cosmic impact or natural fires at the allerod younger dryas boundary a matter of dating and calibration wittke et al 1 present evidence of a major cosmic impact at the onset of the younger dryas yd episode including some markers found in the top of the well known usselo marker horizon uh this finding is contrary to our extensive radiocarbon dating effort from this horizon 2 which shows that the uh at aalsterhut postdates the onset of the yd furthermore wittke et al 1 misinterpret the origin of the uh this horizon is a well defined paleosoil that formed during the allerod and the early yd in the top part of coversand this coversand was deposited before the allerod during cold and dry conditions and is part of the european sandbelt wildfires were common and occurred throughout this period rather than synchronously with the onset of the yd 3

new york alabama lineament a buried right slip fault bordering the appalachians and mid continent north america the new york alabama ny al lineament recognized in 1978 is a magnetic anomaly that delineates a fundamental though historically enigmatic crustal boundary in eastern north america that is deeply buried beneath the appalachian basin data not in the original aeromagnetic data set particularly the lack of any information available at the time to constrain the southern continuation of the anomaly southwest of tennessee left the source of the lineament open to conjecture we use modern digital aeromagnetic maps to fill in these data gaps and for the first time constrain the southern termination of the ny al lineament our analysis indicates that the lineament reflects a crustal scale right lateral strike slip fault that has displaced anomalies attributed to grenville orogenesis by 220 km palinspastic restoration of this displacement rearranges the trace of the grenville belt in southern rodinia and implies only passive influence on later formed appalachian structures the precise timing of dextral movement on the ny al structure is not resolvable from the existing data set but it must have occurred during one of or combinations of the following events 1 a late postcontractional post ottawan stage of the grenville orogeny 2 late neoproterozoic to cambrian rifting of laurentia or 3 right slip reactivation during the late neoproterozoic cambrian rifting of laurentia or during appalachian movements our palinspastic reconstruction also implies that the host rocks for modern earthquakes in the eastern tennessee seismic zone are metasedimentary gneisses and it provides an explanation for the spatial location and size of the seismic zone

deformation induced growth of sillimanite stress minerals revisited a pressure solution process is proposed for the development of monomineralic folia of sillimanite needles that cross cut and anastomose around feldspars and biotite in a weakly deformed granitic pegmatite dike near centerbrook connecticut the truncation of pegmatitic feldspars by sillimanite folia provides strong evidence that these structures and thus also the sillimanite formed in the subsolidus metamorphic regime the universally strong preferred orientation of the sillimanite needles the buckling of some of the folia into tight folds and the truncation of some strands of sillimanite by others are strong evidence that the sillimanite growth was syntectonic textural relations and these high variance assemblages suggest that metasomatic reactions of the type 2 feldspar 2h sillimanite 5 sio 2 aq 2k h 2 o were responsible for sillimanite growth in the pegmatite the very large negative delta v solids indicates that locally high normal stress could have driven these re

tectonic geomorphology of the southeastern mississippi embayment in northern mississippi usa we analyzed drainage basin geometry in part of the southeastern mississippi embayment near the new madrid seismic zone in order to detect areas of lateral stream migration that could indicate recent tectonism toward this goal a field of drainage basin asymmetry vectors was generated from a digital terrain model and compared to geologic structure lithofacies seismicity remotely sensed lineaments and major drainage divides transverse topographical drainage basin asymmetry t index results define geomorphic domains in northern mississippi 33 to 35 n 88 5 to 90 25 w that may reflect deep crustal blocks bounded by active faults or flexures t index measurements a proxy for lateral stream migration of more than 4500 second order drainage basins produced a two dimensional field of 282 spatially averaged vectors from which we interpreted 12 domains some domain boundaries correspond to mapped faults of two paleozoic fault systems in our study area the northeast striking mississippi valley graben system and the northwest striking ouachita thrust belt and foreland faults several other prominent domain boundaries may be previously unmapped faults flexures parallel to these systems this interpretation is supported by satellite image analysis of the northeastern study area which reveals lineaments consistent with our domain boundaries that have strikes that reflect the regional subsurface structural grain

fluid migration and associated diagenesis in the greater reelfoot rift region midcontinent united states the sources and pathways of basinal fluid flow on the north american craton have been the subjects of intense debate regional dolomitization and the distribution of petroleum and sulfide mineralization on the southern midcontinent are believed to be related to migration of warm basinal fluids from and within the arkoma and illinois basins because of its hydrologic connection with these two basins the reelfoot rift has been postulated as a potential source and or conduit for one or more of these fluids cathodoluminescence petrography has revealed nine distinct carbonate cement microstratigraphies in the reelfoot rift and on its adjacent carbonate platforms in missouri western platform and tennessee eastern platform temporal geochemical trends within each of the three areas are distinct eastern platform carbonate cements and their fluid inclusions exhibit decreasing 13c 18o homogenization temperatures t h and last ice melting t m values with decreasing age reelfoot rift dolomite and calcite cement and their fluid inclusions define a trend of decreasing 13c and 18o values and increasing t h and t m values western platform dolomite cements including those in the nearby southeast missouri mississippi valley type mineral district display a paragenetic trend of increasing 13c and 18o values followed by a return to lower values during the final stage of cement deposition distinct geochemical patterns indicate that stratigraphically controlled fluid flow through carbonate aquifers in the reelfoot rift was restricted within individual fault blocks that had limited communication with the adjacent carbonate platforms lateral fluid migration between fault blocks and vertical migration within fault blocks in the reelfoot rift likely was restricted by faults stratigraphic offset and impermeable units if significant fluid movement occurred from the reelfoot rift onto the adjacent western carbonate platform it likely occurred along northwest striking strike slip faults

seismic anisotropy beneath the mississippi embayment and the new madrid seismic zone a study of shear wave splitting shear wave splitting associated with the mississippi embayment me is determined using teleseismic sks phases recorded by the northern embayment lithosphere experiment nele the usarray transportable array ta and the new madrid seismic network for the period 2005 2016 our data set consists of 5900 individual splitting measurements from 257 earthquakes recorded at 151 stations within and outside the me stations outside of the me exhibit significant shear wave splitting with average delay times between 0 4 s and 1 8 s to the northeast and east of the me nearly all observed fast orientations are approximately oriented northeast southwest in agreement with absolute plate motion apm predicted by hs3 nuvel 1a the homogeneity of the fast orientations in this region suggests that the splitting is due to active flow in the asthenosphere a counterclockwise rotation in the splitting orientation is observed moving northeast to northwest across the study area inside the me some stations show large and systematic deviations of the measured fast orientations from the apm the delay times within the entire me range from 0 9 s to 2 1 s splitting complexity is attributed to relic lithospheric fabrics formed during past tectonic events including passage of a hot spot in mid cretaceous time the anisotropy may also be linked to the presence of a southwest dipping region of low p and s wave velocities below the me or to deeper flow in the asthenosphere

archean gold bearing quartz veins at the sigma mine abitibi greenstone belt quebec part i geologic relations and formation of the vein system the sigma deposit consists of an archean gold bearing vein system in metavolcanic rocks intruded by an irregular body of porphyritic diorite both affected by regional deformation and in turn cut by younger undeformed feldspar porphyry dikes the veins cut all rock types and are composed chiefly of quartz and tourmaline with minor amounts of carbonates pyrite chlorite scheelite and free gold they were emplaced during or after a greenschist facies metamorphism postdating the intrusion of feldspar porphyry dikes the veins were not significantly affected by any subsequent metamorphism or tectonism two principal types of veins occur in the mine subvertical veins and subhorizontal veins they are contemporaneous and cogenetic subvertical veins occur within steeply dipping east west trending ductile shear zones along which reverse subvertical displacements have taken place the veins occupy openings created by overriding of undulations and irregularities of foliation and slip planes during progressive shearing movements within the shears subhorizontal veins occupy extensional fractures which developed between ductile shear zones preferentially in the more competent host rocks these veins were formed by one or several episodes of open space filling geologic relations indicate that the mineralized veins were emplaced progressively in a dynamic tectonic environment during and shortly after the formation of ductile shear zones a structural analysis has shown that the ductile shear zones and the two vein types can be related to a single strain ellipsoid of the following orientation the z axis trending north south and horizontal the y axis plunging 10 degrees west and the x axis plunging 80 degrees east the deformation responsible for the generation of ductile shear zones and emplacement of the veins is ascribed to a late north south trending compression postdating regional deformation

calculated mineral equilibria in the pelite system kfmash k 2 o feo mgo al 2 o 3 sio 2 h 2 o

predictive distribution of fault fill and extensional veins example from the sigma gold mine abitibi subprovince canada this paper presents the results of a geometric and structural study aimed at developing a predictive model to account for the distribution of gold bearing veins at the sigma gold deposit abitibi subprovince canada results indicate that third order shear zones define a well organized system comprising two sets of parallel striking moderately to steeply south dipping curviplanar structures intersecting at constant 15 to 25 angles fault fill veins are better developed within the curved segments of the shear zones that represent large zones of enhanced permeability most extensional veins were formed later as tension gashes mainly distributed along conjugate corridors of low reverse faults typical of andersonian type faulting

structural setting hydrothermal alteration and gold mineralisation at the archaean syenite hosted jupiter deposit yilgarn craton western australia the jupiter gold deposit in the northeastern eastern goldfields province of the yilgarn craton of western australia is hosted in greenschist facies metamorphosed tholeiitic basalt quartz alkali feldspar syenite and quartz feldspar porphyry syenite intrudes basalt as irregularly shaped dykes which radiate from a larger stock whereas at least three e w and ne sw striking quartz feldspar porphyries intrude both syenite and basalt brittle ductile shear zones are shallow dipping nw to ne striking or are steep dipping to the south and west quartz carbonate veins that host gold at jupiter occur in all lithologies and are divided into 1 veins that are restricted to the shear zones 2 discrete veins that are subparallel to shear zone hosted veins and 3 stockwork veins that form a network of randomly oriented microfractures in syenite wallrock proximal to shallow dipping shear zones the gold bearing veins comprise mainly quartz calcite ankerite and albite with minor sericite pyrite chalcopyrite galena sphalerite molybdenite telluride minerals and gold proximal hydrothermal alteration zones to the mineralised veins comprise quartz calcite ankerite albite and sericite high gold grades 2 g t au occur mainly in syenite and in the hanging walls to shallow dipping shear zones in syenite where there is a greater density of mineralised stockwork veins the jupiter deposit has structural and hydrothermal alteration styles that are similar to both granitoid hosted but post magmatic archaean lode gold deposits in the yilgarn craton and intrusion related syn magmatic syenite hosted gold deposits in the superior province of canada based on field observations and petrologic data the jupiter deposit is considered to be a post magmatic archaean lode gold deposit rather than a syn intrusion deposit

