the evolution of oceanic and atmospheric anomalies in the northeast pacific during the el nino and la nina events of 1995 2001 abstract from late 1995 through early 2001 three major interannual climate events occurred in the tropical pacific the 1995 97 la nina ln 1997 98 el nino en and 1998 2001 ln we analyze atmospheric and upper oceanic anomalies in the northeast pacific nep during these events and compare them to anomalies both elsewhere in the north and tropical pacific and to typical en and ln anomaly patterns the atmospheric and oceanic anomalies varied strongly on intraseasonal and interannual scales during the 1995 97 ln and 1997 98 en the northeast pacific was dominated by negative slp and cyclonic wind anomalies and by upper ocean temperature and sea surface height ssh anomalies the latter were positive along the north american west coast and in the nep thermal anomaly pool between hawaii vancouver island and baja california and negative in the central north pacific this atmospheric oceanic anomaly pattern is typical of en an eastward shift in the atmospheric teleconnection from east asia created en like anomalies in the nep during the 1995 97 ln well before the 1997 98 en had begun the persistence of negative sea level pressure slp and cyclonic wind anomalies in the nep during the 1997 98 en intensified pre existing upper oceanic anomalies atmospheric anomalies were shifted eastward during late 1996 early 1998 leading to a similar onshore shift of oceanic anomalies this produced exceptionally strong positive upper ocean temperature and ssh anomalies along the west coast during the 1997 98 en and explains the unusual coastal occurrences of several species of large pelagic warm water fishes the growth and eastward shift of these pre existing anomalies does not appear to have been linked to tropical pacific en anomalies until late 1997 when a clear atmospheric teleconnection between the two regions developed prior to this remote atmospheric impacts on the nep were primarily from east asia as the 1998 2001 ln developed nep anomalies began reversing toward the typical ln pattern this led to predominantly negative slp and cyclonic wind anomalies in the nep and upper ocean temperature and ssh anomalies that were mainly negative along the west coast and positive in the central north pacific the persistence of these anomalies into mid 2001 and a number of concurrent biological changes in the nep suggest that a decadal climate shift may have occurred in late 1998 during 1995 2001 nep oceanic anomalies tracked the overlying atmospheric anomalies as indicated by the maintenance of a characteristic spatial relationship between these anomalies in particular wind stress curl and ssh anomalies in the nep maintained an inverse relationship that strengthened and shifted eastward toward the west coast during late 1996 early 1998 this consistent relationship indicates that anomalous ekman transport driven by regional atmospheric forcing was an important contributor to temperature and ssh anomalies in the nep and ccs during the 1997 98 en other studies have shown that coastal propagations originating from the tropical pacific also may have contributed to coastal nep anomalies during this en our results indicate that at least some of this coastal anomaly signal may have been generated by regional atmospheric forcing within the nep

contributions of wind forcing waves and surface heating to sea surface height observations in the pacific ocean the dominant processes affecting sea surface height ssh variability observed by the topex poseidon altimeter vary regionally in the pacific baroclinic rossby waves equatorially trapped kelvin waves steric response to seasonal heating and the response to wind stress curl forcing are all important the steric response to surface heating dominates seasonal ssh variability in the subpolar gyre and the eastern subtropical gyre south of the kuroshio extension and south of 20 n in the eastern pacific the dominant contribution to ssh is from near annual period rossby waves to quantify the wave energy observed ssh was assimilated into a kinematic model of westward propagating waves these waves account for 70 of ssh variance between 10 s and 10 n but only 30 between 10 n and 30 n although wave energy in the eastern pacific is correlated with ssh anomalies at the equator the much larger wave energy in the western pacific is correlated with wind stress curl suggesting that the rossby waves there are locally forced in addition to these planetary waves the ocean response to wind forcing via ekman pumping is observed in several places specifically in the north equatorial current a quasi steady topographic sverdrup balance is detectable over most of the north pacific at latitudes as low as 10 15 n as well as in the south pacific where it is seen north of 50 s the decomposition of the ssh signal into propagating waves an ekman pumping response and sverdrup transport is consistent with the results from an isopycnal numerical model

interannual westward propagating baroclinic long wave activity on line p in the eastern midlatitude north pacific abstract for the period 1959 81 quasi zonal hydrographic sections have been made between the southern coast of vancouver island 49 n 126 w and ocean station p 50 n 145 w approximately every two months along this section called line p hydrographic std stations were repeated at nearly the same locations approximately 2 of longitude apart and at closer intervals near the coast as such 13 stations in all were made nearly every two months over the 23 year period thus the line p dataset has particular value in verifying the existence of interannual baroclinic long waves near 50 n in previous studies white kang and magaard and white and saur had found evidence of annual and interannual baroclinic long wave activity in the eastern midlatitude north pacific over the range 20 40 n but earlier attempts could not discover these westward traveling waves along line p in this study both spectral analysis and complex empirical orthogonal function analysis are used to do just that concentrating u

