erosional processes topographic length scales and geomorphic evolution in arid climatic environments the lluta collapse northern chile the lluta collapse of northern chile is one of the oldest recognizable landslides 2 5 ma in a hyperarid continental setting this paper develops a conceptual landscape evolution model of the lluta collapse and analyzes the controls of mass wasting and erosion sediment transport in channels on the topographic development the data presented here imply that high relief along a topographic scarp surface fracturing elevated groundwater table during a more humid climate and an aquitard underlying permeable ignimbrites are preparatory causal factors for landsliding 2 5 ma ago a strong seismic event then possibly resulted in the displacement of ca 26 km3 of mass subsequent modification of the landslide scar occurred by backward erosion resulting in the establishment of a dendritic drainage network and the removal of an additional ca 24 km3 of material it appears that this mass was produced by mass wasting in the headwaters and exported by high concentrated debris flows in channels in addition morphometric information suggest that whereas the geometrical development of the lluta collapse has been controlled by gravitational mass wasting the rates of the development of this geomorphic unit have been limited by the export rates of mass and hence by the transport capacity of the flows

a threshold in soil formation at earth s arid hyperarid transition the soils of the atacama desert in northern chile have long been known to contain large quantities of unusual salts yet the processes that form these soils are not yet fully understood we examined the morphology and geochemistry of soils on post miocene fans and stream terraces along a south to north 27 to 24 s rainfall transect that spans the arid to hyperarid transition 21 to 2 mm rain y 1 landform ages are 2 my based on cosmogenic radionuclide concentrations in surface boulders and ar isotopes in interbedded volcanic ash deposits near the driest site indicate a maximum age of 2 1 my a chemical mass balance analysis that explicitly accounts for atmospheric additions was used to quantify net changes in mass and volume as a function of rainfall in the arid 21 mm rain y 1 soil total mass loss to weathering of silicate alluvium and dust 1030 kg m 2 is offset by net addition of salts 170 kg m 2 the most hyperarid soil has accumulated 830 kg m 2 of atmospheric salts including 260 kg sulfate m 2 and 90 kg chloride m 2 resulting in unusually high volumetric expansion 120 for a soil of this age the composition of both airborne particles and atmospheric deposition in passive traps indicates that the geochemistry of the driest soil reflects accumulated atmospheric influxes coupled with limited in soil chemical transformation and loss long term rates of atmospheric solute addition were derived from the ion inventories in the driest soil divided by the landform age and compared to measured contemporary rates with decreasing rainfall the soil salt inventories increase and the retained salts are both more soluble and present at shallower depths all soils generally exhibit vertical variation in their chemistry suggesting slow and stochastic downward water movement and greater climate variability over the past 2 my than is reflected in recent 100 y rainfall averages the geochemistry of these soils shows that the transition from arid to hyperarid rainfall levels marks a fundamental geochemical threshold in wetter soils the rate and character of chemical weathering results in net mass loss and associated volumetric collapse after 105 to 106 years while continuous accumulation of atmospheric solutes in hyperarid soils over similar timescales results in dramatic volumetric expansion the specific geochemistry of hyperarid soils is a function of atmospheric sources and is expected to vary accordingly at other hyperarid sites this work identifies key processes in hyperarid soil formation that are likely to be independent of location and suggests that analogous processes may occur on mars

geoacoustic databases for matched field processing preliminary results in shallow water off san diego california detailed seafloor environmental modeling of a moderately complex shallow water area coupled with multitone matched field processing produced reliable and unambiguous source detection and tracking frequency averaging of the matched field output sufficiently reduced sidelobe ambiguities the seabed geologic model is a gridded database containing water depth sediment grain size sediment thickness and acoustic basement type grid cells are squares of side length equal to 2 arc s software separate from the geologic model computes a geoacoustic model for any desired grid square

