dolomitization of the oligocene miocene bluff formation on grand cayman british west indies the oligocene miocene bluff formation on grand cayman is formed of hard white finely crystalline dolostone in which the precursor textures and fossil microstructures are commonly preserved the d

multistage intrusion brecciation and veining at el teniente chile evolution of a nested porphyry system the el teniente copper molybdenum deposit is hosted by the late miocene teniente mafic complex a largely subvolcanic package of primarily basaltic andesite porphyry sills and stocks that were emplaced within the mid late miocene farellones formation in the late miocene pliocene a series of intermediate felsic plutons were intruded into the teniente mafic complex these are spatially associated with magmatic hydrothermal breccias and multiple vein types that form individual mineralized complexes here we present detailed observations on mineralogy textures and intrusion breccias and vein crosscutting relationships to constrain the nature and relative timing of magmatic and hydrothermal events our revised classification defines 13 vein types divided lilt three main stages 1 premineralization biotite and or k feldspar quartz anhydrite albite magnetite actinolite epidote veins that formed prior to emplacement of mineralized intrusions and breccias 2 main mineralization stage veins that grade from gangue dominated quartz anhydrite veins potassic alteration halos into sulfide dominated veins with phyllic alteration halos and 3 late mineralization veins containing sulfosalts five breccia types have been observed in the deposit typically forming individual vertically zoned complexes spatially associated with individual intrusions and overlapping in time i igneous cemented breccias 2 k feldspar cemented breccias 3 biotite cemented breccias 4 anhydrite cemented breccias and 5 tourmaline cemented breccias breccia cements display 1 similar paragenetic evolution to main mineralization stage veins indicating a close genetic link between them a distinction can be made between early premineralization vein types types 1 2 that represent veins formed possibly deposit wide prior to emplacement of intrusion breccia complexes late pre and main mineralization vent types types 3 8 which are interpreted to reflect the repeated cycle of fluid release associated with each mineralized intrusive complex and late mineralization vein types types 9 10 that represent it single event linked to the emplacement of the braden breccia pipe the geologic evidence indicates a close spatial and temporal relationship between emplacement of shallow felsic intermediate pipelike intrusions and the development of igneous mid mineralized magmatic hydrothermal breccias and vein halos the magmatic hydrothermal transitions observed in these complexes indicate that the deposit formed from a series of localized pulses of magmatic hydrothermal activity which followed rather similar evolution paths single deposit wide models of fluid evolution and mineralization are therefore inappropriate we conclude that el teniente represents a nested but otherwise rather typical porphyry cu mo system unusual only in thia the teniente mafic complex pi is it particularly efficient physical trap in terms of pervasive fracturing during intrusion of magmatic hydrothermal breccia complexes and its an effective chemical trap for deposition of sulfides the overlapping of mineralized envelopes from successive fertile intrusions and the absence of barren intermineral porphyries resulted in its unusual size and high hypogene grades

magmatic and structural controls on porphyry style cu au mo mineralization at kemess south toodoggone district of british columbia canada kemess south is the only cu au mo mine in the toodoggone district and a major cu and au producer in british columbia porphyry style cu au mo mineralization is mainly hosted by the tabular sw plunging 199 6 0 6 ma maple leaf granodiorite which intrudes tightly folded sw dipping permian asitka group siltstone and limestone and homogeneous triassic takla group basalt southwest dipping 194 0 0 4 ma toodoggone formation conglomerate volcaniclastic and epiclastic rocks overlie the granodiorite and asitka group rocks minor cu au mo mineralization is hosted by the immediate takla group basalt country rock whereas low tonnage high grade cu zones occur beneath a 30 m thick leached capping in supergene altered granodiorite and in exotic positions in overlying toodoggone formation conglomerate granodiorite has an intrusive contact with mineralized and altered takla group basalt but displays a sheared contact with unmineralized and less altered asitka group siltstone the north block fault is a deposit scale e striking steeply s dipping normal fault that juxtaposes the granodiorite basalt ore body against unmineralized asitka group rocks younger nw and ne striking normal dextral faults cut all rock types orebodies and the north block fault with displacements of up to 100 m and result in the graben and horst style block faulting of the stratigraphy and ore body both basalt and granodiorite host comparable vein sequence and alteration histories with minor variations in hydrothermal mineral assemblages caused by differing protolith chemistry early potassic alteration and associated early stage cu au mo mineralization is partly replaced by phyllic and intermediate argillic alteration associated with main stage cu au mo mineralization two main stage veins have re os molybdenite ages of 201 3 1 2 and 201 1 1 2 ma these mineralization ages overlap the 199 6 0 6 ma u pb zircon crystallization age for the maple leaf granodiorite late stage pyrite rich stringer veins and related phyllic alteration assemblages are cut by anhydrite rich carbonate rich and chlorite veins fluids and metals associated with early main and late stage veins were probably derived principally from the same deep magma chamber as the maple leaf granodiorite these magmatic derived fluids interacted with asitka and takla group country rocks and possibly with meteoric and metamorphic fluids prior to mineralization

