strongly negative δ34s values associated with secondary pyrite above and below the j m reef stillwater complex montana abstract previous sulfur isotope studies of the stillwater complex and in particular the j m reef have noted the generally restricted range of 34s values in the range of 0 2 5 e g zientek and ripley 1990 ripley et al 2017 detailed studies of high s zones in both the hanging wall of the reef package which contains the j m reef and the footwall within the reef package but below the mineralized j m reef indicate the presence of significant quantities of secondary pyrite the pyrite rich intervals are characterized by 34s values that strongly differ from those of the j m reef in the range of 0 2 to 17 4 the pyrite rich rocks above and below the j m reef are both serpentinized lherzolites although the j m reef is locally strongly altered to clinozoisite epidote rich assemblages only the sepentinized lherzolites show anomalously low 34s values the highly negative 34s values of the pyrite rich assemblages result from oxidation processes and s isotope fractionation between reduced and oxidized s species one possible process involves the production of a mixed sulfate sulfide fluid with pyrite produced from h2s with low 34s values a second alternative is that infiltration of an oxidized fluid and interaction with sulfide bearing rocks of the complex produced a sulfate rich fluid that then underwent partial reduction to sulfide via reaction with reduced fe bearing minerals strongly negative 34s values would characterize secondary pyrite produced as a result of either process due to the large s isotope fractionation between oxidized and reduced s species at hydrothermal temperatures of 400 c or below the varying degrees of alteration and restricted pyrite rich intervals with highly negative s isotope signatures suggest that fluid infiltration was not pervasive secondary pyrite that was produced as a result of redox driven reactions is preserved only in olivine rich lithologies characterized by a high degree of serpentinization

stress directions in eastern north america determined to 4 5 km from borehole elongation measurements stress directions have been determined to depths of 4 5 km in eastern north america from borehole elongation measured by dipmeter calipers in 47 wells the average maximum horizontal stress directions are eastern canada n54 e 7 appalachian basin n58 e 8 and the illinois basin n89 w 5 stress directions determined for the illinois and appalachian basins are in agreement with published interpretations from hydraulic fractures and earthquake fault plane solutions new results from eastern canada and the appalachian basin show that stress directions are everywhere consistent with the midcontinent stress province these data suggest that the maximum horizontal stress does not rotate from the ene midcontinent trend to nw near the atlantic coast a directional analysis of hydraulic fractures petal centerline fractures and natural fractures from the appalachian basin shows that borehole elongation is systematically aligned perpendicular to the directions of hydraulic fractures centerline fractures and northeast striking natural fractures but is not systematically aligned with northwest striking natural fractures these observations indicate that borehole elongations provide reliable stress orientations

sulfur isotope studies of the stillwater complex and associated rocks montana the stillwater complex in south central montana is a late archean layered ultramafic to mafic intrusion emplaced into middle to late archean metagraywacke metashale and ironformation sulfide minerals are concentrated near the base of the intrusion in some chromitite layers podiform pegmatoids discontinuous discordant pods or pipes and numerous thin stratiform layers this investigation focuses on the isotopic composition of sulfur in metamorphosed sedimentary rocks stillwater associated sills and dikes sulfide accumulations near the base of the complex and platinum group element enriched sulfide mineralization that constitutes the j m reef and the picket pin deposit the delta 34 s values for 233 samples analyzed in this study have a mean of 0 4 per mil a standard deviation of 1 7 and a maximum and minimum of 3 8 and 7 8 per mil respectively despite the very narrow range of values analysis of variance calculations confirms that the data comprise three statistically distinct groups these groups are 1 metagraywackes and metashales mean 2 8ppm 2 iron formation stillwater associated sills and dikes and basal sulfide accumulations in the complex mean 0 7ppm and 3 disseminated sulfide minerals that occur above the basal accumulations in the stillwater complex mean 1 2ppm the isotopic composition of sulfur in the basal sulfide accumulations indicates that assimilation of substantial amounts of sulfur from iron formation may have occurred the low tenor of the basal sulfide ores is also consistent with assimilation of sulfur into a limited volume of magma near the base of the intrusion

physical models of magmatic intrusion during thrusting to investigate how magma rises through the brittle upper crust in a context of compressional tectonics we have performed experiments on scaled physical models powdered silica having a cohesion of 300 pa and an angle of internal friction of 38 was used to represent brittle crust a vegetable oil with a newtonian viscosity of 10 2 paae sa t 50c was used to represent magma a moving piston shortened the models in a box while oil was injected steadily at the base on cooling to room temperature the oil solidified the resulting intrusions were thin sills dykes and laccoliths their shapes and emplacement modes depended on the ratio r between rates of shortening and injection from shapes and orientations of intrusions we infer that hydraulic fracturing was one mechanism of emplacement unconsolidated intrusions strongly influenced thrust formation on the basis of our experiments we suggest that magmas in orogenic belts can rise along thrust faults

