cooling and uplift patterns in the lepontine alps south central switzerland and an age of vertical movement on the insubric fault line 96 new fission track ft apatite and zircon k ar and rb sr biotite and muscovite ages are presented for 19 samples mainly acid gneisses from a 40 km traverse through the lepontine alps in the maggia valley south central switzerland plotting measured mineral ages against assumed system closure temperatures yields cooling rates for each sample the entire profile shows a fairly uniform late neogene recent mean uplift rate of 0 5 mm a confirmed by a gradient of ft apatite age with elevation cooling from higher temperatures occurred earlier in the south where uplift rates of 2 2 mm a in the steep belt root zone indicate 9 km early miocene uplift of the northern pennine block this uplift started before 23 ma and is interpreted as resulting from a major phase of backthrusting along the insubric line and as dating the formation of the mylonite belt estimated cooling rates constrain the timing of lepontine mid tertiary metamorphism 3 schematic models are proposed which also consider published rb sr k ar mica and hornblende and u pb monazite ages slow cooling differential initial heating and subsequent cooling of different parts of the central alps and post 38 ma cooling with syntectonic metamorphism at 27 ma are postulated as alternative interpretations of isotopic data and geologic evidence from extrapolation between k ar and rb sr mica ages and apatite ft ages 240 50 c is proposed as the closure temperature for the retention of fission tracks in zircon

the soledad caldera bolivia a miocene caldera with associated epithermal au ag cu pb zn mineralization the soledad caldera is a low volume nonresurgent ash flow type caldera located on the altiplano northwest of oruro in the central andes of bolivia the first stage in the evolution of the composite volcanic field was the intrusion of a cluster of small dacitic stocks around la joya on the southwest edge of the caldera at ca 15 ma these have related strong hydrothermal alteration and epithermal vein and disseminated au ag cu pb zn mineralization with minor as sb mn bi w and sn at 8 8 ma a line of small rhyolite domes formed along an outer ring fracture 15 km west of the caldera margin caldera collapse took place at ca 5 4 ma accompanied by the eruption of the soledad tuffs these make up a sequence of interbedded dacitic air fall and nonwelded ash flow tuffs they crop out in a ring around the nonresurgent caldera center which is elongated northwest and is 22 km 14 km 240 km 2 post collapse ring fracture volcanism formed the voluminous dacitic lavas of the esquentaque massif which overlie the tuffs on the south side of the caldera there is a zone of advanced argillic alteration in the lavas formed beneath a hot spring system the caldera is considered to be the volcanic manifestation of the episodic rise and emplacement of a batholith

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

a gis based method for evaluating sediment storage and transport in large mining affected river systems currently the evaluation of sediment storage and transport for large river systems with variable flow has proven to be a challenge that often requires complex numerical models to be applied and sometimes costly direct data acquisition to be made the present study proposes a low cost widely available gis based method through which an initial assessment can be made on areas of sediment storage transport and deposition as well as possible environmental risks that the accumulation of contaminated material may pose to riverine communities along the rivers channel the method mainly relies on satellite imagery and elevation data to devise a quick model of the channel the model has been successfully applied to study the bolivian sector of the well known mining contaminated rio pilcomayo the analysis shows that the portion of the channel from villamontes to d orbigny accounts for more than half of the total c 314 km2 sedimentation area of the pilcomayo in bolivia and that the most environmentally problematic area is centreed around puente sucre where agriculture is practiced on the contaminated floodplain combined with supplementary bathymetric data on the depth of the river in various points around the channel the method could offer further insight into the sediment fluxes and transport capacity of the pilcomayo in various sectors and could thus be successfully used to assess other large mining contaminated river channels around the world

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

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

pb isotopes define basement domains of the altiplano central andes detailed pb isotopic maps of the central andes based on 345 163 previously published 182 new analyses of ores volcanic rocks and their host rocks elucidate the gross structure of the basement and reveal that several isotopically distinct basement domains are juxtaposed in this region the data clearly show that most of the pb in central andean igneous and ore samples is derived from the local basement including pb in ore deposits of the bolivian tin belt some of the isotopic domain boundaries correspond to geologic structures and the residual gravity pattern as well as to metallogenic boundaries such as the western edge of the bolivian tin belt

