the bergslagen ore province sweden review and update of an accreted orocline 1 9 1 8 ga bp abstract we have revisited and re interpreted deformation structures in the classical ore province of west bergslagen part of the 1 9 1 8 ga old svecofennian orogen in south central sweden four successive phases and styles of deformation reflect 1 continental rifting associated with bimodal magmatism in continental back arc basins d1 2 closure of the rift s by upright tight to isoclinal folds on any scale d2 3 renewed rifting d3 with deposition of argillites conglomerates and submarine basalts in restricted rift basins and 4 intense buckling and folding of west bergslagen along variably but generally steeply plunging folds d4 strain localization during d4 initiated a network of conjugate retrogressive shear zones and closely related large scale vertical drag folds with the southwest northeast striking west bergslagen boundary zone system acting as the main tectonic boundary along which west bergslagen slipped relative to the central part of the province d4 transpression and variably oblique d4 shear may have caused the preservation of a low grade domain in west bergslagen as well as the exhumation of a granulite facies domain near the hinge of the orocline d4 structures include a regional scale oroclinal bend of 135 in southwest bergslagen connecting common southwest northeast strikes in west bergslagen to east west strikes to the south and east in central bergslagen the oroclinal bend tightens to 180 to the south it lines up with another orocline the two forming an s shaped pair of large scale buckle folds the orocline finds expression in the regional patterns of form lines and magnetic anomalies and in the spatial distribution of specific intrusive rock suites the northern structural boundary of the province is known as an east west striking zone of dip slip movement and local wrenching which we also interpret as a d4 effect the d4 structures imply the evolution of a separate ribbon like bergslagen micro continent at least 700 km long rifted from the former active continental margin during the d3 event and finally accreted slightly obliquely to pre existing fennoscandia to the present north by correlating d4 deformation structures with the areal distribution and previously determined radiometric ages of late orogenic anatectic granites we derive a provisional estimate 1 83 1 82 ga as the lower age limit to the initiation of terrane accretion d4 accretion is svecofennian orogeny proper in this part of the belt younger than the currently preferred estimate of 1 86 ga west bergslagen s synsedimentary hydrothermal ore deposits originating from 1 91 to 1 89 ga old volcanism and rifting have been remobilized to various extent during the late orogenic d4 phase and show pronounced concentration along d4 shear zones

subseafloor hydrothermal alteration during the early proterozoic at garpenberg central sweden the early proterozoic sulfide deposit at garpenberg is located in the metallogenetic province of central sweden it is a strata bound massive sulfide deposit contained in a supracrustal sequence of mainly acid metavolcanic rocks stratiform zn pb cu mineralization is underlain by cu bearing stockwork ore and an extensive alteration zone the sulfide ores and their altered wall rocks were formed by subseafloor hydrothermal activity the alteration pattern observed in the wall rocks of this deposit is consistent with a hydrothermal system where the fluid consists mainly of seawater and a high water rock mass ratio predominates the hydrothermal activity caused the destruction of the primary mineralogy mainly feldspars and a general redistribution of the chemical elements in the altered wall rocks which were principally depleted in ca na and eu and enriched in mg eu was redeposited with the ore metals near or at the seafloor and ca was deposited as limestone most of the major and trace elements show large mobility during the alteration only ti zr y and ree excluding eu behaved as relatively immobile elements

origin of ree mineralization in the bastnas type fe ree cu mo bi au deposits bergslagen sweden the bastnas type deposits with mineral assemblages of fe oxides ca mg silicates rare earth element ree silicates ree fluorocarbonates and cu fe mo bi sulfides are associated with marble horizons in a strongly na k and or mg altered metavolcanic succession over a distance of at least 80 km in a sw ne trending zone in western bergslagen two subtypes occur 1 enriched relative to the other type in light ree lree and fe exemplified by the bastnas and rodbergsgruvan deposits and 2 enriched in heavy ree hree y mg ca and f represented by deposits in the norberg district bastnasite hosts primary fluid h2o co2 inclusions with salinities of 6 29 eq wt cacl2 and with total homogenization temperatures th tot of ca 300 400 c subtype 2 has late stage fluorite with fluid inclusions that show 1 16 eq wt nacl and th tot of ca 90 150 c molybdenite re os ages obtained from three deposits are 1 904 6 1 863 4 and 1 842 4 ma nd isotopic data from five different ree minerals yielded no defined isochron but a range in end 1 88 ga of 0 2 to 1 6 the oxygen isotope values 18osmow of dolomite and calcite from the associated ree mineralized skarn and recrystallized carbonate assemblages lie in the range 6 1 8 6 overlapping with those of the host marbles carbon isotope values 13cpdb show typical magmatic signatures of 6 7 to 4 4 while the host marbles group around ca 2 4 the sulfur isotope 34scdt values of associated sulfides range between 10 8 and 0 2 the combined evidence suggests ree mineralization beginning at 1 9 ga from mainly svecofennian juvenile magmatic 400 c fluids carrying si f cl s co2 and the ree in addition to other metals mineralization occurred through reactions with dolomitic layers in the supracrustal units coevally with regional metasomatic alteration associated with fluid circulation through an extensive active volcano plutonic complex

