archean lamprophyre dikes of the superior province canada distribution petrology and geochemical characteristics shoshonitic lamprophyre dikes of late archean age are a volumetrically minor yet widespread component of magmatism in the superior province and preferentially occur along major structures that delineate subprovince boundaries lamprophyre swarms are most prominently developed in fault graben where they are associated with alkali gabbro and trachytes and molasse sediments mineralogically the dikes are characterized by zoned phlogopite or hornblende phenocrysts restriction of feldspar to the groundmass globular segregations of k feldspar and calcite panidiomorphic texture and the sporadic presence of crustal xenoliths compositionally fresh dikes have contents of sio2 47 51 wt tio2 0 55 0 60 and p2o5 0 36 1 58 and k2o na2o ratios 1 5 2 3 and mg numbers 78 80 comparable to phanerozoic shoshonitic lamprophyres unaltered dikes are characterized by extreme enrichments of the lithophile elements k rb ba cs th u and light rare earth elements lree and enhanced abundances of cr 650 920 ppm co 37 42 ni 264 355 and sc 22 28 mid ocean ridge basalt normalized plots display 1 consistent enrichments of k rb and ba relative to sr and lree 2 variable enrichments of rb and ba relative to k and 3 troughs at ta nb and ti compared with fresh rocks altered samples have systematically lower k rb ba and sr contents but similar ree distributions and profiles from th yb and sc ni on normalized diagrams suites of post archean ultramafic lamprophyres in the superior province are enriched in large ion lithophile elements lile lree and compatible elements relative to their archean counterparts and lack troughs at ta nb and ti emplacement of the lamprophyres was diachronous from north 2705 ma to south 2674 ma probably being related to localized extension during the successive accretion of individual subprovinces archean lamprophyre magmas were derived from a depleted mantle source variably enriched in lile and lree by sediments and altered oceanic crust with small variable degrees of crustal contamination whereas the post archean lamprophyres lacked a plate margin setting

crustal outgassing and lile enrichment in major lithosphere structures archean abitibi greenstone belt evidence on the source reservoir from strontium and carbon isotope tracers major structural discontinuities in the abitibi greenstone belt acted as conduits for outgassing of the archean crust as reflected in fixation of a select group of lithophile elements including si c k rb ba li cs b and pb in metasomatized faults for two of the largest structures the destor porcupine dp and kirkland lake cadillac kc fault zones 6 1015 g si 3 1015 g co2 and 1015 g k were introduced into the faults during expulsion of an estimated 6 1018 g aqueous fluids strontium isotope ratios of tourmaline piemontite actinolite and scheelite mineral separates characterized by rb sr 0 02 are concordant with respect to 87sr 86sr initial ratios over local sectors of the faults the sr isotope data record geographic variations which from east to west on the kc fault is 0 7031 0 7041 val d or 0 7008 0 7022 bourlemaque 0 7017 0 7019 bousquet 0 7029 0 7031 noranda and 0 7013 to 0 7015 kirkland lake at timmins on the pd fault 87sr 86sr initial ratios cluster at 0 7010 to 0 7020 metasomatised fault zones are systematically more radiogenic than contiguous host lithologies and imply a source reservoir 0 7010 to 0 7041 generally more radiogenic than the upper mantle at 2690 ma 0 700 0 001 or contemporaneous volcanic rocks of mafic to ultramafic composition 0 700 to 0 7012 whereas certain minerals are concordant and retentive rb sr isochrons based on suites of rocks at progressive intensities of metasomatism have been systematically reset over an elpased time of 200 ma after termination of outgassing due to disturbance accompanying incremental displacements on structures

upb zircon and rutile chronology of archean greenstone formation and gold mineralization in the val d or region quebec abstract u pb geochronology provides an absolute time framework for the evolution of the sigma gold deposit and surrounding rocks at val d or southern abitibi subprovince the bourlamaque batholith the largest pluton in the area gives a 2699 8 1 0 ma u pb zircon age this pluton cuts the val d or formation which hosts the mineralization a u pb zircon age of 2704 9 1 1 ma on a felsic volcanic rock the colombiere rhyolite 13 km east of the mine dates that formation the gold bearing quartz vein system at sigma is hosted by andesites and two generations of porphyry intrusions all metamorphosed to the greenschist facies the oldest porphyry porphyritic diorite shows the same deformation as the volcanic rocks and has a 2703 7 2 5 ma u pb zircon age the porphyritic diorite and volcanic rocks are cut by feldspar porphyry dykes which post date regional folding and have a 2694 0 2 2 ma u pb zircon age regional greenschist metamorphism has been dated directly with a u pb date of 2684 7 ma on rutile in the colombiere rhyolite the mineralization and hydrothermal alteration in the mine are superimposed on the metamorphic minerals hydrothermal rutile from an alteration halo around the veins in andesite has a 2599 9 ma u pb age textural evidence clearly indicates that the wall rock alteration and vein filling are contemporaneous and hence the vein system and gold mineralization appear to have developed at least 80 m y after the formation and metamorphism of host greenstones

archean gold bearing quartz veins at the sigma mine abitibi greenstone belt quebec part i geologic relations and formation of the vein system the sigma deposit consists of an archean gold bearing vein system in metavolcanic rocks intruded by an irregular body of porphyritic diorite both affected by regional deformation and in turn cut by younger undeformed feldspar porphyry dikes the veins cut all rock types and are composed chiefly of quartz and tourmaline with minor amounts of carbonates pyrite chlorite scheelite and free gold they were emplaced during or after a greenschist facies metamorphism postdating the intrusion of feldspar porphyry dikes the veins were not significantly affected by any subsequent metamorphism or tectonism two principal types of veins occur in the mine subvertical veins and subhorizontal veins they are contemporaneous and cogenetic subvertical veins occur within steeply dipping east west trending ductile shear zones along which reverse subvertical displacements have taken place the veins occupy openings created by overriding of undulations and irregularities of foliation and slip planes during progressive shearing movements within the shears subhorizontal veins occupy extensional fractures which developed between ductile shear zones preferentially in the more competent host rocks these veins were formed by one or several episodes of open space filling geologic relations indicate that the mineralized veins were emplaced progressively in a dynamic tectonic environment during and shortly after the formation of ductile shear zones a structural analysis has shown that the ductile shear zones and the two vein types can be related to a single strain ellipsoid of the following orientation the z axis trending north south and horizontal the y axis plunging 10 degrees west and the x axis plunging 80 degrees east the deformation responsible for the generation of ductile shear zones and emplacement of the veins is ascribed to a late north south trending compression postdating regional deformation

fluorapatite fenitization and gold enrichment in sheeted trondhjemites within the destor porcupine fault zone taylor township ontario pronounced fluorapatite mineralization is present in sheeted trondhjemites near timmins the trondhjemites intrude a deformed ultramafic to fe tholeiite volcanic sequence disposed in the structural hanging wall of the destor porcupine fault and they bound a domain of high grade gold ore spinifex textured ultramafic flows record variable intensities of secondary alteration to a carbonate cr muscovite assemblage they are characterized by chondritic al2o3 tio2 ratios 19 1 7 enhanced cr 1570 2090 ppm and ni 470 1070 ppm together with low levels of the incompatible elements ta nb and th fresh quartz oligoclase trondhjemitic intrusions are compositionally coherent with respect to sio2 average 63 wt al2o3 16 3 wt and na2o 6 7 wt as well as to the alteration insensitive elements v sc zr and hf whole rock 18o values of 10 0 10 6 combined with magmatic quartz feldspar 0 8 0 9 signify that these are primary high 18o magmas likely derived by partial melting of 18o enriche

