zircons reveal magma fluxes in the earth s crust magma fluxes regulate the planetary thermal budget the growth of continents and the frequency and magnitude of volcanic eruptions and play a part in the genesis and size of magmatic ore deposits however because a large fraction of the magma produced on the earth does not erupt at the surface determinations of magma fluxes are rare and this compromises our ability to establish a link between global heat transfer and large scale geological processes here we show that age distributions of zircons a mineral often present in crustal magmatic rocks in combination with thermal modelling provide an accurate means of retrieving magma fluxes the characteristics of zircon age populations vary significantly and systematically as a function of the flux and total volume of magma accumulated in the earth s crust our approach produces results that are consistent with independent determinations of magma fluxes and volumes of magmatic systems analysis of existing age population data sets using our method suggests that porphyry type deposits plutons and large eruptions each require magma input over different timescales at different characteristic average fluxes we anticipate that more extensive and complete magma flux data sets will serve to clarify the control that the global heat flux exerts on the frequency of geological events such as volcanic eruptions and to determine the main factors controlling the distribution of resources on our planet

migration paths and precipitation mechanisms of ore forming fluids at the shuiyindong carlin type gold deposit guizhou china abstract shuiyindong is one of the largest and highest grade stratabound carlin type gold deposits in china this paper reports on the results of petrographic studies electron microprobe analyses empa of arsenian pyrite and the mass transfer during mineralization and alteration and it presents the deposit scale distributions of au as sb hg tl and trace elements in a representative cross section across the shuiyindong carlin type gold deposit guizhou province the main objectives were to identify the precipitation mechanisms of minerals or elements from fluids and the migration paths of ore forming fluids petrographic and empa studies indicate that gold in the primary ores is mainly hosted by arsenian pyrite mass transfer associated with alteration and mineralization shows that au as sb hg tl and s were significantly added to all mineralized rocks fe 2 o 3 and sio 2 were immobile in the main orebodies that are hosted in bioclastic limestone and cao na 2 o sr and li were removed from country rocks the relations between fe and s indicate that the sedimentary rocks at the shuiyindong deposit contain more iron than is needed to combine with all of their contained sulfur to form pyrite this suggests that sulfidation and decarbonation were the principal mechanism of gold precipitation at the shuiyindong deposit hg sb and as commonly formed sulfide minerals such as stibnite realgar and orpiment in late stage quartz calcite veins or absorbed by organic matter in argillite fluid cooling presumably led to depositions of stibnite realgar and orpiment in late stage quartz calcite veins organic matter likely served as a reductant in argillite for the ore fluids causing the precipitation of as sb hg and s as well as au deposit scale distributions of gold and other relevant elements reflect the passage of fluids through the rocks rock strata and structures allowed the ore forming fluids to migrate horizontally along the unconformity surface of the middle upper permian converge on the high position of an anticline and then ascend into the overlying strata along the anticlinal axis the distributions of the major and trace elements show that elements that accompanied the ore forming fluids include au as sb hg tl and s and that na 2 o and li were exhausted in the longtan formation at the anticlinal core during gold mineralization the enrichment of co cr and ni in the longtan formation at the anticlinal core might be associated with deformation that formed the anticline or with gold mineralization different host rocks were preferentially mineralized by different elements the bioclastic limestone is commonly enriched in au whereas the argillite is preferentially enriched in as hg sb and tl the zonation of ore forming elements in the deposit appears to be sb tl as hg au hg as from bottom to top enrichment of au as sb hg and tl provides useful guidance for the exploration for carlin type gold deposits in guizhou anomalies of as and hg in soil or stream sediment might be an important clue and these elements can be used as indicator elements ore forming fluids migrated along the unconformity surface of the middle upper permian and the anticlinal axis so these are favorable sites for exploration for carlin type gold deposits in guizhou

geology geochronology geochemistry and ore genesis of the wangu gold deposit in northeastern hunan province jiangnan orogen south china abstract the wangu gold deposit in northeastern hunan south china is one of many structurally controlled gold deposits in the jiangnan orogen the host rocks slates of the lengjiaxi group are of neoproterozoic age but the area is characterized by a number of late jurassic cretaceous granites and ne trending faults the timing of mineralization tectonic setting and ore genesis of this deposit and many similar deposits in the jiangnan orogen are not well understood the orebodies in the wangu deposit include quartz veins and altered slates and breccias and are controlled by wnw trending faults the principal ore minerals are arsenopyrite and pyrite and the major gangue minerals are quartz and calcite alteration is developed around the auriferous veins including silicification pyritic arsenopyritic and carbonate alterations field work and thin section observations indicate that the hydrothermal processes related to the wangu gold mineralization can be divided into five stages 1 quartz 2 scheelite quartz 3 arsenopyrite pyrite quartz 4 poly sulfides quartz and 5 quartz calcite the lianyunshan s type granite which is in an emplacement contact with the ne trending changsha pingjiang fracture zone has a zircon la icpms u pb age of 142 2 ma the dayan gold occurrence in the changsha pingjiang fracture zone which shares similar mineral assemblages with the wangu deposit is crosscut by a silicified rock that contains muscovite with a ca 130 ma 40ar 39ar age the gold mineralization age of the wangu deposit is thus confined between 142 ma and 130 ma this age of mineralization suggests that the deposit was formed simultaneously with or subsequently to the development of ne trending extensional faults the emplacement of late jurassic cretaceous granites and the formation of cretaceous basins filled with red bed clastic rocks in northeastern hunan which forms part of the basin and range like province in south china empa analysis shows that the average as content in arsenopyrite is 28 7 atom and the mineralization temperature of the arsenopyrite pyrite quartz stage is estimated to be 245 20 c from arsenopyrite thermometry the high but variable au as molar ratios 0 02 of pyrite suggest that there are nanoparticles of native au in the sulfides an integration of s pb h o he ar isotope systematics suggests that the ore fluids are mainly metamorphic fluids originated from host rocks possibly driven by hydraulic potential gradient created by reactivation of the wnw trending faults initially formed in paleozoic with possible involvement of magmatic and mantle components channeled through regional fault networks the wangu gold deposit shares many geological and geochemical similarities as well as differences with typical orogenic epithermal and carlin type gold deposits and may be better classified as an intracontinental reactivation type as proposed for many other gold deposits in the jiangnan orogen

geology and geochemistry of wall rock alteration at the carlin gold deposit nevada the carlin disseminated gold deposit occurs in an autochthonous sequence of paleozoic sedimentary rocks exposed in a structural window in the roberts mountains thrust in north central nevada the upper 175 m of the silurian roberts mountains formation hosts the majority of ore at carlin and is characterized by laminated fine grained calcareous and or dolomitic argillaceous siltstone with local coarser grained siltstones and 50 cm thick lenticular interbeds of sand and granule sized calcareous bioclastic debris or fossil hash detailed studies of drill core and exposures in the east pit of the carlin mine show that alteration and mineralization are zoned away from crosscutting fault conduits and these more permeable bioclastic beds indicating that these two features were major inflow zones for hydrothermal fluid in unoxidized rocks unaltered calcareous siltstone 1 containing quartz dolomite calcite illite k feldspar and pyrite is progressively converted to assemblages of 2 quartz dolomite calcite illite pyrite 3 quartz dolomite illite k mica pyrite 4 quartz illite k mica pyrite and 5 quartz kaolinite dickite pyrite adjacent to inflow zones where jasperoids are developed gold most consistently enriches the zone of calcite and dolomite removal 3 and 4 above though it occurs in all zones locally in high concentrations this zoned alteration was accomplished by a co 2 rich acidic fluid this acidic alteration enhanced the passage of fluids by extensive carbonate removal to form zones of higher permeability oxidation is wholly a supergene effect related to deep weathering because the oxidation is superimposed on both mineralized and altered rocks with only minor effect on the major element chemistry it has produced low temperature goethitic fe oxides rather than higher temperature hematite and is not spatially related to au distribution at the mine or on a district scale because of extensive carbonate removal leading to local volume reduction through collapse and or compaction geochemical effects are examined using ratios to relatively immobile elements such as al and ti extensive depletion of ca mg aud co 2 and introduction of si au and s have occurred potassium is depleted in the conversion of illite to dickite kaolinite in proximal silicified inflow zones and fe enriches some pyritized rock carbonate removal and silicification are two separate processes both of which are spatially associated with mineralization mineralized decarbonated rocks and barren footwall rocks commonly are not silicified and intensely silicified proximal alteration zones are generally low grade

