origin of iron rich mississippi valley type deposits the abundance of iron in mississippi valley type lead zinc deposits and districts varies greatly some deposits contain large amounts and others are almost free of iron iron in mississippi valley type deposits is largely paragenetically early pyrite or marcasite that was replaced by sphalerite and galena often in the central part of the deposit or district sedimentary exhalative and irish type base metal deposits which also form from basinal brines have similar variations in iron content calculated metal contents of brines in equilibrium with galena sphalerite and pyrite show that iron is significantly more abundant than lead and zinc in high temperature 200 c relatively acid brines with low sulfur contents whereas zinc dominates under most other conditions including brines with high temperatures and high sulfur contents these results suggest that iron rich mississippi valley type deposits form from brines expelled from the deepest hottest parts of sedimentary basins

sediment provenance of pre and post collisional cretaceous paleogene strata from the frontal himalaya of northwest india abstract whereas the timing for india asia collision remains debated contrasting collisional models provide testable predictions in terms of sediment source contributions during the accumulation of paleocene to middle eocene deposits now exposed in the frontal himalayan system within the lesser himalaya and frontal thrust system of northwest india discontinuous exposures of the cretaceous singtali formation and upper paleocene middle eocene subathu formation yield a record of these early collisional stages to test competing collisional models we analyze the provenance of these deposits with new detrital zircon u pb and hf isotopic data which can distinguish among indian plate asian plate kohistan ladahk arc and various himalayan sources detrital zircon age distributions for the singtali formation are dominated by paleoproterozoic zircons with a distinct cretaceous age component whereas age data from the subathu formation record 1 a marked increase in the relative abundance of cambrian neoproterozoic grains 2 a decrease in the proportion of paleoproterozoic grains and 3 distinct permian and late cretaceous paleocene age components all cretaceous grains from the singtali formation yielded crustal hf isotopic signatures indicating a distinctive pre collisional source of cretaceous grains of indian affinity zircon hf isotopic signatures from

quartz sand grain shape and other criteria used to distinguish glacial and non glacial events in a marine core from frobisher bay baffin island n w t canada analyses of quartz sand grain shape sediment influx rates and foraminifera define glacial and non glacial episodes in a 9 69 m core from frobisher bay arctic canada five radiocarbon dates on organic matter provide a preliminary core chronology with a basal date of 11 910 yr bp quartz sand grain morphology is measured for samples at seven core levels using 1 fourier shape analysis 2 percentage of grain surface conchoidally fractured samples at 2 0 and 7 5 m are most fractured and have fourier roughness coefficients similar to particles sampled directly from glacier ice these two samples probably represent glacial events in the core major intervals of non carbonate sand influx occur at 9 0 4 5 m and 3 5 1 5 m separated by several thousand years of slower sedimentation detrital carbonate influx rates are relatively high prior to 4 3 m then decline rapidly indicating a shift in sediment provenance from limestones flooring frobisher bay to rafting from far travelled icebergs bio and lithostratigraphic analysis allows definition of five core units 1 an environment similar to today below 8 5 m 2 glacial conditions from 8 5 6 8 m associated with ice proximal to the core site 3 ameliorating conditions from 6 8 to 3 2 m 4 cooler conditions from 3 2 to 0 5 m probably related to increased iceberg flux rather than neoglacial advances of nearby ice caps 5 an environment similar to today from 0 5 m sand grains sampled at 7 5 and 2 0 m whose shapes indicate they are derived from glacier ice lie within the cooler or glacial units defined from foraminiferal analysis this indicates that quantitative measurement of particle surface morphology can provide useful environmental information in studies of marine cores

paleolatitudes of the tibetan himalaya from primary and secondary magnetizations of jurassic to lower cretaceous sedimentary rocks the tibetan himalaya represents the northernmost continental unit of the indian plate that collided with asia in the cenozoic paleomagnetic studies on the tibetan himalaya can help constrain the dimension and paleogeography of greater india the indian plate lithosphere that subducted and underthrusted below asia after initial collision here we present a paleomagnetic investigation of a jurassic limestones and lower cretaceous volcaniclastic sandstones section of the tibetan himalaya the limestones yielded positive fold test showing a prefolding origin of the isolated remanent magnetizations detailed paleomagnetic analyses rock magnetic tests end member modeling of acquisition curves of isothermal remanent magnetization and petrographic investigation reveal that the magnetic carrier of the jurassic limestones is authigenic mag netite whereas the dominant magnetic carrier of the lower cretaceous volcaniclastic sandstones is detrital magnetite our observations lead us to conclude that the jurassic limestones record a prefolding remagnetiza tion whereas the lower cretaceous volcaniclastic sandstones retain a primary remanence the volcaniclastic sandstones yield an early cretaceous paleolatitude of 55 5 s 52 5 s 58 6 s for the tibetan himalaya suggesting it was part of the indian continent at that time the size of greater india during jurassic time cannot be estimated from these limestones instead a paleolatitude of the tibetan himalaya of 23 8 s 21 8 s 26 1 s during the remagnetization process is suggested it is likely that the remagnetization caused by the oxidation of early diagenetic pyrite to magnetite was induced during 103 83 or 77 67 ma the inferred paleolatitudes at these two time intervals imply very different tectonic consequences for the tibetan himalaya

