application of thermochronology to hydrothermal ore deposits thermochronology finds many applications in economic geology utilizing temperature sensitive radiometric dating techniques to reveal low temperature upper crustal processes can elucidate many aspects of deposit genesis including timing and duration of mineralization processes rate of exhumation and erosion of intrusive ore deposits and comparative preservation potential the tools are utilized to best advantage when combined with other thermochronometry techniques that provide complementary information in addition when thermochronometers are combined with geochronometers e g zircon u pb over 800 c of thermal history is revealed from emplacement to erosion fig 1 with the advent of computational algorithms that provide more accurate and detailed models of thermochronology results the economic geologist has a powerful tool to use when assessing economic favorability of a region or prospect this chapter summarizes the various ways that low temperature thermochronometers have been utilized in studies of economic mineralization and primarily focuses on studies of porphyry ore deposits these deposits are well characterized and provide a good platform from which to demonstrate the extended understanding of ore formation processes that is provided by thermochronology studies in this section we review the fundamental application of u th he fission track and 40ar 39ar thermochronometry methods to mineralized systems we then present a synopsis of how these techniques can be used in thermal history analysis either alone or in combination with other chronometers in order to provide more information on deposit genesis the discussion is presented from low through to higher temperature thermochronometers corresponding to their presentation on figure 1 u th he thermochronology as documented earlier in the volume u th he thermochronology is based on measuring the accumulation of radiogenic 4he produced from u and th decay the daughter helium is retained until the mineral is heated to a temperature at which its structure and helium retentivity change this closure temperature varies from

the impact of hydrodynamics authigenesis and basin morphology on sediment accumulation in an upwelling environment the miocene monterey formation at shell beach and mussel rock pismo and santa maria basins central california u s a abstract in the sections at shell beach pismo basin and mussel rock santa maria basin both along the central california coast the miocene monterey formation is composed of laminated biosiliceous organic rich and phosphate rich sediments which are intercalated with carbonates and volcanogenic deposits the presence of small and large scale scours conglomerates gravity flow deposits slumps and condensed intervals reveals the importance of sediment reworking which is related to the presence of bottom currents the general morphology of the basins and recurrent high energy events of likely seismic origin the lithologies reflect strong upwelling enhanced primary productivity and the presence of an oxygen minimum zone which is indicated by ubiquitous lamination a general absence of bioturbation with the rare exception of traces related to the gravity flow displacement of trace makers high pyrite mo and u concentrations and molar ratios of total organic carbon and organic phosphorus toc p org of up to 2000 oxygen depletion was particularly severe during the early langhian and the late serravallian and early tortonian and favored the preservation of organic matter episodes of deposition of organic rich sediment were followed by longer phases of subdued sediment accumulation and early diagenetic phosphogenesis which resulted in the formation of a low porosity less permeable lid in the near surface sediments that assisted in preserving organic matter phosphogenesis occurred throughout the time interval considered approximately 17 5 6 5 ma and phosphate rich sediments formed especially during periods of sedimentary condensation in the langhian and earliest serravallian and the latest tortonian and early messinian a comparison of temporal trends in the type and quantity of deposited sediments between the two sections at shell beach and mussel rock and three sections of the santa barbara basin el capitan state beach naples beach and haskell beach confirms that towards the top of the monterey formation sediments become enriched in biosilica in all three basins the transition into this lithology occurred diachronously during the serravallian in the pismo and santa maria basins and tortonian in the santa barbara basin and was probably related to a diachronous shift to more intense upwelling in all three basins a phase of important condensation and phosphogenesis terminated monterey type sedimentation during the latest tortonian and early messinian in both the santa maria and santa barbara basins the uniform expression of this phase is interpreted as the result of improved connections between the two basins

argon diffusion in hypogene and supergene alunites implications to geochronology and thermochronometry on earth and mars abstract argon release mechanisms and diffusivity were quantified for coarsely crystalline hypogene alunite kal3 so4 2 oh 6 from marysvale utah and microcrystalline supergene alunite aggregates from coober pedy south australia prior to diffusivity studies all alunite samples sorted in the 500 200 200 100 100 50 50 10 and 460 c isothermal holding time and agesme modelling using our calculated diffusion parameters indicate that alunite should preserve ar quantitatively for long periods 4 0 ga at earth and mars surface conditions and both hypogene and supergene alunite should preserve original formation ages independently of precipitation mechanism

andean uplift and climate evolution in the southern atacama desert deduced from geomorphology and supergene alunite group minerals abstract supergene alunite group minerals from the late eocene el salvador porphyry cu district the el hueso epithermal gold deposit and the coya porphyry au prospect located in the precordillera of northern chile 26 to 26 30 lat s have been dated by the 40 ar 39 ar method and analyzed for stable isotopes these data support published geomorphologic and sedimentologic evidence suggesting that the precordillera in the southern atacama desert had been uplifted as early as the late eocene and thus significantly prior to the altiplano which attained its high elevation only in the late miocene the oldest supergene alunite from the damiana exotic deposit at el salvador was dated at 35 8 1 ma and yielded a d vsmow value of 74 which indicates elevations of the precordillera near el salvador of at least 3000 m in the late eocene in contrast miocene supergene alunite from el salvador el hueso and coya have less negative d signatures reaching values as high as 23 to 25 at el hueso and el salvador between about 8 2 and 14 ma late miocene to holocene supergene alunite jarosite and natroalunite ages are restricted to el hueso and coya located near 4000 m above sea level in the precordillera roughly 1000 m higher than the present elevation of el salvador the d values of samples younger than 5 ma vary between 57 and 97 the complex evolution of the d signatures suggests that meteoric waters recorded in supergene alunite group minerals were variably affected by evaporation and provides evidence for climate desiccation and onset of hyper arid conditions in the central depression of the southern atacama desert after 15 ma which agrees well with published constraints from the atacama desert at 23 24 lat s our data also suggest that wetter climatic conditions than at present prevailed in the latest miocene and early pliocene in the precordillera the new and previously published age constraints for el salvador indicate that supergene mineralization at the damiana exotic cu deposit occurred periodically over 23 ma in a locally exceptionally stable paleohydrologic and geomorphologic configuration

antiquity of aridity in the chilean atacama desert the atacama desert of northern chile and southern peru has one of the longest histories of arid conditions known although most studies have focused on the hyper aridity prevalent since the miocene all terrestrial sediments in the region from the late triassic onwards also record evaporitic and thus arid climates supergene mineralization in the region did not develop under a more humid climate conducive to deep weathering but under arid conditions these processes may have been facilitated by hydrological changes during miocene uplift and drainage incision but were operational prior to the uplift similarly global cooling and changes to oceanic circulation in the post miocene period only accentuated existing conditions a whole regolith perspective is vital to understanding the history of aridity in the atacama desert and its relevance to arid zone morphogenesis regolith formation and supergene mineralization in particular the long history of aridity raises the possibility that supergene mineralisation under the appropriate conditions form in arid environments instead of requiring humid conditions

