geology of a major new porphyry copper center in the superior pioneer district arizona a new porphyry copper deposit has been discovered in the superior pioneer district of east central arizona lying under thick tertiary cover approximately 2 km south of the historic magma vein the porphyry center was encountered in the course of an exploration drilling campaign conducted from 1992 through 1997 comprising a series of subvertical surface core holes and subhorizontal underground core holes ranging in length from 1 000 to 1 955 m the system is large structurally intact and shows indications of a substantial hypogene mineralized zone that averages between 1 0 and 2 0 percent cu the deposit is hosted by a thick sequence of cretaceous volcaniclastic and siliciclastic sedimentary rocks the mesozoic strata rest disconformably on calcareous units of the uppermost paleozoic naco group and are intruded by an east northeast striking swarm of quartz porphyry dikes the dike swarm is truncated at the basal tertiary unconformity under which the mesozoic section is buried by 600 to 1 200 m of unmineralized overburden composed of the oligocene miocene whitetail conglomerate and the miocene 18 6 ma apache leap dacite tuff the attitude of the basal tertiary beds shows that the porphyry center has been tilted 30 to 35 to the east northeast a broad zone of hydrolytic alteration characterizes the high structural levels in mesozoic wall rocks peripheral propylitic alteration in the volcaniclastic rocks is marked by epidote and chlorite giving way inwardly to a narrow zone of chlorite dominant alteration in which epidote is destabilized in favor of smectite clays and carbonate this chlorite zone corresponds closely to the intermediate argillic alteration type of meyer and hemley 1967 the propylitic fringe of the system is erratically mineralized by vein controlled pyrite sphalerite and specularite with minor chalcopyrite and galena chloritic rocks in the intermediate argillic halo contain pyrite and chalcopyrite the intermediate argillic alteration mantles an inner zone of pervasive quartz sericite pyrite the sericitic zone is strongly pyritic with bornite and lesser chalcopyrite within sericitically altered rock pyrite chalcocite digenite bornite veins exhibit centimeters to meters wide halos of advanced argillic alteration in which sericite is replaced by kaolin group clays the quartz sericite pyrite zone is superimposed on earlier potassic alteration that remains preserved at deeper levels the base of the mesozoic sequence and shaly beds in the upper naco group section are pervaded with secondary biotite calcareous rocks in the naco group are altered to 1 pyroxene epidote hornfelses and minor garnet magnetite skarn overprinted by 2 actinolite and 3 chlorite carbonate clay alteration biotitically altered rocks are mineralized by quartz pyrite chalcopyrite anhydrite veins molybdenite is present in an early generation of quartz rich pyrite chalcopyrite veins hornfelses and skarn in the naco group are strongly mineralized by quartz poor chalcopyrite pyrite anhydrite veins with chloritic envelopes surface and underground drilling into the deposit has outlined a zone of 1 0 percent hypogene cu that is at least 750 m long by 250 m wide by 300 m high elongated to the east northeast the same volume of rock also averages 0 02 percent mo and 2 ppm ag this zone apexes approximately 300 m below sea level slightly more than 1 5 km below the ground surface and remains open in all directions laterally and at depth four of the last five drill holes penetrating this zone ended in rock assaying 2 0 percent hypogene cu

geochemistry of vanadium in an epigenetic sandstone hosted vanadium uranium deposit henry basin utah the epigenetic tony m vanadium uranium orebody in south central utah is hosted in fluvial sandstones of the morrison formation upper jurassic although the deposit is mined for uranium vanadium has a higher average abundance in the ore thus the geochemistry of vanadium in the orebody was studied to characterize ore forming processes within the inferred ground water flow regime measurements of the relative amounts of v super 3 and v super 4 in ore minerals show that v super 3 is more abundant thermodynamic calculations show that vanadium was more likely transported to the site of mineralization as v super 4 the ore formed as v super 4 was reduced by hydrogen sulfide followed by hydrolysis and precipitation of v super 3 in oxide minerals e g montroseitc or paramontroseite or chlorite uranium was transported as uranyl ion u super 6 or some complex thereof and reduced by hydrogen sulfide forming coffinite detrital organic matter in the rocks served as the carbon source for sulfate reducing bacteria it was this bacteriogenic h 2 s that reduced the metals in the mineralization process possible sources for the v and u in this deposit have been identified previously vanadium most likely was derived from the dissolution of iron titanium oxides which liberated fe as well as v a zone of titanium rich remnants is observed updip and up the hydrologic gradient from the deposit m goldhaber and r l reynolds unpub data uranium probably was derived from the overlying brushy basin member of the morrison formation northrop 1982 a preliminary age date for the deposit of 115 ma k ludwig 1986 pers commun indicates that the ore formed after deposition of the brushy basin member previous studies have shown that the ore formed at the density stratified interface between a basinal brine and dilute meteoric water the mineralization processes described above occurred within the mixing zone between these two fluids stable isotope analyses of ore stage dolomite show a progressively heavier carbon and oxygen isotope signature with increasing depth through an ore horizon consistent with the two solution interface model

tectonic setting of synorogenic gold deposits of the pacific rim abstract more than 420 million oz of gold were concentrated in circum pacific synorogenic quartz lodes mainly during two periods of continental growth one along the gondwanan margin in the palaeozoic and the other in the northern pacific basin between 170 and 50 ma these ores have many features in common and can be grouped into a single type of lode gold deposit widespread throughout clastic sedimentary rock dominant terranes the auriferous veins contain only a few percent sulphide minerals have gold silver ratios typically greater than 1 1 show a distinct association with medium grade metamorphic rocks and may be associated with large scale fault zones ore fluids are consistently of low salinity and are co2 rich in the early and middle palaeozoic in the southern pacific basin a single immense turbidite sequence was added to the eastern margin of gondwanaland deformation of these rocks in southeastern australia was accompanied by deposition of at least 80 million oz of gold in the victorian sector of the lachlan fold belt mainly during the middle and late devonian lesser devonian gold accumulations characterized the more northerly parts of the gondwanan margin within the hodgkinson broken river and thomson fold belts additional lodes were emplaced in this flyschoid sequence in devonian or earlier palaeozoic times in what is now the buller terrane westland new zealand minor post devonian growth of gondwanaland included terrane collision and formation of gold bearing veins in the permian in australia s new england fold belt and in the jurassic early cretaceous in new zealand s otago schists collision and accretion of dozens of terranes for a 100 m y long period against the western margin of north america and eastern margin of eurasia led to widespread latest jurassic to eocene gold veining in the northern pacific basin in the former location late jurassic and early cretaceous veins and related placer deposits along the western margin of the sierra nevada batholith have yielded more than 100 million oz of gold additional significant ore forming events during the development of north america s cordilleran orogen included those in the klamath mountains region california in the late jurassic and early cretaceous the klondike district yukon by the early cretaceous the nome and fairbanks districts alaska and the bridge river district british columbia in the middle cretaceous and the juneau gold belt alaska in the eocene gold bearing veins deposited during the late jurassic and early cretaceous terrane collision that formed the present day russian far east have been the source for more than 130 million oz of placer gold the abundance of gold bearing quartz carbonate veins throughout the gondwanan north american and eurasian continental margins suggests the migration and concentration of large fluid volumes during continental growth such volumes could be released during orogenic heating of hydrous silicate mineral phases within accreted marine strata the common temporal association between gold veining and magmatism around the pacific rim reflects these thermal episodes melting of the lower thickened crust during arc formation slab rollback and extensional tectonism and subduction of a slab window beneath the seaward part of the forearc region can all provide the required heat for initiation of the ore forming processes

multiple fluid components of salt diapirs and salt dome cap rocks gulf coast u s a abstract salt diapirs contain a few percent of anhydrite that accumulated as residue to form anhydrite cap rocks during salt dissolutions reported 87sr 86sr ratios of these salt hosted and cap rock anhydrites in the gulf coast u s a indicate their derivation from middle jurassic seawater however a much wider range of 87sr 86sr ratios incorporating a highly radiogenic component in addition to the middle jurassic component has been found in several gulf coast salt domes this wide range of 87sr 86sr ratios of anhydrite within the salt stocks records sr contributions from both marine water and formation water that has equilibrated with siliciclastics during cap rock formation this anhydrite either recrystallized in the presence of or was cemented by a low sr fluid with a late cretaceous seawter type sr isotope ratio or simply lost sr during recrystallization later the cap rock was invaded by warm saline brines with high sr isotope ratios from which barite and metal sulfides were precipitated subsequently low salinity water hydrated part of the anhydrite bringing to six the total number of fluids that interacted througout the history of salt dome and cap rock growth the progenitor of these salt diapirs the louann formation is generally thought to have formed from marine water evaporated to halite and rarely higher evaporite facies salt domes in the east texas north louisiana and mississippi salt basins have 87sr 86sr and 34s values that corroborate a mid jurassic age for the mother salt however salt domes in the houston and rio grande embayments of the gulf coast basin have 87sr 86sr ration ranging to values higher than both middle jurassic seawater and all rb free marine phanerozoic rocks these anomalous 87sr 86sr ratios are probably derived from radiogenic sr bearing fluids that equilibrated with siliciclastic rocks and invaded the salt either prior to or during diapirism potential sources of the radiogenic 87sr component include clay and or feldspar located either in older units beneath the louann formation or younger units flanking the salt diapirs and k salts within the louann evaporites because partial sr exchange in anhydrite had to take place in a fluid medium admittance of radiogenic 87sr bearing fluids into the salt may have led to diapirism by lowering the shear strength of the crystalline salt the slight number of anomalous 87sr 86sr values in the interior basins indicates that anomalous values are related to areally discrete structural or stratigraphic controls that affected only the gulf coast basin

geochronology geochemistry and tectonic evolution of the western and central cordilleras of colombia abstract autochthonous rocks of the pre cretaceous continental margin of nw south america the tahami terrane are juxtaposed against a series of para autochthonous rock units that assembled during the early cretaceous allochthonous oceanic crust of the caribbean large igneous province collided with and accreted onto the margin during the late cretaceous we present the first regional scale dataset of zircon u pb la icp ms ages for intrusive and metamorphic rocks of the autochthonous tahami terrane early cretaceous igneous para autochthonous rocks and accreted oceanic crust the u pb zircon data are complemented by multiphase 40ar 39ar crystallization and cooling ages the geochronological data are combined with whole rock major oxide trace element and ree data acquired from the same units to constrain the tectonic origin of the rock units and terranes exposed in the western cordillera cauca patia valley and the central cordillera of colombia the tahami terrane includes lower paleozoic orthogneisses 440 ma that may have erupted during the active margin stage of the rheic ocean basement gneisses were intruded by permian continental arc granites during the final assembly of pangea triassic sedimentary rocks were subsequently deposited in rift basins and partially melted during high t metamorphism associated with rifting of western pangea during 240 220 ma continental arc magmatism during 180 145 ma is preserved along the whole length of the central cordillera and was followed by an early cretaceous out board step of the arc axis and the inception of the quebradagrande arc that fringed the continental margin back stepping of the arc axis may have been caused by the collision of buoyant seamounts which were coeval with plateau rocks exposed in the nicoya peninsular of costa rica rapid westward drift of south america closed the quebradagrande basin in the late aptian and caused medium high p t metamorphic rocks of the arquia complex to exhume and obduct onto the continental margin subduction beneath hot spot derived rocks of the caribbean plateau 100 92 ma formed an intra oceanic arc 92 75 ma which collectively comprise the late cretaceous caribbean large igneous province the remnant ocean basin located between south america and the caribbean large igneous province was partly consumed via continental subduction forming the large antioquia batholith the caribbean large igneous province collided and accreted to south america during 75 70 ma along the cauca almaguer fault resulting in the cessation of both arcs and the paleocene onset of subduction beneath the accreted oceanic crust

felsic magmatic phases and the role of late stage aplitic dykes in the formation of the world class cantung tungsten skarn deposit northwest territories canada abstract a field and petro chemical classification of felsic magmatic phases fmps at the world class cantung w skarn deposit was undertaken to document the evolution of magmatism and the relationships between different fmps metasomatism and mineralization early fmps include moderately differentiated zr hf 18 26 ti zr 14 15 biotite monzogranitic plutons and early biotite rich granitic dykes and compositionally similar quartz feldspar porphyry dykes late highly fractionated zr hf 8 17 ti zr 3 13 fmps sourced from a deeper monzogranitic intrusion include 1 leucocratic biotite or tourmaline bearing dykes derived from localized entrapments of residual magma and 2 sub vertical ne trending aplitic dykes derived from a larger segregation of residual fluid and incompatible element enriched magma the aplitic dykes have textures morphologies spatial associations and a pervasive calcic metasomatic mineral assemblage ca plagioclase quartz or clinozoisite indicative of syn mineralization emplacement very late stage overpressuring and initiation of sub vertical fractures into the overlying plutonic carapace and country rocks by supercritical magmatic fluid led to an interaction with calcareous country rocks that resulted in an increased aca2 in the fluid and the concurrent precipitation of w skarn residual magma also ascended with and quenched in equilibrium with the magmatic fluid to from the aplitic dykes then was metasomatized by the fluid as it interacted with calcareous country rocks overall highly fractionated and moderately to very highly undercooled fmps at cantung provide evidence for a large and evolving felsic magmatic system at depth that segregated and maintained a stable fluid and incompatible element enriched residual magma until the latest stages of crystallization the detailed study of fmps associated with magmatic hydrothermal mineral deposits allow us to refine our understanding of these mineralizing systems and better define metallogenic and exploration models for intrusion related mineralization

rock quality designation rqd time to rest in peace rock quality designation rqd was introduced by don deere in the mid 1960s as a means of using diamond core to classify rock for engineering purposes subsequently it was incorporated into the ro

mesozoic plate motion history below the northeast pacific ocean from seismic images of the subducted farallon slab the high resolution seismic imaging of subducted oceanic slabs1 2 has become a powerful tool for reconstructing palaeogeography3 the images can now be interpreted quantitatively by comparison with models of the general circulation of the earth s mantle4 here we use a three dimensional spherical computer model of mantle convection5 6 to show that seismic images of the subducted farallon plate provide strong evidence for a mesozoic period of low angle subduction under north america such a period of low angle subduction has been invoked independently to explain rocky mountain uplift far inland from the plate boundary during the laramide orogeny7 the computer simulations also allow us to locate the largely unknown kula farallon spreading plate boundary the location of which is important for inferring the trajectories of suspect terrain across the pacific basin8

