a reinterpretation of the delta d h 2 o of inclusion fluids in contemporaneous quartz and sphalerite creede mining district colorado a generic problem for shallow orebodies water extracted from fluid inclusions in quartz from shallow epithermal ore deposits often has a hydrogen isotope composition delta d different from that of water extracted from inclusions in associated minerals this difference is usually attributed to the involvement of primary fluids from multiple sources isotopic and homogenization and freezing temperature determinations on fluid inclusions from contemporaneous quartz and sphalerite from the epithermal silver and base metal orebodies of the oh vein creede district colorado suggest an alternative explanation in near surface deposits differences between delta d sub h 2 o of inclusion fluids in ore minerals and quartz may result instead from contamination during extraction of the fluids contained in primary inclusions by shallow ground water trapped in pseudosecondary inclusions in quartz quartz from the oh vein contains two principal petrographically distinct populations of fluid inclusions primary and pseudosecondary the primary inclusions have salinities ranging from 5 to 10 equiv wt percent nacl and the salinities of pseudosecondary inclusions cluster between 0 and 1 percent primary inclusions in quartz from one locality have a measured delta d sub h 2 o value of 69 per mil while pseudosecondary inclusions at the same locality have a delta d sub h 2 o value of 102 per mil both salinity and isotopic values for primary inclusions in quartz are similar to those for primary inclusions in contemporaneous sphalerite homogenization temperatures for primary and pseudosecondary inclusions in quartz range from 191 degrees to 280 degrees c and from 199 degrees to 278 degrees c respectively the delta d sub h 2 o value measured on fluid inclusions from bulk crystals ranges between 97 and 85 per mil and represents a mixture of fluids from both primary and pseudosecondary inclusions we interpret the data to indicate that one or more episodes of abrupt incursion of cooler overlying ground water into the ore zone caused thermal cracking of the quartz crystals during the time interval of mineralization subsequent healing of the fractures trapped heated low salinity ground water in pseudosecondary inclusions the abrupt incursions of overlying ground water are speculated to have resulted from either collapse of a transient vapor dominated region of the ore zone or catastrophic venting of the system through hydrothermal eruption s the unusually high contrast between the salinities of the ore depositing fluids and the ground water overlying the ore zone allowed recognition of this phenomenon at creede it is likely however that creede is not unique similar phenomena may be common in shallow ore zones where rapid fluctuation of an interface between a deep high temperature thermal plume and an overlying cooler ground water may be expected to occur careful study of the origins of fluid inclusions particularly in quartz is essential to characterize the primary ore fluids and to assess the role of ground water in the hydrology of shallow ore deposits

fluid chemistry structural setting and emplacement history of the rosario cu mo porphyry and cu ag au epithermal veins collahuasi district northern chile the rosario cu mo ag deposit is located in the collahuasi district of northern chile it comprises high grade cu ag au epithermal veins superimposed on the core of a porphyry cu mo orebody rosario has mining reserves of 1 094 million metric tons mt at 1 03 percent copper an additional 1 022 mt at 0 93 percent copper occurs in the district at the nearby ujina and quebrada blanca porphyry deposits the rosario reserve contains over 95 percent hypogene ore whereas supergene sulfide ores dominate at ujina and quebrada blanca mineralized veins are hosted within lower permian volcanic and sedimentary rocks lower triassic granodiorite and late eocene porphyritic quartz monzonite the rosario fault system a series of moderate southwest dipping faults has localized high grade cu ag au veins at cerro la grande similar high grade cu ag au veins are hosted in north northeast trending sinistral wrench faults normal movement in the rosario fault system is interpreted to have been synchronous with sinistral strike slip deformation at la grande hydrothermal alteration at rosario is characterized by a k feldspar core focused in the rosario porphyry that grades out to a secondary biotite albite magnetite assemblage paragenetic relationships indicate that magnetite was the earliest formed alteration product but has been replaced by biotite albite vein crosscutting relationships indicate that k feldspar formed during and after biotite albite alteration chalcopyrite and bornite were deposited in quartz veins associated with both k feldspar and biotite albite assemblages the early hydrothermal fluid was a hypersaline brine 40 45 wt nacl that coexisted with vapor between 400 degrees and 600 degrees c weakly mineralized illite chlorite intermediate argillic alteration of the early k and na silicate assemblages was caused by moderate temperature 250 degrees 350 degrees c moderate salinity brines 10 15 wt nacl molybdenite was precipitated in quartz veins that formed between the potassic and intermediate argillic alteration events these fluids were 350 degrees to 400 degrees c with salinities between 10 and 15 wt percent nacl porphyry style ore and alteration minerals were overprinted by structurally controlled quartz alunite pyrite pyrophyllite dickite and muscovite quartz phyllic alteration assemblages the quartz alunite pyrite alteration formed at 300 degrees to 400 degrees c from fluids with a salinity of 10 wt percent nacl the pyrophyllite dickite assemblage formed between 250 degrees and 320 degrees c from dilute 5 wt nacl fluids an upward flared zone of muscovite quartz pyrite altered rocks surrounds the fault controlled domain of advanced argillic alteration thick veins 0 5 2 m wide of fault hosted massive pyrite chalcopyrite and bornite precipitated brines with a salinity of 30 wt percent nacl at temperatures of 250 degrees to 300 degrees c pressure depth estimates indicate that at least 1 km of rock was eroded at rosario between formation of the k na silicate and advanced argillic assemblages this erosion was rapid occurring over a period of 1 8 m y the rosario porphyry intruded immediately after the incaic tectonic phase implying that it was emplaced as the domeyko cordillera underwent gravitational collapse expressed as normal faults in the upper crust gravitational sliding potentially accelerated exhumation and helped to promote telescoping of the high sulfidation environment onto the rosario porphyry the hydrothermal system responsible for porphyry cu mineralization at rosario was partially exhumed prior to the formation of high sulfidation ore and alteration assemblages this implies that emplacement of a second blind intrusion occurred somewhere beneath the rosario and cerro la grande high sulfidation vein systems and is supported by the fault geometry and zoning of precious metals and sulfosalts at the district scale