genesis of acid sulfate alteration and au cu ag mineralization at summitville colorado the summitville au cu ag deposit occurs within a porphyritic quartz latite primarily in a zone of vuggy silica alteration characterized by leaching of all major elements except silica and iron vuggy silica is best developed between an elevation of 3 700 and 3 500 m where it occurs in irregular pipes and lenticular pods up to 70 m wide and is enclosed by up to 30 m of intense quartz alunite alteration the vuggy silica zones diminish in size with decreasing elevation and rarely exceed a width of 2 m below an elevation of 3 400 m these deep vuggy silica zones are enclosed by quartz kaolinite alteration with little or no alunite sulfide mineralization occurs primarily within the vuggy silica the sulfide assemblage changes with decreasing elevation from covellite luzonite or native gold to chalcopyrite tennantite sulfides occur lining voids or in microveinlets and are accompanied by minor kaolinitc but not by alunite gold also occurs in a near surface barite goethite jarosite assemblage that crosscuts the vuggy silica consideration of alunite kaolinitc and pyrite stability relations and of aluminum speciation at 250 degrees c indicates that a ph of 2 or below and a log f sub o 2 of 31 or 1 at 250 degrees c were necessary to produce vuggy silica alteration the chalcopyrite tennantite mineralization occurring at depth in the vuggy silica zone is indicative of less oxidizing and less acidic conditions the covellite luzonite or gold assemblage requires conditions intermediate between those necessary for vuggy silica alteration and for deep sulfide mineralization barite gold mineralization is interpreted as a near surface assemblage produced under highly acidic oxidizing conditions these textural and geochemical relationships indicate that sulfide and gold mineralization postdate the formation of vuggy silica alteration in the deposit and are not directly related to the highly acidic solutions necessary for this early acid leaching event the change from extreme acid leaching to copper sulfide and precious metal deposition may represent a transition from a magmatic vapor dominated to a liquid dominated system leading to more reducing and less acidic conditions during sulfide deposition

the pallacos of cerro rico de potosi bolivia a new deposit type pallacos are a new kind of ore deposit and are described here in detail for the first time they are found on the flanks of the cerro rico de potosi bolivia the world s largest silver deposit fundamentally pallacos are gravels whose clasts contain sufficient disseminated silver so as to be bulk minable the silver bearing clasts originated from the summit of cerro rico and are composed of dacite porphyry or air fall tuffs or arkosic sands of the caracoles formation that have been altered to vuggy silica or jasperoid analysis of facies bedforms depositional textures and clast sizes and percentages suggests that there are two types of pallacos each with a different depositional mechanism one kind the huacajchi type formed from multiple water transported debris flows the other kind the potosina type is interpreted as catastrophic landslides metal zoning clast alteration style and silver grade vary between the two pallaco types

thermochronology of economic mineral deposits dating the stages of mineralization at panasqueira portugal by high precision 40 39 ar age spectrum techniques on muscovite 40 ar 39 ar age spectrum dates for 13 muscovites have been used to reconstruct the thermal history thermochronology of the panasqueira portugal tin tungsten deposit a deposit spatially associated with a belt of hercynian plutons muscovite samples with an age difference as small as 2 2 m y 0 7 of the age are statistically distinct statistics are even better for comparison of multiple samples from separate events that is a difference of 0 9 m y 0 3 can be resolved in this approximately 300 m y old deposit the major tin and tungsten ore forming stages which are the oxide silicate stage the main sulfide stage and greisenization occurred between 296 3 or 0 8 1 sigma and 291 6 or 0 8 m y 1 sigma the first substage of the oxide silicate stage was a short lived thermal pulse at 296 3 or 0 6 m y the fluids responsible may have emanated from the known granite cupola the main sulfide stage was active at 294 5 or 0 9 m y as a slightly longer lived pulse with oldest evidence for this stage 295 8 or 0 6 m y coming from areas farthest away from the known cupola and youngest evidence 293 5 or 0 8 m y closest to the cupola a second substage of the oxide silicate stage occurred as a short lived thermal pulse at 292 9 or 0 7 m y synchronous with greisenization of the cupola and alteration of the silica cap at 292 1 or 0 4 m y the duration of activity of the oxide silicate stage the main sulfide stage greisenization and alteration of the silica cap based on the ages of all 13 muscovites was greater than 4 2 or 0 5 m y 1 sigma minor argon loss from all dated muscovites occurred during later reheating probably during the longer lived pyrrhotite alteration stage a single center the known cupola had a prolonged role and was the source for main sulfide stage oxide silicate stage ii greisenization and alteration of the silica cap and possibly oxide silicate stage i and the pyrrhotite alteration stage however a separate source for these latter two stages cannot be ruled out this study is an example of a new and powerful application of 40 ar 39 ar age spectrum dating of muscovite because of the high precision demonstrated in this study it is now possible to establish time constraints necessary for solving some of the long standing problems in economic geology beyond this the unique geologic situation of panasqueira has allowed us to quantify the thermal characteristics of muscovite published fluid inclusion data have been used to estimate a muscovite argon closure temperature of approximately 325 degrees c during rapid cooling or short reheating and a temperature of approximately 270 degrees c during slow cooling or extended reheating argon loss patterns displayed by all dated muscovites resulted from reheating after original closure the mechanism for this argon loss appears to have been argon transport by volume diffusion thus 40 ar 39 ar age spectrum dating of muscovite can be used to evaluate thermal conditions controlling argon diffusion as well as age duration and number of episodes of mineralization

