iron oxide copper gold iocg deposits through earth history implications for origin lithospheric setting and distinction from other epigenetic iron oxide deposits the iron oxide copper gold iocg group of deposits initially defined following discovery of the giant olympic dam cu u au deposit has progressively become too embracing when associated deposits and potential end members or analogs are included the broader group includes several low ti iron oxide associated deposits that include iron oxide p rich iron oxide f and ree rich fe or cu au skarn high grade iron oxide hosted au cu carbonatite hosted cu ree and f rich and iocg sensu stricto deposits consideration of this broad group as a whole obscures the critical features of the iocg sensu stricto deposits such as their temporal distribution and tectonic environment thus leading to difficulties in developing a robust exploration model the iocg sensu stricto deposits are magmatic hydrothermal deposits that contain economic cu and au grades are structurally controlled commonly contain significant volumes of breccia are commonly associated with presulfide sodic or sodic calcic alteration have alteration and or brecciation zones on a large commonly regional scale relative to economic mineralization have abundant low ti iron oxides and or iron silicates intimately associated with but generally paragenetically older than fe cu sulfides have lree enrichment and low s sulfides lack of abundant pyrite lack widespread quartz veins or silicification and show a clear temporal but not close spatial relationship to major magmatic intrusions these intrusions where identified are commonly alkaline to subalkaline mixed mafic even ultramafic to felsic in composition with evidence for mantle derivation of at least the mafic end members of the suite the giant size of many of the deposits and surrounding alteration zones the highly saline ore fluids and the available stable and radiogenic isotope data indicate release of deep volatile rich magmatic fluids through devolatization of causative mantle derived magmas and variable degrees of mixing of these magmatic fluids with other crustal fluids along regional scale fluid flow paths precambrian deposits are the dominant members of the iocg group in terms of both copper and gold resources the 12 iocg deposits with 100 tonnes t resources are located in intracratonic settings within about 100 km of the margins of archean or paleoproterozoic cratons or other lithospheric boundaries and formed 100 to 200 m y after supercontinent assembly their tectonic setting at formation was most likely anorogenic with magmatism and associated hydrothermal activity driven by mantle underplating and or plumes limited amounts of partial melting of volatile rich and possibly metal enriched metasomatized early precambrian subcontinental lithospheric mantle sclm fertilized during earlier subduction probably produced basic to ultrabasic magmas that melted overlying continental crust and mixed with resultant felsic melts with devolatilization and some penecontemporaneous incorporation of other lower to middle crustal fluids to produce the iocg deposits preservation of near surface deposits such as olympic dam is probably due to their formation above buoyant and refractory sclm which resisted delamination and associated uplift most precambrian iron oxide p rich or magnetite apatite kiruna type deposits have a different temporal distribution apparently forming in convergent margin settings prior to or following supercontinent assembly it is only in the phanerozoic that iocg and magnetite apatite deposits are roughly penecontemporaneous in convergent margin settings the phanerozoic iocg deposits such as candelaria chile occur in anomalous extensional to transtensional zones in the coastal cordillera which are also the sites of mantle derived mafic to felsic intrusions that are anomalous in an andean context this implies that special conditions possibly detached slabs of metasomatized sclm may be required in convergent margin settings to generate world class iocg deposits it is likely that formation of giant iocg deposits was mainly a precambrian phenomenon related to the extensive mantle underplating that impacted on buoyant metasomatized sclm generally smaller and rarer phanerozoic iocg deposits formed in tectonic settings where conditions similar to those in the precambrian were replicated

globular ferropicritic rocks at pechenga kola peninsula russia liquid immiscibility versus alteration abstract the uppermost volcanic unit of the early proterozoic pechenga group comprises ferropicritic volcanic and hypabyssal rocks which possess well developed light weathering globular up to 10 cm in size or layer like structures mesoscopicly similar to those found in some archean ocellar komatiites these structures are superficially reminiscent of features attributed to liquid immiscibility including spheroidal aggregates in all stages of coalescence however several lines of field microscopical mineralogical and chemical evidence argue against the immiscibility hypothesis these include identical modal proportions grain sizes morphologies and orientations of crystalline phases in the globules and matrix as strikingly demonstrated for example by the spinifex textured varieties of the globular rocks a solid state hydrothermal metamorphic diffusional process that affected the originally glassy mesostasis is concluded to be the only feasable mechanism for the formation of the globular structures in the pechenga ferropicrites

