed caledonian sinistral strike slip in the minches intensities and susceptibilities of magnetization are enhanced in zones of laxfordian reworking relative to the relict scourian zones their distributions in the outer hebrides are reproduced in the mainland outcrop and can be matched following correction for the same sinistral strike slip a third line of evidence suggesting this motion is the offset of the deep crustal contribution to the aeromagnetic anomalies the fault zone accommodating this motion may now be represented by one or more of the low angle detachments bordering the western margin of the minches basin post laxfordian remanences in the outer hebrides are rotated by an average of 4 with respect to equivalent vectors on the mainland and may record later block rotations a mid proterozoic reconstruction of the lewisian complex derived from the magnetic evidence is presented

history of rift propagation and magnetization intensity for the cocos nazca sspreading center analysis of magnetic anomaly profiles collected nearly parallel to tectonic flow lines allows detailed interpretation of the complicated tectonic history of the cocos nazca spreading center forward models of the magnetic anomalies accounting for spreading rate variations ridge jumps asymmetric spreading magnetization intensity variations and bathymetry show excellent agreement with observed anomalies spreading rates can be constrained to a common finite rotation history through anomaly 4a with three changes in rates rate changes at about 1 5 ma and 4 1 ma correspond to changes in rate gradients and occur during the well calibrated part of the reversal timescale so they can unquestionably be identified as true changes in plate motion a 15 rate decrease at about 5 2 ma could be interpreted either as a change in plate motion or as an artifact of poor calibration of the older part of the timescale the change at 4 1 ma is especially important because many timescales are based on the assumption of constant spreading rate for this plate pair for 0 6 ma rift propagation has played a dominant role in the continuous reorganization of the geometry of the ridge axis propagation has been predominantly away from the hotspot with jumps predominantly south ward propagation rates have ranged from 30 to 120 mm yr commonly near 70 mm yr origin of most propagation sequences is difficult to interpret but many appear to involve discrete southward ridge jumps forming a new segment near the hotspot magnetic anomaly amplitude appears to be a reliable tracer of fe content of lavas several generalizations can be drawn about along axis variations in magnetization intensities since 8 ma high magnetizations are only observed at the far ends relative to the galapagos hotspot of segments at least 150 km long offset at the end of a high magnetization segment is at least 15 km and there are no offsets larger than 30 45 km between high magnetization segments and the reconstructed position of the hotspot we interpret these patterns to indicate that fractionated lavas erupt where gradients in magma supply cause along axis flow of evolved magma the gradients in supply result from subaxial flow of hotspot derived asthenosphere in a narrow conduit this flow is only partly obstructed by an offset of 20 30 km but entirely blocked by an offset of 50 km

oxygen isotope exchange and mineral alteration in gabbros of the lower layered series kap edvard holm complex east greenland multiple intrusions of gabbros mafic dikes and syenites in the kap edvard holm complex gave rise to prolonged circulation of meteoric hydrothermal solutions and extreme isotope exchange and mineral alteration in the 3600 m thick lower layered series gabbros in the lower layered series 18o of plagioclase varies from 0 3 to 5 8 and it decreases with an increase in the volume of secondary talc chlorite and actinolite in the same gabbros pyroxenes have a more restricted range in 18o from 5 0 to 3 8 and values of 18opyroxeneare independent of the abundance of secondary minerals which ranges from 14 to 30 these relations indicate that large amounts of water continued to flow through the rocks at temperatures of 500 600 c altering the gabbros to assemblages of talc chlorite actinolite epidote albite and causing significant oxygen isotope exchange in plagioclase but not in pyroxene the extensive low temperature secondary mineralization and 18o depletion of plagioclase in the lower layered series are associated with the later emplacement of dikes and gabbros and syenites which created new fracture systems and provided heat sources for hydrothermal fluid circulation this produced subsolidus mineral alteration and isotope exchange in the lower layered series that are distinct from those in the skaergaard and cuillin gabbros of the north atlantic tertiary province but are similar to those observed in some oceanic gabbros

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

metamorphic fluids and anomalous porosities in the lower crust abstract because of the plastic behavior of rocks at high temperature and confining pressure the porosity of lower crustal rocks is a variable dependent on the amount of free fluid present on the basis of hydrologic theory it is predicted that the permeability k of such rocks can be expressed by a relationship of the form k n where and n are material specific constants the exponent n is likely to vary between 3 and 6 whereas which is ultimately related to grain size and shape probably varies over several orders of magnitude consequently substantial contrasts between the k relationships of lower crustal rocks are to be expected the case in which thin crustal layers aquitards are characterized by relatively low values of or n with respect to the crust in general is of particular interest such layers would acquire high porosities in the presence of miniscule steady state crustal fluid fluxes and are a possible explanation for anomalous seismic brightspots and high electrical conductivities in the lower crust modelling of collision belt metamorphism suggests that even long after the cessation of active metamorphism and isostatic reequilibration relict metamorphic fluid fluxes could generate aquitard porosities between 0 1 and 0 4 adequate to explain such geophysical anomalies

evolution of regoliths and landscapes in deeply weathered terrain implications for geochemical exploration abstract thick commonly lateritic regoliths are widespread in inter tropical regions of the world and present particular challenges in exploration these are best tackled through a sound understanding of the evolution of the landscapes in which they occur the regoliths formed under humid warm to tropical conditions and although they may have been modified by later climatic changes i e to more humid or more arid conditions many chemical and mineralogical characteristics are retained these include the geochemical expressions of concealed mineralization erosional and depositional processes control the preservation and occurrence of specific regolith units that may be used as sample media and in turn target size element associations and contrast thereby influencing sampling procedures analysis and data interpretation these parameters are best summarized in terms geochemical dispersion models based on the degree of preservation of the pre existing lateritic regolith regolith landform mapping permits an assessment of the terrain in terms of such models in relict regolith landform regimes in which the lateritic regolith is largely preserved broad multi element anomalies in the upper ferruginous horizons lateritic residuum can be detected using sample intervals of 1 km or more in contrast in erosional regimes where this material is absent anomalies in upper saprolite and the soil and lag derived from it are more restricted in area and closer sampling intervals 200 40 m or less may be necessary lag and soil are generally ineffective in depositional areas except where the sediments are very thin e g

pacific antarctic australia motion and the formation of the macquarie plate summary magnetic anomaly and fracture zone data on the southeast indian ridge seir are analysed in order to constrain the kinematic history of the macquarie plate the region of the australian plate roughly east of 145 e and south of 52 s finite rotations for australia antarctic motion are determined for nine chrons 2ay 3ay 5o 6o 8o 10o 12o 13o and 17o using data limited to the region between 88 e and 139 e these rotations are used to generate synthetic flowlines which are compared with the observed trends of the easternmost fracture zones on the seir an analysis of the synthetic flowlines shows that the macquarie plate region has behaved as an independent rigid plate for roughly the last 6 myr finite rotations for macquarie antarctic motion are determined for chrons 2ay and 3ay these rotations are summed with australia antarctic rotations to determine macquarie australia rotations we find that the best fit macquarie australia rotation poles lie within the zone of diffuse intraplate seismicity in the south tasman sea separating the macquarie plate from the main part of the australian plate motion of the macquarie plate relative to the pacific plate for chrons 2ay and 3ay is determined by summing macquarie antarctic and antarctic pacific rotations the pacific macquarie rotations predict a smaller rate of convergence perpendicular to the hjort trench than the pacific australia rotations the onset of the deformation of the south tasman sea and the development of the macquarie plate appears to have been triggered by the subduction of young buoyant oceanic crust near the hjort trench and coincided with a clockwise change in pacific australia motion around 6 ma the revised pacific australia rotations also have implications for the tectonics of the alpine fault zone of new zealand we find that changes in relative displacement along the alpine fault have been small over the last 20 myr the average rate of convergence over the last 6 myr is about 40 per cent smaller than in previous models

