controls on the siting and style of volcanogenic massive sulphide deposits abstract volcanogenic massive sulphide vms deposits form in subaqueous environments from circulating hydrothermal fluids heated by volcanic activity these deposits form as sulphide mounds stratiform exhalative and or replacive bodies and commonly have stockwork vein mineralization in their immediate footwall these various styles are essentially facies of mineralization each one being the product of a particular set of conditions that control the ore forming processes and the consequent geometry and architecture style of the deposits these controls include the physical and or chemical nature of the host rocks the temperature and composition of the hydrothermal fluids and the redox state of the depositional environment the style of exhalative deposits is controlled by the salinity of the vented fluids and the redox state at the seafloor hydrothermal fluids with salinities less than twice that of seawater that vented into open oxic oceanic environments typically formed small mound and chimney complexes unless they were rapidly covered by sediments or volcanic rocks the massive sulphides were rapidly oxidized and partly dissolved by seawater in contrast stratiform sheet like deposits are typically formed in anoxic bottom waters anoxic marine conditions were periodically of global extent particularly prior to 2 4 ga or of a regional nature local anoxic conditions can also be self induced by the exhalation of saline and reducing hydrothermal fluids that ponded in bathymetric depressions such as second or third order basins to form a brine pool these exhalative systems may have been initiated as chimney vent complexes and subsequently overlain by stratiform sulphides formed under the self induced anoxic conditions deposits formed in anoxic environments can be significantly larger than those in oxic settings and this is attributed to several factors that include longer lived hydrothermal circulation more efficient sulphide precipitation and reduced or inhibited oxidation thereof replacement of volcanic and sedimentary strata by sulphide typically occurs within the feeder zones beneath the exhalative mineralization however successions with abundant porous permeable and or reactive rocks such as glassy and or pumiceous volcaniclastic rocks and in some cases limestone favoured the development of large replacive deposits that may have had little surficial expression on the sea floor vms deposits at spreading centres within oceanic crust formed almost exclusively as mounds most of them have not been preserved likely due to oxidation of the sulphides in the prevailing oxic environment and or destruction of oceanic crust during subsequent subduction intra continental rifts arc rifts and back arc rifts commonly have more complexity in their structure and facies architecture and environments and can host all styles of vms mineralization in these settings early extension favoured the formation of restricted basins with ideal conditions for the onset of hydrothermal activity and development of anoxic bottom waters whereas in mature rifts the conditions were less conducive for the formation of regionally extensive anoxic environments formation of replacive deposits was permissible in all settings with porous or reactive subsea floor strata replacive mineralization is the most likely to be preserved in the geological record due to the sulphides being physically shielded from oxidative weathering and mechanical erosion at the seafloor the various styles of vms mineralization can rarely be distinguished using a single criterion in most cases multiple criteria are required mound style mineralization is distinguished by a mound or lens shaped morphology b presence of chimney fragments c presence of abundant sedimented sulphide breccias d location on a stratigraphic boundary ore horizon and e association with a thin horizon or thicker stratigraphic interval of fine grained clastic rocks e g shale mudstone that accumulated at slow sedimentation rates stratiform exhalative mineralization is distinguished by a sheet like morphology prior to deformation b presence of fine grained clastic host rocks that accumulated at relatively slow rates e g mudstone c presence of local or extensive planar stratification replacive mineralization is characterized by a irregular geometry and distribution of sulphide bodies b gradation from massive sulphides to semi massive sulphides and disseminated mineralization with relict textures of the host rock and c originally pumiceous glassy or reactive host rocks emplaced at high depositional rates mass flow deposits lavas carbonate altered mass flow deposits or limestone one deposit or district may comprise two or more of these main styles of mineralization in many cases the main styles of vms mineralization present in a particular region can be predicted from examination of the facies architecture and depositional environments of the host succession recognition of the style s of mineralization that occur in a particular basin or mineral belt enables exploration models to be improved and should influence the strategy of exploration for vms deposits

constraints on the energy and chemical balances of the modern tag and ancient cyprus seafloor sulfide deposits the size chemical composition energy flux and fluid composition of the tag hydrothermal sulfide deposit at the mid atlantic ridge and the size chemical composition and reaction zone characteristics of the skouriotissa volcanogenic massive sulfide deposit of the troodos ophiolite in cyprus are used to examine the energy requirements and chemical balances associated with the generation of a large volcanogenic massive sulfide deposit we conclude that formation of large sulfide deposits from oceanic hydrothermal systems is a geologically rapid process and occurs on timescales of hundreds of years with episodes of activity as short as a few tens of years separated by thousands of years of inactivity about 2 1019 j of energy supplied at high temperature is required to form a deposit the size of the tag mound metals up to 4 times the mass of any element present in the sulfide deposit are leached out of relatively small reaction zones most likely at the base of the sheeted dikes chemical balance can be struck for all elements except sulfur with a reaction zone 1 2 km3 in volume from which a small proportion of iron and large proportions of copper zinc and manganese are removed a sulfur balance requires that a significant fraction of sulfur be derived from reduction of seawater sulfate as suggested by stable isotope analyses we argue that the principal source of energy that drives hydrothermal circulation is latent heat of crystallization of magma close to the top of the plutonic section furthermore we speculate that activity of the tag hydrothermal system is related to periods of more rapid magma supply from the mantle at magma supply rates similar to those observed in volcanoes in hawaii and iceland

spreading structure of the troodos ophiolite cyprus orientations of dikes within the sheeted dike complex of the troodos ophiolite reveal primary spreading structure produced at a complex ridge transform intersection three structural grabens are defined by listric and planar normal faults and rotated dikes that dip symmetrically toward graben axes faults flatten at depth into a detachment within the upper parts of the plutonic complex large exhalative massive sulfide deposits occur within the pillow sections of two of the grabens and appear to be associated with underlying altered and mineralized normal fault zones that channeled hydrothermal fluids we suggest that the grabens are fossil axial valleys produced by successive eastward jumps of an approximately north trending present coordinates slow spreading ridge crest a simple model for ridge migration indicates eastward jumps of 8 13 km changes in ridge orientation are suggested by changes in trends of dikes and graben axes comparison of the pattern of dikes near the arakapas fault zone with the structure of active ridge transform intersections suggests that the fault is a right offset sinistral transform in contrast to earlier models in which a ridge to the west of the exposed troodos complex was proposed