sulfur isotope exchange and metal enrichment in the formation of magmatic cu ni pge deposits sulfur isotope measurements provide a useful means of evaluating the source of sulfur in magmatic cu ni pge deposits although low sulfur pge rich deposits e g j m reef of the stillwater complex merensky reef of the bushveld complex show little or no isotopic evidence for the involvement of crustally derived sulfur several high sulfur deposits e g noril sk duluth complex show strong evidence for contamination of mafic magmas by sulfur of crustal origin processes such as magma degassing or redox changes accompanying crystallization are ineffective in significantly modifying sulfur isotope signatures of sulfides that crystallize from immiscible liquids that exsolve from the mafic magmas for this reason 34s values are primarily dependent on sulfur source characteristics and variations can often be treated using a two component mixing approach however deposits such as those at noril sk and voisey s bay that appear to have formed in magma conduit systems present another possibility for modification of sulfur isotope signatures these deposits are thought to have been upgraded in metal tenor by reaction with uncontaminated mantle derived melts that traversed the accumulated sulfide on their way to the surface our calculations illustrate the efficiency of reactions that involve the exchange of fe for cu ni or pge in producing increases in the metal tenor of the accumulated sulfide sulfur isotope exchange between the crustally contaminated sulfide ore accumulation and sulfur of mantle origin may accompany chalcophile element exchange and is capable of masking or limiting the degree of isotopic evidence for the initial stage of ore genesis that involved crustal sulfur sulfur isotope exchange is distinct from the addition of sulfur in the form of immiscible sulfide droplets from a sulfide saturated magma to the accumulated sulfide although both additive mixing and exchange can lead to similar isotopic effects stable isotopic exchange provides a mechanism in addition to additive mixing that may lead to decoupling of different isotopic systems

the disturbed zone around tunnels in jointed rock masses this paper presents an investigation of the effect of joint spacing on the size and shape of the disturbed zone around a tunnel a two dimensional distinct element simulation program udec was used to model the tunnel excavation in a simply jointed rock mass and an analytical method verified the numerical results rock masses were studied ranging from intact rock to heavily jointed rock with rock spacing less than 1 16 of the tunnel diameter four udec models were used to study the effect of joint spacing the disturbed zone was divided into 1 the failure zone where loose rock blocks fall into the tunnel 2 the open zone where joints open and 3 the shear zone numerical results were obtained for disturbed zones for different block sizes boundary conditions stress state and joint orientation an analytical study was made of the shear zones around the tunnel the failure zone is found in the immediate vicinity of the tunnel local block falling occurs there only when the joint spacing is small enough to create loose blocks the open zone enlarges with reducing joint spacing and stress ratio the shear zone around the tunnel can extend a considerable distance from the tunnel boundary the division of the disturbed zone into these zones makes it easier to study short term and long term tunnel instability

acid mine drainage at cerro rico de potosi i unabated high strength discharges reflect a five century legacy of mining intensive mining and processing of ag sn pb and zn ores have occurred in various locations within and around the city of potosi bolivia since 1545 surface and subsurface waters stream sediments and soils are contaminated with various ecotoxic metals in the headwaters of the economically vital yet highly impacted upper rio pilcomayo watershed previous studies have documented downstream trace metal contamination however not addressed specific sources the amd discharges identified in this study help link downstream pollution to primary origins the majority of amd would be considered high strength due to metal concentrations and acidity orders of magnitude greater than typical amd discharges from both operating and abandoned portals as well as tailings related deposits displayed a high degree of heterogeneity with total metal concentrations ranging from 0 11 to 7480 0 022 to 889 0 0006 to 65 3 0 001 to 310 0 12 to 72 100 0 3 to 402 0 012 to 34 8 and 0 24 to 19 600 mg l of al as cd cu fe mn pb and zn respectively net acidity and ph ranged from 10 to 246 000 mg l as caco3 equivalent and 0 90 6 94 standard units respectively data were gathered during two sampling events centered around the most extreme periods of the dry and wet seasons of one water year loadings to local streams were marginally greater for most metals in the wet season if observed loadings are historically representative cerro rico amd has contributed thousands of tonnes of ecotoxic metals to the upper rio pilcomayo over the last five centuries metal and hydrogen ion concentrations in the majority of amd sampled were several orders of magnitude above discharge limits set by the bolivian government yet no action has historically or contemporarily been taken

u pb shrimp geochronology of th poor hydrothermal monazite an example from the llallagua tin porphyry deposit bolivia in situ u pb shrimp analysis of hydrothermal monazite virtually free of th and poor in u 0 2 ppm th 40 103 ppm u from the world class llallagua tin porphyry deposit in bolivia defines a mineralization age of 23 4 2 2 ma mswd 0 48 confirming earlier k ar sericite alteration age data these ages are however in contrast with a weighted mean single crystal 207pb 206pb evaporation age of 39 3 6 0 ma and a related pb pb inverse isochron age of 42 4 4 0 ma mswd 0 66 on zircon from a post porphyry dike as well as with an earlier single crystal sm nd apatite isochron age our data points to a significant time gap between emplacement of the ore hosting porphyry intrusion magmatism and its hydrothermal overprint tin mineralization suggesting long lived magmatic hydrothermal activity in this part of the andean back arc crust the decoupling of porphyry magmatism and hydrothermal activity may explain the unusual occurrence of relatively little fractionated felsic rocks together with extensive tin mineralization our study demonstrates the usefulness of the application of the u pb shrimp method to direct age determination of ore mineralization using th poor hydrothermal monazite even when dealing with geological young events the common assumption of synchronous magmatism and hydrothermal ore formation in porphyry systems may not always be warranted

