regional geologic setting of epithermal gold deposits chile epithermal gold deposits in chile are cretaceous to cenozoic in age the ores and the volcanic rocks with which they are associated were generated at a noncollisional ocean continent convergent plate margin they are related to long slightly sinuous north northeast trending magmatic belts roughly parallel to the pacific coast most of the deposits are associated with eroded volcanic centers and subvolcanic porphyries north south variations in the nature extent and timing of magmatism control the succession of mineralization events in the late cretaceous vein districts e g el bronce and hydrothermal alteration zones were formed in central chile paleocene eocene volcanic rocks constitute a large diffuse volcanic field in the antofagasta region 21 degrees 26 degrees s which contains epithermal precious metal and porphyry copper deposits associated with coeval extrusive rocks e g faride san cristobal guanaco and cachinal de la sierra in the late eocene early oligocene volcanism diminished and finally ceased in northern chile probably as a consequence of highly oblique plate convergence oligocene magmatism was restricted to emplacement of porphyries carrying copper deposits along transtensional zones within a strike slip fault system one of these deposits potrerillos hosted by sedimentary rocks developed epithermal gold mineralization in its cupola el hueso the late oligocene to mid miocene magmatic province of north central chile 26 degrees 31 degrees s coincides approximately with the modern nonvolcanic fiat slab segment of the chilean andes and hosts the largest number of known epithermal developments deposits such as el indio tambo nevada la pepa lobo marte and la coipa are hosted by oligocene miocene volcanics the deposits are associated with large zones of argillic and advanced argillic alteration in some places related to shallow level stocks and flow domes a younger event of alteration mineralization is recognized in the eroded cores of middle miocene composite volcanoes which cap some of the older altered areas e g refugio la laguna the main alteration event began at 23 ma continued after a major deformation event initiated at 18 to 16 ma and ended at 11 to 8 ma the precious metal deposits and mineralized zones of the oligo miocene province were exposed by erosion after the cessation or eastward shift of volcanism in the late miocene which was probably due to shallowing of the subduction zone between 26 degrees 30 and 33 degrees s late miocene to holocene magmatism in chile is present in two segments beneath which the dip of the benioff zone remains steep north of 27 degrees s central volcanic zone cvz and south of 33 degrees s southern volcanic zone svz the central is characterized by widespread neogene hydrothermal alteration zones and native sulfur deposits associated with composite volcanoes calderas subvolcanic intrusions and active geothermal fields which are superimposed on the oligocene miocene volcanic belt these features are believed to represent a very shallowly eroded epithermal environment in this setting the late miocene choquelimpie deposit is located in the exposed core of a volcano

effects of experimental deformation on the remanent magnetization of sediments abstract changes in direction and in intensity of the remanent magnetization of sediments were studied during experimental deformation leading to an axial shortening of 20 30 the remanence was produced before shortening by anhysteretic magnetization in an alternating field of 90 mt superimposed on a parallel steady field of 1 mt at various inclinations with respect to the sample the samples were sandstones marls and volcanic sediments the results show that the direction of magnetization is stable during the deformation within the experimental precision the intensity of magnetization decreases during shortening by an amount which depends on the type of rock about 30 for sandstones and about 5 for marls and volcanic sediments the march model does not account for the data and a discontinuous deformation must be invoked the evolution of the young modulus of sandstones during cyclic loading and unloading shows first a decrease in relation to crack density growth then an increase showing a localization of the fracturation and finally a deformation by displacement of microblocks an interpretation is given after a discussion of the results obtained by other authors with plasticine and artificial clays the amplitude of the decrease and of the rotation of the remanent magnetization could be a function of the size of the magnetic grains compared with the scale of the fracturation during non homogeneous processes of deformation

re os ages for the erdenet and tsagaan suvarga porphyry cu mo deposits mongolia and tectonic implications the erdenet and tsagaan suvarga deposits are the two largest known porphyry cu mo deposits in mongolia replicate re os analyses of molybdenite from these deposits yield ages of 240 7 0 8 and 240 4 0 8 ma for erdenet and 370 1 1 2 and 370 6 1 2 ma for tsagaan suvarga 2 errors the re os age of erdenet is significantly older and that of tsagaan suvarga is slightly older than published 40ar 39ar ages for sericite associated with mineralization in these deposits 207 4 5 0 and 364 9 7 0 ma respectively these re os results suggest that in the hydrothermal environment 40ar 39ar ages may be partly to significantly reset whereas re os ages record and maintain the age of primary mineralization events these re os ages indicate that porphyry mineralization at erdenet and tsagaan suvarga are related to porphyritic intrusions representing the youngest event during regional magmatism in northern and southern mongolia respectively the re os age of erdenet marks the final stage of arc magmatism in northern mongolia followed by late permian collision between the mongolian north china and siberian blocks the re os age of tsagaan suvarga indicates that devonian arc magmatism in southern mongolia was active at the present latitude of 44 n at least until 370 ma