contemporary stress orientations in the faroe shetland region the faroe shetland region of the ne atlantic continental margin contains a number of complexly structured mesozoic palaeogene age rift basins but in comparison with the contiguous british isles and north sea basin the state of crustal stress in the faroe shetland region is poorly understood the orientation of maximum horizontal compressional stress hmax across most of nw europe is roughly nw se which is considered to be controlled by forces acting at the plate boundaries we have determined 16 b d quality hmax orientations based on borehole breakouts interpreted in petroleum wells and define three distinct stress provinces within the faroe shetland region stress orientations in the ne are roughly nw se consistent with the regional pattern of stresses in nw europe and local neotectonic structural trends however contemporary stress orientations in the central and sw of the faroe shetland region exhibit short wavelength distances hmax orientation that is controlled by both plate boundary forces and regional scale sources of stresses

a rapid mechanism to remobilize and homogenize highly crystalline magma bodies the largest products of magmatic activity on earth the great bodies of granite and their corresponding large eruptions have a dual nature homogeneity at the large scale and spatial and temporal heterogeneity at the small scale this duality calls for a mechanism that selectively removes the large scale heterogeneities associated with the incremental assembly of these magmatic systems and yet occurs rapidly despite crystal rich viscous conditions seemingly resistant to mixing here we show that a simple dynamic template can unify a wide range of apparently contradictory observations from both large plutonic bodies and volcanic systems by a mechanism of rapid remobilization unzipping of highly viscous crystal rich mushes we demonstrate that this remobilization can lead to rapid overturn and produce the observed juxtaposition of magmatic materials with very disparate ages and complex chemical zoning what distinguishes our model is the recognition that the process has two stages initially a stiff mushy magma is reheated from below producing a reduction in crystallinity that leads to the growth of a subjacent buoyant mobile layer when the thickening mobile layer becomes sufficiently buoyant it penetrates the overlying viscous mushy magma this second stage rapidly exports homogenized material from the lower mobile layer to the top of the system and leads to partial overturn within the viscous mush itself as an additional mechanism of mixing model outputs illustrate that unzipping can rapidly produce large amounts of mobile magma available for eruption the agreement between calculated and observed unzipping rates for historical eruptions at pinatubo and at montserrat demonstrates the general applicability of the model this mechanism furthers our understanding of both the formation of periodically homogenized plutons crust building and of ignimbrites by large eruptions

gold bearing arsenian pyrite and marcasite and arsenopyrite from carlin trend gold deposits and laboratory synthesis invisible gold in natural and synthetic arsenian pyrite and marcasite correlates with anomalous as content and fe deficiency and high contents of invisible gold in most natural and all synthetic arsenopyrite correlate with excess as and fe deficiency as rich fe deficient arsenopyrite synthesized hydrothermally contains up to 3 0 wt au uniformly distributed in growth zones of light backscattered electron contrast at the deep star gold deposit carlin trend nevada the sulfide compositions apparently span the full range of metastability from fesz to near feass 40 at s arsenian pyrite contains up to 0 37 wt au but arsenopyrite has excess s and is relatively au poor observed minimum fe contents are 29 1 at in arsenian pyrite and marcasite from the deep star deposit and 31 3 at in synthetic arsenopyrite we suggest that invisible gold in arsenian pyrite and marcasite and arsenopyrite from sediment hosted gold deposits represents au removed from ore fluids by chemisorption at as rich fe deficient surface sites and incorporated into the solids in metastable solid solution however the oxidation state of invisible gold aua au1 remains uncertain because the chemisorption process is intrinsically nonsystematic in terms of crystal chemical parameters and does not result in definitive atomic substitution trends

geochemistry and textures of gold bearing arsenian pyrite twin creeks nevada implications for deposition of gold in carlin type deposits this study relates morphology size texture and chemistry of arsenian pyrite in the twin creeks carlin type gold deposits to the chemical position and amount of gold that it contains arsenian pyrite is confirmed as the most important gold hosting mineral and four types of gold bearing arsenian pyrite are recognized among these four types the lowest amount of gold and arsenic 1 wt as 17 60 ppm au is contained in arsenian pyrite with relatively large grain sizes 10 30 mu m it formed at relatively high temperatures 250 degrees c and k super h super ratios adularia stable probably relatively slowly the largest amount of gold and arsenic 1 05 2 43 wt as 595 1 465 ppm au is contained in arsenian pyrite with small grain sizes up to 2 mu m that formed at low temperatures 120 degrees 200 degrees c and moderate k super h super ratios illite stable probably by relatively rapid deposition xanes exafs and sims analyses show that gold in arsenian pyrite is both structurally bound au i and in submicroscopic inclusions of free gold au highest amounts of structurally bound gold were observed in the relatively coarse grained arsenian pyrite noted above whereas most inclusions of free gold were observed in fine grained arsenian pyrite in general there is a good correlation between gold and arsenic in arsenian pyrite although au as ratios vary from one pyrite to another it is suggested that gold in arsenian pyrite was deposited by direct deposition of native gold and by adsorption of au i from hydrothermal solutions adsorption of gold by arsenian pyrite can take place from solutions that are not saturated with respect to native gold relative volumes of gold bearing pyrite from different paragenetic stages at twin creeks suggest that as much as 50 percent of the total gold in the deposit could have formed from solutions that were not saturated with respect to native gold because both au and au i are found together in arsenian pyrite it is likely that they were deposited by the same process consideration of possible depositional reactions suggests that a decrease in the activity of reduced sulfur is most likely to cause gold deposition

