post depositional tectonic modification of vms deposits in iberia and its economic significance the original stratigraphic relationships and structure of vms deposits are commonly obscured by deformation this can also affect their economic significance as shown by several iberian pyrite belt ipb sw iberia examples the contrasting rheologic properties of the different lithologies present in an orebody massive sulphide feeder stockwork alteration envelope volcanic and sedimentary rocks play a major role in determining its overall behaviour variscan thin skinned tectonics led to stacking of the massive pyrite and stockwork bodies in duplex structures resulting in local thickening and increased tonnage of minable mineralization furthermore differential mechanical behaviour of the different sulphide minerals localised the detachments along relatively ductile sulphide rich bands the result was a geochemical and mineralogical reorganisation of most deposits which now consist of barren massive pyrite horses bounded by base metal rich ductile shear zones metal redistribution was enhanced by mobilisation of the base metal sulphides from the initially impoverished massive pyrite through pressure solution processes to tensional fissures within the already ductile shear zones in nw iberia vms deposits were also strongly overprinted by the variscan deformation during emplacement of the cabo ortegal and ordenes allochthonous nappe complexes but no stacking of the orebodies was produced original contacts were transposed and the orebodies their feeder zones and the country rock acquired pronounced laminar geometry in lower grade rocks greenschist facies cabo ortegal complex solution transfer mechanisms are common in pyrite which remains in the brittle domain while chalcopyrite shows ductile behaviour in higher grade rocks amphibolite facies ordenes complex metamorphic recrystallisation overprints earlier deformation textures the contrasting behaviour of the ipb and nw iberian deposits is explained by key factors that affect their final geometry composition and economics such as pre deformation structure size and mineralogical composition of the orebody and associated lithologies temperature crustal level deviatoric stress and availability of a fluid phase during deformation and the style and rate of deformation

geochemistry and petrogenesis of intrusive and extrusive ophiolitic plagiogranites central anatolian crystalline complex turkey plagiogranites associated with the sarikaraman ophiolite of the central anatolian crystalline complex turkey closely resemble other plagiogranites from supra subduction zone type ophiolites of neotethys the ophiolite is remarkable in displaying a higher proportion of the plagiogranite suite ca 10 by volume than is usually associated with such bodies the sarikaraman plagiogranites are represented by intrusive sheets and netvein trondhjemites largely developed at the top of the upper gabbros and as multiphase dykes within the sheeted dyke complex the plagiogranite dykes are considered to feed extrusive silicified rhyolites associated with the basaltic lavas in the volcanic section of the ophiolite field relations suggest that the trondhjemites were probably generated from the roof section of a dynamic and evolving gabbroic magma chamber both the deep seated trondhjemites and the volcanic rhyolites constitute the sarikaraman plagiogranite suite geochemically there is complete overlap between the intrusive trondhjemites and extrusive rhyolites which are characterised by morb normalized low hfs element contents with small negative nb ta anomalies and variably enhanced lil element abundances unlike other plagiogranites however the sarikaraman suite is not characterized by consistently low k2o contents a feature that reflects the variable mobilization of the lil elements under lower greenschist facies conditions the ree are uniformly enriched relative to the basic components of the complex but have similar normalized patterns exhibiting mild light ree depletion in terms of their origin the initial or most primitive plagiogranite melts could have been generated by either fractional crystallization 70 85 of clinopyroxene feldspar amphibole or partial melting 5 15 batch melting of a gabbroic source material although only the first process can produce most of the range of the plagiogranite compositions as a group the plagiogranites exhibit some degree of internal variation which can be generated by further fractionation largely dominated by feldspar with minor apatite and amphibole

a permian kuroko type hydrothermal system afterthought ingot area shasta county california lateral and vertical sections and geochemical evolution the afterthought and smaller kuroko deposits in the east shasta mining district formed at the culmination of a late permian mafic to felsic volcanic cycle they were succeeded by triassic elastic sediments and reworked tuff in a presumed rifting environment with local evidence of renewed mafic volcanism and large scale slumping the afterthought host horizon is repeated in outcrop by folding fold closures are tight east of ingot where preore volcanic rocks are more altered they are open basins and domes west of ingot where the preore volcanic rocks are less altered igneous pyroxene and titanomagnetite and secondary pumpellyite occur only in preore andesite and basalt more than 2 5 km from known kuroko deposits in outcrops closer to the deposits disseminated pyrite type e is locally abundant in preore dacite the kuroko deposits comprise three ore types a banded fine grained sulfides and gangue b recrystallized coarser grained sulfides and gangue with a consistent paragenetic sequence and c coarse grained pyrite and barite b and c appear to replace type a a further type of mineralization type d consists of fine grained pyrite occurring locally within silicified dacite on the afterthought host horizon the paragenetic sequence of type b implies a sequential introduction of metals in a fluid evolving toward higher h 2 s activity cu was introduced early as chalcopyrite and bornite later fluids altered preexisting cu minerals rather than introducing significant new cu type b fluids probably cooled from 300 degrees c chalcopyrite stage to about 270 degrees c sphalerite barite stage the last type b fluids dissolved much base metal sulfide and deposited calcite quartz and sericite types a b c and e sulfides have a delta 34 s value range of 5 to 12 per mil implying a common origin barites from type a and b ore have delta 34 s values near 30 per mil partially reduced contemporaneous marine sulfate 10 12ppm is a possible sulfur source but a concealed rock sulfur source of high delta 34 s value is more probable type d pyrite with delta 34 s values between 28 and 0 per mil contains sulfur of a different probably biogenic origin hydrothermal carbonates formed from fluids with delta 13 c values from near 0 to 12 per mil consistent with oxidation of organic c in the hydrothermal system hydrothermal and postore carbonates have delta 18 o values of 17 to 19 per mil and probably equilibrated with a single fluid after the deposition of the hosselkus limestone