a shear hosted au cu bi metallogenic event at 1660 ma in the tennant creek goldfield northern australia defined by in situ monazite u pb th dating abstract high grade au cu and bi ores in the tennant creek goldfield have been mined from hydrothermal magnetite and or hematite rich ironstone bodies less well known is a style of au cu bi mineralisation hosted by quartz vein systems within shear zones outside ironstones we report ion probe shrimp u pb th ages of 1659 13 ma and 1659 15 ma for hydrothermal monazite associated with this mineralisation style at the orlando east au cu bi deposit and navigator 6 au prospect respectively the dated monazite at orlando east is intergrown with cu and bi sulfides and au in quartz chlorite veins within a shear zone at navigator 6 the monazite occurs both in late stage chalcopyrite bearing quartz hematite veins and within earlier formed sheared chloritic breccia that also contains anomalous au although the full significance of the 1660 ma monazite ages has yet to be determined the results indicate the introduction of au cu and bi in quartz vein systems at 1660 ma or remobilisation of earlier 1850 ma main stage au cu bi from mineralised ironstones and into quartz veins 10 s 100 s of metres from the initial mineralisation in either case hydrothermal fluids capable of transporting significant concentrations of ore metals were present in the tennant creek goldfield at 1660 ma synchronous with deformation a key implication is that rock sequences up to 200 million years younger than the ironstone hosted au cu bi deposits may now be considered prospective for a second stage of au cu bi mineralisation the results also validate non ironstone shear hosted deposits in the tennant creek region as an alternative exploration target to the well known high grade but low tonnage tennant creek ironstone hosted deposits

monazite u pb and th pb geochronology by ion microprobe with an application to in situ dating of an archean metasedimentary rock abstract monazite grains from nine different precambrian rock samples and having concordant isotope dilution u pb and th pb ages were used to develop and evaluate techniques for u pb th pb and pb pb isotopic dating using an ion microprobe operating at high mass resolution r 5500 1 an unidentified isobar at 203 960 amu complicated the determination of 204 pb in the monazites with higher th but it appeared to have been eliminated when using a slight energy offset the 207 pb 206 pb ratios of individual spots were determined in 10 min analyses to 0 3 0 5 1 of the true values using a 20 m wide o 2 primary beam whereas the uncertainty in the mean of several spots on a single generation of monazite was typically 0 1 0 4 2 instrumental bias in the measured pb uo and pb tho values of individual analyses was monitored using linear relationships between 206 pb uo vs uo 2 uo and 208 pb tho vs uo 2 uo the bias appears to vary somewhat with monazite th content requiring the use of compositionally matched standards individual spot measurements of the 206 pb 238 u and 208 pb 232 th ratios in unknown monazites of low to moderate th can be determined to 2 1 the dating technique was applied to in situ analysis of small monazite inclusions hosted in an amphibolite grade metapelite of archean origin both the contextual setting and the morphology of the monazite grains were found to be important factors in interpreting the timing of growth of the host metamorphic mineral e g garnet staurolite plagioclase euhedral monazite grains armoured in low cao garnet and matrix plagioclase yielded an age of 2548 17 ma interpreted as dating an early episode of low pressure metamorphism partially resorbed monazite within a high cao garnet rim yielded ages of ca 2500 ma but are interpreted as inherited from the first event rather than syn genetic with respect to the growth of this garnet an age of 1755 30 ma was obtained for anhedral monazite associated with late chlorite filled fractures apparently reflecting post tectonic retrogression

constraints on the timing of the tennant event and associated au cu bi mineralisation in the tennant region northern territory abstract detrital zircon dating of the warramunga formation that hosts iron oxide au cu bi mineralisation in the tennant region indicates that this unit was deposited after ca 1860 ma intrusive felsic porphyries within the warramunga formation have u pb zircon ages of 1847 5 2 4 1847 4 2 8 1846 3 2 9 and 1848 9 2 2 ma consistent with a single magmatic event at about 1848 ma peperitic margins of some of these intrusions indicate that deposition of the warramunga formation continued to about the time of porphyry emplacement the tennant creek and hill of leaders granites have ages of 1851 1 3 5 ma and 1845 5 2 4 ma respectively indistinguishable from those of the felsic porphyries and consistent with previous interpretations that the porphyries are the high level equivalents of the granitoids of the tennant supersuite felsic porphyry at the white devil mine with an age of 1847 4 2 8 ma cross cuts ironstone that hosts mineralisation and the s 1 foliation indicating that ironstone formation and deformation occurred soon after deposition of the host succession alteration associated with au cu bi mineralisation overprints the white devil porphyry providing a maximum age for mineralisation minimum ages of 1851 1847 ma for mineralisation are provided by previous 40 ar 39 ar age determinations indicating that mineralisation deformation and magmatism took place during the ca 1850 1845 ma tennant event contemporaneous with or shortly after the last stages of deposition of the warramunga formation although there is a close temporal association between mineralisation and felsic intrusive rocks of the tennant supersuite the intrusive rocks are only weakly fractionated suggesting they were not a significant source of au and cu a felsic volcaniclastic rock from the rover gold field 90 km southwest of tennant creek yielded a unimodal zircon age group of 1842 3 4 5 ma allowing correlation with the younger parts of the warramunga formation or the unconformably overlying ca 1840 ma ooradidgee group although a detailed correlation remains uncertain the simplest interpretation is that the au cu bi mineralisation at the rover field occurred at a broadly similar time to that at tennant creek