response of supergene processes to episodic cenozoic uplift pediment erosion and ignimbrite eruption in the porphyry copper province of southern peru the jurassic to middle eocene porphyry copper deposits and prospects exposed on the pacific slopes of the central andean cordillera occidental of southern peru between latitudes 16 309 and 18 s record a protracted ca 30 m y history of supergene processes that were fundamentally controlled by the evolving local geomorphologic environment itself a response to successive regional tectonic events including the late eocene incaic the late oligocene to earliest miocene aymara and the middle to late miocene quechuan events weathering of the porphyry centers also overlapped temporally with the local resumption of arc volcanism in southern peru at 25 5 ma following a 27 m y amagmatic interval and supergene processes were variously interrupted or terminated by ignimbrite blanketing although in several locations supergene profiles were preserved by such cover the landform chronology for the area surrounding the cuajone quellaveco and toquepala deposits ca 17 s is revised and extended northwestward through field mapping to the cerro verde santa rosa district ca 16 309 s the 40 ar 39 ar incremental heating dates of supergene alunite group minerals from the angostura 38 1 and 38 8 ma and posco 38 8 ma prospects and the cerro verde deposit 36 1 38 8 ma demonstrate that supergene processes were underway in the late eocene beneath a subplanar topography resulting from uplift and erosion during the incaic orogeny now represented by a regional unconformity in the cenozoic volcanic sedimentary rock succession broadly contemporaneous supergene processes were probably active in the cuajone quellaveco toquepala district slow erosion and the accumulation of clastic sediments through the tectonically quiescent early to mid oligocene are envisaged to have caused a rise in the water table and the widespread preservation of the incaic supergene profiles aymara uplift subsequently led to the incision of the 23 8 to 24 ma altos de camilaca and the 18 8 to 19 1 ma pampa lagunas pediplains and their regional correlatives the ensuing water table lowering was associated with intense leaching and sulfide enrichment from the late oligocene 24 4 28 ma natroalunite at cerro verde 26 27 ma natroalunite at santa rosa and 28 6 ma jarosite at la llave to the early miocene 23 ma alunite and 21 ma natroalunite at cerro verde and 19 2 ma jarosite at la llave and was plausibly responsible for much of the upgrading of the cuajone and toquepala deposits and thr quellaveco prospect which are intersected by both the altos de camilaca pediplain and erosional features representing upslope extensions of the pampa lagunas pediplain the younger supergene profiles were widely superimposed on the remnants of those generated during the incaic orogeny middle miocene 14 2 ma biotite age chuntacala formation flows protected the cuajone supergene profile from destruction by erosion but at 13 0 ma interrupted supergene processes at quellaveco revision of volcano stratigraphic relationships in the latter area reveals that subsequent erosion of the chuntacala formation ignimbrites and part of the supergene profile took place prior to the deposition of a 10 1 ma ash flow tuff of the asana formation elsewhere supergene activity persisted at the cachuyito prospect through 11 4 ma and minor jarosite development occurred at least until 4 9 ma both there and at cerro verde during and following the multiple pediment landform stage ca 7 9 15 0 ma the occurrence of relics of late eocene alunite group minerals within considerably younger late oligocene to late miocene supergene alteration profiles suggests that the overall physiographic configuration of the pacific piedmont of southern peru remained remarkably consistent from the late eocene to the middle miocene moreover the new age data confirm that as in northern chile semiarid climatic conditions prevailed along much of the plate boundary from the mid eocene until the late miocene or early pliocene onset of hyperaridity the local geomorphologic and volcanic conditions in southern peru however conspired to generate more complex supergene profiles with lower aggregate enrichment factors relative to the strongly enriched profiles in the late eocene to early oligocene porphyry copper belt of northern chile which underwent supergene upgrading over relatively brief periods

stony iron meteorites history of the metal phase according to tungsten isotopes abstract the global composition of the early solar system is thought to be roughly chondritic in terms of refractory components and this means that metal and silicate should be present together in early planetesimals to fully understand the metal silicate differentiation process within the eucrite parent body epb it is important to try and identify the metal reservoir that is complementary to the silicate part the isotope 182 of tungsten w a siderophile element is partly formed from the decay of 182 hf and w isotopes are useful for examining metal silicate segregation the w isotopic composition expected for the metal that is complementary to eucrites falls in the range of iron meteorites however mesosiderites seem to be genetically linked to eucrites based on petrologic and oxygen isotopic similarities therefore we undertook the analysis of the metal phase of these stony irons here we present tungsten isotopic data for mesosiderite and pallasite metal to characterize their parent body bodies and to assess possible relationships with eucrites all stony iron metals are depleted in radiogenic tungsten by 1 3 to 4 2 e units relative to the terrestrial standard while chondrites for comparison are depleted by 1 9 e units in addition to w isotopic heterogeneity from one stony iron to another there is also w isotopic heterogeneity within individual meteorites a formation model is tentatively proposed where we show that mesosiderites pallasites and eucrites could possibly come from the same parent body several hypotheses are discussed to explain the isotopic heterogeneity the production of cosmogenic tungsten the in situ decay of hafnium present in inclusions and tungsten diffusion processes after metal silicate mixing during the cooling of the meteorites the two latter hypotheses provide the best explanation of our data

sedimentation tectonism and volcanism relationships in two andean basins of southern peru the intermontane basins of the central andes formed from the first episode of the andine tectonic peruvian phase santonian are characterized by a clastic continental sedimentation settled in a compressive tectonic context and containing volcanic intercalations

