low temperature metamorphism and secondary components in the portage lake volcanics a reassessment we studied the effects of hydrothermal alteration on magnetic properties of the magnetite and hematite components in the 1 1 ga portage lake volcanics upper peninsula michigan results show the importance of the intensity and direction of the hematite magnetization in the detection of secondary magnetite overprints in basalts we collected samples from 20 flows in two stratigraphic sections one from outcrops and the other from drill core the remanence unblocked below 590 c during thermal demagnetization is considered to be due to magnetite and that unblocked between 610 690 c is considered to be due to hematite hematite remanent intensities in the flow interiors are an order of magnitude less intense than those of magnetite and correlate with magnetite intensities from the top of the volcanics to the lower prehnite pumpellyite zone 5 5 km depth in the outcrop section and to the epidote pumpellyite zone 7 0 km depth in the core section these magnetic properties are very similar to those of re

refinements for footwall red bed diagenesis in the sediment hosted stratiform copper deposits model the emplacement of copper in sediment hosted stratiform copper deposits requires the circulation of a low temperature chloride rich brine in rift filling footwall red beds probably driven by meteoric recharge in adjacent highlands the descending oxygen rich meteoric water may become saline probably by leaching of footwall evaporites or possibly by mixing with brines draining down from contemporaneous evaporite pans and evolve toward a moderately oxidized brine as oxygen is consumed during the diagenetic reddening of the initially nonred coarse grained clastic rift sediments under these conditions the pore solution attains its maximum ability to leach and transport trace amounts of copper from the red beds sediment hosted stratiform copper deposits may form where the brine crosses a redox boundary into reduced sulfide rich gray beds with further consumption of oxygen during reddening the brine may become highly reduced and able to take iron into solution possibly explaining synsediment hosted stratiform copper iron oxide alterations and deposits the timing of copper deposition is clearly linked to the diagenetic reddening of immature footwall sediments in rift basins a process considered to extend over millions of years

paleomagnetism of 1 09 ga lake shore traps keweenaw peninsula michigan new results and implications we report paleomagnetic data from a new section of the 1 09 ga lake shore traps exposed on silver island 10 flows and on the adjacent mainland two flows along the northwestern coastline of the

rubidium strontium dating of ore deposits hosted by rb rich rocks using calcite and other common sr bearing minerals the authors have tested a rb sr technique that permits ore deposits to be dated using common gangue minerals such as calcite and fluorite the only conditions the deposit must meet are that 1 it have minerals with a low rb sr ratio and 2 it be enclosed by wall rock with a high rb sr ratio because hydrothermal minerals acquire a strontium isotope composition that is usually similar to that of the wall rock minerals with low rb sr ratio should record and retain the isotopic composition that the wall rock had at the time of mineralization the difference between that ratio and that of the wall rock at present is a function of time and the rb sr composition of the wall rock the technique was tested fusing fluorite and calcite from three deposits ranging in age from tertiary to precambrian in all cases the age determined here closely resembles that obtained by conventional k ar and rb sr dating methods the precision however can be poor and depends chiefly on the strontium isotope heterogeneity of the wall rock and its rb sr enrichment 36 references 1 figure 1 table

u th pb zircon ages of some keweenawan supergroup rocks from the south shore of lake superior new single crystal zircon u th pb ages for plutonic and rhyolitic keweenawan supergroup rocks from the south shore of lake superior provide geochronological constraints on magmatic evolution associated with the 1 1 ga midcontinent rift analyses of a granophyric phase of the mineral lake intrusion and the mellen granite both parts of the mellen intrusive complex and a laterally extensive rhyolite from the top of the kallander creek volcanics have weighted average 207pb 206pb ages of 1102 0 2 8 ma n 2 1100 9 1 4 ma n 5 and 1098 8 1 9 ma n 4 respectively analyses of a pyroclastic rhyolite flow at the top of the porcupine volcanics result in variable 207pb 206pb ages that range from 1080 to 1137 ma this rhyolite exhibits a continuum between morphologically complex and simpler prismatic zircon crystals the latter yielding concordant analyses having a weighted average 207pb 206pb age of 1093 6 1 8 ma n 2 four prismatic zircons from an aphyric rhyolite of the chengwatana volcani

extraction and carbon isotope analysis of co2 from scapolite in deep crustal granulites and xenoliths abstract carbon isotope compositions of scapolite from granulite facies gneisses and lower crustal xenoliths document the composition and constrain the source of carbon in scapolite from the lower crust co 2 is extracted from scapolite without fractionating carbon isotopes by reaction with phosphoric acid at 25 or 75 c thus partial yields of co 2 from scapolite are sufficient for accurate carbon isotopic analysis isotopic compositions of coexisting scapolite and calcite in high grade calc silicate gneisses and marbles and consideration of the crystal chemical environment of co 3 in the scapolite structure indicate little fractionation of 13 c 12 c between scapolite and calcite 0 1 1 2 at equilibrium conditions of 650 800 c the carbon isotope composition of co 2 extracted from scapolite in twenty nine samples of regional granulite facies gneisses amphibolites calc silicate gneisses and crustal xenoliths yield values of 13 c that range from 10 to 1 pdb high grade marbles and graphitic paragneisses are precluded as major sources of carbon for scapolite in the high grade rocks analyzed in this study as the former are isotopically enriched and the latter isotopically depleted in 13 c 12 c relative to the range of isotopic compositions determined here the 13 c values for mafic granulites and amphibolites in granulite terranes composed of supracrustal sequences 10 1 to 4 0 may reflect the isotopic composition of diagenetic carbonate present in their basaltic protoliths the values of 13 c for scapolite in mafic xenoliths and some granulite facies orthogneisses 8 2 to 1 2 are consistent with crystallization of the scapolite from a mafic melt or derivation of co 2 from mafic melts emplaced in the lower crust or upper mantle the values of 13 c for scapolites from calc silicate gneisses and calc silicate xenoliths 10 0 to 2 9 may result from depletion of 13 c 12 c as a result of decarbonation of calcite bearing protoliths during prograde metamorphism