magma mixing and constitution zone refining in the lac des iles complex ontario genesis of platinum group element mineralization the lac des iles complex is a 2 74 ga old composite intrusion emplaced into a granite and or granite gneiss terrane of the wabigoon subprovince northwestern ontario the complex consists of a gabbroic and ultramafic part the latter has been subdivided into northern and southern intrusive phases both the gabbroic and ultramafic parts contain platinum group element bearing ni cu sulfide mineralization although only the roby zone in the gabbro is of economic importance the southern ultramafic complex consists of a wehrlite to olivine clinopyroxenite core which is surrounded by websterite and gabbronorite cumulates petrographic and geochemical studies suggest that the northern ultramafic complex consists of at least eight cyclic units which ideally are composed of dunite and wehrlite to olivine clinopyroxenite cumulates at the base followed by clinopyroxenite and websterite locally grading into gabbronorite this indicates similar crystallization sequences in both ultramafic centers olivine olivine clinopyroxene clinopyroxene clinopyroxene orthopyroxene or plagioclase in the ultramafic complex platinum group element bearing sulfide mineralization occurs at the top of cyclic units in websterite and orthopyroxenite cumulates sulfide bearing samples up to 1 wt sulfide are enriched in au pd pt and cu relative to ni ir os and ru with pd ir ratios on the order of 250 sulfide immiscibility probably was caused by mixing of fractionated residual melt with more primitive liquid which periodically intruded the magma chamber evidence of mixing is seen in gradational chemical variations across the interface of cyclic units and in the occurrence of inclusions of olivine crystals and crystal aggregates in websterite and gabbronorite cumulates these inclusions are not in equilibrium with their present host rock and presumably formed before mixing in the more primitive magma a mineralized orthopyroxenite cumulate at the top of cyclic unit 2 could not have been produced by the normal crystallization sequence consideration of the phase relationships in the ternary system olivine clinopyroxene plagioclase quartz suggests that these cumulates are the crystallization product of a hybrid magma formed during magma mixing at the interface of cyclic units the gabbroic complex consists mainly of largely altered gabbro and gabbronorite cumulates and is intruded by mafic ultramafic dikes the center of the gabbroic complex contains the economically important roby zone with its platinum group element bearing cu ni sulfide mineralization this is a lithologically and texturally very complex zone in which high grade platinum group element mineralization is associated with pegmatite and mafic ultramaflc dikes the roby zone is characterized by strong enrichments of au pd and pt relative to ir and os and by variable but extremely high pd ir ratios of greater than 10 000 high au pt and pd pt ratios indicate a fractionation of pt from au and pd it is suggested that the high volatile content of the residual magma of the gabbroic complex triggered partial remelting of surrounding gabbro cumulates a process called constitutional zone refining by mcbirney 1987 during the partial melting event compatible elements such as ir and os are buffered by the residual silicate and oxide mineralogy whereas incompatible elements such as au pd and pt as well as volatile components such as h 2 o and h 2 s become highly concentrated in the partial melt phase eventually this newly formed magma may become sulfide saturated either by cooling and fractional crystallization or due to the continuous dissolution of sulfides when the zone of melting proceeds through the gabbro cumulates ratios such as pd ir and pd pt would strongly depend on the degree of remelting and fractional crystallization until the time of sulfide saturation the matrix of the varitextured zone is interpreted as the residuum of this melting process whereas pegmatoids and pegmatites segregationlike gabbro phases and quartz bearing plagioclase and incompatible element rich dikes are regarded as the final crystallization products of the partial melting

using the high mobility of palladium in surface media in exploration for platinum group element deposits evidence from the lac des iles region northwestern ontario platinium group element pge mineralization is commonly accompanied by large amounts of ni fe sulfides which are explored for using a variety of geophysical and geochemical methods appreciation of a new type of pge mineralization is emerging hosted by gabbroic rocks with low concentrations 5 vol of disseminated sulfides the host rocks commonly display textural complexity such as brecciation pods and veins of pegmatite and magma mingling of different lithological units examples include the lac des iles deposit in northwestern ontario the east bull lake suite in eastern ontario and several intrusions in the kola peninsula and northern finland such mineralization does not result in prominent geophysical and geochemical anomalies thus posing a challenge for exploration we initiated a project to develop new geochemical exploration approaches for such pge deposits with low sulfides and selected the lac des iles district northern ontario canada which contains one producing mine and many pd showings mineralization is hosted by gabbroic rocks with low concentrations 5 vol of sulfides enriched elements include pd pt au ni cu and co in separate sulfide rich mineralization base metals can be abundant up to 1 wt percent but pd is insignificant 200 ppb this reduces the relevance of ni cu and co as possible pathfinders for sulfide poor pge mineralization leaving pd pt and au a detailed study in the baker zone at the lac des iles mine and the legris lake property indicates the essentially immobile nature of pt in these surface environments by contrast pd is depleted in b horizon soil suggesting its removal during the conversion of c to b horizon soils the loss of pd in b horizon soil reflects the transport of pd as neutral and anionic complexes probably with oh and possibly with organic ligands which are not adsorbed by fe oxyhydroxides in the soils instead pd migrates in solution to accumulate in humus in swamps and organic rich lake sediments downslope from mineralization the distribution of pd in humus therefore provides a distal indicator of nearby mineralization total digestion of humus using fire assay or aqua regia yielded up to 160 ppb pd whereas a variety of commercially available leaching methods extracted few ppb pd the results indicate that pd is tightly held in organic matter this behavior of pd contrasts with many metallic elements such as cu and ni that are enriched in b horizon soil and may be extracted by a variety of leaching techniques palladium arsenides tellurides antimonides and sulfides are unstable in weathering environments and their dissolution releases pd to surface waters therefore pd can be used to indicate other pge mineralization as it is always abundant among pge where pge mineralization is associated with high concentrations of fe sulfides their oxidation produces acidic waters which further promotes pd dispersion thus the findings of this study are applicable to exploration for both sulfide rich and poor pge mineralization

the plate tectonic approximation plate nonrigidity diffuse plate boundaries and global plate reconstructions the central assumption of plate tectonics that plate interiors are rigid remains a useful but uncertain approximation strain rates of stable plate interiors are bounded between 10 i12 10 i11 year i1 and 4 10 i10 year i1 the narrowness of all plate boundaries the other main assumption of plate tectonics as originally conceived is contradicted by many observations both in the continents and in the oceans some diffuse plate boundaries in both continents and oceans exceed dimensions of 1000 km on a side diffuse plate boundaries cover 15 of earth s surface the maximum speed of relative plate motion across any one diffuse plate boundary ranges from 2t o 15 mm year which is faster than some upper bounds on intraplate motion across stable plate interiors 2m m year i1 strain rates in diffuse plate boundaries can be as high as 10 i8 year i1 25 times higher than the upper bound on strain rates of stable plate interiors but 600 times lower than the lowest strain rates across typical narrow plate boundaries the poles of rotation of the plates flanking a diffuse oceanic plate boundary tend to be located in the diffuse boundary which is a consequence of the strong coupling across the boundary