origin and timing of the post variscan gabbro granite complex of porto western corsica the post variscan complex of porto consists of metaluminous to slightly peraluminous a type biotite granites mingled with gabbro dioritic rocks and late dykes with basaltic to trachyandesitic composition u pb zircon dating by la icp ms on two mafic intrusive samples constrains the time of the gabbro granite crystallisation at 281 3 ma and 283 2 ma hornblende 40ar 39ar ages from a late trachyandesite dyke date the dyking event at 280 2 ma which is within error the u pb zircon ages of the intrusives biotite granites show variable major and trace element compositions and similar initial end 0 3 to 0 9 whole rock chemistry variations and trace element compositions of plagioclase and allanite indicate that the granites are genetically linked essentially through fractional crystallisation of feldspars and minor allanite on the basis of whole rock chemistry e g initial end 4 9 to 1 7 and trace element clinopyroxene compositions we have ascertained that the mafic intrusives and basic dykes formed from isotopically depleted mantle source derived melts with similar trace element signature these basic melts experienced slightly different evolutionary histories controlled by fractional crystallisation and crustal contamination mainly by the acid magma that gave rise to the associated biotite granites but also by the enclosing older variscan granitoids u pb zircon data suggest that the porto complex was affected by hydrothermal fluid circulation at 259 9 ma

volatile control of contrasting trace element distributions in peralkaline granitic and volcanic rocks anomalous enrichments of zr 500 ppm zn 100 ppm nb 25 ppm y 60 ppm th 20 pm u 5 ppm lree 230 ppm and hree 35ppm and high rb sr 5 characterize peralkaline granites in contrast to their peraluminous and calc alkaline equivalents within the peralkaline suite comenditic and pantelleritic volcanics exhibit two to five fold increases in the concentrations of these trace elements over comagmatic granites these cannot be explained by crystal liquid fractionation processes and require the evolution of a sodium enriched fluid corresponding trace element increases in the granites in areas of alkali metasomatism support this argument and reflect the partial confinement of this volatile phase within the high level magma chambers ree studies in particular might eventually allow an evaluation of the role of cl versus f and co3 complexing in the evolution of the volatile fluid

physical chemical properties and metal budget of au transporting hydrothermal fluids in orogenic deposits we use a set of analytical techniques for fluid inclusion analysis to determine the bulk properties of the ore fluids from five orogenic gold deposits the most common ore fluid is a low salinity 0 4 6 5 wt nacl twoto three phase aqueous carbonic fluid its th total range is 200 400 8c and total homogenization occurs by bubble or liquid disappearance a less common aqueous fluid of similar bulk salinity and t h total is also documented microchemical data show that the ore fluid in the five deposits is chemically uniform and made of na subordinate k and b and minor to trace amounts of cu as li sr rb ba cs sb and au range 0 5 5 mg g thermodynamic modelling carried out for one of the studied deposits indicates that the ore fluid was in equilibrium with vein minerals at the time of gold deposition and that its vapour phase was distinctly enriched in au b as and sb the proposed mechanism of gold precipitation is a combination of fluid decompression and boiling combining these results with 3d geometrical reconstructions of one of the studied deposits sigma we estimate that a minimum ore fluid volume of c 0 1 1 0 km is sufficient to generate a world class orogenic deposit orogenic deposits provide about a third of the global gold production goldfarb et al 2001 frimmel 2008 these deposits are formed within active continental margins in orogenic belts where major tectonic boundaries juxtapose metamorphosed volcanic plutonic and sedimentary terranes the ore bodies consist of au mineralized vein networks filled with quartz carbonates and other silicates that in places crosscut the stacked terranes of the continental margins the veins are dilated portions of vertically extensive brittle ductile to ductile fracture networks that form due to the advection of aqueous hydrothermal fluids within the continental crust ridley et al 1996 groves et al 2003 these ore fluids are generated in still undefined source regions deep in the crust and transported to shallower levels 3 15 km depth through the vein networks along which they deposit gold a number of studies kerrich et al 2000 ridley diamond 2000 groves et al 2003 showed that although these networks do not occur systematically in all orogens they have a set of important geological features in common which include 1 formation during accretionary orogenic events 2 occurrence relatively close to regional tectonic lineaments especially along lower order splays of these structures 3 spatial association with granitoid intrusions 4 occurrence within some metamorphic belts with gold precipitation being synto postmetamorphic in age 5 relatively low sulphide abundance within the veins 6 carbonate sericitealbite sulphide hydrothermal alteration of host rocks postdating metamorphism 7 poorly developed mineral and metal zoning within the ore bodies and 8 genesis from low to moderate salinity ore fluids over a large temperature range 250 700 8c but mostly within 300 350 8c these common traits occur systematically in deposits from different terrains having distinct ages suggesting that a common geological process and au transporting fluid are responsible for their formation here we present a database of fluid inclusion properties from five typical orogenic deposits which show most of the geological characteristics listed above the database was constructed with the aim of determining as accurately as possible the properties of the au transporting fluid in this geological environment in particular laser ablation inductively coupled plasma mass spectrometry la icp ms of single fluid inclusions paragenetically coeval with vein gold provides the concentration of a suite of metals including au in this fluid this dataset when combined with 3d geometrical reconstructions of one of these deposits offers new constraints on the volume of the ore from garofalo p s ridley j r eds gold transporting hydrothermal fluids in the earth s crust geological society london special publications 402 http dx doi org 10 1144 sp402 8 the geological society of london 2014 publishing disclaimer www geolsoc org uk pub ethics by guest on april 3 2014 http sp lyellcollection org downloaded from

40ar 39ar geochronological evidence for multiple postmetamorphic hydrothermal events focused along faults in the southern abitibi greenstone belt thermal histories in the abitibi and pontiac subprovinces vary for three general domains 1 zones of impermeable rock with cooling rates of 8 c ma 2 zones of impermeable rock that are spatially associated with monzogranite plutons with apparent cooling rates of 1 2 cima and 3 permeable high strain zones with apparent cooling rates of 2594 ma from the volcanogenic massive sulphide related sericite alteration with the coarser mica yielding the younger date based on radiometric dating impermeable rocks were affected weakly by postmetamorphic hydrothermal events minerals in zones of higher permeability were reset during the intrusion of monzogranite plutons highly permeable zones such as the larder lake cadillac and porcupine destor faults were overprinted repeatedly this produces a drawn out series of dates from minerals with varying closure temperatures radiometric ages from fault zones or ore deposits cannot be used to interpret the cooling history and evolution of the abitibi belt as a whole