mineral paragenesis alteration and geochemistry of the two types of gold ore and the host rocks from the carlin type deposits in the southern part of the goldstrike property northern nevada implications for sources of ore forming elements ore genesis and mineral exploration this study was undertaken to characterize the mineral paragenesis and metal zoning at the property scale evaluate the potential sources of ore related metals quantify the relationship between intensity of alteration and gold grade and propose a comprehensive genetic model for the carlin type au deposits at the southern part of the goldstrike property nevada mineralogy textural relationships whole rock composition and spatial distribution of the studied samples revealed two types of gold ore ore i and ii the former which is hosted by the roberts mountains and rodeo creek formations and the wispy planar and upper mud units of the popovich formation is the most abundant and widespread in the property ore i is characterized by intense hydrothermal alteration e g carbonate dissolution silicification and precipitation of pyrite and high amounts of trace elements e g ag as au ba cd cu hg mo ni s sb se te tl and zn on the other hand ore ii which is hosted in the wispy planar and soft sediment deformation units of the popovich formation is mainly confined to the central north northwest portion of the screamer deposit and is weakly altered with low concentration of trace elements both ores i and ii contain similar average concentrations of au in whole rock 14 and 19 g t au respectively and in pyrite 290 and 540 ppm respectively however auriferous pyrite from ore i has higher trace element as ag cu hg ni sb se and tl au ratios than ore ii the sedimentary units are interpreted to be the major local source of cd mo ni u v and zn and minor as cu hg and se as denoted by the composition of least altered samples and diagenetic pyrite and sphalerite this study reveals that al2o3 and tio2 are the most immobile compounds and their distribution indicates a homogeneous source for the detrital components in the sedimentary rocks among the ore related trace elements tl best correlates with au grade r 2 0 69 and shows some relationship with the calculated amount of pyrite r 2 0 49 indicating that tl would be the best element to vector toward zones of high grade carlin type au mineralization gold grades do not correlate with the amount of pyrite degree of alteration or organic c our results integrated with available thermodynamic data for au ore related elements and sio2 lead us to suggest that the formation of ore i occurred more proximal to the major mineralizing conduits as the hot more acid sio2 and trace element rich auriferous fluids interacted with fe bearing impure carbonate host rocks intensely dissolving the carbonate rocks and precipitating quartz and auriferous pyrite in the betze post and rodeo deposits as the fluids moved laterally throughout the favorable host rocks the ph increased leading to a decreasing in the rate of carbonate dissolution and in the solubility of silica favoring the formation of more distal ore ii in the central northern part of the screamer deposit significantly the gold concentrations in whole rock and in pyrite are in some way very similar in both ore types being slightly higher in ore ii suggesting that less acidic conditions were still favorable for the incorporation of gold in the structure of pyrite even at lower concentrations of other trace elements

alteration associated with gold deposition at the getchell carlin type gold deposit north central nevada wall rock alteration at the getchell underground deposit was examined to determine the effects of au bear ing fluids on host lithologies and the relationship between k bearing alteration minerals and au deposition the major minor and trace element geochemistry of highly altered and mineralized to unmineralized rocks from the getchell deposit was quantified for more than 50 samples collected along 13 transects through calcareous siltstone and carbonaceous limestone and along one transect through a rhyodacite dike each transect in sedimentary rocks was collected along a single homogeneous bed that could be followed from high grade ore to moderately altered rock or waste rock analyses were obtained for 39 elements 10 oxides and loss on ignition using multiple techniques petrographic studies were integrated with geochemistry and x ray diffraction and electron microbeam analyses to identify ore and alteration minerals and to correlate mineralogy with geochemical fluxes geochemical analyses indicate that ti al zr and th behave as immobile elements immobility isocon diagrams show that significant hg sb se te tl and cs are typically added to the wall rocks with au fe is im mobile in some transects added to wall rocks in others and removed from rhyodacite host rocks minor s and w are usually added to the rock whereas minor cu and mo are variably added or removed sio 2 appears to be either added during alteration or remain constant elements that are consistently removed during alteration include ca mn sr and sc and minor mg ba and k na was significantly removed from the rhyodacite au correlates positively with ag hg sb se si te and tl and negatively with ca mn and sc element fluxes correspond with observations of mineral abundances and show that sulfidation and decar bonitization are spatially associated with gold mineralization au hg sb tl cu and possibly te and w were incorporated in trace element rich pyrite as host rock fe was sulfidized fe was added to mineralized rocks in some transects indicating that pyritization occurred in addition to sulfidation cs is a component of galkhaite a trace mineral that is relatively common in some parts of the getchell system decarbonitization removed as much as 95 wt percent calcite from mineralized rocks in some transects and is responsible for loss of ca mn sr and sc although sio 2 as and s are significantly added to carlin type systems they do not appear as major added components in the isocon diagrams this is because the halos for these components are much larger than the areas examined by the sample transects x ray diffraction analyses identified minerals in ore and waste samples and quantified clay minerals including illite and montmorillonite kaolinite was not identified or is present in low abundances in both mineralized and unmineralized rocks montmorillonite does not correlate with gold and textures are consistent with pore filling deposition of montmorillonite following au deposition illite abundance exhibits a positive correlation with au r 2 0 80 intergrowths of illite with ore stage pyrite support a genetic relationship between the two minerals and indicate that illite formed as part of the au related hydrothermal event mineral abundances and textures suggest that illite formed by alteration of k feldspar during au deposition results show that dating of k bearing phases in sedimentary rocks other than illite would provide ages unrelated to gold deposition

geophysical and isotopic constraints on crustal structure related to mineral trends in north central nevada and implications for tectonic history we combined information from sr and pb isotope data and magnetotelluric models to develop a new magnetic and gravity interpretation of the crustal structure of north central nevada to better understand the origin of mineral trends the new interpretation suggests a crustal structure that is composed of precambrian continental crust transitional crust and primarily oceanic crust that are separated by northwest and northeast striking fault zones the magnetic expression of the buried precambrian continental crust is recognized for the first time low magnetic values primarily reflect magnetite poor crystalline crust rather than elevated temperatures at depth northwest and northeast striking crustal boundaries are defined by isotopic data and abrupt gradients in gravity and magnetic data the carlin and battle mountain eureka mineral trends are associated with two of three northwest striking boundaries the carlin boundary is primarily defined by a change in density and isotopic character of the lower to middle crust the battle mountain eureka boundary coincides with a density contrast in the upper crust and a change in isotopic character in the lower to middle crust magnetotelluric models suggest that the battle mountain eureka boundary represents a crustal fault zone for most of its extent but that deep rooted faulting is more complex near and northwest of battle mountain crustal fault zones inferred from the magnetotelluric models near the carlin trend are oblique to it suggesting that they may not have been controlled by the deep boundary seen in the gravity and isotopic data the third northwest trending boundary is related to the western edge of the buried precambrian continent in west central nevada but lacks an associated mineral trend a northeast striking boundary forms the northern limit of precambrian continental and transitional crust the boundaries may have originated as rift or transform faults during precambrian breakup of rodinia or as faults accommodating lateral movements or accretion during later paleozoic tectonic events comparing the crustal structure to tectonic elements produced by successively younger events shows that it had a profound influence on subsequent sedimentation deformation magmatism extension and most important mineralization

