precambrian accretionary tectonics in the bou azzer el graara region anti atlas morocco ophiolites of the anti atlas of morocco occur as highly dismembered slivers of mafic and ultramafic rocks sandwiched between a precambrian craton and a magmatic arc tectonic slices of accretionary melange ophiolites and forearc basins have been juxtaposed by oblique subduction and transpression in the forearc region of a late proterozoic subduction zone synkinematic to postkinematic calc alkalic magmatism in these terranes indicates migration of the magmatic front over the deformed forearc terranes

silicification hydrothermal alteration in an archean geothermal system within the amulet rhyolite formation noranda quebec silicified andesitic flows divided into two alteration facies mottled epidote quartz altered andesite and silicified andesite silicification was contemporaneous with volcanism silicified andesites of the amulet rhyolite are products of extensive regional alteration and present a larger exploration target for massive sulfide deposits than does proximal chloritic and sericitic alteration beneath orebodies modified journal abstract

origin and timing of the metasomatic silicification of an early archean komatiite sequence barberton mountain land south africa abstract stratiform units of pervasively silicified early archean ultramafic rocks occur near the boundary between the onverwacht and fig tree groups in the barberton mountain land republic of south africa the origin of these units has been variously ascribed to early archean subaerial weathering or submarine alteration to cataclastic metamorphism and to the alteration of silicic tuffs an exceptionally well exposed example of such an alteration sequence along the skokohla river valley in the central barberton mountain land was studied to determine the nature of the precursor rocks and the timing and nature of the alteration processes well preserved ghosts of spinifex and culmulate textured olivines and pyroxenes establish the komatiitic ancestry of the skokohla rocks rock units of several distinctive lithofacies can be correlated over the entire 1 5 km strike of the sequence and were derived from the metasomatic conversion of a series of ultramafic flow units to rocks dominated by quartz dioctohedral mica and chlorite the original ultramafics have been strongly enriched in si k rb and ba and depleted in fe mg ca na mn sr zn and ni the elements al ti p cr zr y and nb were immobile alteration apparently occurred soon after extrusive activity and before any significant tectonic deformation it is most probable that alteration resulted from the dynamic hydrothermal upwelling of fluids through original fracture porosity in the ultramafic flows

formation of sulfide deposits and its relation to sodic and potassic alteration of proterozoic metabasites in the sax rift basin bergslagen sweden a number of cu zn fe pb as sulfide mineralizations spatially associated with metabasic sills dikes and flows of originally continental tholeiitic composition occur in the 1 9 1 8 ga old metavolcano sedimentary sequence in the sax sedimentary rift basin in higher stratigraphic levels weak sulfide mineralizations occur just below narich albite chlorite calcite quartz actinolite bearing metabasic flows in the lower stratigraphic levels sulfide mineralizations occur a spatially associated with potassiumrich microcline scapolite or biotite phlogopite rich metabasic sills and dikes and b below metabasic flows in which albite and calcite are replaced by microcline and scapolite the sodium and potassium rich metabasites are demonstrated to represent products of spilitization and potassic alteration respectively during spilitization fe cu co and hree are leached from the basic flows whereas during potassic alteration fe cu y and possibly zn and zr are leached from the basic rocks indicating a genetic relationship between the metabasites and the cu fe sulfide mineralizations spilitization probably took place due to interaction of the flows with descending sea water dominated fluids in a subseafloor hydrothermal system with temperatures roughly between 250 and 400 c potassic alteration probably took place by ascending fluids at higher temperatures than for spilitization

the europium anomalies in banded iron formations and the thermal history of the oceanic crust abstract the ree rare earth element patterns in banded iron formations bif s from various sites in north america two sites in finland and three drillcores of the hamersley basin in western australia have been investigated using icp ms inductively coupled plasma mass spectrometry and inaa instrumental neutron activation analysis the results of this study as well as existing data show trends in the eu anomalies bif s older than 2 5 ga exhibit positive eu anomalies while younger ones exhibit none in chemical sediments between 2 5 and 2 3 ga both trends can be found because of the worldwide distribution of bif s this feature can be considered as a global change in the conditions controlling ree mobilization consideration of the redox conditions of bif formation in comparison with the calculated and measured eu redox equilibria suggests that a eu anomaly points to a dominantly hydrothermal source of ree s with solution temperatures above 175 c we suggest that the disappearance of the eu anomaly towards the proterozoic marks a change from mainly high temperature hydrothermal to dominantly low temperature alteration

geochemistry of upper proterozoic rift related volcanics northern utah and southeastern idaho mafic volcanic rocks associated with upper proterozoic diamictites are widespread in western north america and a continental rift setting has been proposed for their origin we report trace element and rare earth element geochemistry for these volcanics from northern utah and southeastern idaho the analyzed samples are high ti within plate basalts and they vary from transitional tholelitic alkaline in the south to alkaline in the north the geochemistry together with the geologic setting of the volcanics indicates a rift setting but the timing of this rifting and its relationship to final continental separation are still controversial

iron and carbon isotope evidence for microbial iron respiration throughout the archean abstract banded iron formations bifs are voluminous chemical sediments that are rich in iron oxide carbonate and silica and whose occurrence is unique to the precambrian their preservation in the geological record offers insights to the surface chemical and biological cycling of iron and carbon on early earth however many details regarding the role of microbial activity in bif deposition and diagenesis are unresolved laboratory studies have shown that reaction between carbon and iron through microbial iron respiration 2fe2o3 nh2o ch2o 7h 4fe2 hco3 2n 4 h2o chemical energy can impart fractionation to the isotopic compositions of these elements here we report iron 56fe vs irmm 014 and carbon isotopic 13c vs v pdb compositions of magnetite and of iron rich and iron poor carbonates in bifs from the late archean 2 5 ga hamersley basin australia and the early archean 3 8 ga isua supracrustal belt isb greenland the range of 56fe values measured in the hamersley basin including light values in magnetite and heavy values in iron rich carbonates up to 1 2 are incompatible with their precipitation in equilibrium with seawater rather the data together with previously reported light 13c values in iron rich carbonates record evidence for diagenetic reduction of ferric oxide precursors to magnetite and carbonate through microbial iron respiration i e dissimilatory iron reduction dir iron and carbon isotope data of iron rich metacarbonates from the isb are similar to those of late archean bifs the isotopic signatures of these metacarbonates are supportive of an early diagenetic origin despite metasomatic overprint and preserve evidence of microbial iron respiration within the oldest recognized sedimentary rocks on earth

potassic and sodic metasomatism in the southern province of the canadian shield evidence from the paleoproterozoic serpent formation huronian supergroup canada abstract major element geochemistry petrographic observations and microprobe analyses of the paleoproterozoic serpent formation huronian supergroup canada suggest that these strata experienced both potassium and sodium metasomatism major element data plotted in al2o3 cao na2o k2o compositional space form an array nearly identical to data from sub huronian paleosols reported elsewhere suggesting that serpent fluvial systems captured materials from entire weathering profiles potassium metasomatism was concentrated in shales and represents the illitization of kaolinite which is a residual product of plagioclase weathering sodium metasomatism principally affected silt and sand sized particles microprobe analyses of plagioclase grains indicate that they are nearly pure albite ab96 corresponding whole rock major element data plot in bulk albite space on an al2o3 k2o cao na2o ternary diagram whereas basement rocks plot in bulk andesine and oligoclase space end member compositions and the absence of luminescence when bombarded with an electron beam indicate that the albites are of secondary origin extremely pure albite cements ab99 commonly fringe detrital grains there has been little replacement of detrital k feldspars despite the complete albitization of plagioclase grains the timing of metasomatism at least the last stage is constrained to the period between 1700 and 1750 ma based on geochronologic data reported elsewhere when the southern province of the canadian shield experienced considerable felsic plutonism possibly related to arc collision this plutonism may have resulted in the heating of deep basinal brines stresses associated with orogenic build up or gravity driven groundwater circulation working in concert with the plutonism expelled these brines through the serpent formation and the entire huronian supergroup thoroughly metasomatizing the region such processes associated with collisional tectonics are akin to the development of mississippi valley type pb zn mineralization in the mid continent of the united states