u pb geochronology of hydrothermal zircons from the early cretaceous iron skarn deposits in the handan xingtai district north china craton the handan xingtai district in the north china craton is one of the most important concentrations of iron skarn deposits in china with proven reserves of 900 to 1 000 mt at an average of 40 to 55 wt fe the iron mineralization occurs predominantly along contact zones between early cretaceous intermediate silicic intrusions and middle ordovician marine carbonates intercalated with numerous evaporite beds in this paper we present textural features and laser ablation icp ms u pb dating results of hydrothermal zircon from five major iron skarn deposits to place tight constraints on the timing and duration of the district scale iron mineralization zircon grains from the mineralized skarns are anhedral to subhedral crystals and euhedral tetragonal bipyramids they are closely intergrown or texturally associated with diopside garnet epidote calcite albite and phlogopite other common minerals in the skarn assemblages include f rich hornblende wilkeite f f apatite and fluorite zircon grains typically contain abundant inclusions of skarn minerals and daughter mineral rich mostly magnetite halite and sylvite fluid inclusions compositionally these zircon grains have moderately to extremely high th 518 7 477 ppm and u 109 25 610 ppm contents with highly variable th u ratios ranging from 0 01 to 5 23 the morphological textural and geochemical features of the zircons confirm their hydrothermal origin and indicate that they most likely precipitated from high temperature f rich magmatic derived ore forming fluids the hydrothermal zircon grains yield well defined concordant u pb ages for the five studied iron skarn deposits with weighted mean 206 pb 238 u dates ranging from 133 6 0 9 to 128 5 1 4 ma 2 these ages are remarkably consistent with u pb ages 134 1 1 2 to 128 5 0 9 ma 2 of magmatic zircon grains from the ore related intrusions in each deposit demonstrating that iron skarn mineralization was genetically related to the coeval magmatism our new geochronological data when combined with existing isotopic ages indicate that iron mineralization and associated magmatism in the handan xingtai district took place mainly at the ca 137 133 and 131 128 ma intervals iron skarn deposits of similar ages also occur widely in other parts of the eastern north china craton forming the only known giant mesozoic iron skarn province in a cratonic block on the earth the formation of these iron skarn deposits and associated intrusions coincided in time with lithospheric thinning or destruction of the north china craton strongly suggesting a causal link between the two processes

lode gold deposits and archean mantle plume island arc interaction abitibi subprovince canada abstract in combination with seismic interpretations and geochronological constraints the association of juvenile arc type low ti tholeiitic basalts with komatiites in the southeastern abitibi subprovince canada supports a history of subduction step back following late archean mantle plume island arc interaction the resulting paired collision zones preserved abundant komatiites and numerous massive sulphide deposits and established the critical metallogenic features to concentrate the majority of canada s precambrian gold resources in a small area of the southern abitibi subprovince

archaean hydrothermal zircon in the abitibi greenstone belt constraints on the timing of gold mineralisation abstract hydrothermal zircons have been found in archaean mesothermal au veins and altered wallrock selvages at val d or in the abitibi greenstone belt of canada the zircons are paragenetically associated with vein quartz tourmaline mica carbonate scheelite pyrite and gold zirconium mobility and the consequent occurrence of hydrothermal zircon may be associated with the intense tourmalinisation characteristic of archaean gold deposits in this district the shrimp ion microprobe has been used to analyse hydrothermal zircons from four separate mines spatially associated with the bourlamaque batholith and has yielded ages constraining formation of the au bearing vein systems to within 20 ma of emplacement of the pluton the ion microprobe data reveal multiple stages of hydrothermal zircon growth in the vein systems contemporaneous with the regional metamorphic peak and late kinematic activity along regional structures younger including proterozoic ages previously obtained for the veins using other minerals and isotopic schemes must reflect either alteration or renewed mineral growth during much later reactivation of fluids along the same structures up to 400 ma after initial formation of the veins

geologic evolution of the late archean abitibi greenstone belt of canada the southern volcanic zone svz of the late archean abitibi belt of the superior province of canada is dominated by komatiitic to tholeiitic volcanic plateaus and large bimodal mafic felsic volcanic centers these volcanic rocks were erupted between about 2717 and 2700 ma in a series of rift basins that formed as a result of wrench fault tectonics they overlie and juxtapose a volcano plutonic assemblage characterized in the northern abitibi belt the age of the assemblage is about 2720 ma or older and it comprises basaltic to andesitic and dacitic subaqueous massive volcanics cored by comagmatic sills and layered anorthositic complexes and overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 2 ma a tectonic model is proposed in which the svz formed in a series of rift basins that dissected an earlier formed volcanic arc comparisons are made with rift environments that have been postulated for phanerozoic areas such as the hokuroko basin of japan the taupo volcanic zone of new zealand and the sumatra and nicaragua arcs

tracing the fluid evolution of the kiruna iron oxide apatite deposits using zircon monazite and whole rock trace elements and isotopic studies the ore genesis of the paleoproterozoic iron oxide apatite deposits in the vicinity of kiruna in northern sweden is poorly understood despite a century long mining history and 2500 mt of iron ore with grades of 30 to 70 wt fe produced in the region to date zircon grains from the ore recently dated at ca 1874 ma show very different appearances compared to zircon from surrounding host rocks ca 1880 ma and related intrusions ca 1880 and ca 1874 ma particularly an inclusion rich rim in contrast zircon from the host rocks and a proximal granite intrusion exhibit typical igneous growth zoning electron microprobe results show near stoichiometric composition for zr si and hf in the host rock zircon grains the ore zircon crystals have low analytical totals with significant concentrations of ca fe y and p and infrared spectroscopy showed several weight percent of water these ore zircon grains further show fe rich inclusions zones and or veins in elemental x ray maps and light rare earth elements lree enrichment transmission electron microscopy tem shows that the lree are not due to micro or nano inclusions in the zircon but are likely hosted as lree oxides in amorphous regions of the grains based on these characteristics the rims on ore zircon grains are interpreted to be of hydrothermal origin uranium pb in monazite from the ore measured by sims suggests a secondary event influencing the area at ca 1624 ma a period of known geologic activity in fennoscandia electron microprobe x ray mapping of these monazite grains shows no zoning and relatively low u and th concentrations stark contrasts are visible between the ore depleted mantle influence and host rocks crustal influence in the whole rock lu hf and sm nd data the depleted mantle signature of the ore could be related to the kiruna greenstone group as a potential source region for the iron the sm nd isotopic composition of monazite from the ore shows a crustal influence and indicates that the younger event has not disturbed the whole rock sm nd signature of the ore the hydrothermal nature of the ore zircon grains and the isotopic signatures point to a hydrothermal influence on the ore formation with a high temperature magmatic fluid related to the intrusions as most likely heat and fluid source

