quantitative mineralogical and chemical assessment of the nkout iron ore deposit southern cameroon abstract the nkout deposit is part of an emerging iron ore province in west and central africa the deposit is an oxide facies iron formation comprising fresh magnetite banded iron formation bif at depth which weathers and oxidises towards the surface forming caps of high grade hematite martite goethite ores the mineral species compositions mineral associations and liberation have been studied using automated mineralogy qemscan combined with whole rock geochemistry mineral chemistry and mineralogical techniques drill cores saprolitic lateritic bif grab and outcrop samples were studied and divided into 4 main groups based on whole rock fe content and a weathering index the groups are enriched material em weathered magnetite itabirite wmi transitional magnetite itabirite tmi and magnetite itabirite mi the main iron minerals are the iron oxides magnetite hematite and goethite and chamosite the iron oxides are closely associated in the high grade cap and liberation of them individually is poor liberation increases when they are grouped together as iron oxides chamosite significantly lowers the liberation of the iron oxides automated mineralogy by qemscan or other similar techniques can distinguish between fe oxides if set up and calibrated carefully using the backscattered electron signal electron beam techniques have the advantage over other quantitative mineralogy techniques of being able to determine mineral chemical variants of ore and gangue minerals although reflected light optical microscopy remains the most sensitive method of distinguishing closely related iron oxide minerals both optical and electron beam automated mineralogical methods have distinct advantages over quantitative xrd in that they can determine mineral associations liberation amorphous phases and trace phases

major and trace element geochemistry of the archaean ntem plutonic complex south cameroon petrogenesis and crustal evolution the ntem complex crops out in south cameroon and belongs to the congo craton it corresponds to a 2 8 ga basement which in the northern part is made up of plutonic rocks 1 charnockites and 2 soo tonalites both have similar compositions and define a typical tonalite trondhjemite and granodiorite ttg suite the study of isotopes isr major elements and ree allows us to propose a petrogenetic model that may summarized as follows 1 1 partial melting of the mantle gave rise to tholeiites 2 2 tholeiites transformed into garnet bearing amphibolites underwent partial fusion thus generating the parental magma of the ttg suite 3 3 fractional crystallization mainly of hornblende and plagioclase produced the differentiated ttg suite this scheme is typical of archaean ttg suites and is supposed to have occurred in a subduction like geodynamic environment where because of the high archaean geothermal gradients the subducted oceanic crust underwent fusion before dehydration operated it is also possible to show that the ttg suite was not emplaced during the granulite facies metamorphism but that this occurred later affecting only some parts of the basement consequently the ntem complex appears to be an archaean high grade terrane where the granulitic assemblages may be early proterozoic in age characteristic that seems to be more widespread among archaean high grade terranes

u pb zircon chronology geochemistry and isotopes of the changyi banded iron formation in the eastern shandong province constraints on bif genesis and implications for paleoproterozoic tectonic evolution of the north china craton abstract the changyi banded iron formation bif in the eastern north china craton ncc occurs within the paleoproterozoic fenzishan group the bif shows alternating quartz rich light and magnetite rich dark bands with magnetite 15 65 vol quartz 25 65 vol and amphibole 15 30 vol constituting the major minerals minor garnet epidote chlorite calcite biotite and pyrite occur locally the bif bands are interlayered with amphibolite hornblende gneiss biotite quartz schist garnet biotite schist biotite gneiss and leptynite and are intruded by granites la icp ms u pb dating on zircons separated from the bif bands and the wallrocks constrains the depositional age as 2240 2193 ma and metamorphic age as 1864 ma the dominant composition of sio 2 fe 2 o 3 t average value of 92 3 wt of the bif bands suggests their formation mainly through chemical precipitation however the widely varying contents of major elements such as al 2 o 3 0 58 6 99 wt mgo 1 00 3 86 wt cao 0 22 4 19 wt and trace elements such as rb 2 06 40 4 ppm sr 9 36 42 5 ppm zr 0 91 23 6 ppm hf 0 04 0 75 ppm cr 89 1 341 ppm co 2 94 30 4 ppm and ni 1 43 52 0 ppm clearly indicate the incorporation of clastics especially continental felsic clastics as also confirmed by the presence of ancient detrital zircons in the bif bands when normalized against post archean average shale paas the seawater like signatures of ree distribution patterns such as lree depletion positive la and y anomalies and superchondritic y ho ratios average value of 36 3 support the deposition in seawater strong positive eu anomalies eu eu paas 1 14 2 86 also suggest the participation of hydrothermal fluids in addition the sympathetic correlation between cr co and ni as well as the co ni cu vs ree and the al 2 o 3 vs sio 2 relations further indicates that the iron and silica mainly originated from hydrothermal fluids combined with regional geological investigation and protolith restoration of the wallrocks a continental rift environment is suggested for the changyi bif deposition the appearance of negative ce paas anomalies might suggest the influence of the great oxidation event at the time of deposition the changyi bif witnessed the major paleoproterozoic rifting collision events in the ncc and their unique distribution in the ncc contrasts with other examples elsewhere in the world