high cu grades in porphyry cu deposits and their relationship to emplacement depth of magmatic sources observations at several porphyry cu deposits indicate that in those formed above source magma chambers emplaced to relatively shallow levels roofs 3 4 km deep zones of higher cu grade are associated with the oldest intrusions of a central porphyry cluster abundant early a type quartz veins secondary feldspar alteration and usually high fe fe cu low s fe cu sulfide oxide mineral assemblages in deposits for which source magma chambers were deeper roofs 5 km porphyries closest in age to ore are spatially less directly related to mineralization in these higher cu grades in early mineralization comprise swarms of veinlets with alteration halos that include sericite as well as secondary feldspar and biotite sulfide oxide assemblages are usually of lower fe fe cu and higher s fe cu these differences in deposit characteristics appear to correspond with a change from two phase chloride bearing aqueous fluids that would exsolve from magma at shallow depths to one phase fluids deeper

hydrothermal alteration and mineralization of the galore creek alkalic cu au porphyry deposit northwestern british columbia canada the late triassic galore creek porphyry district is the largest accumulation of cu au prospects in british columbia canada in terms of contained metal the principal economic resource is the central zone deposit it comprises three high grade centers the au rich north and south gold lenses and the cu dominated central replacement zone alteration footprints ore shells and porphyry intrusions in these centers dip 45 to 60 to the west in the north gold lens and southwest in the south gold lens and central replacement zone indicative of significant postmineral tilting this inclination created an oblique vertical expression of the central zone at surface and at depth providing a 700 m depth profile through the deposit galore creek is considered to be the end member of the silica undersaturated class of alkalic porphyry cu au deposits it displays a unique hydrothermal alteration footprint generally devoid of quartz veining despite the lack of crosscutting veins a paragenetic sequence of several discrete potassic and calcic alteration events has been established during main stage alteration two mineralization events occurred that formed the economic resource in the central zone highly oxidizing fluids derived from porphyritic syenite to monzonite intrusions produced initial potassic alteration and sulfide mineralization in the north gold lens and south gold lens cu au ore shells are characterized by a gold rich core dominated by bornite associated with hematite dusted orthoclase specular hematite and anhydrite the second period of hydrothermal activity caused brecciation and calcic alteration in the central replacement zone the calcic fluids formed hydrothermal cement grading from ti rich andraditic to grossularitic garnet in the core of the breccia to a diopside and magnetite dominated assemblage at the margins calcic alteration also formed in the surrounding host rock this alteration event was followed by potassic alteration and mineralization characterized by biotite anhydrite and chalcopyrite in the cu rich ore shell of the central replacement zone most of the au is hosted in bornite that is spatially associated with oxidized reddened rock units dominated by an alteration assemblage of orthoclase hematite specular hematite garnet anhydrite high grade cu in the central zone is related to abundant chalcopyrite that has commonly replaced an assemblage of biotite magnetite diopside this distinct spatial separation strongly suggests that the local redox environment within the wall rocks governed sulfide and gold distributions the switch from early au rich mineralization to late stage cu dominated mineralization appears to have been controlled by varying redox conditions within the magma the change from more oxidized alteration assemblages fe3 fe2 to more reduced assemblages fe3 fe2 is indicative of a change in oxidation potential that may have propelled sulfide saturation in the magma and thus the subsequent depletion of most of the au

dike surface lineations as magma flow indicators within the sheeted dike complex of the troodos ophiolite cyprus mesoscopic flow lineations and anisotropy of magnetic susceptibility ams have been measured for dikes within the cretaceous age troodos ophiolite with the goal of comparing the direction of initial magma flow through dike conduits immediately following crack propagation with that of flow of subsequent magma emplaced during later stages of dike growth dike margin indicators of flow include cusp axes and elongate vesicles found high in the ophiolite pseudostratigraphy and ridge and groove structures termed hot slickenlines found throughout the complex a unique flow direction is determined where elongate vesicles near dike margins display imbrication with respect to the margin significant changes in vesicle elongation directions across dikes likely indicate either changes in magma flow direction after dike propagation or back flow of magma during the waning stages of intrusion surface lineations generally lie subparallel to the direction of flow inferred from ams determinations on cores within 5 cm of dike margins surface lineations also lie subparallel to the long axis e1 of the orientation ellipsoid defined by long axes of groundmass plagioclase phenocrysts measured in sections from ams cores correlation of surface lineations with interior indicators of flow ams plagioclase trachytic texture indicate that the surface features are good proxies for grain scale magma flow directions during dike propagation in troodos dikes orientations of surface flow features in the dikes of the troodos ophiolite indicate an approximately equal mix of subhorizontal to near vertical magma flow contradicting the paradigm of primarily vertical flow of magma beneath continuous axial magma chambers at oceanic spreading centers our data are consistent with a model of magma emplacement both vertically and horizontally away from isolated magma chambers beneath axial volcanoes spaced along a ridge crest