enrichment of trace elements in garnet amphibolites from a paleo subduction zone catalina schist southern california the abundance p t stability solubility and element partitioning behavior of minerals such as rutile garnet sphene apatite zircon zoisite and allanite are critical variables in models for mass transfer from the slab to the mantle wedge in deep regions of subduction zones the influence of these minerals on the composition of subduction related magmas has been inferred and disputed from inverse modelling of the geochemistry of island arc basalt or by experiment although direct samples of the dehydration partial melting region of a mature subduction zone have not been reported from subduction complexes garnet amphibolites from melanges of circumpacific and caribbean blueschist terranes reflect high t 600 c conditions in shallower regions such rocks record geochemical processes that affected deep seated high t portions of paleo subduction zones in the catalina schist a subduction zone metamorphic terrane of southern california metasomatized and migmatitic garnet amphibolites occur as blocks in a matrix of meta ultramafic rocks this mafic and ultramafic complex may represent either slab derived material accreted to the mantle wedge of a nascent subduction zone or a portion of a shear zone closely related to the slab mantle wedge contact or both the trace element geochemistry of the complex and the distribution of trace elements among the minerals of garnet amphibolites were studied by inaa xrf electron microprobe and sem in order of increasing alteration from a probable metabasalt protolith three common types of garnet amphibolite blocks in the catalina schist are 1 non migmatitic clinopyroxene bearing blocks which are compositionally similar to morb that has lost an albite component 2 garnet amphibolite blocks which have rinds that reflect local interaction between metabasite metaperidotite and fluid and 3 migmatites that are extremely enriched in th hfse lree and other trace elements these trace element enrichments are mineralogically controlled by rutile garnet sphene apatite zircon zoisite and allanite alkali and alkaline earth elements are much less enriched in the solid assemblage and thus appear to be decoupled from the other elements in the inferred metasomatic process es the compositions of migmatitic garnet amphibolite blocks seem to complement that of average island arc tholeiite trace element metasomatism reflects fluid solid rather than melt solid interaction the metasomatic effects indicate that h2o rich fluid perhaps with a significant component of na al silicate and alkalis carried th u sr ree and hfse fractionations of lree in migmatites resemble those of migmatitic metasedimentary rocks underlying the mafic and ultramafic complex exotic lree deposited in allanite in migmatites could have been derived from fluids in equilibrium with subducted sediment if the paleo subduction zone represented by the mafic and ultramafic complex of the catalina schist had continued its thermal and fluid evolution a selvage of similarly enriched rocks might have been generated along the slab mantle wedge contact between 30 and 85 km depth rocks affected by subduction zone metasomatism although rarely recognized at the surface could be volumetrically significant products of the initiation of subduction and may prove to be geochemical probes of convergent margins that approach the significance of xenoliths in the study of other magmatic environments

magmatism along the southeast margin of the yangtze block precambrian collision of the yangtze and cathysia blocks of china the time and the environment of formation of igneous rocks along the southern margin of the yangtze continental block in southern anhui province and northeastern jiangxi province china are discussed sm nd dating for samples from two ophiolite suites give ages of about 1 ga late proterozoic granodiorites in the study area have ages of about 0 9 ga and have normal intrusive relations against banxi group metamorphic country rocks trace element abundances and initial nd isotope ratios for ophiolite and proterozoic granodiorites are consistent with formation in an island arc environment in contrast sr and nd isotopes for mesozoic granitoids of the region suggest that they are products of remelting of crustal sources similar to the basement of the yangtze block these relations do not support a recently proposed mesozoic overthrust tectonic model for southeast china but rather favor a late precambrian collision history for the yangtze and cathysia blocks