genesis of native copper lodes in the keweenaw district northern michigan a hybrid evolved meteoric and metamophogenic model the keweenaw peninsula of northern michigan is well known for early diagenetic sulfide dominant sediment hosted stratiform copper mineralization hosted mainly by basal siltstone shale units of the nonesuch formation in the white pine district and later native copper lodes hosted mainly by flow top breccias and amygdaloidal portions of flood basalts and interbedded conglomerates of the stratigraphically lower portage lake volcanics the native copper mineralization is generally attributed to downdip burial metamorphism of the host strata an alternative model is proposed here involving a complementary influx of evolved meteoric water elevated in salinity and progressively equilibrating with its aquifers similar to the fluid which formed the white pine mineralization this model has several advantages over the metamorphogenic model alone it would provide the salinity by dissolution of evaporites and moderately oxidizing conditions as a result of hematitization i e reddening of coarse continental rift sediments necessary for copper transport and the evolved meteoric water would have been an important cupriferous brine owing to the leaching of trace amounts of copper from the voluminous aquifers further reduction of the evolved fluid would also explain the updip deposition of native copper in association with dehematitized pumpellyite and chlorite dominant i e reduced hydrothermal mineral assemblages at the same time this influx of meteoric water from highlands adjacent to the keweenaw rift basin would have provided a mechanism to move the deep seated hot dense ore brine upward along the aquifers of the portage lake volcanics on its ascent from deep portions of the rift basin the gravity driven evolved meteoric brine would have mixed with the fluid generated by burial metamorphism to form a hybrid evolved meteoric metamorphogenic oreforming hydrothermal brine

the north american midcontinent rift beneath lake superior from glimpce seismic reflection profiling the midcontinent rift system is a 1 1 b y old structure extending from kansas through the lake superior region and into southern michigan the rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments for most of its extent it is buried beneath paleozoic rocks but can be traced by its strong gravity and magnetic anomalies the rocks of the rift system are exposed only in the lake superior region and comprise the keweenawan supergroup much of the geology of the keweenawan is beneath lake superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on shore geology seismic reflection surveys by the great lakes international multidisciplinary program on crustal evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail the reflection profiles across the rift reveal a deep asymmetrical central graben whose existence and magnitude was not previously documented they show that in addition to crustal sagging documented by previous investigations normal faulting played a major role in subsidence of the axial region of the rift a sequence of volcanic and sedimentary rocks in places greater than 30 km thick fills the graben thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben near the axis the prerift crust is thinned to about one fourth of its original thickness apparently by a combination of low angle extensional faulting and ductile stretching or distributed shear the sense of asymmetry of the central graben changes along the trend of the rift documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments the location of the accommodation zones is inferred from abrupt disruptions in the bouguer gravity signature of the rift uplift of the central graben occurred when the original graben bounding normal faults were reactivated as high angle reverse faults with throws of 5 km or more in places the midcontinent rift has some striking similarities to some younger passive continental margins we propose that it preserves a record of nearly complete continental separation which had it not been arrested would have created a middle proterozoic ocean basin

mineral deposits of the mesoproterozoic midcontinent rift system in the lake superior region a space and time classification abstract the mesoproterozoic midcontinent rift system mrs of north america hosts a diverse suite of magmatic and hydrothermal mineral deposits in the lake superior region where rift rocks are exposed at or near the surface historically hydrothermal deposits such as michigan s native copper deposits and the white pine sediment hosted stratiform copper deposit were major mrs metal producers on going exploration for and potential development of copper nickel sulfide deposits hosted by the duluth complex of minnesota and the opening of the eagle nickel mine in michigan indicate an expanding interest in mrs magmatic deposits mrs hydrothermal and magmatic mineral deposits many of which are significant past present and likely future providers of critical minerals here are placed into a space and time metallogenic framework to construct this framework regional mrs mineral deposits extracted from the u s geological survey mineral resources data system mrds and the ontario ministry of energy northern development and mines mineral deposit inventory mdi were supplemented by other known and recently recognized mineral deposits described in the literature all mineral deposits were classified by deposit type host rock age and type and estimated timing of mineralization deposits were then put into a tectonic evolutionary framework stages for the mrs which shows that deposits formed within discrete spatial and temporal stages of rift evolution each stage of rift evolution is characterized by specific mineral deposit types that are largely confined both by their physical location in rift rocks and type and timing of mineralization examples include mrs nickel rich conduit type magmatic sulfide deposits which are restricted to an early magmatic stage mrs history when magma compositions were characterized by ni rich picrites and high mg basalts in contrast contact type magmatic sulfide deposits with cu ni were derived from more evolved al rich tholeiitic magmas that dominated a later time of voluminous magmatic activity hydrothermal sediment hosted stratiform chalcocite mineralization along the margins of western lake superior in michigan is economic only where fluid flow through red beds was concentrated by structures along the margins of a post volcanic mrs sedimentary basin widespread native cu and native ag mineralization occurred about 40 million years after formation of host basalt lava flows and interflow sedimentary rocks these descriptions of the diverse mineral deposits in the lake superior region cataloged within the space of the local and regional geology and over the more than 60 million year mineralizing history of the mrs are indicative of the complex and at times overlapping magmatic and hydrothermal mineral systems that operated within this major large igneous province