examining potential genetic links between jurassic porphyry cu au mo and epithermal au ag mineralization in the toodoggone district of north central british columbia canada the toodoggone district comprises upper triassic to lower jurassic hazelton group toodoggone formation volcanic and sedimentary rocks which unconformably overlie submarine island arc volcanic and sedimentary rocks of the lower permian asitka group and middle triassic takla group some of which are intruded by upper triassic to lower jurassic plutons and dikes of the black lake suite although plutonism occurred episodically from ca 218 to 191 ma the largest porphyry cu au mo systems formed from ca 202 to 197 ma with minor mineralization occurring from ca 197 to 194 ma porphyry style mineralization is hosted by small volume 468 c saline and deposited metals at deep crustal depths 2 km sulfur c o and pb isotope data confirm the involvement of a magmatic fluid but also suggest that the ore fluid interacted with asitka and takla group country rocks prior to metal deposition in contrast in the shasta lawyers and griz sickle low sulfidation epithermal systems there is no clear association with magmatic fluids instead their fluid inclusion data indicate the involvement of low temperature 175 to 335 c low salinity 1 to 11 equiv wt nacl fluids that deposited metals at shallow depths 850 m their isotope i e o h pb data suggest interaction between meteoric and or metamorphic ore fluids with basement country rocks

the bingham canyon porphyry cu mo au deposit i sequence of intrusions vein formation and sulfide deposition the bingham canyon porphyry copper gold molybdenum deposit is one of the largest and highest grade porphyry orebodies in the world this study focused on the northwest side of the deposit where quartz mon zonite porphyry qmp the first and largest porphyry intrusion hosts the bulk of the high grade copper gold ore 1 0 cu 1 0 ppm au the north northeast trending high grade zone had pre mining dimensions of 1 500 m strike 300 m vertical and 500 m width and contained more than 500 million tonnes mt of ore associated with potassic alteration and abundant quartz veins the lack of superimposed sericitic alteration yielded ideal exposures in which to study the early high temperature stages of ore formation a style of mineralization that in many porphyry deposits represents the major period of copper introduction we mapped multiple porphyry dikes in the sequence 1 qmp 2 latite porphyry lp 3 biotite porphyry bp 4 quartz latite porphyry breccia qlpbx and 5 quartz latite porphyry qlp porphyry dikes faults and quartz veins are steeply dipping and have two dominant orientations north northeast and northwest striking dikes have a north northeast strike but they thicken and develop northwest trending apophyses and host high grade copper gold zones at intersections with northwest faults indicating that magmatic hydrothermal fluids were focused by these structural intersections each porphyry intrusion was accompanied by a similar sequence of veins potassic alteration and sulfides biotite veinlets were followed by fractures with early dark micaceous edm halos of sericite k feldspar biotite andalusite and local corundum containing disseminated bornite chalcopyrite gold edm halos are cut by multiple generations of a quartz veins representing the main cu au ore forming event postdating all intrusions are quartz molybdenite veins followed by quartz sericite pyrite veins cathodoluminescence cl petrography identified distinct a quartz veinlets consisting of dark luminescing quartz filling fractures and dissolution vugs in earlier a quartz veins and adjacent porphyry wall rock these veinlets contain abundant bornite and chalcopyrite and minor k feldspar and are closely linked in time to the introduction of the bulk of the copper and gold although a similar sequence of veins was repeated on emplacement of all porphyry intrusions the vein density and intensity of potassic alteration declined with time the youngest porphyry qlp is mostly weakly mineralized and locally unaltered these observations indicate that magmatic hydrothermal fluids underwent a similar physiochemical evolution during and immediately following emplacement of each of several porphyry dikes the relationship between edm veins and a quartz veins requires that the flux of magmatic fluid from the magma chamber occurred in an episodic manner as opposed to a continuous discharge vein truncation relationships coupled with abrupt changes in copper gold grades sulfide ratios and potassic alteration intensity at porphyry intrusive contacts indicate that the mass of introduced copper and gold decreased significantly during successive porphyry intrusive hydrothermal cycles presumably due to depletion of metals and volatiles in the underlying magma chamber