atacamite in the oxide zone of copper deposits in northern chile involvement of deep formation waters atacamite a copper hydroxychloride is an important constituent of supergene oxide zones of copper deposits in northern chile whereas in similar deposits elsewhere it is rare in chile it has generally been assumed to be a primary constituent of the supergene zones there are two difficulties with this supposition the first is that atacamite requires saline water for its formation whereas supergene oxidation was caused by percolating oxygenated meteoric water mainly rainwater the second is that atacamite dissolves rapidly or undergoes phase change when exposed to fresh water supergene enrichment of copper deposits in northern chile extended over a long period 44 to 9 ma being terminated by the onset of hyperaridity during this period there was at least intermittent rainfall exposing previously formed atacamite to dissolution or phase change furthermore atacamite bearing oxide zones in several deposits are directly overlain by thick miocene alluvial gravels the stream waters that transported these gravels would have permeated the oxide zones in some deposits atacamite bearing assemblages occur both in the oxide zones and in contiguous gravels we suggest that atacamite bearing oxide assemblages are more likely to have been a replacement of preexisting oxide phases after the onset of hyperaridity at about 9 ma a hyperarid climate made possible evaporation and concentration of chloride in meteoric waters in this paper we discuss another source of saline waters to modify oxide zones dewatering of the domeyko basin expelled brines along faults some of which had earlier guided the location of porphyry deposits at the spence porphyry copper deposit saline waters which d vs 18o isotope analyses identify as basinal brines are presently rising through the deposit then flowing away along the base of the covering gravels compositions of these waters lie within the stability fields of atacamite and brochantite the two minerals that comprise the oxide zone evidence is presented for other porphyry deposits radomiro tomic and gaby sur that basinal brines may have been involved in the late formation of atacamite

porphyry copper and tourmaline breccias at los bronces rio blanco chile the los bronces rio blanco deposit is located on the west side of the andes in central chile about 69 km from santiago los bronces is a breccia complex superimposed on the west side of an earlier major porphyry copper system the rio blanco mine currently exploits the north central part of this porphyry deposit and has started operating a large copper bearing tourmaline breccia sur sur about 2 km south of the present mine the los bronces rio blanco deposit was formed on the east side of the san francisco batholith this intrusion is strongly peraluminous and has a calc alkaline composition with an alkali calcic affinity the batholith took a minimum of 11 5 m y to form from the early miocene 20 1 m y to the late miocene 8 6 m y the porphyry copper mineralization alteration and copper tourmaline breccias were formed over a period of at least 2 5 m y between 7 4and 4 9 m y ago a postmineral volcanic neck or diatreme at la copa erupted within and removed a large segment of the northern part of the porphyry copper system thus marking the last evidence of magmatic activity in the area k ar age determinations of biotites indicate the diatreme erupted in the early pliocene between 4 9 and 3 9 m y ago the prebreccia porphyry system exhibits propylitic sericitic silicic and potassic alteration a unique alteration feature of this system is the replacement of mafic minerals by specularitc and or tourmaline within the propylitic zone the porphyry system contains disseminated and stockwork copper iron molybdenum sulfide mineralization within an area of about 12 km 2 los bronces is composed of at least seven different copper bearing tourmaline breccias that form one large contiguous kidney shaped body about 2 km long and 0 7 km wide at the present erosion surface the breccia body crops out at elevations between 4 150 and 3 450 m the various breccias are characterized by their locations matrices clasts shapes types and degrees of mineralization and alteration the breccias are usually monolithic but in some cases are bilithic or heterolithic with most clasts consisting of quartz monzonite or andesitc with locally minor amounts of quartz latite porphyry monzodiorite and vein quartz the breccia matrices consist of variable amounts of quartz tourmaline specularitc anhydrite pyrite chalcopyrite bornitc molybdenite sericite chlorite and rock flour the seven different breccias types are identified from oldest to youngest as ghost central western infiernillo anhydrite fine gray and donoso the breccia complex has sharp contacts with the surrounding intrusive rocks and andesites internally the breccia contacts are locally well defined but elsewhere they coalesce interfinger or display gradational contacts breccias at los bronces are interpreted as being emplaced explosively followed by collapse after pressure release of hydrothermal fluids the primary mineral distribution is best known in the donoso breccia which has been the center of mining activity since its discovery in 1864 in spite of the coarse and irregular nature of the sulfides in the matrix chalcopyrite pyrite and specularitc at the 3 670 m open pit operating level show a tendency to be distributed in irregular shells in which one of the three minerals predominates in any one shell the transitions between shells are rapid several semiellipsoidal shells of alternating high and low copper grades are also apparent from the copper distribution of underground level 3640 and from various cross sections the shells are approximately vertical subparallel to the donoso breccia contacts which dip inward secondary enrichment enhanced the primary grade in the southern two thirds of the los bronces breccia complex and in much of the surrounding porphyry copper system the degree and depth of enrichment are a function of breccia and fracture permeability and extend to a depth of more than 500 m in certain favorable sectors the shape and depth of the enrichment blanket and overlying leached capping suggest that the enrichment process is related to the present ground water regime and is still active