petrography geochemistry and origin of burial diagenetic facies siluro devonian helderberg group carbonate rocks central appalachians petrographic and geochemical studies of pore filling cements and replacement products in the helderberg group upper silurian lower devonian central appalachians document the diagenetic history of these rocks shallow ramp skeletal limestones and buildups were partially lithified by marine cements however most pore filling cement formed under shallow 300 m to deep burial 300 4 000 m conditions regional cathodoluminescent zonation patterns in early calcite cements indicate meteoric ground waters were involved in shallow burial cementation early zoned calcite cements consist of updip nonluminescent cements with thin luminescent laminae in limestone or sandstone that pass downdip into subzoned dull cements interlayered bright and dull laminae and then into nonzoned dull cement in basinward limestone the regional distribution stable isotopic compositions and timing of early zoned cements relative to other diagenetic events suggest that all of the early cements formed synchronously from meteoric ground waters that became progressively more reducing as they flowed downdip at least 150 km helderberg sandstone tongues probably were preferential conduits for meteoric gro nd waters because early calcite cements formed only on scattered skeletal grains in sandstones late void filling dull cement formed from deep burial pore fluids at burial depths of 300 to 4 000 m deep burial pore fluids were dilute to saline na ca cl brines from primary fluid inclusion data with stable isotopic compositions that probably were similar to isotopic compositions of formation waters from modern oil fields void filling dull calcite cements occluded nearly all remaining porosity before late paleozoic fracturing and deformation latest diagenetic phases include dolomite fluorite silica and fracture filling dull calcite this study shows that integrated petrographic and geochemical approaches to diagenetic studies are useful for relating various diagenetic phases and fabrics to burial history and paleoaquifer distribution

uranium and gold deposits in the pine creek orogen north australian craton a link at 1 8 ga abstract the pine creek orogen pco in the north australian craton nac hosts world class unconformity related u deposits and numerous orogenic or intrusion related au deposits whose timing and genesis are the subject of ongoing controversy this study reports new geochronology results for an unconformity related u deposit ranger 1 number 3 orebody to constrain the timing of processes leading to alteration development and u mineralisation within a regional geological and metallogenic context it also examines why some u deposits in the eastern pco contain elevated au concentrations unlike their unconformity related u counterparts in canada and elsewhere key results are as follows i the ranger 1 deposit and the pco au deposits share several characteristics of hydrothermal mineralogy including chlorite white mica monazite xenotime bearing alteration assemblages some sulfides pyrite chalcopyrite silica quartz veining or silicification and au ii the primary host structure at the ranger 1 deposit is a shear zone which was reactivated at low grade metamorphic conditions as during formation of many of the syn tectonic au deposits in the pco iii ion probe u pb dating of low th monazite in an assemblage of hydrothermal white mica and fe rich chlorite from an outer alteration zone at the ranger 1 no 3 orebody yielded an age of 1800 9 ma this age is 60 million years earlier than the oldest previously reported age of unconformity related u mineralisation in the pco and is within error of several u pb ages reported for monazite and xenotime associated with au mineralisation in the pco iv iron rich chlorite intimately associated with the dated monazite is similar compositionally and texturally to chlorites in alteration zones at other u deposits in the pco two scenarios for the formation of the ranger 1 deposit are consistent with the available evidence we prefer a two stage model where an fe chlorite white mica monazite hydrothermal alteration assemblage with probable gold was introduced replacing medium grade metamorphic assemblages within a reactivated shear zone at ca 1800 ma contemporaneous with the formation of some au deposits within the pco subsequently u ore was formed within this environment at 1740 ma and or later via the reaction of oxidised u and mg bearing basin derived fluids with chemical reductants in the basement the new u pb geochronology result for monazite points to an important hydrothermal event at the site of the ranger 1 deposit during regional tectonothermal activity of the shoobridge event at 1 8 ga in the pco and suggests that this pre ore alteration and tectonism was a key factor in localising later unconformity related u mineralisation the pre ore alteration at 1800 ma at the ranger 1 deposit therefore provides a link between the genesis of the unconformity related u deposits and the orogenic or intrusion related au deposits that formed in the pco during the shoobridge event at the broader scale the newly identified 1800 ma hydrothermal activity at the ranger 1 deposit may be part of a craton wide metallogenic and tectonothermal event at 1 8 ga that includes au mineralisation in the pco and in the granites tanami orogen as proposed in other studies

reduced and oxidized au cu bi iron oxide deposits of the tennant creek inlier australia an integrated geologic and chemical model iron oxide associated au cu bi deposits of the tennant creek gold field range from magnetite pyrrhotite pyrite reduced cu rich deposits through sulfide poor magnetite rich high grade au bi intermediate f o2 deposits to hematite rich high grade au oxidized deposits discordant magnetite hematite chlorite quartz ironstone bodies that host the ore formed hydrothermally during the 1840 to 1860 ma barramundi orogeny at temperatures up to 350 to 400 c and 2 5 to 5 kbars deposition of ironstone minerals was localized by dilatency that was generated during d1 reverse shearing and folding a separate au cu bi hydrothermal stage overprinted selected ironstones during d2 at or before 1825 to 1830 ma where favorable d1 structures and fluid pathways were reactivated the extremes of the reduced to oxidized deposit spectrum represent divergent chemical conditions that cannot be reconciled by the action of a single fluid we have identified both reducing and oxidizing fluids that helped to form the au cu bi deposits at the west peko deposit which represents the reduced end member subtype constraints from oxide silicate sulfide mineral stabilities fluid inclusions and chlorite thermochemistry indicate that au cu bi ore was introduced into ironstone at 300 to 340 c by a reducing pyrrhotite stable weakly acidic sulfur rich and n2 ch4 rich aqueous fluid containing 3 to 10 wt percent nacl equiv an ore fluid source outside the relatively oxidized metasedimentary host sequence warramunga formation is implied we suggest similar ore fluids for other reduced cu and sulfide rich deposits in the district chemical mass transfer modeling of reactions between reducing fluid and ironstone incorporating estimated thermodynamic properties for aqueous bismuth complexes indicates that highly efficient gold and native bi deposition was predominantly in response to desulfidation and oxidation of the fluid chalcopyrite precipitated mainly in response to increased ph gold bismuth ore in the eldorado deposit an oxidized and hematitic end member is interpreted to have involved mixing of an oxidizing hematite stable ca na cl brine with an intermediate f o2 low to moderate salinity ore fluid as well as reaction of both fluids with ironstone furthermore the mass transfer modeling suggests that mixing of reducing and oxidizing fluids in the absence of ironstones could have resulted in high grade au deposits lacking fe oxides a subtype that has received comparatively little exploration attention in the tennant creek district