on the initiation of metamorphic sulfide anatexis mineral assemblages in common sulfide ore deposits are examined together with phase relations to 1 investigate the pressure temperature conditions required for the onset of metamorphically induced partial melting involving economic minerals and 2 place constraints on the amount of melt produced deposits that contain sulfosalt or telluride minerals may start to melt at conditions ranging from lowest greenschist facies to amphibolite facies deposits lacking sulfosalt and or telluride minerals may begin to melt once p t conditions reach the upper amphibolite facies if galena is present or well into the granulite facies if galena is absent the result is two broad melting domains a lowto medium temperature low melt volume domain involving melting of volumetrically minor sulfosalt and or telluride minerals and a high temperature potentially higher melt volume domain involving partial melting of the major sulfide minerals epithermal gold deposits which are especially rich in sulfosalt minerals are predicted to commence melting at the lowest temperatures of all sulfide deposit types massive pb zn cu deposits may start to melt in the lower to middle amphibolite facies if pyrite and arsenopyrite coexist at these conditions and in the upper amphibolite facies if they do not excepting sulfosalt bearing occurrences massive ni cu pge platinum group element deposits will show little to no melting under common crustal metamorphic conditions whereas disseminated cu deposits are typically incapable of generating melt until the granulite facies is reached when partial melting commences in bornite bearing rocks the volume of polymetallic melt that can be generated in most deposit types is therefore largely a function of the abundance of sulfosalt minerals even at granulite facies conditions this volume is usually less than 0 5 the exception is massive pb zn deposits where melt volumes significantly exceeding 0 5 vol may be segregated into sulfide magma dykes allowing mobilization over large distances

joint local and teleseismic tomography in the central united states exploring the mantle below the upper mississippi embayment and the illinois basin three dimensional high resolution crustal and upper mantle p and s wave velocity vp and vs models are presented for the central united states the study utilizes local and teleseismic data reco

uniformity in sulfur isotope composition in the orogenic gold deposits from the dharwar craton southern india the sulfur isotope composition of sulfides mainly pyrite and arsenopyrite from gold deposits prospects of the dharwar craton such as hutti hira buddini uti kolar chigargunta ajjanahalli and jonnagiri has a narrow range 34s 1 1 to 7 1 such craton scale uniformity of the above gold camps is noteworthy in spite of the wide diversity in host rock compositions and their metamorphic conditions and suggests a magmatic or average crustal source of sulfur for all deposits studied in addition our study points towards gold precipitation from reduced ore fluids with near homogeneous sulfur isotope compositions

stability of andalusite and sillimanite and the al_ 2 sio_ 5 triple point constraints from the ballachulish aureole scotland andalusite and and sillimanite sil occur in well defined zones in the ballachulish aureole in most of the aureole s sillimanite bearing rocks sillimanite grain diameters range from 2 m to 10 m the boundary between prograde andalusite and prograde sillimanite is constrained to a zone of max 30 m width upgrade of the muscovite quartz andalusite k feldspar h 2 o reaction isograd this boundary is interpreted to correspond to the and sil equilibrium the common occurrence of andalusite well upgrade of the first occurrence of sillimanite is ascribed to sluggish kinetics of the polymorphic inversion the p t position of the and sil equilibrium in the aureole is estimated as 645 20 c and 3 0 0 4 kbar based on a combination of 1 location of the ms qtz and kfs h 2 o reaction corrected for non end member fluid and mineral compositions and 2 independent pressure estimates obtained from equilibria in metapelites marbles calcsilcates and granitoids all in exce

hotspot origin of the mississippi embayment and its possible impact on contemporary seismicity abstract previous authors have related the late cretaceous early tertiary subsidence of the mississippi embayment to the opening of the gulf of mexico but the gulf opened earlier in triassic jurassic time we offer an alternative hypothesis that development of the embayment was coeval with the passage of the mississippi valley graben system over the bermuda hotspot about 90 ma several lines of evidence of significant uplift of the embayment axis accompanying mid cretaceous magmatism and prior to late cretaceous subsidence support this proposal first reactivation of the pascola arch in the northern embayment is recorded by flanking deposits of basal upper cretaceous gravel second beneath a regional mid cretaceous unconformity subcrops of jurassic and early cretaceous strata define a pronounced southwest plunging arch in the southern embayment this arch is collinear with an arch revealed in paleozoic rocks after restoration to mid cretaceous structural geometries third a deep weathering profile on mid cretaceous alkalic plutons along the western embayment margin is nonconformably overlain by paleocene sediments and rapid mid cretaceous cooling of these intrusions has been interpreted from apatite fission tracks moreover exploratory holes along the embayment axis encountered similar weathered alkalic intrusions nonconformably overlain by basal upper cretaceous strata fourth there was an anomalous influx of clastic sediment into the northern gulf of mexico during mid cretaceous time and subsequent clastic facies patterns suggest the mississippi river drainage began to enter the gulf in the late cretaceous passage of the mississippi valley graben over the bermuda hotspot during elevated hotspot activity of cretaceous time may have significantly weakened the previously rifted lithosphere rifted continental margin at charleston south carolina also passed over this hotspot in latest cretaceous time similarly the st lawrence rift system passed over the great meteor hotspot during the cretaceous it is important to note that these rift systems are the principal loci of strong seismicity in eastern north america and thus weakening by increased cretaceous hotspot activity may be an important common factor for these seismic rift zones

metallogeny of the marco zone corvet est disseminated gold deposit james bay quebec canada the corvet est gold deposit is hosted by archean rocks of the superior province in the james bay region northern quebec canada the marco zone is hosted by amphibolite grade strongly foliated volcanic rocks and consists of disseminated gold with an apparent thickness ranging from 1 8 to 39 5 m and gold grades up to 23 g t 1 over 1 m that is continuous along strike for 1 3 km the lithotectonic sequence comprises footwall basaltic andesite amphibolite overlain by a lenticular unit of metadacite and then by hanging wall basaltic andesite amphibolite all intruded by quartz feldspar porphyry dikes dacite basaltic andesite amphibolite and quartz feldspar porphyry show a calc alkaline to transitional affinity and plot in the plate margin arc basalt field with typical volcanic arc trace element patterns mineralization consists of pyrite arsenopyrite pyrrhotite chalcopyrite and gold disseminated in deformed dacite in andesite amphibolite and in quartz feldspar porphyry dikes dacite and andesite display weak alteration characterized by silicification native gold forms inclusions in metamorphic quartz garnet feldspar arsenopyrite and pyrite or free grains interstitial to quartz feldspar pyrite chalcopyrite and arsenopyrite free gold in late quartz veins cut the serici tized metamorphic fabric inclusion and interstitial native gold within minerals annealed during metamorphism shows that gold mineralization is pre to syn metamorphic with some gold remobilized in later veins rhenium osmium dating of arsenopyrite yields an isochron age of 2663 13 ma for mineralization and a weighted average model age of 2632 7 ma for arsenopyrite formed during peak metamorphism the 2663 ma arsenopyrite has a low initial 187 os 188 os of 0 19 0 10 suggesting a juvenile crust or a mantle os source the sulfur isotope composition of marco zone pyrite and arsenopyrite shows that sulfur could have been leached from its volcanic host rocks or from reduction of archean seawater the corvet est deposit is interpreted to be an oro genic gold deposit 2663 ma deformed and recrystallized during amphibolite grade metamorphism 2632 ma

the silidor deposit rouyn noranda district abitibi belt geology structural evolution and paleostress modeling of an au quartz vein type deposit in an archean trondhjemite late orogenic gold bearing quartz vein deposits within the rouyn noranda mining district occur mainly within archean tonalitic plutons of the blake river group southern abitibi belt the silidor deposit is a representative example of a pluton hosted lode gold deposit the silidor mine contained 2 95 million tons mt mined and estimated reserves grading 5 1 g t au 15 t au the mineralized zone is 900 m in length and has a vertical extent of 900 m an average thickness of 3 5 m and trends northwest southeast with a dip of 50 to 70 ne its alteration envelope consists of a red hematite altered trondhjemite measured 18 o values for quartz veins 7 7 10 9 and 34 s values for pyrite 6 1 to 9 4 imply oxidizing conditions during gold deposition the mineralized zone comprises vein quartz white gray and smoky varieties beige mineralized trondhjemite and green carbonate sericite fuchsite breccia which resulted from shearing and metasomatism of an early northwest southeast dioritic dike a quantitative microtectonic study 400 measurements was carried out on the powell tonalitic sill in the silidor mine area to reconstruct the deformation before during and after the mineralizing events the reduced stress tensors display large variations in 1 orientation trending successively northwest southeast northeast southwest and finally north south the silidor deposit is the result of several vein filling events which occurred during evolution from strike slip faulting to reverse faulting regimes with 1 remaining northeast southwest but with exchange of orientation of 2 and 3 such variations may reflect an oblique collision and processes of stress permutation paleostress mapping of archean terranes can be used as a targeting tool for mesothermal lode gold deposits reduced stress tensors were used for the computation of paleostress maps using a distinct element model the silidor au quartz mineralization appears on these paleostress maps in an area characterized by low mean stress throughout the deformation history of the area near the horne creek fault this study emphasizes the role of second order faulting in the location of low and high pressure zones in the archean crust and the possible role of a tectonic indentor in the location of au mineralization

late cretaceous and cenozoic geology of the new madrid seismic zone structure contour maps constructed from well seismic reflection and outcrop data of the tops of the paleozoic section upper cretaceous section paleocene midway group and eocene section illustrate the post paleozoic structure of the new madrid seismic zone region isopach maps of the late cretaceous section midway group and eocene section help constrain the timing of the structural events these maps which encompass much of the northern mississippi embayment reveal reactivation of the underlying late precambrian cambrian reelfoot rift during midway group deposition but no reactivation during late cretaceous or eocene deposition the structure contour maps also indicate a subtle south plunging depression on the tops of the paleozoic upper cretaceous and midway group along the axis of the northern mississippi embayment that we have called a trench this trench is 50 km wide has a maximum depth of 100 m and appears to have formed during the eocene the trench s western boundary coincides with the blytheville arch lake county uplift and its southeastern margin underlies memphis tennessee the blytheville arch lake county uplift is the structure responsible for the new madrid seismic zone and thus it is possible that the southeastern margin of the trench is also a fault zone northern mississippi embayment post paleozoic stratigraphy consists of sands silts and clays that thicken from 477 m at new madrid missouri to 987 m near memphis tennessee the uniformity of these sediments indicates their elastic properties and therefore seismic velocities are very similar however variations in cementation and unconformities within the section may influence seismic wave propagation manuscript received 17 june 1999

stratigraphic and structural setting of the hemlo gold deposit ontario canada results of detailed surface and underground mapping and a compilation of data from the three gold mines in the hemlo area ontario are presented in particular the lithostratigraphy of the area is established the nature of the lithologic units spatially closely associated with the ore and the protolith of the ore are determined and the deformation history and the three dimensional geometry of the area are elucidated the genesis of the gold deposit is discussed in context of these results four generations g 1 to g 4 of ductile structures as well as brittle faulting are recognized g 2 deformation is the strongest and the geometry of the hemlo camp is dominated by macroscopic camp scale s shaped f 2 folds the two ore zones at hemlo the main and lower ore zones are spatially associated with the two limbs of a newly recognized camp scale f 2 fold the moose lake fold and the orebody is folded by f 2 at outcrop scale mine scale and possibly camp scale g 2 deformation is most intense in the hemlo shear zone interpreted to be a sinistral transpressional zone the hemlo gold deposit is hosted in the shear zone mainly in the segment that trends east southeasterly where deformation is stronger the main gold mineralization at hemlo occurred before g 2 or early during g 2 deformation and before peak metamorphism the latter took place during late g 2 to after g 2 deformation the ore and alteration zones are dominantly but not exclusively spatially associated with the stratigraphically lower contact of a volcanic quartz feldspar porphyry the moose lake porphyry a mafic fragmental unit altered felsic fragmental rock and a barite horizon are spatially closely associated with the ore zones the protolith of the ore is mainly the fragmental rock and the barite the stratigraphically lower contact of the moose lake porphyry and the fragmental rock at the contact probably served as mechanical traps and the barite horizon as a chemical trap

