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

copper mineralization prevented by arc root delamination during alpine himalayan collision in central iran contrasting geochemical signatures of copper ore hosting eocene and some barren undeformed miocene diorites to granites in central iran temporally overlap with the alpine himalayan collision and provide key implications on the existence and lack of cu mineralization during collisional magmatism high sr and low y and yb contents of eocene arc rocks in the natanz arc segment reflect thickened andean type orogenic arc crust 45 km whereas barren miocene natanz arc rocks 21 19 ma indicate thin arc crust similar to collisional volcanism in anatolia geochemical modeling indicates a change in the mineralogy of the melt residual from precollisional eocene basaltic garnet bearing 5 30 amphibolite to syn or postcollisional miocene metasomatized mantle peridotite which can be explained by collision induced delamination of the arc lithospheric root subsequent recharge of hot asthenosphere and melting of metasomatized mantle peridotite and lack of interaction with a garnet bearing arc crustal keel explain the low sr and high y and yb contents the relatively enriched initial sr isotope ratios of postcollisional miocene natanz rocks and the lack of copper mineralization in postcollisional miocene natanz arc rocks arc root delamination removes the copper and sulfur enriched metasomatized lithospheric arc root and hydrous cumulate reservoir required to form copper ore deposits lack of the dense melt residues also provides an alternative explanation for the elevated thin crustal iranian back arc plateau 38 km as a result of uplift by isostatic rebound rather than uplift by anomalous shortening miocene arc root delamination implies a minimum age of 21 ma for the alpine himalayan collision in central iran

application of thermochronology to hydrothermal ore deposits thermochronology finds many applications in economic geology utilizing temperature sensitive radiometric dating techniques to reveal low temperature upper crustal processes can elucidate many aspects of deposit genesis including timing and duration of mineralization processes rate of exhumation and erosion of intrusive ore deposits and comparative preservation potential the tools are utilized to best advantage when combined with other thermochronometry techniques that provide complementary information in addition when thermochronometers are combined with geochronometers e g zircon u pb over 800 c of thermal history is revealed from emplacement to erosion fig 1 with the advent of computational algorithms that provide more accurate and detailed models of thermochronology results the economic geologist has a powerful tool to use when assessing economic favorability of a region or prospect this chapter summarizes the various ways that low temperature thermochronometers have been utilized in studies of economic mineralization and primarily focuses on studies of porphyry ore deposits these deposits are well characterized and provide a good platform from which to demonstrate the extended understanding of ore formation processes that is provided by thermochronology studies in this section we review the fundamental application of u th he fission track and 40ar 39ar thermochronometry methods to mineralized systems we then present a synopsis of how these techniques can be used in thermal history analysis either alone or in combination with other chronometers in order to provide more information on deposit genesis the discussion is presented from low through to higher temperature thermochronometers corresponding to their presentation on figure 1 u th he thermochronology as documented earlier in the volume u th he thermochronology is based on measuring the accumulation of radiogenic 4he produced from u and th decay the daughter helium is retained until the mineral is heated to a temperature at which its structure and helium retentivity change this closure temperature varies from

metamorphic evolution of the attic cycladic metamorphic belt on naxos cyclades greece utilizing 40ar 39ar age spectrum measurements 40ar 39ar age spectrum analysis of phengite separates from naxos part of the attic cycladic metamorphic belt in greece indicates that cooling following high pressure low to medium temperature metamorphism m1 occurred about 50 ma ago phengite has 40ar gradients that suggest that part of the scatter observed in conventional k ar ages was caused by diffusion of radiogenic argon from the minerals during a younger metamorphism m2 in central naxos this metamorphism m2 has overprinted the original mineral assemblages completely and is associated with development of a thermal dome excellent 40ar 39ar plateaus at 15 0 0 1 ma 11 8 0 1 ma and 11 4 0 1 ma obtained on hornblende muscovite and biotite respectively from the migmatite zone indicate that relatively rapid cooling followed the m2 event and that no significant thermal overprinting occurred subsequent to m2 toward lower m2 metamorphic grade 40ar 39ar plateau ages of hornblendes increase to 19 8 0 1 ma concomitantly the proportion of excess 40ar in the spectra increases as well we propose that the peak of m2 metamorphism occurred beween 15 0 and 19 8 ma ago k ar ages of biotites from a granodiorite on the west coast are indistinguishable from those found in the metamorphic complex and hornblende k ar ages from the same samples are in the range 12 1 13 6 ma as the latter ages are somewhat younger than most ages obtained from the metamorphic complex intrusion of the granodiorite most likely followed the peak of the m2 metamorphism the metamorphic evolution of naxos is consistent with rapid crustal thickening during the cretaceous or early tertiary causing conditions at which supracrustal rocks experienced pressures in the range 900 1500 mpa transition to normal crustal thicknesses ended the m1 metamorphism about 50 ma ago the m2 metamorphism and granodiorite intrusion occurred during a period of heat input into the crust possibly related to the migration of the hellenic volcanic ar c in a southerly direction through the area

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

the mont saint hilaire plutonic complex occurrence of excess 40ar and short intrusion history mont saint hilaire is an approximately 10 km2 alkaline plutonic complex in the monteregian petrographic province of quebec the complex consists of an older western half of alkali gabbros and an eastern portion of nepheline syenites and magmatic breccias the intrusives were emplaced at depths of no more than a few kilometres into paleozoic sedimentary rocks through the underlying grenville basement 40ar 39ar analyses of amphibole yield complex age spectra in which the apparent ages decrease with increasing heating temperature conventional k ar analyses of amphibole pyroxene and feldspar yield anomalously old dates these data indicate the presence of small yet significant amounts up to 8 10 11 mol g of excess 40ar the quantities of excess argon are variable between rocks and minerals from the same rock in contrast biotites yield concordant plateaus and consistent ages ranging only from 124 1 to 124 6 ma these data define the age of the complex 124 4 1 2 ma and imply a short time span of

excess argon in amphiboles from fluid interaction and short intrusion interval at the epizonal marangudzi complex zimbabwe the epizonal marangudzi complex of the karoo associated nuanetsi igneous province in southern zimbabwe was emplaced into precambrian gneisses and consists of a gabbro mass intruded in turn by ring dikes of quartz and nepheline syenites amphibole and biotite from various lithologies have been examined using 40ar 39ar techniques in order to investigate the temporal distribution of intrusions and the possible occurrence of excess ar with one important exception samples give internally concordant incremental release spectra average 40ar 39ar dates range from 177 6 to 181 3 ma three amphiboles give dates which are significantly older than the other samples and contain small yet discernible quantities of excess 40ar excess ar is restricted to amphiboles displaying substantial fe mg zoning which is interpreted as reflecting late magmatic or early sub solidus reaction with fluid phases argon was apparently acquired from the fluid during alteration and ultimately could have been magmatic or could have come from degassing of the country rocks the remaining samples both amphibole and biotite show only a restricted range 1 m y in apparent age and are essentially analytically indistinguishable implying rapid cooling of the complex the average for these analyses 178 1 ma reflects the mean age of emplacement of cogenetic intrusions within a short interval 1 m y in duration this revised age of the marangudzi complex fits well within the temporal framework of karoo province magmatism in the region

the partitioning behavior of silver in a vapor brine rhyolite melt assemblage abstract the partitioning of silver in a sulfur free rhyolite melt vapor brine assemblage has been quantified at 800 c pressures of 100 and 140 mpa and f o 2 nno nickel nickel oxide silver solubility 2 in rhyolite increases 5 fold from 105 21 to 675 98 g g as pressure increases from 100 to 140 mpa nernst type partition coefficients d ag i j 2 describing the mass transfer of silver at 100 mpa between vapor and melt brine and melt and vapor and brine are 32 30 1151 238 and 0 026 0 004 respectively at 140 mpa values for d ag i j 2 for vapor and melt brine and melt and vapor and brine are 32 10 413 172 and 0 06 0 03 respectively apparent equilibrium constant values 2 describing the exchange of silver and sodium between vapor and melt k ag na v m at 100 and 140 mpa are 105 68 and 14 6 the average values 2 for silver and sodium exchange between brine and melt k ag na b m at 100 and 140 mpa are 313 288 and 65 12 these data indicate that the mass transfer of silver from rhyolite melt to an exsolved volatile phase s is enhanced at 100 mpa relative to 140 mpa suggesting that decompression increases the silver ore generative potential of an evolving silicate magma model calculations using the new data suggest that the evolution of low density aqueous fluid i e vapor may be responsible for the the silver tonnage of many porphyry type and perhaps epithermal type ore deposits for example halter et al halter w e pettke t and heinrich c a 2002 the origin of cu au ratios in porphyry type ore deposits science 296 1842 1844 used detailed silicate and sulfide melt inclusion and vapor and brine fluid inclusions analyses to estimate a melt volume on the order of 15 km3 to satisfy the copper budget at the bajo de la alumbrera copper gold silver ore deposit using their melt volume estimate with the data presented here model calculations for a 15 km3 felsic melt saturated with pyrrhotite and magnetite suggest that a low salinity magmatic vapor may scavenge on the order of 7 1012 g of silver from the melt this quantity of silver exceeds the discovered 2 109 g of ag at alumbrera calculated tonnages for numerous other deposits yield similar results the excess silver in the vapor remaining after porphyry formation is then available to precipitate at lower ptconditions in the stratigraphically higher epithermal environment these data suggest that silver and perhaps other ore metals in the porphyry epithermal continuum may be derived solely from the time integrated flux of dominantly low salinity vapor exsolved from a series of sequential magma batches

tectonic evolution and copper gold metallogenesis of the papua new guinea and solomon islands region papua new guinea and the solomon islands are in one of the most prospective regions for intrusion related mineral deposits however because of the tectonic complexity of the region and the lack of comprehensive regional geological datasets the link between mineralization and the regional scale geodynamic framework has not been understood here we present a new model for the metallogenesis of the region based on a synthesis of recent studies on the petrogenesis of magmatic arcs and the history of subduction zones throughout the region combined with the spatio temporal distribution of intrusion related mineral deposits and six new deposit ages convergence at the pacific australia plate boundary was accommodated from at least 45 ma by subduction at the melanesian trench with related melanesian arc magmatism the arrival of the ontong java plateau at the trench at ca 26 ma resulted in cessation of subduction immediately followed by formation of cu au porphyry epithermal deposits at 24 20 ma throughout the melanesian arc late oligocene to early miocene tectonic reorganization led to initiation of subduction at the pocklington trough and onset of magmatism in the maramuni arc the arrival of the australian continent at the pocklington trough by 12 ma resulted in continental collision and ore deposit formation from 12 to 6 ma this is represented by cu au porphyry deposits in the new guinea orogen and epithermal au systems in the papuan peninsula from 6 ma crustal delamination in papua new guinea related to the prior pocklington trough subduction resulted in adiabatic mantle melting with emplacement of diverse cu and au porphyry and epithermal deposits within the papuan fold and thrust belt and papuan peninsula from 6 ma to the present day subduction at the new britain and san cristobal trenches from ca 10 ma resulted in an escalation in tectonic complexity and the onset of microplate tectonics in eastern papua new guinea and the solomon islands this is reflected in the formation of diverse and discrete geodynamic settings for mineralization within the recent to modern arc setting primarily related to upper plate shortening and extension and the spatial relationship to structures within the subducting slab

porphyry cu mo au deposits related to melting of thickened mafic lower crust examples from the eastern tethyan metallogenic domain abstract most porphyry cu deposits in the world occur in magmatic arc settings and are formed in association with calc alkaline arc magmas related to subduction of oceanic lithosphere this contribution reviews a number of significant porphyry cu deposits in the eastern tethyan metallogenic domain they widely occur in a variety of non arc settings varying from post late collisional transpressional and extensional environments to intracontinental extensional environments related to orogenic and anorogenic processes their spatial temporal localization is controlled by strike slip faults orogen transverse normal faults lineaments and their intersections in these non arc settings these deposits are dominated by porphyry cu mo deposits with minor porphyry cu au and epithermal au deposits and exhibit a broad similarity with those in magmatic arcs the associated magmas are generally hydrous relatively high fo2 high k calc alkaline and shoshonitic and show geochemical affinity with adakites they are distinguished from arc magmas and or oceanic slab derived adakites by their occurrence as isolated complexes high k2o contents 1 2 8 5 and much wider range of end t values 10 to 3 and positive ehf t values 4 6 to 6 9 these potassic magmas are most likely formed by partial melting of thickened juvenile mafic lower crust or delaminated lower crust but also involving various amounts of asthenospheric mantle components key factors that generate hydrous fertile magmas are most likely crust mantle interaction processes at the base of thickened lower crust in non arc settings rather than oceanic slab dehydration as in arc settings breakdown of amphibole in thickened lower crust e g amphibole eclogite and garnet amphibolite during melting is considered to release fluids into the fertile magmas leading to an elevated oxidation state and higher h2o content necessary for development of porphyry cu mo au systems copper and au in hydrous magmas are likely derived from mantle derived components and or melts which either previously underplated and infiltrated at the base of the thickened lower crust or were input into the primitive magmas by melt mantle interaction in contrast mo and part of the s in the fertile magmas are probably supplied by old crust during melting and subsequent ascent