zinc copper and lead in mid ocean ridge basalts and the source rock control on zn pb in ocean ridge hydrothermal deposits the contents of zn cu and pb in mid ocean ridge basalts morb and the morb source rock control on zn pb in ocean ridge hydrothermal deposits are examined the values of zn cu and pb for submarine mid ocean ridge basalts morb are respectively in ppm average morb 75 75 and 0 7 west valley juan de fuca ridge jfr 87 64 and 0 5 southern jfr 120 and 0 5 and 21 n east pacific rise epr 73 78 and 0 5 values of zn pb range from about 100 240 and cu pb from 100 156 in this study zn is found to correlate positively with tio2 feo mean square of weighted deviates mswd of 1 6 for jfr basalt and inversely with mg number mswd of 3 5 therefore contrary to statements in the literature that zn should be compatible in morb zn is a mildly incompatible element and must be enriched in the glass phase relative to olivine as zn does not fit into the other major phenocryst phase plagioclase in the source of morb zn likely is most enriched in oxides spinel magnetite and titanomagnetite copper generally does not correlate well with other elements in most morb data examined when differentiation is dominated by olivine cu has a tendency to behave incompatibly e g at mg numbers 70 but overall cu shows some tendency towards being a compatible element particularly along the mid atlantic ridge a behavior presumably due to separation of sulfides in which cu but not zn is markedly enriched copper thus may be in dispersed sulfides in the source of morb ocean ridges provide important data on source rock controls for sulfide deposits because in sediment starved ridges much is known about the possible source rocks and mineralization is presently occurring in contrast to zn pb 5 in continental hot cl rich brines zn pb in the hottest sediment starved ridge black smoker hydrothermal fluids at 21 n epr is about 110 similar to local morb 145 but cu pb is closer to 30 possibly due to subsurface deposition of cu at the jfr the best value of zn pb in the hydrothermal fluids is about 175 again similar to local morb 240 but cu is very low in the fluids that are at temperatures less than 300 c the large morb like zn pb in the hottest black smoker fluids suggests a source rock control for the metals that prohibits significant galena in the black smoker deposits of sediment starved ridges in contrast exhalative deposits on sediment swamped ridges have significant galena its presence is suggestive of pb derivation from sediments an origin supported by pb isotope studies of lehuray and colleagues in 1988

temporal association of arc continent collision progressive magma contamination in arc volcanism and formation of gold rich massive sulphide deposits on wetar island banda arc whole rock 87sr 86sr and 18o analyses of volcanic rocks and 3he 4he analyses of sulphides and sulphates from mineralized rocks on wetar indonesia indicate a variable contribution of assimilated crustal material or sediment sourced from the subducted australian craton to the south these new data support the idea of progressive source contamination with precisely dated events showing that wetar island hosts the most extreme examples of crustal assimilation in the region the increased continental contamination occurs during the pliocene zanclian to piacenzian during distinct magmatic events between 5 and 4 ma and at 2 4 ma when 87sr 86sr ratios in unaltered lavas with whole rock 18o values between 5 7 and 9 6 increase from 0 707484 to extreme radiogenic values of 0 711656 the earlier of these magmatic events is important in the generation of the hydrothermal systems responsible for the mineralization recorded on wetar samples from this yield radiogenic 3he 4he ratios between 0 5 and 1 4 r ra similar to the data from volcanic rocks on nearby romang the later magmatic event coincides with the arrival of the australian continental margin at the subduction zone along the banda arc progressive incorporation of continental sourced components into the source region below the wetar island edifice coincides with the formation of gold rich volcanogenic massive sulphide deposits hosted within the contaminated volcanic pile

shallow intrusive directions of sheeted dikes in the troodos ophiolite anisotropy of magnetic susceptibility and structural data sheeted dikes play a central role in the formation of oceanic crust it is commonly assumed that sheeted dikes intrude vertically upward from elongated mid ocean ridge mor magma chambers but there are no direct observational data bearing on this hypothesis this assumption contrasts with the intrusive behavior of subaerial volcanoes where magmas rise into shallow central magma chambers that laterally feed vertically oriented dikes the authors have studied intrusive directions of sheeted dikes in a structural analogue to oceanic crust the troodos ophiolite structural and magnetic fabric data of 65 dikes provide consistent results and suggest a broad distribution of shallow 20 degree to nearly vertical upward magma transport directions these data suggest that horizontal emplacement has to be considered for sheeted dikes at mors implying more centralized mor plumbing systems than previously thought such plumbing systems provide ample opportunity for complex mixing fractionation and contamination of mor lavas in magma chambers and tabular magma storage volumes whether the mor magma supply is linear or centralized also has a fundamental effect on crustal accretion processes and the geometry of hydrothermal convection systems

hydrothermal sulfides from a submarine caldera in the shichito iwojima ridge northwestern pacific abstract a large mineralized block of two pyroxene andesite was dredged from a 913 m deep wall of a submarine caldera of the kaikata seamount about 180 km north of iwojima island the kaikata caldera 26 42 n 141 05 e is a seamount in active volcanic front schichito iwojima ridge to the west of ogasawara bonin trench sulfide minerals observed in the sample are dominantly pyrite with minor amount of sphalerite and cu fe sulfide chalcopyrite they occur in 5 mm thick veinlets together with euhedral quartz and chlorite the high filling temperatures 290 c of fluid inclusions in the quartz crystals and enrichment of gold max 142 ppb in vein materials both indicate a hydrothermal origin of the sulfides this is to our knowledge the first discovery of hydrothermal precious and base metal mineralizations associated with front volcanism in the marine environment

tectonic and petrogenetic implications of major and rare earth element chemistry of troodos glasses cyprus chilled pillow margins dike margins and interpillow hyaloclastites from the extrusive sequences of the troodos ophiolite cyprus reveal two compositional groups of glasses 1 andesites to dacitic andesites containing plagioclase ca rich pyroxene and ca poor pyroxene as phenocrysts occur in a stratigraphically low position and 2 basaltic andesites containing olivine and ca rich pyroxene phenocrysts occur in an upper position in the lava sequence the glasses contain between 52 and 62 wt sio2 and have mg numbers between 65 and 27 the troodos glasses are very similar to basaltic andesite or boninitic glasses from the mariana trench and together these two groups define a trend intermediate between typical calcalkaline and tholeiite evolutions the troodos glasses are inferred by difference to contain 3 4 5 wt h2o which is taken to be of primary origin and to reflect the ocean depth of extrusion and quenching based on normative projections in the basalt phase diagram there are indications that elevated water pressures during formation of the phenocryst assemblages may have shifted the isobaric pseudo quaternary peritectic towards silica quantitative crystal fractionation models fail to explain the observed chemical variation it is suggested that the chemical variation may result from high pressure melts equilibrated to lower pressure 2 and 3 phase cotectic relations some mariana and the troodos glasses have similar ree patterns with relatively low abundances depletion in lree and flattening in the hree region the close similarities between the troodos and the mariana glasses point to an identical tectonic and petrogenetic control on their origin