re os isotope systematics of sulfides from felsic igneous rocks application to base metal porphyry mineralization in chile dating sulfides and determining the source of ore forming metals have been limiting factors in our understanding of processes that produce hydrothermal ore deposits here we analyze sulfides from base metal porphyry deposits from chile to demonstrate the potential of the re os system to determine both the age and the source of metals for hydrothermal mineralization cogenetic chalcopyrite bornite pyrite and sphalerite from el teniente ca 5 ma have 187 re 188 os ratios from 0 3 to 21 8 and initial 187 os 188 os ratios from 0 17 to 0 22 a paragenetically late pyrite that has an initial 187 os 188 os of 0 88 indicates that the ore deposit is not isotopically homogeneous in os throughout the formation of the deposit pyrite samples from andacollo ca 100 ma have 187 re 188 os ratios from 15 to 3600 and different isochrons yield ages between 87 and 103 ma consistent with different sericite k ar ages initial 187 os 188 os ratios of 0 2 to 1 1 are similar to those of el teniente the relatively homogeneous 187 os 188 os ratios 0 19 for cogenetic sulfides from el teniente suggest that most of the os in the ore was from the causal intrusive the latestage pyrite probably incorporated os leached from the country rock by meteoric fluids which mixed with magmatic fluids at the periphery of the hydrothermal system we demonstrate that the re os isotope system can be a powerful geochronological tool in hydrothermal ore deposits the large range in re os ratios of sulfides permits the age of mineralization to be well constrained despite isotopic heterogeneities of the hydrothermal fluids

the rotation of magnetic grains abstract it has been questioned whether magnetic remanence rotates as a rigid marker or as a passive marker with no material properties during tectonic strain the remanence of a rock is actually the sum of the moments of individual grains so we must first understand their rotation simple shear provides a simple strain history which may be used to distinguish between the two extreme possibilities a passive marker cannot rotate through the shear plane but a rigid marker can this is a useful criterion to distinguish between the two extreme models however for reasonable strains 10 the passive model is successful in explaining the rotation of these grains even though their behaviour is mechanistically closer to that of a rigid marker this explains the success of field studies in which the remanence of redbeds has been de strained using the hypothesis of passive behaviour notwithstanding the reality that the natural iron oxide grains do not rotate in that manner

punctuated magmatism associated with porphyry cu mo formation in the paleocene to eocene of southern peru the paleocene to eocene southern peru porphyry belt contains three significant porphyry cu mo deposits at cuajone quellaveco and toquepala ten new zircon u pb sensitive high resolution ion microprobe reverse geometry shrimp rg ages for cuajone and toquepala together with published ages for quellaveco establish a magmatic history characterized by episodic events punctuated magmatism at cuajone is distributed over approximately 13 m y at toquepala over 8 m y and at quellaveco over 6 m y the ages of the porphyry intrusions hosting or associated with the introduction of cu and mo at the three deposits show remarkable similarity with emplacement beginning and ending at approximately 56 5 to 53 0 ma at cuajone 57 0 to 54 0 ma at toquepala and at 58 4 to 54 3 ma at quellaveco field relations coupled with the u pb ages for synmineral intrusions suggest very similar timing of the cupriferous hydrothermal systems with the youngest pyritiferous and cu poor hydrothermal systems being associated with porphyry intrusions as much as 2 m y younger than significant cu introduction overall the porphyry cu mo intrusive complexes represent the youngest magmatic complexes formed during the late cretaceous and early tertiary arc having formed prior to eastward migration of the magmatic locus

paleoproterozoic high sulfidation mineralization in the tapajos gold province amazonian craton brazil geology mineralogy alunite argon age and stable isotope constraints the brazilian tapajos gold province contains the first evidence of high sulfidation gold mineralization in the amazonian craton the mineralization appears to be in large nested calderas the tapajos parima or ventuari tapajos geological province consists of a metamorphic igneous and sedimentary sequence formed during a 2 10 to 1 87 ga ocean continent orogeny the high sulfidation mineralization with magmatic hydrothermal alunite is related to hydrothermal breccias hosted in a rhyolitic volcanic ring complex that contains granitic stocks ranging in age from 1 89 to 1 87 ga cone shaped hydrothermal breccias which flare upward contain vuggy silica and have an overlying brecciated cap of massive silica the deposits are located in the uppermost part of a ring structure volcanic cone drill cores of one of the hydrothermal breccias contain alunite natroalunite pyrophyllite andalusite quartz rutile diaspore woodhouseite svanbergite kaolinite and pyrite along with inclusions of enargite luzonite chalcopyrite bornite and covellite the siliceous core of this alteration center is surrounded by advanced argillic and argillic alteration zones that grade outward into large areas of propylitically altered rocks with sericitic alteration assemblages at depth several occurrences and generations of alunite are observed alunite is disseminated in the advanced argillic haloes that envelop massive and vuggy silica or that underlie the brecciated silica cap coarse grained alunite also occurs in branching veins and locally is partly replaced by a later generation of fine grained alunite silicified hydrothermal breccias associated with the alunite contain an estimated reserve of 30 tonnes of gold in rock that grades up to 4 5 g t 1 au seven alunite samples gave 40ar 39ar ages of 1 869 to 1 846 ga with various degrees of apparent minor ar loss stable isotopic data require a magmatic hydrothermal origin for the alunite typical for high sulfidation mineralization the 34s values of most samples of alunite range from 14 0 to 36 9 sulfur isotopic alunite pyrite and oxygen isotopic alunite so4 oh temperatures range from 130 to 420 c the dh2o and 18oh2o values for alunite forming hydrothermal fluids suggest a predominance of magmatic water with a small meteoric contribution a rare sample of supergene alunite has a 34s value of 4 1 and an 40ar 39ar age of 51 3 0 1 ma other than local foliation in the volcanic rocks and recrystallization of alunite near faults the mineralization and associated alteration appears to have been remarkably undisturbed by later metamorphism and by supergene alteration the au mineralization was preserved because of burial by sediments and tuffs in taphrogenic basins that probably developed shortly after mineralization and were probably first exhumed at about 60 ma because high sulfidation mineralization forms at relatively shallow crustal levels the discoveries in tapajos province provide new perspectives for mineral exploration for the amazonian and perhaps for other precambrian cratons