polymetamorphism in the archean hemlo heron bay greenstone belt superior province p t variations and implications for tectonic evolution the tectono metamorphic history of the late archean 2800 2600 ma hemlo heron bay greenstone belt in the superior province has been delineated from textural relationships mineral chemistry and p t paths in metapelites cordierite orthoamphibole rocks and metabasites from the white river exploration property hemlo area ontario an early low temperature medium pressure metamorphism about 500 c and 6 6 5 kbar 1 kbar 100 mpa is indicated by the occurrence of relict kyanite and staurolite porphyroblasts and zoned garnet porphyroblasts in metapelites and the presence of zoned calcic amphiboles in metabasites this early metamorphism appears to have been coeval with the previously documented d1 deformation that is associated with for example low angle thrusts a second regional metamorphism predominates in the hemlo heron bay greenstone belt and is generally of relatively low grade at about 510 530 c and 3 2 3 5 kbar over most of the study area and increases to medium grade 550 650 c and

stable isotope geochemistry of the archean val d or canada orogenic gold vein field a systematic study of the auriferous quartz veins of the val d or vein field abitibi quebec canada demonstrates that the c o s isotope composition of silicate carbonate borate oxide tungstate and sulphide minerals have a range in composition comparable to that previously determined for the whole superior province the oxygen isotope composition of quartz from early quartz carbonate auriferous veins ranges from 9 4 to 14 4 whereas later quartz tourmaline carbonate veins have 18oquartz values ranging from 9 2 to 13 8 quartz carbonate veins have carbonate 18o 6 9 12 5 13c 6 2 1 9 and pyrite 34s 1 2 and 1 9 isotope compositions comparable to those of quartz tourmaline carbonate veins 18o 7 9 11 7 13c 8 0 2 4 34s 0 6 6 0 18oquartz values in quartz tourmaline carbonate veins have a variance comparable to analytical uncertainty at the scale of one locality irrespective of the type of structure the texture of the quartz or its position along strike across strike or down dip a vein in contrast the oxygen isotope composition of quartz in quartz tourmaline carbonate veins displays a regional distribution with higher 18o values in the south central part of the vein field near the cadillac tectonic zone and which 18o values decrease regularly towards the north another zone of high 18o values in the northeast corner of the region and along the trace of the senneville fault is separated by a valley of lower 18o values from the higher values near the cadillac tectonic zone oxygen isotope isopleths cut across lithological contacts and tectonic structures this regional pattern in quartz tourmaline carbonate veins is interpreted to be a product of reaction with country rocks and mixing between 1 a deep seated hydrothermal fluid of metamorphic origin with minimum 18o 8 5 13c 0 6 and 34s 0 4 and 2 a supracrustal fluid most likely archean seawater with a long history of water rock exchange and with maximum 18o 3 9 13 c 5 6 and 34s 5 0

conditions and timing of metamorphism in the southern abitibi greenstone belt quebec regional metamorphism ranging in grade from the subgreenschist facies to the greenschist amphibolite facies transition affects all archean supracrustal rocks 2677 ma in the rouyn noranda area contact metamorphic minerals associated with the posttectonic preissac lacorne batholith 2643 ma show no evidence of a regional retrograde event accordingly the age of regional metamorphism can be bracketed between 2677 and 2643 ma three reaction isograds were mapped in subgreenschist facies metabasites dividing the low grade rocks into three metamorphic zones the pumpellyite actinolite zone the prehnite pumpellyite zone and the prehnite epidote zone in addition the pumpellyite actinolite epidote quartz bathograd corresponding to a pressure of approximately 200 mpa occurs on both sides of the porcupine destor fault low pressure regional metamorphism is also indicated both by the occurrence of an actinolite oligoclase zone and the persistence of pre regional metamorphic andalusite the coincidence

possible neoproterozoic early paleozoic grabens in mississippi alabama and tennessee low intensity magnetic and negative gravity anomalies in northern mississippi north western alabama and western tennessee are viewed as being similar to the magnetic and gravity low associated with the mississippi valley graben the anomalies are interpreted as shallow grabens and it is proposed that the grabens like the mississippi valley graben resulted from northwest southeast extension during neoproterozoic early paleozoic fragmentation of laurentia if real the grabens indicate that extension affected a larger part of laurentia than has been previously documented they imply hitherto unsuspected oil and gas potential and if the area is coupled to the stress field affecting the mississippi valley graben represent structures that warrant evaluation for seismic risk

unraveling the geologic history of the hemlo archean gold deposit superior province canada a u pb geochronological study the hemlo deposit is one of canada s largest gold camps gold is both structurally and lithologically controlled but shows unusual relationships relative to many other archean lode deposits field evidence suggests that gold was emplaced within mechanical and chemical traps along a jog in a major sinistral shear zone most likely during early formation of second generation g2 structures which represent the most intense phase of regional deformation mineralization was followed by amphibolite facies regional metamorphism u pb geochronology has been carried out on zircons from rocks associated with the deposit in order to determine the precise chronology of geologic events and their relationship to mineralization zircon populations are commonly affected by both inheritance and hydrothermal metamorphic crystal growth meaningful and precise age information can only be determined by precise single zircon dating on rocks from a well constrained structural and stratigraphic context gold is mostly found near the deformed contact between the moose lake quartz porphyry volcanic complex and stratigraphically underlying metasedimentary rocks these rocks have been folded by a camp scale noncylindrical f2 fold zircon from the moose lake quartz porphyry is dominated by inheritance and gives ages that extend from 2800 to 2690 ma metasedimentary rocks that underlie and overlie the moose lake porphyry show a much narrower range of provenance ages and are dominated by 2 690 to 2 693 million year old zircon two detrital zircons from a metasedimentary rock directly beneath the porphyry define an age of 2685 4 ma this should be an older age limit on volcanism deformation and gold mineralization the four youngest near concordant zircons from the cedar lake pluton define a magmatic age of 2680 1 ma this pluton was emplaced prior to or early during g2 deformation while emplacement of gold was most likely controlled by g2 structures therefore 2680 1 ma probably also represents an older age limit on gold mineralization a feldspar porphyry dike that cuts gold ore gives a younger age bracket of 2677 1 ma the results of zircon dating suggest that a volcanosedimentary complex was constructed in the area over the period of 2693 to 2685 ma this developed on earlier crust ca 2720 ma in age but there is evidence from detritus and xenocrysts for older rocks dating back to over 2800 ma deposition of supracrustal rocks was followed by granodiorite plutonism at 2680 1 ma then major deformation g2 and gold mineralization overprinted by amphibolite facies metamorphism previously dated at 2676 ma from titanite ages late tectonic sanukitoid suite plutons were emplaced at 2677 1 ma thus the primary gold forming event occurred near the beginning of a period of plutonism and crustal reworking that lasted at most a few million years this time association supports the view that granitoid magmas were the source of auriferous fluids plutons intruded into an actively deforming crust and were probably also the heat source for regional metamorphism the most likely environment was a tectonically active timiskaming type sedimentary basin similarly aged basins are found in the timmins area to the east and the shebandowan greenstone belt to the west

magmatic and tectonic emplacement of the pukaskwa batholith superior province ontario canadathis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh the neoarchean pukaskwa batholith consists of pre syn and post tectonic phases emplaced over an interval of 50 million years pre tectonic phases are broadly synvolcanic and have a high al tonalite trondhjemite granodiorite ttg affinity interpreted to reflect derivation by partial melting of basaltic crust at lower crustal or upper mantle depths minor syn tectonic phases slightly post date volcanism and have geochemical characteristics suggesting some involvement or interaction with an ultramafic mantle source component magmatic emplacement of pre and syn tectonic phases occurred in the midcrust at paleopressures of 550 600 mpa and these components of the batholith are thought to be representative of the midcrust underlying greenstone belts during their development subsequent to emplacement of the syntectonic phases and likely at approximately 2680 ma the pukaskwa batholith was uplifted as a structural dome relative to flanking greenstone belts synchronously with ongoing regional sinistral t

identification of possible quaternary deformation in the northeastern mississippi embayment using quantitative geomorphic analysis of drainage basin asymmetry to investigate neotectonism in the mississippi embayment east of the new madrid seismic zone we identified geomorphic domains that show evidence of ground tilting during quaternary time transverse basin profiles were converted to two dimensional vectors that denote channel position with respect to basin divides these basin asymmetry vectors record the net direction and degree of lateral migration of trunk streams more than 2500 vectors were measured and spatially averaged within 400 km 2 bins this field of 300 mean vectors delineates several domains that show preferred directions of stream migrations possibly driven by ground tilting the timing of stream migration was interpreted using across valley distributions of quaternary alluvial terraces comparison of our mean vector field with subsurface structures suggests that some domain boundaries may be related to reactivated faults late quaternary activity is suggested for two northeast striking faults of the southeastern reelfoot rift margin we acquired two seismic profiles showing near surface faulting beneath scarps that follow the domain boundary associated with one of these northeast striking faults big creek fault zone reelfoot thrust seismicity ends on the south against this fault suggesting that the rift margin has dextral slip accommodating northeastward movement of the thrust hanging wall our vector field also suggests late quaternary movement on the reelfoot thrust and on two other northwest striking faults here termed the hatchie river fault and the wolf river fault several other weak domains may imply minor elements of neotectonism our results demonstrate that morphometric analysis of drainage basin asymmetry can be an effective reconnaissance tool within neotectonic settings

stratigraphy and structure of the eocene memphis sand above the eastern margin of the reelfoot rift in tennessee mississippi and arkansas usa in total 1249 geophysical well logs were interpreted to map the stratigraphy and structure of the eocene memphis sand in west tennessee northwest mississippi and crittenden county arkansas where it overlies the southeastern margin of the reelfoot rift the memphis sand is a 300 m thick reworked fluvial sand deposited on a low relief subsiding plain with limited accommodation space the memphis sand has experienced syn and postdepositional faulting of four types in the region 1 west trending grabens were active during eocene claiborne deposition these grabens also appear to control modern drainage and thus may extend to the surface 2 the n33 e shelby faults pass through shelby county tennessee into northwest mississippi and extend to or near the surface they were right lateral normal faults during eocene memphis sand deposition but they became inverted reverse faults in the late eocene 3 the n67 e striking gibson faults are high angle normal and reverse faults with a component of right lateral strike slip these faults were active during memphis sand through lower cockfield deposition north of the hatchie graben and appear to be due to faulting above an outboard fault of the reelfoot rift 4 the n43 e striking lauderdale faults are high angle normal and reverse faults that reflect right lateral strike slip reactivation of the southeastern margin of the reelfoot rift the structure and stratigraphy revealed in this mapping indicate that the geology of the memphis sand is more complex than previously recognized faulting of the memphis sand and overlying sediments reveals potentially seismogenic faults in western tennessee eastern arkansas and northwestern mississippi this study provides fundamental data for future groundwater modeling and seismic hazard analysis