a sulfur isotopic profile through the troodos ophiolite cyprus primary composition and the effects of seawater hydrothermal alteration the sulfide mineralogy sulfur contents and sulfur isotopic compositions were determined for a section through the troodos ophiolite as represented by drillcore from holes cy1a cy2a and cy4 plus selected outcrops with the goals of understanding the geochemistry of s during hydrothermal alteration of the crust and obtaining a mass balance for sulfur in altered ophiolitic crust primary 34s values of the ophiolite are 0 1 consistent with a morb or bab like sulfur source the volcanic rocks have low sulfur contents mean 40 ppm and generally negative 34s values to 26 1 these are the results of sulfur loss through oxidation by seawater at low temperatures 100 c and isotopic fractionation during partial oxidation of igneous sulfides the sheeted dike complex is uniformly enriched in 34s 34s 5 4 as the result of mixing of reduced cretaceous seawater sulfate 17 with primary crustal sulfide sulfate was reduced through oxidation of ferrous iron in the dikes and conversion of igneous pyrrhotite to secondary pyrite decreasing water rock ratios with depth in the plutonic section led to a general decrease in 34s values downward to igneous values in the lower gabbros seawater effects are present locally to the base of the section however cooling of diffuse upwelling hydrothermal fluids t 250 c produced s and metal enrichments in the upper dikes 1 26 wt s whereas sulfur was lost from the lower dikes and upper plutonic rocks which contain 200 ppm and 10 1640 ppm s respectively through higher temperature 350 c alteration reactions epidosites at the dikes gabbro transition contain 30 ppm s with 34s 5 2 6 5 these rocks lost sulfur and metals during reaction with 34s enriched deep hydrothermal fluids at high temperatures 350 400 c and high water rock ratios in basal hydrothermal upflow zones the ultimate effect of alteration of ophiolitic crust is redistribution of igneous sulfur within the crust and exchange of crustal sulfur for seawater sulfur with little net change in the sulfur content of the crust overall the bulk altered troodos crust is enriched in 34s having 34s 3 6 fluxes of sulfur between seawater and ophiolitic crust are an order of magnitude smaller than modern riverine input to and sedimentary sulfide output from the oceans the amount of seawater sulfur that reacted with troodos ophiolite crust is approximately twice that for a composite section of oceanic crust consistent with the generally greater intensity of alteration of ophiolitic crust compared to modern oceanic crust

pb in the pacmanus sea floor hydrothermal system eastern manus basin numerical modeling of a magmatic versus leached origin mass balance calculations of metal leaching and deposition in the pacmanus submarine hydrothermal vent field in the eastern manus basin in the bismarck sea off the east coast of papua new guinea indicate that simple leaching of the underlying volcanic pile by hydrothermal fluids cannot account for the pb found in associated sulfide deposits and pb enriched altered volcanic rocks with 100 percent leaching transport and precipitation efficiencies 1 69 km3 of volcanic rock would need to be leached to account for pb applying realistic leaching efficiencies of 50 to 60 percent and transport precipitation efficiencies of 10 to 20 percent 14 to 34 km3 of volcanic rock would need to be leached assuming a static reaction zone the estimated available volume of leachable rock is 0 144 km3 much less than required applying a dynamic reaction zone model 98 to 235 volumes of fresh leachable rock 1 volume 0 144 km3 would need to be exposed to hydrothermal fluids this translates into a 588 to 1 410 m migration of the reaction zone a model incorporating such values would not be tenable previously published compositions of melt inclusions in volcanic rocks from pacmanus indicate an aver age of 199 ppm cu 68 ppm zn and 5 2 ppm pb in the melt and 1 3 to 1 7 wt percent of degassed h2o these data suggest that 37 7 million tonnes mt of magmatic fluid containing 7 2 wt percent cu and 2 3 wt percent zn could be released from a 1 km3 body of magma using the same metal transfer efficiency of 80 percent the degassed fluid would have a maximum concentration of 0 17 wt percent pb which translates to 58 000 t of pb from the melt for the highest known fluid discharge rate of 92 000 g s and over a 100 000 year period a concentration of only 12 ppm pb in the original magmatic component could account for all the pb in the pacmanus deposits assuming that 1 percent of the mixed end member fluid is of magmatic origin a wide range of durations of hydrothermal activity and discharge rates assuming a maximum 0 17 wt percent pb concentration in the degassed magmatic fluid can account for the total pb in the deposit

the gold content of volcanogenic massive sulfide deposits volcanogenic massive sulfide deposits contain variable amounts of gold both in terms of average grade and total gold content with some vms deposits hosting world class gold mines with more than 100 t au previous studies have identified gold rich vms as having an average gold grade expressed in g t exceeding the total abundance of base metals expressed in wt however statistically meaningful criteria for the identification of truly anomalous deposits have not been established this paper presents a more extensive analysis of gold grades and tonnages of 513 vms deposits worldwide revealing a number of important features in the distribution of the data a large proportion of deposits are characterized by a relatively low gold grade 2 g t with a gradual decrease in frequency towards maximum gold grades defining a log normal distribution in the analysis presented in this paper the geometric mean and geometric standard deviation appear to be the simplest metric for identifying subclasses of vms deposits based on gold grade especially when comparing deposits within individual belts and districts the geometric mean gold grade of 513 vms deposits worldwide is 0 76 g t the geometric standard deviation is 2 70 g t au in this analysis deposits with more than 3 46 g t au geometric mean plus one geometric standard deviation are considered auriferous the geometric mean gold content is 4 7 t au with a geometric standard deviation of 26 3 t au deposits containing 31 t au or more geometric mean plus one geometric standard deviation are also considered to be anomalous in terms of gold content irrespective of the gold grade deposits with more than 3 46 g t au and 31 t au are considered gold rich vms a large proportion of the total gold hosted in vms worldwide is found in a relatively small number of such deposits the identification of these truly anomalous systems helps shed light on the geological parameters that control unusual enrichment of gold in vms at the district scale the gold rich deposits occupy a stratigraphic position and volcanic setting that commonly differs from other deposits of the district possibly due to a step change in the geodynamic and magmatic evolution of local volcanic complexes the gold rich vms are commonly associated with transitional to calc alkaline intermediate to felsic volcanic rocks which may reflect a particularly fertile geodynamic setting and or timing e g early arc rifting or rifting front at the deposit scale uncommon alteration assemblages e g advanced argillic aluminous strongly siliceous or potassium feldspar alteration and trace element signatures may be recognized e g au ag as sb bi hg te suggesting a direct magmatic input in some systems