contrasting chemical and isotopic signatures from neoproterozoic metasedimentary rocks in the central iberian zone spain of pre variscan europe implications for terrane analysis and early ordovician magmatic belts neoproterozoic metasedimentary units from central spain and the northern central iberian zone ciz show distinctive geochemical signatures with respect to the schist greywacke complex which is dominant in the southwestern part of the ciz significant differences occur in major and trace element composition and initial nd isotope ratios which cannot be explained in terms of metamorphic grade and or anatexis in some layers of these metasedimentary sequences nd540 values from metapelites of central spain range from 5 6 to 10 4 whereas those from the schist greywacke complex span a narrower and higher range of 0 8 to 3 1 this difference suggests different source regions the presence of two contrasting early ordovician magmatic belts in both subzones reinforces the idea of different precambrian basement terranes in the ciz these data are crucial for pre variscan paleogeographic reconstructions of the westernmost european terranes and for checking the validity of current subdivisions of the iberian massif

magnetic studies of magma supply and sea floor metamorphism troodos ophiolite dikes abstract dikes of the eastern troodos ophiolite of cyprus intruded at slow ocean spreading axes with dips ranging up to 15 from vertical and with bimodal strikes now ne sw and n s due to post 88 ma sinistral microplate rotation varied dike orientations may represent local stress fields during dike crack propagation but do not influence the spatial distributions or orientation distributions of dikes magnetic fabrics nor of their palaeomagnetic signals anisotropy of magnetic susceptibility ams integrates mineral orientation distributions from each of 1289 specimens sampled from dikes at 356 sites over 400 km2 in the eastern troodos ophiolite of cyprus in 90 of dikes ams fabrics define a foliation kmax kint parallel to dike walls and a lineation kmax that varies regionally and systematically magma flow alignment of accessory magnetite controls the ams with a subordinate contribution from the mafic silicate matrix that is reduced in anisotropy by sea floor metamorphism titanomagnetite has less influence on anisotropy occasionally intermediate and minimum susceptibility axes are switched so as to be incompatible with the kinematically reasonable flow plane but maximum susceptibility kmax still defines the magmatic flow axis such blended subfabrics of kinematically compatible mafic silicate and misaligned multidomain magnetite subfabrics are rare areas of steep magma flow kmax plunge 70 and of shallow magma flow alternate in a systematic and gradual spatial pattern foci of steep flow were spaced 4 km parallel to the spreading axes and 6 km perpendicular to the spreading axes ridge parallel separation of steep flow suggest the spacing of magma feeders to the dikes whereas ridge perpendicular spacing of 6 km at a spreading rate of 50 mm a implies the magma sources may have been active for 240 ka the magma feeders feeding dikes may have been 2 km in diameter stable paleomagnetic vectors in some cases verified by reversal tests are retained by magnetite and titanomagnetite in all specimens the stable components were isolated by three cycles of low temperature demagnetization ltd followed by 10 steps of incremental thermal demagnetization td 47 of primary a components 338 2 57 2 n 207 95 3 9 mean tub 397 8 c are overprinted by a b component 341 4 63 5 n 96 95 8 7 mean tub 182 11 c a and b components are ubiquitous and shared equally by the n s and ne sw striking dikes a component unblocking temperatures tub are zoned subparallel to the fossil spreading axis their spatial pattern is consistent with chemical remagnetization at some certain off axis distance determined by sea floor spreading a components indicate less microplate rotation and more northerly palaeolatitudes that are consistent with metamorphic remagnetization after some spreading from the ridge axis thus their magnetizations are younger than those of the overlying volcanic sequence for which chrms are commonly reported as 274 33 88 ma

deformation of remanent magnetism in a synthetic aggregate with hematite a hematite calcite aggregate bonded with portland cement is deformed triaxially at 200 mpa confining pressure and at a constant strain rate of 10 5 s 1 in some cases with a pore fluid pressure of 120 mpa this aggregate deforms in a ductile fashion at room temperature and is a good textural analog of naturally penetratively strained hematite bearing rocks the changes in inclination of an isothermal remanent magnetisation are indistinguishable from the behaviour expected of a homogeneously strained non material line rotating according to wettstein s equation this is encouraging for paleomagnetists who might wish to restore remanence in tectonically strained hematite bearing rocks an unknown combination of rigid grain rotation and intracrystalline stress must be responsible for rotation of the grains that carry remanence and thus for the changes in inclination of remanence the reason that wettstein s equation is approximately valid may be because the large aspect ratios of rigid hematite grains cause them to rotate nearly as quickly as a passive line p hydrostatic compaction wet deformation and dry deformation in that order are progressively more effective in reducing the intensity of remanence however wet deformation is more effective than dry deformation in reducing the intensity of the most coercive components presumably by spinning the finest grains in the cement matrix and scattering their individual moments p where multiple direction components of remanence are present deformation reduces the components with higher coercivities 70 mt more than those with lower coercivities in previous experiments on magnetite bearing samples the lower coercivity components were most reduced thus the natural tectonic deformation of an nrm may be expected to preserve selectively low coercivity components in hematite and high coercivity components in magnetite this may be used to predict the success of preservation of a characteristic nrm and conversely the success of overprinting by syntectonic remagnetisation