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

a primary platinum occurrence in the freetown layered intrusion sierra leone the primary source of the alluvial platinum group minerals pgm of the freetown layered intrusion sierra leone has been unknown since their discovery 70 years ago this is a report on work in progress to establish a primary occurrence related to the igneous layering new whole rock major and minor element analyses and precious metal assays show that at key horizons there is a significant change in the silicate mineral compositions and in cu cr and ni as well as in pt and pd comparable to those encountered in other pgm bearing layered intrusions anomalous 0 1 to 0 4 g t pt levels occur in pyroxene troctolites which maintain these levels along strike

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

primary cumulus platinum minerals in the monts de cristal complex gabon magmatic microenvironments inferred from high definition x ray fluorescence microscopy an unusual occurrence of pt enriched pyroxenites in the monts de cristal igneous complex is characterized by unusually high ratios of pt to other platinum group elements pges and very low cu and sulfide contents synchrotron x ray fluorescence microscopy was used to identify over a hundred discrete grains of platinum minerals and relate their occurrence to textural associations in the host heteradcumulate orthopyroxenites element associations backed up by fib sem and pixe probe observations indicate that most of the pt is associated with either as or trace cu ni rich sulfides or both some of the pt as grains can be identified as sperrylite and most are likely to be pt fe alloy the relative abundances and volumes of pt minerals to sulfide minerals are very large compared with typical magmatic sulfides almost all of the grains observed lie at or within a few tens of m of cumulus orthopyroxene grain boundaries and there is no significant difference between the populations of grains located inside or outside plagioclase oikocrysts these oikocrysts are inferred to have crystallized either at the cumulus stage or very shortly thereafter on the basis of their relationship to ti enrichment in the margins of pyroxene grains not enclosed in oikocrysts this relationship precludes a significant role of trapped intercumulus liquid in pt deposition or mobilization and also allows a confident inference that pt rich and pt as enriched phases precipitated directly from the magma at the cumulus stage these observations lead to the conclusion that fractionation of pt from other pges in this magmatic system is a consequence of a solubility limit for solid pt metal and or pt arsenide

the origin of platinum group minerals from the freetown intrusion sierra leone inferred from osmium isotope systematics the origin of certain alluvial platinum group mineral pgm grains is controversial with two major proposed models one involves primary formation within ultramafic intrusions followed by weathering into laterites with no further alteration prior to alluvial concentration the other involves in addition solution of the platinum group elements pge during weathering of the ultramafic source rocks probably by organic reactions followed by the supergene growth of the macroscopic grains within laterites in order to provide further evidence concerning the formation of macroscopic m in size platinum group mineral grains the authors have studied the osmium isotope ratios in pgm grains from the freetown peninsula in sierra leone using an ion microprobe indirect isotopic evidence and modeling are required to infer their origin as no pgm grains larger than 10 mm have ever been discovered in situ within the source rocks unlike virtually all other alluvial pgm grains that have been studied the freetown ones are distinguished by 187 os 188 os ratios that are much higher than canonical mantle values and vary considerably between different grains 0 12 187 os 188 os 0 28 the 187 os 188 os ratios are with one exception the same for individual osmium rich inclusions within the platinum iron alloy host of each grain although different grains have significantly different 187 os 188 os ratios in the one significant exception there are two os ru ir alloy inclusions within a single platinum iron alloy host which have 187 os 188 os ratios that are different from each other various models are considered to explain these observations 1 the elevated osmium isotope ratios could be primary and derived from a mantle inhomogeneity such as a mantle plume 2 the intruded material with a canonical mantle signature may have reacted with the host rocks at the time of the intrusion or at a later stage during postulated metamorphic or hydrothermal activity or 3 the supergene remobilization could have modified the osmium isotope signature of the pgm interpretation of the osmium isotopes is by no means unambiguous the range of 187 os 188 os ratios is far higher than known mantle sources which combined with the grain to grain variation seems to rule out the primary formation model sr nd and o isotope evidence indicates no reaction with the host rocks during intrusion or by metamorphic or hydrothermal activity and mixing calculations show that enhancement of the radiogenic os during intrusion could only have been small the evidence points to some extremely localized process which could however have acted in two ways either a preexisting high concentration of rhenium and pge a xenolith was incorporated into the magma chamber remained essentially unmixed with the majority of the material in the chamber and after extrusion of the gabbro pge nuggets were weathered out into laterites or alternatively concentration processes within the magma chamber produced a localized concentration of rhenium possibly in copper sulfides or late magmatic iron oxides in acid intrusions which when chemically dissolved in the supergene environment precipitated pge nuggets with extremely radiogenic 187 os 188 os signatures the data presented in this paper in conjunction with work elsewhere have been used to demonstrate that the processes producing highly radiogenic 187 os 188 os signatures cannot have resulted from large scale processes and have been used to dismiss a number of previously proposed models however a resolution between the two alternative models proposed is not possible using the present data

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

geoelectromagnetic measurements across the moine thrust and the great glen in northern scotland summary results are presented for a magnetotelluric and magnetovariational study in northern scotland for nine of the 20 sites at which observations were made the results of the analysis are presented fully together with one dimensional 1 d monte carlo inversions for representative sites from the lewisian foreland the north west highlands the great glen region and the grampian highlands for the remaining sites the results are summarized the 1 d models of all sites situated in a traverse extending from the lewisian foreland to the neighbourhood of the highland boundary fault have been used as the basis for preliminary 2 d modelling maps of magnetic field induction vectors and transfer functions for an inducing field perpendicular to the average geological strike are presented for periods of 23 819s the 2 d conductivity models were constrained by both the magnetotelluric and magnetovariational responses the period range of observation in this study was too limited to resolve structural differences in the upper crust but adequate to reveal a moderately good conductor at upper mantle lower crustal depths in the caledonian metamorphic zone in contrast the whole of the crust in the lewisian foreland is highly resistive and there is no evidence of an intermediate conducting layer between the crust and mantle in this region

horizontal thermal contraction of oceanic lithosphere the ultimate limit to the rigid plate approximation average strain rate inferred from seismic moment release newly created i e 0 1 ma old lithosphere is displaced by thermal contraction toward lithosphere in old ocean basins at a rate that is 1 35 of the half rate of seafloor spreading giving displacement rates of 0 1 to 1 1 km ma 1 or equivalently 0 1 to 1 1 mm a 1 predicted displacement rates parallel to midocean ridges depend strongly on lithospheric age and are proportional to the distance along which contractional strain rates are integrated displacement rates can be significant 1k m ma 1 for young lithosphere in particular the displacement of oceanic lithosphere adjacent to southern baja california relative to lithosphere near the pacific antarctic rise has an indicated north south displacement rate between 3 and 10 km ma 1 this suggests that global plate motion circuits based on the assumption of plate rigidity may be biased by a substantial velocity