late pleistocene ice age scenarios based on observational evidence ice age scenarios for the last glacial interglacial cycle based on observations of boyle and keigwin concerning the north atlantic thermohaline circulation and of barnola et al concerning atmospheric co sub 2 variations derived from the vostok ice cores are herein analyzed northern hemisphere continental ice sheets are simulated with an energy balance model ebm that is asynchronously coupled to vertically integrated ice sheets models based on the glen flow law the ebm includes both a realistic land sea distribution and temperature albedo feedback and is driven with orbital variations of effective solar insolation with the addition of atmospheric co sub 2 and ocean heat flux variations but not in their absence a complete collapse is obtained for the eurasian ice sheet but not for the north american ice sheet we therefore suggest that further feedback mechanisms perhaps involving more accurate modeling of the dynamics of the mostly marine based laurentide complex appears necessary to explain termination i 96 refs 12 figs 2 tabs

application of strontium isotopes to late miocene early pliocene stratigraphy measurements of sup 87 sr sup 86 sr ratios of planktonic foraminifers from paleomagnetically dated deep sea drilling project dsdp cores indicate that the sup 87 sr sup 86 sr ratio of seawater increased linearly between 6 and 4 ma with a slope of 87 times 10 sup minus 6 m y this suggets that sup 87 sr sup 86 sr variation in this time interval are useful for estimating ages in marine sequences the theoretical stratigraphic resolution of the technique is 0 23 m y but the practical resolution is estimated to be 0 3 0 5 m y strontium isotope ratios of five samples at the base of the miocene pliocene m p boundary stratotype at capo rossello sicily have a mean sup 87 sr sup 86 sr value of 0 708995 plus minus 0 00002 applying the regression equation derived from our sup 87 sr sup 86 sr vs age data set for deep sea sequences yields an age of 4 94 plus minus 0 50 ma 95 confidence interval for the m p boundary stratotype similar to ages estimated by biostratigraphic and magnetostratigraphic methods correlation of this mean sup 87 sr sup 86 sr ratio to the section at dsdp site 519 suggests that the boundary is coincident with the onset of a major early pliocene transgression as recorded by a decrease in oxygen isotope values for benthic foraminifers this studymore demonstrates the potential application of strontium isotope stratigraphy for correlating and dating problematic sequences such as those deposited in restricted shallow water or high latitude environments less

stratigraphy of the upper hazelton group and the jurassic evolution of the stikine terrane british columbia11ess contribution 20120051 the lower to middle jurassic hazelton group represents the final stage of magmatic arc activity in the intraoceanic stikine terrane which was followed by accretion within the cordilleran terrane c

the nuevo chaquiro cu au mo porphyry deposit middle cauca belt colombia geology alteration mineralization nuevo chaquiro is a newly discovered blind porphyry cu au mo deposit located in the middle cauca belt colombia approximately 60 km south southeast of medellin the most recent resource estimate was stated as 566 m tonnes t averaging 0 64 cu 0 31 g t au 4 34 g t ag and 127 ppm mo the geology of nuevo chaquiro consists of a miocene volcanic sequence of andesitic tuffs agglomerates and flows combia formation intruded by small stocks and dikes of diorite and quartz diorite also of miocene age sparsely occurring outcrops at nuevo chaquiro consist of a 1 2 0 8 km zone of pervasive sericitic alteration with locally well developed quartz fe oxide stockworks the sericitic alteration grades downward into sericite chlorite alteration and then predominantly potassic biotite magnetite alteration at depths of 250 to 400 m calcic potassic alteration containing actinolite and epidote occurs at greater depths a high grade sheeted vein complex occurs immediately adjacent to the upper contact of an early quartz diorite intrusion quartz veinlet density in this area reaches up to 80 quartz by volume accompanied by up to 5 vol chalcopyrite later intermineral porphyries created a second orebody slightly to the west but contiguous with the early orebody here cu au mo mineralization in an inverted bowl shape is associated with potassic alteration that manifests itself as secondary flooding of the tuff wall rock matrix by hydrothermal biotite accompanied by disseminations and hair veinlets of chalcopyrite magnetite pyrite and molybdenite the porphyry system is also accompanied by intermediate sulfidation carbonate quartz base metal veins typically with sericitic alteration selvages which occur on the upper western flank of the deposit at nuevo chaquiro multiple intrusions of early and intermineral porphyry contain calcic potassic and potassic alteration with a later overprint by sericite chlorite alteration centered on intrusion apexes the amount of sericite chlorite overprinting varies with each intrusion collectively these intrusions are all overprinted by a single later sercitic phyllic alteration event that is superimposed as a broad cap upon the entire system there is a consistent paragenesis for the alteration mineralization associated with each intrusion suggesting a relatively stable underlying shallow magma chamber that fluxed geochemically similar pulses of magmatic hydrothermal fluid with each porphyry in many aspects nuevo chaquiro is a typical au rich porphyry copper system however the contrast between the mineralization styles between the early and intermineral orebodies as well the timing differences in copper deposition suggest that differences in porphyry mineralization styles also occur at nuevo chaquiro