pb sr and nd isotopic and chemical evidence for a primitive island arc emplacement of the el arco porphyry copper deposit baja california mexico the el arco porphyry copper deposit is located in central baja california and is a resource containing 600 mt of ore with 0 6 copper it was emplaced within a relatively primitive jurassic island arc and it was subsequently metamorphosed and intruded by the cretaceous peninsular ranges batholith the porphyritic stock intrusion and ore formation at el arco has recently been dated at 165 ma valencia et al geos 24 189 2004 this age is much older than aptian albian k ar ages previously reported from el arco barthelmy geology of el arco calmalli area baja california mexico msc thesis san diego state university ca 1975 baja california geology san diego state university ca pp 127 138 1979 the copper mineralization at el arco is concentrated in a core of potassic alteration in a dioritic porphyritic stock surrounded by propylitic alteration in andesitic lavas mafic dikes that intruded the deposit are not mineralized but they are affected by post ore low grade metamorphism the dikes are compositionally the most primitive rocks while host rock andesites and the porphyry stock display typical volcanic arc characteristics the pb isotope data from sulfides feldspars quartz and whole rock samples indicate that 1 the copper bearing porphyry stock and the surrounding andesites evolved from a similar source with an average value of 9 43 2 no external pb was added during mineralization 3 some pb isotope compositions were slightly disturbed by a later metamorphic event strontium and nd isotopes show that the magmas evolved from a depleted mantle reservoir with no involvement of older continental crust our data favor a model for the formation of the el arco deposit linked to a triassic to jurassic intra oceanic arc system cropping out at the western margin of central baja california in the cedros vizcaino region the intra oceanic arc together with the el arco deposit was accreted to the active continental margin of north america and metamorphosed during the early cretaceous this model is in disagreement to earlier models that favor the el arco deposit formation being linked to the cretaceous continental margin

mineralization of breccia pipes in northern arizona the paleozoic sedimentary rocks on the colorado plateau of northern arizona are host to hundreds ofbreccia pipes the uranium and copper deposits in these breccia pipes transgress formation boundaries from the mississippian redwall limestone to the triassic chinle formation they are not classic breccia pipes in that there is no volcanic rock associated with them in time or space they are the result of solution collapse within the redwall limestone and stoping of the overlying strata the karst development in the redwall limestone began in the mississippian and apparently either continued to the triassic or was at least once again active during that time the mineralization apparently occurred shortly thereafter sometime during the mesozoic mining activity in breccia pipes of the grand canyon region began during the nineteenth century and continues today with the operation of the hack i ii and iii mines although the exploited commodity has changed from cu to u although small in size these pipes contain samples with up to 55 percent u 3 o 8 and can yield ore averaging between 0 30 and 0 60 percent u 3 o 8 mineralization at the surface commonly occurs within nodules and concretions associated with pyrite and goethite and along fractures while the primary ore of the unoxidized zones is commonly within a comminuted sandstone matrix surrounding breccia fragments of overlying formations the ore mineral is uraninite although associated with it are sphalerite galena chalcopyrite tennantite millerite siegenite and molybdenite some of the surface nodules are encrusted with malachite and are exceptionally enriched in ag pyrite is abundant and the organic carbon content of some rocks is high enough to suggest that it along with the pyrite may be a reductant for uranium in contrast it is possible if uranium were transported as a bicarbonate or carbonate complex that only a conduit of brecciated rock was necessary to release co 2 thus disrupting the equilibrium and allowing uraninite to precipitate an extensive suite of elements is significantly enriched in the mineralized rock ag as ba cd co cr cs cu hg mo ni pb sb se sr u v zn and the rare earth elements of these cu pb zn ag and particularly as appear to be the best geochemical indicators of mineralized pipes at present the origin of the mineralizing fluids is not known the lack of extensive silification within the breccia along with the 80 degrees to 173 degrees c fluid inclusion filling temperatures on sphalerite dolomite quartz and calcite suggests relatively low temperature mineralizing fluids although heated in excess of what would be expected from the normal geothermal gradient on the colorado plateau with the exception of the u mineralized rock the mineral assemblage and geochemistry is similar to mississippi valley type deposits

lead isotope compositions of late cretaceous and early tertiary igneous rocks and sulfide minerals in arizona implications for the sources of plutons and metals in porphyry copper deposits porphyry copper deposits in arizona are genetically associated with late cretaceous and early tertiary igneous complexes that consist of older intermediate volcanic rocks and younger intermediate to felsic intrusions the igneous complexes and their associated porphyry copper deposits were emplaced into an early proterozoic basement characterized by different rocks geologic histories and isotopic compositions lead isotope compositions of the proterozoic basement rocks define from northwest to southeast the mojave central arizona and southeastern arizona provinces porphyry copper deposits are present in each pb isotope province lead isotope compositions of late cretaceous and early tertiary plutons together with those of sulfide minerals in porphyry copper deposits and of proterozoic country rocks place important constraints on genesis of the magmatic suites and the porphyry copper deposits themselves the range of age corrected pb isotope compositions of plutons in 12 late cretaceous and early tertiary igneous complexes is 206 pb 204 pb 17 34 to 22 66 207 pb 204 pb 15 43 to 15 96 and 208 pb 204 pb 37 19 to 40 33 these pb isotope compositions and calculated model th u are similar to those of the proterozoic rocks in which the plutons were emplaced thereby indicating that pb in the younger rocks and ore deposits was inherited from the basement rocks and their sources no pb isotope differences distinguish late cretaceous and early tertiary igneous complexes that contain large economic porphyry copper deposits from less rich or smaller deposits that have not been considered economic for mining lead isotope compositions of late cretaceous and early tertiary plutons and sulfide minerals from 30 metallic mineral districts furthermore require that the southeastern arizona pb province be divided into two subprovinces the northern subprovince has generally lower 206 pb 204 pb and higher model th u and the southern subprovince has higher 206 pb 204 pb and lower model th u these pb isotope differences are inferred to result from differences in their respective post 1 7 ga magmatic histories throughout arizona pb isotope compositions of late cretaceous and early tertiary plutons and associated sulfide minerals are distinct from those of jurassic plutons and also middle tertiary igneous rocks and sulfide minerals these differences most likely reflect changes in tectonic setting and magmatic sources within late cretaceous and early tertiary igneous complexes that host economic porphyry copper deposits there is commonly a decrease in pb isotope composition from older to younger plutons this decrease in pb isotope values with time suggests an increasing involvement of crust with lower u pb than average crust in the source s of late cretaceous and early tertiary magmas lead isotope compositions of the youngest porphyries in the igneous complexes are similar to those in most sulfide minerals within the associated porphyry copper deposit this pb isotope similarity argues for a genetic link between them however not all pb in the sulfide minerals in porphyry copper deposits is magmatically derived some sulfide minerals particularly those that are late stage or distal to the main orebody or in proterozoic or paleozoic rocks have elevated pb isotope compositions displaced toward the gross average pb isotope composition of the local country rocks the more radiogenic isotopic compositions argue for a contribution of pb from those rocks at the site of ore deposition combining the pb isotope data with available geochemical isotopic and petrologic data suggests derivation of the young porphyry copper related plutons most of their pb and other metals from a hybridized lower continental crustal source because of the likely involvement of subduction related mantle derived basaltic magma in the hybridized lower crustal source an indiscernible mantle contribution is probable in the porphyry magmas clearly in addition pb was contributed from the local country rocks this is most evident in sulfide minerals in veins that are late stage hosted in proterozoic gneiss and or peripheral to the porphyry copper deposit

mesozoic transpression transtension subduction and metallogenesis in northern and central california middle paleozoic to middle jurassic terrane assemblies in the klamaths and sierran foothills consist of mafic ultramafic complexes fine grained terrigenous strata derived from previously accreted continental margin belts sutured oceanic terranes reflect c 230 myr of margin parallel slip involving chiefly transtension and transpression quartzofeldspathic clastic rocks and blueschists eclogites are very rare little devolatilization occurred at magmagenic depths hence coeval hydrothermal ore deposits and granitoids are uncommon in contrast nearly head on cretaceous subduction of the farallon plate generated the massive klamath sierra nevada volcanic plutonic arc reflecting dewatering of the eastward descending oceanic lithosphere in the magmagenic zone immature great valley forearc and franciscan trench deposits shed from the arc record c 70 myr of rapid crustal growth au bearing solutions rising from magmagenic depths exsolved from plutons and expelled from heated wall rocks were mobilized attending arc construction precipitation of gold bearing quartz veins occurred where h2o co2 bearing fluids encountered major geochemical discontinuities in the wall rocks

re os and u pb geochronology of the clear creek dublin gulch and mactung deposits tombstone gold belt yukon canada absolute timing relationships between plutonism and mineralization new re os molybdenite and u pb zircon dates from deposits of the tombstone gold belt provide direct timing constraints for sulphide mineralization and crystallization of spatially associated plutons at clear creek re os molybdenite dates 93 6 0 3 to 92 4 0 4 ma are in excellent agreement with the host intrusion u pb zircon age 92 3 0 3 ma consequently we propose the nominally younger existing 40 ar 39 ar hydrothermal mica ages 91 7 0 4 and 90 0 0 3 ma for clear creek are a result of slow cooling at dublin gulch the re os molybdenite date 93 2 0 3 ma for a late stage vein is nominally younger than the host intrusion u pb zircon age 94 0 0 3 ma in agreement with the deposits paragenesis the u pb zircon date 92 1 0 2 ma for the mactung pluton which lies adjacent to the w skarn has a younger age than the re os molybdenite dates 97 5 0 5 ma fluid inclusion geochemical data for ore fluid indicate a magmatic origin for the w skarn however the u pb and re os age data suggest that the exposed mactung stock is not the source of ore fluid for the w skarn and that the progenitor pluton for ore fluid is unknown the robust age constraints obtained for mineralization and magmatism in this study are in accord with the hypothesis that these deposits are intrusion related ore systems the results are also in agreement with previous studies demonstrating that the re os molybdenite chronometer can provide accurate and precise dates for the timing of metallic mineralization with rigorous sampling protocol

petrology u pb dating and c o stable isotope constraints on the source and evolution of the adakite related mezcala fe au skarn district guerrero mexico the mezcala gold district guerrero mexico is a cretaceous to early paleocene oxidized fe au skarn deposit genetically associated with an adakitic magmatic event of 63 2 ma dated by ion probe u pb on zircons

geochronology and geochemistry of the max porphyry mo deposit and its relationship to pb zn ag mineralization kootenay arc southeastern british columbia canada the max porphyry mo deposit is located at the northern end of the kootenay arc in southeastern british columbia canada molybdenum mineralization is genetically related to the variably altered trout lake granodiorite stock which has lithogeochemical affinities to the low fluorine calc alkaline granodiorite hosted porphyry mo deposit type three molybdenite samples from early and late mo bearing veins yield re os dates that overlap within analytical error with a weighted average age of 80 3 0 2 ma these dates are in close agreement with two 206pb 238u weighted average zircon ages for phases of the trout lake stock at 80 3 1 6 and 79 2 1 0 ma indicating that crystallization of the stock and hydrothermal molybdenite formation were coeval and likely cogenetic 40ar 39ar plateau ages for primary biotite in igneous dikes secondary biotite in early potassic alteration and muscovite in late phyllic alteration range from 80 to 76 ma the younger 40ar 39ar plateau ages may reflect unusually slow cooling of the magmatic hydrothermal system or a late thermal disturbance pb zn ag vein deposits are common throughout the kootenay arc and several such veins are located within and proximal to the max porphyry mo deposit the presence of rare molybdenite associated with sphalerite and argentiferous galena in some of these pb zn ag veins suggests that they represent a late stage event related to the max magmatic hydrothermal system fluid inclusions in quartz from mo and pb zn ag veins are similar and can be divided into two compositional groups type 1 aqueous liquid rich fluid inclusions and type 2 aqueous carbonic liquid rich fluid inclusions pressure corrected 1 4 1 7 kbar trapping temperatures of typical type 1 inclusions from mo bearing veins range from 430 to 460 c and for pb zn ag veins from 335 to 365 c the ranges of salinity 0 12 wt nacl equiv in fluid inclusions from both vein types are almost identical and type 2 inclusions from both vein types contain similar amounts of co2 avg xco2 0 08 with traces of ch4 the similarity of chemistry in the two vein types suggests a genetic link between porphyry mo mineralization and adjacent pb zn ag deposits however muscovite from the alteration halo around a pb zn ag vein within the max deposit yielded a 40ar 39ar plateau age of 72 2 0 5 ma this date is significantly younger than the 80 ma age of porphyry magmatism and mo mineralization suggesting either that this event is in fact unrelated to porphyry mo mineralization at 80 ma or that the k ar isotope system has been disturbed by a later thermal event

examining potential genetic links between jurassic porphyry cu au mo and epithermal au ag mineralization in the toodoggone district of north central british columbia canada the toodoggone district comprises upper triassic to lower jurassic hazelton group toodoggone formation volcanic and sedimentary rocks which unconformably overlie submarine island arc volcanic and sedimentary rocks of the lower permian asitka group and middle triassic takla group some of which are intruded by upper triassic to lower jurassic plutons and dikes of the black lake suite although plutonism occurred episodically from ca 218 to 191 ma the largest porphyry cu au mo systems formed from ca 202 to 197 ma with minor mineralization occurring from ca 197 to 194 ma porphyry style mineralization is hosted by small volume 468 c saline and deposited metals at deep crustal depths 2 km sulfur c o and pb isotope data confirm the involvement of a magmatic fluid but also suggest that the ore fluid interacted with asitka and takla group country rocks prior to metal deposition in contrast in the shasta lawyers and griz sickle low sulfidation epithermal systems there is no clear association with magmatic fluids instead their fluid inclusion data indicate the involvement of low temperature 175 to 335 c low salinity 1 to 11 equiv wt nacl fluids that deposited metals at shallow depths 850 m their isotope i e o h pb data suggest interaction between meteoric and or metamorphic ore fluids with basement country rocks

