rare earth elements systematics of west shasta metavolcanic rocks petrogenesis and hydrothermal alteration west shasta district bimodal volcanics copley greenstone and balaklala rhyolite are members of a low k calc alkaline suite they have rare earth element distributions characterized by pronounced light rare earth element depletion and unfractionated heavy rare earth elements the striking similarities of the rare earth element patterns between the two compositions suggest derivation by partial melting of the same previously depleted source region in the devonian upper mantle or lower crust these sources are even more depleted in incompatible elements than the sources for similar low k suites in the southwest pacific and caribbean the basic volcanics appear to be similar to low ti normal or i type midocean ridge basalts clinopyroxene wo 42 en 51 fs 7 now partially altered to chlorite mg mg fe 0 73 0 95 was the apparent high pressure liquidus phase in the basalts the absence of an eu anomaly indicates that the source had no intrinsic anomaly and that no significant low pressure fractionation of plagioclase took place prior to or during the emplacement of the andesitic lavas the balaklala extrusions have a pronounced negative eu anomaly whereas the intrusive coeval mule mountain trondhjemite does not consequently the anomaly in the balaklala could not be inherited from the source region but must instead reflect the fractionation of plagioclase in shallow magma chambers prior to eruption hydrothermal fluids associated with the formation of volcanogenic massive sulfide deposits have modified the rare earth patterns of the acid extrusions in the immediate vicinity of apparent feeder systems indicated by the presence of stockworks europium has been selectively mobilized probably as a consequence of reduction to the divalent state and deposited around the vents the light rare earth elements have as a group been selectively extracted by the hydrothermal fluids resulting in their severe depletion relative to the heavy rare earth elements oxygen isotope signatures are dominated by regional low to moderate temperature exchanges with seawater copley greenstone delta 18 o values reflect the opposing effects of chloritization decreasing delta 18 o and albitization increasing delta 18 o the balaklala rhyolite tends to be enriched in 18 o relative to the copley greenstone due to a higher content of quartz hydrogen isotopes show the regional effects of chloritization light hydrogen except in the immediate area of hydrothermal vents where a pronounced increase in delta d is observed the increase in delta d is a consequence of the formation of coarse muscovite in the altered vent rocks

platinum group minerals in the skouries cu au pd pt te porphyry deposit the skouries deposit is a platinum group element pge enriched cu au porphyry system located in the chalkidiki peninsula greece with associated ag bi and te enrichment the deposit is hosted by multiple porphyritic monzonite and syenite intrusions which originated from a magma chamber at depth an initial quartz monzonite porphyritic intrusion contains a quartz magnetite chalcopyrite pyrite vein stockwork with intense potassic alteration the quartz monzonite intrusion is cross cut by a set of syenite and mafic porphyry dykes and quartz chalcopyrite bornite magnetite veins which host the majority of the cu and au mineralisation late stage quartz pyrite veins with associated phyllic alteration crosscut all previous vein generations electron microprobe and scanning electron microscopy shows that the pge are hosted by platinum group minerals pgm in the quartz chalcopyrite bornite magnetite veins and within potassic alteration assemblages the pge mineralisation in skouries is therefore part of the main high temperature hypogene mineralisation event platinum group minerals at skouries include sopcheite ag4pd3te4 merenskyite pd pt te bi 2 and kotulskite pd te bi with rare telargpalite pd ag 3te isomertieite pd11sb2as2 naldrettite pd2sb testibiopalladite pdte sb te and sobolevskite pdbi the most common platinum group mineral is sopcheite the pgm in skouries are small 52 m2 on average and occur as spherical grains on the boundaries between sulphides and silicates and as inclusions within hydrothermal quartz and sulphides these observations support a semi metal collector model whereby an immiscible bi te melt acts as a collector for pge and other precious metals in high temperature hydrothermal fluids this mechanism would allow the formation of pgm in porphyries without pt and pd fluid saturation

self destructive sulfide segregation systems and the formation of high grade magmatic ore deposits the metal concentrations of sulfide liquids depend upon the mass of silicate magma that they are able to process for metals some high grade magmatic ni cu pge deposits and many magmatic platinum group element pge ni cu deposits demand very high magma sulfide ratios that seem improbable in the light of physical and kinetic constraints recent models for some high grade deposits invoke multistage upgrading processes in which an early formed sulfide liquid reacts with multiple later batches of silicate magma quantitative models of this open system process demonstrate that it is indeed more efficient than a closed system however it is likely that most later magmas in such a system will be sulfur undersaturated and will thus partly redissolve preexisting sulfide liquids further increasing metal concentrations in the remaining sulfide liquids this combined process is termed multistage dissolution upgrading and quantitative models show that it could reduce the mass of silicate magma that must be processed by as much as two orders of magnitude furthermore if sulfide liquids are extensively dissolved during enrichment their base metal concentrations will generally stabilize at a limiting value whereas their pge concentrations will generally increase without limit this divergence could account for unusually high pge concentrations and high pge base metal ratios in many pge dominated deposits the multistage dissolution upgrading model is tested using reasonable degrees of dissolution in the context of natural deposits in the noril sk area and the bushveld intrusion the models reproduce the observed sulfide compositions significantly better than previous models and are also consistent with other aspects of the geology of these deposits thus dynamic sulfide forming magmatic systems may be intrinsically self destructive but this apparently undesirable attribute could play an important role in forming high grade deposits however dissolution could lead to the complete destruction of sulfide liquids and the return of all metals to later magma batches in this case no sulfide deposit would remain but metal depletion signatures indicating sulfide liquid segregation would be preserved in rocks that crystallized from early magmas caution is therefore advised in the use of magmatic depletion signatures to infer overall mineral potential or to estimate the possible sizes of undiscovered magmatic sulfide deposits

precious and base metal geochemistry and mineralogy of the grasvally norite pyroxenite anorthosite gnpa member northern bushveld complex south africa implications for a multistage emplacement the grasvally norite pyroxenite anorthosite gnpa member within the northern limb of the bushveld complex is a mineralized layered package of mafic cumulates developed to the south of the town of mokopane at a similar stratigraphic position to the platreef the concentration of platinum group elements pge in base metal sulfides bms has been determined by laser ablation inductively coupled plasma mass spectrometry these data coupled with whole rock pge concentrations and a detailed account of the platinum group mineralogy pgm provide an insight into the distribution of pge and chalcophile elements within the gnpa member during both primary magmatic and secondary hydrothermal alteration processes within the most unaltered sulfides containing pyrrhotite pentlandite and chalcopyrite only the majority of ipge rh and some pd occur in solid solution within pyrrhotite and pentlandite with an associated pt as and pd bi te dominated pgm assemblage these observations in conjunction with the presence of good correlations between all bulk pge and base metals throughout the gnpa member indicate the presence and subsequent fractionation of a single pge rich sulfide liquid which has not been significantly altered in places the primary sulfides have been replaced to varying degrees by a low temperature assemblage of pyrite millerite and chalcopyrite these sulfides are associated with a pgm assemblage characterized by the presence of pd antimonides and pd arsenides which are indicative of hydrothermal assemblages the presence of appreciable quantities of ipge pd and rh within pyrite and to a lesser extent millerite suggests these phases directly inherited pge contents from the pyrrhotite and pentlandite that they replaced the replacement of both the sulfides and pgm occurred in situ thus preserving the originally strong spatial association between pgm and bms but altering the mineralogy precious metal geochemistry indicates that fluid redistribution of pge is minimal with only pd au and cu being partially remobilized and decoupled from bms this is also indicated by the lower concentrations of pd evident in both pyrite and millerite compared with the pentlandite being replaced the observations that the gnpa member was mineralized prior to intrusion of the main zone and that there was no local footwall control over the development of sulfide mineralization are inconsistent with genetic models involving the in situ development of a sulfide liquid through either depletion of an overlying magma column or in situ contamination of crustal s we therefore believe that our observations are more compatible with a multistage emplacement model where preformed pge rich sulfides were emplaced into the gnpa member such a model explains the development and distribution of a single sulfide liquid throughout the entire 400 800 m thick succession it is therefore envisaged that the gnpa member formed in a similar manner to its nearest analogue the platreef notable differences however in pge tenors indicate that the ore forming process may have differed slightly within the staging chambers that supplied the platreef and gnpa member

discriminating hydrothermal and terrigenous clays in the okinawa trough east china sea evidences from mineralogy and geochemistry abstract the okinawa trough ot in the east asian continental margin is characterized by thick terrigenous sediment and ubiquitous volcanic hydrothermal activities in this study the clays collected during iodp expedition 331 to the middle ot iheya north knoll were analyzed for mineralogical and geochemical compositions by comparing with the clays from the east china sea shelf and surrounding rivers we examine different clay origins the hydrothermal field in the mid ot is dominated by mg rich chlorite while the recharge zone has clay mineral assemblages similar to the shelf and rivers showing high content of illite subordinate chlorite and kaolinite and scarce smectite compared to the terrigenous clays the hydrothermal clays in the ot have high concentrations of mg mn and zr but low fe na k ca ba sr p sc and ti while the hydrothermal clays in the mid ocean ridge are relatively enriched in fe and v and depleted in al mg zr sc and ti different fractionation patterns of rare earth elements also register in the terrigenous and hydrothermal clays diagnostic of variable clay origins we infer that the ot hydrothermal clay was primarily formed by the chemical alteration of detrital sediments subject to the hydrothermal fluids the remarkably different compositions of hydrothermal clays between the sediment rich back arc basin like ot and the sediment starved ocean ridge suggest different physical and chemical processes of hydrothermal fluids and fluid rock sediment reactions under various geologic settings

acoustic multipole logging in transversely isotropic poroelastic formations the properties of the modes generated by multipole sources in a fluid filled borehole embedded in radially layered formations that include transversely isotropic poroelastic layers are investigated these layers are modeled following biot s and homogenization theories under the assumption that the principal axes of symmetry of both the transversely isotropic skeleton and the complex permeability tensor are parallel to the vertical axis of the borehole a general formulation based on the thomson haskell method accounts for any combination of fluid elastic and poroelastic layers either isotropic or transversely isotropic the investigation is achieved through the computation of dispersion and attenuation curves and frequency dependent sensitivity coefficients with respect to the parameters in the presence of a fast or slow radially semi infinite transversely isotropic poroelastic formation with an impermeable borehole wall the shear wave transverse isotropy of the skeleton can be determined using t

the transversely isotropic poroelastic wave equation including the biot and the squirt mechanisms theory and application the transversely isotropic poroelastic wave equation can be formulated to include the biot and the squirt flow mechanisms to yield a new analytical solution in terms of the elements of the squirt flow tensor the new model gives estimates of the vertical and the horizontal per meabilities as well as other measurable rock and fluid properties in particular the model estimates phase ve locity and attenuation of waves traveling at different an gles of incidence with respect to the principal axis of anisotropy the attenuation and dispersion of the fast quasi p wave and the quasi sv wave are related to the vertical and the horizontal permeabilities modeling sug gests that the attenuation of both the quasi p wave and quasi sv wave depend on the direction of permeability for frequencies from 500 to 4500 hz the quasi p wave attenuation will be of maximum permeability to test the theory interwell seismic waveforms well logs and hydraulic conductivity measurements recorded in the fluvial gypsy sandstone reservoir oklahoma provide the material and fluid property parameters for exam ple the analysis of petrophysical data suggests that the vertical permeability 1 md is affected by the presence of mudstone and siltstone bodies which are barriers to vertical fluid movement and the horizontal permeabil ity 1640 md is controlled by cross bedded and planar laminated sandstones the theoretical dispersion curves based on measurable rock and fluid properties and the phase velocity curve obtained from seismic signatures give the ingredients to evaluate the model theoreti cal predictions show the influence of the permeability anisotropy on the dispersion of seismic waves these dispersion values derived from interwell seismic signa tures are consistent with the theoretical model and with the direction of propagation of the seismic waves that travel parallel to the maximum permeability this analy sis with the new analytical solution is the first step toward a quantitative evaluation of the preferential directions of fluid flow in reservoir formation containing hydrocar bons the results of the present work may lead to the development of algorithms to extract the permeability anisotropy from attenuation and dispersion data de rived from sonic logs and crosswell seismics to map the fluid flow distribution in a reservoir