zircon solubility in alkaline aqueous fluids at upper crustal conditions field evidence and thermodynamic data at ambient conditions suggest that complexing of zr with hydroxyl ions at high ph enhances the solubility of zircon we tested this hypothesis by measuring the solubilities of the assemblages zircon zrsio 4 baddeleyite z b and zircon quartz z q in neutral to alkaline fluids at 0 2 gpa and 450 750 c in neutral to alkaline fluids 0 0 1 1m naoh zircon dissolved incongruently in quartz undersaturated fluids to form baddeleyite at 450 c various na zr silicates precipitated from fluids saturated and undersaturated in quartz we observed no significant dependence of solubility on temperature the measured solubilities of zircon in pure h 2o at 600 and 750 c are only slightly higher than the procedural blank of 3 3 10 7 m zr however the measured solubility of zircon increases with increasing aqueous silica suggesting that zirconium complexes with silica in the fluid at 600 c linear regression of experimental results yielded ln zr 0 25 2 9 ln si where terms in parentheses represent molal concentrations zircon solubility also seems to increase with increasing hydroxyl concentration in the fluid enhanced solubility in silica rich alkaline fluids may cause dissolution reprecipitation of zircon and resetting of its isotopic clocks suggesting that some zircon u pb ages may correspond to fluid events

stable isotope geochemistry of the archean val d or canada orogenic gold vein field a systematic study of the auriferous quartz veins of the val d or vein field abitibi quebec canada demonstrates that the c o s isotope composition of silicate carbonate borate oxide tungstate and sulphide minerals have a range in composition comparable to that previously determined for the whole superior province the oxygen isotope composition of quartz from early quartz carbonate auriferous veins ranges from 9 4 to 14 4 whereas later quartz tourmaline carbonate veins have 18oquartz values ranging from 9 2 to 13 8 quartz carbonate veins have carbonate 18o 6 9 12 5 13c 6 2 1 9 and pyrite 34s 1 2 and 1 9 isotope compositions comparable to those of quartz tourmaline carbonate veins 18o 7 9 11 7 13c 8 0 2 4 34s 0 6 6 0 18oquartz values in quartz tourmaline carbonate veins have a variance comparable to analytical uncertainty at the scale of one locality irrespective of the type of structure the texture of the quartz or its position along strike across strike or down dip a vein in contrast the oxygen isotope composition of quartz in quartz tourmaline carbonate veins displays a regional distribution with higher 18o values in the south central part of the vein field near the cadillac tectonic zone and which 18o values decrease regularly towards the north another zone of high 18o values in the northeast corner of the region and along the trace of the senneville fault is separated by a valley of lower 18o values from the higher values near the cadillac tectonic zone oxygen isotope isopleths cut across lithological contacts and tectonic structures this regional pattern in quartz tourmaline carbonate veins is interpreted to be a product of reaction with country rocks and mixing between 1 a deep seated hydrothermal fluid of metamorphic origin with minimum 18o 8 5 13c 0 6 and 34s 0 4 and 2 a supracrustal fluid most likely archean seawater with a long history of water rock exchange and with maximum 18o 3 9 13 c 5 6 and 34s 5 0

differential response of zircon u pb isotopic systematics to metamorphism across a lithologic boundary an example from the hope valley shear zone southeastern massachusetts usa a detailed morphological chemical and isotopic study of zircons from a single outcrop of two mineralogically and chemically distinct units of the late precambrian ponaganset gneiss was undertaken to investigate the effects of mylonitization and metamorphism on u pb isotopic systematics late paleozoic amphibolite grade approx 600 c mylonitization of the ponaganset gneiss at this locality is associated with movement along the hope valley shear zone the response of zircon to metamorphism in each gneiss unit is distinct zircons in gray augen gneiss are uncorroded and not overgrown whereas zircons from fluorite bearing pink granitic gneiss are variably corroded and over 50 bear opaque overgrowths the zircon overgrowths are chemically distinct from the primary cores and contain high conentrations of hf u hree and th mylonite derived from the gray gneiss contains only a small population of hf u rich metamorphic zircon but zircons in the pink gneiss derived mylonite are dominated by the hf u rich metamorphic component in terms of their u pb isotopic systematics overgrowth free zircons from both units are markedly discordant gray 10 20 pink 35 but overgrown zircons from the pink gneiss are up to 70 discordant zircons from the mylonites yield younger pb pb and u pb ages than those of the protolith gneisses and isotopic data from each gneiss mylonite pair define a linear array on concordia plots upper intercept ages of the gray gneiss 621 27 ma and the pink gneiss 635 50 ma indicate that the crystallization of both units was coeval and the lower intercept ages gray 270 92 ma pink 285 26 ma fall within the range of other published age estimates for alleghanian metamorphism in southeastern new england e g zartman et al 1988 new growth of zircon suggests that zr was mobile during metamorphism the presence of fluorite in the pink gneiss and a discontinuity in log left f text h text 2 text o mathord left vphantom f text h text 2 text o f text hf right kern nulldelimiterspace f text hf right values obtained from biotite across the pink gneiss gray gneiss contact indicates that dissolution and reprecipitation of zircon may be related to local variations in hf fugacity zircon dissolution reprecipitation in the pink gneiss and the lack of similar features in the contiguous gray gneiss suggests that the degree of isotopic perturbation of zircon during metamorphism is related to bulk chemistry fluid chemistry and or the degree of fluid rock interaction

timing of structural deformation and age of mineralization in the northern shiquan hanyin gold orefield south qinling belt china constraints from zircon u pb and biotite 40ar 39ar isotope dating abstract the northern shiquan hanyin gold orefield in southern shaanxi china is located within the south qinling belt north dabashan thrust nappe system the ore bearing rocks in the orefield comprise low grade metamorphic rocks and intensely deformed schist of the lower silurian meiziya formation s1m and the structural framework includes five e w trending brittle ductile shear zones however important aspects of the orefield are still uncertain such as the genetic type of the deposits the characteristics and development of brittle ductile shear zones the division of structural stages and the ages of structural deformation and mineralization the present study uses large scale structure alteration mapping zircon u pb chronology and biotite 40ar 39ar isotope dating to characterize the structural development and mineralization of the orefield three stages of structural deformation are identified the second stage s2 is dominant and included the formation of thrust nappe ore controlling brittle ductile dextral shear zones a zircon u pb age 180 2 3 6 ma of a granite dike in a brittle ductile shear zone and biotite 40ar 39ar plateau ages 191 0 1 1 178 4 0 8 170 4 0 9 and 168 4 0 9 ma of lamprophyre and altered mineralized rocks indicate that the main structural magmatic mineralization event occurred during the early middle jurassic coeval with intracontinental orogenesis in the south qinling belt ore formation occurred in three stages in stage 1 190 ma the upper crust underwent a transition from a compressive to an extensional tectonic environment and the initial deposition of gold likely occurred in stage 2 180 ma under an intracontinental orogenic collapse regime several brittle ductile shear zones and a series of extensional strike slip normal faults developed making stage 2 the main stage of mineralization in stage 3 170 ma the late stage of intracontinental orogeny the main tectonic style gradually transformed from extensional collapse to tectonic extrusion related nappe fold deformation inducing late alteration mineralization that superimposed and enriched the existing mineralization

