evaporation from cultivated soils containing rock fragments abstract evaporation from bare soil surfaces represents an important water loss for agriculture in semi arid regions numerous efforts have been undertaken to modify the topsoil characteristics mulching tillage in order to create a thin dry topsoil that reduces evaporation however little attention was paid to the role of natural rock fragments in topsoils with respect to evaporation this paper presents the results of laboratory experiments simulating evaporation from initially wet and air dry soils containing a range of rock fragment contents and compares them to field conditions evaporation was stimulated by blowing fans at one high evaporative demand eo 7 7 9 2 mm day 1 time domain reflectometry tdr was used to investigate the relation between the water content of the topsoil and the actual evaporation rate for soils at field capacity initial fine earth water content decreases with rock fragment content and consequently evaporation rates decrease in the same order for air dry soils that received a limited amount of rain 10 and 20 mm an opposite behaviour was observed initial fine earth water content and evaporation rates increase with rock fragment content a strong positive sigmoidal relation between relative evaporation rate actual over open pan evaporation rates and fine earth water content in the centre of the wetted soil section was observed during the laboratory experiments except for the columns covered with a mulch there were no systematic differences in this relation between the treatments a rock fragment mulch reduces evaporation rate at a given soil water content significantly these trends explain the often ambivalent effects of rock fragments on evaporation rates in the field

late cenozoic structure and tectonics of the northern mojave desert in the fort irwin region of the northern mojave desert late cenozoic east striking sinistral faults predominate over northwest striking dextral faults of the same age kinematic indicators and offset marker units indicate dominantly sinistral strike slip on the east striking portions of the faults and sinistral thrust slip on northwest striking moderately dipping segments at the east ends of the blocks crustal blocks 7 10 km wide by 50 km long are bounded by complex fault zones up to 2 km wide at the edges and ends of each block faulting initiated after 11 ma and quaternary deposits are faulted and folded we document a minimum of 13 km cumulative sinistral offset in a north south transect from south of the bicycle lake fault to north of the drinkwater lake fault paleomagnetic results from 50 sites reveal two direction groups in early and middle miocene rocks the north to northwest declinations of the first group are close to the middle miocene reference pole however rock magnetic studies suggest that both primary and remagnetized directions are present in this group the northeast declinations of the second group are interpreted as primary and 63 5 7 6 clockwise from the reference pole and suggest net post middle miocene clockwise rotation of several of the east trending blocks in the northeast mojave domain the jurassic independence dike swarm in fort irwin may be rotated 25 80 clockwise relative to the swarm north of the garlock fault thus supporting the inference of clockwise rotation using a simple shear model that combines sinistral slip and clockwise rotation of elongate crustal blocks we predict 23 clockwise rotation using the observed fault slip or one third that inferred from the paleomagnetic results the discrepancy between slip and rotation may reflect clockwise bending at the ends of fault blocks where most of our paleomagnetic sites are located however at least 25 40 of clockwise tectonic rotation is consistent with the observed slip on faults within the domain plus possible rigid body rotation of the region evidenced by clockwise bending of northwest striking domain bounding faults our estimates of sinistral shear and clockwise rotation suggest that approximately half of the 65 km of dextral shear in the eastern california shear zone over the last 10 m y occurred within the northeast mojave domain the remainder must be accommodated in adjacent structural domains e g east of the avawatz mountains and west of the goldstone lake fault supporting appendices 1 and 2 are available on diskette or via anonymous ftp from kosmos agu org directory apend username anonymous password guest diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 or by phone at 800 966 2481 15 00 payment must accompany order

fluid inclusion characteristics of intrusion related gold mineralization tombstone tungsten magmatic belt yukon territory canada intrusion hosted low sulfide sheeted vein systems are common within many plutons and stocks of the middle cretaceous tombstone tungsten magmatic belt yukon territory and host significant gold mineralization fluid inclusion characteristics of five such systems namely emerald lake dublin gulch scheelite dome mike lake and mactung constrain the vein forming fluid composition formation temperatures and pressures hydrothermal fluid processes and potential fluid sources the veins contain a wide range of fluid inclusion types ubiquitous type 1a and 1b inclusions are low salinity 1a xnacl 1 kbar at emerald lake and dublin gulch and 2 kbar at mactung and scheelite dome that potentially control fluid composition and evolution a variety of fluid processes may have been responsible for gold precipitation including immiscibility and or release of an evolving magmatic fluid

the use of palaeomagnetism and rock magnetism to understand volcanic processes introduction this special publication provides a snapshot of our understanding of volcanic processes through the use of palaeomagnetic and rock magnetic techniques here we provide a context for the book placing individual chapters within the milieu of previous work including some magnetic techniques that were not used in the particular studies described herein thermoremanent magnetization is a powerful tool to understand processes related to heating and cooling of rocks including estimating the temperature of emplacement of pyroclastic deposits which may allow us to better understand the rates of cooling during eruption and transport anisotropy of magnetic susceptibility and anisotropy of remanence are used primarily to investigate rock fabrics and allow the interpretation of flow dynamics in dykes lava flows and pyroclastic deposits as well as the location of the eruptive vents rock magnetic characteristics can help in the correlation of volcanic deposits but also provide means to date volcanic deposits and to better understand the processes of cooling of the deposits as the magnetic minerals can change with temperature in addition volcanic rocks may be key recorders of past magnetic fields allowing a better understanding of changes in field intensity and perhaps providing clues of how the magnetic field is formed over the past several decades palaeomagnetic and rock magnetic techniques have been applied to numerous problems in volcanology since aramaki akimoto s pioneering study in 1957 which used natural remanent magnetization nrm data to qualitatively estimate the temperature of emplacement of selected pyroclastic deposits in japan aramaki akimoto 1957 many studies have utilized and refined the approach of using progressive thermal demagnetization data to estimate emplacement temperatures of pyroclastic deposits and have demonstrated the utility of this approach in many settings another extensively utilized technique is that of anisotropy of magnetic susceptibility ams which was first applied to pyroclastic volcanic rocks by ellwood 1982 on ignimbrites in the san juan mountains sw colorado usa to estimate flow fabrics and in that study to infer source locations through triangulation it is staggering to think of the number and diversity of ams studies on volcanic rocks that have been undertaken since that time in a bit more than three decades remanent magnetization characteristics have long been used to correlate volcanic rocks and such data together with rock magnetic properties have been employed to study crystallization and cooling processes e g schlinger et al 1988 wells hillhouse 1989 as techniques have been refined and the sensitivity precision reliability and or speed of measurements for an array of modern instrumentation all have improved workers have been able to study volcanic problems in more detail pulling out details of the processes that have not previously been considered in addition the types of problems addressed have diversified considerably we intend this special publication to provide a representative suite of contributions that help to summarize the current array of studies that apply palaeomagnetism and rock magnetism to volcanic processes and to stimulate further research in this exciting science this book is a result of two sessions on volcanic processes and palaeomagnetism held at the international union of geodesy and geophysics congress in melbourne australia in july 2011 and at the american geophysical union annual meeting in san francisco usa in december 2011 from ort m h porreca m geissman j w eds 2015 the use of palaeomagnetism and rock magnetism to understand volcanic processes geological society london special publications 396 1 11 first published online january 20 2015 http dx doi org 10 1144 sp396 17 2015 the geological society of london for permissions http www geolsoc org uk permissions publishing disclaimer www geolsoc org uk pub ethics angharad hills at the geological society of london graciously and tactfully guided this book and the editors through the entire process from chapter solicitation to publication thermoremanent magnetization in volcanic rocks and emplacement temperature

structural and mechanical controls on intrusion related deposits of the tombstone gold belt yukon canada with comparisons to other vein hosted ore deposit types abstract intrusion related gold deposits at the clear creek scheelite dome and dublin gulch properties of the tombstone gold belt tgb yukon territory have dominantly e striking steeply dipping auriferous quartz extension veins within intrusions in adjacent metasedimentary rocks gold is hosted in subvertical nw to nnw striking sinistral faults as veins and breccias in e striking extension veins and locally in e to ene striking fault veins these structural relationships indicate low magnitude broadly e w directed shortening and n s extension during stock emplacement and gold mineralisation at 92 ma the lack of any deviation or deflection of the extension vein orientations in the country rocks with respect to their orientation within the stocks indicates consistent stress trajectories in both rock types these tgb deposits formed at 5 8 km depth where mean and differential stresses may be greater in magnitude than in shallower porphyry environments many porphyry systems feature magmatic related stresses that dominated the local stress field with more variable vein orientations the result conversely orogenic gold systems usually exhibit strong dimensionality in vein orientations fault hosted mineralisation in metasedimentary rocks of the tgb deposits in this study is comparable in geometry but generally smaller in size than in many orogenic gold systems intrusion related systems of the tgb exhibit intermediate structural styles of mineralisation that provide a useful bridge in understanding the diversity of mechanically controlled structural styles in otherwise mostly unrelated gold deposit types

geology and geochemistry of the clear creek intrusion related gold occurrences tintina gold province yukon canada the clear creek gold occurrences lie within deformed lower greenschist facies rocks of the western selwyn basin they consist of auriferous sheeted quartz veins that cut six cretaceous stocks and their hornfels the veins contain 1 2 combined pyrite and arsenopyrite with lesser pyrrhotite bismuthinite and scheelite as well as 2 5 g t au new 40ar 39ar geochronology of hydrothermal micas indicates that the veins formed within 1 2 million years of granitoid emplacement fluid inclusion microthermometry defines a parent ore fluid of 81 mol h2o 14 mol co2 4 mol nacl kcl and 1 mol ch4 which unmixed into a low and high salinity immiscible pair this suggests a more saline parent fluid and a greater degree of fluid unmixing relative to the other occurrences in the eastern tintina gold province inclusions trapped in as and bi rich high gold grade veins have homogenization temperatures of 300 350 c whereas inclusions found in more ag and pb rich veins are characterized by temperatures of

age of the peach springs tuff southeastern california and western arizona sanidine separates from pumice of the early miocene peach springs tuff are concordantly dated at 18 5 0 2 ma by two isotopic techniques the peach springs tuff is the only known unit that can be correlated between isolated outcrops of miocene strata from the central mojave desert of southeastern california to the western colorado plateau in arizona across five structural provinces a distance of 350 km thus the age of the peach springs tuff is important to structural and paleogeographic reconstructions of a large region biotite and sanidine separates from bulk samples of the peach springs tuff from zones of welding and vapor phase alteration have not produced consistent ages by the k ar method published ages of mineral separates from 17 localities ranged from 16 2 to 20 5 ma discordant 40ar 39ar incremental release spectra were obtained for one biotite and two of the sanidine separates ages that correspond to the last gas increments are as old as 27 ma the 40ar 39ar incremental release determinations on sanidine separated from blocks of peach springs tuff pumice yield ages of 18 3 0 3 and 18 6 0 4 ma laser fusion measurements yield a mean age of 18 51 0 10 the results suggest that sanidine and biotite k ar ages older than about 18 5 ma are due to inherited ar from pre tertiary contaminants which likely were incorporated into the tuff during deposition sanidine k ar ages younger than 18 ma probably indicate incomplete extraction of radiogenic 40ar whereas laser fusion dates of biotite and hornblende younger than 18 ma likely are due to postdepositional alteration laser fusion ages as high as 19 01 ma on biotite grains from pumice suggest that minerals from pre tertiary country rocks also were incorporated in the magma chamber

a fluid inclusion and isotopic study of an intrusion related gold deposit irgd setting in the 380 ma south mountain batholith nova scotia canada evidence for multiple fluid reservoirs a set of sheeted quartz veins cutting 380 ma monzogranite at sandwich point nova scotia canada provide an opportunity to address issues regarding fluid reservoirs and genesis of intrusion related gold deposits the quartz veins locally with arsenopyrite 5 and elevated au bi sb cu zn occur within the reduced south mountain batholith which also has other zones of anomalous gold enrichment the host granite intruded p 3 5 kbars lower paleozoic metaturbiditic rocks of the meguma supergroup well known for orogenic vein gold mineralization relevant field observations include the following 1 the granite contains pegmatite segregations and is cut by aplitic dykes and zones 1 2 m of spaced fracture cleavage 2 sheeted veins containing coarse comb textured quartz extend into a pegmatite zone 3 arsenopyrite bearing greisens dominated by f rich muscovite occur adjacent the quartz veins and 4 vein and greisen formation is consistent with riedel shear geometry although these features suggest a magmatic origin for the vein forming fluids geochemical studies indicate a more complex origin vein quartz contains two types of aqueous fluid inclusion assemblages fia type 1 is a low salinity 3 wt equivalent nacl with minor co2 2 mol and has th 280 340 c in contrast type 2 is a high salinity 20 25 wt equivalent nacl ca rich fluid with th 160 200 c pressure corrected fluid inclusion data reflect expulsion of a magmatic fluid near the granite solidus 650 c that cooled and mixed with a lower temperature 400 c wall rock equilibrated ca rich fluid evidence for fluid unmixing an important process in some intrusion related gold deposit settings is lacking stable isotopic o d s analyses for quartz muscovite and arsenopyrite samples from vein and greisens indicate the following 1 18oqtz 11 7 to 17 8 and 18omusc 10 7 to 11 2 2 dmusc 44 to 54 and 3 34saspy 7 8 to 10 3 these data are interpreted in conjunction with fluid inclusion data to reflect contamination of a magmatic derived fluid delta 18 hbox o rm h 2 rm o 10 by an external fluid delta 18 hbox o rm h 2 rm o 15 the latter having equilibrated with the surrounding metasedimentary rocks the 34s data are inconsistent with a direct igneous source based on other studies for the host intrusion delta 18 hbox o rm h 2 rm o 5 and are instead consistent with an external reservoir for sulphur based on 34sh2s data for the surrounding metasedimentary rocks divergent fluid reservoirs are also supported by analyses of pb isotopes for pegmatitic k feldspar and vein arsenopyrite collectively the data indicate that the vein and greisen forming fluids had a complex origin and reflect both magmatic and non magmatic reservoirs thus although the geological setting suggests a magmatic origin the geochemical data indicate involvement of multiple reservoirs these results suggest multiple reservoirs for this intrusion related gold deposit setting and caution against interpreting the genesis of intrusion related gold deposit mineralization in somewhat analogous settings based on a limited geochemical data set