geomorphologic environment and age of supergene enrichment of the cuajone quellaveco and toquepala porphyry copper deposits southeastern peru copper ore grades of the cuajone quellaveco and toquepala porphyry cu mo deposits situated at 3 000 to 4 000 m a s l on the pacific slope of the cordillera occidental of southernmost peru lats 17 degrees 02 17 degrees 15 s have been markedly increased by supergene sulfide enrichment following the emplacement of the hypogene mineralization in the early eocene 52 57 ma as the terminal stage in the development of the toquepala group continental volcano plutonic terrane this andean transect was gradually reduced by erosion in a semiarid climate to a low altitude topography unroofing of the deposits had taken place by the mid oligocene the initiation of cordilleran uplift at ca 25 to 26 ma accompanied by the episodic eruption of felsic ash flow tuffs at the oceanward front of the volcanic arc led to more rapid erosion and the conversion of the mid tertiary landscape into the subplanar altos de camilaca surface this regionally extensive pediplain constitutes the major landform component of the present precordillera surrounding the porphyry centers its final configuration was attained at ca 18 to 19 ma ash flow tuff eruption was widespread and frequent at this time supergene enrichment began in the late oligocene during the progressive lowering of topography and continued through the more abrupt water table depression resulting from the latest oligocene to early miocene uplift however the distribution of chalcocite sensu lato in the three porphyry deposits and the local postmineralization landforms and volcanic histories differed significantly during the latter interval at toquepala and to a lesser extent quellaveco the landform regimes were dominated by open valleys and ignimbrite blanketing was short lived whereas the exposed cuajone deposit was both the site of aggressive early miocene valley incision and the deposition of an unusually thick in part welded ash flow tuff at 22 8 or 0 7 ma as a result the enrichment blanket at cuajone remained relatively thin and was even partially eroded in the early miocene whereas the blankets at quellaveco and toquepala were thickened continued strong uplift in the mid miocene ca 8 15 ma generated the array of apron and terrace pediments of the multiple pediment stage which dominates the lower cordilleran slopes and in the mineralized areas led to the deepening of existing valleys and the development of new fluvial channels ignimbrite eruption persisted throughout the miocene again local regimes of erosion and volcanism proved inimical to supergene activity at cuajone but enrichment continued at quellaveco and particularly toquepala where there is no record of later miocene ash flow accumulation uplift in this period was apparently also most extensive in the vicinity of the toquepala deposit which experienced the formation of a deep chalcocite blanket while the extant enrichment zone at quellaveco was thickened the development of enriched assemblages containing massive chalcocite had clearly terminated by 13 1 or 0 4 ma at cuajone and by 9 5 or 0 5 ma at quellaveco the timing of later enrichment at toquepala is less constrained but a mid miocene age is probable supergene upgrading of the three peruvian deposits was contemporaneous with that of copper mineralization in northernmost and northern chile as at chuquicamata and la escondida and in the copiapo mining district in all of which areas a late oligocene to mid miocene age has been inferred for supergene alteration this major and regionally developed metallogenic episode took place during and between two major periods of cordilleran uplift under semiarid climatic conditions the termination of intense enrichment along this 2 000 km stretch of the cordillera occidental in the late miocene was a direct result of marked climatic desiccation which not only reduced the overall supergene activity but focused fluvial erosion leading to the incision of steep walled canyons less favorable for supergene alteration than the earlier subplanar landforms

40ar 39ar geochronology of supergene k bearing sulfate minerals cenozoic continental weathering and its paleoclimatic significance in the tu ha basin northwestern china abstract 40ar 39ar incremental heating analyses of supergene jarosite and yavapaiite were conducted for three weathering profiles at the hongshan liuhuangshan and caihuagou deposits in the tu ha basin the analyses provide numerical constraints on the timing and duration of weathering and supergene enrichment well constrained plateau ages and best fitting inverse isochrons yield 40ar 39ar ages ranging from 33 3 0 5 ma to 3 3 0 4 ma 1 our 40ar 39ar ages combined with the published ages indicate that a protracted history of weathering and supergene enrichment and by inference arid semiarid climate with at least a moderate amount of precipitation 10 cm y favorable to intense chemical weathering emerged at 33 3 ma 27 7 23 3 ma and 16 4 14 7 ma and prevailed from 11 to 7 8 ma then a progressive change from arid semiarid toward hyperarid conditions and predominantly hyperarid conditions may have persisted since at least ca 3 3 ma the climatic implications inferred from the weathering geochronology are in agreement with the chemical parameters and isotopic compositions of the cenozoic sedimentary sequence from the lianmuqin section in the tu ha basin attesting to the reliability of weathering geochronology by the 40ar 39ar method as an indicator of paleoclimate in arid areas our results suggest that the retreat of the paratethys sea which would have reduced eastward water vapor transport by the westerlies to the tu ha basin led to its aridification in the oligocene and that increased rain shadow effects resulting from uplift of the tibetan plateau and tian shan mountains played important roles in the aridification history of the tu ha basin since the late miocene