manganese in monoclinic members of the epidote group piemontite and related minerals according to mayo 1932 who provided a brief historical review of the names used for piemontite the first researcher who described this mineral has been cronstedt in 1758 who named it rod magnesia in 1790 chevalier napione analyzed the sample described by cronstedt and termed it manganese rouge on the basis of his chemical data hauy designated the substance as manganese oxide violet silicifere in 1801 later in his traite de mineralogie hauy 1822 adopted the name proposed by cordier 1803 who first recognized the mineral as an epidote manganesifere the name piedmontite was proposed in 1853 by kenngott the basis of the type locality and more recently transformed into piemontite according to the standard guidelines for mineral nomenclature the name piemontite should be reserved to members of the ternary solid solution ca2al2 mn fe al si2o7 sio4 o oh that basically contains mn3 dominant at one site nonetheless the use of this name for any monoclinic manganiferous epidote group members showing the characteristic strong red yellow violet pleochroism is very common and probably convenient with special regard to petrographic purposes indeed the color of manganian i e mn3 bearing epidote or clinozoisite ranges to red to pinkish while manganoan i e mn2 bearing members do not exhibit the characteristic reddish hue the discredited name withamite was used to describe poorly manganiferous piemontite hutton 1938 yoshimura and momoi 1964 but corresponds on the basis of the current nomenclature to a manganian clinozoisite the name thulite sometimes erroneously used for pinkish clinozoisite should be reserved to mn3 bearing orthorhombic members in this chapter we focus on piemontite sensu stricto with the ideal formula ca2al2mn3 si2o7 sio4 o oh but also include for the reasons explained above those members of the clinozoisite epidote piemontite

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

fluid inclusion studies of the polymetallic hydrothermal ore deposits in bolivia homogenization temperature and salinity were determined for fluid inclusions in mostly quartz and partly sphalerite cassiterite and barite from the 28 tin polymetallic ore deposits in bolivia generally the homogenization temperatures and salinities of these fluid inclusions are comparatively high for ore deposits formed by cassiterite mineralization such as morococala and avicaya in the oruro district frequently indicating a temperature higher than 300 c and salinity higher than 20 equiv wt nacl particularly it is quite possible that tin deposits associated with the w bi and tourmaline mineralizations such as viloco and caracoles have been produced by such high temperature hypersaline fluid ranging up to 500 c and 56 equiv wt nacl similar to the porphyry copper type this feature reveals that the hydrothermal fluid related to the sn w bi mineralization may be of magmatic origin homogenization temperatures for the pb zn deposits with no tin minerals are low mostly ranging 170 300 c at the avicaya bolivar mining area in the oruro district as well as at the tasna and chocaya animas mining areas in the quechisla district temperature gradients consistent with the zonal distributions of ore minerals were confirmed