age constraints on jerritt canyon and other carlin type gold deposits in the western united states relationship to mid tertiary extension and magmatism carlin type gold deposits are difficult to date and a wide range of ages has been reported for individual deposits therefore several methods were employed to constrain the age of the gold deposits in the jerritt canyon district dated igneous rocks with well documented crosscutting relationships to ore provided the most reliable constraints k ar and 40 ar 39 ar dates on igneous rocks are as follows andesite dikes 324 ma sericitic alteration in andesite dikes 118 ma basalt dikes 40 8 ma quartz monzonite dikes 39 2 ma and calc alkaline ignimbrites 43 1 to 40 1 ma of these only the andesite and basalt dikes are clearly altered and mineralized the gold deposits are therefore younger than the 40 8 ma basalt dikes the sericitic alteration in the andesite dikes is unrelated to the gold deposits a number of dating techniques did not work k ar and 40 ar 39 ar dates on mica from mineralized ordovician to devonian sedimentary rocks gave misleading results the youngest date of 149 ma from the smallest 187 os 188 os initial 0 9 3 6 but did not yield a meaningful isochron due to very low re and os concentrations and large analytical uncertainties paleomagnetic dating techniques failed because the hydrothermal fluids sulfidized nearly all of the iron in the host rocks leaving no remnant magnetism when published isotopic dates from other carlin type deposits in nevada and utah are subject to the rigorous evaluation developed for the jerritt canyon study most deposits can be shown to have formed between 42 and 30 ma k ar and 40 ar 39 ar dates on the youngest preore igneous rocks range from 41 to 32 ma whereas the oldest postore igneous rocks range from 35 to 33 ma hydrothermal adularia from the twin creeks deposit yields similar 40 ar 39 ar dates of 42 ma k ar dates on supergene alunite range from 4 to 30 ma k ar and 40 ar 39 ar dates on micas separated from sedimentary 395 43 ma and igneous 145 38 ma rocks are usually much older than the gold deposits and most are suspect because they are from incompletely reset preore micas or from mixtures of preore and ore stage mica fission track dates on zircons are also generally older than the deposits 169 35 ma and are not completely reset by mineralization apatites are likley to be reset by the hydrothermal systems and by younger thermal events and yield dates 83 22 ma that are younger than those from zircon independent support of a mid tertiary age is provided by the low delta d sub h 2 o values of hydrothermal fluids from 10 widely separated carlin type deposits 134 or 23ppm the low delta d sub h 2 o values are consistent with a mid tertiary age when the climate was cool but are inconsistent with the cretaceous and jurassic ages favored by some workers when the climate was warm the age and distribution of carlin type deposits coincides with areas that underwent extension and calc alkaline magmatism over the same time period despite this relationship the deposits lack clear spatial or genetic relationships to mid tertiary epizonal plutons rather many deposits are localized along preexisting crustal fault zones in the getchell trend structures tapped deep sourced metamorphic or magmatic fluids and subsequently variably exchanged meteoric water in the other trends and districts only variably exchanged meteoric water has been detected these relationships suggest either that all of the deposits formed from deep sourced fluids and were subsequently flooded by meteoric water or that fluids from different sources evolved to produce deposits with similar charateristics

lithogeochemistry of carlin type gold mineralization in the gold bar district battle mountain eureka trend nevada abstract the gold bar district contains five carlin type gold deposits and four resources for a combined gold endowment of 1 6 m oz 50 t the gold deposits are hosted in devonian carbonate rocks below parautochthonous and allochthonous paleozoic siliciclastic rocks emplaced during the early mississippian antler orogeny the district is in the battle mountain eureka trend a long lived structural feature that localized intrusions and ore deposits of different types and ages the whole rock geochemistry of four different mineralized and unmineralized devonian carbonate rock units two favorable and two unfavorable were determined and interpreted in the context of the regional geology a combination of basic statistics r mode factor analysis isocon plots and alteration diagrams were utilized to 1 identify favorable geochemical attributes of the host rocks 2 characterize alteration and associated element enrichments and depletions and 3 identify the mechanism of gold precipitation this approach also led to the recognition of other types of alteration and mineralization in host rocks previously thought to be solely affected by carlin type mineralization unit 2 of the upper member of the denay formation with the highest al 2 o 3 fe 2 o 3 and sio 2 contents and the lowest cao content is the most favorable host rock based on the high regression coefficients of data arrays on x y plots that project toward the origin al 2 o 3 and tio 2 were immobile and k 2 o and fe 2 o 3 were relatively immobile during alteration and mineralization specific element associations identified by factor analysis are also prominent on isocon diagrams that compare the composition of fresh and altered equivalents of the same rock units the most prominent associations are au as sb sio 2 tl cao and loi the main gold mineralizing event and related silicification and decalcification cd zn ag p ni and tl an early base metal event and mgo early dolomitization alteration diagrams consisting of x y plots of sio 2 al 2 o 3 k 2 o al 2 o 3 co 2 al 2 o 3 s al 2 o 3 fe 2 o 3 al 2 o 3 provide evidence for progressive silicification decarbonation decalcification and dedolomitization argillization illite and sulfidation as a function of gold mineralization the latter process is identified as the principal mechanism of gold precipitation the lithogeochemistry of the ores in the gold bar district is typical of that documented in classic carlin type gold deposits in the region but the size of the deposits and the intensity of alteration and mineralization are less the presence of other types of mineralization in the gold bar district is also common to most of the other carlin type districts located in major mineral belts the approach used in this study is well suited to the interpretation of multi element geochemical data from other study areas with superimposed alteration and mineralization

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

regional crustal structures and their relationship to the distribution of ore deposits in the western united states based on magnetic and gravity data upgraded gravity and magnetic databases and associated filtered anomaly maps of western united states define regional crustal fractures or faults that may have guided the emplacement of plutonic rocks and large metallic ore deposits fractures igneous intrusions and hydrothermal circulation tend to be localized along boundaries of crustal blocks with geophysical expressions that are enhanced here by wavelength filtering in particular we explore the utility of regional gravity and magnetic data to aid in understanding the distribution of large mesozoic and cenozoic ore deposits primarily epithermal and porphyry precious and base metal deposits and sediment hosted gold deposits in the western united states cordillera on the broadest scale most ore deposits lie within areas characterized by low magnetic properties the mesozoic mother lode gold belt displays characteristic geophysical signatures regional gravity high regional low to moderate background magnetic field anomaly and long curvilinear magnetic highs that might serve as an exploration guide geophysical lineaments characterize the idaho montana porphyry belt and the la caridad mineral park belt from northern mexico to western arizona and thus indicate a deep seated control for these mineral belts large metal accumulations represented by the giant bingham porphyry copper and the butte polymetallic vein and porphyry copper systems lie at intersections of several geophysical lineaments at a more local scale geophysical data define deep rooted faults and magmatic zones that correspond to patterns of epithermal precious metal deposits in western and northern nevada of particular interest is an interpreted dense crustal block with a shape that resembles the elliptical deposit pattern partly formed by the carlin trend and the battle mountain eureka mineral belt we support previous studies which on a local scale conclude that structural elements work together to localize mineral deposits within regional zones or belts this study of mineral deposits of the western united states demonstrates the ability of magnetic and gravity data to elucidate the regional geologic framework or structural setting and to contribute in locating favorable environments for hydrothermal mineralization

primary geochemical characteristics of mineral deposits implications for exploration extract the old adage mineral exploration is like finding a needle in a haystack recaps the difficulty of finding economic mineral deposits near or beneath the surface in general two main approaches can be used to detect the presence of mineral deposits near or beneath the ground surface one employs a mineral deposit model and the other aims at the identification of a primary halo about a mineral deposit a mineral deposit model provides systematic empirical descriptive and or theoretical genetic descriptions of the essential geological geochemical and geophysical features of a type of mineral deposits cf cox and singer 1986 and roberts et al 1988 the primary halo of a mineral deposit as defined originally by safronov 1936 is an area including rock surrounding mineral deposit ore bodies and enriched elements that make up that deposit thus descriptions of primary haloes may form part of a mineral deposit model but both deposit model and primary halo approaches to mineral exploration are based on studies of primary geochemical characteristics of mineral deposits because chemical processes during mineralization are the ones that ultimately bring about metal precipitation or mineral formation however a deposit model approach to mineral exploration is apt for identifying which haystacks contain a needle whereas a primary halo approach to mineral exploration is proper for deducing where in a needle bearing haystack is the needle located