sources of metals in the porgera gold deposit papua new guinea evidence from alteration isotope and noble metal geochemistry the porgera gold deposit is spatially and temporally associated with the late miocene mafic alkalic epizonal porgera intrusive complex pic located in the highlands of papua new guinea png the highlands region marks the site of a tertiary age continent island arc collision zone located on the northeastern edge of the australasian craton the pic was emplaced within continental crust near the lagaip fault zone which represents an oligocene suture between the craton and volcano sedimentary rocks of the sepik terrane magmatism at porgera probably occurred in response to the late miocene elimination of an oceanic microplate and subsequent early pliocene collision between the craton margin and an arc system located on the bismarck sea plate gold mineralization occurred within 1 ma of the time of magmatism metasomatism accompanying early disseminated au mineralization in igneous host rocks resulted in additions of k rb mn s and co2 and depletions of fe mg ca na ba and sr rare earth and high field strength elements remained largely immobile pervasive development of illite k feldspar quartz carbonate alteration assemblages suggests alteration by mildly acidic 200 to 350 c fluids at high water rock ratios strontium and lead isotopic compositions of minerals from early base metal sulphide veins associated with k metasomatism and later quartz roscoelite veins carrying abundant free gold and tellurides are remarkably uniform e g 87sr86sr 0 70745 0 00044 n 10 207pb204pb 15 603 0 004 n 15 these compositions fall between those of unaltered igneous and sedimentary host rocks and specifically sedimentary rocks from the jurassic om formation which underlies the deposit igneous rocks 87sr86sr 0 7035 207pb204pb 15 560 om formation 87sr86sr t 0 7153 207pb204pb 15 636 it is therefore suggested that the hydrothermal fluids acquired their sr and pb isotopic signatures by interaction with or direct derivation from a plutonic root of the pic and host sedimentary rocks of the om formation it is likely that au was also derived from one or both of these two sources concentrations of au in unaltered igneous and sedimentary rocks from porgera 10 ppb au do not indicate that either lithology represents a significantly enriched protore although au and platinum group element pge abundances in the igneous rocks suggest a mild primary magmatic enrichment of au relative to the pge average au pt pd mantlenormalized 14 0 6 5 n 8 evidence that the porgera magmas were rich in volatiles permits speculation that au may have been concentrated in a magmatic fluid phase but alternative possibilities such as derivation of au by hydrothermal leaching of solidified igneous materials or sedimentary rocks cannot be excluded at this time

geochronology of the porgera gold deposit papua new guinea resolving the effects of excess argon on k ar and age estimates for magmatism and mineralization abstract mesothermal epithermal gold mineralization at porgera in the highlands of papua new guinea png occurs in structurally controlled veins and disseminations which overprint and cross cut a suite of shallow level comagmatic mafic alkaline stocks and dykes the porgera intrusive complex pic and their sedimentary host rocks conventional k ar apparent ages of twelve hornblende separates from eight different intrusions scatter between 7 and 14 ma but four biotite separates are concordant at 6 02 0 29 ma 2 40 ar 39 ar step heating experiments on six of the hornblende separates reveal saddleshaped age spectra which indicate the presence of excess 40ar one of these samples yields a well defined plateau with an apparent age of 5 96 0 25 ma 2 this age is identical to that of the biotites and suggests an age of 6 0 0 3 ma 2 for emplacement of the pic older apparent ages obtained from conventional k ar and 40 ar 39 ar analyses are explained by excess 40ar contamination conventional k ar analyses of six separates of hydrothermal illite and roscoelite associated with gold mineralization yield apparent ages of between 5 1 and 6 1 ma and indicate that ore deposition occurred within 1 ma of magmatism at porgera evidence for the evolution of a magmatic volatile phase and the presence of excess 40ar both in the intrusives and in hydrothermal fluids associated with the orebody suggest that magmatic fluids may have had some involvement in metallogenesis but the exact nature of this involvement is not yet clear late miocene magmatism and mineralization at porgera are thought to have occurred shortly prior to or during the initiation of continent arc collision and to pre date associated pliocene uplift and foreland deformation in the highlands

sm nd and rb sr dating of an archean massive sulfide deposit kidd creek ontario highly altered felsic metavolcanics associated with the kidd creek ontario cu zn massive sulfide deposit show a large range of sm nd ratios and yield a sm nd isochron of 2674 40 ma initial ratio nd 1 55 0 30 which represents the time of rare earth element redistribution during intense hydrothennal alteration that the sm nd age is consistent with age constraints on ore deposition provided by precise u pb zircon data indicates contemporaneity of ore deposition hydrothennal alteration and rare earth mobility the age is therefore interpreted as a minimum age of ore deposition in contrast the rb sr age of the altered rocks as well as the metavolcanic rocks outside the alteration zone has been reset at 2576 26 ma most likely as a result of widespread low temperature metasomatism unrelated to ore deposition our results suggest that sm nd dating could be a useful tool in the study of ore deposits and potentially in the study of a wide range of mineralizations initial nd t values for massive ore altered felsic volcanics and their weakly altered precursors are identical indicating derivation and redistribution of light rare earth elements within the altered footwall volcanics these data suggest that the footwall volcanics have also supplied part of the base metals to the stratiform ore

sasanian glass from veh ardasīr investigated by strontium and neodymium isotopic analysis abstract a collection of 31 glass fragments dated to the 3rd to 7th century ad and excavated in veh ardas r modern iraq were analysed for their sr and nd isotopic compositions with the aim of deepening our knowledge on glass circulating within central mesopotamia during sasanian times and to highlight the merits of isotope ratios for the definition of the production scenery based on the elemental composition of the samples two groups related to the exploitation of different plant ashes were previously distinguished and also a change in the silica source in the course of the 4th century ad emerged the determination of isotopic distributions of sr and nd offers a deeper insight the 87 sr 86 sr isotope ratio together with the sr elemental concentration confirms this use of different plant ashes and permits one to distinguish distinct glass batches although the isotopic sr signature does not allow us to highlight evidences of different provenances for the samples of different composition moreover the 143 nd 144 nd isotope ratio suggests the use of sands belonging to a common geological area and also allows the identification of a set of samples that were possibly produced exploiting a further different sand source as a whole sr and nd isotope ratio values for the samples considered i e 87 sr 86 sr 0 70833 to 0 70916 and nd 8 69 to 4 98 differ from the published isotopic data on middle eastern glass samples and can therefore define a characteristic sr nd signature for plant ash sasanian glass excavated in veh ardas r

new 40ar 39ar ages from the kalatag district in the eastern tianshan nw china constraints on the timing of cu mineralization and stratigraphy abstract the kalatag region in the eastern tianshan of nw china contains a number of economically important cu deposits among these the south meiling vms deposit is hosted in the late ordovician to early silurian dacitic tuff belonging to the daliugou formation the deposit is composed of two domains a concordant massive sulfide ore domain and a discordant vein type ore domain located in the footwall phyllic alteration zone here we report new 40 ar 39 ar ages on sericite from the phyllic alteration zone to better constrain the timing and genesis of mineralization the sericite displays markedly flat 40 ar 39 ar age spectra yielding a 40 ar 39 ar plateau age of 438 8 3 0 ma and an isochron age of 443 0 11 0 ma the 40 ar 39 ar age is in good agreement with the re os age reported earlier placing the timing of cu zn mineralization at 434 2 3 9 ma the age is also consistent with the field relationships that concordant massive sulfide orebody formed later than discordant vein type orebody another deposit at hongshi is hosted in basaltic andesites of the daliugou formation the orebodies cut the phyllic alteration zone indicating that the cu mineralization formed later than the phyllic alteration hydrothermal sericite from the phyllic alteration zone from this deposit yielded flat 40 ar 39 ar age spectra with 40 ar 39 ar plateau ages of 441 5 3 2 ma and 446 4 4 6 ma and isochron ages of 445 8 5 6 ma and 448 8 7 3 ma respectively this age is obviously older than the re os age which is also in conformity with the field relations that quartz sulfide veins cut the sericitization zone our 40 ar 39 ar ages offer precise constraints on early paleozoic cu mineralization in the eastern tianshan orogen these results indicate that the lower unit basaltic andesites of the daliugou formation formed earlier than 446 ma we thus consider that the daliugou formation formed during middle ordovician to early silurian we also propose several metallogenic events in the kalatag area including 1 an early vms cu zn and hydrothermal vein cu mineralization at 440 430 ma 2 porphyry cu and skarn fe mineralization at 390 380 ma and 3 a late cu au mineralization at ca 300 ma these three metallogenic events were coeval with the distinct magmatic pulses in the kalatag area

reliability of k ar dating of clays and silicifications associated with vein mineralizations in western europe k ar ages were determined on fine fractions and one massive silicification associated with vein mineralizations in uranium barium fluorine and related base metals in western europe the reliability of the potassic materials analysed once the mineralogical and geochemical relationships to the low temperature mineralization have been established is demonstrated the isotopic ages are concentrated between nearly 190 and 170 ma this epoch corresponds to that of the rifting of the future atlantic and tethys oceans this type of low temperature mineralization thus seems to be better explained by extensional phenomena

saddle shaped 40ar39ar age spectra from young microstructurally complex potassium feldspars abstract a suite of young potassium feldspars show markedly saddle shaped 40 ar 39 ar age spectra as a result of incorporating 10 10 to 10 9 mol g of excess 40ar the minima of these age spectra record reasonable cooling ages based on the known thermal history and geology of the samples acid etching of one sample indicates that excess 40ar is concentrated near grain margins the release of a substantial portion of this excess ar at high temperatures in the laboratory requires that this component be situated in a more retentive site than radiogenic 40ar anion vacancies have been proposed to act in this role in plagioclase and we speculate that this is so in k feldspar as well such a mechanism would explain the observation that relative to radiogenic 40ar excess 40ar is incorporated at low temperatures in nature but is released at high temperatures in the laboratory oxygen diffusion provides an appropriate analogy for this phenomenon being relatively fast under natural hydrothermal conditions but extremely slow in anhydrous environments such as an ar extraction system tem observations made on two of the samples confirm that their effective grain sizes for diffusion are likely to be on the order of ten microns due to the presence of such microstructures as incoherent exsolution lamellae dislocations and stepped twins tem observations also reveal the presence in one sample of orthoclase enclaves in a microcline host

geochronology and geochemistry of the early cretaceous jigongshan and qijianfeng batholiths in the tongbai orogen central china implications for lower crustal delamination the jigongshan and qijianfeng batholiths in the tongbai orogen consist mainly of porphyritic hornblende biotite monzogranite biotite monzogranite and biotite syenogranite which are variably intruded by lamprophyre diorite and syenogranite dykes mafic microgranular enclaves commonly occur in the hornblende biotite monzogranite whereas surmicaceous enclaves are found in the biotite monzogranite both batholiths have zircon u pb ages ranging from ca 139 to 120 ma indicating their emplacement in the early cretaceous the hornblende biotite monzogranite has an adakitic affinity marked by relatively high sr y and la yb n ratios lack of eu anomalies low mgo and ni contents and na2o k2o its chemical compositions combined with enriched sr nd isotopic signatures suggest formation by dehydration melting of mafic rocks in a thickened lower crust this thickened crust resulted from the permo triassic subduction collision between the north china and south china blocks and persisted until the early cretaceous the biotite monzogranite and biotite syenogranite have low al2o3 cao and sr contents low rb sr feot mgo and na2o k2o cao ratios and flat hree patterns with moderate to weak eu anomalies they were produced by partial melting of crustal materials under relatively low pressure partial melting at different crustal levels could have significantly contributed to mechanical weakening of the crust the diorite and lamprophyre dykes show linear trends between sio2 and major or trace elements on harker diagrams with two lamprophyre samples containing normative nepheline and olivine these rocks have high la yb and dy yb ratios both displaying co variation with contents of yb they were originated from relatively deep lithospheric mantle followed by fractionation of olivine clinopyroxene apatite fe ti oxides extensive partial melting in the lithospheric mantle indicates relatively high temperatures at this level we suggest that the presence of adakitic magmas thickened but weakened crust and high temperatures in the lithosphere mantle point to lower crustal delamination in the early cretaceous in the tongbai orogen