neogene landscape evolution in the andes of north central chile between 28 and 32 s interplay between tectonic and erosional processes

k ar and 40ar 39ar geochronology of supergene processes in the atacama desert northern chile tectonic and climatic relations new k ar and 40 ar 39 ar data obtained for 29 samples of supergene alunite jarosite and hollandite group minerals from the central depression of northern chile document almost continuous supergene oxidation from 33 to 9 ma the downward migration of supergene oxidation zones is used to calculate maximum denudation rates of 16 35 m ma 1 these calculated rates are sensitive to pre existing topography and show that the present relief observed in the central depression represents a modified miocene landscape the results presented here are in good agreement with previously published geochronological data for the atacama desert the combination of all published data including the results presented here defines a continuous skewed distribution with a maximum between 21 and 14 ma the relative paucity of ages greater than 30 ma reflects the removal of supergene profiles by erosion during and immediately after the incaic compressive deformation from 29 to 9 ma supergene oxidation profiles were generated under semi arid conditions throughout the atacama region ages between 9 and 5 ma on the northern and southern margins of the desert mark the end of supergene oxidation processes consistent with a pliocene onset of present day hyperaridity

analysis of supergene ore forming processes and ground water solute transport using mass balance principles analysis of metal distribution in weathering profiles developed above ore deposits provides a simple and direct method to determine the geologic controls on surficial leaching and enrichment of metals by supergene descending fluids this approach offers an understanding of the interrelationships of metal concentration zone thicknesses and bulk rock density of the various zones within a weathered rock column analytical solutions to mass balance equations provide expressions describing the chemical evolution paths of enrichment blanket metal grade b in terms of original protore metal grade p leached zone metal grade 1 and the ratio of total leached zone column height to blanket thickness l o t b neglecting density terms for the sake of simplicity this function is expressed as b p l o t b p 1 linear data arrays in the coordinate system b versus p 1 for groups of drill holes in individual geologic domains e g wall rock types strongly suggest that enrichment blanket metal grades evolve from a protore metal grade p with a fairly constant slope of l o t b to a final value b determined by the extent of metal leaching in the zone of oxidation the constancy of the ratio l o t b within geologic domains indicates a simultaneous deepening of the zone of oxidation and thickening of the enrichment blanket under conditions of a descending ground water table analysis of metal mass balance in drilling profiles also provides a means of reconstructing paleosurficial topography at the time of active oxidation even in highly eroded terrains application of these methods using rock density and assay data from the butte district of montana shows the paleo oxidation surface to have been eroded by as much as 450 ft down to the present unconformity between weathered bedrock and overlying tertiary gravels geomorphic stability and slow erosion rates often thought necessary for effective chemical weathering are therefore questioned we propose instead a general balance of rates of erosion and descent of the ground water table for optimal enrichment preservation of the fossil weathering profiles occurs by submergence of the zone of oxidation below the paleoground water table resulting in cessation of supergene oxidation and preservation of the blanket the mass balance methods derived in this study also relate overall leaching efficiency to protore characteristics showing a strong hypogene alteration control at butte as described by mcclave 1973 lateral copper transport is evaluated by identifying local anomalies in the computed oxidation surface zones of copper introduction near faults and zones of high permeability appear as apparent topographic highs whereas zones of depletion appear as topographic depressions preliminary analysis of this type at the porphyry copper deposit at la escondida chile indicates deeper oxidation and leaching effects than at butte the assumption of vertically homogeneous protore copper grades leads to the conclusion that erosion of at least 200 m of leached capping has occurred throughout much of the la escondida district geologic domains defined by computed values of l o t b correlate well with hypogene alteration zoning computation of the magnitude and direction of overall subsurface lateral metal fluxes indicates transport distances of at least 1 km in directions consistent with present surface topography and fracture patterns there is a strong correlation of the thickest and highest grade enrichment zones with inferred positive lateral fluxes whereas the most thoroughly leached cap rocks make up zones of inferred negative lateral flux which are likely source regions alternative explanations to the lateral flux hypothesis are evaluated including assessment of variations in protore metal grade projections both in vertical and horizontal directions despite the approximations involved in the present calculations the lateral flux pattern appears to be the most likely interpretation of the present metal distribution in the supergene system at la escondida