cherts of the barberton greenstone belt interpreted as products of submarine exhalative activity cherts studied from the southern barberton greenstone belt are identified as replacement products of various pillow lavas pyroclastic deposits and sedimentary deposits silica filled fracture systems within the cherts have been found to be associated with this silicification process these systems include 1 silica filled fractures 2 silica vein stockworks 3 silica cemented breccias and 4 pseudoconglomerate bodies some of the silica filled fracture systems can be shown to have formed soon after sedimentation silica vein stockworks can be seen to develop laterally into silica cemented breccias the fragments of which have sometimes become rounded within the fracture systems to form pseudoconglomerate the cherts are interpreted as silicified hemi pelagic sediments that overlie pillow lavas some of which are now also cherts cells of convective sea water interacted with oceanic crust to produce silica rich fluids these fluids silicified lava and sediments on re entering the near surface env

subaqueous calderas in the archean abitibi greenstone belt an overview and new ideas abstract the 300 700 km abitibi greenstone belt in canada contains numerous world class archean volcanogenic massive sulfide vms deposits yet documentation of archean subaqueous calderas hosting such deposits is lacking the modern sunrise deposit in the myojin knoll caldera of the izu bonin arc shows that submarine calderas are first order sites for vms the hunter mine and normetal calderas as well as the blake river megacaldera complex are reviewed with respect to geometry regional geology physical volcanology dyke emplacement and hydrothermal carbonate alteration these subaqueous calderas are placed into the geodynamic context of the abitibi greenstone belt the abitibi belt displays a complex history of oceanic arc formation evolution collision and fragmentation with numerous types of volcanic edifices and sedimentary basins controlled by oblique subduction and mantle plumes analogue models yielding new insights into the sequence of caldera forming events and their synvolcanic fault patterns are applied to explain archean examples underpressure experiments develop a consistent sequence of fault patterns that compare favourably with natural caldera events during subsidence two sets of faults propagate from the magma chamber margin 1 early reverse outward dipping faults and 2 normal inward dipping faults the largest massive sulfide deposits form preferentially at the caldera margin based on experimental studies and detailed facies mapping the hunter mine caldera is of the segmented piecemeal variety and the normetal caldera is akin to a piston structure the blake river megacaldera contains 1 the e w trending misema 2 the nw trending new senator and 3 the ne trending noranda calderas with the latter two being nested graben type calderas consistent with oblique archean subduction the poorly documented hydrothermal carbonate alteration is discussed and a new exploration model for calderas is presented the alteration study was evaluated at three levels microprobe analysis geochemical staining of drill core and whole rock analyses there were two principal stages of alteration related to vms formation an early silicification phase developed either at depth and or at the seawater interface whereby silicification was bedding parallel in fine grained volcaniclastic deposits near the water surface a pervasive and extensive semi conformable carbonate alteration zone with a discordant focused root zone along faults evolved subsequently three distinct carbonate pairings were observed 1 proximal siderite sideroplesite fe ankerite next to the vms deposit 2 an intermediate ankerite fe dolomite zone and 3 a distal calcite dolomite zone the carbonate phase is associated with the mineralizing event transitions between carbonate species are subtle and changes are indicated by mineral assemblage overlap the silica carbonate hydrothermal alteration pattern is a replacement product and is far more widespread than has been documented this alteration can be easily confused with primary carbonates and chemical precipitation deposits e g banded iron formation if detailed volcano sedimentary facies mapping with alteration has not been conducted and if outcrop is insufficient

petrology and geochemistry of the neoproterozoic siroua granitoids central anti atlas morocco evolution from subduction related to within plate magmatism the siroua massif includes many plutons of neoproterozoic age the mineralogical and geochemical character of the plutons allows us to describe an evolution of the magmatism in space and time from a subduction related type in the northern part to a within plate subalkaline type in the southern part the first magmatic activity coeval with the closing of the khzama oceanic basin in the north is little evolved and of oceanic type dominantly gabbros and basalts it is followed by a low potassic calc alkaline magmatism gabbro diorites tonalites and trondhjemites of nebdas pluton and by a voluminous highly potassic calcalkaline magmatism askaoun and ifouachguel plutons that marks the collisional period the end of crustal uplift and the beginning of the extension is marked in the south by a sub alkaline magmatism corresponding to the ida ou illoun imdghar and affela n ouassif granites magmatic activity in the siroua massif is marked at the end of the neoproterozoic piii by a continental tholeiite with an alkaline affinity which occurs as dykes crosscutting the neoproterozoic granites and later by dominantly alkaline granites

development of a mixed seawater hydrothermal fluid geochemical signature during alteration of volcanic rocks in the archean 2 7 ga abitibi greenstone belt canada abstract we investigated a group of silicified volcanic rocks from the 2 72 ga hunter mine group hmg abitibi greenstone belt canada in order to document progressive compositional change associated with alteration in a subaqueous caldera system rocks of the hmg divide into three groups based on mineralogy and texture for petrographic and geochemical analyses volcanic features phenocrysts pseudomorphs after primary glass shards lapilli volcanic clasts are preserved in all groups despite changing mineralogy from primarily quartz feldspar chlorite groups 1 and 2 to quartz hematite and carbonate groups 2 and 3 compositionally group 1 rocks resemble volcanic rocks in the region while group 2 and 3 rocks show a change in mineralogy to iron silica and carbonate minerals which is associated with depletion of many major and trace elements associated with volcanic rocks al2o3 na2o k2o zr in addition rare earth elements display a clear progression from volcanic signatures in group 1 prsn ybsn 1 7 2 96 eusn eusn 0 84 1 72 y ho 25 20 27 41 lasn lasn 0 97 1 29 and zr hf 38 38 42 09 to transitional mixed volcanic hydrothermal and seawater signatures in group 2 prsn ybsn 1 33 2 89 eusn eusn 1 33 2 5 y ho 23 94 30 lasn lasn 0 93 1 34 and zr hf 40 70 to mixed hydrothermal and seawater signatures in group 3 prsn ybsn 0 62 2 88 eusn eusn 1 30 7 15 lasn lasn 1 02 1 86 y ho 25 56 55 and zr hf 35 50 we interpret that silicification of volcanic rocks group 1 produced transitional altered volcanic rocks group 2 and siliceous and jaspilitic rocks group 3 based on preservation of delicate volcanic features building on this explanation we interpret that major trace and rare earth element mobility occurred during the process of silicification during which siliceous and jaspilitic rocks group 3 acquired aspects of the rare earth element geochemical signatures of marine chemical precipitates we conclude that seafloor silicification in hydrothermal depositional settings is capable of producing rocks that resemble marine chemical precipitates such as banded iron formation and could be a process that is widespread in the archean consequently because silicified volcanic rocks from the hmg possess mixed seawater and hydrothermal rare earth element characteristics similar to archean iron formations and cherts we suggest caution must be exercised when interpreting the geochemical information preserved in metamorphosed rocks where original genesis is unknown

structural and geochronological constraints on the evolution of the bou azzer neoproterozoic ophiolite anti atlas morocco abstract the bou azzer ophiolite in the moroccan anti atlas represents the suture of the neoproterozoic pan african orogen the bou azzer inlier includes from ne to sw three main geological units a volcanic arc an ophiolite and a continental platform several subduction related plutons intrude the ophiolite and the volcanic arc and syn to late orogenic deposits unconformably cover the three terranes the eburnean 2 ga continental basement observed in other inliers of the anti atlas appears not to crop out at bou azzer where rocks traditionally attributed to that old basement are in fact neoproterozoic rocks either metagranitoids of the continental margin or ophiolitic metagabbros the age of a metagabbro of the ophiolite is reported here for the first time as 697 8 ma shrimp u pb on zircons u pb geochronological data are also presented for two syn kinematic subduction related plutons which together with previous data establish a time span of ca 655 635 ma for these arc type rocks moreover the oumlill granite previously attributed to the eburnean basement has been dated as 741 9 ma the earliest tectonometamorphic event affecting the ophiolite is recorded as a relic foliation and a garnet rutile metamorphic assemblage which might attest to an early peak pressure exceeding the 5 6 kbar medium pressure reported so far thus we suggest a first stage of ophiolite underthrusting below a volcanic arc followed by ophiolite exhumation and the concomitant development of the main low grade planar linear tectonic fabric the ages of the calc alkaline syn kinematic subduction related plutons 655 635 ma might be taken as the end of subduction and the beginning of the ophiolite exhumation obduction the author s work in the bou azzer region supports the ophiolite continental platform interpretation of leblanc 1975 and leblanc and moussine pouchkine 1994 rather than the tectonic melange interpretation proposed by saquaque et al 1989b and others a recumbent anticlinal and the basal thrust of the ophiolitic terrane are the main macrostructures formed during the obduction stage characterized by top to the w sw kinematics this deformational event is also recorded in the rocks of the underlying continental platform a syn orogenic unit namely the tiddiline formation unconformably overlies the arc volcanics the ophiolite and the continental platform further collisional deformation affected the tiddiline sediments giving way to ese wsw striking upright folds and oblique reverse faults the latest structures in this neoproterozoic suture were left lateral strike slip faults pointing to an increase in the left lateral component of the continental collision