crustal structure and evolution of the southern voring basin and voring transform margin ne atlantic abstract the dominantly passive volcanic voring and more margins ne atlantic are separated by the 200 km long voring transform margin vtm the southern voring basin and the vtm have been studied by use of four regional ocean bottom seismograph obs profiles combined by gravity modelling the models demonstrate a complex pattern of magmatism along the transform margin the distribution of magmatism seems to be related to the existence and trend of a lower crustal 8 km s body interpreted as eclogitized rocks present in the southern voring basin early tertiary breakup related magmatic leakage across the continent ocean transition cot appears to be facilitated where this layer is absent these results support earlier workers who have concluded that the jan mayen fracture zone originated from a caledonian zone of weakness we propose that partly eclogitized rocks were uplifted into the lower crust close to this zone during the caledonian orogeny and that this body acted as a barrier to magma emplacement during the late cretaceous early eocene phase of rifting breakup the eclogitized terrain also appears to have caused northeastward channeling of the late cretaceous early tertiary intrusions within the voring basin an up to 10 km thick pre cretaceous sedimentary basin in the southern voring basin may be genetically related to the ns trending late paleozoic and mesozoic rift basins in north east greenland

geochemistry and intrusive directions in sheeted dikes in the troodos ophiolite implications for mid ocean ridge spreading centers 1 sheeted dikes at mid ocean ridge volcanoes represent the link between deep magma production and storage processes and shallow processes such as volcanism and hydrothermal activity as such they are crucial for the interpretation of many observations at mid ocean ridges or other volcanoes with pronounced rift zones including topography hydrothermal systems petrology and geochemistry we carried out a structural magnetic and chemical investigation of a 4 10 km sheeted dike section in the troodos ophiolite cyprus on the basis of major and trace element geochemistry we distinguish dikes that may be correlated with the basal high ti series hts lavas from those of the overlying low ti series lts lavas all dikes studied are nearly parallel to each other with vertical or steeply dipping planes whose strike likely indicates the orientation of the spreading center anisotropy of magnetic susceptibility measurements suggests that the hts and lts dikes intrude in fundamentally different ways hts dikes reflect the intrusive behavior of dikes in the vicinity of a magma supply system and define ridge parallel intrusive sheets that radiate out from the magma chamber lts dikes show a bimodal orthogonal set of intrusive directions one shallow and one near vertical near lateral propagating dikes provide a means for delivery of magma into distant portions of a rift system and near vertical dike propagation directions are probably associated with feeder dikes to down rift surface flows our study suggests that the types of dike intrusive behavior in the troodos ophiolite may also be typical for normal mid ocean ridges or other major shield volcanoes with well developed rift zones

constrains on the origin and history of the eastern mediterranean basin abstract the basin formed along the northern passive margin of gondwanaland as a relict of the mesozoic neotethys in early mesozoic time though direct information from the basin itself is meagre the geology of its passive margins and the regional framework of the tethys evolution constrain the formation and history of the basin the eastern mediterranean and its passive margins are interpreted to have formed as a result of several faulting and continental breakup phases from early permian to middle jurassic times before pangaea s complete disintegration this led to the detachment of the eratosthenes and tauride blocks from the margins of the levant and herodotus basins respectively the latter merged in the northeast with the cyprus basin that extended along arabia s northern margin the main rifting probably occurred in the triassic leading to seafloor spreading in the herodotus and cyprus basins while along the levant margin important rifting also occurred in the jurassic during the initial rifting stages extension perpendicular to the levant margin was important but subsequently the tauride block drifted mainly northward the existence of the passive margins and slopes of the eastern mediterranean is documented at least from the end of the middle jurassic until then the basin was probably narrow 200 300 km but during the main phase of the africa laurasia separation it grew to twice its present width or more this scenario depends critically on the interpretation that the tauride block was originally located next to northeastern africa on the western side of the eratosthenes block plate convergence and consumption of the northern portion of the basin have taken place since mid cretaceous times but the active convergent boundary on its northern side formed only in the neogene by modification of earlier structures some intra plate deformation also took place especially in the neogene but it hardly changed the original configuration of the remaining portion of the basin

petrogenesis of a tholeiite boninite sequence from ayios mamas troodos ophiolite evidence for splitting of a volcanic arc lavas and intrusives from ayios mamas at the western end of the limassol forest complex south of the arakapas fault belt troodos comprise three groups a a basal sequence of pillow lavas intruded by dikes and sills with the phenocryst assemblage plag cpx mt ol b a middle sequence of pillows and massive flows cpx plag ol and c an upper sequence mostly of pillows and breccia ol opx cpx sp the lower group is tholeiitic and the upper two groups boninitic group c whole rock and glass mg numbers exceed 72 and range up to 78 with corresponding olivine compositions of fo92 they are strongly depleted in high field strength elements and on the basis of cr y variation and major element mass balances are interpreted to reflect partial melting of a basalt depleted cpx harzburgite source group a tholeiites in contrast are interpreted to derive from a more fertile lherzolite source magmaphile and compatible trace element variation is best modelled after an open magma system involving periodic replenishment tapping and fractionation although neither tholeiites nor boninites represent steady state compositions the boninites belong to a cao rich sio2 poor variant resembling those from guam but contrasting with those from bonin and cape vogel morb normalized enrichment of low field strength elements k sr ba and rb in all groups indicates subduction derived source contamination it is proposed that the ayios mamas section reflects splitting of a volcanic arc prior to back arc spreading in an environment analogous to that of the present day andaman sea

a chemical model for the concentration of gold in volcanogenic massive sulphide deposits abstract study and review of the gold distribution in selected volcanogenic massive sulphide deposits rosebery and hellyer in tasmania mt chalmers in queensland and millenbach in quebec indicates that gold has two distinct occurrences in these deposits 1 a gold zinc lead silver association which is commonly found in the upper part of zinc rich massive sulphide lenses e g rosebery and hellyer and 2 a gold copper association which is commonly present in the stringers and lower portions of copper rich deposits e g mt chalmers and millenbach in each deposit one of these occurrence dominates almost to the exclusion of the other thermodynamic modelling of the volcanogenic fluid which is based on previous work by large 1977 eldridge et al 1983 and pisutha arnond and ohmoto 1983 indicates that gold in the upper part of the zinc rich massive sulphide lenses was transported as au hs 2 while gold in the stringers and bases of copper rich massive sulphide deposits was transported as aucl 2 the ph and o 2 of the ore forming fluid have an important control on gold deposition and therefore gold grades high gold grades in zinc rich massive sulphide are promoted by near neutral oxidised and low temperature 150 c to 275 c fluids as indicated by the presence of barite carbonate feldspar gangue high gold grades in the copper rich stringers and base of the massive sulphide are promoted by higher temperatures 275 c to 350 c relatively oxidised fluids as indicated by bornite or by barite or hematite gangue and relatively acid fluids as indicated by kaolinite gangue in o 2 t space significant gold transport by au hs 2 occurs only in the pyrite stability field at relatively low temperatures 2 is not restricted by iron mineral equilibria but is greatest in the hematite magnetite and pyrite fields at high temperatures 300 c and high o 2 10 30 under conditions of au hs 2 transport bonanza type gold deposition may be found in areas where the hydrothermal fluids passed from the field of pyrite stability to that of hematite stability oxidation of hydrothermal fluids by seawater mixing and the rapid precipitation of sulphide minerals at the top of the massive sulphide lens may cause the precipitation of gold via the mechanisms of either h 2 s oxidation or sulphide gold coprecipitation other types of gold deposits may be classified into either the gold zinc or gold copper association the gold copper bismuth association is developed in higher temperature systems of moderate to high salinity where aucl 2 is the dominant complex e g porphyry cu au deposits and au rich skarns whereas the gold zinc lead silver association is developed in systems of low salinity and high h 2 s where au hs 2 is the dominant complex e g epithermal au ag and archean greenstone hosted au quartz