tectonic and magmatic evolution of the andes of northern argentina and chile abstract two orogenic cycles both with different evolution are developed in the western margin of the south american continent in northern argentina and chile the paleozoic hercynic cycle and the meso cenozoic andean cycle the hercynic cycle a wide marine basin extending westward of the cordillera oriental which developed in cambrian ordovician times marks the beginning of this cycle in contrast to peru and bolivia where this basin developed between two precambrian blocks the western margin of this basin in northern argentina and chile is still unknown the ordovician sedimentation and accompanied volcanism ends with the ocloyic deformation phase and its synkinematic granitic plutonism two basins were developed in the silurian devonian separated by the puna arc uplifted during this ocloyic phase the shallow water marine terrigeneous sediments which were deposited in them were deformed by a new tectono magmatic associated phase chanic phase carboniferous to lower permian marine carbonates were deposited west of the puna arc and red beds east of it later on during the permian to triassic a magmatic belt was developed along the cordillera occidental the rhyolites ignimbrites and the granitic to granodioritic related intrusives are well represented in chile although the overall geologic history of this period is known many problems concerning its origin and relations to plate tectonics are still unsolved the andean cycle during this cycle a series of magmatic arc systems related to the subduction of the pacific crust was built up along the western margin of south america huge volumes of calc alkaline lavas and related plutons were emplaced since the jurassic along belts parallel to the present coastline showing a general eastward migration trend up to the lower cretaceous an ensialic back arc basin was formed east of this magmatic arc thousands of meters of marine and continental sediments were deposited in it this basin disappeared during the middle cretaceous probably as a result of the final opening of the atlantic and the active westward movement of the south american plate since middle cretaceous times the magmatic arc has been the fundamental paleogeographic element the arc progressively migrated stepwise eastward each step marked by a tectonic phase subduction related crustal erosion could explain the lack of fore arc petrotectonic assemblages the eastward magmatic polarity which characterizes this section of the andes could also be explained by such a mechanism

a genetic model based on evapoconcentration for sediment hosted exotic cu mineralization in arid environments the case of the el tesoro central copper deposit atacama desert chile although the formation of exotic cu deposits is controlled by multiple factors the role of the sedimentary environment has not been well defined we present a case study of the el tesoro central exotic cu deposit located in the atacama desert of northern chile this deposit consists of two mineralized bodies hosted within late cenozoic gravels deposited in an arid continental environment dominated by alluvial fans with sub surficial ponded water bodies formed at the foot of these fans or within the interfan areas both exotic cu orebodies mostly consist of chrysocolla copper wad atacamite paratacamite quartz opal and calcite the most commonly observed paragenesis comprises chrysocolla silica minerals and calcite and records a progressive increase in ph which is notably influenced by evaporation the results of stable isotope analyses 13c and 18o and hydrogeochemical simulations confirm that evapoconcentration is the main controlling factor in the exotic cu mineralization at el tesoro central this conclusion complements the traditional genetic model based on the gradual neutralization of highly oversaturated cu bearing solutions that progressively cement the gravels and underlying bedrock regardless of the depositional environment this study concludes that in exotic cu deposits formed relatively far from the source a favorable sedimentary environment and particular hydrologic and climatic conditions are essential to trap accumulate evapoconcentrate neutralize and saturate cu bearing solutions to trigger mineralization thus detailed sedimentological studies should be incorporated when devising exploration strategies in order to discover new exotic cu resources particularly if they are expected to have formed relatively far from the metal sources

giant porphyry deposits characteristics distribution and tectonic controls more than half of the 25 largest known porphyry copper deposits defined in terms of contained copper metal formed during three time periods the paleocene to eocene eocene to oligocene and middle miocene to pliocene these giant deposits are clustered within three provinces central chile northern chile and southwest arizona northern mexico other giant deposits occur in montana utah panama peru argentina irian jaya mongolia and iran compressive tectonic environments thickened continental crust and active uplift and erosion were associated with the formation of many of these deposits calc alkalic magmas are most favorable for the formation of giant porphyry copper deposits although several of the largest systems are associated with high k calc alkalic intrusions the 25 largest gold rich porphyry deposits are concentrated in the southwest pacific and south america with other occurrences in eurasia british columbia alaska and new south wales many of the deposits formed in the last 13 m y the largest of the deposits are associated with high k calc alkalic intrusions many calc alkalic porphyritic intrusions have also produced giant gold rich porphyries in the last 20 m y the formation of giant porphyry copper molybdenum and copper gold deposits in the circum pacific region has been closely associated with subduction of aseismic ridges seamount chains and oceanic plateaus beneath oceanic island and continental arcs in several examples these tectonic perturbations have promoted flat slab subduction crustal thickening uplift and erosion and adakitic magmatism coeval with the formation of well endowed porphyry and or epithermal mineral provinces similar tectonic features are inferred to be associated with the giant porphyry copper molybdenum provinces of northern chile eocene oligocene and southwest united states cretaceous paleocene topographic and thermal anomalies on the downgoing slab appear to act as tectonic triggers for porphyry ore formation other factors such as sutures in the overriding plate permeability architecture of the upper crust efficient processes of ore transport and deposition and in some cases formation and preservation of supergene enrichment blankets are also vital for the development of high grade giant ore deposits a low grade geochemical anomaly may be the final product of mineralization if ore forming processes do not operate efficiently even in the most favorable geodynamic settings