tourmaline in the central swedish ore district more than 40 recently discovered tourmaline occurrences have been investigated in the mid proterozoic bergslagen ore district of central sweden some are spatially associated with ores others with zones of leaching remobilization and migmatization among the tourmaline bearing ore deposits are the dammberg znpb fe sulphide deposit the sala pb zn ag deposit the dalkarlsberg pershyttan and haksberg fe oxide deposits the leja cu deposit and the zinkgruvan zn pb ag deposit tourmaline has been recorded a as tourmalinites and tourmaline bearing chemical sediments b in tourmaline bearing skarns c in tourmaline quartz veins d as disseminations along the foliation in schists e in tourmaline pegmatites f in tourmalinized haloes in metavolcanites along tourmaline pegmatites and g in late joints tourmalinites tourmaline bearing chemical sediments and tourmaline bearing skarns are spatially associated with sulphide and oxide mineralizations the dravite components in these tourmalines are proportional to the size of zn pb sulphide mineralizations tourmalines from quartz veins close to and within ore deposits contain high zr and cr contents with increasing distance away from these deposits the zr and cr contents fall significantly tourmalines from pegmatites have inherited a number of trace element enrichments through partial melting and assimilation of volcaniclastic sediments into granitic melts despite magmatic homogenization zn contents in these tourmalines reflect the proximity of zn pb sulphide deposits decreasing away from them tourmalines from late joints with zn contents above the 100 ppm level are also indicative for the proximity of zn pb sulphide mineralizations thus some trace elements in these tourmalines may represent suitable exploration tools

the aqueous geochemistry of the rare earth elements and yttrium 4 monazite solubility and ree mobility in exhalative massive sulfide depositing environments abstract although there is considerable evidence that rare earth elements ree and particularly the light ree lree have been mobile in the alteration zones below many massive sulfide deposits there is some disagreement over the cause of this mobility most researchers have argued or implied that the ree were mobilized by analogues of modern seafloor hydrothermal vent fluids however the few measurements that have been made of the ree concentrations of these modern fluids suggest that their ability to mobilize ree is negligible in order to shed further light on this problem we have carried out calculations of monazite solubility monazite is thought to be the principal host for the lree in a model vent fluid at temperatures from 200 to 300 c these calculations show that at 300 c the model fluid will contain 0 08 ppb ce 0 07 ppb la and 0 03 ppb nd at 200 c the concentrations increase to 0 54 ppb 0 37 ppb and 0 15 ppb respectively a decrease in ph of one unit at 300 c also increases ree solubility significantly 7 4 ppb ce 5 7 ppb la and 1 1 ppb nd these solubilities are similar to the concentrations of ree measured in seafloor hydrothermal vent fluids however they are considerably lower than required to account for the scale of ree mobility in alteration zones associated with massive sulfide deposits if the mass ratio of typical hydrothermal vent fluid to rock reached 1000 concentrations of ree in the altered rock the predicted to be in the range

geochemistry and origin of cordierite orthoamphibole orthopyroxene phlogopite rocks from namaqualand south africa abstract a suite of cordierite orthoamphibole cordierite orthopyroxene and cordierite phlogopite gneisses are described from the amphibolite and granulite facies region of the western namaqualand metamorphic complex the gneisses are directly associated with underlying and overlying metapelitic rocks to which they show certain geochemical affinities using overall geochemical trends and comparisons between amphibolite and granulite facies samples particularly for na 2 o feo and tio 2 a residual origin by extraction of granite melt can be excluded and a premetamorphic origin proposed for the mg al rich bulk compositions the formation of these specific cordierite orthoamphibole orthopyroxene rocks by metamorphism of carbonate associated evaporite sequences is not supported by their field relationships or by their al fe rich character the bulk of the cordierite orthoamphibole orthopyroxene rocks have trace element contents that strongly resemble those of underlying mafic rocks an al rich group of cordierite phlogopite bearing rocks shows marked enrichment in zr ce and th suggesting residual concentration following weathering major element modelling of postulated premetamorphic constituents indicates that the majority of cordierite orthoamphibole orthopyroxene rocks comprise former mixtures of chlorite with lesser quartz and other minor clay components phlogopite rich rocks can be modelled as mixtures of chlorite illite and kaolinite the suite probably formed by weathering of mafic volcanic material and its local redeposition in an environment where relatively warm saline conditions favoured the development of chlorite rich assemblages