rapid remobilization of magmatic crystals kept in cold storage we lack thermal histories for magma reservoirs but here the magma under mount hood oregon usa is shown to have been too cold to mobilize for most of the time it has been stored which implies that magma mobilizes at which point it can be imaged geophysically very quickly prior to eruption this paper presents a novel approach to constraining the thermal history of the subsurface magma bodies that feed eruptions by combining temporal information from a variety of sources kari cooper and adam kent combine timescales derived from uranium series disequilibria textural information and trace element zoning in crystals they show that at mount hood oregon only a small fraction of the total storage duration has been spent at temperatures above the critical crystallinity the point at which magma is easily mobilized they conclude that largely liquid bodies that could be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption the processes involved in the formation and storage of magma within the earth s upper crust are of fundamental importance to volcanology many volcanic eruptions including some of the largest result from the eruption of components stored for tens to hundreds of thousands of years before eruption1 2 3 although the physical conditions of magma storage and remobilization are of paramount importance for understanding volcanic processes they remain relatively poorly known4 5 eruptions of crystal rich magma are often suggested to require the mobilization of magma stored at near solidus conditions6 7 8 however accumulation of significant eruptible magma volumes has also been argued to require extended storage of magma at higher temperatures7 8 9 what has been lacking in this debate is clear observational evidence linking the thermal and therefore physical conditions within a magma reservoir to timescales of storage that is thermal histories here we present a method of constraining such thermal histories by combining timescales derived from uranium series disequilibria crystal sizes and trace element zoning in crystals at mount hood oregon usa only a small fraction of the total magma storage duration at most 12 per cent and probably much less than 1 per cent has been spent at temperatures above the critical crystallinity 40 50 per cent at which magma is easily mobilized partial data sets for other volcanoes also suggest that similar conditions of magma storage are widespread and therefore that rapid mobilization of magmas stored at near solidus temperatures is common magma storage at low temperatures indicates that although thermobarometry calculations based on mineral compositions may record the conditions of crystallization they are unlikely to reflect the conditions of most of the time that the magma is stored our results also suggest that largely liquid magma bodies that can be imaged geophysically will be ephemeral features and therefore their detection could indicate imminent eruption

rapid cooling and cold storage in a silicic magma reservoir recorded in individual crystals silicic volcanic eruptions pose considerable hazards yet the processes leading to these eruptions remain poorly known a missing link is knowledge of the thermal history of magma feeding such eruptions which largely controls crystallinity and therefore eruptability we have determined the thermal history of individual zircon crystals from an eruption of the taupo volcanic zone new zealand results show that although zircons resided in the magmatic system for 103 to 105 years they experienced temperatures 650 to 750 c for only years to centuries this implies near solidus long term crystal storage punctuated by rapid heating and cooling reconciling these data with existing models of magma storage requires considering multiple small intrusions and multiple spatial scales and our approach can help to quantify heat input to and output from magma reservoirs

origin of high grade gold ore source of ore fluid components and genesis of the meikle and neighboring carlin type deposits northern carlin trend nevada the meikle mine exploits one of the world s highest grade carlin type gold deposits with reserves of ca 220 t gold at an average grade of 24 7 g t locally gold grades exceed 400 g t several geologic events converged at meikle to create these spectacular gold grades prior to mineralization a devonian hydrothermal system altered the bootstrap limestone to fe rich dolomite subsequently the rocks were brecciated by faulting and late jurassic intrusive activity the resulting permeability focused flow of late eocene carlin type ore fluids and allowed them to react with the fe rich dolomite fluid inclusion data and mineral assemblages indicate that these fluids were hot ca 220 c of moderate salinity 2 s rich gold rich pyrite formed by dissolution of dolomite and sulfidation of its contained fe where dissolution and replacement were complete ore stage pyrite and other insoluble minerals were all that remained locally these minerals accumulated as internal sediments in dissolution cavities to form ore with gold grades 400 g t petrographic observations geochemical data and stable isotope results from the meikle mine and other deposits at the goldstrike mine place important constraints on genetic models for meikle and other carlin type gold deposits on the northern carlin trend the ore fluids were meteoric water d 135 18 o 5 that interacted with sedimentary rocks at a water rock ratio of ca 1 and temperatures of ca 220 c the absence of significant silicification suggests that there was little cooling of the ore fluids during mineralization these two observations strongly suggest that ore fluids were not derived from deep sources but instead flowed parallel to isotherms the gold was transported by h 2 s 34 s 9 which was derived from paleozoic sedimentary rocks the presence of auriferous sedimentary exhalative mineralization in the local stratigraphic sequence raises the possibility that preexisting concentrations of gold contributed to the carlin type deposits taken together our observations suggest that meteoric water evolved to become an ore fluid by shallow circulation through previously gold and sulfur enriched rocks carlin type gold deposits formed where these fluids encountered permeable reactive fe rich rocks

structural controls on carlin type gold mineralization in the gold bar district eureka county nevada the gold bar district in the southern roberts mountains 48 km northwest of eureka nevada contains one main deposit gold bar five satellite deposits and other resources approximately 0 5 moz of gold have been recovered from a resource of 1 639 000 oz of gold in carlin type gold deposits in lower plate miogeoclinal carbonate rocks below the roberts mountains thrust host rocks are unit 2 of the upper member of the devonian denay formation and the bartine member of the mccolley canyon formation spatial and temporal relations between structures and gold mineralization indicate that both pre tertiary and tertiary structures were important controls on gold mineralization gold mineralization occurs primarily along high angle tertiary normal faults some of which are reactivated reverse faults of paleozoic or mesozoic age most deposits are localized at the intersection of northwest and northeast striking faults alteration includes decalcification and to a lesser extent silicification along high angle faults jasperoid pervasive silicification which formed along most faults and in some strata bound zones accounts for a small portion of the ore in every deposit in the gold canyon deposit a high grade jasperoid pipe formed along a tertiary normal fault which was localized along a zone of overturned fault propagation folds and thrust faults of paleozoic or mesozoic age

dating gold deposition in a carlin type gold deposit using rb sr methods on the mineral galkhaite significant effort has been expended in an attempt to date hydrothermal activity that generated carlin type gold deposits ctds in the great basin of nevada thus far these efforts have been only partially successful because the relationship s between the dated mineral and hydrothermal activity are equivocal in many cases galkhaite a trace component of at least four ctds in nevada contains significant amounts of rb and virtually no sr making it an ideal candidate for radiometric dating at the getchell deposit galkhaite is paragenetically late but clearly associated with gold mineralization our data place gold mineralization at getchell at 39 0 2 1 ma this is the first unequivocally gold related date produced for any of the carlin type systems galkhaite also has been reported at the carlin rodeo and betze deposits and is likely present in other ctds in nevada this mineral may provide a solution to the conundrum of dating of ctds