remnants of a fossil alluvial fan landscape of miocene age in the atacama desert of northern chile using cosmogenic nuclide exposure age dating we have used cosmogenic nuclides to estimate limits on the surface exposure durations and erosion rates of alluvial fans and bedrock surfaces in the atacama desert in chile the oldest landforms we studied are extensive alluvial fans referred to as atacama gravels with the exception of samples collected in antarctica the cobbles collected on these alluvial surfaces have the lowest erosion rates of any samples as determined by cosmogenic nuclides analyzed to date the oldest cobble has a model surface exposure age of 9 myr based on combined measurements of cosmogenic 10be 26al and 21ne concentrations cobbles from the alluvial fans are eroding slower than the surrounding steep mountainous bedrock surfaces maximum erosion rates for cobbles on alluvial surfaces are uniformly 0 1 m myr the survival of these gravels specifically and more generally the stability of landform features in this geographic area is made possible by the attainment of hyperarid conditions in the atacama desert resulting from global climatic cooling about 15 myr ago combined with the rain shadow effect caused by uplift of the central andes the landform features observed presently in the atacama desert are remarkably stable and despite the inevitable erosion that is detectable using cosmogenic nuclides undoubtedly bear considerable resemblance to conditions as they existed in the miocene over geologic time the atacama landscape is evolving in such a manner as to erode the higher bedrock ridges relative to the more stable but stratigraphically lower depositional surfaces through which clastic detritus now travels occasionally along the floors of incised drainage systems leaving the older permeable alluvial fan surfaces largely intact as widespread remnants of a miocene fossil landscape

phosphogenesis and organic carbon preservation in the miocene monterey formation at naples beach california the monterey hypothesis revisited the middle part of the miocene monterey formation at naples beach west of santa barbara california is predominantly composed of organic rich mudstone interstratified with phosphatic laminae minor lithologies include volcanic ash dolomite porcelanite and chert and condensed phosphatic beds sediments dated as 14 3 13 5 ma have average total organic carbon toc values around 8 5 wt and organic carbon oc accumulation rates are around 565 mg cm2 k y sediments dated as 13 5 13 ma are characterized by average toc values of 12 6 wt and oc accumulation rates of around 1130 mg cm2 k y the interval between 13 and 10 6 ma is marked by condensation average toc values are around 8 6 wt and oc accumulation rates diminished to around 55 mg cm2 k y the last interval studied is dated as 10 6 9 4 ma and average toc values are around 6 wt whereas oc accumulation rates rose again to 320 mg cm2 k y the presence of erosional surfaces angular unconformities and reworked clasts and nodules suggests that bottom current activity and gravity flow deposition have been instrumental in sediment accumulation the phosphatic laminae were precipitated at a very early stage of diagenesis during periods of nonsedimentation they formed less permeable sedimentary lids and may as such have contributed to enhanced oc preservation between 13 and 10 6 ma the thus formed phosphatic laminae were frequently subjected to subsequent sediment winnowing and reworking resulting in the formation of condensed phosphatic beds calculated p c molar ratios suggest that 1 the measured section is highly enriched in phosphorus p relative to oc 2 regeneration of organic p from organic matter decomposition was negligible and 3 the source of p was external likely upwelled bottom water rich in inorganic p in spite of good preservation conditions and correspondingly high toc contents the overall oc accumulation rates are moderate in comparison to those of actual high productivity areas which is mainly due to the episodic character of depositional processes and the intervening long periods of nondeposition and sediment reworking they preclude this section and by extrapolation the monterey formation in general from being an important oc sink during the middle miocene alternatively large oc sinks were probably created on the continent lignite deposits and in sedimentary depocenters which received increasing amounts of detrital sediments due to a combination of climate change spreading of grasslands and the increasing importance of mountain chains such as the himalaya the associated high nutrient fluxes may have been involved in the backstepping and drowning of carbonate platforms and in the generation of widespread phosphat rich deposits during the late early and early middle miocene

chronology of exotic mineralization at el salvador chile by 40ar 39ar dating of copper wad and supergene alunite exotic copper mineralization is a complex hydrochemical process linking supergene enrichment lateral copper transport and precipitation of copper oxide minerals in the drainage network of a porphyry copper deposit at the el salvador porphyry copper deposit in northern chile the majority of the exotic ore comprised a mixture of copper bearing manganese oxyhydrates termed copper wad x ray diffraction scanning electron microscopy and electron probe microanalysis show that the copper wad is composed of copper bearing cryptomelane k1 2 mn3 mn4 8o16 x h2o and birnessite k0 33 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document mn 7 3 9 end document o14 7h2o structures these natural occurrences within the exotic ore provide the opportunity to directly date the formation of these deposits using recent advances in 40ar 39ar geochronology of supergene k mn oxides formed by weathering a suite of copper bearing cryptomelane and birnessite samples from exotic deposits within the el salvador district were characterized and dated by 40ar 39ar laser step heating supergene alunite kal3 so4 2 oh 6 found in paleospring feeder systems leading from the source zones of copper outward to the exotic mineralization was dated to independently constrain the age of exotic ore formation although the ar retentivity of the layered birnessite structure has been questioned by others the ar retentivity of these samples is thought to be a function of their natural preservation and limited postcrystallization ground water interaction in the hyperarid atacama desert the 40ar 39ar analytical results show that ar and or k losses after crystallization excess 40ar and 39ar recoil do not pose significant problems therefore in the context of exotic copper deposits within hyperarid environments this dating method is applicable to both cryptomelane and birnessite within copper wad the 40ar 39ar dating of exotic mineralization at el salvador indicates that supergene and exotic mineralization processes were active at 35 ma about 5 m y after the emplacement of hydrothermal mineralization and continued until the middle miocene the majority of exotic mineralization extends from the oligocene miocene 24 ma boundary through the middle miocene 11 ma and relates to supergene fluid emanating in multiple directions from the source of copper in indio muerto which represents a topographic high above the exotic deposits one reconnaissance date of exotic mineralization at the chuquicamata porphyry copper deposit yielded an age of 17 03 0 03 ma coincident with the known supergene alunite dates for this deposit reconnaissance dating at exotica mina sur and el abra proved problematic due to the presence of contaminant silicate minerals within the copper wad ages derived here for exotic mineralization are similar to the known supergene ages throughout northern chile the dates indicate that supergene exotic mineralization began at the eocene oligocene boundary and continued through the oligocene eventually ceasing in the middle miocene due to desiccation of the atacama desert a series of 10 cm thick supergene alunite veins were microsampled across their widths and dated to address the kinetics of alunite vein growth apparent growth rates of the veins vary from 71 to 100 mm m y in the horizontal direction to 24 mm m y in the vertical presumably caused by differences in pressure regimes due to their orientation

isotopic dating of supergene manganese oxides from the groote eylandt deposit northern territory australia a total of 51 conventional k ar dates on 22 manganese oxide samples from the groote eylandt manganese deposit is presented because most of the samples contained detrital mica from the bedrock k ar ages and their standard errors were corrected using material balance equations following determination of the contribution of radiogenic argon and potassium from the silicate residue in this way the times of formation of cryptomelane ba todorokite and romanechite and probably also vernadite were estimated in addition high resolution 40 ar 39 ar dating was used to date growth bands of pure void filling cryptomelane corrected k ar ages show that cryptomelane and ba todorokite the latter previously thought to be of diagenetic origin were precipitated between 18 and 6 ma early to late miocene as a result of chemical weathering through widespread dissolution and in situ replacement of pisoliths and ooliths romanechite in the lower part of the profile in the g south quarry must be older than 43 7 or 1 2 ma in the upper part of the g south quarry profile romanechite formed at about 30 ma these ages together with the cryptomelane and ba todorokite ages appear to define several episodes of intense chemical weathering in this deposit during the tertiary a pre late eocene episode older than 43 7 or 1 2 ma an oligocene episode around 30 ma and a miocene weathering megacycle 6 18 ma it is suggested that these weathering related processes had a much more significant effect on development of the deposit than previously thought the present mineralogy was produced by replacement processes during the tertiary associated with intense chemical weathering and not by sedimentary or diagenetic processes isotopic ages on manganese oxides from groote eylandt are similar to those determined on manganese oxides from northwestern australia and brazil indicating that periods of intense weathering in the tertiary recording humid climatic conditions were relatively widespread