rossby waves and the variability of the north pacific current during 2002 03 abstracta numerical model the parallel ocean program pop with 0 25 horizontal spatial resolution and 28 vertical levels is used to simulate the circulation of the north pacific ocean for the time period 1960 2006 spectral nudging is used so that model drift of the mean state over the 46 yr time period of the simulation is prevented while allowing for the prognostic evolution of the circulation at time scales that are not nudged the simulation successfully reproduces a southward shift in the north pacific current in 2002 03 as calculated from scalar observations it is suggested that this calculated shift may not be solely due to meridional current drift but also a consequence of the shifting intensity of two eastward moving current bands separated by 300 500 km a distance consistent with the rhines scale the scale at which the 2d turbulence cascade tends to be arrested which implies an influence from rossby waves that are heavily affected by nonlinearities the simulation suggests that the nort

styles of lode gold mineralization contributing to the placers of the indian river and black hills creek yukon territory canada as deduced from microchemical characterization of placer gold grains between 1978 and 2009 approximately 430 000 oz of placer gold were obtained from the indian river and black hills creek which equates to roughly 20 of the production for the entire yukon territory during that period the area is unglaciated exposure is poor and there are few known lode gold occurrences present the technique of microchemical characterization of placer gold grains has been applied to illuminate the style s of source mineralization and their relationship to placer gold from the klondike gold district immediately to the north a total of 2 613 placer gold grains from 22 localities were characterised in terms of the au ag cu and hg content of their alloy and associated suite of opaque mineral inclusions a combination of alloy and inclusion mineralogy was used to define gold signatures which augmented the previous classification of orogenic gold in the klondike gold type 3b 8 25 ag is the main component of the placers in lower dominion creek but is augmented and eventually replaced by type 3a gold 10 40 ag in placers in the main indian river valley probably through erosion of gold bearing veins in the valley floor type 4 gold exhibits highly variable ag which may contain hg to a maximum of 11 wt this gold type also hosts a distinctive inclusion assemblage of complex polymetallic sulphides tellurides sulfotellurides and sulfosalts and has previously been ascribed to local low sulfidation epithermal mineralization placer gold in drainages radiating from eureka dome exhibits various proportions of types 3 and 4 gold depending on location but type 3 gold forms the major component in black hills creek and northerly flowing tributaries of the indian river with the exception of eureka and montana creeks type 5 gold is found only in placers in the middle and lower indian river it is distinguished by slightly elevated 0 05 0 17 cu in the gold alloy together with low 5 9 ag contents inclusions of bi minerals cr bearing magnetite and molybdenite within type 5 gold suggest derivation from an intrusion related source candidates for such a source include undiscovered lode occurrences associated with cretaceous age intrusions to the south of the indian river or deformed cu au mo porphyry occurrences which are known to be present in the same area this analysis of placer gold has indicated that the contribution of low sulfidation epithermal gold from eureka creek to the larger placers of the indian river is minor consequently the placer gold inventory of the indian river is primarily orogenic in origin similarly the characterization of placer gold in blackhills creek strongly suggests an orogenic source this study has demonstrated for the first time that orogenic lode gold mineralization extends a considerable distance to the south of the southern klondike goldfield this information contributes to the regional models of gold mineralization in an area which is currently the focus of intensive exploration

delayed accumulation of placers during exhumation of orogenic gold in southern new zealand abstract the giant gold placer system on the otago schist of southern new zealand was derived from mesozoic orogenic gold deposits in the underlying schist basement the core of the schist basement was exhumed in the middle cretaceous coeval with the accumulation of the oldest preserved nonmarine sedimentary rocks in the area ca 112 ma those sedimentary rocks contain quartz clasts with distinctive ductile deformation textures that were derived from structural zones in or adjacent to major orogenic gold deposits quartz textures in these structural zones are readily distinguishable from the rest of the schist belt and hence provide a fingerprint for erosion of gold the earliest sedimentary rocks on the margins of the gold bearing schist belt are immature and were derived from unoxidised outcrops in areas of high relief gold was not liberated from unoxidised basement rocks during erosion and was removed from the system without placer concentration placer concentration did not begin until about 20 million years later when oxidative alteration of gold deposits had facilitated gold grain size enhancement from micron scale primary to millimetre scale secondary subsequent erosion and recycling of gold in the early cenozoic and again in the late cenozoic caused additional concentration of gold in progressively younger deposits the klondike giant placer goldfield of canada had a similar geological history to the otago placer field and klondike placer accumulation occurred in the late cenozoic at least 70 million years after mesozoic exhumation of orogenic gold the giant placer deposit on the western slopes of the sierra nevada in california occurs in eocene and younger sedimentary rocks at least 40 million years younger than the timing of major exhumation of the source rocks circum pacific giant gold placers formed under entirely different tectonic regimes from the emplacement of their source orogenic deposits and these giant placer deposits do not form in foreland basins associated with convergent orogens formation of giant placers requires less active erosion and more subdued topography than the collisional orogenic activity that accompanied emplacement of source gold deposits in basement rocks as well as oxidative alteration of the primary deposits to liberate gold from sulfide minerals and enhance secondary gold grain size