high precision pb isotope measurements reveal magma recharge as a mechanism for ore deposit formation examples from lihir island and conical seamount papua new guinea abstract lihir island part of the tabar lihir tanga feni island chain in the sw pacific hosts the giant ladolam gold deposit polymetallic gold mineralization also was discovered offshore at conical seamount in 1998 during rv sonne cruise so 133 the seamount and the island are composed mainly of trachybasalts and basaltic trachyandesites although some monzonites are found on lihir sr isotopic analyses of mineralized samples from the island and the seamount suggest that most of the sr is derived from the local alkaline lavas elevated 87sr 86sr ratios in some of the samples suggest that during the waning stages of the hydrothermal system part of the sr was contributed either from seawater or the thick sequence of marine sediments underlying the island high precision pb isotopic analyses conducted with mc icp ms show that the ores and volcanic rocks share similar pb isotopic compositions suggesting that the pb in the mineralized zones was ultimately derived from local magmatic sources the pb isotopic data however reveal small but significant differences between the mineralized zones and the associated host lavas mineralized samples from lihir have slightly less radiogenic lead isotopic ratios than their host lavas these lead isotopic compositions are similar however to some of the fresh lavas recovered from conical seamount and to a monzonite intrusion underlying the ladolam deposit lead isotopic ratios in mineralized samples from conical seamount are slightly more radiogenic than their host lavas and similar to those of fresh lavas recovered from nearby tubaf and edison seamounts petrographic observations and microprobe elemental analyses reveal a complex magmatic history for the magma chamber inferred beneath conical seamount based on zoning patterns in the conical seamount clinopyroxenes it appears that a sub seamount magma chamber was recharged with a mafic magma similar to the most primitive volatile rich and xenolith bearing lavas recovered from tubaf and edison seamounts rapid cooling of this mafic magma accompanied by exsolution of metal bearing fluids in the relatively shallow magma chamber is proposed as the mechanism that ultimately led to the formation of the gold mineralization similarity of the lead isotopic ratios between the ores and the mafic magmas in the area indicate that the ore metals were primarily derived from the recharging magma not by hydrothermal leaching of the host lavas suggesting that ore deposit genesis can be related to a single volatile rich magmatic pulse within otherwise barren polyphased volcano magmatic complex

glass production in late antiquity and the early islamic period a geochemical perspective first millennium ad glass production was divided between a relatively small number of workshops that made raw glass and a large number of secondary workshops that fabricated vessels glass compositions reflect the primary glassmaking source for most of the period egyptian mineral soda was fused with lime bearing siliceous sand to produce soda lime silica glass the location of the belus glassmaking sand which is known from the classical literature is located on that part of the levantine coast where iron contents are lowest sr 87 sr 86 of primary glass from workshops in the levantine region is close to that of modern seawater and confirms the use of beach sand which contained shell heavy mineral assemblages of levantine beach sands are dominated by hornblende hence the primary glasses are characterized by very similar trace element signatures glasses believed on archaeological grounds to have been made in other regions for example in inland egypt may have higher sr 87 sr 86 reflecting terrigenous sources of lime and have different trace element signatures compositional data for glasses from as far away as britain suggest origins of the glass material in the eastern mediterranean recycling of old glass may be recognized by the presence of elevated transition metals the use of plant ash as a flux became dominant practice in the ninth century and preliminary data for plant ash glasses from the early islamic world indicate that primary production centres may be separated using strontium and oxygen isotopes as well as by major and trace elements

intraplate type magmatism in a continent island arc collision zone porgera intrusive complex papua new guinea cogenetic volatile rich alkali basalts and gabbros hawaiites and mugearites occur in the late miocene age epizonal porgera intrusive complex which is temporally and spatially associated with rich gold silver mineralization the least evolved rocks show enrichments in light rare earth elements la yb sub cn 15 19 and other incompatible elements e g ba la approx 8 10 la nb approx 0 6 0 7 sr nd approx 25 characteristic of intraplate alkalic basalts and have isotopic compositions sup 87 sr sup 86 sr approx 0 7035 epsilon sub nd approx 6 sup 206 pb sup 204 pb approx 18 66 sup 207 pb sup 204 pb approx 15 56 sup 298 pb sup 204 pb approx 38 55 consistent with derivation from a time averaged depleted mantle source the porgera intrusive complex was emplaced at 6 ma in jurassic cretaceous shelf facies sedimentary rocks near the edge of the australasian plate intrusion appears to have occurred in a back arc environment during subduction of an oceanic microplate segment on two sides beneath the continental margin and an island arc we suggest that this unusual tectonic setting promoted partial melting of asthenospheric source materials that were perhaps modified by deep 150 km subduction processes to form alkalic magmas with intraplate character

helium and argon isotope geochemistry of alkaline intrusion associated gold and copper deposits along the red river jinshajiang fault belt sw china abstract the red river jinshajiang strike slip fault zone on the eastern margin of the tibetan plateau was originally produced by the india eurasia collision 60 70 myr ago numerous post collisional mantle derived alkaline igneous rocks with ages of 40 30 ma have been intruded along this fault zone in recent years several copper and gold deposits associated with the alkaline intrusions of this region were discovered such as the yao an and beiya gold deposits and the yulong and machangqing copper deposits studied in this paper the mineralised intrusions are felsic with sio 2 ranging from 61 4 to 67 7 wt k 2 o na 2 o from 8 1 to 11 5 wt and k 2 o na 2 o 1 the deposits are located at both the exo and endo contact zones of the intrusions the mineral deposits are of hydrothermal origin with the ore forming temperatures mainly in the range 150 450 c this paper presents he and ar isotope analyses of these four deposits the concentrations of 4 he trapped in fluid inclusions of pyrites from the ores are 0 7 54 1 10 6 cm 3 stp g 1 and those of 40 ar are 0 6 7 3 10 6 cm 3 stp g 1 3 he 4 he ratios are 0 3 2 5 ra ra represents the 3 he 4 he ratio of air 1 39 10 6 40 ar 36 ar ratios are 316 1736 and 3 he 36 ar ratios are 0 2 11 2 10 3 generally the 3 he 4 he 40 ar 36 ar and 3 he 36 ar ratios for the gold deposits are higher than those for the copper deposits we suggest that the ore forming fluids of both gold and copper deposits were differentiated from the mantle derived alkaline magmas but were diluted by modified air saturated water masw that experienced intensive interaction with crustal rocks however the magmatic fluids responsible for the gold deposits were less extensively diluted by masw resulting in higher 3 he 4 he 40 ar 36 ar and 3 he 36 ar ratios than the copper deposits

active deformation in the new guinea fold and thrust belt seismological evidence for strike slip faulting and basement involved thrusting the new guinea fold and thrust belt forms a 1000 km long mountain chain running the length of new guinea but its trend is oblique to the predicted convergence direction a large component of left lateral shear across the mountain belt is expected but geological evidence for such motion is ambiguous surface structures in the fold and thrust belt show mountain building by folding and thrusting supposedly thin skinned of pre pliocene continental shelf sediments although some evidence indicates that active faulting penetrates the basement we examine the active deformation of this mountain belt by using teleseismic long period p and sh waveforms short period p waveforms and p wave first motions to determine reliable focal mechanisms and source depths for the 18 largest earthquakes in the new guinea fold and thrust belt since 1964 eight thrust events with seismic moments m0 between 1017 and 2 1018 n m are distributed throughout the thrust belt at depths ranging from 0 5 to 45 km and have steeply dipping fault planes that strike parallel to surface structural trends six of the eight events are between 11 and 25 km deep the steep dips of nodal planes the depths of these earthquakes and the known thicknesses of the sedimentary section show that thrust faulting is not exclusively thin skinned i e confined above a shallow detachment surface but penetrates the crystalline basement at high angles most earthquakes in the western half of the thrust belt including the three largest earthquakes examined m0 5 1018 n m show strike slip faulting with the probable fault plane oriented east west so that displacement is left lateral the nine strike slip earthquakes account for almost 90 of the observed seismic moment in the period 1964 1985 and indicate a seismic slip rate of 5 25 mm a all but one of these strike slip earthquakes are shallower than the thrust earthquakes suggesting that east northeast convergence across the fold and thrust belt is accommodated by a decoupled system of strike slip and dip slip faults while the great height of the mountain belt is an obvious indication that crustal thickening is an important consequence of plate motion in new guinea seismological evidence suggests that translation by strike slip faulting may play a much greater role than previously recognized by analogy in older collisional orogens whose motions can be inferred only from remnant structures and not from seismicity strike slip faulting may have been equally important in accommodating convergence we estimate from seismic moments of earthquakes and from the volume of the mountains that deformation of the new guinea highlands may account for 5 20 of the total convergence between the pacific plate and australia in new guinea

small scale variations of 87 sr 86 sr isotope composition of barite in the madjarovo low sulphidation epithermal system se bulgaria implications for sources of sr fluid fluxes and pathways of the ore forming fluids the madjarovo ore district is located in the volcanic rocks of the oligocene madjarovo shield volcano and underlying precambrian and palaeozoic high grade metamorphic rocks of the rhodope massif bulgaria it is situated in the eastern part of a large tertiary magmatic metallogenic belt which resulted from the cretaceous collision of serbo macedonian and rhodope massifs with the pelagonian microplate the low sulphidation base and precious metal mineralisation at madjarovo 32 1 ma formed shortly after the cessation of the shoshonitic and high k calc alkaline volcanic and intrusive activity 32 6 32 2 ma and is spatially and temporally related to late monzonite trachyte intrusions the strontium isotope composition of five barite samples hosted in the volcanic sequence is used to characterise the source s of sr in associated base and precious metal mineralisation and to infer fluid fluxes and pathways of the ore forming fluids the 87sr 86sr compositions for barite samples from ore veins fall in the range 0 70854 0 70955 with the lowest and highest values found in the smaller brecciated veins barite from a thin vein in the basal latite lava flow has the most radiogenic 87sr 86sr isotope composition likely reflecting the involvement of more radiogenic metamorphic basement rocks the least radiogenic composition was determined for barite from a similar vein high in the volcanic pile 500 600 m above the metamorphic basement barites from large veins taken from different stratigraphic positions in the host lavas have almost identical isotopic compositions in the range 0 70883 0 70899 the 87sr 86sr values for barite within the deposit are higher than those for host magmatic rocks 0 70775 0 70863 but lower than those of the metamorphic basement 0 7110 0 7278 it is concluded that strontium in the fluids is a mixture of predominantly magmatic strontium and a variable amount of more radiogenic sr from the metamorphic basement rocks the ore bearing fluid may have been derived from a shallow 1 4 km silicic pluton with an initial 87sr 86sr value 0 7080 because of the short distance to the site of deposition the fault focused ore fluids maintained homogeneous strontium predominantly magmatic strontium isotope ratios with slight contribution s of more radiogenic strontium from the metamorphic basement rocks larger variations in the small brecciated veins can be explained by more extensive fluid rock interaction with local host metamorphic or volcanic rocks

lead in the getchell turquoise ridge carlin type gold deposits from the perspective of potential igneous and sedimentary rock sources in northern nevada implications for fluid and metal sources lead isotope compositions of bulk mineral samples fluorite orpiment and realgar determined using conventional techniques and of ore stage arsenian pyrite using the sensitive high resolution ion microprobe shrimp in the getchell and turquoise ridge carlin type gold deposits osgood mountains require contribution from two different pb sources one pb source dominates the ore stage it has a limited pb isotope range characterized by 208pb 206pb values of 2 000 to 2 005 and 207pb 206pb valuesof 0 8031 to 0 8075 as recorded by 10 mm diameter spot shrimp analyses of ore stage arsenian pyrite these values approximately correspond to 206pb 204pb of 19 3 to 19 6 207pb 204pb of 15 65 to 15 75 and 208pb 204pb of 39 2 to 39 5 this pb source is isotopically similar to that in average neoproterozoic and cambrian clastic rocks but not to any potential magmatic sources whether those clastic rocks provided pb to the ore fluid cannot be unequivocally proven because their pb isotope compositions over the same range as in ore stage arsenian pyrite are similar to those of ordovician to devonian siliciclastic and calcareous rocks the pb source in the calcareous rocks most likely is largely detrital minerals since that detritus was derived from the same sources as the detritus in the neoproterozoic and cambrian clastic rocks the second pb source is characterized by a large range of 206pb 204pb values 18 34 with a limited range of 208pb 204pb values 38 1 39 5 indicating low but variable th u and high and variable u pb values the second pb source dominates late and postore stage minerals but is also found in preore sulfide minerals these pb isotope characteristics typify ordovician to devonian siliciclastic and calcareous rocks around the carlin trend in northeast nevada petrologically similar rocks host the getchell and turquoise ridge deposits lead from the second source was either contributed from the host sedimentary rock sequences or brought into the hydrothermal system by oxidized ground water as the system collapsed late ore and postore stage sulfide minerals pyrite orpiment and stibnite from the betze post and meikle deposits in the carlin trend and from the jerritt canyon mining district have pb isotope characteristics similar to those determined in getchell and turquoise ridge this observation suggests that the pb isotope compositions of their ore fluids may be similar to those at getchell and turquoise ridge two models can explain the pb isotope compositions of the ore stage arsenian pyrite versus the late ore or postore sulfide minerals in either model pb from the ordovician to devonian siliciclastic and calcareous rock source enters the hydrothermal system late in the ore stage but not to any extent during the main stage of ore deposition in one model ore stage pb was derived from a source with pb isotope compositions similar to those of the neoproterozoic and cambrian clastic sequence transported as part of the ore fluid and then deposited in the ore stage arsenian pyrite and fluorite the second model is based on the observation that the pb isotope characteristics of the ore stage minerals also are found in some ordovician to devonian calcareous and siliciclastic rocks hence ore stage pb could have been derived locally and simply concentrated during the ore stage critical to the second model is the removal of all high 206pb 204pb 20 material during alteration it also requires the retention of only the low 206pb 204pb component of the ordovician to devonian sedimentary rocks this critical step is possible only if the high 206pb 204pb values are contained in readily dissolvable mineral phases whereas the low 206pb 204pb values are found only in refractory minerals that released pb during a final alteration stage just prior deposition of auriferous arsenian pyrite distinguishing between pb transported with the ore fluid or inherited from the site of mineral deposition is not straightforward however it is simpler to explain the pb isotope compositions of ore stage arsenian pyrite and fluorite in two different but spatially related carlin type deposits getchell and turquoise ridge with different host rocks by input of pb with the ore fluid once the limited pb in the hydrothermal fluid was exhausted by incorporation in ore stage arsenian pyrite or other ore stage minerals pb from the second source the ordovician to devonian sedimentary rock sequences became available for incorporation in some but not all of the late stage sulfide minerals