porphyry epithermal transition maricunga belt northern chile the refugio aldebaran and la pepa districts in the maricunga belt of northern chile contain advanced argillic alteration zones that locally host high sulfidation epithermal gold deposits in proximity to porphyry gold copper deposits the spatial association suggests a genetic link mineralized zones are characterized by four main vein types that formed at different times and have specific zonal relationships a veinlets are the earliest and deepest vein type they are restricted to potassic alteration zones in intrusive rocks a veinlets contain variable amounts of quartz magnetite biotite and chalcopyrite and locally have k feldspar halos they have nonmatching irregular vein walls and lack internal symmetry hypersaline liquid rich inclusions coexisting with vapor rich inclusions in a veinlets indicate temperatures as high as nearly 700 c and pressures between 200 and 400 bars assuming a lithostatic load depths of 0 8 to 1 6 km are inferred zones of abundant a veinlets contain mostly 1 ppm gold and 0 1 to 0 4 percent hypogene copper banded quartz veinlets occur mostly above a veinlets and cut a veinlets where they overlap dark gray bands the color resulting from a high density of vapor rich fluid inclusions and micron sized grains of magnetite commonly occur as symmetric pairs near the vein walls vein walls are parallel and slightly wavy vuggy vein centers are common and alteration envelopes are absent data from rare liquid rich inclusions in banded quartz veinlets indicate temperatures 350 c at pressures between 20 and 150 bars assuming a hydrostatic load depths of 0 2 to 1 5 km are inferred zones of abundant banded quartz veinlets generally contain 0 5 to 2 ppm gold and 0 1 percent hypogene copper d veins are pyrite veins with quartz sericite pyrite halos they are widespread and crosscut a veinlets and banded quartz veinlets the brittle nature of d veins and limited fluid inclusion data suggest temperatures 400 c d veins serve as important time lines they are nowhere truncated or crosscut by intrusions a veinlets or banded quartz veinlets quartz alunite replacement veins referred to as ledges in this paper are typical of the high sulfidation epithermal environment they are mostly limited to overlying volcanic rocks they contain local core zones of vuggy residual quartz that can contain enargite or at higher elevations barite of the three districts studied only la pepa has mineable quartz alunite ledges which contain an average gold grade of about 20 ppm a spectrum of porphyry style deposits exists cerro casale at aldebaran shares many characteristics of porphyry copper deposits worldwide whereas verde at refugio is a true porphyry gold deposit potassic alteration zones and a veinlets are strongly developed at cerro casale whereas they are absent at verde banded quartz veinlets predominate at verde whereas they occur only at the upper levels of cerro casale the pancho deposit at refugio and the cavancha deposit at la pepa are telescoped systems in which banded quartz veinlets overprint potassic alteration zones and a veinlets a veinlets and banded quartz veinlets cut and are cut by intrusions indicating multiple cycles of intrusion potassic alteration a veinlets banded quartz veinlets during formation of porphyry style mineralization banded quartz veinlets are thought to have formed by flashing of magmatic fluids during episodic transitions from lithostatic to hydrostatic pressure loss of sulfur to the vapor phase during flashing inhibited formation of copper sulfides in banded quartz veinlets and therefore resulted in high gold copper ratios where rising magmatic vapors condensed into overlying meteoric water along faults barren quartz alunite ledges formed this conclusion is supported by equivalent 40ar 39ar dates on hydrothermal biotite associated with porphyry style ore and alunite from barren ledges at aldebaran 40ar 39ar dates at la pepa indicate alunite formed at least 140 000 years to as long as 900 000 years after hydrothermal biotite within the high sulfidation epithermal environment the development of ore depends on the ability of late moderate salinity magmatic fluids to reach the surface without condensing a brine upon ascent cooling and boiling of the moderate salinity fluid below its critical temperature results in the formation of sericite at depth and alunite near the surface that is essentially synchronous with high sulfidation ore formation the timing of the switch from lithostatic pressures to brittle hydrostatic conditions relative to the life of the hydrothermal system might determine how much porphyry style ore forms relative to high sulfidation epithermal ore

comment relationship between copper tonnage of chilean base metal porphyry deposits and os isotope ratios researchers and mineral explorationists have long searched for a silver bullet to find giant world class ore deposits because of the ability of the rhenium re osmium os isotopic system to directly trace the origin of metals in ore deposits it has been suggested that os isotopes may