paleomagnetism of middle miocene volcanic rocks in the mojave sonora desert region of western arizona and southeastern california paleomagnetic directions have been obtained from 190 early to middle miocene 12 20 ma mafic volcanic flows in 16 mountain ranges in the mojave sonora desert region of western arizona and southeastern california these flows generally postdate early miocene tectonic deformation accommodated by low angle normal faults but predate high angle normal faulting in the region after detailed demagnetization experiments 179 flows yielded characteristic directions interpreted as original thermal remanent magnetizations trm because of the episodic nature of basaltic volcanism in this region the 179 flows yielded only 65 time distinct virtual geomagnetic poles vgps the angular dispersion of the 65 vgps is consistent with the angular dispersion expected for a data set that has adequately averaged geomagnetic secular variation the paleomagnetic pole calculated from the 65 cooling unit vgps is located at 85 5 n 108 9 within a 4 4 circle of 95 confidence this pole is statistically indistinguishable at 95 confidence from reference poles calculated from rocks of similar age in stable north america and from a paleomagnetic pole calculated from rocks of similar age in baja california the coincidence of paleomagnetic poles from the mojave sonora desert region with reference poles from the stable continental interior indicates that 1 significant vertical axis net tectonic rotations have not accompanied post middle miocene high angle normal faulting in this region 2 there has been no detectable post middle miocene latitudinal transport of the region and 3 long term nondipole components of the middle miocene geomagnetic field probably were no larger than those of the recent 0 5 ma geomagnetic field in contrast paleomagnetic data indicate vertical axis rotations of similar age rocks in the transverse ranges the eastern transverse ranges and the mojave block we speculate that a major structural discontinuity in the vicinity of the southeastward projection of the death valley fault zone separates western areas affected by vertical axis rotations from eastern areas that have not experienced such rotations

effect of shifting sand burial on soil evaporation and moisture salt distribution in a hyper arid desert it is a serious threat for the sustainable protection of the taklimakan desert highway shelterbelt tdhs from shifting sand burial to explore the effects of shifting sand burial on soil evaporation salt accumulation and their distribution micro lysimeters were used under sand burial of different thickness 1 3 and 5 cm and particle size 0 063 0 063 0 20 and 0 20 2 00 mm the results demonstrated that 1 soil daily evaporation and accumulative evaporation decreased with the sand burial thickness and thus evaporative inhibiting efficiency increased consequently the soil water content increased with sand burial thickness 1 48 12 70 than the control 2 finer textured 0 063 mm sand burial 0 2 cm depth promoted soil evaporation and salt accumulation at topsoil accumulative evaporation increased 5 1 mm and electrical conductivity ec of topsoil increased 13 30 ds m 1 compared with the control and 3 while topsoil ec decreased 1 65 6 46 ds m 1 with the increase in sand burial thickness soil ec beneath the sand burial interface showed a reverse trend we concluded that shifting sand burial has obvious effects on soil water evaporation salt accumulation and water salt redistribution and it could be considered to save water and reduce salt accumulation in arid desert areas like the tdhs with saline irrigation

mid tertiary structural evolution of the old woman mountains area implications for crustal extension across southeastern california investigations in the old woman mountains area provide insights into mid tertiary extensional tectonism in southeastern california the old woman mountains area is a moderately extended region that lies between two highly extended terranes the central mojave extensional complex and the colorado river extensional corridor two normal faults and a high angle fault that we interpret to be a transfer structure divide the area into four distinct structural blocks these blocks correspond to the four mountain ranges of the area the old woman piute little piute and ship mountains other major faults involved in tilting are inferred to be buried beneath alluvium in surrounding valleys deformation occurred in part before but primarily after deposition of the peach springs tuff an 18 5 ma regional stratigraphic marker transport directions of hanging walls are westward on the west side of the old woman mountains area and east southeastward on the east side we suggest that the old woman mountains area is a less extended portion of a continuous extensional terrane that stretches from the central mojave desert across the colorado river to the transition zone of the colorado plateau lower strain in the old woman mountains area than in most of the colorado river extensional corridor and central mojave extensional complex and age relations across the terrane can be explained by a model in which the principal locus of upper crustal breakaway to an east dipping low angle simple shear zone shifted at about 18 20 ma from a position far to the west to just east of the old woman mountains area

sulphur isotopic composition of sulphides from the beaver dam and other meguma group hosted gold deposits nova scotia implications for genetic models sulphur isotopic compositions were determined for sulphide mineral phases in meguma group hosted gold deposits nova scotia in order to resolve i potential source regions for sulphur in these de

grain boundary migration of water and carbon dioxide during uplift of garnet zone alpine schist new zealand deformed quartz veins in garnet zone schist adjacent to the active alpine fault new zealand have fluid inclusions trapped along quartz grain boundaries textures suggest that the inclusions formed in their present shapes during annealing of the deformed veins many of the inclusions are empty but some contain carbon dioxide with densities that range from 0 16 to 0 80 g cm 3 no water nitrogen or methane was detected the inclusions are considerably more co2 rich than either the primary metamorphic fluid 5 co2 or fluids trapped in fracture related situations in the same or related rocks 50 co2 enrichment of co2 is inferred to have resulted from selective migration wicking of saline water from the inclusions along water wet grain boundaries after cooling induced immiscibility of a water co2 mixture inclusion volumes changed after loss of water non wetting co2 remained trapped in the inclusions until further percolation progressively removed co2 in solution this mechanism of fluid migration dominated in ductile quartz rich rocks near but below the brittle ductile transition at deeper levels hydraulic fracturing is also an important mechanism for fluid migration whereas at shallower levels advection through open fractures dominates the fluid flow regime

integrated model for meguma group lode gold deposits nova scotia canada meguma group lode gold deposits represent concentrations of a variety of auriferous quartz veins in lower paleozoic metaturbidites of the meguma terrane of southern nova scotia our studies incorporating extensive field regional and detailed mapping and laboratory petrography fluid inclusions geochemistry isotopes studies of 30 of these deposits in the eastern meguma terrane suggest that mineralization occurred at ca 370 ma 40 ar 39 ar dating of hydrothermal vein minerals during widespread incursion of mafic and felsic magmatism into the crust concurrent with subvertical shear zone development thus postdating regional acadian deformation by some 30 40 m y isotopic data s c o sr indicate that vein forming fluids were not solely magmatic and were in part exotic to the meguma group thus earlier magmatic and metamorphogenic lateral secretion models that considered either late stage magmatic fluids or the enveloping meguma group metasedimentary rocks as the sole source of vein components are invalidated

magnetic structural and geochronologic evidence bearing on volcanic sources and oligocene deformation of ash flow tuffs northeast nevada magnetic properties of mid tertiary volcanic rocks west of jiggs in northeast nevada were investigated for the purposes of interpreting igneous structural and tectonic processes in this part of the basin and range province anisotropy of magnetic susceptibility ams patterns reflect flow fabrics and suggest previously unknown sources for these ash flow tuffs paleomagnetic and structural evidence suggest counterclockwise relative rotation of the southern part of the area with respect to the north new stratigraphic chemical and k ar isotopic data support these interpretations dacite to rhyolite ash flow tuffs of the indian well formation were subdivided into two main units the lower and predominant tuff of jiggs 35 8 37 0 ma and the unconformably overlying but areally restricted tuff of hackwood 30 8 ma the jiggs unit has two polarities whereas the hackwood has only a reversed polarity remanence together these units with tilt correction define a pole 92 e 87 n dp 6 dm 8 for approximately 30 37 ma this pole is concordant with coeval north american reference poles indicating that this zone of approximately 30 km ns extent has not undergone significant vertical axis rotation relative to the north american reference andesite lavas of normal magnetic polarity and of 32 5 ma age characterize the diamond hills immediately to the south we interpret this region from both structural evidence and discordant paleomagnetic direction to have rotated approximately 25 counterclockwise relative to the indian well volcanic units to the north the apparent rotation of the diamond hills is possibly the result of drag on the left lateral garcia fault which limits the diamond hills on the southwest analysis of ams data suggests by patterns of the k1 axes two distinct sources for the jiggs unit a northern buried source and a central partially buried source lithologic evidence consistent with proximal vent facies is found near the latter source an unusual pattern in the ams k3 axes suggested as resulting from the tilt correction applied to initial dip is consistent with the interpreted flow directions tight age constraints can be placed on the deformation of the main ash flows of the jiggs unit in the 35 8 to 30 8 ma interval because the diamond hills unit is slightly younger and appears to have been affected by the same deformative episode as the jiggs the regional deformation affecting both units can be more narrowly confined to the 32 5 30 8 ma interval this deformation is inferred to coincide with the major early tertiary episode of crustal extension in this region of the basin and range

geochemical behavior of sulfur in granitoids during intrusion of the south mountain batholith nova scotia canada abstract a detailed stable isotope s o and elemental major trace s content study of peraluminous s type granites and granodiorites of the south mountain batholith and the meguma group metasediments into which the batholith was intruded has been conducted on a regional and local scale near mt uniacke sulfur contents of the metasediments range from 5 to 2440 ppm and 34 s ranges from 3 7 to 26 3 sulfur contents of the igneous rocks range from 6 to 570 ppm with the granites 20 ppm having lower sulfur contents than the granodiorites 300 ppm approximately 30 to 60 of the sulfur in the granodiorites is present as pyrrhotite a late phase with most of the remainder being present as sulfur substituted into biotite an early phase 18 oq quartz values of the mt uniacke granodiorites are heavy a 12 2 indicating formation of the magmas by partial melting of sedimentary rocks major element variations suggest that the granites are related to the granodiorites by fractional crystallization of plagioclase quartz and biotite comparison of the sulfur contents of the granodiorites with experimental determinations of sulfur solubility suggest that the distribution coefficient of sulfur d s between the cumulate and the magma during fractional crystallization 4 the regional granodiorites have a relatively restricted range of 34 s values 5 4 to 8 4 while the regional granites have a wide range of 34 s values 1 6 to 15 0 granodiorites at mt uniacke with normal magmatic values 0 76 to 1 42 for 18 o quartz feldspar have 34 s values between 6 6 and 12 3 and show good negative correlations between 34 s and s feo and mgo contents some granodiorites at mt uniacke have anomalously low sulfur contents for their major element composition and 18 o quartz feldspar values 0 45 to 0 67 indicate that these samples have undergone hydrothermal alteration and this alteration event is probably responsible for the low sulfur contents a number of processes have been considered in order to understand the variations of 34 s partial melting of inhomogeneous source rocks may be responsible for some of the regional 34 s variations but this implies that the granites are not genetically related to the granodiorites inhomogeneous source rocks are an unlikely explanation for the systematic variations of 34 s observed over only a few kilometres at mt uniacke rayleigh fractionation accompanying the fractional crystallization of biotite 34 s pyrrhotites silicate s 0 5 to 1 0 can account for only a small component of the observed 34 s variations the assimilation of isotopically variable sulfur derived from the meguma group metasediments during fractional crystallization combined assimilation fractional crystallization or afc provides the best explanation for the 34 s variations transfer of sulfur from the country rock may have been possible by either bulk afc where all of the country rock material is incorporated into the magma or by selective afc where sulfur and other volatile elements only are transferred from the country rock into the magma selective afc could have been effected by a sloping mechanism of intrusion followed by the heating of country rock xenoliths to the point where extensive desulfidation reactions took place with sulfur being transferred from the country rock to the magma by a fluid phase

chemical and isotopic pb sr zonation in a peraluminous granite pluton role of fluid fractionation the davis lake pluton dlp 800 km2 of southwestern nova scotia canada part of the large peraluminous south mountain batholith of ca 380 ma u pb zircon ar ar mica consists of granite and subordinate topaz muscovite leucogranite that hosts greisen tin base metal mineralization a new pb pb isochron age for leucogranite from the most evolved part of the dlp indicates a crystallization age of 378 3 6 ma coincident with other radiometric ages of the dlp rb sr re os pb pb the intrusion displays a compositional zonation defined by lead and strontium isotopic ratios as well as some major elements e g si f incompatible trace elements e g li rb ta u sn and elemental ratios e g k rb and nb ta the greisens and the leucogranites that host them are characterized by extreme radiogenic compositions for pb and sr and their chemical isotopic trends are extensions of the trends displayed by the less evolved granites the covariations of the isotopic ratios with several major and trace elements and elemental ratios as well as the pb pb and rb sr isochrones indicate that all phases of the intrusion originated from a homogeneous parental magma the granitoid magma underwent extensive fractional crystallization of feldspars minor biotite and accessory minerals monazite apatite and zircon in a compositionally zoned magma chamber that was subsequently accompanied by fluid fractionation during which time the internally derived fluorine rich fluids modified the rb sr u pb and th pb ratios leading to distinct variations of 87sr 86sr 206pb 204pb 238u 204pb and 232th 204pb isotopic ratios these data therefore document the evolution of a granitic magma through magmatic i e crystal fractionation orthomagmatic i e crystal fluid fractionation and hydrothermal i e fluid fractionation stages that culminated in the formation of a tin base metal deposit the pb isotope data also constrain the source region for the dlp as being avalonian basement that by inference must underlie much of the meguma terrane

origin and evolution of the greisenizing fluid at the east kemptville tin deposit nova scotia canada the process responsible for greisen hosted tin mineralization at east kemptville was investigated using petrographic chemical and fluid inclusion analyses of samples from a single deep drill hole 90 1 in the western part of the deposit baby zone based on their trace element chemistry the greisens intersected in the hole appear to have all formed from a single leucogranite protolith alteration developed as symmetrically zoned halos around central fractures as a result of the interaction of a fluoride rich orthomagmatic fluid with evolved leucogranite during alteration k feldspar was first replaced by albite with further alteration albite was replaced by muscovite resulting in the formation of quartz sericite greisen ore minerals are locally present in this zone closer to veins muscovite is replaced by topaz and quartz producing a quartz topaz greisen with which the most intense mineralization is associated this latter greisen is enriched in f fe s zn and sn suggesting that these elements were added by the mineralizing fluid next to the vein dissolution of pyrrhotite sphalerite and cassiterite characterizes quartz greisen in which the concentration of ore forming elements sn fe s f zn cu is lower than in the quartz topaz greisen microthermometric measurements of fluid inclusions in quartz from the various alteration zones show that this alteration sequence was formed by only one greisenizing event and that temperature was approximately constant 450 degrees c the fluid responsible for greisen formation was an nacl brine containing subordinate and variable concentrations of fe mn and k measured eutectic temperatures of fluid inclusions are lowest in quartz topaz greisen as a result of an increase in the fe concentration due to pyrrhotite dissolution oxygen fugacity which was calculated from the co 2 ch 4 ratio in gases released by crushing fluid inclusion rich samples displays a corresponding minimum since pyrrhotite dissolution releases fe super 2 the distribution of pyrrhotite and the fluid inclusion data indicates that pyrrhotite was precipitated close to the vein during early stages of the alteration and reprecipitated farther out as alteration progressed this suggests that alteration zones moved away from the vein and widened with time the salinity of fluid inclusions varies between 27 and 41 wt percent nacl equiv and increases linearly with increasing distance from the vein even in the absence of na bearing phases it therefore follows that sodium was transported toward the fracture vein down a chemical potential gradient this occurred through compensated infiltration i e a regime in which flow was dominantly parallel to the fracture but individual aliquots of fluid followed complex paths back and forth between the fracture and the rock cassiterite precipitated in quartz topaz greisen in response to a ph increase of the mineralizing fluid due to its interaction with the wall rock other components affecting cassiterite solubility were either constant temperature or acted against its precipitation f sub o 2 and a sub cl super