controls on the disseminated ni cu pge sulfide mineralization at the tubane section northern molopo farms complex botswana implications for the formation of conduit style magmatic sulfide ores abstract the molopo farms complex mfc is located near the western margin of the archean kaapvaal craton in southern botswana the complex hosts low grade disseminated ni cu and platinum group elements pge mineralization restricted to the lower ultramafic cumulates notably in the feldspathic orthopyroxenites the mineralized feldspathic orthopyroxenites contain up to 0 47 ppm pt pd au manifested in the form of platinum group mineral pgm assemblages dominated by pge bismuthotellurides asernides and minor au and ag alloys the pgm encountered are froodite pdbi2 moncheite ptte2 michenerite pdbite sperrylite ptas2 largely associated with altered silicates and sulfide minerals the association of pgm with sulfides suggests that they were originally concentrated by sulfide melt while late stage reaction of sulfides with hydrothermal fluids resulted in the observed spatial proximity of pgm with altered silicates the cu pd ratios of most of the rocks fall within the primitive mantle range however the cu pd ratios of the feldspathic orthopyroxenites are more variable with few samples falling above and below the primitive mantle range suggesting the prior sulfide extraction from the magmas and the presence of cumulus sulfides this is supported by the cu zr ratios of these rocks which are also variable above and below unity trace element ratios on the th yb pm versus nb th pm plot indicate strong crustal contamination across all units including those not in proximity to the country rocks the 34s of the magmatic rocks are variable between 6 3 and 11 4 in contrast the 34s of dolomitic siltstones of the transvaal supergroup which constitute the immediate country rocks vary from 4 1 to 0 6 these data indicate that crustal contamination largely occurred prior to magma emplacement it is inferred that the most important contaminant and source of crustal s were the ramotswa and ditlhojana formations that include sulfidic shales at depth we consider that magnesian basaltic magmas of b1 bushveld magma composition underwent crustal contamination assimilation of crustal s and segregation of sulfide melt in the feeder conduit or staging chamber prior to final magma emplacement the feeder conduit or staging chamber was subsequently invaded by further b1 magma this magma entrained some of the sulfides lodged in the plumbing system and flushed them to the final mfc chamber studied here after the staging chamber had already partially solidified it was invaded by yet another influx of magma possibly even more primitive than the b1 bushveld magma being a crystal mush this magma assimilated and entrained some of the previously formed orthopyroxenites manifested by fragments of orthopyroxenite in the harzburgites of the final mfc chamber the presence of centimeter sized globular sulfides and fragments of orthopyroxenite within harzburgites suggests that the complex was emplaced as separate pulses of magmas therefore it is considered that sulfide accumulations may remain undiscovered in the feeder zones or the staging chamber

a record of assimilation preserved by exotic minerals in the lowermost platinum group element deposit of the bushveld complex the volspruit sulphide zone abstract low gradeplatinum group element mineralisation in the volspruit sulphide zone is sulphide poor here we present a petrological investigation on the distribution of pges and chalcophile metals in mineralised pyroxenite cumulates from the volspruit sulphide zone to determine the origin of the pge mineralisation in ultramafic cumulates and evaluate whether volspruit style mineralisation could occur in the stratigraphically lowest ultramafic portions of other layered intrusions electron microscopy of pyroxenite cumulates revealed 1 chromite inclusions containing dolomite albite monazite pb chlorides base metal sulphides and pt as minerals 2 the presence of exotic microxenocrysts systematic mapping of high density mineral assemblages in pyroxenite cumulates across the volspruit sulphide zone identified 196 precious metal mineral grains pt pd rh au or ag minerals 98 pb sulphide grains se cl 27 pb chloride grains k se te s as well as 1 grain of pb telluride 1 monazite grain and 1 grain of u pb th oxide trace element analyses of base metal sulphides reveal the highest s se values in pyrrhotite and chalcopyrite yet recorded in the bushveld complex while some base metal sulphides are enriched in pges the overall low grade of the deposit and inferred fertile ultramafic magma s require relatively low r factors mass of silicate to sulphide melt compared to other sulphide poor pge deposits with a calculated r factor of 500 3000 we consider that the presence of exotic inclusions in chromite exotic microxenocrysts and pb zn cl grains enclosed within primary base metal sulphide assemblages provide strong evidence for crustal contamination in the volspruit sulphide zone the malmani dolomite and the black reef quartzite within the lower chuniespoort group 2 2 2 4 ga are the most likely source of xenocrysts assimilated in a staging chamber beneath the main grasvally chamber in which the volspruit sulphide zone developed it is possible that the malmani dolomite contained an enrichment of pb zn cl and s minerals prior to assimilation the assimilation of dolomite and limestone would locally increase the fo2 of the magma triggering chromite crystallisation the sudden removal of fe from the melt coupled with the addition of external sulfur triggered saturation of an immiscible sulphide melt in the ultramafic volspruit magma chromite and base metal sulphides were subsequently emplaced into the main grasvally magma chamber as a crystal bearing slurry therefore we consider it is possible for pge mineralisation to occur in the ultramafic portion of any layered intrusion intruding in the vicinity of carbonate units even if this style of mineralisation in the lowermost portions of layered intrusions is sub economic it may reduce the grade or opportunity for pge mineralisation higher up in the local magmatic stratigraphy or in later magma emplacement events sourced from the same reservoir the technique of specifically searching for microxenocrysts could be applied beyond layered intrusion research to identify the range of crustal contaminants in other magmatic systems where macro scale xenoliths are neither sampled nor preserved

trace and rare earth element geochemistry in tourmaline and cassiterite from the yunlong tin deposit yunnan china implication for migmatitic hydrothermal fluid evolution and ore genesis abstract yunlong tin deposit is a medial sized hydrothermal deposit in western yunnan province of china this deposit is unusual because the tin mineralization occurs exclusively within the migmatite bodies the genesis of the ore deposit has been hotly debated and various models have been proposed including granite related magmatic hydrothermal and migmatitic hydrothermal origins this paper reports a systematic trace and rare earth elements ree geochemical study in tourmaline and cassiterite from the orebodies and associated migmatites from the yunlong tin deposit tourmaline and cassiterite from this deposit show light ree lree enriched patterns with variable eu and ce anomalies consistent with hydrothermal fluids being derived from local high grade metamorphism and migmatization the data suggest that tourmaline and cassiterite were likely precipitated from migmatitic hydrothermal fluids and indicate that the yunlong tin deposit was probably formed during migmatitic hydrothermal process the high concentrations of transition metals e g sc ti v cr mn and zn in tourmalines are also consistent with this model the high concentrations of sn sr and especially sc v and cr in tourmalines can be used to discriminate tourmaline from barren deposits relative to those associated with economic tin mineralization

platinum group minerals in the limoeiro ni cu pge sulfide deposit brazil the effect of magmatic and upper amphibolite to granulite metamorphic processes on pgm formation the limoeiro ni cu platinum group elements pge deposit is a recent discovery associated with an igneous tubular conduit system in northeastern brazil representative ores from the deposit have been used for platinum group minerals pgm identification and for pge in base metal sulfides bms quantification ninety eighty percent of the pgm in the massive sulfide ores is homogeneous pt ni bi bearing merenskyite pdte2 enclosed primarily by pyrrhotite suggesting that it is formed by exsolution from monosulfide solid solution mss merenskyite gradually but systematically becomes poorer in pt and ni with increasing fractionation which is interpreted to reflect a transition to a more evolved sulfide liquid that segregated in the eastern parts of the intrusion in massive sulfide ores merenskyite forms unusually large up to 5000 m2 euhedral grains commonly in contact with spherical silicate inclusions bms hosts 12 16 of the pd with the remainder hosted by pgm which is interpreted to indicate that merenskyite recrystallized from a pge bearing bismuthotelluride metamorphic melt formed during high grade metamorphism sperrylite ptas2 is the second most abundant pgm 18 of pgm in disseminated ore and in contrast to merenskyite occurs mainly as very small median of 25 m2 inclusions in high temperature silicates and oxides interpreted to have crystallized at high temperatures directly from sulfide blebs that formed and were transported within the limoeiro magma conduit

geochronological evidence for an extension of the northern lobe of the bushveld complex limpopo province south africa abstract we have conducted the first geochronological study of the ultramafic mafic succession the waterberg project located in the southern marginal zone of the limpopo belt north of the northern lobe of the bushveld complex using u pb dating on zircons extracted from both mineralized reefs as well as on detrital zircons of the waterberg group unconformably overlying the mafics this contribution is the first in a series of publications describing the well mineralized succession and will therefore give an overview of the local geology u pb baddeleyite ages for the main mafic sill intruding the area supplement the data the succession is extremely well mineralized and therefore of interest to the wider geological community the intrusion ages for the lobe are 2059 3 and 2053 5 ma and within error coeval with the intrusion ages of the bushveld complex detrital zircon age probability density distributions indicate a maximum age of deposition of c 2045 ma for the waterberg group and show abundant peaks of archean age paleoproterozoic zircons are interpreted to represent zircon from the detritus of the now eroded roof rocks of the lobe while the archean zircons are most likely sourced from the limpopo complex to the northeast the major dolerite sill intruding the project area has been confirmed to belong the umkondo igneous province with a baddeleyite age of 1125 5 ma the combination of these new age dates local geology and existing literature is used to argue that i the ultramafic mafic succession is related to the bushveld complex ii a significant unconformity is present between igneous rocks and the overlying sediments and iii the area was exposed to a level of paleoproterozoic tectonism not previously observed in the bushveld complex

characterization of fractured low q zones at the buena vista hills reservoir california we show that attenuation of high resolution interwell seismic and acoustic waves based on velocity dispersion analysis relates to fluid flow effects in fractured and shale sand sequence formations at the buena vista hills reservoir california fractured low quality factor q factor zones in the brown shale and antelope shale reservoir intervals in the monterrey formation correlate with a system of fractures having permeabilities of 2 5 to 5 md vertical fractures oriented at azimuths from 0 to 30 are detected in the frequency range of 1 to 10 khz we establish that a poroelastic model based on the biot squirt flow bisq mechanism can be used to relate the low q factor zones in the brown and antelope shales because the brown shale has no sands we use it to evaluate a fracture system s response to attenuation we adapt the bisq mechanism to simulate fluid flow in fracture induced anisotropy which provides flow properties parallel and perpendicular to fractures in the siliceous shale formations the

tectonic setting of the permo triassic chiang khong volcanic rocks northern thailand based on petrochemical characteristics abstract the inferred permo triassic chiang khong volcanic belt is composed of felsic to mafic volcanic rocks and their pyroclastic equivalents almost all the least altered mafic volcanic rocks are lava flows a few might have occurred as dykes these mafic volcanic rocks are non foliated to weakly foliated and mostly have porphyritic textures the phenocrysts microphenocrysts in porphyritic samples are commonly plagioclase and may include clinopyroxene olivine fe ti oxide apatite and amphibole the matrix of lava flows ranges texturally from felty to trachytic but a few samples show felty to ophitic subophitic and glassy textures whereas that of possible dyke samples is holocrystalline the primary matrix constituents are largely plagioclase and variable proportions of clinopyroxene fe ti oxide amphibole olivine apatite quartz alkali feldspar and or glass all the studied samples have been subjected to greenschist facies regional metamorphism chemically the samples show narrow ranges of least mobile incompatible element ratios and range compositionally from dacite to basalt of tholeiitic series these samples are chemically analogous to those of the tertiary andesite from sardinian rift sardinia italy particularly in terms of least mobile incompatible element ratios accordingly the studied mafic volcanic rocks are interpreted to have formed in a continental volcanic arc however the problem related to the geometry of plate convergence giving rise to the continental volcanic arc still exists

the petrogenesis of platinum group element reefs in the upper main zone of the northern lobe of the bushveld complex on the farm moorddrift south africa boreholes have intersected the interval hosting the pyroxenite marker at the base of the upper main zone on the farm moorddrift in the northern bushveld complex the concentrations of the major and compatible trace elements in the rocks are in the range of other pyroxenite marker intervals elsewhere in the bushveld complex mgo up to 22 wt cr up to 300 ppm over a stratigraphic height of some 300 m the interval is characterized by a broad but irregular reversal in mg number and cr v analogous to stratigraphically equivalent horizons elsewhere in the bushveld complex the rocks have distinctly higher concentrations in incompatible trace elements particularly large ion lithophile elements lile and light rare earth elements lree than other main zone rocks and thus appear to contain an elevated crustal component the interval hosts several thick meters to 10s of meters but mostly relatively low grade and laterally discontinuous pge reefs that have been delineated over a strike length of ca 1 km pge contents locally reach approximately 16 ppm pt pd au in rocks containing up to approximately 4 to 5 percent sulfides the pge tenors of most sulfides are 100 ppm pt pd ratios are above unity and pd ir is 200 based on well defined positive correlations between pt and pd with ir we interpret the mineralization as being largely of primary magmatic origin most previous models interpreted the pyroxenite marker to have formed from a major replenishment to the bushveld magma chamber with relatively primitive magma the new magma would have been emplaced on top of an approximately 4 km thick cumulate package consisting of the ultramafic mafic lower and critical zones and the lower to central portions of the gabronoritic main zone here we propose that the upper main zone including the pyroxenite marker did not result from a magma replenishment instead it crystallized from the s saturated residual magma of the upper critical zone cumulates and originally directly overlaid the latter the approximately 1 to 2 km thick lower central main zone package formed from a crystal charged magma emplaced at a relatively late stage in the solidification history of the bushveld chamber between the largely consolidated upper critical and upper main zones we envisage that the emplacement of this large magma batch resulted in subsidence of the center of the layered suite perhaps in response to the evacuation of a putative lower staging chamber from which the bushveld complex was fed semiconsolidated portions of the cumulate package sagged and slumped toward the center of the complex and the flowing crystal mushes underwent mechanical unmixing of light relatively buoyant plagioclase from denser pyroxenes oxides and sulfides this resulted in the formation of an interlayered sequence of relatively melanocratic sulfide enriched with relatively leucocratic sulfide poor layers reef formation is thus partly the result of cumulate reorganization in response to the emplacement of the main zone magma