saline fluid inclusions in sphalerite from the broadlands ohaaki geothermal system a coincidental trapping of fluids being boiled toward dryness report of a fluid inclusion study of sphalerites from the broadlands ohaaki geothermal system the findings indicate that brine filled fluid inclusions do exist in sphalerite from one location but that their origin is a consequence of local boiling toward dryness rather than introduction of a deep brine

geology of the escondida porphyry copper deposit antofagasta region chile the escondida porphyry copper deposit located in northern chile was one of the two largest copper producers of the world in the 1990s the hydrothermal evolution of this deposit is associated with the emplacement of a late eocene oligocene quartz monzonitic to granodioritic intrusive stock complex composed of at least three intrusive phases and hosted by paleocene andesite this paleocene andesite is underlain by mesozoic and paleozoic sedimentary and volcanic rocks which are characteristic of the basement of the precordillera de domeyko province in northern chile we propose that the intrusive stock associated with the mineralization at escondida was emplaced in a tensional gash formed between sinistral strike slip faults of the regional domeyko fault system the complete evolution of the porphyry system is characterized by overprinting of pervasive and vein associated alteration mineralization styles grouped in three main hydrothermal stages the early stage includes a zone of pervasive biotitization of andesite and development of a silicification shell around the intrusive complex propylitic alteration around the biotitic zone and a vein associated orthoclase quartz anhydrite biotite alteration that mark the end of stage one this early alteration contains magnetite chalcopyrite and bornite with less than 0 5 vol percent of sulfides and a hypogene copper grade of 0 2 wt percent the second hydrothermal stage is represented by vein and vein selvage associated chlorite sericite quartz and by quartz sericite with sulfides including chalcopyrite pyrite and molybdenite in the chlorite sericite and quartz sericite zones the content of sulfides ranges from less than 0 5 to 2 vol percent with a chalcopyrite to pyrite ratio of 3 to 1 and copper grades that range between 0 4 and 0 6 wt percent the intrusion of a rhyolite dike and a dome are at least 3 m y younger than the first and second hydrothermal stages and separate them in time from the late hydrothermal stage the late hydrothermal stage is represented by an acid sulfate mineral association that includes pyrophyllite alunite and quartz as alteration minerals and a variety of sulfides that include bornite chalcopyrite pyrite chalcocite covellite enargite sphalerite tennantite and galena this acid sulfate event occurred mainly in west northwest striking veins and also along the contact of the rhyolite and its host rock where sulfides from this event overprint previous sulfides the primary copper grades range from 0 6 to higher than 1 0 wt percent in the escondida deposit the highest hypogene and supergene copper grades occur in areas where all three hydrothermal stages are present