spatial variations in biosphere 87sr 86sr in britain abstract strontium isotopes are a powerful tool for investigating the geographical origins of people and animals but assignment of provenance requires reference maps and databases this paper presents a map of strontium isotope variations across britain for use in archaeological and other studies and is produced by direct measurement of strontium isotope compositions of biosphere components predominantly plants the map characterizes the biosphere signature of specific lithologies a technique that is applicable worldwide marine derived strontium from both rainwater and coastal sea splash and spray is an important contribution to the british biosphere particularly on the western seaboard which is subject to high rainfall and prevailing westerly winds supplementary material methods full data table and full size version of figure 1b are available at http www geolsoc org uk sup18388

rb sr and k ar dating of precambrian clays and glauconies abstract isotopic dating of sedimentary material requires a different approach from the isotopic dating of plutonic volcanic and high grade metamorphic rocks the present contribution presents mineralogical criteria such as clay composition illite or glauconite crystallinity indices illite or glauconite polymorphic types sedimentological criteria such as mechanisms of genesis as well as chemical criteria such as the sr composition and content of the carbonates associated with the studied clays and glauconies these criteria help in the interpretation of the numerical results and should always be considered in attempts to obtain radiometric ages of sedimentary clays and or glauconies it is emphasized that a difference between measured and expected age seldom means that the chronometer used is inaccurate but it commonly means that a parameter was not recognized or even disregarded a critical review of the uncertainties typically related to sedimentary dating is also proposed they may be grouped into four types 1 genetic uncertainties dependent on the genetic mechanisms of the studied minerals 2 historic uncertainties dependent on the alteration events imprinted in the sediments since deposition weathering diagenesis and or tectono metamorphism 3 stratigraphic uncertainties dependent on the precision of the sample location in the sedimentary sequence and 4 analytic uncertainties dependent on the representativeness and the preparation of the samples as well as the accuracy of the measurements a reliability test of the rb sr and k ar methods as applied to dispersed precambrian clays and glauconies is discussed from an extensively studied sequence from west africa it shows that the k ar system of these minerals is much more sensitive to later modifications than the rb sr system therefore the two techniques may be complementary in geochronological studies of precambrian sediments

multiple mineralization events in the zacatecas ag pb zn cu au district and their relationship to the tectonomagmatic evolution of the mesa central mexico abstract mineralization in the zacatecas district is polymetallic ag zn pb cu and au and occurs as skarn type and epithermal veins formed in different metallogenetic stages the oldest mineralization in the district is skarn type cu rich with lesser zn pb ag and is considered to be close in age to felsic dikes and plugs dated at 51 ma epithermal mineralization occurs in both low and intermediate sulfidation styles intermediate sulfidation veins the veta grande mala noche el bote and la cantera veins are polymetallic ag rich hosted in ese to se striking structures and were formed at 42 ma adularia 40ar 39ar isochron age from veta grande of 42 36 0 18 ma 2 mswd 0 76 low sulfidation au ag mineralization occurs in the n s trending el orito vein system which yielded an adularia 40ar 39ar isochron age of 29 19 0 20 ma 2 mswd 1 8 these ages and the differences in structural orientation indicate that the two styles of epithermal mineralization are temporally and tectonically unrelated the mineral paragenesis of the mala noche deposit consists of early skarn type cu mineralization overprinted by later epithermal pb zn ag veins skarn type minerals include relicts of prograde silicate minerals hedenbergite and garnets retrograde silicate minerals ilvaite grunerite stilpnomelane epidote clinochlore and ore minerals chalcopyrite pyrite sphalerite galena magnetite wolframite and minor bismuthinite epithermal mineralization is characterized by layered to vuggy quartz veins and breccias with phyllic wallrock alteration quartz sericite illite the veins consist of quartz calcite dolomite and ankerite with variable amounts of base metal sulfides sphalerite galena pyrite minor chalcopyrite and rare acanthite and stromeyerite the veta grande epithermal mineralization was emplaced in two main stages of ag rich quartz veining with narrow selvedges of phyllic quartz sericite wallrock alteration stage i consist of quartz calcite and minor adularia intergrown with pyrite followed by sphalerite galena and lesser chalcopyrite acanthite pyrargyrite and jamesonite stage ii mineral paragenesis is similar to stage i but is characterized by amethystine quartz and contains less abundant sulfide minerals the ore mineral paragenesis of the el compas vein within the el orito system consists of quartz adularia calcite and chalcedony with minor pyrite followed by rare aguilarite naumannite electrum and native gold the skarn type and intermediate sulfidation mineralization is coeval with eocene subduction related magmatism in the zacatecas area which is constrained by zircon u pb ages for igneous rocks between 51 and 42 ma the emplacement of these magmas was controlled by the same regional scale ese to se trending transtensional structures that controlled the skarn type and intermediate sulfidation deposits this mineralization is thus interpreted to be related to the last stages of subduction volcanism in central mexico under transtensional stress conditions in contrast no nearby magmatism is clearly related to the oligocene low sulfidation system however its age and structural orientation n s combined with a regional change in magma composition from eocene calc alkaline to oligocene bimodal volcanism in central mexico suggest that the low sulfidation mineralization is related to post subduction continental extension processes reflecting the beginning of basin and range tectonics

mafic dike swarms in the south shetland islands volcanic arc unravelling multiepisodic magmatism related to subduction and continental rifting eight groups of mafic dikes and related high level stocks cut triassic accretionary complex and mesozoic magmatic arc formations on livingston island some are affected by silicic sericitic alteration related to cretaceous hydrothermal activity and propylitic epidosite alteration analogous to that in ocean floor sheeted dikes alteration was accompanied by major and trace element metasomatism ar ar analysis of the freshest rocks indicates five intrusive events some of which are unexpectedly young groups 1 3 were intruded in the mid to late cretaceous 108 74 ma and were coeval with the calc alkaline arc between 70 and 50 ma relatively rapid and oblique plate convergence led to strike slip tectonism and a pause in magmatism at 52 ma orthogonal slow convergence resulted in extensional faulting and emplacement of calc alkaline group 2 and primitive tholeiitic dikes groups 4 6 between 51 and 45 ma extension of antarctic peninsula southern south american crust culminated in emplacement of mafic to intermediate medium grained plutons and group c porphyries between 44 and 36 ma localized hydrothermal flow along fault zones resulted in partial to complete argon loss from nearby cretaceous lavas and ar ar reset ages of 40 ma in mid cretaceous hydrothermal k feldspar primitive olivine basalts group d and epithermal carbonate veins 31 29 ma were emplaced during along arc extension accompanying the opening of drake passage and powell basin excess argon occurs in two forms strongly held in melt inclusions in the primitive tholeiites and weakly held in some secondary alteration there is no radiometric evidence in the area studied for magmatism related to late cenozoic subduction nor to the pleistocene recent opening of the back arc bransfield rift

gold deposit continuum a genetic model for lode au mineralization in the continental crust a model is presented dealing with the relations between convection in the continental crust and the formation of lode au deposits sufficient permeability exists in the brittle rheologic regime in large sections of the continental crust to permit convection of meteoric water on both local and regional scales the solubility of au in this fluid is high enough to permit the mobilization of quantities of au which are sufficient to form ore grade deposits this general process is shown to be responsible for a continuous series of deposit types ranging rom 1 2 km deep epithermal deposits to intermediate depth carlin type deposits to 10 12 km deep mesothermal deposits major geologic and geochemical differences among the deposit types are products of fluid convection at variable depths and the resulting effects on water rock ratios and gas solubilities

sm nd direct dating of the collins bay hydrothermal uranium deposit saskatchewan primary pitchblende mineralization from the collins bay unconformity type uranium deposit in northern saskatchewan has been dated using the sm nd method the mineral isochron age of 1281 80 ma is in good agreement with other geologic and geochronological data and demonstrates the potential of the sm nd technique for the direct dating of hydrothermal mineral deposits 23 references 5 figures 7 tables

increasing magmatic oxidation state from paleocene to miocene in the eastern gangdese belt tibet implication for collision related porphyry cu mo au mineralization in the gangdese belt of southern tibet paleocene eocene magmas record the final stage of neo tethyan subduction and are associated with few economic porphyry deposits in contrast magmas formed during later stages of the india asia collision in the oligo miocene are associated with several large porphyry cu mo au deposits especially in the eastern part of the belt 89 e 93 e in a previous study we showed that these oligocene miocene magmas were more hydrous than the earlier paleocene eocene magmas in this study we show that the later magmas were also more oxidized paleocene eocene rocks from the eastern gangdese belt are characterized by low zircon ce4 ce3 ratios mostly 50 32 3 141 9 average 74 3 30 1 n 33 and higher f o2 values fmq 0 8 2 9 average 1 8 0 8 n 6 estimated from magnetite ilmenite pairs estimates of magmatic oxidation state from amphibole compositions also show an increase from fmq 1 2 to 2 1 average 1 6 0 2 n 40 in the paleocene eocene to 2 0 to 2 8 average 2 5 0 2 n 58 in the miocene sparse whole rock fe3 fe2 ratios show the same general trend the amphibole results are systematically shifted to higher fmq compared to data from magnetite ilmenite pairs but their trend is internally consistent the higher oxidation states and water contents of miocene igneous rocks from the eastern gangdese belt may explain their unique association with large porphyry deposits in tibet because it has been shown from other studies that the potential for hydrous calc alkaline magmas to transport au is maximized near fmq 1 0 and cu at higher f o2 fmq 1 0 in comparison the somewhat less oxidized and less hydrous paleocene eocene magmas would have been less fertile for the formation of such deposits we suggest that these differences reflect the tectonomagmatic evolution of the cenozoic gangdese collisional orogen from early collision related magmatism in the paleocene eocene to late collisional magmatism in the miocene asthenospheric upwelling following slab breakoff in the miocene caused interaction between mantle derived magmas and previously subduction modified tibetan lithosphere the resultant evolved magmas had high oxidation states and water contents which are favorable properties for the subsequent formation of magmatic hydrothermal porphyry cu mo au deposits

high precision 40ar 39ar dating of two consecutive hydrothermal events in the chuquicamata porphyry copper system chile 40ar 39ar data have been used to resolve the relatively high temperature 350 c potassic alteration event at chuquicamata from the lower temperature 350 c quartz sericite alteration phase four k feldspars from the potassic zone yielded plateau large domain ages in the range 32 9 33 8 ma mean 33 4 0 3 ma the fact that apparent ages of co existing biotites span a similar range suggests that there was very rapid cooling through feldspar and biotite closure temperatures especially if these minerals closed at substantially different temperatures multi domain diffusion modelling of the feldspar data suggests that at this time cooling was rapid ca 60 80 my through closure temperatures of 330 440 c as these closure temperatures are substantially higher than the ones normally assumed for biotite i e about 300 c they and or the calculated cooling rates may be unreliable four sericites from the quartz sericite zone yielded variably discordant age spectra the least discordant of these has a plateau at 31 1 0 3 ma and this is interpreted as the time of this alteration event some k feldspars in the quartz sericite zone were completely overprinted at this later time and most of the other ones studied show partial overprinting overprinting appears to decrease as distance from the central quartz sericite zone increases in the more remote areas effects of the later event appear constrained to narrow fault controlled alteration zones the data are consistent with a conceptual model proposed by zentilli et al zentilli m leiva g rojas j graves m c 1994a the chuquicamata system revisited extended abstract society of economic geologists symposium copper deposits of the andes new developments concepcion chile october 17 18 1994 proceedings 2 1647 1651 wherein potassic alteration by late magmatic fluids occurred at depth prior to rapid exhumation of the system quartz sericite alteration perhaps caused by a new pulse of porphyry intrusion at depth occurred some 2 3 my later principally in the core of the deposit although alteration did propagate outwards through the action of hydrothermal fluids passing through a brittle relatively shallow fracture system