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

fluid inclusions and hear isotopes in pyrite from the yinjiagou deposit in the southern margin of the north china craton a mantle connection for poly metallic mineralization abstract the yinjiagou poly metallic deposit is located at the southern margin of the north china craton smncc pyrite the most economically important mineral is associated with cu mo au pb zn and ag mineralization two main mineralization stages have been identified and studied a porphyry stage including the quartz molybdenite and quartz sericite pyrite qsp veins and a vein stage including the early and the late economically most important pyrite stages three types of fluid inclusions are distinguished in quartz and pyrite including liquid rich vapor rich biphase lv and halite bearing inclusions the transmitted and infrared microthermometry produced several results 1 each of the three types of fluid inclusions is trapped in the quartz of the porphyry stage with homogenization temperatures of 335 419 c and salinities of 4 7 49 6 wt nacl equiv the coexistence of halite bearing inclusions and low density vapor rich biphase lv inclusions indicates that the fluid was boiling the boiling elevated the fe and s levels in the hyper saline fluid as reflected by the development of pyrite chalcopyrite and s daughter minerals in the halite bearing inclusions 2 the fluid in the early pyrite also displays features characteristic of boiling with homogenization temperatures of 352 400 c and salinities of 3 7 42 4 wt nacl equiv whereas the late pyrite only contains liquid rich biphase lv inclusions with homogenization temperatures of 263 354 c and salinities of 6 0 21 3 wt nacl equiv the relationship between the decreasing salinity and the corresponding decreasing homogenization temperature clearly indicates extensive mixing and dilution 3 finally the infrared calibration indicates that the impact of infrared light intensity on the microthermometric results for the yinjiagou pyrite hosted fluid inclusions is limited the he ar isotopic analysis of the fluid inclusions in the pyrite indicates two different fluid sources that involve abundant mantle input the 3he 4he ratios in the porphyry stage are relatively uniform ranging from 1 39 to 1 78 ra ra 1 39 10 6 for air corresponding to 16 22 mantle 4he contribution this relatively constant range represents the actual helium isotopic composition of the fluids emanating from the cooling intrusions at depth the range of 3he 4he ratios in the vein stage is wide varying from 0 80 to 5 26 ra corresponding to 9 65 mantle 4he the significant variation in the 3he 4he ratios in the vein stage reflects mixing between two fluids these data indicate that there are two different fluid sources and separate evolutionary processes for the porphyry and vein stages and support a multi stage mineralization model the veinlet molybdenite mineralization was induced by the boiling and associated cooling of a magma sourced fluid that emanated from intrusions at depth the qsp veins are likely to be pressure release zones and late fluid flow channels the early pyrite mineralization may be attributed to the influx of a dominantly mantle derived fluid with high 3he 4he ratio 5 26 ra which is most likely related to a later more mafic magmatic event the precipitation of the most economically important pyrite resulted from the mixing and diluting of this mantle derived fluid with a surface derived fluid additionally the high 3he 4he ratios indicate a strongly extensional setting which is most likely related to the late jurassic to early cretaceous lithospheric modification and thinning of the smncc

paleomagnetism of the native copper mineralization keweenaw peninsula michigan the age and genesis of michigan s world class native copper deposits are poorly constrained the copper is hosted by basaltic flow tops and conglomeratic interbeds of the 1095 2 ma keweenawan por

first u pb shrimp zircon and 40ar 39ar ages of metarhyolites from the afyon bolkardag zone sw turkey implications for the rifting and closure of the neo tethys abstract the afyon bolkardag zone abz is one of the most important metamorphic zones in turkey this zone consists of metasedimentary and metavolcanic rocks which have undergone regional greenschist facies metamorphism the metavolcanic rocks are dominated by rhyolite dacite and trachyandesite in the ilgin konya area the low grade metavolcanics were originally rhyolitic lavas and tuffs we present new precise sensitive high resolution ion microprobe shrimp 206 pb 238 u age analyses of zircons and 40 ar 39 ar age data of phengite from metarhyolites that enable a more precise reconstruction of the abz shrimp analyses of euhedral or sub euhedral magmatic zircons from two metarhyolite samples yield weighted means 230 2 ma and 229 2 ma these ages are interpreted as formation age of rhyolitic volcanics implying they were formed in the late triassic time carnian we suggest that these upper triassic magmatic rocks were formed in an extensional tectonic setting on the northern active margin of gondwana which led to the development of the northern branch of the neo tethys 40 ar 39 ar dating of two phengite samples from metarhyolites gave plateau ages of between 63 7 0 1 ma and 62 6 0 1 ma these dates are interpreted as the metamorphic age of these volcanic rocks and as the age of metamorphism that affected the abz consequently these earliest tertiary ages also constrain the timing of the closure of the neo tethys