formation of epithermal sn ag zn vein type mineralization at the pirquitas deposit nw argentina fluid inclusion and noble gas isotopic constraints abstract the pirquitas sn ag zn deposit in northwestern argentina is thought to be an analogue to the miocene polymetallic epithermal sn ag deposits of the southern bolivian tin belt but little is known in detail about the origin and evolution of ore forming fluids at pirquitas this paper reports on a microthermometric study of fluid inclusions in quartz sphalerite ag sn sulfides and ag rich sulfosalts using transmitted near infrared and visible light combined with noble gas isotope analyses of fluids released from mineral separates the study focused on the vein hosted mineralization which formed during two major mineralization events whereby the first event i comprises two stages i 1 and i 2 all studied minerals exclusively contain aqueous two phase inclusions indicating that the ore forming fluids did not undergo two phase phase separation boiling salinity of fluid inclusions in i 1 quartz that precipitated along with pyrite and pyrrhotite ranges between 0 and 7 5 wt nacl equiv and homogenization temperatures th are between 233 and 370 c stage i 2 is characterized by abundant sn ag pb zn sulfides and a variety of ag rich sulfosalts fluid inclusions in stage i 2 ag sn sulfides have salinities up to 10 6 wt nacl equiv and th between 213 and 274 c the deposition of stage i 2 ore is likely related to a new pulse of saline magmatic fluids to the hydrothermal system the mineralization event ii deposited the richest ag ores at pirquitas colloform sphalerite and pyrargyrite deposited during event ii contain two phase aqueous fluid inclusions with homogenization temperatures between 190 and 252 c and salinities between 0 9 and 4 3 wt nacl equiv noble gas concentrations and isotopic compositions of ore hosted fluid inclusions were determined from crushing hand picked ore minerals from both mineralization events with one exception all samples yielded 3he 4he ratios between 1 9 and 4 1 ra which is within the range of published data from the volcanic arc and somewhat higher than typical values of meteoric water derived hot springs in the region this demonstrates a significant contribution of magmatic fluids to the pirquitas mineralization although no intrusive rocks are exposed in the mine region taking the noble gas evidence for a magmatic fluid source we interpret the trends of decreasing th and salinity values in fluid inclusions from events i and ii to represent waning of the magmatic hydrothermal system and or increased admixing of meteoric water to the magmatic fluids

precise determination of earth s center of mass using measurements from the global positioning system global positioning system gps data from a worldwide geodetic experiment were collected during a 3 week period early in 1991 geocentric station coordinates were estimated using the gps data thus defining a dynamically determined reference frame origin which should coincide with the earth center of mass or geocenter the 3 week gps average geocenter estimates agree to 7 13 cm with geocenter estimates determined from satellite laser ranging a well established technique the rms of daily gps geocenter estimates were 4 cm for x and y and 30 cm for z

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

resources of gold in phanerozoic epithermal deposits we have estimated earth s endowment of gold in phanerozoic epithermal deposits using a tectonic diffusion model which simulates the emplacement of deposits at a shallow crustal depth and their subsequent vertical tectonic migration in the crust the calculation was calibrated by least squares comparison of a calculated age frequency distribution to the age frequency distribution for 448 epithermal deposits of phanerozoic age results indicate that 17 percent of the epithermal deposits that formed through phanerozoic time remain in the crust today whereas 83 percent have been removed by erosion assuming a similar age distribution for all 1 181 epithermal deposits in our compilation indicates that 307 000 deposits formed throughout phanerozoic time that 63 000 of these remain in the crust and that 244 000 have been eroded grade and tonnage data of gold in 757 epithermal deposits in the compilation have an arithmetic average of 34 7 t and yield an estimate of 2 2 106 t of gold for epithermal deposits remaining in the crust using the model based vertical distribution of deposits remaining in the crust we estimate that epithermal deposits in the upper kilometer of the united states contain 2 9 104 t of gold adjusting for areas of outcropping volcanic rocks in each continent increases our estimate to 4 0 104 t these estimates are similar to the 1 7 104 t gold resource estimated by the u s geological survey using completely different methods several comparisons based on these results provide insights into the efficiency of processes that form epithermal deposits and the outlook for sustainability for instance all of the epithermal deposits that formed through phanerozoic time represent only about 0 03 percent of the gold in the crust only about 0 007 percent of crustal gold remains in epithermal deposits the rest has been eroded and recycled this is consistent with comparisons based on the flux of gold in the taupo volcanic zone which indicate that epithermal deposits trap only 0 1 to 0 2 percent of the gold that moves through convergent margin hydrothermal systems epithermal gold deposits that remain in the crust can supply current production from them for 1 400 to 5 000 years depending on success of exploration and depth of mining finally we are consuming gold from epithermal deposits about 17 000 times faster than earth is replenishing the supply