local and regional mass transfer during thrusting veining and boudinage in the genesis of the giant shale hosted paracatu gold deposit minas gerais brazil multiscale structural and geochemical studies have been applied to understand the genesis of the paracatu deposit in brazil a shallow dipping bulk tonnage vein style low grade au ag pb zn orebody hosted in 1000 ma black phyllites of the paracatu formation associated with intense shearing accompanying thrusting during the 680 ma brasiliano orogeny massive to laminated quartz sulfide carbonate veins and associated alteration and au mineralization formed early in the deformation history and these veins were boudinaged during subsequent progressive increase in shear strain geochemical profiles across the ore allow recognition of a relatively homogeneous protolith at 100 m scales with respect to ti al zr v and rare earth elements including a footwall with protolith attributes similar to those of the ore oxygen hydrogen and sulfur isotopes do not reveal a distinctive external fluid source rather they reflect fluid rock equilibration with the host phyllites during greenschist facies regional metamorphism detailed geochemical sampling around a prominent population of smaller boudinaged veins shows that si ca and sr were removed and al k and ti were residually concentrated during formation of synboudinage dark selvages this process of mass transfer occurred at scales more local than the processes responsible for ore genesis despite the local processes of chemical equilibration and redistribution many of the main ore components appear externally derived at scales broader than the orebody gold as ag sb bi and to some extent pb and zn show a strong spatial and statistical correlation with boudinaged quartz sulfide carbonate veins and these are surrounded by a broader 10 50 m vertical scale halo of enriched k ba and volatiles loss on ignition with depleted na and sr the majority of gold is present as inclusions in pyrite and arsenopyrite within and immediately surrounding the veins and gold bearing pyrite shows mineral chemistry consistent with a hydrothermal origin the main population of large veins locally preserve preboudinage complex internal vein textures including sulfide rich wall rock laminae similar to laminated fault fill veins in many orogenic style vein deposits these larger veins contain most of the gold in the paracatu resource metals were most likely sourced distal to the paracatu formation and were precipitated in sheeted shear or tensile veins during the early stages of the brasiliano thrusting the oblate strain recorded by the boudinaged mineralized veins is anomalous and distinctive to paracatu at km scales even though the thrusting is regional although the orebody may have been physically dislocated by thrusting from the place where the early veins formed the anomalous strains recorded suggest a perturbation in the footwall to the regional thrust or perhaps a spatially restricted competency contrast within the stratigraphy which contributed to the localization of the present mineralization during the thrusting

structural controls on tertiary orogenic gold mineralization during initiation of a mountain belt new zealand two types of structurally controlled hydrothermal mineralization have occurred during folding of fissile schist in southern new zealand fold related mineralization and normal fault related mineralization both types have the same mineralogy and textures and are dominated by quartz ankerite veins and silicified breccias with ankeritic alteration most mineralized zones are thin centimetre scale although host schist is commonly impregnated with ankerite up to 20 m away thick up to 5 m wide mineralized zones are generally gold bearing and contain pyrite and arsenopyrite with stibnite pods locally some of these auriferous zones have been extensively mined historically despite rugged topography and difficult access mineralization occurred during regional tectonic compression in the initial stages of development of the southern alps mountain belt at the pacific australian plate boundary in the miocene most of the gold bearing deposits occur in east to south east striking normal faults that cut across mesoscopic folds in a belt that coincides with the southern termination of a regional scale north trending antiform mineralized zones have similar structural control and relative timing to a nearby swarm of miocene lamprophyre dykes and carbonatites limited stable isotopic data c and o and trace element geochemistry suggest that there was probably no genetic link between the igneous activity and gold mineralization however these two types of fluid flow have been controlled by the same tectonically created crustal plumbing system this miocene hydrothermal activity and gold deposition demonstrates that orogenic mesothermal mineralization can occur during the inception of an orogenic belt not just in the latter stages as is commonly believed these miocene structures have been preserved in the orogen because the locus of uplift has moved northwards so the early formed gold deposits have not yet been structurally overprinted or eroded