geology geochemistry and 40ar 39ar ages of the cerro millo epithermal high sulfidation gold prospect southern peru abstract the high sulfidation system of cerro millo is hosted in a late miocene andesitic paleo stratovolcano in the high andes of southern peru very pronounced advanced argillic alunite silicic and argillic kaolinite and smectite alteration characterizes the central part of the hydrothermal system propylitic alteration is developed in a 3 to 4 km wide outer halo abundant alunite occurs as hypogene acicular crystals and very fine grained aggregates the latter formed during near surface steam heated overprinting hypogene alunite has an ar ar plateau age of 10 8 0 9 ma 2 and is synchronous with the andesitic volcanism ar ar on biotite 11 0 0 5 ma a second ill defined alunite age plateau of 8 0 0 9 ma is probably related to steam heated overprint and points to major erosion in between both hydrothermal events telescoping is also evident by a series of silicified horizons which mark the paleo groundwater table these units have elevated mercury antimony and arsenic levels late phase barite occurs in some structurally controlled advanced argillic altered envelopes the hypogene alteration mineralogy points to temperatures at 250 c hydraulic fracturing and steam heated overprinting suggest a shallow boiling environment slight gold enrichment is observed in the lowermost exposed parts of the system

acid mine drainage at cerro rico de potosi ii severe degradation of the upper rio pilcomayo watershed ag pb sn and zn ores have been intensively mined and processed at cerro rico de potosi bolivia since 1545 acid mine drainage amd and mineral processing plant effluent are prime sources of water contamination in the headwaters of the upper rio pilcomayo watershed streams receiving amd drainage from the slopes of cerro rico and surrounding landscapes were sampled during the dry july august 2006 and wet march 2007 seasons of one water year in stream waters contained total metal concentrations of up to 16 mg l as 4 9 mg l cd 0 97 mg l co 1 100 mg l fe 110 mg l mn 4 1 mg l pb and 1 500 mg l zn with ph ranging from 2 8 to 9 5 amd impacted streams contained elevated concentrations of the same major ecotoxic constituents present in amd discharges at concentrations orders of magnitude greater than in those streams unimpacted by amd many of the amd impacted water bodies are more degraded than class d of the bolivian receiving water body criteria rendering them unfit for domestic or agricultural use natural attenuation is insufficient to render waters safe for use however some of these waters are currently being utilized for irrigation and livestock watering the data indicate that historic and current mining activities have transformed these key natural resources into potential human and environmental health hazards

multiscale insights into borehole instabilities in high porosity sandstones this paper presents a multiscale analysis of the classical borehole instability problem in high porosity sandstones using a hierarchical multiscale approach a rigorous two way message passing coupling of finite element method and discrete element method is employed where the finite element method is employed to solve a boundary value problem and the constitutive material responses required at each of its integration points are derived by the discrete element method solution to an embedded representative volume element instead of by using an assumed phenomenological model we employ this multiscale approach to examine the successive failure of a borehole subjected to gradually decreased support stresses at the inner borehole wall or increased far field stresses to reproduce the material behavior of high porosity sandstone representative volume elements with a high porosity structure and interparticle bonds are generated it is found that stress concentration triggers the initial failure at the borehole wall and the subsequent failure in form of deformation bands is driven by further stress concentration ahead of the band tips the failure pattern around the borehole varies with initial stress state and global loading path the comparison of the local stress paths of the initial failure points in various cases reveals that the change of failure pattern from shear failure to mixed mode failure and to compaction failure is dominated by the increased mean stress cross scale parametric studies show that the failure mode changes from clear compaction bands to multiple arrays of shear bands by changing the high porosity specimen to a low porosity one the increase in cohesion strength expands the yield locus and enlarges the critical mean stress between different failure patterns hence the failure mode may change from compaction failure to mixed mode failure or from mixed mode failure to shear failure due purely to the increased cohesion strength the qualitative comparisons indicate greater length larger area and more severe damage of the diametrically opposite failure with the increase in minor principal stress 0 and principal stress ratio the diametrically opposite failure mode is found a good indicator for 0 direction but it may also occur under hydrostatic far field stress due to material anisotropy