sodium calcium metasomatism chemical temporal and spatial relationships at the yerington nevada porphyry copper deposit tilting associated with extensional tectonics has exposed 1 800 m of continuous palcovertical relief through the jurassic age yerington porphyry copper deposit the rarely observed root zones of a porphyry system are characterized at yerington by sodium calcium metasomatism the principal hydrothermal reactions involve the replacement of primary k feldspar by oligoclase and of primary biotite by actinolite copper has been removed sodic calcic alteration developed at deep intrusive contacts nearly simultaneously with potassic alteration and copper mineralization at shallow intrusive contacts the reactions accompanying potassic alteration are the reverse of those accompanying sodic calcic alteration plagioclase reacted to form k feldspar hornblende reacted to form biotite the potassic and sodic calcic alteration couplet principally accompanied each of two separate intrusive events 1 luhr hill granite stock and associated porphyry dikes and 2 walker river granite porphyry dikes the younger walker river intrusion was emplaced at slightly deeper levels than the older luhr hill complex vertically stacked and partially superimposed alteration and mineralization patterns resulted from differences in depth and in timing of emplacement at lower temperatures late sodic alteration was superimposed on early sodic calcic alteration at deeper levels in the system whereas late quartz sericite pyrite alteration was superimposed on potassic alteration at higher levels sodic alteration is characterized by the replacement of k feldspar by albite and of biotite by chlorite copper deposited during potassic alteration was remobilized the transition to lower temperature alteration is associated only with the walker river intrusion the hydrothermal system associated with the emplacement of the luhr hill complex had not evolved to lower temperature assemblages prior to emplacement of the walker river dikes phase equilibria and geologic reconstruction of depth of emplacement suggest that temperatures during early sodic calcic alteration were limited to the range of 360 degrees to 480 degrees c at pressures between 300 and 800 bars late sodic alteration took place at similar pressures but at temperatures less than 360 degrees c nearly simultaneous development of potassic and sodic calcic alteration resulted from the flow of fluids away from intrusions at shallow levels potassic alteration and from the flow of fluids toward intrusions at deeper levels sodic calcic alteration because k na and na ca activity ratios of a fluid in equilibrium with granitic rocks decrease with decreasing temperature cooling of high temperature fluids can generate potassic alteration and heating of fluids to high temperatures can generate sodic or sodic calcic alteration

discrimination of productive and nonproductive porphyritic intrusions in the chilean andes a geochemical comparison between productive intrusions those associated with mineralization and nonproductive intrusions those unrelated to mineralization has been made on the basis of over 100 fresh granitoid rocks from the chilean porphyry copper belt the study was carried out on the scale of a mining district el salvador potrerillos then on the scale of a single traverse el salvador chilean coast and finally on the scale of the andes itself the productive intrusions are best characterized by large negative anomalies in y mn th and the heavy rare earth elements a discriminant diagram of y against mno is recommended for predicting whether an apparently fresh intrusion has associated peripheral mineralization the low values of y and mn in productive intrusions can be partly explained by the early crystallization of y and mn rich hydrous phases such as hornblende and by the loss of mn rich fluids from the magma the same elements make useful discriminants in other porphyry copper provinces although the exact position of the discriminant boundary varies from island arcs through continental margin to continental settings