sequence architecture during the holocene transgression an example from the great barrier reef shelf australia abstract the application of sequence stratigraphic analysis to post glacial sediments requires the correct identification of sequence boundaries flooding surfaces and systems tracts the nature of coastal sediments deposited during sea level cycles is complex and misinterpretation is easy particularly in ancient sequences for which eustasy is inferred rather than demonstrated a large core seismic and radiocarbon database from cleveland bay in tropical queensland allows detailed analysis of the systems tracts and parasequences developed during the late holocene despite the large database a unique sequence stratigraphic interpretation is not possible because oscillations in the sea level curve apparently occurred at a frequency higher than the resolving power of radiocarbon dating our best estimate is that the final stages of the holocene sea level rise comprise four parasequences each deposited during episodic slowdowns in sea level rise and or a pulse of rapid sediment supply sea level pauses probably occurred at 12 10 m below modern levels at 8 5 ka bp followed by a fall to ca 17 m at 8 1 ka bp a very rapid transgression to ca 10 m at 7 9 ka bp a further transgression to ca 5 m at 6 8 ka bp and a final rise to the holocene highstand at 1 65 m at 5 5 ka bp the stratigraphic record of these sea level rises is complex occurs within the modern shore connected sedimentary wedge and comprises parasequences associated with shorelines at 11 m p11 17 m p17 10 m p10 5 m p5 and 1 65 m p 1 7 each parasequence comprises in ascending order onlapping estuarine shoreline sediment including mangrove mud and beach sand and progradational shallow bay muddy sand across the bay units up to and including the lower parts of the shallow bay mud correspond to the transgressive systems tract tst of the last 18 ka glacial interglacial cycle the particular parasequence represented being dependent upon location especially depth bay mud above this level corresponds to the post 5 5 ka bp highstand systems tract hst which comprises sand cheniers on the coastal plain and bioturbated muddy sand in the shallow bay the junction between the tst and the hst lies within the bay mud and corresponds approximately to the local flooding surface for the p 1 7 parasequence deposited at the 1 65 m shoreline offshore terrigenous tst strata do not occur over most of the middle and outer shelf which instead is veneered with a thin mid cycle shellbed the lower part of this shellbed is time equivalent to the later part of the tst and the upper part is time equivalent to the hst offshore the maximum flooding horizon mfh and peak eustatic sea level horizon pesh of the late pleistocene holocene sea level cycle lie at some level within the mid cycle shellbed nearshore the mfh and pesh lie within the shore connected sediment wedge at levels near the tst hst boundary however at no locality do either the mfh and pesh necessarily correspond to a physical boundary within the stratigraphy

upb and smn edvidence for eburnian and pan african high grade metamorphism in cratonic rocks of southern cameroon abstract u pb analyses of zircons and sm nd mineral and whole rock analyses of samples from southern cameroon document three distinct rock forming or high grade metamorphic events 2900 ma 2050 ma and 600 ma zircons from the ebolowa charnockite in southern cameroon yield an age of 2896 7 ma consistent with published rb sr total rock data for the region sm nd crustal formation ages for these rocks are 3000 ma indicating that the granitoids and ntem series gneisses in the north end of the congo craton formed 2900 ma ago and do not contain major contributions from older archean crust a second metamorphic event of eburnian age is identified from re equilibration of sm and nd in gneisses and metadolerites of the ntem complex at 2050 ma zircons from the younger nyong series gneisses are a complex mixture of cores and overgrowths the overgrowths in most cases formed during a high grade metamorphic event between 2100 and 2000 ma and clearly postdate the ebolowa charnockite the ages of the cores are not precisely known but we believe most represent detrital zircons from 2900 ma protoliths sm nd crustal formation ages for samples of the nyong series gneisses are generally 2500 to 2900 ma indicating that their precursors were apparently derived mainly from the archean craton with variable amounts of younger component sm nd garnet total rock isochrons for metamorphosed mafic rocks in the nyong series yield ages of 2050 ma and zircons from one of them yield an upper intercept age of 2037 10 ma further documenting the existence of a major event with magmatism and charnockitization at 2000 to 2100 ma we interpret the nyong series metasedimentary gneisses as post 2900 ma cratonic cover that was deformed and metamorphosed during the 2000 to 2100 ma eburnian orogeny we cannot define precise limits on the ages of sedimentation preliminary data on metamorphosed intrusive rocks associated with the yaounde series yield u pb concordant ages around 620 ma consistent with sm nd garnet whole rock ages from the same series this dates the pan african tectono metamorphic activity in the southern end mobile belt the pan african lower intercepts of zircons and sm nd garnet whole rock ages from thnyong series are the consequences of this event which is further represented by a 590 ma zircon age from a late syenitic pluton results from cameroon are consistent with recent results from the western part of the congo craton farther south and from the northeastern part of the sao francisco craton in brazil one interpretation of these results is that the congo and sao francisco cratons were joined during the ebumian trans amazonian orogeny 2100 ma and are bordered to the north by a late mesoproterozoic rift basin that was closed during the pan african brasiliano orogeny 600 ma

geodynamic evolution of the pan african belt in central africa with special reference to cameroon the pan african belt in central africa has benefited from the many petrographic structural and geochronological studies in the recent years that have improved our understanding of the belt however those studies have also produced various and often divergent evolutionary models for the belt some of which do not even involve well defined cratons following a review of the available data in cameroon we propose a model of continent continent collision that involved the congo craton and the north central cameroon active margin showing archean to paleoproterozoic inheritances this model is based among others on i the prominent role of the congo craton as demonstrated by the regional extension of external nappes on its northern edge and the concomitant exhumation of the 620 ma granulitic rocks believed to have formed at the root of the collision zone and ii the late development of a strike slip fault system in central cameroon as the result of horizontal movement following the multistage collision