unabated acid mine drainage from cerro rico de potosi bolivia uncommon constituents of concern impact the rio pilcomayo headwaters intensive mining and processing of the polymetallic sulfide ore body of cerro rico de potosi bolivia has occurred since 1545 leading to severe degradation of surface and subsurface waters stream sediments and soils at the headwaters of the economically vital yet highly impacted rio pilcomayo previous studies have documented extremely elevated concentrations of a limited suite of metals in local waterways from acid mine drainage amd terrestrial and in stream tailings and ore processing plant discharges however contamination from a wider variety of ecotoxic metals metalloids was considered likely due to the highly mineralized polymetallic nature of the ore body to screen for this broader range of ecotoxic elements in amd and receiving streams data were gathered during two sampling events timed for the most extreme periods of the dry and wet seasons of one water year concentrations of ag b ba mo sb se sn and v in amd and receiving streams were greater than bolivian discharge limits and receiving water body guidelines as well as international agricultural use standards locally results indicate that contamination from mining cerro rico has a larger scope than previously thought and underscore the importance of remediation globally the results raise the possibility that other mining regions could have unquantified hazards from overlooked ecotoxic elements and that screening for a broader range of contaminants may be warranted

transient poroelastic response of equivalent porous media over a mining panel a transient poroelastic formulation based on the biot consolidation theory is presented to study the impact of underground longwall mining of coal on strata deformation and the subsequent modification of hydrogeological properties a finite element model is used where the geometric conditions may be complex the important parameters in this model are identified as pore pressure ratio 1 and relative compressibility 2 with the latter exerting far greater influence on the resulting fluid pressure field than the former this study indicates that the fluid pressure distribution is strongly controlled by the stress distribution or the strata deformation induced by mining has a great impact on the change in hydrogeological environment in the mining region

compressibility of porous rocks four compressibilities are defined for porous rock relating changes in the bulk and pore volumes to changes in the pore and confining pressures using a micromechanical theory based on classical linear elasticity three relations are found between these compressibilities two of these relations are verified experimentally for berea and bandera sandstone bounds are derived for these compressibilities involving only the porosity and the elastic moduli of the rock forming minerals for the strains to be unique functions of the stresses the compressibilities must be functions only of the difference between the confining and pore pressures this dependence is verified for berea bandera and boise sandstone while the strains cannot be expressed as functions of an effective stress regardless of how it is defined the elastic volumetric behavior of porous rocks can be studied through tests conducted at zero pore pressure

insights into ore genesis of ni cu pge sulfide deposits of the noril sk province russia evidence from copper and sulfur isotopes abstract combined cu s isotope data for ni cu pge sufides from the economic deposits hosted in the noril sk 1 talnakh and kharaelakh intrusions from the subeconomic deposits in the chernogorsk zub marksheider and vologochan intrusions and from the non economic occurrences in the nizhny talnakh intrusion in the noril sk province russia and the j m reef in the stillwater complex usa are presented for the first time most sulfide samples including the ores from the talnakh intrusion and the stillwater complex have a restricted range of 65 cu values from 1 1 to 0 whereas those from the kharaelakh and noril sk 1 intrusions have more variable 65 cu values of 2 3 to 1 0 samples from the three economic deposits in the noril sk province have distinct mean 65 cu values of 1 56 0 27 for kharaelakh 0 55 0 41 for talnakh and 0 23 0 28 for noril sk 1 consistent with those for carbonaceous chondrite and iron meteorites the variation of 65 cu values is interpreted to represent a primary signature of the ores although a magmatic fractionation of cu isotopes and or assimilation from an external source cannot be ruled out particularly in the case of the kharaelakh ores collectively the eight investigated intrusions have an overall sulfur 34 s range from 0 7 to 13 8 disseminated and massive ores of the same intrusion have similar 34 s values the disseminated and massive ores of the khraelakh intrusion have a mean 34 s value of 12 57 0 54 and 12 76 0 42 respectively which are distinctly different from those of the disseminated 10 97 0 44 and massive ores 10 88 0 77 of the talnakh intrusion the different 34 s values of the intrusions may be attributed to sulfur fractionation in deep seated magma chambers rather than at shallow levels or the present emplacement site the mean 34 s value of the disseminated ores from the subeconomic ni cu pge deposits and non economic occurrences spans over 10 5 averaging 0 39 with a standard deviation of 1 55 for the highly contaminated zub marksheider rocks 6 38 1 89 for nizhny talnakh 6 90 1 17 for vologochan and 10 86 0 43 for chernogorsk samples from the zub marksheider intrusion have mantle like s isotope signatures consistent with a deep seated origin although individual mineralized instrusions have a restricted range the overall trend they define of falling 34 s with rising 65 cu values might be a useful indicator of the economic potential of ni cu pge sulfide deposits