mobility of au and related elements during the hydrothermal alteration of the oceanic crust implications for the sources of metals in vms deposits volcanogenic massive sulphide vms deposits are commonly enriched in cu zn and pb and can also be variably enriched in au as sb se and te the behaviour of these elements during hydrothermal alteration of the oceanic crust is not well known ocean drilling program odp hole 1256d penetrates a complete in situ section of the upper oceanic crust providing a unique sample suite to investigate the behaviour of metals during hydrothermal alteration a representative suite of samples was analysed for au as sb se and te using low detection limit methods and a mass balance of metal mobility has been carried out through comparison with a fresh mid oceanic ridge basalt morb glass database the mass balance shows that au as se sb s cu zn and pb are depleted in the sheeted dyke and plutonic complexes by 46 12 27 5 2 5 0 5 27 6 8 4 0 7 9 6 1 6 7 9 0 5 and 44 6 respectively arsenic and sb are enriched in the volcanic section due to seawater derived fluid circulation calculations suggest that large quantities of metal are mobilised from the oceanic crust but only a small proportion is eventually trapped as vms mineralisation the quantity of au mobilised and the ratio of au to base metals are similar to those of mafic vms and ten times enrichment of au would be needed to form a au rich vms the cu rich affinity of mafic vms deposits could be explained by base metal fractionation both in the upper sheeted dykes and during vms deposit formation

mineralogical zonation and metasomatism of the alteration pipes of cyprus sulfide deposits the stockwork mineralized lavas associated with the cupriferous pyrite deposits of cyprus occur in the cores of concentrically zoned alteration pipes these represent the channels of ascending hydrothermal solutions this paper reports a detailed mineralogical and geochemical study of the pitharokhoma alteration pipe and reconnaissance studies of other pipes together with complementary work on the agrokipia b pipe robinson et al 1987 and the mathiati pipe lydon and galley 1986 these studies allow three types of pipe to be distinguished namely p pitharokhoma k kokkinopezoula and m mathiati types all the pipes have similar outer alteration zones the outermost being characterized by smectite rich chlorite smectite mixed layer minerals farther in the layer silicates are increasingly dominated by chlorite and plagioclase is altered to albite giving rise to a chlorite albite quartz sphene assemblage chlorite albite facies the major differences between pipe types are in the intensely mineralized and metasomatized centers in the p type pipes most of the mineralized zone is composed of lavas altered to an mg chlorite illite quartz pyrite anatase assemblage chlorite illite facies whereas in k type pipes the assemblage is mg chlorite rectorite quartz pyrite anatase chlorite rectorite facies in both types chlorite abundance decreases as mineralization increases toward the pipe center at pitharokhoma the pipe center is exposed and consists of lavas altered to illite quartz pyrite anatase the m type pipes have mineralized zones in which the outer parts are altered to chlorite rectorite facies but the central parts are altered to fe rich chlorite quartz pyrite anatase all types of pipe show mg enrichment in the peripheral zones of chlorite smectite and chlorite albite facies alteration and also in the outer parts of the mineralized zones the most intensely leached lavas at pitharokhoma are enriched in k rb and ba and are depleted in mg ca sr and na and al has been significantly mobile in the k type pipes the metasomatism is in many respects similar to the p type but the composition of rectoritc results in there being no enrichment in k rb and ba in m type pipes mg is not depleted in the pipe center whereas ca sr na k rb and ba are all strongly depleted the alteration pipes have all been affected to some degree by postmineralization low temperature alteration effects of this alteration include formation of smectite k feldspar and calcite and possibly alteration of original hydrothermal sericite to illite in p type pipes and illite to rectoritc in k type pipes if all the rectorite in k type pipes has been derived in this way then k type alteration would represent p type alteration overprinted by low temperature alteration and so only p and m type pipes would be the result of differing conditions during sulfide mineralization one of the chief differences was probably in the s fe ratios in the hydrothermal fluids in p type pipes the calculated ratios of s addition to fe addition in the mineralized lavas are greater than the ratio for pyrite addition whereas the reverse is true at mathiati this difference is paralleled in the modern black smoker fluids of which some have an excess of dissolved sulfide over dissolved fe whereas some have an excess of fe over sulfide

constraints on water depth of massive sulfide formation evidence from modern seafloor hydrothermal systems in arc related settings the results of three decades of seafloor research provide the most reliable information on the importance of water depth in massive sulfide formation available data from over 130 occurrences show that water depths of seafloor vent sites vary with plate tectonic setting and the regional magmatic and volcanic environment the shallowest hydrothermal systems in subduction related settings are hosted by arc volcanoes these shallow vent sites have a number of features in common with subaerial epithermal systems massive sulfide occurrences in arc related rifts the most likely setting for many ancient analogs are generally restricted to water depths from 700 to 2 000 m with rifts developing within old arc crust at the deeper end of this range back arc spreading centers proximal to arcs host massive sulfide deposits at depths of 1 500 to 2 000 m the deepest hydrothermal systems occur along mature back arc spreading centers distal to volcanic arcs where water depths range from 2 000 to 3 700 m these deeper vent sites probably represent the best modern analogues of ophiolite hosted massive sulfide deposits boiling of the hydrothermal fluids is common at volcanic arcs and in arc related rifts in these environments elevated magmatic gas contents of the hydrothermal fluids can contribute to the widespread occurrence of phase separation and associated gas loss by contrast the high ambient pressures in deep marine hydrothermal systems along mature back arc spreading centers prevent fluids from boiling during their ascent to the seafloor boiling controls the maximum temperature at which hydrothermal fluids discharge at the seafloor and therefore influences the metal content of seafloor sulfide deposits copper rich massive sulfides typically occur at water depths exceeding 1 000 m whereas zn and pb rich occurrences may form at any water depth boiling can be an important control on ag and au grades but is not the only factor controlling precious metal enrichment in massive sulfides shallow marine hot spring deposits can be highly enriched in trace metals such as as hg and sb submarine volcanic arc and back arc settings are geologically complex and significant variations in water depth can occur over short distances paleoenvironmental reconstruction of these environments in ancient volcanic terranes is hampered by the lack of unequivocal volcanological or sedimentological criteria that indicate water depth the relationships established here using modern seafloor observations provide important constraints on the paleoenvironmental setting of ancient volcanic hosted massive sulfide deposits