the dewulu reduced au cu skarn deposit in the xiahe hezuo district west qinling orogen china implications for an intrusion related gold system abstract the triassic west qinling orogen which formed by the continental collision between the south china block and north china craton following the subduction and closure of the paleo tethyan ocean hosts numerous gold deposits that have commonly considered as products of metamorphic dehydration during the oceanic subduction and subsequent continental collision however whether or not there are gold deposits that are genetically related to regional magmatism remains poorly understood here we present mineralogical compositional and geochronological data of the dewulu au cu skarn deposit in the xiahe hezuo district to understand the ore genesis and its genetic link to many coeval sediment hosted disseminated and magmatic hosted vein gold deposits in the same district which collectively reveal a possible intrusion related gold system the dewulu au cu skarn deposit in the eastern zone of the xiahe hezuo district is associated with the early triassic i type ilmenite series dewulu quartz diorite pluton that intrudes permian marine clastic and carbonate rocks ore related hydrothermal biotite separates yield a well defined 40ar 39ar plateau age of 239 9 1 4 ma 2 which agrees with a previous zircon u pb age 238 6 1 5 ma at 2 of the dewulu quartz diorite and thus demonstrates a temporal and likely genetic link between the two both prograde and retrograde skarn assemblages are well developed in the dewulu au cu deposit prograde garnet and pyroxene are compositionally dominated by grossularite and hedenbergite respectively indicating a reduced skarn system au cu mineralization is largely represented by the arsenopyrite loellingite chalcopyrite pyrrhotite bornite assemblage and is associated with retrograde skarns gold is mostly hosted in arsenopyrite loellingite and chalcopyrite and has close textural relations with native bismuth or bismuthides mineral stability relationships and fluid inclusion microthermometric data confirm that the prograde skarns formed at a low oxygen fugacity high temperature 630 c and a depth range of 2 9 to 6 5 km arsenopyrite and chlorite geothermometers indicate that the retrograde skarns and late quartz sulfide calcite assemblages formed at 550 to 345 c and

mass transfer and the path of metasomatic reactions in mesothermal gold deposits an example from flambeau lake ontario gold mineralization in the flambeau lake area is found in and adjacent to quartz carbonate albite veins surrounded by halos of intense ankerite albite alteration these features are the result of the passage of hydrothermal fluid through tension fractures and shear zones developed in response to regional dextral shearing wall rock alteration is zoned consisting of four distinct alteration assemblages distal weak isochemical alteration a transitional zone where chlorite was gradually removed and replaced by carbonates and albite a zone of mainly ankerite and albite and a rare marginal zone of mainly albite and quartz a detailed analysis of potential mass balance indicators from a single alteration halo in quartz diorite revealed that zr ti light ree u th and to some extent nb remained immobile during alteration a mass balance study of this halo shows that sio 2 al 2 o 3 fe 2 o 3 mgo and k 2 o were removed from the wall rock and that cao co 2 s and na 2 o were added these changes are attributed to the breakdown of chlorite sericite calcite and quartz and their replacement by albite and ankerite vein mineral textures show that the veins were open allowing the passage of the substantial amounts of hydrothermal fluid required to produce the alteration and to remove large amounts of sio 2 from the rock up to 30 wt the behavior of perfectly mobile elements indicates that the fluid rock ratio increased toward the vein a model is proposed in which the observed alteration is explained by the interaction of the wall rock with a quartz undersaturated alkaline co 2 bearing fluid the study emphasizes the importance of quartz undersaturation as a mechanism for the enhancement of hydrothermal alteration through porosity modification in addition it demonstrates how fluid rock reaction paths can be investigated using simple mass balance calculations

hydrothermal characterization of the west gore sb au deposit meguma terrane nova scotia canada the west gore sb au deposit is anomalous in the megurea terrane of nova scotia because of its enrichment in sb a metal that is essentially absent from other megurea gold deposits the deposit is hosted by graphitic and sulfide bearing slates of the lower paleozoic halifax formation that were deformed into a northeast trending upright closed syncline and metamorphosed to the greenschist facies during the regional acadian orogeny ca 400 ma mineralized veins at the deposit define a single structure trending 110 degrees that probably formed the dextral component of a conjugate shear system as part of regional northwest directed compression the veins crosscut a penetrative regional schistosity s 1 in the host slates and vein formation is constrained by 1 the presence of cleaved wall rock slates in the veins 2 vein related sulfides overgrowing the s 1 fabric in wall rock fragments and 3 a 370 ma 40 ar 39 ar plateau age for hydrothermal muscovite mineralization occurs as stibnite native antimony aurostibnite au sb alloys and au sb o phases in vein quartz with associated fe as pb zn cu sulfides and chlorite carbonate gangue wall rock alteration is variably developed as narrmv zones peripheral to veins enriched in sericite calcite sulfides tourmaline and chlorite early stages of mineralization are constrained to maximum temperatures of 495 degrees c from arsenopyrite geothermometry but lower temperatures are recorded by chlorite geothermometry 350 degrees 390 degrees c textures of vein quartz include comb and plumose varieties with a bimodal grain size and also a coarser anhedral quartz fluid inclusion studies indicate that the fluid was a mixed h 2 o nacl cacl 2 co 2 ch 4 type and that fluid unmixing occurred albeit in minor amounts thermometric measurements reveal maximum t h of 375 degrees c and a range of salinities 0 4 28 1 wt nacl equiv the volumetric properties of fluid inclusions reflect cycling of fluid pressures with supralithostatic pressures to 4 5 kbars p fluid recorded by the presence of h 2 o co 2 inclusions to 20 mole co 2 which are interpreted to have resulted from fluid rock interaction subsequent reduction in fluid pressures to ca 2 kbars resulted in decreasing amounts of co 2 1 2 mole and fluid unmixing that generated higher and lower salinity trends observed in t h salinity plots vein forming fluids are characterized by delta 13 c sub sigma c 19 5 to 22 6 per mil delta 18 o sub h 2 o 9 to 15 per mil delta d 42 to 64 per mil and delta 34 s sub h 2 s 9 7 to 10 3 per mil based on analysis of calcite quartz fluid inclusion extracts in quartz and stibnite vein tourmaline associated with stibnite has 87 sr 86 sr measured of 0 71707 to 0 72284 these data are consistent with a metamorphic origin for the fluid with local inheritance of c and s isotope signatures from interaction of the ore fluid with graphite and sulfide wall rock slates of the halifax formation the west gore deposit originated from infiltration of metamorphic derived fluids generated during the waning stages of the acadian orogeny contemporaneous with generation and emplacement of felsic and mafic magmas the mineralizing fluids were focused to higher crustal levels p fluid ca 2 kbars where brittle ductile conditions prevailed and veins were localized to the dextral component of a conjugate shear system related to movement along a major dextral strike slip fault or shear zone cobequid chedabucto fault system deposition of vein constituents resulted from a decrease in temperature and changes in fluid chemistry decrease in f sub s 2 and f sub o 2 induced by fluid unmixing and interaction with the wall rock lithogeochemistry of the local stratigraphy does not indicate any regional enrichment in sb au or other metals the occurrence of the stibnite may therefore reflect either telescoping of metals in a au w sb province or enrichment of sb in the source area or fluid conduit relative to other meguma gold deposits

the active southwest margin of the colorado plateau uplift of mantle origin during cenozoic time the colorado plateau was raised about 2 km above sea level the most recent and best documented uplift of the plateau 1 km has been concentratedatitssouthwestmarginbetween6 and 1 ma whereas the eastern colorado plateau may have been at high elevations sinceeocenetime tobetterunderstandthe recent tectonic activity at the southwest marginofthecoloradoplateau wecompile detailed crustal thickness and density informationfromseismicandgravitydatafor a region that includes northwest arizona and the southern tip of nevada this information is used to isolate the mantle contribution to uplift wefind that there is relatively low density mantle underlying the southernmarginoftheplateauinnorthwest arizona which could result from about 60 80 km of thinning of the dense mantle lithosphere combined with about 100 cof heating through a 100 km thick mantle layer the available estimates from earthquake source seismology in or near the studyareaarecompatiblewiththisestimate of lithospheric thinning we speculate that uplift may result from subduction related thinning of the continental lithosphere

active faulting and block rotations in the western transverse ranges california the geometry of faulting slip vectors of major earthquakes and results of very long baseline interferometry vbli are consistent with active clockwise rotation of crustal blocks in the western transverse ranges about vertical axes oblique left lateral thrust faults trending roughly east west across the western transverse ranges apparently separate long east west blocks of upper crust and the large components of lea lateral shear on east striking planes suggest that the blocks rotate with respect to the san andreas fault in the mojave region between the western transverse ranges and the mojave desert paleomagnetic declinations indicate some 35 and possibly 60 of clockwise rotation since about 10 ma and more importantly the relative velocity between the only two frequently measured vbli sites in the western transverse ranges is consistent with a rotation rate of 6 ma 3 ma during a 5 year period in the 1980s azimuths of slip vectors of major earthquakes are essentially orthogonal to the segment of the san andreas fault across the western mojave desert indicating that crustal shortening across the region is absorbed by crustal thickening and not by lateral translation extension of material parallel to that segment of the fault plate motions late quaternary offsets on the san andreas and san jacinto faults and geodetic measurements in the mojave region yield a calculated direction of relative motion between the san gabriel mountains and the pacific plate across the western transverse ranges that also is perpendicular to the san andreas fault in the mojave region rotation rates associated with simple mechanisms for applying essentially normal tractions to the lateral margins of blocks are not consistent with the observed geometry of faulting and with average rotation rates the rotation rate of 2 8 5 3 ma calculated assuming that the long east west blocks rotate in response to a uniform compression of a viscous substratum to the western transverse ranges however overlaps the range of observed average rotation rates thus the hypothesis that blocks of continental crust move as if carried passively by a substratum undergoing continuous deformation passes one test that can be made of it

intrusion related high temperature gold quartz veining in the cosmopolitan howley metasedimentary rock hosted gold deposit northern territory australia in the proterozoic cosmopolitan howley gold deposit northern australia quartz vein hosted gold mineralization occurs predominantly within 50 m of the noncarbonaceous footwall of carbonaceous slates although abundant veins farther away from or within carbonaceous slate are barren except for hydrofracture stock works in the hinge zone the gold bearing quartz veins qv1 are bedding concordant and occur in hornfels and slate in the steeply dipping fold limbs of the howley anticline the qv1 veins formed near the peak of contact metamorphism 550 degrees 620 degrees c and 240 200 mpa following the intrusion of the cullen granite batholith the vein alteration assemblage is k feldspar biotite andalusite or cordierite and mineralization was accompanied by large scale potassium metasomatism whereas sodium and calcium were leached from the country rocks the qv1 veins were overprinted first by superposed folding and then by reverse faulting later transpressional strike slip faulting was accompanied by extensive retrograde alteration the precipitation of sulfide lenses delta 34 s arsenopyrite 0 3 to 4 8 quartz veins qv2 with subeconomic gold enrichments and barren pegmatitic veins qv3 the involvement of a magmatic fluid in ore genesis at the cosmo howley deposit is indicated by 1 high fluorine contents in biotite from the alteration selvages of the gold quartz veins qv1 f 5 573 6 616 ppm 49 000 ppm at mt shoobridge 7 km west 2 mutual crosscutting relations of gold quartz veins with aplite dikes 3 the location of the deposit above i type granite at a vertical distance of less than 2 km most likely or 1 km and 4 the high salinity of the mineralizing fluid as estimated from the cl content of biotite in the alteration halos of the veins a magmatic derivation of the sulfur in the auriferous sulfides is consistent with isotopic data 0 3 to 7 2 5a4s calculated oxygen isotope compositions of the fluids which precipitated the successive generations of quartz veins qvi delta 18 o sub h 2 o 13 6 qv3 delta 18 o sub h 2 o 11 8 indicate that the isotopically heavy country rock chert nodules delta 18 o or 18 5 was infiltrated by an isotopically light

a petrological and geochronological study of a 360 ma metallogenic event in maritime canada with implications for lithophile metal mineralization in the canadian appalachians the occurrence of ca 360 ma greisen related molybdenite mineralization in the clayton hill biotite muscovite leucomonzongranite 1 km2 is unusual for the southwestern meguma terrane of the maritime appalachians where 380 ma granite related sn mineralization dominates the presence of extensive hornfelsing and occurrence of mineralized sn w cu zn skarns in the area suggests a larger intrusion occurs at depth the intrusion and mineralization are constrained to 361 ma based on concordant re os molybdenite and 40ar 39ar muscovite ages which equates to other ca 360 ma granites in the area hence magmatism of this age is more widespread than previously considered the intrusion is highly evolved 74 wt sio2 metaluminous a cnk 1 1 05 a cnk molar ratio al2o3 cao na2o k2o and depleted in cafemic and transition elements rare earth elements ree 106 23 ppm are strongly fractionated with light rare earth elements lree enriched la lun 10 and negative eu anomalies biotite is

tectonic evolution of fault bounded continental blocks comparison of paleomagnetic and gps data in the corinth and megara basins greece 1 we report on new paleomagnetic and anisotropy of magnetic susceptibility ams data from plio pleistocene sedimentary units from corinth and megara basins peloponnesus greece paleomagnetic results show that megara basin has undergone vertical axis cw rotation since the pliocene while corinth has rotated ccw during the same period of time these results indicate that the overall deformation in central greece has been achieved by complex interactions of mostly rigid rotating fault bounded crustal blocks the comparison of paleomagnetic results and existing gps data shows that the boundaries of the rigid blocks in central greece have changed over time with faulting migrating into the hanging walls sometimes changing in orientation the megara basin belonged to the beotia locris block in the past but has now been incorporated into the peloponnesus block possibly because the faulting in the gulf of corinth has propagated both north and east paleomagnetic and gps data from megara and corinth basins have significant implications for the deformation style of the continental lithosphere in areas of distributed deformation the continental lithosphere behaves instantaneously like a small number of rigid blocks with well defined boundaries this means that these boundaries could be detected with only few years of observations with gps however on a larger time interval the block boundaries change with time as the active fault moves paleomagnetic studies distinguishing differential rotational domains provide a useful tool to map how block boundaries change with time index terms 1208 geodesy and gravity crustal movements intraplate 8110 1518 geomagnetism and paleomagnetism magnetic fabrics and anisotropy 1525 geomagnetism and paleomagnetism paleomagnetism applied to tectonics regional global 8107 tectonophysics continental neotectonics 9335 information related to geographic region europe