the metallogeny of late triassic rifting of the alexander terrane in southeastern alaska and northwestern british columbia a belt of unusual volcanogenic massive sulfide vms occurrences is located along the eastern margin of the alexander terrane throughout southeastern alaska and northwestern british columbia and exhibits a range of characteristics consistent with a variety of syngenetic to epigenetic deposit types deposits within this belt include greens creek and windy craggy the economically most significant vms deposit in alaska and the largest in north america respectively the occurrences are hosted by a discontinuously exposed 800 km long belt of rocks that consist of a 200 to 800 m thick sequence of conglomerate limestone marine clastic sedimentary rocks and tuff intercalated with and overlain by a distinctive unit of mafic pyroclastic rocks and pillowed flows faunal data bracket the age of the host rocks between anisian middle triassic and late norian late late triassic this metallogenic belt is herein referred to as the alexander triassic metallogenic belt the vms occurrences show systematic differences in degree of structural control chemistry and stratigraphic setting along the alexander triassic metallogenic belt that suggest important spatial or temporal changes in the tectonic environment of formation at the southern end of the belt felsic volcanic rocks overlain by shallow water limestones characterize the lower part of the sequence in the southern and middle portion of the belt a distinctive pebble conglomerate marks the base of the section and is indicative of high energy deposition in a near slope or basin margin setting at the northern end of the belt the conglomerates limestones and felsic volcanic rocks are absent and the belt is composed of deep water sedimentary and mafic volcanic rocks this northward change in depositional environment and lithofacies is accompanied by a northward transition from epithermal like structurally controlled discontinuous vein and pod shaped pb zn agba cu occurrences with relatively simple mineralogy to sulfosalt enriched vms occurrences exhibiting characteristics of vein diagenetic replacement and exhalative styles of mineralization and finally to cu zn co au occurrences with larger and more clearly stratiform orebody morphologies occurrences in the middle of the belt are transitional in nature between structurally controlled types of mineralization that formed in a shallow water near arc setting to those having a more stratiform appearance formed in a deeper water rift basin setting the geologic setting in the south is consistent with shallow subaqueous emplacement on the flanks of the alexander terrane northward the setting changes to an increasingly deeper back or intra arc rift basin igneous activity in the alexander triassic metallogenic belt is characterized by a bimodal suite of volcanic rocks and a previously unrecognized association with mafic ultramafic hypabyssal intrusions immobile trace and rare earth element ree geochemical data indicate that felsic rocks in the southern portion of the belt are typical calc alkaline rhyolites which give way in the middle of the belt to peralkaline rhyolites rhyolites are largely absent in the northern part of the belt throughout the belt the capping basaltic rocks have transitional geochemical signatures radiogenic isotope data for these rocks are also transitional basalts and gabbros end 4 9 and 87sr 86sr initial at 215 ma 0 7037 0 7074 together these data are interpreted to reflect variable assimilation of mature island arc crust by more primitive melts having the characteristics of either mid ocean ridge morb or intraplate within plate basalts wpb the ore and host rock geochemistry and the sulfosalt rich mineralogy of the deposits are strikingly similar to recent descriptions of active sea floor hydrothermal white smoker systems in back arcs of the southwest pacific ocean these data in concert with existing faunal ages record the formation of a belt of vms deposits and occurrences in a propagating intra to back arc rift tectonic setting during the late triassic a modern analogue having similar tectonic and metallogenic features is the southward projection of the lau basin from the active sea floor hydrothermal vents of the valu fa ridge to the taupo volcanic zone of the north island new zealand

source of fluorine and petrogenesis of the rio grande rift type barite fluorite galena deposits abundant fluorite in the rio grande rift barite fluorite galena deposits is anomalous in comparison to typical mississippi valley type mvt deposits the source of fluorine in these deposits is controversial we have tested two hypothesized sources for the origin of fluorine in these deposits including release of gaseous hf from magmas associated with rifting and the leaching of fluorine from proterozoic basement granites that underlie the deposits in this study chlorine isotopes were measured from fluorite fluid inclusions from eight rio grande rift deposits stable chlorine isotope ratios were used as a proxy for determining the possible source of the fluorine because chlorine unlike fluorine has more than one isotope and thus can be used as an isotopic tracer chlorine and fluorine exhibit chemically similar behavior and therefore are likely to be derived from the same source if chlorine is associated with rift related magmatism as a complement to the chlorine isotope measurements br cl ratios were measured from the fluorite fluid inclusions and applied as additional tracers in addition sr and nd isotopes were measured from fluorites regionally related granites carbonates and asthenospheric basalts to aid in understanding the petrogenesis of rio grande rift deposits sr and nd isotope ratios of the fluorites 87 sr 86 sr 0 71986 0 74078 143 nd 144 nd 0 51177 0 51234 are distinctly different from local basalts 87 sr 86 sr 0 70304 0 70465 143 nd 144 nd 0 51277 0 51297 and pennsylvanian limestones 87 sr 86 sr 0 70858 0 72926 143 nd 144 nd 0 51198 0 51245 but similar to the proterozoic bingham granite 87 sr 86 sr 0 73285 143 nd 144 nd 0 51208 the relatively radiogenic sr and unradiogenic nd signatures of the fluorites indicate that the sr and nd were derived largely from a granitic source with some influence from a carbonate and or asthenospheric source this is consistent with previously published pb isotope data on galena from hansonburg mining district 37 cl values from fluorite fluid inclusions range from 0 66 to 3 07 per mil relative to smoc and the br cl ratio by weight for all the fluorite samples ranges between 0 00008 and 0 00050 except for san diego mountain which has a relatively high br cl ratio of 0 00242 the positive correlation between fluorite 37 cl and br cl ratio is attributed to mixing of cl from asthenospheric magmatic and evaporite sources the calculated range of cl derived from an asthenospheric source is 40 to 49 percent for the mex tex mine sample and between 13 and 35 percent for the sunshine no 2 sample these ranges in the asthenospheric component of cl probably represent minimum possible values because f and cl closely follow one another during magma degassing and exhibit similar chemical behavior the presence of asthenospheric cl is consistent with an asthenospheric magmatic source of f in the rio grande rift deposits

brine history indicated by argon krypton chlorine bromine and iodine analyses of fluid inclusions from the mississippi valley type lead fluorite barite deposits at hansonburg new mexico argon krypton chlorine bromine and iodine were measured in a homogeneous population of high salinity hydrothermal fluid inclusions from the tertiary age mississippi valley type mvt lead fluorite barite deposits at hansonburg new mexico to establish new types of evidence for the history of both the fluid and the major dissolved salts noble gases and halogens in fluid inclusions containing 10 10 10 9 l of brine cl 3 molal were analyzed by laser microprobe noble gas mass spectrometry lmngms on neutron irradiated samples the concentrations of36ar 4 7 10 8 molal and84kr1 8 10 9 molal in the fluid inclusions are equal to those of fresh surface waters in equilibrium with air at approximately20 5 the mole ratios ofbr cl 1 2 10 4 andi cl 1 2 10 6 are among the lowest measured in any natural waters similar to those of modern brines formed by dissolution of permian nacl bearing evaporites in southeast new mexico 40ar 36ar ratios 600 are twice that of air and indicate that the fluid inclusions had excess radiogenic40ar 1 4 10 5 molal when trapped the amount of excess40ar appears to be too large to have been acquired with cl by congruent dissolution of halite bearing evaporites and possibly too small to have been acquired with pb by congruent dissolution of granitic basement rocks with proterozoic kar ages from thelmngms data combined with published pb and s isotope data we infer the following sequence of events in the history of the hansonburg mvt hydrothermal brine 1 the brine originated as relatively dilute meteoric water and it did not gain or lose atmospheric ar or kr after recharge 2 the originally dilute fluid acquired the bulk of its cl and sulfate in the subsurface after recharge by dissolving halite bearing permian marine evaporites 3 the high salinity brine then acquired most of its pb and excess radiogenic40ar from interactions with aquifer rocks other than evaporites possibly clastic sedimentary rocks or basement rocks with phanerozoic kar ages and 4 the brine deposited fluorite without having boiled or degassed

revised mesozoic cenozoic orogenic architecture and gold metallogeny in the northern circum pacific abstract the orogenic collages of the northern circum pacific between japan and alaska revealed an endowment of about 450 moz au in various deposit types and diverse mesozoic cenozoic tectonic settings the area consists of predominantly late paleozoic to cenozoic turbidite to island arc terranes as well as precambrian cratonic terranes that can be grouped into the kolyma alaska kamchatka aleutian and nipponide collages the latter can be linked via the mongol okhotsk suture with the late paleozoic to early mesozoic terranes in the mongolides the early yanshanian magmatic arc terranes in the fossil kolyma alaska collage host copper gold porphyry deposits which have only recently received much attention exploration has revealed a large and growing gold endowment of more than 30 moz au in some individual deposits with smaller role of epithermal deposits this mineralization formed at 140 125 ma is partly coeval with the collisions of magmatic arcs with the passive margin sequences of the siberian craton and related granitoid magmatism about 200 moz of gold is known in the kolyma alaska collage in the mesozoic orogenic gold deposits and related quaternary placers the central kolyma indigirka south verkhoyansk and north chukotka subprovinces of the collage revealed an endowment of more than 10 moz au each a similar and coeval event in the mongolides in relation to the collision between siberia and north china is largely reflected in still poorly dated intrusion related gold deposits clustered along the mongol okhotsk suture the overlapping yanshanian magmatic arcs in transbaikalia and northeast china and the okhotsk chukotka magmatic arc in the russian far east stitch the kolyma alaska collage with the paleozoic central asian supercollage and adjacent cratons while the okhotsk chukotka arc reveals a relatively simple and broad oroclinal pattern the yanshanian arcs in mongolia and ne china form a tightly deformed giant z shaped feature that was bent in response to the southward movement of the siberian craton and northward translation of the nipponides and north china craton to close the mongol okhotsk suture in late jurassic to cretaceous times the yanshanian arcs host mostly small to medium sized 100 70 ma au ag deposits with the largest endowment discovered in the baley district in transbaikalia and at kupol in the northern part of the okhotsk chukotka arc some intrusion related gold deposits were formed synchronously with this arc magmatism with the largest known examples in the tintina belt in alaska formed at 104 and 93 91 ma the kamchatka aleutian collage is still evolving in front of the westward subducting pacific plate it s late cretaceous to paleogene magmatic arc rocks form immature island arc terranes extending from the aleutian islands towards the nipponides via kamchatka peninsula kuril islands and eastern sakhalin however in the nipponides the sikhote alin portion of the magmatic arc overlaps the mesozoic turbidite terranes the oroclinal pattern of this more than 8000 km long magmatic arc indicates its westward translation in agreement with the movement of the pacific plate so that the arc is presently colliding with itself along the island of sakhalin a seismically active intraplate lineament and a boundary between the nipponide and kamchatka aleutian collages this magmatic arc is usually interpreted to be of intra oceanic origin with subsequent docking to asia from the south however presence of the sea of okhotsk cratonic terrane between sakhalin and kamchatka suggests that it may be rather considered as an external arc system that separated from the rest of asia due to backarc spreading events therefore forming the most external arc system at the active margin with the pacific plate the subduction related events in the collage produced numerous late mesozoic to cenozoic 1 3 moz gold epithermal deposit in kamchatka and sikhote alin as well as au cu porphyry deposits with currently largest gold endowment in the pre tertiary pebble copper deposit in alaska the westward translation of the kamchatka aleutian collage might have controlled the emplacement of this porphyry deposit as well as up to 30 moz into intrusion related gold deposits at 70 65 ma in the kuskokwim belt immediately north from the porphyry cluster

a lithologic tectonic framework for the metallogenic provinces of california the lithologic tectonic framework of california developed principally during mesozoic time when various terranes of oceanic crust and island arc crust were accreted to older sialic crust resulting in westward growth of the continent emplacement of great batholithic masses of granitoid rocks cutting all these crustal types also took place during the mesozoic period the discrete tectonostratigraphic terranes that resulted from these events and subsequent tertiary and quaternary volcanic events are characterized by specific types of metallic mineral deposits or in some terranes by the virtual absence of deposits lead silver zinc replacement type deposits are common in the paleozoic carbonate terrane in the eastern part of the state and occur sporadically elsewhere in the miogeoclinal and cratonal terranes but are absent from the oceanic and island arc terranes the vast majority of contact metasomatic tungsten deposits including all the large ones are in pendants of miogeoclinal rocks in the sierra nevada batholith but the important atolia deposits reside in granitoid rocks that invade oceanic terrane molybdenum distribution closely follows that of tungsten all the large contact metasomatic iron deposits in california are in craton and miogeoclinal terranes but sparse small deposits of this type also occur in island arc terranes of the northern sierra nevada and eastern klamath mountains lode gold deposits although widely scattered show a marked preference for oceanic and island arc terranes that have been invaded by granitoid plutons all the major deposits including late tertiary bonanza deposits such as bodie are in such terranes it appears that magmatic processes were responsible for mobilizing and transporting the gold but the metal was perhaps derived from the eugeosynclinal rocks notably the mafic volcanics most mercury deposits are found in the coast ranges where they commonly occur in silica carbonate rock an alteration product of serpentinite the deposits appear to be spatially related to the coast range thrust and the source of the mercury may have been sedimentary rocks of the underlying franciscan assemblage epigenetic mineralization occurred at several different times during the mesozoic and again during miocene and pliocene time the timing of mineralization events and the distribution of various deposit types indicate that no broad scale zoning of epigenetic deposits exists around the sierra nevada batholith syngenetic deposits are represented mainly by massive sulfides chert associated manganese and chromite the massive sulfide deposits with one exception are restricted to island arc terranes and nearly all of these deposits are in silicic volcanic rocks they are interpreted to be syngenetic with the enclosing rocks although some redistribution of metals may have occurred after the original deposition the deposits occur in volcanic sequences of at least five different ages ranging from early devonian to late jurassic or early cretaceous and along with their enclosing rocks were probably formed at some distance from their present sites chert associated manganese deposits occur mainly in exotic blocks of oceanic crust in melange and probably formed in fairly deep ocean environments chromite is confined to ultramafic rock much of which occupies suture zones separating various accreted terranes

bald mountain gold mining district nevada a jurassic reduced intrusion related gold system the bald mountain mining district has produced about 2 million ounces moz of au geologic mapping field relationships geochemical data petrographic observations fluid inclusion characteristics and pb s o and h isotope data indicate that au mineralization was associated with a reduced jurassic intrusion gold deposits are localized within and surrounding a jurassic 159 ma quartz monzonite porphyry pluton and dike complex that intrudes cambrian to mississippian carbonate and clastic rocks the pluton associated dikes and au mineralization were controlled by a crustal scale northwest trending structure named the bida trend gold deposits are localized by fracture networks in the pluton and the contact metamorphic aureole dike margins high angle faults and certain strata or shale limestone contacts in sedimentary rocks gold mineralization was accompanied by silicification and phyllic alteration argillic alteration at shallow levels although au is typically present throughout the system exhibits a classic concentric geochemical zonation pattern with mo w bi and cu near the center ag pb and zn at intermediate distances and as and sb peripheral to the intrusion near the center of the system micron sized native au occurs with base metal sulfides and sulfosalts in peripheral deposits and in later stages of mineralization au is typically submicron in size and resides in pyrite or arsenopyrite electron microprobe and laser ablation icp ms analyses show that arsenopyrite pyrite and bi sulfide minerals contain 10s to 1 000s of ppm au ore forming fluids were aqueous and carbonic at deep levels and episodically hypersaline at shallow levels due to boiling the isotopic compositions of h and o in quartz and sericite and s and pb in sulfides are indicative of magmatic ore fluids with sedimentary sulfur together the evidence suggests that au was introduced by reduced s bearing magmatic fluids derived from a reduced intrusion the reduced character of the intrusion was caused by assimilation of carbonaceous sedimentary rocks tertiary faults dismember the area and drop down the upper part of the mineralizing system to the west the abundant and widespread kaolinite in oxide ores is relatively disordered 1a polytype and has d and 18o values suggestive of a supergene origin the deep weathering and oxidation of the ores associated with exhumation made them amenable to open pit mining and processing using cyanide heap leach methods