distribution of alteration zones in the upper oceanic crust aging of the upper oceanic crust produces alteration zones that have distinct mineralogical and geochemical characteristics five zones are recognized in the troodos ophiolite and in situ oceanic crust 1 a sea floor weathering zone swz 2 a low temperature zone ltz 3 a transition zone tz 4 an upper dike zone udz and 5 a mineralized zone mz the mz may be overlain by the swz ltz or tz but in the known examples it is always underlain by the udz the development of the swz is dependent upon the duration of cold seawater circulation which is dependent upon seafloor topography and the rate and nature of sedimentation the boundary between the low and high temperature zones is largely controlled by permeability contrasts lateral variation in the depth of this boundary indicates that the thermal gradient is not uniform within the crust this along with the evidence for disequilibrium assemblages implies that regional metamorphic zones are not appropriate to describe the distribution of alteration effects

channelling of hydrothermal fluids during the accretion and evolution of the upper oceanic crust sr isotope evidence from odp hole 1256d abstract odp hole 1256d in the eastern equatorial pacific is the first penetration of a complete section of fast spread ocean crust down to the dike gabbro transition and only the second borehole to sample in situ sheeted dikes after dsdp hole 504b here a high spatial resolution record of whole rock and mineral strontium isotopic compositions from site 1256 is combined with core observations and downhole wireline geophysical measurements to determine the extent of basalt hydrothermal fluid reaction and to identify fluid pathways at different levels in the upper ocean crust the volcanic sequence at site 1256 is dominated by sheet and massive lava flows but the sr isotope profile shows only limited exchange with seawater however the upper margins of two anomalously thick 25 m massive flow sequences are strongly hydrothermally altered with elevated sr isotope ratios and appear to be conduits of lateral low temperature off axis fluid flow elsewhere in the lavas high 87 sr 86 sr are restricted to breccia horizons mineralised hyaloclastic breccias in the lava dike transition are strongly altered to mg saponite silica and pyrite indicating alteration by mixed seawater and cooled hydrothermal fluids in the sheeted dike complex 87 sr 86 sr ratios are pervasively shifted towards hydrothermal fluid values 0 705 dike chilled margins display secondary mineral assemblages formed during both axial recharge and discharge and have higher 87 sr 86 sr than dike cores indicating preferential fluid flow along dike margins localised increases in 87 sr 86 sr in the dike gabbro transition indicates the channelling of fluids along the sub horizontal intrusive boundaries of the 25 to 50 m thick gabbroic intrusions with only minor increases in 87 sr 86 sr within the cores of the gabbro bodies when compared to the pillow lava dominated section from hole 504b the sr isotope measurements from site 1256 suggest that the extent of hydrothermal circulation in the upper ocean crust may be strongly dependent on the eruption style sheet and massive flow dominated lava sequences typical of fast spreading ridges may experience relatively restricted circulation but there may be much more widespread circulation through pillow lava dominated sections in addition the hole 1256d sheeted dikes display a much greater extent of sr isotopic exchange compared to dikes from hole 504b because seawater derived hydrothermal fluids must transit the dikes during their evolution to black smoker type fluids the different sr isotope profiles for holes 504b and 1256d suggest there are significant variations in mid ocean ridge hydrothermal systems at fast and intermediate spreading ridges which may impact geochemical cycles of elements mobilised by fluid rock exchange at different temperatures

petrological and chemical evolution of the northeastern troodos extrusive series cyprus four magma series are distinguished in the northeastern troodos extrusive series a a lower low ti series lo lts of basaltic andesites b a high ti series hts of basaltic andesites to rhyodacites c a low ti series lts and d a depleted low ti series dlts the last two being of basaltic andesite trace element characteristics vary systematically from series a to d and are interpreted in terms of a variable contribution of three major source components scs lile enriched water rich fluids sci derived from dehydration of a subducted lithosphere slab were continuously added to the overlying mantle wedge increasing lile hfse and lile ree ratios and decreasing absolute hfse and ree concentrations from series a to d indicate progressive depletion of the actual mantle source scii a negative ta anomaly in the lavas decreases from series a to d and is interpreted to have resulted from partial melting of the lower crust scii where ta nb ti may be fractionated by ti rich accessory phases the contribution of sciii decreases when the eruptive sites successively move away from the central axial zone and the temperature of the lower crust decreases preventing partial melting of the lower crust chemical compositions of fresh glass separates and phenocrysts indicate a change of major petrogenetic processes from series a to d lo lts and hts lavas are intrepreted to be directly related by open system fractional crystallization in crustal magma chambers removal of observed phenocryst phases clinopyroxene orthopyroxene plagioclase and magnetite and repeated subsequent mixing of dacitic to rhyodacitic magmas with batches of replenishing basaltic andesites are the major processes possibly induced by vesicle formation in the mafic layer after a period of some crystallization lts and dlts magmas were directly fed to the surface without stagnating at crustal levels with feeder dykes positioned marginal to the central rift zone and thus by passing the central magma chambers these magmas apparently experienced only limited fractional crystallization of 10 15 wt olivine clinopyroxene chromite probably at the mantle crust boundary

geochemistry of hydrothermal fluids from axial seamount hydrothermal emissions study vent field juan de fuca ridge subseafloor boiling and subsequent fluid rock interaction hydrothermal fluids collected from the ashes vent field in 1986 1987 and 1988 exhibit a very wide range of chemical composition over a small area 60 m in diameter compositions range from a 300 c gas enriched 285 mmol kg co2 low chlorinity 33 of seawater fluid to a 328 c relatively gas depleted 50 mmol kg co2 high chlorinity 116 of seawater fluid the entire range of measured compositions at ashes is best explained by a single hydrothermal fluid undergoing phase separation while rising through the ocean crust followed by partial segregation of the vapor and brine phases other mechanisms proposed to produce chlorinity variations in hydrothermal fluids precipitation dissolution of a chloride bearing mineral or crustal hydration cannot produce the covariation of chlorinity and gas content observed at ashes there is good agreement of the measured fluid compositions with compositions generated by a simple model of phase separation in which gases are partitioned according to henry s law and all salt remains in the liquid phase significant enrichments in silica lithium and boron in the low chlorinity fluids over levels predicted by the model are attributed to fluid rock interaction in the upflow zone depletions in iron and calcium suggest that these elements have been removed by iron sulfide and anhydrite precipitation at some time in the history of the low chlorinity fluids the distribution of low and high chlorinity venting is consistent with mechanisms of phase segregation based on differential buoyancy or relative permeability the relatively shallow depth of the seafloor 1540 m and the observed chemistry of ashes fluids are consistent with phase separation in the sub critical or near critical region

