epithermal gold mineralisation in the active aegean volcanic arc the profitis ilias deposit milos island greece the profitis ilias gold deposit located on the western part of milos island greece is the first epithermal gold deposit discovered in the pliocene pleistocene aegean volcanic arc estimated ore reserves are 5 million tonnes grading 4 4 g tonne au and 43 g tonne ag the deposit is closely associated with a horst and graben structure and occurs in a series of steep interconnected crustiform banded quartz veins up to 3 m wide extending to depths of at least 300 m the mineralisation occurs in three stages and is hosted by 3 5 2 5 ma old silicified and sericitised rhyolitic lapilli tuffs and ignimbrites it consists of pyrite galena chalcopyrite electrum and native gold additionally adularia occurs with quartz mainly in veins homogenisation temperatures of primary liquid rich inclusions vary from 145 to 399 c for the ore stage and 112 to 263 c for the post ore stage salinities range between 0 1 and 11 4 wt nacl equiv and 0 93 to 8 5 wt nacl equiv for the ore stage and the post ore stage respectively rare vapour rich inclusions in ore stage quartz homogenise between 368 and 399 c and estimates of eutectic melting 25 to 38 c indicate the presence of ca and mg in the ore fluids sample elevation versus fluid inclusion th salinity relationships show 1 a high salinity trend where moderate temperature 300 250 c and moderate salinity brines 3 wt nacl equiv trend to high salinity up to 15 wt nacl equiv fluids with lower 25 50 c homogenisation temperatures and 2 a high th trend where moderate salinity and moderate temperature brines 200 250 c 3 wt nacl equiv develop into low salinity 350 c fluids these trends are best explained by extreme boiling and vapourisation phenomena between 200 and 250 c the 430 450 m asl metres above sea level level marks the transition between a lower liquid dominated segment of the system where only the steep high salinity trend is seen and an upper vapour dominated segment where the high th trend or a combination of both are seen there is a close spatial association between mineable gold grades and the upper segment of the system depth to boiling curves suggest that the paleo surface was 200 m above the present summit of profitis ilias comparison of the mineralisation and fluid geochemistry at profitis ilias with that of the nearby modern geothermal system indicates that the processes of metal mineralisation have probably been continuous since the late pliocene

thermochronology of the high pressure metamorphic rocks of crete greece implications for the speed of tectonic processes new fission track thermochronologic data from the high pressure p low temperature t rocks of crete greece combined with pressure temperature and stratigraphic constraints reveal that their subduction began between 36 and 29 ma metamorphism took place in western crete at peak conditions of 10 2 kbar and 400 50 c between 24 and 19 ma and rapid exhumation to 10 km and 300 c at a minimum rate of 4 km m y was completed before 19 ma constraints from the thermal history of the plate above the inferred extensional detachment reveal that tectonic unroofing contributed 85 to 90 of the overall exhumation of the high p low t rocks of crete we propose that the hellenic subduction zone has acted as a retreating plate boundary since at least the early oligocene and collision and extension during this time were driven by roll back associated with slab pull rather than by gravitational collapse as a consequence of crustal thickening the speed of subduction and exhumation of the high p low t rocks of crete within 10 m y has important implications for other orogenic belts showing that rocks can be subducted metamorphosed at high pressure and exhumed despite slow overall plate convergence within the uncertainties of many paleontologic and isotopic age data

dextral rotations and tectonomagmatic evolution of the southern rhodope and adjacent regions greece abstract palaeomagnetic measurements deduced from eocene oligocene miocene and pliocene formations exposed in the greek rhodope and surrounding areas reveal significant clockwise rotations displaying an apparent systematic increase in magnitude from thrace westwards and a decrease northwards the pelagonian zone in the west has suffered no rotation at least since the late miocene a transition from rotated to unrotated regions seems to exist close to the western boundary of the vardar zone no rotations are apparent earlier than the mid oligocene parts of the rhodope may have been rotated clockwise by 12 during the late oligocene additional clockwise rotations occurred after the early miocene it is proposed that at least the post early miocene rotations were the result of plate tectonic motions in the north aegean area initiated by middle miocene times and continuing to the present day southward stretching and bending of the ductile part of the lithosphere was accomplished at brittle upper crustal levels by the detachment translation and rotation of the upper plate of the rhodope core complex the serbomacedonian element possibly followed by further fragmentation and rotation of individual blocks

erosion rates for taiwan mountain basins new determinations from suspended sediment records and a stochastic model of their temporal variation abstract we estimate erosion rates from suspended sediment records for 11 basins in the eastern central range ecr of taiwan using methods based on mean measured sediment discharge a rating curve of sediment and water discharge and a rating curve corrected for periods of limited sediment due to the lack of landslide supplied sediment the preferred method for any basin depends on record length and sampling frequency with higher quality records being analyzed by the latter method erosion rate estimates range from 2 2 to 8 3 mm yr for records with varying sampling frequencies and durations between 8 and 27 yr this variation in erosion rates does not seem to reflect lithology tectonic environment or climate we interpret the variation in terms of natural stochastic variation in water discharge and sediment supply to assess the quality of the erosion rate estimates and to better understand the dependence of uncertainty on the duration and frequency of sampling we construct a stochastic model of sedi

late cretaceous structural control and alpine overprint of the high sulfidation cu au epithermal chelopech deposit srednogorie belt bulgaria the chelopech epithermal high sulfidation deposit is located in the panagyurishte ore district in bulgaria which is defined by a nnw alignment of upper cretaceous porphyry cu and cu au epithermal deposits and forms part of the eastern european banat srednogorie belt detailed structural mapping and drillcore descriptions have been used to define the structural evolution of the chelopech deposit from the late cretaceous to the present the chelopech deposit is characterized by three fault populations including n55 n110 and n155 trending faults which are also recognized in the entire panagyurishte district mapping and 3 d modeling show that hydrothermal alteration and orebody geometry at chelopech are controlled by the n55 trending and n110 trending faults moreover the n155 trending faults are parallel to the regional ore deposit alignment of the panagyurishte ore district it is concluded that the three fault populations are early features and late cretaceous in age and that they were active during high sulfidation ore formation at chelopech however the relative fault chronology cannot be deduced anymore due to late cretaceous and tertiary tectonic overprint structurally controlled ore formation was followed by senonian sandstone limestone and flysch deposition the entire late cretaceous magmatic and sedimentary rock succession underwent folding which produced wnw oriented folds throughout the panagyurishte district a subsequent tectonic stage resulted in overthrusting of older rock units along ne trending reverse faults on the upper cretaceous magmatic and sedimentary host rocks of the high sulfidation epithermal deposit at chelopech the three fault populations contemporaneous with ore formation i e the n55 n110 and n155 trending faults were reactivated as thrusts or reverse faults dextral strike slip faults and transfer faults respectively during this event previous studies indicate that the present day setting is characterized by dextral transtensional strike slip tectonics the ne trending overthrust affecting the chelopech deposit and the reactivation of the ore controlling faults are compatible with dextral strike slip tectonics but indicate local transpression thus revealing that the chelopech deposit might be sited at a transpressive offset within a generally transtensional strike slip system the early wnw trending folds require a roughly nne ssw shortening which is incompatible with the present day dextral strike slip tectonic setting and the ne trending thrust formed during the tectonic overprint of the chelopech deposit this reveals a rotation of the principal stress axes after late cretaceous high sulfidation ore formation and post ore deposition of sedimentary rocks the nature of the sedimentary rocks interlayered and immediately covering the upper cretaceous magmatic rocks hosting the chelopech deposit indicates sedimentation and associated volcanism in an extensional setting immediately before ore formation it is concluded that the chelopech deposit was formed when the tectonic setting changed from extensional during late cretaceous basin sedimentation and magmatism to compressional producing wnw trending folds under a roughly nne ssw compression possibly in a sinistral strike slip system thus like other world class high sulfidation epithermal deposits the chelopech deposit was formed at the end of an extensional period or during a transient period of stress relaxation which are particularly favorable tectonic settings for the formation of high sulfidation epithermal deposits the exceptional preservation of the upper cretaceous chelopech epithermal deposit is explained by the combined deposition of a thick senonian sedimentary sequence on top of the upper cretaceous magmatic host rocks of the deposit and the later overthrust of older rock units on top of the deposit our study at chelopech supports previous studies stating that post ore basin sedimentation and tectonic processes provide the favorable environment to preserve old epithermal deposits from erosion the tectonic evolution of the chelopech deposit is similar to that of the entire panagyurishte ore district this coherence of the magmatic hydrothermal and tectonic events from north to south suggests that the ore deposits of the entire panagyurishte ore district were formed in a similar tectonic environment

age of the camaguey gold silver district cuba tectonic evolution and preservation of epithermal mineralization in volcanic arcs epithermal precious metal deposits commonly form in middle to late cenozoic volcanic arcs several kilometers above the level at which batholiths are emplaced this relationship has led to the guideline that epithermal deposits should not be sought in volcanic arcs where batholiths are widely exposed the camaguey district in central cuba violates this tenet and provides important insights into arc forming processes that create and expose epithermal deposits the camaguey district hosts the jacinto and florencia low sulfidation vein deposits and the deseada and golden hill high sulfidation prospects all of which are within a few kilometers of the large camaguey batholith some of the deposits such as jacinto lie immediately below a local paleosurface that exposes the batholith and probably formed when rhyolite domes were emplaced onto this surface a second group of deposits such as the deseada high sulfidation system is hosted by volcanic rocks that overlie the camaguey batholith and stand above the paleosurface new ar ar age measurements indicate that jacinto type deposits and prospects cooled through ar ar closure temperatures for adularia and muscovite at about 71 ma and are slightly younger than the widespread granodiorite facies of the camaguey batholith in contrast deseada type deposits and mineralized zones cooled through roughly similar closure temperatures for sericite illite and alunite at about 80 ma which is older than the age of the immediately underlying camaguey batholith these ages are best explained if the camaguey batholith and surrounding volcanic rocks underwent rapid uplift probably by tectonic unroofing to form the paleosurface below which jacinto type deposits formed and if the older deseada type deposits were thrust into place over this surface this tectonic unroofing probably took place in an extensional regime generated during oblique convergence of cuba with yucatan as the greater antilles arc moved north into the caribbean region during late cretaceous time thrusting probably accompanied late stages of this convergence and or later collision with the bahamas bank in eocene time all of the epithermal deposits were covered by late cretaceous eocene sedimentary rocks that have been removed only recently by erosion thus accounting for the preservation of these unusually old epithermal deposits extensional events causing tectonic unroofing are probably common in arcs that undergo oblique convergence and could be an important agent for formation of epithermal deposits in other parts of the world preservation of these deposits might be aided by deposition and later erosion of sedimentary cover related to later collisions