experimental tests of simple models for the dynamics of diffuse oceanic plate boundaries summary diffuse plate boundaries which are zones of deformation hundreds to thousands of kilometres wide occur in both continental and oceanic lithosphere here we build on our prior work in which we use analytical and numerical models to investigate the dynamics of diffuse oceanic plate boundaries assuming that the vertically averaged rheology of deforming oceanic lithosphere is characterized by a power law rheology that is n where 1 n is strain rate and is deviatoric stress a major conclusion of this prior work is that the pole of relative rotation of plates adjoining a diffuse oceanic plate boundary is predicted to be confined to the diffuse oceanic plate boundary especially if n 3 here we use laboratory experiments to test the general validity of this conclusion in these analogue experiments deforming oceanic lithosphere is modelled alternately with a newtonian fluid n 1 and a power law fluid n 6 10 the experimental results are consistent with the analytical and numerical models in particular the experimental results confirm that for a given applied force the pole of rotation is more strongly confined to the laboratory analogue of a diffuse oceanic plate boundary for a power law fluid than it is for a newtonian fluid for forces applied far enough from the centre of the analogue diffuse plate boundary however results for newtonian and power law fluids are similar

seismicity of the caribbean nazca boundary constraints on microplate tectonics of the panama region there are at least three kinds of models that have been used to explain plate motions and boundaries in the panama region these models suggest that the major caribbean nazca boundary is 1 a thrust fault along which the panama deformed belt has developed 2 a left lateral strike slip fault along the southern margin of panama or 3 a diffuse array of mostly thrust faults which cause internal deformation to occur throughout panama in this paper we use joint epicenter location methods to obtain improved locations for earthquakes that occurred in this region between 1964 and 1985 and we use waveform modeling to obtain more precise depths for five events hypocenters of 13 events beneath the panama deformed belt suggest that overthrusting of panama upon the caribbean plate causes active deformation which supports model 1 but does not rule out models 2 and 3 the nature of seismic faulting along the panama deformed belt is complex and supports the premise that slow north south convergence loads the cold thick caribbean lithosphere and causes it to fail seismically epicenters of 64 events along the panama fracture zone indicate that this north south trending rightlateral transform fault is splayed north of 6 n overall the seismicity throughout suggests that the panama arc can be treated as a microplate in accordance with models 1 or 2 similarly a possible second microplate may exist south of panama but if so it does not affect regional plate motions significantly the strongest evidence for model 3 occurs in the easternmost and westernmost portions of panama where the diffuse seismicity prevents us from resolving specific microplate triple junction boundaries

platinum group elements pge in basalts of the seaward dipping reflector sequence se greenland coast the rift related seaward dipping reflector sequence sdrs se of introduction greenland consists of basaltic lavas that exhibit variable degrees of seaward dipping reflector sequences sdrss are charmagmatic differentiation derived from a heterogeneous mantle source acteristic features of volcanic rifted margins the sdrs platinum group elements pge are used to provide insights into se of greenland formed during the initial break up of the petrogenetic evolution of the sdrs and to characterize the the north atlantic in the late paleocene early eocene magma sources noble metal concentrations correlate well with duncan et al 1996 and spans the whole period from indicators for magmatic differentiation mg number mgo excontinental break up to seafloor spreading brooks hibiting two distinct trends concentrations of ir ru and rh tend nielsen 1982 saunders et al 1997 large volumes of to decrease with progressive differentiation indicating compatible lava erupted in a short period of time under subaerial behaviour of these elements during fractional crystallization processes or shallow water conditions along the continental margins the variation of pt and pd shows segmented trends in primitive of norway and greenland the high magma production magmas pt and pd are incompatible and become enriched in the rates which led to the formation of magmatic sequences melt the primitive magma is s undersaturated despite derivation up to 6 km thick are inferred to require the presence of from a depleted mid ocean ridge basalt source at a moderate degree a mantle plume white mckenzie 1989 of melting reflecting enhanced s solubility in the melt caused by the aims of ocean drilling program odp legs 152 high fe content and elevated temperature in the more evolved lavas and 163 were to investigate the origin and nature of the pt and pd decrease with decreasing mgo and mg number this sdrs with regard to the influence of a mantle plume indicates that s saturation had occurred with pt and pd being larsen et al 1994a duncan et al 1996 nine sites were incorporated in sulphides which probably segregated during ascent drilled in two transects distal to the ancient plume track bulk partition coefficients for the pge during partial melting are at 63 n and 66 n fig 1 calculated based on data from a primitive basaltic unit with mgo it has been shown that platinum group element pge 20 wt representing a near primary magma composition the systematics can provide important information about determined bulk partition coefficients for an s undersaturated melt the petrogenesis of basaltic magmas such as magmatic are about 2 ir 4 ru 1 2 rh 0 5 pt and 0 4 pd this differentiation sulphide segregation and mantle source indicates that ir ru and rh are compatible during partial melting characteristics e g brugmann et al 1987 greenough whereas pt and pd are incompatible owen 1992 barnes picard 1993 seitz keays

re os isotope geochemistry of tertiary picritic and basaltic magmatism of east greenland constraints on plume lithosphere interactions and the genesis of the platinova reef skaergaard intrusion abstract re os abundance and isotopic studies on a small number of samples from the lowermost part of the east greenland lava pile and the nearby skaergaard intrusion show that picrite and ankaramite lavas tend to have high os abundances and low re os ratios with most rocks having near chondritic values for initial 187 os 188 os one olivine basaltic sample with very high 187 os 188 os has most likely been affected by upper crustal contamination there is no clear evidence for recycled crustal material in the ancestral iceland plume source or for a significant degree of interaction of the picrites with ancient subcontinental lithospheric mantle of the north atlantic craton the platinova reef the precious metal enriched zone in the skaergaard intrusion has very high au and pd contents but very low pt and os contents compared to platiniferous horizons in other layered intrusions these characteristics are almost certainly due to the very late stage in the evolution of the skaergaard magma chamber at which the platinova reef formed the platinova reef yields a re os tchur model age that is younger than the age of the intrusion which was most likely produced by some form of post depositional disturbance to the re os isotopic system the high pge contents of the east greenland volcanic rocks indicate that they were formed from magmas that had the potential to form major accumulations of ni cu pge sulphide deposits had similar magmas undergone significant interaction with continental crust elsewhere in east greenland they may have formed massive magmatic sulphide deposits thus the potential for the discovery of large magmatic sulphide deposits of the noril sk talnakh type in east greenland must be considered excellent

statistical tests for closure of plate motion circuits the standard method of testing whether data describing the relative motion of three plates meeting at a triple junction are consistent with rigid plate tectonics uses the condition of plate circuit closure the euler vectors or angular velocities describing the relative motion of each plate pair must sum to zero deviations from closure suggest that the data are systematically in error or that the plate boundaries have been identified incorrectly or that the plates are non rigid we examine two methods one based on a chi square test and the other based on an f ratio test of testing whether plate motion circuits close we find the latter test is useful but the former is of limited value because of the common practice of systematically overestimating errors in plate motion data we apply the f ratio test to minster and jordan s data for the boundaries dividing the pacific cocos and nazca plates which prove consistent with plate circuit closure application of the test to their data for the boundaries dividing the australian antarctic and african plates shows nonclosure even after deletion of data from the carlsberg ridge which recently has been suggested to divide the african plate from an indo arabian plate this nonclosure suggests systematic errors in the data or internal deformation of one or more of the plates meeting at the indian ocean triple junction

crustal structure from two ship multichannel seismic profiles on the continental shelf off northwest scotland abstract we report a series of two ship multichannel seismic experiments carried out on the continental shelf off northwest scotland in february 1981 both expanding spread and constant offset profiles were shot using explosives and a 16 4 1 airgun the expanding spread shot receiver configurations yield detailed information on the variation of seismic velocity with depth whilst constant offset profiles allow us to monitor lateral variations in the upper crustal velocity structure we show an example of marked variations in the efficiency of compressional to shear wave conversion along a constant offset profile over shallow lewisian basement west of the outer hebrides which we interpret as due to changes in the uppermost basement velocity structure from profiles north of cape wrath we show that where the precambrian lewisian basement is buried beneath mesozoic sediments of variable thickness we can use the constant offset profile of the basement refractor to strip off the effect of the sediments and so correct deeper crustal returns on the coincident expanding spread profiles