mineralogic and stable isotope zonation at the surface over the el salvador porphyry copper deposit chile we examined in detail the mineralogic and stable isotope characteristics of alteration minerals exposed at the surface of the el salvador porphyry copper deposit a total of 276 samples was collected from 203 localities over an area of 5 km 2 at elevations between 2 900 and 3 300 m the alteration assemblages can be separated into two groups broadly corresponding to the stages previously identified from underground studies although the early stage k silicate alteration identified underground is not evident at the surface a transitional to late stage is characterized at the surface by assemblages of 1 muscovite andalusite with trace diaspore including some areas rich in andalusite and 2 muscovite with trace diaspore there is a sharp contact outward with a marginal propylitic assemblage chlorite calcite epidote pyrophyllite occurs as a retrograde overprint of muscovite in both of the transitional to late stage assemblages these assemblages are concentrically zoned and centered on a core of least altered granodiorite porphyry a late intramineral intrusion the muscovite andalusite assemblage occurs up to 500 m away from the granodiorite porphyry and andalusite rich zones are located near the brecciated margin of the porphyry further outward is the muscovite zone from 500 to 1 500 m wide the pyrophyllite overprint overlaps the contact of the muscovite andalusite and muscovite zones and occurs mainly above 3 000 m elevation such that its present distribution is controlled largely by topography very late alteration minerals occur in radial pebble dikes of hydrothermal breccia and their extensions and they are dominated by an advanced argillic assemblage of alunite aluminum phosphate sulfate minerals diaspore zunyite pyrophyllite dickite with residual muscovite in one case a breccia extends upward and appears to be the root of a 300 500 m outcrop 80 m thick of massive quartz alunite replacement of rhyolite pure mineral separates were prepared from these samples for o h and s isotope analysis including 48 samples of silicate minerals muscovite pyrophyllite dickite and supergene kaolinite and 20 samples of alunite some supergene in origin information on mineral assemblage was used to estimate temperatures of formation which then allowed calculation of the isotopic composition of associated hydrothermal fluids the fluid responsible for forming the muscovite had a relatively narrow range of isotopic composition 6 5 1 5 per mil 18 o and 40 10 per mil d the heaviest 18 o and d values about 8 and 35 per mil respectively were obtained from the samples with andalusite muscovite diaspore assemblage this composition is close to that expected for a magmatic fluid with 10 to 20 percent meteoric water component responsible for forming the other muscovite samples the fluid compositions calculated for the very late stage alunite range from 5 and 30 per mil to 1 and 40 per mil 18 o and d respectively this range in composition agrees with the trend predicted for magmatic vapor absorbed by local groundwater the latter similar in composition to that responsible for forming the supergene kaolinite there is no systematic spatial zonation of the alunite samples in terms of isotopic composition or paleotemperature the temperature estimates from s isotope geothermometry are higher than those based on mineral assemblage possibly caused by partial supergene replacement of some hypogene alunites there is no strong isotopic zonation of hydrothermal minerals from surface alteration assemblages at el salvador that can be applied to generate clear exploration guidelines by contrast mapping of alteration mineralogy in the field backed up by laboratory checking of mineralogy and examination of relict minerals and replacement textures in thin section clearly indicates the center of the late intramineral intrusion

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

quantification of bioturbation in hemipelagic sediments via thin section image analysis traditional ichnofabric descriptive semiquantitative l schemes rely upon subjective estimates of the degree to which original sedimentary fabrics have been disrupted this paper presents an objective method of quantifying bioturbation in hemipelagic sediments based on a thin section image analysis technique in order to perform these analyses optical or backscattered electron micrographs are processed in order to obtain black and white images in these images black pixels are silt size larger grains and the clay size sedimentary matrix are white pixels once the initial processing has been accomplished an index h is defined to quantify the horizontal orientation of the larger grain size fraction of the sediment h is low for bioturbated fabric and high for laminated sediments in hemipelagic suspension deposits h can he used to quantify bioturbation assuming that no other sedimentary processes have disrupted the original depositional fabric the quantitive bioturbation scheme described here is consistent with existing semiquantitative bioturbation schemes in addition it is objective and provides higher resolution descriptions of biological processes that disrupted the sediment after deposition applying this quantitative scheme should refine future paleoecological reconstruction derived from lacustrine and marine successions

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

opal and organic carbon in laminated diatomaceous sediments saanich inlet santa barbara basin and the miocene monterey formation abstract laminated biosiliceous sediments from saanich inlet british columbia si the monterey formation california mf and santa barbara basin california sbb are organic rich and were studied to assess the suitability of si and sbb as modern analogs of the miocene mf and to explore the controls on distributions of opal and organic carbon to this end individual laminae and groups of laminae extracted from odp cores and outcrop samples were analyzed using a multi tracer geochemical approach total organic carbon opal c n ratios 13 c of organic matter om kerogen type and major and minor element abundances were measured si sediments are 52 84 detrital 15 45 opal and 1 2 organic carbon 13 c and c n ratios indicate a marine source of the om present in the sediments samples from the upper siliceous member of the mf are 6 41 detritus 28 87 opal and 1 5 organic carbon kerogen types c n ratios and 13 c data are suggest mixed but predominantly marine om sbb samples are 59 90 detritus 6 34 opal 3 22 calcite and 1 5 percent organic carbon si and sbb have similar bulk compositions with the exception of the presence of calcite in sbb samples mf sediments have significantly lower detrital and higher opal contents than si and sbb samples these large differences suggest that si and sbb are not appropriate modern analogs for the mf in si sediments organic carbon is concentrated in opal rich detritus lean laminae and intervals this distribution suggests that the coupling of opal and om created in the surface ocean by diatom productivity is preserved in the sediment record in the mf opal and toc are negatively correlated implying that the coupling of opal and organic carbon did not survive the processes of settling sedimentation diagenesis and or weathering this decoupling of organic carbon and opal in the mf is attributed to dilution of om by high opal flux rates and or the respiration and or redistribution of om during diagenesis and or weathering