exhumation and preservation of porphyry cu au deposits at oyu tolgoi south gobi region mongolia field relationships and u pb data zircon are used to constrain the timing of tectonomagmatic events that occurred shortly after the formation of late devonian 372 ma porphyry cu au deposits at oyu tolgoi mongolia in the two northernmost of the several discrete porphyry centers hugo dummett deposits upper devonian rocks positioned stratigraphically above copper gold mineralization preserve a record of uplift exhumation and subsequent burial shortly after mineralization strongly altered and mineralized quartz monzodiorite and volcanic rocks at hugo dummett are overlain by an unmineralized dacite sequence 369 2 ma the basal breccia and conglomerate of the dacite sequence contain mineralized porphyry clasts derived from erosion of the porphyry deposits two zircon grains recovered from one of these mineralized porphyry clasts yielded chemical abrasion thermal ionization mass spectrometry ca tims 206 pb 238 u dates of 374 0 1 9 and 371 4 1 6 ma which are consistent with previously published ages for mineralized intrusions this implies that oyu tolgoi or other nearby porphyry deposits were exhumed shortly after the main mineralization event weakly mineralized granodiorite dikes intruded the postmineralization dacite sequence as well as devonian sedimentary and volcaniclastic rocks that structurally overlie the dacite a weighted mean 206 pb 238 u age of 370 6 0 5 ma for this rock is based on the three youngest grains analyzed by the ca tims technique mswd 1 14 burial of the cu au deposits at oyu tolgoi soon after their exhumation was critical to their preservation exhumation and subsequent burial occurred at 371 ma during a short time frame within the overlapping u pb zircon ages for the quartz monzodiorite mineralized porphyry clast unmineralized dacite sequence and granodiorite unit

pliocene pleistocene asymmetric unroofing of the irian fold belt irian jaya indonesia apatite fission track thermochronology the central range of new guinea is an orogenic belt up to 100 km wide and more than 1300 km long with numerous peaks over 3 km high the orogen was generated since the middle miocene by the collisional underthrusting of the northern passive margin of the australian continent into a north dipping subduction zone beneath the melanesian island arc south of puncak jaya the highest peak in new guinea pliocene pleistocene uplift and unroofing have exposed tertiary carbonates and mesozoic clastic rocks of a rift and passive margin sequence above unmetamorphosed paleozoic strata and greenschist facies slates probably of precambrian age eight magmatic apatite samples from pliocene intrusions of intermediate composition in the gunung bijih ertsberg mining district at the crest of the central range in irian jaya indonesia yield pooled fission track ages ranging between 3 7 0 9 and 2 0 0 3 ma 1 long etchable mean track lengths 14 m and narrow track length distributions indicate rapid cooling as expected for shallow level intrusions these track length data and the observation that the grasberg pluton was emplaced into its own volcanic cover indicate that 2 km of material has been eroded from the top of this part of the range since the pliocene seven detrital apatite samples from the triassic jurassic tipuma and the carboniferous permian aiduna formations and apatite from two igneous dikes exposed halfway down the southern slope of the range yield pooled fission track ages ranging between 2 7 0 7 and 2 0 0 5 ma 1 etchable mean track lengths of 9 12 m and wide track length distributions indicate slower cooling compared to the pliocene intrusions complete resetting of provenance fission track ages in detrital apatite requires burial deeper than 4 km unroofing rates are 1 7 km m y on the southern flank of the range but 0 7 km m y at the crest these fission track data combined with local and regional geologic relationships indicate that unroofing of the irian fold belt occurred at least two and a half times faster and perhaps more than five times faster on the lower slope than at the crest this difference is probably due to regional weather patterns that result in up to 11 m yr of orographically induced precipitation on the southern slope of the irian central range but only 3 m yr near the highest elevations

geology and geochemistry of sulfide mineralization underlying the pueblo viejo gold silver oxide deposit dominican republic the large pueblo viejo oxide gold deposit was developed by weathering of a gold quartz pyrophyllite deposit that formed during sedimentation in a small basin in the upper part of the los ranchos formation one of the oldest volcanic units in the greater antilles the basin is floored by conglomerates and agglomerates which grade upward into standstones and carbonaceous sediments that include chert with thin pyrite layers plant fossils which are common in the carbonaceous sediments indicate that the basin was near a land mass the mineralized zone is roughly funnel shaped and expands to its maximum horizontal dimensions in the carbonaceous sediments the lower part of the funnel tube contains alunite and diaspore pervasive pyrophyllite alteration makes up most of the overlying tube and upper part of the zone and locally extensive silicification makes up the top of the funnel supergene kaolinite is present in the upper part of the funnel metallic minerals consisting mostly of pyrite are found as disseminations layers and veins with base and precious metal values being highest in the veins veins are most common in the carbonaceous sediments and diminish in abundance downward into the coarser clastic units the veins contain abundant pyrite sphalerite and quartz minor barite enargite and pyrophyllite and traces of electrum argentite colusite tetrahedrite tennantite geocronite galena and tellurides containing gold silver and copper there is a strong correlation between au and zn in the upper parts of the mineralized zone and the au ag ratio which averages 1 7 decreases slightly with depth au is in electrum in the lower part of the mineralized zone and partly in tellurides in the upper part fluid inclusions indicate that fluids in the veins were very dilute boiled at least locally and reached temperatures of 130 degrees to 190 degrees c pyrophyllite stability requires minimum temperatures of about 260 degrees c for the hydrothermal system isotopic data indicate that most of the sulfur in the mineralization was derived from seawater whereas the strontium and lead came from the los ranchos formation which probably also supplied the precious metals the pueblo viejo system is similar to other gold quartz pyrophyllite deposits except that it contains no known mineable vein orebodies and instead contains numerous narrow closely spaced irregular veins that can be mined by bulk methods these veins are thought to have formed by hydraulic fracturing of the carbonaceous sediments caused by fluid pressures as the hot spring system developed the size and shape of the mineralized system at pueblo viejo resemble active hot spring systems such as wairakei new zealand and its environment of formation probably resembled that of the quaternary volcano chain which crosses lake managua in nicaragua

geochemistry of the mabujina complex central cuba implications on the cuban cretaceous arc rocks the margins of the caribbean plate are marked by cretaceous island arc basalts associated with accreted fragments of the cretaceous colombian caribbean oceanic plateau in cuba the cretaceous volcanic island arc rocks are in fault contact with the mabujina complex interpreted as an oceanic jurassic to early cretaceous arc basement with local island arc rocks the cuban cretaceous island arc consists of early to late cretaceous volcanic series associated with limestones while the pre albian arc rocks consist of tholeiitic basalts and rhyolites the post albian volcanic series is characterized by calc alkaline andesites the cretaceous lavas have sr and nd isotopic compositions similar to the intraoceanic arcs and the pb isotopic initial ratios are close to the east pacific rise mid ocean ridge basalt field according to our data the mabujina arc rocks are tholeiites and calc alkaline basalts developed in a jurassic and or early cretaceous intraoceanic island arc their nd sr and pb isotopic compositions indicate that they derive from a depleted mantle source contaminated by sediments this subduction magmatism is not related to the classic early cretaceous caribbean tholeiitic series but is similar to the late jurassic to early cretaceous guerrero arc terrane from mexico and may represent its southernmost extension thus the different tectonic units of central cuba cannot be easily correlated with those of hispaniola our data also indicate that two different island arcs were tectonically juxtaposed in central cuba the classical lower and upper cretaceous suites of the greater antilles arc and a jurassic to early cretaceous island arc suite with a pacific provenance

origin of the mclaughlin mine sheeted vein complex metal zoning fluid inclusion and isotopic evidence the mclaughlin deposit is a large hot spring type gold deposit located in the northern coast ranges of california at the faulted lithologic contact between the coast range ophiolite and the great valley sequence the mclaughlin deposit is centered around the sheeted vein complex a large multistage vein swarm localized in a dilation zone formed by rheologic contrasts in the footwall polymictic melange the surface expression of the sheeted vein complex is a subaerial sinter terrace which merges into the sheeted vein complex metal zoning in the sheeted vein complex is highly telescoped the sinter is enriched in mercury whereas gold and silver are restricted to the upper 350 m with the proportion of gold to silver decreasing with depth gold is typically present as electrum and is associated with silver and base metal bearing sulfosalts below 350 m mineralization is dominated by small quartz veins with base metal sulfides fluid inclusion studies indicate that nacl is the main dissolved salt in concentrations averaging 2 4 wt percent nacl equiv the bulk of the salinity data can be explained by variable amounts of adiabatic boiling and mixing with steam heated ground water fluid inclusion homogenization data ranges from 121 degrees to 263 degrees c with the deepest portion of the system examined having the highest temperatures and the shallowest samples having the lowest temperatures fluid inclusion gas concentrations vary with depth below the sinter co 2 and to a lesser extent ch 4 shows a systematic increase with depth fluid pressures calculated from fluid inclusion homogenization temperatures and gas analysis are in excess of the hydrostatic pressure and approach or exceed lithostatic pressure self sealing of the system and tectonic dilation most likely resulted in catastrophic phase separation and hydrothermal eruption breccias geothermometry results based on fluid inclusion gas ratios show a reasonable correspondence with the highest fluid inclusion homogenization temperatures there is a strong trend for decreasing delta 18 o quartz values with depth due to the effect of temperature the calculated delta 18 o sub h 2 values are fairly constant at about 9 to 10 per mil oxygen and hydrogen isotope values as well as geologic relationships are consistent with those of hydrothermal fluids originating as meteoric water these evolved to increasingly high ones by interaction with great valley sequence sedimentary rocks lead isotope compositions of sulfides and potential source rocks indicate that lead and by inference other metals was derived from a crustal source the lead source may have been detritus from the sierra nevada batholith that comprises the great valley sequence or alternatively it may have been derived from a thorough mixture of radiogenic lead and nonradiogenic lead that occurred prior to mineralization

epithermal gold mineralization in an old volcanic arc the jacinto deposit camagueey district cuba the jacinto vein system in the camaguey district of central cuba is a low sulfidation adularia sericite epithermal gold deposit it is hosted by volcanic and volcaniclastic rocks of cretaceous age along the margin of the camaguey batholith jacinto is unusually old and much more closely associated with a large batholith than would be expected for an epithermal vein deposit in an island arc this study was undertaken to determine how jacinto could have been preserved for so long the jacinto system includes at least five veins that range in length up to 1 km have maximum widths of 60 m and extend to depths of at least 150 m the veins consist almost entirely of quartz with minor amounts of adularia calcite commonly bladed pyrite and gold which form at least 20 growth zones that can be divided into five main stages and can be correlated from one vein to another gold ore shoots are restricted vertically and are more continuous horizontally their present configuration suggests that the entire system has been tilted about 5 degrees to the northeast wall rock alteration is limited and consists of minor k feldspar and sericite fluid inclusions in vein quartz homogenize at temperatures of 200 degrees to 320 degrees c and freeze at temperatures characteristic of salinities of about 0 to 1 wt percent nacl equiv the presence of bladed calcite and silica pseudomorphs after bladed calcite suggests that the hydrothermal fluids boiled stable isotope analyses of quartz calcite and inclusion fluids indicate that the veins were deposited by waters with delta 18 o and delta d values that range from values just above those of magmatic water to those typical of cretaceous meteoric water 40 ar 39 ar analyses of adularia in the jacinto veins indicate an age of about 72 ma for mineralization and geologic relationships suggest that the vein system is related to la sierra rhyolite flows and domes of that approximate age la sierra rhyolite was extruded onto an erosion surface that exposed the camaguey batholith and related volcanic rocks indicating that erosion rates were relatively rapid immediately prior to formation of the deposit shortly after the deposit formed however it was tilted and then buried beneath late cretaceous to eocene sediments of the maraguan basin which are being eroded to expose the system today this fortuitous sequence of volcanism batholith emplacement rapid erosion to deep batholith levels extrusion of rhyolite and local sedimentation appears to be the result of collision between the cuban arc and the bahamas platform similar collisions between arcs and continental masses could have formed other epithermal deposits