saline brines and metallogenesis in a modern sediment filled rift the salton sea geothermal system california u s a the salton sea geothermal system ssgs is the site of active hydrothermal metamorphism and metallogenesis in the delta of the colorado river which partially fills the salton trough rift zone at the head of the gulf of california growth of the delta across the rift has isolated the northern part of the salton trough since the pleistocene forming the evaporative salton sea basin whose sediments host the ssgs more than 70 commercial geothermal wells including a 3 2 km deep research borehole drilled as part of the salton sea scientific drilling project sssdp are yielding a wealth of new data from this system within the ssgs active greenschist facies metamorphism is occuring at temperatures 365 c at only 2 3 km depth by reaction of nacakfemncl brines with the deltaic and lacustrine pilo pleistocene sediments two kinds of base metal ore mineralization occur at depth stratabound diagenetic fe sulfides and epigenetic vertical veins containing fezncupb sulfides and fex oxides the vein mineralization occurs as two types an older reduced carbonate sulfide assemblage and a modern oxidized silicate hematite sulfide assemblage the earlier assemblage formed at temperatures up to 100 c higher than the ambient temperatures measured in the wells today implying that cooler oxidized fluids have displaced hot reduced fluids a sharp interface between shallow fluids containing 12 wt tds and deep hypersaline brines containing 15 27 wt tds exists in the ssgs the deeper hypersaline brines are rich in base metals fe 1500 ppm mn 1000 ppm zn 500 ppm whereas the overlying lower salinity fluids contain less than 100 ppm each of fe mn and zn the modern silicate hematite sulfide vein assemblage is precipitating where the two fluids appear to be mixing the two fluid types also have distincty different 18o and d relations the shallow lower salinity fluids have only partially exchanged oxygen with deltaic and lacustrine host rocls the deep hypersaline brines have exchanged oxygen extensively at 250 c with the deltaic sediments d values of the hypersaline brines are typically lighter than either the lower salinity fluids or the modern evaporating groundwaters occupying the salton trough implying a different source for the original fluids the high salinities of the geothermal brines are derived from a combination of evaporation of fossil lakewaters groundwater dissolution of shallow lacustrine evaporites and subsurface hydrothermal metamorphism of buried lacustrine evaporites episodic filling and desiccation of the closed salton sea basin has allowed cold saline brines to form and percolate down into the sedimentary section in some wells anyhydrite meta evaporites and interbedded solution collapse breccias occur at 1 km depth the anyhydrite contains fluid inclusions that are saturated in halite at their homogenization temperatures of 300 c recording the hydrothermal dissolution of bedded salt based on sr and pb isotopic data and whole rock chemical data the source of the metals in the hypersaline brines is from leaching of the host sediments however the origin of reduced s for ore mineralization remains an enigma 34s values for vein sulfides cluster uniformly around zero per mil implying that an isotopically homogeneous source of reduced sulfide exists in the brines it is possible that the vein sulfides receive a constribution from a magmatic s source movement and mixing of brines of different chemistry and oxidation states play a major role in ore genesis our model envisages an early stage in which a deep brine pool accumulated at depth in the sedimentary section by partial section by partial evaporation of basin and dissolution of bedded salts intrusion of rift related basaltic magma into the base of the sedimentary caused heatingand fracturing of the sediments resulting in precipitation of the carbonate sulfide veins during pore fluid expulsion heating also caused a diapir of the hypersaline brine to rise and displace colder less saline shallower pore fluids this brine intrusion was accompanied by pervasive and extensive mineralization as this diapir cooled it began to move downward drawing in shallow more oxidized fluids and causing the formation of the modern silicate hematite sulfide vein ore zone

regional trends in water chemistry smackover formation southwest arkansas geochemical and physical controls we present results on regional geochemical analyses of formation waters from jurassic carbonate strata of the upper smackover formation from the southwest arkansas shelf gulf coast basin these formation waters have been noted in earlier studies for their exceptional salinities and abnormally br rich composition our detailed and regionally extensive database shows that chemical heterogeneities occur on a relatively small scale and have spatial relations to faults which may act as pathways for fluid migration from deeper and hotter portions of the stratigraphic section one of the most pronounced heterogeneities in water chemistry is an h 2 s rich anomaly in the center of the shelf enrichments in h 2 s are accompanied by large increases in alkali elements li k and b and b which could be produced by higher temperature diagenesis of clay minerals and feldspars in deeper seated siliciclastic strata bromide enrichments in all smackover formation waters are apparent from relations between br and cl contents as well as br and divalent cation carpenter function values as suggested by earlier studies of these waters evaporatively concentrated seawater expelled from associated thick monomineralic evaporite sections may be an important source of br cl and na whereas ca mg and sr concentrations have been modified by carbonate mineral recrystallization and dolomitization mineral equilibria exert significant controls on concentrations of ca mg sr and so 4 in these brines br enrichment likely was produced by halite recrystallization in fluids already enriched in br from seawater evaporation elevated cl br ratios occur in regional association with lower salinity water near t e south arkansas fault system consistent with halite dissolution during meteoric water recharge thus despite relatively uniform reservoir lithology and simple structural setting processes on small spatial scales have resulted in significant heterogeneity in water chemistry within a small area of the gulf coast sedimentary basin although movement of fluids along these fault systems is not demonstrated in our study the regional trends in geochemistry are strongly suggestive of their important role in controlling chemical heterogeneities in smackover formation waters

palaeomagnetism of cretaceous to tertiary sediments from southern tibet evidence for the extent of the northern margin of india prior to the collision with eurasia abstract palaeomagnetic studies have been carried out on mid cretaceous to latest palaeocene sediments from the tethyan himalaya on the northern margin of the indian plate near the localities of gamba 28 3 n 88 5 e and duela 28 0 n 89 2 e in southern tibet a total of 61 sites more than 700 cores have been sampled mainly from limestones zongshan formation jidula formation member ii zongpu formation sandstones jidula formation member iii and graywackes gamba group member ii with ages well defined from detailed lithological and biostratigraphic analyses carriers of characteristic remanences are magnetite in limestones and graywackes and hematite in sandstones reliable palaeodirections were obtained from 35 sites 356 specimens positive fold tests antiparallel normal and reverse polarities a northward trend of the palaeolatitudes with decreasing age and absence of any metamorphic overprint indicate a primary origin for the characteristic remanences the results have been compared with expected palaeodirections according to available data for the indian subcontinent based on its present day geometry the late maastrichtian sites fourteen sites 156 specimens and the middle late palaeocene sites fourteen sites 113 specimens yield palaeolatitudes of 5 7 s and 4 0 n for the sampling localities in southern tibet respectively these palaeolatitudes are much further north than expected and imply that crustal shortening between the tethyan himalaya and the indian shield since the time of india eurasia collision has occurred on the order of 1500 km at the longitude of gamba duela comparison of our observed palaeolatitudes with palaeolatitudes for the lhasa block during cretaceous and palaeocene times indicate that initial contact of the northern indian margin with the lhasa block was already established at 65 60 ma with suturing completed at about 55 50 ma