district scale concentration of native copper lodes from a tectonically induced thermal plume of ore fluids on the keweenaw peninsula northern michigan economic native copper lodes in the lake superior district are confined to a 45 km long portion of the portage lake volcanics on the keweenaw peninsula despite similar host rocks extending for more than 1 000 km around lake superior the focus of ascending ore forming brine into a narrow segment of portage lake strata may be attributed to upward thrusting on the keweenaw fault during closure of the midcontinent rift forming anomalously hot steeply dipping aquifers on the keweenaw promontory and to the development of a single dominant thermal plume within the incipiently buoyant brine at the scale of the mine district on that promontory

primary curvature in the mid continent rift paleomagnetism of the portage lake volcanics northern michigan usa abstract rocks of the north american mid continent rift mcr in the keweenaw peninsula of northern michigan display a change in structural trend from east to west varying in strike from 100 in the east to 35 farther west before returning to a more east west trend of 80 near wisconsin in general curvature can be described either as of primary origin meaning that the arc developed in its present curved state or as of secondary origin when the arc formed from an initially straighter geometry a powerful tool in evaluating the origin and degree of curvature in any deformed belt is paleomagnetism where coincident change in declination and strike is evidence for secondary curvature several paleomagnetic studies have been completed on rift related sedimentary rocks as well as lava flows from the mcr in the lake superior region paleomagnetic directions for the sedimentary rocks vary much more in declination than in inclination possibly reflecting a vertical axis rotation if secondary rotation has affected the sedimentary rocks then underlying volcanic rocks should also demonstrate a similar rotation thirty one sites were collected from the portage lake volcanics in the most highly curved part of the keweenaw peninsula in the upper peninsula of michigan sites were chosen to maximize the variation in structural trend thermal demagnetization results showed two components in samples from most sites a lower temperature a component 580 c both components were tested for primary remanence using a conglomerate test the a component carried by magnetite passes the conglomerate test at a 95 confidence level and is therefore considered a primary remanence the b component carried by hematite fails at the 95 level and is considered a secondary magnetization most importantly declination of the primary a magnetization between sites shows no correlation with strike demonstrating that vertical axis rotations cannot explain the curvature of the mid continent rift we conclude that curvature in the lake superior region is a primary feature likely reflecting a pre existing zone of weakness that was exploited during rifting

closing of the midcontinent rift a far field effect of grenvillian compression the midcontinent rift formed in the laurentian supercontinent between 1109 and 1094 ma soon after rifting stresses changed from extensional to compressional and the central graben of the rift was partly inverted by thrusting on original extensional faults thrusting culminated at about 1060 ma but may have begun as early as 1080 ma on the southwest trending arm of the rift the crust was shortened about 30 km on the southeast trending arm strike slip motion was dominant the rift developed adjacent to the tectonically active grenville province and its rapid evolution from an extensional to a compressional feature at ca 1080 ma was coincident with renewal of northwest directed thrusting in the grenvllle probably caused by continent continent collision a zone of weak lithosphere created by rifting became the locus for deformation within the otherwise strong continental lithosphere stresses transmitted from the grenville province utilized this weak zone to close and invert the rift

age of native copper mineralization keweenaw peninsula michigan amygdaloidal flood basalts and conglomerates are the host for substantial deposits of native copper within the portage lake volcanics in the keweenaw peninsula of michigan a wide variety of gangue minerals are associated with the regional hydrothermal alteration mineralization event application of the rb sr method to amygdule filling microcline calcite epidote and chlorite suggests an age of mineralization between 1 060 and 1 047 m y or asymp 20 m y these results are supported by a fission track age on epidote of 1 044 or 169 m y the age of native copper mineralization determined in this study is consistent with geologic evidence which suggests that mineralization postdated the deposition of most or all of the overlying freda sandstone variable initial 87 sr 86 sr ratios between the low rb sr phases suggest mixing of isotopically distinct sources of sr during the generation and equilibration of the hydrothermal solutions