geology and reconnaissance stable isotope study of the oyu tolgoi porphyry cu au system south gobi mongolia the oyu tolgoi porphyry cu au system in the south gobi desert mongolia comprises five deposits that extend over 6 km in a north northeast oriented zone they occur in a middle to late paleozoic arc terrane and are related to late devonian quartz monzodiorite intrusions the hugo dummett deposits are the northernmost and deepest with up to 1 000 m of premineral sedimentary and volcanic cover rock remaining they are the largest deposits discovered to date and characterized by high grade copper 2 5 cu and gold 0 5 2 g t mineralization associated with intense quartz veining and several phases of quartz monzodiorite intruded into basaltic volcanic host rocks sulfide minerals in these deposits are zoned outward from a bornite dominated core to chalcopyrite upward to pyrite enargite and covellite at shallower depth the latter high sulfidation state sulfides are hosted by advanced argillic alteration mineral associations this alteration is restricted mainly to dacitic ash flow tuff that overlies the basaltic volcanic rock and includes ubiquitous quartz and pyrophyllite kaolinite plus late dickite veins as well as k alunite al phosphate sulfate minerals zunyite diaspore topaz corundum and andalusite a reconnaissance oxygen hydrogen and sulfur isotope study was undertaken to investigate the origin of several characteristic alteration minerals in the oyu tolgoi system with particular emphasis on the hugo dummett deposits based on the isotopic composition of o h and s 18o so4 8 8 20 1 d 73 to 43 34s 9 8 17 9 the alunite formed from condensation of magmatic vapor that ascended to the upper parts of the porphyry hydrothermal system without involvement of significant amounts of meteoric water the isotopic data indicate that pyrophyllite 18o 6 5 10 9 d 90 to 106 formed from a magmatic fluid with a component of meteoric water muscovite associated with quartz monzodiorite intrusions occurs in the core of the hugo dummett deposits and isotopic data 18o 3 0 9 0 d 101 to 116 show it formed from a magmatic fluid with water similar in composition to that which formed the pyrophyllite mg chlorite 18o 5 5 d 126 is a widespread mineral retrograde after hydrothermal biotite and may have formed from fluids similar to those related to the muscovite during cooling of the porphyry system by contrast paragenetically later and postmineralization alteration fluid which produced dickite 18o 4 1 to 3 3 d 130 to 140 shows clear evidence for mixing with substantial amounts of meteoric water relatively low d values 140 for this meteoric water component may indicate that its source was at high elevations the geologic structure nature of alteration styles of mineralization and stable isotope data indicate that the oyu tolgoi deposits constitute a typical porphyry system formed in an island arc setting the outward zonation of sulfide minerals for the hugo dummett deposits from a bornite dominated core to chalcopyrite and pyriteenargite can be interpreted to be related to a cooling magmatic hydrothermal system which transgressed outward over enclosing advanced argillic alteration this resulted in some unusual alteration and sulfide parageneses such as topaz or pyrite enargite and tennantite entrained by high grade bornite

the red chris porphyry copper gold deposit northern british columbia canada igneous phases alteration and controls of mineralization red chris is a late triassic porphyry cu au mo deposit in the stikinia island arc terrane in northern b c late triassic sedimentary and volcanic rocks of the stuhini group host a series of late triassic to early jurassic 204 198 ma diorite to quartz monzonite stocks and dikes the red chris deposit is hosted in the 204 ma red stock the largest 6 5 1 5 km and most altered intrusion in the area volcanic rocks approximately time equivalent with the stock are exposed 2 km to the southwest and west postmineral early and middle jurassic sedimentary rocks unconformably overlie the stuhini group and red stock to the south and southeast the composite red stock consists of multiple phases of crowded porphyries 30 65 plagioclase hornblende biotite phenocrysts including abundant premineral p1 leucodiorite multiple subphases of synmineral p2 quartz monzonite porphyries and volumetrically minor late mineral monzonite p3 and monzodiorite p4 porphyries the stock evolved from subalkaline to silica saturated alkalic composition with the mineral related p2 porphyries being high k calcalkalic and high sr y common septa of stuhini biotitic hornfels are entrapped within the stock minor postmineral weakly altered mafic dikes cut the stock the zone of 0 25 cu equiv cu au is 2 km long wsw ene and up to 650 m wide early a type quartz veins with disseminated bornite accompanied biotitic and k feldspar magnetite alteration which was coeval with p2 porphyries higher cu and au grades correlate in a general way with zones of more abundant a veins chlorite carbonate epidote actinolite replaced hornblende peripheral to biotitic alteration and chlorite overprinted much of the secondary biotite late alteration affected all porphyry phases and is subdivided into earlier sericite clay pyrite and later low sulfide carbonate clay hematite late sericite clay pyrite alteration sulfidized much of the primary bornite to chalcopyrite with or without pyrite and carbonate clay hematite alteration converted most magnetite to hematite highest cu au grades 2 cu equiv occur in and around the early and intermediate stage p2 porphyries zoning of cu au au cu ratios sulfides and quartz veins is centered and mostly symmetrical around a core of high grade cu au within and surrounding apices of p2 porphyry contact relationships confirm the close association of p2 porphyry phases with cu au zones of 50 ppm mo surround the highest grade cu au zone at depth west northwest and east northeast strike slip to oblique slip faults offset sulfide cu au and mo patterns several lines of evidence indicate 15 to 20 of southward tilt since the system formed due to glaciation oxidation is very thin and supergene enrichment negligible red chris was previously considered by most workers as an alkalic or hybrid alkalic deposit new chemical data on synmineral porphyry phases instead suggest high k calc alkalic composition similar to the monzonitic cu mo au class of porphyry deposits which includes bingham and bajo de la alumbrera red chris is also classified as an a vein type deposit due to strong control of higher grade cu au by a vein stockworks with abundant disseminated cu sulfides