positive relief over buried post glacial channels great barrier reef province australia abstract continuous seismic profiling on the continental shelf central great barrier reef province australia has delineated a major sub bottom reflector interpreted as the pre holocene surface channels eroded in this surface are infilled and overfilled by post glacial sediments filling is ascribed to estuarine backfilling during sea level rise followed by deposition of delta front deposits in many cases these fills create positive relief over buried channels hence modern shelf morphology may not be an accurate guide to the positions of pleistocene channels across the shelf

geology of the gouap iron deposit congo craton southern cameroon implications for iron ore exploration abstract the ntem complex represents the northwestern edge of the archean congo craton in southern cameroon where numerous banded iron formation bif hosted iron deposits have been discovered the gouap iron deposit which is located on the western edge of the ntem complex is one of the recent discoveries of iron deposits in this area in this study geological mapping detailed drillcore logging and petrographical studies reveal that the gouap iron deposit comprises of two lithostratigraphic units namely the bif and the country rocks the bif unit consists of amphibole bif and amphibole pyrite bif whereas the country rocks unit is made up of gneisses garnet amphibole magnetite and amphibole garnet pyrite gneiss schists amphibole magnetite and garnet magnetite micaschist quartzites and dolerite sills the bifs are well interlayered and observed to be in sharp or gradational contacts to their associated country rocks the mineral assemblages of the bifs and wallrocks except the dolerites indicate that both units have been metamorphosed under amphibole facies conditions two main iron mineralization types are identified in the gouap deposit the soft iron type which is mainly located in the western and northern parts and the hard iron type mostly confined to the southern sector of the deposit the structural controls of the gouap iron deposit are better observed in drillcore samples other than on the outcrop scale there are three principal deformation events d1 d2 and d3 that led to the development of foliation folding and faulting respectively the iron mineralization are mostly concentrated on the f2 fold hinge zones and fractures that acted as passage ways for the circulation of meteoric water at surface we suggest that the following three major processes that led to the formation of the gouap deposit i the chemical precipitation of chert and iron oxide followed by sedimentation and diagenesis that formed chert magnetite bifs ii secondly the amphibolite facies metamorphism and deformation that formed quartz magnetite bifs which constitute the main iron ridge located on thicker zones of fold noses and iii finally the gouap bifs have been upgraded to medium grade iron ore due to supergene enrichment processes which are restricted to the modern land surface and locally reached deeper levels through fault and fracture zones the western and northern parts of the gouap deposit have been identified as potential exploration target areas for iron mineralization even though the iron formation processes of the gouap deposit are not yet completely understood and many questions remain unanswered regarding the genesis of the soft and hard iron mineralization these new insights have important implications on the exploration drilling strategies of this deposit

geochemistry and pb pb geochronology of the neoarchean nkout west metamorphosed banded iron formation southern cameroon the nkout west iron deposit of the northern edge of the congo craton in southern cameroon forms part of the northwest plunging isoclinal mega fold composed of metamorphosed banded iron formations bifs which have been deformed and metamorphosed resulting in recrystallized magnetite quartz and silicates here we present whole rock geochemistry together with pb pb isotopic compositions and dating of metamorphosed bif with the aim to constrain the genesis and elucidate the metamorphic resetting age based on the mineral composition two types of bif magnetite bif garnet bif have been distinguished the magnetite bif shows high sio2 and fe2o3 and low al2o3 tio2 hfse and ree contents similar to detritus free chemical sediments in contrast their high concentrations in garnet bif indicate clay rich contaminated input the co zn ratios are consistent with trace metals of hydrothermal sources 0 14 for magnetite bif or with minor hydrogenous component 0 27 for garnet bif the paas normalized ree y patterns of the studied metamorphosed bifs show lree depletion relative to hree and positive eu anomalies the magnetite bif exhibits positive y anomaly y y paas 1 3 which is the signature inherited from sea water but its y ho 39 93 ratio suggests a mixture of hydrothermal solution with sea water the garnet bif shows positive eu negative y anomalies and chondritic y ho ratio 27 49 which is consistent with hydrothermal solution with significant clastic contamination whole rock pb concentration is roughly on average of 2 0 6 0 ppm suggesting a hydrothermal pb source the pb pb dating yielded an age of 2679 ma interpreted as the neoarchean metamorphic event in the ntem complex

post glacial seismic stratigraphy central great barrier reef australia a regional programme of continuous seismic boomer profiling in the central great barrier reef province has identified a widespread shallow seismic discontinuity reflector a which is interpreted as the pre holocene surface nine seismic facies units are distinguished primarily on the basis of the seismic records but also with the aid of additional criteria such as location and surface sediment types two units underlie reflector a and are pre holocene these units are interpreted as a permo carboniferous bedrock and b pleistocene tertiary sediments consisting of both shelf wide terrigenous units and carbonate mounds and platforms under present reefs seven units are post glacial and overlie reflector a these units are interpreted as c fluvial estuarine channel fill d relict delta front deposits e relict transgressive veneer f coastal coarse and g fine deposits h modern reef and i reef talus in general post glacial sediment cover is very thin and in many places on the mid shelf the pre holocene units crop out substantial post glacial accumulations are limited to protected coastal embayments and to offshore reef masses