quaternary intra arc extension in the central trans mexican volcanic belt the trans mexican volcanic belt is an active volcanic arc related to subduction along the middle america trench and characterized by shallow seismicity and synvolcanic to postvolcanic extensional arc parallel faulting major intra arc basins within the central part of the belt between long 998w and 1028w are from west to east the cuitzeo and acambay grabens the aljibes half graben and the mezquital graben in this region 100 eastwest2striking 2 km long steeply dipping normal faults expressed by pronounced multi event fault scarps have been mapped of the 100 faults 65 displace rocks of known quaternary age younger than 1 6 ma and 22 cut rocks with a documented age younger than 750 ka known historical surface ruptures are limited to faults of the acambay graben overall the faults cause north south to north northwest2south southeast2oriented extension of 1 km which is distributed over a 30250 km wide zone that has 5210 faults in cross section and a relative extension of 3 quaternary vertical slip rates estimated for 13 of the faults have a mean of 0 07 mm yr they are highest at the southern margin of the cuitzeo graben the northern margin of the acambay graben and in the aljibes half graben where they measure between 0 16 and 0 18 mm yr the north south to north northwest2south southeast2oriented quaternary bulk extension rate of the system is likely to be 0 2 6 0 05 mm yr this fault system is in an initial stage of coalescence its western and eastern parts consist mostly of isolated fault segments whereas in the central part deformation is localized onto a few through going faults the longest of these are the venta de bravo fault 45 km and the acambay tixmadeje fault 34 km the short fault traces have a simple structure whereas the longer ones are commonly composed of two or more segments there is no obvious migratory pattern of quaternary fault activity which suggests that the entire region is tectonically active

the porphyry gold deposit at marte northern chile a porphyry gold deposit has been documented for the first time at marte in the maricunga belt of the andean cordillera northern chile exploration conducted from 1981 through 1987 resulted in definition of 66 metric tons of contained gold the deposit entered production in late 1989 as an open pit heap leach operation the gold deposit is part of a linear calc alkaline volcanoplutonic arc constructed during the mid to late miocene as eastward directed subduction was shallowing the consequent compression of continental basement underlying the deposit and its environs created a series of high angle reverse faults stock emplacement and gold mineralization took place within a coeval andesitic stratovolcano at 13 to 14 ma the hornblende biotite diorite stock is subdivided into three phases two mineralized porphyries and a weakly porphyritic late mineralization microdiorite much of the microdiorite constitutes the matrix of an intrusion breccia which is transected locally by a closely related hydrothermal breccia gold mineralization is coincident with a stockwork of quartz veinlets surrounded by chloritesericite clay alteration of intermediate argillic type pyrite and iron oxides are both present in dominantly disseminated form with each making up as much as 10 vol percent of the rock hematite exceeds magnetite in abundance and was generated in large part by hypogene martitization minor chalcopyrite and even lesser quantities of molybdenite bornite tennantite and enargite also occur gypsum after anhydrite commonly exceeds 5 vol percent and calcite and tourmaline exist as traces isolated remnants of hydrothermal biotite and alkali feldspar attest to the former presence of k silicate alteration which is believed responsible for introduction of much of the veinlet quartz magnetite and gold chlorite dominated alteration affected the late stage microdiorite and its associated breccias an erosional remnant of the andesitic volcanic roof to the stock is largely replaced by a chalcedony and alunite rich advanced argillic assemblage which is part of a formerly more extensive tabular alteration zone abundant pyrite gypsum native sulfur and barite and trace amounts of enargite and subeconomic gold are present in this advanced argillic cap the relative impermeability of the cap caused development of an inverted supergene profile in which sulfide bearing ore is underlain by jarositic leached ore the steep elongate orebody is made up of both ore types which do not differ in gold content however the hypogene copper content about 550 ppm was more than halved by supergene oxidation the gold ore is poor in silver ag au 0 44 but highly anomalous in molybdenum as well as copper and weakly anomalous with respect to lead arsenic and mercury the advanced argillic cap is marked by pronounced au bi hg tl pb mo and as anomalies which contrast with a broad zinc halo to the orebody preliminary fluid inclusion studies suggest that the auriferous stockwork is a product of boiling fluids which ranged in temperature from 155 degrees to 375 degrees c and in salinity from 2 to 20 wt percent nacl equiv a reconstruction of the volcanic edifice combined with pressure estimates based on the fluid inclusion data implies that mineralization took place about 600 to 700 m below the paleosurface the geologic alteration and mineralization characteristics of the marte gold deposit are closely similar to those of gold rich porphyry copper deposits especially those in the philippines the only significant mineralogic difference is a deficiency of copper at marte the fluid inclusion population at marte is also more reminiscent of porphyry rather than epithermal environments although high salinity inclusions common in porphyry copper deposits were not encountered in the few samples studied

the timing of adularia sericite type mineralization and alunite kaolinite type alteration mount skukum epithermal gold deposit yukon territory canada 40 ar 39 ar and u pb geochronology the lower eocene mount skukum epithermal au deposit southwestern yukon territory 60 degrees 13 n 135 degrees 28 w exhibits both hypogene alteration and mineralization facies characteristic of the adularia sericite deposit type and barren advanced argillic alteration of the alunite kaolinite type the two being separated by only 750 m on the present erosional surface the advanced argillic alteration forms a crudely cylindrical possibly downward tapering body with an alunite quartz core and surrounding pyrophyllite or kaolinitc zones and is centered on a small or 150 m diam rhyolitic stock the delta d and delta 34 s values of the alunite which average 175 or 0 5 1sigma per mil and 2 93 or 0 43 1sigma per mil respectively suggest that this alteration conforms to the hypogene type formed by magmatic so 2 mixing with meteoric water an inference that is consistent with mineral associations and textures that imply fairly high temperatures pyrophyllite stable the 40 ar 39 ar age spectra for alunite generated by furnace heating exhibit good multi step plateaus and show that during slow heating with small increment steps alunite releases virtually all of its argon through volume diffusion at temperatures below 540 degrees c the preferred age for the alunitic alteration is 55 69 or 0 24 2sigma ma the error weighted average of four plateau dates that range from 55 00 or 0 55 to 56 12 or 0 45 ma calculated closure temperatures for alunite average 280 degrees or 20 degrees c 2sigma at a cooling rate of 50 degrees c m y gold bearing quartz calcite adularia veins are hosted by andesitic volcanic strata of the eocene skukum group and by rhyolitic dikes the mineralized veins conform entirely to the adularia sericite class and were controlled by strike slip faults and fracture zones a wide rhyolite dike cut by the most important ore zone was emplaced at 56 3 or 0 4 2sigma ma earliest eocene as defined by u pb zircon dating step heating 40 ar 39 ar dating of vein adularia with orthoclase structure yields plateaus which suggest that no significant ar loss occurred the error weighted average of two plateau dates for the lake zone and the lowest age steps in two slightly saddle shaped plateaus for the cirque zone 54 05 or 0 31 ma is taken as the age of adularia sericite type mineralization which is slightly but significantly younger than the alunite kaolinite type alteration a swarm of intermediate dikes some of which cut ore veins was intruded over an interval that embraced the time of mineralization one of these dikes which cuts the wide rhyolite dike yielded a u pb zircon age of 55 7 or 0 3 ma but this does not further constrain the age of the mineralization the ages of alunite and adularia from mount skulmm indicate a succession from the barren alunite kaolinite type hydrothermal system to the gold bearing adularia sericite type system both of which formed during ongoing magmatism in the mount skukmn volcanic complex

mechanisms of argon retention in clays revealed by laser 40ar 39ar dating a method for dating clays is important for studies of weathering diagenesis hydrocarbon migration and the formation of major metalliferous deposits however many attempts have produced imprecise or inaccurate results data from shales show that contrary to expectations the 40ar 39ar dating technique can be successfully used to determine the diagenetic age of ancient sediments because 39ar losses during irradiation are controlled by release from low retentivity sites in illite equivalent to those that have lost radiogenic 40ar in nature rather than by direct recoil as is generally assumed

the role of exhumation in the temporal distribution of ore deposits we show here that the temporal age frequency distribution of ore deposits is controlled largely by exhumation the combination of burial uplift and erosion that exposes subsurface rocks and ore deposits exhumation rates calculated from modal ages and emplacement depths for epithermal porphyry copper and orogenic gold deposits of phanerozoic age are 167 158 and 63 m m y respectively these rates agree with rates estimated for continent scale terranes by numerous independent methods thus the scarcity of precambrian epithermal and porphyry copper deposits is probably due to their removal by exhumation archean and pale oproterozoic orogenic gold deposits are more abundant than indicated by the exhumation record of their phanerozoic counterparts suggesting that the precambrian deposits formed in greater numbers recognition of the pivotal role of exhumation requires that the temporal distribution of ore deposits be corrected for this effect before it can be used as an indicator of large scale changes in earth history

petrochemistry and tectonic significance of cretaceous island arc rocks cordillera oriental dominican republic abstract cretaceous island arc rocks of the caribbean island arc system have been exposed by cenozoic faulting in the cordillera oriental in eastern hispaniola high k 2 o intermediate to felsic volcanic rocks loma la vega volcanics are interbedded with marine epiclastic sedimentary rocks and tuffs las guajabas tuffs and unconformably overlie pre aptian sedimentary rocks low k 2 o volcanic rocks guamira volcanics and a granodioritic to tonalitic intrusion el valle pluton the petrology and geochemistry of these units in conjunction with regional stratigraphic data are used to speculate on the tectonics of the newly developing caribbean island arc system during early and late cretaceous time the loma la vega volcanics are characterized by the presence of large phenocrysts of sanidine and minor amounts of clinopyroxene opaque oxides and rare leucite in a devitrified matrix of chlorite and clay although the volcanic rocks have undergone some low temperature alteration metamorphism which redistributed some major elements and large ion litho phile trace elements the high field strength elements rare earth elements and radiogenic isotopes appear to have been minimally affected based on abundances of the relatively immobile elements trace element enrichment patterns and isotopic compositions the loma la vega volcanics are considered part of the high k calc alkaline ca or shoshonitic island arc volcanic series in contrast pre aptian early cretaceous volcanic and plutonic rocks of the underlying los ranchos formation have chemical characteristics similar to rocks in the island arc tholeiitic or primitive island arc pia series that form coeval and along strike sections of the early cretaceous caribbean island arc in other parts of present day hispaniola cuba puerto rico and the virgin islands an abrupt and regional change in composition from island arc tholeiites to high k calc alkaline rocks is coincident with a hypothesized reversal in subduction polarity in pre aptian time as inferred from previously published tectonic models polarity reversal may have been triggered by attempted subduction of the caribbean oceanic plateau beneath this segment of the caribbean island arc the observed magmatic and tectonic effects of the proposed cretaceous caribbean arc reversal are similar to the better documented neogene subduction reversal event in the solomon islands arc in the southwest pacific