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

tertiary plutons monitor climate change in east greenland the d values of amphibole and biotite in tertiary plutons in central eastern greenland decreased by 30 between 55 and 50 ma and then increased by 35 between 50 and 43 ma the d values of amphibole and biotite in in the plutons range from 160 to 123 these low values reflect isotopic exchange between the igneous rocks and hydrothermal fluids that were dominated by meteoric water the changes in mineral d values between 55 and 43 ma are explained most plausibly by shifts in the isotopic composition of meteoric waters that fed hydrothermal recharge areas at 50 ma uplift related to the iceland mantle plume elevated hydrothermal recharge areas by 1 2 km high altitude low d meteoric waters collected by the uplifted recharge areas caused the 30 decrease in mineral d values between 55 and 50 ma the subsequent increase in mineral d values from 50 to 43 ma corresponds to widening of the north atlantic ocean and a change from continental to maritime climate thus tertiary plutons in central eastern greenland represent a network of paleoclimate stations that monitored the d of meteoric water during opening of the north atlantic ocean basin

oblique seafloor spreading across intermediate and superfast spreading centers abstract we show that oblique seafloor spreading occurs in several regions where obliquity was not recognized before these include the slow spreading centers of the red sea 20 intermediate spreading centers of the cocos nazca plate boundary between 91 w and 94 w 9 and superfast spreading centers of the east pacific rise at the nazca pacific plate boundary between 29 s and 32 s 10 and perhaps between 16 s and 22 s 4 thus oblique spreading occurs across slow intermediate and superfast spreading centers but not across fast spreading centers across intermediate and superfast spreading centers oblique spreading does not occur in staircase mode plate geometries but does occur in oblique mode plate geometries across slow and intermediate spreading centers obliquity tends to decrease with increasing spreading rate while across fast and superfast spreading centers it tends to increase with increasing spreading rate oblique spreading at intermediate and superfast spreading centers may be related to magma overpressure or to unusual directions of remote tectonic stress or to ongoing plate boundary reorganizations or to some combination of these using prior methods we estimate the ratio of overpressure to remote tectonic stress for several spreading centers in particular we show along a segment of the cocos pacific plate boundary that magma overpressure is only one fourth as large as remote tectonic stress consistent with a prior inference from other observations the highest obliquity occurs along ridge segments lying 200 km to 1500 km from a mantle plume but not all ridge segments near plumes spread obliquely for one set of estimates of plume fluxes the rate of plume flux delivered to ridges correlates positively and significantly with spreading rate

birps deep seismic reflection studies of the british caledonides the western isles north channel winch traverse fig 1 an extension of the successful moist profile1 was recorded in 1982 at sea along the west coast of britain for birps british institutions reflection profiling syndicate by the geophysical company of norway geco the purpose of the winch traverse was to study crustal structure of the caledonian foreland to cross the caledonian orogen and to establish the three dimensional geometry of mantle reflectors originally seen on moist we describe here the data first emphasizing british caledonian structures and then discussing features of the deep crust of wider significance the data are of very good quality and contain clear moho and upper mantle reflections the lower crust is surprisingly reflective deeply penetrative thrusts are observed and there is firm evidence that several of the mesozoic basins round the shores of the united kingdom were formed by reactivation of older features

palaeomagnetism and magnetic fabric in the freetown complex sierra leone summary about six separately orientated cores were collected at each of 14 sites distributed throughout the arcuate west dipping 6 km thick freetown layered igneous complex alternating field and thermal demagnetization both isolate a stable component of remanent magnetism which corresponds to a palaeomagnetic south pole from 13 sites nine reverse four normal polarity at 82 9 s 32 7 e a 95 5 6 this is indistinguishable from that reported in 1971 based on alternating field demagnetization of cores from 10 orientated hand samples the diverence between the freetown pole age 193 3 ma and other mid jurassic poles from west africa could be due to its greater age the diverence between the whole west african jurassic pole group and the karoo pole from southern africa however suggests moderate 10 d i verential rotation of west africa relative to the kaapvaal craton a prevalent magnetic foliation fabric coincides generally with the petrological layering as might be expected but a ubiquitous magnetic lineation is predominantly down dip this is compatible with a down dip pyroxene lineation reported to be present in some field outcrops and interpreted in terms of late stage deformation during the slow crystallization and cooling of the large igneous body however a fold test shows that the igneous layering had already achieved its present attitude before the complex cooled to 570 c the maximum blocking temperature of the characteristic remanence

40ar39ar geochronology of tertiary mafic intrusions along the east greenland rifted margin relation to flood basalts and the iceland hotspot track abstract the east greeland tertiary igneous province includes the largest exposed continental flood basalt sequence within the north atlantic borderlands more than ten layered gabbro complexes including the 55 ma skaergaard intrusion and a large dolerite sill complex are the plutonic equivalents of flood basalts both lavas and intrusions have been regarded as synchronous with continental breakup at 57 54 ma we report ten new ages of the mafic intrusions determined by40ar 39ar incremental heating experiments demonstrating that the mafic intrusions formed in two distinct time windows only intrusion ii of the imilik gabbro complex the skaergaard intrusion and the sorgenfri glestcher sill complex formed at 57 55 ma coeval with the eruption of regional flood basalts and continental breakup other layered gabbro intrusions at imilik intrusion iii kruuse fjord igtutarajik nordre aputite q kap edvard holm and lilloise are distinctly younger and formed between 50 and 47 ma plate kinematic models indicate the axis of the ancestral iceland mantle plume was located under central greenland at 60 ma and subsequently crossed the east greenland rifted continental margin we propose that tholeiitic magmatism along the east greenland rifted margin largely occurred in three distinct pulses at 62 59 ma lavas and dykes 57 54 ma lavas dykes sills and some gabbros and 50 47 ma gabbros dykes and rare lavas related to discrete mantle melting episodes triggered by plume impact continental breakup and passage of the plume axis respectively this model implies northwestward continental drift of greenland relative to the plume axis by 3 9 5 0 cm yr between 60 and 49 ma consistent with estimates from seismic studies of submerged flood basalts