synchronous records of pco2 and δ14c suggest rapid ocean derived pco2 fluctuations at the onset of younger dryas just before the onset of the younger dryas yd cold event several stomatal proxy based pco2 records have shown a sharp increase in atmospheric co2 concentration pco2 of between ca 50 and 100 ppm followed by a rapid decrease of similar or even larger magnitude here we compare one of these records a high resolution pco2 record from southern sweden with the intcal13 record of radiocarbon d 14 c the two records show broadly synchronous fluctuations at the yd onset specifically the intcal13 record documents decreasing d 14 c just before the yd onset when pco2 peaks consistent with a source of old co2 from the deep ocean we propose that this fluctuation occurred due to a major ocean flushing event the cause of the flushing event remains speculative but could be related to the hypothesis of the glacial ocean as a thermobaric capacitor we confirm that the earth system can produce such large multidecadal timescale fluctuations in pco2 through simulating an artificial ocean flushing event with the genie earth system model we suggest that sharp transitions of pco2 may have remained undetected so far in ice cores due to inter firn gas exchange and time averaging the stomatal proxy record is a powerful complement to the ice core records for the study of rapid climate change

two phase syntectonic gold mineralization and barite remobilization within the main ore body of the golden giant mine hemlo ontario canada abstract structural analysis of the central part of the hemlo gold deposit in the hemlo heron bay greenstone belt has revealed four generations of deformation structures d1 d4 the second d2 and fourth d4 deformations significantly influenced the orebody and controlled the localization of gold mineralization development of d3 dextral shearing during retrogressive conditions resulted in strain partitioning around the feldspathic orebody such that this deposit appears to be controlled by the hemlo shear zone however the orebody dominantly preserves vertical d2 parasitic folds rather than d3 structures the earliest occurrence of gold is with molybdenite as s1 inclusion trails in vanadium muscovite grains that grew during d2 this suggests that the gold deposit was emplaced by at least early d2 syngenetic banded barite was partially dissolved transported and redeposited during each deformation event and in particular in association with brittle deformation that occurred during d4 microstructural relationships indicate two distinct generations of gold the first generation is associated with molybdenite and potassic alteration and was deposited before or early in d2 during progressive amphibolite facies metamorphism the second generation formed by redistribution and concentration of gold together with stibnite in ductile brittle d4 structures after peak metamorphic conditions

magmatism metamorphism and deformation at hemlo ontario and the timing of au mo mineralization in the golden giant mine the golden giant mine is developed in the archean hemlo gold molybdenum deposit hosted within the hemlo heron bay greenstone belt in northwestern ontario canada the ore occurs in steeply dipping tabular zones within metasedimentary rocks and within quartz eye muscovite schists in association with microcline mica pyrite alteration and a variety of other minerals the hemlo area has been intruded by quartz feldspar porphyries the golden giant porphyry and sceptre porphyry these porphyries are altered mineralized and highly deformed the calc alkalic granodioritic cedar creek stock and cedar lake pluton occur near the deposit feldspar porphyry sills and dikes generated from the stock and pluton cut the earlier porphyries mineralized rocks and many of the fabric elements the golden giant rocks have been regionally metamorphosed to amphibolite grades this early m 1 metamorphisin t 500 degrees 600 degrees c p 5 6 kbars probably accompanied the intrusion of the cedar lake pluton and produced mineral assemblages in metasedimentary rocks containing hornblende kyanite staurolite garnet and micas a second m 2 metamorphic event t 300 degrees 400 degrees c p 1 3 kbars produced mineral assemblages containing actinolite fibrolite andalusite calcite garnet epidote chlorite and muscovite the golden giant mine rocks are strongly foliated and commonly exhibit mylonitic and transposed fabrics isoclinal folds and boudinage in both mineralized and nonmineralized rocks it is proposed that the primary au mo mineralization occurred before the m 1 metamorphic event possibly related to emplacement of the golden giant quartz porphyry in a premineralization structural zone the mineralization was modified by the m 1 event and severely deformed during mylonitization dikes from the cedar creek stock intruded the deposit sequence and overprinted the m 1 event with an m 2 metamorphism

genesis of the auriferous c quartz tourmaline vein of the siscoe mine val d or district abitibi subprovince canada structural mineralogical and fluid inclusion constraints the c quartz tourmaline vein of the siscoe deposit is a classic example of a high grade archean gold lode and averages 45 g t au locally attaining 221 g t au this vein cuts all dikes the main regional foliation s2 and several auriferous quartz carbonate veins and is interpreted to be a shear vein which formed during the development of a late to post d2 reverse fault the vein is composed of alternating tourmaline and quartz rich layers with minor amounts of rutile scheelite apatite chlorite muscovite carbonate mainly calcite pyrite chalcopyrite tetradymite and native gold vein filling occurred during the development of a reverse fault through a process involving repeated episodes of opening mineral deposition and plastic and brittle deformation the vein geometry was controlled by local rheologic heterogeneities or by a west northwest east southeast compressive event the wall rock alteration is characterized by the occurrence of tourmaline pyrite and calcite the timing of gold deposition and the gold fineness in the c quartz tourmaline vein differ from those of the wall rock in the vein native gold 4 7 7 3 wt ag was the latest phase to deposit and commonly occurs in fractures that cut across the quartz and tourmaline layers whereas in the wall rock gold 7 9 13 2 wt ag started to precipitate just before or synchronously with pyrite consequently the textural and mineral chemical evidence suggests that the fluids in the vein and wall rock were not in equilibrium with each other during gold deposition and that the mechanisms of precious metal precipitation were different in the two environments in the wall rock gold started to precipitate synchronously with sulfides due to reaction of hydrothermal fluids with magmatic ilmenite this reaction caused the destabilization of gold bisulfide complexes and prompted gold deposition however sulfides are very rare in the c quartz tourmaline vein and gold deposited after the main episode of vein filling suggesting that its precipitation was not controlled by pyrite deposition the occurrence of aqueous carbonic ac fluid inclusions in healed fractures that radiate from gold grains suggests that these fluids were associated with the ore forming process two types of aqueous carbonic fluid inclusions were identified h2o nacl co2 bearing salinities between 4 and 7 wt nacl equiv and h2o nacl co2 ch4 bearing salinities varying between 7 and 9 wt nacl equiv both types homogenize at t 190 c based on structural textural and fluid inclusion evidence we propose that gold deposition in the c quartz tourmaline vein took place after the main episode of vein filling as a result of continued fracturing and mixing of these two fluids

the geology and mineralogy of the loulo mining district mali west africa evidence for two distinct styles of orogenic gold mineralization loulo is a world class orogenic gold mining district in the birimian terrane of western mali orebodies are located along second or higher order shears associated with the senegal mali shear zone with gold mineralization largely linked to a transtensional event the mine camp is divided into two distinct styles of gold deposit on the basis of differing geologic characteristics one group is typified by the gara deposit whereas the other by the yalea deposit both deposit styles are hosted by similar rock types calcareous graywackes and calciticdolomitic marbles gara style orebodies occur as sulfide disseminations or ankerite rich shear vein stockworks hosted in folded tourmalinized sandstones and breccias mainly within 2 km of the senegal mali shear zone these deposits are characterized by intense multi stage albitization and tourmalinization pre syn and postmineralization gold lodes are fe rich dominated by nickeloan pyrite contain cu ni co minor and trace sulfides e g chalcopyrite gersdorffite pentlandite cobaltite millerite and show consistently high levels of p ree w bearing phases apatite monazite xenotime and scheelite base metal concentrations show a marked increase in marble host rocks with the formation of nickeloan pyrite cobaltite clausthalite ores in contrast yalea style deposits are associated with quartz ankerite vein lodes and disseminated sulfide stringer zones mineralization occurs along highly altered tourmaline absent brittle ductile shears up to 8 km away from the senegal mali shear zone wall rock alteration is characterized by addition of k 2 o cao co 2 h 2 o and sio 2 with mineral assemblages consisting of chlorite sericite carbonate quartz albite ore paragenesis is enriched in as mainly as multistage growth of arsenopyrite and arsenian pyrite base metal sulfides scheelite and ree phosphates are extremely rare the diversity in the ore paragenesis is controlled by a dynamic hydrothermal system that sourced fluids and metals from different reservoirs within the region the as rich yalea style deposits have characteristics typical of birimian gold mineralization in ghana and auriferous fluids are likely derived from the dewatering of sedimentary rocks during regional metamorphism on the other hand the polymetallic fe b rich gara style orebodies show atypical features for birimian gold mineralization instead field relations and the mineralogy and geochemistry of the gara style lodes indicate a strong hydrothermal influence from surrounding intrusive stocks with possible links between gold mineralization and iron oxide skarn development in the region the data collected at loulo highlight the diverse nature of orogenic gold deposits especially in west africa this style of ore deposit can form from a variety of fluid sources both metamorphic and magmatic

chronological evidence fails to support claim of an isochronous widespread layer of cosmic impact indicators dated to 12 800 years ago according to the younger dryas impact hypothesis ydih 12 800 calendar years before present north america experienced an extraterrestrial impact that triggered the younger dryas and devastated human populations and biotic communities on this continent and elsewhere this supposed event is reportedly marked by multiple impact indicators but critics have challenged this evidence and considerable controversy now surrounds the ydih proponents of the ydih state that a key test of the hypothesis is whether those indicators are isochronous and securely dated to the younger dryas onset they are not we have examined the age basis of the supposed younger dryas boundary layer at the 29 sites and regions in north and south america europe and the middle east in which proponents report its occurrence several of the sites lack any age control others have radiometric ages that are chronologically irrelevant nearly a dozen have ages inferred by statistically and chronologically flawed age depth interpolations and in several the ages directly on the supposed impact layer are older or younger than 12 800 calendar years ago only 3 of the 29 sites fall within the temporal window of the yd onset as defined by ydih proponents the ydih fails the critical chronological test of an isochronous event at the yd onset which coupled with the many published concerns about the extraterrestrial origin of the purported impact markers renders the ydih unsupported there is no reason or compelling evidence to accept the claim that a cosmic impact occurred 12 800 y ago and caused the younger dryas

comprehensive analysis of nanodiamond evidence relating to the younger dryas impact hypothesis during the end of the last glacial period in the northern hemisphere near 12 9k cal a bp deglacial warming of the bolling allerod interstadial ceased abruptly and the climate returned to glacial conditions for an interval of about 1300 years known as the younger dryas stadial the younger dryas impact hypothesis proposes that the onset of the younger dryas climate reversal pleistocene megafaunal extinctions and disappearance of the clovis paleoindian lithic technology were coeval and caused by continent wide catastrophic effects of impact bolide events in north america while there are no known impact structures dated to the younger dryas onset physical evidence of the impact bolide events is argued to be present in sediments spanning several continents at stratigraphic levels inferred to date to the bolling allerod younger dryas boundary ydb reports of nanometer to submicron sized diamonds in ydb sediments in particular the rare 2h hexagonal polytype of diamond lonsdaleite have been presented as strong evidence for shock processing of crustal materials we review the available data on diamonds in sediments and provide new data we find no evidence for lonsdaleite in ydb sediments and find no evidence of a spike in nanodiamond concentration at the ydb layer to support the impact hypothesis