the alkaline porphyry associated yao an gold deposit yunnan china rare earth element and stable isotope evidence for magmatic hydrothermal ore formation the yao an vein type gold deposit is located in the ailaoshan jinshajiang alkaline intrusive belt yunnan province china and is associated both in time and space with 33 5 1 0 ma old alkaline intrusions the gold mineralization is associated with potassic wall rock alteration the ree distribution patterns of secondary k feldspar are generally similar to those of the igneous perthite but with about seven times higher total ree abundances the alteration is ascribed to a high ree magmatic fluid derived from the yao an alkaline intrusive suite the hydrothermal yao an gold deposit formed during two gold bearing stages i e a sulfide pyrite stage and a sulfide oxide stage pyrite specularite the ree abundance of early stage i pyrite is relatively high with strong enrichment in lree la yb n of 40 290 generally positive eu anomalies eu eu 0 86 1 55 and ree patterns very similar to those of secondary feldspar in contrast the later stage ii pyrite has much lower ree concentrations and lower la yb n of 5 5 11 8 eu eu of 0 49 0 76 and flat chondrite normalized spidergram patterns the stage i pyrite has 34s in the range of 2 2 to 3 2 and overlaps with regionally distributed pyrite in least altered syenite porphyry in contrast stage ii pyrite has much higher 34s values between 7 8 and 16 5 carbon isotope data for four samples from stage ii revealed 13c pdb values between 6 and 8 these stable isotope and ree data suggest that magmatic fluids of the alkaline intrusions caused both potassic alteration and stage i sulfide mineralization the system evolved from stage i to stage ii mineralization by influx of meteoric fluids with relatively heavier sulfur although calcite carbon isotope data suggest that the co2 remained dominantly of magmatic origin

subduction related hot spring type gold mineralization in the central tengchong block southwestern china abstract numerous present day hot springs and associated ancient lianghe gold deposit occur in the central tengchong block sw china associated with oceanic crust subduction from the center outwards the alteration zones in the hot spring of the rehai consist of alunite and silica in a breccia zone a kaolinite and illite zone and a chlorite illite kaolinite zone the gas phase in the hot spring is dominated by co 2 with 13 c ranging from 4 5 to 7 2 these features indicate a major magmatic origin of the hot spring gas the 3 he 4 he ratios in the gas from central tengchong vary from 1 56 to 5 88 approaching to those of the morb this is probably related to the ongoing subduction of the indian plate beneath the tengchong block the hot spring waters in the central tengchong block have much higher content of au and other elements e g ag as sb and tl than other typical hot springs such as hasbrouck sulfer etc in the world the hot springs with more mantle gas contribution and higher water temperature show more elevated au contents which implies that the metal in the hot spring was most likely contributed from the magmatic gas the compiled d o isotopes of the hot spring waters from previous studies suggest that they are primarily derived from meteoric water a genetic model for the hot spring system in the central of tengchong has been tentatively established the gold charged hot spring is genetically connected to an active magma chamber which was produced by subduction of oceanic crust the chamber is driving long term surface meteoric circulation and constantly releasing magmatic volatiles enriched with metals as hot spring water mixed with magmatic volatiles causing decompression boiling and forming hydraulic fractures to penetrate the roof of siliceous sinter which induced the continuing metal deposition and formed the lianghe hot spring

contrasting platinum group element concentration patterns in cumulates of the bushveld complex chromite rich lithologies in both the lower and the critical zones of the bushveld complex in the potgietersrus area display flat chondrite normalized platinum group element pge concentration patterns whereas those of associated sulphide bearing but chromite poor rocks are considerably steeper the low pt pd os ir ru ratio in the chromite bearing rocks is maintained irrespective of the amount of sulphide or chromite in the rock this feature suggests that the partitioning of the individual pge into pge bearing phases during conditions in the magma under which crystallization of chromite in excess of the normal cotectic amounts was favoured differed from conditions under which an immiscible sulphide liquid separated from the same magma in the absence of enhanced chromite crystallization these changes in the partitioning coefficients of the individual pge are considered to reflect changes in the solubility of these elements in response to variations in the intensive parameters in the magma necessary to bring about the enhanced crystallization of chromite

40ar 39ar ages for plutons of the monteregian hills quebec evidence for a single episode of cretaceous magmatism 40ar 39ar data for biotite and amphibole from six plutonic complexes of the monteregian petrographic province in quebec canada are presented generally uncomplicated and concordant incremental heating spectra are observed although there is evidence for minor excess 40ar at several localities biotites and amphiboles are concordant consistent with rapid cooling of the high level complexes ages for different phases of individual complexes are analytically indistinguishable and suggest that the various principal lithologies are cogenetic the 40ar 39ar ages of six monteregian complexes st hilaire rougemont johnson shefford brome megantic all fall within the very restricted range of 124 1 m y it is suggested that all of the monteregian plutons with the exception of oka were formed within a short interval 1 2 m y during the cretaceous consistent with paleomagmetic results the new ages along with geochemical characteristics support derivation of the alkaline magmas by a fixed mantle hot spot which also produced broadly similar complexes in new england and the chain of new england seamounts

seismotectonics of new guinea a model for arc reversal following arc continent collision the structure and evolution of the northern new guinea collision zone is deduced from international seismological center isc seismicity 1964 1985 new and previously published focal mechanisms and a reexamination of pertinent geological data a tectonic model for the new guinea margin is derived which illustrates the sequential stages in the collision and suturing of the bewani toricelli adelbert finisterre huon new britain arc to central new guinea followed by subduction polarity reversal in the west east of 149 e the solomon plate is being subducted both to the north and south bringing the new britain and trobriand forearcs toward collision west of 149 e the forearcs have collided and together they override a fold in the doubly subducted solomon plate lithosphere which has an axis that is parallel to the strike of the ramu markham suture and that plunges westward at an angle of 5 beneath the coast ranges of northern new guinea active volcanism off the north coast of new guinea is related to subduction of the solomon plate beneath the bismarck plate active volcanism of the papuan peninsula and quaternary volcanism of the new guinea highlands are related to slow subduction of the solomon plate beneath the indo australian plate along the trobriand trough and the trough s former extension to the west respectively from 144 148 e seismicity and focal mechanisms reveal that convergence between the sutured bismarck and indo australian plates is accommodated by thrusting within the finisterre and adelbert ranges and compression of the new guinea orogenic belt together with basement involved foreland folding and thrusting to the south the finisterre block overthrusts the new guinea orogenic belt whereas the adelbert block is sutured to new guinea and overthrusts the oceanic lithosphere of the bismarck sea along the new guinea trench west of 144 e seismicity defines a southward dipping wadati benioif zone and focal mechanisms indicate oblique subduction only this oldest westernmost portion of the collision has progressed past suturing to a full reversal in subduction polarity

comparison of metasomatic reactions between a common co 2 rich vein fluid and diverse wall rocks intensive variables mass transfers and au mineralization at alleghany california the gold deposits at alleghany california are typical of many epigenetic gold bearing hydrothermal vein systems in metamorphic terranes worldwide detailed analyses of alteration halos in serpentinite mafic amphibolite and granite wall rocks at alleghany indicate that widely contrasting deposit types ranging from fuchsite carbonate schists to pyrite albitites resulted when different wall rocks interacted with the same externally derived co 2 rich hydrothermal vein fluid patterns of element redistribution within halos and among lithologic units suggest a complex process involving fluid flow along vein fractures and diffusion or infiltration normal to the veins wall rocks locally controlled both the directions and magnitudes of chemical fluxes across vein walls vein fluids at constant temperature and pressure with relatively constant ratios of mobile species na super k super h super and co 2 h 2 s h 2 caused contrasting divergent reactions in proximal alteration zones because the wall rocks controlled the ratios of relatively immobile species al cr fe mg and fe mg and the component activities of hydrothermal mineral solid solutions thus pyrite and albite were stable with respect to the relatively pure siderite and k mica that occur sparingly in proximal granite zones but they were not stable with respect to the relatively impure siderite and k mica that characterize proximal serpentinite zones partly as a result of divergent reactions altered rocks exhibit contrasting mobile element concentration gradients e g ca ba sr rb na k c s and imply local sources and sinks and possibly short residence times for some of these components along discordant fluid flow paths calculated reaction coefficients for h super h 2 and h 2 s indicate that different wall rocks at different stages of alteration may have inhibited or promoted gold precipitation the distribution of gold and its calculated solubility are consistent with its having been precipitated from au hs 2 super complexes in response to pyritization of granite fluid desulfidation and reduction and possibly in part in response to dolomitization and sericitization of serpentinite fluid acidification strata bound disseminated orebodies may be expected in a variety of other host rocks with higher than average fe mg ratios where pyrite was stable with respect to intermediate magnesite siderite as in the granite it is proposed that the co 2 rich vein fluids approached partial equilibrium with alteration assemblages in the average or dominant rocks along their flow paths simpler but radically different proximal assemblages were then produced where the fluids encountered chemically extreme rock types fluid rock interaction equilibria can account for much of the diversity in the mineralogy and chemistry of gold deposits in heterogeneous metamorphic terranes while maintaining a simple genetic classification based on the distinctive chemistry of the hydrothermal fluids comparative studies of multicomponent phase equilibria and mass exchanges can provide both rationalizations and predictions of local wall rock effects on geochemical anomalies and the locations of ore deposits in lithologically heterogeneous terranes simultaneous consideration of contrasting high variance phase deficient assemblages along discordant veins is suggested as a general method for quantifying fluid intensive variables in metasomatic systems

trace element discriminants between egyptian and mesopotamian late bronze age glasses the origins of late bronze age glass artefacts found throughout the mediterranean and near and middle east remain controversial previous analyses of major and minor elements in both egyptian and mesopotamian glasses have proven equivocal revealing no significant compositional differences between glasses from these two regions here we present new laser ablation icpms analyses of 32 trace elements in 54 samples of both egyptian and mesopotamian blue and colourless glasses our results show that there are consistent differences between these two suites which are not related to the colorant and clearly indicate the use of different raw materials and or manufacturing processes compositional variations are related to geological controls hence this method holds promise for the development of a minimally destructive technique for discriminating between archaeological glasses of different provenance which will be essential in the interpretation of ancient trade patterns and contacts

pleistocene volcanic rocks from the fly highlands province of western papua new guinea a note on new sr and nd isotopic data and their petrogenetic implications abstract rb sr and sm nd isotopic and trace element abundance values have been determined for 15 mafic and intermediate rocks from six pleistocene volcanic centres of the fly highlands province 87sr 86sr and n d values range from 0 70362 to 0 70540 and 1 9 to 5 9 respectively these new data can be accounted for by contamination of mantle derived magmas by the continental crust through which the magmas have risen they do not however preclude derivation of some of the sr and nd from subducted crust nor are they inconsistent with sr and nd enrichments having taken place by means of mantle metasomatic events nevertheless there is no benioff zone beneath the fly highlands province although there is geological evidence for cretaceous subduction a preferred interpretation is that uncontaminated mantle derived magmas are related to the pliocene crustal uplift that caused the development of the highlands and which formed in response to a mid tertiary continent island arc collision

linking high grade gold mineralization to earthquake induced fault valve processes in the porgera gold deposit papua new guinea processes responsible for high grade gold enrichment are of significant economic interest and have long been elusive structural and geophysical models postulate that earthquake induced fault valve processes are frequently responsible for gold mineralization in structurally controlled hydrothermal systems although a comprehensive geochemical record of these processes has been lacking the porgera gold deposit papua new guinea provides an opportunity to examine the complex processes surrounding high grade au mineralization in structurally controlled hydrothermal systems new high resolution trace element and sulfur isotope data obtained in situ by laser ablation inductively coupled plasma mass spectrometry and sensitive high resolution ion microprobe stable isotope analysis of pyrite across a well constrained paragenetic sequence reveal a temporal evolution in fluid chemistry the data uncover a stratigraphy of repeated high au negative 34 s and low au positive 34 s zones within individual pyrite crystals present in the highest grade gold event stage ii results provide the first clear geochemical documentation of rapid complete switching in ore forming processes preserved in individual pyrite crystals throughout an evolving hydrothermal system it is postulated that geochemical variations are attributable to rapid au deposition during pressure release due to fault failure followed by switching to more reduced rock buffered metal poor fluids from adjacent sediments as the rupture seals the cyclicity of this process is reflected in the pyrite zonation seen at the porgera deposit this study demonstrates the usefulness of applying contemporary analytical techniques to pyrite in hydrothermal systems to gain new insights into ore genesis and fluid pathways revealing that deposition of exceptionally high grade ore at the porgera deposit was the result of multiple events in a fluctuating ore forming environment

rifting of the northern margin of the australian continent and the origin of some microcontinents in eastern indonesia abstract continental australia is bounded on the east south and west sides by passive margins and the geological histories of these are well documented falvey and mutter 1981 the northern margin of the australian continent is now an active collision margin its previous history as a passive margin has rarely been examined this paper shows how the late palaeozoic to mesozoic sequence which forms the northern margin of the australian continent in the island of new guinea is readily related to the tectonic stages of a rift drift sequence rifting start of breakup began at about 230 m y ago at the permian triassic boundary the onset of seafloor spreading is marked by a post breakup unconformity and ranges in age along the northern margin of the continent from 185 m y in papua new guinea to 170 m y in irian jaya from there the age of the post breakup unconformity continues to young in a southwesterly direction along the western margin of the australian continent reflecting the opening of the indian ocean off western australia the timing of the onset of spreading in central papua new guinea is consistent with the timing of the initiation of spreading in the proto pacific ocean proposed by nur and ben avraham 1977 by the end of the jurassic the northern margin of the australian continent faced a seaway which linked the proto indian and proto pacific oceans this newly formed ocean was separated from the pre existing oceans of the neo tethys and panthalassa by a screen of continents or microcontinents the identity of this screen is discussed and it is suggested that part of it is preserved in the microcontinents of eastern indonesia