stress field determinations in france by hydraulic tests in boreholes the stress field has been determined at eight different sites in france four in crystalline or metamorphic rocks four in sedimentary formations by hydraulic tests in boreholes for seven of these sites the complete stress field has been determined using the hydraulic tests on pre existing fractures htpf inversion method validity of the results is demonstrated by the good fit between a priori and a posteriori values for the data and by the low values of the a posteriori standard deviation on the unknowns for four sites results have been compared with those derived according to the hydraulic fracturing theory the two methods yield comparable results for the orientation of the maximum horizontal principal stress h a generally satisfactory fit for the magnitude of the minimum horizontal principal stress h but a very poor agreement for the magnitude of h this latter misfit has been attributed to the effect of fluid percolation prior to the actual opening of the fractures below a depth of 500 m a homogeneous h direction n150 e has been determined at the crystalline sites auriat echassieres le mayet de montagne chassoles all located in the northern massif central for three of these sites the vertical stress is significantly lower than the weight of overburden as computed from the rock density and the depth of the corresponding measurement at auriat and echassieres the stress field is consistent with a mostly strike slip faulting regime at le mayet de montagne and chassoles the maximum stress is vertical but nearly equal to h the stress field has been found to be much more heterogeneous at two of the four sites in sedimentary rocks because of the large variability in mechanical rock properties in such heterogeneous formations inversion with the htpf method must be limited to those data which pertain to the same rock horizon however because of its structure this heterogeneity does not restrict the possibility of defining large scale uniform principal stress directions all the results are consistent with the local seismotectonics

re computing palaeopoles for the effects of tectonic finite strain abstract the pre messinian limestone cover 58 8 ma to the troodos ophiolite 88 ma of southern cyprus is penetratively strained as shown by ubiquitous magnetic fabrics and in many sites stylolitic cleavage these define a gently n dipping foliation and an n plunging extension south vergent folding and thrusting is well known in very localized whereas the bulk of the strained limestone cover dips gently south disturbed by faulting the magnetic fabrics and stylolitic cleavage define the axes of finite strain in all sites studied and the calcite matrix was suitably ductile to permit the original palaeomagnetic directions to be de strained assuming continuum behaviour the optimum de straining 30 40 shortening in a flattening strain is compatible with the stylolitic cleavage development restores bedding to the near horizontal and restores the characteristic remanent magnetization vectors chrms to concentrated symmetrical fisherian distributions the strain corrected chrms yield more reasonable palaeopole locations for the lefkara and pakhna limestone and more uniform micro plate rotation rates corrected palaeopoles reveal a relatively uniform anticlockwise rotation of the troodos plate since the creation of the late cretaceous 88 ma ocean lithosphere it did not accelerate during the deposition of the limestone cover as required by palaeopoles calculated from data not corrected for finite strain but turned at 1 5 ma 1 since 58 ma

magnetic fabrics may proxy as neotectonic stress trajectories polis rift cyprus 1 the miocene polis rift opened along a nnw axis obviously in response to wsw relative extension and relative tension starting 5 myr ago however geologically young plate movements are believed to be closing the rift by sinistral transpression producing minor younger faults since 1 ma modern earthquake solutions give e w compression compatible with the younger faults magnetic fabrics using anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanent magnetization are kinematically compatible with stress axes required by the fault and rift geometry and thus were acquired in 5 ma however by different statistical treatments and selecting samples from different subareas it is possible to reveal sedimentary depositional fabrics and perhaps also a composite ams fabric related to young transpression 1 ma

porphyry gold deposits of the refugio district maricunga belt northern chile the porphyry gold deposits of the refugio district and similar deposits in the maricunga belt contain the lowest known copper to gold ratios cu ppm au 0 03 of any porphyry type deposit the gold deposits are associated with subvolcanic andesitic to dacitic intrusions emplaced into coeval volcanic rocks both the verde and pancho deposits are zoned in space from a deeper zone of banded quartz veinlets associated with chlorite magnetite albite and or pyrite albite clay alteration to a shallow zone of pyrite albite clay and local quartz alunite ledges pancho contains an additional deepest porphyry copperlike zone with quartz veinlets a veinlets and potassic alteration relative to verde pancho is telescoped with all three zones present within a 400 m vertical interval the porphyry copperlike zone at pancho is characterized by a veinlets and pervasive potassic alteration both restricted to intrusive rocks a veinlets range from hairline streaks of magnetite biotite with minor quartz and chalcopyrite and k feldspar alteration envelopes to sugary quartz veinlets 1 cm in width with magnetite and chalcopyrite and no alteration envelopes hypersaline liquid inclusions coexisting with vapor rich inclusions indicate temperatures above 600 c and salinities as high as 84 wt percent nacl equiv a pressure estimate of 250 bars indicates a depth of 1 000 m assuming lithostatic pressure potassic alteration consists of a central zone of magnetite k feldspar oligoclase that changes outward to a biotite rich zone total sulfide content predominately as chalcopyrite is generally 1 vol percent whereas magnetite content is 2 to 5 percent where a veinlets and potassic alteration predominate grades are typically 0 1 wt percent hypogene copper and 0 5 to 1 ppm gold banded quartz veinlets are present at both verde and pancho where they occur mostly above a veinlets and cut a veinlets where they overlap they are less than 2 cm in thickness and lack alteration envelopes dark gray bands whose color is due to abundant vapor rich fluid inclusions and micrometer sized grains of magnetite commonly occur as symmetric pairs near the vein walls the bands are commonly botryoidal and are continuous through quartz grains suggesting that the quartz recrystallized from a silica gel rare liquid rich fluid inclusions in quartz indicate temperatures 350 c and salinities 35 wt percent nacl equiv estimated pressures are 200 bars suggesting depths of 190 to 1 500 m under hydrostatic pressure gold occurs both in the dark bands with magnetite and outside the dark bands with pyrite chlorite illite and k feldspar banded veinlets occupy steeply dipping radial and shallowly dipping concentric fractures zones of abundant banded veinlets without early a veinlets generally contain 0 5 to 2 ppm gold and 0 05 wt percent hypogene copper most of the differences between porphyry gold deposits at refugio and porphyry copper deposits can be attributed to shallower depths of formation less than 1 km compared to 1 5 to 4 km that is typical for porphyry copper deposits shallower depths resulted in lower sulfide concentrations local garnet veinlets widespread albite bearing alteration and most importantly banded quartz veinlets which are unique to porphyry gold deposits banded quartz veinlets are a direct result of episodic intrusion of magma to within 1 km of the surface and exposure of high temperature magmatic fluids to hydrostatic pressures episodic rupturing of a brittle ductile boundary surrounding the intrusive centers at verde and pancho led to flashing of magmatic fluids loss of sulfur to vapor and low sulfide gold ratios in ore