archean terrane docking upper crust collision tectonics abitibi greenstone belt quebec canada abstract the northern nvz and southern volcanic zones svz of the abitibi greenstone belt are separated by the major e trending destor porcupine manneville fault zone dpmfz the dpmfz is interpreted to be the locus of archean terrane docking between the older diffuse volcanic arc of the nvz 2730 2710 ma and the younger arc segments of the svz 2705 2698 ma two distinct evolutionary phases can be documented along the dpmfz of the abitibi greenstone belt and include 1 arc arc collision occurring between 2697 and 2690 ma and 2 arc fragmentation between 2689 and 2680 ma identification of these two events along the dpmfz is based on detailed structural studies sedimentary basin analysis and precise u pb age determinations the thrusting event representative of the arc arc collision phase is characterized by shallow north dipping foliations 20 40 and dip parallel stretching lineations in the eastern manneville segment of the dpmfz local overturned mafic pillowed units suggest recumbent folding late strike slip or transcurrent movement displayed in the late orogenic sedimentary duparquet basin records the arc fragmentation phase basin geometry e trending en echelon folds shallow e plunging stretching lineations and a late ne striking cleavage cross cutting the folds support a dextral shear sense along the western destor porcupine segment of the dpmfz the sedimentary facies observed in the basin are consistent with those of modern strike slip basins located along the east anatolian fault turkey hazar lake and the hope fault new zealand hanmer basin precise u pb zircon age determinations from porphyry stocks located at the northern and southern limits of the duparquet basin yielded 2681 1 ma and 2689 3 2 2 9 ma respectively these ages constrain the rapid change from thrusting to transcurrent movement it is apparent that once thrusting ceased the response to oblique subduction continued in the form of strike slip displacement modern fold and thrust belts commonly show this evolution the deformation pattern is the result of oblique convergence the abitibi greenstone belt is considered to be an archean analogue of modern subducting oceanic plates such as those found in the western pacific

experimental investigation of flow past submerged vanes in this paper the physics of the flow past a submerged vane at high angles of attack is presented the data presented were collected during an experimental study of flow past vanes at various angles of attacks in a deformable bed straight rectangular channel the first few experimental runs were undertaken to establish the optimal angle of attack required to generate the strongest secondary circulation in the flow at this optimal angle of attack the flow features responsible for the transverse movement of bed sediment are clearly identified the differences between the flow features observed near the vanes at both low and high angles of attack are also outlined these include among others two counterrotating vortices for vanes at high angles of attack as opposed to the widely acclaimed hypothesis of a single vortex for low angles of attack

apatite in early archean isua supracrustal rocks southern west greenland its origin association with graphite and potential as a biomarker abstract rare earth element ree abundances in individual apatite crystals in banded iron formations bifs metacherts metacarbonates and mafic dykes in the isua supracrustal belt isb have been determined by laser ablation inductively coupled plasma mass spectrometry the results together with petrographic observations on the distribution of graphite have been used to track the origin of the different compositional types of apatite and to evaluate the potential proposed in earlier studies for use of the apatite graphite association as a biomarker the chondrite normalized distribution patterns of apatite in metasedimentary bifs and metacherts fall into three groups relatively flat profiles with distinct positive eu anomaly are interpreted as characterizing sedimentary diagenetic apatite that carry the ree signature of the archean ocean secondary apatite in isua metasdiments with either middle ree enriched profiles or with light ree depleted profiles is interpreted to have crystallized from percolating carbonate rich metasomatic fluids or from fluids derived from cross cutting mafic dykes respectively the occurrence together of these different genetic types of apatite with distinct ree signatures within cm scale samples shows the immobility of ree in preexisting apatite during metamorphic episodes apatite crystals in isua rocks of uncontested chemical sedimentary origin bif and metachert samples do not have graphite inclusions or coatings graphite inclusions and coatings on the other hand characterize apatite in secondary metacarbonate rocks in these rocks graphite is produced by thermal metamorphic reduction of carbonate ion derived from dissociation of the metasomatic ferrous carbonate where iron serves as electron donor oxidizing to form magnetite in view of the non sedimentary metasomatic origin of isua metacarbonates and the abiogenic source of graphite the apatite graphite assemblage can not be considered as a biomarker and does not provide information on early archean life in the isb

pre 3750 ma supracrustal rocks from the nuvvuagittuq supracrustal belt northern quebec geochemistry and u pb ion microprobe zircon geochronology guided by high resolution mapping 1 50 scale was used to define a minimum age of ca 3750 ma for supracrustal rocks from the nuvvuagittuq supracrustal belt nsb in the northern superior province quebec canada mineralogy and geochemistry of critical field relationships preserved at the porpoise cove locality describe a supracrustal succession of mafic amphibolites and ultramafic rocks finely banded quartz magnetite units with intermixed coarse grained ferruginous quartz pyroxene rocks and quartz biotite schists that superficially resemble polymict metaconglomerates with large up to 10 cm deformed polycrystalline quartz and mafic clasts all units in the mapped outcrop have sharp lithological contacts narrow dominantly trondhjemitic orthogneiss sheets locally preserve intrusive contact relationships to the supracrustals the total extent of the supracrustal enclave is 8k m 2 it is strongly deformed and the full deformation history appears to be shared by all units with later modifications from leucogranitoid intrusions the quartz biotite schists record complex zircon growth at 3500 and 2800 ma interpreted to reflect metamorphic events zircons separated from orthogneisses in the enclave including one sheet that transects a banded quartz pyroxene magnetite unit yield magmatic 207 pb 206 pb ages of ca 3750 ma these ages are slightly younger than earlier provisional reports for an nsb orthogneiss from porpoise cove the nuvvuagittuq supracrustals are the oldest rocks thus far reported for the minto block they overlap in age with the ca 3 70 3 83 ga isua supracrustal belt and akilia association rocks in west greenland and they represent an important new area for exploration of the early earth

geochemistry of precambrian ophiolites from bou azzer morocco the upper proterozoic ophiolite complex of bou azzer morocco includes ultramafic rocks cumulate gabbros sheeted dykes pillow lavas and diorite quartz diorite intrusions and an overlying volcano sedimentary sequence the gabbroic cumulates basaltic flows and dykes have compositions similar to recent ocean floor rocks n and or t type among other features they have comparable light ree depleted patterns and relations of ti zr and la nb although fractional crystallization played an important role in the evolution of these rocks the large variations in their chemical compositions require generation from a heterogeneous upper mantle source and or by a dynamic partial melting process diorites quartz diorites and the volcanic rocks of the overlying sequence are calc alkaline genetically unrelated to the tholeiitic suite and indicative of an island arc setting a possible tectonic model for the ophiolite complex is a marginal basin just behind a still active island arc