carbon isotope evidence for microbial involvement in exotic copper silicate mineralization huinquintipa and mina sur northern chile exotic copper silicate and oxide mineralization a peripheral facies of several cenozoic porphyry cu mo systems in northern chile is traditionally ascribed to the lateral migration of cu rich initially highly acidic waters away from active zones of supergene leaching and sulfide enrichment copper precipitation as cu fe silicates and cu mn fe oxides in permeable accumulating pediment gravels or underlying fractured bedrock resulted in part from fluid neutralization attending hydrolytic alteration supergene activity was most intense in the late oligocene early miocene during a protracted middle cenozoic semiarid interval along the lower pacific slope of the central andean cordillera occidental samples from the huinquintipa and mina sur deposits contiguous with respectively the 32 7 to 34 4 ma rosario collahuasi and 31 1 to 34 6 ma chuquicamata porphyry centers were examined for carbon isotope evidence of the conditions of ore formation carbon was extracted as co2 from representative high grade chrysocolla rich samples using four different techniques combustion pyrolysis crushing and edta leaching and has a widely varying isotopic composition consistent with mixing of atmosphere derived carbon with a 13c value of 0 per mil plant derived carbon with 13c values between 25 and 15 per mil and carbon with exceptionally low 13c values of 47 per mil the extremely light carbon sampled primarily by leaching with a 10 percent edta solution is most plausibly ascribed to a microbial consortium including anaerobic methanogenic microbes the presence of mineralized carbon having such low 13c values provides evidence that microbial metabolism may have contributed directly to copper silicate precipitation in these deposits lower 13c values at huinquintipa than at mina sur could be attributed to higher water tables and higher toxic metal contents in the mineralizing fluids the new data reinforce the role played by microbes in the economically critical supergene modification of porphyry and epithermal ore deposits in the semiarid environment

late pliocene age for the atacama desert implications for the desertification of western south america the atacama desert forms one of the major hyperarid deserts of the world previous studies suggest that desertification commenced at 14 ma during global climate desiccation sedimentologic data from middle miocene to upper pliocene successions in the modern atacama desert indicate that a semiarid climate persisted from 8 to 3 ma punctuated by a phase of increased aridity at ca 6 ma as such hyperaridity did not commence until the late pliocene implications are 1 that the rain shadow generated by the andean cordillera has had a minor influence on climate change and 2 that the upwelling north flowing cold humboldt current although important in establishing the generally arid climate of western south america did not control the shift to hyperaridity the formation of the hyperarid atacama desert in the late pliocene accompanied the development of the current phase of aridification in the sahara and namib deserts and is attributed to a phase of global climate cooling

middle miocene climatic change in the atacama desert northern chile evidence from supergene mineralization at la escondida geochronology and paleotopographic reconstruction of the porphyry copper deposit at la escondida chile are used to calculate long term erosion rates and to deduce the timing of tertiary climatic change for a portion of the atacama desert region hypogene hydrothermal alteration and protore mineralization at la escondida took place between 33 7 1 4 and 31 0 1 4 ma based on k ar dating of hydrothermal biotite and sericite supergene weathering and copper sulfide en richment processes were active from 18 0 0 7 to 14 7 0 6 ma based on k ar dating of supergene alunite distinguished from hypogene alunite by grain size color and sulfur isotopic composition reworked lenses of volcanic ash in the vicinity of la escondida provide useful time stratigraphic markers at 8 7 0 4 6 5 0 2 and 4 2 0 2 ma within present soil profiles long term average rates of erosion are determined by these age dates and quantitative calculation of eroded leached capping thickness at la escondida using mass balance analysis of geochemical profiles coupled with an estimate of unmineralized lithocap thickness based on alteration petrology and fluid inclusion geobarometry at similar deposits the observed trend of decreasing long term average erosion rates with time is consistent with arid to semiarid conditions in the early miocene changing to hyperarid conditions during the middle miocene this climatic desiccation caused termination of significant supergene copper suffide enrichment at la escondida and elsewhere in the atacama region and preservation of surficial features including the ash horizons and the leached capping middle miocene climatic desiccation in northern chile and southern peru was probably related to a pronounced decrease in temperature of coastal waters supplied by an ancestral humboldt current and an increase in upwelling intensity as the antarctic ice cap became established at approximately 15 to 13 ma the central andes cordillera which now provides a rain shadow protecting the atacama region from precipitation from the east must have attained at least half its present elevation prior to about 15 ma to have played a similar role in the middle miocene