roman and late roman glass from north eastern italy the isotopic perspective to provenance its raw materials in this study the strontium neodymium and oxygen isotopic composition of roman 1st 3rd century ad and late roman glass 4th 6th century ad from adria and aquileia two of the most important archaeological sites of north eastern italy is discussed the majority of glass analysed independent from age shows values of strontium isotope ratios close to that of modern seawater indicating that the source of lime in the glass was marine shell and likely coastal sands were used in its production the nd signature of all late roman glasses from aquileia and of the majority of the roman ones from adria independent from their chemical composition is homogeneous and higher than 6 end supporting the hypothesis of an eastern mediterranean origin probably located on syro palestinian coast however the composition of late roman samples with himt signature with lower 87sr 86sr values correlated to higher contents in fe2o3 tio2 mgo and lower contents in cao suggests an area of origin for this glass on the egyptian coast in addition the different nd signatures of two adria roman glasses end 7 suggest their primary production in western mediterranean oxygen isotopes proved to be a further diagnostic method to discriminate natron and soda plant ash glass and different silica sources in the case of the soda plant ash glass the combination of isotopic and chemical data supports the hypothesis of an eastern mediterranean origin for late roman glass which may be produced in few primary workshops on the syro palestinian and egyptian coast although not necessarily in the same ateliers as have been identified so far in the case of the roman glass investigated although the majority of data suggests an eastern mediterranean origin on the basis of nd isotopes and chemical compositions the existence of other primary glass producers located in the western mediterranean can be suggested

new radiometric dating of volcanic ash layers in periadriatic foredeep basin system italy abstract new geochronological data are reported for two key ash beds interbedded in upper miocene messinian early pleistocene rocks at maccarone bellante and mosciano s angelo on the adriatic side of the italian peninsula major element chemistry of glass shards was determined with electron microprobe analysis on the younger key bed the older messinian age ash bed sampled at maccarone yields a corrected glass fission track age of 5 38 0 42 ma the younger ash bed sampled at mosciano s angelo and at two localities at bellante yields corrected glass ages of 2 1 ma similar to ages previously determined on pliocene pleistocene ash collected along the ionic coast and on the tyrrhenian side of italy apatite fission track dating of one of the bellante samples confirms the glass age whereas apatite from the mosciano s angelo sample yields an excessively old age due to the presence in the sample of a significant component of detrital grains an unusual bimodal spontaneous track diameter distribution is found in the maccarone glass similar to the distributions previously obtained on other messinian age tephra samples from the same region this anomalous track diameter distribution is interpreted as a thermal disturbance coeval with the volcanic activity that produced the pliocene pleistocene key volcanic ash bed sampled at bellante and mosciano s angelo the contemporaneity of this ash with the event detected by the maccarone glass suggests that it was related to the tectonic phase which marked with numerous erosive levels distributed along the whole apennine region the end of the pliocene sedimentary cycle and the onset of the pleistocene cycle

principles of mobilization dissolution of metals in mafic and ultramafic rocks the role of immiscible magmatic sulphides in the generation of hydrothermal gold and volcanogenic massive sulphide deposits abstract immiscible magmatic sulphides exert a major control over both the absolute precious and base metal contents of magmas and on the availability of these metals from rocks during ore formation it is concluded that only those mafic magmas that attained s saturation at a late stage are not depleted in the chalcophile elements and are hence suitable source rocks for metals in hydrothermal ore deposits immiscible magmatic sulphides are not only strongly enriched in the ore forming metals but they are also much more reactive than silicates the ore forming metals may be liberated from the sulphides at the sea floor alteration stage and deposited in interflow chemical sediments au deposits or volcanogenic massive sulphide deposits subsequent mobilization remobilization of au from the interflow sediments may concentrate au within the interflow sediments to form economic au deposits the mobilized remobilized au may also be transported to form epigenetic style gold deposits in archaean greenstone belts and in palaeozoic slate belts

the miocene gangdese porphyry copper belt generated during post collisional extension in the tibetan orogen abstract the mid miocene gangdese orogenic belt in southern tibet contains porphyry cu deposits that developed after the indian asian continental collision in the early cenozoic field work confirms that these porphyry cu deposits are temporally and spatially associated with mid miocene monzogranitic and quartz monzonitic stocks which intruded mesozoic cenozoic gangdese granitoid batholiths and surrounding triassic tertiary volcano sedimentary sequences in the lhasa terrane and formed a 50 km wide 400 km long e w belt magmatism occurred during the interval 11 2 to 19 7 ma peaking at 16 ma for the porphyry stocks this is consistent with the age 10 to 18 ma of the east west extension and subsequent ns striking normal fault systems 13 5 ma in the tibetan orogen the stocks typically occur as multiple small diameter intrusions 50 km breakdown of hornblende in the juvenile lower crust source during melting is regarded as fundamental for the formation of the fertile adakite and porphyry cu systems in south tibet the gangdese belt contains four large and three intermediate and small sized porphyry cu mo deposits and shows a continuous spectrum from typical porphyry type cu mo to skarn type zn pb mineralization twenty three samples collected from four deposits yielded a limited range of molybdenite re os isochron ages from 15 2 to 16 0 ma similar to previously reported re os ages 13 8 to 14 9 ma for other porphyry cu mo deposits in the belt suggesting that cu mo mineralization in the belt occurred in a post collisional extension environment the characteristics of porphyry cu mo deposits in the collisional zone are similar in many aspects to those in arc settings i e mineralization style alteration zoning and metal association preliminary fluid inclusion studies indicate that the early hydrothermal fluids are preserved as high temperature 450 c high salinity 40 to 47 wt nacl equiv and high pressure 1100 to 1500 bar fluid inclusions these fluids are interpreted to represent an orthomagmatic fluid that boiled episodically due to fluid overpressuring and hydrofracturing resulting in potassic alteration and precipitation of chalcopyrite molybdenite assemblages on the basis of comparison with porphyry cu deposits in arc settings we proposed that melting of thickened newly formed basaltic crust resulted in generation of cu bearing magmatic systems in the tibetan collision zone and both orogen transverse normal faults and orogen parallel thrust zones during the miocene post collisional extension constrain the temporal and spatial localization of the porphyry cu systems in the collision zone

the porgera gold deposit papua new guinea the gold deposit at porgera is located in the highlands of papua new guinea alluvial gold was first recognized in 1939 but systematic exploration did not begin until 1961 the middle miocene porgera intrusive system which was derived from melting of thickened crust was emplaced within late cretaceous sediments on the northern margin of the papuan platform about 25 km south of the central orogenic belt mineralization is associated with several medium to fine grained high level porphyritic intrusions of mafic diorite composition with alteration assemblages of calcite dolomite chlorite and sericite plus subordinate clays and quartz sediments on the margin of the intrusions are also altered sulfide mineralization consisting of pyrite sphalerite and galena is hosted by both igneous and sedimentary rocks and occurs as veins disseminations and breccia matrix the distribution of sulfides hydrothermal alteration and associated gold and silver mineralization are largely a function of the intensity and extent of fracturing which is best developed at intrusive contacts and in breccias most of the gold mineralization is hosted by sulfides in particular pyrite and arsenical pyrite about 60 percent of the gold is apparently submicroscopic and not readily recoverable on the basis of the au ag cu zn and pb contents of drill core samples five mineralization types have been defined they reflect several episodes of gold mineralization from older highly refractory to younger less refractory types although overprinting is evident discrete zones dominated by one mineralization type occur and commonly transgress rock types and alteration assemblages subdivision of the deposit on the basis of mineralization type and the use of geostatistical procedures has resulted in improved precision of reserve estimates

geochemistry of the kalatongke ni cu pge sulfide deposit nw china implications for the formation of magmatic sulfide mineralization in a postcollisional environment the kalatongke also spelt as karatungk ni cu platinum group element pge sulfide deposit containing 33 mt sulfide ore with a grade of 0 8 wt ni and 1 3 wt cu is located in the eastern junggar terrane northern xinjiang nw china the largest sulfide ore body which occupies more than 50 vol of the intrusion y1 is dominantly comprised of disseminated sulfide with a massive sulfide inner zone economic disseminated sulfides also occur at the base of the intrusions y2 and y3 the main host rock types are norite in the lower part and diorite in the upper part of each intrusion enrichment in large ion lithophile elements and depletion in heavy rare earth elements relative to mid ocean ridge basalt indicate that the mafic intrusions were produced from magmas derived from a metasomatized garnet lherzolite mantle the average grades of the disseminated ores are 0 6 wt ni and 1 1 wt cu whereas those of the massive ores are 2 wt ni and 8 wt cu the pge contents of the disseminated ores 14 69 ppb pt and 78 162 ppb pd are lower than those of the massive ores 120 505 ppb pt and 30 827 ppb pd however on the basis of 100 sulfide pge contents of the massive sulfides are lower than those of the disseminated sulfides very high cu pd ratios 4 5 104 indicate that the kalatongke sulfides segregated from pge depleted magma produced by prior sulfide saturation and separation a negative correlation between the cu pd ratio and the pd content in 100 sulfide indicates that the pge content of the sulfide is controlled by both the pge concentrations in the parental silicate magma and the ratio of the amount of silicate to sulfide magma the negative correlations between ir and pd indicate that the massive sulfides experienced fractionation

isotopic mapping reveals the location of crustal fragments along a long lived convergent plate boundary abstract new guinea has acted as the boundary between the australian and pacific plates for hundreds of millions of years strike slip movement and arc continent collisions along this boundary during the cenozoic have shuffled rocks of different age and composition in a series of terranes along the plate boundary making mapping them a considerable challenge here we report results of sr nd isotopic data obtained from rock samples from western new guinea that are representative of the different terranes these isotopic data reveal the crustal affinity of the terranes and we have used these data to map their spatial distribution the isotopic data show three distinct crustal domains underlying western new guinea palaeozic mesozoic australian continental crust 87sr 86sr 0 719594 to 0 710921 end 13 85 to 1 373 thinned transitional crust intruded by miocene pleistocene magmatic rocks 87sr 86sr 0 706524 to 0 704019 end 6 67 to 2 13 and accreted island arc crust 87sr 86sr 0 704053 to 0 703759 end 6 63 to 4 97 these data together with crustal contamination models indicate that the northern most extent of australian continental crust exists beneath the northern most section of western new guinea we also combined our isotopic data with existing data across new guinea and used these to develop an isotopic map that shows the position of the ancient australian pacific plate boundary producing results that are also consistent with broad scale seismic tomography imagery our findings provide a framework for mapping other plate boundaries particularly ancient systems where only fragmentary data exist

bio and magneto stratigraphy in the tiber valley revised abstract the stratigraphy of three sections of marine deposits in the tiber valley north of rome was investigated in detail biostratigraphic investigation covered the valle ricca tini quarry fosso liano and lugnano sections the valle ricca and lugnano sections show continuity in sedimentation from the latest pliocene to early pleistocene in the fosso liano section upper pliocene sediments are missing as in other outcrops in central italy and the lower pleistocene deposits lie unconformably on the middle pliocene ones possible causes of these features are discussed magnetostratigraphy and rock magnetism analyses were performed only in the valle ricca section the data shows that the previously recognised reversal around a volcanic level is the effect of strong changes in the magnetic mineralogy and it is not representative of true geomagnetic changes

excess40ar in metamorphic rocks from broken hill new south wales implications for40ar 39ar age spectra and the thermal history of the region 40ar 39ar age spectrum analyses of samples from broken hill new south wales indicate that the region has experienced a complex thermal history following high grade metamorphism 1660 ma ago the terrain cooled slowly 3 c ma 1 until about 1570 ma ago when the temperature fell below about 500 c following granitoid emplacement 1500 ma ago the region remained relatively cold until affected by a thermal pulse 520 40ma ago causing temperatures to rise to 350 c in some places during this event accumulated40ar was released from minerals causing a significant ar partial pressure to develop laboratory ar solubility data combined with the40ar 39ar age spectra gives a local estimate of this partial pressure of 10 4atm the region finally cooled below 100 c about 280 ma ago 40ar 39ar age spectrum analyses of hornblende plagioclase and clinopyroxene containing excess40ar are characterized by saddle shaped age spectra detailed analysis of plagioclase samples reveals a complex diffusion behaviour which is controlled by exsolution structures this effect in conjunction with the presumed different lattice occupancy of excess40ar with respect to radiogenic40ar appears to be responsible for the saddle shaped age spectra