timing of palaeoproterozoic au cu bi and w mineralization in the tennant creek region northern australia improved constraints via intercalibration of 40ar 39ar and u pb ages abstract the magnitude of systematic uncertainties inherent in comparisons of proterozoic 40ar 39ar and u pb based ages is illustrated via a consideration of age constraints on two contrasting styles of palaeoproterozoic mineralization in northern australia published 40ar 39ar ages for au cu bi mineralization in the tennant creek region range from 1825 to 1829 ma apparently younger than the timing of deformation bracketed by 207pb 206pb zircon ages of igneous rocks at 1850 ma despite this apparent age offset propagation of systematic uncertainties in the 40ar 39ar ages shows that they are indistinguishable from the 1850 ma zircon ages recalculation using revised estimates of the 40k decay constant and age of the 40ar 39ar standard shifts the 40ar 39ar ages to 1847 1851 ma in excellent agreement with 207pb 206pb zircon ages from tennant supersuite igneous rocks and with local geological constraints to the south of tennant creek in the davenport ranges numerous small w prospects have been interpreted to be coeval with post tectonic granites of the devils suite at 1710 1720 ma new 40ar 39ar ages from muscovite selvages on wolframite bearing veins range from 1697 to 1703 ma these ages shift to 1711 to 1717 ma when recalculated using revised estimates for 40k decay constant and age of standard material bringing them into agreement with existing 207pb 206pb zircon ages for devils suite granitoids and consistent with field and geochemical interpretations these examples highlight the importance of considering systematic sources of uncertainty when comparing radiometric ages determined via different isotopic methods particularly in the proterozoic and archean parts of the timescale ongoing work to calibrate the 40ar 39ar and u pb based timescales will further enhance the application of multiple isotopic methods to constrain geological processes

the origin of large varioles in flow banded pillow lava from the hooggenoeg complex barberton greenstone belt south africa exceptionally well preserved pillowed and massive phenocryst free metabasaltic lava flows in the uppermost part of the palaeoarchaean hooggenoeg complex of the barberton greenstone belt exhibit both flow banding and large leucocratic varioles the flow banding is defined by blebs and bands of pale and dark green metabasalt and was the result of mingling of two types of basalt robins et al in bull volcanol 72 579 592 2010a varioles occur exclusively in the dark chlorite mgo and feo rich metabasalt varioles are absent in the outermost rinds of pillows and increase in both abundance and size towards the centres of pillows in the central parts of some pillows they impinge to form homogeneous pale patches bands or almost homogenous cores individual varioles consist essentially of radially orientated or outwardly branching dendritic crystals of albite many varioles exhibit concentric zones and finer grained rims some varioles seem to have grown around tiny vesicles and vesicles appear to have been trapped in others between a core and a finer grained rim the matrix surrounding the ocelli contains acicular pseudomorphs of actinolite and chlorite after chain like skeletal ca rich pyroxenes that are partly overgrown by the margins of varioles varioles are enriched in the chemical constituents of feldspar but contain concentrations of immobile tio2 cr zr and ree that are similar to the host metabasalts the shape distribution texture and composition of the varioles exclude liquid immiscibility and support an origin by spherulitic crystallisation of plagioclase from severely undercooled basalt melt and glass nucleation of plagioclase was strongly inhibited and took place on vesicles on the bases of drainage cavities and along early fractures eruption in deep water and retention of relatively high concentrations of volatiles in the melt may be the principal cause of spherulitic crystallisation in the interiors of pillows rather than only in their margins as in younger submarine flows