stable isotope geochemistry of the pierina high sulfidation au ag deposit peru influence of hydrodynamics on so42 h2s sulfur isotopic exchange in magmatic steam and steam heated environments abstract the pierina high sulfidation au ag deposit formed 14 5 my ago in rhyolite ash flow tuffs that overlie porphyritic andesite and dacite lavas and are adjacent to a crosscutting and interfingering dacite flow dome complex the distribution of alteration zones indicates that fluid flow in the lavas was largely confined to structures but was dispersed laterally in the tuffs because of a high primary and alteration induced permeability the lithologically controlled hydrodynamics created unusual fluid temperature and ph conditions that led to complete so 4 2 h 2 s isotopic equilibration during the formation of some magmatic steam and steam heated alunite a phenomenon not previously recognized in similar deposits isotopic data for early magmatic hydrothermal and main stage alunite 34 s 8 5 to 31 7 18 o so 4 4 9 to 16 5 18 o oh 2 2 to 14 4 d 97 to 39 sulfides 34 s 3 0 to 4 3 sulfur 34 s 1 0 to 1 1 and clay minerals 18 o 4 3 to 12 5 d 126 to 81 are typical of high sulfidation epithermal deposits the data imply the following genetic elements for pierina alteration mineralization 1 fluid and vapor exsolution from an i type magma 2 wallrock buffering and cooling of slowing rising vapors to generate a reduced h 2 s so 4 6 highly acidic condensate that mixed with meteoric water but retained a magmatic 34 s s signature of 1 3 so 2 disproportionation to hso 4 and h 2 s between 320 and 180 c and 4 progressive neutralization of laterally migrating acid fluids to form a vuggy quartz alunite quartz clay intermediate argillic propylitic alteration zoning magmatic steam alunite has higher 34 s 8 5 to 23 2 and generally lower 18 o so 4 1 0 to 11 5 18 o oh 3 4 to 5 9 and d 93 to 77 values than predicted on the basis of data from similar occurrences these data and supporting fluid inclusion gas chemistry imply that the rate of vapor ascent for this environment was unusually slow which provided sufficient time for the uptake of groundwater and partial to complete so 4 2 h 2 s isotopic exchange the slow steam velocities were likely related to the dispersal of the steam column as it entered the tuffs and possibly to intermediate exsolution rates from magmatic brine the low d values may also partly reflect continuous degassing of the mineralizing magma similarly data for steam heated alunite 34 s 12 3 to 27 2 18 o so 4 11 7 to 13 0 18 o oh 6 6 to 9 4 d 59 to 42 are unusual and indicate a strong magmatic influence relatively high temperatures 140 to 180 c based on 18 o so 4 oh fractionations and partial to complete sulfur isotopic exchange between steam heated sulfate and h 2 s restricted lithologically controlled fluid flow in the host tuffs allowed magmatic condensate to supplant meteoric groundwater at the water table and create the high temperature low ph conditions that permitted unusually rapid so 4 2 h 2 s isotopic equilibration 50 300 days and or long sulfate residence times for this environment late void filling barite 34 s 7 4 to 29 7 18 o so 4 0 4 to 15 1 and later void filling goethite 18 o 11 8 to 0 2 document a transition from magmatic condensate to dominantly meteoric water in steam heated fluids during cooling and collapse of the hydrothermal system these steam heated fluids oxidized the top 300 m of the deposit by leaching sulfides redistributing metals and precipitating barite acanthite gold and goethite hematite gold steam heated oxidation rather than weathering was critical to forming the orebody in that it not only released encapsulated gold but likely enriched the deposit to ore grade au concentrations

relative age of cordilleran base metal lode and replacement deposits and high sulfidation au ag epithermal mineralization in the colquijirca mining district central peru at colquijirca central peru a predominantly dacitic miocene diatreme dome complex of 12 4 to 12 7 ma 40ar 39ar biotite ages is spatially related to two distinct mineralization types disseminated au ag associated with advanced argillic alteration and local vuggy silica typical of high sulfidation epithermal ores are hosted exclusively within the volcanic center at marcapunta a second economically more important mineralization type is characterized as cordilleran base metal lode and replacement deposits these ores are hosted in mesozoic and cenozoic carbonate rocks surrounding the diatreme dome complex and are zoned outward from pyrite enargite quartz alunite to pyrite chalcopyrite dickite kaolinite to pyrite sphalerite galena kaolinite siderite alunite samples related to the au ag epithermal ores have been dated by the 40ar 39ar method at 11 3 11 6 ma and those from the cordilleran base metal ores in the northern part of the district smelter and colquijirca at 10 6 10 8 ma the significant time gap 0 5 my between the ages of the two mineralization types in the colquijirca district indicates they were formed by different hydrothermal events within the same magmatic cycle the estimated time interval between the younger mineralization event base metal mineralization at 10 6 ma and the ages of 12 5 ma obtained on biotites from unmineralized dacitic domes flanking the vicinity of the diatreme vent suggest a minimum duration of the magmatic hydrothermal cycle of around 2 ma this study on the colquijirca district offers for the first time precise absolute ages indicating that the cordilleran base metal lode and replacement deposits were formed by a late hydrothermal event in an intrusive related district in this case post au ag high sulfidation epithermal mineralization

laser probe 40 ar 39 ar ages of early and late stage alteration assemblages rosario porphyry copper molybdenum deposit collahuasi district i region chile sensibly identical laser probe 40 ar 39 ar dates of 32 9 or 0 5 2sigma and 32 9 or 0 6 ma are determined for respectively early stage hydrothermal biotite and late stage hydrothermal muscovite from the giant rosario porphyry copper deposit in the collahuasi district of northern chile the age determinations were carried out on ca 1 cm 2 1 to 0 5 mm thick wafers of altered rock or on smaller chips broken therefrom obviating the requirement for mineral separation and permitting direct petrographic control of the analytical procedure the age data although not unambiguous imply that this extremely large hydrothermal center was emplaced over a short interval hydrothermal activity in this central andean transect migrated eastward over a period of ca 3 to 4 m y from copaquire through quebrada blanca to rosario the intensity of mineralization increasing with time rosario chuquicamata and la escondida the largest of the west fissure domeyko fault system array were apparently emplaced coevally in the terminal stage of the late eocene early oligocene metallogenic episode