archaean hydrothermal zircon in the abitibi greenstone belt constraints on the timing of gold mineralisation abstract hydrothermal zircons have been found in archaean mesothermal au veins and altered wallrock selvages at val d or in the abitibi greenstone belt of canada the zircons are paragenetically associated with vein quartz tourmaline mica carbonate scheelite pyrite and gold zirconium mobility and the consequent occurrence of hydrothermal zircon may be associated with the intense tourmalinisation characteristic of archaean gold deposits in this district the shrimp ion microprobe has been used to analyse hydrothermal zircons from four separate mines spatially associated with the bourlamaque batholith and has yielded ages constraining formation of the au bearing vein systems to within 20 ma of emplacement of the pluton the ion microprobe data reveal multiple stages of hydrothermal zircon growth in the vein systems contemporaneous with the regional metamorphic peak and late kinematic activity along regional structures younger including proterozoic ages previously obtained for the veins using other minerals and isotopic schemes must reflect either alteration or renewed mineral growth during much later reactivation of fluids along the same structures up to 400 ma after initial formation of the veins

hydrothermal zircon and baddeleyite in val d or archean mesothermal gold deposits characteristics compositions and fluid inclusion properties with implications fortiming of primary gold mineralization zircon and baddeleyite occur within quartz tourmaline veins at four gold deposits in the val d or district of the archean abitibi southern volcanic zone host rocks have experienced intense metasomatic enrichment of zr hf y and rare earth elements the zircons contain primary inclusions of quartz tourmaline pyrite albite k mica scheelite and gold and gold occurs in primary fluid inclusions in zircons magmatic zircons in host rocks do not have this suite of inclusions consequently a wall rock inheritance model for the vein zircons is implausible compositionally the zircons feature pronounced interzone and intergrain variations of hf y yb th and u and sporadic anomalous ce contents of 1100 ppm distinct from magmatic counterparts two principal types of primary fluid inclusion occur in the vein zircons type 1 h2o co2 inclusions have low salinities variable quantities of co2 and homogenization temperatures of 260 380 c and type 2 co2 rich inclusions contain minor h2o and ch4 the vei

sulfur and oxygen isotopic evidence of country rock contamination in the voisey s bay ni cu co deposit labrador canada abstract the emplacement of basaltic magma into sulfide bearing country rocks provides a favorable geologic environment for magmatic sulfide ore formation related either directly to assimilation of country rock sulfur or indirectly to a depression of sulfide solubility caused by assimilation induced changes in magma composition pelitic country rocks of the proterozoic tasiuyak gneiss in the area of the voisey s bay ni cu co deposit contain sulfidic layers that may have provided sulfur to basaltic magmas during emplacement of the voisey s bay intrusion sulfur isotopic compositions of the tasiuyak gneiss range from 0 9 to 17 0 values typical for sulfides produced via bacterial sulfate reduction in an open marine environment archean gneisses in the area contain low amounts of sulfide and are less likely to have served as a source of externally derived sulfur sulfur isotopic compositions of the sulfide minerals from the voisey s bay deposit show consistent variations both spatially and with rock types disseminated and massive sulfides show a decrease in 34 s to the west with values typically between 0 and 2 in the eastern deeps ovoid and discovery hill zone and between 2 and 4 in the reid brook zone 34 s values of the mushua intrusion to the north and the normal troctolite in the eastern deeps are more positive ranging between 0 5 and 1 8 this range is taken to represent the isotopic composition of primary mantle derived sulfur in the area because the mushua intrusion and normal troctolite show the least geochemical evidence for contamination by country rocks sulfur isotopic data from the reid brook zone are consistent with up to a 50 sulfur contribution from the tasiuyak gneiss correspondingly lower proportions are indicated for the eastern portion of the deposit where country rocks are predominantly low sulfide enderbitic and quartzofeldspathic gneisses oxygen isotopic values of gneiss fragments in the basal breccia sequence and feeder breccia suggest that the assimilation process involved a greater proportion of high 18 o contaminant to the west 18 o values of the tasiuyak gneiss 5 9 to 14 0 enderbitic gneiss 6 4 to 8 7 and archean quartzofeldspathic gneiss 9 5 to 9 7 are consistent with an increased proportion of tasiuyak gneiss contaminant to the west isotopic data strongly indicate that sulfur from the tasiuyak gneiss has been involved in ore deposition at the voisey s bay deposit however sulfur and oxygen isotopic data also strongly suggest that the addition of externally derived sulfur was not the sole process responsible for mineralization and that assimilation of both proterozoic and archean country rocks played a key role in depressing sulfide solubility prior to sulfide localization via dynamic physical mechanisms

borehole stability analysis accounting for anisotropies in drilling to weak bedding planes abstract borehole instabilities pose significant challenges to drilling and completion operations particularly in regions with weak bedding planes and pre existing fractures where formations have strong anisotropies the bedding planes rock anisotropy and their impacts on horizontal stresses are considered in the proposed model to improve borehole stability modeling this improved model enables to calculate borehole failures and minimum mud weight along borehole trajectories with various drilling orientations versus bedding directions laboratory test data of rock compressive strengths are analyzed and a new correlation is developed to allow for predicting uniaxial compressive strengths in weak rocks from sonic velocities time dependent rock compressive strength is also examined to analyze the wellbore failure evolution with time the slip failure gradient in the weak planes is derived which can be used to model wellbore sliding shear failure in the planes of weakness the mud weight applied to prevent borehole shear failures in both intact rocks and ones with weak bedding planes can be obtained from the proposed model