geochronological constraints on the timing of magmatism deformation and gold mineralization in the red lake greenstone belt northwestern ontario the paper presents u pb ages on zircon baddeleyite titanite rutile and monazite from volcanic and plutonic rocks of the red lake greenstone belt in the northern superior province the purpose of the study was to refine a previously outlined time frame for the volcanic evolution of the belt define the time of major plutonism and deformation and constrain the age of formation of gold deposits volcanism spanned at least 270 ma between 3000 and 2730 ma new ages of 2989 3 and 2894 2 ma for units of a lower volcanic sequence and and 2744 1 ma for units of an upper volcanic sequence corroborate and refine the previously established relationships local gabbroic and felsic intrusions cutting through the lower sequence yield ages of 2870 15 and respectively volcanism was succeeded by major plutonism deformation alteration and metamorphism between about 2730 and 2700 ma these processes were interrelated and progressed as a relatively continuous sequence of events accentuated by major activity d

formation of archean batholith hosted gold veins at the lac herbin deposit val d or district canada mineralogical and fluid inclusion constraints the lac herbin deposit consists of a network of mineralized parallel steep reverse faults within the synvolcanic bourlamaque granodiorite batholith at val d or in the archean abitibi greenstone belt there are two related quartz tourmaline carbonate fault fill vein sets in the faults which consist of subvertical fault fill veins associated with subhorizontal veins the paragenetic sequence is characterized by a main vein filling ore stage including quartz tourmaline carbonate and pyrite hosted gold chalcopyrite tellurides pyrrhotite and cubanite inclusions most of the gold is located in fractures in deformed pyrite and quartz in equilibrium with chalcopyrite and carbonates with local pyrrhotite sphalerite galena cobaltite pyrite or tellurides petrography and microthermometry on quartz from the main vein filling ore stage reveal the presence of three unrelated fluid inclusion types 1 gold bearing aqueous carbonic inclusions arranged in three dimensional intragranular clusters in quartz crystals responsible for the main vein filling stage 2 barren high temperature aqueous moderately saline inclusions observed in healed fractures postdating the aqueous carbonic inclusions and considered as a remobilizing agent of earlier precipitated gold in late fractures and 3 barren low temperature aqueous high saline inclusions in healed fractures similar to the crustal brines reported throughout the canadian shield and considered to be unrelated to the gold mineralization at the lac herbin deposit the aqueous carbonic inclusions are interpreted to have formed first and to represent the gold bearing fluid which were generated contemporaneous with regional greenschist facies metamorphism in contrast the high temperature aqueous fluid dissolved gold from the main vein filling ore stage transported and reprecipitated it in late fractures during a subsequent local thermal event

the age of the kambalda greenstones resolved by ion microprobe implications for archaean dating methods abstract a chert horizon within the archaean basalt pile at kambalda western australia contains zircons from air fall tuffs erupted as the sediment was laid down ion microprobe analyses of these magmatic zircons constrain the long sought age of the kambalda lavas to 2692 4ma 2 and show that regional metamorphism and deformation of the greenstone pile occurred within 30 ma of deposition rare zircon xenocrysts are evidence of pre greenstone crust as old as 3450 ma exposed remnants of which are yet to be identified subsets of both the magmatic and xenocryst zircons show evidence of hitherto unsuspected low temperature u and or pb mobility in the proterozoic long after stabilisation of the craton the ion microprobe data unravel complexities of isotopic inheritance and alteration illustrating the hazards of using bulk or whole rock samples in archaean isotopic investigations identification of interflow cherts as a precisely dateable rock type opens the way to direct ion probe dating of mafic ultramafic piles and close numerical age control of the complex but ill exposed stratigraphic and structural relations in this richly mineralised greenstone belt

geochronological and mineralogical constraints on mineralization of the hetai goldfield in guangdong province south china abstract the hetai goldfield located in the southern segment of the qinzhou bay hangzhou bay juncture orogenic belt qhjb is the largest concentration of gold deposits in guangdong province south china the gold mineralization is hosted within the late neoproterozoic to early paleozoic yunkai group and strictly confined to mylonite ductile shear zones the nature of the structural control of mineralization in particular the role of ductile versus brittle deformation and their ages which remain unclear despite numerous previous studies are examined in this paper through an integrated study of geochronology and mineralogy lamellar and filament structures shown by pyrite and pyrrhotite in the ores suggest that sulfidation took place during ductile deformation and syntectonic metamorphism but the majority of the ores are associated with brittle deformation features in combination with macroscopic and microscopy observations on shear fabrics la icp ms u pb dating on zircons of hydrothermal origin from mylonites suggests that the hetai goldfield was subjected to two shearing events an early sinistral ductile shearing at ca 240 ma and a late dextral ductile brittle shearing at ca 204 ma indosinian these ages are ca 90 30 ma older than the previously published gold mineralizing ages of ca 175 152 ma yanshanian suggesting that the main gold mineralization and related brittle deformation significantly postdate the ductile deformation this inference is supported by the mineralization temperatures estimated from geothermometers of arsenopyrite ca 350 290 c chlorite ca 260 230 c and sphalerite ca 230 170 c intergrown with native gold which are considerably lower than that for the ductile deformation 500 300 c or higher based on these data we propose that the gold mineralization in the hetai goldfield predominantly occurred during the yanshanian event and only minor gold mineralization and associated sulfidation took place during the earlier indosinian ductile deformation

pb isotope compositions of pyrite from the c quartz tourmaline vein of the siscoe gold deposit val d or quebec constraints on the origin and age of the gold mineralization the siscoe mine was one of the richest in the val d or mining camp producing 27 5 tonnes t of gold and 9 5 t of silver from 1929 to 1949 the c quartz tourmaline vein at the siscoe mine which is the youngest auriferous vein on the property is an excellent example of a high grade archean lode gold deposit it contains an average of 45 g t au and locally up to 221 g t au this vein cuts all dikes the main regional foliation s2 and several auriferous quartz carbonate veins and is interpreted to be a shear structure that formed during the development of a late to post d2 reverse fault the vein comprises mainly alternating tourmaline and quartz rich layers and gold occurs commonly in late fractures cutting the vein the visible wall rock alteration is characterized by the occurrence of tourmaline pyrite and calcite in this study we report the pb isotope compositions of hydrothermal pyrite separates taken from the c quartz tourmaline vein and its wall rock pyrite is abundant in the wall rock where it occurs as grains with inclusions of chalcopyrite gold tetradymite pyrrhotite rutile silicates and carbonates in the vein it forms euhedra generally devoid of inclusions locally the euhedra are fractured and the fractures are filled with native gold calcite and tetradymite textural relationships suggest that pyrite precipitated synchronously with and or after gold in the wall rock but predated gold in the vein pyrite separates from the vein and its wall rock yielded similar 206pb 204pb 207pb 204pb and 208pb 204pb ratios 13 63 13 83 14 58 14 62 and 33 42 33 76 respectively which are more radiogenic than those reported for volcanic supracrustal and syntectonic plutonic rocks of the southern abitibi subprovince but are very similar to those of the late to posttectonic s type granites in the pontiac subprovince and lacorne block this suggests that the pb originated from a reservoir similar to that of the magmas which produced these granites or alternatively that it was leached from the latter the pb ratios yield a model age of 2 57 0 07 ga which is interpreted to date the cooling of the hydrothermal system that introduced and or remobilized gold into the c quartz tourmaline vein these results when considered in the context of the evolution of the southern abitibi indicate that widespread hydrothermal circulation occurred between 2 64 and 2 55 ga after the collision of the pontiac and abitibi subprovinces and was caused by dewatering and s type magmatism in the thickened unstable continental crust this long lived hydrothermal system promoted remobilization of gold from earlier veins and possibly introduction of gold from deeper zones enhancing gold resources in the siscoe deposit