advanced argillic lithocaps in the bolivian tin silver belt zones of advanced argillic alteration constituting lithocaps are commonplace in the shallow parts of porphyry copper systems similar lithocaps are also recognized in the shallowly eroded southern part of the bolivian tin silver belt where mineralization typically is centred on felsic volcanic domes a well preserved lithocap at potosi is dominated by vuggy residual quartz and contained the world s largest silver resource whereas the basal remnant of a lithocap at pulacayo is composed of barren quartz alunite minor gold occurs in the structurally controlled roots of a lithocap at tasna the mineralization in these lithocaps is of high sulphidation type and was generated in the epithermal environment in contrast the tin and base metal bearing massive sulphide veins and associated sericitic and quartz tourmaline alteration that underlie the bolivian lithocaps are typified by sulphides of low sulphidation state but are dominantly mesothermal rather than epithermal in character magma chemistry is believed to account for the characterization of bolivian lithocaps by silver tin antimony and porphyry copper lithocaps by gold copper arsenic low grade bulk tonnage silver mineralization within bolivian lithocaps and high grade tin bearing veins concealed beneath them both constitute attractive exploration objectives

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

magmatism in the huarina belt bolivia and its geotectonic implications abstract the huarina belt is a tectono stratigraphic unit extending along the western side of the eastern cordillera and southern altiplano it includes the well known bolivian tin belt both geophysical and isotopic evidence suggest that the huarina belt is located between three cratonic blocks in a major lithospheric weakness zone that has probably been inherited from the proterozoic most of the phanerozoic backarc magmatism in the bolivian andes occurred along this belt local ordovician basalts and andesites and clasts of devonian granitoids occurring in late devonian early carboniferous sedimentary units are testimonies of a scarce palaeozoic igneous activity during the mesozoic rift related magmatism resulted in peraluminous to metaluminous high k calcalkaline triassic granitoids as well as discrete cretaceous centers of alkaline and carbonatitic rocks in the cenozoic huge high k generally peraluminous ignimbrite sheets along with numerous volcanic and intrusive centers were generated along the belt shoshonitic alkaline and ultrapotassic rocks have been locally described on the basis of both geochemical and geophysical evidence a general evolution model is proposed for the huarina belt during paleozoic and mesozoic times the predominant extensional regime of the continental crust led to lithospheric thinning basin generation and production of within plate magmas during the cenozoic prevailing compressional conditions led to a gradual thickening of the lithosphere inducing delamination of its denser and less viscous root the mantle generated magmas interplayed widely with crustal melts giving rise to the dominantly peraluminous magmas

lewisian gneiss geochemistry and archaean crustal development models abstract the geochemistry of lewisian amphibolite facies gneisses from northwest scotland is described with particular reference to the rare earth elements ree and compared with the geochemistry of lewisian granulite facies gneisses the results show that there are no significant differences between laxfordian amphibolite facies and scourian granulite facies gneisses in terms of ree and other immobile trace elements at equivalent silica levels although the mobile radioactive heat producing elements k rb th u are significantly lower in the granulites in both types the basic gneisses have moderately fractionated ree patterns while the intermediate and acid gneisses have strongly fractionated ree patterns with low heavy ree abundances and decreasing levels of total ree with increasing sio 2 the most silicic gneisses develop large positive europium anomalies these gross chemical similarities between gneisses from intermediate amphibolite facies and lower granulite facies crustal levels constrain models for the evolution of the archaean crust the depletion of k rb cs th and u in granulites but not other incompatible trace elements cannot be explained by magmatic processes the positive eu anomaly in the more siliceous gneisses of both facies is a function of the primary processes of crustal generation and not secondary processes such as intracrustal melting or fractional crystallisation fractionation of radioactive heat producing elements from other trace elements is a result of granulite facies metamorphism with these elements being removed by an active fluid phase the apparent lack of partial melting in lower crustal granulites suggests a model for archaean crustal growth largely through underplating by primary tonalitic magmas

the kari kari caldera bolivia and the cerro rico stock abstract the rocks comprising the kari kari massif southeast of the city of potosi bolivia consist entirely of welded ignimbrites it is argued that the massif constitutes the resurgent centre of a 20 m y old resurgent caldera plutonic rocks are exposed in the south but volcanic rocks of the caldera rim are exposed in the north and indicate a shallower erosion level there the volcanic rocks consist of a coarse moat deposit consisting of angular fragments of basement material and juvenile clasts overlain by an extensive garnet bearing ignimbrite a plant fossil bearing lacustrine deposit was laid down in a lake within the caldera the cerro rico stock noted for its silver tin mineralisation may be a late intrusion along the caldera ring fractures