upper cenozoic volcanism and tectonism in the andes of northern chile abstract recent geological and radiometric information of the upper cenozoic volcanic sequences of northern chile in conjunction with previous evidences point out the complex evolution of the central andes volcanism volcanic activity developed along the andean range during the last 23 m y appears to have been temporally and genetically related to tectonic movements that shaped the major morphostructural features of this region since the middle miocene 12 m y there has been a constant overlap both in time and space between lavas and ignimbrites however ignimbrite eruptions seem to predominate up to the upper miocene while lava eruptions are dominant from the pliocene to the quaternary small scale chronological and volumetric variations of the volcanic activity in the studied areas make difficult the correlations with volcanic episodes through long distances units older than pliocene have been folded and thrusted as a result of compressive movements that occurred ca 7 4 m y ago this deformation is mostly remarkable at the borders of major faulted blocks these blocks were structured along pre existing fractures caused by tectonic movements during the lower miocene pliocene and quaternary block movements developed mainly through n s fault systems these faults together with nw se and ne sw systems control the spatial distribution of most volcanic vents in this region locally the n s faults may correspond to reactivations of previous fault zones this allowed successive effusions of volcanic material at least during the last 17 m y

controls on supergene enrichment of porphyry copper deposits in the central andes a review and discussion the central andes host some of the world s largest porphyry copper deposits the economic viability of these deposits is dependent on the size and quality of their supergene enrichment blanket published models that have strongly influenced exploration policy suggest that supergene enrichment ceased at 14 ma due to an increase in aridity here we discuss these models using published geochronological geomorphological and geological data geochronological data indicate that supergene oxidation and enrichment has been active between 17 and 27 s across the forearc of northern chile and southern peru from 44 to 6 ma and on the bolivian altiplano and eastern cordillera of argentina from 11 ma to present there is evidence for cessation at 20 14 and 6 ma however a major problem is that as more geochronological data become available the age ranges and periods of enrichment increase this suggests that the full spectrum of enrichment ages may not have been sampled the relationship between supergene enrichment and the age of regional pediplain surface development is not well constrained only in two areas have surfaces related to enrichment been directly dated southern peru and south of 26 s in chile and suggest formation post 14 ma sedimentological data indicate that a fluctuating arid semi arid climate prevailed across the atacama desert until between 4 and 3 ma climatic conditions that are thought to be favourable for supergene enrichment the balance between uplift erosion burial and sufficient water supply to promote enrichment is complex this suggests that a simple model for controlling supergene enrichment is unlikely to be widely applicable in northern chile general models that involve climatic desiccation at 14 ma related to rainshadow development and or the presence of an ancestral cold upwelling humboldt current are not supported by the available geological evidence the integration of disparate sedimentological geomorphological and supergene age data will be required to fully understand the controls on and distribution of supergene oxidation and enrichment in the central andes

ela icp ms u pb zircon geochronology of regional volcanism hosting the bajo de la alumbrera cu au deposit implications for porphyry related mineralization ela icp ms u pb zircon geochronology has been used to show that the porphyritic intrusions related to the formation of the bajo de la alumbrera porphyry cu au deposit nw argentina are cogenetic with stratigraphically well constrained volcanic and volcaniclastic rocks of the late miocene farallon negro volcanic complex zircon geochronology for intrusions in this deposit and the host volcanic sequence show that multiple mineralized porphyries were emplaced in a volcanic complex that developed over 1 5 million years volcanism occurred in a multi vent volcanic complex in a siliciclastic intermontane basin the complex evolved from early mafic intermediate effusive phases to a later silicic explosive phase associated with mafic intrusions zircons from the basal mafic intermediate lavas have ages that range from 8 46 0 14 to 7 94 0 27 ma regionally extensive silicic explosive volcanism occurred at 8 0 ma 8 05 0 13 and 7 96 0 11 ma which is co temporal with intrusion of the earliest mineralized porphyries at bajo de la alumbrera 8 02 0 14 and 7 98 0 14 ma regional uplift and erosion followed during which the magmatic hydrothermal system was probably unroofed shortly thereafter dacitic lava domes were extruded 7 95 0 17 ma and rhyolitic diatremes 7 79 0 13 ma deposited thick tuff blankets across the region emplacement of large intermediate composition stocks occurred at 7 37 0 22 ma shortly before renewed magmatism occurred at bajo de la alumbrera 7 10 0 07 ma the latest porphyry intrusive event is temporally associated with new ore bearing magmatic hydrothermal fluids other dacitic intrusions are associated with subeconomic deposits that formed synchronously with the mineralized porphyries at bajo de la alumbrera however their emplacement continued from 7 10 0 06 to 6 93 0 07 ma after the final intrusion at bajo de al alumbrera regional volcanism had ceased by 6 8 ma 6 92 0 07 ma