dolomitization of the pedro castle formation pliocene cayman brac british west indies abstract the pedro castle formation pliocene on cayman brac is variably dolomitized by texture preserving but non mimetic and texture destructive replacive dolomite mimetic replacement of skeletal grains is limited to echinoderm plates and with few rare exceptions there is no mimetic replacement of red algae foraminifera green algae or any other type of skeletal grain the lack of mimetic dolomite is atypical of island dolostones found in the caribbean sea and the pacific ocean dolostones in the pedro castle formation are formed entirely of high ca calcian dolomite average of 57 4 mol caco 3 oxygen isotopes mean 1 25 pdb from the dolomite indicate that dolomitization was mediated by seawater or modified seawater carbon isotopes in the dolomite which range from 1 81 to 1 42 pdb were probably inherited from the precursor limestone the average sr content in the dolomite 360 ppm is higher than that found in most other island dolomites the sediments that now form the pedro castle formation were deposited in shallow water on an open bank during the early pliocene pre dolomitization diagenesis of those sediments included syntaxial overgrowths around echinoderm fragments dissolution of aragonitic bioclasts stabilization to low magnesium calcite and local precipitation of vadose cements thus the limestones had been extensively stabilized by the time that dolomitization took place during the late pliocene the general paucity of mimetic replacement in these dolostones can probably be attributed to the calcite stabilization that took place before dolomitization

evolution and development of miocene island dolostones on xisha islands south china sea on xisha islands located in the south china sea the neogene succession includes the unconformity bounded huangliu formation that is 210 5 m thick in well ck 2 and formed almost entirely of dolostones the diverse biota in the huangliu formation which includes corals algae bivalves and foraminifera indicates that the original carbonate sediments accumulated in water that was 30 m deep the dolostones are formed of various mixtures of low and high calcium calcian dolomite with limpid dolomite lining the walls of many cavities the o 18 and c 13 stable isotopes suggest that dolomitization was mediated by slightly modified seawater the sr 87 sr 86 ratios from the dolostones suggest that dolomitization took place similar to 9 4 and 2 3 ma ago with the age of dolomitization becoming progressively younger towards the top of the formation island dolostones like these found on many islands throughout the pacific ocean and the caribbean sea have commonly been linked to eustatic changes in sea level with dolomitization taking place during lowstands highstands or transgressive phases data from the huangliu formation in well ck 2 suggests that dolomitization was associated with semi continuous transgressive conditions that were controlled by the interaction of tectonic subsidence and eustatic changes in sea level

giant versus small porphyry copper deposits of cenozoic age in northern chile adakitic versus normal calc alkaline magmatism cenozoic magmatic activity in northern chile led to the formation of two contrasting porphyry copper belts 1 a paleocene early eocene belt comprising small porphyry copper deposits e g lomas bayas of normal calc alkaline affinity and 2 a late eocene early oligocene belt hosting huge porphyry copper deposits e g chuquicamata of adakitic affinity although the first belt comprises both volcanic and plutonic rocks andesitic basaltic and rhyolitic lavas and tuffs and associated sub volcanic porphyries and felsic stocks the latter only includes intrusions mostly granodioritic types including porphyry copper deposits we suggest that the late eocene early oligocene belt formed when fast and oblique convergence between the south america and farallon plates led to flat subduction and direct melting of the subducting plate hence giving rise to plutonic rocks of adakitic affinity the absence of volcanism under prevailing compressional conditions prevented the escape of so2 from the adakitic sulfur rich highly oxidized magmas closed porphyry system which allowed formation of huge mineral deposits on the contrary coeval volcanic activity during formation of the paleocene early eocene calc alkaline porphyries allowed development of open systems hence to outgassing and therefore to small mineral deposits

evidence from magnetic fabric for the flow pattern of magma in the mackenzie giant radiating dyke swarm continental flood basalts and their associated swarms of mafic dykes are thought to result from melting in mantle plumes1 3 but the processes by which the resulting magma makes its way from mantle to crust remain obscure here we use magnetic anisotropy measurements to reconstruct the pattern of magma flow in the proterozoic mackenzie dyke swarm our results indicate that magma was injected vertically within 500 km of the focal point of the dyke swarm and then travelled horizontally at all further distances out to at least 2 100 km the sharp transition from vertical to horizontal flow at between 500 and 600 km may correspond to the outer boundary of melt generation in the mantle plume believed to be responsible for the mackenzie igneous events