geochemistry and depositional environment of mn oxide deposits in the tokoro belt northeastern hokkaido japan numerous early cretaceous mn oxide and mn fe deposits are distributed in the tokoro belt northeastern hokkaido japan the mn oxide deposits usually occur in the southwestern part of the belt and within bedded cherts whereas the mn fe deposits are restricted to the northeastern part of the belt and occur between bedded cherts and underlying basaltic pillow lava the mn oxide ores are characterized by low fe mn ratios and the trace element concentrations and rare earth element ree patterns are similar to those of the submarine hydrothermal mn deposits distribution patterns of trace elements co ni cu and zn and ree in the mn oxide ores determine two types of deposits hydrogenous deposits which have relatively high concentrations and submarine hydrothermal deposits which have low concentrations these compositional trends and geologic evidences of the tokoro mn deposits suggest that the fe phases precipitated first closer to their source whereas the mn phases deposited later at a distance from their source some hydrogenous effects on the mn oxide deposits are due to the adsorption of trace elements from seawater trace element concentrations of tokoro mn oxide deposits show that they formed within 40 km from their source

some geochemical indicators for discrimination between diagenetic and hydrothermal metalliferous sediments abstract two types of metalliferous sediments are known hydrothermal associated with tectonic faults and diagenetic enriched in precipitates from seawater they are similar in their appearance and chemical composition only the first type is an indicator for hydrothermal discharge therefore in exploration criteria for differentiating between the two are needed four possible indicators have been investigated and are discussed a differences in p y correlation b differences in cr zr correlation c different distribution patterns of ree and d different as contents the as content is easy to determine and seems to be at the present time the most suitable criterion for determining the genesis of the metalliferous sediments

geology and geochemical constrains on the origin and depositional setting of the kpwa atog boga banded iron formations bifs northwestern congo craton southern cameroon abstract the kpwa atog boga kab area lies within the paleoproterozoic nyong unit which defines the northwestern edge of the archean congo craton in southern cameroon rock types in the area are mainly comprises of garnet gneiss associated with banded iron formations bifs the bifs and its host rocks were subjected to amphibolite facies metamorphism the rare earth elements ree patterns and the discrimination ratios e g al2o3 tio2 cr th th sc la sc of the garnet gneiss suggest that they are metasedimentary rocks derived from a felsic parent sources that have undergone intensive weathering the kab bifs exhibit significant concentration of tfe2o3 sio2 93 wt and lower al2o3 and tio2 contents and high field strength elements e g hf nb ta and zr this involves little contamination with continental detritus at their depositional environment the post archean australian shale paas normalized ree patterns of the kab bifs show enrichment in heavy ree and the depletion in light ree with positive la anomalies which imply that these bifs precipitated in ambient seawater moreover the contribution of low temperature hydrothermal fluids during the deposition of the studied bifs were highlighted through their high superchondritic y ho ratio mean 31 48 weak positive eu eu sn ratio mean 1 83 and the low contents of transition metals this suggests that the geochemical characteristics of the kab bifs and those of the paleoproterozoic superior type bifs are almost similar in summary low temperature hydrothermal fluids mixed with seawater contributed to the deposition of the kab bifs on the continental margin this depositional setting similar to its host rocks is consistent with the paleoproterozoic geological history recorded within the nyong unit this preliminary study reveals that the kab bifs are associated with low grade gold

hydrothermal origin for the 2 billion year old mount tom price giant iron ore deposit hamersley province western australia giant iron ore deposits such as those in the hamersley province of northwestern australia may contain more than a billion tonnes of almost pure iron oxides and are the world s major source of iron it is generally accepted that these deposits result from supergene oxidation of host banded iron formation bif accompanied by leaching of silicate and carbonate minerals new textural evidence however shows that formation of iron ore at one of those deposits mount tom price involved initial high temperature crystallisation of magnetite siderite iron silicate assemblages this was followed by development of hematite and ferroan dolomite bearing assemblages with subsequent oxidation of magnetite leaching of carbonates and silicates and crystallisation of further hematite preliminary fluid inclusion studies indicate both low and high salinity aqueous fluids as well as complex salt rich inclusions with the range of fluid types most likely reflecting interaction of hydrothermal brines with descending meteoric fluids initial hematite crystallisation occurred at about 250 c and high fluid pressures and continued as temperatures decreased although the largely hydrothermal origin for mineralisation at mount tom price is in conflict with previously proposed supergene models it remains consistent with interpretations that the biosphere contained significant oxygen at the time of mineralisation