the precambrian nonesuch formation of the north american mid continent rift sedimentology and organic geochemical aspects of lacustrine deposition abstract the precambrian keweenawan trough in the lake superior region of north america is part of a failed intractonic rift that is filled with a thick sequence of volcanic and sedimentary rocks one unit in this sequence the none such formation is composed of black to green siltstones and shales and is conformable with the underlying alluvial copper harbor conglomerate and the overlying fluvial freda sandstone the formation is exposed as a lens in northern michigan and wisconsin and attains a maximum thickness of 200 m based on the integration of outcrop and core data nine facies have been recognized in the formation each has been assigned to one of three genetic facies assemblages a marginal lacustrine assemblage characterized by interbedded lithic sandstones siltstones mudstones and sandstone shale couplets is interpreted as a record of deposition on a sandflat mudflat complex a lacustrine assemblage is characterized by massive to well laminated dark shaly siltstones carbonate laminites shales siltstones and mudstones these sediments were probably deposited in a progressively shallowing perennial lake bottom conditions were anoxic and became increasingly more oxic as lake regression progressed a gradual transition from a lacustrine environment to a fluvial environment is represented by the red to brown massive horizontally laminated and rippled fine sandstones siltstones and mudstones of the fluvial lacustrine assemblage interactions among rates of subsidence rates of sedimentation fluctuations in lake level possible changes in climate and differences in tectonic setting controlled sedimentation in the nonesuch lake and resulted in variable vertical facies sequences total organic carbon toc analyses show a strong correlation between organic richness and the shale facies lacustrine assemblage and mudstone facies marginal lacustrine assemblage detailed petrographic analysis using incident white light and reflected blue light fluorescence revealed two prevalent organic petrographies which could be distinguished using combined pyrolysis gas chromatography mass spectrometry py gc ms and to a limited extent by stable carbon isotope mass spectrometry the geographic and stratigraphic distribution of rocks containing the two organic petrographies suggests that differential preservation e g microbial degradation accounts for the difference rather than maturity or variable source input

crustal scale thrusting and origin of the montreal river monocline a 35 km thick cross section of the midcontinent rift in northern michigan and wisconsin a structurally simple 35 km thick north facing stratigraphic succession of late archean to middle proterozoic rocks is exposed near the montreal river which forms the border between northern wisconsin and michigan this structure the montreal river monocline is composed of steeply dipping to vertical sedimentary rocks and flood basalts of the keweenawan supergroup middle proterozoic along the south limb of the midcontinent rift and disconformably underlying sedimentary rocks of the marquette range supergroup early proterozoic these rocks lie on an archean granite greenstone complex about 10 km of which is included in the monocline this remarkable thickness of rocks appears to be essentially structurally intact and lacks evidence of tectonic thickening or repetition tilting to form the monocline resulted from southward thrusting on listric faults of crustal dimension the faults responsible for the monocline are newly recognized components of a well known regional fault system that partly closed and inverted the midcontinent rift system resetting of biotite ages on the upper plate of the faults indicates that faulting and uplift occurred at about 1060 20 ma and followed very shortly after extension that formed the midcontinent rift system

the identification of the guqiong orogenic ag au polymetallic deposit in southern tibet evidences from mineralogy geochronology and fluid evolution abstract guqiong is an ag au polymetallic deposit located in the eastern part of the au sb ore belt along the indus yarlung zangbo suture in the north himalaya in this study we have identified three major alteration mineralization stages for guqiong based on the mineral assemblages and their paragenetic relationships namely the i pre ore alteration forming mainly ore barren thick quartz veins and vuggy quartz ii main ore stage that comprises quartz veins with sulfide mineralization including ag au bearing minerals in this stage sericite occurs closely associated with sulfides iii the post ore stage characterized by the formation of calcite and quartz with trace sulfides fluid inclusion assemblage fia microthermometry was conducted on barren and mineralized quartz and sphalerite three types of inclusions are identified based on their petrographic features and laser raman spectra i e the pure carbonic pc aqueous carbonic three phase c1 and two phase c2 and aqueous w types the fias in stage i ore barren quartz are mainly pc type and c1 type which are characterized by being high temperatures 260 to 300 c low salinities 0 8 to 3 5 wt nacl eqv and with abundant co2 sphalerite of main ore stage contains mainly w type with limited c2 type of low temperatures 158 to 180 c and medium salinities 5 6 to 6 7 wt nacl eqv meanwhile the fia in mineralized quartz is featured by the general coexistence of c2 type and w type with local pc type and c1 type inclusions the fluid system may have been a mixture of the two aforementioned fluids in barren quartz and sphalerite as indicated by two significantly different fias c type and w type with contrasting homogenization temperatures 260 to 290 c and 180 to 220 c respectively but comparable salinities 0 2 to 8 2 and 0 2 to 8 1 wt nacl eqv respectively the fluid d 18o and 13c values of the main ore stage quartz are 172 to 114 8 8 to 13 1 and 13 4 to 7 7 respectively the negative 13c values are interpreted as devolatilization products of biogenic carbon with a metamorphic origin while the d and 18o values likely reflect meteoric water involvement into metamorphic fluids we propose that fluid mixing had occurred and caused sulfide precipitation at the main ore stage additionally the estimated fluid trapping depth of 5 to 6 km obtained using 3 d phase diagram is consistent with the formation depth of many orogenic type deposits worldwide 40ar 39ar dating on sericite yielded a well defined plateau age of 42 03 0 42 ma with normal and inverse isochron ages of 42 8 1 6 ma and 42 9 1 9 ma respectively these ages bracket the ag au mineralization at guqiong place it in the main stage india asia collision and further support its orogenic type origin

keweenaw hot spot geophysical evidence for a 1 1 ga mantle plume beneath the midcontinent rift system the proterozoic midcontinent rift system of north america is remarkably similar to phanerozoic rifted continental margins and flood basalt provinces like the younger analogues the volcanism within this older rift can be explained by decompression melting and rapid extrusion of igneous material during lithospheric extension above a broad asthenospheric thermal anomaly which the authors call the keweenaw hot spot great lakes international multidisciplinary program on crustal evolution seismic reflection profiles constrain end member models of melt thickness and stretching factors which yield an inferred mantle potential temperature of 1 500 1 570c during rifting combined gravity modeling and subsidence calculations are consistent with stretching factors that reached 3 or 4 before rifting ceased and much of the lower crust beneath the rift consists of relatively high density intruded or underplated synrift igneous material the isotopic signature of keweenawan volcanic rocks presented in a companion paper by nicholson and shirey this issue is consistent with the model of passive rifting above an asthenospheric mantle plume