hydrochemical signatures of deep groundwater circulation in a part of the himalayan foreland basin groundwater samples were analyzed from 71 springs and wells as part of a larger study in a region of compressional tectonic regime the study site covers the peshawar basin and surroundings in the himalayan foreland of pakistan the northern portion is mountainous and the water table is discontinuous in different intermontane valleys with abundant springs with normal and anomalous temperatures and composition the southern part is divided into isolated basins with a number of drilled deep and dug shallow wells hydrochemical signatures of elevated strontium sr sio2 boron b and the geothermometric signatures all indicate a deep circulation of the emerging groundwater moreover for several of the sample sites water chemical compositions measured spring and water well temperatures and reservoir temperatures calculated for spring waters all point to origin from deep horizons within the basin remarkable proximity of all the thermal and hydrochemical anomalies to major faults suggests that the waters ascended along these faults from greater depths the area is a natural western extension of the himalayan geothermal belt described in earlier literature for the eastern and central himalayas

hydrologic implications of alteration and fluid inclusion studies in the fresnillo district mexico evidence for a brine reservoir and a descending water table during the formation of hydrothermal ag pb zn orebodies ag pb zn mineralization in the fresnillo district formed from a large magma related hydrothermal system between approximately 32 to 28 ma a partial record of this hydrothermal activity is preserved by hydrothermal alteration mineral assemblages and fluid inclusions occurring in barren and mineralized parts of the district these features provide constraints on the hydrologic interpretation of the mineralizing system surficial exposures contain alteration assemblages which reflect two distinct fluid types that are known to occur in the shallow parts of active hydrothermal systems the hydrothermal mineral assemblage comprising quartz calcite illite or interlayered illite montmorillonite plus or minus chlorite pyrite or adularia occurs in scattered exposures around the district and indicates the former widespread presence of near neutral ph chloride waters fluid inclusion data in barren exposures south of the district center indicate that these fluids were dilute containing less than 5 wt percent nacl equiv and had temperatures in the range of 160 degrees to 240 degrees c the assemblage of kaolinite plus or minus alunite and natroalunite occurs locally above blind orebodies in the southern part of the district and in outlying exposures and indicates the former presence of acid sulfate waters geologic and fluid inclusion evidence strongly suggests that these waters originated as steam heated condensates which formed above the water table at plateros which lies north of fresnillo a hotter thermal environment is exposed where fluid inclusion data range from 220 degrees to 300 degrees c in contrast to other surficial exposures in the fresnillo district those that crop out in the center of the district on cerro proano contain significant silver mineralization associated with stockwork quartz veins fluid inclusion data from two quartz crystals indicate part of a complex hydrothermal history in which fluids of 17 10 and 200 degrees c this last feature explains why the tops of late formed orebodies are concealed at 150 to 200 m depth this model implies that dilute hydrothermal fluids devoid of ore metals were the fluids mostly responsible for hydrothermal alteration and vein fillings at the present day surface and that changes to the hydrology significantly altered the position of the ore depositional environment

a hydrogen and oxygen isotope study of the san cristobal mine peru implications of the role of water to rock ratio for the genesis of wolframite deposits oxygen and hydrogen isotope ratios were measured from vein minerals and extracted fluid inclusion waters from the san cristobal wolframite base metal deposit in peru oxygen isotope values for quartz range from 9 8 to 14 4 per mil and for wolframite from 0 6 to 4 6 per mil the calculated delta values at 300 degrees c for water in equilibrium with quartz and wolframite are delta 18 o 2 8 to 7 3 per mil for quartz and 1 4 to 5 4 per mil for wolframite hydrogen isotope values show an overall range of delta d 58 to 148 per mil but the range for any given mineral is much smaller these data are interpreted in light of a model which examines the effect of temperature and water to rock ratio on the isotopic compositions of meteoric waters exchanging with granite in a rock dominated system an isotopically exchanged water becomes depleted in delta 18 o and enriched in delta d as the temperature of exchange decreases when the water to rock ratio is also allowed to vary a wide variety of water compositions is generated the isotopic composition of minerals from san cristobal can be interpreted as the result of deposition from a meteoric water which has undergone isotopic exchange with a granite at 400 degrees c with a water to rock ratio ranging from 0 01 to 0 003 isotopic data taken from two other wolframite deposits pasto bueno landis 1972 and panasquiera kelly and rye 1979 are reinterpreted using the model developed for san cristobal both sets of data are consistent with the hypothesis that wolframite deposits are formed by meteoric waters which have experienced exchange at very low water to rock ratios 0 05 the importance of the low water to rock ratio may be related to the way in which tungsten is leached from a crystallized pluton