the geochemistry of ca sr ba and ra sulfates in some deep brines from the palo duro basin texas the geochemistry of ca sr ba and ra sulfates in some deep brines from the palo duro basin of north texas was studied to define geochemical controls on radionuclides such as 90sr and 226ra published solubility data for gypsum anhydrite celestite barite and raso4 were first reevaluated in most cases using the ion interaction approach of pitzer to determine solubility products of the sulfates as a function of temperature and pressure ionic strengths of the brines were from 2 9 to 4 8 m their temperatures and pressures up to 40 c and 130 bars saturation indices of the sulfates were computed with the ion interaction approach in one brine from the arkosic granite wash fades and four from the carbonate wolfcamp formation all five brines are saturated with respect to gypsum anhydrite and celestite and three of the five with respect to barite all are undersaturated by from 5 to 6 orders of magnitude with respect to pure raso4 226ra concentrations in the brines which ranged from 10 11 3 to 10 12 7 m are not controlled by raso4 solubility or adsorption but possibly by the solubility of trace ra solid solutions in sulfates including celestite and barite

an exhumed late paleozoic canyon in the rocky mountains abstract landscapes are thought to be youthful particularly those of active orogenic belts unaweep canyon in the colorado rocky mountains a large gorge drained by two opposite flowing creeks is an exception its origin has long been enigmatic but new data indicate that it is an exhumed late paleozoic landform its survival within a region of profound late paleozoic orogenesis demands a reassessment of tectonic models for the ancestral rocky mountains and its form and genesis have significant implications for understanding late paleozoic equatorial climate this discovery highlights the utility of paleogeomorphology as a tectonic and climatic indicator

shallowing and steepening subduction zones continental lithospheric loss magmatism and crustal flow under the central andean altiplano puna plateau

age of the sherman type zn pb ag deposits mosquito range colorado the sherman type zn pb ag dolomite deposits in central colorado are hosted in dolostones of the early mississippian leadville formation paleomagnetic analysis using progressive alternating field and thermal demagnetization and isothermal remanent magnetization acquisition methods was performed on specimens from samples at 37 sites in the sherman type continental chief peerless ruby sacramento and sherman deposits in their host rocks in the 72 ma pando porphyry sill s and in the 40 ma leadville type black cloud massive sulfide deposit paleomagnetic fold contact and breccia tests were performed to test for the antiquity of the magnetizations the results are interpreted to indicate that the leadville carbonates were regionally dolomitized at 308 6 1 ma in the early pennsylvanian and that the sherman type deposits were emplaced at 272 18 1 ma during the early permian after northeast trending block faulting karstification and 4 1 km of sedimentary burial possibly as the result of subsurface gravity driven fluid flow related to the ouachita marathon orogen following late ouachita marathon or earliest laramide late cretaceous folding the remanence in the sherman type deposits and the leadville dolostone rocks within the contact alteration zone of the 72 ma pando porphyry sill s was reset to acquire a late cretaceous normal characteristic remanent magnetization thereafter the black cloud leadville type massive sulfide deposit was magnetized in the eocene to acquire a reversed polarity characteristic remanent magnetization that was not found in the sherman type deposits

age and paleotectonic setting of volcanogenic massive sulfide deposits in the guerrero terrane of central mexico constraints from u pb age and pb isotope studies the guerrero terrane of central and western mexico comprises volcanic and sedimentary strata of mainly mesozoic age with geochemical signatures indicating formation in juvenile back arc to slightly more evolved arc environments at least 60 volcanogenic massive sulfide vms deposits have been discovered thus far within the guerrero terrane they comprise two main belts the coastal belt mainly comprising deposits in the zihuataneo subterrane including the cuale bramador district and the central belt which includes deposits in the teloloapan subterrane campo morado tlanilpa azulaquez tizapa rey de plata etc the guanajuato subterrane el gordo las gavalinas and the zacatecas subterrane san nicolas and el salvador deposits the age of host rocks for the vms mineralization in the various subterranes was poorly constrained prior to this study eleven new u pb zircon ages for volcanic host rocks from the campo morado tlanilpa azulaquez leon guanajuato and san nicolas el salvador vms districts together with recently determined u pb ages from the cuale area demonstrate that vms deposits in the guerrero terrane range from latest middle jurassic to early early cretaceous callovian to valanginian in age the oldest ages were obtained from the cuale district total age range of 157 4 4 1 ma for three units and the youngest are from the tlanilpa azulaquez district total age range of 139 7 2 5 ma for two samples volcanogenic massive sulfide deposits in the central belt resemble those of the bimodal siliciclastic deposit type whereas those in the coastal belt are more similar to the bimodal felsic type published geochemical and radiogenic isotope data from the various subterranes suggest that vms deposits in the coastal belt and most of those in the central belt are hosted within juvenile to slightly evolved arc settings the san nicolas and el salvador deposits of the zacatecas subterrane are the only deposits that unequivocally formed in a back arc setting our new u pb dating results together with existing stratigraphic geochemical and radiogenic isotope data from throughout the guerrero terrane are consistent with the guerrero terrane having developed as a west facing continental margin arc that was built on mainly oceanic crust and overlying early mesozoic siliciclastic fans shed from the east along the western margin of nuclear mexico slab rollback in the middle jurassic led to the development of a rocas verdes type continental back arc inboard of the arc which is now preserved as the arperos basin in the central and southern guerrero terrane and the host rocks for the san nicolas and el salvador deposits farther to the north in southeastern zacatecas state we have determined pb isotope compositions of sulfides from vms deposits and several other styles of mineralization from throughout the guerrero terrane a compilation of these and previously published data shows that isotopic compositions from the various vms deposits fall in a relatively confined field that overlaps the average upper crustal growth curve this suggests that metals were mainly sourced from continentally derived sedimentary strata that are interlayered with or immediately underlie the host volcanic section and or from the host volcanic rocks themselves or subvolcanic intrusive equivalents that had assimilated significant quantities of upper crustal material either at the point of origin or during subsequent ascent and eruption lead isotope compositions from the francisco i madera deposit in zacatecas state indicate that most of the sulfides in this deposit are epigenetic rather than syngenetic in origin and are likely tertiary in age

geology and uranium thorium mineral deposits of the bokan mountain granite complex southeastern alaska abstract the bokan mountain kendrick bay uranium thorium deposits are associated with a late jurassic peralkaline granite ring dike complex the uranium thorium deposits form pipe like bodies along contacts or occur as pods in en echelon northwest striking shear zones the host intrusive is a multiphased body consisting mainly of riebeckite granite porphyry and subordinately of pegmatite aplite aegirine granite porphyry and eight other lesser rock types the ore is closely associated with an albitized aegirine syenite the ore consists of thorium and rare earth rich rock containing uranothorite uraninite and generally less than 2 of sulfide species about 100 000 tons 89 000 t of ore have been mined at an average grade of about 1 u3o8 and nearly 3 tho2 mainly from the ross adams pipe the pipe itself lies along the contact of aegirine syenite and aegirine granite porphyry it is as much as 24 m across and was mined along strike for over 300 m the ore in shear zones occurs in lenses as much as 3 m thick and 30 m in strike length wallrock alteration within and adjacent to orebodies consists of pervasive hydrothermal albite and lesser amounts of chlorite fluorite calcite quartz sericite and tourmaline hematite is present in the outer distal parts of the ore zones the minerals produced to date occur within the outer annular zone of the bokan granite complex in which aegirine is present as the major alkali ferro magnesian mineral ore emplacement occurred during the first subsidence event in association with devolatilization of the magma chamber the pod like ore zones appear to have formed during regional faulting synchronous with magma crystallization and subsequent hydrothermal events subsequent magma crystallization yielded riebeckite granites that were depleted in u and th filling temperatures in ore stage fluid inclusions within quartz range from 320 to 331 c with pressure corrections increasing the temperature of formation to 420 c or higher depending on dissolved gas content sulfur isotopic analyses on pyrite galena sphalerite and pyrrhotite indicate disequilibrium but 34sh2s is estimated at 7 6 carbon and oxygen isotopic analyses on ore stage calcite yield 18oh2o of 6 8 to 8 1 and 13c c of 4 3 to 7 0 the oxygen and carbon isotopic data support a magmatic origin for the calcite and associated thorium uranium deposits

paleogeographic and paleotectonic setting of laramide sedimentary basins in the central rocky mountain region in the rocky mountain region between central montana and central new mexico sedimentologically isolated nonmarine basins were produced by basement deformation during the laramide orogeny within the area formerly occupied by a broad late cretaceous foreland basin in which laterally continuous marine facies had accumulated previously laramide structures of varying trend and scale reflect heterogeneity of crustal strain caused by shear between the continental lithosphere and an underlying subhorizontal slab of subducted oceanic lithosphere laramide basins include perimeter basins along the cratonic periphery of the arcuate laramide province axial basins along a north south intramontane trend and ponded basins located farther west closer to the overthrust belt twelve specific stratigraphic and sedimentologic criteria for the onset duration and termination of laramide deformation allow the chronology of basin development to be inferred independently for each basin maastrichtian initiation of laramide deformation was approximately synchronous throughout the laramide province but termination of laramide deformation was systematically diachronous from north to south between early and late eocene time widespread eocene erosion surfaces truncate syntectonic laramide sequences and are overlain by largely volcanic and volcaniclastic post laramide strata of eocene age in the north and oligocene age in the south fluvial depositional systems draining toward the great plains were dominant in perimeter and axial basins but ponded basins were occupied at times by large lakes that served as regional sediment traps paleocene drainages from ponded basins also led eastward toward the continental interior but partly interconnected lakes that developed within several ponded basins by mid eocene time were either closed hydrologically or else they drained westward into the tyee paleoriver of the pacific northwest

geologic framework and geochronology of ultramafic complexes of southern mexico in southern mexico discrete ultramafic intrusive bodies larger than 4 km2 are genetically related to their enclosing volcano sedimentary terranes these terranes are the cuicateco and guerrero which include the cuicateco and tierra caliente metamorphic complexes respectively their basement is largely unknown and the ultramafic masses previously have been interpreted as allochthonous dismembered ophiolites to constrain the age of these accreted terranes the geologic setting and 40ar 39ar ages are presented from the localities of loma baya el tamarindo guerrero and san pedro limon state of mexico in the tierra caliente complex and teotitlan conception papalo oaxaca in the cuicateco complex the teotitlan conception papalo area is characterized by a sequence of metamorphosed intermediate granitoid rocks andesitic lavas tuffs and psammites the intrusive rocks and lavas are pervasively mylonitized and show abundant ultramafic segregations the largest ultramafic bodies were emplaced int

crystallization and alteration of quartz monzonite iron springs mining district utah relation to associated iron deposits the iron springs mining district of southwestern utah contains the largest iron deposits in the western united states these are related to emplacement at minimum depths of 1 to 2 3 km of three miocene laccoliths of porphyritic quartz monzonite when emplaced the magma was nearly half crystallized on the verge of brittle behavior and contained phenocrysts of plagioclase amphibole biotite and clinopyroxene granophyric dikes in quartz monzonite formed from residual liquid but miarolitic cavities in these dikes are lined with alpha quartz indicating that deposition continued from a volatile rich phase below the solidus pervasive high temperature alteration led to replacement of augite by diopside oxidation of magnetite and ilmenite replacement of amphibole completely and biotite partially by aggregates of diopside alkali feldspars smectite chlorite magnetite ilmenite and calcite albitization of groundmass alkali feldspar exchange of mg for fe and of f for oh in biotite and exchange of f and oh for cl in apatite this alteration produced whole rock enrichment in fe super 3 na and h 2 o and losses in fe super 2 ca and k total fe was effectively unchanged a chilled relatively impermeable margin surrounding the pervasively altered interior of each laccolith was mechanically bonded to wall rock more strongly than to the magmatic interior and deformed with the wall rock during subsequent intrusion and structural readjustment in this peripheral shell clinopyroxene amphibole and biotite were less altered where the roof of each laccolith had a radius of curvature of less than 2 km a heterogeneous jointed zone formed below the peripheral shell in this zone pervasively altered quartz monzonite was fractured by renewed intrusion or by structural readjustment of roof rocks along the resulting joints quartz monzonite was altered to bleached zones selvages during a second stage of alteration within these selvages complete decomposition of biotite that had survived pervasive alteration was accompanied by further growth of diopside albitization of feldspars and dissolution of some apatite unlike early pervasive alteration selvage alteration was fracture controlled and caused strong whole rock depletion in light ree total fe mg p rb y ba and h 2 o super but caused enrichment in na magnetite apatite veins commonly formed along the joint adjacent to zones of selvage joints strata bound orebodies of magnetite hematite and apatite formed in limestone around the three laccoliths iron bearing fluid was derived by subsolidus alteration from quartz monzonite there is no evidence supporting immiscible separation of iron oxide rich liquid from silicate magma mass balance calculations indicate that the fe p s al and mg in the orebodies could have been entirely supplied by selvage alteration of 40 km 3 of quartz monzonite in agreement with the exposed and confidently inferred volume of altered intrusive rock a fourth pluton lacking magnetite veins orebodies and selvages was emplaced closer to the surface and against sandstone and share rather than limestone iron liberated from hydrous mafic phenocrysts in this body was oxidized in place forming disseminated hematite

iron oxide copper gold deposits an andean view iron oxide copper gold iocg deposits defined primarily by their elevated magnetite and or hematite contents constitute a broad ill defined clan related to a variety of tectono magmatic settings the youngest and therefore most readily understandable iocg belt is located in the coastal cordillera of northern chile and southern peru where it is part of a volcano plutonic arc of jurassic through early cretaceous age the arc is characterised by voluminous tholeiitic to calc alkaline plutonic complexes of gabbro through granodiorite composition and primitive mantle derived parentage major arc parallel fault systems developed in response to extension and transtension induced by subduction roll back at the retreating convergent margin the arc crust was attenuated and subjected to high heat flow iocg deposits share the arc with massive magnetite deposits the copper deficient end members of the iocg clan as well as with manto type copper and small porphyry copper deposits to create a distinctive metallogenic signature