tectonic setting for the genesis of oceanic plagiogranites evidence from a paleo spreading structure in the oman ophiolite abstract in the oman ophiolite detailed structural and lithological mapping of the 60 45 km maqsad area has revealed several plagiogranite intrusions ranging from metre scale dykes to hectometre scale bodies these plagiogranites are spatially related to kilometre scale mafic plutons displaying generally pegmatitic textures and locally deformed in amphibolite facies conditions along shear zones up to a few hundred metres thick isotope data demonstrate the mantle origin of these pegmatites and plagiogranites nd ranges from 6 3 to 9 9 and has an average value 26 samples of 8 1 within the field of present day morbs and similar to the average nd 7 8 of the oman ophiolite primary igneous sequence these intrusions are not restricted to a given structural level their paleovertical extent exceeds 5 6 km from 2 km below the moho up to the base of the sheeted dyke complex their modal composition evolves up section from predominantly pyroxenites and gabbro norites in the mantle harzburgites to gabbro norites diorites and quartz diorites at crustal level petrography whole rock and mineral chemistry show that this vertical succession of lithologies can be simply explained by low pressure fractional crystallization of a hydrated basaltic melt plagiogranites although more abundant at upper crustal level are not restricted to this horizon plagioclase composition in plagiogranites presents a remarkable evolution with paleodepth from highly calcic 70 95 an in the mantle section intermediate and highly scattered in the lower crust 10 85 an to quite sodic at upper crustal level 5 20 an highly calcic plagioclases in quartz bearing rocks is a puzzling character inconsistent with fractional crystallization even if high degrees of fractional crystallization of a hydrated basaltic melt remain the simplest way to account for the chemistry of the upper crustal plagiogranites processes such as fluid induced remelting or assimilation of country rocks must be invoked to account for the chemistry and field characteristics of the deeper plagiogranites mantle and crustal structures in the maqsad area indicate that the pegmatites and plagiogranites emplaced in an ocean ridge setting when a mantle diapir soaked with basaltic melt intruded a cool hydrothermally altered lithosphere they were eventually transposed in an off axis setting as the ascension of the diapir progressed up to moho level it is concluded that the formation of large volumes of buoyant leucocratic rocks is possible at ocean ridges provided periods of amagmatic spreading are long enough to induce the growth and alteration of an axial lithospheric lid this is likely more frequent at slow spreading centres where mantle upwelling is discontinuous in space and time

the petrogenesis of leucogranitic dykes intruding the northern semail ophiolite united arab emirates field relationships geochemistry and sr nd isotope systematics the khawr fakkan block of the semail ophiolite united arab emirates exhibits a suite of 10 100 m scale metaluminous to peraluminous granitic intrusions ranging from cordierite andalusite biotite monzogranites to garnet tourmaline leucogranites which intrude mantle sequence harzburgites and lower crustal sequence cumulate gabbros structural constraints suggest that the subduction of continental sedimentary material beneath the hot proto ophiolite in an intra oceanic arc environment led to granulite facies metamorphism at the subduction front and the generation of granitic melts which were emplaced up to the level of the ophiolite moho compositions indicate the analysed granitoids were largely minimum melts that crystallised at variable a h2o and pressures of 3 to 5 kbar the lile sr rb and ba covariation modelling suggests that the granitoids formed largely by the dehydration melting of muscovite rich metasediments initial 87sr 86sr ratios of analysed dykes vary between 0 710 and 0 706 at initial nd values of between 6 3 and 0 5 cogenetic units of a composite sill from ra s dadnah yield a sm nd isochron age of 98 8 9 5 ma mswd 1 18 geochemical and isotopic characteristics of the analysed granitic intrusions indicate that the subducted continental material was derived from oceanic trench fill haybi complex sediments preserved as greenschist asimah area to granulite facies bani hamid area ophiolitic metamorphic sole terranes the sr nd isotope systematics suggest that hybrid granitic melts were derived from pre magmatic mixing of two contrasting subduction zone sources

the origin of geochemical variations in a late permian volcanic arc eastern klamath mountains california abstract volcanic are remnants present in the eastern klamath terranes record an intermittent volcanism from ordovician to early jurassic times the upper permian volcanics nosoni dekkas and bully hill formations consist mainly of andesites with minor basalts and rhyolites metamorphosed in the prehnite pumpellyite facies their petrographic and geochemical characters indicate a mature island arc setting but their tholeiitic affinity suggests an intra oceanic environment nd isotopic compositions of the nosoni dekkas lavas e nd t 4 to 8 as well as a preferential enrichment of light rare earth over high field strength elements and systematic negative ce anomalies point to morb type mantle sources contaminated by a few percent of slab derived sediments large variations in the chondrite normalized la yb ratio 0 95 to 3 7 are interpreted in terms of variable depth and degree of partial melting two distinctive basalt types are found at the top of the nosoni formation dekkas saddle and the bully hill formation the former are high mg basalts showing a strong enrichment in incompatible elements la yb 50 8 7 ppm th 50 ppm nb but low tio 2 al 2 o 3 yb zr and sr contents with a pronounced positive eu anomaly they may result from small degrees of partial melting of a refractory peridotitic source enriched by felsic melts derived from subducted morb with garnet in the residue these lavas probably record a transient anomalous thermal regime in the deepest part of the subduction zone and can be compared to some extent to the arc alkali series on the other hand the bully hill basalt shows boninitic affinities and probably originated from a shallower lithospheric mantle source the large range of compositions recorded by the permian arc lavas gives evidence of efficient extraction of melts segregated at different depths

volatile phase fluxed anatexis of metasediments during late caledonian ophiolite obduction evidence from the sogneskollen granitic complex west norway in the west norwegian caledonides minor granitic sheets were intruded during obduction of the c 443 ma solund stavfjord ophiolite complex the sogneskollen granitic complex is the largest intrusive complex and consists of several minor lenses and one larger lens of granodiorite the sogneskollen granitic complex samples generally have high ba sr and na 2 o very low y values nd 420 ma values c 7 to 10 1 and 87 sr 86 sr 420 ma ratios of 0 70437 0 70705 the ree patterns are strongly hree depleted and la yb n varies from 0 3 to 36 most samples display small positive eu anomalies but those that are most fractionated have small negative anomalies the main part of the sogneskollen granitic complex is a white composite granodiorite which is intruded by a suite of grey biotite granite dykes it is suggested that this and other similar granitic sheets in the area most probably formed by melting of sediments in the presence of a h 2 o bearing volatile phase during obduction of the ophiolite the quartzofeldspathic source sediments most probably contained detritus from long term rb depleted lower continental crust and from an island arc the sediments were deposited in large amounts in the marginal basin now represented by the solund stavfjord ophiolite complex during obduction the sediments were buried beneath the hot overriding ophiolite complex and the associated island arc and yielded granitic magmas during partial melting