skarn manto and breccia pipe formation in sedimentary rocks of the cananea mining district sonora mexico skarns mantos and breccia pipes occur at cananea in a 2 by 4 km horst of paleozoic carbonate rock and minor quartzite surface mapping and core logging reveal a sequence of events beginning with early metamorphism that converted impure carbonate lithologies to iron poor garnet pyroxene or idocrase hornfels subsequent metasomatism formed garnet py roxene skarn along the pre cretaceous elisa fault contact between carbonate rock and mesozoic volcanic rock skarn is zoned from an andradite rich center through a zone containing both andraditic garnet and salitic pyroxene to a mineralogic sequence near the marble front which is largely a function of the sedimentary protolith chert nodules are rimmed by wollastonite dolomite is converted to massive phlogopite magnetite skarn calc silicate hornfels is over printed by veins of skarn garnet and relatively pure marble is replaced by coarse blades of iron and manganese rich pyroxene multiple generations of garnet and pyroxene can be distinguished by subtle variations in color and texture age classification on the basis of crosscutting vein and overgrowth relationships indicates that late pyroxenes are more iron and manganese rich than the early generations garnets show a less regular iron enrichment with time the spatial distribution of the different garnet and pyroxene generations is irregular garnet and pyroxene in late veins have iron rich compositions both near the skarn center and near the marble front pyrite and minor chalcopyrite are the only sulfides associated with this stage of metasomatism following the main stages of garnet and pyroxene formation veins and orbicular patches of amphibole or quartz or calcite occur replacing pyroxene and in some cases garnet most of the amphibole is actinolitic in composition and is associated with pyrite and minor amounts of chalcopyrite about 20 percent of the amphibole is subcalcic and is associated with or replaced by massive calcite the alteration of prograde skarn to amphibole or quartz or calcite can be best explained by a general temperature decline the destruction of skarn by alteration related to brecciation and breccia pipe formation and the replacement of previously unaltered carbonate rocks by stratiform blankets mantos of iron oxides and sulfides resulted in some of the highest grade orebodies breccia pipe and manto formation appears to have been largely contemporaneous with emplacement and subsequent sericitic alteration of a series of quartz monzonite porphyry stocks where breccia pipes crosscut skarn garnet pyroxene and amphibole are converted to mixtures of calcite quartz chlorite hematite siderite and sulfides where breccia pipes or porphyries crosscut previously unaltered carbonate rock mixtures of magnetite sulfides chlorite siderite calcite quartz and serpentine form massive mantos veinlets of chlorite and magnetite or pyrite or serpentine extend tens of meters beyond the zones of massive replacement there is a rough vertical and lateral zonation of sulfide minerals with respect to the democrata breccia pipe from chalcopyrite or bornite or pyrite in the core and at depth to sphalerite pyrite chalcopyrite to pyrite sphalerite galena distal to the center of mineralization mineral stability relations suggest that brecciation and mineralization took place at lower temperatures 275 degrees 25 degrees c and possibly under lower x sub co 2 conditions than the earlier skarn formation the high zn cu ratios in skarn the zonation of most hypogene mineralization relative to the breccia pipes which crosscut skarn and the lack of skarn spatially associated with the quartz monzonite porphyry stocks which intrude carbonate rock all suggest that unlike skarn in most porphyry copper districts skarn at cananea is not related to the quartz monzonite porphyry stocks that are mined elsewhere in the district for disseminated supergene enriched sulfide mineralization rather the skarn may be related to a deeper magmatic system which has not yet been encountered in subsurface exposures

microthermometry of enargite hosted fluid inclusions from the lepanto philippines high sulfidation cuau deposit abstract the spatial relation between porphyry and high sulfidation epithermal deposits is particularly well revealed in the mankayan mineral district of northern luzon philippines where the lepanto high sulfidation cu au deposit lies over and adjacent to the far southeast fse porphyry cu au deposit consequently a study was undertaken to characterize the fluids responsible for epithermal mineralization in this environment the ore stage at lepanto consists of enargite luzonite cu 3 ass 4 pyrite tennantite tetrahedrite and chalcopyrite infrared petrography of the enargite reveals variable transparency with growth banding and twinning visible in euhedral specimens two phase liquid vapor fluid inclusions occur as primary and secondary types ranging from t h of fluid inclusions in enargite were measured from within the lateral 3 0 km and vertical 0 5 km extent of the enargite mineralization these values show a cooling trend toward the northwest away from the area over the porphyry deposit with average t h ranging from 285 c proximal to 166 c distal ice melting temperatures t m were measured using a cycling technique as ice was usually not visible in frozen inclusions apparent salinities range from 4 5 to 0 2 eq wt nacl with samples from the margins of the deposit showing a general decrease in apparent salinity with lower t h secondary fluid inclusions in quartz phenocrysts tend to have a higher average t h and lower apparent salinities compared to enargite hosted inclusion fluids from the same locations several samples of pyrite are also transparent to ir radiation and show internal features such as growth banding and in one instance a two phase liquid vapor fluid inclusion this inclusion yielded a salinity of 1 2 eq wt nacl there is a large discrepancy in t h and apparent salinities between the enargite mineralization and the subjacent porphyry deposit despite contemporaneous formation suggesting that the hot 500 c hypersaline magmatic brines associated with porphyry mineralization did not directly ascend to the elevation of the enargite deposit however absorbtion of magmatic vapors into overlying meteoric water may have created the mineralizing fluid of the lepanto deposit as this liquid moved to the northwest along the lepanto fault it was cooled and diluted by mixing with groundwater resulting in enargite deposition