chemistry of micas and chlorite in proterozoic acid metavolcanics and associated rocks from the hastefalt area norberg ore district central sweden microprobe analyses are performed on micas biotite muscovite and phlogopite and chlorite from 1 9 1 8 ga acid k or na rich metavolcanics cordierit emica schists and manganiferous rocks from the hastefalt area in central sweden the results indicate that fe rich biotites and muscovites containing 10 to 25 celadonite and or pyrophyllite are common in the k and na rich metavolcanics in the cordierite mica schists the biotites are mg rich and the muscovites contain less than 10 celadonite and or pyrophyllite the predominant mica in the manganiferous rocks are phlogopite and less frequent rather pure muscovite the chlorites show a wide range in composition but principally those occurring in the k and na rich metavolcanics are brunsvigite and diabantite and those in the cordierite mica schists and the manganiferous rocks are mainly sheridanite and clinochlore the chlorites of the manganiferous rocks show enrichment in mn compared to those in other rock types in general the compositional variations in the micas and less commonly chlorites are strongly controlled by rock type and fluid chemistry particularly with respect to the ratio of feo feo mgo estimates of maximum prograde metamorphic temperature based on phyllosilicates and co existing cordierite and garnets indicate a value of up to 500 c

the behaviour of so called immobile elements in hydrothermally altered rocks associated with volcanogenic submarine exhalative ore deposits evidence is available that some elements notably zr tio2 y sc ce and nb are largely immobile during the alteration of volcanic rocks owing to metamorphism hydrothermal events and weathering e g floyd and winchester 1978 however it is shown by reference to analyses of rocks from the environment of five volcanogenic massive sulphide bodies that while zr tio2 and ce are mostly immobile even during intense hydrothermal alteration y and particularly sc and nb may be extremely mobile when elements are removed by solution in a hydrothermal fluid it seems that reaction rates are such that these elements are almost totally removed from the rock therefore of the so called immobile trace elements only zr and tio2 may be used with any reliability to identify the degree of magmatic differentiation in an hydrothermally altered rock however if an element has been mobile it is usually readily identified as having moved

early proterozoic continental tholeiites from western bergslagen central sweden iii geodynamic inferences abstract the 1 88 ga old within plate continental tholeiites belong to the mainly metavolcanic bimodal bergslagen supracrustal series bss in the svecofennian domain of the baltic shield the bss evolved in an intracontinental rift despite only moderate lithospheric extension the tholeiites are the near surface expression of a 10 20 km thick magmatic crustal underplate that probably underlies most of the sw svecofennian domain and caused the svecofennian crust to be three layered thick but yet isostatically stable underplating strengthened the lithosphere sufficiently to control the time of cratonization geodynamic models for the origin of the bss consistent with our interpretations are gravitational collapse of an overthickened continental accreted terrane margin or back arc processes related to a marginal volcanic arc 250 km further south a tentative reconstruction of relative plate motions during svecofennian time indicates predominantly longitudinal motion of the plates in question and complies with the relatively slow latitudinal drift of the shield at the time recent ion microprobe u pb ages of proterozoic zircons in svecofennian metasediments imply about 0 2 ga of pre svecofennian crustal evolution while the nd model ages of the most primitive metatholeiites may be indicative of initial mantle differentiation as early as 2 4 ga bp accompanying protocrust formation

geochemical characteristics of granulite facies rocks in the archean varpaisjarvi area central fennoscandian shield abstract archean lower crustal granulites are exposed as fault bounded blocks in the varpaisjarvi area central finland the prevailing rock types in these blocks are igneous enderbites tonalitic trondhjemitic granodioritic migmatites and mafic granulites although metamorphosed at moderate pressures 8 10 kbar the varpaisjarvi granulites are not in a geochemical sense good representatives of depleted restitic lower crust having invariably negative or negligible eu anomalies only the enderbites have k rb ratios showing depleted granulite trends the enderbites are orthopyroxene bearing igneous rocks ranging in composition from diorite to tonalite they have distinctive flat lree patterns with la sm from 2 03 2 86 which may be due to the removal of a lree rich phase during fractionation if not the result of infiltration by a co 2 rich fluid mafic granulites occur as intercalations within the tonalitic trondhjemitic granodioritic gneisses and while all are tholeiitic in composition they fall into two groups on the basis of major and trace element abundances the group 1 tholeiites are mostly basaltic with steeply increasing alkali contents over a short range in sio 2 ti zr ratios are close to the chondritic value of 110 the group 2 tholeiites vary from picrobasaltic to andesitic in composition with the latter being dominant ti zr ratios lie below the chondritic value the group 1 rocks have higher ni and cr and lower zr y ratios than their group 2 counterparts the varpaisjarvi area represents at least two accreted blocks or terranes that exhibit marked lithological and geochemical differences with the jonsa block being the most distinctive the main differences are the two contrasting groups of tholeiitic mafic granulites the jonsa block having only the group 2 metatholeiites and the presence of quartz cordierite and cordierite orthoamphibole orthopyroxene rocks only in the jonsa block where they are found as interlayers amongst the group 2 tholeiites the ti zr and zr y ratios of these latter lithologies are closely comparable with those of the group 2 tholeiites the jonsa block cordierite orthoamphibole orthopyroxene rocks have high mgo mgo feo tot ratios compared to most cordierite orthoamphibole rocks produced by seawater basalt interaction their major and trace element geochemistry nevertheless indicates that they were derived from a mafic igneous precursor