synthetic magnetic fabrics in a plasticene medium mixtures of ferrimagnetic grains with a supporting medium of paramagnetic plasticene permit hypotheses concerning the interpretation of assemblages of different minerals to be confirmed the deformation of such plasticene mixtures in pure shear transpression and simple shear emphasize the dominance of the initial anisotropy over the end result for low strains 25 shortening in pure shear for minor amounts of multidomainal ferrimagnetic grains the deformation experiments indicate that the magnetic lineation rotates rapidly into the maximum extension direction for the three strain histories tested although the magnetic fabric directions are reliable kinematic indicators after modest strain in simple shear the symmetry of the fabric is not a plane strain fabric great discrepancies occur in the case of assemblages single domain femmagnetic grains in plasticene in pure shear these produce inverse fabrics in which the magnetic lineation spins towards parallelism with the maximum shortening direction

experimental and numerical modeling of irm rotation in deformed synthetic samples abstract the effect of strain upon isothermal remanent magnetization irm carried by hematite particles embedded in a plasticine matrix has been investigated magnetized artificial hand samples were deformed by simple coaxial shortening after deformation the hand samples were cut into 8 15 specimens and each specimen s irm measured hence a mean irm direction was obtained for each shortening step the main results are 1 when the initial direction of irm is parallel to the shortening axis 3 the hand sample mean irm stays parallel to 3 during deformation however individual magnetization directions are deflected towards the flattening plane and thus the specimen directions are scattered by strain this is accompanied by a significant decrease of magnetization intensity 2 in contrast when the initial direction of irm is perpendicular to 3 the shortening results in a clustering of specimen magnetization directions the mean direction being unchanged this is accompanied by a slight increase of magnetization intensity 3 in all other cases where initial irm direction makes an intermediate angle with shortening direction deformation induces a deviation of each specimen irm toward the flattening plane this results in an overall deflection of hand sample mean irm the amount of rotation increases with strain intensity changes of grouping and intensity depend on the initial angle between irm and 3 defining a continuum in behavior between irm directions being either parallel to 3 or perpendicular to 3 finally it is shown that all the experimental changes in irm due to deformation direction grouping intensity can be modeled by a passive marker rotation of hematite planar particles

magnetic fabric methods and applications fabric is a ubiquitous and significant feature of geological materials the processes involved in the formation and deformation of rocks and sediments leave their mark on the orientations of the constituent mineral grains petrofabrics thus provide essential keys to understanding the history of geological materials magnetic anisotropy is directly related to petrofabric and has become one of the most rapid sensitive and widely used tools for its characterization the relationship between magnetic fabric and petrofabric is complex and depends on various factors including the composition concentration and grain size of mineral grains ongoing research in geological applications is paralleled by studies of the fundamental mineral magnetic phenomena involved the papers in this book represent the current state of investigations in magnetic anisotropy studies as a discipline that integrates geological interpretations mineral fabric development technical advances and rock magnetic properties

viscous magnetization at 300 k in a profile through troodos type oceanic crust abstract this study examines the question of the relative importance of viscous magnetism vm at 300 k to the total magnetization of older oceanic crust a section through the troodos cyprus ophiolite now recognized as a good proxy for in situ oceanic crust has been used as the source of samples the section extends downwards continuously from little altered submarine extrusives through greenschist facies sheeted dikes to mafic and variously serpentinized ultramafic cumulate intrusives the principal result of the investigation is that vm is not expected to be the dominant magnetization and is often relatively negligible at all crustal levels vm acquisition varies irregularly with depth with predicted maximum values equivalent to about one third of the total magnetization in the extrusives and sheeted dikes the contribution of vm in two samples of serpentinized ultramafics is insignificant vm acquisition does not show any simple relationships with primary lithology alteration history or magnetic properties or history the occurrence of two stage vm growth mechanisms is widespread the second stage typically has an onset time of about 103 min and an acquisition rate three times that of the initial growth mechanism the physical origin of this two stage mechanism and its significance in predicting vm growth over geologic time intervals are considered to be important problems for future work

geological development and mineralization in the atacama segment of the south american andes northern chile 26 15 27 25 s between the late jurassic and the middle miocene widespread magmatism tectonic events and hydrothermal mineralization characterized the geological evolution of the atacama segment of the south american andes a characteristic feature of this zone is the coincidence in time and space between subduction generated igneous activity crustal deformation and mineralization in the magmatic arcs which formed longitudinal belts migrating eastward