evolution of metal ratios and δ34s composition of sulfide mineralization during anhydrite cap rock formation hockley dome texas u s a metal zonation and sulfur isotope patterns of cap rock hosted sulfides reveal a complex evolution of the sulfide depositing system at the hockley salt dome the general metal trend observed is an overall increase of zn relative to pb downward in the anhydrite cap rock in the upper cap rock section the pb pb zn ratios commonly range from 0 25 to 0 40 ratios of 0 20 and commonly 0 10 are typical of the lower cap rock section the concentration of ag also appears to increase in the lower part of the anhydrite cap rock textural evidence indicates that some of the metal concentrations represent stratiform sulfides that were precipitated at the salt cap rock contact as the anhydrite zone accumulated by sequential underplating the metal ratio data suggest that the sulfide precipitating fluid became more zn and possibly ag rich during anhydrite cap rock accumulation 34s values of sulfide minerals within the hockley dome cap rock range from 30 to 4 cdt 59 analyses a profile of 34s values from a single drill hole indicates that sulfide minerals become progressively heavier with depth to the approximate middle of the cap rock where the isotopic trend reverses the inflection point corresponds with a local highly mineralized biogenic calcite zone a mixing model involving variable contributions of isotopically heavy and light sulfur end members appears to best account for the observed isotopic trends fault leaked h2s from mature oil and gas reservoirs is a possible extrinsic source of heavy sulfur but local thermochemical sulfate reduction of cap rock sulfate cannot be eliminated by existing data bacterial reduction of cap rock sulfate is believed to be the source of light h2s the relative abundance of deep sourced metalliferous brines appears to have increased with time during the accretion of the upper part of the anhydrite cap rock biogenic h2s subsequently began to dominate the mixed sulfur reservoir resulting in a 15 shift and relatively light 34s values of pyrite to the present cap rock salt contact

a diatreme hosted gold deposit at montana tunnels montana a bulk minable gold silver deposit at montana tunnels southwestern montana is located in the central part of a diatreme this diatreme was emplaced along the faulted contact between andesitic volcaniclastic rocks assigned to the late cretaceous elkhorn mountains volcanics and a sequence of quartz latitic ignimbrites which are part of the lowland creek volcanics of middle eocene age a north northeast striking swarm of quartz latite porphyry dikes was emplaced during the waning stages of lowland creek volcanism the two largest dikes dated at 45 to 50 m y middle eocene were intruded into the diatreme prior to the cessation of brecciation and mineralization the diatreme underlies a 1 km 2 area and is known from drilling to extend steeply downward for at least 310 m the principal rock type in the diatreme is a matrix rich breccia it is characterized by a sand size tuffaceous matrix of quartz latitic composition containing fragments of contiguous volcanic wall rocks and intrusive rocks derived from the late cretaceous boulder batholith the batholith is inferred to underlie the montana tunnels area foundered blocks of volcanic wall rocks lie at various attitudes against the walls of the diatreme and within it those composed of elkhorn mountains volcanics underwent hydraulic brecciation after they sank into the diatreme blocks interpreted to be composed of pyroclastic base surge deposits from a subaerial tuff ring and a piece of carbonized wood subsided into the diatreme from the palcosurface the diatreme is separated into two unequal parts by a west northwest striking oblique slip fault sulfide minerals in the diatreme occur as disseminations in the breccia matrix and in sub ordinate amounts as widely spaced multidirectional veinlets and as the matrix to brecciated blocks of elkhorn mountains volcanics all forms of mineralization consist of pyrite sphalerite galena minor chalcopyrite and rare electrum accompanied by a gangue of manganocalcite siderite and minor quartz clastic grains and fragments of the sulfides and gangue minerals are common and attest to multiple alternating episodes of brecciation and mineralization in the ore zone the introduction of abundant manganocalcite and siderite was accompanied by pervasive sericitic alteration but only weak kaolinization and silicification sericitization grades outward beyond the ore zone to chlorite montmorillonite carbonate alteration an assemblage that also characterizes the interiors of the late mineral dikes in the diatreme gold occurs in electrum with a fineness of about 550 as inclusions of less than 200 mu m in pyrite and sphalerite inclusions of electrum and galena were precipitated together silver is present mainly in solid solution in galena the montana tunnels gold silver deposit is characterized geochemically by anomalously high concentrations of zinc lead and manganese and in comparison to many volcanic hosted precious metal deposits is low in arsenic antimony and mercury this metal association and the low fineness of the gold are features shared by the wau gold deposit in papua new guinea it is suggested that the mineralized diatreme observable at montana tunnels was emplaced several hundred meters beneath the palcosurface upon which a gold bearing maar volcano like that partly preserved at wau was constructed

fluid inclusion delta 18 o and 87 sr 86 sr evidence for the origin of fault controlled copper mineralization lisbon valley utah and slick rock district colorado

early late cretaceous early tertiary gold mineralization a galena lead isotope study of the bridge river mining camp southwestern british columbia canada the bridge river gold camp 180 km north of vancouver canada produced more gold than any other camp in british columbia over its 70 years of operation 130 metric tons or 4 million oz mainly from the bralorne and pioneer deposits a compilation of new and existing lead isotope ratios from 20 widespread and diverse deposits in the bridge river camp shows a distinct clustering of data this is interpreted to be the expression of a single protracted but episodic mineralizing event coinciding with the emplacement of the coast plutonic complex during early late cretaceous to early tertiary time 90 45 ma the timing of this event is constrained at bralorne by a premineral albitite dike set 91 4 or 1 4 ma by u pb on zircon and a late intramineral hornblende porphyry dike set 85 7 or 3 ma by k ar on hornblende late lamprophyre dikes that crosscut the mineralized veins dated at 43 5 or 1 5 ma by k ar on biotite set a lower age limit for the mineralization the host rocks the bralorne intrusions have no genetic relation to the mineralization since they are early permian 270 or 5 ma by u pb on zircon the conclusion that all mineralization is related to a coherent event is in accord with recognition of a districtwide mineral zoning scheme by woodsworth pearson and sinclair in this scheme a sequence from high temperature au as w mo through intermediate sb ag au as to low temperature sb hg vein assemblages as generated from southwest to northeast within the thermal aureole of the coast plutonic complex stable isotope and temperature data from other workers support this northeastly decreasing gradient in mineralization temperature clustered lead isotope data define a trend that plots between model pb isotope curves of doe and zartman for the upper crust and the mantle this trend is interpreted to be a mixing line between primitive mantle type lead and more radiogenic lead by one or a combination of two postulated processes between two groups of rocks more primitive oceanic volcanics and more radiogenic upper crustal volcanic arc clastic rocks by a meteoric thermal system generated by the emplacement of the coast plutonic complex or between less and more radiogenic magmas during the emplacement of the mesozoic cenozoic volcanics and intrusion hosting the deposits the accreted nature of the host oceanic and arc terranes is emphasized by the differences between the model proposed and the upper crustal shale curve model of godwin and sinclair which represents much of the locally adjacent autochthonous portion of the canadian cordillera

the late cretaceous donlin creek gold deposit southwestern alaska controls on epizonal ore formation the donlin creek gold deposit southwestern alaska has an indicated and inferred resource of approximately 25 million ounces moz au at a cutoff grade of 1 5 g t the ca 70 ma deposit is hosted in the late cretaceous kuskokwim flysch basin which developed in the back part of the arc region of an active continental margin on previously accreted oceanic terranes and continental fragments a hypabyssal mainly rhyolitic to rhyodacitic and commonly porphyritic 8 3 km dike complex part of a regional ca 77 to 58 ma magmatic arc formed a structurally competent host for the mineralization this deposit is subdivided into about one dozen distinct prospects most of which consist of dense quartz carbonate veinlet networks that fill north northeast striking extensional fractures in the northeast trending igneous rocks the sulfide mineral assemblage is dominated by arsenopyrite pyrite and typically younger stibnite gold is refractory within the arsenopyrite sericitization carbonatization and sulfidation were the main alteration processes fluid inclusion studies of the quartz that hosts the resource indicate dominantly aqueous ore fluids with also about 3 to 7 mol percent co2 ch4 and a few tenths to a few mole percent nacl kcl the gold bearing fluids were mainly homogeneously trapped at approximately 275 to 300 c and at depths of 1 to 2 km some of the younger stibnite may have been deposited by late stage aqueous fluids at lower temperature measured 18o values for the gold bearing quartz range between 11 and 25 per mil the estimated 18o fluid values range from 7 to12 per mil suggesting a mainly crustally derived fluid a broad range of measured d values for hydrothermal micas between 150 and 80 per mil is suggestive of a contribution from devolatilization of organic matter and or minor amounts of mixing with meteoric fluids gold associated hydrothermal sulfide minerals are characterized by 34s values mainly between 16 and 10 per mil with the sulfur derived from diagenetic pyrite and organic matter within the flysch basin a smaller group of 34s measurements which shows values as depleted as 27 per mil suggests a different local sulfur reservoir in the basin for the later hydrothermal episode dominated by stibnite initial nd of 8 7 to 3 1 and 87sr 86sr measurements of 0 706 to 0 709 for the ore hosting dikes also indicate a crustal reservoir for some of the late cretaceous magmatism overlapping lead isotope data for these intrusive rocks and for sulfide minerals suggest a crustal contribution for the lead in both copper and gold bearing stockwork veinlets in hornfels occur at dome a prospect located at the northern end of the donlin creek deposit these stockworks are cut by the younger auriferous gold veins that define the main donlin creek gold mineralization highly saline gas rich heterogeneously trapped fluids deposited the stockworks at temperatures approximately 100 c hotter than those of the main gold forming event at donlin creek the genetic relationship of the dome prospect to the main donlin creek gold resource is equivocal the epizonal donlin creek deposit shows affinities to the gold systems interpreted by various workers as orogenic or intrusion related it shows important differences from typical epithermal and carlin like deposits the ore forming fluids were derived by either broad scale metamorphic devolatilization above rising mantle melts or exsolution from a magma that was dominated by a significant flysch melt component

volcanogenic massive sulfide deposits and the missing complement to the calc alkaline trend evidence from the jurassic talkeetna island arc of southern alaska

special paper major gold deposits and belts of the north and south american cordillera distribution tectonomagmatic settings and metallogenic considerations a compilation of economically viable gold concentrations containing 10 moz in the north and south american cordillera reveals the existence of 22 discrete belts in addition to five major isolated deposits most formed over the last 150 m y the gold concentrations are attributed to eight widely recognized deposit types of which porphyry sediment hosted and high sulfidation epithermal are economically the most important individual gold belts are typically several tens to hundreds of kilometers long dominated by single deposit types and metallogenically active for relatively brief periods 5 20 m y many of the gold belts and major isolated deposits were generated under extensional or transtensional tectonic conditions in either arc or back arc settings nevertheless the two main high sulfidation epithermal gold belts were generated in thickening or already thickened crust during low angle subduction eight other gold belts or districts also accompanied compression or transpression with two of them the main orogenic gold belts occupying fore arc sites there is a strong suggestion that the preeminent cordilleran gold concentrations formed either during or immediately following prolonged contraction the major gold deposits and belts occur along the craton edge as well as in adjoining accreted terranes but almost all are of postaccretionary timing many of the gold belts and isolated deposits were localized by crustal scale faults or lineaments which may be either parallel or transverse to the cordilleran margin the gold concentrations accumulated during active subduction commonly in close spatial and temporal association with intermediate to felsic medium to high k calc alkaline igneous rocks by contrast the low sulfidation epithermal gold deposits accompany bimodal volcanic pulses of calc alkaline tholeiitic or alkaline affinity however a gold alkaline rock association is uncommon a genetic link between gold mineralization and coeval magmatism is widely accepted for most of the deposit types the exceptions being the sediment hosted and orogenic gold deposits notwithstanding the small cumulative extent of the gold concentrations relative to the entire cordilleran margin there is a marked tendency for two or more belts or isolated deposits of different ages and genetic types to occur in close proximity within relatively restricted arc including fore and back arc segments in the case of the western united states for example six belts and four isolated major deposits make up a particularly prominent cluster if fortuity is discounted this clustering or pairing of gold concentrations must imply a predisposition of certain arc segments to gold mineralization an analogous situation is evident for other metals particularly copper and tin the reason for the recurrent generation of major deposits and belts dominated by one or more metals remains uncertain although heterogeneously distributed metal preconcentrations favorable redox conditions or other parameters somewhere above the subducted slab between the mantle wedge and upper crust are widely contemplated possibilities elucidation of the reason s for this metallogenic inheritance at the scale of limited arc segments poses an important and challenging series of research questions as well as being critical to the planning of potentially successful greenfield exploration programs

stratigraphy structure and geochronology of the anyox pendant northwest british columbia and implications for mineral exploration anyox pendant contains complexly deformed greenschist facies volcanic strata overlain by a thick succession of turbidite sedimentary rocks at the contact between the two are well studied copper rich volcanogenic massive sulphide deposits however the age and correlation of the volcanic strata including the ore horizon are poorly known new stratigraphic structural and geochronology studies elucidate the regional stratigraphic tectonic and metallogenic settings and significance of these strata the thick turbidite succession is correlated with bowser lake group and conformably overlies the volcanic succession volcanogenic units include pillowed basalt volcaniclastic rocks chlorite phyllite tuffaceous siltstone and abundant gabbroic sills and dykes strained leucocratic intrusions yielded u pb zircon crystallization ages of 176 9 0 2 ma and 185 6 0 3 ma volcaniclastic rocks yielded detrital zircons that constrain the maximum ages of the volcanic succession the youngest zircon grains from four samples in different structural segments limit strata to younger than ca 186 ma ca 178 ma ca 183 ma and ca 186 ma the results combined with the regional oldest age limit of bowser lake group constrain the volcanic succession to late early jurassic and early middle jurassic age and support correlation with hazelton group of stikinia furthermore they enable comparisons with the coeval au ag rich stratiform eskay creek deposit expanding southward the zone of extension in western stikinia which tapped primitive mafic magmas and accompanied the last vestiges of stikinian volcanism in the early middle jurassic