dike hosted ores of the beast deposit and the importance of eocene magmatism in gold mineralization of the carlin trend nevada a reply we appreciate the comments of g d langstaff and the opportunity to elaborate on the origin of carlin type gold deposits langstaff focuses on two of the critical issues regarding these deposits their age and spatial association with igneous rocks although his comment mentions almost every known carlin type deposit our reply focuses on deposits in major districts direct dating of hydrothermal minerals and indirect dating based on crosscutting relations with pre syn or postmineral rocks in nevada have demonstrated that major deposits in all trends are eocene emsbo et al 1996 leonardson and rahn 1996 phinisey et al 1996 groff et al 1997 hall et al 1997 2000 hofstra et al 1999 hofstra and cline 2000 johnston 2000 ressel et al 2000a b tretbar et al 2000 although many carlin type deposits are not dated we emphasize that no deposit has been demonstrated to be any other age preliminary 40ar 39ar dating of white micas from mineralized eocene dikes at meikle and griffin in the carlin trend gives eocene ages ressel et al 2000b any discussion of the origin especially spatial association of carlin type deposits with igneous rocks must emphasize the northern and central carlin trend fig 1 deposits there constitute 60 percent of the total estimated contained gold in carlin type deposits of nevada and utah hofstra and cline 2000 we interpreted the 700 km2 aeromagnetic anomaly fig 1 there to reflect an underlying composite eocene intrusion emplaced between about 40

genesis and evolution of framboidal pyrite and its implications for the ore forming process of carlin style gold deposits southwestern china abstract southwestern guizhou an indispensable component of the golden triangle area of southwestern china is famous for its numerous carlin style gold deposits unlike other sulfides in the deposits that have been well studied the genesis of framboidal pyrite is still controversial because of its tiny size here we first determined the chemical and in situ sulfur isotopic compositions of framboidal pyrites from three typical carlin style gold deposits namely shuiyindong taipingdong and nibao using secondary electron image sei back scattered electron bse electron probe micro analysis epma and nanometer scale secondary ion mass spectroscopy nano sims methods our epma data show that the framboidal pyrites contain a minor amount of trace elements ni

evaluation of the role of sulfidation in deposition of gold screamer section of the betze post carlin type deposit nevada this study was undertaken to evaluate the relation between sulfidation and gold deposition in the screamer section of the betze post carlin type deposit we also attempted to determine the source of sulfur in the deposit and the possibility that more than one gold depositing event contributed metal to the screamer system gold ore at screamer forms a generally stratiform body hosted largely by the wispy member of the devonian popovich formation and lacks obvious alteration of wall rock gangue minerals gold at screamer is hosted by arsenian pyrite that forms disseminated grains and overgrowths on diagenetic pyrite a strong correlation is observed between the gold content of rock samples and their proportion of ore related arsenian pyrite as determined by point counts isocon plots show that mineralization at screamer involved addition of both sulfur and iron along with gold arsenic antimony tungsten and local silica barium and phosphorus fe al vs s al plots show that most ore samples at screamer do not contain enough sulfur to account for all of their iron as pyrite petrographic examination shows that the iron occurs in pyrite ferroan dolomite and iron bearing micas in order of decreasing abundance the fe al vs s al plots also show that screamer samples with high gold contents contain more pyrite than samples with low gold contents and that samples inside the ore zone have more pyrite than those in the surrounding area these relations are interpreted to indicate that screamer has undergone at least two pyrite forming events the first event which probably took place during diagenesis involved incomplete sulfidation that left some iron in carbonate and silicate minerals the second event which probably took place during gold mineralization sulfidized most remaining iron and added pyrite in the screamer ore zone the 34 s values of chemically extracted sulfur from disseminated pyrite at screamer range from 13 8 to 16 5 per mil and 34 s values for hand picked separates of pyrite and other sulfides in veins cutting these rocks range from 21 2 to 11 7 per mil a significant fraction of samples with high gold contents and of samples in the ore zone regardless of gold content have 34 s values between 1 and 5 per mil these data suggest that early diagenetic sulfur with a wide range of 34 s values was overprinted by sulfur with isotopic compositions in the 1 to 5 per mil range that was associated with gold mineralization the 34 s values for gold related sulfur at screamer are lower than those reported for bulk mineral separates from most other carlin type deposits and from sims analyses of sulfides from the proximal post part of the betze post system and could be magmatic limited evidence can be found for multiple gold forming events in the screamer zone tungsten which might have been introduced by the jurassic age goldstrike stock is widespread in the deposit and correlates closely with gold a few samples with high tungsten gold ratios found along fault zones might be part of an earlier phase of mineralization related to the goldstrike stock a 34 s value for pyrite in one of these samples is similar to the high values reported for sulfides in auriferous polymetallic mineralization in the post section of the deposit but this mineralization is not reported to contain tungsten other veins containing sphalerite have lower 34 s values and lack consistent gold values whereas there is no correlation between gold values and the degree to which the host rocks have undergone sulfidation there is a strong and highly significant correlation between gold values and the abundance of ore related pyrite this shows that simple sulfidation of immediately adjacent wall rock cannot account alone for gold deposition at screamer and it might not be the only ore depositing process in some other carlin deposits a more general extended sulfidation process could be important however one likely process involves mixing of an invading mineralizing fluid containing sulfur and gold with a wall rock fluid containing iron derived from adjacent or distal wall rocks recognition of the source s for this iron and fluid flow pathways responsible for introducing it to the ore zone could provide useful guidance in exploration for the next carlin trend

gold deposition by sulfidation of ferrous fe in the lacustrine sediments of the pueblo viejo district dominican republic the effect of fe c s diagenesis on later hydrothermal mineralization in a maar diatreme complex abstract the pueblo viejo district located in the cordillera central of the dominican republic contains large au ag deposits associated with acid sulfate alteration within spilites conglomerates and carbonaceous sedimentary rocks that were deposited in a maar diatreme complex much of the au mineralization occurs in pyritic carbonaceous siltstones of the pueblo viejo maar diatreme member of the cretaceous los ranchos formation pyrite is the only fe bearing phase in mineralized rock whereas siderite is the dominant fe bearing phase in siltstones distal to mineralization disseminated pyrite occurs as framboids cubes pyritohedra concretions and cement early framboids occur throughout the district au occurs as inclusions in later non framboid disseminated pyrite nfdp an occurrence that is interpreted to be indicative of contemporaneous deposition pyrite framboids exhibit a wide range of 34s cdt values 17 5 to 4 8 and are interpreted to have formed during biogenic reduction of pore water sulfate the nfdp yield restricted 34s cdt values x 5 2 s 2 4 n 43 similar to those obtained from later vein pyrite x 6 4 s 1 5 n 12 alunite and barite have 34s values ranging from 18 8 to 21 6 the interpretation that the nfdp vein pyrite alunite and barite and possibly even the framboidal pyrite share a common source of igneous sulfur is supported by the 34s data siderite occurs as concretions and cement contains abundant mg fe0 75mg0 19mn0 03ca0 02co3 and has 13c pdb and 18o smow values ranging from 2 5 to 1 1 and 14 6 to 19 5 respectively these data are consistent with the interpretation that the siderite formed in lacustrine sediments and that the carbonate in the siderite is probably methanogenic although contributions from oxidation of organic matter during biogenic sulfate reduction thermal decarboxylation of organic matter or magmatic vapor cannot be ruled out disseminated au mineralization in the sedimentary rocks formed when a hydrothermal fluid encountered reactive fe2 in diagenetic siderite the ensuing pyrite deposition consumed h2s and destabilized the au hs 2 complex leading to precipitation of au the capacity of the sedimentary rocks to consume h2s and precipitate au was controlled by the amount of non pyrite fe present as siderite the abundance of siderite was controlled by the extent of pyrite formation during diagenesis