hercynian age of the cobalt nickel arsenide gold ores bou azzer anti atlas morocco re os sm nd and u pb age determinations the age of the hydrothermal co ni as au mineralization in the bou azzer el graara inlier was determined using different isotopic systems on ore and mineralization related gangue minerals namely re os on molybdenite sm nd on carbonates and u pb on brannerite re os dating of molybdenite was difficult owing to the intimate intergrowth of molybdenite with co arsenides and the fact that sampling by drilling complex sulfide mixtures resulted in poor concentration of molybdenite accordingly re os dating of the impure molybdenite sulfide mixtures yielded inconsistent ages in the range 400 to 350 ma this scatter is attributed to 1 inadequate concentration of pure molybdenite 2 complex re os systematics possibly involving excess radiogenic os derived from older generation s of sulfide and or 3 mixture of at least two re os systems consisting of a component with pre hercynian history and later disturbance by hercynian metamorphism carbonates and brannerite that coexist with molybdenite gave more consistent ages of 308 31 ma sm nd and 310 5 ma u pb respectively because both ages agree within their errors we regard the brannerite u pb age of 310 5 2 ma as the best and most precise estimate for the age of the co ni as au mineralization although an earlier onset of mineralization cannot be ignored totally the new age data underline that the principal co ni as au mineralization at bou azzer occupying the main ore bearing structures was driven by and formed during the end of the hercynian orogeny

chert in the iberian pyrite belt since lenses of chert are common within the volcano sedimentary succession hosting the massive sulphide deposits of the iberian pyrite belt spain and portugal we examined numerous chert occurrences both petrographically and geochemically to test their possible value for massive sulphide exploration the chert is found at two main lithostratigraphic levels upper and lower that are also interpreted as massive sulphide bearing in both cases the chert is located at the top of acidic volcanic sequences or in the associated sediments we have not been able to observe the relationships between massive sulphides and chert but some of the large orebodies of the province lousal la zarza tharsis planes san antonio body of rio tinto neves are described as being locally capped by chert facies four main types are recognized among the chert and associated facies 1 red hematitic chert magnetite 2 radiolarian and or sedimentary textured conglomeratic chert with hematite and or mn oxides 3 pale sulphidic chert 4 rhodonite and or mn carbonate magnetite facies in the spanish part of the province the radiolarian chert is confined to the upper level the distribution of the other types appears to be haphazard the hydrothermal origin of the south iberian chert is shown by its high fe mn and low co ni cu contents the presence of small positive ce anomalies indicates a shallow marine environment shelf or epicontinental sea which is consistent with the volcanological and sedimentological data the chert was emplaced below the sea floor through chemical precipitation and or through alteration and replacement of the country rock residual traces of which are ghost phenocrysts and high al ti and rare earth contents macro and microscopic relationships indicate that the oxide facies hematite magnetite formed first probably providing a protective insulating cover against the marine environment and enabling an evolution towards sulphide facies a phase of mn carbonate and silicate quartz chlorite sulphides appears to be even later it was not possible through discrimination to isolate a chert that could be considered as representing a lateral marker of massive sulphides moreover both field observations and geochemical data seem to indicate a relative independence of this siliceous sulphide hydrothermal activity from the hydrothermal activity giving rise to the massive sulphides such is also indicated by the lead isotopic signature of the chert which is appreciably more radiogenic than that of the massive sulphides the lead enrichment in the sulphidic chert facies indicates the participation of a different source sediments sea water from that of the massive sulphides the hypothesis of an independent hydrothermal chert event can thus be envisaged wherein the chert reflects submarine low temperature hydrothermal activity that is most apparent during a break within the volcano sedimentary succession and which may locally have competed with the high temperature hydrothermal activity giving rise to the massive sulphides the interest of the chert thus rests in its palaeodynamic significance as a marker of periods of volcanic quiescence and in its possible role as a protective insulating cap favourable to the deposition of massive sulphides

an electron microprobe study of detrital feldspar microprobe analysis of detrital alkali feldspar and plagioclase is useful in provenance studies detrital grains of alkali feldspar more sodic than 50 weight percent albite molecule are derived almost exclusively from volcanic sources alkali feldspar grains more potassic than 87 percent orthoclase molecule are derived from metamorphic and plutonic rocks for the more sodic varieties of detrital plagioclase it is possible to distinguish on the basis of potassium content between volcanic and non volcanic sources non volcanic albite and volcanic and non volcanic oligoclase and andesine are abundant forms of detrital plagioclase in tertiary sandstones from the southern colorado plateau area labradorite and bytownite are present as detrital grains in some deposits from this region most of the detrital alkali feldspar in these rocks is highly potassic containing more than 90 percent or molecule intermediate alkali feldspar is common in sandstones of volcanic and mixed provenance detrital plagioclase from several sandstones of mixed provenance contains between 0 to 0 7 percent fe and 0 to 0 5 percent sr alkali feldspar from the same samples contains between 0 and 0 5 percent fe and 0 and 1 5 percent ba detrital plagioclase interpreted as volcanic in origin generally contains more fe and sr than non volcanic plagioclase other approaches to the study of detrital feldspar with the microprobe are useful in provenance analysis chemical zoning of plagioclase established previously as a provenance tool can be studied more accurately and in more detail with a microprobe than with a petrographic microscope also sand sized volcanic lithic fragments can be classified as mafic intermediate or silicic in composition by analyzing their feldspar microphenocrysts

the subsea floor formation of volcanic hosted massive sulfide evidence from the ansil deposit rouyn noranda canada the ansil cu zn volcanic hosted massive sulfide deposit is located approximately 14 km north northwest of rouyn noranda quebec within the blake river group of the archean southern abitibi greenstone belt the deposit is the second lowest within the central mine sequence a series of bimodal rhyolite andesite formations that fill in the noranda cauldron host to 17 volcanic hosted massive sulfide deposits the deposit consists of a single massive sulfide lens situated at the contact between rhyolite of the northwest formation and overlying andesite of the rusty ridge formation the rhyolite flow forms a ridge up to 500 m high which was inundated by the andesite flows the orebody is hosted within a unit of quartz porphyritic finely layered felsic volcaniclastite cranston tuff that fills in an east west trending graben formed on the upper surface of the rhyolite flow at the east end of the graben the cranston tuff is interlayered with a thin unit of dacite flows the deposit lies within an extensive discordant alteration zone characterized by feldspar destruction and na depletion the zone extends from 400 m below the deposit at the contact of the synvolcanic flavrian intrusive complex to over 400 m above the deposit this alteration zone is controlled by a fault system that served as a focus for hydrothermal fluid upflow from the beginning of cauldron formation until the end of the first cauldron cycle within the na depletion zone there are three distinct alteration facies associated with the orebody early hydrothermal activity is defined by hydrothermal explosion breccias that formed along the base of the east west trending graben walls they were filled with finely banded quartz albite sphalerite pyrite and the wall rock altered to sericite quartz with finely disseminated sphalerite and pyrite pyrrhotite the graben was then filled with the cranston tuff which was selectively altered and mineralized by the zn rich fluid subsequent development of northerly trending faults was accompanied by the upflow of cu rich fluid that selectively chloritized the footwall rhyolite below the graben floor and formed a

rare earth element systematics of the chemically precipitated component in early precambrian iron formations and the evolution of the terrestrial atmosphere hydrosphere lithosphere system irrespective of provenance age and metamorphic grade the chemically precipitated component in early precambrian 2 3 ga iron formations ifs displays smyb cn 1 which reflects the corresponding ratios of contemporaneous seawater in conjunction with end if end shale jacobsen and pimentel klose 1988a b this rare earth element ree signature reveals that the ree distribution in early precambrian ifs must be explained by mixing between a marine bottom and a surface water component and that the rees and by analogy the fe cannot be derived from weathering of a continental source eusm n ratios of detritus free ifs are controlled by the marine bottom water component where eusm cn 1 results from ree input from high temperature high t hydrothermal fluids which received their ree signature during high t alteration of oceanic crust at spreading centres decreasing eusm cn of ifs with decreasing depositional age reflect the vaning importance of high t compared to low t 200 c hydrothermal fluids hence oceanic crust alteration for the ree budget of marine bottom waters and are related to decreasing upper mantle temperatures smyb n ratios of detritus free ifs are controlled by the marine surface water component whose ree distribution is affected by the same processes which operate today in the entire ocean mixing calculations reveal that smyb cn in early precambrian marine surface waters was significantly lower than it is today to explain this difference two mechanisms are discussed on the basis of higher pco2 and lower po2 levels of the precambrian atmosphere 1 1 higher co32 in precambrian river and coastal waters and 2 2 redox cycling of the rees at an oceanwide chemocline which separated anoxic and oxic water masses however these processes are synergetic and could have operated in unison the ree distribution in precambrian ifs is described as a result of mixing in a multicomponent system where high t and low t hydrothermal fluids contributed to the marine bottom water and ree input from the dissolved ree pool in river waters after some modification in estuaries dominated the ree distribution in marine surface waters after mixing of bottom and surface waters their combined ree load was quantitatively scavenged onto precipitating iron oxihydroxides or iron carbonates and deposited with the chemically precipitated component after addition of variable amounts of clastic detritus this eventually resulted in the ree distribution observed in precambrian ifs