rare earth element systematics of the chemically precipitated component in early precambrian iron formations and the evolution of the terrestrial atmosphere hydrosphere lithosphere system irrespective of provenance age and metamorphic grade the chemically precipitated component in early precambrian 2 3 ga iron formations ifs displays smyb cn 1 which reflects the corresponding ratios of contemporaneous seawater in conjunction with end if end shale jacobsen and pimentel klose 1988a b this rare earth element ree signature reveals that the ree distribution in early precambrian ifs must be explained by mixing between a marine bottom and a surface water component and that the rees and by analogy the fe cannot be derived from weathering of a continental source eusm n ratios of detritus free ifs are controlled by the marine bottom water component where eusm cn 1 results from ree input from high temperature high t hydrothermal fluids which received their ree signature during high t alteration of oceanic crust at spreading centres decreasing eusm cn of ifs with decreasing depositional age reflect the vaning importance of high t compared to low t 200 c hydrothermal fluids hence oceanic crust alteration for the ree budget of marine bottom waters and are related to decreasing upper mantle temperatures smyb n ratios of detritus free ifs are controlled by the marine surface water component whose ree distribution is affected by the same processes which operate today in the entire ocean mixing calculations reveal that smyb cn in early precambrian marine surface waters was significantly lower than it is today to explain this difference two mechanisms are discussed on the basis of higher pco2 and lower po2 levels of the precambrian atmosphere 1 1 higher co32 in precambrian river and coastal waters and 2 2 redox cycling of the rees at an oceanwide chemocline which separated anoxic and oxic water masses however these processes are synergetic and could have operated in unison the ree distribution in precambrian ifs is described as a result of mixing in a multicomponent system where high t and low t hydrothermal fluids contributed to the marine bottom water and ree input from the dissolved ree pool in river waters after some modification in estuaries dominated the ree distribution in marine surface waters after mixing of bottom and surface waters their combined ree load was quantitatively scavenged onto precipitating iron oxihydroxides or iron carbonates and deposited with the chemically precipitated component after addition of variable amounts of clastic detritus this eventually resulted in the ree distribution observed in precambrian ifs

the pb pb age of the minas supergroup carbonate rocks quadrilatero ferrifero brazil abstract pb pb isochron ages determined on carbonate rocks from the minas supergroup quadrilatero ferrifero qf show that the deposition of the gandarela formation the intermediate unit of the minas supergroup took place at 2420 19 ma the entire sedimentary sequence and underlying basement rocks have been deformed at a regional scale and data from greenschist facies carbonate rocks from the piracicaba group indicate that the pb isotopes were reset at about 2100 ma this age is in agreement with whole rock rb sr and u pb titanite ages determined by many authors for metamorphism of the surrounding archean granite gneiss terrane

a mixed seawater and hydrothermal origin of superior type banded iron formation bif hosted kouambo iron deposit palaeoproterozoic nyong series southwestern cameroon constraints from petrography and geochemistry abstract the kouambo iron deposit contains banded iron formations bifs and is located in the northwestern margin of the congo craton the bifs are hosted in palaeoproterozoic nyong series a dominantly metasedimentary formations which were metamorphosed into greenschist to granulite facies the kouambo bifs are medium to coarse grained banded rocks consisting of alternation of si rich and fe rich mesobands and belong to oxide facies iron formations geochemistry analyses reveal that these iron formations are composed of 96 wt fe2o3 and sio2 and have low concentrations of al2o3 tio2 and trace hfse suggesting chemical precipitates of silica and iron moreover these bifs have low concentrations of al2o3 tio2 and trace hfses high field strength elements e g zr hf ta pb and th suggesting that terrigenous detrital materials contributed insignificantly to the sedimentation the post archean australian shale paas normalized ree y patterns display seawater like profile minor lree depletion and hree enrichment positive y anomalies however they display positive eu and negative ce anomalies and low y ho ratio average 29 which suggest the influence of the hydrothermal fluids the weak positive eu eu paas ratio average 1 5 associated with the low v 17 5 ppm co 6 1 ppm and ni 27 5 ppm contents similar to other superior type bifs worldwide are consistent with the deposition of the kouambo bifs in continental marginal sea or back arc basin environment in summary the kouambo bifs show a seawater like ree y signature however the positive eu anomalies and reduced y ho ratios relative to seawater indicates a possible mixing with hydrothermal fluids 0 5

geochemistry and sedimentology of a facies transition from microbanded to granular iron formation in the early proterozoic transvaal supergroup south africa abstract a transition from microbanded kuruman to granular griquatown iron formation is described in terms of sedimentological petrographic and geochemical characteristics major minor trace and rare earth elements as well as whole rock carbon and oxygen isotopic compositions transitions between microbanded and granular iron formation are uncommon in the global stratigraphic record with microbanding absent from most known granular iron formations the kuruman griquatown transition zone differs from other granular iron formation sequences in being non oolitic i e peloidal and magnetite rich instead of hematite rich it is concluded that the coarsening upward microbanded granular iron formation units in the kuruman griquatown sequence represent upward shallowing storm dominated deposits that is primary microbanded relatively deep water fair weather deposits having been intermittently reworked by storm wave action form wavy sharp based graded and poorly sorted irregular interbeds of granular iron formation this is in contrast to typical hematitic oolitic iron formation units which represent true shoal deposits geochemical results strongly support the storm reworking hypothesis microbanded and granular iron formation beds have similar ree patterns with negative ce anomalies and depletion in light ree reflecting deposits in a marine setting below the zone of shallow surface water positive eu anomalies in the ree patterns suggest admixture of a hydrothermal component of the ocean water carbonates in the iron formation are depleted in 13c indicating involvement of organic matter during diagenetic alteration of the iron formations there are also indications that total dissolved carbon dioxide in deeper water was more depleted in 13c than in shallower water deposition of the iron formations was sporadically interrupted by influx of tuffaceous material now represented by centimeter thick shard and lapilli bearing stilpnomelane lutite beds these beds differ chemically from the iron formations in being enriched in elements such as al sc y zr nb hf ta th u and the lanthanides and displaying ree patterns with strong negative eu and slight positive ce anomalies the composition of the parent magma of the volcanics remains uncertain because of probable post depositional alteration of the volcanic glass however it was most probably of intermediate to acidic composition as concluded from the widespread distribution of the ash beds typical of plinian type deposits chemical variations in the minor trace and ree compositions of the iron formations are closely correlated with the amounts of admixed volcanic ash material most of the iron formations appear chemically very uncontaminated being highly depleted in all minor trace and ree and organic carbon contents the iron formations were therefore most probably deposited as a chemical precipitate on a submerged shallow cratonic platform with volcanic ash as the only exogenic material sporadically reaching the depository