duration of magmatic hydrothermal and supergene activity at cerro rico de potosi bolivia new high precision 40ar 39ar dating of sanidine and biotite from two rhyolitic domes and an ignimbrite combined with existing fission track data and a hydrothermal sericite age suggests that the world class ag deposit at cerro rico was emplaced during a protracted period of magma related hydrothermal activity beginning at 13 77 0 03 ma and continuing for at least 0 2 m y this may have been sustained by a large single injection or repeated injections of fractionated ag enriched magma into a high level magma chamber k ar dating of alunite indicates that supergene oxidation had begun by about 13 5 ma soon after dome emplacement and progressed semicontinuously for at least 7 5 m y this oxidation while not leading to significant enrichment has significantly enhanced the economic viability of the disseminated part of the orebody

sources of lead in the san cristobal pulacayo and potosi mining districts bolivia and a reevaluation of regional ore lead isotope provinces new lead isotope data on ores crustal rocks and leachates of crustal rocks combined with data in the literature provide important new constraints on the sources of ore metals in southwest to south central bolivia including the very large recently discovered silver zinc deposit at san cristobal the pulacayo polymetallic district and the giant potosi silver tin base metal deposit lead isotope ratios of ores and igneous rocks from the san cristobal deposit and from paleozoic and cretaceous sedimentary rocks are compared with published data on high grade middle proterozoic metamorphic basement rocks these data constrain the major source of lead and by inference of other ore metals at san cristobal to be the metamorphic basement rocks leaching experiments on samples of paleozoic and cretaceous sedimentary rocks show that the easily leachable lead from these rocks is much less radiogenic than the whole rock compositions however lead isotope ratios of both whole rocks and leachates of these upper crustal rocks are too radiogenic for them to be major sources of ore lead at san cristobal lead isotope ratios of ores from pulacayo and potosi are similar to each other and lie within the range of paleozoic and cretaceous sedimentary whole rock compositions leaching of pb from the sedimentary rocks cannot explain the isotopic compositions of the pulacayo and potosi ores and the isotopic homogeneity of the potosi ores also argues against mixing of lead from diverse sources in the hydrothermal system lead from the sedimentary rocks may have been incorporated by magmatic assimilation followed by extraction of ore metals from the resulting magma lead isotope ratios of san cristobal ores are different from those of pulacayo potosi and other deposits to the east but resemble the compositions of ores and volcanic rocks in western bolivia on this basis we identify a new ore lead isotope province extending from san cristobal northward across the eastern altiplano and into southern peru this province is coincident with but smaller than the extent of the proposed arequipa antofalla metamorphic basement craton the degree of incorporation of ore metals from the metamorphic basement appears to depend on the timing and or location of the mineralizing event ore deposits in the northern part of province iv formed before the thickening of andean crust beginning around 20 ma and incorporated minor amounts of metals from the metamorphic basement younger deposits farther to the south contain major to dominant components of basement lead

fully coupled analysis of interaction between the borehole and pre existing fractures abstract the coupling of rock and thermal stresses along with fluid pressure are particularly important in fractured rock masses since stress induced changes in permeability can be large and irreversible under perturbations resulting from various natural and induced activities a new method is presented to model fracture permeability changes during drilling in fractured rocks the approach includes finite element method fem for fully coupled thermo poroelastic analysis of stress distribution around borehole and displacement discontinuity method ddm to model fracture deformation three cases of overbalanced underbalanced and balanced drilling fluid pressure conditions are employed the application of the approach illustrates that the maximum variation of aperture occurs near to the borehole and become negligible at large distances away from the borehole it was shown that mechanical stresses caused by excavation of the rock contribute to short time while fluid pressure and thermal stresses are responsible for long term permeability variation of fractures the results show that the fracture permeability decreases with depletion of the fracture and the rock matrix while increases with pressurization and cooling of fracture during overbalanced condition the fracture permeability reduces during balanced fluid pressure condition within a short time then enhances by cooling of the rock surrounding the borehole

boron isotope composition of melt inclusions from porphyry systems of the central andes a reconnaissance study quartz hosted melt inclusions from latite dykes of the eocene el salvador copper porphyry system in northern chile display wide ranges in both boron concentration 15 155 p p m b and isotope composition 11b 7 to 12 n 10 likely reflecting slab derived fluid input from seawater altered oceanic crust in contrast the major miocene tin silver and tin porphyry systems in the bolivian back arc region cerro rico de potosi chorolque llallagua have distinctly different melt inclusion compositions with 11b of 11 4 2 7 n 10 and magmatic boron enrichment up to several hundred p p m b the seawater signature in the el salvador melt inclusions explains the oxidized mineral assemblage of the copper porphyry system as opposed to the more reduced nature of the bolivian tin porphyry systems which reflect intracrustal melting of pelitic rocks