a fluid inclusion and stable isotope study of au quartz veins in the klondike district yukon territory canada a section through a mesothermal vein system gold bearing quartz veins in the klondike district of yukon territory are of earliest cretaceous age and are hosted by permian middle to upper greenschist facies metasedimentary and meta igneous lithologies belonging to the yukon tanana terrane the veins occur in brittle extensional structures which may have developed during extensional deformation during uplift of the klondike region in the late jurassic to early to middle cretaceous following thrust imbrication in the early jurassic the quartz veins are massive and unbanded with localized carbonate flooding and or pyritization of the wall rocks fluid inclusion studies have delineated significant regional p t x trends in primary inclusion fluids from the au quartz veins the fluids have low salinities 6 wt nacl equiv but variable co 2 concentrations decreasing from approximately 0 13 to 0 18 mole fraction co 2 in the southeastern klondike through variable concentrations in the central klondike x sub co 2 0 01 to asymp 1 00 to dominantly aqueous fluids x sub co 2 0 01 in veins from the northwest portion of the region estimated trapping pressures decrease from 2 300 or 700 bars to between 300 to 900 bars from southeast to northwest estimated fluid temperatures range from 300 degrees to 350 degrees c in central and southern lodes to 200 degrees to 250 degrees c in the northwest klondike the p x trends are interpreted as evidence for 1 effervescence of co 2 from the mineralizing fluid most likely in response to regional uplift during the life of the hydrothermal system and 2 that the au quartz vein system represents a section through a mesothermal vein system with deeper structural levels exposed in the southeastern klondike convection of the hydrothermal fluid may have been driven by an elevated geothermal gradient resulting from the rapid uplift the calculated isotopic composition of the au quartz vein fluids delta 18 o fluid 8 3 or 1 2ppmsmow delta d fluid asymp 150 or 18ppmsmow indicates that the mineralizing fluid was meteoric in origin but underwent considerable isotopic evolution prior to mineralization additionally the delta d fluid values of extracted inclusion fluids 166 or 11ppm and from hydrothermal muscovites 174 or 3ppm from the sheba vein system are in good agreement and indicate that the deuterium signature reflects that of the primary mesothermal fluid

fusion by earthquake fault friction stick or slip field observations of pseudotachylites and experimental studies of high speed friction indicate that melting on a slipping interface may sig nificantly affect the magnitude of shear stresses resisting slip we investigate the effects of rock melting on the dynamic friction using theoretical models of shear heating that couple heat transfer thermodynamics of phase transitions and fluid mechanics results of laboratory experiments conducted at high order of m s slip velocities but low order of mpa normal stresses suggest that the onset of frictional melting may give rise to substantial increases in the effective fault strength presumably due to viscous effects however extrapolation of the modeling results to in situ conditions suggests that the efficiency of viscous braking is significantly reduced under high normal and shear stresses when transient increases in the dynamic fault strength due to fusion are not sufficient to inhibit slip decreases in the effective melt viscosity due to shear heating and melting of clasts drastically decrease the dynamic friction resulting in a nearly complete stress drop thermal runaway the amount of energy dissipation associated with the formation of pseudo tachylites is governed by the temperature dependence of melt viscosity and the average clast size in the fault gouge prior to melting clasts from a coarse grained gouge have lower chances of survival in a pseudotachylite due to a higher likelihood of non equilibrium overheating the maximum temperature and energy dissipation attainable on the fault surface are ultimately limited by either the rock solidus via viscous braking and slip arrest or liquidus via thermal runaway and vanishing resistance to sliding our modeling results indicate that the thermally activated fault strengthening and rupture arrest are unlikely to occur in most mafic protoliths but might be relevant for quartz rich rocks especially at shallow 5 7 km depths where the driving shear stress is relatively low

gold in the hills patterns of placer gold accumulation under dynamic tectonic and climatic conditions formation of placer accumulations in fluvial environments requires 103 106 or even greater times concentration of heavy minerals for this to occur regular sediment supply from erosion of adjacent topography is required the river should remain within a single course for an extended period of time and the material must be reworked such that a high proportion of the sediment is removed while a high proportion of the heavy minerals remains we use numerical modeling constrained by observations of circum pacific placer gold deposits to explore processes occurring in evolving river systems in dynamic tectonic environments a fluvial erosion transport model is used to determine the mobility of placer gold under variable uplift rate storm intensity and rock mass strength conditions gold concentration is calculated from hydraulic and bedload grain size conditions model results suggest that optimal gold concentration occurs in river channels that frequently approach a threshold between detachment limited and transport limited hydraulic conditions such a condition enables the accumulation of gold particles within the framework of a residual gravel lag an increase in transport capacity which can be triggered by faster uplift rates more resistant bedrock or higher intensity storm events will strip all bedload from the channel conversely a reduction in transport capacity triggered by a reduction in uplift rate bedrock resistance or storm intensity will lead to a greater accumulation of a majority of sediments and a net decrease in gold concentration for our model parameter range the optimal conditions for placer gold concentration are met by 103 times difference in strength between bedrock and fault uplift rates between 1 and 5 mm a 1 and moderate storm intensities fault damage networks are shown to be a critical factor for high au concentrations and should be a target for exploration

contrasting fluids in gold bearing quartz vein systems formed progressively in a rising metamorphic belt otago schist new zealand four different quartz vein systems occur within 600 km 2 in east otago the vein systems have formed within fault systems which crosscut regional schistosity the macraes system formed from almost pure water at about 3 kbars and 350 degrees c data from fluid inclusions and arsenopyrite geothermometer the bonanza system formed from immiscible h 2 o co 2 nacl fluids at about 1 000 bars nenthorn formed under near surface conditions from boiling low salinity water with some co 2 present barewood most closely resembles macraes but is thought to have formed at slightly shallower levels the veins formed at different times during mesozoic cenozoic uplift of the otago schist belt c and o isotope data and the high co 2 content of some fluids suggest that the vein forming fluids were metamorphogenic a component of downward percolating meteoric water was probably involved in shallower vein formation vein formation is considered to be an inevitable consequence of devolatilization of uplifting metamorphic rocks with focusing of fluids into available structural features similar fluid flow has occurred within the last 1 m y in the northwestern extension of the otago schist where alpine fault uplift has induced gold mineralization on joint surfaces