composition volume changes during hydrothermal alteration of andesite at buttercup hill noranda district quebec abstract hydrothermally altered andesites in the upper member of the amulet formation at buttercup hill noranda quebec represent part of the aquifer and cap of a self sealing geothermal system that focussed the discharge of hydrothermal fluids during the formation of massive cu zn sulfide deposits five alteration facies are recognized 1 pervasive greenschist facies regional metamorphism least altered andesite 2 epidotization silicification 3 albitization silicification 4 chloritization 5 sericitization silicification alteration is localized on permeable zones such as amygdules fractures flow tops discordant breccia dikes and conformable breccia horizons epidotized silicified andesite is enriched in ca sr eu and depleted in mg and first transition series metals ftsm relative to least altered andesite albitized silicified andesite is significantly enriched in na and depleted in most ftsm relative to least altered andesite the abundances and inter element ratios of the rare earth elements ree and most high field strength elements hfs y zr th u hf ta are similar in least altered epidotized silicified and albitized silicified andesites the most silicified andesites are strongly enriched in na si strongly depleted in mg and divalent ftsm and slightly but systematically depleted in ree and most hfs elements serialized andesites were previously silicified they are very strongly enriched in k rb cs ba very strongly depleted in na ca sr eu and slightly depleted in light ree relative to silicified andesite chloritized andesitic rocks exhibit heavy ree and hfs element ratios similar to those of leastaltered andesite but are relatively strongly enriched in mg and divalent ftsm strongly depleted in si and large ion lithophile lil elements and slightly depleted in light ree the coupled behavior of the heavy ree and most hfs elements during epidotization albitization silicification chloritization and serialization suggests that they were inert during hydrothermal modification of the andesite mass balance calculations suggest that volume was conserved during epidotization silicification and albitization silicification but that intense silicification was accompanied by volume increases up to 30 percent

an experimental investigation of high temperature interactions between seawater and rhyolite andesite basalt and peridotite natural seawater was allowed to react with rhyolite andesite basalt and peridotite at 200 500 c and 1 000 bars at water rock mass ratios of 5 and 50 in order to investigate the effects of rock type water rock ratio and temperature on solution chemistry and alteration mineralogy the results indicate that interactions of seawater with various igneous rocks are similar in the production of a hydrous mg silicate and anhydrite as major alteration products fluids involved in the interactions lose mg to alteration phases while leaching fe mn and si from the rocks the ph of the solutions is primarily controlled by mg oh silicate formation and therefore varies with mg and si concentration of the system other reactions which involve mg such as mg ca exchange or which produce free h cause major differences in fluid chemistry between different seawater rock systems high water rock ratio systems 50 1 are generally more acidic and more efficient in leaching than low ratio systems 5 1 due to relatively more seawater mg available for mgsilicate production the experiments show that large scale seawater rock interaction could exert considerable control on the chemistry of seawater as well as producing large bodies of altered rock with associated ore deposits

the provenance of low calcic black shales the elemental concentration of sedimentary rocks depends on the varying reactivity of each element as it goes from the source through weathering deposition diagenesis lithification and even low rank metamorphism however non reactive components of detrital particles ideally are characteristic of the original igneous source and thus are useful in provenance studies to determine the source of detrital granitic and volcanic components of low calcic 1 caco3 marine black shales the concentrations of apparently non reactive i e unaffected by diagenetic redox and or low rank metamorphic processes trace elements were examined using standard trace element discrimination diagrams developed for igneous rocks the chemical data was obtained by neutron activation analyses of about 200 stratigraphically well documented black shale samples from the cambrian through the jurassic a la th sc ternary diagram distinguishes among contributions from the upper and bulk continental crust and the oceanic crust taylor and mclennan 1985 all the low calcic black shales cluster within the region of the upper crust th hf co ternary diagrams also are commonly used to distinguish among the upper and bulk continental crust and the oceanic crust taylor and mclennan 1985 as co is redox sensitive in black shale environments it was necessary to substitute an immobile element i e example rb in the diagram with this substitution of black shales all cluster in the region of the upper continental crust to determine the provenance of the granitic component pearce et al 1984 plots of ta vs yb and rb vs yb ta shows a cluster at the junction of the boundaries separating the volcanic arc granite vag syn collision granite syn colg and within plate granite wpg fields the majority fall within the vag field there are no occurrences of ocean ridge granite org the minimal contribution of basalts to marine black shales is confirmed by the ternary wood diagram th hf 3 ta wood et al 1979 the black shales plot in a cluster in a high th region outside the various basalt fields which suggests contribution from the continental crust