genesis of the late triassic southwest zone breccia hosted alkalic porphyry cu au deposit galore creek british columbia canada the late triassic southwest zone alkalic porphyry cu au deposit in the galore creek district northwestern british columbia is hosted in hydrothermally cemented breccias that overprinted older matrix rich breccia and megacrystic orthoclase phyric syenite and monzonite dikes synmineralization cemented breccias and biotite phyric monzodiorite dikes are localized at the contact of the matrix rich breccia and host syenite and monzonite dikes as well as in crosscutting faults regional shortening tilted the southwest zone deposit so that the cemented breccias now dip 50 to 60 southwest four paragenetic stages formed the deposit a premineralization stage 1 consists of the emplacement of coherent syenite and monzonite host rocks that are cut by a matrix rich breccia the synmineralization stage 2 produced potassic altered rocks and phlogopite magnetite k feldspar cemented breccia and veins and most of the cu fe sulfides a halo of stage 2 diopside magnetite grandite phlogopite calcic potassic veins and altered rocks formed around the potassic domains and cu ore zones alteration during stage 2 probably formed at temperatures of about 500 to 425 c from oxidized saline magmatic hydrothermal fluids contemporaneous high aca2 ah 2 conditions peripheral to the potassic domains are required to form the marginal calcic potassic and calcic assemblages the potassic and calcic potassic alteration domains were overprinted by propylitic and calcic alteration assemblages due to the retrograde influx of an admixture of cooler magmatic and nonmagmatic fluids sphene from a stage 2 calcic potassic vein yielded a 202 8 2 1 ma age u pb shrimp rg that is interpreted to reflect the approximate time of hydrothermal alteration late mineralization stage 3 alteration overprinted the cemented breccias and comprises sericite anhydrite sericitic grandite calcic and chlorite epidote carbonate pyrite propylitic assemblages localized late mineralization and cu poor stage 4 k feldspar fe carbonate specularite carbonate potassic and quartz pyrite veins quartz veins cut stage 2 and 3 assemblages chalcopyrite and bornite dominate the core of the deposit and grade outward to chalcopyrite pyrite and to pyrite chalcopyrite flanked by a pyrite only halo the core of the system is cu au rich grading outward to an au enriched cu poor halo coincident with pyrite bearing propylitic alteration gold copper ratios increase upward within stage 2 potassic zones and outward from the core variations in ph redox and temperature along and across fluid flow paths best explain the change to higher au cu values upward and away from the cu rich potassic domains

constraints and interpretation of 87sr 86sr ratios in cenozoic dolomites replacement dolomitization by seawater has been modeled in order to quantify the sr isotope signature in cenozoic dolomites as a function of precursor mineralogy and 87sr 86sr ratio reaction stoichiometry and 87sr 86sr ratio of the dolomitizing fluids high sr carbonates such as aragonite may introduce a significant precursor memory into an otherwise seawater dominated sr isotope signature if small quantities of seawater per unit volume of precursor carbonate are involved dolomitization of low sr carbonates i e low mg calcite are shown to create an isotopic signature indistinguishable from that of the seawater involved in the reaction therefore by comparison with the sr isotope evolution curve of seawater the 87sr 86sr ratios of the dolomites can be used to record the oldest possible age of dolomitization and the youngest age of deposition the implications for this approach have been applied to data obtained from a dolomitized core from little bahama bank bahamas two periods of dolomitization are recognized one in the early late miocene involving middle miocene or older rocks and a second one around 2 4 ma ago affecting early pliocene carbonates

high sr y arc magmas and porphyry cu mo au deposits just add water porphyry cu mo au deposits are commonly associated with intermediate composition arc related igneous rocks with high sr y and la yb ratios and such rocks are equated by some researchers with melts derived from subducted oceanic crust undergoing eclogite facies metamorphism adakites however similar geochemical characteristics are readily developed in normal asthenospheric mantle wedge derived magmas by fractionation of amphibole and or garnet and or by interaction with crustal materials during ascent through the upper plate lithosphere while there is widespread evidence for such fractionation and contamination processes in porphyry magmas there is little direct evidence for an origin by slab melting the enhanced fertility of arc magmas relates primarily to their high water content which is required for the formation of magmatic hydrothermal systems upon shallow crustal emplacement magmatic water contents 4 wt percent h 2 o cause abundant amphibole garnet fractionation but suppression of plagioclase crystallization at deep crustal levels resulting in increasing sr y and la yb ratios with differentiation commonly into adakitic ranges but not reflecting slab melting additional factors that affect arc magma fertility are relatively high oxidation states and sulfur content but these are secondary to the requirement for sufficient water thus arc magmatic suites with high sr y ratios and evidence for the presence of hydrous phenocryst phases hornblende and or biotite are indeed prospective for porphyry cu mo au deposits but only because these parameters indicate high magmatic water content