slip rate of the western garlock fault at clark wash near lone tree canyon mojave desert california the precise tectonic role of the left lateral garlock fault in southern california has been controversial three proposed tectonic models yield significantly different predictions for the slip rate history orientation and total bedrock offset as a function of distance along strike in an effort to test these models we present the first slip rate estimate for the western garlock fault that is constrained by radiocarbon dating a channel referred to here as clark wash incised into a latest pleistocene alluvial fan has been left laterally offset at least 66 6 m and no more than 100 m across the western garlock fault indicating a left lateral slip rate of 7 6 mm yr 95 confidence interval of 5 3 10 7 mm yr using dendrochronologically calibrated radiocarbon dates the timing of aggradational events on the clark wash fan corresponds closely to what has been documented elsewhere in the mojave desert suggesting that much of this activity has been climatically controlled the range front fault located a few hundred meters northwest of the gar lock fault has probably acted primarily as a normal fault with a holocene rate of dip slip of 0 4 0 7 mm yr the record of prehistoric earthquakes on the garlock fault at this site though quite possibly incomplete suggests a longer interseismic interval 1200 2700 yr for the western garlock fault than for the central garlock fault the relatively high slip rate determined here indicates that the western and central segments of the garlock fault show similar rates of movement that are somewhat faster than rates inferred from geodetic data the high rate of motion on the western garlock fault is most consistent with a model in which the western garlock fault acts as a conjugate shear to the san andreas fault other mechanisms involving extension north of the garlock fault and block rotation at the eastern end of the fault may be relevant to the central and eastern sections of the fault but they cannot explain a high rate of slip on the western garlock fault

structural control magmatic hydrothermal evolution and formation of hornfels hosted intrusion related gold deposits insight from the thaghassa deposit in eastern anti atlas morocco in the moroccan eastern anti atlas the thaghassa intrusion related gold deposit is hosted in hornfelsed metasedimentary rocks that lie adjacent to the ikniwn granodiorite fields studies reveal three tectono magmatic stages controlling the formation of the deposit i the first stage refers to the top to the south asymmetry and the syn kinematic ikniwn pluton emplacement controlled by a compressional or transpressional strain regime ii the second stage is characterized from older to younger and further away from the intrusion by metatexite with leucocratic stromatic bands aplo pegmatite sills intermediate veinlets composed of quartz k feldspar and muscovite and then gold bearing striped foliation veins all these features are assumed to have been emplaced during a large scale ene wsw dextral shearing process that results from an ese wnw shortening direction during transtensive tectonics we suggest that the progressive and continuous shearing was initiated since the aplo pegmatite stage and achieved during the hydrothermal phase the existence of intermediate veins characterized by quartz rich core and apatite muscovite feldspar rich rims demonstrates the progressive evolution from the magmatic to the hydrothermal stage and evidence for the persistence of the magmatic character at least until the onset of the hydrothermal process iii the late stage developed large volcanic dyke swarm and brittle faulting zircon u pb la icp ms dating yields a concordia age of 563 5 6 3 ma for the ikniwn granodiorite intrusion the fluid inclusions data besides the mineral thermometry indicate that two main types of fluids can been highlighted i a hot aquo carbonic h2o nacl co2 fluid with n2 and ch4 evolving from vapour rich n2 and ch4 inclusions for the magmatic stage 550 c to ch4 co2 biphased inclusions for the ongoing hydrothermal stage 450 300 c and ii an always secondary low t 200 c saline aqueous type probably nacl free from volatiles with very variable salinity eventually we highlight that in the hornfels hosted irg deposits fluid sources may originate from both magmatic processes and devolatization of the metamorphic host rocks

localized rotation during paleogene extension in east central idaho paleomagnetic and geologic evidence we collected paleomagnetic data from 137 sites in middle eocene volcanic rocks of the northeastern basin and range province in order to characterize the three dimensional kinematics of continental extension in the lost river and lemhi ranges idaho and to test previous block rotation models statistically significant flattening of directions at three of 10 localities reflects either insufficient averaging of paleosecular variation or erroneous tilt corrections five of the seven remaining locality means are statistically indistinguishable from the middle eocene reference direction this indicates that vertical axis rotation did not accompany post 50 ma normal faulting discordant declinations from two localities in the northern lost river range indicate 30 40 of counterclockwise rotation of a large domain of nnw striking volcanic rocks inclination flattening at one of the two rotated localities is moderate but statistically significant f f 9 9 6 5 in total the paleomagnetic data conflict with earlier predictions of uniform clockwise rotation in response to right lateral transtension adjacent to the eastern snake river plain there is a close correspondence between counterclockwise rotated localities and domains of nnw striking volcanic rocks and between unrotated localities and domains of north striking volcanic rocks new mapping shows that counterclockwise rotated localities are all in the hanging wall of a major west dipping low angle normal fault the donkey fault named for the first time this fault loses displacement southwards was active in eocene oligocene time and is probably a tilted high angle fault the spatial distribution of rotated and unrotated localities is best explained by counterclockwise rotation of the hanging wall of the donkey fault during opening of a southward tapering half graben according to this hanging wall rotation model the footwall of the donkey fault remained fixed relative to its rotating hanging wall our proposed kinematic model is generally applicable to extending continental lithosphere and provides a mechanism for coeval crustal thinning and vertical axis rotation thus vertical axis rotation may occur in the absence of strike slip faults in extensional orogens

hydrogen isotope systematics of submarine basalts the d h ratios and water contents in fresh submarine basalts from the mid atlantic ridge the east pacific rise and hawaii indicate that the primary d h ratios of many submarine lavas have been altered by processes including 1 outgassing 2 addition of seawater at magmatic temperature and 3 low temperature hydration of glass decreases in d and h2o from exteriors to interiors of pillows are explained by outgassing of water whereas inverse relations between d and h2o in basalts from the galapagos rise and the famous area are attributed to outgassing of ch4 and h2 a good correlation between d values and h2o is observed in a suite of submarine tholeiites dredged from the kilauea east rift zone where seawater added directly to the magma affected only the isotopic compositions of hydrogen and argon analyses of some glassy rims indicate that the outer millimeter of the glass can undergo lowtemperature hydration by hydroxyl groups having d values as low as 100 d values vary with h2o contents of subaerial transitional basalts from molokai hawaii and subaerial alkali basalts from the society islands indicating that the primary d values were similar to those of submarine lavas extrapolations to possible unaltered d values and h2o contents indicate that the primary d values of most thoteiite and alkali basalts are near 80 5 the weight percentages of water are variable 0 15 0 35 for mor tholeiites about 0 25 for hawaiian tholeiites and up to 1 1 for alkali basalts the primary d values of 80 for most basalts are comparable to those measured for deep seated phlogopites these results indicate that hydrogen in marked contrast to other elements such as sr nd pb and o has a uniform isotopic composition in the mantle this uniformity is best explained by the presence of a homogeneous reservoir of hydrogen that has existed in the mantle since the very early history of the earth

anisotropy of magnetic susceptibility data bearing on the transport direction of mid tertiary regional ignimbrites candelaria hills area west central nevada in west central nevada the oligocene candelaria pyroclastic sequence reaches a local thickness of up to 1 3 km in what has been referred to as the candelaria trough but more generally the accumulation of ash flow tuffs and related volcanic rocks is less than 300 m thick complete to near complete outcrops are scattered over about 3200 km2 in the candelaria hills and surrounding ranges of the southern walker lane structural zone three regionally extensive compound cooling units within the overall sequence 25 8 ma metallic city 24 1 ma belleville and 23 7 ma candelaria junction tuffs have distinguishing characteristics and are the focus of study at 106 sites anisotropy of magnetic susceptibility ams data provide an estimate of transport direction of each tuff inferred transport directions based on the ams data are corrected for a modest clockwise yet variable magnitude vertical axis rotation that affected these rocks in late miocene to pliocene time as revealed by paleomagnetic studies the ams data show a somewhat orderly pattern of magnetic fabrics that we interpret to define unique transport directions for the metallic city and candelaria junction tuffs the low susceptibility and degree of anisotropy of the belleville tuff limits our interpretation from this pyroclastic deposit the metallic city and candelaria junction tuffs typically show gentle south southeast and southeast dipping magnetic fabric imbrication respectively and very gently plunging magnetic lineations these ams fabric elements indicate the tuffs were transported to the north northwest and northwest respectively the ams fabric data from the metallic city and candelaria junction tuffs suggest relatively unrestricted flow during emplacement evidence across the 3 200 km2 area to support more regionally controlled channelized flow into and or flow along the east northeast west southwest axis of the candelaria trough is lacking the ignimbrites clearly filled a topographic depression inferred to have formed concurrent with early localized basin and range extension during pyroclastic emplacement but based on the uniformity of ams fabric data we infer that the depression quickly filled and did not hinder flow across the region unrecognized eruptive centers for the three ignimbrites may lie buried beneath neogene basin fill sediments south southeast of the candelaria hills or concealed below younger deposits farther southeast into the palmetto mountains

the berezovsk giant intrusion related gold quartz deposit urals russia evidence for multiple magmatic and metamorphic fluid reservoirs abstract the berezovsk gold deposit in the middle urals has been mined for 270 years its endowment past production and gold reserves is estimated to be 490 t of gold the deposit is located in the greenschist metamorphosed silurian volcanogenic sedimentary rocks intruded by granitoid dykes to the north east of late carboniferous shartash granite massif mineralisation is represented by sulphide quartz veins in the granitoid dykes ladder veins and in the host rocks krassyk veins formed in the following four stages ankerite quartz quartz pyrite gold polymetallic and carbonate ore veins are accompanied by halos of gumbeite quartz orthoclase carbonate beresite quartz sericite ankerite pyrite and listvenite quartz fe mg carbonate fuchsite pyrite the veins mainly consist of quartz with sulphide minerals commonly 3 5 vol about 180 minerals have been identified in ores but the most abundant minerals are quartz calcite ankerite pyrite galena tennantite chalcopyrite aikinite native gold and sphalerite native gold was deposited during quartz pyrite au i and gold polymetallic au ii stages fineness of gold ranges from 863 to 984 and from 723 to 848 for au i and au ii respectively the mineral and metal zoning was identified relative to the roof of the shartash granite massif the fluid inclusion study revealed that the gold mineralisation at the berezovsk deposit was formed at 300 230 c and 0 3 2 3 kbar mostly 0 5 1 2 kbar from a h2o co2 nacl fluid with salinity of 7 3 18 2 wt nacl equiv the fluid was separated into h2o co2 nacl and co2 rich fluids due to temperature and or pressure drop at the deposition site calculated 18o and d values are 5 2 8 1 and 39 to 63 respectively for the fluid in equilibrium with alteration assemblages the average 13c value for the fluid equilibrated with carbonates from the inner zones of metasomatic halos is 5 3 the calculated 18o and 13c values are 3 0 9 6 and 3 to 9 respectively for ore forming fluids the 34s values are 1 4 12 9 and 1 6 to 11 7 for the fluid in equilibrium with early and late sulphides respectively in addition to the isotopic data the geological mineralogical and fluid inclusion data confirmed the predominant contribution of the magmatic fluid to formation of the berezovsk hydrothermal system the light c o and s isotope enrichment of the fluid was mainly caused by fluid phase separation fluids generated by decarbonation and dehydration reactions due to the contact metamorphism of the host rocks during the shartash massif emplacement were responsible for additional 34s input the ore forming fluid was enriched in the light 16o isotope on the deposit flanks indicating the mixing with heated meteoric water

geological characteristics tectonic setting and preliminary interpretations of the jilau gold quartz vein deposit tajikistan the southern tien shan metallogenic province of central asia hosts a number of important gold resources including the jilau gold quartz vein system in western tajikistan these deposits were formed at the late stages of continent continent collision in association with subduction related magmatism metamorphism and continental margin deformation attributed to the central asian hercynian orogeny jilau is hosted by a hercynian syntectonic granitoid intrusive that was emplaced into bituminous dolomite country rocks economic mineralisation is associated with a dilational jog within a high angle oblique dextral reverse slip shear zone that was undergoing brittle ductile deformation the orebody takes the form of shear zone subparallel quartz veins and lenses that emanate from a steeply plunging ore shoot of veins and stringers within a silicified and sulphidised granodiorite core it is thought to have formed by a dynamic process in which fluid flow was governed by a fault valve mechanism numerous cycles of fluid pressure build up fault failure jog dilation fluid flow phase separation of low salinity h2o co2 ch4 n2 fluids and sealing took place gold appears together with scheelite and bismuth minerals predominantly as inclusions in arsenopyrite in quartz veins and altered wall rock and is mainly associated with quartz containing fluid inclusions enriched in ch4 the correlation between high gold grades and high ch4 concentrations suggests that components of the mineralising fluids were derived from or passed through the reducing carbonaceous rocks in the contact aureole of the intrusive the occurrence of au and w in an adjacent hercynian skarn deposit and in the jilau orebody infers that the ore metals in both these systems were ultimately derived from a magmatic source