a hydroclimatic threshold for landslide initiation on the north shore mountains of vancouver british columbia landslides triggered by rainfall are the cause of thousands of deaths worldwide every year one possible approach to limit the socioeconomic consequences of such events is the development of climatic thresholds for landslide initiation in this paper we propose a method that incorporates antecedent rainfall and streamflow data to develop a landslide initiation threshold for the north shore mountains of vancouver british columbia hydroclimatic data were gathered for 18 storms that triggered landslides and 18 storms that did not discriminant function analysis separated the landslide triggering storms from those storms that did not trigger landslides and selected the most meaningful variables that allow this separation discriminant functions were also developed for the landslide triggering and nonlandslide triggering storms the difference of the discriminant scores cs for both groups is a measure of landslide susceptibility during a storm the variables identified that optimize the separation of the two storm groups are 4 week rainfall prior to a significant storm 6 h rainfall during a storm and the number of hours 1 m3 s discharge was exceeded at mackay creek during a storm three thresholds were identified the landslide warning threshold lwt is reached when cs is 1 the conditional landslide initiation threshold ctli is reached when cs is zero and it implies that landslides are likely if 4 mm h rainfall intensity is exceeded at which point the imminent landslide initiation threshold itli is reached the lwt allows time for the issuance of a landslide advisory and to move personnel out of hazardous areas the methodology proposed in this paper can be transferred to other regions worldwide where type and quality of data are appropriate for this type of analysis

the upper miocene diatomaceous sediments of the northernmost mediterranean region a lamina scale investigation of an overlooked palaeoceanographic archive

fossil whale preservation implies high diatom accumulation rate in the miocene pliocene pisco formation of peru diatomaceous deposits in the miocene pliocene pisco for mation contain abundant whales preserved in pristine condition bones articulated or at least closely associated in some cases in cluding preserved baleen the well preserved whales indicate rapid burial the 346 whales within 1 5 km 2 of surveyed surface were not buried as an event but were distributed uninterrupted through an 80 m thick sedimentary section the diatomaceous sediment lacks repeating primary laminations but instead is mostly massive with irregular laminations and speckles there is no evidence for bioturbation by invertebrates in the whale bearing sediment cur rent depositional models do not account for the volume of diato maceous sediments or the taphonomic features of the whales these taphonomic and sedimentary features suggest that rapid burial due to high diatom accumulation in part by lateral advection into pro tected shallow embayments is responsible for the superb preser vation of these whales leading to a higher upper limit on phyto plankton accumulation rates than previously documented

climate shift at 4400 years bp evidence from high resolution diatom stratigraphy effingham inlet british columbia canada abstract diatom paleoecology and climatic interpretations were assessed from a 15 cm long laminated sediment slab extracted from an anoxic fjord in southwest british columbia the slab spans at least 62 years of deposition determined from counting varves and is dated at approximately 4400 years bp the slab shows a sedimentation pattern where thick diatom rich varves at the bottom become thinner and more silty toward the top thin section analysis reveals that the thicker varves contain a distinct succession of diatoms representing seasonal deposition throughout each year annual scale subsampling shows that the abundance of coastal marine diatoms namely a weakly silicified form of skeletonema costatum decreased over the 62 year period while benthic and brackish water diatoms such as planothidium delicatulum and achnanthes minutissima increased with the concomitant increase in silt the increase in such benthic species and silt along with the presence of 1 cm thick nonlaminated intervals is interpreted to represent deposition during progressively increasing precipitation over time these sedimentation patterns and changes in diatom assemblages may signify a change in the relative intensities of the aleutian low al and north pacific high nph atmospheric pressure systems thicker diatomaceous varves at the bottom of the slab reflect a stronger nph system with associated coastal upwelling and enhanced diatom production the thinner silty varves at the top of the slab suggest that the al system was prevalent resulting in greater amounts of precipitation and reduced upwelling the findings of this study show that significant natural environmental change can occur within a twenty year time frame and can provide a basis for the study of modern change in the ocean atmosphere system over the northeast pacific ocean

sediment flow and deformation sfd layers evidence for intrastratal flow in laminated muddy sediments of the triassic osawa formation northeast japan abstract thin chaotic layers containing abundant intraformational mud clasts are found in the offshore laminated mudstone of the triassic osawa formation in northeast japan the geometry and internal structures of the layers are somewhat similar to those of slump slide or debris flow deposits however the layers have both upper and lower erosional contacts projections of host sediments and mud clasts with planed surfaces are identified beneath the upper contact of the layers closely packed sheet form or ellipsoidal mud clasts are embedded in a poorly sorted fine to coarse grained sandy matrix some mud clasts are tightly folded or show imbricated stacking internal layering structures are also well developed in the layers the above features strongly suggest soft sediment deformation as horizontal displacement of sand and mud under buried conditions here we term these layers sediment flow and deformation sfd layers deformation layers with similar attributes have been identified by some prior authors as in the case of flow of liquefied and or fluidized host sand and clastic sill intrusion however the sfd layers described herein have more complex structure indicating advanced deformation occurrence of the sfd layers is stratigraphically limited to the uppermost part of the osawa formation which is overlain by sand rich submarine fan deposits the fukkoshi formation it suggests that the stress field caused by differential loading of overlying deposits facilitated intrastratal flow and sill intrusions of sand the vertical confinement was also controlled by the anisotropy in physical properties such as tensile strength of laminated mud and or conditions of gentle slope

age and duration of weathering by 40k 40ar and 40ar 39ar analysis of potassium manganese oxides supergene cryptomelane k1 2 mn3 mn4 8o16 h2o samples from deeply weathered pegmatites in southeastern brazil subjected to 40k 40ar and 40ar 39ar analysis yielded 40k 40ar dates ranging from 10 1 0 5 to 5 6 0 2 ma million years ago laser probe 40ar 39ar step heating of the two most disparate samples yielded plateau dates of 9 94 0 05 and 5 59 0 10 ma corresponding within 2 to the 40k 40ar dates the results imply that deep weathering profiles along the eastern brazilian margin do not reflect present climatic conditions but are the result of a long term process that was already advanced by the late miocene weathering ages predate pulses of continental sedimentation along the eastern brazilian margin and suggest that there was a time lag between weathering and erosion processes and sedimentation processes

high precision 40ar 39ar dating of two consecutive hydrothermal events in the chuquicamata porphyry copper system chile 40ar 39ar data have been used to resolve the relatively high temperature 350 c potassic alteration event at chuquicamata from the lower temperature 350 c quartz sericite alteration phase four k feldspars from the potassic zone yielded plateau large domain ages in the range 32 9 33 8 ma mean 33 4 0 3 ma the fact that apparent ages of co existing biotites span a similar range suggests that there was very rapid cooling through feldspar and biotite closure temperatures especially if these minerals closed at substantially different temperatures multi domain diffusion modelling of the feldspar data suggests that at this time cooling was rapid ca 60 80 my through closure temperatures of 330 440 c as these closure temperatures are substantially higher than the ones normally assumed for biotite i e about 300 c they and or the calculated cooling rates may be unreliable four sericites from the quartz sericite zone yielded variably discordant age spectra the least discordant of these has a plateau at 31 1 0 3 ma and this is interpreted as the time of this alteration event some k feldspars in the quartz sericite zone were completely overprinted at this later time and most of the other ones studied show partial overprinting overprinting appears to decrease as distance from the central quartz sericite zone increases in the more remote areas effects of the later event appear constrained to narrow fault controlled alteration zones the data are consistent with a conceptual model proposed by zentilli et al zentilli m leiva g rojas j graves m c 1994a the chuquicamata system revisited extended abstract society of economic geologists symposium copper deposits of the andes new developments concepcion chile october 17 18 1994 proceedings 2 1647 1651 wherein potassic alteration by late magmatic fluids occurred at depth prior to rapid exhumation of the system quartz sericite alteration perhaps caused by a new pulse of porphyry intrusion at depth occurred some 2 3 my later principally in the core of the deposit although alteration did propagate outwards through the action of hydrothermal fluids passing through a brittle relatively shallow fracture system