sulfur redox reactions hydrocarbons native sulfur mississippi valley type deposits and sulfuric acid karst in the delaware basin new mexico and texas hydrocarbons native sulfur mississippi valley type mvt deposits and sulfuric acid karst in the delaware basin southeastern new mexico and west texas usa are all genetically related through a series of sulfur redox reactions the relationship began with hydrocarbons in the basin that reacted with sulfate ions from evaporite rock to produce isotopically light 34s 22 to 12 h2s and bioepigenetic limestone castiles this light h2s was then oxidized at the redox interface to produce economic native sulfur deposits 34s 15 to 9 in the castiles paleokarst and along graben boundary faults this isotopically light h2s also migrated from the basin into its margins to accumulate in structural anticlinal and stratigraphic yates siltstone traps where it formed mvt deposits within the zone of reduction 34s 15 to 7 later in time in th zone of oxidation this h2s reacted with oxygenated water to produce sulfuric acid which dissolved the caves e g carlsbad cavern and lechuguilla cave guadalupe mountains massive gypsum blocks on the floors of the caves 34s 25 to 4 were formed as a result of this reaction the h2s also produced istopically light cave sulfur 34s 24 to 15 which is now slowly oxidizing to gypsum in the presence of vadose drip water

dynamic subsidence and uplift of the colorado plateau we use inverse models of mantle convection to explore the vertical evolution of the colorado plateau by satisfying multiple constraints seismic tomography stratigraphy in the western united states and great plains and other structural and volcanic data adjacent to the plateau the model provides predictions on the continuous history of colorado plateau vertical motion since 100 ma with the arrival of the flat lying farallon slab dynamic subsidence swept from west to east over the plateau and reached a maximum ca 86 ma two stages of uplift followed the removal of the farallon slab below the plateau one in the latest cretaceous and the other in the eocene with a cumulative uplift of 1 2 km both the descent of the slab and buoyant upwellings raised the plateau to its current elevation during the oligocene a locally thick plateau lithosphere enhances the coupling to the upper mantle so that the plateau has a higher topography with sharp edges the models predict that the plateau tilted downward to the northeast before the oligocene caused by northeast trending subduction of the farallon slab and that this northeast tilting diminished and reversed to the southwest during the miocene in response to buoyant upwellings

anomalous cold in the pangaean tropics comment comment soreghan et al 2008 1 present the hypothesis that glaciation existed to near sea level in the tropical region of pangaea in late paleozoic time their most important evidence for this comes from unaweep canyon a wind gap that crosses the uncompahgre plateau colorado united states the

anomalous cold in the pangaean tropics reply we thank w hood et al 2009 1 for their comment on our geology paper soreghan et al 2008 2 we recognize that our hypothesis of episodic cold in the pangaean tropics which relies in part on our proposal of a late paleozoic age and glacial origin for unaweep canyon is radical hence

skarn manto and breccia pipe formation in sedimentary rocks of the cananea mining district sonora mexico skarns mantos and breccia pipes occur at cananea in a 2 by 4 km horst of paleozoic carbonate rock and minor quartzite surface mapping and core logging reveal a sequence of events beginning with early metamorphism that converted impure carbonate lithologies to iron poor garnet pyroxene or idocrase hornfels subsequent metasomatism formed garnet py roxene skarn along the pre cretaceous elisa fault contact between carbonate rock and mesozoic volcanic rock skarn is zoned from an andradite rich center through a zone containing both andraditic garnet and salitic pyroxene to a mineralogic sequence near the marble front which is largely a function of the sedimentary protolith chert nodules are rimmed by wollastonite dolomite is converted to massive phlogopite magnetite skarn calc silicate hornfels is over printed by veins of skarn garnet and relatively pure marble is replaced by coarse blades of iron and manganese rich pyroxene multiple generations of garnet and pyroxene can be distinguished by subtle variations in color and texture age classification on the basis of crosscutting vein and overgrowth relationships indicates that late pyroxenes are more iron and manganese rich than the early generations garnets show a less regular iron enrichment with time the spatial distribution of the different garnet and pyroxene generations is irregular garnet and pyroxene in late veins have iron rich compositions both near the skarn center and near the marble front pyrite and minor chalcopyrite are the only sulfides associated with this stage of metasomatism following the main stages of garnet and pyroxene formation veins and orbicular patches of amphibole or quartz or calcite occur replacing pyroxene and in some cases garnet most of the amphibole is actinolitic in composition and is associated with pyrite and minor amounts of chalcopyrite about 20 percent of the amphibole is subcalcic and is associated with or replaced by massive calcite the alteration of prograde skarn to amphibole or quartz or calcite can be best explained by a general temperature decline the destruction of skarn by alteration related to brecciation and breccia pipe formation and the replacement of previously unaltered carbonate rocks by stratiform blankets mantos of iron oxides and sulfides resulted in some of the highest grade orebodies breccia pipe and manto formation appears to have been largely contemporaneous with emplacement and subsequent sericitic alteration of a series of quartz monzonite porphyry stocks where breccia pipes crosscut skarn garnet pyroxene and amphibole are converted to mixtures of calcite quartz chlorite hematite siderite and sulfides where breccia pipes or porphyries crosscut previously unaltered carbonate rock mixtures of magnetite sulfides chlorite siderite calcite quartz and serpentine form massive mantos veinlets of chlorite and magnetite or pyrite or serpentine extend tens of meters beyond the zones of massive replacement there is a rough vertical and lateral zonation of sulfide minerals with respect to the democrata breccia pipe from chalcopyrite or bornite or pyrite in the core and at depth to sphalerite pyrite chalcopyrite to pyrite sphalerite galena distal to the center of mineralization mineral stability relations suggest that brecciation and mineralization took place at lower temperatures 275 degrees 25 degrees c and possibly under lower x sub co 2 conditions than the earlier skarn formation the high zn cu ratios in skarn the zonation of most hypogene mineralization relative to the breccia pipes which crosscut skarn and the lack of skarn spatially associated with the quartz monzonite porphyry stocks which intrude carbonate rock all suggest that unlike skarn in most porphyry copper districts skarn at cananea is not related to the quartz monzonite porphyry stocks that are mined elsewhere in the district for disseminated supergene enriched sulfide mineralization rather the skarn may be related to a deeper magmatic system which has not yet been encountered in subsurface exposures

release of uranium from volcanic glass in sedimentary sequences an analysis of two systems uranium can be released from volcanic glass shards by solution of the shards studies of the tertiary tascotal formation of trans pecos texas suggest that total solution of shards during diagenesis in an open meteoric flow system does not release uranium for migration instead uranium is only redistributed locally in the tertiary catahoula formation of the texas coastal plain uranium also was not mobilized for long distance migration and ore formation during diagenesis of glass in the zone of saturation within an open meteoric flow system but was released during solution and argillation of glass by subaerial soil forming processes and diagenesis in the vadose zone tuffaceous facies are depleted in proportion to the intensity of vadose and pedogenic processes it is unclear why uranium mobility is limited within meteoric flow systems release for long distance migration can be documented by chemical evidence for depletion of uranium in potential source beds soil processes impart typical vadose microscopic textures and cause formation of distinctive outcrop features diagenesis in open meteoric flow systems occurs below the water table producing characteristic isopachous rims of secondary minerals together such field textural and geochemical criteria allow recognition of functional sources of uranium which in turn provides a rapid inexpensive evaluation of regional resource and exploration potential

anomalous cold in the pangaean tropics the late paleozoic archives the greatest glaciation of the phanerozoic whereas high latitude gondwanan strata preserve widespread evidence for continental ice the permo carboniferous tropics have long been considered analogous to today9s warm and shielded from the high latitude cold here we report on glacial and periglacial indicators that record episodes of freezing continental temperatures in western equatorial pangaea an exhumed glacial valley and associated deposits record direct evidence for glaciation that extended to low paleoelevations in the ancestral rocky mountains furthermore the permo carboniferous archives the only known occurrence of widespread tropical loess in earth9s history the volume chemistry and provenance of this loess ite is most consistent with glacial derivation together with emerging indicators for cold elsewhere in low latitude pangaea these results suggest that tropical climate was not buffered from the high latitudes and may record glacial interglacial climate shifts of very large magnitude coupled climate ice sheet model simulations demonstrate that low atmospheric co 2 and solar luminosity alone cannot account for such cold and that other factors must be considered in attempting to explain this best known analogue to our present earth

geochemical and geochronological constraints on mineralization within the hilltop lewis and bullion mining districts battle mountain eureka trend nevada the hilltop lewis and bullion mining districts located within the northern shoshone range lander county nevada are part of the greater battle mountain eureka trend historic production of mostly ag with au cu pb and zn from all three districts exploited 3 m wide quartz veins hosted mainly within upper plate ordovician devonian siliceous and siliciclastic rocks of the roberts mountains allochthon eocene felsic intrusive rocks located both proximal and distal to the vein deposits were emplaced throughout the northern shoshone range along a west northwest trend molybdenite mineralization is associated with some intrusive rocks and some intrusive rocks host vein type mineralization new and previously published radiometric ages for primary igneous minerals from unaltered rocks within and or proximal to mineralized areas are nearly coincident with published re os ages for several northern shoshone range molybdenite occurrences 40 1 0 6 ma avg supporting a relationship between pluton emplacement and molybdenite mineralization age differences between quartz vein hosted gold base metal mineralization e g 38 3 0 1 ma gray eagle mine bullion district and nearby intrusive rocks e g 39 0 0 1 ma granite mountain imply that vein mineralization probably formed during prolonged hydrothermal activity synchronous with or subsequent to igneous rock emplacement or from heat associated with a second slightly younger phase of intrusive igneous activity primary fluid inclusion salinity 0 6 4 wt nacl equiv and th 115 425 c data from ore bearing vein quartz in conjunction with measured 18ovsmow data 0 97 to 17 3 indicate that vein forming fluids contained variable mixtures of magmatic and meteoric components calculated 18ovsmow values of 16 0 to 13 3 and spanned the epithermal porphyry temperature continuum new and previously published stable isotope c o and s data from vein hosted ore and gangue minerals unaltered and altered siliceous upper plate host rocks and unaltered carbonate lower plate and upper plate rocks indicate mixed and variable sources of vein hosted ore and gangue components and variable interaction between some vein forming fluids and nonsiliceous rock types carbonates bedded barite that underlie the northern shoshone range large vein widths high vein densities and lack of evidence supporting fluid boiling in primary fluid inclusions suggest that vein deposits of the northern shoshone range probably formed at shallow depths 1 km under lithostatic conditions geochronology and stable isotope data support a temporal and genetic link between eocene magmatism quartz vein hosted precious and base metal mineralization and molybdenite porphyry mineralization within the hilltop lewis and bullion mining districts

structural controls and evolution of gold silver and ree bearing copper cobalt ore deposits blackbird district east central idaho epigenetic origins the cu co au ag ni ree ore deposits of the blackbird district east central idaho have previously been classified as besshi type vms sedex and iocg deposits within an intact stratigraphic section new studies indicate that across the district mineralization was introduced into the country rocks as a series of structurally controlled vein and alteration systems quartz rich and biotite rich veins and alteration zones and minor albite and siderite veinlets maintain consistent order and sulfide mineral associations across the district both early and late quartz veins contain chalcopyrite and pyrite whereas intermediate stage tourmaline biotite veins host the cobaltite barren early and late albite and late carbonate generally siderite form veins or are included in the quartz veins ree minerals principally monazite allanite and xenotime are associated with both tourmaline biotite and late quartz veins the veins are in mineralized intervals along axial planar cleavage intrafolial foliation and shears mineralized intervals are hosted by a variety of metasedimentary rocks including three phyllitic units of mesoproterozoic age and two schistose units all of these units are s tectonites in the footwall of a regional thrust fault specifically the district lies within an oblique thrust ramp containing a series of structural horses three domains in a duplex system the deposits span the three domains and are hosted by metamorphic rocks that range from lower amphibolite facies in the structurally upper domain to lower middle greenschist facies in the lower domain an inverted metamorphic sequence early quartz and biotite veins were introduced during progressive folding and prolonged peak metamorphic conditions and they underwent late tectonic retrograde recrystallization and metamorphic mineral growth to the same extent as the country rocks in each domain where little subsequent deformation occurred early veins are discordant to bedding but where folding was polyphase and fabrics are penetrative mineralized zones are concordant with metamorphic compositional layering late quartz veins in the zones are associated with retrograde minerals and textures and are only locally deformed 40 ar 39 ar dating of unoriented muscovite from the selvage of a late quartz vein yields a late cretaceous age of about 83 ma the time of retrograde metamorphism associated with introduction of late quartz veins textural data at all scales indicate that the host sites for veins and the tectonic evolution of both host rocks and mineral deposits were kinematically linked to late cretaceous regional thrust faulting heat fluids and conduits for generation and circulation of fluids were part of the regional crustal thickening the faulting also juxtaposed metaevaporite layers in the mesoproterozoic yellowjacket formation over blackbird district host rocks we conclude that this facilitated chemical exchange between juxtaposed units resulting in leaching of critical elements cl k b na from metaevaporites to produce brines scavenging of metals co cu etc from rocks in the region and finally concentrating metals in the lower plate ramp structures although the ultimate source of the metals remains undetermined the present cu co au ag ni ree blackbird ore deposits formed during late cretaceous compressional deformation

geochronology of the porphyry copper deposits and related mineralization of mexico k ar dating demonstrates that all but eight of 41 dated porphyry copper and related ore deposits of mexico were emplaced during the laramide episode of maximum plate convergence one older deposit is related to the jurassic volcanic arc of western north america one is pre laramide cretaceous four are oligocene in age and two late cenozoic deposits are within the modern trans mexican chiapenecan volcanic arc thirty three of the deposits lie within a long narrow belt that continues into arizona and new mexico and widens from 100 km to over 300 km in the region of maximum extension in the southern basin and range province eighty five percent of the deposits were emplaced during the eastward transgression of the cordilleran volcanic arc in middle cretaceous through eocene time the occurrence of the porphyry copper deposits of mexico appears to be independent of the terrane intruded and the copper content of the wall rocks where the wall rocks predate the volcanic arc which is syngenetic with the porphy

geological setting u pb geochronology and radiogenic isotopic characteristics of the permo triassic kutcho assemblage north central british columbia the kutcho assemblage is a latest permian to early triassic volcano sedimentary sequence within the fault bounded king salmon allochthon volcanic and volcaniclastic rocks consist of massive to pillowed flows and tuffs of basaltic to basaltic andesitic composition as well as flows mass flows and pyroclastic flows of rhyodacitic to rhyolitic composition the volcanic sequence is intruded by gabbro diorite trondhjemite and quartz plagioclase porphyry volcanic and intrusive rocks have tholeiitic magmatic affinities with die exception of the gabbro sills and dykes which are chemically similar to alkaline arc magmas cu zn volcanogenic massive sulphide mineralization at die kutcho creek deposit is hosted by rhyolite mass flows near me top of the kutcho assemblage rhyolite mass flows from the hanging wall and footwall to mineralization have u pb ages of 242 1 and respectively whereas a quartz plagioclase porphyritic intrusion to the south of the mineralization has a u pb age of 244 6 ma the da