geology mineralization and fluid inclusion characteristics of the lermontovskoe reduced type tungsten cu au bi skarn deposit sikhote alin russia abstract the lermontovskoe deposit 48 kt wo 3 average 2 6 wo 3 0 24 cu 0 23 g t au is situated in a w sn au metallogenic belt that formed in a collisional tectonic environment this tungsten skarn deposit has a w au as bi te sb signature that suggests an affinity with reduced intrusion related au deposits the deposit is associated with an intrusion that is part of the ilmenite series high k peraluminous granitoid granodiorite to granite suite these rocks formed via mantle magma induced melting of crustal sources the deposit comprises reduced type pyroxene dominated prograde and retrograde skarns followed by hydrosilicate amphibole chlorite pyrrhotite scheelite quartz and phyllic muscovite sericite carbonate albite quartz scheelite sulfide with abundant apatite alteration assemblages fluid inclusions from the skarn assemblages indicate high temperature 500 c high pressure 1400 1500 bars and high salinity 53 60 wt nacl equiv magmatic hydrothermal fluids they were post dated by high carbonic methane dominate low salinity fluid at the hydrosilicate alteration stage these fluids boiled at 360 380 c and 1300 1400 bars the subsequent phyllic alteration started again with a high temperature 450 c high pressure 1000 1100 bars and high salinity 42 47 wt nacl equiv fluid with further incursion of high carbonic methane dominated low salinity fluid that boiled at 390 420 c and 1150 1200 bars the latest phyllic alteration included the lower temperature 340 360 c lower pressure 370 400 bars high carbonic methane dominated but with higher co 2 fraction low salinity fluid and then the low temperature 250 300 c h 2 o co 2 ch 4 nacl fluid with both fluids boiled at the deposit level the high salinity aqueous fluids are interpreted to have come from crystallizing granitoid magma whereas the reduced high carbonic fluids probably came from a deeper mafic magma source both of these fluids potentially contributed to the w au as bi te sb metal budget decreasing temperatures coupled with high aca 2 and fluid boiling promoted scheelite deposition at all post skarn hydrothermal stages the deposit is characterized by limited downdip extent of mineralized zones and abundant coarse grained muscovite quartz apatite scheelite aggregates that formed at the phyllic alteration stage together with presence of high temperature high pressure and high salinity fluids directly exsolving from crystallizing magma this suggests a root level of the mineralized magmatic hydrothermal system of reduced w skarn deposits

paleomagnetism and tectonic rotation of the lower miocene peach springs tuff colorado plateau arizona to barstow california we have determined remanent magnetization directions of the lower miocene peach springs tuff at 41 localities in western arizona and southeastern california an unusual northeast and shallow magnetization direction confirms the proposed geologic correlation of isolated outcrops of the tuff from the colorado plateau to barstow california a distance of 350 km the peach springs tuff was apparently emplaced as a single cooling unit about 18 or 19 ma and is now exposed in 4 tectonic provinces west of the plateau including the transition zone basin and range colorado river extensional corridor and central mojave desert strike slip zone as such the tuff is an ideal stratigraphic and structural marker for paleomagnetic assessment of regional variations in tectonic rotations about vertical axes from 4 sites on the stable colorado plateau we have determined a reference direction of remanent magnetization i 36 4 d 33 0 95 3 4 that we interpret as a representation of the ambient magnetic field at the time of eruption a steeper direction of magnetization i 54 8 d 22 5 95 2 3 was observed at kingman where the tuff is more than 100 m thick and similar directions were determined at 7 other thick exposures of the peach springs tuff the steeper component is presumably a later stage magnetization acquired after prolonged cooling of the ignimbrite when compared to the plateau reference direction tilt corrected directions from 3 of 6 sites in the central mojave strike slip zone show localized rotations up to 13 in the vicinity of strike slip faults the other three sites show no significant rotations with respect to the colorado plateau both clockwise and counterclockwise rotations were measured and no systematic regional pattern is evident our results do not support kinematic models which require consistent rotation of large regions to accommodate the cumulative displacement of major post middle miocene strike slip faults in the central mojave desert most of our sites in the transition zone and basin and range province have had no significant rotation although small counterclockwise rotation in the mccullough and new york mountains may be related to sinistral shear along en echelon faults southwest of the lake mead shear zone the larger rotations occur in the colorado river extensional corridor where 8 of 14 sites show rotations ranging from 37 clockwise to 51 counterclockwise these rotations occur in allochthonous tilt blocks which have been transported northeastward above the chemehuevi whipple mountains detachment fault upper plate blocks within 1 km of the exposed detachment unexpectedly show no significant rotation from this relation we infer that rotations are accommodated along numerous low angle faults at higher structural levels above the detachment surface

quantifying dextral shear on the bristol granite mountains fault zone successful geologic prediction from kinematic compatibility of the eastern california shear zone abstractfor regional kinematic compatibility to be a valid boundary condition for continental tectonic reconstructions there must be tests that validate or invalidate kinematic model predictions in several reconstructions of western north america the displacement history of the mojave block continues to be unresolved the magnitude of displacement along the bristol granite mountains fault zone bgmfz which is the eastern margin of the eastern california shear zone ecsz in the mojave block is a key example of a long standing kinematic prediction that has defied a positive field test until now the ecsz is a network of late neogene and quaternary right lateral strike slip faults that extend from the gulf of california north through the mojave desert linking pacific north america plate motion with basin and range extension this network of faults accounts for 15 of post 16 ma plate transform motion geologic estimates of net dextral offset along the mojave portion of the ecsz 53 6 km are appro

magnetic fabric flow directions and source area of the lower miocene peach springs tuff in arizona california and nevada we have used anisotropy of magnetic susceptibility ams to define the flow fabric and possible source area of the peach springs tuff a widespread rhyolitic ash flow tuff in the mojave desert and great basin of california arizona and nevada the tuff is an important stratigraphic marker from the colorado plateau to barstow california a distance of 350 km however the location of its source caldera is unknown dated at 18 5 ma by 40ar 39 ar the tuff erupted during the early stages of miocene extension along the lower colorado river the thicker accumulations 100 m occur at kingman arizona and in the piute mountains california on opposite sides of the colorado river extensional corridor our ams studies produced well defined magnetic lineations in 30 of 42 sites distributed throughout the tuff typical ratios of the principal ams axes are 1 01 for the magnetic lineation kmax kint and 1 02 for the foliation kint kmin the bulk magnetic susceptibility of the peach springs tuff averages 2 0 10 3 in the si unit system the subhorizontal lineations which presumably parallel the flow directions form a pattern radiating outward from the approximate center of the outcrop area magnetic foliations define an imbrication that generally dips away from the distal margins and toward the center of the outcrop of the tuff the lineation and imbrication indicate a source region near the southern tip of nevada defining the best intersection of the ams lineations required restoration of major extension strike slip faulting and associated tectonic rotation in the disrupted tuff the optimum intersection of magnetic lineations lies in the southern black mountains of arizona on the eastern side of the colorado river extensional corridor no caldera structures are known from that area but the area contains thick sections of the peach springs tuff above a silicic volcanic center the caldera may be buried under younger deposits in the mohave valley of arizona tertiary granite in the newberry mountains may represent a deeper level of the peach springs tuff vent that has been exhumed by detachment faulting

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

effects of hydrothermal alteration on the magnetization of the oligocene carpenter ridge tuff bachelor caldera san juan mountains colorado intracaldera oligocene carpenter ridge tuff fills the bachelor caldera in the central san juan caldera complex and hosts mineral deposits of the creede mineral district the carpenter ridge tuff and unaltered portions of its intracaldera bachelor mountain member have strong high coercivity reverse magnetizations with average magnetic susceptibility ms and natural remanent magnetization nrm of 6 10 3 volume si and 8 a m respectively oxide phenocrysts in these rocks are titanomagnetite and ilmenite however magnetization appears to be controlled by microcrystic titanomaghemite based on thermal demagnetization unblocking temperatures and curie temperatures between 580 c and 620 c much of the intracaldera tuff was affected by potassic metasomatism a type of hydrothermal alteration characterized by addition of k and loss of ca and na between 27 3 ma and 25 1 ma potassic metasomatism resulted in the replacement of original feldspars by potassium feldspar and quartz oxidation of the original oxide phenocrysts to hematite and rutile and consequent suppression of ms and nrm by a factor of 5 also present however are metasomatized rocks that have high magnetizations even though their original oxide phenocrysts were destroyed values of ms and nrm are similar to those of outflow tuff such rocks are suspected of containing secondary magnetite on the basis of 1 curie and thermal unblocking temperatures at and below 580 c indicating that magnetization is carried by magnetite but that primary microcrysts of maghemite were destroyed and 2 observation in one sample of magnetite rimming cores of hematite rutile the presence of secondary magnetite in metasomatized rocks requires a local shift to more reducing fluid chemistry although we are unable to determine whether this shift occurred during potassic metasomatism or during later alteration or ore deposition these altered rocks may record a change from a pervasive alteration by alkaline oxidizing fluids to a vein controlled alteration dominated by more reduced fluids in which magnetite was stable

enrichment of co2 in fluid inclusions in quartz by removal of h2o during crystal plastic deformation abstract ductile strain induced leakage of h 2 o from mixed co 2 h 2 o fluid inclusions in quartz is proposed as a mechanism for producing occurrences of pure co 2 fluid inclusions in metamorphic rocks the h 2 o needed for hydrolytic weakening of quartz under stress may be provided by the h 2 o in fluid inclusions with dislocation creep as dislocations nucleate on the walls of fluid inclusions or intersect them h 2 o can be transported with the dislocations from the inclusion to the grain boundaries the process should continue so long as there is stress on the host quartz and the inclusion contains h 2 o the h 2 o taken to the grain boundaries would due to its wetting properties be wicked out along the grain boundaries if the h 2 o is totally removed from an inclusion by a flux of dislocations through the quartz during the crystal plastic flow then a residual inclusion bearing components other than water should remain a mixed co 2 h 2 o inclusion would as a consequence of the process become a pure co 2 fluid inclusion during dynamic recrystallization the co 2 would collect at grain boundary triple junctions and following grain boundary migration would become incorporated into the recrystallized quartz as fluid inclusions this mechanism of generating pure co 2 inclusions could result in co 2 fluid inclusions having densities appropriate for the pressure temperature conditions during deformation using estimates of temperature of deformation the total confining pressure during deformation could be determined from the density of the co 2 fluid in these inclusions

magnetic anisotropy in rhyolitic ignimbrite snake river plain implications for using remanent magnetism of volcanic rocks for correlation paleomagnetic studies and geological reconstructions some of the field and laboratory expenses for this work were covered by the natural environment research council nerc grant ne g005372 1 the salary and school fees for the first author were covered in part by a national science foundation nsf grant ear 443549 81179 awarded to x zhao

paleomagnetism of the hog heaven volcanic field montana difficulties in averaging paleosecular variation in volcanic fields twelve virtual geomagnetic poles from 25 paleomagnetic sites in the oligocene hog heaven volcanic field of northwestern montana yield a mean pole at 74 4 n 71 5 e alpha 95 5 9 this pole indicates clockwise rotation of 10 5 7 4 and flattening of 13 2 4 6 at the 95 confidence level assuming the axial geocentric dipole hypothesis criteria have been satisfied at 95 confidence rotation and flattening are statistically different from zero these results indicate clockwise rotation of the hog heaven volcanic field as expected for the sanpoil block farther west however under these assumptions this farsided pole requires northward transport of nearly 1400 km about 100 times too much given the geologic constraints thus although radiometric dates indicate 5 million years of volcanism several igneous rock types are present there are interbedded sediments and erosional intervals during volcanism and both stable states of the geomagnetic field are recorded this pole is not a reliable estimate of the oligocene rotation axis perhaps the volcanic events were not distributed independently with respect to paleosecular variation or a long term nondipole field component persisted during much of the evolution of the hog heaven volcanic field these results and previous studies indicate paleomagnetic data from one volcanic field may not be sufficient to estimate tilt translation or rotation of a tectonic block

paleomagnetic evidence for the age and extent of middle tertiary counterclockwise rotation dixie valley region west central nevada paleomagnetic data obtained from oligocene to lower miocene igneous rocks and middle miocene basaltic rocks at fifteen localities from a region surrounding dixie valley in west central nevada indicate that parts of the area experienced counterclockwise vertical axis rotation and these data provide constraints on the extent and timing of rotation counterclockwise vertical axis rotation probably exceeding 30 is indicated for oligocene to lower miocene rocks in the central part of the study area calculated rotations increase with increasing age of the oligocene to lower miocene rocks indicating that vertical axis rotation was concurrent with ash flow deposition about 23 33 ma paleomagnetic data indicate that middle miocene basaltic rocks about 10 17 ma postdate significant counterclockwise vertical axis rotation although this interpretation is complicated because the rocks were erupted episodically in pulses that apparently were short lived with respect to geomagnetic secular variation counterclockwise rotation was related to a deformation event that predated development of the present basin and range physiography of the area the total amount of crustal rotation in tertiary time in the area is poorly known rotation estimates calculated from time averaged mean directions that incorporate data from a broad range of the oligocene to lower miocene units probably underestimate total tertiary rotation because the mean directions include data from units that postdate much of the rotation for example a rotation estimate for a composite sequence from the central part of the study area is 23 15 whereas estimates from older and younger halves of the sequence are 37 21 and 11 16 respectively paleomagnetic data indicate that oligocene to lower miocene rocks at some localities in the northern and southern parts of the study area e g the golconda canyon locality probably did not experience significant tertiary counterclockwise rotation although the paleomagnetic data alone are insufficient to fully define the geographic area subjected to tertiary counterclockwise rotation the data suggest that a discrete subregion was affected rather than the entire basin and range province

late cretaceous paleomagnetism of the tucson mountains implications for vertical axis rotations in south central arizona the tucson mountains of southern arizona are the site of an upper cretaceous caldera from which the rhyolitic cat mountain tuff was erupted at about 72 ma two magnetic units within the cat mountain tuff are distinguished by paleomagnetic data in both the northern and southern tucson mountains the resurgent amole pluton 72 ma in the northern tucson mountains was emplaced soon after eruption of the cat mountain tuff but cooled and was magnetized after northeastward tilting 50 85 of the adjacent caldera fill sequence petrologic and paleomagnetic data indicate that the lower magnetic unit of the cat mountain tuff caps the silver bell mountains to the northwest a previous paleomagnetic investigation n 34 indicates that the silver bell mountains have been rotated clockwise 30 16 95 confidence level about a vertical axis relative to cratonic north america a similar paleomagnetic study of upper cretaceous volcanic volcaniclastic and intrusive units in the tucson mountains n 26 indicates that these rocks have been rotated 7 14 clockwise relative to stable north america a direct comparison of paleomagnetic directions for the lower unit of the cat mountain tuff shows a 17 10 clockwise rotation between the silver bell mountains and the tucson mountains which supports the relative accuracy of the absolute rotations determined for these two mountain ranges preliminary paleomagnetic directions for middle tertiary units from the silver bell and tucson mountains are consistent with clockwise rotation having occurred prior to deposition of these rocks clockwise rotation of crustal blocks in southern arizona likely was associated with strike slip movement on major northwest trending faults in the region and this movement may have been related to oblique subduction of oceanic plates along the western continental margin in late cretaceous and early tertiary time the available paleomagnetic data indicate that rocks in southern arizona have not remained unrotated with respect to north america since late cretaceous time and that vertical axis rotations may have played an important role in the region during laramide deformation