geology of the hope brook mine newfoundland canada a preserved late proterozoic high sulfidation epithermal gold deposit and its implications for exploration the hope brook gold mine is a high sulfidation epithermal gold deposit hosted by late proterozoic rocks of the northern appalachian avalon zone it is located within the whittle hill sandstone which is intruded by a late proterozoic quartz feldspar porphyry sill dike complex of the roti intrusive suite the deposit 45 t au is enclosed within a zone of hydrothermal alteration 3 km long and up to 400 m wide this zone of alteration is characterized by extensive advanced argillic alteration with pyrophyllite kaolinite andalusite and alunite which is developed mostly in the structural hanging wall of the ore zone and two main stages of massive silicic alteration the first a buff colored stage of massive silicic alteration extends for or 3 km laterally away from the deposit it constitutes a barren to weakly auriferous unit and most likely results from the pervasive acid leaching of the original host s the second stage of gray massive silicic alteration is spatially coincident with gold mineralization and is characterized by vuggy quartz the mineralization consists of several percent of pyrite and lesser amounts of chalcopyrite and bornite and some tennantite with local traces of enargite in addition to au and cu there is a very limited amount of other metals in the deposit although it has anomalously high amounts of sb bi pb and as the ages of altered pre and late ore quartz feldspar porphyry and of an unaltered postore intermediate dike cutting altered rocks bracket the age of mineralization alteration between 578 and 574 ma this temporally and genetically links mineralization alteration and roti intrusive suite plutonism despite a strong spatial relationship between a major fault zone cinq cerf fault zone and gold mineralization the hope brook deposit bears no genetic relationship to major ductile deformation the deposit was formed approximately 150 m y prior to the development of this fault zone the existence of hope brook along with a few mesozoic pueblo viejo paleozoic temora neoproterozoic brewer and paleoproterozoic deposits enaasen highlights the potential for discovery of such au cu intrusion related deposits in older terranes despite their emplacement at high levels in the crust the potential for preservation of such high sulfidation deposits in older terranes may be high if tilting and or burial took place early in the geologic evolution of the host volcano plutonic arc although the nature and distribution of the initial buff silicic stage of alteration are critical indications of a large scale hydrothermal system the ore at hope brook is only localized in rocks displaying the gray colored second silicic stage of alteration consequently the recognition of the two stages of massive silicic alteration was critical in understanding the genesis of the hope brook deposit and constitutes an important exploration tool in discriminating between potentially mineralized and barren zones

overlapping cretaceous and eocene alteration twin creeks carlin type deposit nevada we report here new ar ar dates for adularia illite and muscovite from the twin creeks carlin type deposit all illite samples were obtained from altered intrusive and volcanic rocks that are interlayered with mineralized sedimentary rocks thus largely avoiding complications caused by detrital illite and muscovite in sedimentary rocks interpretation of the illite age spectra was based in part on newly developed models for the behavior of argon in fine grained illite the results indicate that all adularia in the megapit area formed at about 42 ma eocene whereas most illite in the megapit area and muscovite in alteration zones beneath chimney creek formed at about 109 to 103 ma cretaceous additional events involving formation of illite at 200 ma and muscovite at about 310 ma were also observed but were not widespread enough to be considered significant our eocene date for adularia is in complete agreement with previous measurements on adularia from twin creeks but our cretaceous dates are considerably older than dates determined in previous studies of the nearby getchell district particularly the osgood stock which has a date of 98 to 92 ma our results suggest that the osgood stock is part of a more protracted cretaceous igneous hydrothermal event that took place between about 109 and 83 ma the relationship of adularia and illite to gold mineralization is not completely clear making it impossible to use our data to assign a unique age to the twin creeks deposit we favor an eocene age for some gold mineralization because the adularia that we dated is intimately intergrown with auriferous arsenian pyrite typical of carlin type mineralization however adularia has a very restricted distribution in the deposit and temperatures at which it formed were probably not great enough to reset cretaceous and older ages of illite illite is much more widespread and its association with gold is supported by correlation between gold content and k al ratios of mineralized material thus cretaceous illitization was at least an important ground preparing event at twin creeks and it is not possible to show from our data that gold ore or protore were not deposited during this event

dating of alteration episodes related to mercury mineralization in the almaden district spain the almadin mercury deposits represent one of the largest geochemical anomalies on earth but the mode and timing of their formation remains a matter of controversy there are two main possibilities hydrothermal solutions associated with alkali basalt volcanism in the silurian and devonian or regional hydrothermal activity during later hercynian metamorphism although these models can be distinguished by determining the age of ore deposition no suitable isotopic method had been applied to the problem prior to this study we report here on vacuum encapsulated and more traditional laser ar ar dating performed on illite concentrates and cr rich micas that are intimately associated with mercury mineraiization in both devonian and silurian host rocks illite is associated with a later stage of hg mineralization cinnabar kaolinite pyrophyllite chlorite which locally replaces an earlier episode associated with carbonates and cr mica the dating results from illite separates suggest that the later stages of hg mineralization or remobilization occurred about 360 ma which is about 20 ma younger than any mercury hosting rocks in the district and coincides with the onset of regional deformation during the hercynian orogeny cr mica ages are variable and range from 365 ma to 427 ma this age range spans the period between the deposition of the criadero quartzite the oldest sedimentary unit hosting mercury and the later episode of mineralization this suggests that mineralization started at least by the mid to lower silurian and the younger argon ages for cr mica could represent partial to nearly total argon loss caused by the hercynian metamorphic event it appears that both postulated styles of mercury mineralization occurred separated in time by up to 80 million years

erosion and exhumation in the himalaya from cosmogenic isotope inventories of river sediments the outward erosional flux is a key factor in the tectonic evolution of mountain belts and there is much debate about the feedbacks between tectonics erosion and climate here we use cosmogenic nuclides 10be and 26al analysed in quartz from river sediments from the upper ganges catchment to make the first direct measurements of large scale erosion rates in a rapidly uplifting mountain belt the erosion rates are highest in the high himalaya at 2 7 0 3 mm yr 1 errors fall to 1 2 0 1 mm yr on the southern edge of the tibetan plateau and are 0 8 0 3 to 0 6 mm yr in the foothills to the south of the high mountains these relative estimates are corroborated by the nd isotopic mass balance of the river sediment analysis of sediment from an abandoned terrace suggests that similar erosion rates have been maintained for at least the last few thousand years the data presented here along with data recently published for european river catchments demonstrate that a log linear relationship between relief and erosion rate holds over three orders of magnitude variation in erosion rate and between very different climatic and tectonic regimes the erosion rate estimates from cosmogenic nuclides correlate well with exhumation rates calculated from previously published apatite fission track ages in the indian himalaya this confirms that much of the exhumation in the himalayan mountain chain is now balanced by erosion however exhumation rates calculated from high blocking temperature systems such as 40ar 39ar in muscovite imply lower exhumation rates rocks presently at the surface must have undergone a three to six fold increase in exhumation rate within the last few million years we show how this could be explained either by climatic forcing of erosion rate changes or by tectonics published evidence for equally rapid changes of exhumation rate in the past and the probable diachroneity in the time at which the present exhumation rates accelerated imply that tectonics has moderated at least some of the change in exhumation rates

rb sr and 40ar 39ar mineral ages of granitoid intrusives in the mabujina unit central cuba thermal exhumation history of the escambray massif abstract we present new and critical isotopic data for late to postkinematic granitoid intrusions and their epidote amphibolite facies country rocks mabujina unit in central cuba the mabujina unit tectonically overlies a nappe sequence of high pressure hp metamorphic rocks in the escambray massif these granitoid rocks have a variable granodioritic to granitic composition are generally rich in large ion lithophile elements and resemble normal midocean ridge basalts in their high field strength element concentrations on the basis of low initial 87sr 86sr ratios it is inferred that sr is very likely to be mantle derived primary coarse grained muscovite from pegmatitic intrusions yields rb sr crystallization ages of 86 88 ma rio jicaya region and 80 82 ma yayabo region laser 40ar 39ar ages for coarse and fine grained white mica 72 74 ma and rb sr ages for biotite 73 ma outline the cooling history of these granitoids and their country rock dating of pegmatites within the mabujina unit

brief duration of hydrothermal activity at round mountain nevada determined from ar 40 ar 39 geochronology the round mountain gold silver deposit is one of the world s largest volcanic hosted precious metal deposits reserves plus production total at least 16 million oz 500 000 kg au the deposit occurs in ash flow tuff that is ponded within its source caldera along and overlying the caldera ring fracture host rocks have undergone propylitic potassic and high level silicic and intermediate argillic alteration these are interpreted either as a temporal sequence from early propylitic to late silicic and argillic or as a spatial progression from a potassic core outward to a propylitic halo 40 ar 39 ar ages determined by single crystal fusion of sanidine from host tuff and incremental heating of adularia indicate a most likely duration of hydrothermal activity of approximately 0 1 m y possibly no more than 0 05 m y and a maximum of 0 5 m y eruption of host ash flow tuff and caldera collapse occurred at 26 5 ma a late or postalteration tuff that rests upon mineralized rock is 26 0 ma hydrothermal circulation is unlikely to have been established immediately after caldera collapse so duration was probably considerably less than 0 5 m y eight adularia samples give ages between 25 94 or 0 09 and 26 09 or 0 05 ma and one is 26 20 or 0 05 ma the mean of all nine is 26 03 or 0 08 ma the maximum and minimum values and their uncertainties allows a duration of as much as 0 4 m y but for the following reasons the longevity is probably much less the absolute ages of seven samples that can be placed in the possible temporal alteration sequence do not correlate with that sequence this does not disprove the temporal sequence but does indicate that alteration occurred in a time span which is not resolvable even with precise 40 ar 39 ar dating furthermore all samples show flat age spectra which themselves argue against a long continuously active system kinetic studies of two adularia reveal diffusion domains with ar closure temperatures between approximately 230 degrees and approximately 390 degrees c adularia that crystallized and remained at temperatures of 275 degrees c the maximum temperature inferred from fluid inclusion data would yield age spectra with age gradients spanning the lifetime of the system no spectra are consistent with a thermal history that incorporates sustained 0 1 m y temperatures at or above 275 degrees c episodic activity to account for the one older adularia date cannot be precluded but is inconsistent with most data therefore the hydrothermal system probably was active for less than the uncertainty in the ages of individual samples possibly as little as 50 000 years we conclude that hydrothermal activity at round mountain was a brief event at approximately 26 0 ma and was closely linked to caldera structure and magmatism this activity followed initial ash flow eruption by 0 5 m y and may have been driven by a ring fracture intrusion