physiographic and tectonic settings of high sulfidation epithermal gold silver deposits of the andes and their controls on mineralizing processes abstract gold and silver ores in the vast majority of andean high sulfidation epithermal au ag deposits occur at high present day elevations and typically 200 500 m below low relief landforms situated at 3500 to 5200 m a s l most deposits are middle miocene and younger and include el indio tambo pascua lama veladero el indio belt chile argentina cerro de pasco central peru pierina lagunas norte yanacocha northern peru quimsacocha ecuador and the california vetas mining district santander colombia jointly accounting for 130 moz au resources slightly older examples are only preserved in the atacama desert and include the middle eocene el guanaco and el hueso and the late oligocene early miocene la coipa deposits the absence of paleocene and older high sulfidation epithermal deposits can be explained by limited preservation potential imposed by transpressional tectonics within overall contractile episodes and surface uplift these conditions prevailed predominantly in segments of shallow angle subduction of the nazca or caribbean plate below the south american continent a tectonic setting also common for porphyry style cu au mo deposits stratovolcanoes are uncommon ore hosts and volcanic rocks coincident with mineralization are in most cases volumetrically restricted or absent recording the terminal stages of local arc magmatism however dacitic domes are important at e g yanacocha and la coipa at lagunas norte a small stratovolcano largely pre dating but temporally overlapping with mineralization occurs immediately east of the deposit and volcanic sector collapse may have occurred during hydrothermal activity mineralization is typically located near the backscarp of pediments or the heads of valleys incising now high elevation low relief surfaces in the california vetas mining district and el indio belt hydrothermal alunite ages become generally younger upstream along the incising valleys indicating that hydrothermal activity and by inference ore deposition were facilitated by erosion the lowering of the water table and reduction of hydrostatic and lithostatic pressure at these sites of high local relief are believed to have enhanced both boiling and mixing of magmatic with meteoric fluids ultimately enhancing ore deposition the host rock composition permeability and location of the water table control the distribution of alteration zones and ore intermediate volcanic rocks are the most common ore hosts but they typically pre date mineralization by several ma however high sulfidation epithermal mineralization can be hosted in any conceivable rock type including high grade metamorphic rocks california vetas mining district significantly older plutonic rocks pascua lama or quartzites lagunas norte large vuggy quartz alteration zones and commonly oxidized low grade large tonnage mineralization are best developed in relatively permeable volcaniclastic rocks or hydrothermal breccia bodies whereas coherent volcanic plutonic or metamorphic rocks may host fault and breccia controlled ores the near surface steam heated zone can attain a thickness of several hundred meters in dry climates e g veladero pascua lama tambo but is typically poorly developed and less than 20 m thick in humid climatic zones the physiographic and tectonic settings of high sulfidation epithermal deposits are distinct from low sulfidation epithermal districts such as those of patagonia el penon chile or fruta del norte ecuador the latter range to significantly older ages jurassic to early eocene occur at mainly lower elevations and were emplaced in extensional settings a temporal coincidence between uplift erosion and mineralizing processes as well as a spatial and temporal association with porphyry style mineralization is not evident for these low sulfidation districts

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

the relationship of diagenesis with a complex microbial ecosystem in the phosphatic interval of the miocene monterey formation evidence from stable isotopes and mineralogy abstract a combination of evidence from mineralogy sulfur and oxygen stable isotope ratios from early diagenetic carbonate associated sulfate cas and structurally substituted sulfate in phosphate both termed cas here and carbon and oxygen stable isotope ratios from carbonate describe a complex redox system and complex microbial ecosystem within the phosphate rich interval of the monterey formation 34s and 18o of cas are lower than miocene seawater sulfate as reconstructed from marine evaporite and barite minerals combined with mineralogical speciation modeling and mineralogical observations isotopically low cas isotope values suggest mixing from three isotopically characterized sulfate pools two isotopically higher sulfate pools one with the composition of miocene seawater sulfate and a second isotopically evolved porewater modified by sulfate reducing microbes srm and a third isotopically lower porewater sulfate pool resulting from the oxidation of h2s produced by srm by sulfide oxidizing bacteria sob high carbonate 13c and diagenetic dolomite validates prior claims of methanogenesis differences in mineralogy are consistent with differences in isotope ratios e g higher 13c in dolomites suggesting products from a vertical distribution of oxic to anoxic redox environments and associated ecosystems are preserved these mineralogical and isotopic fluctuations repeat in various facies changes suggesting that redox environments fluctuated over time this interpretation is supported by prior sedimentologic analysis which interpreted that deposition of the phosphatic interval of the monterey formation was influenced by gravity deposition processes promoted by tectonic activity gravity deposition would serve to transport more oxygenated waters to the deep water depositional environment and support microbially mediated sulfide oxidation at depth whereby h2s generated by srm in the absence of sufficient iron could diffuse upward to a more oxygenated zone near the suboxic oxic boundary h2s oxidation would decrease porewater ph slightly and inhibit the preservation of carbonate minerals and promote precipitation of phosphate increasing depth in sediments generates successively more reducing conditions promoting sulfate reduction which may have been coupled with the anaerobic oxidation of methane and carbonate mineral precipitation the most reducing conditions would support methanogenesis and dolomite production after consumption of the majority of sulfate

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

age of supergene oxidation and enrichment in the chilean porphyry copper province twenty five samples of supergene alunite collected from deeply developed supergene profiles in porphyry copper deposits and prospects between latitudes 20 degrees and 27 degrees s in northern chile yield k ar ages ranging from about 34 to 14 ma therefore supergene oxidation and enrichment processes were active from the early oligocene to the middle miocene a minimum of 20 m y supergene activity at individual deposits lasted for at least 0 4 to 6 2 m y the early oligocene supergene activity affected deposits in the paleocene porphyry copper belt whereas early and middle miocene supergene processes are documented in the early cretaceous paleocene and late eocene to early oligocene porphyry copper belts middle miocene oxidation also affected the oldest epithermal gold silver deposits in the maricunga belt farther east supergene activity commenced no less than 11 m y after generation of each porphyry copper deposit because of the time required to unroof the copper bearing parts of the system supergene activity throughout northern chile ceased at approximately 14 ma the geologic features of deposits and prospects and their morphotectonic positions present latitudes and present elevations display no obvious correlations with the supergene chronology exploration for major cumulative enrichment blankets should not be carried out either beneath thick sequences of piedmont gravels or ignimbrites of oligocene through middle miocene age unless their accumulation is demonstrably late in the documented history of supergene activity or in porphyry copper provinces such as those of central chile and northwestern argentina which formed after approximately 14 ma the uplift responsible for efficient cumulative copper enrichment is difficult to correlate convincingly with the brief pulses of compressive tectonism postulated for northern chile and contiguous areas unless their effects were much more prolonged intensifying aridity is confirmed as the likely reason for the cessation of supergene activity in northern chile and tectonic uplift was its most probable cause however more fundamental global controls producing a period of chemical weathering followed by worldwide dessication also may have played a role