paleomagnetism and upb geochronology of volcanic rocks from michipicoten island lake superior canada precise calibration of the keweenawan polar wander track abstract paleomagnetic poles have been determined for normally magnetized samples of late proterozoic volcanic rocks from 23 sites on michipicoten island lake superior canada the results are in general accord with an earlier less precise study by one of the authors and indicate a small polar shift between rocks of the mamainse point formation 186 e 38 n and the michipicoten island formation 175 e 26 n a quartz feldspar porphyry intruding the lower mamainse point formation but unconformably overlain by the michipicoten island formation was sampled for u pb zircon dating the zircons define an age of emplacement of 1086 5 1 3 3 0 ma this age and the pole positions are consistent with and supplement ages previously determined for the keweenawan paleomagnetic polar wander curve these data define an apparent polar wander rate of 14 cm a 1 this age combined with previously published u pb ages indicates a time span of 22 ma for most keweenawan igneous activity in addition the time of the keweenawan paleomagnetic reversal is constrained to be between 1086 5 1 3 3 0 and 1097 6 3 6 ma the age of the upper part of the reversely magnetized osler group the zircon population contained an inherited component from a source 1403 50 ma old this suggests that the porphyry magma was at least partly derived by melting of an anorogenic pluton emplaced during the mid proterozoic

a process based approach to estimating the copper derived from red beds in the sediment hosted stratiform copper deposit model most estimates of the total tonnage of copper metal leached from footwall red beds to form sediment hosted stratiform copper deposits are based on the amounts of copper at ppm levels that might have been derived from the volume of red beds underlying given sediment hosted stratiform copper deposits that static calculation is replaced here with a process based approach in which the release of copper results from the progressive diagenetic alteration of the footwall aquifers by oxygen rich meteoric water driven by topographic recharge in highlands adjacent to the intracontinental rift basins hosting these deposits the total amount of liberated copper depends on the volume of meteoric water its oxygen content and the duration of the diagenetic leaching of copper estimates suggest that adequate amounts of copper may be derived from footwall red beds within reasonable periods of early and possibly more advanced diagenesis

latent thermal effects from porcupine volcanics calderas underlying the white pine presque isle stratiform copper mineralization northern michigan economic stratiform copper mineralization in the fine grained carbonaceous basal nonesuch formation of the 50 km wide white pine presque isle mining district northern michigan has long been explained by the essentially vertical infiltration of cupriferous brine across the full width of basal nonesuch strata from the underlying copper harbor conglomerate aquifer followed by copper precipitation mainly by reactions with in situ iron sulfide based on that model why do lesser amounts of total copper measured in vertical sections occur in coarser grained presumably more permeable basal nonesuch strata situated laterally to the east and west of the mining district the answer may lie in increased infiltration rates for copper harbor hosted ore forming brine warmed locally by latent volcanic heat from a resurgent caldera or calderas located within the large porcupine volcanics dome underlying the mine district not only would caldera heat have warmed the brine and lowered its density but more importantly it would have produced a remarkable decrease in the brine viscosity an increase in temperature from 15 to 100 c could have increased the brine infiltration rate by more than four times in very early diagenetic time when the sediments were poorly compacted and highly porous because progressive compaction would have opposed the temperature effect a very early diagenetic timing is suggested for main stage copper mineralization over the porcupine volcanics caldera textural evidence has also long supported an early mineralization event at the rift basin scale the exceptionally rapid upward flux of warmed brine into fine grained nonesuch over the caldera would have required equivalent volumes of replacement brine which would in turn have required a lateral convergence of brine within the footwall copper harbor conglomerate aquifer toward the white pine presque isle district this lateral focus of brine toward the warm caldera area would have created a more efficient basin scale ore forming system than that resulting from a linear hydrogeologic flow most concepts described here deserve further evaluation through quantitative hydrogeologic modeling thermal effects on ore brine viscosities and densities developed here for the sediment hosted stratiform copper mineralization of the white pine presque isle district could be considered in genetic models for other deposits and deposit types the effects are especially strong for solutions warmed from low temperatures e g 20 c or lower to 100 c and could also be important to ore fluid flow for ores involving much higher hydrothermal temperatures if solutions with initially low temperatures played a role in their genesis

slip distribution of the february 27 2010 mw 8 8 maule earthquake central chile from static and high rate gps insar and broadband teleseismic data 1 the shallow depth underthrust earthquake of february 27 2010 mw 8 8 ruptured the subduction plate interface in central chile between 34 s and 38 s we retrieve the spatial and temporal distribution of slip during this mega earthquake through a joint inversion of teleseismic records insar and high rate gps hrgps data additionally our model is shown to agree with broadband surface waves rupture initiated at about 32 km depth and propagated bilaterally resulting in two main slip zones located ssw and nne of the hypocenter nucleation did not take place within or at the edge of one of these main asperities but in between during the first 30s slip propagated predominantly southwards later on the rupture evolved more slowly and more symmetrically eventually the northern asperity became predominant with maximum slip reaching about 20 m most of the seismic moment was released within 110s a relatively short time explained by the bilateral propagation the overall average rupture velocity is 2 6 km s but propagation occurred initially faster towards the south 3 2 km s large slip did not reach the trench a result consistent with the moderate size of the tsunami down dip rupture stopped at about 50 km depth in agreement with the lower limit of the locked zone inferred by ruegg et al 2009 from pre seismic gps data