the role of migrating oil field brines in the formation of sediment hosted cu rich deposits oil field brines commonly contain ppm quantities of zn pb and h 2 s but negligible quantities of cu theoretical analysis of the water rock interactions expected during migration through evaporitic and red bed sequences suggests that these reducing oil field brines can become transformed into far more oxidized fluids capable of transporting cu pb zn and sulfate and of forming sediment hosted cu rich deposits calculation of the chemical mass transfer accompanying reaction of a typical reducing brine with anhydrite hematite and quartz followed by the assemblage quartz k feldspar muscovite hematite chlorite and cuprite reveals that the aqueous phase becomes greatly undersaturated with respect to sphalerite galena chalcopyrite bornite pyrite and pyrrhotite as it achieves log f sub o 2 values well above the magnetite hematite buffer consistent with saturation with respect to anhydrite hematite and chalcocite and ore forming concentrations of copper and sulfate consequently evaporites are not only possible sources of salinity and oxidized sulfur for evolving basinal brines but may be crucial to the development of the f sub o 2 conditions required to transport cu in ore forming concentrations reaction of the oxidized brine with organic matter approximated by graphite and diagenetic pyrite results in precipitation of the zonation sequence chalcocite bornite chalcopyrite and pyrite with variable amounts of hematite covellite sphalerite and galena depending on the relative rates of reaction of the graphite and pyrite

the timing of india asia collision onset facts theories controversies abstract the timing of initial collision between india and asia has remained controversial for half a century this paper attempts to review this crucial and hotly debated argument describing first the different methods used to constrain the age of collision and discussing next the rationale results inferences and problems associated with each we conclude that stratigraphy represents the best direct way to unravel collision chronology other methods focusing on the magmatic metamorphic or paleomagnetic record provide additional fundamental constraints but cannot provide a robust direct estimate of collision onset initial collision in the central eastern himalaya is dated directly at the middle paleocene 59 1 ma by the abrupt change in sediment provenance recorded in trench settings the quasi synchronous unconformities documented along both tethyan passive margin of india and active margin of asia from tibet to zanskar ladakh confirm that orogeny was underway at the close of the paleocene 56 ma well before the disappearance of marine seaways in the himalaya during the early middle eocene 50 45 ma sedimentary evolution and provenance changes in marine to fluvio deltaic successions are recorded synchronously within error from the western to the central eastern himalaya failing to provide conclusive evidence for diachronous collision these coherent observations are hard to reconcile with three widely cited hypotheses invoking either paleogene arc continent collision or late cretaceous ophiolite obduction or the protracted existence of a greater india basin which are all not favored after discussing the geological evidence critically point by point a scenario no more complex than the one involving solely the passive continental margin of india and the active continental margin of asia is needed to explain the geological evolution of the nascent himalaya the collision between the tethys himalaya and the transhimalayan arc trench system does represent the collision between india and asia because the yarlung zangbo ophiolite is the forearc basement of the asian active margin its obduction onto india could not have preceded the initial closure of neo tethys ophiolite obduction began when collision began in the middle paleocene

a synvolcanic origin for magnetite rich orebodies hosted by bif in the weld range district western australia abstract iron ore deposits hosted by archean banded iron formation bif in the weld range greenstone belt are representative of most of the documented iron ore deposits in the yilgarn craton they include near surface supergene goethite hematite orebodies that overlie and partly modify deeper occurrences of hypogene magnetite and specular hematite ores the cenozoic goethite hematite rich orebodies in these deposits are unequivocally the product of meteoric fluid alteration affecting bif in the near surface supergene environment however the deeper and likely older magnetite and specular hematite rich orebodies have a more contentious origin this study is the first to present a fluid alteration model for hypogene iron mineralization in the yilgarn craton that uses fluid inclusion and mineral chemistry data to constrain the physical chemical characteristics and source of hypogene fluids responsible for mineralization high grade 57 wt fe magnetite rich iron ore at the beebyn deposit defines a discontinuous series of 440 c and co2 rich paired o and c stable isotope data for stage 1 ferroan dolomite suggest that these fluids had a magmatic source while stage 1 magnetite chemistry e g enrichments in mg mn ca and na indicates chemical exchange took place between the fluids and mafic igneous rocks prior to crystallization of magnetite the presence of monophase carbonic fluid inclusions in stage 1 ferroan dolomite suggest that phase separation of a bicarbonate rich aqueous fluid took place in deeper parts of the hydrothermal system which led to the separation of the resultant volatile rich and brine phases during transport of the stage 1 fluid to shallower crustal levels cooling of the hydrothermal system during the stage 2 fluid event involved i an early brine 275 327 c 36 40 equiv wt nacl with cl br and c and o isotopes values that overlap the ranges for magmatic fluids with minor involvement of archean seawater and extensive chemical exchange with country rocks followed by ii pulses of moderate and lower temperature stage 2 brines 125 260 c 2 24 equiv wt cacl2 with cl br o and c isotope na br and ca ca na signatures that suggest cooling of magmatic derived fluids that mixed with archean seawater and reacted with mafic igneous country rocks in areas more distal to fluid pathways the last stage of formation of magnetite rich ore at the beebyn deposit involved the flow of stage 3 fluids through the existing fault network that controlled earlier fluids stage 3 fluids are lower temperature 98 175 c low to high salinity brines with cl br values that overlap reported ranges for 3 2 ga vent fluids and seawater thus they are likely the product of heated archean seawater that was chemically modified through interaction with mafic country rocks magnetite talc veins at the madoonga deposit have fluid halogen ratios for fluid inclusions hosted by magnetite that are compatible with a range of possible sources including low grade metamorphic fluids geothermal brines or oil field formation waters although their common spatial association with semi massive sulfides suggests their likely precipitation from brines derived from heated archean seawater more locally developed specular hematite quartz veins that cut folded magnetite rich ores at the beebyn and madoonga deposits are the product of iron redistribution in bif by heated meteoric fluids or seawater with precipitation of specular hematite as a consequence of oxidation and cooling of the fluid

remagnetization of carbonate rocks in southern tibet perspectives from rock magnetic and petrographic investigations netherlands organization for scientific research nwo with a rubicon grant 825 15 016 institute for rock magnetism irm at the university of minnesota instruments and facilities program of nsf erc starting grant 306810 nwo vidi 864 11 004

reply to comment on comparing paleomagnetic study means with apparent wander paths a case study and paleomagnetic test of the greater india versus greater indian basin hypotheses

late holocene syngenetic ice wedge polygons development bylot island canadian arctic archipelago the initial configuration of the syngenetic ice wedge polygons that developed in the outwash plain of glacier c 79 after 6000 bp was modified by the accumulation of wind blown and organic sediments that began after 3670 110 bp the late holocene sedimentation led to an increase in the thermal contraction coefficient of the soil and the formation of third and fourth order contraction cracks partially explaining the current configuration of the polygonal network the upturning of the sedimentary strata bordering the ice wedges was associated with the summer thermal expansion and resulting internal creep of the soil the mean annual soil displacement coefficient was in the order of 2 5 2 7 10 5 c at the thousand year scale the late holocene sedimentary strata under the centre of the polygons were undisturbed which will make it possible to use this sedimentary record in further studies to attempt paleoenvironmental reconstructions from cores