patterns of mineralization and alteration below the porphyry copper orebody at el salvador chile three diamond drill holes angled below the lowest haulage level at el salvador have doubled the vertical exposure of the deposit and revealed very different features of alteration and mineralization below this major porphyry copper orebody sulfide assemblages persist with depth but the total sulfide content diminishes magnetite becomes a part of all sulfide assemblages except very late pyritic d veins residual traces of pyrrhotite chalcopyrite found locked in quartz and as abundant and wide spread inclusions in pyrite apparently represent the remains of an early prograde mineralization obliterated by intense sulfidation of subsequent events relicts of specularite veinlets may be a similar phenomenon vein types change newly recognized early biotitic eb veinlets with and without sulfides quartz albite anhydrite and actinolite have varied alteration halos containing albite k feldspar biotite green sericite anhydrite and andalusite they appear to be deeper equivalents of a quartz veins veinlets descriptively similar to both eb and a quartz veins formed as a second generation within the young intramineral l porphyry complex which truncates similar veinlets in older and better mineralized rocks granular a quartz k feldspar sulfide anhydrite veins diminish in abundance and in content of sulfide and k feldspar with depth and are hard to distinguish from b quartz anhydrite veins with characteristic molybdenite the latter have much better developed k feldspar alteration halos than seen above younger g sulfide veins with green sericite biotite and anhydrite and halos with green sericite alkali feldspar and andalusite cut b veins they are older than relatively sparse d pyrite quartz veins with sericite pyrite calcite anhydrite halos and occasional tourmaline pervasive sericite chlorite in the pyritic fringe terminates downward and biotitic alteration of andesite diminishes revealing more restricted and residual actinolite hornfels ilmenite and then sphene appear as residual accessory minerals and minor vein constituents minor andalusite with alkali feldspar extends to deepest exposures mostly within halos of b and c veins traces of corundum and cordierite occur with andalusite

direct dating of weathering phenomena by 40ar39ar and k ar analysis of supergene k mn oxides abstract potassium bearing manganese oxides cryptomelane k1 2 mn3 mn4 8 o16 xh2oand hollandite k ba 1 2 mn3 mn4 8 o16 xh2o are often authigenically precipitated in weathering profiles the presence of structural potassium in manganese oxides makes these minerals datable by the k ar and 40 ar 39 ar methods dating of these phases allows us to time the progression of oxidation fronts during weathering and pedogenic processes detailed age resolution of the weathering history allows us to use deep weathering profiles as land based stratigraphic records of past climatic and geomorphic events complementing evidence from the marine sedimentary and paleontological record potential problems in manganese oxide dating such as ar and or k losses excess argon 39ar loss by recoil during neutron irradiation contamination by primary minerals resistant to weathering the presence of multiple generations of manganese oxides and proportionately low radiogenic ar yield due to adsorbed atmospheric gases are addressed the k ar and 40 ar 39 ar analytical results indicate that ar and or k losses excess 40ar and 39ar recoil seem not to pose problems in manganese oxide dating this investigation suggests that fine scale laser probe 40 ar 39 ar technique is most appropriate for dating of weathering phenomena because this technique permits us to identify contaminating phases and the presence of multiple generations of weathering minerals in the inherently complex mineral assemblage characteristic of weathering profiles k ar and 40 ar 39 ar dating of supergene k bearing manganese oxides formed during lateritization of archean and proterozoic bedrocks in the caraj s region amazonia brazil indicates that weathering started before 72 6 ma petrographic electron microscope and electron microprobe investigation reveal multiple generations of manganese oxide precipitation k ar and 40 ar 39 ar laser probe step heating results independently confirm the presence of these various generations of manganese oxides age clusters at 65 69 51 56 40 43 33 35 20 24 12 17 ma and zero age 0 2 0 2 ma suggest episodic precipitation of k mn oxides resulting from changing weathering conditions in the amazon throughout the cenozoic k ar and 40 ar 39 ar dating of supergene cryptomelane from weathering profiles in eastern minas gerais southeastern brazil suggests continuous weathering from 10 to 5 6 ma ago possibly reflecting local climatic conditions due to the proximity with the atlantic ocean

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

40ar 39ar ages of hypogene and supergene mineralization in the cerro verde santa rosa porphyry cu mo cluster arequipa peru the contiguous cerro verde and santa rosa porphyry copper deposits are hosted by paleogene granitoid rocks and precambrian gneiss and spatially associated with 61 1 ma u pb zircon mukasa 1986 dacitic porphyry stocks the age of hydrothermal activity is constrained by laser induced incremental heating 40ar 39ar sericite muscovite 2m1 dates of 61 8 0 7 2 and 62 0 1 1 ma for cerro verde and 62 2 2 9 ma for santa rosa representing the terminal event in the arequipa segment of the coastal batholith the deposits crop out on the santa rosa erosional pediment which itself is incised into the older la caldera surface two populations of ages 36 1 to 38 8 ma and 24 4 to 28 0 ma are identified by multiple analyses of a sample from cerro verde comprising alunite partially replaced by natroalunite demonstrating that supergene activity had commenced by the latest eocene during the incaic orogeny thereafter continuing through the oligocene in the santa rosa deposit deep ca 300 350 m leaching in the late oligocene is recorded by ca 26 ma natroalunite that is inferred to have formed beneath the la caldera surface at the top of the cerro verde pit 2738 m bench veins of alunite ca 23 ma and natroalunite ca 21 ma in a hematitic leached zone are truncated by the santa rosa surface which is inferred to have developed after 21 ma decreasing ages of alunite group minerals with increasing depth in the cerro verde pit e g ca 12 ma at the 2648 m level and 4 9 6 7 ma at the 2618 m level are evidence for deepening of the supergene profile through the miocene beneath this pediment jarosite dates 0 7 1 3 ma record the persistence of minor supergene activity into the pleistocene

comment on late holocene rupture of the northern san andreas fault and possible stress linkage to the cascadia subduction zone by chris goldfinger kelly grijalva roland burgmann ann e morey joel e johnson c hans nelson julia gutierrez pastor andrew ericsson eugene karabanov jason d chaytor jason patton and eulalia gracia goldfinger et al 2008 conclude that in the northern san andreas fault and cascadia margins holocene turbidites i e deposits of turbidity currents were triggered by earthquakes however their conclusion falters because of 1 selective data sets that were used for establishing seismicity as the sole triggering mechanism of turbidites 2 flawed sedimentological concepts and criteria that were applied for interpreting turbidites and 3 faulty methodologies that were employed for correlating turbidites over long distances this same turbidite i e sedimentological theme has also been published previously by goldfinger et al 2003a b 2006 2007 as with the recent article in bssa their previous publications on the subject matter were published in journals that are concerned primarily with geophysical topics as a result critical sedimentological conclusions which are an integral part of the articles by goldfinger et al have not been subjected to a rigorous scrutiny therefore i would like to comment on the following three issues along the washington oregon california margin potential triggering mechanisms of sediment failures and related turbidity currents are 1 earthquake adams 1990 2 tectonic oversteepening greene et al 2006 3 tsunami adams 1990 geist 2005 4 cyclone adams 1990 shanmugam 2008a fig 4 5 ebb tidal current puig et al 2003 6 submarine volcanic activity davis et al 2002 7 sediment loading adams 1990 and 8 gas hydrates chapman et al 2004 the link between gas hydrate decomposition and mass movements on the norwegian margin was discussed by mienert et al 2005 in the cascadia margin a total of 40 cyclones along the oregon coast were reported for the 1984 2006 period moritz and moritz 2006 a total of 24 tsunamis were reported along the oregon coast for the 1868 1994 period table 1 cyclones and tsunamis are powerful agents of sediment failures shanmugam 2008a on the outer continental shelf of the gulf of mexico for example the