the behaviour of platinum group elements in basalts from the east greenland rifted margin the continental flood basalts of the east greenland volcanic rifted margin were extruded during continental breakup above the ancestral iceland mantle plume at 55 ma three distinct magma types the low ti high ti and very high ti series lts hts and vhts respectively are found intercalated in the 6 km thick plateau lava sequence incompatible trace elements indicate that the lts are derived from a more depleted mantle source compared to hts and vhts the lts is characterised by increasing cu 105 to 248 ppm and pd 7 to 24 ppb constant cu pd ratio 10 000 and decreasing ir 1 1 to 0 05 ppb and ru 1 8 to 0 3 ppb concentrations during magmatic differentiation 16 to 7 wt mgo the constant cu pd ratio reflects silicate and chromite dominated fractionation without concurrent segregation of sulphide s undersaturated differentiation is also indicated in the hts which also displays increasing pd 6 16 ppb and decreasing ir concentrations 1 to 0 05 ppb during differentiation and the cu pd ratios for the entire series average 21 000 however some hts samples have elevated cu pd ratios up to 33 000 cu pd ratios in the hts do not correlate with mgo and this is interpreted to reflect varying cu pd ratios of hts parental magmas rather than s saturated differentiation during s undersaturated differentiation of the lts and hts pt pd ratios decrease from 1 3 to 0 11 and 1 1 to 0 2 respectively which indicates that pd is much more incompatible than pt during s undersaturated differentiation the vhts consists exclusively of highly evolved samples with low mgo 6 6 6 1 wt and pd ir ratios 98 228 here cu pd ratios increase from 17 500 to 35 000 with decreasing cr concentrations which indicate that these magmas experienced silicate fractionation with concurrent segregation of sulphide the lts represent melting of a depleted source and show high pge concentrations and constant cu pd ratios during s undersaturated differentiation melting of a normal depleted upper mantle source generates s saturated melts morb and the depleted lts source is therefore considered an extraordinary s poor component within the ancestral iceland plume of the three series the vhts contain the largest contribution from enriched mantle portions the vhts have similar pge but much higher nb concentrations for instance compared to the most evolved lts and hts samples indicating that the enriched source contributes nb but not pge

the rigid plate and shrinking plate hypotheses implications for the azimuths of transform faults the rigid plate hypothesis implies that oceanic lithosphere does not contract horizontally as it cools hereinafter rigid plate an alternative hypothesis that vertically averaged tensional thermal stress in the competent lithosphere is fully relieved by horizontal thermal contraction hereinafter shrinking plate predicts subtly different azimuths for transform faults the size of the predicted difference is as large as 2 44 with a mean and median of 0 46 and 0 31 respectively and changes sign between right lateral rl slipping and left lateral ll slipping faults for the morvel transform fault data set all six plate pairs with both rl and ll slipping faults differ in the predicted sense with the observed difference averaging 1 4 0 9 95 confidence limits which is consistent with the predicted difference of 0 9 the sum squared normalized misfit r to global transform fault azimuths is minimized for 0 8 0 4 95 confidence limits where is the fractional multiple of the predicted difference in azimuth between the shrinking plate 1 and rigid plate 0 hypotheses thus observed transform azimuths differ significantly between rl slipping and ll slipping faults which is inconsistent with the rigid plate hypothesis but consistent with the shrinking plate hypothesis which indicates horizontal shrinking rates of 2 ma 1 for newly created lithosphere 1 ma 1 for 0 1 ma old lithosphere 0 2 ma 1 for 1 ma old lithosphere and 0 02 ma 1 for 10 ma old lithosphere which are orders of magnitude higher than the mean intraplate seismic strain rate of 10 6 ma 1 5 10 19 s 1

the triple group and the platinova gold and palladium reefs in the skaergaard intrusion stratigraphic and petrographic relations this paper describes the setting structure and textural relations of the platinova reefs in the skaergaard intrusion the reefs comprise a strata bound occurrence of au pge estimated at 91 million tons of rock with 1 8 ppm au and similar amounts of pd the precious metals occur in two major zones a lower major pd reef which is concordant with the modal layering and followed by at least ten minor pd reefs also concordant and an au concentration which fringes the pd reefs outward and upward through the triple group units the major pd concentration and the au concentration are separated in a systematic way across the intrusion near the center of the intrusion the au concentration occurs some 60 m higher in the stratigraphy than the main pd reef but the separation decreases gradually outward being only a few meters near the margins the dominant precious metal minerals are small grains of cu fe au pd pt alloys which typically are included in interstitial sulfides bornite digenite and smaller amounts of chalcopyrite the textural relations suggest a close genetic association between the sulfides and au pge minerals an association which is confirmed by whole rock compositional data our observations demonstrate that the platinova reefs formed after significant concentration of the au pge and s in the residual magma during fractional crystallization before it became saturated with respect to a sulfide fe cu s o liquid this sulfide liquid acted as a collector of au pge and settled into the layered series cumulates we propose that the structure of the deposit evolved through fractional crystallization of the au pge bearing sulfide liquid as it was filter pressed along with the interstitial silicate melt upward into the overlying unconsolidated cumulates the fractional crystallization resulted in an evolution of the au pge bearing phases with progressive fractionation from pd rich alloys at the base of the deposit to au rich alloys at the top we argue that the sulfides evolved to their present extreme cu rich fe poor composition during oxidation desulfurization reactions in the same event which oxidized the iron titanium oxides of the intrusion this oxidation desulfurization probably had no overall effect on the distribution of au and pge

speciation and solubility of palladium in aqueous magmatic hydrothermal solutions combining thermodynamic data at 25c and 1 bar with correlation algorithms enables prediction of equilibrium dissociation constants for aqueous platinum group element pge complexes at high pressures and temperatures we used these data 1 to calculate the speciation of pd ii in nacl solutions at temperatures to 750c pressures to 2 kbar and values from 3 to 8 and 2 to predict the solubility of palladium at 2 kbar as a function of temperature oxygen fugacity f sub o2 ph and total chloride concentration m sub t cl sup minus calculations indicate that as ph increases there is a sharp transition from chloride to hydroxide complex dominance at all chloride concentrations this shift occurs at or below neutral ph in dilute solution but at m sub t cl sup minus ge 1 0 pd ii chloride complexes account for the majority of dissolved pd ii well into alkaline conditions solubility of palladium increases with increasing temperature m sub t cl sup minus and f sub o2 at 700c 2 kbar and the f sub o2 set by the fmq buffer palladium solubility in a 3 0 m nacl solution is approximately 0 3 ppm at a ph of 4 and one unit ph variation can change palladium solubility by two orders of magnitude these results suggest that pge can be transported as chloridemore complexes in cl rich magmatic hydrothermal solutions like those inferred from petrologic studies of the stillwater and bushveld complexes less

seismic refraction shooting on the continental margin west of the outer hebrides northwest scotland seventeen sonobuoy refraction profiles have been shot to determine the nature of the basement and the broad pattern of sedimentation on the continental margin west of the outer hebrides nw scotland under much of the shelf crystalline rocks vp 5 1 km s lie within 100 m of the seafloor the basement being largely an extension of the precambrian lewisian metamorphic complex of western scotland vp vs gives poisson s ratios of 0 26 0 30 for the lewisian values which are significantly higher than in the deep crust under northern britain implying important compositional differences comparisons with ultrasonic velocities in rocks from the scourian 2700 ma and laxfordian 2200 1500 ma belts of the scottish mainland suggest that the lewisian on the inner continental shelf is predominantly laxfordian vp 5 5 km s higher velocity rocks probably scourian with only a moderate degree of laxfordian reworking vp 5 9 km s and cenozoic intrusions occur locally two seismic profiles indicate that the outer continental shelf may be underlain by a zone of dense scourian early laxfordian granulites whose presence possibly influenced the siting of the continental slope the sediments covering the basement are generally thin thicknesses exceeding 1 km are restricted to a fault bounded trough off the isle of lewis and to the outer shelf and continental slope the deposits can be divided into cenozoic 1 7 1 9 km s and mesozoic 3 0 4 4 km s units velocity variations in the latter probably reflecting the abundance of early cenozoic basic intrusions the distribution of the mesozoic is partly controlled by faults which appear to be related to early precambrian shear zones in the basement these highly foliated belts seem to have facilitated stress relief by normal faulting during permo triassic rifting activity the general lack of subsidence of the outer hebridean block is attributed to the buoyancy of granitic material incorporated at an early stage of its history

magma chamber processes in the formation of the low sulphide magmatic au pge mineralization of the platinova reef in the skaergaard intrusion east greenland