hydrothermal mobilisation of au and other metals in supra subduction oceanic crust insights from the troodos ophiolite the troodos ophiolite is an ideal location to investigate the relationships between the mobilisation of metals by hydrothermal fluids from the lower oceanic crust and the formation of volcanogenic massive sulphide vms deposits the ophiolite hosts the classic cyprus type cu rich vms deposits as well as abundant zones of epidosite alteration in the lower sheeted dyke section that are significantly depleted in base metals including cu and zn and are considered to be the source of the metals enriched in the overlying deposits previous research indicates that the troodos vms deposits are irregularly enriched in au and related elements as sb and se but the behaviour of these elements during the hydrothermal alteration of the troodos ophiolite hitherto has been poorly investigated low detection limit whole rock analyses of fresh glass samples reveal that the troodos primitive crust has a similar metal content and distribution to modern day arc related environments such as the manus basin compared to mid oceanic ridge basalt morb the troodos primitive crust is enriched in as sb and pb most likely due to addition from a subducting slab during crustal formation during early stages of magmatic differentiation 9 3 5 wt mgo au as sb se cu zn and pb behave as incompatible elements due to the sulphide undersaturated nature of the melt the onset of magnetite crystallisation however at 3 5 wt mgo leads to sulphide segregation and depletion of strongly chalcophile elements au cu and se during continued differentiation 3 5 wt mgo whereas poorly chalcophile elements as sb zn and pb remain incompatible these differences in metal behaviour can account for the cu rich zn pb poor of the cyprus type vms deposits as the source area rocks show high cu fertility compared to zn and pb mobilisation of metals during hydrothermal alteration of the troodos ophiolite is more extensive than observed in hydrothermally altered morb mass balance calculations show that the epidosite zones are significantly depleted in au 88 16 as 89 23 sb 60 12 se 91 20 cu 84 18 zn 63 9 and pb 60 8 background altered diabase from outside epidosite zones shows similar metal depletions which suggests that the source areas of vms are not restricted to epidosite zones but are extended to the lower sheeted dyke section the masses of metals mobilised from a source area of 10 9 km3 composed of a 5 km3 epidosite zone and 5 9 km3 of background altered diabase in the solea graben are 47 t au 21 kt as 1200 t sb 3100 t se 2 4 mt cu 1 8 mt zn and 27 kt pb comparison of metal quantities mobilised from lower sheeted dike section in the solea graben with those hosted in vms deposits shows trapping efficiencies ranging from 4 to 37 indicating that most of the metals is lost by other processes

metal depleted root zones of the troodos ore forming hydrothermal systems cyprus the cupriferous pyrite deposits of cyprus were precipitated from hydrothermal solutions derived by interaction of contemporaneous seawater with hot mafic rock at the ancient troodos spreading centre here we identify the zones in which this interaction took place the zones occur in the lower part of the sheeted dyke complex and within them 30 50 of the rock is made up of epidosite an epidote quartz rock replacing the dykes as sheets and pipes the epidosites contain abundant fluid inclusions which give trapping temperatures of 350 400 c or even higher and contain water normally near seawater in salinity zones of epidosite are elongate parallel to the strike of the sheeted dykes and are up to 1 km wide the rocks throughout these zones are strongly depleted in cu and zn and the metals removed are sufficient to supply the ore deposits in fact several large ore deposits lie along strike from zones of epidosite all of these features support the identification of the epidosites as the hydrothermal reaction zones the location of the epidosite zones immediately above the gabbros of the plutonic complex supports the hypothesis that the heat to drive the ore forming systems came from the underlying magma as is also likely for modern black smoker springs

submarine hydrothermal activity and gold rich mineralization at brothers volcano kermadec arc new zealand brothers volcano of the kermadec intraoceanic arc is host to a hydrothermal system unique among seafloor hydrothermal systems known anywhere in the world it has two distinct vent fields known as the nw caldera and cone sites whose geology permeability vent fluid compositions mineralogy and ore forming conditions are in stark contrast to each other the nw caldera site strikes for 600 m in a sw ne direction with chimneys occurring over a 145 m depth interval between 1 690 and 1 545 m at least 100 dead and active sulfide chimney spires occur in this field and are typically 2 3 m in height with some reaching 6 7 m their ages at time of sampling fall broadly into three groups 4 23 and 35 years old the chimneys typically occur near the base of individual fault controlled benches on the caldera wall striking in lines orthogonal to the slopes rarer are massive sulfide crusts 2 3 m thick two main types of chimney predominate cu rich up to 28 5 wt cu and more commonly zn rich up to 43 8 wt zn geochemical results show that mo bi co se sn and au up to 91 ppm are correlated with the cu mineralization whereas cd hg sb ag and as are associated with the dominant zn rich mineralization the cone site comprises the upper cone site atop the summit of the recent main dacite cone and the lower cone site that straddles the summit of an older smaller more degraded dacite cone on the ne flank of the main cone huge volumes of diffuse venting are seen at the lower cone site in contrast to venting at both the upper cone and nw caldera sites individual vents are marked by low relief 0 5 m mounds comprising predominately native sulfur with bacterial mats vent fluids of the nw caldera field are focused hot 300 c acidic ph 2 8 metal rich and gas poor calculated end member fluids from nw caldera vents indicate that phase separation has occurred with cl values ranging from 93 to 137 of seawater values by contrast vent fluids at the cone site are diffuse noticeably cooler 122 c more acidic ph 1 9 metal poor and gas rich higher than seawater values of so4 and mg in the cone vent fluids show that these ions are being added to the hydrothermal fluid and are not being depleted via normal water rock interactions iron oxide crusts 3 years in age cover the main cone summit and appear to have formed from fe rich brines evidence for magmatic contributions to the hydrothermal system at brothers includes high concentrations of dissolved co2 e g 206 mm kg at the cone site high co2 3he negative d and 18oh2o for vent fluids negative 34s for sulfides to 4 6 sulfur to 10 2 and 15n2 to 3 5 vent fluid ph values to 1 9 and mineral assemblages common to high sulfidation systems changing physicochemical conditions at the brothers hydrothermal system and especially the cone site occur over periods of months to hundreds of years as shown by interlayered cu au and zn rich zones in chimneys variable fluid and isotopic compositions similar shifts in 3he 4he values for both cone and nw caldera sites and overprinting of magmatic mineral assemblages by water rock dominated assemblages metals especially cu and possibly au may be entering the hydrothermal system via the dissolution of metal rich glasses they are then transported rapidly up into the system via magmatic volatiles utilizing vertical 2 5 km long narrow 300 m diameter pipes consistent with evidence of vent fluids forming at relatively shallow depths the nw caldera and cone sites are considered to represent stages along a continuum between water rock and magmatic hydrothermal dominated end members