petrology and mineralisation of the southern platreef northern limb of the bushveld complex south africa the platreef the putative local analogue of the merensky reef forms the floor to the mafic succession in the northern limb of the bushveld complex we define the platreef as the lithologically variable unit dominated by pyroxenite which is irregularly mineralised with pge cu and ni between the transvaal metasedimentary footwall or archaean basement and the overlying main zone gabbronorite we define the mineralisation around calcsilicate xenoliths within the main zone in the far north of the limb as a platreef style mineralisation the platreef ss has a strike extent of 30 km whereas platreef style mineralisation occurs over a strike length of 110 km the platreef varies from 400 m thick in the s to 30 modal in some intersections these are dominated by pyrrhotite with lesser pentlandite and chalcopyrite minor pyrite and traces of a wide compositional range of sulphides in the southern sector mineralised zones have cu grades of 0 1 0 25 and ni 0 15 0 36 massive sulphides are localised commonly but not exclusively towards the contact with footwall metasedimentary rocks magmatic sulphides are disseminated or net textured ranging from a few microns to 2 cm grains of pyrrhotite and pentlandite with chalcopyrite and minor pyrite much of the sulphide is associated with intergranular plagioclase or quartz feldspar symplectites along the margins of rounded cumulus orthopyroxenes the pges in the southern sector occur as tellurides bismuthides arsenides antimonides bismuthoantimonides and complex bismuthotellurides pgm are rarely included in the sulphides but occur as micron sized satellite grains around interstitial sulphides and within alteration assemblages in serpentinised zones the pt pd ratio 1 and pge grade may be decoupled from s and base metal abundance

rare earth element mobility around the volcanogenic polymentallic massive sulfide deposit at que river tasmania australia abstract the que river volcanogenic base and precious metal deposit is located in the cambrian mt read volcanics of western tasmania the host andesites and dacites adjacent to mineralization have been hydrothermally altered during mineralization and the entire sequence has undergone devonian regional prehnite pumpellyite facies metamorphism rocks petrographically recognizable as andesites exhibit chondrite normalized ree patterns similar to modern orogenic high k andesitic rocks with ce yb n ratios ranging from 6 4 to 8 6 dacites are more fractionated with ce yb n ranging from 10 6 to 16 0 like the andesites the dacites are characterized by negative eu anomalies eu eu 0 73 0 85 except for a more extreme eu anomaly in one sample eu eu 0 57 polymict volcaniclastic rocks have patterns very similar to those of the andesites the chondrite normalized ree patterns for the hydrothermally altered volcanics fall into three groups that can also be distinguished petrographically a single chloriterich sample has a substantial eu anomaly and shows less relative depletion in the middle ree than do either the andesites or the dacites highly silicified samples are all characterized by large negative eu anomalies eu eu 0 37 0 46 and many are relatively unfractionated with ce yb n ratios ranging from 3 5 to 7 7 a third group of samples characterized by the development of phengitic mica and carbonate show ree patterns similar to the andesites although their eu anomalies are generally smaller a sample of massive zn 1bpb ore has very low ree abundances showing light ree enrichment and a small negative eu anomaly eu eu 0 82 massive pyrite has a similar pattern except for higher overall abundances and a small positive eu anomaly eu eu 1 10 except for the eu anomalies the andesites and dacites show no evidence for ree mobility eu anomalies in the andesites are probably due to alteration of phenocrystic plagioclase and subsequent removal of divalent eu substantial ree mobility has occurred in the hydrothermally altered volcanic rocks the degree of mobility correlating with alteration style and intensity eu has been leached from both former plagioclase phenocrysts and the groundmass some of the eu leached from zones of the most intense alteration may have been redeposited in lower temperature more distal alteration zones

precious metals in magmatic fe ni cu sulfides from the potosi chromitite deposit eastern cuba abstract the moa baracoa ophiolite in eastern cuba is one of the few known ophiolites that display sulfide mineralization attributable to a magmatic origin in association with podiform chromite ores hosted in the mantle crust transition these sulfide ores chiefly consist of fe ni cu sulfides namely pyrrhotite pentlandite chalcopyrite and cubanite partly altered to valleriite the sulfide mineralization is located along the contact between the podiform like chromite ores and intruding pegmatitic gabroic dykes the detailed mineralogical study of the sulfide mineralization coupled with the first ever laser ablation icp ms analysis reveals that this sulfide mineralization show contents of the precious metals os ir ru pt re au ag and other semi metals co ni cu se te bi pb as sb comparable to those sulfides from the magmatic sulfide deposits associated with mafic complexes hosted in the continental crust the results obtained from this study confirm that fe ni cu sulfides at potosi are magmatic in origin and very likely derived from the solidification of droplets of sulfide melt segregated by immiscibility from the intruding mafic melts once they interacted with the pre existing chromitite at the mantle crust transition zone of the ophiolite the immiscibility of sulfide melt was achieved as a result of a progressive increase of fs2 very likely triggered by a set of circumstances including the progressive fractionation of the intruding mafic melt leading to increase of asio2 and accumulation of volatiles as well as the crystallization of oxides two main generations of pentlandite were observed one generation is primary in origin and it was locally exsolved along with pyrrhotite from monosulfide solid solution mss during low temperature cooling the second type of pentlandite resulted from the reaction of mss with coexisting droplets of cu and ni rich sulfide melt la icp ms analysis reveals that most precious metals ru os ir re au ag were concentrated along with the base metal sulfides bms although their distribution among the different bms pyrrhotite pentlandite chalcopyrite and cubanite does not strictly follow the expected distribution according to the known melt solid and solid solid partition coefficients unlike the other analyzed pges pt was not preferentially concentrated in bms but as discrete micrometer sized sperrylite grains the crystallization of sperrylite took place before and contemporaneous to sulfide segregation and pt as nanoparticles probably played an important role in the pt uptake as nucleation seeds for the formation of micron sized sperrylite grains these observations highlight the open system nature of the ore forming system as well as the important role of arsenic in concentrating pge in high temperature silicate and sulfide melts

petrology geochemistry and the mechanisms determining the distribution of platinum group element and base metal sulphide mineralisation in the platreef at overysel northern bushveld complex south africa platinum group element pge mineralisation within the platreef at overysel is controlled by the presence of base metal sulphides bms the floor rocks at overysel are archean basement gneisses and unlike other localities along the strike of the platreef where the floor is comprised of transvaal supergroup sediments the intimate pge bms relationship holds strong into the footwall rocks decoupling of pge from bms is rare and the bms and platinum group mineral assemblages in the platreef and the footwall are almost identical there is minimal overprinting by hydrothermal fluids therefore the mineralisation style present at overysel may represent the most primary style of platreef mineralisation preserved anywhere along the strike chondrite normalised pge profiles reveal a progressive fractionation of the pge with depth into the footwall with ir ru and rh dramatically depleted with depth compared to pt pd and au this feature is not observed at sandsloot and zwartfontein to the south of overysel where the footwall rocks are carbonates there is evidence from rare earth element abundances and the amount of interstitial quartz towards the base of the platreef pyroxenites that contamination by a felsic melt derived from partial melting of the gneissic footwall has taken place textural evidence in the gneisses suggests that a sulphide liquid percolated down into the footwall through a permeable inter granular network that was produced by partial melting around grain boundaries in the gneisses that was induced by the intrusion of the platreef magma pge were originally concentrated within a sulphide liquid in the platreef magma and the crystallisation of monosulphide solid solution from the sulphide liquid removed the majority of the ipge and rh from it whilst still within the mafic platreef transport of pge into the gneisses via downward migration of the residual sulphide liquid fractionated out the remaining ipge and rh in the upper parts of the gneisses leaving a slick of disseminated sulphides in the gneiss with the residual liquid becoming progressively more depleted in these elements relative to pt pd and au highly sulphide rich zones with massive sulphides formed where ponding of the sulphide liquid occurred due to permeability contrasts in the footwall this study highlights the fact that there is a fundamental floor rock control on the mechanism of distribution of pge from the platreef into the footwall rocks where the floor rocks are sediments fluid activity related to metamorphism assimilation and later serpentinisation has decoupled pge from bms in places and transport of pge into the footwall is via hydrothermal fluids in contrast where the floor is comprised of anhydrous gneiss such as at overysel there is limited fluid activity and pge behaviour is controlled by the behaviour of sulphide liquids producing an intimate pge bms association xenoliths and irregular bands of chromitite within the platreef are described in detail for the first time these are rich in the ipge and rh and evidence from laurite inclusions indicates they must have crystallised from a pge saturated magma the disturbed and xenolithic nature of the chromitites would suggest they are rip up clasts either disturbed by later pulses of platreef magma in a multi phase emplacement or transported into the platreef from a pre existing source in a deeper staging chamber or conduit

rare earth elements in volcanic rocks of the buchans area newfoundland rare earth elements ree were determined for 57 samples representative of the range of stratigraphic units both mineralized and unmineralized associated with the high grade polymetallic volcanogenic sulphide deposits at buchans newfoundland these data do not indicate any features indicative of magmatic fractionation processes e g enrichment of total ree or any europium depletion anomaly in the mineralized relative to the unmineralized volcanics suggesting that such processes did not play an important role in the formation of these ore deposits these results also emphasize the need for caution in any attempts to use the rare earth elements as a general tool for discrimination between barren and mineralized volcanic sequences

continuity between eastern and western bushveld complex south africa confirmed by xenoliths from kimberlite the three dimensional shapes of mafic layered intrusions have to be inferred from surface outcrops in some cases aided by drilling and or geophysical data however geophysical models are often equivocal for the 2 06 ga bushveld complex of south africa early geological models proposed a shape of a single gently inward dipping lopolith subsequent resistivity and gravity data were interpreted to suggest that the eastern and western limbs were discrete dipping wedge shaped intrusions separated by 150 km a more recent gravity model that takes crustal flexure into account allows continuity and the reversal to the original model distinguishing between these possibilities is difficult from surface based studies because the central regions of the complex are obscured by large volumes of younger granites and sedimentary volcanic cover rocks here we describe xenoliths from the cretaceous palmietgat kimberlite pipe located mid way between the exposed western and eastern lobes of the complex they are chromite bearing feldspathic pyroxenites considered equivalent to those of the typical outcropping critical zone of the bushveld complex this result provides strong support for a regionally interconnected bushveld complex implying its emplacement as a single sill like body confirming the continuity of the bushveld complex greatly expands exploration opportunities and implies that other layered mafic intrusions could have similar geometry