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

variation in the composition of apatite in the munni munni complex and associated intrusions of the west pilbara block western australia abstract apatite is a ubiquitous intercumulus mineral in the archean munni munni radio hill and mount sholl layered igneous complexes of the west pilbara block in western australia apatite compositions from the three intrusions are relatively rich in the fluorapatite endmember and become more so in cumulates derived from compositionally evolved magmas the most extensive stratigraphic coverage is from the munni munni complex where the mole fraction of fluorapatite increases from approximately 60 70 in cumulates of the ultramafic zone to over 90 near the top of the gabbroic zone the chlorapatite component decreases from a high of 20 30 mol in the ultramafic zone to less than 10 in the uppermost part of the gabbroic zone the ree content of apatite parallels stratigraphic changes in whole rock incompatible element zr rb concentrations and is consistent with equilibration with the bulk assemblage the stratigraphic decrease in the cl f and oh f ratio of apatite can be modeled by assuming that fluid saturation of intercumulus liquids is a function of pressure and occurs prior to apatite saturation imposed on the stratigraphic trend are the effects of the combined processes of continuing fluid loss and fractional crystallization of apatite and locally biotite during solidification of the intercumulus liquid these processes produced relative compositional variations in endmember components on the order of 50 mol or more in individual thin sections because apatite crystallizes late in the solidification of intercumulus liquids it is estimated that the cl f ratio of the original trapped parent liquid prior to degassing could have been more than 2 orders of magnitude higher than is now recorded by intercumulus apatite compositions a comparison of literature apatite compositions from other intrusions shows that with the exception of the unusually cl rich assemblages of the stillwater and bushveld complexes processes affecting halogen geochemistry of layered intrusions do not appear to have changed significantly over the past 3 ga

halogen geochemistry of the great dyke zimbabwe apatite from the great dyke of zimbabwe is relatively rich in the hydroxy fluorapatite end members the mole fraction of fluorapatite increases from approximately 40 in cumulates of the ultramafic sequence to over 60 in a sample near the top of the exposed mafic sequence the chlorapatite component decreases from a typical high of 10 20 mole in the ultramafic sequence to about 1 in the uppermost part of the mafic sequence however within sample variation may be as great as the entire stratigraphic variation halogen contents of marginal samples generally are similar to axial samples but tend not to have as high cl concentration and tend to oh enrichment biotite compositions approach hydroxyl end member compositions and apatite biotite oh f exchange geothermometers give an average closure temperature of 564 c apatite from the umvimeela dyke an unlayered dike that parallels the great dyke over much of its length contains less cl than is seen in the ultramafic sequence cumulates of the great dyke while the overall stratigraphic trend is characterized by a decrease in the cl f ratio with stratigraphic height within the p1 unit at the top of the ultramafic sequence there is a positive correlation between cl and other incompatible elements such as na and ce the apparent contradiction between the general stratigraphic trend of decreasing cl f ratio with fractionation and the apparent increase in cl and other incompatible elements seen in the p1 unit can be explained by assuming that the great dyke magma chamber was degassing near its top where confining pressure was lowest and cl was preferentially lost to a separating volatile rich fluid as cumulates formed on the floor they entrapped liquid that was increasingly depleted in cl at the higher stratigraphic levels however at any given stratigraphic interval either local fluid enrichment or the eventual crystallization of halogen bearing minerals that incorporate the smaller f ion in preference to the larger cl ion led to a local increase in the cl f ratio

the malpelo plate hypothesis and implications for nonclosure of the cocos nazca pacific plate motion circuit using global multiresolution topography we estimate new transform fault azimuths along the cocos nazca plate boundary and show that the direction of relative plate motion is 3 3 1 8 95 confidence limits clockwise of prior estimates the new direction of cocos nazca plate motion is moreover 4 9 2 7 95 confidence limits clockwise of the azimuth of the panama transform fault we infer that the plate east of the panama transform fault is not the nazca plate but instead is a microplate that we term the malpelo plate with the improved transform fault data the nonclosure of the nazca cocos pacific plate motion circuit is reduced from 15 0 mm a 1 3 8 mm a 1 to 11 6 mm a 1 3 8 mm a 1 95 confidence limits the nonclosure seems too large to be due entirely to horizontal thermal contraction of oceanic lithosphere and suggests that one or more additional plate boundaries remain to be discovered

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

shear waves in a seismic survey of lewisian basement an extra control on lithological variation and porosity the study of shear s wave velocities in the lewisian metamorphic complex of sutherland nw scotland shows their potential value in modelling lithological variation and porosity the interpretation of a 40 km refraction line across a major lineament separating different rock units produced distinctive lateral and vertical velocity variations due to compositional change and closing of cracks respectively analysis of the vertical velocity variation shows water saturation of cracks except in the top 400 m where cracks may he partially saturated the lateral velocity variations which show very good correlation with lithology have been modelled using effective velocities of essential minerals and by regression analysis of longitudinal p and s wave velocity against composition adding s wave data to p wave data improves the modelling capability here only modestly but is potentially more useful in identifying separately the proportions of mafic v felsic and hydrated v unhydrated minerals

the skaergaard pge and gold deposit the result of in situ fractionation sulphide saturation and magma chamber scale precious metal redistribution by immiscible fe rich melt the skaergaard intrusion greenland is the type locality for skaergaard type mineralizations mineralization levels are perfectly concordant with igneous layering up to 5 m thick internally fractionated and contain crystallized sulphide droplets and precious metal alloys sulphides arsenides and telluride immiscible cu rich sulphide droplets formed in a mush zone below the roof scavenged precious metals they were subsequently dissolved and transported to the floor in late formed immiscible fe rich mush melts mineralized stratigraphic intervals of floor gabbro formed in proto macrolayers owing to local sulphide saturation in melt concentrated between floating plagioclase and sinking clinopyroxene the floor mineralization is divided into four stratigraphic sections formation of the lower platinum group element mineralization lpgem involved 1 crystallization of the bulk liquid liquidus paragenesis and in situ fractionation 2 sulphide saturation and formation of sulphide droplets in melt in the upper part of proto macrolayers after further in situ fractionation the following steps occurred 3 the onset of silicate silicate immiscibility and the consequent loss of buoyant and immiscible si rich melt 4 dissolution of unprotected droplets of sulphide melt present in the fe rich mush melt 5 compaction driven upwards loss of residual mush melt enriched in for example au the lpgem preserves upward increasing bulk pd pt 6 13 owing to a continued supply of pge and au with high pd pt the further development of the lpgem ceased as the supply of precious metals to the floor waned the upper pge mineralization upgem subsequently formed from precious metals recycled in the floor the upgem is characterized by increasing au substitution in pge phases and a decrease in total pge and pd pt owing to upward fractionation in migrating mush melts and exhaustion of pd and pt an upper au rich mineralization level uaum was caused by late remobilization of au and deposition on grain boundaries in fully crystallized gabbro cu concentrations 150 ppm are not correlated with pge and au repeated cu mineralization levels cum attaining 1000 ppm occur above the au levels caused by local mush layer sulphide saturation pge au and cu distributions in the floor mineralization reflect sub liquidus but supra solidus processes and reactions in mushes at the roof wall and floor constraints provided by a new model for the mineralization provide the basis for re evaluation of the solidification processes in the skaergaard intrusion we have identified the importance of extensive in situ fractionation and intrusion wide elemental redistributions in immiscible fe and si rich silicate melts our model characterizes the floor cumulates as bulk liquid orthocumulates containing an upwards increasing proportion crystallized from fe rich immiscible mush melt the roof rocks are complementary to the floor with downwards increasing proportions crystallized from the conjugate si rich melt petrographic observations and the relative timing of crystallization support the hypothesis that crystallization was restricted to marginal mush zones bulk melt remaining in the magma chamber evolved not as generally assumed as a result of loss of crystals grown from the bulk melt but as the consequence of mixing with recycled and evolved melt expelled from the mush by compaction redistribution of fe in immiscible melts may be common to mafic intrusions and puts into question the validity of petrogenetic modelling of bulk liquids in mafic intrusions based only on consideration of floor cumulates