halogens and noble gases in sedimentary formation waters and zn pb deposits a case study from the lennard shelf australia abstract halogen ratios br cl and i cl and concentrations provide important information about how sedimentary formation waters acquire their salinity but the possible influence of organic br derived from sedimentary wall rocks is rarely quantified here it is demonstrated that br cl versus i cl mixing diagrams can be used to deconvolve organic br contributions that organic matter has a limited range of br i ratios and that organic br is a more significant component in zn pb deposit ore fluids than previously recognised the significance of these findings is illustrated for the lennard shelf zn pb deposits of western australia fluid inclusions related to lennard shelf zn pb mineralisation have variable salinity and hydrocarbon contents the halogen data from these fluid inclusions require mixing of three fluid end members 1 an evaporated seawater bittern brine 30 wt nacl equiv with greater than seawater br cl ratio 2 a lower salinity pore fluid 5 wt nacl equiv with moderately elevated br cl and i cl and 3 fluids with br cl ratios of 5 times seawater and extremely elevated i cl ratios of 11 500 times seawater the first two fluids have 40 ar 36 ar of 300 400 and greater than air saturated water 36 ar concentrations that are typical of fluid inclusions related to zn pb mineralisation the third organic rich fluid has the highest 40 ar 36 ar ratio of up to 1500 and a depleted 36 ar concentration mineralisation is interpreted to have resulted from mixing of zn rich evaporitic brines and h 2 s present in hydrocarbons it is suggested that aqueous fluids acquired organic br and i from hydrocarbons and that hydrocarbons exsolving from the aqueous fluid removed noble gases from solution interaction of variably saline brines and hydrocarbons could account for the variable br cl and i cl composition and 36 ar concentrations recorded by lennard shelf fluid inclusions the distinct 40 ar 36 ar signature of the fluid with the highest i cl ratio suggests the hydrocarbons and brines were sourced independently from different parts of the sedimentary basin these data indicate the complementary nature of halogen and noble gas analysis and provide new constraints on important mixing processes during sediment hosted zn pb mineralisation

crustal origin of early proterozoic syenites in the congo craton ntem complex south cameroon two petrographically different syenite suites can be distinguished in the mostly archean ntem complex of cameroon a suite of two pyroxene syenites and a suite of clinopyroxene syenites the syenites occur as discrete intrusions surrounded by archean gneisses based on structural relationships the syenites are of late to post orogenic origin the pb pb zircon evaporation ages indicate that the syenites intruded at ca 2 3 ga and thus postdate significantly the archean episode of crust formation and differentiation that generated the surrounding gneiss terrane the isotope signatures and the trace elements provide unambiguous evidence that the two syenite suites are of crustal origin the syenite suites do not show simple chemical trends have highly variable trace element contents and display only a small variation in their major element contents particularly in their sio2 contents the isotope characteristics including 143nd 144nd 87sr 86sr and common pb point to a heterogeneous source or multiple sources for the syenites the sr isotope compositions calculated for the time of intrusion vary over a wide range from an unrealistically low 87sr 86sr ratio of 0 690 to high values of 0 713 in contrast the initial 143nd 144nd ratios are more homogenous with end 2320 from 5 8 to 7 5 the pb isotopes from leached k feldspars indicate that the sources of the syenites had a history of u depletion and th u enrichment the syenites show the same isotope characteristics as the surrounding gneiss terrane thus the deep continental crust of the ntem complex itself is the most likely source for the syenite melts this conclusion is also supported by the heterogeneity of the trace elements in the syenites that exclude a homogenous source in addition inherited zircons from the syenites yield pb pb evaporation ages that are consistent with ages obtained for zircons extracted from the surrounding gneisses in contrast to many other syenites worldwide the syenites in the ntem complex do not contain juvenile mantle material but are the sole products of crustal reworking thus the syenites did not contribute to the crustal growth in the ntem complex however this conclusion may not preclude the possibility that the syenites were generated in the deep crust during a time of addition of mantle material to the continental crust the cause for the generation of these large syenite bodies in the ntem complex is most likely an episode of deep seated thermal activity that was so far not known from this part of the congo craton