flow banding in basaltic pillow lavas from the early archean hooggenoeg formation barberton greenstone belt south africa well preserved pillow lavas in the uppermost part of the early archean volcanic sequence of the hooggenoeg formation in the barberton greenstone belt exhibit pronounced flow banding the banding is defined by mm to several cm thick alternations of pale green and a dark green conspicuously variolitic variety of aphyric metabasalt concentrations of relatively immobile tio2 al2o3 and cr in both varieties of lava are basaltic compositional differences between bands and variations in the lavas in general have been modified by alteration but indicate mingling of two different basalts one richer in tio2 al2o3 mgo feot and probably ni and cr than the other as the cause of the banding the occurrence in certain pillows of blebs of dark metabasalt enclosed in pale green metabasalt as well as cores of faintly banded or massive dark metabasalt suggest that breakup into drops and slugs in the feeder channel to the lava flow initiated mingling the inhomogeneous mixture was subsequently stretched and folded together during laminar shear flow through tubular pillows while diffusion between bands led to partial homogenisation the most common internal pattern defined by the flow banding in pillows is concentric in some pillows the banding defines curious mushroom like structures commonly cored by dark variolitic metabasalt which we interpret as the result of secondary lateral flow due to counter rotating transverse dean vortices induced by the axial flow of lava towards the flow front through bends generally downward in the tubular pillows other pillows exhibit weakly banded or massive dark variolitic cores that are continuous with wedge shaped apophyses and veins that intrude the flow banded carapace these cores represent the flow of hotter and less viscous slugs of the dark lava type into cooled and stiffened pillows

a comparison of mineral deposits at the gecko and white devil deposits implications for ore genesis in the tennant creek district northern territory australia detailed geologic metal zonation paragenetic fluid inclusion and stable isotope studies have been undertaken of the cu rich gecko k44 and the au rich white devil deeps zone orebodies in the proterozoic tennant creek district of the northern territory australia despite differences in the grade and mineral abundances in the ores three paragenetic stages are present in both deposits 1 an early synkinematic oxide stage at which time the ironstones formed 2 a syn to postkinematic sulfide stage at which time cu au and bi were introduced and 3 a late minor carbonate stage with minor pb and zn the oxide stage involved precipitation of magnetite hematite quartz and chlorite this stage was subsequently foliated the sulfide stage involved the precipitation of gold chalcopyrite bi minerals chlorite and quartz and the hematization of magnetite the carbonate stage involved precipitation of carbonate talc galena and sphalerite and the alteration of chalcopyrite to bornite and covellite fluid inclusion studies indicate that oxide stage minerals formed from low temperature 250 degrees c high salinity 15 20 wt nacl equiv fluids whereas the sulfide stage formed from high temperature 350 degrees c high salinity 20 wt nacl equiv fluids that show evidence of phase separation sulfur isotope studies indicate that lower delta 34 s values occur in au rich pods whereas higher delta 34 s values occur in cu rich halos metal zonation studies indicate that in the white devil deeps zone orebody a basal au rich zone is surrounded by a bi rich shell this zonation is broadly similar to zonation at the juno deposit au bi cu pyrite large 1975 owing to its very low grades the distribution of cu in the white devil deeps orebody is erratic conversely the gecko k44 orebody is only weakly zoned with au enrichment at the top of the orebody in the sulfide tongue mineral deposition occurred in well zoned au rich e g juno and white devil and weakly zoned chalcopyrite hematite rich e g gecko deposits owing to the reduction of highly oxidized sulfide stage fluids through hematization of magnetite in ironstones the fluids evolved through the hematite field in f sub o 2 ph space to the hematite pyrite join and then followed the hematite pyrite join toward the hematite pyrite magnetite triple point this fluid evolution caused early gold precipitation in the hematite field but chalcopyrite precipitated only along the hematite pyrite join which accounts for metal zonation in well zoned au rich deposits in chalcopyrite hematite rich deposits fluid evolution was restricted to the hematite pyrite join and minor gold precipitated with chalcopyrite the k44 sulfide tongue is inferred to be a small zoned au rich system adjacent to the main k44 orebody differences in metal abundance and zonation can be interpreted in terms of varying proportions of highly oxidized magmatic hydrothermal fluids relative to more reduced connate basinal fluids introduced into the ore forming systems a large component resulted in well zoned au rich deposits an intermediate component resulted in chalcopyrite hematite rich au poor deposits and a small component resulted in unzoned chalcopyrite rich pyrrhotite bearing deposits with intermediate au grades e g peko

volcanic evolution of the upper onverwacht suite barberton greenstone belt south africa abstract the volcanic stratigraphy of the upper onverwacht suite in the southeastern part of the paleoarchean barberton greenstone belt has been investigated in 18 sections through parts of the hooggenoeg kromberg and lower mendon complexes the ca 2700 m thick volcanic sequence of the lowest tectonostratigraphic unit the hooggenoeg complex hc can be subdivided into 9 stratigraphic units representing major eruptive phases each separated by silicified sedimentary and volcaniclastic rocks the thicknesses of the units vary from the lavas of the hc and kc are predominantly pillowed and massive flows whereas those of the ncakini section are exclusively massive the massive and pillowed lava are commonly organized in cyclic units from 3 m to 32 m thick and consisting of massive lava or large pillows that are succeeded by progressively smaller pillows cyclic units are inferred to have resulted from individual eruptions with decreasing rates of effusion the pillow lavas of both the hc and kc contain

five generations of monazite in langtang gneisses implications for chronology of the himalayan metamorphic core monazite grains from greater himalayan sequence gneisses langtang valley nepal were chemically mapped and then dated in situ via th pb ion microprobe analysis correlation of ages and chemistry reveals at least five different generations of monazite ranging from c 9 to 300 ma petrological models of monazite chemistry provide a link between these generations and the thermal evolution of these rocks yielding an age for the melting of greater himalayan rocks within the main central thrust sheet c 16 ma and for the timing of thrust sheet emplacement that are younger than commonly viewed chemical characterization of monazite is vital prior to chronological microanalysis and many ages previously reported for monazite from the greater himalayan sequence are interpretationally ambiguous