paleocene eocene foreland basin evolution in the himalaya of southern tibet and nepal implications for the age of initial india asia collision siliciclastic sedimentary rocks derived from the southern lhasa terrane sitting depositionally upon rocks of the northern indian passive continental margin provide an estimate of the age of initial contact between the continental parts of the indian and asian plates we report sedimentological sedimentary petrological and geochronological data from upper cretaceous paleocene strata in the sangdanlin section located along the southern flank of the indus yarlung suture zone in southern tibet this is probably the most proximal and therefore the oldest record of the india asia collision these strata were deposited by high density turbidity currents or concentrated density flows and suspension settling of pelagic biogenic debris in a deep marine setting an abrupt change from quartz arenitic to feldspatholithic sandstone compositions marks the transition from indian to asian sediment provenance the abrupt compositional change is accompanied by changes in u pb ages of detrital zircons diagnostic of a sediment provenance reversal from indian to asian sources the timing of the transition is bracketed between 60 ma and 58 5 0 6 ma by detrital zircon u pb ages and zircon u pb ages from a tuffaceous bed in the upper part of the section in the context of a palinspastically restored regional paleogeographic framework data from the sangdanlin section combined with previously published data from the northern tethyan himalaya and the frontal nepalese lesser himalaya and subhimalaya suggest that a flexural wave migrated 1300 km southward across what is now the himalayan thrust belt from paleocene time to the present

geochemistry of metal rich brines from central mississippi salt dome basin u s a abstract oil field brines are the most favored ore forming solutions for the sediment hosted mississippi valley type ore deposits detailed inorganic and organic chemical and isotope analyses of water and gas samples from six oil fields in central mississippi one of the very few areas with high metal brines were conducted to study the inorganic and organic complexes responsible for the high concentrations of these metals the samples were obtained from production zones consisting of sandstone and limestone that range in depth from 1900 to 4000 m 70 120 c and in age from late cretaceous to late jurassic results show that the waters are dominantly bittern brines related to the louann salt the brines have extremely high salinities that range from 160 000 to 320 000 mg l total dissolved solids and are na ca cl type waters with very high concentrations of ca up to 48 000 mg l and other alkaline earth metals but with low concentrations of aliphatic acid anions the concentrations of metals in many water samples are very high reaching values of 70 mg l for pb 245 mg l for zn 465 mg l for fe and 210 mg l for mn the samples with high metal contents have extremely low concentrations 2 s samples obtained from the smackover formation limestone have low metal contents that are more typical of oil field waters but have very high concentrations up to 85 mg l of h 2 s computations with the geochemical code solmineq 87 give the following results 1 both pb and zn are present predominantly as aqueous chloride complexes mainly as pbcl 4 2 and zncl 4 2 respectively 2 the concentrations of metals complexed with short chained aliphatic acid anions and reduced s species are minor 3 organic acid anions are important in controlling the concentrations of metals because they affect the ph and buffer capacity of the waters at subsurface conditions and 4 galena and sphalerite solubilities control the concentrations of pb and zn in these waters

reconsidering the uplift history and peneplanation of the northern lhasa terrane tibet the elevation history of the tibetan plateau promises insight into the mechanisms and dynamics that develop and sustain high topography over tens of millions of years as well as the contribution of uplift related erosive flux to cenozoic global cooling the elevation history of the center and northern margin of the plateau have been historically less well constrained than the southern margin a diverse suite of techniques each with their own biases and uncertainties yield discrepant mid cenozoic elevation estimates 0 5 km we reconstruct paleogene to miocene elevations of the lunpola basin on the northern lhasa terrane the southernmost crustal block of the tibetan plateau using stable isotope paleoaltimetry and clumped isotope paleothermometry on lacustrine and pedogenic carbonates integrated with previously published compound specific n alkane derived hydrogen isotopes paleo elevation estimates for the lunpola basin 3 1 4 7 km demonstrate that the northern edge of the lhasa block attained high elevation prior to 24 ma and potentially by the early eocene

comment on comparing paleomagnetic study means with apparent wander paths a case study and paleomagnetic test of the greater india versus greater indian basin hypotheses by david b rowley

genesis of the mississippi valley type zn pb deposit of san vicente central peru geologic and isotopic sr o c s pb evidence the san vicente zn pb ore deposit is situated 300 km east of lima in central peru within the upper triassic lower jurassic carbonate platform pucara group at the western margin of the brazilian shield production during the last 20 years and present reserves exceed 12 million tons of ore assaying about 12 percent zn and 1 percent pb sphalerite and galena the only ore minerals occur as lens shaped bodies generally parallel to the bedding within the 1 400 m thick pucara sequence three ore bearing dolomite units occur the ore lenses are bound to dolomitized tidal flat and lagoon facies with cryptalgal lamination and evaporite molds and to adjacent oolithic grainstones of barrier facies strontium carbon oxygen and sulfur isotope geochemistry was carried out on consecutive crystallization generations the results obtained display systematic trends the 87 sr 86 sr ratios range between 0 7077 and 0 7084 the first generations display values very similar to those of upper triassic lower jurassic ocean water later generations are slightly enriched in radiogenic strontium the degree of enrichment in radiogenic strontium is low compared to other mississippi valley type deposits the delta 18 o values range between 6 and 10 per mil pdb and the delta 13 c values between 2 and 1 per mil pdb the late crystallization generations are enriched in the light isotopes of oxygen and carbon compared to the first generations the sulfur isotope ratios of sphalerite are relatively homogeneous ranging between 9 9 and 13 0 per mil a trend to lighter sulfur isotope ratios with advancing diagenetic stage can also be recognized the san vicente lead zinc deposit formed during burial diagenesis the temperatures indicated by sulfur isotope geothermometry 75 degrees 92 degrees c would be consistent with temperatures reached at a burial depth of about 2 to 3 km this might have been reached by the end of the jurassic a model based on abiogenic reduction of sulfates at or near the ore site and the introduction of a zinc and lead bearing basinal brine characterized by strontium isotope ratios slightly higher than contemporaneous seawater and highly radiogenic lead is favored the association of the orebodies with peritidal facies with abundant sulfate molds is important evidence favoring the existence of two separate reservoirs for sulfur and metals leaching of detrital material derived from precambrian upper crust from the brazilian shield would explain both strontium and lead isotope ratios