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

the origin of the calcites from pb zn veins in the harz mountains federal republic of germany abstract studies of the ree distribution in calcites from hydrothermal banded pb zn mineralizations as well as in calcites from pelitic and psammitic unmetamorphosed rocks and magmatites of the harz mountains revealed that the calcite of the banded ores crystallized from hybrid magmatic metamorphic solutions these were rather rich in ree3 fe3 and probably zn2 and pb2 ion concentrations the absence of any negative eu anomalies points to oxidizing conditions the sulphides have crystallized from a different solution it is suggested that the necessary hs rich solutions stem from near surface hydrothermal convection cells set up by nearby granitic intrusions the alternating deposition of calcite and sulphide banded ores may be explained by assuming that farther away from the intrusions the hs rich solutions were intermittedly admixed with the hybrid fluids this caused a decrease in temperature an interruption of calcite deposition and sulphide mineralization near the intrusion continuous mixing of these two fluids became more common and consequently banded ores are rather rare

poroelastic model to relate seismic wave attenuation and dispersion to permeability anisotropy a transversely isotropic model with a horizontal axis of symmetry based on the biot and squirt flow mechanisms predicts seismic waves in poroelastic media the model estimates velocity dispersion and attenuation of waves propagating in the frequency range of crosswell and high resolution reverse vertical seismic profiling vsp 250 1250 hz for vertical permeability values much greater than horizontal permeability parameters the model assumes the principal axes of the stiffness constant tensor are aligned with the axes of the permeability and squirt flow tensors in addition the unified biot and squirt flow mechanism bisq model is adapted to simulate cracks in permeable media under these conditions the model simulations demonstrate that the preferential direction of fluid flow in a reservoir containing fluid filled cracks can be determined by analyzing the phase velocity and attenuation of seismic waves propagating at different azimuth and incident angles as a result the fast compressional wave

hydrothermal alteration in volcanic rocks eastern part of the lukavice group železne hory mountains czech republic many rocks mapped as felsic metavolcanics in the eastern part of the lukavice group are shown to be altered mafic metavolcanics similar to those in the noranda and flin flon snow lake mining districts canada the relatively fresh rocks of the lukavice group are rhyolite dacite andesite and andesite basalt of calcalka line character assuming no substantial volume change during alteration ti p la ce yb lu th partly sc and v contents remained unchanged altered rocks are enriched in fe mg k and si and depleted in na ca and zr some elements show both increased and decreased contents in altered rocks mg ba sm eu tb and hf although hydrothermal alteration in the lukavice group is of large extent it is of the proximal kuroko style and not of regional amulet rhyolite style implications for a large hydrothermal system within a volcanic pile are discussed in relation to the ordovician lukavice group and its mineral deposits and to some other parts of the bohemian massif with volcanosedimentary sequences of the same age

geochemistry and mineralogy of the platreef and critical zone of the northern lobe of the bushveld complex south africa implications for bushveld stratigraphy and the development of pge mineralisation the northern lobe of the bushveld complex is currently a highly active area for platinum group element pge exploration this lobe hosts the platreef a 10 300 m thick package of pge rich pyroxenites and gabbros that crops out along the base of the lobe to the north of mokopane formerly potgietersrus and is amenable to large scale open pit mining along some portions of its strike an early account of the geology of the deposit was produced by percy wagner where he suggested that the platreef was an equivalent pge rich layer to the merensky reef that had already been traced throughout the eastern and western lobes of the bushveld complex wagner s opinion remains widely held and is central to current orthodoxy on the stratigraphy of the northern lobe this correlates the platreef and an associated cumulate sequence that includes a chromitite layer known as the grasvally norite pyroxenite anorthosite gnpa member directly with the sequence between the ug2 chromitite and the merensky reef as it is developed in the upper critical zone of the eastern and western bushveld implicit in this view of the magmatic stratigraphy is that similar critical zone magma was present in all three lobes prior to the development of the merensky reef and the platreef however when this assumed correlation is examined in detail it is obvious that there are significant differences in lithologies mineral textures and chemistries mg of orthopyroxene and olivine and the geochemistry of both rare earth elements ree and pge between the two sequences this suggests that the prevailing interpretation of the stratigraphy of the northern lobe is not correct the critical zone of the northern lobe cannot be correlated with the critical zone in the rest of the complex and the simplest explanation is that the gnpa platreef sequence formed from a separate magma or mixture of magmas chilled margins of the gnpa member match the estimated initial composition of tholeiitic main zone type magma rather than a critical zone magma composition where the gnpa member is developed over the ultramafic lower zone hybrid rocks preserve evidence for mixing between new tholeiitic magma and existing ultramafic liquid this style of interaction and the resulting rock sequences are unique to the northern lobe the gnpa member contains at least seven sulphide rich horizons with elevated pge concentrations some of these are hosted by pyroxenites with similar mineralogy crystallisation sequences and pd rich pge signatures to the platreef chill zones are preserved in the lowest main zone rocks above the gnpa member and the platreef and this suggests that both units were terminated by a new influx of main zone magma this opens the possibility that the platreef and gnpa member merge laterally into one another and that both formed in a series of mixing quenching events involving tholeiitic and ultramafic magmas prior to the main influx of tholeiitic magma that formed the main zone

an effective medium inversion algorithm for gas hydrate quantification and its application to laboratory and borehole measurements of gas hydrate bearing sediments the presence of gas hydrate in marine sediments alters their physical properties in some circumstances gas hydrate may cement sediment grains together and dramatically increase the seismic p and s wave velocities of the composite medium hydrate may also form a load bearing structure within the sediment microstructure but with different seismic wave attenuation characteristics changing the attenuation behaviour of the composite here we introduce an inversion algorithm based on effective medium modelling to infer hydrate saturations from velocity and attenuation measurements on hydrate bearing sediments the velocity increase is modelled as extra binding developed by gas hydrate that strengthens the sediment microstructure the attenuation increase is modelled through a difference in fluid flow properties caused by different permeabilities in the sediment and hydrate microstructures we relate velocity and attenuation increases in hydrate bearing sediments to their hydrate content using an effective medium inversion algorithm based on the self consistent approximation sca differential effective medium dem theory and biot and squirt flow mechanisms of fluid flow the inversion algorithm is able to convert observations in compressional and shear wave velocities and attenuations to hydrate saturation in the sediment pore space we applied our algorithm to a data set from the mallik 2l 38 well mackenzie delta canada and to data from laboratory measurements on gas rich and water saturated sand samples predictions using our algorithm match the borehole data and water saturated laboratory data if the proportion of hydrate contributing to the load bearing structure increases with hydrate saturation the predictions match the gas rich laboratory data if that proportion decreases with hydrate saturation we attribute this difference to differences in hydrate formation mechanisms between the two environments

multi stage hydrothermal processes involved in low sulfide cu ni pge mineralization in the footwall of the sudbury igneous complex canada amy lake pge zone east range the amy lake pge zone is a low sulfide type cu ni pge mineralization in the east range footwall of the 1 85 ga sudbury igneous complex occurring in a 100 m wide sudbury breccia belt that coincides with an impact related major fracture zone bay fault zone detailed hydrothermal alteration mapping fluid inclusion trace element and stable isotope studies revealed a complex alteration and mineralization history in a multi source multi stage sudbury related hydrothermal system the two major stages of syn sudbury hydrothermal activity are characterized by similarly high salinity high temperature fluids that are 1 locally derived from footwall granophyre bodies and typified with high ni cu and pge s ratios and high ree contents magmatic hydrothermal stage and 2 a more voluminous cu ni pge rich fluid flux probably originated from the sudbury igneous complex footwall contact hydrothermal stage the second hydrothermal flux was introduced by brittle fractures in the area and resulted in a complex zonation of alteration assemblages and mineralization governed by local footwall composition the sudbury related hydrothermal event was overprinted by shear related epidote veining and calcite chlorite replacement both regionally present in the sudbury structure based on analogies the most important factors involved in the formation of hydrothermal low sulfide mineralization are proposed to be 1 accumulation of pge enriched fluids 2 large scale brittle structures as conduits to these fluids and 3 adequate host rock composition as a chemical trap resulting in sulfide and pgm precipitation in environments meeting these criteria hydrothermal pge mineralization is known to have formed not only in the sudbury footwall but also from mafic ultramafic intrusions associated with primary magmatic pge from several locations around the world

the rare earth element geochemistry of acid sulphate and acid sulphate chloride geothermal systems from yellowstone national park wyoming usa rare earth element ree concentrations have been determined by inductively coupled plasma mass spectrometry icp ms in acid sulphate and acid sulphate chloride waters and the associated sinters and volcanic rocks from the yellowstone national park ynp wyoming usa geothermal system ree concentrations in the volcanic rocks range from 222 to 347 ppm their chondite normalised ree patterns are typical of upper continental crust with lree hree and negative eu anomalies total ree concentrations in the fluids range from 3 to 1133 nmol kg 1 162 ppm and ree concentrations in sinter are 181 ppm ree abundances and patterns in drill core material from ynp indicate some ree mobility normalisation of ree concentrations in altered lava creek tuff lct from y 12 drill core to ree concentrations in fresh lct indicate that the ree overall have been depleted with the exception of eu which has been decoupled from the ree series and concentrated in the altered rocks relative to the host rocks the ree patterns of the fluids are variably depleted in hrees and lrees and usually have a pronounced positive eu anomaly this decoupling of eu from the ree suite suggests that 1 eu has been preferentially removed either from the host rock glass or from the host rock minerals or 2 the waters are from a high temperature or reducing environment where eu2 is more soluble than the trivalent rees since the latter is inconsistent with production of acid sulphate springs in a low temperature oxidising near surface environment we suggest that the positive eu anomalies in the fluids result from preferential dissolution of a eu rich phase in the host rock spatial and temporal variations in major element chemistry and ph of the springs sampled from norris geyser basin and crater hills accompany variations in ree concentrations and patterns of individual geothermal springs these are possibly related to changes in subsurface plumbing which results in variations in mixing and dilution of the geothermal fluids and may have lead to changes in the extent and nature of ree complexing

the mineralogy and mineral associations of platinum group elements and precious metals in the aurora cu ni au pge deposit northern limb bushveld complex abstract aurora is a platinum group element pge prospect hosted in the northern limb of the bushveld complex south africa it is one of only three deposits discovered in the northern limb so far to be hosted in the melanocratic leucocratic gabbroic cumulates of the main zone of the rustenberg layered suite aurora moorddrift and waterberg t zone deposits rather than in predominantly ultramafic rocks e g platreef the host cumulates at aurora have been divided into three principal units and they intrude the dolomites of the lower transvaal supergroup base metal sulphide bms mineralisation with pge is present in the leucogabbronorites and gabbronorites of unit 2 and in coarse grained gabbronorite veins which intrude the peridotites of unit 1 these veins contain up to 50 interstitial pyrrhotite pentlandite chalcopyrite pyrite unit 2 contains 1 3 pentlandite pyrrhotite chalcopyrite assemblages and 1 5 chalcopyrite pyrite pyrrhotite associated with hydrothermal alteration the pge content of aurora however is predominantly controlled by the presence of platinum group minerals pgm not bms la icp ms analysis of sulphides shows the bms in aurora have lower pge concentrations than other bushveld magmatic sulphides with pentlandite carrying much lower concentrations of pd average 23 ppm than the platreef or the merensky reef sem eds analysis of 26 sections characterised 995 platinum group minerals pgm and precious metal bearing minerals pmm with a total area of 27850 m2 and an average size of 28 2 m2 of the pgm and pmm identified in aurora 85 by area are pd te bi minerals with 6 pd te minerals 4 electrum and 3 ag te minerals along with minor pd bi pd as pt te bi pt as and pt s minerals that collectively comprise 2 of total area only 25 of the pgm and pmm in aurora are bms hosted with the rest hosted in silicates of the total pgm and pmm area 22 are hosted in alteration silicates quartz chlorite or actinolite in an alteration halo around sulphides unusually 52 of the pgm and pmm are spatially removed from bms instead hosted in alteration silicates and within cracks in primary silicates away from any bms this indicates a multi stage ore genesis model with hydrothermal remobilisation of pge important for ore formation the style and host rocks for mineralisation in the aurora deposit are fundamentally different from other deposits in the northern limb of the bushveld hosted in ultramafic rocks such as the platreef gnpa member deposits and the f zone of the waterberg deposit all of which contain a greater diversity of pgm and bms with higher precious metal contents the mineralisation most similar to aurora is the t zone of the waterberg deposit located to the north of aurora which been suggested to be an along strike equivalent of the aurora main zone mineralisation however despite strong similarities in pgm mineralogy and s isotope signatures there are significant differences in bms mineralisation and host lithology meaning it is unlikely they are directly linked stratigraphically at present it seems more likely that aurora and the waterberg t zone reflect similar fluid influenced processes operating in different parts of the main zone perhaps at different times and in different structural basins rather than a continuous mineralised zone along strike

a hydrothermal ni as pge geochemical halo around the miitel komatiite hosted nickel sulfide deposit yilgarn craton western australia the remobilization of metals during postdeposition hydrothermal alteration of magmatic sulfide ores has the potential to result in large haloes the recognition of which could potentially enlarge the detectable footprint of this ore type the miitel komatiite hosted nickel sulfide deposit in western australia was used as a case study to investigate the nature and 3 d geometry of the geochemical halo created by the remobilization of base metals gold and platinum group elements pge into the rocks surrounding the mineralization at miitel anomalous metal enrichment is found in the country rocks surrounding the massive sulfides up to 250 m away from the ore this enrichment detected using portable x ray fluorescence pxrf and backed up by laboratory analyses occurs in the mount edwards footwall basalt within decimeters of the contact with the overlying widgiemooltha komatiites it is associated with the presence of nickel arsenides gersdorffite and minor nickeline are concentrated within small quartz and carbonate veinlets and are interpreted to form during the circulation of arsenic rich hydrothermal fluids results of lead fire assay analyses and in situ laser ablation inductively coupled plasma mass spectrometer la icp ms analyses indicate high pge concentrations pd and pt and minor gold associated with the observed nickel and arsenic enrichment results from a larger regional scale study combined with this pge enrichment suggest that the massive nickel sulfides from the miitel ore are the source of the remobilized nickel in the country rocks the presence of pd and pt enriched trace arsenide phases in country rocks and shear zones may be a generally applicable proximity indicator for nickel sulfides in hydrothermally altered terranes