magnetization age from paleomagnetism of the copper harbor red beds northern michigan usa and its keweenawan geologic consequences the copper harbor formation on lake superior s keweenaw peninsula records the transition from volcanic to sedimentary infilling of north america s 1 1 ga keweenawan rift radiometric dating shows t

the copper harbor conglomerate a late precambrian fining upward alluvial fan sequence in northern michigan the keweenawan copper harbor conglomerate exposed in northern michigan is part of a thick succession of volcanics and terrigenous clastics deposited in the keweenawan trough a failed intracontinental rift it is a fining upward wedge of volcanogenic conglomerate and sandstone deposited as proximal to distal braided stream and sheet flood facies on coalesced alluvial fans and sand flats this unit along with the lower portion of the overlying nonesuch shale consists of an alluvial fan shallow lake sequence that filled the rift during and following volcanic activity the fining upward nature of the sequence reflects a waning sediment supply probably due to lowering of the source area following faulting the copper harbor conglomerate exhibits attributes of both arid fan and humid fan alluvial models coarse grained facies are lithologically similar to alluvial fan units deposited in modern humid regions conversely finer distal copper harbor facies exhibit numerous features indicating flashy discharges and desiccation processes typical of deposition in modern arid regions ambiguity of sedimentary features useful for climatic inference probably reflects the limited applicability of holocene models to more ancient alluvial sequences because precambrian hydraulic regimes developed under various climatic settings may have differed from their holocene counterparts

an isotopic c o sr study of vein gold deposits in the meguma terrane nova scotia implication for source reservoirs vein quartz carbonate and tourmaline from 19 meguma gold deposits in the meguma terrane of nova scotia have been analyzed for stable delta 18 o delta 13 c and radiogenic 87 sr 86 sr isotopes in order to assess the nature and origin of the vein forming fluids as with other mesothermal gold provinces the meguma gold deposits are well suited to such a study because carbonate is the next most abundant phase after quartz in these mesothermal lode gold deposits all vein types have been sampled for quartz and carbonate and in addition all compositional and textural varieties of carbonate have been sampled vein quartz is of uniform isotopic composition with most 18 o values between 10 2 to 17 6 per mil avg ca 14 per mil except for one deposit west gore sb au where values go to 19 4 per mil there is no systematic variation for quartz within a deposit or position within the meguma group stratigraphy vein carbonate 18 o values range from 11 8 to 27 5 per mil with most in the 13 to 16 per mil range the relatively 18 o enriched carbonates reflect exchange with low temperature fluids based on analyses of quartz carbonate pairs that indicate disequilibrium fractionation delta sub quartz carbonate or 0 this is best illustrated by the strong negative correlation between delta 18 o carbonate and delta sub quartz carbonate the 18 o water is estimated at 10 or 3 per mil for a temperature of vein formation of 350 degrees to 400 degrees c and using the appropriate mineral water fractionation equations whereas delta 18 o water values partly overlap the field for magmatic fluids the values are wholly consistent with a metamorphic fluid and it is considered unlikely that a primary magmatic fluid signature has been substantially modified due to wall rock influences given that mesothermal gold systems are sites of high fluid rock ratios vein carbonate has delta 13 c values of 13 1 to 25 9 per mil but a slight negative correlation between delta 13 c carbonate and delta sub quartz carbonate suggests that the primary values lie in the range 20 to 25 per mil thus the delta 13 c values indicate a reduced biogenic source for the carbon oxidation of the reduced carbon as indicated by co 2 in fluid inclusions may have occurred via hydrolysis of graphite or dissolution of carbonate minerals both of which occur in the wall rock lithologies of the meguma group the latter of which has the appropriate isotopic composition the initial 87 sr 86 sr of the vein fluid estimated from the analyses of 52 vein carbonates 17 deposits and four tourmalines three deposits ranges from 0 70118 to 0 72284 and within deposits considerable variation is observed there is insufficient data to quantify the extent of the low temperature overprint which has modified the c and o isotope data although it is likely that some influence is present nevertheless the data cannot be reconciled by a source confined exclusively to the meguma group which suggests therefore involvement of another reservoir s the isotopic heterogeneity can be explained by variable amounts of contamination of a primary fluid with radiogenic sr derived from meguma group lithologies by interaction along the fluid path or at the site of vein formation concomitant with wall rock alteration as discussed above a dominantly magmatic source is not considered feasible this fluid source is suggested to be within the structural basement to the meguma group and the liscomb gneisses are the favored source based on the combined results of the sr isotope data presented herein and previously published pb isotope data collectively the data indicate that a primary fluid of metamorphic origin has had its isotopic signature variably modified due to interaction with different reservoirs the most affected isotopic systems are c and s based on earlier work on delta 34 s values which are abundant as graphite carbonate and sulfides in the meguma wall rock respectively the range in 87 sr 86 sr values of the fluid also reflects contamination but this was quite variable the uniform delta 18 o water value for the fluid indicates that this was the least affected isotopic system except for the later exchange of carbonate at low temperatures