determination of the rare earth element abundances in 29 international rock standards by instrumental neutron activation analysis a critical appraisal of calibration errors abstract a standard additions method has been used to calibrate an in house multi element rock standard for the analysis of the rare earth elements ree and th ta hf and co by instrumental neutron activation analysis this preliminary calibration with a precision of typically better than 7 1 has been refined by comparing our analyses of the four u s g s standard rocks agv 1 bcr 1 g 2 and gsp 1 with compiled analyses obtained by a critical survey of over 70 published references our standard calibration was adjusted by the average discrepancy between our analyses and published analyses for each element for all four u s g s standards the accuracy of this final calibration is estimated to be better than 5 absolute and it has been used to determine the above elemental abundances in 29 international rock standards five of the ree have been determined by interpolation from chondrite normalised plots

nd isotopic systematics and chemistry of central australian sapphirine granulites an example of rare earth element mobility abstract lower proterozoic sapphirine bearing and associated granulites from central australia exhibit the greatest range of present day 143 nd 144 nd ratios nd o 26 5 to 112 3 yet reported for rocks believed to be cogenetic the nd isotopic data and ree abundances of these rocks demonstrate extreme fractionation of the rare earths during the formation of stratiform cu pb zn sulfide deposits with which they are closely associated field relationships petrography and chemistry of the sapphirine granulites suggest that their protoliths comprised chlorite rich rocks which were generated by hydrothermal alteration of a range of rock types prior to metamorphism calculations employing ree abundances of the sapphirine granulites and associated rocks combined with bulk solid fluid distribution coefficient data yield high fluid rock ratios consistent with a pre metamorphic hydrothermal origin for the unusual ree patterns the sm nd data for these rocks define an age of 1760 75ma which is significantly younger than the crust formation age of the terrain 2070 125ma but indistinguishable from the rb sr whole rock age for granulite facies metamorphism 1790 35ma these data are interpreted in terms of major hydrothermal fractionation of the rare earths shortly perhaps tens of millions of years before granulite facies metamorphism followed by redistribution of nd isotopes or small fractionations of the sm nd ratio during the granulite facies event and possibly also during intense retrogression which reset rb sr whole rock and u pb zircon and monazite systematics at about 1700 ma

behaviour of ree during thermal metamorphism and hydrothermal infiltration associated with skarn and vein type tungsten ore bodies in central morocco abstract the analysis of ree in tungsten ore bodies and surrounding rocks from paleozoic schist sandstone and limestone series in central morocco shows two kinds of behaviour 1 no significant modification of lanthanide distribution occurs during thermal and hydrothermal metamorphic events and 2 important variations of ree patterns characterize highly infiltrated and mineralized samples corresponding to the mixing of scheelite and wall rock spectra a water rock ratio of 2 4 is estimated for the paleogeothermal continental system by comparison of ree patterns hydrothermal mineralizing fluids are assumed to be derived from blind evolved late acidic magmas rather than from outcropping aouam granitic stocks

polygenetic titanite records the composition of metamorphic fluids during the exhumation of ultrahigh pressure metagranite in the sulu orogen

rare earth element abundances and distribution patterns in hydrothermally altered basalts experimental results oceanic tholeiites that have been experimentally reacted with seawater at 500 600 c 800 1000 bars have rare earth element ree abundances and distribution patterns that are essentially identical to those of the unaltered basalts although the data indicate the possibility of some ree leaching and redistribution the net effects are small and the ree patterns of basalts affected by submarine hydrothermal processes may still be valid petrogenetic indicators

early proterozoic continental tholeiites from western bergslagen central sweden i petrology geochemical petrogenesis and geotectonic setting abstract the early proterozoic bergslagen supracrustal series bss of western bergslagen consists of a pile of felsic volcanites and intercalated carbonates with in the upper part an alternating succession of volcaniclastics and basic flows this pile is intruded by a bimodal sequence of granites and basic sills and dikes major and trace element data reveal the influence of several processes that have acted upon the basic magma crustal contamination or supra subduction zone ssz modification fractional crystallization and potassic and sodic alteration due to sub seafloor processes are recognized using geochemical and petrological criteria the basic rocks are divided in two least altered la and two alkali altered aa sets geochemically the least altered basalts resemble phanerozoic continental within plate tholeiites and thus contradict the interpretation by many previous authors of bergslagen as an active margin volcanic arc alkali alteration leads to pseudo calc alkaline or pseudo alkaline signatures however a ssz component observable in part of the la set indicates a relation in space and time to a convergent margin