sulfur isotopes in early proterozoic volcanogenic massive sulfide deposits new data from arizona and implications for ocean chemistry the 34s range of sulfides from early proterozoic volcanogenic massive sulfides of arizona is narrow 129 out of 133 measurements lying within a range of 2 8 to 3 2 other early proterozoic and archean deposits of comparable type and mineralogy have similar s isotope characteristics these deposits unlike phanerozoic analogues do not appear to have incorporated reduced sulfur derived from marine sulfate early proterozoic oceans are known to have contained sulfate in this paper it is a major tenet that sulfate was confined to an upper water layer it is proposed that most volcanogenic massive sulfide hydrothermal systems prior to 1 7 ga were recharged with sulfate free water from an anoxic lower layer and in general discharged into reducing environments rare deposits with exceptional mineralogy or s isotope compositions may reflect hydrothermal recharge from the oxidized upper layer or deposition from fluids discharging into and mixing with the upper water layer

petrology and u pb geochronology of the telohat migmatites aleksod central hoggar algeria the aleksod region is composed of metasedimentary rocks and large areas of biotite and hornblende bearing migmatites anatexis associated with the main deformation stages occurred under high pressure and temperature conditions estimated at 13 2 kbar and 750 50 c the bulk mineralogical composition of the telohat migmatites shows that their protolith was granodioritic internal structures of zircons and u pb data suggest a polyphased evolution with a 2131 12 ma age for the protolith and a 609 17 ma age for the pan african tectono metamorphic evolution thus precluding any kibaran event in the aleksod area leucosomes are richer in sr and display lower rb zr nb y th u and ree contents than melanosomes wherein accessory phases are stored eu contents are also lower in the leucosomes but in lesser proportion than the other rare earth s leading to a significant positive anomaly petrogenetic modelling accounting for accessory mineral phases clearly shows that the trace element contents of leucosomes and melanosomes follow a distribution law consistent neither with equilibrium nor fractional melting their trace element patterns are best explained by the model of disequilibrium melting with mixing of a few residual phases the present results and previous sr isotopic data as well raise the question of disequilibrium melting in anatexis of crustal material

urucum neoproterozoic cambrian manganese deposits ms brazil biogenic participation in the ore genesis geology geochemistry and depositional environment abstract the urucum region mato grosso do sul brazil contains neoproterozoic cambrian manganese ore in the layers mn 1 mn 2 and mn 3 which are interbedded with iron formations ifs from the santa cruz formation jacadigo group and surrounded by carbonate rocks from the bocaina and tamengo formations corumba group these rocks fill an ancient graben with sediments from the santa cruz formation overlying fluvial deposits from the urucum formation and limestones from the bocaina and tamengo formations deposited in the shallow marginal regions as the graben deepened it was flooded with microbe rich seawater and hydrothermalism began iron and manganese precipitated within the pores in arkosic sandstones thereby forming the clastic lower mn 1 ore layer the remaining manganese was simultaneously deposited on the basin floor with clastic clay and silt from turbidity currents thereby forming the upper mn 1 clastic chemical ore layer the sedimentation of mn 1 and its host ifs was controlled by seawater saturation and homogeneous oxidation which caused manganese and iron sedimentation and the death of microorganisms generating mangano ferriferous sediments with high p co and ni contents mn 2 formed during periods of tectonic quiescence through water influxes that carried abundant microbes fe ii mn ii and sio 2 under uv radiation these microbes oxidized mn ii and fe ii to form hydroxides and died thereby forming biomass that was enriched in p co and ni and minor carbonate adjacent to and below the oxycline these microbes also formed kremydilites oblate structures that resemble manganese stromatolites within the biomass and produced ch 4 and or h 2 gas bubbles during early diagenesis wormlike organisms inhabited the biomass and probably separated iron from manganese to produce cryptomelane microbialites which agglutinated to form manganese micronodules that coalesced to form the massive layer mn 2 this process ended with the disappearance of microorganisms and the sedimentation of the ifs and a new water influx generated layer mn 3 via the same process the final seawater upwelling precipitated ifs in the basin depocenter whereas limestone from the tamengo formation was deposited along the basin s edges the diagenesis initially transformed the manganese oxides into cryptomelane and then into hollandite