internal zonation in a carbonate hosted zn pb ag deposit nanisivik baffin island canada zn pb ag ores at nanisivik northwest baffin island are hosted by upper proterozoic laminated dolostone of the society cliffs formation sulfide orebodies containing pyrite sphalerite galena sparry dolomite and pyrite pseudomorphous after marcasite are characterized by well banded ore textures considered to have resulted from the progressive replacement of carbonate wall rock textural and mineralogical variations enable the main orebody to be divided into three vertical and four horizontal ore zones the upper lens of the main orebody may be further subdivided into six texturally distinct laterally extensive mine units in the eastern upper lens where textural variations are best developed fluid inclusions indicate that most ore mineralization took place over the temperature range of 165 degrees to 210 degrees c and involved a brine containing 20 to 38 wt percent nacl equiv the sulfur isotope compositions of main and late stage pyrite crystals range from delta 34 s 27 4 to 28 0 per mil iron contents of sphalerite vary from 14 to 0 mole percent fes corresponding to well developed color zoning and constrain the oxygen activity of the ore fluid to 10 super 46 to 10 super 41 at 200 degrees c during sphalerite precipitation interbanded primary marcasite and sparry dolomite gangue indicates that the ore fluid fluctuated around ph 5 ore formation is modeled based on processes involving in situ reduction of sulfate by hydrocarbons and minor h 2 s encountered at the site of ore deposition the location and configuration of the orebodies was probably controlled in part by the former position of a horizontal fluid interface banding is likely the result of repetitive sulfate reduction metal precipitation wall rock dissolution and dolomite recrystallization in response to the pulsatory influx of ore fluid textural and mineralogic variations probably resulted from slight variations in the availability of reduced sulfur

palaeomagnetic estimates of crustal shortening in the himalayan thrusts and zangbo suture this article reports new palaeomagnetic results from precisely dated late palaeocene limestones from the former northern margin of the indian subcontinent now in tibet between the major himalayan thrusts and the india tibet suture zone convergence in the suture and intracontinental thrusting can be estimated separately and are respectively found to be of the order of 550 km 650 km and 400 km 400 km comparison with kinematic models derived from magnetic anomalies constrains the age of the collision at about 50 myr

kinematic evolution of a continental collision constraining the himalayan tibetan orogen via bulk strain rates abstract we investigated temporal variations of the bulk strain rate across the himalayan tibetan orogen which impact the magnitudes rates and distribution of deformation across the orogen the present day strain rate estimated from geodesy is 5 6 10 16 s 1 and estimation of past rates depends on the orogenic shortening rate and effective orogen width plate circuit reconstructions provide constraints on time varying india asia convergence rates showing a marked deceleration since initial collision at ca 58 ma geologic evidence suggests that most of the tibetan crust started deforming shortly after initial collision which simplifies estimation of the initial orogen width we examined several kinematic models 1 greater indian and tibetan crust started deforming immediately after collision 2 only tibetan crust deformed initially 3 plate convergence was decoupled from crustal shortening with tibetan crust deforming slower than plate rates or 4 a hard india asia collision at ca 45 ma following closure of the xigaze backarc models 1 3 and 4 yield early cenozoic bulk strain rates that match present day rates 5 6 10 16 s 1 whereas model 2 yields faster rates 1 2 10 15 s 1 that must have decreased through time to avoid shortening exceeding total plate convergence using these bounds we constrain crustal shortening across the entire orogen and along its southern and northern margins defined by the himalaya and qilian shan respectively application of these models to tibetan crustal thickening suggests that the plateau reached its present day value 70 km by ca 15 ma which corresponds to the onset of lateral deformation e g strike slip and normal faulting lateral deformation in the himalayan tibetan orogen may have resulted from progressive crustal thickening and reorientation of the intermediate principal stress axis at the thrust belt scale our modeling yields shortening rates and magnitudes that match geologic observations this approach provides testable external kinematic constraints to guide future geological and geophysical investigations

thermal comparison of mississippi valley type lead zinc deposits and their host rocks using fluid inclusion and conodont color alteration index data fifteen north american mississippi valley type districts distributed from the arctic archipelago to arkansas and in host rocks ranging in age from late cambrian to early carboniferous were selected for abroad test survey of mississippi valley type district thermal histories for each district in which data were collected histograms of homogenization temperatures for the ore minerals were compared with the temperature range of the corresponding host rocks as indicated by color alteration index cai determinations on conodonts in the host rock carbonates although a special effort was made to collect conodonts from the upper cambrian bonneterre formation host rock to the southeast missouri district all samples were devoid of conodonts thus only 14 districts were used in the final analysis comparison of fluid inclusion homogenization temperatures in main stage ore minerals with color alteration index determined host rock temperatures reveals that a majority of mississippi valley type districts pine point newfoundland zinc mascot jefferson city copper ridge sweetwater central missouri northern arkansas and tri state are representative of group 1 i e they are in thermal equilibrium with their host rocks insofar as could be determined by the color alteration index method ore minerals in group 2 districts upper mississippi valley polaris and central tennessee are significantly hotter than surrounding host rocks and as such define a positive thermal anomaly host rocks to the austinville ivanhoe northern newfoundland and robb lake deposits group 3 revealed the highest color alteration index values of north american mississippi valley type districts with host rock temperatures greatly exceeding ore mineral fluid inclusion temperatures negative thermal anomaly with respect to major tectonic elements of north america group 3 deposits cool ore in hot rocks are positioned within orogenic belts but on the continental side some group 1 deposits ore temperature host rock temperature are well within the continental interior but others are adjacent to orogenic belts on the continental side group 2 deposits hot ore in cool rocks are all within the continental interior current hypotheses regarding the formation of mississippi valley type deposits and simultaneous heating of their host rocks invoke the migration of hot brines set into motion craton ward by the hydraulic gradient produced by orogenic uplift at the craton edge once the ore bearing fluids entered the foreland carbonate platforms they traveled through sedimentary aquifers of the continental interior raising the temperature of the thinly covered carbonate rocks the fluids left as evidence of their passage elevated color alteration index values and group 1 mississippi valley type deposits distributed with decreasing homogenization temperatures away from the orogenic front the correspondence between host rock and ore temperatures suggests that group 1 deposits were deposited essentially within their own aquifers group 2 deposits were formed when the same or genetically similar fluids rose from the regional aquifer s into structurally controlled conduits in the continental interior possibly developed as a result of synorogenic doming and arching this upward channeling of the fluids resulted in rapid and localized fluid flow which precluded or at least inhibited wide spread heating of the host rocks by ore fluids consequently color alteration index values in group 2 districts record only maximum burial depths of the host rocks and bear little relation to temperatures of the ore fluids although the cause of the unusually high color alteration index values 4 5 of group 3 districts is not fully understood it is noted that these districts occur near the extreme outward edge of foreland platforms near or within the leading margins of orogenic belts whether these group 3 host rocks were heated by simple tectonic burial or by passage of heated fluids under shallower conditions is not known