the anatomy of a carboniferous epithermal ore shoot at pajingo queensland setting zoning alteration and fluid conditions the paleozoic scott lode gold silver epithermal deposit situated at pajingo northeast queensland is hosted by feldspathic volcanolithic sandstones quartzose sandstones ignimbrites andesitic voltanits with interbedded block and ash deposits lapilli tuffs and andesitic to dioritic intrusive rocks the scott lode deposit contains 1 23 million metric tons of 9 9 g t gold and 38 9 g t silver there are six principal types of alteration assemblages which are zoned with respect to the main vein structure propylitic potassic adularia intermediate argillic silicic kaolinitic and ferroan carbonate overprinting textural relationships indicate that the propylitic type is earliest progressively followed by potassic adularia then intermediate argillic and finally silicic alteration auriferous veins are largely coeval with the silicic and intermediate argillic alteration types kaolinitc and ferroan carbonate occur as late stage alteration products postdating the precious metal mineralization principal ore and sulfide minerals include pyrite sphalerite galena chalcopyrite hessitc tennantite tetrahedrite argentitc petzite electrum and native gold gangue minerals in the veins are quartz illitc adularia calcite and locally kaolinitc the main stage quartz is zoned with respect to mineralogy mineralization styles metal distribution patterns and vein textres three major zones can be recognized zone i an upper precious metal moss chalcedonic crustiform colloform quartz zone zone ii intermediate precious base metal crystalline quartz coarse grained sulfide dominated crustiform colloform quartz zone and zone iiideep sparse base metal weakly banded crystalline comb quartz zone the ore interval occurs in zones i and ii with the transition to zone iii effectively defining the limit of mineralization economic mineralization at the scott lode deposit was laid down from dilute fluids 0 7 2 5 wt nac1 equiv with a very low concentration of co2 homogenization temperatures of 170 o to 315oc exhibit vertical variation with higher temperatures at greater depth the presence of coexisting liquid and gas rich inclusions in mineralized quartz together with adularia vein brecciation and pronounced crustiform colloform banding indicates hydrothermal boiling as the principal precipitation mechanism for gold and silver deposition thermodynamic modeling of adiabatic boiling of a fluid with an initial concentration of 2 to 5 ppb gold at 300oc indicates that such a fluid can deposit ores containing 10 ppm gold and 260 ppm silver at the temperature intervals recorded by the fluid inclusion work

grenvillian age decompression of eclogites in the glenelg attadale inlier nw scotland the glenelg attadale inlier is the largest basement inlier within the caledonian moine nappe of nw scotland in the eastern part of the inlier amphibolite facies retrogression of the eclogites is associated with tectonic fabrics and p t estimates indicate significant decompression c 20 km previous sm nd mineral whole rock dates indicated that peak eclogite facies metamorphism occurred around c 1 08 ga which was correlated with the grenvillian orogeny however the middle ree enrichment of the analysed garnets suggests the influence of apatite inclusions it is therefore likely that the interpretation of the c 1 08 ga age is complex possibly reflecting re equilibration at lower temperatures sampled eclogites contain zircon in a number of distinct textural forms that are mainly associated with pargasite and plagioclase and are part of the retrograde amphibolite facies assemblages titanite extensively replaces rutile and is clearly associated with the retrograde amphibolite facies event a second textural type of titanite forms anhedral grains with plagioclase and pargasite which is developed where the retrograde amphibolite facies assemblage overprints the eclogite mineralogy u pb dating has yielded the following ages zircon age of 995 8 ma and variably discordant rutile ages between 416 and 480 ma u pb and pb pb isochrons on titanite and plagioclase quartz separates yielded ages of 971 65 ma and 945 57 ma respectively in agreement with the zircon age analysed zircons and titanites are texturally part of the amphibolite facies assemblage the new zircon age demonstrates that amphibolite facies metamorphism during exhumation occurred at 995 8 ma the titanites could have closed with respect to pb at this time or alternatively at some time between c 1000 and 900 ma these data clearly demonstrate that parts of the scottish basement underwent major thick skinned tectonics during the grenvillian orogeny rutile is part of the eclogite facies paragenesis and yet has young ages these data are best explained by reheating producing near total pb loss related to emplacement of the late to post tectonic ratagain granite complex at c 425 ma at the end of the caledonian orogeny

the la union au cu prospect camaguey district cuba fluid inclusion and stable isotope evidence for ore forming processes the camaguey district cuba is known for its epithermal precious metal deposits in a cretaceous volcanic arc setting recently the la union prospect was discovered in the southern part of the district containing gold and minor copper mineralization interpreted as porphyry type mineralization is hosted in a 73 0 1 5 ma calc alkaline i type oxidized porphyry quartz diorite intrusive within volcanic and volcaniclastic rocks of the early cretaceous guaimaro formation the porphyry is affected by propy litic alteration and crosscut by a network of quartz and carbonate veinlets and veins chlorite epidote sericite quartz and pyrite are the main minerals in the early veins which are cut by late carbonate and zeolite veins late barite pseudomorphously replaces pyrite gold is associated with pyrite as disseminations in the altered quartz diorite and in the veins occurring as inclusions or filling fractures in pyrite with 4 g t au in bulk samples and up to 900 ppm au in in pyrite fluid inclusion and oxygen isotope data are consistent with a h2o nacl kcl mineralizing fluid derived from the quartz diorite magma and trapped at least at 425 c and 1 2 kbar this primary fluid unmixed into two fluid phases a hypersaline aqueous fluid and a low salinity vapor rich fluid boiling during cooling may have played an important role in metal precipitation pyrite 34 s values for the la union prospect range between 0 71 and 1 31 consistent with a homogeneous magmatic sulfur source the fluids in equilibrium with the mineralized rocks have estimated 18 o values from 8 to 11 8 calculated for a temperature range of 480 505 c the tectonic environment of the la union prospect its high gold and low copper contents the physical chemical characteristics of the mineralizing fluids and the isotopic signature of the alteration minerals and fluids indicate that the la union gold mineralization is similar to the porphyry gold type even though the ore related epidote chlorite alteration can be classified as propylitic and not the classic potassic and or phyllic alteration the low copper contents in the prospect could be due to a mineralizing fluid previously saturated in copper which is indicated by trapped chalcopyrite crystals in high temperature fluid inclusions the low temperature paragenesis represented by carbonate zeolite and barite indicates epithermal overprint the study shows the potential for other gold porphyry type deposits in the cretaceous volcanoplutonic arc of cuba

hydrothermal alteration and hydrologic evolution of the golden cross epithermal au ag deposit new zealand golden cross located in the coromandel peninsula new zealand is a classic example of a volcanic rock hosted low sulfidation epithermal gold silver deposit gold and silver ore is confined to the empire vein system and shallow level stockwork the veins are hosted by miocene to early pliocene andesites and dacites of the waipupu formation and the waiharakeke dacite that are unconformably overlain by the postore whakamoehau andesite hydrothermal alteration minerals display distinct spatial and temporal zonation around veins as defined along three cross sections 1 000 m long 450 m deep located 200 m apart which transect the empire vein system along these sections replacement quartz chlorite and pyrite are ubiquitous with the abundance of quartz veinlets increasing toward major veins replacement adularia envelops the empire vein system and shallow stockwork in each section it is coextensive with and is variably replaced by illite that progressively grades upward and laterally into a zone of interstratified illite smectite that mantles the deposit replacement calcite and minor siderite formed contemporaneously with and also overprint the above minerals whereas late barren calcite veins crosscut mineralized quartz veins kaolinite pyrite veinlets together with rare very local alunite formed during late stage hydrothermal activity clay mineral zonation is well developed illite occurs at depth and close to the veins grading outward and upward into illite smectite with minor smectite occurring 600 m east of the empire vein system this overall zonation reflects paleothermal gradients of 150 c on the periphery to 220 c near the veins consistent with the observed t h range of 150 to 240 c for fluid inclusions in quartz platy calcite and late barren calcite veins final ice melting temperatures for inclusions mostly range from 0 0 to 1 4 c corresponding to apparent salinities of less than 2 4 wt percent nacl equiv ice melting temperatures combined with vapor bubble expansion on crushing indicate the presence of dissolved co 2 in some platy calcite and late stage barren calcite the co 2 content is estimated to range from 0 35 to 3 5 wt percent with the lower limit set by fluid inclusion vapor expansion during crushing and the upper limit by the absence of any observable clathrates depth estimates based on inclusions in platy calcite suggest that the shallow level stockwork zone formed about 100 m below the paleowater table under hydrostatic conditions veins and alteration minerals at golden cross formed in the shallow part 400 m of a hydrothermal system analogous to geothermal systems in the nearby taupo volcanic zone the assemblage of quartz adularia chlorite illite calcite and pyrite reflects the upflow of near neutral ph to weakly alkaline chloride waters contemporaneously steam heated co 2 rich waters formed on the margins and reacted with the wall rock to form the assemblage of calcite siderite smectite illite smectite and kaolinite eventual thermal collapse and invasion of these peripheral co 2 rich waters into the former upflow zone produced late stage barren calcite veins and an overprint of clay carbonate alteration minerals at the same time localized steam heated acid sulfate waters generated above the water table descended to form the late overprint of kaolinite pyrite and alunite

tracking the multi stage exhumation history of the western chinese tianshan by apatite fission track aft dating implication for the preservation of epithermal deposits in the ancient orogenic belt abstract the western chinese tianshan located in the southern domain of the central asian orogenic belt caob was originally established by multiple accretion collision processes in the paleozoic and was modified by complicated intracontinental tectonic evolution in the mesozoic to cenozoic understanding the timing and mechanism of the latter geological processes is critical to unravel the preservation conditions of the epithermal deposits in the western chinese tianshan this work presents new apatite fission track aft data for three mountain ranges keguqinshan wusunshan and nalatishan of the western chinese tianshan to track their exhumation history our aft data gave a wide range of ages from 76 8 5 5 ma to 182 3 9 9 ma and the mean confined fission track lengths are between 9 8 0 5 m and 12 3 0 2 m the new data in combination with the thermal history modeling results allow us to summary three main episodes of exhumation in the western chinese tianshan including the early permian 300 280 ma late triassic late jurassic 230 140 ma and late oligocene early miocene 30 20 ma the first episode may have been caused by terrane accretion collision in the late paleozoic leading to uplift of the keguqinshan that supplied abundant sediments to form a critical cover of ca 4 km thick on these epithermal deposits the second episode was likely related to closure of the mongol okhotsk ocean in the mesozoic and an extremely slow exhumation rate ca 0 0083 mm y as well as a moderate erosion of ca 0 8 km thick are estimated the last episode is regarded to be the result of collision between the indian plate and the eurasia plate in the cenozoic and this significant tectonic event triggered rapid exhumation up to ca 0 1 mm y of the whole region in the western chinese tianshan which removed more than ca 3 2 km pre cenozoic sedimentary cover and sent the epithermal deposits to the subsurface level