sulfur carbon hydrogen and oxygen isotope geochemistry of the idaho cobalt belt cobalt copper gold deposits of the idaho cobalt belt including the deposits of the blackbird district have been analyzed for their sulfur carbon hydrogen and oxygen isotope compositions to improve the understanding of ore formation previous genetic hypotheses have ranged widely linking the ores to the sedimentary or diagenetic history of the host mesoproterozoic sedimentary rocks to mesoproterozoic or cretaceous magmatism or to metamorphic shearing the 34 s values are nearly uniform throughout the blackbird district with a mean value for cobaltite coass the main cobalt mineral of 8 0 0 4 n 19 the data suggest that 1 sulfur was derived at least partly from sedimentary sources 2 redox reactions involving sulfur were probably unimportant for ore deposition and 3 the sulfur was probably transported to sites of ore formation as h 2 s hydrogen and oxygen isotope compositions of the ore forming fluid which are calculated from analyses of biotite rich wall rocks and tourmaline do not uniquely identify the source of the fluid plausible sources include formation waters metamorphic waters and mixtures of magmatic and isotopically heavy meteoric waters the calculated compositions are a poor match for the modified seawaters that form volcanogenic massive sulfide vms deposits carbon and oxygen isotope compositions of siderite a mineral that is widespread although sparse at blackbird suggest formation from mixtures of sedimentary organic carbon and magmatic metamorphic carbon the isotopic compositions of calcite in alkaline dike rocks of uncertain age are consistent with a magmatic origin several lines of evidence suggest that siderite postdated the emplacement of cobalt and copper so its significance for the ore forming event is uncertain from the stable isotope perspective the mineral deposits of the idaho cobalt belt contrast with typical vms and sedimentary exhalative deposits they show characteristics of deposit types that form in deeper environments and could be related to metamorphic processes or magmatic processes although the isotopic evidence for magmatic components is relatively weak

the origin of oceanic plagiogranites from the karmoy ophiolite western norway both field relationships and geochemical characteristics indicate two suites of plagiogranitic and related rocks coexisting in the higher parts of the karmoy ophiolite of western norway the plutonic zone of this ophiolite can be subdivided into three complexes the east karmoy igneous complex the visnes high level complex and the veavagen igneous complex and plagiogranitic rocks are well developed in the first two of these within the east karmoy igneous complex plagiogranites are associated with high temperature pre basic dyke shear zones rare earth element modelling indicates that these plagiogranites were derived by anatexis of amphibolite hydrated diabase assuming a starting material consisting of 40 hornblende and 60 plagioclase and that batch melting occurred within the stability field of hornblende in comparison plagiogranite occurs in a number of bodies in the upper part of the visnes high level complex and forms a sandwich horizon together with biotite diorites and epidosites between a roof assemblage of dykes microgabbros and magnetite gabbros and a floor assemblage of layered and non layered gabbros the r e e modelling of the petrogenesis of this series of plagiogranites indicates that they were derived by filter pressing of a differentiated interstitial liquid to the vari textured gabbros although the distribution of highly hygromagmatophile elements such as k rb ba etc cannot be explained satisfactorily by this model alone depletion in these elements appears to be an autometasomatic effect

redistribution of cu zn pb and ba in the bollibokka group east shasta california u s a implications for metallogeny and geochemical balances abstract basalt and andesite of the bollibokka group underlie stratigraphic horizons with volcanogenic massive sulfide vms deposits in the east shasta district but with no known deposits to the north least altered rock remote from known deposits commonly retains clinopyroxene and titanomagnetite phenocrysts and contains abundant secondary pumpellyite and prehnite in contrast to altered rock near known deposits least altered rock contains on average 89 ppm cu 100 ppm zn 10 ppm pb and 160 260 ppm ba alteration within a few kilometers of the bully hill rising star mines led to removal of 59 ppm cu and possibly 7 ppm zn 2 ppm pb and significant ba from basalt one km 3 of altered basalt can account for 7 the cu and 1 7 the zn production of the mines basaltic volcaniclastic wacke in the area contains framboidal pyrite and kerogen and was enriched in cu zn and pb intense leaching within 1 km of the afterthought mine led to removal of 74 ppm cu 34 ppm zn and 4 ppm pb relative to unaltered basalt alteration of 1 km 3 of basalt could supply 40 the cu 8 the zn and 14 the pb produced by the afterthought mine zones of maximum leaching of metals underlie known deposits yet it cannot be proven that leached metals contributed to the deposits the oceanic inventories of cu and zn can be established in thousands of years by hydrothermal and river water fluxes these fluxes cannot account for the rate of fixation of zn in shale a dominant detrital flux is required a dominant detrital flux of cu is permitted but not required