metamorphic history of the hemlo gold deposit from al2sio5 mineral assemblages with implications for the timing of mineralization textural relations between al2sio5 phases and deformation fabrics provide constraints on the metamorphic history of the hemlo gold deposit kyanite in the deposit is most common within and on the margins of boudinaged quartz realgar veins and less commonly as rotated porphyroblasts within the matrix of schistose rocks kyanite predates the main d2 schistosity sillimanite postdates kyanite occurring irregularly as discrete knots and foliae that run parallel to but sometimes cut across the principal d2 foliation indicating that sillimanite postdates the d2 foliation we regard kyanite to be part of the peak metamorphic assemblage with sillimanite representing a partial later overprint most likely related to fluid infiltration rare andalusite occurs in two associations as late stage clean idioblastic crystals and as large fractured grains locally overprinted by sillimanite in boudinaged quartz realgar veins we suggest two possible origins for this second form of andalusite one involving generally late growth the second involving early growth prior to the development of peak metamorphic kyanite although not unambiguous we prefer the second scenario p t conditions from petrogenetic grid constraints and new geothermobarometric estimates indicate 6 7 gpa 600 650 c for the peak kyanite grade metamorphism ca 2677 ma and 4 5 gpa 600 c for the later sillimanite overprint ca 2672 ma we see these two events as part of an evolving p t path in a single metamorphic event in our early andalusite scenario the andalusite may have formed from pyrophyllite breakdown at 2 4 gpa 450 c possibly associated with emplacement of the regional suite of granodiorite plutons ca 2686 ma late andalusite formed sporadically on the retrograde path the occurrence of deformed andalusite and aligned kyanite in and on the margins of boudinaged auriferous realgar stibnite quartz veins provides evidence in support of a premetamorphic mineralization event

structural setting for canadian malartic style of gold mineralization in the pontiac subprovince south of the cadillac larder lake deformation zone quebec canada abstract the structural setting of the pontiac subprovince in the vicinity of the world class canadian malartic gold deposit has been revisited by combining and reinterpreting airborne geophysical surveys together with a century of structural observations felsic intermediate intrusive bodies are a key component of this deposit defining the regional and local favorable structural setting for intrusive rock emplacement within the clastic meta sedimentary rocks of the pontiac group south of the cadillac larder lake deformation zone may further assist gold exploration in similar tectonic environment three structural domains are interpreted in the area based on the geometry of the bedding folds and structural fabrics related to the three major phases of deformation during these events four phases of magmatism and one metamorphic episode occurred the north domain which hosts the canadian malartic deposit is characterized by highly variable bedding orientations produced by the interference patterns of isoclinal f 1 folds overprinted by open to tight steeply dipping f 2 folds the bedding in this domain is cut by a penetrative s 2 biotite foliation which possibly built on rheological changes enhanced by metasomatism in the footprint of the canadian malartic deposit by contrast the central and south domains display homogeneous bedding orientations cross cut by a discrete s 2 biotite foliation and syn to late d 2 metamorphic porphyroblasts in all domains the l 2 stretching lineation consistently plunges at about 60 degrees toward the east d 3 is a minor deformation event in the pontiac subprovince which possibly correlates with late dextral transcurrent movement along the cadillac larder lake deformation zone intrusive bodies were emplaced throughout the first and second deformation events phase 1 monzonite quartz monzodiorite and granodiorite plutons ca 2683 2680 ma intruded into consolidated pontiac sedimentary rocks during d 1 phase 2 quartz monzodiorite bodies ca 2679 2676 ma predominantly formed in the north domain along f 1 fold axial surfaces prior to or at the onset of d 2 phase 3 basic dykes ca 2675 2673 ma cross cut earlier felsic intermediate intrusions across all domains and subsequently developed an s 2 metamorphic hornblende foliation phase 4 magmatism ca 2672 2662 ma is related to the decelles batholith s type granite and pegmatite which is interpreted to be contemporaneous with the peak of regional metamorphism the decelles batholith may project at depth underneath the canadian malartic deposit and could be associated with magmatic hydrothermal mineralizing fluids in the canadian malartic deposit gold mineralization at canadian malartic is spatially located on the contact of phase 2 quartz monzodiorite bodies the proximity to the cadillac larder lake deformation zone combined with the rheological contrast between steeply dipping clastic meta sedimentary rocks and quartz monzodiorite intrusions favored the protracted failure of the contacts between these two rock masses thus forming favorable conduits for hydrothermal fluids e g the sladen fault zone such specific rheological behavior is demonstrated by domains of structural complexity emphasized by the variance of the bedding dip these structurally complex zones systematically host gold mineralization in the canadian malartic district of the pontiac subprovince proximal to the cadillac larder lake deformation zone

structural evolution of a gold bearing transtensional zone within the archean porcupine destor deformation zone southern abitibi greenstone belt eastern ontario canada abstract the garrison camp comprises four structurally distinct orogenic gold deposits that formed in different host lithologies during progressive deformation detailed field mapping drill core logging and geochronological constraints suggest that the 2678 2 ma garrison granitic stock played a fundamental rheological role in the location of the four deposits initial local shear movement occurred along the southwestern margin of the stock leading to the development of the nw trending sinistral ne side up buffonta shear zone which hosts the buffonta deposit subsequently a transtensional zone formed between the ne trending sinistral porcupine destor and munro deformation zones which host the 903 and jonpol deposits respectively finally a local change in shortening orientation from ne to nnw caused westerly directed extension resulting in the formation of the younger gold bearing veins composing the garrcon deposit in situ u pb laser ablation inductively coupled plasma mass spectrometry performed on monazite grains formed within the shear fabric of the munro deformation zone indicates that transtension occurred at 2657 15 ma therefore at least three of the four deposits formed subsequent to crystallization of the garrison granitic stock the reported u pb dates represent the first direct age constraints on the movement along a gold bearing deformation zone in the southern abitibi greenstone belt

the hemlo gold deposit ontario canada principal deposit characteristics and constraints on mineralization abstract the archean world class hemlo lode gold deposit has been overprinted by deformation and metamorphism resulting in part in several proposed depositional models for its emplacement no single model clearly accounts for all of the characteristics of the deposit most proposed models are inappropriate in terms of timing with respect to host rock deformation intrusion or metamorphism or require inappropriate geometry metal tenors or metal ratios the deposit is spatially associated with high strain zones a restraining bend in the hemlo greenstone belt and the volcanic sedimentary contact of the moose lake volcanic complex mlvc the restraining bend correlates with changes in the style of mineralization and or alteration hence the deposit is subdivided into two segments the west segment is west striking and consists of numerous low grade erratically mineralized west to west northwest striking zones mineralization is locally dominantly fracture controlled or disseminated the main segment strikes approximately 290 contains the majority of the ore and consists of two main tabular zones of dominantly disseminated mineralization the main mineralized zone within the lake superior shear zone and the lower mineralized zone within the moose lake fault zone a broad zone of alteration about 4 km long and as much as 400 m wide asymmetrically surrounds the deposit there is ample evidence at a variety of scales of d 2 structural control on the various mineralized zones tabular zones of alteration and or disseminated mineralization are parallel to the predominant s 2 fabric in the west segment and a composite mylonitic s 2 s 2m s 3c fabric in the main segment tabular mineralized zones of fracture controlled alteration and pervasive alteration are paraconcordant with lithotectonic units the deposit is ductilely deformed by d 2 and d 3 strains which are associated with sinistral and dextral transpression respectively the deposit was emplaced after f 2 folding synchronous with the incipient development of high strain zones during mid d 2 strain post deposit overprinting by deformation and regional metamorphism has resulted in remobilization and modification of some of the original deposit characteristics the hemlo gold deposit is interpreted as an atypical mesozonal orogenic disseminated replacement stockwork deposit broadly synchronous with d 2 and middle stage granitoid plutonism prior to or synchronous with peak regional metamorphism and involving magmatic metamorphic fluids its emplacement is poorly constrained between 2694 and 2677 ma u pb systematics are consistent with field evidence for a history of protracted episodic progressive strain intrusive activity and an evolving hydrothermal system the chief emplacement controls specifically appear to have been the restraining bend competency contrast at a major lithologic contact and permeability in a fragmental unit with general more broadly developed steady state permeability in ductile zones enigmatic barite associated with the deposit is considered an intrinsic product of the hydrothermal system the orientation of the major part of the deposit is consistent with statistically preferred orientations for large gold deposits in other gold camps of the superior province other examples of alteration and mineralization in the hemlo greenstone belt share some common attributes with the hemlo deposit these factors provide a basis for selecting favourable exploration targets in the greenstone belt

host rock and structural controls on the nature and timing of gold mineralization at the holloway mine abitibi subprovince ontario the holloway mine a mesothermal gold deposit is located within the western abitibi subprovince ontario mineralized zones that contain microscopic gold within pyrite are hosted by a sequence of metakomatiites mafic to felsic metavolcanic and metasedimentary rocks there were two mineralization events however the early one was the main event in terms of the amount of gold mineralization this event was associated with albitization silicification chloritization and sericitization in addition to pyritization the later mineralization is associated with sericitization and pyritization carbonate and sericite alterations are associated with both mineralizing events results of u pb zircon age dating of rocks within the sequence constrains the main mineralization event to 2672 ma ore lenses of the holloway deposit are enveloped by 080 striking steeply south dipping zones of intense deformation and fabric development within the porcupine destor deformation zone the shear zones are characterized by a strong foliation and locally an associated extension lineation the ore lenses are interpreted to represent lithons that were formed by boudinage of the more competent mineralized zones which are pervasively albitized analysis of geochemical data demonstrates that the ree th nb hf ti fe and al were largely immobile with respect to the albite quartz chlorite sericite and hematite alterations locally preserved textures such as abundant varioles and spherulites indicate that the host metavolcanics were relatively evolved rocks geochemical data show they have an iron enrichment trend typical of tholeiitic mafic to felsic suites and that they were derived through fractional crystallization of parent basalt because of their high fe mg ratios the rocks reacted with hydrothermal solutions carrying gold presumably as thio complexes to form pyrite which scavenged the gold

evidence for deposition of 10 million tonnes of impact spherules across four continents 12 800 y ago airbursts impacts by a fragmented comet or asteroid have been proposed at the younger dryas onset 12 80 0 15 ka based on identification of an assemblage of impact related proxies including microspherules nanodiamonds and iridium distributed across four continents at the younger dryas boundary ydb spherule peaks have been independently confirmed in eight studies but unconfirmed in two others resulting in continued dispute about their occurrence distribution and origin to further address this dispute and better identify ydb spherules we present results from one of the largest spherule investigations ever undertaken regarding spherule geochemistry morphologies origins and processes of formation we investigated 18 sites across north america europe and the middle east performing nearly 700 analyses on spherules using energy dispersive x ray spectroscopy for geochemical analyses and scanning electron microscopy for surface microstructural characterization twelve locations rank among the world s premier end pleistocene archaeological sites where the ydb marks a hiatus in human occupation or major changes in site use our results are consistent with melting of sediments to temperatures 2 200 c by the thermal radiation and air shocks produced by passage of an extraterrestrial object through the atmosphere they are inconsistent with volcanic cosmic anthropogenic lightning or authigenic sources we also produced spherules from wood in the laboratory at 1 730 c indicating that impact related incineration of biomass may have contributed to spherule production at 12 8 ka an estimated 10 million tonnes of spherules were distributed across 50 million square kilometers similar to well known impact strewnfields and consistent with a major cosmic impact event