platinum group element behaviour and thermochemical constraints in the ultrabasic basic complex of the vizcaino peninsula baja california sur mexico the vizcaino complex is characterized by tectonised harzburgites overlain by serpentinized dunite bodies and wherlites layers containing several cumulate chromitite deposits and by fractional crystallization sequences cyclic repetititons of dunites pyroxenites with pegmatoidal textures and gabbros overlain by a succession of gabbronorites to diorites in which olivine is followed by pyroxene and by plagioclase enriched sulfide horizons generally associated with pegmatoidal rocks are lacking the fractionation of platinum group element pge displays evidence of erratic dispersion throughout the intrusion pge patterns of chromitites show high ir and ru contents relatively high rh content and an unusual pt pd depletion lower than in ophiolitic chromitites these chromitites do not contain laurite type sulfides which are commonly observed in association with os ir ru alloys dunites show relatively flat patterns and gabbros present typical trends with depletions in the iridium group ipge relative to ppge ru rh and especially ir are very low in these silicate rocks indicating the precipitation of these elements during the earliest stages of fractional crystallization and their concentration in chromitites spilitized dolerite dykes from the metamorphic basement show significant pge enrichments similar to those of gabbros based on our previous thermochemical data an extremely low fs2 10 2 atm was in equilibrium with the silicate melt as indicated by the absence of laurite included in chromitites in the absence of complexation of pt pd by sulfur in the early stage of magmatic differentiation a complexation of these elements by cl as chloride complexes in a cl rich fluid phase may be expected this hypothesis is reinforced by the notable absence of any sulfide horizon throughout the intrusion part of precious metals pt pd au may have been remobilized at low temperature and transported as chloride or hydroxide complexes by fluid phases introduced during low grade metamorphic processes or later during deformation events in this interpretation the erratic behaviour of pge at vizcaino was partially controlled by solubilities of pt and pd as chloride complexes at magmatic temperatures as early as chromite crystallization and the control of local saturation of the magma by variation of fs2 was overshadowed

jurassic ash flow sheets calderas and related intrusions of the cordilleran volcanic arc in southeastern arizona implications for regional tectonics and ore deposits volcanologic petrologic and paleomagnetic studies of widespread jurassic ash flow sheets in the huachuca southern dragoon mountains area have led to identification of four large source calderas and associated comagmatic intracaldera intrusions stratigraphic facies and contact features of the caldera related tuffs also provide constraints on the locations lateral displacements and very existence for some major northwest trending faults and inferred regional thrusts in south eastern arizona for example the intricate cochise thrust system as mapped by others in the southern dragoon mountains consists instead of primary depositional contacts within caldera fill megabreccia and the inferred regional thrusts do not exist at least as previously interpreted silicic alkalic compositions of the jurassic caldera related ash flow tuffs bimodal associated mafic magmatism and interstratified coarse sedimentary deposits provide evidence for synvolcanic extension and rifting within the cordilleran magmatic arc gold copper mineralization is associated with subvolcanic intrusions at several of the jurassic calderas

anatomy and global context of the andes main geologic features and the andean orogenic cycle the andes make up the largest orogenic system developed by subduction of oceanic crust along a continental margin subduction began soon after the breakup of rodinia in late proterozoic times and since that time it has been intermittently active up to the present the evolution of the pacifi c margin of south america during the paleozoic occurred in the following stages 1 initial proterozoic rifting followed by subduction and fi nal re amalgamation of the margin in early cambrian times as depicted by the puncoviscana and tucavaca basins and related granitoids in southern bolivia and northern argentina 2 a later phase of rifting in the middle cambrian and subsequent collisions in middle ordovician times of parautochthonous terranes derived from gondwana such as paracas arequipa and antofalla and exotic terranes originating in laurentia such as cuyania chilenia and chibcha 3 fi nal permian collision between south america and north america to form pangea during the alleghanides orogeny leaving behind rifted pieces of laurentia as the tahami and tahuin terranes in the northern andes and other poorly known orthogneisses in the cordillera real of ecuador in the late permian early triassic and 4 amalgamation of the mejillonia and patagonia terranes in early permian times representing the last convergence episodes recorded in the margin during the gondwanides orogeny these rifting episodes and subsequent collisions along the continental margin were the result of changes of the absolute motion of gondwana related to global plate reorganizations during proterozoic to paleozoic times generalized rifting during pangea breakup in the triassic concentrated extension in the hanging wall of the sutures that amalgamated the paleozoic terranes the opening of the indian ocean in early jurassic times was associated with a new phase of subduction along the continental margin the northeastward absolute motion of western gondwana produced a negative trench roll back velocity that controlled subduction under an extensional regime until late early cretaceous times the northern andes of venezuela colombia and ecuador record a series of collisions of island arcs and oceanic plateaus from the early cretaceous to the middle miocene as a result of interaction with the caribbean plate the remaining central and southern andes record periods of orogenesis and mountain building alternating with periods of quiescence and absence of deformation as recorded in parts of the oligocene based on the generalized occurrence of fl at slab subduction episodes through time as recorded in most ramos v a 2009 anatomy and global context of the andes main geologic features and the andean orogenic cycle in kay s m ramos v a and dickinson w r eds backbone of the americas shallow subduction plateau uplift and ridge and terrane collision geological society of america memoir 204 p 31 65 doi 10 1130 2009 1204 02 for permission to copy contact editing geosociety org 2009 the geological society of america all rights reserved

the origin and evolution of saline formation water lower cretaceous carbonates south central texas u s a abstract land l s and prezbindowski d r 1981 the origin and evolution of saline formation water lower cretaceous carbonates south central texas u s a in w back and r letolle guest editors symposium on geochemistry of groundwater 26th international geological congress j hydrol 54 51ndash 74 systematic chemical variation exists in formation water collected from a dip section through lower cretaceous rocks of south central texas chemical variation can be explained by an interactive water rock diagenetic model the cyclic lower cretaceous shelf carbonates of the edwards group dip into the gulf of mexico coast geosyncline and can be considered to a first approximation as part of a complex aquifer contained by paleozoic basement beneath and by relatively impermeable upper cretaceous clay and chalk above the hydrodynamic character of this carbonate system is strongly controlled by major fault systems major fault systems serve as pathways for vertical movement of basinal brines into the lower cretaceous section formation water movement in this sytem has strong upfault and updip components the parent na ca cl brine originates deep in the gulf of mexico basin at temperatures between 200 and 250 c by the reaction halite detrital plagioclase quartz water albite brine other dissolved components originate by reaction of the fluid with the sedimentary phases k feldspar calcite dolomite anhydrite celestite barite and fluorite significant quantities of pb zn and fe have been mobilized as well as the brine moves updip out of the overpressured deep gulf of mexico basin and encounters limestones of the stuart city reef trend the buried platform margin small amounts of galena precipitate in late fractures continuing to rise upfault and updip the brine becomes progressively diluted on encountering significant quantities of dolomite in the backreef facies the ca rich brine causes dedolomitization although thermochemical consideration suggests that small amounts of several authigenic phases should precipitate most have yet to be found minor amounts of several kinds of calcite spar are present however as the brine evolves by dilution and by cooling no systematic changes in any cation cl ratio occur except for regular updip gain in mg as a result of progressive dedolomitization the formation water highly diluted by meteoric water eventually discharges along faults as hot mineral water

a re os study of sulfide minerals from the bagdad porphyry cu mo deposit northern arizona usa rhenium and osmium isotopes in sulfide minerals from the bagdad porphyry cu mo deposit have been used to determine timing of mineralization and the source of osmium and by inference ore metals molybdenite chalcopyrite and pyrite were analyzed mainly from the quartz monzonite and porphyritic quartz monzonite units which are characterized by moderate to strong potassic alteration secondary biotite and k feldspar rhenium concentrations in molybdenite are between 330 and 642 ppm four re os analyses of two molybdenite samples from the quartz monzonite and porphyritic quartz monzonite yield a weighted average age of 71 8 0 2 ma 2 analyses of a third sample from a molybdenite vein in precambrian rocks outside of the main ore zone yield a weighted average age of 75 9 0 2 ma 2 and provide evidence of two separate mineralization episodes chalcopyrite samples contain 6 to 12 ppt os and 1 7 to 4 1 ppb re 187os 188os initial ratios are between 0 1 and 0 8 pyrite samples have osmium and rhenium concentrations varying in the range 8 17 ppt and 3 9 6 8 ppb respectively analyses from these pyrite samples yield an eight point isochron with an age of 77 15 ma 2 and an initial 187os 188os ratio of 2 1 0 8 mswd 0 90 the results presented here add to the growing body of work indicating that porphyry type mineralization is produced by long term multiple episodes of magmatism and associated mineralization the data also support the hypothesis that a significant part of the metals and magmas may have a crustal source as has been suggested for other copper deposits and districts in arizona

zinc germanium ores of the tres marias mine chihuahua mexico the tres marias carbonate hosted zn ge deposit in chihuahua mexico contains sphalerite with the highest average ge 960 ppm and willemite with the highest reported ge contents of mississippi valley type mvt deposits worldwide this has prompted current exploration efforts to focus on the deposit as a high grade source of germanium the sulfide rich ore type 125 000 t at 20 zn and 250 g t ge contains fe rich botryoidal sphalerite type i associated with solid hydrocarbons this type exhibits distinctive intimately intergrown lamellar texture of high fe sphalerite average 9 9 wt fe and 800 ppm ge and a somewhat less fe rich sphalerite phase average 5 5 wt fe and 470 ppm ge reddish brown banded sphalerite type ii average 5 7 wt fe and 1 320 ppm ge is subordinately followed by galena and pyrite the sulfide poor oxidized zinc ore up to 50 wt zn 250 to 300 ppm ge is a fine grained often friable alteration product of the sulfide ore and associated limestone and breccia host while some areas are dominated by carbonates and sulfates others are enriched in silicates such as hemimorphite and willemite the gangue assemblage includes goethite hematite and amorphous silica or quartz minor wulfenite greenockite cinnabar and descloizite also occur willemite occurs as interstitial replacement of sphalerite and fracture fillings in the oxidized ore and can be unusually rich in pb up to 2 0 wt and ge up to 4 000 ppm oscillatory zonation reflects trace element incorporation into willemite from the oxidation of primary ge bearing sphalerite and galena by siliceous aqueous fluids the tres marias deposit has hybrid characteristics consisting of a primary low temperature mvt ge rich zn pb sulfide ore body overprinted by ge rich hemimorphite willemite and fe oxide mineralization

origin and significance of postore dissolution collapse breccias cemented with calcite and barite at the meikle gold deposit northern carlin trend nevada the final event in a complicated hydrothermal history at the meikle gold deposit was gold deficient but caused extensive postore dissolution of carbonate collapse brecciation and precipitation of calcite and barite crystals in the resulting cavities although previously interpreted to be part of the carlin type hydrothermal system crosscutting relationships and u th pb geochronology constrain this hydrothermal event to late pliocene time ca 2 ma nearly 36 ma after ore formation mineralogic fluid inclusion and stable isotope data indicate that postore hydrothermal fluids were reduced h2s rich unevolved meteoric waters 18o 17 of low temperature ca 65 c the 18o values of barite and calcite indicate that these minerals were in isotopic equilibrium requiring that barite so4 was derived from the oxidation of reduced sulfur however preexisting sulfides in breccia cavities were not oxidized the 34s 15 values of barite are higher than those of local bulk sulfide and supergene alunite indicating that so4 was not derived from supergene oxidation of local sulfide minerals the 15 per mil 34s value suggests that the h2s in the fluids may have been leached from sulfur rich organic matter in the local carbonaceous sedimentary rocks a reduced h2s rich fluid is also supported by the bright cathodoluminescence of calcite which indicates that it is mn rich and fe poor calcite has a narrow range of 13c values 0 3 1 8 that are indistinguishable from those of the host bootstrap limestone indicating that co2 in the fluid was from dissolution of the local limestone these data suggest that dissolution and brecciation of the bootstrap limestone occurred where h2s rich fluids encountered more oxidizing fluids and formed sulfuric acid h2so4 intense fracturing in the mine area by previous structural and hydrothermal events probably provided conduits for the descent of oxidized surface water which mixed with the underlying h2s rich waters to form the dissolving acid the surface derived fluid apparently contained sufficient oxygen to produce h2so4 from h2s but not enough to alter pyrite to fe oxide although h2s is an important gold transporting ligand the temperature was too low to transport a significant amount of gold the presence of analogous calcite and barite lined cavities in other carlin type deposits suggests that the generation and oxidation of h2s rich meteoric waters was a common phenomenon in north central nevada previous sulfur isotope studies have also shown that the paleozoic sedimentary rocks were the principal source of h2s in devonian sedimentary exhalative type jurassic intrusion related eocene carlin type and miocene low sulfidation gold deposits in the region the similar sulfur source in all of these systems suggests that basin brines magmatic fluids and meteoric waters all evolved to be h2s rich ore fluids by circulation through paleozoic sedimentary rocks thus although not directly related to gold mineralization the recent hydrologic history of the deposit provides important clues to earlier ore forming processes that were responsible for gold mineralization