the dublin gulch intrusion hosted gold deposit tombstone plutonic suite yukon territory canada the dublin gulch intrusion is a member of the tombstone plutonic suite a linear belt of middle cretaceous intrusions that extend across the yukon territory like many of the intrusions in this suite the dublin gulch intrusion is associated with several different zones of gold and tungsten mineralization within and immediately adjacent to the intrusion the eagle zone 50 3 mt 0 93 g t gold located in the southwestern part of the dublin gulch intrusion hosts the most significant concentration of gold in the area the gold occurs in a broadly east west striking steeply south dipping series of sheeted veins the veins consist of early quartz scheelite pyrrhotite pyrite arsenopyrite and are associated with k feldspar albite alteration envelopes these grade out to and are overprinted by sericite carbonate chlorite alteration the same assemblage also occurs in veinlets that refracture sheeted quartz veins and contain the majority of the gold the gold occurs with molybdenite lead bismuth antimony sulfosalts galena and bismuthinite gold correlates strongly with bismuth r 2 0 9 a relationship common to several intrusion related gold deposits but has a poor correlation with all other elements tungsten and molybdenum have a weak inter element correlation r 2 0 55 and paragenetically pre date the majority of gold precipitation lead zinc copper silver antimony and arsenic have moderate to strong inter element correlations 0 58 to 0 93 the change from tungsten bearing mineralization through to gold bismuth rich ores with elevated syn to post ore lead zinc copper silver antimony and arsenic can be grossly correlated with a change in hydrothermal fluid composition early scheelite bearing quartz contains primary co2 rich fluid inclusions which are post dated by secondary inclusions with higher salinities up to 15 wt nacl equiv and less co2 these latter inclusions are interpreted to coincide with the later gold bismuth and base metal mineralization the favored genetic model is one in which early co2 rich fluids exsolved from a magma with an initially high co2 content but progressively became more saline and h2o rich as the system evolved

paleomagnetism of three upper jurassic ash flow sheets in southeastern arizona implications for regional deformation a sequence of three petrologically distinct upper jurassic ash flow tuffs has been sampled for palcomagnetic analysis in the huachuca mountains canelo hills and mustang mountains of southeastern arizona site mean palco magnetic directions for these units indicate significant ver tical axis rotations between sampling localities 15o 40o and a comparison of these and previously published data with reference directions from the colorado plateau implies that the jurassic rocks in southeastern arizona have been rotated in a clockwise sense with respect to stable north america these new data are consistent with palcomagnetic results showing clockwise rotation of upper cretaceous rocks in south central arizona and further indicate that studies of north american apparent polar wander based on rocks from southern arizona could be subject to error the clockwise rotations in southern arizona are likely related to late cretaceous and early tertiary strike slip movement on northwest trending high angle faults in the region and this movement may be linked to an increase in oblique farallon north american plate conver gence at that time

faults in the mojave desert california as revealed on enhanced landsat images previously unknown strike slip and normal faults in the central and eastern mojave desert have been revealed on landsat thematic mapper images enhanced by four component processing this method provides color images on which lithologies are discriminated by their contrasting absorption and reflection primarily at infrared wavelengths and particularly with regard to their ferric iron ferrous iron and hydroxyl contents while retaining landform depiction these discriminants represent a new type of geophysical display for geologic mapping in regions of well exposed bedrock faults are revealed on the images by abrupt spectral and textural contrasts that coincide with aligned topographic features the newly discovered faults form part of an extensive regional network of right shear that connects faults in the death valley region with the san andreas fault system they support a heterogeneous strain model for late cenozoic tectonic evolution of the region regional structural relations indicate a westward migration of the locus of strain through time some of the newly identified faults bound blocks that have experienced contrasting rotational histories since early miocene time

elevated shear zone loading rate during an earthquake cluster in eastern california we compare geodetic velocity to geologic fault slip rates to show that tectonic loading was doubled across the eastern california shear zone ecsz during a cluster of major earthquake activity new slip rates are presented for six dextral faults that compose the ecsz in the central mojave desert these rates were determined from displaced alluvial fans dated with cosmogenic 10 be and from a displaced lava flow dated with 40 ar 39 ar we find that the sum geologic mojave ecsz slip rate 6 2 1 9 mm yr is only half the present day geodetically measured velocity of 12 2 mm yr these rates account for cumulative fault slip and geodetic observations that span the 60 km wide shear zone therefore this difference cannot be attributed to postseismic relaxation redistribution of tectonic loading over the earthquake cycle at a regional scale suggests that earthquake clustering may be enhanced via feedback with weakening of ductile shear zones

paleomagnetic evidence of localized vertical axis rotation during neogene extension sierra san fermin northeastern baja california mexico paleomagnetic data from sierra san fermin in the gulf of california extensional province indicate that localized clockwise rotations about vertical axes occurred during pliocene through recent extension and dextral shear relative declination discordances in upper miocene and pliocene ash flow tuffs indicate a net clockwise rotation of 30 16 clockwise rotation between 12 5 and 6 ma is statistically insignificant 11 17 structural observations and geochronological data suggest that rotations in this area began post 6 ma comprising uniform sense block rotations oblique divergence associated with extension and dextral slip in the northwest striking boundary between the pacific and north american plates northeast striking sinistral slip faults and north striking normal faults accommodate distributed dextral shear in this area allowing fault blocks to rotate in a clockwise sense a model for oblique divergence predicts 21 km of shear in the direction of relative plate motion and 20 7 km ene directed extension perpendicular to the main gulf escarpment a broad region of northeastern baja california may have undergone similar distributed shear two possible dynamic models may explain this shear in one model rotation accumulates above a deep subhorizontal basal shear zone rotating blocks may extend downward to a detachment beneath the extensional province either a low angle eastward continuation of the san pedro martir fault or to a basal shear surface on top of a subducted remnant of the farallon plate alternatively distributed dextral shear may be the surface manifestation of a deep vertical shear zone linking transform faults in the northern gulf with dextral transpeninsular faults in either case shear may have transferred northward onto faults west of the san andreas fault contributing to late miocene to recent clockwise rotation of the western transverse ranges this shear is not accounted for in the 300 km of dextral slip computed from cross gulf geologic tie points

paleomagnetic study of upper miocene rocks from northern chile implications for the origin of late miocene recent tectonic rotations in the southern central andes paleomagnetic studies in the southern central andes have shown the widespread presence of clockwise vertical axis rotations rock units sampled in these studies however are heterogeneously distributed in stratigraphic age with most paleomagnetic data from northern chile being from mesozoic and lower tertiary rocks whereas most data in the southern altiplano puna and cordillera oriental are from upper tertiary rocks in this paper we present the results of a paleomagnetic study on upper miocene sedimentary rocks and ignimbrites from the precordillera of northern chile at 22 s these rocks are coeval with the initiation of crustal shortening in the eastern foreland fold thrust belt which some tectonic models relate to oroclinal rotation of northern chile primary magnetizations in rocks from widely distributed sites in two ignimbrites indicate that no relative rotations have occurred between sites suggesting the study area has acted as a single coherent block with respect to vertical axis rotational deformation although minor inadequate sampling of paleosecular variation can affect our data set its time averaged paleomagnetic direction is indistinguishable from the expected late miocene reference direction indicating no paleomagnetically significant rotation of the study area since circa 11 ma this suggests that late miocene recent oroclinal rotation of the northern chilean forearc if present must be either very low or nonuniform a direct implication of this result is that much of the unquestionable tectonic rotations detected from upper miocene rocks in the southern central andes is of local origin available structural data permit us to relate several of these rotations with shear in dextral transfer zones in the foreland thrust belt on the east side of the andes

style and age of late oligocene early miocene deformation in the southern stillwater range west central nevada paleomagnetism geochronology and field relations paleomagnetic and geochronologic data combined with geologic mapping tightly restrict the timing and character of a late oligocene to early miocene episode of large magnitude extension in the southern stillwater range and adjacent regions of west central nevada the southern stillwater range was the site of an oligocene to early miocene volcanic center comprising 1 28 3 to 24 3 ma intracaldera ash flow luffs lava flows and subjacent plutons associated with three calderas 2 24 8 to 20 7 ma postcaldera silicic dikes and domes and 3 unconformably overlying 15 3 to 13 ma dacite to basalt lava flows plugs and dikes the caldera related luffs lava flows and plutons were tilted 60 70 either west or east during the initial period of cenozoic deformation that accommodated over 100 extension directions of remanent magnetization obtained from these extrusive and intrusive caldera related rocks are strongly deflected from an expected miocene direction in senses appropriate for their tilt a mean direction for these rocks after tilt correction however suggests that they were also affected by a moderate 33 4 11 8 component of counterclockwise vertical axis rotation paleomagnetic data indicate that the episode of large tilting occurred during emplacement of 24 8 to 20 7 ma postcaldera dikes and domes in detail an apparent decrease in rotation with decreasing age of individual isotopically dated bodies of the postcaldera group indicates that most tilting occurred between 24 4 and 24 2 ma the onset of tilting immediately following after the final caldera eruptions suggests that the magmatism and deformation were linked deformation was not driven by magma buoyancy however because tilting equally affected the caldera systems of different ages including their plutonic roots it is more likely that regional extension was focused in the southern stillwater range due to magmatic warming and reduction of tensile strength of the brittle crust faults that accommodated deformation in the southern stillwater range initially dipped steeply and cut deeply to expose more than 9 km of crustal section the exposed crustal sections are probably rotated blocks above an unexposed basal detachment that lay near the early miocene brittle ductile transition

a sulfur isotope study of main stage tin and base metal mineralization at the east kemptville tin deposit yarmouth county nova scotia canada evidence for magmatic orgin of metals and sulfur

40ar 39ar dating of ribbon textured veins and wall rock material from meguma lode gold deposits nova scotia implications for timing and duration of vein formation in slate belt hosted vein gold deposits results of 15 40ar 39ar age spectra for whole rock argillite samples collected from within and adjacent to veins from eight meguma gold districts in the meguma terrane of southern nova scotia are presented the samples give excellent plateau ages i e concordance of plateaux correlation and integrated ages that range from ca 379 to ca 403 ma and there is no apparent correlation of age with bulk rock composition k2o al2o3 or sampling localities in addition apparent ages for samples from a district are similar within analytical error except for one deposit and there is no difference for samples from within ribbon textured veins versus samples collected outside of the gold districts i e in areas free of quartz veining the results of this work compare well to previous whole rock 40ar 39ar dating of meguma group samples and we concur with previous workers that the range in dates i e 380 410 ma reflects diachronous cooling of the area through the intracrystalline retention temperature fo

geochemical constraints on the genesis of the scheelite dome intrusion related gold deposit tombstone gold belt yukon canada the scheelite dome intrusion related gold deposit western selwyn basin yukon is hosted in hornfelsed metasedimentary strata that lie adjacent to the exposed apices of a monzogranite to quartz monzonite plutonic complex of the mid cretaceous tombstone tungsten magmatic belt tintina gold province alaska and yukon a variety of mineralization styles occur throughout a 10 3 km east trending corridor and include reduced au and w rich skarns au w and ag pb zn sb rich quartz tension vein arrays and multiphase fault veins and isolated zones of au rich sericite carbonate altered rock integrated u pb shrimp data for magmatic zircon and ar ar data for magmatic and hydrothermal biotite indicate that gold mineralization occurred within 1 to 2 m y of magma emplacement fluid inclusion oxygen isotope and arsenopyrite geothermometry data indicate that hydrothermal minerals formed at depths of 6 to 9 km over a temperature range from 550 c high temperature au rich skarns formed at 400 c whereas vein hosted mineralization formed at 280 to 380 c in skarns au is strongly associated with enrichments of bi te w and as whereas a variety of au rich veins occur with as rich type 1 and te and w rich type 2 end members silver pb zn sb veins are typically au poor and represent the latest and lowest temperature phase in the hydrothermal paragenesis the fluid inclusion data indicate that all mineralization styles were formed from low salinity 4 wt nacl equiv aqueous carbonic fluids consistent with the composition of fluid inclusions within infilled miarolitic cavities in the intrusive rocks however the nonaqueous fluid was predominantly ch 4 in skarn co 2 in au te and au w veins and a fluid with roughly equal amounts of co 2 ch 4 and n 2 in au as and ag pb zn sb veins oxygen isotope data are consistent with a mineralizing fluid of predominantly magmatic origin that was variably modified to more positive 18 o values during interaction with 18 o enriched metasedimentary strata sulfur isotope data suggest two possible sources of sulfur a magmatic source characterized by 34 s values of approximately 5 to 0 per mil and sulfur from the metasedimentary country rocks characterized by more negative 34 s values of approximately 15 to 10 per mil collectively the data indicate that gold at scheelite dome was deposited from a magmatic hydrothermal system interaction of magmatic fluids with graphitic hornfels rocks resulted in reduction of the ore fluids higher ch 4 co 2 ratios and modification of the oxygen and sulfur isotope values of the ore fluids toward those of the metasedimentary hornfels progressive reduction and cooling of hydrothermal fluids in addition to phase separation in vein hosted mineralization were the mechanisms for gold deposition compared to other intrusion related gold deposits associated with the tombstone tungsten magmatic belt magmatism exposed mineralization at scheelite dome is predominantly hosted by hornfelsed metasedimentary rocks this results in more diverse mineralization styles and a greater spread of isotope and fluid inclusion data

emplacement depth and carbon dioxide rich fluid inclusions in intrusion related gold deposits phanerozoic intrusion related gold deposits have many consistent characteristics such as a spatial temporal and geochemical association with moderately reduced predominantly ilmenite series i type intrusions the deposits exhibit a range of characteristics that vary over a wide range of emplacement depths 7 km deposits in shallow crustal settings 5 km have characteristics of mesothermal environments and are hosted by plutons containing sheeted veins greissen and disseminated gold fluid characteristics also vary with depth deposits in shallow environments contain high temperature 350 c immiscible brine 30 wt nacl equiv and low salinity 5 wt nacl equiv vapor that commonly contains carbon dioxide deposits in deeper environments contain abundant low salinity carbon dioxide rich aqueous fluids 10 wt nacl equiv that in some deposits are postdated by moderate to high salinity brines 10 40 wt nacl equiv these contrasting fluid types are interpreted to be magmatic in origin and are the result of the complex interplay between exsolution of different volatiles carbon dioxide water and chlorine from felsic magmas emplaced at different crustal levels experimental studies have shown that carbon dioxide will exsolve from felsic magmas at much higher pressures than water and chlorine due to its lower solubility in the melt thus deposits formed from felsic magmas in deeper environments will contain abundant carbon dioxide rich fluids subsequent fluids released from the magma are likely to be water rich and more saline felsic magmas in shallow environments will contain lower concentrations of carbon dioxide that will partition into the vapor phase during phase separation between brine and vapor