dacitic ocelli in mafic lavas 3 8 3 7 ga isua greenstone belt west greenland geochemical evidence for partial melting of oceanic crust and magma mixing abstract mafic volcanic rocks in the 3 8 3 7 ga isua greenstone belt southern west greenland contain randomly distributed 1 to 10 centimeters long white spheroidal structures in this study these structures are called ocelli in the western part of the belt ocelli bearing lavas are enclosed in basaltic to picritic flows mgo 9 21 wt with a subduction zone geochemical signature the ocelli are composed predominantly of polycrystalline na plagioclase and quartz with minor hornblende and biotite whereas the surrounding amphibolite matrix basaltic host is composed mainly of hornblende ca plagioclase and quartz the ocelli are devoid of radial or concentric internal structure and display all stages of coalescence contacts between the ocelli and surrounding amphibolite matrix are sharp to gradational compositionally the ocelli are calc alkaline dacites sio2 62 9 72 0 wt mgo 0 60 3 50 wt ni 58 143 ppm cr 250 510 ppm whereas the surrounding matrix is tholeiitic basalt sio2 46 6 50 6 wt mgo 8 70 12 30 wt ni 119 175 ppm cr 330 600 ppm in terms of major element composition the isua ocelli closely resemble plagiogranites in phanerozoic supra subduction zone ophiolites field and petrographic observations and geochemical data sio2 54 2 60 7 wt mgo 3 95 7 72 wt ni 127 158 ppm cr 500 570 ppm on the transitional areas between the ocelli and the matrix suggest magma mixing between dacitic and basaltic melts on a chondrite normalized diagram the basaltic host is characterized by variably depleted lree patterns la smcn 0 30 0 94 gd ybcn 1 03 1 45 whereas the dacitic ocelli display lree enriched patterns la smcn 1 30 2 60 gd ybcn 1 32 2 58 the strongly depleted ree patterns in the basaltic host are attributed to lree loss during carbonate alteration partial melting of a forearc mantle wedge is favoured for the origin of the protolith of the basaltic host the geochemical characteristic of the ocelli cannot be explained by post magmatic alteration slab melting fractional crystallization of tholeiitic melts or liquid immiscibility we suggest that the dacitic ocelli might have been derived from hydrous melting of the fragments of oceanic crust high mg volcanic rocks that fell into the magma chamber suggesting magma crust interaction in the early earth formation of dacitic volcanic rocks by partial melting of altered oceanic crust may have played an important role in the generation of felsic crust in the early archean

shrimp ionprobe dating of short lived proterozoic tectonic cycles in the northern arunta inlier central australia structural and metamorphic evidence from low p high t metamorphic terranes within the northern arunta inlier central australia indicates that two discrete tectonic cycles affected the region each cycle was associated with compressive deformation coincident with peak granulite facies metamorphism and granite intrusion shrimp u th pb analysis of zircons from early syn and late tectonic granitoids allows the age of the two tectonic cycles to be directly measured the age of the first cycle d1 is determined by emplacement of the mount stafford granite dated at or before 1818 15 ma after the first two foliations of the cycle had formed but before the axial plane cleavage associated with later upright folding the regional extent of this tectono magmatic event is confirmed by an emplacement age of 1818 8 ma measured on the large discordant harverson granite 40 km away the second cycle d2 consisted of four distinct deformation stages d2a 2d the ages of which are constrained by 1785 1775 ma old early syn and late tectonic granitoids the warimbi granite emplaced at or before 1785 22 ma and the granitic protolith to the napperby gneiss inferred to be 1780 10 ma pre date or formed during the earliest foliation s2a zircon considered to have grown at peak granulite facies metamorphic conditions during generation of the dominant s2b foliation gives an age of 1774 6 ma in the possum creek charnockite leucogranite in the napperby gneiss which post dates s2b and large scale upright nw se trending f2d folds yields an age of 1775 12 ma and confirms that the entire tectonic cycle d2a 2d formed in a rapid 10 ma period younger zircons which formed at 1659 6 ma in the napperby gneiss cannot be related to any local structural event but are similar in age to a major magmatic metamorphic episode in the southern arunta black and shaw 1992 150 km away they may be the peripheral manifestation of that event further zircon growth at 1600 ma in the napperby gneiss and possum creek charnockite was caused by hydrothermal fluid fluxing associated with post tectonic pegmatite emplacement these dykes were deformed by amphibolite facies retrograde shear zones ion probe dating of zircon stratigraphy from the granitoids has confirmed the discrete successive tectonic cycles identified by earlier structural and metamorphic studies for the northern arunta inlier at least those cycles or orogenies are associated with short lived 10 ma thermal pulses related to granite magmatism not prolonged thermal perturbations spanning 100 ma or more cf dirks and wilson 1990 dirks and hand 1991 such short lived pulses are intimately associated with granite intrusion and may be a general feature of low p high t metamorphism