silica alteration zones in the barberton greenstone belt a window into subseafloor processes 3 5 3 3 ga ago abstract silicification of volcanic rocks ranging from komatiitic to dacitic in composition is a common phenomenon of the 3 5 to 3 2 ga old volcano sedimentary succession of the barberton greenstone belt silica enrichment occurs in zones several tens of metres thick at the top of volcanic sequences that are capped by sedimentary chert horizons sio2 contents increase upsection from the original igneous value up to 92 silicification is associated with a depletion of most elements commonly mobile during water rock interaction while k2o rb and ba are enriched in these zones some transition metals sc v and cr were immobile whereas ni and co and some other metals cu zn were mobile and are depleted the 18o values of volcanic rocks show a positive linear relationship with silica content and range from 9 0 to 17 3 the element depletion enrichment patterns and oxygen isotope data indicate low temperature c 100 150 c hydrothermal processes for the origin of the alteration zones the loss of most elements coupled with the addition of sio2 as well as ree systematics indicate high water rock ratios and a fluid ree composition typical of archaean seawater that had a 18o value of 0 hydrothermal activity is attributed to heat derived from cooling of volcanic rocks and a high regional heat flow in an oceanic plateau like setting and resulted in the establishment of shallow subseafloor convection cells diffuse venting of hydrothermal fluids over broad areas of the ancient ocean floor provided an ideal environment for both seafloor and subsurface biota 3 5 ga ago

native silver mineralization along a rodingite tectonic contact between serpentinite and quartz diorite bou azzer morocco the newly discovered silver mineralization of bou azzer anti atlas morocco is located along a major quartz diorite serpentinite tectonic contact in an upper proterozoic ophiolite complex 780 m y from the quartz diorite to the serpentinite the following succession is observed massive and retromorphosed quartz diorite cataclastic quartz diorite and k na metasomatized cataclasite rodingite chloritite serpentinite rodingite resulted from mg ca mn metasomatic processes related to serpentinization the rodingite mineral assemblage grossularite salite piemontite prehnite overprinted the cataclastic quartz diorite releasing k na rich fluids in a temperature range 350 degrees 200 degrees c and under pressure conditions 2 kbars which are in agreement with pan african greenschist metamorphism 685 m y the bou azzer silver mineralization is generally restricted to the k na metasomatic zone along the rodingite front in the form of a network of very thin ore veinlets comprising a native silver and co fe arsenides assemblage with accessory sulfosalts and copper sulfides in a silicatecalcite gangue the co fe arsenides crystallized with the earlier silicates salite piemontite prehnite in rodingite k feldspar and albite in the k na metasomatic zone chlorite in cataclastic quartz diorite whereas the native silver is associated with late gangue minerals prehnite and k feldspar in rodingite calcite and quartz in the k na zone and in cataclastic quartz diorite the investigated silver mineralization is clearly connected to the metasomatic processes related to serpentinization rodingitization and resulting k na metasomatism a serpentinite source rock is suggested for this silver mineralization

the relation between iron formation and low temperature hydrothermal alteration in an archean volcanic environment abstract the uppermost section of the hunter mine group hmg 2728 ma a bimodal volcanic complex in the abitibi greenstone belt contains both oxide and carbonate facies banded iron formation bif this paper explores the relationship between volcanic activity and the development of the two types of iron formation the oxide facies represented by chert jasper magnetite iron formation is widespread and recurs at several stratigraphic levels field evidence suggests it deposited directly on the sea floor during periods of volcanic quiescence and is most probably derived from fluids seeping from volcaniclastic sediments and the compaction of felsic shard rich tuffs seeping occurred along synsedimentary and synvolcanic faults and then along bedding planes carbonate siderite facies iron formation in contrast formed locally below a silica cap rock by in situ low temperature hydrothermal replacement of chert tuff beds and even of the oxide formation thus the two types of iron formation although spatially and stratigraphically juxtaposed are not the result of the same process and only the carbonate facies appears to be directly related to a low temperature hydrothermal volcanic process the contrasting genesis of oxide and carbonate iron formations indicates that current simplistic models relating banded iron formation to volcanic massive sulphide deposits need to be re evaluated the notion that oxide iron formation represents a distal manifestation of volcano generated hydrothermal activity may be the exception and not the rule because as indicated by this study bif occurs in the central part of a volcanic edifice rather than on the flanks

neoproterozoic hydrothermally altered basaltic rocks from rajasthan northwest india implications for late precambrian tectonic evolution of the aravalli craton abstract the isolated volcano sedimentary sequences of the punagarh and sindreth groups occur along the western flank of the delhi fold belt in northwest india and include mafic rocks pillow basalts and dolerite dykes that are dominantly olivine tholeiites with minor quartz normative and alkali basalts sindreth samples appear to have higher primary tio 2 and p 2 o 5 abundances relative to those from punagarh both suites of mafic rocks show variable but profound hydrothermal alteration effects with loss on ignition loi values up to 10 3 wt and extensive secondary minerals including albite sericite chlorite and calcite despite this there is excellent preservation of magmatic textures but there has been extensive albitization of plagioclase phenocrysts a hallmark of hydrothermal alteration processes in oceanic crust supporting evidence for such hydrothermal alteration comes from correlations of loi abundances with cao na 2 o and evidence for u mobility is apparent on diagrams of nb th vs nb u felsic volcanic rocks rhyolite dacite interlayered with the sindreth basalts yield u pb zircon ages tims method between 761 16 and 767 3 ma which we interpret as representing the time of primary magmatic activity we infer that the volcano sedimentary rocks of the punagarh also formed at this time on the basis of similarities in lithology stratigraphy field relations and geochemistry intermediate granitoid rocks yield older u pb ages of 800 2 and 873 3 ma which we correlate with the post delhi supergroup erinpura granites taken together the features of the punagarh and sindreth groups are consistent with their formation in a back arc basin setting their coevality with other magmatic systems in nw india malani igneous suite the seychelles and madagascar for which a continental arc setting has also been proposed supports the notion of an extensive convergent margin in western rodinia at 750 770 ma

deep sea fan deposits in the macigno formation middle upper oligocene of the gordana valley northern apennines italy abstract facies characteristics are described for a 2300 m thick turbidite succession of the macigno formation middle upper oligocene which crops out as a continuously exposed section in the gordana valley northern apennines in the study area the macigno formation consists of two sandy members separated by a shaly member the sandy members contain lithic associations characteristic of submarine fan deposits and form distinct fan complexes the middle shaly member represents a period of basin starvation three main facies associations are recognized in the macigno fan deposits fan fringe outer fan lobe and braided midfan facies associations the fan fringe deposits consist of monotonous or small scale cyclically arranged thin bedded turbidites the outer fan lobe deposits are composed of medium to very thick bedded classical turbidites internally organized in thickening upward cycles with minor intercalations of thin bedded sandstone beds two distinct types of outer fan lobe deposits distal and proximal are recognized on the basis of rock type internal organization and vertical facies relationships the midfan deposits consist of amalgamated massive sandstone and pebbly sandstone which display repeated thinning upward cycles the three main facies associations are superposed t define major thickening and coarsening upward cycles reflecting distinct episodes of fan progradation the relationships between the facies associations of the macigno deposits and the existing models of submarine fan sedimentation are discussed after the deposition of the fan complexes fan sedimentation in this part of the basin was replaced by movements of allochthonous units heralded by large olistostromes