evolution of early cenozoic marine temperatures the equator to high southern latitude sea surface and vertical temperature gradients are reconstructed from oxygen isotope values of planktonic and benthic foraminifers for the following five time intervals late paleocene early eocene early middle eocene late eocene and early oligocene paleotemperatures are calculated using standard oxygen isotope temperature equations with adjustments to account for 1 variations in sea water 18o related to changes in global ice volume over time and 2 latitudinal gradients in surface water 18o these reconstructions indicate that sea surface temperatures sst of the southern oceans in the early eocene were as high as 15 c whereas temperatures during the late paleocene and early middle eocene reached maximum levels of 10 12 c by the late eocene and early oligocene high latitude sst had declined to 6 and 4 c respectively for most of the early paleogene low latitude sub tropical temperatures remained constant and well within the range of holocene temperatures 24 ndash 25 c but by the late eocene and early oligocene declined to values in the range of 18 to 22 c the late paleogene apparent decline in tropical temperatures however might be artificial because of dissolution of near surface foraminifera tests which biased sediment assemblages toward deeper dwelling foraminifera moreover according to recent plate reconstructions it appears that the majority of sites upon which the late eocene and early oligocene tropical temperatures were previously established were located either in or near regions likely to have been influenced by upwelling global deepwater temperature on average paralleled southern ocean sst for most of the paleogene we speculate based on the overall timing and character of marine sea surface temperature variation during the paleogene that some combination of both higher levels of greenhouse gases and increased heat transport was responsible for the exceptional high latitude warmth of the early eocene

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

a sedimentological model of organic matter preservation and phosphogenesis in the miocene monterey formation at haskells beach goleta central california abstract at haskells beach west of santa barbara ca u s a the upper part of the miocene monterey formation is predominantly composed of organic rich mudstone which is interstratified with phosphatic laminae and lenses minor lithologies consist of dolomite volcanic ash porcelanite chert and condensed phosphate these sediments date from the end of the serravallian and almost the entire tortonian 11 05 7 85 ma based on calcareous nannofossils and the 13 c org record which correlates well with the global record sediments have total organic carbon values between 2 75 and 9 15 wt average value 6 48 wt n 61 rock eval analyses show the dominance of type ii kerogen the sediment accumulation rate varied between 5 9 and 84 7 m ma correspondingly organic carbon accumulation rates range between 0 08 and 1 43 g cm 2 ky average value 0 48 g cm 2 ky total phosphorus contents in the organic rich mudstone vary between 0 001 and 3 48 wt and c org p org ratios show high values of up to 4727 average value 2232 whereas c org p total values are low up to 185 average value 52 this indicates that early diagenetic organic matter degradation was a source of phosphate in phosphogenesis in addition other sources of phosphate were necessary in order to explain the abundance of phosphate relative to organic matter such as the transfer of dissolved inorganic phosphate into the sediments slumps angular unconformities erosive surfaces reworked clasts and nodules and condensed phosphatic layers suggest that hydrodynamic conditions were important and likely variable leading to frequent erosion and sediment reworking under these circumstances organic matter was predominantly delivered during gravity flow events which were followed by longer periods of low sediment accumulation and phosphogenesis of the uppermost sediment layer associated pore occlusion by phosphate minerals may have considerably enhanced the preservation of organic matter during the early late miocene a progressive change is observed from the mixed accumulation of organic phosphate biosilica and carbonate sediments to the predominant accumulation of biosilica rich sediments this change is related to more intense upwelling a shift of the upwelling center to the californian coastline and a leveling out of basin topography leading to a more even spread of biosilica

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

40ar 39ar ages of precambrian manganese ore minerals from sweden india and morocco in order to test their chronometric potential 40ar 39ar stepheating and 4he analyses have been carried out on five manganese ore minerals of the hollandite cryptomelane series from three precambrian manganese deposits ultevis sweden sitapar india bachkoun morocco samples from the metamorphic occurrences ultevis and sitapar yielded ar ages of 1 8 ga and 0 95 ga interpreted as the age of postmetamorphic cooling hollandites ultevis and of an early k introducing alteration process subsequent to amphibolite facies metamorphism cryptomelanes sitapar both data are consistent with known chronologies of the svecokarelian and satpura orogenic cycles a date of 670 ma obtained for a hollandite from a volcanogenic vein deposit bachkoun however contrasts with published extrusion ages of 580 560 ma for the volcanic host rocks ouarzazate series probably due to incorporation of excess argon the use of the 40ar 39ar technique together with multiple isotope systematics made it nevertheless possible to establish a reasonable estimate of a mineralization age close to 580 ma measurement of fractional ar losses during vacuum step heating 500 1600 c although indicating good ar retentivities failed to define model diffusion parameters because of non linear arrhenius arrays helium diffusion results 200 1200 c indicated retention of radiogenic 4he by the samples corroborated by u he mineral dates between 0 96 and 0 31 ga potassium bearing manganese oxides are therefore able to retain argon possibly also 4he through geological times and may thus provide ages of ore forming processes and perhaps later cooling and alteration stages

carbonaceous and phosphate rich sediments of the miocene monterey formation at el capitan state beach california u s a abstract the organic and phosphate rich interval of the monterey formation at el capitan state beach west of santa barbara california u s a late early to early late miocene in age is composed of a carbonaceous marl toc contents between 1 2 and 23 2 wt with intercalated phosphate rich laminae lenses and layers subordinate lithologies include ash layers dolomitized horizons and siliceous beds we distinguished five lithological units 1 a gray marl unit lacking major phosphate accumulations 16 3 ma average toc content 2 by weight average sedimentation rate 75 m my average toc accumulation rate 0 19 mg cm2 yr 2 a black marl unit including light colored phosphatic laminae lenses and discrete particles 16 3 14 5 ma average toc content 7 5 by weight average sedimentation rate 20 m my average toc accumulation rate 0 19 mg cm2 yr 3 a red marl unit including light colored phosphatic laminae lenses and commonly reworked particles 14 5 12 7 ma average toc content 15 by weight average sedimentation rate 20 m my 14 5 13 3 ma and 2 m my 13 3 12 7 ma respectively average toc accumulation rate 0 39 mg cm2 yr 14 5 13 3 ma and 0 04 mg cm2 yr 13 3 12 7 ma respectively 4 a unit of complex and condensed phosphatic beds interbedded with red marl 12 7 10 8 ma average sedimentation rate 3 m my and 5 a black marl unit with intercalated phosphatic laminae and lenses 10 8 ma average sedimentation rate 9 m my average toc accumulation rate 0 09 mg cm2 yr phosphogenesis and accumulation of phosphate were dynamic processes which started with local phosphogenesis leading to the formation of phosphatized particles as well as stratigraphically bound phosphogenesis leading to the formation of phosphate laminae and lenses phases of subsequent sediment reworking resulted in the concentration of phosphate particles in phosphate rich layers and repeated phases of sediment reworking and phosphogenesis ultimately resulted in the formation of the complex phosphate condensed horizons preservation of organic matter was favored by high productivity rates and by the development of dysaerobic bottom water conditions the dynamic sedimentary environment likely led to the formation of early diagenetic phosphatic lids which may have sealed off subjacent organic rich layers as well as to the rapid deposition of entire layers in the form of mud flows thereby eventually enhancing the potential of organic matter preservation our new age data suggest that at the el capitan state beach section the intervals characterized by high toc values and maximum toc accumulation rates red marl as well as significant quantities of in situ phosphates appeared in the late middle miocene i e during and after the major cooling phase at around 14 5 ma this implies that deposition of phosphate and organic carbon continued well after this cooling phase thereby underlining the observation that preservation of organic carbon in the monterey formation is not only dependent on climate change during the mid miocene but also on regional conditions