flexural slip folding in the meguma group nova scotia canada abstract flexural slip folding has been established for two anticlines in the meguma group nova scotia flexural slip postdates the main acadian episode of flexural flow fold growth and represents a late brittle ductile reactivation of the fold belt flexural slip structures are dominated by bedding parallel movement horizons but include a linked system of frontal and lateral ramps and conjugate movement horizons slip amount calculated from displacement of discordant quartz veins combined with vein spacing indicates an average local shear strain of approximately 0 5 however because flexural slip occurred when the fold limbs were steep the shear strain accounts for a small change in limb dip 4 8 variation of slip amount with spacing of movement horizons suggests progressive flexural slip was accommodated by continuous initiation of new movement horizons as well as continuing slip on existing movement horizons the presence of quartz veins along flexural slip structures together with evidence for ductile deformation coeval with flexural slip suggests that slip episodes were fluid assisted and intermittent separated by periods of ductile deformation and build up of fluid pressure flexural slip veins in the ovens anticline are auriferous and a flexural slip model may be appropriate for other similar meguma gold deposits thrust sheets up to 10 m thick occurring on a fold limb are interpreted to have originated in flat box fold hinges and to have been placed onto the steep limbs of the chevron folds during the flexural slip episode thrusting may have assisted the transformation of a box fold into a chevron

syn acadian emplacement model for the south mountain batholith meguma terrane nova scotia magnetic fabric and structural analyses the anisotropy of magnetic susceptibility and the structural geology of the late devonian south mountain batholith meguma terrane nova scotia are used to characterize its emplacement and structural evolution the south mountain batholith is the largest peraluminous granitoid complex within the appalachian orogen it is a composite batholith made up of stage 1 granodiorites and monzogranites and stage 2 leucomonzogranites and leucogranites the anisotropy of magnetic susceptibility was used to map the biotite petrofabric at the scale of the batholith the biotite fabric pattern is similar to the pattern of folding and shearing of the feldspar megacryst foliation documented at the outcrop scale in the stage 1 plutons the magnetic foliation is deformed into a girdle about the horizontal northeast southwest trending magnetic lineation which is parallel to regional fold axes and extension lineations in the country rocks in the stage 2 plutons a horizontal northeast southwest to east west trending magnetic lineation is also present but the horizontal orientation of the magnetic foliation is widely preserved the folding and shearing of the foliations and the horizontal northeast southwest trending lineation that is pervasive throughout the south mountain batholith are interpreted to record acadian tectonic deformation of the plutons as they crystallized acadian deformation continued during the latest stages of crystallization resulting in the localization of mineral deposits and greisens along shear zones faults and joints a model is presented for the syntectonic emplacement of the south mountain batholith as a laccolithic complex

an oxygen isotope study of two contrasting orogenic vein gold systems in the meguma terrane nova scotia canada with implications for fluid sources and genetic models sampling of quartz vein material from two gold deposits of similar geological setting but different ages the ovens 408 ma dufferin 380 ma in the meguma terrane of nova scotia has been done to compare and contrast their 18oquartz signatures despite different ages of formation quartz from all vein types in each of the deposits i e saddle reef bedding concordant leg reefs en echelon discordant shows limited intra deposit variation with similar average 18o values of 15 2 0 9 n 16 for the ovens and 15 7 0 6 n 12 for dufferin using an average 18o value of 15 4 for the two deposits the corresponding 18oh2o values calculated for 400 c and 350 c based on constraints from mineral assemblages and fluid inclusion studies indicate averages of 11 4 0 2 and 10 2 0 2 respectively the isotopic data indicate that 1 the vein forming fluids have a metamorphic signature 2 all vein types in the two deposits represent formation from a single isotopically homogeneous fluid and 3 a fluid of similar isotopic signature was generated by two contrasting tectono thermal events in the meguma terrane that were separated by 30 ma integration of these results with previously published data for 14 meguma gold deposits indicate a general stratigaphic dependence in 18oh2o values for deposits when arranged in their relative stratigraphic position such that 18oh2o values increase upwards in the stratigraphy this apparent trend cannot be explained by models involving either fluid mixing or cooling of the vein forming fluids but instead is modelled using fluid rock interaction taking into account a change in the modal mineralogy of the metasedimentary rocks upwards in the stratigraphy

late paleozoic collision delamination short lived magmatism and rapid denudation in the meguma terrane nova scotia canada constraints from 40ar 39ar isotopic data the meguma terrane consists of 8 15 km of early paleozoic stratified rocks deposited on a continental basement that were deformed and metamorphosed during the late paleozoic as a result of lithospheric plate collision the oldest cleavage previously published whole rock 40ar 39ar plateau ages of 415 395 ma dates the onset of crustal thickening which was followed by voluminous but short lived late devonian granitic and minor mafic magmatism 380 370 ma this magmatism may have been the product of delamination of the lower lithosphere and upwelling of asthenosphere which effected melting above the new moho and resulted in intrusion at depths of 5 12 km 40ar 39ar plateau ages of hornblende muscovite and biotite indicate that at the present erosion level most of the meguma terrane cooled through 300 c by 368 360 ma slightly earlier than for the southwestern meguma terrane ca 345 ma the present erosion level was exhumed by the latest devonian early carboniferous visean the age of the

cretaceous eocene laramide landscape development and oligocene pliocene drainage reorganization of transition zone and colorado plateau arizona landscape development of central and northern arizona can no longer be ascribed mainly to events of miocene and pliocene age new information on the age and distribution of older cenozoic deposits has led to the recognition of a regional cretaceous paleocene surface of erosion that conforms to major elements of the present topography and to the recognition that a formerly thick deposit of gravel accumulated on this regional surface of erosion these relations cast new light on the history of evolution of the landscape and indicate a much greater age for the main landscape elements and a more complicated and prolonged history of erosion and deposition than has been previously supposed the timing of events postulated for development of drainage on the colorado plateau can now be compared and partly reconciled with events recognized in the adjacent closely related mountain region transition zone of central arizona as a consequence of late cretaceous paleocene laramide compression central and northern arizona underwent at least 1200 m of uplift documented by paleochannels cut into erosionally truncated paleozoic strata on the hualapai plateau of the southwestern colorado plateau during this time a highly irregular erosion surface was developed on proterozoic rocks across the transition zone south of the mogollon rim the scarp of the mogollon rim was eroded to its present height 600 900 m and an extensive stripped surface was developed on resistant upper paleozoic strata north of the rim deposition of several hundred meters of paleocene eocene rim gravels derived from highlands south and west of the region followed covering much of the cretaceous paleocene erosion surface nearly complete burial of the rim is suggested by the distribution of remnants of the rim gravels across the erosional scarps and on high plateau areas north of the rim a second increment of uplift apparently occurring in late eocene time and apparently recorded by a series of fission track cooling ages from the marble and grand canyons is inferred to have been responsible for ending deposition of the rim gravels for initiating differential uplift of contemporaneous deposits canaan peak and claron formations to their positions in the high plateaus of central utah and for causing the drainage reorganization required to explain the extensive removal of rim gravels from much of the region a southerly flowing ancestral verde river related to the drainage reorganization removed much of the older gravel cover from the transition zone of central arizona resulting in a younger regional erosion surface having 600 900 m of relief a surface closely approximating the cretaceous paleocene erosion surface late oligocene and early miocene rocks locally rest unconformably on remnants of rim gravels in the transition zone indicating that the second episode of regional erosion had been completed by late oligocene time north of the mogollon rim a west flowing ancestral colorado river is inferred to have become established on the rim gravels draining the interior parts of the colorado plateau and transporting detritus off the plateau exhumation of the mogollon rim and development of 600 900 m of topographic relief in the transition zone by an ancestral verde river system suggests the potential for a comparable coeval entrenchment of an ancestral colorado river in paleozoic strata north of the mogollon rim regional extension and volcanic activity ensued in late oligocene to pliocene time the oligocene erosion surface in the extensional basins of central arizona became largely concealed by miocene and pliocene deposits as the neogene climate became drier in late miocene and pliocene time perennial streams appear to have been lacking transport of detritus appears to have been principally by flash flooding little or no detritus appears to have been removed from the region and much of the precipitation presumably moved by groundwater flow through the deposits of aggradation a coeval episode of aggradation in the grand canyon is suggested by deposits that appear to have once choked much of the canyon if this event parallels the episode of late miocene and pliocene aggradation recorded east south and west of the grand canyon the colorado river could have been incised to its present level by late miocene time a return to wetter conditions in late pliocene time presumably was responsible for renewed erosion and reexcavation of older drainages and basins an understanding of this tertiary structural erosional and depositional history can be important for the geological analysis of geophysical transects across the region

deformation across the western united states a local estimate of pacific north america transform deformation we obtain a locally based estimate of pacific north america relative motion and an uncertainty in this estimate by integrating deformation rate along three different paths leading west across southwestern north america from east of the rio grande rift to near the continental escarpment data are primarily quaternary geologic slip rate estimates and resulting deformation determinations therefore are instantaneous in a geologic sense but long term with respect to earthquake cycles we deduce a rate of motion of the pacific plate relative to north america that is 48 2 mm yr a rate indistinguishable from that predicted by the global kinematic models rm2 and nuvel 1 however we obtain an orientation that is 5 9 counterclockwise of these models a more westerly motion of the pacific plate with respect to north america is calculated from all three paths the relatively westerly motion of the pacific plate is accommodated by deformation in the north american continent that includes slip on relatively counterclockwise oriented strike slip faults including the san andreas fault which is especially relevant in and south of the transverse ranges and a margin normal component of net extension across the continent which is especially relevant north of the transverse ranges deformation of the sw united states occurs in regionally coherent domains within which the style of deformation is approximately uniform in the vicinity of the transverse ranges two important shear systems splay from the san andreas fault the eastern california shear zone trending nnw from the eastern transverse ranges and the trans peninsular faults trending sse from the western and central transverse ranges within the transverse ranges the right lateral san andreas fault steps left seemingly requiring large amounts of convergence there however most of this convergence is avoided through a funneling flow of the crust toward the western transverse ranges and into the relatively narrow central california coast ranges and the northern motion of the mojave the former process involves an alternation of rotation direction from counterclockwise in and south of the central transverse ranges to clockwise in the western transverse ranges

geology and genesis of variscan porphyry style gold mineralization petrackova hora deposit bohemian massif czech republic a large number of variscan mesothermal gold deposits are located in the central part of the bohemian massif close to the central bohemian plutonic complex the petrackova hora deposit has many features that distinguish it from other deposits in the region and suggest its mineralization is closely related to the late magmatic processes associated with the petrackova hora granodiorite the gold ores occur as sheeted arrays of quartz veins and veinlets hosted by the small petrackova hora granodiorite stock gold is found mainly as free grains of 900 fineness and is accompanied by abundant pyrrhotite and chalcopyrite and accessory pyrite arsenopyrite loellingite and molybdenite molybdenite from the petrackova hora deposit has been dated by the re os method at 344 4 2 8 ma hydrothermal alteration in the petrackova hora deposit exhibits a distinct temporal paragenesis selectively pervasive early k alteration and silicification are the oldest hydrothermal phases these were followed by early quartz veins q1 to q4 that contain most of the gold mineralization late quartz veins q5 and fracture controlled silicification are gold poor or barren barren calcite veins are the youngest hydrothermal product extensive low temperature meteoric water dominated alteration as is typical of classic porphyry deposits is absent however the lower 18o whole rock values for petrackova hora granodiorite and aplite 2 4 to 5 1 smow compared to other intrusions in the region reflect either interaction with isotopically light external fluids or magma assimilation of small volumes of hydrothermally altered country rock the 18o isotopic compositions for quartz scheelite and hornblende 7 7 to 13 4 smow and the 34s compositions for sulfide minerals 1 to 3 5 cdt from early gold rich quartz veins indicate formation at high temperatures 590 to 400 c from fluids with a magmatic isotopic signature 18ofluid of 5 7 to 7 2 fluids related to late quartz veins q5 suggest the presence of a significant component of non magmatic water 18ofluid 2 5 to 4 0 the 34s values of post q5 sulfide minerals 4 5 to 3 5 reflect at least partial derivation of late stage sulfur from a source external to the intrusions aqueous aqueous carbonic and nitrogen bearing fluid inclusions were identified in hydrothermal and igneous quartz with the aqueous inclusions being the most common in hydrothermal vein quartz the salinity of primary aqueous inclusions falls into ranges 6 to 23 and 33 to 41 equiv wt nacl in igneous quartz populations in salinity were observed between 5 to 16 35 to 40 and 62 to 70 equiv wt nacl the salt component of these fluids is best and minimally approximated by the nacl kcl cacl2 system low and high salinity aqueous carbonic inclusions are accessory in many of the analyzed samples three large successive pulses of fluids are recognized each pulse begins with a high salinity 30 equiv wt nacl magmatic fluid and evolves toward a lower salinity 5 equiv wt nacl fluid data suggest that external meteoric water s were significant for only the third fluid pulse which formed the late q5 quartz veins and the calcite veins polyphase fluid inclusions hosted by igneous quartz of the petrackova hora granodiorite indicate minimum trapping conditions of about 3 kbar and 550 c the gold rich q1 to q4 veins may have formed along a quasi isobaric cooling path at 2 5 to 1 5 kbar and 590 to 400 c this was followed by uplift and formation of late q5 quartz veins 0 5 to 1 5 kbar 300 c and post ore calcite veins 0 5 kbar 100 to 140 c the characteristics of the petrackova hora deposit suggest that it may represent a position intermediate between intrusion related gold systems e g fort knox deposit alaska and gold rich copper poor porphyry deposits e g maricunga belt in chile as such the petrackova hora deposit might be an example of the reduced gold sub type of porphyry deposit

new evidence on the age of the taklimakan desert the taklimakan desert is the world s second largest shifting sand desert located in the rain shadow of the tibetan plateau the initiation of desert formation in the asian interior is one of the most prominent climate changes in the northern hemisphere during the cenozoic era dating the earliest formation of this desert is important for understanding the climatic effects of the uplifted tibetan plateau here we report 1071 m thick neogene deposits with inter calated eolian dune sands from the only outcrop in the central taklimakan desert based on paleo magnetic measurements biostratigraphic age control and previous electron spin resonance dating results the oldest in situ eolian dune sands were confi rmed to be ca 7 ma much older than the previous reported wind blown siltstone from the southern edge of the taklimakan desert this new result indicates that desert conditions initiated ca 7 ma in the center of the tarim basin we attribute this event to the late cenozoic climatic deterioration and or to the rain shadow effect caused by late miocene uplift of the northern tibetan margin