ore forming fluids in archean gold bearing quartz veins at the sigma mine abitibi greenstone belt quebec canada archean gold bearing quartz veins at the sigma mine occur in deformed metavolcanics intruded by two generations of preore porphyries the veins were emplaced during a late compressional tectonic event and they consist primarily of quartz tourmaline carbonates pyrite and scheelite vein minerals have been deformed and are also cut by abundant healed fractures outlined by fluid inclusions in the veins gold typically occurs in late fractures and in recrystallized quartz microthermometric sem eda leachate analysis and gas chromatographic techniques were used to study fluid inclusions in quartz most fluid inclusions at sigma occur in late fractures and are secondary the fluid inclusions are commonly found in fractures parallel to or less commonly within gold bearing fractures and are genetically related to the gold three types of fluid inclusions are identified 1 high salinity na ca cl aqueous inclusions 25 34 wt salt with small amounts of co 2 some containing halite calcite and nahcolite as daughter minerals 2 co 2 rich inclusions containing small amounts of h 2 o and ch 4 and 3 low salinity h 2 o co 2 inclusions 10 equiv wt nacl with co 2 contents which are variable but commonly in the range of 15 to 30 mole percent co 2 leachate analyses show that in addition to na and ca the aqueous fluids contain minor amounts of k fe mg and li homogenization temperatures of h 2 o co 2 inclusions with constant phase ratios range from 285 degrees to 395 degrees c those of aqueous inclusions range from 60 degrees to 295 degrees c the aqueous and co 2 rich inclusions are interpreted to have trapped two coexisting fluids resulting from the unmixing of a homogeneous h 2 o co 2 parent fluid itself occasionally trapped in late fractures the ore bearing fluid is interpreted to be a low salinity 10 equiv wt nacl fluid containing 15 to 30 mole percent co 2 at minimum temperatures in the range of 285 degrees to 395 degrees c the relatively low homogenization temperatures of aqueous inclusions are interpreted to be the result of co 2 effervescence from a co 2 saturated aqueous fluid due to pressure fluctuations at the time of entrapment it is also proposed that the onset of gold deposition is related to the onset of fluid unmixing

zircons from syros cyclades greece recrystallization and mobilization of zircon during high pressure metamorphism zircons were studied from high pressure low temperature metamorphosed meta igneous lithologies from syros these rocks carry several zircon generations related to each other by dissolution reprecipitation processes one generation is pristine zircon that shows growth zoning relatively elevated contents of trivalent cations and high th u ratios the other end member is a skeletal zircon generation with negligible trivalent cation contents and low th u ratios 0 1 texturally between these two there is a range of zircon crystals with complex inclusion populations of y hree th phases and fluid inclusions showing variable progress of replacement recrystallization both end members yield distinct sensitive high resolution ion microprobe shrimp u pb ages the pristine generation has an age of 80 2 1 6ma from a metagabbro and 76 4 2 1ma from a meta plagiogranite dyke the skeletal low th u zircon generation yields an age of 52 4 0 8ma the older late cretaceous zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust the younger eocene age however dates a zircon recrystallization event which possibly coincides with high solubility and mobility of high field strength elements in a high pressure aqueous fluid phase intergrowth relations between zircon and peak metamorphic garnet and excellent agreement of the u pb ages with white mica ar ar ages for the same samples support the conclusion that eocene is the true age of high pressure metamorphism on syros

synthesis of the structural evolution and associated gold mineralization of the cadillac fault abitibi canada abstract the cadillac fault zone cfz is a 250 km long east trending crustal discontinuity associated with numerous world class gold deposits in the archean abitibi subprovince in quebec the fault follows the boundary between the abitibi and the pontiac subprovinces and can be divided into four segments 1 rouyn 2 joannes 3 malartic and 4 val d or each displays different attitudes and structural styles the malartic segment strikes se whereas the other segments are e trending abrupt changes in strike or smaller scale faults represent limits between the segments structural styles include four generations of foliation with numerous shear sense indicators and folding specifically related to each foliation structural features can be correlated from one segment to another but display clear variations in attitude or expression a stretching lineation signature is definitely associated with each segment in the malartic segment the predominant e trending schistosity displays an angular relationship with the contact between groups and bedding suggesting that the se trending architecture of this segment was already emplaced prior to ductile deformation the evidence suggests a model where the cfz originates from several independent structures inherited from volcanism prior to timiskaming deformation and abitibi pontiac accretion the proto cfz was then linked up during the timiskaming deposition forming high strain zones inherited structures form the frame on which each deformation event was overprinted with different responses depending on segmentation the structural styles of each segment can be integrated in a structural evolution model for the cfz the n s shortening led to the development of a dominantly e striking penetrative foliation and folding during the subsequent extensional stage the cfz was reactivated with normal kinematics during the pontiac uplift a third deformation event is represented by the local development of nw trending cleavage axial planar to the s shaped minor folds finally a late nw se shortening event produced a dextral strike slip movement with ne striking vertical foliation and development of z shaped folds on bedding surfaces and anterior fabrics linkage of the cfz had a major impact on hydrothermal activity in the southern abitibi as it channeled fluids along large scale pathways distribution of gold bearing intrusions can also be related to segmentation and inherited structures of the cfz this association is expressed by gold mineralization styles present in several longitudinal fields reflecting fault segmentation

uthpb zircon dating of the 13 8 ma dacite volcanic dome at cerro rico de potosi bolivia abstract the temporal relationship between the extrusion of the miocene dacite volcanic dome at cerro rico de potosi bolivia and the associated ag sn mineralization has an important bearing on the heat and metal sources for this world class mineral deposit the highly altered nature of the volcanic rock however has prevented previous efforts from definitively determining the emplacement age of the dome rather than subsequent episodes of alteration the present study uses u th pb dating of sparse zircon contained in the dacite to demonstrate that at most only several hundred thousand years separate dome emplacement from main stage mineralization the dacite contains a morphologically heterogeneous population of zircon with considerable variation in size shape and color preliminary analyses revealed the presence of a significant component of inherited zircon most likely as older cores that were overgrown by rims during the crystallization of the dacite magma morphological distinctions shape size color within the zircon population provided the basis for separating the zircon into fractions that reduced or enhanced the inherited component in composite grains by analyzing the tips of zircon with very large length to width ratios it was possible to constrain the crystallization age of the dacite to 13 8 0 2 ma the dispersion of data on a concordia diagram indicates at least two ages of inherited zircon one mesozoic associated with yellow grains and another proterozoic associated with colorless grains such xenocrystic components have useful petrogenetic implications for the source and travel path of the magma