the role of exhumation in the temporal distribution of ore deposits we show here that the temporal age frequency distribution of ore deposits is controlled largely by exhumation the combination of burial uplift and erosion that exposes subsurface rocks and ore deposits exhumation rates calculated from modal ages and emplacement depths for epithermal porphyry copper and orogenic gold deposits of phanerozoic age are 167 158 and 63 m m y respectively these rates agree with rates estimated for continent scale terranes by numerous independent methods thus the scarcity of precambrian epithermal and porphyry copper deposits is probably due to their removal by exhumation archean and pale oproterozoic orogenic gold deposits are more abundant than indicated by the exhumation record of their phanerozoic counterparts suggesting that the precambrian deposits formed in greater numbers recognition of the pivotal role of exhumation requires that the temporal distribution of ore deposits be corrected for this effect before it can be used as an indicator of large scale changes in earth history

the role of exhumation in the temporal distribution of ore deposits a reply sir we welcome this opportunity to clarify points raised by groves and goldfarb 2007 in their discussion of our recent express letter kesler and wilkinson 2006 their first concern centers on our modal age of 160 ma for orogenic gold deposits they state that it is unclear how the stated theoretical modal age was calculated the modal ages that we gave for the three deposit types are not theoretical as explained in the text they are simply the most common age in a best fit curve to the age frequency distribution for each deposit type for orogenic gold deposits the curve was fit to ages of deposits younger than 1900 ma using their age data goldfarb et al 2005 and

observation of mesoscale ocean features in the northeast pacific using geosat radar altimetry data in this paper geosat altimetry data from the northeast pacific are examined the data are 1 binned data on 1 latitude by 2 longitude blocks and 2 collinear track data the binned data cover the region from 20 n to 60 n from the north american coast westward to 150 w and span from december 15 1986 to june 1 1989 to examine the data for evidence of westward propagation of sea level anomalies three zonal transects 45 n 49 n and 57 n are made complex empirical orthogonal function analysis of the data transects at these latitudes reveals phase speed and amplitude information the first eigenmode at each latitude has a very strong annual signal two dimensional spectral analysis of the first eigenmode at 45 n and 57 n shows westward propagation of sea level anomalies at 45 n only the propagating features are annual in period and of order 1000 km in length at 57 n westward propagation is evident as at 45 n but there is also an eastward component to examine the variability of the coastal sea level signal collinear track data from three ascending tracks crossing the gulf of alaska are analyzed revealing mesoscale features of the order of 100 km that parallel the canadian and alaskan coast the most notable feature is the sitka eddy westward propagation of several of these features is observed the results from the analysis of both types of geosat data are compared with other observational data

the origin and evolution of the klondike goldfields yukon canada abstract the world famous klondike goldfields are located in the unglaciated part of west central yukon canada since their discovery over 100 years ago they have produced an estimated 311 tonnes of gold primarily from bench and creek placers that are fluvial in origin and range from pliocene to holocene in age historically the placers are classified into three levels of gravel with four main units these include the high level white channel gravel pliocene presently the most important gold bearing unit which sits nonconformably on an erosional bedrock surface i e the white channel strath and is overlain and interbedded with the glaciofluvial klondike gravel pliocene the intermediate level gravel pleistocene the least important economically and the low level gravel pleistocene holocene historically the most important gold bearing unit but it has been mined three or four times now the goldfields originated from the weathering and erosion of early cretaceous discordant mesothermal quartz veins and the light grey color of the matrix of the white channel gravel is due mainly to weathering and diagenetic alteration by groundwater flow the concentration of placer gold is related to a hierarchy of physical scales at the lithofacies scale metres bed roughness determined sites of gold deposition at the element scale tens of metres gravel bars were preferentially enriched in gold at the reach scale hundreds of metres stream gradient was an important factor at the system scale hundreds of km braided river environments transported large amounts of gold and at the sequence scale thousands of km economic placers formed initially in the high level white channel gravel and later in the intermediate level and low level gravel the white channel strath is interpreted as an erosional tectonic terrace that formed during isostatic uplift and under conditions of dynamic equilibrium the high level white channel gravel and klondike gravel are interpreted as a depositional climatic terrace that formed during a reversal in the tectonically induced downcutting which is attributed to the initial and most extensive of the pre reid glaciations 3 ma in the yukon the intermediate level gravel is interpreted as minor erosional complex response terraces that formed during static equilibrium when there were pauses in valley floor degradation which are attributed to the subsequent and less extensive pre reid glaciations the low level gravel formed also during valley floor degradation and may represent a return to dynamic equilibrium conditions hence the dominant forcing mechanisms controlling the evolution of the goldfields were isostatically compensated exhumation and climatic change related to the repeated glaciation of the yukon in addition the lowering of baselevel from high level to intermediate level and finally to low level gravel was accompanied by a decrease in accommodation space as indicated by a decrease in gravel thickness which resulted in an increase in the concentration of the placer gold