sour gas hydrothermal jarosite ancient to modern acid sulfate mineralization in the southern rio grande rift abstract as many as 29 mining districts along the rio grande rift in southern new mexico contain rio grande rift type rgr deposits consisting of fluorite barite sulfide jarosite and additional rgr deposits occur to the south in the basin and range province near chihuahua mexico jarosite occurs in many of these deposits as a late stage hydrothermal mineral coprecipitated with fluorite or in veinlets that crosscut barite in these deposits many of which are limestone hosted jarosite is followed by natrojarosite and is nested within silicified or argillized wallrock and a sequence of fluorite barite sulfide and late hematite gypsum these deposits range in age from 10 to 0 4 ma on the basis of 40ar 39ar dating of jarosite there is a crude north south distribution of ages with older deposits concentrated toward the south recent deposits also occur in the south but are confined to the central axis of the rift and are associated with modern geothermal systems the duration of hydrothermal jarosite mineralization in one of the deposits was approximately 1 0 my most 18oso4 oh values indicate that jarosite precipitated between 80 and 240 c which is consistent with the range of filling temperatures of fluid inclusions in late fluorite throughout the rift and in jarosite 180 c from pena blanca chihuahua mexico these temperatures along with mineral occurrence require that the jarosite have had a hydrothermal origin in a shallow steam heated environment wherein the low ph necessary for the precipitation of jarosite was achieved by the oxidation of h2s derived from deeper hydrothermal fluids the jarosite also has high trace element contents notably as and f and the jarosite parental fluids have calculated isotopic signatures similar to those of modern geothermal waters along the southern rift isotopic values range from those typical of meteoric water to those of deep brine that has been shown to form from the dissolution of permian evaporite by deeply circulating meteoric water jarosite 34s values range from 24 to 5 overlapping the values for barite and gypsum at the high end of the range and for sulfides at the low end most 34s values for barite are 10 6 to 13 1 and many 34s values for gypsum range from 13 1 to 13 9 indicating that a component of aqueous sulfate was derived from permian evaporites 34s 12 2 the requisite h2so4 for jarosite formation was derived from oxidation of h2s which was likely largely sour gas derived from the thermochemical reduction of permian sulfate the low 34s values for the precursor h2s probably resulted from exchange deeper in the basin with the more abundant permian so42 at 150 to 200 c jarosite formed at shallow levels after the ph buffering capacity of the host rock typically limestone was neutralized by precipitation of earlier minerals some limestone hosted deposits contain caves that may have been caused by the low ph of the deep basin fluids due to the addition of deep seated hf and other magmatic gases during periods of renewed rifting caves in other deposits may be due to sulfuric acid speleogenesis as a result of h2s incursion into oxygenated groundwaters the isotopic data in these sour gas jarosite occurrences encode a record of episodic tectonic or hydrologic processes that have operated in the rift over the last 10 my

relation of flat subduction to magmatism and deformation in the western united states flat subduction of the farallon plate beneath the western united states during the laramide orogeny was caused by the combined effects of oceanic plateau subduction and unusually great suction in the mantle wedge the latter of which was caused by the shallowing slab approaching the north american craton root once in contact with basal north america the slab cooled and hydrated the lithosphere upon removal asthenospheric contact with lithosphere resulted in magma production that was especially intense where the basal lithosphere was fertile in what now is the basin and range province and this heating weakened the lithosphere and made it convectively unstable small scale convection has since affected many areas with slab sinking and unloading the western united states elevated into a broad plateau and the weak portion gravitationally collapsed with development of a transform plate boundary the western part of the weak zone became entrained with the pacifi c plate and deformation there is dominated by shear 2 humphreys mwr204 04 1st pages e surface heat flow p wave velocity d seismic velocity at 100 km background s wave velocity 2 100 175 km c s wave velocity

jurassic to holocene tectonics magmatism and metallogeny of northwestern mexico the present metallic distribution in northwestern mexico is the culmination of superposed magmatism tectonism erosion and burial over more than 150 m y detailed palinspastic reconstructions of preextensional configurations the first study of its kind for this region clarify the interplay among these features on the present distribution and character of mineralized geologic systems this new synthesis goes beyond previous metallogenic investigations of northwestern mexico by separating events into specific timing and structural relationships and by restoring the geology to its preextensional configuration metallogenic factors such as enrichment preservation and erosion play major roles in the present distributions and for the first time are related to the overall metallogenic framework of northwestern mexico the analysis concludes that modern metallogenic patterns are the result of the complex superposition and subsequent redistribution of geologic systems in a way that is related directly to the regional history rather than simply metallic belts or interpreted angle of a subducting slab three main extensional events in the oligocene holocene have been restored and the palinspastic distributions have been analyzed reconstructions reveal the following 1 mineralization events igneous centers and sedimentary sequences are continuous across the gulf of california and other areas with large amounts of extension 2 middle tertiary gold silver mineralization in baja california may be the western part of the sierra madre oc present address bhp billiton avenida america vespucio sur 100 8th floor las condes santiago chile e mail john mark g staude bhpbilliton com cidental metallogenic province thus expanding the previously recognized extent of this province s mineralization 3 late cretaceous early tertiary porphyry copper deposits and intrusive centers form a narrower belt than previously noted and are traceable for over 400 km with parts of the belt buried beneath the younger sierra madre occidental volcanic fields 4 interpreted alignments of older geologic features including lineaments of ore deposits are displaced in the reconstructions 5 sedimentary rock hosted gold deposits and low angle detachment gold systems are closely related and occur around core complexes by using structurally restored time slices it becomes clear that older deposit types tend to be those formed at greater depths and more proximal to intrusions whereas younger deposits formed at shallower depths are less eroded and are more commonly volcanic rock hosted these characteristics express themselves in the regional distribution of deposit types second mineralization of widely differing ages is spatially superposed commonly associated with coeval magmatic and tectonic events the structural and magmatic events together with paleodistribution of ore deposits define a new framework to interpret the metallogenic history of northwestern mexico

structural and tectonic evolution of the acatlan complex southern mexico its role in the collisional history of laurentia and gondwana 1 correlation of deformational phases and thermal events in the acatlan complex permits definition of nine major tectonic events seven events are related to the evolution of the iapetus and rheic oceans by the early ordovician the xayacatlan suite of laurentian affinity was metamorphosed to eclogite facies and exhumed before colliding with the el rodeo suite from the late ordovician to silurian the upper ordovician ixcamilpa suite with peri gondwanan affinity was metamorphosed to blueschist facies exhumed and subsequently was overthrust by the xayacatlan el rodeo block part of the esperanza suite is an early silurian intra iapetian continental arc that collided with and subducted beneath continental crust generating a silurian eclogitic event by the late devonian collision of the three high p t suites closed the iapetus ocean the resulting composite terrain was thrust over the gondwana bordering cosoltepec suite to close the rheic ocean during the earliest mississippian the collision of the acatlan complex and the oaxaca terrane uplifted and deformed the composite terrane by the early permian deposition of the chazumba suite occurred in a pull apart basin a latest early permian event produced thrust shear zones with n s directed shortening during the middle jurassic plume rise caused regional doming magmatism and amphibolite facies metamorphism during the late cretaceous paleogene folds and thrust faults overprint the acatlan complex most events in the acatlan complex correlate with events in the appalachian chain suggesting that the acatlan complex was located near the appalachians until the jurassic when it was displaced by the gulf of mexico opening

geology and geochemistry of the kokomo mining district colorado the ores of the kokomo mining district colorado are ag pb zn massive sulfide replacement deposits mantos controlled by northeast striking high angle faults and hosted by limestone beds in the pennsylvanian minturn formation the minturn formation is as much as 1 830 m thick and consists of interbedded nonmarine coarse clastic sedimentary rocks and thin marine limestones the sedimentary section is greatly expanded by cretaceous tertiary porphyritic granodiorite and quartz monzonite sills dikes and stocks hydrothermal alteration is dominated by skarns in calcareous sedimentary rocks centered on the tucker mountain area manto orebodies occur to the south in an updip direction distal to the skarns intrusive rocks throughout the kokomo district have been affected by propylitic alteration argillic and sericitic alteration of sedimentary and intrusive rocks is restricted to areas immediately around the mantos and along faults that served as conduits for hydrothermal solutions mantos are surrounded by halos of siderite manganosiderite hydrothermal dolomite and or jasperoid in the limestone massive sulfide replacement ore exhibits banded and tabular textures very similar to ores at leadville and gilman colorado ore deposition occurred in the general sequence jasperoid iron sulfides pyrrhotite marcasite and pyrite siderite quartz high iron sphalerite ferroan dolomite galena chalcopyrite and sulfosalts barite ferroan calcite and calcite fluid inclusion analyses of quartz and carbonate gangue minerals indicate that temperatures averaged 350 degrees c during the early stages of ore deposition and decreased to about 160 degrees c during the late stages ore fluid salinities also decreased through the ore deposition event from 4 6 to 18 o and delta 18 c values compared to diagenetic dolomite due to isotopic exchange between limestones and hydrothermal fluids carbonate gangue minerals within the ore zone exhibit lower delta 18 o and delta 13 c values and these values yield a calculated fluid delta 18 o sub h 2 o 5 0 to 6 9 per mil within the range for magmatic water sulfur isotope values of sulfide minerals range from 0 6 to 0 8 per mil the narrow range of delta 34 s values around 0 per mil suggests a magmatic sulfur source the fission track age of the kokomo ore system is about 40 ma fission track dates are younger to the north and indicate a thermal center around tucker mountain a porphyritic rhyolite plug on tucker mountain described as a pennsylvanian volcanic feature in previous literature yields a fission track age of 39 5 ma the kokomo stable isotope values fluid inclusion homogenization temperatures and salinities are comparable to data from other leadville type magmatic hydrothermal carbonate hosted sulfide replacement deposits in the colorado mineral belt including leadville and gilman

geology of the delamar silver mine idaho a volcanic dome complex and genetically associated hydrothermal system the delamar silver mine located in southwest idaho produced over 17 million ounces of silver and 230 thousand ounces of gold by bulk mining methods between 1977 and late 1987 the mine occurs within the silver city rhyolite an extensive and complex sequence of middle miocene silicic volcanic flows and domes that unconformably overlie a thick pile of miocene alkali olivine basalt the basalt overlies late cretaceous granite which is correlative with the idaho batholith all three major rock types contain epithermal silver gold veins although rhyolites such as those at delamar are the only hosts for low grade large tonnage mineralization the delamar mine occurs within a sequence of six pervasively altered silicic volcanic units from oldest to youngest these units have the following informal designations 1 porphyritic latite 2 quartz latite 3 tuff breccia 4 porphyritic rhyolite 5 banded rhyolite and 6 millsite rhyolite the porphyritic rhyolite unit is spatially temporally and genetically related to silver gold mineralization and is the principal host rock at delamar this unit was emplaced as coalescing and overlapping lava domes and was preceded by cogenetic eruption of tuff breccia in the form of tuff rings the emplacement of tuff rings and lava domes was controlled by structures related to a shallow level magma body and regional basin and range faulting onlap of banded rhyolite preceded hydrothermal activity and served as an important control on localizing ore deposition the mineralogic character of ore and alteration at delamar is similar to adularia sericite type volcanic hosted epithermal deposits but the geologic setting is more like that of acid sulfate deposits

shotgun deposit granite porphyry hosted gold arsenic mineralization in southwestern alaska usa the shotgun deposit 30 tonnes gold inferred resource using 0 5 ppm cutoff consists of stockwork au as mineralization in a late cretaceous 69 7 0 3 ma 40ar 39ar granite porphyry stock located approximately 150 km north of dillingham sw alaska preliminary metallurgical results indicate 90 gold is recoverable by cyanide leaching with average estimated concentrations of 1 4 ppm au 6 1 ppm ag 0 5 as 0 07 cu 40 ppm mo and 8 ppm bi shotgun is a low fo2 porphyry au as deposit that shares characteristics with both intrusion related gold and porphyry copper deposits the bulk of gold occurs in intense quartz stockwork and breccias it is texturally identical to cu mo porphyries but has different mineralization and alteration assemblages shotgun contains arsenopyrite pyrite pyrrhotite chalcopyrite lacks k alteration and instead contains vein and pervasive albite quartz sericite carbonate as temperature decreased early high temperature low fs2 mineralization arsenopyrite loellingite pyrrhotite evolved to later lower temperature moderately high fs2 conditions pyrite bornite chalcopyrite in addition ores at shotgun contain primary native gold native bismuth bi te sulfides cubanite maldonite scheelite and sphalerite as well as supergene covellite chalcocite marcasite and native copper arsenopyrite geothermometry yields deposition temperatures of 470 to 630 c fluid inclusion microthermometry and laser raman spectroscopy indicate that vapor rich inclusions h2o co2 ch4 homogenize to vapor at 360 c and saline aqueous inclusions contain 40 to 60 wt nacl equiv probably nacl kcl and homogenize to liquid at 280 to 600 c coexisting vapor and liquid rich fluid inclusions in quartz suggest boiling occurred stable isotope data include 18o of 16 4 to 17 1 quartz d of 124 to 105 sericite and 34s of 5 5 to 5 0 asp cpy at 380 c fluid compositions of 18o of 10 4 to 11 1 and d of 95 to 74 are compatible with a magmatic fluid source gold correlates fairly well r 0 60 with ag bi mo and te when combined with the evidence for high temperature fluids the fragmental morphology of the intrusion stockwork veining and exsolution of vapor rich and aqueous saline rich fluid inclusions indicate a direct relationship between early mineralization and magmatism the shotgun deposit shares similarities with porphyry cu mo au deposits stockwork breccia and reduced nonporphyry intrusion related au as bi deposits sulfide mineralogy geochemistry and oxidation state they appear to be products of the same style of magmatic hydrothermal systems and differ only in depth and thus volatile release history the shotgun deposit is essentially an intrusion related au as bi deposit emplaced at pressures and temperatures typical of a porphyry system

regional stratigraphy and distribution of epigenetic stratabound celestine fluorite barite and pb zn deposits in the mvt province of northeastern mexico northeastern mexico hosts numerous epigenetic stratabound carbonate hosted low temperature hydrothermal deposits of celestine fluorite barite and zinc lead which formed by replacement of mesozoic evaporites or carbonate rocks such deposits can be permissively catalogued as mississippi valley type mvt deposits the deposits studied in the state of coahuila are associated with granitic and metasedimentary basement highs horsts marginal or central to the mesozoic sabinas basin these horsts controlled the stratigraphy of the mesozoic basins and subsequently influenced the laramide structural pattern the sabinas basin consists of 6 000 m thick jurassic to cretaceous siliciclastic carbonate and evaporitic series the mvt deposits are mostly in barremian and in aptian albian to cenomanian formations and likely formed from basinal brines that were mobilized during the laramide orogeny although earlier diagenetic replacement of evaporite layers barite and celestine deposits and lining of paleokarstic cavities in reef carbonates zn pb deposits is observed fluid inclusion microthermometry and isotopic studies suggest ore formation due to mixing of basinal brines and meteoric water homogenization temperatures of fluid inclusions range from 45 c to 210 c salinities range from 0 to 26 wt nacl equiv and some inclusions contain hydrocarbons or bitumen sulfur isotope data suggest that most of the sulfur in barite and celestine is derived from barremian to cenomanian evaporites regional geology and a compilation of metallogenic features define the new mvt province of northeastern mexico which comprises most of the state of coahuila and portions of the neighboring states of nuevo leon durango and perhaps extends into zacatecas and southern texas this province exhibits a regional metal zonation with celestine deposits to the south fluorite deposits to the north and barite and zn pb deposits mostly in the central part