fluid inclusion geochemistry of high grade vein hosted gold ore at the pamour mine porcupine camp ontario this study was undertaken to characterize fluid inclusions associated with high grade vein hosted gold at the pamour mine in the porcupine camp of ontario and to compare them with fluid inclusions in quartz veins associated with wall rock hosted gold which is much more common the pamour mine contains two principal types of orebodies metasediment hosted fracture zones and larger metavolcanic or metasediment hosted quartz carbonate veins gold in the veins is distributed erratically attains very high grades locally and increases in grade at higher levels zoned alteration around the veins is dominated by addition of k 2 o co 2 as s and cao and depletion of na 2 o k 2 o co 2 and s are concentrated in the hanging wall and the intensity of co 2 and h super metasomatism increases upward in the mine roughly parallel to the trend seen in gold values gold was introduced along with sphalerite galena calcite chlorite and chalcopyrite late in the paragenetic history after brittle deformation of earlier vein hosted quartz tourmaline feldspar and pyrite sphalerite that is directly associated with high grade gold contains two types of primary inclusions a co 2 ch 4 fluid 20 45 mole ch 4 and an h 2 o rich liquid with a carbonaceous phase co 2 ch 4 as well as muscovite and graphite daughter minerals aqueous phase salinity estimates range from 2 to 9 equiv wt percent nacl and sem analyses of evaporitic residues of thermally decrepitated inclusions contain nacl and cacl 2 with lesser amounts of kcl mgcl 2 s zn and fe these two inclusion types appear to represent immiscible fluids trapped along the solvus in the h 2 o co 2 ch 4 salt system at temperatures of 325 degrees or 25 degrees c and pressures of 1 to 2 kbars these fluids are similar to those observed in vein quartz at the mcintyre and hollinger deposits where most gold is hosted by wall rocks except that co 2 immiscibility appears to have been more widespread at pamour the separation of immiscible fluids of the type identified in this study would increase the f sub o 2 and ph of the residual fluid owing to the partitioning of h 2 s ch 4 and co 2 into the carbonaceous phase the net effect of these changes would be to decrease the solubility of bisulfide complexed gold suggesting that co 2 immiscibility is an important contributor to gold deposition within veins where wall rock interaction is limited it is not clear whether all vein hosted gold formed from such unmixing or only the higher grade ore shoots

in situ zircon u pb dating and o isotopes of the neoarchean hongtoushan vms cu zn deposit in the north china craton implication for the ore genesis abstract the hongtoushan volcanogenic massive sulfide vms deposit is the largest archean cu zn deposit in china located in the qingyuan greenstone belt on the northern margin of the north china craton the cu zn mineralization was stratigraphically controlled by the interbeds 100 m in thickness of mafic felsic volcanic sets and overlain by banded iron layers however the relationship between vms deposits and associated volcanics has not been examined this study ultimately clarifies the times and sources of the volcanics and mineralization based on in situ zircon u pb and o isotope on vms hosting mafic felsic volcanic rocks banded and massive sulfide ores and postmineralization pegmatite vein we considered that there were two main formation stages for the qingyuan cu zn deposits one was exhalative hydrothermal sedimentation and another was further cu zn enriched by later hydrothermal processes the timing of the first stage occurred at 2571 6 ma based on the magmatic zircons in the vms hosting mafic volcanic rocks from which the inherited zircons also indicate the existence of 2 65 3 12 ga ancient supercrustal rocks in the qingyuan district a modern mantle like 18 o zircon value of 5 5 0 1 2sd for this volcanism was well preserved in the inherited core domains of ore samples it suggests that the mafic volcanics was most likely sourced from partial melting of juvenile crust e g ttg granites a large scale metamorphic or hydrothermal event is documented by the recrystallized zircons in sulfide ores the timing is tightly constrained by the hydrothermal zircon u pb ages they are 2508 4 ma for the banded ore 2507 4 ma for the massive ore and 2508 2 ma for the postmineralization pegmatite vein these indistinguishable ages indicate that the 2507 ma hydrothermal systems played a significant role in the upgrading of the vms cu zn orebodies the weighted 18 o values of hydrothermal zircons show a successively increasing trend from 6 0 0 1 2 for the banded ore 6 6 0 2 2 for the massive ore to 7 3 0 2 2 for the later pegmatite vein this variation might be induced by gradual inputting of the 18 o rich oceanic crust and or oceanic sediment during the hydrothermal cycling system considering its modern mantle like oxygen isotope composition of 2571 ma volcanism a submarine volcanic hydrothermal system involving mantle plumes is a preferred setting for the neoarchean vms cu zn deposits in the qingyuan greenstone belt

u pb zircon geochronology in the southwestern abitibi greenstone belt superior province the abitibi belt is the largest continuous greenstone belt in the superior province of the canadian shield it comprises several composite komatiitic tholeiitic calc alkalic and sedimentary sequences that are folded transected by major faults and intruded by various generations of plutonic rocks precise u pb geochronology has been carried out in the belt for the past decade to solve chronostratigraphic and metallogenetic problems this paper presents new zircon ages and reassesses previously published ones now refined by the addition of abraded and concordant zircon analyses volcanic and subvolcanic units of the timmins area yield the following ages 2727 1 5 ma for a tuff of the upper deloro formation 2703 1 5 ma for a tuff of the upper tisdale formation and 2698 4 ma for the krist fragmental assigned to the top of the tisdale formation the age of a dunite intrusive into the upper deloro formation is revised at 2707 3 ma whereas the aquarius diorite east of timmins yields a poorly define

sm nd and rb sr isotope systematics of scheelites possible implications for the age and genesis of vein hosted gold deposits a positive correlation between present day sup 143 nd sup 144 nd and sup 147 sm sup 144 nd ratios from scheelite samples from the hollinger mcintyre coniaurum gold deposit of ontario canada corresponds to an age of 2403 plus minus 47 ma and may date the age of gold mineralization initial isotope ratios of scheelites suggest that the fluid responsible for the scheelite and gold mineralization was probably of mantle origin and that it subsequently interacted with continental crust

meandering subtropical jet and precipitation over summertime east asia and the northwestern pacific abstractit has been revealed that in summertime precipitation is enhanced to the south of the upper level tropopausal potential vorticity contours which are accompanied by instantaneous jets over the eastern coastal region of china to the northwestern pacific it is frequently exhibited as precipitation bands ranging in size from over a thousand to several thousands of kilometers long in this study an analysis was conducted to quantify the relationship depending on the phase of upper level disturbances with composite analysis it is shown that the enhancement along the contours occurs at all phases it occurs not only to the east but also to the west of the upper level troughs although it is weaker the midtropospheric distributions of upwelling and the q vector convergence are collocated with the precipitation enhancement suggesting the importance of dynamical induction by geostrophic flow at all phases the effects of upper level disturbances and low level jets lljs with a southerly component