iron deposits hosted by banded iron formations in the yilgarn craton products of sequential iron enrichment by magmatic marine and meteoric fluids abstract mesoarchean algoma type bif in the yilgarn craton are locally enriched by secondary processes to iron concentrations approaching 72 wt fe while their near surface goethite hematite ores are unequivocally the product of supergene fluid alteration their deeper magnetite and crystalline hematite martite ores have a contentious origin this study shows that magnetite ores formed after diagenesis and peak greenschist facies metamorphism of bif coinciding with regional deformation events the ores formed by a multi stage process involving initial replacement of primary quartz bands in bif by hypogene carbonate minerals followed by the dissolution of the carbonate and concentration of magnetite at the beebyn deposit the earliest fluids were hot 440 c co2 vapor rich and were likely derived from felsic magmatic fluids in contrast the fluids responsible for carbonate alteration at the matthew ridge koolyanobbing and windarling deposits were cooler 366 to 125 c and aqueous rich with salinities from 0 3 to 43 9 wt nacl equiv their cl br and c o isotopic ratios suggest they are the product of mixing between magmatic fluids and heated seawater in all deposits later cooling 178 to 98 c of these fluids led to dissolution of carbonate minerals and concentration of magnetite dated monazite and xenotime from magnetite ores demonstrates that fluid alteration of bif began in the mesoarchean and continued episodically through to the paleoproterozoic later specular hematite rich veins that cut and modify magnetite ores crystallized from aqueous rich fluids that are more oxidized with salinities from 0 to 28 7 wt nacl equiv and crystallization temperatures of 243 to 85 c their cl br and c o isotopic fluid ratios indicate that they formed from mixing between heated seawater and meteoric waters the bif hosted iron deposits in the yilgarn craton show remarkable similarities with the well studied iron deposits in the carajas mineral province and hamersley basin they all show the same transition through time from magnetite to hematite rich ores subsequent episodic tectonically driven fluid alteration events led to further iron enrichment in these deposits the transition in ore types most likely corresponds to a common guiding process namely the uplift of crustal rocks containing bif resulting in the progressive shallowing of fluid alteration pathways and the greater influence of cooler more oxidized hydrothermal fluids unlike iron deposits hosted by algoma type bif in the yilgarn and carajas iron deposits hosted by superior type bif in the hamersley basin display a greater influence of oxidized marine and meteoric waters without involvement of magmatic fluids

the cadjebut deposit as an example of mississippi valley type mineralization on the lennard shelf western australia single episode or multiple events the cadjebut zn pb deposit is one of several mississippi valley type resources hosted in devonian carbonates of the lennard shelf western australia several dolomitization events and at least three pulses of mineralization are documented in the cadjebut area indicating multiple episodes of fluid expulsion during basin evolution the data contrast with previous models which propose a single dolomitization and mineralization event for the lennard shelf two major mineralization events are recorded in the cadjebut area the earlier more weakly developed mineralization event is characterized by finely crystalline sphalerite and marcasite laths disseminated in dolomite cements that occlude primary porosity in evaporitic sequences the later mineralizing event forming the cadjebut orebody is represented in terms of diagenesis either in cloudy calcites that replace evaporitic minerals or within blocky calcite cements in large secondary vugs and cavities timing relationships relative to diagenetic events in the area indicate that the first mineralization event occurred early in the diagenetic history prior to significant burial and is interpreted to be middle devonian late givetian early frasnian in age the later event occurred subsequent to significant chemical compaction and maximum burial and is dated as mid carboniferous late tournaisian westphalian anomalous values of zn and pb in early carboniferous sediments of the lennard shelf and the apparent absence of mineralization in late carboniferous to permian sediments support a shelfwide mid carboniferous event stratigraphic constraints suggest an absolute age for cadjebut of 325 or 25 ma this second mineralization event coincides with a period of reefal platform emergence second order cycle regression and minor uplift of the lennard shelf all temporally coinciding with the waning stages of the alice springs orogeny centered in central australia 700 km to the southeast of cadjebut the two main pulses of mississippi valley type mineralization on the lennard shelf have distinctive patterns of pb isotope compositions the early burial small resource mineralization has heterogeneous pb isotope compositions whereas later economic and subeconomic mineralization has more homogeneous signatures the pb isotope data and the overall intensity of mineralization suggest that large volumes of fluid were expelled during the later period of platform emergence sea level regression distal orogenic events and mineralization

air temperature snow cover creep of frozen ground and the time of ice wedge cracking western arctic coast the time of ice wedge cracking is examined for several sites with young and old ice wedges along the western arctic coast the correlation between sharp air temperature drops and ice wedge cracking is highest where the snow cover is thin and least where the snow cover is thick the favoured duration and rate of a temperature drop that results in cracking is about 4 days at a rate of about 1 8 c d such temperature drops have a minimal effect in cooling the top of permafrost wherever there is an appreciable snow cover since short duration temperature drops often result in ice wedge cracking the thermal stresses that trigger cracking probably originate more within the frozen active layer than at greater depth in permafrost although most ice wedges tend to crack during periods of decreasing air temperatures about one third of those monitored have cracked during periods of increasing air temperatures long term measurements show that the active layer and top of permafrost move differentially all year in