zoned and recurrent deposition of na mg fe si exhalites and cu fe sulfides along synsedimentary faults bleida morocco in the upper proterozoic marginal basin of bleida morocco several horizons of copper mineralization are present in the main orebody a hydrothermal synsedimentary system has been reconstructed with evidence from thickness and facies variations it includes two sulfide rich basins on both sides of a half horst along a synsedimentary normal fault trending orthogonally to the continental margin three successions of ferruginous jasper sulfide bearing felsic tuffs and turbidites occur in adjacent basins whereas only chloritite and albitite beds are present on the axial half horst toward the fault zone the ferruginous jasper becomes apatite and chlorite rich then grades laterally to a chlorite and k feldspar rich albitite whereas in the same direction sulfide bearing layers show a mineral zoning pyrite chalcopyrite bornite then grade to a bornite and chlorite rich albitite chlorite is more magnesian toward the axial half horst it crystallized around 250 degrees c the axial zone is characterized by high b ba and zr contents zircon crystals and tourmaline needles are present toward the axial zone there is also a strong ree enrichment up to a hundred times and a marked light ree fractionation the ree patterns correspond to hydrothermal fluids linked to felsic volcanism the synsedimentary faults channeled first k na mg rich fluids which deposited chemical sediments on the sea floor then cu fe bearing fluids migrated through the more permeable laminations of the overlying felsic tuffs resulting in syndiagenetic crystallization of sulfides the lateral mineral zoning results mostly from decreasing temperature and increasing f sub o 2 away from the axial zone these long lived hydrothermal flows were muffled by the deposition of turbidite material although recurrent volcanic and hydrothermal activity occurred during stages of stagnating turbidite sedimentation these complex relationships between synsedimentary faulting felsic volcanism turbidite deposition and fluid expulsion from compressed sediments were controlled by the rapid and episodic subsidence of the marginal basin of bleida

precambrian superplumes and supercontinents a record in black shales carbon isotopes and paleoclimates prominent maxima in black shale abundance and in black shale total shale ratio occur at 2 0 1 7 ga with less prominent peaks in the late neoproterozoic 800 600 ma and in the late archean 2 7 2 5 ga peaks in chemical index of alteration cia of shales at the same times suggest corresponding warm paleoclimates the peaks in cia and black shale abundance are correlated in time at a 94 confidence level the black shale and cia peaks may reflect the combined effects of mantle superplume events and supercontinent formation at 2 7 and 1 9 ga mantle superplume events may have introduced large amounts of co2 into the atmosphere ocean system increasing depositional rates of carbon and increasing global warming increased black shale deposition may reflect some combination of 1 increased oceanic hydrothermal fluxes introducing nutrients 2 anoxia on continental shelves and 3 disrupted ocean currents the apparent absence of carbon isotope anomalies at these times reflects an increase in the deposition and burial rate of both reduced and oxidized carbon peaks in black shale abundance at 2 1 ga and 800 600 ma correlate with peaks in 13 c in marine carbonates increases in atmospheric oxygen and with high cia values in shales these are all consistent with higher rates of organic carbon burial in black shales at these times these peaks may record the breakup of supercontinents at 2 2 2 0 ga and again at 800 600 ma which resulted in increased numbers of partially closed marine basins disruption of ocean currents and increased hydrothermal vents at ocean ridges all of which led to widespread anoxia 2001 elsevier science b v all rights reserved

geochemistry and significance of metavolcanic rocks from the bou azzer el graara ophiolite morocco abstract the bou azzer el graara ophiolite has been interpreted as oceanic or backarc crust overlain by a calc alkalic arc sequence or as a composite forearc and accretionary melange seventy nine new major and trace element whole rock analyses of the metavolcanic rocks from throughout the complex support the latter interpretation the results of this study indicate that the bou azzer metavolcanics comprise three petrographically and geochemically distinct groups 1 a a siliceous metavolcanic series ranging from basaltic andesites to rhyolites with calc alkalic characteristics is the principal component of the northern section of the complex it includes distinct quartz bearing rhyolite flows isolated massive pyroclastic layers are typically associated with this series these rocks are interpreted as forearc basin volcanics and pyroclastics produced during late proterozoic north dipping subduction 2 b low ti metabasalts which display island arc tholeiitic affinities occur as isolated flows or blocks in most of the ophiolite these rocks may represent fragments of the forearc basement below the siliceous series 3 c high ti metabasalts which range from enriched tholeiitic zr y 4 compositions are the principal volcanic block in the southern melange like terrane of the complex these have high ba and sr contents 100 200 ppm average ba 260 ppm average sr they are identical to basaltic compositions from tachdamt and bleida on the southern margins of the ophiolite which have previously been interpreted as a rifted continental margin sequence these rocks could represent seamount volcanic products which have been thrust into an accretionary prism along the north dipping subduction zone or rift margin basalts which have been assembled in a tectonic melange as a consequence of a forearc continent collision both the low ti and high ti metabasalt groups include samples previously identified as ocean ridge or back arc volcanics however there is no compelling evidence for the occurrence of these lava compositions in this region the distribution of rock types within the ophiolite is most consistent with a previously proposed model which views the complex as three allochthonous terranes which were juxtaposed during collision to form the bou azzer el graara complex

geochronological constraints on late precambrian intrusion metamorphism and tectonism in the anti atlas mountains the bou azzer inlier anti atlas morocco is a critical element for understanding gondwanan geology because it exposes rocks formed during the paleosuturing of the gondwanan margin with peri gondwanan arc terranes numerous intrusions within the inlier allow specific tectonic events associated with the tectonic evolution of the gondwanan margin to be bracketed detailed examination of plutons within the inlier reveal down temperature magmatic to solid state fabrics and contact relationships indicative of emplacement during oblique collision between the margin and one or more arc terranes u pb geochronological data for the ao t abdulla diorite ma and 653 8 1 6 bou offroh granodiorite ma provide a limit to the onset of collision and regional metamorphism a 653 0 1 3 precise age of ma for the ousdrat quartz diorite provides an indication of the lower limit of the collision 640 8 1 4 in the region tectonothermal activity in the anti atlas at ca 650 ma correlates with similar periods of tectonism in the peri gondwanan terranes of avalonia and cadomia and points to the emergence of continent wide tectonism in western gondwana as early as 650 ma

depositional gaps in abitibi greenstone belt stratigraphy a key to exploration for syngenetic mineralization models of greenstone belt development are crucial for exploration allochthonous models predict belts to be a collage of unrelated fragments whereas autochthonous models allow for prediction of syngenetic mineral deposits within specific stratigraphic intervals superior province greenstone belts consist of mainly volcanic units unconformably overlain by largely sedimentary timiskaming style assemblages and field and geochronological data indicate that the abitibi greenstone belt developed autochthonously we describe major revisions to stratigraphy of the abitibi greenstone belt and the implications of an autochthonous development of the volcanic stratigraphy for exploration for syngenetic mineralization the abitibi greenstone belt is subdivided into seven discrete volcanic stratigraphic episodes on the basis of groupings of numerous u pb zircon ages of pre 2750 2750 to 2735 2734 to 2724 2723 to 2720 2719 to 2711 2710 to 2704 and 2704 to 2695 ma we present revised lithotectonic and or stratigraphic nomenclature using these time intervals including 1 isotopic inheritance in younger episodes which indicates that the older episodes 2750 2735 and 2734 2724 ma had greater extent than is presently seen 2 dikes feeding younger volcanic episodes 2706 ma cutting older volcanic units 2734 2724 ma and 3 2710 to 2704 ma mafic to ultramafic sills intruding the 2719 to 2711 ma episode changes to the nomenclature include the identification of pre 2750 ma volcanic episode supracrustal fragments in the northern and southern abitibi greenstone belt and subdivision of the 2719 to 2711 ma 2710 to 2704 ma and 2704 to 2695 ma episodes into lower and upper parts we present the results of this lithostratigraphic subdivision as the first geochronologically constrained stratigraphic and or lithotectonic map of the abitibi greenstone belt many of the volcanic episodes are intercalated with and capped by a relatively thin sedimentary interface zone dominated by chemical sedimentary rocks stratigraphic and geochronological analysis of these zones indicates discontinuous deposition with localized gaps of 2 to 27 m y between volcanic episodes the zones consist of up to 200 m of iron formation chert breccia heterolithic debris flows of volcanic provenance sandstone and or argillite and conglomerate modeling of the time required for deposition of the volcanic units based on rates of magma production in modern arc and plume environments is on the order of 103 to 104 years whereas the time interval between basalt rhyolite cycles is 106 years the sedimentary interface zones are therefore interpreted as condensed sections zones with very low rates of sedimentation in a basinal setting or zones with negligible rates of sedimentation marked by silicification of existing rock types the sedimentary interface zones are therefore considered submarine correlative conformities disconformities or unconformities separating the equivalent of group level stratigraphic and lithotectonic units the unconformity bounded stratigraphic model provides a new regional to deposit scale interpretive model for use in exploration for syngenetic mineralization