petrogenetic evolution of felsic volcanic sequences associated with phanerozoic volcanic hosted massive sulphide systems the role of extensional geodynamics abstract felsic volcanic hosted massive sulphide deposits form in many different geotectonic environments usually associated with episodes of extension however empirical geochemical links between felsic volcanism geotectonic environment and extensional geodynamics have not been adequately documented so as to distill the fundamental relationships between these features and formation of volcanic hosted massive sulphide vhms mineralization which is important considering most vhms deposits are hosted in felsic volcanic bearing sequences using felsic geochemical data from six different vhms mineralized phanerozoic geotectonic settings it seems that the felsic geochemical data are consistent with the overall geotectonic interpretations for these areas which in turn helps in stratigraphic and geotectonic interpretations for detailed vhms exploration in extensional geodynamic settings bimodal volcanism is common compared to the continuum of compositions observed under compressional to neutral high stress regimes in calc alkaline arc building environments the empirical geochemical metallogenic vhms relationships evident in the archean felsic volcanic rocks in particular the low zr y n yb n 2 o conditions in different source areas and geotectonic environments they seem to be the predominant controls on the compositional evolution of felsic magmas in extensional settings because of the limited degree of assimilation and fractional crystallization afc affecting their compositions with rapid intrusion into the uppermost crust to positions were they achieve neutral buoyancy in the volcanic to subvolcanic environment within plate felsic or a type anorogenic type and anomalous ocean ridge or type felsic magmas that have tholeiitic affinities relatively low alkali elements are common within parts of the volcanic sequences hosting massive sulphide deposits reflecting in part a relationship to extension the high contents of high field strength elements zr ga lree hree y nb etc and other lines of evidence indicate they are relatively high temperature relatively reduced magmas probably derived from previously dehydrated granulitic or melted oceanic simatic or continental sialic crust low p granulites or amphibolites but also in some cases possibly mantle rocks such that they are significantly water undersaturated during melting i e a h 2 o 1 reduced viscosities typical of high temperature melts enable rapid intrusion to higher levels diking vs diapirism higher rates of heat advection from mantle mafic magmas into the thickened oceanic and or continental crust during lithospheric extension are needed to facilitate melting of these rock to form a type and or or type partial melts tholeiitic felsic this usually results in bimodal magmatism as magma mixing is not favored in extensional settings especially at high magma production intrusion and extrusion rates i e high heat flow this would inhibit ferromagnesian phase in particular hornblende and magnetite fractionation especially at low f o 2 in the felsic magmas that would deplete the melt in y hree as well as ore related elements particularly zn which is 2 to 3 times higher in a type and or type rocks compared to other magma types therefore the empirical felsic igneous geochemical metallogenic relationships observed by others low zr y n yb n

plagiogranites from the ocean crust and ophiolites rock types similar to those found in ophiolites have been recovered from the floors of ocean basins1 3 this is critical evidence for the theory that ophiolites represent the ocean crust4 5 aumento6 has described diorites quartz diorites and trondhjemites dredged from the mid atlantic ridge mar near 45 n and noted that these were the first rocks analogous to the low k acid differentiates in ophiolites collectively referred to as oceanic plagiogranites 7 8 to be recognized amongst those recovered from the modern ocean floor only a few plagiogranites from the ocean floor ocean crust plagiogranites have since been described these were dredged from widely spaced localities on the mar9 11 the amani plateau and the kyushu palau ridge north west pacific 12 however the kyushu palau ridge could be the product of destructive plate margin magmatism13 as are the potassic granites from the aves ridge caribbean 14 plagiogranite xenoliths also occur in extrusives on surtsey and askja iceland 15 that only some of the petrological compositions of ophiolite plagiogranites are represented in the ocean crust is not surprising in view of the small sample of ocean crust plagiogranites it is less obvious however if ophiolites represent the ocean crust why certain acidic rock types from the oceans have not been found in ophiolites i show here that the difference between the two groups of plagiogranites is evidence that ophiolites do not represent all kinds of ocean crust and that a greater range of magma compositions and or conditions of magmatic differentiation is represented by the ocean crust

the paleoproterozoic aripuana zn pb ag au cu volcanogenic massive sulfide deposit mato grosso brazil geology geochemistry of alteration carbon and oxygen isotope modeling and implications for genesis aripuana is a paleoproterozoic 1 76 1 75 ga stratiform volcanogenic zn pb ag au cu massive sulfide deposit has an estimated resource of 11 6 million metric tons mt of ore and is located on the south southwest border of the amazon craton brazil aripuana resides in a caldera composed of strongly fractionated felsic metavolcanic rocks transitional between calc alkaline and tholeiitic compositions the entire region was metamorphosed into lower greenschist facies 1 68 1 63 ga and the deposit was subsequently thermally metamorphosed and metasomatized 1 56 1 53 ga the mineralized bodies are lenticular and elongated they are located at the base of a sequence of turbidites which overlie dacitic and rhyolitic ignimbrites the rocks of the footwall are silicified sericitized and chloritized and the hanging wall rocks are carbonate rhythmites and turbidites hydrothermal marbles carbonate breccias and breccia like rocks carbonate talc tremolite chlorite rock talc tremolite and biotite rich rocks and carbonate and fluorite bearing cherts the deposit was formed during at least four submarine exhalative hydrothermal cycles in which siliceous marbles and marls represent the initial phase of each cycle followed by the coprecipitation of pyrite pyrrhotite sphalerite argentiferous galenas chalcopyrite which were disseminated in clay carbonate breccias or formed massive lenses this phase was followed by fluoritic cherts and argillites which were followed by carbonate and fluorite rhythmites and turbidites to close the cycle the variation in total rare earth element ree contents and in the eu and ce anomalies indicates that the original sediments were mixtures of hydrothermal carbonate and smectitic clay and were deposited below a redoxcline isotopic analysis identified 18osmow values between 8 and 13 and 13cpdb values between 7 and 1 modeling isotopic data showed that mineralizing fluids were distinct based on the proportion of magmatic fluid that mixed with the seawater carbonate rocks and sulfide minerals precipitated while hydrothermal activity and volcanism evolved from an exhalative to an exhalative explosive character with temperatures increasing between 100 and 200 c the regional metamorphism of the deposit formed carbonate talc tremolite chlorite rocks tremolite biotite and talc rich rocks it recrystallized the minerals foliated and folded all of the rocks and caused limited decarbonation at the sites where carbonate rocks contained silica and smectite however the original isotopic signatures were maintained the thermal metamorphism erased the metamorphic foliation and the metasomatism generated veinlets of chlorite with sulfides