well bore breakouts and in situ stress the detailed cross sectional shape of stress induced well bore breakouts has been studied using specially processed ultrasonic borehole televiewer data we show breakout shapes for a variety of rock types and introduce a simple elastic failure model which explains many features of the observations both the observations and calculations indicate that the breakouts define relatively broad and flat curvilinear surfaces which enlarge the borehole in the direction of minimum horizontal compression this work supports the hypothesis that breakouts result from shear failure of the rock where the compressive stress concentration around the well bore is greatest and that breakouts can be used to determine the orientation of the horizontal principal stresses in situ

poroelastic response of a borehole in a non hydrostatic stress field abstract this paper deals with the solution of the stress displacement and pore pressure field induced by the drilling and or the pressurization of a vertical borehole the rock which is permeated by fluid is assumed to behave as a poroelastic material with compressible constituents following the biot theory the analytical solution is derived in the laplace transform space and is transformed to the time domain using an approximate numerical inversion technique the solution reveals a full range of coupled poroelastic effects which provide potential mechanisms for delayed borehole instability and shear fracture initiation inside the rock

uthpb zircon dating of the 13 8 ma dacite volcanic dome at cerro rico de potosi bolivia abstract the temporal relationship between the extrusion of the miocene dacite volcanic dome at cerro rico de potosi bolivia and the associated ag sn mineralization has an important bearing on the heat and metal sources for this world class mineral deposit the highly altered nature of the volcanic rock however has prevented previous efforts from definitively determining the emplacement age of the dome rather than subsequent episodes of alteration the present study uses u th pb dating of sparse zircon contained in the dacite to demonstrate that at most only several hundred thousand years separate dome emplacement from main stage mineralization the dacite contains a morphologically heterogeneous population of zircon with considerable variation in size shape and color preliminary analyses revealed the presence of a significant component of inherited zircon most likely as older cores that were overgrown by rims during the crystallization of the dacite magma morphological distinctions shape size color within the zircon population provided the basis for separating the zircon into fractions that reduced or enhanced the inherited component in composite grains by analyzing the tips of zircon with very large length to width ratios it was possible to constrain the crystallization age of the dacite to 13 8 0 2 ma the dispersion of data on a concordia diagram indicates at least two ages of inherited zircon one mesozoic associated with yellow grains and another proterozoic associated with colorless grains such xenocrystic components have useful petrogenetic implications for the source and travel path of the magma

hot dry rock geothermal reservoir testing 1978 to 1980 abstract the phase i hot dry rock geothermal energy reservoirs at the fenton hill field site grew continuously during run segments 2 through 5 january 1978 to december 1980 reservoir growth was caused not only by pressurization and hydraulic fracturing but also by heat extraction and thermal contraction effects reservoir heat transfer area grew from 8000 to 50 000 m2 and reservoir fracture volume grew from 11 to 266 m3 despite this reservoir growth the water loss rate increased only 30 under similar pressure environments for comparable temperature and pressure conditions the flow impedance a measure of the resistance to circulation of water through the reservoir remained essentially unchanged and if reproduced in the phase ii reservoir under development could result in self pumping geochemical and seismic hazards have been nonexistent in the phase i reservoirs the produced water is relatively low in total dissolved solids and shows little tendency for corrosion or scaling the largest microearthquake associated with heat extraction measures less than minus one on the extrapolated richter scale

effect of anisotropy and hydro mechanical couplings on pore pressure evolution during tunnel excavation in low permeability ground abstract continuous monitoring around drifts excavated in the underground research laboratory url of the french national radioactive waste management agency andra in callovo oxfordian claystone revealed the development of a fractured zone extensional and shear fractures induced by the excavation moreover the field monitoring during the excavation showed an anisotropic pore pressure field around the drifts with zones of marked overpressure for drifts parallel to the horizontal major principal stress even though the initial stress state in the drift section is quasi isotropic the pore pressure evolution and the mechanical response are anisotropic these observations suggest that the intrinsic anisotropy of the material plays a key role in the response of rock formation a poroelastic analysis of the pore pressure change induced by drift excavation is performed the principal objective is to simulate the main trends of the pore pressure evolution with a simple model taking into account the elastic anisotropy of the material it is shown that depending upon the permeability and the degree of anisotropy of the rock strong overpressures can be induced by the excavation furthermore it is observed that the distribution of overpressures around drifts depends also on the drifts orientation finally an analysis of the onset of failure shows the key role of the hydro mechanical coupling on the extension of the failed zone around the drifts

stress orientations from borehole wall fractures with examples from colorado east texas and northern canada azimuthally aligned breakouts in oil wells are explained as shear fractures in the zone of amplified stress difference near the borehole in a stress field having unequal horizontal principal stres