analytic models for the dynamics of diffuse oceanic plate boundaries summary some plates deform across zones that are many hundreds to thousands of kilometres wide much broader than traditional boundaries such as mid ocean ridges deep sea trenches and oceanic transform faults across which most deformation is concentrated in a zone just a few kilometres wide these wide zones of deformation commonly referred to as diffuse plate boundaries occur in both continental and oceanic lithosphere composite plates are composed of two or more rigid or nearly rigid component plates separated by one or more diffuse plate boundaries royer gordon 1997 such complete diffuse boundaries are terminated at both ends by triple junctions although there are other diffuse boundaries that transform into narrow boundaries here we consider the dynamics of complete diffuse oceanic plate boundaries by constructing simple analytical models on a flat earth and on a spherical earth assuming that the viscous force resisting deformation is described by either a linear newtonian law or a high exponent power law we investigate the observed tendency for the pole of relative motion between component plates separated by a diffuse plate boundary to lie within the diffuse boundary itself we show that this tendency is due to geometrical effects that make it unlikely that the total torque acting between plates at a diffuse boundary could be oriented such that relative rotation occurs between the plates about a pole lying outside the boundary this is demonstrated for both flat and spherical earth cases assuming that resistance to strain along the diffuse boundary increases linearly with stress newtonian rheology we further show that the pole of rotation is even more likely to lie in the diffuse plate boundary if the viscous force resisting deformation is described by a high exponent power law rather than a newtonian law

biological role in the transformation of platinum group mineral grains platinum group elements are strategically important metals finding new deposits is becoming increasingly difficult owing to our limited understanding of the processes that affect their mobility in surface environments microorganisms have been shown to promote the mobility of metals around ore deposits here we show that microorganisms influence the mobility of platinum group elements in mineral grains collected from brazil australia and colombia scanning electron microscopy showed biofilms covering the platinum group mineral grains the biofilms contained abundant platinum group element nanoparticles and microcrystalline aggregates and were dominated by proteobacteria many of which were closely related to known metal resistant species some platinum group mineral grains contained carbon nitrogen sulfur selenium and iodine suggesting the grains may be biogenic in origin molecular analyses show that brazilian platinum palladium grains hosted specific bacterial communities which were different in composition from communities associated with gold grains or communities in surrounding soils and sediments nano phase metallic platinum accumulated when a metallophillic bacterium was incubated with a percolating platinum containing medium suggesting that biofilms can cause the precipitation of mobile platinum complexes we conclude that biofilms are capable of forming or transforming platinum group mineral grains and may play an important role for platinum group element dispersion and re concentration in surface environments

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

pacific plate deformation from horizontal thermal contraction the central approximation of plate tectonics is that the plates are rigid which gives the theory its rigor and predictive power space geodetic measurements are consistent with the rigidity of stable plate interiors but some failures of plate circuit closure in particular of oceanic plates indicate that plates may be measurably non rigid we explore the hypothesis that horizontal thermal contraction causes deformation of oceanic plates here we show significant expected displacement fields due to thermal contraction for the pacific plate based on a previously proposed relationship between seafloor age and strain rate and on two end member assumptions on how strain compatibility is enforced the predicted maxi mum 2 2 mm yr southeastward motion of the northeastern part of the plate relative to the pacific antarctic rise may contribute to a large part of the non closure of the pacific north america plate motion circuit our predicted displacement rates cannot yet be confirmed by current space geodetic data and will require seafloor geodesy with 1 mm yr accuracy the spatial distribution of predicted moment rate agrees reasonably well with that of intraplate earthquake epicenters similar to what is observed for plate boundary zones our results suggest that plate scale horizontal thermal contraction is significant and that it may be partly released seismically

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

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

postmagmatic sulphur loss in the skaergaard intrusion implications for the formation of the platinova reef abstract the stratabound platinova au pd reef in the skaergaard intrusion is intimately linked to cu fe sulphide minerals and occurs at a level that shows changes in the sulphur concentration and the cu s ratio these features suggest that the reef is related to silicate sulphide liquid immiscibility in the skaergaard magma however although the evidence is strong present day sulphur concentrations are too low to support such a model the detailed knowledge of the shape volume and compositional relations of the skaergaard lithologies allows for numerical modeling of sulphur and copper concentrations during fractionation of the magma these data can be evaluated with the current models for sulphur saturation in iron rich mafic magmas to assess the original sulphur concentrations the extent of fractionation the composition of sulphide liquids and the extent of postmagmatic sulphur loss mass balance modeling yields initial copper and sulphur concentrations of 250 and 894 ppm for the parental magma to the lower zone a prior to sulphur saturation cu behaves as an incompatible element with a bulk partition coefficient in the cumulus material dcumuluscu of 0 13 indicating some incorporation of cu into mafic silicates probably augite sulphur can be assumed to be perfectly incompatible in the cumulus minerals although it will be included as a component of the interstitial liquid these values yield sulphur saturation at around 635 ppm cu and 2607 ppm s assuming the sulphur to originally be present as a mixture of monosulphide mss and intermediate solid solution sulphide iss the primary sulphides in the cumulates would amount to 0 07 modal through lower and middle zones and 0 53 modal on average through upper zone the expected weight proportion of primary chalcopyrite pyrrhotite evolves from 30 70 at the base of lza through 26 74 at the level of the platinova reef to 18 82 at the sandwich horizon the present day bornite magnetite mineral assemblage is likely to have been derived from a primary pyrrhotite chalcopyrite assemblage through reequilibration and postmagmatic oxidation significant sulphur appears to have been lost from the lower and middle parts of the layered series lower zone a to upper zone b whereas the sulphides of the upper zone c can be explained in the context of a sulphur gain local scale reequilibration of cu from mafic silicates into sulphides may have contributed to the sulphide mineralogy of the lower and middle zones the structure of the platinova reef probably formed from an original single stratabound reef sulphur loss through oxidation in upward migrating deuteric or hydrothermal fluids could have caused dissolution and reprecipitation of pd and au in a cumulate with several stratiform redox barriers this process can thus explain the successive pd peaks of the reef as well as the variable stratigraphic offset of pd and au across the intrusion