new mineralogical and isotopic constraints on main zone hosted pge mineralisation at moorddrift northern bushveld complex the northern limb of the bushveld complex south africa contains a number of occurrences of platinum group element pge mineralisation within main zone rocks whereas the rest of the complex has pge depleted main zone units on the farm moorddrift cu ni pge sulphide mineralisation is hosted within the upper main zone in a layered package of gabbronorites mottled anorthosites and thin pyroxenites our observations indicate that a 10 m thick reef style package of mineralisation has been extensively disturbed forming a mega breccia which in some localities may distribute mineralised rocks over intersections of over 300 m the sulphides are made up of pyrrhotite pentlandite and chalcopyrite heavily altered around their margins and overprinted by secondary pyrite platinum group mineral assemblages typical of primary magmatic deposits with pt and pd tellurides and sperrylite are present in the reef style package whereas there is a decrease in tellurides and an increase in antimonides in the disturbed package interpreted to be related to hydrothermal recrystallization during veining and brecciation sulphur isotopes show that all sulphides within the mineralised package on moorddrift have a crustal signature consistent with local country rock sediments of the transvaal supergroup we interpret the mineralisation at moorddrift as a primary sulphide reef likely produced as a result of the mixing of crustally contaminated magmas in the upper main zone which has been locally disrupted post crystallisation at present there are no firm links between moorddrift and the other known pge occurrences in the main zone at the aurora and waterberg projects although the stratigraphic position of all may be similar and thus intriguing nonetheless they do demonstrate that the main zone of the northern limb of the bushveld complex unlike the eastern and western limbs can be considered a fertile unit for potential pge mineralisation

partial least squares discriminant analysis of trace element compositions of magnetite from various vms deposit subtypes application to mineral exploration abstract the petrography and mineral chemistry of magnetite from fifteen volcanogenic massive sulfide vms deposits in canada and the lasail vms deposit in oman as well as from two vms associated banded iron formations bif austin brook new brunswick canada and izok lake nunavut canada were investigated using optical microscopy electron probe micro analyzer epma and laser ablation inductively coupled plasma mass spectrometry la icp ms the method of robust estimation for compositional data robcompositions was applied to investigate geochemical censored data among thirty seven elements analyzed by epma and or la icp ms in magnetite from the studied deposits bedrock lithologies only the results for si ca zr al mg ti zn co and ni contain pls da shows that variable compositions of magnetite slightly discriminate different studied deposits bedrock lithologies the geochemical observations suggest that the variation in magnetite chemistry from different vms settings might be sourced from differences in 1 the composition and temperature of parental magmas or hydrothermal fluids 2 the composition of host bedrocks 3 the composition of co forming minerals and 4 oxygen fugacity pls da distinguishes magnetite compositions from the studied vms deposits and bifs from that of the other ore deposit types including ni cu porphyry cu mo au iron oxide copper gold iron oxide apatite and the bayan obo ree fe nb deposit magnetite from the vms settings on average contains lower concentrations of si zr al mg ti zn co and ni relative to that from the other mineral deposit types pls da of magnetite data from vms deposits and bifs of the bathurst mining camp as well as pls da of magnetite compositions from various mineral deposit types yield discrimination models for application to mineral exploration for vms deposits using indicator minerals in quaternary lithified sedimentary rocks

geochemistry and pge of the lower mineralized zone of the waterberg project south africa the waterberg deposit is located north of the northern lobe of the bushveld complex in south africa and represents a large high grade new platinum group element pge discovery the first comprehensive study of the lower ultramafic section of the area has been completed and whole rock chemical analyses and pge data are presented the section studied comprises mineralized harzburgite and marginal orthopyroxenite overlain by troctolite grading into gabbroic rocks whole rock analyses show geochemical variations typical of differentiated assemblages of cumulus olivine plagioclase and pyroxene normalized trace element data display hree depletion strong positive eu and sr anomalies and lree enrichment the negative anomalies for th rb nb and ta are typical for rocks of the bushveld complex normalized pge distributions are fractionated pd ir 177 pdenriched and au poor emplacement of the magmas is believed to have commenced with west east trending finger like intrusions followed by lateral dilation and emplacement of sulfide droplet bearing ultramafic magmas this was followed by a second phase of intrusions characterized by sheet like bodies of troctolite fractionation of these magmas led to the development of gabbroic rocks that make up the top of the succession the waterberg project is located in the southern marginal zone of the limpopo belt this position in a structurally active area may have facilitated the creation of space for initial magmas it is argued that the mafic to ultramafic succession of the waterberg project does not represent a simple marginal extension of the northern lobe nor does it directly correlate with the platreef it shares geological features but represents a separate magmatic basin

rare earth element distribution in exhalites associated with broken hill type mineralisation at the pinnacles deposit new south wales australia abstract rare earth element ree concentrations in exhalites and pelites associated with strata bound base metal broken hill type mineralisation have been measured for the early proterozoic pinnacles deposit in western new south wales garnetites garnet rich rocks and sulphide rich garnet gahnite bearing rocks exhibit a wide variety of ree concentrations particularly lree and eu which cannot be related entirely to changes in mineralogy but show some correlation with stratigraphic position relative to the main orebody exhalites lying in the footwall to the mineralised rocks have negative eu anomalies and steep lree enriched patterns whereas exhalites in the hanging wall to the ores and silicate rich rocks from within the ore lenses have zero or positive eu anomalies and less lree enriched patterns hree have more consistent values throughout the deposit but in places are strongly depleted non mineralised pelites are slightly enriched in the ree relative to shale composite values and have negative eu anomalies it is argued in this paper that because the variations in data can be related to the stratigraphic location of samples relative to the orebodies that the observed ree distributions are due to fluctuations in ree concentrations in the primary exhalative fluids rather than differential mobilisation of ree during a second post depositional episode of hydrothermal alteration or mobilisation of the ree during prograde granulite facies metamorphism or retrograde amphibolite facies metamorphism which has affected the area the lack of association between metamorphic mineral assemblages and ree distribution and the absence of any homogenisation of the ree concentrations suggest that the data reflect the primary exhalative fluid composition and that the variations in ree data reflect changes in palaeoenvironmental conditions such as f o 2 and temperature a corollary to this conclusion is that there was minimal mobilisation of the ree during subsequent metamorphism

sulfur isotope variations within the platreef ni cu pge deposit genetic implications for the origin of sulfide mineralization the platreef the world s third largest platinum group element pge deposit is a 10 400 m thick pyroxenitic unit at the base of the northern limb of the mafic bushveld complex hosting pge mineralization in association with base metal sulfides the sulfide mineralization is thought to have been either largely magmatic in origin with contamination by assimilation of local floor rocks considered an ore modifying process or a product of assimilation of country rock s and silicic contamination which caused s saturation in the magma with the contamination being fundamental to the ore forming process we have performed an extensive and detailed sulfur isotope study of the platreef which indicates that magmatic signatures 34s 0 to 2 are preserved in early formed sulfide droplets within the platreef pyroxenites in the area from sandsloot to witrivier these values are comparable to sulfide inclusions in diamonds in the nearby klipspringer kimberlite and are considered to have a primary mantle origin there is no indication of any significant external s in any of the primary sulfides in the sandsloot witrivier area however later sulfides found in calc silicate floor rocks at sandsloot and zwartfontein and in xenoliths of calc silicate throughout the section indicate an input of country rock s anhydrite bearing horizons in the malmani dolomites may have exchanged s with magmatic platreef sulfide during platreef intrusion imparting a heavier s isotope signature in the fluid affected rocks than those in the early sulfides the archean basement although containing minor amounts of sulfide with a negative 34s value is not a significant contributor to the s budget of the platreef previous studies have indicated that in areas where the sedimentary floor rocks contain appreciable sulfides rather than sulfates such as at turfspruit the platreef sulfides are extensively contaminated with country rock s assimilation of pyrite bearing shales in the turfspruit area has locally upgraded the s content of the platreef and given the basal platreef sulfides a heavier isotopic signature sulfur saturation in the platreef magma took place before contamination probably in a staging chamber prior to intrusion a major pulse of magma entrained the preformed pge rich sulfides and was injected to form the platreef where assimilation of country rock sulfides upgraded the s content on a strictly local scale and hydrothermal leaching introduced s from country rock sulfates into later stage sulfides again on a local scale south of zwartfontein it is proposed that the platreef was intruded into sediments of the transvaal supergroup which formed the floor and the roof of the platreef north of zwartfontein the platreef intruded the boundary between the malmani supergroup sediments and the archean basement this could have potentially mineralized the roof calc silicates north of zwartfontein in a manner similar to the floor in the sandsloot area the later intrusion of the main zone magma formed a magmatic unconformity on top of the cold platreef and entrained large rafts of mineralized calc silicate for tens of kilometers north of the last footwall outcrop of calc silicate

wall rock alteration and metamorphism of a volcanogenic massive sulfide deposit at que river tasmania petrology and mineralogy the rocks in the alteration halo surrounding the polymetallic massive sulfide deposit at que river preserve primary hydrothermal mineral textures and assemblages despite the imprint of later prehnite pumpellyite facies regional metamorphism their textural preservation is attributed to the mechanical strength of many of the hydrothermal minerals such as pyrite carbonates except calcite and quartz the competency of some of the host rocks and the low strain that has affected many rocks at the margin of the halo assemblages include quartz white mica carbonates albitized plagioclase pyrite and chlorite closer to the ore as intensity of alteration increases albite disappears pyrite apatite rutile and anatase become more common white mica quartz carbonates and chlorite vary in abundance and are distributed irregularly these minerals also occur in hydrothermal veins throughout the stockwork zone several generations of veins are commonly developed with each generation having its own distinctive hydrothermal assemblage both hydrothermal and metamorphic phases are compositionally variable despite some overlap there appear to be systematic differences in composition x ray diffraction and microprobe analyses show that the hydrothermal white micas are variable in composition ranging from illites or muscovites to fuchsites their metamorphic counterparts contain less pyrophyllite and more celadonite the hydrothermal carbonates are most commonly ferroan dolomite magnesian siderite and ferroan magnesite the latter two are more abundant than dolomite where alteration is intense calcite and ferroan dolomite formed during metamorphism and the composition of the hydrothermal and metamorphic carbonates may be different in individual specimens both hydrothermal and metamorphic chlorites are ferro clinochlores and magnesio chamosites metamorphic chlorites outside the halo are more fe rich and more al vi deficient than those within the hydrothermal chlorites exhibit fe mg and al iv al vi si iv mg fe vi tschermakite substitutions and contain al iv contents suggesting temperatures of crystallization of 290 degrees to 320 degrees c similar to the temperature of 280 degrees c inferred from fluid inclusions the chemistry of the hydrothermal phases varies throughout the halo overall fe and mg do not show a systematic trend toward mineralization however fe enrichment of hydrothermal phases in the ore is apparent as is mg enrichment of locally developed chlorite carbonate rocks na and celadonite in white micas and ca in carbonates on the other hand decrease with increasing alteration mg only became an important component in the fluid when seawater infiltrated into the hydrothermal system the heterogeneous distribution of the hydrothermal phases is attributed to successive influxes of hydrothermal fluids of varying composition into the alteration pipe variable water rock ratios and seawater entrainment

distribution of platinum group elements in magmatic and altered ores in the jinchuan intrusion china an example of selenium remobilization by postmagmatic fluids the division of platinum group elements pge between those hosted in platinum group minerals pgm versus those in solid solution in base metal sulfides bms has been determined for ores from the pge bearing ni cu rich jinchuan intrusion in northwest china all the bms are devoid of pt and ir and magmatic bms are also barren of rh these pge may have been scavenged by arsenic to form pgm during magmatic crystallization of the bms pd os and ru are recorded in bms and pd is predominantly in solid solution in pentlandite unlike the fresh magmatic ores in altered or serpentinized ores pd pgm are present froodite is hosted in magnetite formed during alteration of bms accompanied by sulfur loss and liberation of pd michenerite pd pt bite sperrylite ptas2 and au bearing pgm are located in altered silicates irarsite irass occurs mainly enclosed in bms padmaite pdbise identified at the junctions of magnetite and bms was the last pgm to form and locally partially replaces earlier non se bearing pgm we propose that padmaite formed under oxidizing conditions during late local remobilization of se from the bms se bearing pgm are rare and our review shows they are frequently associated with carbonate suggesting that pd and se can be mobilized great distances in low ph oxidizing fluids and may be precipitated on contact with carbonate s se ratios are used by researchers of magmatic ni cu pge ores to determine sulfur loss assuming se is immobile and representative of magmatic sulfur content this study shows that se as well as s is potentially mobile and this should be considered in the use of s se ratios