large scale subduction of continental crust implied by india asia mass balance calculation continental crust is buoyant compared with its oceanic counterpart and resists subduction into the mantle when two continents collide the mass balance for the continental crust is therefore assumed to be maintained here we use estimates of pre collisional crustal thickness and convergence history derived from plate kinematic models to calculate the crustal mass balance in the india asia collisional system using the current best estimates for the timing of the diachronous onset of collision between india and eurasia we find that about 50 of the pre collisional continental crustal mass cannot be accounted for in the crustal reservoir preserved at earth s surface today represented by the mass preserved in the thickened crust that makes up the himalaya tibet and much of adjacent asia as well as southeast asian tectonic escape and exported eroded sediments this implies large scale subduction of continental crust during the collision with a mass equivalent to about 15 of the total oceanic crustal subduction flux since 56 million years ago we suggest that similar contamination of the mantle by direct input of radiogenic continental crustal materials during past continent continent collisions is reflected in some ocean crust and ocean island basalt geochemistry the subduction of continental crust may therefore contribute significantly to the evolution of mantle geochemistry buoyant continental crust is thought to resist subduction calculation of the crustal mass balance during the collision between india and eurasia indicates that about 50 of pre collisional continental crust has been subducted into the mantle

the frequency of ice wedge cracking 1967 1987 at garry island western arctic coast canada the frequency of ice wedge cracking has been studied at garry island northwest territories for the 1967 1987 period sites have included low centre polygons intermediate centre polygons and polygons that do not fit any classification system analyses of crack frequency have included trough characteristics polygon characteristics and ice wedge types the results show that crack frequencies are highly variable within one site and also between two adjacent sites the correlation between crack frequency and a low air temperature is poor crack frequencies for a site with 59 wedges ranged from 8 to 42 between 1967 and 1979 and for a nearby site with 32 wedges from 22 to 75 between 1967 and 1987 in view of the wide range in crack frequencies at a given site the use of mean ice wedge growth rates for estimating ages of ice wedges and their casts in environmental reconstruction may be hazardous the data show that the common twofold classification into active and inactive wedges is difficult to apply be

greater india basin hypothesis and a two stage cenozoic collision between india and asia cenozoic convergence between the indian and asian plates pro duced the archetypical continental collision zone comprising the himalaya mountain belt and the tibetan plateau how and where india asia convergence was accommodated after collision at or be fore 52 ma remains a long standing controversy since 52 ma the two plates have converged up to 3 600 35 km yet the upper crustal shortening documented from the geological record of asia and the himalaya is up to approximately 2 350 km less here we show that the discrepancy between the convergence and the shortening can be explained by subduction of highly extended continental and oceanic indian lithosphere within the himalaya be tween approximately 50 and 25 ma paleomagnetic data show that this extended continental and oceanic greater india promontory resulted from 2 675 700 km of north south extension between 120 and 70 ma accommodated between the tibetan himalaya and cratonic india we suggest that the approximately 50 ma india asia collision was a collision of a tibetan himalayan microconti nent with asia followed by subduction of the largely oceanic greater india basin along a subduction zone at the location of the greater himalaya the hard india asia collision with thicker and contiguous indian continental lithosphere occurred around 25 20 ma this hard collision is coincident with far field deforma tion in central asia and rapid exhumation of greater himalaya crys talline rocks and may be linked to intensification of the asian monsoon system this two stage collision between india and asia is also reflected in the deep mantle remnants of subduction imaged with seismic tomography

paleomagnetic results from the early cretaceous lakang formation lavas constraints on the paleolatitude of the tethyan himalaya and the india asia collision abstract to better constrain the early cretaceous paleogeographic position of the tethyan himalaya and the india asia collision process a paleomagnetic study was performed on the lakang formation lava flows in the cuona area in the southeastern tethyan himalaya stepwise thermal and alternating field demagnetizations successfully isolated reliable characteristic remanent magnetization chrm directions that include antipodal dual polarities and pass positive fold tests at the 99 confidence level and reversal tests at 95 confidence level indicating prefolding primary magnetizations the distribution patterns of chrm directions from the lakang formation lava flows are consistent with young lava flows at similar latitudes suggesting that secular variation has likely been averaged out the tilt corrected site mean direction for 31 sites is d 261 6 i 68 5 with 95 3 6 which provides a paleopole at 26 8 s 315 2 e a 95 5 7 corresponding to a paleolatitude of 52 2 5 7 s for the study area comparison of the paleolatitude observed from the lakang formation lava flows with that expected from the apparent polar wander paths of india at 130 ma show a paleolatitude difference of 2 1 230 km indicating that neither a great north south continental crustal shortening occurred between the indian craton and the tethyan himalaya after 130 ma nor that a wide ocean separated them at that time comparison with reliable cretaceous paleocene paleomagnetic results observed from the tethyan himalaya and the lhasa terrane indicates that the latitudinal width of the neo tethyan ocean could have been up to 7000 km at 134 130 ma and an extension should have existed between the indian craton and the tethyan himalaya during the late cretaceous and paleocene furthermore reliable paleomagnetic results suggest that the india asia collision was a dual collision process consisting of a first collision of the tethyan himalaya with the lhasa terrane asia at 54 9 2 3 ma and a final continent continent collision of the indian craton with the tethyan himalaya at 40 0 3 3 ma