proterozoic seawater felsic volcanics interaction w bergslagen sweden evidence for high ree mobility and implications for 1 8 ga seawater compositions mg enrichment in diffusely bounded schistose alteration zones occurring in proterozoic submarine felsic volcanics ne of hjulsjo w bergslagen is shown to result from felsic rock seawater interaction in a sub seafloor hydrothermal system the alteration is apparently in two stages feldspar being altered first to a kaolinite type clay which is then replaced by mg chlorite

hydrothermal mobility of ti zr and ree examples from the bergell and adamello contact aureoles italy contact metamorphic marbles affected by metasomatic fluids at the contact to the bergell and adamello intrusives contain various accessory minerals zirconolite allanite titanite rutile geikielite hoegbomite which provide new evidence for the hydrothermal mobility of ti and zr in both examples ti and zr migrated along with u th y and ree in a metasomatic fluid rich in potassium the composition of the main minerals fluorine rich phlogopite pargasite and titanian clinohumite and the abundance of fluor apatite demonstrate that fluorine and phosphorus were important components of the fluid the textural relationships indicate that the formation of the accessory phases is linked to the crystallization of the hydrous minerals

mineralogy and genesis of cordierite anthophyllite rocks from the sulfide deposit of falun sweden abstract cordierite anthophyllite rocks from the aureole of the falun sulfide deposit are characterized by the assemblages cordierite anthophyllite garnet gahnite quartz dierite staurolite quartz and cordierite andalusite quartz metamorphic conditions are estimated to be 550 50 c 2 5 1 kbar bulk rock chemistry trace element concentration and the variance of the system as defined by the parageneses hint at an origin from acid rocks altered metasomatically at rather low temperatures to chlorite quartz muscovite and subsequently metamorphosed under essentially isochemical conditions

ore deposition in a proterozoic incipient rift zone environment a tentative model for the filipstad grythyttan hjulsjo region bergslagen sweden the 1 9 1 8 ga bergslagen supracrustal series comprises an early volcanic stage represented by the lower leptite group an initial rift stage by the middle leptite group a rift stage by the upper leptite halleflinta and slate group metabasites and the granite granophyre suite and a post rift stage by conglomerate beds remobilized granite granophyres and the hyttsjo gabbro tonalite suite the formation and subsequent alteration of iron manganese and sulfide skarn ores in the supracrustal series involve

hydrothermal alteration as a control of regional geochemistry and ore formation in the central baltic shield in a geochemical study of the proterozoic volcano sedimentary bergslagen ore province in central sweden more than 1000 eruptive rocks were analyzed for main and trace elements the volcanic rocks form a bimodal suite dominated by rhyolites but deficient in andesites igneous geochemical variation bears little relevance to the occurrence and distribution of ores which mostly appear related to secondary alteration patterns hydrothermal metasomatic alteration occurs on a regional scale and has affected markedly more than half of the 732 analyzed macroscopically fresh porphyritic volcanic rocks metasomatism produced extreme na and k differentiation and fe mg si relationships incompatible with igneous variation patterns enrichment in na and mg and leaching of ore forming metals are prominent in the lower parts of the volcanic pile whereas meta volcanics enriched in k and volcanogenic sediments associated with ores and marbles dominate the upper levels the metasomatic alteration of the volcanic rocks and the formation of most ores are two aspects of a comprehensive process of hydrothermal convection which circulated sea water extracted the ore forming elements from the lower levels of the volcanic pile and enriched them at or close to the top of the volcanic sequence

hydrothermal alteration of basalt by seawater under seawater dominated conditions fresh mid ocean ridge basalt glass and diabase have been reacted with seawater at 150 300 c 500 bar and water rock mass ratios of 50 62 and 125 using experimental apparatus which allowed on line sampling of solution to monitor reaction progress these experiments characterize reaction under what we have called seawater dominated conditions of hydrothermal alteration in an experiment at 300 c basalt glass undergoing alteration removed nearly all mg2 from an amount of seawater 50 times its own mass in the process the glass was converted entirely to mixed layer smectite chlorite anhydrite and minor hematite removal of mg from seawater occurred as a mg oh 2 component incorporated into the secondary clay this produced a precipitous drop in solution ph early in the experiment accompanied by a dramatic increase in the concentrations of fe mn and zn in solution as mg removal neared completion and the glass was hydrolyzed ph rose again and heavy metal concentrations dropped at water rock ratios of 62 and 125 and 150 300 c the mineral assemblage produced was similar to that at a water rock ratio of 50 solution chemistry however contrasted with the earlier experiment in that mg concentrations in solution were greater and ph lower this caused significant leaching of heavy metals at 300 c nearly all of the na ca cu zn and co2 and most of the k ba sr and mn were leached from the silicates h2s al si and possibly co were also significantly mobilized whereas v cr and ni were not little or no seawater sulfate was reduced although submarine hot spring solutions sampled to date along mid ocean ridges clearly come from rock dominated hydrothermal systems evidence from ocean floor metabasalts and from heat flow studies indicates that seawater dominated conditions of alteration prevail at least locally both in axial hightemperature systems and in ridge flank systems at lower temperatures