the subsea floor replacement origin of the ordovician highway reward volcanic associated massive sulfide deposit mount windsor subprovince australia the highway reward massive sulfide deposit is hosted by a submarine below storm wave base silicic synsedimentary intrusion dominated volcanic succession the succession includes at least 13 porphyritic units in a volume of 1 x 1 x 0 5 km peperitic upper margins suggest that most of the rhyolites rhyodacites and dacites were emplaced as small 300 degrees c h 2 s dominant fluids by replacement of rhyolite rhyodacite and peperite lower temperature 300 degrees c fluids that diffused from the margins of the pipes deposited a halo of sphalerite rich ore

textural geochemical and isotopic data from silicified rocks and associated chemical sedimentary rocks in the 2 7 ga abitibi greenstone belt canada insight into the role of silicification abstract silica rich precambrian rocks often preserve geochemical information and microfossil remnants from the early biosphere and could play a critical role in the formation of early crust because these rocks are important geochemical and paleontological archives we need to better constrain their geochemical and isotopic attributes and generate a refined picture of the evolving archean silica cycle here we investigate a series of sub to greenschist facies si rich archean rocks from the 2 7 ga abitibi greenstone belt canada that represent chemical sedimentary rocks and rocks formed via silica addition through the process of silicification we report data for major and trace element geochemistry multi crystal silicon and oxygen isotopes of quartz using isotope ratio mass spectrometry and texture specific silicon isotope values measured using secondary ion mass spectrometry on neoarchean chemical sedimentary rocks their silicified equivalents and associated silicified volcanic rocks we find that in such a well preserved terrane we can utilize petrographic textures and geochemical attributes to establish rock origin distinguishing siliceous rocks that form via chemical sedimentation from those that form via silicification chemical sedimentary rocks display a wide range of 30si depleted silicon isotopes values that vary with stratigraphy similar to other archean iron formation silicified volcanic rocks possess 30si enriched values similar to archean silicified basalts we conclude that because silicon isotope values of iron formation shift toward 30si enriched values up stratigraphy basinal changes in the composition of the silicon isotope reservoir may be preserved silicon isotope values of silicified volcanic rocks by contrast likely represent precipitation from an isotopically heavy silicon reservoir influenced by downward percolating seawater and upward moving convecting fluids interacting with host volcanic rock basalt or andesite overall we confirm that neoarchean silicified rocks are 30si enriched like their paleoarchean counterparts

ferruginous and manganiferous haloes around massive sulphide deposits of the urals proximal brecciform ferruginous and manganiferous rocks related to vms deposits of the urals are subdivided into jasperites gossanites and umbers in addition to thin bedded jaspers and cherts the coherence of host rock composition and mn fe fertility of the sediments have been established fe poor pink hematitic and gray sulphidic chert are typical of the felsic class of vms deposits in contrast the contents of fe vary from high to moderate in ferruginous rocks enclosed in basaltic units associate with vms deposits fe and mn rich ferruginous rocks and umbers occur in association with limestones and calcareous sedimentary rocks in both types of volcanic sequences a common feature of jasperites and umbers is the abundance of replacement textures of hyaloclastites and carbonates by hematite and silica in addition replacement of clastic sulphides by hematite and magnetite is a characteristic genetic feature of gossanites all of these sedimentary rocks are accompanied by pseudomorphs of hematite and quartz formed after bacterial filaments the abundance of replacement textures are supportive of the halmyrolysis model in addition to hydrothermal sedimentary and sub seafloor hydrothermal replacement theories study of chemical zonation of altered hyaloclasts shows depletion of their rims not only in mobile na k mg but also in immobile al ti and ree whereas si and fe are concentrated in situ the halmyrolysis model presented here involving organic rich calcareous hyaloclastic sediments resolves the problem of subtraction of al ti ree and other elements which are commonly immobile under hydrothermal conditions the evolution of the halmyrolysis process from acidic reducing to alkaline oxidized conditions infers a possible range in transformation from fe ii mg smectites to fe silicates and fe si oxides as precursors of brecciform jasperite and thin bedded jasper the higher acidic initial stage of gossanite formation seems to be required for oxidation of organic matter and or pyrite the acidic condition facilitates the temporal preservation of immobile elements al ti ree in immature chlorite hematite gossanites another peculiarity of the gossanite forming processes is the likely sorption of p u and v by iron hydroxides displacing sulphides the general evolution of all ferruginous sediments results in complete fe 2 oxidation and silicification accompanied by subtraction of other elements the vertical diagenetic differentiation leads to concentration of mn carbonates silicates and oxyhydroxides into the tops of jasperite and gossanite layers mn oxyhydroxides scavenge positively charged hydrated cations like co and ni near vent bacterial communities may activate the processes of volcanic glass and sulphide degradation the proposed processes of halmyrolysis followed by silicification in situ may resolve the enigma of silica rich sediment formation in a silica undersaturated ocean the discrimination between gossanite and jasperite is useful for elaboration of new criteria for local exploration of vms and mn deposits halo dispersion of gossanites covering an area about two to three times that of the massive sulphide deposit is a good vector for ore body discovery proximal gossanites can be differentiated from jasperites by presence of relic sulphide clasts or elevated contents of chalcophile elements cu fe zn pb bi te as sb ba noble metals au ag and distinct ree patterns with la and eu positive anomalies the development of halmyrolysis and biomineralization models merit further elaboration and testing in on going research in order to add or revise theories of iron and manganese deposit formation 2012 elsevier b v

sedimentary and volcanic evolution of a neoproterozoic continental margin bleida anti atlas morocco abstract a tentative geodynamic evolution of the neoproterozoic continental margin of the northern border of the west african craton is reconstructed using both sedimentological observations and geochemical arguments from volcanic rocks the rifting stage was marked first by intermittent subsidence of a very shallow shelf environment leading to deeper water sedimentation conditions with local surges of deltaic channels this rifting stage was also characterized by the eruption of a 500 m thick pile of submarine basic lava flows with geochemical characteristics similar to those generally observed in basaltic lavas erupted during continental rifting this first rifting stage 790 ma clauer 1976 was succeeded by a drastic change both in sedimentation and volcanism the younger sediments are deep water distal turbidites with associated pyritic black shales they include a few highly differentiated tholeiitic flows and sedimentary exhalative copper deposits linked to felsic volcanism these changes are attributed to the development of deep water oceanic basins along the northern border of the west african craton the closure of this pan african oceanic domain occurred by transpression tectonics along this major lithospheric transform zone which is roughly e w trending

seafloor silicification and hardground development during deposition of 2 5 ga banded iron formations banded iron formations bifs are important archives of the ancient oceans atmosphere and biosphere but fundamental questions remain about their origin it is widely assumed that bifs were derived from layers of ferric oxyhydroxides and silica that precipitated directly from a water column that was enriched in dissolved iron and silica the reported lack of current generated structures and clastic particles beyond mud grade and the perceived basin scale extent of laminae is regarded as evidence for uninterrupted pelagic settling with no sedimentary reworking new sedimentological and petrographic results show that laminated cherts in the 2 5 ga dales gorge member of the brockman iron formation western australia preserve textures indicative of in situ brecciation immediately below the seafloor and the deposition of intraformational sandstones composed of chert clasts in a chert matrix chert intraclasts have two sedimentary components silt sized microgranules and submicron sized particles indicating that the original sediment comprised iron rich silicate muds that were cemented on or just below the seafloor by pore filling silica silicified muds were episodically eroded by density currents and the resulting detritus was transported as sand sized clasts and locally deposited in a matrix of microgranules and mud our results support the hypothesis that high concentrations of silica in early precambrian seawater favored episodic silica cementation of sediments on the seafloor we suggest that competition between sediment accumulation and seafloor silica cementation with subsequent differential compaction explains primary layering in bifs between beds of relatively thickly laminated chert and beds of thinly laminated iron rich minerals the thickest laminated chert beds are interpreted to represent intervals when seafloor silicification outpaced deposition of hydrothermal muds forming the equivalent of phanerozoic hardgrounds at sequence boundaries