lode and placer gold composition in the klondike district yukon territory canada implications for the nature and genesis of klondike placer and lode gold deposits we have determined the major and trace element composition of approximately 2 700 gold particles from 21 gold bearing mesothermal quartz veins and 35 placer gold samples from the klondike district in western yukon territory measured au ag cu and hg contents were used to define a characteristic geochemical signature or fingerprint for each of the vein samples these signatures were then compared with the various compositional populations that we have distinguished within each of the placer samples preliminary conclusions derived from the study include 1 placer gold in both recent stream deposits and in the pliocene to pleistocene white channel gravels is detrital in origin 2 the placer gold is mainly if not entirely derived from mesothermal quartz veins 3 all lode sources for the placer gold have not yet been located and 4 gold composition data can be used to identify the lode signature for placer gold and if it has not been removed by erosion help locate and link placer gold to its specific lode source

the role of exhumation in the temporal distribution of ore deposits a discussion sir kesler and wilkinson 2006 have taken an interesting approach to understanding the temporal distribution of ore deposits in this case epithermal porphyry copper and orogenic gold deposits by examining the role of exhumation the conjunction of burial uplift and erosion that exposes ore deposits and their host rocks at the surface calculation of exhumation rates of phanerozoic deposits which developed in crust over negatively buoyant subcontinental lithospheric mantle sclm poudjom djomani et al 2001 shows an excellent match with the known distribution of epithermal and porphyry copper deposits which are generally accepted to have formed at less than 1 km and 5 km depths respectively seedorff et al 2005 simmons et al 2005 however there appear to be some problems with the methodology as applied to orogenic gold deposits first kesler and wilkinson 2006 use a modal age of 160 15 ma for exposed orogenic gold deposits which is a period in earth history when there are few if any absolute dates for large orogenic gold deposits appendix in goldfarb et al 2005 in fact without a description of how the best fit curves were applied to the age frequency histograms fig 2 kesler and wilkinson 2006 it is unclear how the stated theoretical modal age for orogenic gold deposits as well as for porphyry copper ores was calculated second a review of the literature is stated to lead to a modal formation depth for orogenic gold deposits of 10 km and this is shown to fit with the goldfarb et al 2005 estimate of 5 to 20 km which is thus nicely separated from the 0 5 to 5 km range for porphyry deposits unfortunately the referenced paper clearly gives a range of 2 to 20 km for orogenic gold deposits and we would argue that phanerozoic orogenic gold deposits are as

gold dispersal and placer formation in an active oblique collisional mountain belt southern alps new zealand the southern alps of new zealand constitute part of an orogenic belt forming on the western margin of the pacific plate due to collision along the pacific australian plate boundary the topography of the mountain range is asymmetrical with a steep inboard zone adjacent to the plate boundary where very high rainfall causes rapid erosion and a relatively gentle outboard zone in the rain shadow east of the topographic divide detritus including gold shed from the rising mountains are transported eastward from the outboard zone into a foreland basin and westward from the inboard zone across the plate boundary to the australian plate rivers in the outboard zone are widely spaced 20 km and structurally controlled oblique to the trend of the orogen these rivers are long 150 km with relatively low gradients and form braid plains showing net aggradation over the past million years gold placer formation in outboard rivers draining the central southern alps requires concentration factors of ca 10 5 to 10 6 which has occurred only during localized postdepositional uplift and recycling of sediments tectonic shortening coupled with low outboard erosion rates results in capture of some outboard rivers by inboard rivers with consequent redirection of gold into the inboard region inboard rivers are generally short 20 km and closely spaced 10 km and oriented perpendicular to the plate boundary aggradation of sediments is temporary and periodic scouring of the valleys to bedrock by floods and glaciers results in ephemeral placer deposits in river bed lags and flood gravels on the pacific plate gold is ultimately transported across the plate boundary on to the indentor australian plate in till or river gravels where it is concentrated in degradational lags in fluvial sediments and in marine beach placers dextral tectonic motion at the plate boundary has transported detrital gold up to about 180 km laterally from southern alps sources during the past 5 m y

interdecadal variations in the alaska gyre abstract climatic dynamic topography variations in the alaska gyre during the period 1968 1990 are described with an objective analysis of more than 12000 std and xbt stations and coads wind stress data interannual dynamic height and sst variations were correlated and were consistent with recently described large scale climatic shifts in the north pacific the gyre was cantered more to the east circulation appeared stronger and sst was lower during the early to mid 1970s than during the 1980s the aleutian low np and pna indices intensified during the interim but the response did not appear as a gyre spinup instead the associated wind stress anomalies forced a slowly varying dynamic height anomaly across the eastern and northern part of the gyre through ekman convergence which had the effect of displacing the gyre s low somewhat to the wsw in the 1980s the wind curl spectrum was white and the slow oceanic response was modelled as stochastic forced climate variability with a simple first order m