maceration of calcareous skeletons in the december issue of sedimentology 1979 26 two papers on the disintegration of calcareous skeletons appeared in both articles the authors alexandersson 1979 fitzgerald parmenter milliman 1979 describe skeletal degradation producing fragments and even single calcium carbonate crystallites in the present marine environment at least part of this maceration alexandersson 1978 is attributed to the breakdown of the skeletal organic matrix on the sea bottom both studies were carried out on samples from the sediment surface of marine cold water regions there another destructive process is active etching and dissolution of the calcareous particles by the ambient carbonate under saturated water e g alexandersson 1975 1978 the possible coexistence of these biological and physicochemical destructive processes and the lack of information from cores have left open questions in the above mentioned two studies some of these questions were discussed by me lewy 1975 my observations and conclusions may now have a wider significance

petrology of philippine geothermal systems and the application of alteration mineralogy to their assessment abstract philippine geothermal systems occur in the vicinity of large holocene calc alkaline volcanic complexes wells drilled in these areas encountered multiple intrusions the latest dikes are the subsurface manifestations of the youngest heat source commonly at least two hydrothermal regimes are juxtaposed in a single area with the latest being in equilibrium with the present temperature and chemical regime alteration by neutral ph water is pervasive and abundant a contact metamorphic aureole also occurs near intrusives alteration due to acid sulfate fluids is generally confined to permeable structures neutral ph alteration is divided into four zones on the basis of key clay minerals and two subzones are defined by calc silicates these are the smectite ambient to 180 c transition 180 230 c illite 230 320 c and biotite 270 340 c zones subzones are defined by epidote 250 340 c and amphibole 280 340 c the four main zones of acid alteration are kaolinite ambient to 120 c dickite kaolinite 120 200 c dickite pyrophyllite 200 250 c and pyrophyllite illite 230 320 c where relict high temperature alteration reaches the surface the area being drilled is usually the outflow zone of the present system these hydrothermal mineral assemblages are used 1 as geothermometers 2 to assist in determining the depth at which the production casing will be set during drilling 3 to estimate fluid ph and other chemical parameters 4 to predict possible corrosion and scaling tendencies of the fluids 5 as a measure of permeability and possible cold water influx into wells 6 as a guide to field hydrology and 7 to estimate roughly the thickness of the eroded overburden

alunite in the pascua lama high sulfidation deposit constraints on alteration and ore deposition using stable isotope geochemistry the pascua lama high sulfidation system located in the el indio pascua belt of chile and argentina contains over 16 million ounces moz au and 585 moz ag the deposit is hosted primarily in granite rocks of triassic age with mineralization occurring in several discrete miocene age phreatomagmatic breccias and related fracture networks the largest of these areas is brecha central which is dominated by a mineralizing assemblage of alunitepyrite enargite and precious metals several stages of hydrothermal alteration related to mineralization are recognized including all types of alunite bearing advanced argillic assemblages magmatic hydrothermal steam heated magmatic steam and supergene the occurrence of alunite throughout the paragenesis of this epithermal system is unusual and detailed radiometric mineralogical and stable isotope studies provide constraints on the timing and nature of alteration and mineralization of the alunite pyrite enargite assemblage in the deposit early preore alteration occurred prior to ca 9 ma and consists of intense silicic and advanced argillic assemblages with peripheral argillic and widespread propylitic zones alunite of this stage occurs as fine intergrowths of alunite quartz kaolinite dickite and pyrophyllite that selectively replaced feldspars in the host rock stable isotope systematics suggest a magmatic hydrothermal origin with a dominantly magmatic fluid source alunite is coeval with the main stage of au ag cu mineralization alunite pyrite enargite assemblage ore which has been dated at approximately 8 8 ma ore stage alunite has an isotopic signature similar to preore alunite and 34salun py data indicate depositional temperatures of 245 to 305 c the d and 18o data exclude significant involvement of meteoric water during mineralization and indicate that the assemblage formed from h2sdominated magmatic fluids thick steam heated alteration zones are preserved at the highest elevations in the deposit and probably formed from oxidation of h2s during boiling of the magmatic ore fluids coarsely crystalline magmatic steam alunite 8 4 ma is restricted to the near surface portion of brecha central postmineral alunite jarosite were previously interpreted to be supergene crosscutting veins and overgrowths although stable isotope data suggest a mixed magmatic meteoric origin for this late stage alteration only late jarosite veinlets 8 0 ma associated with fine grained pseudocubic alunite have a supergene isotopic signature the predominance of magmatic fluids recorded throughout the paragenesis of the pascua system is atypical for high sulfidation deposits which typically involve significant meteoric water in near surface and peripheral alteration and in some systems even ore deposition at pascua the strong magmatic signature of both alteration and main stage alunite pyrite enargite assemblage ore is attributed to limited availability of meteoric fluids this is in agreement with published data for the el indio pascua belt indicating an event of uplift and subsequent pediment incision as well as a transition from semiarid to arid climatic conditions during the formation of the deposit in the mid to late miocene