intrusive hyaloclastite and peperitic breccias associated to sill and cryptodome emplacement on an early paleocene polymagmatic compound cone dome volcanic complex from el guanaco mine northern chile abstract this work explores the textural characteristics morphology and facies architecture of well preserved paleocene hyaloclastic and peperitic breccias associated with subvolcanic intrusions at el guanaco gold mine northern chile the el guanaco mine volcanic sequence is part of a polymagmatic compound cone dome volcanic complex grouping several dacitic domes and maar diatremes and subordinated subvolcanic intrusions of basaltic andesitic and dacitic compositions the soledad penafiel fault system is a first order regional structure controlling the location and style of the volcanism in the region three different intrusive bodies basaltic sills dacitic cryptodomes and andesitic cryptodomes were found to intrude into a wet and poorly consolidated pyroclastic sequence representing the upper portions of a maar diatreme consequently extensive quench fragmentation and fluidization along their contacts occurred leading to the formation of widespread breccia bodies enclosing a coherent nucleus differences in matrix composition allows to define two main breccias types 1 poorly sorted monomictic breccias intrusive hyaloclastites and 2 poorly sorted tuff matrix breccias peperites the observed facies architecture is interpreted as the result of the interplay of several factors including 1 magma viscosity 2 the geometry of the intrusives and 3 variations on the consolidation degree of the host rocks additionally the overall geometry of each intrusive is interpreted to be controlled by the effective viscosity of the magmas along with the available magma volume at the time of the intrusions the presence of three compositionally different subvolcanic bodies with intrusive hyaloclastite and peperite envelopes indicate not only that all these intrusions occurred in a short period of time probably less than 2 3 ma but also that the volcaniciclastic pile suffer little or none compaction nor consolidation during that time the presence of three compositionally variated synvolcanic intrusives can be explained either by the presence of a zoned magmatic chamber feeding the volcanic complex or more likely by the influence of the soledad penafiel fault zone acting as a preferential pathway for different magma compositions sources to rise to the surface

the cretaceous iron belt of northern chile role of oceanic plates a superplume event and a major shear zone the cretaceous constitutes a turning point in the tectonic magmatic and metallogenic history of chile the geological evidence indicates that a major change occurred in late neocomian time when superplume emplacement mid pacific superplume and plate reorganization processes took place in the pacific the superplume event resulted in a major ridge push force resulting in increased coupling between the subducting and overriding plates this completely changed the tectonic setting of chile ending the early cretaceous extensional period aborted rifting in the back arc basin and increasing stress at a crustal scale as a consequence overpressurized dioritic magmas were pushed up mainly along the best possible structural path in northern chile i e the atacama fault zone eventually forming a 500 km long belt of kiruna type iron deposits with reserves of 2 000 mt 60 fe a unique case in chile s geological history

magnetic anomaly interpretation across the southern central andes 32 34 s the role of the juan fernandez ridge in the late tertiary evolution of the margin marine and terrestrial magnetic surveys have been integrated to study the tectonic structure of the convergent margin of chile between 32 34 s three magnetic domains have been identified oceanic continental margin and subaerial the oceanic domain has seafloor spreading anomalies 16 37 ma to 18 39 5 ma disturbed by anomalies of the juan fernandez hot spot chain in the continental margin the most prominent fabric are e w anomalies in the upper slope corresponding to onshore e w anomalies of large intrusive bodies onshore a n s lineament of short wavelength anomalies defines the roots of a cretaceous volcanic arc a resembling lineament offshore indicates a submerged older volcanic arc and that continental basement extends to 50 km landward of the trench absolute cenozoic plate motion for nazca and south american plates and dating of the juan fernandez chain provide a kinematic model of ridge continent collision the reconstruction indicates rapid southward migration of the collision point along 1400 km of the margin from 20 to 11 ma 20 cm yr 1 from 11 ma to present the collison point has migrated at a slower rate along 375 km of the margin 3 5 cm yr 1 the predicted location of the subducted portion of the juan fernandez chain coincides with the south edge of the southward migrating flat slab segment of the subducted lithosphere and with a cluster of deep earthquakes indicating a causal relationship in the last 10 myr the ridge has separated a sediment starved trench to the north where subduction erosion may dominate from a sediment filled trench to the south where recent sediment accretion dominates these observations indicate that subduction of the juan fernandez chain plays a major role in arc forearc tectonics

geology petrography and geochemistry of igneous rocks related to mineralized skarns in the nw neuquen basin argentina implications for cordilleran skarn exploration abstract five mineralized cu au and fe skarns and related igneous rocks of the andes cordillera of nw neuquen paleogene campana mahuida caicayen and cerro nevazon and sw mendoza neogene vegas peladas and hierro indio provinces of argentina 34 38 s are reviewed to demonstrate that geochemical signatures of igneous rocks can be used to predict the metal potential of skarn prospects of the andes cordillera of argentina these igneous rocks are calc alkaline metaluminous and derived from a sub arc mantle source without residual garnet they were emplaced at shallow depths the vegas peladas hierro indio and cerro nevazon igneous rocks have similar major and trace element contents and are typical of primitive i type plutons associated with fe skarns worldwide the vegas peladas and hierro indio plutons are however less reduced and they have lower ni concentration than the cerro nevazon rocks whose skarns have higher au concentrations in the caicayen district small skarns with sub economic cu concentrations and abundant pyrite are associated with porphyry copper style alteration in igneous rocks that underwent amphibole fractionation their chemical compositions are intermediate between the most primitive plutons associated with fe skarns and the most evolved plutons associated with cu skarns both with high f o 2 at campana mahuida a small porphyry copper deposit and associated cu skarns are linked to andesite dikes and pluton with similar sio 2 contents but richer in incompatible trace elements k rb sr ba la ce and th than the mendoza cerro nevazon and caicayen igneous rocks this suggests that their parental magmas evolved in an open system and were contaminated with crustal material thus resulting in the strongest fractionation of amphibole and accessories phases these intrusions share many features with typical cu skarns related plutons