paleomagnetism and the age of copper mineralization in the portage lake volcanics upper peninsula michigan the close association of hematite with native copper in the flow tops of the portage lake volcanics invites the opportunity to establish the relative age of copper mineralization for this purpose

orogenic gold common or evolving fluid and metal sources through time abstract orogenic gold deposits of all ages from paleoarchean to tertiary show consistency in chemical composition they are the products of aqueous carbonic fluids with typically 5 20 mol co 2 although unmixing during extreme pressure fluctuation can lead to entrapment of much more co 2 rich fluid inclusions in some cases ore fluids are typically characterized by significant concentrations of ch 4 and or n 2 common estimates of 0 01 0 36 mol h 2 s a near neutral ph of 5 5 and salinities of 3 7 wt nacl equiv with na k ca mg this fluid composition consistency favors an ore fluid produced from a single source area and rules out mixing of fluids from multiple sources as significant in orogenic gold formation nevertheless there are broad ranges in more robust fluid inclusion trapping temperatures and pressures between deposits that support a model where this specific fluid may deposit ore over a broad window of upper to middle crustal depths much of the reported isotopic and noble gas data is inconsistent between deposits leading to the common equivocal interpretations from studies that have attempted to define fluid and metal source areas for various orogenic gold provinces fluid stable isotope values are commonly characterized by the following ranges 1 18 o for precambrian ores of 6 to 11 and for phanerozoic ores of 7 to 13 2 d and 34 s values that are extremely variable 3 13 c values that range from 11 to 2 and 4 15 n of 10 to 24 for the neoarchean 6 5 to 12 for the paleoproterozoic and 1 5 to 10 for the phanerozoic secular variations in large scale earth processes appear to best explain some of the broad ranges in the o s and n data fluid rock interaction particularly in ore trap areas may cause important local shifts in the o s and c ratios the extreme variations in d mainly reflect measurements of hydrogen isotopes by bulk extraction of waters from numerous fluid inclusion generations many which are not related to ore formation radiogenic isotopes such as those of pb sr nd sm and os measured on hydrothermal minerals are even more difficult to interpret for defining metal source particularly as the low salinity ore fluids transport limited amounts of these elements and significant amounts of these may be locally added to the minerals during alteration reactions at the sites of gold deposition noble gas and halogen data are equally equivocal fluid exsolution from granitoids emplaced into the upper and middle crust metamorphism of the crust or fluids entering trans crustal fault zones from below the crust all remain as permissive scenarios associated with orogenic gold formation as the abundant geochemical data are equivocal however geological and geochronological data weigh heavily against a magmatic hydrothermal model in the upper to middle crust there is no universal temporal association between orogenic gold and magmatism and where there is an overlap in age there is no specific type of magmatism consistently associated with gold formation nor element zonation around any specific pluton a crustal metamorphic model for fluid and metal sources is very consistent with geological geochronological and geochemical data although metamorphism on a regional scale that releases these components into major fault zones can be associated with many processes along active continental margins these can include crustal thickening and radiogenic heating slab rollback and heating during crustal extension or subduction of a spreading ridge heating the base of an accretionary prism in rare examples where phanerozoic orogenic gold deposits are hosted in precambrian high grade metamorphic terranes fluids and metals must however enter a transcrustal fault system from a sub crustal source this could either be a devolatilized subducted relatively flat perhaps stalled slab and its overlying sediment or the corner of the fertilized mantle wedge that releases a fluid during a thermal event

no asymmetry in geomagnetic reversals recorded by 1 1 billion year old keweenawan basalts 1 1 billion year old volcanic rocks in north america are thought to record asymmetric geomagnetic reversals indicating non axial dipolar behaviour of the magnetic field high resolution data from ontario suggest that the reversals were instead symmetric and that the apparent reversal asymmetry is an aliasing effect of the low resolution of earlier samples combined with the rapid motion of north america

the direction correction tilt test an all purpose tilt fold test for paleomagnetic studies abstract the tilt or fold test in paleomagnetism is used to infer whether paleomagnetic remanence was acquired before or after tectonic tilting while several tilt test formulations have been proposed none fully satisfies the requirements of statistical validity applicability to all bedding geometries and ease of use this paper introduces the direction correction dc test which examines the relationship between the paleomagnetic site mean directions and their corresponding bedding tilt corrections the dc test is similar to mcfadden s correlation test because both test whether or not the site directions contain information about the bedding tilts however the dc test gives greater weight to sites with greater counter bedding the dc test is similar to watson s numerical tilt tests because they determine the degree of untilting which gives optimal concentration of site directions however the dc test uses analytical rather than numerical methods graphical output aids the researcher in recognizing problem sites using both real and simulated data the dc test is demonstrated to be more discriminating than other tilt test formulations for all bedding geometries the simulations show that the power of the tilt test is inversely proportional to the 95 confidence interval 95 of the overall mean as a rule of thumb paleomagnetists should attempt to sample sufficient sites to obtain an 95 less than 1 6 of the bedding attitude difference