platinum group minerals pgm in the falcondo ni laterite deposit loma caribe peridotite dominican republic two ni laterite profiles from the loma caribe peridotite dominican republic have been investigated for their platinum group element pge geochemistry and mineralogy one profile loma peguera i

pueblo viejo high sulfidation epithermal gold silver deposit dominican republic a new model of formation beneath barren limestone cover a discussion sillitoe et al 2006 suggest that the pueblo viejo au ag deposit is younger than its early cretaceous los ranchos formation host rocks this interpretation contrasts with our earlier observations that the mineralization is coeval with the los ranchos formation although a younger age would allow pueblo viejo to be conveniently grouped with many other high sulfidation deposits in a subaerial calc alkaline setting evidence for it conflicts with geologic observations in and around the deposit the new interpretation of the age of mineralization depends largely on the contention that the hatillo limestone which overlies the los ranchos formation was replaced along its basal contact by silica and magnetite that are coeval with pueblo viejo mineralization in their figure 2 p 1429 sillitoe et al show two silicified and iron oxide bearing zones along the poorly exposed base of the hatillo limestone outliers one of these zones is along the southern edge of the pueblo viejo mine where mine workings and exploration drill holes immediately south of the contact provide good exposures in this area the contact which is tectonized locally ranges from a basal conglomerate with silicified cobbles to calcareous sandstone siltstone mudstone with abundant marine fossils silicified cobbles require that silicification preceded deposition of the hatillo limestone similarly carbonaceous sediments containing silicified boulders and cobbles and underlying fragmental rocks with clasts exhibiting different alteration assemblages are widespread in the mine area russell and kesler 1991 fig 3 sillitoe et al 2006 do cite one outcrop containing veinlets of iron oxide limonite with marginal silicification russell and kesler 1991 p 210 211 as evidence of silicification and iron oxides in the basal few meters of the hatillo limestone they acknowledge however that we interpreted these features as paleo supergene which they clearly are furthermore they are

stress field along the ryukyu arc and the okinawa trough inferred from moment tensors of shallow earthquakes abstract we investigated the stress field along the ryukyu arc and the okinawa trough using regional nied mt and global harvard cmt moment tensor catalogs using the combined dataset of 4 years of data from the nied mt catalog and 24 years of data from the harvard cmt catalog we found an arc parallel extensional stress province along the forearc of the ryukyu arc and observed a detailed extensional stress field in the okinawa trough this arc parallel extension is observed in the entire region of the ryukyu arc except at its northeastern end this stress field is clearly separated from the backarc extensional stress field in the okinawa trough by the volcanic chain since it exists in a normal subduction region as well as in a region of weakly coupled subduction the effects of oblique subduction might not be enough to explain the formation of the arc parallel extension thus we consider that the backarc opening process should play an important role in this arc parallel extension along the okinawa trough an arc perpendicular extensional stress field is observed in the southeastern and central ryukyu arc while in the northeastern okinawa trough the direction of the extensional axis is oblique to both the arc normal direction and the direction of plate motion two dimensional mechanical interaction models which consider the interactions between the subducting plate forearc and backarc are inadequate because an arc parallel extensional stress field is observed in the entire ryukyu arc and the extension axis in the northeastern okinawa trough is oblique to the arc trend the oblique backarc stress may originate beneath the backarc or on the continental side of the backarc one of the simplest ways to explain this observation might be active rifting in the northeastern okinawa trough

strike slip fault reactivation as a control on epithermal vein style gold mineralization epithermal precious metal mineralization develops within contemporaneously active tectonic and volcanic terranes in which co active faults focus fluids from deep high temperature reservoirs and magmas into shallower environments recognition of such structural controls through analysis of the architecture of prospective volcanic belts and basins is therefore important in the exploration for epithermal gold deposits field and high resolution aeromagnetic data suggest that gold mineralization at bimurra and wirralie in the late paleozoic drummond basin northeast queensland australia is primarily controlled by reactivation of a northeast striking strike slip fault array that may have developed as a transfer fault in the early history of the basin gold mineralization is hosted by volcaniclastic and sedimentary rocks whose distribution was also controlled by this reactivated structure

structure of the intraplate eastern palmyride fold belt syria the palmyride fold belt is an aulacogen within the arabian plate linked to the levantine continental margin which contains 10 km of phanerozoic sediments inversion of this mesozoic basin began during late cretaceous time although most deformation was clearly post oligocene recent field mapping shows that almost all the structures have resulted from a phase of northwest southeast compression followed by major dextral transpression early northwest southeast compression resulted in northeast southwest aligned fold axes in the central and southeast palmyrides many of which were subsequently rotated by progressive dextral simple shear during neogene to recent time the north south aligned sinistral dead sea fault is the major wrench fault aligned at 45 to the maximum compressive stress the conjugate east west striking dextral jhar fault and the northwest southeast aligned bishri fault have accommodated limited dextral shear and clockwise rotation of northeast southwest aligned folds jebels mrah ash shaer in the northern palmyrides dextral strike slip faults aligned along reidel fractures r1 cut the flanks of earlier domal structures jebel abu rajmein subordinate antithetic r2 reidel shears aligned north northeast south southwest are trans tensional sinistral strike slip faults showing minor extension that cut through the earlier domal structures for example ras al hawa depression in jebel abu rajmein northwest southeast aligned faults for example jebel bilas cross trend faults show northeast southwest extension at right angles to the palmyride shortening direction this extension parallels the major euphrates graben trend in eastern syria and northwestern iraq and also parallels some aligned pliocene pleistocene volcanic rifts south of damascus and in the jebel druze jebel arab region of southern syria and northern jordan a regional palmyride strain ellipse is presented to explain the geometry of all these structures the ad daww depression is a northeast southwest aligned transpressional basin containing around 2 km thickness of tertiary sediments the basin is bounded to the west by a sinistral shear couple along the cherrife structural trend in the central palmyrides along the north by the dextral jhar fault and along the southeast by the compressional southeast palmyrides in the southeast palmyrides structural style is one of a series of northeast southwest aligned narrow anticlines in the mesozoic platform sediments with upright to north west dipping southeast facing axial planes shortening amounts are very small in the eastern palmyrides 1 km and taken up almost entirely by folding but these increase slightly toward the west to probably 20 km in the cherrife region tight box folds and slightly asymmetric folds in the southeast palmyrides have a localized basal detachment along the triassic gypsum horizon which in some examples flows into the fold core jebel hayyan there is no evidence for a major basal detachment underlying the palmyrides all folding and faulting can be related to disharmonic folding of the mesozoic sediments above a triassic evaporite sequence and subsequent dextral transpression the lack of overthrusting explains the absence of any foreland basin caused by lithospheric flexuring on the rutbah platform the major palmyride deformation in miocene pliocene times was concomitant with the opening of the red sea with the closing of the bitlis zagros suture zone in eastern turkey and central iran and with the main sinistral strike slip motion along the dead sea fault

active displacement partitioning and arc parallel extension of the aleutian volcanic arc based on global positioning system geodesy and kinematic analysis global positioning system geodesy and structural analysis of parts of the aleutian island chain support the interpretation of forearc migration westward along right lateral transcurrent faults measured displacements are essentially arc parallel in the eastern unalaska island and western attu island parts of the arc and vary from 3 1 mm yr to 31 4 mm yr respectively at the center of the arc adak subduction of the amlia fracture zone disrupts the pattern of arc parallel displacement and records an upper plate velocity of 9 6 mm yr parallel to plate motion where active displacement partitioning accommodates 30 50 of the strike slip component of plate convergence in the area of impeded subduction partitioning ceases suggesting that the process is sensitive to the degree of coupling across the plate boundary megathrust the differential arc parallel displacements observed at the ends of the arc together with structural relations indicate substantial arc parallel extension which may play an important role in exhumation of high pressure low temperature metamorphic rocks in ancient arc systems

supergene alteration of primary ore assemblages from low sulfidation au ag epithermal deposits at pongkor indonesia and nazareno peru the textures and mineralogical assemblages of high grade ore samples from the low sulfidation au ag epithermal deposits of pongkor 2 ma west java indonesia and nazareno 10 ma cordillera shila southern peru were studied to investigate processes related to post depositional alteration of primary mineral parageneses both deposits are characterized by a late primary high grade base and precious metal stage composed of chalcopyrite sphalerite galena pyrite polybasite pearceite tennantite tetrahedrite electrum minerals of the polybasite pearceite series the main ag carriers were found to be transformed into acanthite acanthite was also found locally in association with native silver as a replacement product of base metal sulfides as veinlets alternating rhythmically with supergene iron oxyhydroxides replacing pyrite as well as associated with barite iron oxyhydroxides and covellite as boxwork fillings primary electrum grains with a composition of about au0 6ag0 4 are partially replaced by au ag sulfide locally identified as uytenbogaardtite ag3aus2 and or by acanthite with both of the replacing minerals commonly associated with vermicular electrum of a greater fineness up to au0 8ag0 2 the occurrence of the replacement textures near the present day surface and the associated secondary minerals strongly suggests a supergene origin for the au and ag sulfide phases the comparison of pongkor and nazareno also suggests that supergene remobilization of au and ag is geochemically significant and can occur in widely differing morphological and climatic contexts

reconnaissance k ar geochronology of the maricunga gold silver belt northern chile the maricunga metallogenic belt in northern chile is defined by the presence of both intrusion related porphyry type and volcanic hosted high sulfidation acid sulfate epithermal deposits containing gold and or silver the nine principal mineralized centers in the belt including the la coipa la pepa marte and lobo deposits were dated radiometrically using the k ar method hypogene alunite from advanced argillic assemblages was dated from eight of the deposits the 18 new radiometric ages permit subdivision of the maricunga belt into two overlapping longitudinal subbelts the western of early miocene including latest oligocene age 25 20 ma and the eastern of middle miocene age 14 12 5 ma this spatial and temporal subdivision of the alteration and mineralization accords well with that defined previously for volcanic rocks in the region and is substantiated further by regional structural and volcanic relationships the migration of the magmatic and metallogenic activity from the western to the eastern subbelt may be attributed to shallowing of the subduction zone which was initiated at 18 to 16 ma the western and eastern maricunga subbelts are broadly correlative with two discrete volcanic alteration events in the el indio precious metal belt 300 km to the south the maricunga subbelts are also integral parts of an eastward younging sequence of gold bearing magmatic metallogenic subbelts of paleocene to late miocene age which span an east west distance of 380 km in this transect 26 degrees 28 degrees s of the central andes