high element mobility in 1 9 1 86 ga hydrothermal alteration zones bergslagen central sweden relationships with exhalative fe ore mineralizations abstract the 1 9 1 86 ga ensialic belt of western bergslagen sweden comprizes a 12 km thick felsic volcano sedimentary sequence intruded by syn volcanic granitic plutons and mafic dykes and sheets developed in a rifted continental setting regional low temperature alteration of the supracrustal sequence produced greenschist to low amphibolite facies mineral assemblages localized cross cutting zones of intense alteration are developed in felsic volcanics which have been affected by the regional alteration producing drastic changes in both mineralogy and chemistry these zones are found close to the hjulsjo granite granophyre intrusive complex most probably the heat source for the hydrothermal system and related to the exhalative sirsjoberg osjoberg fe oxide deposits the hjulsjo zones are subdivided into inner and outer zones based on differences in mineralogy and chemistry resulting from the variable intensity of alteration while the degree to which most elements are leached increases from the outer into the inner zones mg increases with alteration intensity and si and al stay unchanged although they were mobilized during alteration initially k rb fe and mn increase in the outer zones but are depleted in the inner zones rare earth elements ree are depleted with the light ree being more depleted than the heavy ree a pronounced negative eu anomaly is generally present the hydrothermal fluid was enriched in ore forming metals during alteration and leaching ree patterns of exhalative minerals indicate that the fe oxide deposits were derived from these hydrothermal fluids

a semi quantitative model for fractionation of rhyolite from rhyodacite in a compositionally altered archean volcanic complex superior province canada abstract the phinncy dash lakes complex pdlc of the western wabigoon subprovince northwestern ontario consists of subvolcanic and pyroclastic rhyodacite subordinate rhyolite pyroclastic rocks and lava flows minor intercalated basalt lava flows and related clastic and chemical sediments which have been metamorphosed to low greenschist grade the basalt lying stratigraphically below the complex is intruded by tonalite that is coeval with the felsic volcanic rocks rhyolite in pdlc differs from the associated rhyodacite by containing k feldspar and having lower la yb ratios negative eu anomalies and generally lower fe mg ca ti al p zr sc and cr but higher si k rb ba and nb major and trace element calculations using estimates of rhyodacite and rhyolite primary phenocryst compositions and estimates of partition coefficients were done to determine if the rhyolite may have been generated by fractional crystallization from the rhyodacite it was found that fractionation of plagioclase clinopyroxene quartz and small amounts of apatite zircon allanite and fe ti oxides from the rhyodacite would produce a composition similar to the rhyolite only if the abundances of the mobile elements si ca na sr ba and mg in the rhyodacite initial composition were adjusted from the measured values if the rhyolite fractionated from the rhyodacite then there must have been a net addition of si 2 net depletion of ca na sr and ba and 3 net depletion of mg but conservation of fe in the rhyodacite during synvolcanic alteration or regional metamorphism these inferences are supported by thin section evidence of 1 addition of quartz in microfractures and groundmass 2 breakdown of primary plagioclase and 3 breakdown of primary mafic silicates plots of mgo versus tio2 and feo versus tio2 also demonstrate that by comparison with the nearby and coeval kishkutena tonalite mg is depleted and fe is conserved in the pdlc rocks the secondary mineral assemblages combined with the apparent chemical reconstitution suggest that during alteration of the pdlc rocks 1 the water to rock ratio was small 2 the fluid temperature was low probably below about 300 c 3 if the temperature and water to rock ratio were large however then the fluid source was probably not seawater because of the limited precipitation of mg to the rocks

metamorphism of magnesium altered felsic volcanic rocks from bergslagen central sweden a transition from mg chlorite to cordierite rich rocks abstract the ca 1 9 ga old felsic volcanic rocks of the bergslagen province have been affected by synvolcanic hydrothermal mg metasomatism a spatial relationship exists between mg alteration zones and calc silicate carbonate banded iron mineralizations and a genetic link is suggested this is supported by the occurrence of the mg rich tourmaline dravite which exists in the mg alteration zones as well as in the wall rocks to iron oxide mineralizations following later regional metamorphism and deformation these rocks occur as large zones of mg rich schists parallel to sub parallel with the regional foliation s1 the dravite crystals have been affected by this regional deformation a second deformational episode folded s1 along steeply dipping fold axes locally this resulted in a crenulation cleavage and the development of a new generation of micas s2 crosscutting s1 in the westernmost areas the mg schists contain a fine grained network of mg chlorite and muscovite with little or no preferred orientation indicating low pressures and low grade metamorphic conditions the grade of recrystallization and deformation increase towards the east and a boundary between low and medium grade metamorphic terrains is crossed the metamorphic boundary is marked by the disappearance of chlorite and the appearance of cordierite in the mg schists porphyroblasts of mg chlorite or cordierite grew across s1 and cordierite porphyroblasts are transected by s2 micas this indicates that the peak of regional metamorphism occurred between the two deformational episodes retrogression of cordierite to chlorite and muscovite is confined to the vicinity of crosscutting s2 micas