spatial variability in oceanic redox structure 1 8 billion years ago the deposition of iron formations ceased about 1 84 billion years ago reconstructions of ocean chemistry suggest that the advent of euxinic conditions along ocean margins preferentially removed dissolved iron from the water column in the form of the mineral pyrite inhibiting widespread iron oxide mineral deposition

metallogeny of the north china craton link with secular changes in the evolving earth abstract the north china craton ncc has experienced a complex geological evolution since the early precambrian and carries important records of secular changes in tectonics and metallogeny here we synthesize the salient geological and tectonic features of the evolution and destruction of the ncc vis a vis major metallogenic events and the formation of potential ore deposits we identify a close relationship between the major geological events in the ncc and those reported elsewhere on the globe we trace the records of a regular change in the pattern of metallogeny mineral deposit character spatial distribution and genetic mechanisms which closely match the timing and styles of the major geological and tectonic events in this craton the ncc went through five major tectonic cycles 1 neoarchean crustal growth and stabilization 2 paleoproterozoic rifting subduction accretion collision with imprints of the great oxidation event goe 3 late paleoproterozoic neoproterozoic multi stage rifting 4 paleozoic orogenesis at the margins of the craton and 5 mesozoic extensional tectonics associated with lithospheric thinning and decratonization coinciding with these major geological events are five major metallogenic systems identified as follows 1 an archean bif system 2 paleoproterozoic cu pb zn and mg b systems 3 a mesoproterozoic ree fe pb zn system 4 a paleozoic orogenic cu mo system and 5 mesozoic intracontinental au and ag pb zn and mo systems the ore deposit types in each of these metallogenic systems show distinct characteristics and tectonic affinities from early precambrian through late precambrian to paleozoic and mesozoic the ncc records a transition from primitive to modern style plate tectonics evidence for imbricated oceanic plate stratigraphy in a subduction accretion setting and collisional orogenesis along at least three major zones of ocean closure are documented major transitions in tectonic style and surface environmental changes recorded in other parts of the world are also reflected in the geological history and metallogenic events in the ncc large scale gold deposits formed through intraplate tectonics during the mesozoic provide important insights into mantle dynamics and crust mantle interaction associated with lithospheric thinning and craton destruction the ncc provides one of the best examples for documenting secular changes in the geological history and metallogenic epochs of an evolving earth

late archaean high k granite geochronology of the northern metacratonic margin of the archaean congo craton southern cameroon evidence for pb loss due to non metamorphic causes abstract high k granitoids in the archaean congo craton are products of crustal differentiation by partial remelting of archaean ttg and in addition the remelting of charnockites at its northern metacratonic margin the ntem complex in southern cameroon at the sangmelima region of this metacratonized complex high k granites mostly occur as small intrusions and as pods dykes and veins in ttg and charnockites zircon u pb analyses of these high k granites show an important radiogenic pb loss indicated by highly discordant u rich fractions pb loss in these smaller granitic intrusions appears to be due to continuous than discrete pan african diffusion and leaching that characterizes larger charnockitic and tonalitic intrusions in the region or a combination of both the contributive effect of leaching due to later reactivations during the phanerozoic cannot be ruled out these multiple pb loss causes and apparently disproportionate amounts of 206pb and 207pb loses probably explain the irregular clustering of the highly discordant u pb data however an upper intercept age of 2751 32 ma and a lower intercept age of 300 29 ma are obtained most of the zircons especially those from high k granite veins lost pb from the entire grain with no domains remaining that reflect the original pb isotopic composition it was therefore impossible to obtain meaningful 207pb 206pb evaporation ages for such grains thus caution should be taken against use of the evaporation method for zircons that experienced severe pb loss however some whole zircons and zircon domains from larger granite bodies apparently retained their primary pb isotopic composition and were dated successfully by the evaporation technique they yielded reproducible 207pb 206pb ages between 2717 9 ma and 2724 3 ma with a weighted mean of 2722 2 ma interpreted as the minimum estimate of the crystallization age of the sangmelima high k granites similar sr and nd isotope signatures and nd model ages 3 04 3 25 ga as for the ttg and charnockite protoliths indicate that the high k event did not add new juvenile material to the archaean crust of the congo craton but was the result of crustal differentiation

the nature of the pre holocene surface john brewer reef with implications for the interpretation of holocene reef development abstract seismic reflection profiles trace out a steep sided antecedent platform assumed to be the holocene pleistocene interface beneath the post glacial holocene coralline and sediment veneer of john brewer reef a small mid shelf reef within the central great barrier reef province this platform defined by a semi continuous seismic reflector is characterised by three essentially planar surfaces at 20m 32 m and 40 m depth with respect to present low water datum the 20 m surface which exhibits a maximum relief of 2 m is extensive and is postulated to represent either a senile reef which grew to maturity most probably during the 50 kyr interstadial or the erosion and concurrent infilling during this phase of a near flat reefal structure which dates from an earlier 20 m interstadial the deeper surfaces are less extensive that at 32 m is considered to represent a fringing reef which became established on the flanks of the antecedent platform during a brief late pleistocene sea level stillstand the 40 m surface appears to be predominantly erosional in origin and also of late pleistocene age whilst these surfaces are sites of major coral growth holocene reef development has occurred largely independent of antecedent platform morphology present reef morphology reflects the interaction between the rate of post glacial sea level rise prevailing hydrodynamic conditions and biological activity and bears no resemblance to that of the underlying antecedent surface the present morphology of john brewer reef is consistent with a continuous post glacial sea level rise