global geologic maps are tectonic speedometers rates of rock cycling from area age frequencies relations among ages and present areas of exposure of volcanic sedimentary plutonic and metamorphic rock units lithosomes record a complex interplay between depths and rates of formation rates of subsequent tectonic subsidence and burial and or rates of uplift and erosion thus they potentially serve as efficient deep time geologic speedometers providing quantitative insight into rates of material transfer among the principal rock reservoirs processes central to the rock cycle areal extents of lithosomes exposed on all continents from two map sources geological survey of canada gsc and the food and agricultural organization fao of the united nations educational scientific and cultural organization unesco indicate that volcanic sedimentary plutonic and metamorphic rocks occupy 8 73 7 and 12 of global exposures respectively plots of area versus age of all mapped rock types display a power law relation where 6 5 of continental area is resurfaced with younger 10 volcanic 90 sedimentary units every million years and where areas of rock exposure decrease by 0 86 for each 1 increase in outcrop age r 2 0 90 area age relations for volcanic and sedimentary lithosomes are similar to the power law distribution defined by all rock units because 81 of mapped area consists of these two lithologies and reflect progressive decrease in amount of exposure with increasing age over the long term continental surfaces are blanketed by new volcanic rocks and sediments at rates of 1 5 and 12 1 10 6 km 2 ma respectively in contrast to power law distributed volcanic and sedimentary rocks that form at the earth9s surface age frequency distributions for plutonic and metamorphic rocks exhibit lognormal relations with modes at ca 154 and 697 ma respectively a dearth of younger exposures of plutonic and metamorphic rocks reflects the fact that these rock types form at depth and some duration of tectonism is therefore required for their exposure increasing modal ages from quaternary for volcanic and sedimentary successions to early mesozoic for intrusive rocks to neoproterozoic for metamorphic rocks demonstrate that greater amounts of geologic time are required for uplift to bring more deeply formed rocks to the earth9s surface the two different age frequency distributions observed for these major rock types a general power law age distribution for volcanic and sedimentary rocks and a lognormal distribution for plutonic and metamorphic rock ages reflect the interplay between depths of formation and mean rates of vertical tectonic displacement age frequency distributions for each of the major rock types are closely replicated by a model that presumes that individual crustal elements behave as a large population of random walks in geologic time and crustal depth and where the processes of surficial erosion associated with tectonic uplift serve to impose an absorbing boundary on this random walk space comparisons between model predicted age frequencies and those apparent in global map data suggest that mean rates of crustal subsidence and uplift are approximately equal in magnitude with mean rates of vertical tectonic diffusion of lithosomes from crustal depths of formation of about half a kilometer per million years rates of uplift and subsidence are strongly dependent on durations of tectonic dispersion lithosome ages however mean rates on the order of hundreds of meters per million years suggested by map age frequencies are the same as would be anticipated on the basis of hundreds of published rates of erosional uplift and exhumation determined by more conventional geochronometers this agreement suggests that geologic maps serve as effective deep time speedometers for the geologic rock cycle

mineralogy 40ar 39ar dating and apatite fission track dating of rocks along the castle mountain fault alaska abstract the castle mountain fault is a 200 km long right lateral fault that forms the northern boundary of the cook inlet basin and matanuska valley alaska fault gouge and fault rock at six localities contain the clay minerals illite smectite chlorite and interstratified illite smectite at one locality gouge contains deformed illite smectite with very little wall rock chlorite contamination fine 36 24 0 08 ma for fine to coarse sizes respectively argon retention ages obtained from 40ar and 39ar retained in the three size fractions of illite at room temperature during neutron irradiation are 37 36 0 15 42 11 0 14 and 47 20 0 10 respectively apatite fission track ages were measured in arkose at a locality on the fault 60 km west of the gouge locality three samples of arkose were dated one within 10 m of the fault core one 170 m from the fault and one 335 m from the fault the sample nearest to the fault yielded an age of 29 3 2 8 ma but it only had four track lengths at 10 13 m two apatite grains from the intermediate sample yielded a pooled age of 34 3 6 1 ma the distant sample 25 grains counted 101 track lengths yielded an age of 32 0 2 9 ma this sample has a broad distribution of track lengths and a broad distribution of individual grain ages ranging from 14 8 5 1 to 67 8 8 8 ma monte carlo modeling of the apatite age and track length data is consistent with hydrothermal mineralization at 37 39 ma followed by rapid uplift and cooling after 10 ma the 40ar 39ar total gas ages k ar are minimum ages and the argon retention ages are maximum ages the thermal model derived from the fission track data and the argon retention age for the finest illite fraction of 37 ma date a hydrothermal mineralization event on the fault

structure of the accretionary prism and the evolution of the paleogene northern caribbean subduction zone in the region of camaguey cuba the deformation history of sedimentary units incorporated in the north cuban fold and thrust belt in the paleocene to middle late eocene was associated with major shortening between the caribbean and north american plates this led to the formation of an intensely deformed tectonic pile comprising from top to bottom of a volcanic arc nappe a deformed mafic ultramafic complex with mesozoic ophiolite components and a serpentinitic melange with blocks of sedimentary the placetas belt and meta morphic rocks and the structurally lower unit composed of folded and thrusted sediments of the southern promontory of the bahamas platform in this paper we study the deformation history of sedimentary units incorporated in the north cuban fold and thrust belt associated with this shortening history we find that the occurrences of the placetas sedimentary rocks within the foliated serpentinite melange show varying styles and intensity of deformation and varying number of deformation phases they form isolated blocks within the serpentinite melange and do not represent a coherent nappe underlying the allochtonous mafic ultramafic complex the deformation of the remedios belt part of the bahamas platform underwent a single phase of folding and thrusting with shortening perpendicular to the plate contact this folding occurred in the middle to late eocene and marks the arrest of subduction and arc continent collision we find no evidence for a component of strike slip during collision the volcanic arc is thrusted upon the mafic ultramafic complex and the original forearc ophiolite appears to be shortened this shortening may attest to a period of subduction erosion thrusting of the volcanic arc led to deposition of the paleocene lower eocene taguasco olistostrome which may date this event we show that careful analysis of the complexly deformed cuban fold and thrust belt may allow identification of subduction erosion and subduction accretion episodes expanding the analysis carried out in this paper to the scale of the northern caribbean fold and thrust belt may provide a new and independent geological tool to constrain the geodynamic processes associated with subduction and arc continent collision along the northern caribbean margin

brittle frictional mountain building 1 deformation and mechanical energy budget an active fold and thrust belt is analogous to a wedge of soil or snow that forms in front of a moving bulldozer such wedges exhibit a critical taper and a regional state of stress that is everywhere on the verge of coulomb failure the width of such a critically tapered fold and thrust belt does not depend on its brittle strength or frictional properties but rather on the accretionary influx rate of fresh material at its toe and on the rate of erosion a steady state fold and thrust belt is one in which the accretionary influx is balanced by the erosive efflux rocks are accreted at the toe and then horizontally shortened as they are transported toward the rear those that enter lower in the accreted section are more deeply buried before being uplifted by erosion mass balance and isotropy constrain the kinematics of this large scale deformation enabling us to infer the trajectories residence times and stress strain histories of rocks incorporated into eroding fold and thrust belts a typical rock resides in the steady state taiwan wedge for 2 3 m y before it is uplifted and eroded during its motion through the wedge it experiences strain rates in the range 10 13 to 10 14 s 1 the mechanical energy budget of brittle frictional mountain building is described by the equation where is the rate of work performed on the base and front of the fold and thrust belt by the subducting plate is the rate at which energy is dissipated against friction on the decollement fault is the rate at which energy is dissipated by internal frictional processes within the deforming brittle wedge and is the rate of work performed against gravitational body forces in a reference frame attached to the overriding plate the total mechanical power being supplied by the subducting eurasian plate to the active fold and thrust belt in taiwan is slightly over 3 gw approximately 60 of this work of steady state mountain building is being dissipated against friction on the decollement fault and about another 25 is being dissipated against internal friction this leaves only 15 or roughly half a gigawatt available to do useful work against gravity in general fold and thrust belts with moderate pore fluid pressures are dominated by work done against friction on the decollement fault however those with nearly lithostatic pore fluid pressures may be dominated by work done against gravity internal frictional dissipation is always less than basal frictional dissipation as it is in taiwan an alternative and equivalent description of the mechanical energy balance of a steady state fold and thrust belt is provided by the equation in this version the quantity on the left is the rate at which work is performed on the back of the wedge by the overriding plate and is the rate of work performed against gravity in a reference frame attached to the subducting plate the latter quantity is always positive for any critically tapered fold and thrust belt whose decollement fault dips toward its rear in contradiction to the central premise of the gravity spreading theory of fold and thrust tectonics

stratigraphic and petrochemical data support subduction polarity reversal of the cretaceous caribbean island arc stratigraphic and petrochemical data from island arc rocks in the ne caribbean support previously proposed tectonic models of attempted subduction of the caribbean oceanic plateau beneath the caribbean island arc during pre aptian time 119 ma and consequent reversal of arc polarity from sw facing to ne facing by the early late cretaceous 97 ma we propose that reversal in subduction polarity is marked by an abrupt change from relatively depleted and non radiogenic tholeiites of the primitive island arc series to more enriched and radiogenic volcanic rocks of the calc alkaline series the stratigraphic contact between the primitive island arc series and the overlying calc alkaline series in central and eastern hispaniola dominican republic and puerto rico is a major unconformity overlain by a shallow marine limestone of aptian albian age we compare the characteristics of the proposed pre aptian caribbean subduction reversal event to the better documented neogene subduction reversal event in the so

magmatic arc fault systems deformation partitioning and emplacement of granitic complexes in the coastal cordillera north chilean andes 25 30 s to 27 00 s three fault systems were responsible for permian to late cretaceous deformation of the overriding plate of the andean convergent margin in the coastal cordillera of northern chile 25 30 to 27 00 s displacements were linked to crustal growth expressed by the emplacement of a sequence of magmatic arcs the tigrillo fault system active from triassic to early cretaceous time was characterized by arc normal extension with increasing left oblique extension transtension from early jurassic to early cretaceous time stretching of the crust created space for triassic early jurassic and early cretaceous arc basins where epiclastic volcaniclastic and volcanic sequences accumulated in continental to shallow marine environments tabular plutonic complexes were emplaced by roof uplift floor subsidence that allowed a vertical transfer of material in the crust without significant horizontal extension the atacama fault system was initiated at c 132 ma as a mainly left strike slip fault during left oblique extension of the margin elongate tabular plutonic complexes were emplaced within the atacama fault system between c 132 and c 106 ma again by roof uplift floor subsidence mechanisms ductile brittle transitions in synplutonic mylonitic rocks of the atacama fault system provided the setting for kiruna type fe apatite and fe oxide with cu and or au ores the chivato fault system was active as an extensional fault system at the eastern side of the coastal cordillera during displacement on the tigrillo fault system and later between c 125 and c 93 ma as a partitioned left oblique extensional fault system in post early cretaceous time the chivato fault system was inverted by left oblique contraction transpression when nw trending transfer faults some probably reactivated lateral ramps in the tigrillo fault system accommodated clockwise vertical axis rotations of 35 45 contraction inverted the atacama fault system and tigrillo fault system and was responsible for west vergent thin skinned fold thrust deformation in stratified rocks throughout the margin