a petrogenetic grid for pelitic schists in the system sio 2 al 2 o 3 feo mgo k 2 o h 2 o a quantitative petrogenetic grid for pelitic schists in the system kfmash that includes the phases garnet chlorite biotite chloritoid cordierite staurolite talc kyanite andalusite sillimanite and pyrophyllite with quartz h2o and muscovite or k feldspar in excess is presented the grid is based on thermodynamic data of berman et al 1985 and berman 1988 for endmember kfash and kmash equilibria and natural fe mg partitioning for the kfmash system calculation of p t slopes and the change in fe fe mg along reactions in the kfmash system were made using the gibbs method in addition the effect on the grid of mno and cao is evaluated quantitatively the resulting grid is consistent with typical buchan and barrovian parageneses at medium to high grades at low grades the grid predicts an extensive stability field for the paragenesis chloritoid biotite which arises because of the unusual facing of the reaction chloritoid biotite quartz h2o garnet chlorite muscovite which proceeds to the right with increasing t in the kfmash system however the reaction proceeds to the left with increasing t in the mnkfash system so the assemblage chloritoid biotite is restricted to bulk compositions with high fe fe mg mn typical metapelites will therefore contain garnet chlorite at low grades rather than chloritoid biotite

nanodiamonds and wildfire evidence in the usselo horizon postdate the allerod younger dryas boundary the controversial younger dryas impact hypothesis suggests that at the onset of the younger dryas an extraterrestrial impact over north america caused a global catastrophe the main evidence for this impact after the other markers proved to be neither reproducible nor consistent with an impact is the alleged occurrence of several nanodiamond polymorphs including the proposed presence of lonsdaleite a shock polymorph of diamond we examined the usselo soil horizon at geldrop aalsterhut the netherlands which formed during the allerod early younger dryas and would have captured such impact material our accelerator mass spectrometry radiocarbon dates of 14 individual charcoal particles are internally consistent and show that wildfires occurred well after the proposed impact in addition we present evidence for the occurrence of cubic diamond in glass like carbon no lonsdaleite was found the relation of the cubic nanodiamonds to glass like carbon which is produced during wildfires suggests that these nanodiamonds might have formed after rather than at the onset of the younger dryas our analysis thus provides no support for the younger dryas impact hypothesis

pb isotope compositions of pyrite from the c quartz tourmaline vein of the siscoe gold deposit val d or quebec constraints on the origin and age of the gold mineralization the siscoe mine was one of the richest in the val d or mining camp producing 27 5 tonnes t of gold and 9 5 t of silver from 1929 to 1949 the c quartz tourmaline vein at the siscoe mine which is the youngest auriferous vein on the property is an excellent example of a high grade archean lode gold deposit it contains an average of 45 g t au and locally up to 221 g t au this vein cuts all dikes the main regional foliation s2 and several auriferous quartz carbonate veins and is interpreted to be a shear structure that formed during the development of a late to post d2 reverse fault the vein comprises mainly alternating tourmaline and quartz rich layers and gold occurs commonly in late fractures cutting the vein the visible wall rock alteration is characterized by the occurrence of tourmaline pyrite and calcite in this study we report the pb isotope compositions of hydrothermal pyrite separates taken from the c quartz tourmaline vein and its wall rock pyrite is abundant in the wall rock where it occurs as grains with inclusions of chalcopyrite gold tetradymite pyrrhotite rutile silicates and carbonates in the vein it forms euhedra generally devoid of inclusions locally the euhedra are fractured and the fractures are filled with native gold calcite and tetradymite textural relationships suggest that pyrite precipitated synchronously with and or after gold in the wall rock but predated gold in the vein pyrite separates from the vein and its wall rock yielded similar 206pb 204pb 207pb 204pb and 208pb 204pb ratios 13 63 13 83 14 58 14 62 and 33 42 33 76 respectively which are more radiogenic than those reported for volcanic supracrustal and syntectonic plutonic rocks of the southern abitibi subprovince but are very similar to those of the late to posttectonic s type granites in the pontiac subprovince and lacorne block this suggests that the pb originated from a reservoir similar to that of the magmas which produced these granites or alternatively that it was leached from the latter the pb ratios yield a model age of 2 57 0 07 ga which is interpreted to date the cooling of the hydrothermal system that introduced and or remobilized gold into the c quartz tourmaline vein these results when considered in the context of the evolution of the southern abitibi indicate that widespread hydrothermal circulation occurred between 2 64 and 2 55 ga after the collision of the pontiac and abitibi subprovinces and was caused by dewatering and s type magmatism in the thickened unstable continental crust this long lived hydrothermal system promoted remobilization of gold from earlier veins and possibly introduction of gold from deeper zones enhancing gold resources in the siscoe deposit

deposit scale structural architecture of the sigma lamaque gold deposit canada insights from a newly proposed 3d method for assessing structural controls from drill hole data deposit scale structural analysis is a practical method of structurally analyzing 3d drilling data this interpretation method uses readily available distributed 3d data at the most relevant scale for mining and exploration this method does not depend on collecting traditional structural data from core such as orientation of planar and linear data but relies on the premise that grade distributions sampled from drilling provide insight into the structural control of the deposit with the interpretation of structural geometry done using a computer 3d viewer this method is applied to the well studied archean sigma lamaque gold deposit which features late orogenic vein mineralization and which is controlled by three orders of structural features the first order control is the easterly plunging pipe like conduit parallel to f2 fold plunge defined by the intersection of bedding s0 and foliation s2 1 5 km radius 3 km down plunge the second order controls nested within the first order are the younger subvertical late diorite plugs that intruded vertically into the host rocks along the established s2 foliation anisotropy 1250 400 150 m dimension for main plug the third order control laterally limited by first and second order structural controls is the well documented gold bearing vein sets 1 10 m thick and up to 1 5 km in extent that developed in the late diorites and in the older folded volcanogenic stratigraphy and diorites it is proposed that the first order control defined by the moderately plunging f2 fold is the main fluid pathway resulting from structural permeability formed during earlier folding the second order late diorite plugs and other competent lithological units which intersected and fractured during late stage d2 compression served as chemically reactive sites causing gold to precipitate along this first order pathway it is predicted that intrusions that do not intersect the first order structural conduits are less likely to be mineralized greenfield orogenic gold exploration requires that the first order controls are identified and the geometry of hydrothermal alteration is best understood in the context of the structural architecture of the host rocks

rare earth elements protoliths and alteration at the hemlo gold deposit ontario canada and comparison with argillic and sericitic alteration in the highland valley porphyry district british columbia canada rare earth elements ree sc and th are used to identify two volume important protoliths for alteration and mineralization at the hemlo gold deposit ontario quartz feldspar porphyry and greenstone metasedimentary and metavolcanic rocks of predominant intermediate mafic composition the ree geochemistry was essentially passive to the pervasive and characteristic microcline alteration at the deposit however using al 2 o 3 as the immobile component there is evidence for mobility of ree particularly of light ree tio 2 and zr in other styles of alteration dilution of ree abundances by secondary minerals pyrite barite quartz and carbonates averaged x 2 0 and loss of light ree to leaching averaged 56 percent in ores of the porphyry protolith and x 1 6 and 68 percent respectively in ores of the greenstone protolith whereas most of the secondary minerals diluting ree abundances crystallized broadly contemporaneously with microcline leaching of light ree most likely occurred later during the late calc silicate alteration event depletion of light ree as measured by a decrease in la yb n increases with an increase in au content for gold ores derived from both porphyry and greenstone protoliths however the ree geochemistry remains dominated by the protolith composition and thus is not a suitable primary tool for exploration for hemlo type gold deposits argillic alteration in the highland valley porphyry district british columbia is uniformly enriched in ree or x 2 compared with incipiently to intensely altered whole rocks and sericitic alteration the argillic and sericitic alteration separates define a characteristic linear trend in the al 2 o 3 cao na 2 o k 2 o ternary diagram that is distinct from the composition field for aluminous rocks of the hanging wall of the hemlo deposit the geochemistry and regional occurrence of these latter rocks clearly indicate that they are metapelites

the late archean hemlo gold deposit ontario canada a review and synthesis abstract the hemlo gold deposit northwestern ontario canada is situated within the late archean hemlo heron bay greenstone belt of the wawa subprovince of the superior province the greenstone belt represents an ancient volcanic island arc and has subsequently been subjected to a complex history of evolution including two episodes of regional metamorphism three main pulses of granitoid plutonism and four generations of structures related to two major deformational events the main second regional metamorphism was a low p t event and generally belonged to greenschist amphibolite transition facies but climaxed at middle amphibolite facies along both margins and the central axis of the greenstone belt the hemlo gold deposit is located within the central metamorphic zone of middle amphibolite facies the main orebody contains approximately 80 mt of ore averaging 7 7 g t au and is 2900 m along strike 1300 m down dip and 3 to 45 m thick additional gold mineralization extends sporadically and discontinuously westward and includes the c zone of the williams mine and the north south and highway zones of the golden sceptre resources limited property the main orebody and north zone are contained within a major shear zone proximal to a lithological contact between structurally underlying felsic rocks and overlying metasedimentary rocks the c and south zones are entirely within the felsic rocks and commonly are present as intensively sheared and altered lenses the hemlo deposit is enriched in as ba hg mo s sb te tl and v its economic gold concentrations correlate with the isotopic compositions of sulphide minerals but not their total abundances the hemlo gold deposit is characterized by intensive and extensive hydrothermal alteration including potassic alteration calc silicate alteration sulphidation and less significant silicification tourmalinization and carbonatization field occurrence textural relationships quantitative p t x t data from mineral equilibria oxygen isotope geothermometry fluid inclusion microthermometry and u pb and 40 ar 39 ar geochronology suggest a protracted history of hydrothermal activity which commenced with an earlier calc silicate alteration 550 c and 3 4 kbar 2678 2676 ma immediately after the peak of the second regional metamorphism followed by pervasive potassic alteration mica schists and microcline rich rocks 450 500 c 2672 2670 ma and terminated with a later low to very low grade calc silicate alteration 200 400 c 1 2 kbar and 2643 2632 ma there remains uncertainty among investigators regarding the timing and style of gold mineralization of the hemlo deposit consequently many ore forming processes have been proposed for its origin including syngenetic metamorphic replacement and porphyry models suggesting gold mineralization either pre dating or during the second regional metamorphism and also structural and hot spring epithermal models indicating mineralization post dating the second regional metamorphism we suggest that the deposition of metals and sulphides in the hemlo deposit is most consistent with a multistage late replacement model the hemlo shear zone was activated as a regional planar structure during the second regional metamorphism and became a conduit for regional fluids during periods of dilatancy orogenic activity was protracted and fluids focused along the conduit could have been variously of metamorphic and magmatic origin at different stages in the development of the hemlo deposit gold mineralization most likely occurred during the late calc silicate alteration event about 40 ma after the second regional metamorphism