quiruvilca peru mineral zoning and timing of wall rock alteration relative to cu pb zn ag vein fill deposition quiruvilca district cu pb zn ag veins have produced over 8 million tons of ore since 1789 the veins occupy tension gashes associated with left lateral strike slip faults that cut the central facies of a miocene andesitic stratovolcano complex the district has four broadly concentric mineralogic zones from district center to edges which are based on dominant vein fill enargite transition lead zinc and stibnite all veins regardless of zonal position in the district share the same four gross paragenetic stages pyrite base metal sulfosalt and carbonate veins closest to the center of the district contain relatively greater proportions of the earlier assemblages mineralogic zoning commonly is focused on vein intersections and on a local scale reflects changes in vein width wall rock alteration types from the vein margin outward and from most intense to least intense are intense sericitic strong sericitic moderate sericitic strong argillic weak argillic and propylitic petrographic observations indicate that at any given location propylitic alteration is oldest followed by argillic and then sericitic alteration the zoned alteration halos formed as each inner assemblage advanced overprinted and replaced its adjacent outer precedent the width and intensity of alteration halos enclosing veins increase with depth and toward the district center correlation between paragenetic stages of vein fill and wall rock alteration is based upon the mineralogy and wall rock alteration of single stage veinlets that are zoned about major veins the veinlet sequence in time correlates with the veinlet distribution in space this also matches the time space sequence documented for major veins most alteration occurred contemporaneously with the pyrite stage of mineralization ore deposition followed in the base metal stage thus ore deposition in the veins in large part postdated formation of the alteration halos associated with those veins this time sequence is corroborated by breccia dikes which cut alteration halos but precede the base metal stage temperatures of ore deposition deduced from the stability of various mineral assemblages decreased through time from 320 degrees to 230 degrees c fugacity of sulfur f sub s 2 decreased from approximately 10 super 6 to 10 super 12 5 both with time and laterally away from the district center the ph is inferred to have increased laterally away from the center of the district and to have decreased in time at any one spot in the wall rock

gold mineralization in the okanagan valley southern british columbia fluid inclusion and stable isotope studies two styles of precious metal vein mineralization epithermal and mesothermal occur in the okanagan valley of southern british columbia the epithermal deposit dusty mac is composed of quartz stockwork mineralization in eocene volcanic and volcaniclastic rocks the mesothermal deposits fairview and oro fino consist of thick 1 10 m quartz veins in paleozoic and mesozoic metamorphic rocks fluid inclusion and stable isotope studies indicate that two distinct hydrothermal fluids were responsible for the mineralization events at dusty mac the epithermal fluid had a relatively low temperature 230 degrees 250 degrees c extremely low salinity 18 o 7 to 9ppm and delta d 133ppm values at fairview and oro fino the mesothermal fluids had higher temperatures 280 degrees 330 degrees c significant co 2 contents and higher salinities 3 6 equiv wt nacl stable isotope data show that the mesothermal fluids had low delta d 121 to 148ppm and delta 13 c 8 5ppm values but high delta 18 o 4 to 6ppm values the data indicate that the fluids involved in both styles of mineralization are 18 o shifted meteoric water with shallow circulation responsible for the epithermal deposits and deep circulation involved in the formation of the mesothermal deposits although both styles of mineralization occur in the same tectonic regime it appears that they are not temporally or genetically related to each other

mineralization and paragenesis of the huanzala mine central peru the huanzala mine is a zinc and lead mineral deposit found in the lower cretaceous santa formation limestone and has associated copper and accessory polymetallic minerals early stage mineralization began with pyritization and was followed by middle stage zinc and lead mineralization occasionally accompanied by skarnization the lead and zinc mineralization stage was followed by late stage copper ore deposition together with small amounts of silver tin tungsten and miscellaneous minerals the pyrite orebodies can be grouped by texture into two kinds compact and fine grained and loose and coarse grained ore the zinc ores are composed of two types iron rich sphalerite and iron poor sphalerite the iron rich sphalerite contains only a few blebs of copper minerals whereas the iron poor sphalerite is characterized by many inclusions of copper minerals the copper bearing hydrothermal solution of the late stage mineralization permeated the grain boundaries and cracks of the iron rich sphalerite and converted it into iron poor sphalerite petrographic observations suggest that iron poor sphalerite was produced by a process involving extraction of iron from iron rich sphalerite and the combination of this iron with copper in the hydrothermal solution resulting in precipitation of chalcopyrite and bornite the compact fine grained pyrite ore was changed into the loose coarse grained type by the same hydrothermal solution which transformed the iron rich sphalerite into the ironpoor type late stage copper ores were produced by mineralization ofchalcopyrite bornite tennantite and enargite followed by scheelite cassiterite stannite canfieldite hocartite argentite and other miscellaneous minerals mineralization of enargite was accompanied by bornite and was followed by chalcopyrite and tennantite in several stages tennantite was deposited simultaneously with bornite and also with chalcopyrite studies of these paragenetic relationships together with consideration of the orebody formation temperature and thermochemical treatment of the ore forming minerals suggest that with a decline in temperature the mineralization proceeded toward low f sub s 2 and low f sub o 2