40 ar 39 ar k ar and fission track geochronology of sediment hosted disseminated gold deposits at post betze carlin trend northeastern nevada the post betze deposit of nevada is the largest sediment hosted disseminated gold deposit presently known both dimensionally and in terms of contained metal ore occurs primarily as submicron sized gold that is disseminated in altered sedimentary rocks of the lower paleozoic roberts mountains formation however significant portions of the ore are present in altered monzonite of the goldstrike stock alteration and mineralization were controlled by both structure and stratigraphy alteration began with early decarbonatization and was followed by silicification and finally argillization phyllosilicate mineral zoning grades from proximal kaolinitc to kaolinitc sericite to unaltered rock based on geochronologic studies utilizing the k ar 40 ar 39 ar and fission track techniques we suggest that the age of gold mineralization is approximately 117 ma the premineralization goldstrike stock was emplaced at about 158 ma and a postmineralization sill was emplaced at 39 ma these events place clear limits on the age of mineralization fine grained sericite that is interpreted to have formed during hydrothermal events which also generated gold ore was dated by either the k ar method or a modification of the standard 40 ar 39 ar fusion technique age determinations on several samples of fine grained sericite from altered host rocks both sedimentary and igneous yield ages near 117 ma younger dates are interpreted to be the result of either thermal disturbance of the k ar system or mixing of sericites of two ages one sample of coarser grained sericite that was stepheated gave discordant spectra fission track analyses of zircon and apatite from the postore sill clearly document the lack of significant thermal or hydrothermal activity younger than 39 ma although less definitive fission track data on zircon from preore sedimentary and igneous hosts suggest that no widespread hydrothermal activity having temperatures above about 100 degrees c has occurred since approximately 110 ma the 117 ma age is consistent with one of the major magmatic pulses recognized in the northern great basin it is also similar to within analytic uncertainty a k ar date of 109 ma from the welches canyon stock 20 km south of post betze an intrusion of similar age unexposed at present could have been the driving force for the hydrothermal system at post betze published geochronologic data though less well tied to mineralization suggest similar mid cretaceous ages for other sediment hosted disseminated gold deposits in nevada based on these ages we suggest that sediment hosted disseminated gold deposits are not necessarily products of extensional environments and in fact appear to be associated with compressional environments in nevada during cretaceous time the alignment of sediment hosted disseminated gold deposits and other features in the basin and range province of the western united states suggests that zones of long term crustal weakness may have controlled igneous intrusion and consequent hydrothermal gold mineralization although not all igneous episodes necessarily generated sediment hosted disseminated gold ore deposits the strong association between sediment hosted disseminated gold deposits the precambrian craton and these deep seated structures also suggests the possibility of a deep source for gold and other components of these systems

as sb bi au mineralization in the baogutu gold deposit xinjiang nw china here we report the occurrence of some uncommon mineral assemblages including paakonenite aurostibite native arsenic native antimony and native bismuth found in the baogutu gold deposit in the western junggar xinjiang nw china the mineralization could be generally subdivided into two types the gold bearing quartz vein type mineralization and disseminated mineralization in the wall rocks the sulfide minerals in gold lodes commonly include pyrite arsenopyrite marcasite and stibnite however the l7 lode in no 4 orebody and the l1 lode in no 11 orebody of the baogutu gold deposit are quite different in terms of their mineral assemblages the l7 lode contains native arsenic quartz veins in shallow levels and stibnite quartz veins at depth gold bearing minerals electrum native gold and rarely aurostibite mainly coexist with paakonenite stibnite native arsenic and native antimony the crystallization of as and sb bearing minerals was likely to have consumed h2s from the hydrothermal fluid which probably triggered the precipitation of native gold the l1 lode consists of several discontinuous sulfide dominated lensoid orebodies the massive sulfide ores that produced most of the gold resource are characterized by an intimate association between native bismuth and native gold mineralization

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

evaluation of radioisotope dating of carlin type deposits in the great basin western north america and implications for deposit genesis a variety of techniques have been used in attempts to date the mineralization in carlin type deposits in the great basin with highly variable results these techniques and results are reviewed in this paper along with presentation of two new dates for the rodeo deposit on the carlin trend and for the barneys canyon deposit in utah complete resetting of sericite by hydrothermal fluids of the temperature and duration of hydrothermal activity that form carlin type deposits is considered highly unlikely therefore dates from sericite are generally of questionable value unless that sericite can be shown to have formed during the same hydrothermal event during which au was deposited for the deposits that have been investigated to date sericite dates rarely if ever record the age of au mineralization however sericite ages do appear to record pre au events in some districts such events may have contributed to ground preparation and to a much lesser extent to the tenor of the ore fission track and u th he techniques provide important age constraints on mineralization in some districts but also suffer from a less than clear association with au rb sr dating of galkhaite an hg sulfosalt provides the only direct age of mineralization but galkhaite has been recognized in only a few locations and dated in only two deposits in a similar manner adularia is contemporaneous with au at twin creeks but twin creeks is the only carlin type deposit where adularia has been reported several other techniques u pb reos sm nd have been used in attempts to date the deposits but with limited success the best available ages are those from preore and postore igneous events and those from galkhaite galkhaite from getchell indicates a mineralization age of 39 ma and that from rodeo carlin trend indicates a mineralization age of 40 ma a new u th he age 34 ma for apatite at barneys canyon suggests contemporaneity with the nearby bingham porphyry cu system the most reliable data from several deposits and districts in the great basin indicate that carlin type mineralization occurred in a relatively narrow time interval between 33 and 42 ma this interval coincides with the transition from a compressional to an extensional tectonic environment in the region although similar tectonic environments have existed at different times and places in western north america no other carlin type mineralization is known to exist in the region therefore although the age and tectonic environment of these deposits is now reasonably well constrained it is clear that tectonic environment alone is insufficient to explain the genesis of carlin type deposits additional perhaps unique factors such as host rock geochemistry must have contributed to the development of these exceptional deposits

new insight into explosive volcanic eruptions connecting crystal scale chemical changes with conduit scale dynamics explosive volcanic eruptions may be triggered by processes in the conduit including p t variations volatile loss and crystal growth these unobserved processes may be inferred via textural and chemical examination of eruptive products juvenile clasts were examined from vulcanian explosions of the soufriere hills volcano here we show that secondary ion mass spectrometry sims depth profiling into plagioclase phenocrysts provides a nearly continuous record of magma ascent from chamber to surface via high resolution 10 nm chemical profiles of decompression induced crystal growth changes in lithium concentration allow quantification of crystal growth rates which range from 2 10 8 mm s to 7 10 8 mm s anorthite variations suggest conduit temperature increases of 50 to 100 c assuming contemporaneous overpressures approaching 20 mpa these data demonstrate a new means of constraining conduit conditions preceding explosive volcanic eruptions