the evolution of illite to muscovite mineralogical and isotopic data from the glarus alps switzerland thirty five illite and muscovite concentrates were extracted from triassic and permian claystones shales slates and phyllites along a cross section from the diagenetic alpine foreland tabular jura and borehole samples beneath the molasse basin to the anchi and epimetamorphic helvetic zone of the central alps concentrates and thin sections were investigated by microscopic x ray infrared mossbauer thermal dta and tg wet chemical electron microprobe k ar rb sr 40ar 39ar and stable isotope methods with increasing metamorphic grade based on illite crystallinity data xrd and ir the following continuous changes are observed i the 1md 2m1 polymorph transformation is completed in the higher grade anchizone ii k2o increases from 6 8 wt diagenetic zone to 8 5 10 anchizone to 10 11 5 epizone reflecting an increase in the total negative layer charge from 1 2 to 2 0 iii a decrease of the chemical variation of the mica population with detrital muscovite surviving up to the anchizone epizone boundary iv a shift of an endothermic peak in differential thermal curves from 500 to 750 c v k ar and rb sr apparent ages of the fraction 2 m decrease from the diagenetic zone to the epizone k ar ages being generally lower than rb sr ages the critical temperature for total ar resetting is estimated to be 260 30 c k ar and rb sr ages become concordant when the anchizone epizone boundary is approached the stable isotope data on the other hand show no change with metamorphic grade but are dependent on stratigraphic age these results suggest that the prograde evolution from 1 md illite to 2m1 muscovite involves a continuous lattice restructuration without rupture of the tetrahedral and octahedral bonds and change of the hydroxyl radicals however this is not a recrystallization process this restructuration is completed approximately at the anchizone epizone boundary the isotopic data indicate significant diffusive loss of 40ar and 87sr prior to any observable lattice reorganization the restructuration progressively introduces a consistent repartition of ar and k in the mineral lattices and is outlined by the 40ar 39ar age spectra concordant k ar and rb sr ages of around 35 30 ma with concomitant concordant 40ar 39ar release spectra are representative for the main phase of alpine metamorphism calanda phase in the glarus alps a second age group between 25 and 20 ma can probably be attributed to movements along the glarus thrust ruchi phase while values down to 9 ma in regions with higher metamorphic conditions suggest thermal conditions persisting at least until the middle tortonian

recognition of extraneous argon components through incremental release 40ar39ar analysis of biotite and hornblende across the grenvillian metamorphic gradient in southwestern labrador 40ar39ar incremental release ages have been determined for 15 hornblende and 20 biotite concentrates separated from rocks collected across the garnet and kyanite zones of grenvillian metamorphism in southwestern labrador most hornblende spectra from the kyanite zone are slightly discordant with low temperature increments yielding ages older than the ca 1000 ma date suggested for culmination of grenvillian metamorphism in the area however all the hornblende concentrates record well defined plateau ages these range from 968 to 905 ma across the kyanite zone and date times of diachronous post metamorphic cooling the discordant spectra are interpreted to result from low temperature liberation of excess 40ar components from grain margins two hornblende concentrates from the garnet zone display very discordant spectra total gas ages of 2100 and 3017 ma in which incremental dates systematically decrease during analysis this pattern of discordance suggests that excess argon components are inhomogeneously distributed throughout these hornblende grains most biotites from the garnet and kyanite zones record total gas or plateau ages in excess of 1000 ma 2066 857 ma reflecting the widespread presence of excess argon components because most of the 40ar39ar age spectra are internally concordant the ratios of excess 40ar relative to radiogenic 40ar must have been uniform in the various gas fractions liberated from each sample this is also reflected in the inability of isotope correlation diagrams to differentiate between excess radiogenic and atmospheric argon components the biotite total gas or plateau dates show marked local variation this is interpreted to indicate that the biotite grains were in contact with a post metamorphic intergranular vapor phase that was characterized by large and variable 40ar36ar ratios such ratios most likely resulted from widespread diffusion of the argon liberated from adjacent archean basement gneisses during the grenvillian metamorphic overprint

geochronology and thermochronology by the 40ar39ar method ian mcdougall and t mark harrison oxford university press 1988 212p u s 55 00 isbn 0 19 504302 2

recycling of orogenic arc crust triggers porphyry cu mineralization in kerman cenozoic arc rocks southeastern iran pre collisional eocene oligocene arc diorites quartzdiorites granodiorites and volcanic equivalents in the kerman arc segment in central iran lack porphyry cu mineralization and ore deposits whereas collisional middle late miocene adakite like porphyritic granodiorites without volcanic equivalents host some of the world s largest cu ore deposits petrological and structural constraints suggest a direct link between orogenic arc crust evolution and the presence of a fertile metallogenic environment ore hosting kuh panj porphyry intrusions exhibit high sr 400 ppm low y 20 no negative eu anomalies eu eu 1 and relatively non radiogenic sr isotope signatures 87sr 86sr 0 7042 0 7047 relative to eocene oligocene granitoids mainly sr 12 la yb 15 eu eu 1 87sr 86sr 0 7053 0 7068 trace element modeling indicates peridotite melting for the barren eocene oligocene intrusions and a hydrous garnet bearing amphibolite source for middle late miocene ore hosting intrusions the presence of garnet implies collisional arc crustal thickening by shortening and basaltic underplating from about 30 35 to 40 45 km or 12 kbar the changes in residual mineralogy in the source of eocene to miocene rocks in the kerman arc segment reflect probing of a thickening arc crust by recycling melting of the arc crustal keel underplating of cu and sulfur rich melts from fertile peridotite generated a fertile metallogenic reservoir at or near the crust mantle boundary and dehydration melting under oxidizing conditions produced syn and post collisional ore hosting intrusions while the lack of post collisional volcanism prevented the venting of volatiles to the atmosphere from sulfur rich and oxidized adakitic magmas

pliny the elder and sr nd isotopes tracing the provenance of raw materials for roman glass production in roman and byzantine times natron glass was traded throughout the known world in the form of chunks production centers of such raw glass active from the 4th to 8th century ad were identified in egypt and syro palestine however early roman primary glass units remain unknown from excavation or scientific analysis the ancient author pliny described in 70 ad that besides egyptian and levantine resources also raw materials from italy and the gallic and spanish provinces were used in glass making in this study the primary provenance of 1st 3rd century ad natron vessel glass is investigated the use of combined sr and nd isotopic analysis allows the distinguishing and characterizing of different sand raw materials used for primary glass production the isotope data obtained from the glass samples are compared to the signatures of primary glass from known production centers in the eastern mediterranean and a number of sand samples from the regions described by pliny the elder as possible sources of primary glass eastern mediterranean primary glass has a nile dominated mediterranean nd signature higher than 6 0 nd while glass with a primary production location in the western mediterranean or north western europe should have a different nd signature lower than 7 0 nd most roman glass has a homogeneous 87sr 86sr signature close to the modern sea water composition likely caused by the intentional use of shell as glass raw material in this way strontium and neodymium isotopes now prove that pliny s writings were correct primary glass production was not exclusive to the levant or egypt in early roman days and factories of raw glass in the western roman empire will have been at play

zircon u pb and molybdenite re os geochronology and geochemistry of jinchang porphyry gold copper deposit ne china two phase mineralization and the tectonic setting abstract whether multiple mineralization styles in porphyry deposits are the result of one or multiple pulses of magmatism is unknown in this paper we present new age and geochemical data on the jinchang porphyry gold copper au cu deposit an important deposit in ne china and in the eastern central asian orogenic belt caob field geological study suggests that both the mineralization and ore causative magmatism at jinchang are multiphase la icp ms zircon u pb dating of the monzogranite ore host yielded 197 1 ma which is considerably older than the ore causative monzonite 115 3 ma and granodiorite 107 2 ma molybdenite re os dating on the breccia pipe hosted au cu ores yielded a well defined isochron age of 114 3 ma coeval to the monzonite emplacement the au cu mineralization was followed by another phase of crypto explosive breccia pipe hosted disseminated vein type au mineralization this au only mineralization phase was auriferous pyrite re os dated to be 103 101 ma coeval to the granodiorite emplacement geochemically the monzonite and granodiorite are enriched in liles and lrees but depleted in hfses and hrees the ore causative rocks are i type and display continental arc geochemical affinities we suggest that the monzonite and granodiorite were generated in an intracontinental extensional setting probably caused by the early cretaceous subduction roll back of the paleo pacific plate

platinum solubility and partitioning in a felsic melt vapor brine assemblage abstract the solubility and partitioning of pt in a s free vapor brine rhyolite melt pt metal assemblage has been quantified at 800 c f o 2 nno and pressures of 100 and 140 mpa vapor and brine were sampled at run conditions by trapping these phases as glass hosted fluid inclusions as the melt cooled through the glass transition temperature the vapor and brine were in equilibrium with the melt at the time of trapping and thus represent fluids which were sampled at the termination of each experimental run the microthermometrically determined salinities of vapor and brine are 2 and 63 wt nacl eq and 9 and 43 wt nacl eq at 100 and 140 mpa respectively platinum solubilities in vapor brine and glass i e quenched melt were quantified by using laser ablation inductively coupled plasma mass spectrometry la icp ms equilibrium is discussed with reference to the major and trace element concentrations of glass hosted fluid inclusions as well as the silicate melt over run times that varied from 110 to 377 h at 140 mpa and 159 to 564 h at 100 mpa platinum solubility values 1 in h2o saturated felsic melt are 0 28 0 13 g g and 0 38 0 06 g g at 140 and 100 mpa respectively platinum solubility values 1 x at 140 and 100 mpa respectively in aqueous vapor are 0 91 0 29 g g and 0 37 0 17 g g and in are brine 16 10 g g and 3 3 1 0 g g the measured solubility data were used to calculate nernst type partition coefficients for pt between vapor melt brine melt and vapor brine the partition coefficient values 1 x for vapor melt brine melt and vapor brine at 140 mpa are 2 9 1 0 67 27 and 0 13 0 05 and at 100 mpa are 1 0 0 2 6 8 2 4 and 0 15 0 05 the partitioning data were used to model the pt scavenging capacity of vapor and brine during the crystallization driven degassing i e second boiling of a felsic silicate melt over a depth range i e 3 6 km consistent with the evolution of magmatic hydrothermal ore deposits model calculations suggest that aqueous vapor and brine can scavenge sufficient quantities of pt and by analogy other platinum group elements pge to produce economically important pge rich magmatic hydrothermal ore deposits in earth s upper continental crust

the role of preexisting geologic architecture in the formation of giant porphyry related cu au deposits examples from new guinea and chile development of giant porphyry related copper and or gold deposits in new guinea and northern and central chile occurred during tertiary magmatic events that overprinted earlier extensional tectonic settings the earlier tectonic settings consisted of a mesozoic passive margin in new guinea a jurassic cretaceous back arc basin in northern chile and an oligocene intra arc basin in central chile the preexisting architecture of the basement rocks the fault systems and the stratigraphic packages associated with these settings played a strong role in controlling development of the giant deposits although the extensional tectonic settings in these terranes were different they share a number of elements that were used during formation of the tertiary ore deposits these were the presence of coupled systems of deeply detached listric synsedimentary faults and steep transverse faults and the presence of a relatively flat lying heterogeneous volcano sedimentary rock package during tertiary collision the deeply detached listric faults were inverted and strongly focused uplift exhumation and associated overpressuring induced failure and fluid flow in addition steep transverse faults were activated to form wrench systems with attendant steep deeply tapping pathways for magma and or fluid associated with dilatant jogs flexures or fault intersections ore deposits are commonly located in the hanging wall of the thrust faults under compression associated with collision the competent units of flat lying stratigraphic packages formed plates overlying the folded weaker units underneath and separated by a detachment or otherwise faulted contact examples of the competent upper units include the darai mendi limestone or equivalents in new guinea and the lavas of the farellones formation in central chile these plates appear to have impeded magma ascent and formed a cap on a magma and or fluid system pressurized by collision and orogenesis this geometry provided an ideal location for focusing of magmas and magmatic hydrothermal fluids from these three example terranes a set of exploration indicators have been constructed to highlight belts that are prospective for giant porphyry cu systems these indicators include 1 the presence of magmatic arcs that migrated into preexisting extensional architectures 2 coupled systems of deeply detached listric synsedimentary faults and steep transverse faults 3 rigid basement blocks that could form regional stress field perturbations and 4 fold belts with a major plate to 50 km wide of competent relatively undeformed or gently folded stratigraphy overlying more complexly folded and faulted parts of the sequence

experimental study of the solubility of platinum and iridium in basic silicate melts implications for the differentiation of platinum group elements during magmatic processes this study presents evidence for the dissolution of two platinum group elements pge s pt and ir in silicate melts of basic composition in the absence of any associated fluid phase the dissolution of these pge s is attributed to the formation of a complex ion in which the noble metal is uncharged an increase in oxygen fugacity in equilibrium with the melt brings about a considerable decrease in ir solubility in this way we can explain the early enrichment in ir group pge s ipge s ir ru os observed during the crystallization of ultrabasic cumulates by contrast however pt remains soluble in the melt an increase in the fs2 results in a pronounced increase in the solubility of pt in the silicate melt the sulfur saturation which leads to the precipitation of the base metal sulfides bms s also brings about the fixation of pt group pge s ppge s pt pd rh which are dissolved in bms s this accounts for the presence of ppge s in the nickel sulfide horizons of stratiform deposits the occurrence of platiniferous fe crystallizing with chromite in alaskan type complexes at oxygen fugacities of 10 6 bar can be explained by the low activity of fe in fe pt alloys such alloys have been formed in our laboratory at such fo2