geochemistry of late proterozoic basaltic rocks from southeastern new brunswick canada late proterozoic volcanic rocks of the coldbrook group 630 600 ma from the avalon zone of the appalachians in southeastern new brunswick canada are predominantly bimodal and occur in three belts fault blocks eastern central and western the basaltic rocks were emplaced in a convergent plate margin setting and range from incompatible element depleted island arc tholeiites in the eastern belt to calc alkalic types of the western belt the distinct compositional zonation of the basaltic rocks resembles the across arc variation observed in recent volcanic arc systems the trends include an increase of the lile and hfse abundances and the zr sio2 th sio2 la yb th nb th hf and zr y ratios from the eastern belt in the southeast through the central belt to the western belt in the northwest the zonation of the coldbrook group is attributed to a northwest dipping subduction zone during late proterozoic time it is suggested that some of the late proterozoic basins in the avalon zone of the northern appalachians reached a stage of oceanization

the cosmos greenstone succession agnew wiluna greenstone belt yilgarn craton western australia geochemistry of an enriched neoarchaean volcanic arc succession abstract the geodynamic setting of the neoarchaean eastern goldfields superterrane egs of the yilgarn craton is the subject of debate some authors propose plume models while others advocate variants on a subduction accretion model for the origin of mineralised greenstone belt sequences felsic volcanism in the kalgoorlie terrane the westernmost terrane of the egs is considered to have a tonalite trondhjemite granodiorite dacite ttg d geochemical affinity the cosmos greenstone succession which lies in the agnew wiluna greenstone belt awb of the kalgoorlie terrane contains several komatiite hosted nickel sulphide deposits the volcanic footwall to which consists of an intercalated succession of fragmental and coherent rocks ranging in composition from basaltic andesite to rhyolite light rare earth elements lrees and large ion lithophile elements liles are strongly enriched relative to high field strength elements hfses across all volcanic units and the rocks display strong positive pb and negative nb anomalies these geochemical characteristics resemble closely those of modern high k calc alkaline to shoshonite continental arc successions contrasting ree lile and hfse concentrations coupled with assimilation fractional crystallisation afc modelling shows that the intercalated dacitic and andesitic volcanic rocks within the footwall succession are not co genetic xenocrystic zircons within the felsic volcanic lithologies indicate that some assimilation of older continental crust contributed to the generation of the footwall volcanic sequence the geochemical characteristics of the cosmos volcanic succession indicate that parental melts were derived via partial melting of enriched peridotite that had been contaminated by subducted crustal material within the mantle wedge of a subduction zone in contrast two younger felsic porphyry intrusions which cross cut the volcanic succession have a distinct ttg d affinity therefore these intrusions are considered to be generated via partial melting of a subducting slab and are related to local high ca granitoid intrusions the cosmos volcanic succession represents the first extrusive high k calc alkaline to shoshonitic volcanic arc sequence described in the kalgoorlie terrane and coupled with age dating of the stratigraphy is indicative of formation in a long lived volcanic arc setting active from 2736 ma to later than 2724 ma the composition and geochemical affinity of the volcanic footwall succession to the cosmos komatiite hosted nickel sulphide deposits contrasts with the majority of felsic volcanic rocks within the awb and also the wider kalgoorlie terrane suggesting that the overall architecture of this region is more complex than is currently thought our conclusions not only have consequences for recent models of the tectonic evolution of the egs but also contribute to the debate on the operation of plate tectonics during the late archaean in general the arc affinity of the cosmos volcanic succession containing abundant high k calc alkaline andesite lavas provides further support for the operation of plate tectonics in the neoarchaean

comparison of high resolution wax embedded and pneumatically coupled borehole seismic detectors high quality seismic measurements at frequencies up to about 2000hz are needed if projected resolution limits on the order of i m in spatial dimension are to be realized in reservoir structure delineation cross well sonic logging and shallow reverse vsp applications while sources and detectors are critical to this goal we have investigated detector requirements in an objective way to demonstrate a successful design philosophy capable of achieving unprecedented wide band frequency response and data quality in three component shallow borehole sensors two prototype detectors were developed a nearly ideal responding wax embedded reference detector and a pneumati cally coupled detector exhibiting closely comparable performance our approach uses a three axis acceler ometer sensor assembly installed in a borehole drilled through the weathered surface to a depth at which the

seismic wave attenuation and dispersion resulting from wave induced flow in porous rocks a review one major cause of elastic wave attenuation in heterogeneous porous media is wave induced flow of the pore fluid between heterogeneities of various scales it is believed that for frequencies below 1 khz the most important cause is the wave induced flow between mesoscopic inhomogeneities which are large compared with the typical individual pore size but small compared to the wavelength various laboratory experiments in some natural porous materials provide evidence for the presence of centimeter scale mesoscopic heterogeneities laboratory and field measurements of seismic attenuation in fluid saturated rocks provide indications of the role of the wave induced flow signatures of wave induced flow include the frequency and saturation dependence of p wave attenuation and its associated velocity dispersion frequency dependent shear wave splitting and attenuation anisotropy during the last four decades numerous models for attenuation and velocity dispersion from wave induced flow have been developed with varying degrees of rigor and complexity these models can be categorized roughly into three groups according to their underlying theoretical framework the first group of models is based on biot s theory of poroelasticity the second group is based on elastodynamic theory where local fluid flow is incorporated through an additional hydrodynamic equation another group of models is derived using the theory of viscoelasticity though all models predict attenuation and velocity dispersion typical for a relaxation process there exist differences that can be related to the type of disorder periodic random space dimension and to the way the local flow is incorporated the differences manifest themselves in different asymptotic scaling laws for attenuation and in different expressions for characteristic frequencies in recent years some theoretical models of wave induced fluid flow have been validated numerically using finite difference finite element and reflectivity algorithms applied to biot s equations of poroelasticity application of theoretical models to real seismic data requires further studies using broadband laboratory and field measurements of attenuation and dispersion for different rocks as well as development of more robust methods for estimating dissipation attributes from field data

rare earth elements in volcanic rocks associated with cu zn massive sulphide mineralization a preliminary report massive sulphide deposits are closely associated with felsic volcanism this association is believed to be genetic and it forms the cornerstone for most exploration programs but unfortunately not all felsic volcanic rocks contain ore it seems likely that ore bearing felsic volcanic rocks have a different genetic history from those that are barren and if this is so these differences should be reflected in their ree geochemistry a preliminary study of ree in archean felsic volcanic rocks has shown that those associated with ore have flat ree patterns with well developed eu anomalies whereas those from barren volcanic rocks have steep ree patterns with weak or absent eu anomalies the felsic volcanic rocks associated with ore can be subdivided into two types tholeiitic and calc alkaline kam kotia matagami and south bay are tholeiitic whereas sturgeon lake golden grove and kuroko are calc alkaline the well developed eu anomalies in the ore related felsic volcanic rocks indicate that the melt has und

geochemistry and tectonic setting of the central loei volcanic rocks pak chom area loei northeastern thailand abstract the central loei volcanic rocks as evidenced by those in the pak chom area were formed in the late devonian early carboniferous and can be separated into three magmatic groups transitional tholeiitic basalt tholeiitic microgabbro and calc alkalic basalt andesite on the basis of immobile element contents and ratios of least altered samples all the tholeiitic microgabbro possibly occurred as dikes chemically the transitional tholeittic basalt and tholeiitic microgabbro have higher abundances of tio 2 ni and cr relative to the calc alkalic basalt andesite at similar values for feo mgo they also contain higher ti zr but lower zr nb the transitional tholeiitic basalt has higher concentrations of p 2 o 5 and nb relative to the tholeiitic microgabbro at similar levels of feo mgo and also has higher ratios of nb y and ti v but lower values for ti zr and zr nb in terms of chondrite normalized ree and n morb normalized patterns the transitional tholeiitic basalt tholeiitic basalt and calc alkalic basalt andesite are analogous to those from north atlantic southwest indian ridge and new britain arc on this basis the central loei volcanic rocks are comprised of morbs and oceanic island arc lavas these arc lavas may have erupted on an oceanic basement in the same ocean basin as those in the chiang rai chiang mai volcanic belt

trace element stratigraphy of the bellevue core northern bushveld multiple magma injections obscured by diffusive processes the 3 km long bellevue core was recovered from the upper zone uz and main zone mz of the northern limb of the bushveld complex the largest known layered mafic intrusion we present abundance data for 57 trace elements hosted in plagioclase clinopyroxene low ca pyroxene and olivine obtained by laser ablation inductively coupled plasma mass spectrometry these data permit the first comprehensive investigation of trace element distribution in the bushveld complex on a mineral by mineral basis and show that the cr content of pyroxene is the only proxy that records three cryptic magma injections coincident with two pyroxenite horizons and the uz mz boundary on the basis of cr abundance we infer that an anomalous troctolitic horizon at the base of the bellevue core was related to injection of relatively primitive magma analogous to the lower or critical zone magmas in the eastern and western limbs of the bushveld complex this horizon defines the greatest inconsistency of magmatic stratigraphy between the northern limb and the eastern and western limbs of the complex and is prospective for platinum group element exploration however the most significant finding of this study is that plagioclase pyroxene and olivine have undergone extensive diffusive modification the major and trace element geochemistry of these minerals records sub solidus equilibration during the prolonged cooling of the bushveld complex controlled by diffusion rates and local partitioning between adjacent minerals by analogy texturally mature cumulates in any layered intrusion are also likely to be chemically re equilibrated

laser ablation icp ms study of platinum group elements in sulphides from the platreef at turfspruit northern limb of the bushveld complex south africa the platreef unit of the northern bushveld complex comprises a diverse package of pyroxenites peridotites and mafic lithologies with associated ni cu platinum group element pge mineralisation base metal sulphides bms are generally more abundant in the platreef than in other bushveld pge deposits such as the merensky reef and the ug2 chromitite but the platreef though thicker has lower overall pge grades despite a commonly held belief that pges are closely associated with sulphide mineralisation a detailed study by laser ablation icp ms la icp ms on a core through the platreef at turfspruit suggests that this is not strictly the case while a significant proportion of the pd os and ir were found to be hosted by bms pt irrespective of its whole rock concentration was not only at the top of the platreef is pt directly associated with sulphide minerals where pt pd sb te bi bearing inclusions were detected in the chalcopyrite portions of large composite sulphides in contrast pd os and ir occur in solid solution and as discrete inclusions within the bms throughout the core for os and ir this is usually in the form of os ir alloys whereas pd forms a range of pd te bi sb phases scanning electron microscope observations on samples from the top of the core revealed the presence of 0 2 mm long ptpd 2 sb te bi 2 michenerite maslovite laths within the chalcopyrite portions of large composite sulphides additional pt bearing minerals including sperrylite and geversite and a number of pd te bi sb minerals were observed in or close to the alteration rims of these sulphides this textural association was observed throughout the core similar platinum group minerals pgms were observed within the felsic assemblages composed of quartz plagioclase alkali feldspar and clinopyroxene produced by late stage felsic melts that permeated the platreef many of these pgms occur a significant distance away from any sulphide minerals we believe these features can all be linked to the introduction of as sb te and bi into the magmatic system through assimilation of sedimentary footwall rocks and xenoliths where the degree of contamination was high all of the pt and some of the pd formed as and sb bearing pgm that were expelled to the edges of the sulphide droplets many of these were redistributed where they came into contact with late stage felsic melts where no felsic melt interactions occurred the expelled pt and pd arsenides and antimonides remained along the margins of the sulphides at the top of the platreef where the effects of contamination were relatively low some of the pt remained within the sulphide liquids on cooling this formed the micro inclusions and blade like laths of pt pd sb bi te in the chalcopyrite

rare earth element abundances and distribution patterns in hydrothermally altered basalts experimental results oceanic tholeiites that have been experimentally reacted with seawater at 500 600 c 800 1000 bars have rare earth element ree abundances and distribution patterns that are essentially identical to those of the unaltered basalts although the data indicate the possibility of some ree leaching and redistribution the net effects are small and the ree patterns of basalts affected by submarine hydrothermal processes may still be valid petrogenetic indicators