generation of plagiogranite by amphibolite anatexis in oceanic shear zones field geochronological and rare earth element ree evidence obtained from the fournier oceanic fragment of the canadian appalachians indicates that its plagiogranite component was generated by the anatexis of amphibolite and not by the fractionation of basic magma we propose that the process occurred in two stages first gabbro was plastically deformed at high temperature to form dry low angle shear zones that subsequently evolved to amphibolite via the addition of light ree enriched hydrothermal solutions second the amphibolite underwent partial melting during shear to yield a migmatite comprising bands of plagiogranite alternating with amphibolite restite the plagiogranite locally coalesced to form pods dikes and lenses that injected the surrounding undeformed gabbro we attribute the development of the fournier plagiogranite to dynamothermal processes occurring in proximity to a spreading center due to asthenosphere induced shear within ocean layer 3 this serves as an important illustration of the dynamic nature of metamorphism and melting that can occur in the ocean crust

u pb geochronology and origin of granitoid rocks in the thetford mines ophiolite canadian appalachians we integrate new and existing age data from ophiolite and subophiolite rocks in the thetford mines region quebec appalachians to determine the timing of 1 oceanic crust formation 2 oceanic decoupling 3 ophiolite emplacement onto the continent and 4 postemplacement taconic orogenesis few ophiolites possess high precision temporal data for all these events this allows us to develop models to explain the origin of emplacement related granitoids that have intruded the ophiolite u pb zircon ages have been determined for four samples from the thetford mines ophiolite oceanic plagiogranite from the lac de l9est region of the thetford massif yields an age of 480 2 ma this age is identical to that of the adjacent mont ham massif thrust sheet 478 3 2 ma suggesting a close temporal and spatial association of these two units in the source oceanic domain two peridotite hosted granitoid samples yield crystallization ages of 470 5 3 ma and 469 4 ma a precambrian inherited zircon component is also present in one sample these granitoids are strongly peraluminous contain abundant inherited zircon exhibit variable but high 87 sr 86 sr initial ratios and low 208 pb 206 pb ratios in igneous zircon which suggest that they were derived by partial melting of laurentian margin sedimentary rocks the ages indicate that the ophiolite was thrust over the laurentian margin within 1 17 m y of oceanic crustal crystallization and that these obduction related granites were generated during a period of shear heating calculations indicate that shearing can generate the peridotite hosted granitoid melts and their observed contact zone mineralogies under specific conditions although margin sedimentary rock melting in response to west dipping subduction related arc magmatism cannot be excluded

microthermometry and chemical composition of fluid inclusions from the mt chalmers volcanic hosted massive sulfide deposits central queensland australia implications for ore genesis mt chalmers is a mound style volcanic hosted massive sulfide vhms deposit in central queensland australia the ore lenses are hosted by rhyolitic volcanics and sedimentary rocks of early permian age the two ore lenses main lode and west lode consist of cu zn pb massive sulfide underlain by cu rich stringer mineralization textural and petrographic investigations of fluid inclusions indicate that primary type i inclusions up to 25 microns are found in quartz from the stringer mineralized zone and microthermometric studies of these inclusions yielded homogenization temperatures of 160 285 degrees c and salinities of 5 10 5 nacl equiv wt laser raman spectroscopic lrs analysis indicates the presence of co2 0 1 1 mol in the mt chalmers vhms systems semiquantitative sem wds microprobe analyses of fluid inclusion decrepitates indicate that the mt chalmers ore fluids were enriched in potassium and calcium but depleted in magnesium relative to seawater pixe microanalysis of fluid inclusions in quartz from the stringer zone also indicates a significant base metal concentration in these fluids cation composition and higher salinities relative to seawater suggest that recycled seawater cannot be the sole source of the ore fluids high base metal content and the presence of co2 in the fluid inclusions imply that magmatic input of ore metals copper in particular accompanying seawater leaching of the footwall volcanic pile is a distinct possibility in terms of fluid composition the k ca fe variation of the mt chalmers ore fluid is comparable with those of typical epithermal deposits e g thames new zealand and porphyry copper deposits bingham ut the cu zn fe 10 plot of the mt chalmers ore fluids indicates that there is significant copper in the system comparable to copper enrichment in a porphyry copper system the mt chalmers ore fluids also show similar copper content with the cu rich end member ore fluid composition of the mineralized stage 2b veins of the hellyer vhms deposit whereas the stage 2a veins of hellyer are more enriched in lead and zinc overall the ore fluids have a variable chemistry with a continuum of compositional data from vhms to epithermal porphyry style ore fluids shallow water emplacement 300 m for the vhms mineralization has been postulated for the mt chalmers deposit based on the presence of trace fossils in the footwall and hanging wall sedimentary rocks and volcanic facies studies however fluid inclusion studies do not rule out a moderate to deeper submarine environment as there is no fluid inclusion evidence of boiling at mt chalmers boiling of ore fluids would probably have occurred as in most epithermal systems if the ore fluid exhaled onto the seafloor under a shallow environment