detection of inherited monazite in the manaslu leucogranite by 208pb232th ion microprobe dating crystallization age and tectonic implications abstract although leucogranites are among the least petrologically variable of all igneous rocks ironically they are among the most difficult to reliably date the high himalayan leucogranites have been the subject of numerous geochronological investigations because of their interrelationship with the most significant tectonic features of that mountain belt for a variety of reasons linked to the minimum melt composition of these leucogranites these dating studies have not been entirely successful we report results of a new ion microprobe dating method based on the decay of 232 th to 208 pb in monazite that has directly revealed the presence of inherited pb in monazite from the manalsu granite casting doubt on its previously accepted age monazite ages obtained from this leucogranite yield two distinct populations a large number of ages with a normal distribution and mean age of 22 4 0 5 ma 2 s e that we interpret to be the crystallization age and a smaller inherited fraction with an age of ca 600 ma because formation of the granite is thought to be related to slip on the main central thrust both this date and a second less precise result from a structurally similar pluton near mt everest indicate that the main central thrust was active in the interval 24 22 ma but do not constrain its initiation together with crosscutting relationships these data require that movement on the north himalayan fault occurred prior to 22 ma at both locations

interpretation of monazite ages obtained via in situ analysis abstract monazite grains from nepal and vietnam were compositionally analyzed with an electron microprobe and dated th pb with an ion microprobe five sources of uncertainty explain age distributions from single samples that appear inconsistent with a single population 1 pb loss due to diffusion 2 dissolution reprecipitation reactions along a retrograde path 3 analytical uncertainties 4 analyses of overlapping age domains and 5 episodic monazite growth the influence of these factors is sample dependent but can be evaluated 1 using peak metamorphic conditions and x ray element maps to assess potential polymetamorphism or retrogression 2 obtaining other geochronologic data including previous work or dating other minerals in the sample 3 evaluating any method related uncertainty including counting statistics for electron microprobe analyses or calibration reproducibility for ion microprobe analyses and 4 ascertaining the potential growth mechanism of the monazite grain including dissolution of detrital grains or production from rare earth element ree oxide or allanite chemical contents of monazite grains analyzed in this study fail to reflect timing information or mineral growth mechanisms instead of relying on monazite chemical composition major mn fe mg ca and minor y element garnet zoning patterns and peak p t conditions should be used to facilitate age interpretation this thermobarometric data records the sample s thermal history changes in garnet growth rate and mechanisms and accessory mineral breakdown

a textural and geochemical guide to the identification of hydrothermal monazite criteria for selection of samples for dating epigenetic hydrothermal ore deposits hydrothermal monazite is one of the most reliable geochronometers for u pb dating of epigenetic or hydrothermal ore deposits because its blocking temperature at ca 700 c is higher than that of most metamorphic conditions although igneous monazite is routinely used for age dating igneous rocks only a few ore deposits have been dated by hydrothermal monazite this is due to the similar morphology of hydrothermal and igneous monazite which prevents the reliable distinction of hydrothermal monazite in particular we demonstrate that hydrothermal monazite can be distinguished from igneous monazite by its unique geochemical signature its local abundance and to common association with hydrothermal mineral assemblages our data and the data of others reported in the literature suggest that the low tho 2 content of hydrothermal monazite 0 1 wt is distinct from that of igneous monazite 3 to 5 wt and may be used to determine their genesis

matrix effects and calibration limitations in ion probe u pb and th pb dating of monazite abstract monazite can be used for u pb and th pb dating of a wide variety of rocks and geological processes particularly metamorphism in many instances ion microprobe analysis is the preferred method for dating monazite combining good analytical precision with m scale spatial resolution however ion microprobe dates can be compromised by matrix effects on the ionisation efficiencies of different secondary ionic species which can bias monazite pb tho n and pb uo n data by as much as 40 and 30 respectively for known monazite compositions these effects appear to be entirely attributable to the sputtering process in which case similar effects exist in data from all ion probes the effects on data ratios are modified sometimes exacerbated or reversed when data are subjected to the correlation based pb th and pb u calibrations that are now commonly used one dimensional calibrations of 206 pb 270 uo 2 and 208 pb 264 tho 2 are recommended corrections derived from data for a suite of reference materials with diverse compositions can reduce biases in 208 pb 232 th ages to 206 pb 238 u ages to 204 pb can be addressed without iterative data reduction

fabric retentive non luminescent brachiopods as indicators of original delta 13 c and delta 18 o composition a test abstract fabric retentive non luminescent helderbergian brachiopods from open marine deposits analogous to brachiopods used in other studies to determine paleo oceanic stable isotopic composition exhibit depleted 13c values immediately below the erosional contact and more enriched values of 13c at greater stratigraphic distances below this surface in addition 13c values of these brachiopods display a marked shift across the erosional surface and values above it are essentially the same as those 30 m or more beneath the surface complementing these trends whole rock analyses metastable high agnesian calcite constituents echinoderms as well as early equant calcite cement also exhibit an increased depletion trend in 13c values as the helderberg oriskany contact is approached from below fabric retentive luminescent as well as obviously recrystallized brachiopods display stable isotopic values similar to those of fabric retentive non luminescent brachiopods thus best preserved fabric retentive non luminescent brachiopods exhibit values similar to obviously recrystallized brachiopods diagenetically altered metastable constituents echinoderms stable early meteoric cements and whole rock samples helderberg strata of new york state were affected by early meteoric diagenesis during a siegenian exposure event prior to deposition of the overlying marine oriskany sandstone meteoric diagenesis during this exposure gave rise to a characteristic carbon isotopic depletion trend approaching the helderberg oriskany contact all of the skeletal constituents underwent isotopic exchange during early diagenetic exposure to meteoric waters consequently the practice of using fabric retentive non luminescent brachiopods as indicators of their original chemistry and by extension paleo oceanic chemistry is questioned