synthesis of the structural evolution and associated gold mineralization of the cadillac fault abitibi canada abstract the cadillac fault zone cfz is a 250 km long east trending crustal discontinuity associated with numerous world class gold deposits in the archean abitibi subprovince in quebec the fault follows the boundary between the abitibi and the pontiac subprovinces and can be divided into four segments 1 rouyn 2 joannes 3 malartic and 4 val d or each displays different attitudes and structural styles the malartic segment strikes se whereas the other segments are e trending abrupt changes in strike or smaller scale faults represent limits between the segments structural styles include four generations of foliation with numerous shear sense indicators and folding specifically related to each foliation structural features can be correlated from one segment to another but display clear variations in attitude or expression a stretching lineation signature is definitely associated with each segment in the malartic segment the predominant e trending schistosity displays an angular relationship with the contact between groups and bedding suggesting that the se trending architecture of this segment was already emplaced prior to ductile deformation the evidence suggests a model where the cfz originates from several independent structures inherited from volcanism prior to timiskaming deformation and abitibi pontiac accretion the proto cfz was then linked up during the timiskaming deposition forming high strain zones inherited structures form the frame on which each deformation event was overprinted with different responses depending on segmentation the structural styles of each segment can be integrated in a structural evolution model for the cfz the n s shortening led to the development of a dominantly e striking penetrative foliation and folding during the subsequent extensional stage the cfz was reactivated with normal kinematics during the pontiac uplift a third deformation event is represented by the local development of nw trending cleavage axial planar to the s shaped minor folds finally a late nw se shortening event produced a dextral strike slip movement with ne striking vertical foliation and development of z shaped folds on bedding surfaces and anterior fabrics linkage of the cfz had a major impact on hydrothermal activity in the southern abitibi as it channeled fluids along large scale pathways distribution of gold bearing intrusions can also be related to segmentation and inherited structures of the cfz this association is expressed by gold mineralization styles present in several longitudinal fields reflecting fault segmentation

shock synthesized hexagonal diamonds in younger dryas boundary sediments the long standing controversy regarding the late pleistocene megafaunal extinctions in north america has been invigorated by a hypothesis implicating a cosmic impact at the llerod younger dryas boundary or ydb 12 900 100 cal bp or 10 900 100 14c years abrupt ecosystem disruption caused by this event may have triggered the megafaunal extinctions along with reductions in other animal populations including humans the hypothesis remains controversial due to absence of shocked minerals tektites and impact craters here we report the presence of shock synthesized hexagonal nanodiamonds lonsdaleite in ydb sediments dating to 12 950 50 cal bp at arlington canyon santa rosa island california lonsdaleite is known on earth only in meteorites and impact craters and its presence strongly supports a cosmic impact event further strengthened by its co occurrence with other nanometer sized diamond polymorphs n diamonds and cubics these shock synthesized diamonds are also associated with proxies indicating major biomass burning charcoal carbon spherules and soot this biomass burning at the younger dryas yd onset is regional in extent based on evidence from adjacent santa barbara basin and coeval with broader continent wide biomass burning biomass burning also coincides with abrupt sediment mass wasting and ecological disruption and the last known occurrence of pygmy mammoths mammuthus exilis on the channel islands correlating with broader animal extinctions throughout north america the only previously known co occurrence of nanodiamonds soot and extinction is the cretaceous tertiary k t impact layer these data are consistent with abrupt ecosystem change and megafaunal extinction possibly triggered by a cosmic impact over north america at 12 900 100 cal bp

timing of gold mineralization in the val d or district southern abitibi belt evidence for two distinct mineralizing events gold bearing quartz vein deposits in the val d or area of the southern abitibi greenstone belt are classified in two categories based on structural and chronologic relationships young deposits 2690 ma feature commonly deformed orebodies that are cut by dikes of various composition such deposits occur at the siscoe kiena orion and norlartic mines the norlartic main orebody is crosscut by a tonalite dike which has been dated by u pb at 2692 or 2 ma in agreement with structural data these observations show that gold mineralization in the val d or area occurs in at least two different settings and was emplaced at different times

an exsolution origin for low temperature sulfides at the hemlo gold deposit ontario canada many aspects of the timing and genesis of the archean hemlo gold deposit are unresolved pre syn and postmetamorphic models have been proposed the presence of abundant low temperature sulfide minerals within highly deformed sillimanite grade host rocks has been used to support a postmetamorphic origin for the deposit however the present sb as hg ore assemblage including cinnabar realgar orpiment assemblages developed through a sequence of exsolution events during postpeak metamorphic cooling at peak metamorphism most of the as and hg along with tl cu zn and minor fe was incorporated into a high temperature antimonian sulfosalt in the lower amphibolite facies or upper greenschist facies impure hg zn s exsolved from the high temperature phase an impure chalcopyrite and native antimony exsolved from the host sulfosalt soon after leaving behind an sb 2 s 3 as 2 s 3 solid solution further cooling resulted in the exsolution of cinnabar and mercurian sphalerite from the hg zn s impurities in the cinnabar and chalcopyrite exsolved to form aktashite arsenopyrite and native antimony eventually orpiment exsolved from the arsenic bearing stibnite late retrograde events converted the free orpiment to realgar and remobilized the realgar over short distances along with cinnabar and native antimony the history of sequential sulfide exsolution along with the deformed nature of the sb as hg bearing veins and their metamorphosed alteration selvages suggests that the metals were introduced prior to or during peak metamorphism

the barry gold deposit abitibi subprovince canada a greenstone belt hosted gold deposit coeval with late archean deformation and magmatism the barry gold deposit is an example of an archean greenstone hosted lode gold deposit located in the urban barry greenstone belt in the abitibi subprovince of quebec canada the host mafic volcanic rocks are part of the 2717 ma macho formation of the northern volcanic zone they are cut by a series of weakly foliated preore diorite pre and postore quartz feldspar porphyry qfp and quartz monzonite dikes and plugs providing a unique opportunity to evaluate the age of the gold mineralization auriferous zones are spatially associated with northeast trending ductile shear zones with moderate south easterly dip gold mineralization occurs within straight n64 e 64 se and folded n20 e 60 se on average albite carbonate quartz veins and within the surrounding carbonate quartz pyrite and locally within biotite carbonate alteration zones of the host mafic volcanic rocks the auriferous veins comprise 5 to 15 vol percent of the mafic volcanic rocks are 1 to 5 cm wide and are locally boudinaged although the volcanic units strike n55 60 e and dip 40 se the ore envelope 2 g t au is constrained from surface to a depth of 30 m in an antiformal shape gold generally occurs as microinclusions in pyrite but is also present as free gold in albite carbonate quartz veins synmineralization altered host rocks and locally within quartz veins cutting early qfp dikes the deposit has an indicated resource of 309 500 oz au 7 701 000 t at 1 25 g t au and an inferred gold resource of 471 950 oz au 10 411 000 t at 1 41 g t au u pb dating of single zircon grains from the premineralization diorite and postmineralization qfp yielded indistinguishable ages averaging 2697 0 6 ma which is interpreted as the age of gold mineralization at the barry deposit this age and the field relationships reveal that lode gold mineralization was coeval with regional deformation and magmatism similar to that documented at the kiena norlartic and siscoe main zone deposits of the val d or district these deposits represent the earlier orogenic gold event in the abitibi greenstone belt which preceded the postmagmatic quartz tourmaline auriferous veins such as those present at sigma barry kiena norlartic and siscoe share many similarities with other orogenic gold deposits worldwide that are spatially associated with felsic and intermediate intrusions the genesis for these deposits is not well constrained as it is not obvious to distinguish whether the auriferous fluids were derived from the coeval magmas or whether both magmas and fluids were derived from a thermal event from the deep crust or possibly the mantle

the significance of titanite and apatite u pb ages constraints for the post magmatic thermal hydrothermal evolution of a batholithic complex berens river area northwestern superior province canada abstract titanite apatite and in one case rutile from a suite of twenty seven samples representing the major phases of an extensive and well dated archean plutonic domain of the northwestern superior province have been dated by u pb to determine the temporal progress of thermal and hydrothermal events accompanying and following major plutonism and to assess the isotopic behaviour of these geochronometers most samples contain 1 dark brown titanite with 350 20 ppm u showing shapes and textural relationships indicative of a primary magmatic origin and yielding ages corresponding either to the time of magmatic crystallization or to that of secondary metamorphic or magmatic events known in the area 2 pale titanite with 30 2 ppm u showing textural evidence for a secondary origin by breakdown of biotite hornblende oxide or older titanite and yielding ages that in general post date the major plutonic and metamorphic events in the area 3 apatite containing 30 5 ppm u occasionally up to 150 ppm u showing igneous shapes and textural relationships but yielding post magmatic ages similar to those of coexisting pale titanite 4 rutile in one sample postdating the ages of coexisting titanite and apatite the common coexistence of two noncoeval titanite generations demonstrates that the often invoked mechanism of pb diffusion during slow cooling in this case has no general validity the distribution of primary dark titanite ages however could be explained by late closure to diffusion or alternatively as the result of resetting by the thermal or hydrothermal effects caused by younger plutonic activity the ages of the secondary pale titanites probably reflect crystallization during discrete retrogressive events promoted by moderate heating or enhanced fluid activity the correspondence between pale titanite and apatite ages suggests that the same processes also caused the isotopic resetting of apatite on a regional scale the data pattern permits the identification of domains characterized by differences in the length and intensity of the thermal and hydrothermal processes the most protracted activity is recorded in northern domains of the berens river area whereas the gold mineralized red lake area in the south underwent a very rapid decline of the thermal hydrothermal activity these data also highlight important differences between the red lake and other gold camps of the superior province such as hemlo and val d or where hydrothermal activity locally associated with mineralization occurred as much as 30 80 m y after the major orogenic developments

paleoseismology of the southeastern reelfoot rift in western tennessee and implications for intraplate fault zone evolution 1 the southeastern reelfoot rift margin is expressed as a series of collinear scarps across western tennessee and shallow s wave reflection profiles coring and trenches show late quaternary faulting along this lineament at the north end our seismic profile and coring suggest 1 5 m of wisconsin down to the nw faulting coring and trenches across the central lineament show 3 m of up to the nw and 8 15 m of right lateral late wisconsin holocene faulting averaged late quaternary right lateral slip has been 0 85 0 37 mm yr on one fault plane at this locality suggesting that total slip across all southeast rift margin fault planes may accommodate much of the regional strain our seismic profile at this site shows up to the west faulting of eocene strata folded down to the west indicating structural inversion seismic profiles electrical surveys coring and a trench across the southern lineament segment show holocene faulting trenching exposed a vertical fault with 0 5 m of late holocene down to the nw displacement nearby coring and electrical surveys show 0 5 m of late holocene up to the nw faulting we conclude the southeastern reelfoot rift margin is a right lateral system with high angle faults showing both up to the nw and down to the nw separations age constraints permit an earthquake circa 2500 to 2000 years b p on the southwestern central segment of this fault system that ruptured 80 km we suggest the northeastern segment of the southeastern rift margin turned off in holocene when the reelfoot step over thrust turned on

constraints on pb closure temperature in titanite based on rocks from the ungava orogen canada implications for u pb geochronology and p t t path determinations on the basis of multiequilibrium metamorphic pressure temperature p t estimates we have determined that 660 700 c is a reasonable minimum estimate of the closure t of pb diffusion in titanite a temperature significantly higher than previously suggested activity corrected reactions that produce titanite define narrow t stability fields 40 c that correspond closely to metamorphic p t determinations derived for coronitic garnet clinopyroxene hornblende samples growth of titanite occurring between 1789 and 1814 ma in the underthrust archean basement of the paleoproterozoic ungava orogen northern quebec is associated with retrograde metamorphism during underthrusting of the archean basement approx 1 b y after formation of the original granulitic assemblages