speckled beds distinctive gravity flow deposits in finely laminated diatomaceous sediments miocene monterey formation california distinctive nonlaminated intervals we term speckled beds sbs are common in finely laminated diatomites of the monterey formation at celite quarry lompoc california sbs have sharp and bedding parallel upper and lower contacts tabular geometry and are less than or equal to 10 cm thick sbs are characterized by a bedding parallel arrangement of sand to granule size diatomaceous and detrital aggregates that are uniformly distributed in a fine grained matrix of macerated biosilica and detrital mud the unique physical properties of diatomaceous sediments provided cohesive substrates and high fluid content that were prerequisite for the formation of sbs abundant hybrid fault vein structures slump folds and complexly deformed laminae in the quarry attest to syndepositional deformation of laminated diatomaceous sediments on a subaqueous slope within the oxygen minimum zone the unique features of sbs permit an interpretation of stepwise rheological transformation during mass movement via passage through critical point transformations from bingham plastic slump and or slide block to turbulent fluid turbidity current to viscous fluid exhibiting laminar flow debris flow the recognition of sbs clarifies the origin of these distinctive yet problematic deposits and clearly differentiates them from other nonlaminated intervals sb composition indicates provenance from an oxygen minimum zone and further illustrates that a hiatus is not required to form cohesive firmgrounds in diatomaceous sediments the recognition of sbs as recorders of slope failure events complements paleoseismic studies of holocene diatomaceous laminites from saanich inlet british columbia ocean drilling program sites 1033 and 1034

pleistocene recycling of copper at a porphyry system atacama desert chile cu isotope evidence we present cu isotope data of hypogene and supergene minerals from the late paleocene spence cu mo porphyry in the atacama desert of northern chile chalcopyrite displays a restricted range of 65cu values within the values reported for primary porphyry cu sulfides 0 28 to 0 34 n 6 supergene chalcocite samples show heavier and remarkably homogeneous 65cu values between 3 91 and 3 95 n 6 consistent with previous models of cu leaching and enrichment in porphyry systems secondary cu minerals from the oxide zone show a wider range of composition varying from 1 28 and 1 37 for chrysocolla n 6 to very light cu isotope signatures reported for atacamite between 5 72 to 6 77 n 17 these data suggest redox cycling of cu during supergene enrichment of the spence cu deposit characterized by a first stage of supergene chalcocite formation from acidic isotopically heavy leach fluids of meteoric origin down flowing in a semi arid climate 44 to 15 9 ma reworking of the initial supergene copper assemblage during the pleistocene by rising neutral and chlorine rich deep formation waters under well established hyper arid climate conditions lead to the formation of atacamite with extremely fractionated cu compositions essentially coeval chrysocolla formed by dissolution of atacamite during short episodes of wetter climatic conditions occurring in the latest pleistocene

argon release systematics of hypogene and supergene alunite based on progressive heating experiments from 100 to 1000 c abstract few minerals have as many different geological applications for k ar or 39 ar 40 ar dating as alunite kal 3 so 4 2 oh 6 but a basic understanding of the systematics of argon release from this mineral is lacking progressive heating experiments from 100 1000 c at 100 c intervals were conducted in this study to obtain argon release patterns from hypogene and supergene alunite the samples analyzed were selected from well understood deposits and are representative of the principal natural environments of acid sulfate generation and related advanced argillic alteration and alunite formation for gas extraction and purification a low blank metallic extraction purification system used for dating silicates was complemented with a quartz pyrex glass line that included an external resistance furnace the resulting patterns of argon release vs temperature are well defined and similar for all the samples quantitative release of radiogenic 40 ar occurs during the dehydroxylation reaction of alunite at temperatures 400 and 800 c is mainly derived from the extraction system the results indicate that the retentivity of ar in fine grained supergene alunite is similar to that of coarsely crystalline hypogene alunite the data support the reliability of k ar dating of hypogene and supergene alunite and provide practical information to conduct routine high precision analyses

high frequency paleoclimate signals from foulden maar waipiata volcanic field southern new zealand an early miocene varved lacustrine diatomite deposit abstract a freshwater diatomite deposit near dunedin south island new zealand provides an exceptional archive of mid latitude seasonal climate variation during latest oligocene early early miocene time the diatomite accumulated in a small 1 5 km diameter maar floored lake that formed during basaltic volcanism except for exposures of basanite dated at 23 2 ma by 40 ar 39 ar and remnants of a paleogene sandstone and conglomerate cover the diatomite body is surrounded by otago schist geophysical profiles indicate that the total diatomaceous sediment thickness may exceed 100 m two depositional facies are described from 15 5 m of weakly consolidated fresh diatomite accessible in two pits a thinly laminated facies comprising 60 of the section consists of dark brown and white couplets of average thickness 0 5 mm both brown and white laminae are composed essentially of frustules of a single pennate diatom along with 1 2 of siliceous sponge spicules dark laminae also contain abundant 5 7 5 m diameter siliceous chrysophycean stomatocysts and organic matter decalcified complete skeletons of galaxias a southern hemisphere group of freshwater or diadromous fish are found throughout diverse well preserved leaves some with attached scale insects are dominated by lauraceae rare flowers are also present trace fossils preserved in the laminated comprise 1 1 5 mm wide pale diatomaceous strings of probable faecal origin and isolated 20 40 mm diameter sand lenses interpreted as faecal remains of swimming waterfowl the second diatomite facies group comprises dark brown speckled beds interpreted as sediment gravity flows up to 14 cm thick and interspersed throughout the study section they incorporate abundant laminated diatomite flecks and leaves and minor woody plant matter and terrigenous silt almost all are capped by 1 8 mm of white diatomite that is interpreted as post flow fall out of resuspended diatom frustules associated breccia and swirly beds up to 8 cm thick are composed of variably contorted clasts of laminated diatomite in a speckled matrix nodules spheroids and films of authigenic framboidal pyrite are commonly associated with plant and fish remains as in modern lakes white laminae likely formed from diatom blooms when light flux temperature or nutrients were optimal during spring early summer dark laminae accumulating during autumn winter months laterally continuous pinstripe lamination absence of bioturbation and excellent fish and plant preservation indicate that the profundal lake and floor were anoxic periodic variations in couplet thickness of 3 10 years compare with quaternary records of el nino southern oscillation variability and indicate that the early early miocene low altitude new zealand climate was seasonal and strongly ocean influenced as it is today