the duration of the strangways event in central australia evidence for prolonged deep crust processes abstract protoliths of the strangways metamorphic complex smc in the arunta region of central australia are dated to the 1810 1800 ma stafford magmatic event by shrimp u pb dating of relict igneous zircon within granulite facies metamorphic rocks detrital zircons preserved in associated metasediment have a provenance age spectrum similar to that in the lander package sedimentary rocks which are widespread in the region the onset and duration of granulite facies metamorphism is recorded differentially across the smc depending on lithology varying mechanisms of metamorphic zircon crystallisation and the specific p t t paths of individual rocks in different sectors of the complex the earliest recorded thermal process is partial melting in the kanandra granulite at 1734 3 ma zircon ages indicate that melts at edwards creek 1716 3 ma and coles hill 1708 6 ma formed later overgrowths indicate renewed metamorphic zircon precipitation in the kanandra granulite probably from fluids as late as ca 1690 ma a 50 myr continuum of metamorphic zircon ages from ca 1740 to 1690 ma is recorded in a single rock at utnalanama and attributed to prolonged solid state metamorphic zircon formation the strangways event was not a transient and discrete process with a single age there is evidence for prolonged residence at granulite facies conditions with both discrete crystallisation events and prolonged crystallisation responses in different locations termination of the high grade metamorphism coincided with local extension indicated by the intrusion of a dolerite dyke swarm at 1689 8 ma the strangways event coincides with the segment between prominent bends b0 and b1 of the australian palaeomagnetic polar wander path relating the thermal process to major direction changes in the movement of the north australian plate the geographic distribution of high grade metamorphism within the smc shallow crust emplacement of magmas in the tennant region and a low temperature ar isotope record in the tanami region is consistent with effects propagating northwards from a plate boundary south of the smc

new data on relative stability of carbonate minerals implications for diagenesis and cementation abstract redetermination of magnesian calcite solubilities indicates that the constants currently in use overestimate their solubility by 300 the relative chemical stability of aragonite and magnesian calcite plays a fundamental role in controlling the behavior of these metastable minerals during the diagenetic transformation to stable low magnesian calcite limestone via dissolution and reprecipitation reactions temporal evolution of rock and groundwater geochemistry as well as development of limestone fabrics depends on the relative chemical reactivity of carbonate grains during dissolution as well as their receptiveness to cementation in both marine and meteoric diagenetic systems following the laboratory determination of magnesian calcite stabilities by l n plummer and f t mackenzie in 1974 it has been accepted that magnesian calcites containing greater than 7 5 mole mgco3 are more unstable than aragonite and become progressively more unstable with increasing mole mgco3 for example 18 mole magnesian calcite was thought to be seven times more soluble than aragonite the solubilities of several biogenic magnesian calcites echinoid 12 mole mgco3 red algae 18 mole mgco3 have been redetermined the new data for the stability of the magnesian calcites indicate that previously determined solubilities for magnesian calcites were significant overestimates using the new stability constants from the present study 12 5 mole magnesian calcite is equivalent in stability to aragonite and an 18 mole magnesian calcite is only twice as soluble as aragonite thus the solubility of magnesian calcites must be reduced by 300 from existing values in the literature it is believed that the discrepancy with previous work on magnesian calcite stability is due to sample preparation procedure in the present study care was taken both to eliminate submicron fine particles by ultrasonic cleaning and to reduce the crystal strain induced by crushing experiments show samples not so treated yield larger magnesian calcite solubilities because of enhanced reactivity and solubility of adhering submicron particles and strained crystal surfaces the new stability relations have significant implications for the sequence and pathways of diagenetic reactions mineralogic stability and available surface area are the main chemical controls on the rate of diagenetic alteration given the reduced solubility offset between aragonite and magnesian calcite aragonites having more complex microstructures and hence more reactive surface area dissolve more rapidly than coexisting high magnesian calcites the lower solubilities for the magnesian calcites sharply reduce the potential chemical region in ground waters where the incongruent dissolution of magnesian calcite is possible since the specific nature of dissolution is closely related to the rate and style of cementation these findings prompt the reevaluation of possible pathways which may be taken during early diagenesis more work remains to be done to quantify the precise effects of mineralogy and grain microstructure into a complete predictive model for patterns of cementation but it is clear that the reduced mineralogic stability offset between aragonite and magnesian calcite makes microstructural detail and ground water geochemistry prime controls over the reaction paths followed during the early diagenetic stabilization of shallow water carbonate deposits end of article last page 566