structural evolution of the hemlo greenstone belt in the vicinity of the world class hemlo gold deposit a complex history of volcano sedimentary deposition polyphase strain multiple intrusive events and various stages of porphyroblastesis is indicated for the hemlo gold deposit area within the hemlo greenstone belt structural elements can be assigned to at least six stages of development d1 d6 d1 generated small scale folds and low angle faults thrusts with no planar fabric except within strain aureoles around the earliest intrusions d2 was a progressive event resulting from northeast directed compression which generated regional predominantly s shaped folds early d2 penetrative planar and linear fabrics overturned stratigraphy and formation of an inflection in the strike of the greenstone belt mid d2 and development of high strain zones with dominant sinistral and local dextral shear sense late d2 d3 was a distinctly separate progressive event resulting from northwest directed transpression which generated variably penetrative east to northeast striking foliation s3 ductile de

evolution and tectonic significance of intermediate to felsic plutonism associated with the hemlo greenstone belt superior province canada abstract plutonic rocks from the hemlo area in the southern superior province span a period from ca 2720 to 2677 ma early ttg plutons intruded into lower basaltic volcanic sequences and were probably derived from melting of basaltic crust at upper mantle depths they predate major regional deformation and span ages from 2719 to 2697 ma felsic to intermediate volcanism 2697 2688 ma partially overlaps ttg plutonism but is mostly slightly younger and lacking plutonic equivalents regional deformation and metamorphism of the greenstone belt and ttg plutons progressed rapidly following deposition of the youngest supracrustal rocks syn to late tectonic plutons emplaced between 2684 and 2677 ma have a distinctive dual primitive evolved character that is interpreted to reflect their derivation by direct melting of long term depleted mantle that was metasomatically enriched in a wide range of lile and hfse this secular evolutionary change in the nature of intermediate to felsic plutonism at hemlo and its relationship to regional deformation and metamorphism parallels that occurring more regionally within the superior province and is interpreted to be the consequence of a transition in the nature of large scale tectonic processes intermediate to felsic magmatism within a protracted period of volcanism and ttg plutonism originated primarily by melting of subducted basaltic crust probably in an oceanic arc related environment the termination of this style of magmatic activity and initiation of regional deformation and metamorphism at ca 2688 ma is interpreted to be a consequence of collisional orogeny the petrogenetic characteristics of the latest period of magmatism at 2684 2677 ma requires that the melting regime migrate into sub arc lithospheric mantle that was metasomatized during the earlier magmatic stage slab breakoff following terrane accretion is suggested as one possible mechanism for the abrupt termination of ttg magmatism and ensuing rapid and progressive introduction of large amounts of heat into sub arc mantle and basal portions of the crust this pattern of development is seen in many superior province greenstone belts but is older in the north therefore it is probably an inevitable consequence of the tectonic processes that led to accretion of the superior province

multistage gold mineralization at the lapa mine abitibi subprovince insights into auriferous hydrothermal and metasomatic processes in the cadillac larder lake fault zone the lapa gold deposit contains reserves of 2 4 mt at 6 5 g t au and is one of the few deposits located directly within the cadillac larder lake fault zone cllfz a first order crustal scale fault that separates the archean abitibi subprovince from the pontiac subprovince to the south gold mineralization is predominantly hosted in highly strained and altered upper greenschist lower amphibolite facies mafic to ultramafic rocks of the piche group auriferous ore zones consist of finely disseminated auriferous arsenopyrite pyrrhotite pyrite and native gold disseminated in biotite and carbonate altered wall rocks native gold which is also present in quartz dolomite calcite veinlets is locally associated with sb bearing minerals especially at depth 1 km from surface where the deposit is characterized by a au sb as association at vertical depth greater than 1 km gold is associated with arsenopyrite and pyrrhotite au as association the mineralogy and paragenesis of the lapa deposit metamorphosed ore and alteration assemblages record the superposition of three metamorphic episodes m1 m2 and m3 and three gold mineralizing events spatial association between biotitized wall rocks and auriferous arsenopyrite indicates that arsenopyrite precipitation is concomitant with potassic alteration the predominant au as association recognized across the deposit is related to gold in solid solution in arsenopyrite as part of a pre m2 low grade auriferous hydrothermal event however the occurrence of hornblende oligoclase porphyroblasts overprinting the biotite alteration and the presence of porous clusters and porphyroblasts of arsenopyrite with native gold and pyrrhotite indicate an auriferous metasomatic event associated with peak m2 prograde metamorphism late retrograde metamorphism m3 overprints the hornblende oligoclase m2 assemblage within the host rocks proximal to ore by an actinolite albite assemblage by precipitation of free gold and sb sulfosalts at lower p t the complex relationships between ore structural features and metamorphic assemblages at lapa are related to the tectonometamorphic evolution of the cadillac larder lake fault zone at different times and crustal levels and varying heat and fluid flow regimes the lapa deposit demonstrates that early low grade gold mineralization within the cadillac larder lake fault zone has benefited from late gold enrichment s during prograde and retrograde metamorphism suggesting that multi stage processes may be important to form gold rich orogenic deposits in first order crustal scale structures

the hemlo gold deposit ontario an example of melting and mobilization of a precious metal sulfosalt assemblage during amphibolite facies metamorphism and deformation the timing of mineralization at the hemlo gold deposit has long been a controversial issue we present arguments that use partial melting of the ore assemblage to explain the mechanism of ore mobilization and resolve problems with the model of premetamorphic mineralization the results have implications for mine exploitation and exploration strategies we show that the ore mineral assemblage underwent partial melting during middle amphibolite facies metamorphism 600 650 c 6 7 kbars primarily through breakdown of stibnite and arsenopyrite in a high s2 environment concurrent deformation led to segregation of the resulting sb and as rich sulfosalt melt interaction between this melt and a range of unmelted sulfides led to further melting and incorporation of other elements into the melt the gold bearing melt was mobilized from compressional high strain sites into dilational domains such as boudin necks and extensional fractures developed in competent lithologies ore minerals that did not melt significantly pyrite molybdenite pyrrhotite and sphalerite were not extensively mobilized and largely remained within high strain compressional domains this segregation of melt from residue thus resulted in the observed heterogeneous distribution of ore minerals within the deposit crystallization of the sulfosalt melt produced a diverse suite of ore minerals dominated by stibnite and realgar and containing an array of rare sulfosalts native elements intermetallic compounds and tellurides some sulfosalt melt persisted to low temperature 300 c allowing continued small scale localized mobilization during late deformation although gold occurs at moderate concentrations within the compressional high strain domains it is particularly concentrated in the dilational domains a consequence of its mobilization within a sulfosalt melt our model of partial melting of the ore mineral assemblage with consequent mobilization explains how ore minerals that are unstable even at greenschist facies conditions came to be hosted in structures that formed at or near the peak of amphibolite facies metamorphism

geological influences on the lower mississippi river and its alluvial valley abstract studies of photographs maps and channel morphology permit identification of greatly different mississippi river reaches from this it becomes apparent that this large alluvial river is not monotonous in appearance and therefore it is not completely controlled by hydrology and hydraulics in fact the mississippi river has reacted to uplift faults clay plugs outcrops of tertiary clay and pleistocene gravel in its bed and tributaries this classic example of a large alluvial river has major geological controls on its shape pattern and magnitude of change through time in addition the same controls plus the effect of plutonic intrusives and fault zones have significantly affected smaller rivers and the alluvial valley of the mississippi river

late kinematic timing of orogenic gold deposits and significance for computer based exploration techniques with emphasis on the yilgarn block western australia abstract orogenic gold deposits are a widespread coherent group of epigenetic ore deposits that are sited in accretionary or collisional orogens they formed over a large crustal depth range from deep seated low salinity h2o co2 ch4 n2 ore fluids and with au transported as thio complexes regional structures provide the main control on deposit distribution in many terranes first order faults or shear zones appear to have controlled regional fluid flow with greatest ore fluid fluxes in and adjacent to lower order faults shear zones and or large folds highly competent and or chemically reactive rocks are the most common hosts to the larger deposits focusing of supralithostatic ore fluids into dilatant zones appears to occur late during the evolutionary history of the host terranes normally within d3 or d4 in a d1 d4 deformation sequence reactivation of suitably oriented pre existing structures during a change in far field stress orientation is a factor common to many deposits and repeated reactivation may account for multiple mineralization episodes in some larger deposits absolute robust ages of mineralization support their late kinematic timing and in general suggest that deposits formed diachronously towards the end of the 100 to 200 m y long evolutionary history of hosting orogens for example in the yilgarn block a region specifically emphasised in this study orogenic gold deposits formed in the time interval between 40 and 90 m y with most about 60 to 70 m y after the youngest widespread basic ultrabasic volcanism and towards the end of felsic magmatism the late timing of orogenic gold deposits is pivotal to geologically based exploration methodologies this is because the present structural geometries of i the deposits ii the hosting goldfields and iii the enclosing terranes are all essentially similar to those during gold mineralization at least in their relative position to each other thus interpretation of geological maps and cross sections and three dimensional models can be used to accurately simulate the physical conditions that existed at the time of ore deposition it is particularly significant that the deposits are commonly related to repetitive and predictable geometries such as structural heterogeneities within or adjacent to first order structures around rigid granitoid bodies or in specific locked up fold thrust structures importantly the two giant greenstone hosted goldfields kalgoorlie and timmins show a remarkably similar geometry at the regional scale computer based stress mapping and gis based prospectivity mapping are two computer based quantitative methodologies that can utilize and take advantage of the late timing aspect of this deposit type to provide important geological aids in exploration both in broad regions and more localized goldfields both require an accurate and consistent solid geology map stress mapping requires knowledge of the far field stresses during mineralization and the empirical prospectivity mapping requires data from a significant number of known deposits in the terrane the kalgoorlie terrane in the yilgarn block meets these criteria and illustrates the potential of these methodologies in the exploration for orogenic gold deposits low minimum stress anomalies interpreted to represent dilational zones during gold related deformation coincide well with the positions of known goldfields rather than individual gold deposits in the terrane and there are additional as yet unexplained anomalies the prospectivity analysis confirms that predictable and repetitive factors controlling the siting of deposits are i proximity to and orientation and curvature of granitoid greenstone contacts ii proximity to segments of crustal faults which strike in a preferred direction iii proximity to specific lithological contacts which have similar preferred strike iv proximity to anticlinal structures and v the presence of preferred reactive host rocks e g dolerite the prospectivity map defines a series of anomalous areas which broadly conform to those of the stress map 78 correspondence the most prospective category on this map covers less than 0 3 of the greenstone belts and yet hosts 16 of the known deposits which have produced 80 of known gold thus it discriminates in favour of the larger economically more attractive deposits in the terrane the successful application of stress mapping and prospectivity mapping to geology based exploration for orogenic gold deposits indicates that more quantitative analysis of geological map data is a profitable line of research the computer based nature of these methodologies is ideal for the production of an ultimate integrated deposit target map which can be compared to other more conventional targeting parameters such as geophysical and geochemical anomalies such an integrated strategy appears the way forward in the increasingly difficult task of cost effective global exploration for orogenic gold deposits in poorly exposed terranes

tectonic geomorphology of the mississippi valley between osceola arkansas and friars point mississippi abstract a geomorphic study of the mississippi river and its alluvial valley between osceola arkansas and friars point mississippi has identified anomalous surface features that can be linked to known geological structures for example relatively recent deformation along big creek fault zone white river fault zone bolivar mansfield tectonic zone blytheville arch crittenden county fault zone and reelfoot rift margins is suggested by river and topographic anomalies faults and plutons appear to affect drainage networks and the morphology of crowleys ridge suggests significant fault control the many anomalies probably reflect a fractured suballuvial surface although movement along these fractures will most likely occur in seismically active areas the probability of movement elsewhere