geology and ore petrography of permian kuroko type volcanogenic massive sulfide deposits of the bully hill area east shasta district california kuroko type volcanogenic massive sulfide vms deposits occur in the late permian bully hill rhyolite and the basal part of the overlying pit formation in the bully hill area the bully hill rhyolite at bully hill consists of three tuffaceous units and a lava unit that overlie the dekkas andesite two ore host horizons are present the younger containing the bully hill and probably the copper city deposits and the older the rising star deposit a large olistostrome in the basal pit formation contains disrupted shale mudstone and volcanic rock and includes hydrothermally altered and mineralized material the host strata are folded into tight anticlines at bully hill and copper city distinct zones of hydrothermal alteration ranging from intense silicification to a quartz albite chlorite epidote assemblage are found within the bully hill rhyolite at bully hill the most intensely silicified rock occurs within the lava unit stratigraphically beneath the known vms deposits this rock preserves a stockwork of quartz and barite veinlets some bearing sulfides which represent hydrothermal feeders of the syngenetic mineralization the syngenetic ore has been hydrothermally reworked and is characterized by a paragenetic sequence resembling that of japanese kuroko type deposits eldridge et al 1983 with the added complexity of retrograde barite replacing sulfide minerals

mount st helens potential example of the partial melting of the subducted lithosphere in a volcanic arc mount st helens 50 km to the west of mount adams and the main cascade volcanic chain is only 80 km above the subducting oceanic lithosphere the elevated temperatures off the subducting slab because of the close proximity of the juan de fuca ridge to the trench may induce slab melting at a depth of 80 km dacites from mount st helens have geochemical compositions off magmas that are derived by direct partial melting of metamorphosed basalts at high pressure i e relatively high ai al2o3 15 at 70 sio2 low y and yb because of garnet and amphibole stability in the source low sc and high sr and eu trace element modeling of the partial melting of mid oceanic ridge basalt morb from the juan de fuca ridge that yields a hornblende eclogite residue can reproduce the mount st helens data results off the model are quite distinct from data derived from the mount adams volcanic rocks in contrast mount adams is 135 km above the subducting slab and is associated with normal arc magmatism believed to be derived from the mantle above the subducting plate the cascade are has been active in its present locality because of oblique subduction for the past 7 m y the major volcanoes along the arc have existed for at least 500 ka but mount st helens has existed for 40 ka we suggest that the subducting plate may have reached elevated temperatures because of the approach of north america to the juan de fuca ridge at 40 ka which initiated melting of the slab

mio pliocene adakite generation related to flat subduction in southern ecuador the quimsacocha volcanic center abstract new geochemical and geochronological data on the miocene pliocene quimsacocha volcanic center qvc have led to the recognition of adakitic lavas generated by slab melting related to the flat slab subduction in southern ecuador and northern peru the qvc located in the presently inactive southern part of the ecuadorian arc was built up during three distinctive volcanic phases the first phase generated a basal edifice with mainly andesitic lava flows while the second phase is characterized by the emplacement of cryptodomes domes and related outflow breccias comprised of andesites and some dacites the last phase released rhyolitic ignimbrites associated with the formation of a large caldera which was later partly filled by dacitic rhyolitic domes geochemical data for the qvc indicate higher al2o3 tio2 na2o zr and sr contents and lower fe2o3 mgo y mree and hree abundances compared to other eruptive rocks of the plio quaternary volcanic front of ecuador such geochemical features as well as the frequent presence of an associated epithermal gold deposit are characteristic of the involvement of slab melts also known as adakites 1 2 in the generation of these magmas after a calc alkaline arc magmatism phase slab horizontalization in response to the subduction of a buoyant oceanic plateau results in increased involvement of a slab melting component in the magmas produced however pristine adakites were generated and emplaced during a relatively short period as indicated by zircon fission track ages then volcanic activity stopped and a volcanic gap formed the identification of these adakites their location and age support a model of slab melting associated with flat slab subduction m a gutscher et al geology 28 2000 535 538

adakitic granites within the ne jiangxi ophiolites south china geochemical and nd isotopic evidence geochemical and nd isotopic data are reported for the ca 970 ma albite granites within the neoproterozoic ne jiangxi ophiolite south china these granites are high in al2o3 15 17 na2o 7 3 8 5 mg 43 62 sr 383 920 ppm sr y 166 444 and y yb 12 14 but low in k2o 0 2 0 6 tio2 0 3 p2o5 0 1 furthermore they display consistently high morb like initial end values of 4 6 to 6 7 at ca 970 ma these geochemcial and isotopic characters are similar to those of the modern adakites compared with the modern adakites the xiwan albite granites are depleted in middle to heavy ree with y yb ratio of 12 14 which can be attributed to residual amphibole and garnet in the source geological and geochemical features suggested that the granites were generated by low degrees of partial melting of subducted spilitized oceanic crust at pressures high enough to stabilize garnet and amphibole these are the first adakitic granites identified within the ne jiangxi ophiolite their presence shows that granitic rocks that intrude ophiolites can be generated not only during the processes of sea floor spreading oceanic crust shearing and obduction of fragmented oceanic lithosphere as previously thought but also during the process of subduction of oceanic lithosphere the shrimp u pb zircon age of 968 23 ma for the xiwan adakitic granite geochimica 23 1994 117 should be re interpreted as the timing of subduction of oceanic crust rather than as the age of oceanic crustal formation as previously interpreted