noble metals in thetford mines ophiolites quebec canada part i distribution of gold iridium platinum and palladium in the ultramafic and gabbroic rocks the gold iridium palladium and platinum contents of ophiolites from the thetford mines area quebec were determined by radiochemical neutron activation analysis the main lithologies studied are the tectonite peridotites harzburgites of the lower structural unit and the ultramafic mafic plutonic members of the overlying cumulates average noble metal contents of the main rock types are au ppb ir ppb pt ppb pd ppb tectonite peridotitesharzburgite 1 5 3 2 10 0 3 8dunite lenses in harzburgite 0 27 3 6 2 1 0 29cumulate rocksolivine chromitite 0 28 30 0 1 5 0 33dunite 1 3 2 4 28 0 21 0pyroxenite 1 4 0 24 17 0 29 0gabbro 1 7 0 017 4 5 3 0the harzburgites have chondritelike noble metal abundance patterns and relatively uniform noble metal contents both features are compatible with an origin for the harzburgite as a residue from extensive partial melting of mantle peridotite the cumulates are highly variable in noble metal content iridium is enriched in early olivine chromitite and progressively depleted in later rocks palladium increases from early to late ultramafic cumulates and decreases in gabbros whereas pt shows no systematic trend in the cumulates gold increases very slightly from early to late cumulates the noble metal trends in cumulate rocks result mainly from fractional crystallization but other processes including sulfur saturation of magma and adcumulus growth may enrich rocks in noble metals sulfur rich late dunites and early pyroxenites probably have the best mineralization potential of the cumulate rocks there is little preferential partition of noble metals into any ferromagnesian silicate but the platinum group elements particularly ir are strongly associated with chromite acid leaching experiments suggest that in harzburgites ir is concentrated along chromite silicate grain boundaries whereas in cumulus chromitites ir occurs partly within chromite grains it is suggested that ir was originally incorporated into the chromite structure during a high temperature magmatic stage and in the case of the harzburgites has diffused to grain boundaries in response to cooling and to recrystallization induced in part by tectonic stress serpentinization probably occurred in both oceanic and continental environments cumulates suffered intense serpentinization but little noble metal mobility resulted harzburgites were serpentinized in both oceanic and continental environments and the latter process probably caused some mobilization of au in conjunction with formation of asbestos veins

argon retentivity and argon excess in amphiboles from the garbenschists of the western tauern window eastern alps 22 hornblende k ar ages and 10 39ar 40ar spectra were obtained for hornblende garbenschists from the western tauern window the post kinematic amphiboles were produced during the late alpine prograde metamorphism 6 10 kb and 500 570 c two nearly potassiumfree cummingtonites rimming hornblende yield k ar ages of 120 ma while the 20 tschermakitic hornblendes scatter between 17 and 37 ma the reason for this scatter is excess ar possibly incorporated into amphiboles during healing of fractures now traceable by trails of fluid inclusions excess ar is semiquantitatively corrected for by combining cogenetic hornblende and cummingtonite with k ar isochrons it can be quantified in 4 out of 10 hornblendes by 39ar 40ar stepwise heating experiments ages of 18 20 ma result for corrected hornblendes the retentivity of 40ar after correction for excess shows no correlation with chemistry within the narrow compositional range observed rather it shows intriguing correlations with irregularities in ca k spectra pointing to a microstructurally controlled mechanism for ar loss this observation leads to a critical evaluation of the closure temperature constant which apparently depends on an incompletely known number of mineralogical and environmental parameters in particular those 39ar 40ar release spectra which yield low temperature steps with younger ages than the plateaus are not interpretable in terms of a synchronous closure this gives evidence that loss of radiogenic isotopes proceeds by a more complex mechanism than simple volume diffusion through isotropic media

glass production for the silk road provenance and trade of islamic glasses using isotopic and chemical analyses in a geological context abstract we report the first nd and sr results for sands from inland locations in the middle east and new nd and sr results for coastal sand on the syro palestinian coast when the isotopic results for belus sands are combined with our seven new sand samples from the lebanon we have established the likely range of isotopic signatures for levantine sand that was used for the manufacture of hellenistic roman byzantine and some islamic glass in the area the combined isotopic results for sands and sr results for plants growing in syria and the lebanon provides an environmental base line which has allowed us to identify 5 separate isotopic production zones we have made direct comparisons with nd and sr signatures for raw furnace glasses from al raqqa syria and tyre the lebanon secondary furnace glasses from banias israel and with new results for four raw glass fragments from the early 11th century ce serce limani shipwreck excavated off the coast of turkey we have also compared the base line isotopic data with our new isotopic data for twenty one 9th 14th ce islamic plant ash glass vessel fragments from damascus and beirut and with published isotopic data for al raqqa vessel glass for the first time we have been able to provide a more secure provenance for the late 11th century ce serce limani raw glass it was made from levantine coastal sand or an equivalent source of geologically young sand and has a sr signature that suggests a production provenance in the palestinian area but not in established centres such as tyre as has been suggested we are able to suggest an inland isotopic provenance for 9th 12th century islamic vessel fragments from al raqqa beirut and damascus which probably coincides with an area in or near damascus two al raqqa vessel glasses with elevated potassium oxide levels and distinctive nd and sr signatures may have a central asian origin this environmental approach involving isotopic analysis of sands and plants has provided a more sensitive way of provenancing islamic plant ash glasses found in the syro palestinian area and in turkey than the use of trace elements has thus far the five separate production centres zones we have defined have provided more evidence for a decentralised system of islamic plant ash glass production also found across extensive areas of the silk road at least as far as northern iran

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

ela icp ms u pb zircon geochronology of regional volcanism hosting the bajo de la alumbrera cu au deposit implications for porphyry related mineralization ela icp ms u pb zircon geochronology has been used to show that the porphyritic intrusions related to the formation of the bajo de la alumbrera porphyry cu au deposit nw argentina are cogenetic with stratigraphically well constrained volcanic and volcaniclastic rocks of the late miocene farallon negro volcanic complex zircon geochronology for intrusions in this deposit and the host volcanic sequence show that multiple mineralized porphyries were emplaced in a volcanic complex that developed over 1 5 million years volcanism occurred in a multi vent volcanic complex in a siliciclastic intermontane basin the complex evolved from early mafic intermediate effusive phases to a later silicic explosive phase associated with mafic intrusions zircons from the basal mafic intermediate lavas have ages that range from 8 46 0 14 to 7 94 0 27 ma regionally extensive silicic explosive volcanism occurred at 8 0 ma 8 05 0 13 and 7 96 0 11 ma which is co temporal with intrusion of the earliest mineralized porphyries at bajo de la alumbrera 8 02 0 14 and 7 98 0 14 ma regional uplift and erosion followed during which the magmatic hydrothermal system was probably unroofed shortly thereafter dacitic lava domes were extruded 7 95 0 17 ma and rhyolitic diatremes 7 79 0 13 ma deposited thick tuff blankets across the region emplacement of large intermediate composition stocks occurred at 7 37 0 22 ma shortly before renewed magmatism occurred at bajo de la alumbrera 7 10 0 07 ma the latest porphyry intrusive event is temporally associated with new ore bearing magmatic hydrothermal fluids other dacitic intrusions are associated with subeconomic deposits that formed synchronously with the mineralized porphyries at bajo de la alumbrera however their emplacement continued from 7 10 0 06 to 6 93 0 07 ma after the final intrusion at bajo de al alumbrera regional volcanism had ceased by 6 8 ma 6 92 0 07 ma

the source and significance of argon isotopes in fluid inclusions from areas of mineralization argon isotopes in fluid inclusions in quartz veins associated with granite hosted tungsten mineralization in the southwest and north of england have been investigated in detail by the40ar 39ar technique the natural argon is present as a number of discrete components which can be identified through correlations with39ar 38ar and 37ar induced by neutron bombardment of potassium chlorine and calcium the potassium correlated component arises principally from in situ decay of potassium in solid phases in the inclusions in the case of the hemerdon tungsten deposit of southwest england the phases responsible are small 25 m captive authigenic micas which are shown to have been deposited from a fluid 268 20 ma ago shortly after the emplacement of the host granite the chlorine correlated component is present in the brines which constitute the fluid phase of the inclusions the argon in these hydrothermal fluids is made up in part of parentless or excess 40ar leached from surrounding crustal rocks and in part of dissolved ancient atmospheric argon absolute concentrations of both atmospheric and excess components in the brine can be estimated from 40arcl ratios and independent determinations of the salinity of the inclusions the absolute concentrations of the atmospheric argon are close to those found in modern meteoric water while those of the excess component can be interpreted in terms of the degree of interaction betwen the circulating fluids and country rock a calcium correlated component with a much higher ratio of excess to atmospheric argon than that in the brine was found to be a dominant phase in one sample from the hemerdon deposit indicating the presence of a solid phase probably a caso4 daughter mineral inclusions of this composition represent fluids which have had a more prolonged interaction with crustal rocks the results obtained from this study provide a systematization and a framework for future multi component argon studies of fluid inclusions together with an indication of the wide range of information which can be inferred

short life span of the ore forming system at the porgera gold deposit papua new guinea laser 40 ar 39 ar dates for roscoelite biotite and hornblende the porgera gold deposit papua new guinea is associated with sodic alkalic hypabyssal intrusions of alkali basaltic to mugearitic composition the intrusions were emplaced into cretaceous mudstones and siltstones in the latest miocene and both igneous and sedimentary rocks are mineralized three types of veins occur 1 magnetite sulfide au carbonate veins 2 base metal sulfide au carbonate veins and economically most important 3 quartz roscoelite au veins and breccias magmatic hornblende and biotite hydrothermal biotite from alteration selvages associated with the magnetite sulfide au veins and roscoelite were dated using the laser 40ar 39ar step heating technique magmatic biotite yielded a date of 5 99 0 11 ma 2 error mswd 3 7 and two samples of magmatic hornblende provided dates of 6 35 0 23 ma mswd 1 0 and 6 3 0 7 ma mswd 2 6 hydrothermal biotite provided a date of 5 98 0 13 ma mswd 2 7 and analyses of two roscoelite samples yielded dates of 5 92 0 09 ma mswd 2 0 and 5 92 0 17 ma mswd 2 0 the date of the magmatic biotite is interpreted to represent the age of the intrusions more accurately than the hornblende dates because small amounts of excess 40ar were present in the hornblende samples the date for hydrothermal biotite is interpreted to be the age of the alteration associated with the magnetite sulfide au veins the paragenetically earliest veins and thus reflects the age of the onset of the mineralizing activity at porgera based on the two dates for roscoelite the age of the main ore deposition event is interpreted to be 5 92 0 08 ma the apparent ages for the intrusive and the mineralizing events are thus identical within error and suggest that the magmatic and ore forming system at porgera was short lived

rb sr k ar and stable isotope evidence for the ages and sources of fluid components of gold bearing quartz veins in the northern sierra nevada foothills metamorphic belt california gold bearing quartz veins occur in and near major fault zones in deformed oceanic and island arc rocks west of the main outcrop of the sierra nevada composite batholith veins typically occupy minor reverse faults that crosscut blueschist to amphibolite grade metamorphic rocks whose metamorphic ages range from early paleozoic to jurassic vein micas and carbonate quartz mica assemblages that formed by hydrothermal metasomatism of ultramafic wall rocks in the alleghany grass valley washington and mother lode districts yield concordant k ar and rb sr ages the dated veins are significantly younger than prograde metamorphism penetrative deformation and accretion of their host rocks to the continental margin new and previously published mineralization ages from 13 localities in the sierra foothills range from about 140 to 110 m y ago with mean and median between 120 and 115 m y the age relations suggest that mineralizing fluids were set in motion by deep magmatic activity related to the resumption of east dipping subduction along the western margin of north america following the late jurassic nevadan collision event co 2 bearing fluids responsible for metasomatism and much of the vein mica carbonate albite and quartz deposition in several northern mines were isotopically heavy delta 18 o asymp 8 14ppm delta d between about 10 and 50ppm and do not resemble seawater magmatic or meteoric waters metasomatic and vein filling mica dolomite magnesite and quartz in altered ultramafic rocks generally formed from fluids with similar sr and o isotope ratios at a given locality consistent quartz mica delta 18 o fractionations delta 18 o sub q m 4 5 4 9ppm from various localities imply uniform equilibration temperatures probably between 300 degrees and 350 degrees c on a local mine scale fluids responsible for both carbonate alteration of mafic and ultramafic wall rocks and albitic alteration of felsic and pelitic rocks had similar sr isotope ratios samples from three veins in the central alleghany district fit a 115 7 or 3 m y rb sr isochron with a 87 sr 86 sr i value of approximately 0 7119 inferred 87 sr 86 sr ratios of metasomatic fluids from mines in different parts of the foothills region vary considerably 0 704 0 718 suggesting that sr was derived from sources ranging from western assemblage mesozoic ophiolitic or arc volcanic rocks to early paleozoic continent derived clastic rocks of the shoo fly complex systematic geographic variations in both sr and o isotopes can be rationalized by assuming extensive fluid interaction with rocks similar to the ones that are exposed within a few kilometers of the veins but the ultimate sources of the fluids and of au and other constituents may be independent of these isotopically lighter meteoric fluids deposited some late quartz overgrowths and occupied secondary fluid inclusions in earlier vein quartz