rare earth element study of exhalites within the willyama supergroup broken hill block australia a wide variety of unusual rock types exhalites are commonly associated with or host to exhalative mineralisation within the willyama supergroup chondrite normalised ree patterns of feldspar gahnite calcite magnetite and garnet rich lithologies in the vicinity of stratiform broken hill type pb zn ag mineralisation are lree and eu enriched similar to the ree patterns of pure metalliferous sediments and hydrothermal fluids of the east pacific rise and the red sea in contrast tourmaline garnet amphibole feldspar and gahnite rich exhalites in strike extension of broken hill type orebodies possess lree enrichments and negative eu anomalies and also hree enrichments and negative ce anomalies these ree patterns are the result of decreasing temperatures of the hydrothermal fluids changing oxidation reduction conditions and increasing influence of basic volcanism with increasing distance from the sulphide mineralisation

systematics of hydrothermal alteration at the falun base metal sulfide deposit and implications for ore genesis and exploration bergslagen ore district fennoscandian shield sweden the paleoproterozoic falun zn pb cu au ag pyritic sulfide deposit in the bergslagen ore district sweden is enveloped by hydrothermally altered rocks metamorphosed to the lower amphibolite facie

rare earth elements in the sedimentary cycle a pilot study of the first leg the effects of source rock composition and climate on the natural abundances of rare elements ree in the first leg of the sedimentary cycle are evaluated using a study with holocene fluvia sands the medium grained sand fraction of samples collected from first order streams exclusively draining granitic plutons in montana semi arid georgia humid and south carolina humid are analyzed it is found that the ree distribution patterns but not the total absolute abundances of the daughter sands are very similar despite compositional differences between parent plutons averages of the three areas are determined to have a la lu ratio of about 103 showing a depletion of heavy ree with respect to an average granite la lu 79 or the composition of north american shales la lu 55 however the eu sm ratio in sands from these areas is about 0 22 which is very close to this ratio in north american shales 0 21 although the overall ree distribution of these sands is not similar to that of the north american shales in any way it is concluded that the major rock type but neither its minor subdivisions nor the climate controls the ree distribution patterns in first cycle daughter sands although the total and the parent rock normalized abundances of ree in sands from humid areas are much lower than those in sands from arid areas

upwelling of hydrothermal solutions through ridge flank sediments shown by pore water profiles high calcium ion and low magnesium ion concentrations in sediment pore waters in cores from the galapagos mounds hydrothermal field on the flank of the galapagos spreading center are believed to be due to a calcium magnesium exchange reaction between circulating seawater and basement basalt the nonlinearity of the calcium ion and magnesium ion gradients indicates that these discharging hydrothermal solutions on the ridge flank are upwelling at the rate of about 1 centimeter per year through the pelagic sediments of the mounds field and at about 20 centimeters per year through the hydrothermal mounds themselves

rare earth elements and hydrothermal ore formation processes abstract the geochemical behaviour of ree is influenced by nearly all important hydrothermal ore formation processes including fluid rock interactions fluid precipitations adsorption and scavenging onto particles and changes in fluid temperature pressure ph eh alkalinity and ligand concentration destabilization of ree complexes in response to these physicochemical changes and possible chemical crystallographic controls determine the concentration and distribution of ree within hydrothermal minerals mineraloids and amorphous phases alteration assemblages of intrusive related mineralisations may exhibit a wide range of ree distributions and ree fractionation trends the ree distribution within these hydrothermally altered lithologies will depend on the ree concentration in the rock and the fluid the partitioning behaviour of the ree between the rock phases and the fluid and the types of alteration reactions which take place reactive rocks such as carbonate rich lithologies of skarn deposits may gain or loose significant amounts of ree during fluid rock interactions ree geochemical investigations combined with fluid inclusion and stable isotope studies may point to the origin of alteration products and the source s of ore fluids ancient massive sulphide ores commonly exhibit anomalous eu concentrations similar to many recent submarine hydrothermal precipitates the recognition of hydrothermal fluid seawater sedimentary or volcanically derived ree distributions within ores chemical sediments and hydrothermally altered lithologies may help to constrain genetic modelling of massive sulphide deposits care should be taken with the interpretation of ree distributions from metamorphosed hydrothermal ore deposits becase metamorphic and diagenetic alteration reactions may change the ree distributions mobility of ree may occur during diagenesis of carbonate rich rocks and during the development of shear zones and migmatites in contrast very large fluid rock ratios are necessary to cause significant changes in ree patterns of silicate rich rocks during diagenesis and regional metamorphism