the moroccan anti atlas the west african craton passive margin with limited pan african activity implications for the northern limit of the craton abstract the moroccan anti atlas region located south of the south atlas fault has been viewed traditionally as containing two segments separated by the anti atlas major fault these two segments are said to consist of a 600 700 ma pan african segment located in the northeast and b 2 ga eburnian segment situated to the southwest on the basis of observations in the zenaga and saghro inliers and of a recent literature review we suggest that this subdivision is inappropriate in that eburnian and pan african materials occur throughout the anti atlas region the entire anti atlas is underlain by eburnian crust unconformably overlain by a lower neoproterozoic passive margin allochthonous pan african ocean crustal slices were thrust onto the west african craton wac passive margin sequence 685 ma ago as a result of pan african accretion tectonics high level high k calc alkaline and alkaline granitoids locally intruded the anti atlas sequence as a whole at the end of the pan african orogeny at 585 560 ma the intervening 100 m y interval was marked by quiescence this succession of events can be related to the behaviour of one single rigid cratonic passive margin during an orogeny and correlated to the pan african events that occurred to the east in the tuareg shield and to the north in avalonian terranes this model implies that the actual northern limit of the wac is located at the south atlas fault saf and not at the anti atlas major fault aamf we propose that the aamf corresponds to the southwestern boundary of an aulacogen that formed along the northern margin of the wac during early neoproterozoic times this is consistent with the development of the gourma aulacogen on the eastern side of the wac this model further suggests that the northeastern boundary of the wac occurs north and not south of ougarta algeria support for this model is provided by geological and geophysical evidence

age constraints and characteristics of subaqueous volcanic construction the archean hunter mine group abitibi greenstone belt the archean hunter mine group hmg is a 6 7 km thick south facing volcanic sequence at the southern limit of the northern volcanic zone abitibi greenstone belt the hmg is composed of 1 felsic lava flows and endogenous and exogenous domes with autoclastic breccia deposits 2 pyroclastic fire fountain products and high to low concentration volcaniclastic sediment gravity flows of autoclastic and pyroclastic origin interstratified with iron formation 3 mafic pillowed and brecciated flows 4 a prominent felsic dyke swarm and 5 abundant mafic intrusions the stratigraphy is complex as inferred by detailed volcano sedimentary facies mapping a 5 7 km wide n trending felsic dyke swarm coupled with abundant synvolcanic and synsedimentary faults chaotic breccias and mineralization suggest a central volcanic depression which is consistent with a caldera structure voluminous effusive activity and explosive fire fountaining eruptions contributed to caldera development rifting of this arc edifice is supported by the presence of felsic dykes and mafic tholeiitic sills the lithofacies and overlying komatiites flows are consistent with a deep water marine setting u pb age determinations help constrain the hmg caldera formation to 6 million years the sampled quartz feldspar phyric dykes yielded u pb zircon ages of 2728 3 4 3 4 and 2728 9 0 8 ma and together with a previously published dyke age of 2729 6 1 4 ma from the same dyke swarm a case is made that the swarm evolved over 1 million years 2728 6 2729 6 ma a lobate flow and flow breccia revealed similar ages of 2724 6 4 6 1 7 and 2727 6 4 2 2 0 ma whereas the up to 1 km thick gabbro diorite roquemaure sill precedes both felsic dykes and flows as indicated by the 2731 8 2 2 2 0 ma age the 2706 4 5 7 4 3 ma feldspar quartz phyric dykes occurred late in the history of the hmg and were not associated with subaqueous volcanic construction the 207pb 206pb ages of analyzed zircon fractions have an inherited component within error of the oldest crystallization ages obtained for the hmg complex collectively these age determinations and inheritance as well as stratigraphy support a tripartite division into a 2734 2730 ma lower formational stage a 2732 ma middle formational stage and a 2730 2728 ma upper formational stage the study shows that age constraints without detailed facies mapping may lead to erroneous conclusions on volcanic construction and that felsic volcanic dykes and flows may inherit an earlier component of their own volcanic edifice

the replacement of plagioclase feldspars by albite observations from hydrothermal experiments oligoclase and labradorite crystals have been experimentally replaced by albite in an aqueous sodium silicate solution at 600 c and 2 kbars the replacement is pseudomorphic and is characterised by a sharp chemical interface which progresses through the feldspar while preserving the crystallographic orientation reaction rims of albite up to 50 m thick can be readily achieved within 14 days re equilibration of plagioclase in an 18o enriched sodium and silica bearing solution results in oxygen isotope redistribution within the feldspar framework structure the observed characteristics of the reaction products are similar to naturally albitised plagioclase and are indicative of an interface coupled dissolution reprecipitation mechanism chemical analyses demonstrate that the albitisation is accompanied by the mobilisation of major minor and trace elements also including elements such as al and ti which are commonly regarded as immobile during hydrothermal alteration the results contribute to developing our understanding of the close association between large scale albitisation and secondary ore mineralisation which is common in nature

coupled silicon oxygen isotope fractionation traces archaean silicification silica alteration zones and cherts are a conspicuous feature of archaean greenstone belts worldwide and provide evidence of extensive mobilisation of silica in the marine environment of the early earth in order to understand the process es of silicification we measured the silicon and oxygen isotope composition of sections of variably silicified basalts and overlying bedded cherts from the theespruit hooggenoeg and kromberg formations of the barberton greenstone belt south africa the 30si and 18o values of bulk rock increase with increasing amount of silicification from unsilicified basalts 0 64 30si 0 01 and 8 6 18o 11 9 to silicified basalts 30si and 18o values as high as 0 81 and 15 6 respectively cherts generally have positive isotope ratios 0 21 30si 1 05 and 10 9 18o 17 1 except two cherts which have negative 30si values but high 18o up to 19 5 the pronounced positive correlations between 30si 18o and sio2 imply that the isotope variation is driven by the silicification process which coevally introduced both 18o and 30si into the basalts the oxygen isotope variation in the basalts from about 8 6 to 15 6 is likely to represent temperature dependent isotope fractionation during alteration our proposed model for the observed silicon isotope variation relies on a temperature controlled basalt dissolution vs silica deposition process

the composition of earth s oldest iron formations the nuvvuagittuq supracrustal belt quebec canada abstract the composition of iron formations in the 3 75 ga yr old nuvvuagittuq supracrustal belt in northern quebec provides a proxy for seawater composition of the eoarchean and perhaps hadean oceans as well as constraints on the types of nutrients available to earth s earliest life forms integrated petrologic and geochemical relationships mapped between mineral phases in thin section and whole rock chemistry provide a framework for interpreting bulk and micro scale variations in these chemical sedimentary precipitates results show that there are two distinct chemical sedimentary units in the nuvvuagittuq belt i a banded iron formation bif consisting of alternating micro bands of magnetite ca mg fe silicates and quartz and ii a more silicate rich fe poor unit the banded silicate formation bsf of alternating micro bands of quartz and ca mg fe silicates precursor bif and bsf deposits were likely layered amorphous silica and ferric oxyhydroxides fine grained carbonate oozes and or ca mg fe rich silicate gels deposited in a marine setting low al2o3 tio2 and hfse concentrations show that they are relatively detritus free with distinctively seawater like ree y profiles and consistently positive eu anomalies these features suggest that the rocks preserved their seawater like compositions despite metamorphic overprinting the most significant trace elements in the sediments are ni and zn experimentally derived partitioning coefficients show that ni was enriched in eoarchean seawater as compared to today up to 300 nm while zn was fairly similar up to 20 nm compositional resemblances between the nuvvuagittuq sediments and those documented in the ca 3 8 ga isua supracrustals west greenland provide a plausible case that global ocean processes in terms of trace metal abundances had reached steady state by the eoarchean

petrology and geochemistry of late upper proterozoic volcanic rocks from the pan african belt in north western africa geodynamic implications and crustal evolution