synchronous deformation and hydrothermal activity in the shear zone hosted high sulphidation au cu deposit at peak hill nsw australia altered and mineralised rocks at peak hill are confined to a 300 500 m wide north south striking steeply dipping shear zone that is flanked by the mingelo volcanics along its western side and cotton formation siltstones along its eastern side this shear zone is defined by extensive zones of cataclasite and strongly foliated micaceous schists in marked contrast to the largely undeformed nature of the adjacent rocks advanced argillic assemblages quartz kaolinite pyrite alunite illite occur throughout the core of the peak hill deposit propylitic assemblages including albite quartz interlayered chlorite smectite illite and ankerite and a narrow discontinuous zone of argillic quartz illite pyrite alteration are developed in the mingelo volcanics along the western side of the deposit propylitic argillic and advanced argillic assemblages are overprinted by an internally zoned phase of phyllosilicate alteration that grades inwards from a peripheral sericite clay chlorite assemblage through phyllic assemblages muscovite illite pyrite paragonite to a pyrophyllite pyrite diaspore andalusite altered core au cu mineralisation is hosted by barite pyrite veins that cut the advanced argillic assemblage but pre date the phyllosilicate dominated alteration native au lacking ag calaverite te rich tennantite tetrahedrite goldfieldite chalcopyrite covellite and chalcocite occur in the barite pyrite veins no ore bearing minerals were detected in any of the alteration assemblages the total gold content of the peak hill deposit is currently 720 k ounces and this includes 100 k ounces of unmined reserves within the shear zone phyllosilicate minerals are developed in strain shadows and partly define the stretching lineation associated with dip slip movement the zonation within the phyllosilicate assemblages mimics the geometry of bends in the shear zone and minor internal structures these textures indicate that the phyllosilicate alteration developed synchronous with movement on the shear zone earlier advanced argillic alteration and mineralisation are developed in rocks derived from both sides of the shear zone hydrothermal activity associated with the earlier advanced argillic alteration was therefore either synchronous with juxtaposition of these distinct rock units or occurred during a later phase of movement on the shear zone cross cutting fibrous textures in the auriferous barite pyrite veins indicate that repeated fracturing of the advanced argillic altered rocks accompanied development of successive generations of auriferous veins concentrations of auriferous veins are localised in steeply plunging shoots that are oriented parallel to the stretching lineation in the shear zone these features all indicate movement on the host shear zone accompanied each phase of hydrothermal activity in the peak hill deposit the location alteration zonation and distribution of mineralised veins within the deposit are intimately controlled by deformation on the host shear zone synchronous with hydrothermal activity the development of high sulphidation hydrothermal systems synchronous with deformation along brittle ductile shear zones is a predictable consequence of intrusive activity during deformation in areas characterised by a high geothermal gradient the close relationship between tectonism and hydrothermal activity indicates that these deposits are likely to be located in the vicinity of regional scale shear zones deposits are likely to be aligned parallel to the regional scale structural grain and restricted to areas of conspicuous deformation as is the case at peak hill and temora nsw aluminous alteration zones concentrated in the vicinity of regional scale structures in the carolina slate belt may be a further example of this style of hydrothermal activity

jurassic epithermal au ag deposits of patagonia argentina abstract important precious metal deposits have been discovered during the last 10 years in the deseado massif region of patagonia argentina this region is a plateau consisting of middle to upper jurassic volcanic rocks that host fracture controlled epithermal au ag mineralization these mineral deposits represent low sulfidation type hydrothermal systems and formed following the main period of volcanism probably during the late jurassic period the presence of silica sinters and veins interpreted to be feeder structures indicates that the tops of some deposits have been preserved with erosion of younger sedimentary and volcanic rocks now exposing these systems these deposits represent diverse erosional levels ranging from sinter formed at the paleosurface to intermediate au ag rich quartz veins to base metal bearing au ag veins that represent deeper levels of the epithermal systems the best known deposits are in the cerro vanguardia district where vein systems occur over a 350 km 2 area and veins can be traced for more than 14 km geological mapping of the district has resulted in the definition of over 140 km of veins current reserves are over 3 2 million ounces au equivalent with grades of 10 g t au and 120 g t ag the geologic characteristics of the region and the presence of major au ag deposits provide targets for continuing exploration in the deseado massif

lead isotope signatures of epithermal and porphyry type ore deposits from the romanian carpathian mountains lead isotope analyses have been performed on the two major miocene mining districts of romania baia mare and apuseni mountains these two districts have different non overlapping 206pb 204pb isotopic signatures ranging from 18 752 to 18 876 and 18 497 to 18 740 in the baia mare district epithermal deposits are overall homogeneous in their lead isotopic compositions and have values similar to the average of the calc alkaline volcanic rocks these results suggest a magmatic signature for the pb and possibly other metals in the hydrothermal fluids however magmas in this district show isotopic evidence of crustal assimilation in the southern apuseni mountains the lead isotope compositions of sulfide minerals in porphyry copper deposits are clustered confirming that pb and probably other metals were derived principally from associated porphyry stocks on the other hand lead isotope data on sulfides in epithermal ore deposits are much more scattered indicating a notable contribution of pb from local country rocks in the apuseni mountains fertile volcanics are few and appear to come from a more primitive mantle derived source most of the analysed volcanic rocks seem barren differences in lead isotopic compositions between the baia mare district and the apuseni mountains are due to a different basement and probably to variations in crustal assimilation superimposed on variations in the mantle source composition in the apuseni mountains pb may be partly inherited from the previous mesozoic magmatic hydrothermal stage from a geodynamic point of view it seems that the nature and the source of volcanic rocks and their position related to the collision area of the carpathian arc are not the only factors controlling the fertility of a volcanic district

pongkor west java indonesia a pliocene supergene enriched epithermal au ag mn deposit the pongkor gold silver epithermal deposit with reserves of at least 98 tonnes of gold and 1026 tonnes of silver average grades 16 4 g t au and 171 2 g t ag is one of the most recent and largest gold and silver discoveries in indonesia proven within a short period 1988 1991 40ar 39ar dating on adularia samples give an age of 2 05 0 05 ma the deposit is of the low sulfidation epithermal type and consists of four main mineralized quartz veins located close to the internal rim of a volcano tectonic depression caldera this resulted from an explosive ignimbritic eruption that produced pyroclastic flows and accretionary lapilli with rare intercalations of epiclastic rocks this volcanic unit unconformably overlies miocene subaqueous volcanic andesitic rocks with interbedded epiclastic rocks the mineralized bodies are thick average 4 2 m steeply dipping quartz carbonate adularia veins with a very low sulfide content 0 5 wt their genesis is related to an extensional episode within a tectonic corridor showing nw se and nne ssw conjugate strike slip faults the major vein being located on the inner rim of the caldera the vein fill reveals four successive stages of deposition marked by a specific facies 1 carbonate quartz breccia with dominant quartz and calcite and minor kutnahorite rhodochrosite and rhodonite cq facies 2 a network of banded quartz and former carbonate transformed into manganese oxides through supergene alteration moq facies 3 banded opaline milky quartz boq facies and 4 grey locally banded sulfide rich quartz breccia cutting all the other types gsq facies adularia was deposited at the same time as the quartz the mineralogy and internal structures of the veins crustiform banding vugs collapse breccia clearly indicate a dilational context which is common in low sulfidation epithermal systems gold and silver grades as well as sulfide mineral abundances increase steadily through stages 1 to 4 locally reaching 1 kg t in the gsq facies the sulfides are dominated by pyrite accompanied by common acanthite aguilarite polybasite pearceite and electrum in which the gold content ranges from 48 to 74 wt sphalerite galena chalcopyrite and hessite are fairly rare although present within the cq facies the fluid inclusions of the four facies show homogenization temperatures ranging from 150 to 382 c indicating boiling of a hydrothermal fluid with an initial temperature of around 205 c no marked difference is seen in the gsq facies which has the highest gold content salinities are low generally below 1 wt eq nacl lead isotope compositions of the associated volcanic rocks and the mineralization are very similar 206pb 204pb between 18 706 and 18 814 and between 18 744 and 18 801 respectively demonstrating a genetic link between the pliocene volcanism and the auriferous hydrothermal activity the isotopic signature suggests that the source of the mineralization and associated volcanic rocks is an underlying ancient continental crust that melted and remobilized during the pliocene volcanic and hydrothermal events these conclusions seem applicable to the entire bayah dome the existence of both a tectonic corridor and a caldera favoured channelling of the hydrothermal fluids and the deposition of primary ore in the veins late intense weathering of the ore deposit to depths of 250 m below the surface has given rise to manganese oxide layers limonite zones and silver micronuggets within the veins as well as to gold enrichment

structural evolution of a world class epithermal orebody the martha hill deposit waihi new zealand we describe the complex structure in the highest now mined out levels of the world class martha hill gold silver deposit coromandel peninsula which is hosted in late miocene andesitic rocks a northeast striking block of rock 360 m long and up to 100 m wide was occupied by a complex vein network dominated by northeast striking gold silver lodes up to 30 m thick gold production was entirely from quartz veins the mineralization is embedded in the following geological history 1 deposition of the host volcanic rocks 2 initial tectonic fracturing 3 intrusion of clastic dikes and sills 4 start of hydrothermal alteration 5 main phase of faulting 6 main phase of veining 7 erosion and deep oxidation 8 11 deposition of further volcanic material including prominent rhyolites and formation of present topography veins occupied fractures but also followed a network of preexisting mostly normal faults with a small to medium amount of displacement faulting involved northwest southeast north south and east west extension implying complex 3 d strain clastic dikes of carbonaceous sandstone intruded during the faulting phase fault rocks were mostly brittle cataclasites or breccias but some clay rich zones displayed ductile structures many veins overprinted faults with a change from shear mode to opening mode deformation stockwork veins formed a mesh composed of two orthogonal sets one with northeast southwest and northwest southeast striking veins including the dominant mineralized lodes and the other with north south and east west striking veins although northwest striking veins were earliest during the main vein phase opening oscillated between all the vein directions with the two main lodes in the open pit area the martha and the welcome capturing most of the strain a variety of vein and breccia textures indicate repeated structurally controlled vein opening events that were associated with changes in the physical and chemical conditions of the mineralizing fluid a special concentration of structural features facilitated focusing of fluid flow to produce this world class deposit which contrasts with the less well endowed vein systems around it the overall tectonic control was dominated by northwest southeast extension and dip slip deformation the 3 d strain in this area was not only due to local interference of differently oriented structural features but also to the superposition of regional tectonic north northwest and northeast trends associated with migration of a subduction zone past the coromandel peninsula this demonstrates that structural control in an epithermal mineral deposit may originate from a number of different tectonic controls at different scales