elastic anisotropy due to aligned cracks in porous rock all theoretical expressions which relate the characteristics of saturated aligned cracks to the associated elastic anisotropy are restricted in some important way for example to the case of stiff pore fluids or of the absence of equant porosity or of a moderately high frequency band because of these restrictions previous theory is not suitable for application to the upper crust where the pore fluid is brine kf k820 the equant porosity is often substantial p 0 1 and the frequency band is sonic to seismic this work removes these particular restrictions recognizing in the process an important mechanism of dispersion a notable feature of these more general expressions is their insensitivity at low frequency to the aspect ratio of the cracks only the crack density is critical an important conclusion of this more general model is that many insights previously achieved concerning the shear wave splitting due to vertical aligned saturated cracks are sustained however conclusions on crack orientation or crack aspect ratio which were derived from p wave data or from shear wave critical angles may need to be reconsidered further the non linear coupling between pores and cracks due to pressure equalization effects means that the linear schoenberg muir calculus may not be applied to such systems the theory receives strong support from recent data by rathore et al on artificial samples with controlled crack geometry

metasomatic rocks with greisenization associated with the ore bearing zones in the iron mountains železne hory mountains eastern bohemia czechoslovakia in the central iron mountains the ore mineralized deformation zones represent deep reaching faults which possibly reach the upper mantle during geological history they were used by ascending magmas fluids and barren and ore hydrothermal solutions it can be shown that the metasomatism provoked by acid fluids bears the character of greisenization the type of the original greisenized rocks here can be determined on the basis of elements that were immobile during greisenization especially si ti sc and ree after or even during greisenization the metasomatic rocks were affected by shearing metamorphism corresponding in its pt conditions to the middle zone of the almandine amphibolite facies it was possibly in this period that the enigmatic skarn body of sama ov originated owing to its similarity to the dyke skarn of kraskov its similar origin could be assumed the deposition of skarn minerals from circulating fluids which were perhaps released during greisenization

p and s wave anisotropy of a synthetic sandstone with controlled crack geometryl1 a method is reported for the production of synthetic porous sandstones containing cracks of known dimensions and geometry with respect to the matrix a synthetic sandstone was manufactured from sand cemented with an epoxy glue the cracks of known geometry were introduced into the material in the manufacturing stage by emplacing thin metallic discs in the sand epoxy matrix these discs were chemically leached out of the consolidated porous sandstone acoustic anisotropy and shear wave splitting were observed in the synthetic sandstones for the dry sample the observed angular dependence of the p and s wave velocities at 100 khz compares well qualitatively with the theoretical models of hudson and of thomsen quantitatively however the experimental data fits hudson s model better for the case of a saturated sample the experimental results are in excellent agreement with thomsen s model hudson s model on the other hand predicts a different angular dependence for p waves this demonstrates that the concept of fluid transfer between cracks and the ambient porosity can be a significant process the results reported here are from the first successful experiment in which the theoretical models were tested on a porous material containing a known crack geometry

fracture detection using crosswell and single well surveys we recorded high resolution 1 to 10 khz crosswell and single well seismic data in a shallow 15 to 35 m water saturated fractured limestone sequence at conoco s borehole test facility near newkirk oklahoma our objective was to develop seismic methodologies for imaging gas filled fractures in naturally fractured gas reservoirs the crosswell 1 4 m receiver spacing 50 to 100 m well separation surveys used a piezoelectric source and hydrophones before during and after an air injection that we designed to displace water from a fracture zone our intent was to increase the visibility of the fracture zone to seismic imaging and to confirm previous hydrologic data that indicated a preferred pathway for the single well seismic imaging a piezoelectric source and an eight element hydrophone array at 1 4 m spacing we also recorded data before and after the air injection the crosswell results indicate that the air did follow a preferred pathway that was predicted by hydrologic modeling in addition

hydrothermal remobilisation around a deformed and remobilised komatiite hosted ni cu pge deposit sarah s find agnew wiluna greenstone belt yilgarn craton western australia the sarah s find nickel deposit located 4 5 km north of the mount keith nickel mine western australia was chosen as a case study to investigate the nature and three dimensional geometry of a geochemical halo created by the hydrothermal remobilisation of base metals and platinum group elements into the country rock surrounding a small massive ni sulphide orebody portable and laboratory based xrf analyses were carried out on samples from a shear zone localised along the basal komatiite dacite contact that hosts the orebody a geochemical halo was identified that extends along the shear zone up to 1780 m away from the massive sulphides parallel to a prominent stretching lineation elevated ni and pd are associated with high as co cu and s palladium and pt concentrations increase with proximity to massive sulphides from 6 to 1190 ppb pd these anomalous concentrations reflect the presence of sulfarsenides and sulphides either physically remobilised and forming veinlets close to the massive sulphides or hydrothermally transported and redeposited within the foliation in situ laser ablation icp ms indicates that pd and pt are hosted within these nickel sulfarsenides this ni co as pd geochemical halo observed around the sarah s find ore body is interpreted as forming syn deformation by the circulation of as rich hydrothermal fluids dissolving base metals pd and pt from the orebody and redepositing them along the sheared footwall contact similar ni co pd pt as geochemical haloes could potentially exist around any magmatic nickel sulphide mineral system that has undergone a phase of arsenic metasomatism and may be a generally applicable proximity indicator for nickel sulphides in hydrothermally altered terranes

transport and deposition of ree in h2s rich fluids evidence from accessory mineral assemblages abstract it is shown in four examples from geologically different environments that accessory or minor minerals rich in rare earth elements ree are often texturally associated with base metal sulfides the examples demonstrate that ree can migrate in hydrothermal fluids containing large amounts of reduced sulfur s ii species i e h 2 s hs and s 2 in certain fluids complexes of ree with s ii species may be important for the ree transport but most probably only in the absence of other strong ligands such as fluoride carbonate or possibly phosphate textural relationships and geochemical data suggest that the formation of ree bearing minerals in s ii rich environments is genetically related to the precipitation of basemetal sulfides

europium redox equilibria in aqueous solution thermochemical data and recent advances in theoretical aqueous solution chemistry enable prediction of the relative stabilities of aqueous eu2 and eu3 over a wide range of temperatures and pressures at low temperatures near earth surface conditions the aqueous geochemistry of europium should be dominated by the trivalent state except possibly in the most reducing alkaline pore waters of anoxic marine sediments however at temperatures greater than about 250 c and elevated pressures divalent europium should predominate even with significant amounts of complexing trivalent europium is not stable at elevated temperatures and pressures consequently under most hydrothermal and metamorphic conditions europium in aqueous solution should be divalent at intermediate temperatures around 100 c significant activities of both eu2 and eu3 and related complexes can occur in aqueous solutions depending on the oxidation state and the ph of the solutions and the activities of potential ligands such as sulfate carbonate and chloride the predicted stability of divalent europium in aqueous solution at elevated temperatures is consistent with the large positive europium anomalies in rare earth element patterns of high temperature barites of hydrothermal and metamorphic origin reported by guichard et al 5 and with the observed depletion of europium as a result of high temperature sericitization of feldspar bearing assemblages discovered by alderton et al 1 it is suggested that significant fractionation of europium relative to the other rare earth elements may take place during high temperature hydrothermal alteration processes such as in the mid oceanic ridge systems because of the expected stability of divalent europium during these processes

preliminary geochemical study of volcanic rocks in the pang mayao area phrao chiang mai northern thailand tectonic setting of formation abstract the least altered permian mafic volcanic rocks from the pang mayao area phrao district chiang mai province part of chiang rai chiang mai volcanic belt have been analyzed and are found to be mid ocean ridge and ocean island basalts the mid ocean ridge basalts occur as lava flows or dike rocks they are equigranular fine to medium grained and consist largely of plagioclase clinopyroxene and olivine these basalt samples are tholeiitic and have compositions very similar to t morb from the region where the du toit fracture zone intersects the southwest indian ridge the ocean island basalt occurs as pillow breccia and lava flows or dike rocks they are slightly to moderately porphyritic with phenocrysts microphenocrysts of clinopyroxene olivine plagioclase and or fe ti oxide the groundmass is very fine grained and made up largely of felty plagioclase laths with subordinate clinopyroxene these basalt samples are alkalic and chemically analogous to those from haleakala volcano maui hawaiian chain these mafic volcanic rocks may have been formed in a major ocean basin rather than in a mature back arc basin

relationships among compressional wave attenuation porosity clay content and permeability in sandstones anelastic attenuation is the process by which rocks convert compressional waves into heat and thereby modify the amplitude and phase of the waves understanding the causes of compressional wave attenuation is important in the acquisition processing and interpretation of high resolution seismic data and in deducing the physical properties of rocks from seismic data we have measured the attenuation coefficients of compressional waves in 42 sandstones at a confining pressure of 40 mpa equivalent to a depth of burial of about 1 5 km in a frequency range from 0 5 to 1 5 mhz the compressional wave measurements were made using a pulse echo method in which the sample 5 cm diameter 1 8 cm to 3 5 cm long was sandwiched between perspex lucite buffer rods inside the high pressure rig the attenuation of the sample was estimated from the logarithmic spectral ratio of the signals corrected for beam spreading reflected from the top and base of the sample the results show that for these samples compressio

dynamic poroelasticity a unified model with the squirt and the biot mechanisms the velocities and attenuation of seismic and acoustic waves in rocks with fluids are affected by the two most important modes of fluid solid interaction 1 the biot mechanism where the fluid is forced to participate in the solid s motion by viscous friction and inertial coupling and 2 the squirt flow mechanism where the fluid is squeezed out of thin pores deformed by a passing wave traditionally both modes have been modeled separately with the biot mechanism treated in a macroscopic framework and the squirt flow examined at the individual pore level we offer a model which treats both mechanisms as coupled processes and relates p velocity and attenuation to macroscopic parameters the biot poroelastic constants porosity permeability fluid com pressibility and viscosity and a newly introduced microscale parameter a characteristic squirt flow length the latter is referred to as a fundamental rock property that can be determined experimentally we show that the squirt flow mechanism dominates the biot mechanism and is responsible for measured large velocity dispersion and attenuation values the model directly relates p velocity and attenuation to measurable rock and fluid properties therefore it allows one to realistically interpret velocity dispersion and or attenuation in terms of fluid properties changes e g viscosity during thermal enhanced oil recovery eor or to link seismic measurements to reservoir properties as an example of the latter transformation we relate permeability to attenuation and achieve good qualitative correlation with experimental data

distribution of platinum group elements in the platreef at overysel northern bushveld complex a combined pgm and la icp ms study detailed mineralogical and laser ablation inductively coupled plasma mass spectrometry studies have revealed the physical manifestation of the platinum group elements pge within the platreef at overysel northern bushveld complex south africa the pge in the platreef were originally concentrated in an immiscible sulfide liquid along with semi metals such as bi and te as the sulfide liquid began to crystallize virtually all the os ir ru and rh partitioned into monosulfide solid solution mss which on further cooling exsolved to form pyrrhotite and pentlandite with os ir and ru remaining in solid solution in both phases with rh prefentially partitioning into pentlandite platinum some pd and au were concentrated in the residual sulfide liquid after mss crystallization and were then concentrated in an immiscible late stage melt along with semi metals which was expelled to the grain boundaries during crystallization of intermediate solid solution iss to form pt and pd tellurides and electrum around the margins of the sulfide grains tiny droplets of this melt trapped in the crystallizing mss and iss cooled to form pt bi te microinclusions in all sulfide phases whilst the excess pd was accommodated in solid solution in pentlandite minor redistribution and recrystallization by hydrothermal fluids occurred around xenoliths and at the very base of the mineralized zone within the footwall however the overall lack of secondary alteration coupled with the volatile poor nature of the gneissic footwall have allowed the preservation of what may be the most primary style of platreef mineralization the lack of pgm inclusions within early liquidus phases suggests very early sulfur saturation in the platreef lending support to theories involving s saturation occurring prior to intrusion of the platreef possibly within a staging chamber

inversion of 4 c borehole flexural waves to determine anisotropy in a fractured carbonate reservoir observations of anisotropy in fractured carbonate reservoirs can yield important information such as the orientation of aligned crack systems and crack density a new generation of borehole acoustic tools provides the data necessary to observe anisotropy around the borehole the flexural waves excited by two dipole sources operating perpendicularly to each other in these tools are recorded in two perpendicular directions resulting in four component flexural wave recordings superposition of sources and rotation of receivers are the basic elements for the inversion of the waveforms to determine fast or slow mode orientations since the two sources are not necessarily of the same strength a reciprocity based source equalization is carried out as part of the inversion procedure we have inverted four component borehole flexural waves recorded in a vertical well penetrating three lithologic units of a fractured carbonate reservoir in northern oman for the lower unit a northeast southwest azimuth of flexural wave anisotropy is well resolved which is parallel to the strike of open fractures observed in the borehole preferential flow directions and to the regional present day in situ maximum horizontal stress furthermore the northeast southwest direction corresponds with polarization directions of the fast shear wave observed in a multicomponent surface seismic survey over the reservoir the middle unit is most likely isotropic except for a depth interval of low gamma readings where a northeast southwest orientation is found for the fast mode for the upper unit a north south fast mode orientation is found which does not correspond to the expected northeast southwest direction borehole geometry data indicate that the shape of the hole is circular for the middle unit but elongated for the upper and lower unit caliper orientations track the flexural wave anisotropy azimuths in these two units because of the elongation of the borehole the observed flexural mode orientations as well as the magnitude of flexural wave anisotropy cannot be related to formation anisotropy directly which obscures the determination of fracture orientation and fracture density from four component borehole flexural waves