extension and gold mineralisation in the hanging walls of active convergent continental shear zones abstract orogenic gold bearing quartz veins form in mountain belts adjacent to convergent tectonic boundaries the vein systems hosted in extensional structures within compressively deformed rocks are a widespread feature of these orogens in many cases the extensional structures that host gold bearing veins have been superimposed on and locally controlled by compressional structures formed within the convergent orogen exploring these observations within the context of a three dimensional mechanical model allows prediction of mechanisms and locations of extensional zones within convergent orogens our models explore the effect of convergence angle and mid crustal strength on stress states and compare them to the southern alps and taiwan the dilatation zones coincide with the highest mountains in the hanging walls of major plate boundary faults and can extend as deep as the brittle ductile transition extensional deformation is favoured in the topographic divide region of oblique orogens with mid lower crustal rheology that promotes localisation rather than diffuse deformation in the near surface topography influences the stress state to a depth approximately equal to the topographic relief bringing the rock closer to failure and rotating 1 to near vertical the distribution of gold bearing extensional veins may indicate the general position of the topographic divide within exhumed ancient orogens

river drainage reorientation during placer gold accumulation southern new zealand many alluvial placer deposits around the world occur in river systems that have been affected by tectonic events causing drainage reorientation and severance of links between placers and their sources this study documents tectonic rejuvenation of topography in the otago giant placer goldfield new zealand which has resulted in numerous river capture and drainage reorientation events these events have induced changes to gold transport directions and numerous stages of separation of detrital gold from primary sources goldfield wide reconstructions of drainage patterns through time are as yet only possible for miocene recent and numerous earlier drainage changes back to cretaceous primary orogenic mineralisation are probable variations in basement lithologies permit auriferous gravel provenance determinations facilitating paleodrainage pattern reconstruction and documentation of river capture events river capture events and timing of these events for gold bearing paleodrainages have also been documented using genetic divergences of populations of freshwater galaxiid fish that were isolated by drainage reorientation gold bearing quartz pebble conglomerates had a southeastward drainage in the miocene this was disrupted in the pliocene by mountain range uplift and gold placer recycling with deposition of lithic conglomerates containing only minor gold placers the most dramatic changes in gold transport directions occurred through the quaternary as antiformal ranges grew across the pre existing drainages miocene and pliocene placers were recycled with numerous local 1 10 km scale changes in river directions and numerous capture events large axial rivers were segmented into a more complex drainage pattern and on going river capture resulted in growth of the main clutha river catchment at the expense of neighbouring catchments the most productive placers developed in the clutha river in late quaternary when increased discharge from captured mountain catchments enhanced gold transport and concentration similar river drainage reorientation has occurred in other placer fields around the world but the lack of preserved evidence inhibits documentation of most such changes

microchemical studies of placer and lode gold in the klondike district yukon canada 1 evidence for a small gold rich orogenic hydrothermal system in the bonanza and eldorado creek area orogenic gold camps are commonly associated with placer deposits although in many cases the relationship between placer and lode source may be unclear placer gold deposits in bonanza and eldorado creeks in the northern klondike district goldfield yukon canada probably yielded more than 2 million ounces moz from a small geographic area but known occurrences of gold bearing orogenic style veins from which the placers were derived accounted for only a few thousand ounces the alloy chemistry and mineralogy of opaque mineral microinclusions microchemical signatures within these gold grains were compared to that of samples collected during a detailed placer gold sampling program collection of placer samples from the headwaters of watercourses in this unglaciated area allowed identification of microchemical signatures within gold populations from proximal sources which augmented information from lode gold sources to facilitate characterization of local gold mineralization a total of 1 163 gold grains from 18 lode sources and 2 777 grains from 27 placer localities within the bonanza and eldorado creek drainages were characterized in terms of the ag and hg content of the alloy together with the associated inclusion suite and two main compositional ranges along with several subtypes were identified type 1 gold is a simple au ag alloy but various subtypes have been identified on the basis of different ranges of ag and or specific inclusion assemblages type 2 gold is an au ag hg alloy composition the spatial arrangement of the gold types shows a systematic change in microchemical signature the lowest ag gold occurs in the lone star ridge area and the ag content increases both in the northeast and southwest the appearance of te bearing mineral inclusions in populations coincides with increasing ag and argentite inclusions are present only in high ag gold toward the periphery of study area type 2 gold is absent at lone star ridge and all localities to the east but is progressively more abundant in sample localities to the west the geographic distribution of gold types correlates well with placer abundance inferred from the production from previous mining activity and the study area is bounded by creeks and gulches in which there was insufficient placer gold for economic extraction we conclude that the lone star ridge area represents the epicenter of a single hydrothermal system in which spatial and temporal fluid evolution has generated compositional zonation variation in ag is ascribed to decrease in au ag aq following preferential au deposition in the center of the system controls on hg contents within au alloy are unclear but may be a consequence of mineralization temperature the complex but consistent signatures of placer gold samples from the eldorado creek drainage represents liberation of types 1 and 2 gold grains which coexist in some lode occurrences the different proportions of types 1 and 2 gold observed in placer and lode samples from west of lone star ridge reflect both temporal and spatial evolution of the hydrothermal system the main eldorado creek placers exhibit a signature most similar to lode gold from the buckland and nugget zones on lone star ridge placer gold in upper bonanza creek is distinguished by a relatively narrow ag range and the small but consistent presence of hessite in the inclusion suite no corresponding lode occurrence for this gold type has been identified to date this is the first time that the compositional signatures of major placer gold deposits derived from orogenic gold vein systems has been studied in this detail and the application of basic chemical principles to fluid evolution and electrum precipitation has allowed demarcation of an area in which extremely rich gold mineralization was formerly present while simultaneously identifying the most promising areas for further exploration for undiscovered lode gold