multiple s isotope studies of the stillwater complex and country rocks an assessment of the role of crustal s in the origin of pge enrichment found in the j m reef and related rocks abstract the j m reef in the stillwater complex of montana is the highest grade pd deposit in the world averaging approximately 14 ppm pd one suggestion for the origin of the enrichment centers on the sequestration of metals in staging chambers via immiscible sulfide liquids that formed in response to the addition of crustal s because 34s values alone are often insufficient to document in igneous rocks the assimilation of s from archean sedimentary rocks a combined study of both 33s and 34s values and computed 33s values was undertaken 34s and 33s values of cordierite bearing hornfels as well as massive sulfides in the metasedimentary rocks of the contact aureole can be divided into two populations one is characterized by anomalous positive 33s values from 0 to 0 32 and 34s values from 0 5 to 1 6 whereas the other is characterized by anomalous negative 33s values from 0 to 0 22 and 34s values from 1 5 to 5 8 34s and 33s values for rocks from the basal series of the complex range from 0 9 to 3 0 and 0 01 to 0 18 respectively the anomalous s isotope ratios indicate that for some samples at least 50 of the their s was of crustal origin in contrast most of the rocks from the j m reef are characterized by 34s and 33s values between 1 3 and 1 8 and 0 06 to 0 06 respectively these values deviate only slightly if at all from those thought to be representative of mantle s and suggest that i contamination was not an important process for pge enrichment in the complex ii the contaminant was also characterized by near 0 33s values or iii s isotopic exchange in the staging chambers was an important process upgrading of pge values via reaction between sulfide in the chamber and uncontaminated mantle derived magma may also have resulted in the conversion of s isotopic ratios that may have been indicative of crustal s addition to mantle like values rare 34s values up to 7 2 in the j m reef and 8 2 in chromitite from the ultramafic series provide strong evidence for localized assimilation of crustal s which did not experience isotopic homogenization the s isotopic ratios of the j m reef are non unique in terms of a specific genesis as they may be interpreted in either a downward sulfide collection model or an upward fluid transport model

the age and thermal history of cerro rico de potosi bolivia cerro rico de potosi bolivia is the world s largest silver deposit and has been mined since the sixteenth century for silver and for tin and zinc during the twentieth century together with by product copper and lead the deposit consists primarily of veins that cut an altered igneous body that we interpret to be a dacitic volcanic dome and its underlying tuff ring and explosion breccia the deposit is compositionally and thermally zoned having a core of cassiterite wolframite bismuthinite and arsenopyrite surrounded by a peripheral lower temperature mineral assemblage consisting principally of sphalerite galena lead sulfosalt and silver minerals the low temperature assemblage also was superimposed on the high temperature assemblage in response to cooling of the main hydrothermal system both the dacite dome and the ore fluids were derived from a larger magmatic hydrothermal source at depth the dome was repeatedly fractured by recurrent movement on the fault system that guided its initial emplacement the dome was extruded at 13 8 0 2 ma 2 based on u th pb dating of zircon mineralization and alteration occurred within about 0 3 my of dome emplacement as indicated by a 40ar 39ar date of 13 76 0 10ma 1 for sericite from the pervasive quartz sericite pyrite alteration associated with the main stage high temperature mineralization the last thermal event able to reset zircon fission tracks occurred no later than 12 5 1 1 ma 1 as indicated by fission tract dating minor sericite and magmatic steam alunite veins were episodically formed around 11 ma and between 8 3 and 5 7 ma the younger episodes occurring at the time of extensional fracturing at cerro rico and wide spread volcanism in the adjacent los frailes volcanic field none of these younger events appear to be significant thermal mineralizing events the exceptionally flat thermal release pattern of 39ar from sericite and the results of the fission tract dating of zircon show that none of the younger events was hot enough and lasted long enough to cause significant loss of ar or annealing of zircon fission tracks u th pb dating of zircon cores indicates a precambrian progenitor for some zircons and ree analyses of dated samples of hydrothermally altered dacite show the presence of a prominent positive eu anomaly which constrains interpretations of the origin and evolution of the magmatic hydrothermal system

a study of the trace sulfide mineral assemblages in the stillwater complex montana usa the sulfide assemblages of the stillwater complex away from the well studied ore zones are composed mainly of variable proportions of pyrrhotite chalcopyrite pentlandite and pyrite excluding vein assemblages and those affected by greenschist and lower temperature alteration the majority can be classified into two broad assemblages defined here as pristine multiphase often globular in shape or volatile bearing multiphase high temperature volatile rich minerals such as biotite hornblende or an unmixed calcite dolomite assemblage the volatile bearing assemblages are mainly found within and below the j m reef where native copper and sphalerite are also locally present pristine sulfides are found throughout the stratigraphy both groups can be affected by apparent s loss in the form of pyrite being converted to magnetite and chalcopyrite to a cu fe oxide delafossite with little to no silicate alteration an upward trend from pentlandite rich to pyrrhotite rich to pyrite rich assemblages is observed in the footwall rocks in upper gn i and the same trend repeats from just below the reef and continues into the overlying n ii and gn ii modeling suggests that the sulfide ni in the peridotite zone is largely controlled by silicate ni when taken together observations are most readily explained by the remobilization of selected elements by a high temperature fluid with the apparent loss of s cu ni this could concentrate ore metals by vapor refining eventually producing a platinum group element enriched sulfide ore zone such as the j m reef