wolframite deposition in a hydrothermal vein system the grey river tungsten prospect newfoundland canada mineralization in the grey river tungsten prospect newfoundland is contained within a sheared devonian k feldspar megacrystic granite and metamorphic rocks which are typical of the gneissic terrain of the gander zone the mineralization is genetically related to a suite of post tectonic highly differentiated alkali rich leucogranite dikes the tungsten bearing hydrothermal veins range from tensional veinlets to lodes containing several injections of quartz major composite lodes have greisen alteration envelopes which are characterized by enrichment in volatiles k 2 o al 2 o 3 rb li f heavy rare earth elements and w the heavy rare earth elements enrichment appears to be related to deposition of carbonate and fluorite in the greisen the mineralization is divided into four stages stage i characterized by quartz feldspar molybdenite veins stage ii composite lode formation consisting of five vein types in paragenetic order quartz bismuthinite iia quartz fe rich wolframite iib greisen iic quartz sulfide iid and quartz fe poor wolframite veins iie stage iii characterized by silver bearing quartz galena sphalerite veins and stage iv composed of fluorite calcite barite veins a spatial mineral zonation from south to north matches the temporal sequence outlined above with the exception of the stage iv veins which crosscut stage ii veins fluid inclusion data indicate a complex evolutionary history for the hydrothermal fluid the simultaneous trapping of co 2 rich and h 2 o rich fluid inclusions as well as solid inclusions of calcite in quartz of the quartz feldspar molybdenite and quartz bismuthinite vein types is evidence for the existence of a heterogeneous fluid during stage i and early stage ii mineralization fluid phase equilibria indicate that the inclusions were trapped at temperatures of 330 degrees to 360 degrees c and fluid pressures of less than 500 bars in composite lodes quartz sulfide and quartz fe poor wolframite veins were deposited in open spaces created by normal faulting these movements prompted rapid decrease in fluid pressure and temperature and caused retrograde boiling of the hydrothermal fluid at 200 bars and 300 degrees to 330 degrees c during deposition of quartz sulfide veins deposition of wolframite in quartz fe poor wolframite veins occurred at 270 degrees to 300 degrees c from an aqueous fluid of low salinity 0 5 wt nacl and low co 2 content 1 0 wt co 2 and after separation of a co 2 vapor phase by retrograde boiling isotopic and fluid inclusion data suggest that deposition of fe rich wolframite may have occurred at a slightly higher temperature 330 degrees c and from a fluid of higher co 2 content than the bulk of the wolframite which has an fe poor composition progressive co 2 loss from the hydrothermal fluid by immiscibility and retrograde boiling had a marked effect on the solution ph which shifted from 5 weakly acid to 6 weakly alkaline at 300 degrees to 350 degrees c during tungsten deposition thermodynamic data on the solubility of wolframite suggest that a ph increase would provide an effective mechanism for tungsten deposition and explain the variation in wolframite composition from fe rich to fe poor with time the association of hematite fe rich wolframite and muscovite in the greisen suggests high f sub o 2 10 super 20 10 super 30 conditions and is supported by eu behavior during alteration sulfides formed after tungsten deposition at temperatures less than 300 degrees c and under f sub s 2 conditions of less than 10 super 11 co 2 rich fluids are commonly associated with tungsten deposits from a variety of environments and the evidence from the grey river tungsten prospect shows that co 2 plays an important role in the transport and deposition of tungsten in the hydrothermal environment

palladium platinum and gold concentration in porphyry copper systems of greece and their genetic significance compilation of new and published data on precious metal and associated trace element contents in mineralized porphyry copper intrusions of greece indicates that the pd content in the potassic propylitic zone of the skouries deposit is relatively high ranging between 60 and 200 ppb average 110 ppb whereas in the porphyry copper intrusions of gerakario and pontokerasia it is very low 3 ppb pd the oxidized zone of fissoka op 65 exhibits low pd and cu contents but higher au te as pb and zn contents the pt content is much lower than pd in the porphyry copper intrusions studied the pd pt ratio in the skouries deposit ranges from 9 to 60 while it is much lower 2 in the mineralized samples from fissoka gerakario and pontokerasia a decrease of the pd content and pd pt ratio with increasing te as pb and zn content may indicate that the pd distribution in porphyry copper systems is effected by the evolution of the magma hydrothermal system also mineralogical and geochemical data on the skouries deposit coupled with textural relations between base metal sulphides pgm and au ag tellurides indicate that the main pd bearing mineral merenskyite is associated with the vein type chalcopyrite or bornite and was deposited during the main stage of mineralization the high magmatic oxidation state and the magmatic features of the hydrothermal system limited participation of meteoric portion in the skouries intrusion may be a strong control of its pd potential assuming that pd is mainly associated with chalcopyrite in the porphyry copper deposits the calculated pd from measured pd contents in chalcopyrite average 3300 ppb pd are relatively high and comparable to that in the chalcopyrite concentrate made after the processing of large composite drill hole samples 2400 ppb pd to 21 wt cu it is considered to be an encouraging economic factor for pd as a by product probably along with au although the main product is cu

the ug2 merensky reef interval of the bushveld complex northwest of pretoria northwest of pretoria the ug2 merensky reef interval overlies a critical zone lower zone sequence that contains numerous large blocks of floor material nevertheless individual layers can be correlated with equivalent units at crocodile river mine the rustenburg impala union and amandelbult sections concentrations of platinum group elements in two borehole intersections of the ug2 chromitite are 4 ppm over 1 2 m and 2 4 ppm over 2 2 m therefore bulk pge levels appear to be only moderately lower than those at western platinum mine this renders models explaining pge enrichment by upward percolating melt or fluids problematic the merensky reef although containing sulphides is only weakly mineralized with pge 0 6 ppm the ug2 pyroxenite is separated from the ug2 chromitite by a 15 m noritic layer the introduction of feldspathic cumulates between two units that elsewhere directly overly each other may be explained by the more evolved composition of resident magma in those parts of the chamber distally located with regard to a major feeder zone at union section it also suggests that the ug2 unit is a multiple rather than a single cyclic unit

rare earth element mobility in alteration pipes below massive cuzn sulfide deposits abstract a convincing demonstration of trace element mobility should satisfy two criteria 1 the range of trace element concentrations in the rock before and after alteration must be known with confidence and 2 the absolute abundances and the ratios of the mobile trace elements should vary between altered and unaltered rocks these criteria have been applied to the host felsic volcanics and alteration pipes associated with several canadian massive cu zn sulfide deposits although there is no evidence of rare earth element ree mobility in the pervasive low grade alteration envelopes adjacent to the deposits the light and middle ree lree and mree plus y have been systematically mobilized relative to the heavy ree hree and other trace elements in the alteration pipes beneath the ore zones there is evidence of minor lree and mree depletion in the most altered rocks at the south bay mine uchi lake and of significant lree depletion in the alteration pipe beneath the mattabi mine sturgeon lake however the strongest mobility is found at the kidd creek mine where the lree were leached from the rocks in the footwall stringer zone and redeposited in a metasomatic chlorite zone directly beneath the massive ore horizon and the mree and y were preferentially lost from the system producing distinctive chondrite normalized ree patterns with tb yb n ratios less than unity eu2 was found to be more mobile than the trivalent ree especially at kidd creek where there is a marked increase in the magnitude of the eu anomaly in the most altered samples high field strength elements such as u th hf and zr have not decoupled and apparently remained relatively immobile during all but the most intense alteration i e kidd creek preliminary data suggest that the degree of ree mobility increases with the size of the deposit thus ree mobility is a potentially useful method of distinguishing between small and large massive sulfide deposits at an early stage in exploration however this hypothesis requires further testing

litho and chemostratigraphy of the flatreef pge deposit northern bushveld complex the flatreef is a world class platinum group element pge deposit recently discovered down dip from existing mining and exploration operations on the northern limb of the bushveld complex current indicated resources stand at 42 moz pge 346 mt with 3 8 g t pt pd rh au 0 32 ni and 0 16 cu which in the case of pt is equivalent to 10 years global annual production making it one of the largest pge deposits on earth the grade and thickness of the flatreef mineralised interval is highly unusual with some drill core intersections containing up to 4 5 g t pt pd rh au over 90 m in drill core here we document the down dip and along strike litho and chemostratigraphy of the flatreef and its footwall and hanging wall rocks based on a diamond drill core database totalling 720 km at the base of the sequence intersected in the drill cores are up to 700 m thick sills of ultramafic rocks dunite harzburgite pyroxenite emplaced into pelitic dolomitic and locally quartzitic and evaporitic rocks belonging to the duitschland formation of the transvaal supergroup next is an approximately 100 200 m sequence of low grade sulphide mineralised layered mafic ultramafic rocks containing abundant sedimentary xenoliths and in places several chromite seams or stringers this is overlain by a 100 m thick sequence of well mineralised mafic ultramafic rocks the flatreef sensu strictu overlain by a laterally persistent mottled anorthosite layer at the base of 1 km of homogenous main zone gabbronorite based on stratigraphic lithological and compositional analogies to the layered rocks in the eastern and western bushveld complex we correlate the flatreef and its chromite bearing footwall rocks with the upper critical zone notably the interval between the ug2 chromitite and the bastard reef as found elsewhere in the bushveld complex this includes recognition of a merensky reef correlative the ultramafic rocks below the main chromitite seam ug2 correlative in the flatreef footwall are correlated with the lower critical and lower zones however compared to the western and eastern bushveld limbs the studied sequence is strongly enriched in sulphide and pge many of the rocks show elevated cao k2o rb and zr contents and lateral continuity of layers between drill cores is less pronounced than elsewhere in the bushveld whereas ultramafic units are locally considerably thickened these compositional and lithological traits are interpreted to result from a range of processes which include contamination with calcsilicate and hornfels intrusion of granitic magmas and the influence of multiple structural events such as pre to syn emplacement regional scale open folding and growth faults evidence for the existence of potholes also exists in the shallow up dip portions of the project area the entire magmatic sequence below the main zone becomes increasingly contaminated to the extent that individual units are somewhat more difficult to correlate between drill cores this package represents the platreef as exposed in outcrop and shallow bore holes across much of the northern limb of the bushveld complex the new data presented here thus indicate that the platreef is a relatively more contaminated up dip extension of parts of the critical and lower zones

the effect of hydrate content on seismic attenuation a case study for mallik 2l 38 well data mackenzie delta canada 1 observations of velocities in sediments containing gas hydrates show that the strength of sediments increases with hydrate saturation hence it is expected that the attenuation of these sediments will decrease with increasing hydrate saturation however sonic log measurements in the mallik 2l 38 well and cross hole tomography measurements in the mallik field have shown that attenuation increases with hydrate saturation we studied a range of mechanisms by which increasing hydrate saturation could cause increased attenuation we found that a difference in permeability between the host sediment and the newly formed hydrate can produce the observed effect we modelled attenuation in terms of biot and squirt flow mechanisms in composite media we have used our model to predict observed attenuations in the mallik 2l 38 well mackenzie delta canada

zonation of merensky style platinum group element mineralization in turfspruit thick reef facies northern limb of the bushveld complex the stratigraphy whole rock composition and mineralogy from a thick undisturbed section of the platreef at depth on turfspruit in the northern limb of the bushveld complex have been examined in detail this section has much more in common with the critical zone than with the platreef structure updip although the high grade platinum group element pge reefs located close to the boundary with the main zone are much thicker than any known merensky reef facies a key feature that marks the conversion of the platreefstyle mineralization into the merensky style is the regular appearance of chromite seams which become more persistent with decreasing contamination westward and downdip the similar zonal distribution of platinum group minerals pgm in two consecutive thick pge reefs correlates with changes in silicate compositions and vertical zonation of pge concentrations in whole rocks and base metal sulfides this supports orthomagmatic in situ crystallization of ore minerals during mixing of consecutive magmatic influxes the typical merensky style ore assemblages of predominant pt sulfides and alloys are developed within 1 m thick intervals which correspond to the highest temperature mixing zones between two magmas whereas the rest of the reefs host pt pd bismuthotellurides and arsenides pge content and tenor as well as cu pd in major base metal sulfides in the turfspruit reefs are within the range of the typical high grade merensky values suggesting the same enrichment process