40ar 39ar age constraints on the thermal history of the archean abitibi greenstone belt and the pontiac subprovince implications for terrane collision differential uplift and overprinting of gold deposits 40ar 39ar mineral age spectra of granitic and metamorphic rocks in conjunction with existing conventional zircon geochronology indicate that at least two major late archean thermal events affected tectonic blocks of the abitibi southern volcanic zone svz and the juxtaposed pontiac subprovince the earlier thermal activity 2690 2670 ma was accompanied by the intrusion of voluminous syntectonic plutons and caused low pressure greenschist facies metamorphism in the svz and intermediate pressure metamorphism in the pontiac subprovince the second thermal event 2660 2630 ma was coeval with the emplacement of syncollisional s type garnet muscovite granites in the pontiac subprovince and the higher grade lacorne block of the abitibi svz and reset the k ar systems in preexisting rocks magmatic amphibole from the syntectonic round lake batholith 2695 ma u pb zircon age of the abitibi svz has a slightly disturbed ar release spectrum with an upper plateau age of 2669 6 ma signifying that the low gra

hydrothermal zircon geochronology age constraint on nanling range tungsten mineralization southeast china abstract the nanling range southeast china is well known for its wolframite bearing quartz vein wqv tungsten deposit this study focuses on the geochemistry and geochronology of zircons from the wqv and challenges the current view of the tungsten mineralization in the nanling range the features of the wqv zircons include 1 pale brown murky brown or orange red color and translucence under microscope 2 110 101 type crystal form 3 weak cathodoluminescence 4 enrichment of hf ranging from 1 97 to 7 83 wt hfo2 u ranging from 0 02 to 3 97 wt uo2 th ranging from 0 to 0 65 wt tho2 and p ranging from 0 to 1 82 wt p2o5 and 5 presence of solid hydrothermal and ore minerals and fluid inclusions these features indicate that the wqv zircons crystallized from hydrothermal fluids during tungsten mineralization the in situ la icpms u pb results of the wqv zircons from five different tungsten deposits in the nanling range yield similar ages ranging from 134 4 1 9 ma to 132 9 1 5 ma approximately 20 million years younger than proposed tungsten ore ages 155 5 ma several mineralization characteristics and field observations also cast doubt on the current model nanling range tungsten ore is the result of orthomagmatic processes the zircon characterization method provided in this study could be applied to tungsten metallogenic research in other parts of the world

new u pb age constraints for the timing of gold mineralization at the pampalo gold deposit archaean hattu schist belt eastern finland obtained from hydrothermally altered and recrystallised zircon abstract in this paper we present new u pb data on zircon and titanite from the host rocks of the pampalo gold deposit located within the neoarchaean hattu schist belt in eastern finland we also present new u pb data on nearby plutonic rocks plagioclase porphyries at the mine site demonstrate inheritance from 2 82 2 83 ga sources while a suggestive intrusive age is at c 2 76 ga zircon grains extracted from the altered felsic units hosting the gold ore show complex alteration and recrystallisation textures and demonstrate zr mobility this mobility is most probably related to the alteration event although direct link to gold mineralization remains to be shown the preserved or recrystallized parts of the altered zircon grains texturally homogeneous grain aggregates and some overgrowths yield heterogeneous ages which cluster between ca 2 73 and 2 70 ga with a mean age of 2 71 ga this age is considered to place a new constraint on the timing of mobility of au at the pampalo this event postdates the known crustal formation event at 2 76 2 73 ga in the area as recorded by the adjacent plutonic rocks and volcanoclastic rocks within the hattu schist belt the naarva tourmaline granite 10 km nw of pampalo is 2 69 ga and is thus temporally associated with known regional crustal anatexis and metamorphism in the archaean of eastern finland whether this crustal reworking event played a role in the genesis of the gold in eastern finland needs to be further studied

u pb dating of hydrothermal zircon from the dongping gold deposit in north china constraints on the mineralization processes abstract the dongping gold deposit is a large deposit with total gold reserves of 100 tons it is located at the northern margin of the north china craton northwestern hebei province china the ore bodies are hosted by the devonian shuiquangou syenite complex and consist mainly of auriferous quartz veins and disseminated ore in the altered and silicified syenite u pb dating of zircon from hornblende syenite on the western margin of the complex yields a crystallization age of 400 3 5 ma mswd 0 018 morphology cathodoluminescence imaging and geochemical classifications of zircon from the first stage of disseminated ore and gray auriferous quartz veins and from later stage low grade quartz veins point to their newly crystallized hydrothermal origin the hydrothermal zircon from the disseminated ore and auriferous gray quartz vein are dated at 389 1 0 ma and 385 5 7 ma respectively which are detectably younger than but close to the crystallization age of the syenite complex and might have been formed by post magmatic hydrothermal processes both types of ore are dominant in the ore deposit and we propose that the pervasive post magmatic hydrothermal alteration is the main ore forming stage hydrothermal zircon from a low grade auriferous quartz vein yields a u pb age of 140 ma interpreted as forming during a younger period of superimposed yanshanian hydrothermal mineralization thus the ore of the dongping gold mine represents a post magmatic devonian hydrothermal ore deposit with jurassic cretaceous hydrothermal overprinting

constraints of hydrothermal and magmatic zircon on the origin of the yaogangxian tungsten deposit southern china abstract the yaogangxian tungsten deposit located in the central nanling range is a well known vein type tungsten deposit and has been mined for centuries this study focuses on the morphology geochronology and geochemistry of zircon crystals from wolframite bearing quartz veins and coexisting alkali feldspar granite to better understand the source of ore forming materials and the exact timing of tungsten mineralization all zircon grains separated from the muscovite alkali feldspar granite and the wolframite bearing quartz veins show core to rim structure the overgrowth rims of the zircon grains from the muscovite alkali feldspar granite and the wolframite bearing quartz veins have crystallization ages of 133 4 1 3 ma and 133 7 1 3 ma whereas the relict cores have ages of 155 3 1 6 ma and 154 8 1 6 ma respectively the ages of the cores are identical to the crystallization age of the hosting yaogangxian biotite monzogranite thus these cores were derived from the biotite monzogranite whereas the overgrowth rims should have crystallized during the intrusion of muscovite alkali feldspar granite and wolframite bearing quartz veins the geochemical consanguinity and similar ages of the muscovite alkali feldspar granite and the ore veins suggest that they were both formed from the highly fractionated granitic magma in an extensional setting this highly fractionated granitic melt might be extracted from the biotite monzogranitic mush in a long surviving deep seated magma chamber

reply to comment by f corfu and d w davis on archaean hydrothermal zircon in the abitibi greenstone belt constraints on the timing of gold mineralisation

comment on archean hydrothermal zircon in the abitibi greenstone belt constraints on the timing of gold mineralization by j c claoue long r w king and r kerrich

the sequence of magmatic and mineralization events in the abitibi greenstone belt isotopic evidence from taylor township timmins area northern ontario canada samples from the porphyry deposit and the shoot zone prospect of st andrew goldfields ltd in taylor township near matheson ontario have been dated by several different techniques and utilized as a test of the use of pb isotope measurements in determining the time of mineralization in gold deposits of the abitibi greenstone belt clear and abraded zircons from an altered sulfidic porphyry unit yield a well defined age of 2697 3 1 3 ma indicating that the original intrusive rock unit containing these zircons was either latest synvolcanic or earliest syntectonic larger bulk samples of zircon from the same unit contain many altered and cracked grains and yield an age of 2682 4 ma close to the peak of syntectonic igneous activity pb pb isochrons determined from sulfide samples in mineralized material from the taylor porphyry zone yield a two stage model age of 2663 17 ma and suggest that mineralization postdates the syntectonic granitoids these pb isotope data are compared with isotope r