structural controls on orogenic gold mineralisation in the klondike goldfield canada the klondike schist that forms the basement rocks for the famous klondike placer goldfield was emplaced as km scale thrust slices in early jurassic time along with some thin 10 to 30 m scale slices of greenstone and ultramafic rocks permian metamorphic fabrics in the schists were deformed during thrust emplacement by structures formed as the rocks passed through the brittle ductile transition early formed thrust related structures were almost pervasive recumbent folds that affected both the schist and greenstone ultramafic slices and imposed a spaced cleavage with minor recrystallisation of micas these structures gave way to shallow dipping phacoidal cleavage near within 100 m of thrust structures thrust related structures have been overprinted locally by well defined steeply dipping reverse fault fold zones and associated upright folding on regional km to mesoscopic m scales the fold fault zones occur as two orthogonal sets of structures oriented nw to n and ne to e some of these steeply dipping fault zones have been reactivated by late cretaceous normal faulting orogenic mesothermal gold bearing veins were emplaced in local sites of extension during or after formation of the compressional fault fold zones and before normal fault reactivation over 400 veins m to cm scale observed in this study imply a general nw strike for mineralised structures w to n but with a broad scatter of orientations vein emplacement was controlled principally by fold axial surfaces of kink folds of the fault fold generation however some other local extension sites have opened along preexisting structures to host veins locally including metamorphic foliation and spaced cleavage planes in addition irregular extensional fractures with no obvious structural control host some veins the klondike mineralised veins formed as swarms with broad regional structural control but represent relatively diffuse mineralised zones with numerous scattered small veins compared to most orogenic vein systems these diffuse vein swarms appear to be sufficient sources for the rich and geographically localised placer gold deposits that formed in overlying gravels during erosion of the klondike schist basement

the influence of an alaskan stream eddy on flow through amchitka pass flow into the bering sea through amchitka pass in the central aleutian islands is shown to respond to the presence of a large meander eddy in the alaskan stream south of the pass maximum flow into the aleutian basin of the bering sea occurs when the eddy center lies nearest to the axis of amchitka pass a conceptual model of low frequency circulation variability within the bering sea based upon results of empirical orthogonal function analyses of sea surface height anomalies measured by the geosat radar altimeter is presented

low frequency isopycnal variability in the alaska gyre from argo abstract the deployment of a large number of argo floats in the northeast pacific ocean since 2001 has led to a great increase in the volume of data available to study the regional variability on interannual to decadal time scales in this paper gridded fields of hydrographic data from argo are used to examine the low frequency variability of representative isopycnal surfaces over the alaska gyre in the eastern subpolar north pacific anomalies in the depth of isopycnal surfaces and spice anomalies on these surfaces are considered over the 14 year period 2004 2017 an eof analysis is used to characterize the variability occurring at large scales over the region the leading mode of isopycnal depth anomalies through the upper 1000 m of the water column is found to be related to the pacific decadal oscillation the response of a stochastic climate model driven by local ekman pumping is shown to give a reasonable account of variability in the depth of the shallow main pycnocline over the alaska gyre although ocean dynamics must play a role in shaping the spatial response to the wind forcing evidence is presented for counter propagating isopycnal depth anomalies in the subarctic current at the depth of the main pycnocline propagation is dominated by disturbances that are advected eastward by the mean flow at greater depths isopycnal depth anomalies propagate westward at approximately twice the speed of first mode long baroclinic rossby waves consistent with the propagation of rossby waves seen in altimetric observations of sea surface height variability the eastward advection of spice anomalies is also evident in the upper ocean however the dominant signal in the record is a large scale positive spice anomaly that forms locally on subsurface isopycnals evidence is presented that this anomaly arises mainly from the downward diffusion across the pycnocline of heat associated with the marine heatwave of 2013 14 over the northeast pacific the extent to which anomalies are generated locally may limit the potential for decadal prediction of upper ocean conditions over the region