an isotopic c o sr study of vein gold deposits in the meguma terrane nova scotia implication for source reservoirs vein quartz carbonate and tourmaline from 19 meguma gold deposits in the meguma terrane of nova scotia have been analyzed for stable delta 18 o delta 13 c and radiogenic 87 sr 86 sr isotopes in order to assess the nature and origin of the vein forming fluids as with other mesothermal gold provinces the meguma gold deposits are well suited to such a study because carbonate is the next most abundant phase after quartz in these mesothermal lode gold deposits all vein types have been sampled for quartz and carbonate and in addition all compositional and textural varieties of carbonate have been sampled vein quartz is of uniform isotopic composition with most 18 o values between 10 2 to 17 6 per mil avg ca 14 per mil except for one deposit west gore sb au where values go to 19 4 per mil there is no systematic variation for quartz within a deposit or position within the meguma group stratigraphy vein carbonate 18 o values range from 11 8 to 27 5 per mil with most in the 13 to 16 per mil range the relatively 18 o enriched carbonates reflect exchange with low temperature fluids based on analyses of quartz carbonate pairs that indicate disequilibrium fractionation delta sub quartz carbonate or 0 this is best illustrated by the strong negative correlation between delta 18 o carbonate and delta sub quartz carbonate the 18 o water is estimated at 10 or 3 per mil for a temperature of vein formation of 350 degrees to 400 degrees c and using the appropriate mineral water fractionation equations whereas delta 18 o water values partly overlap the field for magmatic fluids the values are wholly consistent with a metamorphic fluid and it is considered unlikely that a primary magmatic fluid signature has been substantially modified due to wall rock influences given that mesothermal gold systems are sites of high fluid rock ratios vein carbonate has delta 13 c values of 13 1 to 25 9 per mil but a slight negative correlation between delta 13 c carbonate and delta sub quartz carbonate suggests that the primary values lie in the range 20 to 25 per mil thus the delta 13 c values indicate a reduced biogenic source for the carbon oxidation of the reduced carbon as indicated by co 2 in fluid inclusions may have occurred via hydrolysis of graphite or dissolution of carbonate minerals both of which occur in the wall rock lithologies of the meguma group the latter of which has the appropriate isotopic composition the initial 87 sr 86 sr of the vein fluid estimated from the analyses of 52 vein carbonates 17 deposits and four tourmalines three deposits ranges from 0 70118 to 0 72284 and within deposits considerable variation is observed there is insufficient data to quantify the extent of the low temperature overprint which has modified the c and o isotope data although it is likely that some influence is present nevertheless the data cannot be reconciled by a source confined exclusively to the meguma group which suggests therefore involvement of another reservoir s the isotopic heterogeneity can be explained by variable amounts of contamination of a primary fluid with radiogenic sr derived from meguma group lithologies by interaction along the fluid path or at the site of vein formation concomitant with wall rock alteration as discussed above a dominantly magmatic source is not considered feasible this fluid source is suggested to be within the structural basement to the meguma group and the liscomb gneisses are the favored source based on the combined results of the sr isotope data presented herein and previously published pb isotope data collectively the data indicate that a primary fluid of metamorphic origin has had its isotopic signature variably modified due to interaction with different reservoirs the most affected isotopic systems are c and s based on earlier work on delta 34 s values which are abundant as graphite carbonate and sulfides in the meguma wall rock respectively the range in 87 sr 86 sr values of the fluid also reflects contamination but this was quite variable the uniform delta 18 o water value for the fluid indicates that this was the least affected isotopic system except for the later exchange of carbonate at low temperatures

a reinterpretation of the role of granitoids in the genesis of neoproterozoic gold mineralization in the telfer dome western australia the telfer gold mine in the paterson province of western australia is one of australia s largest gold deposits with more than 4 million oz m oz or 125 t of gold produced since mining commenced in 1977 hosted by low grade dominantly marine metasedimentary sequences of the upper yeneena group approximately 1 000 750 ma the neoproterozoic mineralization 700 600 ma is localized within the northwest trending telfer dome which consists of two en echelon asymmetric doubly plunging anticlines called the main dome and the west dome within these two domes gold and minor copper occur as a series of vertically stacked strata bound to stratiform gold copper sulfide quartz reefs up to several meters in thickness linked by zones of intense stockwork and sheeted veins carbon oxygen boron lead and sulfur isotope data including shrimp microanalysis from ore sulfides and alteration minerals carbonates and tourmalines in conjunction with tourmaline and pyrite compositional data indicate that the ore fluid solutes were derived chiefly from sedimentary host rocks and not neoproterozoic granitoids a model for telfer mineralization is proposed such that granitoids act primarily as sources of heat that drive thermal convection cells in which heated saline formational contact metamorphic fluids scavenge gold copper and sulfur from the surrounding sedimentary wall rocks at depth this style of ore fluid generation shares some similarities with a convective porphyry copper gold model although the lack of hornfelses proximal granitoids and types of alteration characteristic of porphyry copper deposits implies that the telfer dome deposits did not form in the near porphyry environment these data do suggest however highly channelized fluid flow to the domal sites of mineralization after an initial period of extensive deep fluid circulation and leaching of sulfur and metals from the sedimentary wall rocks adjacent to the pluton s a regional scale deep structure capable of focusing these deeper fluids to higher stratigraphic levels is the north northwest trending telfer lineament upon which the telfer dome is situated periodic reactivation of this structure during the the paterson orogeny would allow for episodes of widespread fluid advection at depth under essentially lithostatic fluid pressures followed by rapid ingress of fluids into the structure triggered by a decrease in fluid pressure during reactivation

bulk microanalysis of assemblages of small fluid inclusions by la icp ms methodology and application to orogenic gold systems abstract microanalysis of individual fluid inclusions by laser ablation inductively coupled mass spectrometry la icp ms is a powerful tool for reconstructing the composition of hydrothermal fluids but it demands a sample quality that is unattainable in many cases in orogenic gold deposits the need for direct fluid microanalysis has been present for several decades but due to the high fluid flux and prolonged hydrothermal and tectonic history that typifies these systems most samples do not meet the criteria of fluid inclusion size and distribution that allow la icp ms analysis of individual isolated fluid inclusions to overcome this difficulty a method has been developed and tested whereby areas of quartz densely populated with fluid inclusions e g growth zones secondary planes are analyzed along a single continuous laser ablation profile the generated signals are subsequently converted to time slice datasets and plotted as element ratios in ternary diagrams to reconstruct specific major and trace element ratios the estimated fluid compositions are in good agreement with previous analytical results of the same material microthermometry evaporate mound and conventional la icp ms and are shown to be geologically viable on a boarder scale when compared to literature data from similar ore environments the method has high spatial and chemical resolution which allows the reconstruction of micro scale fluid chemical changes such as significant fluctuation in the relative element concentration e g k rb ba sr and v versus na li and as during crystal growth as observed in one of the test samples the significance of this bulk la protocol is that it allows the quick easy and cost effective microanalysis of samples that are typically inundated with fluid inclusions such as those in orogenic and epithermal systems which would otherwise not be amenable for further quantitative analysis to constrain fluid chemistry

textural evolution of synthetic fluid inclusions in quartz during reequilibration with applications to tectonic reconstruction experimental reequilibration of synthetic 10 wt nacl h2o inclusions in natural quartz reveals that reequilibration textures show distinct differences depending upon the p t path followed by the inclusion after formation these differences combined with other geological information may be used to determine whether the sample rock followed a dominantly isothermal or isobaricp t path following entrapment the intensity and style of inclusion reequilibration features is related to the direction and magnitude of the departure of thep t path from the original isochore for the inclusion thus fluid inclusion reequilibration textures not only permit inclusionists to determine whether the rocks followed an isothermal or isobaric retrogradep t path but also the magnitude of departure of this path from one that is isochoric

rhenium osmium geochronology of arsenopyrite in meguma group gold deposits meguma terrane nova scotia canada evidence for multiple gold mineralizing events rhenium osmium geochronology using arsenopyrite was undertaken for three gold deposits in the meguma terrane nova scotia canada in order to better constrain their age of formation and to assess the utility of arsenopyrite for dating similar deposits globally analyses of arsenopyrite from bedding concordant veins from the ovens locality southwest meguma terrane yield a precise re os isochron age of 409 5 ma arsenopyrite analyses from a bedding discordant vein at this locality indicate an identical re os age of 407 4 ma saddle reef veins from the dufferin deposit northeast meguma terrane contain arsenopyrite with a precise re os isochron age of 380 3 ma the lack of common os in some arsenopyrite samples from both the ovens and dufferin permit calculation of single mineral model ages for each deposit which are identical to those determined using the isochron method initial os compositions for the two vein types at the ovens suggest a predominately crustal source of os in the mineralizing fluids whereas a less radiogenic initial os composition for arsenopyrite from dufferin does not as clearly define a crustal metal source at a third locality the touquoy deposit the re os systematics of arsenopyrite associated with disseminated gold mineralization do not define a precise formation age possibly as a result of mixing of re and os derived from the mineralizing fluid and the shale host rock the re os ages of arsenopyrite indicate that there were at least two distinct periods of gold deposition in the meguma terrane coinciding with widespread tectonothermal events regional deformation and metamorphism associated with acadian orogenesis and widespread generation of meta and peraluminous granites and high grade metamorphism within the basement rocks under the meguma terrane the ca 407 ma age for the ovens arsenopyrite provides the best estimate for the timing of regional acadian deformation in the meguma terrane and is slightly older than previous estimates based solely on 40ar 39ar dating

geological and geochemical studies of a metaturbidite hosted lode gold deposit the beaver dam deposit nova scotia ii isotopic studies the beaver dam gold deposit is located within metaturbidites of the lower paleozoic megurea group of southern nova scotia part of the meguma terrane of the canadian appalachians which records a mid devonian metamorphic and deformational history the deposit one of several hosted by the meguma group is dominated by bedding concordant veins with lesser discordant or fissure veins that record a two stage paragenesis stage 1 quartz calcic plagioclase biotite mg mg fe super 2 or mg no 0 50 tourmaline or amphibole mg no 0 54 or garnet or epidote or apatite or ilmenite and stage 2 quartz calcite chlorite mg no 0 50 albite scheelite muscovite and sulfides cu fe zn pb transcend the two stages and gold is late pardgenetically mineralization formed from a low salinity h 2 o co 2 x sub co 2 0 15 fluid and vein formationis constrained geochronologically at 370 ma from 40 ar 39 ar dating of hydrothermal amphibole muscovite and biotite isotopic studies o c h sr of mineral phases quartz amphibole feldspar mica carbonate tourmaline indicate the following 1 the veins were emplaced over a temperature interval of ca 500 degrees to 300 degrees c based on oxygen isotope geothermometry of silicate phases this range is comparable to temperatures inferred from silicate sulfide geothermometry kontak and smith 1993 2 the stable isotope composition of the parental fluid is delta d 40 or 6 and delta 18 o 6 to 12 per mil based on mean values of analyzed phases and inferred temperatures of formation the delta d water values calculated from mica phases concur with extracts from vein quartz samples dominated by a single generation of primary fluid inclusions these results demonstrate that a delta d signature of the primary fluid can be obtained via crushing of bulk samples where the potential influence of secondary inclusions has been eliminated 3 interaction of the ore forming fluid with wall rock lithologies i e meguma group at the site of deposition has reatly influenced the s and c isotope signatures of the carbonates delta 13 c 21 1 or 1 1ppm and sulfides delta 34 s 10 or 1ppm kontak and smith 1989 4 87 sr 86 sr measured ratios of calcite 0 70787 0 71358 n 8 and tourmaline 0 713055 indicate modification of the primary signature of the fluid ca or 0 708 due to contamination probably by meguma group lithologies collectively the isotopic data indicate that the ore forming fluids were not generated within the meguma group sr also see pb isotope data of chatterjee et al 1992 but that subsequent interaction with the meguma group has modified some isotopic signatures c s sr the o and h isotope data are equivocal with regard to the meguma group and do not permit a definitive distinction between magmatic and metamorphic reservoirs although the latter are preferred on the basis of geologic grounds and analogies with other similar deposit types we conclude that the isotopic data for the beaver dam deposit are most consistent with a model invoking generation of fluids from metamorphic devolatilization reactions deep within the crust and structural focusing of such fluids in contrast a magmatic source is incompatible with the isotopic data unless such a fluid has been highly modified by fluid rock interaction finally there is no support for a meteoric component to the ore fluid

can economic porphyry copper mineralization be generated by a typical calc alkaline melt numerical simulation of chlorine and copper partitioning between a crystallizing melt and exsolving aqueous fluids indicates that typical calc alkaline magmas contain sufficient copper chlorine and water to produce economic porphyry copper mineralization neither an elevated copper content in the magma nor an additional large volume of magma are required to provide metals or volatiles the most important variables that determine the volume of melt necessary to produce sufficient copper are the degree of compatible behavior displayed by copper the ratio of initial water in the melt to the water saturation level and the initial chlorine water ratio of the melt the absolute values for initial water in the melt and water content at saturation are relatively unimportant in determining the required melt volume the bulk salinity of the exsolved fluid may vary from 2 0 wt nacl to saturation levels 84 wt nacl at 700 c indicating that boiling is not necessary to produce high salinity brines at appropriate p t xnacl conditions the magmatic aqueous fluid separates into a saline liquid which transports most of the copper and a low salinity vapor the salinities of the two immiscible phases are governed by the p t conditions while the bulk fluid salinity determines the mass fractions of liquid and vapor formed pressure quenching causes rapid crystallization of the aplitic groundmass in porphyritic rocks associated with copper mineralization and significantly reduces the amount of chlorine and copper partitioning to the aqueous fluid this results in abrupt and possibly large reductions in fluid salinity and causes copper to become concentrated in the melt as copper is transported from the melt by the earliest exsolving fluids in deep 2 0 kbar systems and by late exsolving fluids in shallow 0 5 kbar systems the relative timing of pressure quenching aplite formation and fluid transport of copper from the melt can vary significantly in systems produced under different confining pressures model results incorporating petrologic constraints determined for yerington nevada are in good agreement with observed mineralization