high grade gold deposition and collapse breccia formation cortez hills carlin type gold deposit nevada usa the cortez hills breccia zone a nevada carlin type gold deposit located in the cortez district on the battle mountain eureka trend formed within a high grade polylithic breccia and grades at the center of the breccia zone are locally in excess of 1 oz ton au sample transects from low grade or below detection 5 ppb to high grade were collected and analyzed using petrography x ray diffraction xrd scanning electron microscopy sem electron probe microanalysis epma and laser ablation inductively coupled plasma mass spectrometry la icp ms analyses to correlate changes in ore pyrite geochemistry and alteration mineralogy with grade and to relate ore deposition to breccia formation ore related minerals textures and analyses are consistent with fluid rock reaction and replacement at relatively low hydrothermal temperatures 260 c during a single heating and cooling event and over a relatively short time frame coincident in time and space with regional magmatism prior to carlin mineralization roberts mountains wenban and horse canyon formations were affected by burial diagenesis and low grade contact metasomatism and metamorphism producing host rocks composed of calcite dolomite and quartz with generally minor dickite pyrite chalcopyrite sphalerite clinochlore tremolite and organic material and minor to trace k feldspar muscovite and biotite acidic ore fluids sulfidized fe in carbonate rock to form rims of au and trace element rich pyrite on earlier formed pyrite or produced small generally 10 m spheres of fuzzy ore pyrite with no apparent core trace elements in ore stage pyrite are dominated by as with strongly correlated hg tl and cu all greater than au lesser and variable sb and minor to undetected ag bi ti w and te ore fluids also variably dissolved calcite and dolomite and replaced carbonate minerals with jasperoid or produced minor pore space ore pyrite and jasperoid after calcite are enclosed in illite that replaced dickite clinochlore and tremolite and filled minor open space primarily between 227 and 277 c for most analyzed samples based on illite chemistry as hydrothermal collapse began ore pyrite chemistry transitioned to an evolved ore stage chemistry with abundant as low to undetected au lesser pb cu sb and diminished hg and tl compared to the ore stage evolved ore stage fluids coincidently precipitated sulfosalt minerals initially unusual cu and hg rich aktashite cu6hg3as4s12 and tl and hg rich christite tlhgass3 and later realgar as cu and zn in hydrothermal fluids and chalcopyrite and sphalerite in the host rocks respectively were consumed calcite only veins indicate termination of the hydrothermal system small fragmented clasts of realgar with chemically distinct au bearing evolved ore stage pyrite rims found in brecciated rock along a faulted dike wenban contact provide the first evidence in a carlin type gold deposit of precipitation of an au bearing pyrite rim on a late ore stage mineral these rare rims likely formed in response to fault movement along the dike wenban contact or by dike injection that allowed deep au bearing fluids to access a higher and cooler part of the system where late ore stage minerals had begun to form these rims further highlight the normal textures in carlin type gold deposits where ore stage pyrite was overgrown by late ore stage sulfosalt minerals demonstrating a single hydrothermal event followed by a single cooling event this paragenesis is consistent with previously modeled deposit formation time frames of 45 000 to 15 000 years two styles of breccia were identified within the deposit replacement matrix supported breccia in which an illite rich punky matrix encloses au bearing pyrite jasperoid and variable relict calcite formed during intense ore stage alteration and mineralization the fine grained ore and alteration replacement minerals did not require significant open space to form later evolved and late ore stage sulfosalt minerals and calcite that are visible at the hand sample scale provide the earliest evidence for the development of significant open space hydrothermal collapse allowed the incursion of cooler meteoric water into the system which along with the retrograde solubility of calcite provided coupled mechanisms driving significant calcite dissolution collapse brecciation and formation of the cortez hills breccia zone dissolution and collapse brecciation increasingly concentrated ore stage pyrite as the hydrothermal system transitioned to the late ore stage contributing to high au grades

hydrogen incorporation and charge balance in natural zircon the water and trace element contents of natural igneous zircons were determined to constrain the mechanism of hydrogen incorporation the low radiation damage zircons were derived from fe ti oxide gabbros from the vema fracture zone 11 n mid atlantic ridge they contain up to 1212 ppmw h2o 1 9 wt y2o3 and 0 6 wt p2o5 and are generally strongly zoned ree y are partially charge balanced by p y ree3 p5 zr4 si4 but a large ree excess is present on an atomic basis this excess is closely approximated by the amount of h present in the zircons we therefore conclude that h is incorporated by a charge balance mechanism h ree3 zr4 this interpretation is consistent with ftir data of the vema zircons which shows a strongly polarised main absorption band at ca 3100 cm 1 similar to experimentally grown lu doped hydrous zircon the size of this 3100 cm 1 band scales with h and ree contents apart from a small overlapping band at 3200 cm 1 no other absorption bands are visible indicating that a hydrogrossular type exchange mechanism does not appear to be operating in these zircons because of charge balanced uptake of h p and ree in zircon the partitioning of these elements into zircon is dependent on each of their concentrations for instance dreezrc melt increases with increasing h and p contents of the melt whereas dhzrc melt increases with increasing ree content but decreases with increasing p content in addition h p ree systematics of sector zoning indicate kinetic effects may play an important role hence using h in zircon to determine the water content of melts is problematic and ree partitioning studies need to take into account p and h2o contents of the melt

fault controlled lateral fluid flow underneath and into a carlin type gold deposit isotopic and geochemical footprints many carlin type gold deposits in northern nevada are found adjacent to steeply dipping faults that channeled hydrothermal fluids upward into mineralized regions however some carlin deposits comprise gently dipping tabular ore zones lacking any obvious large steeply dipping feeder structure beneath them the goal of this paper is to test whether fluid flow into such ore zones also involved largely vertical upwelling or whether lateral fluid flow particularly along low angle fault structures may have played a significant role in their formation in this study we use carbon and oxygen isotopes in conjunction with trace element geochemistry to investigate pathways taken by hydrothermal ore forming fluids into the tabular subhorizontal ore zones of the pipeline deposit a giant carbonate rock hosted carlin type deposit in nevada we sampled deep drill holes within below and to the side of the main ore zone to assess the extent of lateral versus vertical fluid flow up to 600 m below mineralization the main ore zone at the pipeline deposit is dominated by carbonate host rocks having 18 o and 13 c values that are depleted in the heavier 18 o and 13 c isotopes compared to global background values for rocks of the same age depletion results from the progressive buffering of the rock by an infiltrating auriferous hydrothermal fluid there is significant heterogeneity in the spatial pattern and relative degree of 18 o and 13 c depletion through the ore zone most likely reflecting multiple flow paths with different total integrated fluid fluxes path lengths rates of fluid cooling and possibly fluid mixing a lack of a pervasive reduction in 18 o and 13 c values and low concentrations of trace elements in the rocks immediately beneath the main ore zone indicate that the deposit was not a product of large scale vertical upwelling of auriferous hydrothermal fluid directly into the mineralized region rather flow was focused along preexisting low angle thrusts particularly the abyss fault with fluids flowing laterally underneath and then up into the area of the main ore zone enhanced permeability along these low angle structures most likely derived from fault reactivation and the generation of fracture networks in damage zones peripheral to a relatively impermeable fault core such reactivation would have required suprahydrostatic fluid pressures relatively shallow crustal depths and a lack of more optimally oriented high angle faults 50 that would otherwise have preferentially failed and focused fluid flow steeply upward

diffusion coupling between trace and major elements and a model for calculation of magma residence times using plagioclase residence times of plagioclase crystals in magma reservoirs can be determined by modeling the compositional zoning of trace elements in these crystals we present a formulation to model diffusion of trace elements in plagioclase paying special attention to certain thermodynamic and kinetic aspects in particular we account for the compositional dependence on anorthite content of the chemical potential and diffusion coefficients of trace elements e g mg the choice of suitable boundary conditions and potential effects of diffusion in more than one dimension we show that contrary to intuition diffusive fluxes of trace elements may be coupled to major element concentration gradients and ignoring such coupling can lead to incorrect estimates of timescales we illustrate application of the model using plagioclase crystals from a suite of gabbroic xenoliths from a holocene dacitic lava flow of volcan san pedro chilean andes 36 s the inferred timescale for metasomatism of the xenoliths by evolved liquids is on the order of 100 30 to 148 yr and serves to illustrate how trace element zoning in plagioclase provides a window into timescales of magmatic processes inaccessible by isotopic or other methods