the mount gibson banded iron formation hosted magnetite deposit two distinct processes for the origin of high grade iron ore the mount gibson banded iron formation lies within the windanning formation of the luke creek group which is found in almost all greenstone belts throughout the murchison province the banded iron formation bif typically consists of alternating bands of magnetite and microcrystalline quartz chert with rare carbonaceous and iron silicate rich shale partings and layers and rare thin fine grained tuff bands owing to repetition by isoclinal folding and attenuation by faulting the true thickness of the mount gibson bif is unknown but appears to be on the order of 100 m although the mount gibson bif is typical of many algoma type iron formations situated within a greenstone belt on an archean craton it is also similar to hamersley type bif in petrology areal extent and ore genesis high grade hematite deposits formed within bif were thought to have formed by the supergene leaching of chert from typical cherty bif recent evidence suggests that at least some of these deposits are formed by hypogene replacement of chert by carbonates with subsequent supergene leaching of the carbonate and accessory minerals and oxidation of magnetite to hematite magnetite carbonate bif in which there is clear evidence of hydrothermal replacement of chert by carbonate forms distinctive magnetite goethite ore with magnetite locally persisting to the surface mount gibson shows clear evidence of the formation of high grade ore by this process but also contains high grade hematite ore and chert free bif that show no evidence of the hypogene replacement of chert high grade hematite occurrences up to 1 km in strike length are found within the weathered zone overlying the magnetite bif at mount gibson and continue into unweathered chert free bif at depth that show no evidence of hydrothermal carbonate or supergene enrichment the cherty bif shows sharp contacts against chert free bif and high grade ore even when strongly weathered this suggests that deep saprolitic in situ high grade ore may be produced by different processes including hydrothermal replacement of chert mesobands by carbonates with subsequent supergene leaching of the carbonate and by the oxidation of chert free bif in which chert bands either never developed or were apparently removed during diagenesis neither model requires supergene selective leaching of quartz chert during deep weathering

a review on the huoqiu banded iron formation bif southeast margin of the north china craton genesis of iron deposits and implications for exploration abstract the huoqiu iron ore field in northwest anhui province is located in the north china craton ncc as a large banded iron formation bif iron ore field ore bodies occur in a middle high grade of neoarchean metamorphic formation forming a banded silicon iron series from north to south the main ore bodies can be divided into two sub belts from bottom to upper layers i e the a b ore belt consisting of leptynite schist magnetite quartz formation and the d ore belt consisting of schist marble hematite quartz formation based on a dataset from geological settings geophysical and geochemical exploration ore forming conditions and structural analysis of the iron deposit we discuss structural types sedimentary environments deep tectonic and ore controlling factors as well as characteristics and distribution of this colossal bif ore field in the huoqiu region using la icp ms techniques we obtained the oldest u pb age of ca 2 7 ga for plagioclase amphibolite as its original rock and 1 8 ga for magmatic granite in the huoqiu group the hf isotopes of zircon were also determined resulting in the oldest hf model age of 3 5 ga geochemical data indicate that the protolithes of amphibolites belong to a series of subalkaline rocks with enrichments of large ion lithophile elements and depletions of high field strength elements which are typical volcanic arc rocks the amphibolites have low k 2 o concentrations with low ratios of ti v 22 7 to 25 9 averaging 24 5 similar to island arc tholeiite this suggests that the iron deposit and bif are of the superior type in the huoqiu region

origin and evolution of the late precambrian high grade yaounde gneisses cameroon abstract the yaounde series is composed of low to high grade garnet bearing schists and gneisses belonging to the pan african north equatorial fold belt the high grade rocks consist of kyanite garnet gneisses and garnet plagioclase gneisses containing layers of calcsilicate rocks marbles quartzites and magnetite rich orthopyroxenites the chemical patterns of these rocks are those of a sedimentary sequence of shales and greywackes and minor dolomite rocks dolomitic marls evaporitic beds quartzites and iron rich sediments some volcanogenic greywackes may have been of alkaline affinity this sequence was probably deposited in shallow water near shore semi barred conditions which can be related either to an intracontinental distensive environment or to a passive margin at the northern edge of the congo craton the yaounde gneisses were deformed during two main tectonic phases the first phase d1 is marked by a layering s0 s1 resulting from tectonic transposition the second one d2 corresponds to tangential tectonics with isoclinal folds and flat lying shear zones associated with a s2 schistosity and a l2 lineation a third phase late d2 is marked by the development of low temperature mylonitic shears scarce wrench faults indicate later e w compression the d1 d2 transition is characterized by the emplacement of mafic and ultramafic rocks and by the development of high pressure granulite facies conditions t 750 800 c p 10 12 kbar these peak metamorphic conditions triggered a widespread migmatization dated at 565 22 ma and coeval with the d2 event the gradual decrease of the metamorphic conditions toward the south and the existence of a syn d2 retrograde inverse metamorphic gradient suggest that the overall structure of the yaounde series is a large tectonic nappe thrust onto the congo craton a geotectonic scheme for the north equatorial fold belt is discussed