fluid evolution in the w cu zn pb san cristobal vein peru fluid inclusion and stable isotope evidence abstract the zn pb ag cu san cristobal district is located 100 km east of lima in the western cordillera of peru it is centred around the chumpe intrusion and is composed of vein and carbonate replacement ore types the main san cristobal vein presents a paragenesis that can be divided into three stages a an early wolframite quartz pyrite stage b a quartz base metal stage and c a late quartz carbonate barite stage fluid inclusions in quartz from the tungsten stage are biphase lv at room temperature and homogenise to the liquid phase between 146 and 257 c their salinities range between 2 1 and 5 1 wt nacl equiv rare inclusions contain an additional crystal of halite and have salinities of 46 54 wt nacl equiv data of the first two stages show a decrease in homogenisation temperatures concomitant with a salinity decline fluid inclusions in quartz from the late stage homogenise at higher temperatures between 252 and 323 c with salinities ranging between 4 6 and 6 7 wt nacl equiv hydrogen and oxygen isotope data indicate a two stage evolution isotopic compositions of the fluid associated with the first two stages define a trend with constant 18 o values and decreasing d values 18 o 3 2 to 5 0 v smow and d 60 to 112 v smow which is interpreted as mixing of a dominantly magmatic component with minor meteoric water that had equilibrated with the host rocks this interpretation is supported by sulphur and lead isotopic data from previous studies by contrast the quartz carbonate barite stage bears isotopic characteristics defining a trend with a coupled decrease of 18 o and d 18 o 8 1 to 2 5 v smow and d 57 to 91 v smow and is explained by addition of meteoric water to the system and subsequent mixing with a less important magmatic component different fluid origins are confirmed by laser ablation icp ms analyses of the triphase lvh and biphase lv primary inclusions the concentrations of the major ore elements i e w cu zn and pb decrease throughout the paragenesis w and to a lesser extent cu show significant variations associated with a steep decrease in their concentration the decreasing concentrations can be explained by mineral deposition and dilution by the meteoric fluid differences in the rate of decrease indicate selective precipitation of w fluid inclusions of the quartz carbonate stages show an abrupt increase in ba and sr concentrations this is interpreted to reflect a higher volume of host rock silicate alteration probably due to the increasing size of the fluid flow cell and is explained by the input of a third fluid of unknown origin la icp ms analyses show that the fluids were already depleted in w and cu before reaching the emplacement of carbonate replacement ore type whereas zn and pb were still present in considerable amounts this is again due to selective precipitation and is consistent with the interpretation that the economically interesting metals were dominantly introduced by magmatic fluids

fluid inclusion and sulfur isotope studies of the tintic mining district utah implications for targeting fluid sources the silver city stock one of several resurgent intrusions along the margin of the tintic caldera has been historically accepted as the source for ore fluids at tintic or at least responsible for the heat to drive the hydrothermal system fluid inclusions in quartz from fissure veins however reveal a thermal gradient that traverses exposures of the silver city stock reaching a thermal maximum 3 km south of the exposed stock sulfur isotope values from sulfide minerals in the vein and replacement deposits are typical of igneous values about 1ppm near the thermal high but are distinctly lower as low as 6 8ppm to the north though still located in the silver city stock main stage mineralization at tintic may have been the product of fluids emanating from a very late resurgent granitoid intrusion in the tintic district one that is still largely unexposed an apophysis of this intrusion may approach the surface near the thermal high delineated by fluid inclusion homogenization temperatures additional evidence for the existence of such an intrusion includes widespread propylitic alteration of the silver city stock and surrounding volcanic rocks a ladder style pattern of fissure veins traversing the district and rare exposures of granitic aplite dikes crosscutting the silver city stock

new 40ar 39ar alunite ages from the colquijirca district peru evidence of a long period of magmatic so2 degassing during formation of epithermal au ag and cordilleran polymetallic ores we present 40ar 39ar data acquired by infra red co2 laser step heating of alunite crystals from the large miocene colquijirca district in central peru combined with previously published data our results show that a long at least 1 3 my and complex period of magmatic hydrothermal activity associated with epithermal au ag mineralization and base metal cordilleran ores took place at colquijirca the new data indicate that incursion of magmatic so2 bearing vapor into the colquijirca epithermal system began at least as early as 11 9 ma and lasted until 10 6 ma four alunite samples associated with high sulfidation epithermal au ag ore gave 40ar 39ar plateau ages between 11 9 and 11 1 ma compared to the previously documented 11 6 to 11 3 ma by combining individually these new ages with crosscutting relationships the duration of the au ag deposition period can be estimated to at least 0 4 my three new 40ar 39ar plateau ages on alunite associated with the base metal cordilleran ores are consistent with previously obtained ages all of them between 10 83 0 06 and 10 56 0 06 ma suggesting that most of the sulfide rich polymetallic deposits of smelter and colquijirca formed during this short period the recognition of consecutive alunite bearing and alunite free mineral assemblages within both the au ag and the base metal cordilleran ores may suggest that so2 bearing magmatic vapor entered the epithermal environment as multiple discontinuous pulses a number of which was not necessarily associated in time with ore fluids it is likely that a period of so2 bearing vapor degassing longer than 11 9 to 10 6 ma may be recognized with further more detailed work