the patricia zn pb ag epithermal ore deposit an uncommon type of mineralization in northeastern chile the patricia ore deposit represents an unusual example of economic zn pb ag mineralization at the northernmost end of the late eocene oligocene metallogenic belt in chile it is hosted by volcano sedimentary units which are typically tuffaceous and andesitic breccias the ore body consists of a set of subvertical e w vein systems developed under a sinistral strike slip regime that included transtensive domains with generalized extensional structures where the ores were deposited the deposit is divided into two blocks by a set of nnw ese trending reverse faults which uplifted the eastern block and exhumed thicker and deeper parts of the deposit at least 200 m of volcano sedimentary pile hosting the mineralization has been eroded in this block by contrast the western block exposes a shallower part of the system where cherts amorphous silica and jasperoids occur three main stages of mineralization have been defined 1 pre ore stage is characterized by early quartz pyrite and arsenopyrite 2 base metal and silver stage characterized by sphalerite 6 to 15 mol fes galena chalcopyrite pyrrhotite and ag bearing minerals freibergite polybasite stephanite pyrargyrite freieslebenite and acanthite and 3 post ore stage characterized by late quartz kutnohorite and minor sulfides arsenopyrite sphalerite pyrite galena ag bearing minerals and pb sulfosalts whole ore geochemistry shows two groups of elements that are positively correlated 1 ag cd cu pb zn related to the base metal sulfides and 2 au as ge sb w related to arsenopyrite and pyrite hydrothermal alteration is pervasive in the outcropping mineralized areas including silicification and locally vuggy silica textures at depth chloritic and sericitic alteration is developed along vein selvages and is superimposed to the regional propylitic alteration fluid inclusions indicate that the base metal ores were deposited from 250 to 150 c moderate salinity fluids 1 9 wt nacl the pre ore stage is characterized by a saline fluid 6 22 wt nacl and between 210 and 250 c whereas the post ore stage has salinity of 4 8 wt and temperature from 175 to 215 c cooling was the mechanism of ore mineral precipitation in the patricia deposit although mixing of fluids could have occurred in the pre ore stage mineralogical geochemical and fluid inclusion evidence is consistent with an intermediate sulfidation is epithermal deposit type this study highlights the high potential for hidden economic mineralization at depth in the western block and for extension of the ore body both to the south and to deeper levels in the eastern block of the patricia ore deposit to a larger extent the implications of finding such polymetallic epithermal style of mineralization in the northern chile precordillera is relevant both to the regional metallogenic perspective and to the exploration potential of the region where the late eocene early oligocene metallogenic belt apparently disappears

porphyry epithermal and orogenic gold prospectivity of argentina this paper presents a review of the available information on the significant porphyry epithermal and orogenic gold districts in argentina including the tectonic geological and structural settings of large deposits or deposits that have been exploited in the past based on this review of the geology and mineralization targeting models are developed for epithermal and orogenic gold systems in order to produce gis based prospectivity models using publically available digital geoscience data weights of evidence and fuzzy logic prospectivity maps were generated for epithermal and orogenic gold mineralization in argentina the results of the prospectivity mapping highlight existing gold deposits within known mineralized districts throughout argentina as well as other highly prospective areas with no known deposits within these districts additionally areas within argentina that have no known gold mineralization based on publically available information were highlighted as being highly prospective based on the models used

magma sources and tectonic setting of central andean andesites 25 5 28 s related to crustal thickening forearc subduction erosion and delamination

paleomagnetic vectors and tilted dikes abstract where tectonic deformation reorients rocks without penetrative strain their paleomagnetic vectors may be restored to their original attitudes by untilting for strata paleomagnetic inclination is readily restored but the tilt axis must be precisely known if paleodeclination is required for dikes without the knowledge of the rotation s neither declination nor inclination of the paleomagnetic vector can be uniquely defined furthermore back rotating dike orientations to an upright attitude assumes primary verticality whereas primary dike dips are bimodal across the spreading axes e g troodos ophiolite cyprus in the cyprus ophiolite the dikes of the limassol forest transform zone are tilted due to uplift of the mantle sequence rocks and deflected against the arakapas fault their paleomagnetic vectors may be restored rotating about the two axes defined by the strike and the vertical or about a net axis that is possibly the actual tectonic rotation axis this net axis is determined from the tectonic regional dispersion of the dike orientations in this test case the results of the restorations differ slightly but underline the difficulty in selecting the best restoration procedure and the greater difficulty of restoring the paleomagnetic data from dikes vis a vis strata for dikes it is recommended that the paleomagnetic vectors are restored using average dike orientations to minimize the inaccuracies due to the large primary variation in dike orientation