isotopic and trace element composition of volcanic glasses from the akaki canyon cyprus implications for the origin of the troodos ophiolite abstract we report major elements k rb cs sr ba sc cr ni hf ta th and ree concentrations and isotopic compositions of sr nd pb and o of carefully handpicked volcanic glasses from the akaki river section of the troodos ophiolite complex on the basis of sr and o isotopic composition and fe 2 o 3 feo ratios nine of the ten glasses analyzed are considered to be fresh even with respect to those elements that are easily affected by alteration the glasses range in composition from basaltic andesite to dacite incompatible element concentrations in the mafic compositions are low 10 chondritic and the light ree are strongly depleted however the ratios of low field strength elements k rb cs sr th to high field strength elements ta hf ti and rare earth elements are higher than those in n morb nd isotopic compositions show a very small range with e nd t 80ma varying from 6 5 to 7 5 initial 87 sr 86 sr ratios range from 0 7033 to 0 7040 pb isotope ratios have a restricted range with 206 pb 204 pb 18 60 18 70 207 pb 204 pb 15 55 15 59 and 208 pb 204 pb 38 08 38 51 the oxygen isotopic composition for fresh glasses falls within the range reported for fresh mafic mantle derived volcanics with 18 o varying from 5 4 to 6 5 all geochemical characteristics of the troodos volcanic glasses are similar to those observed in supra subduction zone volcanics but we cannot distinguish between an early stage island arc or back arc origin for the troodos ophiolite on a geochemical basis only however considering our data and previously published geologic and geochemical data we suggest that the troodos ophiolite formed in the earliest stages of island arc development

processes of brine generation and circulation in the oceanic crust fluid inclusion evidence from the troodos ophiolite cyprus detailed temporal thermal and compositional data on aqueous fluid inclusions from a suite of plutonic and diabase samples from the troodos ophiolite cyprus provide the first documentation that generation of high temperature brines may be common at depth in the oceanic crust anastomosing arrays of fluid inclusions in rocks of the upper intrusive sequence record episodic fracturing events the earliest fracturing event at temperatures 450 600 c resulted in entrapment of brine rich aqueous fluids with salinities of 36 61 wt nacl equivalent homogenization of the brine inclusions by haute dissolution the virtual absence of vaporrich fluid inclusions throughout the upper level plutonic sequence and the restriction of brine inclusions to the most evolved plutonic rocks suggests that exsolution of brines off of the late stage gabbro and plagiogranite melts played a significant role in generating the quartz hosted high salinity inclusions cooling of the fluids during pulses of fluid migration associated with episodic fracturing events resulted in entrapment of the brines in the deep seated high temperature portion of the hydrothermal system in localized areas the high temperature brines nacl kcl cacl2 caused extreme alteration of the plagiogranite bodies and in the formation of podiform epidosites arrays of low temperature low salinity fluid inclusions which in some samples crosscut fractures dominated by brine inclusions indicate downward propagation of a cracking front subsequent to collapse of the high temperature magmatic system resulting in penetration of seawaterlike fluids into the plutonic sequence at temperatures 200 400 c hydration reactions under greenschist facies conditions or limited mixing with brine rich fluids may have resulted in salinity variations from 70 below to 200 above seawater concentrations temperatures and compositions of the low salinity inclusions are similar to those found in stockwork systems beneath troodos ore deposits and to those of fluids exiting active submarine hydrothermal vents at mid ocean ridge spreading centers the low temperature fracture networks may represent an extensive deep seated feeder system which coalesced to form zones of concentrated hydrothermal upflow

multiple deformations with successive subvertical and subhorizontal axial planes in the mount isa region their impact on geometric development and significance for mineralization and exploration the first orogeny to affect most rocks exposed in the mount isa province caused significant repetition of stratigraphy through north south shortening geological maps are dominated however by younger folds with subvertical north south striking axial planes these folds were produced by east west shortening during a second orogeny both orogenies produced several foliations however some foliations have negligible geometric consequences allowing the geometry of the region to be described generally in terms of just the three deformations d 1 d 2 and d 3 which produced macroscopic folds and the younger deformations which produced brittle faults one of the weak events that appears superficially to have had little or no geometric impact played a highly significant role in the generation of structurally controlled mineralization that role has been identified following advances in conceptual understanding of fold development and associated foliation the deformation which occurred between the d 2 and d 3 events produced geometric effects that were masked by overprinting of north south trending d 2 folds by similarly trending d 3 folds significantly these d 3 folds which control the localization or upgrading of much mineralization in the mount isa province did not form in rocks unaffected by this intermediate event because the near parallelism of s 2 and s 3 in such rocks resulted in intensification of s 2 rather than development of a new s 3 the intermediate deformation event called d sub 2 5 locally produced highly asymmetric deflections in bedding and s 2 with subhorizontal axial planes that were subsequently overprinted by d 3 folds and an s 3 cleavage no mesoscopic foliation was produced during d sub 2 5 and its rotational effects on bedding were commonly subsumed within younger d 3 structures thus d 3 disguises the presence of d sub 2 5 except in relatively rare locations where d 2 folds were isoclinal such as in the tommy creek block and on the northern end of the snake creek anticline southeast of cloncurry and northwest of mount isa in these three locations in the latter locations both limbs were affected resulting in large scale overturning of bedding on one limb prior to overprinting by d 3 the asymmetric folds developed during d sub 2 5 created sites where bedding could be pulled apart during d 3 resulting in implosion because of the high fluid pressure in the surrounding country rock with brecciation jostling fluid access and mineralization large scale zones of mineralization form where d sub 2 5 was localized at an earlier formed heterogeneity such as a thrust surface e g at mount isa mine or a granite country rock contact in these locations early formed angular bedding relationships were locally rotated into orientations which created sites that could implode during d 3 this role of d sub 2 5 in the generation of d 3 allows recognition of regional zones for exploration that previously have received little attention recognition of the role of d sub 2 5 reveals a cyclicity in successive subvertical and subhorizontal events not recognized by most workers in this terrain that may occur in other orogens around the world but has similarly gone unnoticed after the first orogeny finished a succession of three near vertical and two subhorizontal events occurred during the east west shortening orogeny at mount isa these were succeeded by younger kinks with east west to northeast southwest trending axial planes that appear to be the youngest ductile deformation to have affected the mount isa province