structural control hydrothermal alteration zonation and fluid chemistry of the concealed high grade 4ee iron orebody at the paraburdoo 4e deposit hamersley province western australia high grade iron ore of the 4ee orebody of the 4e deposit 200 mt at 63 5 wt fe occurs as a southerly dipping sheet within banded iron formation bif of the paleoproterozoic dales gorge and joffre members of the brockman iron formation structural reconstruction of the 4e deposit shows that reactivation of the 18e fault and development of the nw striking steeply sw dipping 4e and 4ee normal faults resulted in preservation of the 4ee orebody below the 4e deposit and 400 m below the modern topographic surface three hypogene alteration zones between low grade bif and high grade iron ore are observed 1 distal magnetite quartz dolomite stilpnomelane hematite pyrite 2 intermediate magnetite dolomite hematitechlorite quartz stilpnomelane and 3 proximal hematite dolomite chlorite pyrite magnetite hydrothermal alteration is temporally and spatially constrained by nw trending dolerite dikes that intruded the 4e and 4ee faults prior to hypogene alteration six vein types v1 v6 are recognized at the 4e deposit the veins both cut and parallel the primary bif layers and were emplaced contemporaneously with the hydrothermal alteration zones that record the transformation of low grade bif to high grade iron ore our integrated structural hydrothermal alteration and fluid flow model proposes that during early stage 1a hypogene fluid flow in the 4e orebody occurred during a period of continental extension and enhanced heat flow within sedimentary basins to the south of the paraburdoo range heated basinal brines were focused by the nw striking steeply sw dipping 4e and 4ee normal faults and reacted with bif of the dales gorge and joffre members the warm to hot 160 255 c ca rich 26 6 31 9 equiv wt cacl2 basinal brine interacted with magnetite chert layers transforming them into magnetite quartz dolomite stilpnomelane hematite pyrite bif the iron rich brine up to 2 8 wt fe likely originated from evaporated seawater that had lost mg and na and gained li and ca through fluid rock reactions with volcaniclastic rocks and carbonate successions within the wittenoom formation the first incursion of deeply circulating low salinity 5 8 9 5 wt nacl equiv heated 106 201 c modified meteoric water is recorded in late stage 1a minerals this modified meteoric water had lost some of its na through wall rock interaction with plagioclase possibly by interaction with dolerite of the weeli wooli formation that directly overlies the joffre and dales gorge members stage 1b involved continuing reactions between the hydrothermal fluids and the magnetite quartz dolomite stilpnomelane hematite pyrite bif and produced both the intermediate magnetite dolomite hematite chlorite pyrite and the proximal hematite dolomite magnetite stilpnomelane alteration assemblages microplaty 10 80 m platy 100 250 m and anhedral hematite increasingly replace magnetite in the intermediate alteration zone forming the proximal alteration zones that consist of microplaty platy anhedral hematite and magnetite the intermediate and proximal alteration zones represent the mixing of a hot 250 400 c high salinity ca rich 30 40 wt cacl2 equiv sr rich basinal brine with low temperature and low salinity 5 wt nacl equiv modified meteoric water that was heated 100 200 c during its descent into the upper crust heterogeneous mixing of the two end member fluids resulted in the trapping of primary fluid inclusion assemblages containing a wide range of trapping temperatures up to 200 c and salinities up to 25 wt nacl equiv stage 1c of the hypogene hydrothermal fluid is characterized by low temperature 110 c low salinity 5 wt nacl meteoric water that interacted with the proximal hematite dolomite magnetite stilpnomelane altered bif leaving a porous hematite apatite high grade ore supergene alteration affected the orebody since the cretaceous and produced a hematite goethite alteration assemblage resulting in destruction of the hypogene alteration zones that are only preserved below the depth of modern weathering discovery of the concealed 4ee orebody of the 4e deposit demonstrates that structural geology plays a critical role in the exploration for high grade iron orebodies structural reconstruction should be considered a critical exploration activity in structurally complex terranes where concealed orebodies may exist

origin of late dolomite cement by co2 saturated deep basin brines evidence from the ozark region central united states studies of fluid inclusions in regionally extensive late dolomite cement ldc throughout the ozark region show that co2 effervescence was widespread during dolomite precipitation on the basis of quantitative analyses of inclusion fluids reaction path modeling shows that ldc with trace amounts of sulfides can be deposited by effervescence of a co2 saturated basin brine as it migrates to shallower levels and lower confining pressures this precipitation mechanism best explains occurrences of ldc in the ozark region and may account for ldc found in sedimentary basins worldwide

petrology mineral chemistry and exploration significance of fe sulfides from the metal dispersion halo surrounding the cadjebut znpb mvt deposit western australia abstract the cadjebut mvt mineralization is hosted by carbonate evaporite units in the givetian lower dolomite sequence of the pillara platform northern western australia and occurs as 2 ore types 1 an early zn rich stratiform and rhythmically banded ore and its laterally equivalent halo of banded marcasite barite calcite and 2 a later cross cutting breccia fill pb rich ore the 2 ore types are exploited through 2 mining lenses an upper lens 1 and lower lens 2 the orebody is sited n of the normal basin side down cadjebut fault rhythmically banded marcasite completely envelopes the orebody in both lenses whereas banded barite occurs n of the lower lens 2 only where the barite is partially to completely replaced by late cloudy calcite marcasite barite replacement by marcasite and cloudy calcite increases northwards petrographic and trace element data indicate 3 types of fe sulfides 1 low pb fe sulfides with pb contents 30 000 ppm pb only the colloform pyrites exhibit pb contents above 30 000 ppm and sem images show these elevated pb contents are due to micron scale galena inclusions mineral chemical data from sulfides in the stratiform fe sulfide and barite halos indicate that main ore stage fluids of the rhythmically banded ore event were enriched in zn pb cd as and to a minor extent ni and cu late stage fluids of the rhythmically banded ore event were not saturated with respect to galena precipitating low pb marcasites along the very outer fringes of the orebody and replacing rhythmically banded barite in lens 2 only the low pb marcasites show various stages of pyrite inversion petrographic and trace element mineral chemistry of the 2 stratiform halos are best explained by a model in which main ore stage fluids of the early rhythmically banded ore event emanated from below the cadjebut orebody along an inferred antithetic reverse fault the presence of a laterally zoned alteration halo with a pattern of proximal higher pb and trace element bearing fe sulfides provides a vector towards the ore zone and has important exploration significance for this style of base metal mineralization

iron rich and iron poor mississippi valley type mineralization metaline district washington two texturally and mineralogically distinct types of mississippi valley type mineralization are hosted by the upper cambrian metaline formation in northeastern washington josephine type mineralization is iron poor and coarse grained whereas yellowhead type mineralization is iron rich fine grained and commonly botryoidal josephine mineralization is found largely in the josephine lithofacies a fragmental unit near the contact between dolomites of the metaline formation and shales of the overlying ledbetter formation yellowhead mineralization forms several planar zones several hundred meters below largely in the light gray bedded dolostone lithofacies of the metaline formation about 12 7 million tonnes mt of josephine type ore 1 3 pb 3 0 zn and 730 000 t of yellowhead type ore 4 5 zn 0 5 pb were produced from the district before 1977 and about 5 5 mt of ore 7 2 zn 1 3 pb remain largely in the yellowhead zone ore textures in both types of mineralization indicate a complete lack of equilibrium among adjacent minerals and this is confirmed by geochemical measurements although fluid inclusion homogenization and freezing temperatures are similar for both types of ore fluid inclusion leachate and gas analyses differ for each mineral compositions for josephine minerals appear to form a cluster that differs from compositions for yellowhead minerals suggesting that ore minerals were deposited from two parent brines that changed composition slightly to deposit each mineral isotopic analyses indicate that sulfur in both types of mineralization was probably derived from the same source whereas lead definitely came from two distinct sources these observations indicate that ore formed when two parent metal rich brines mixed with the same source of sulfur abundant pyrite that retains the form of marcasite suggests that the original brine in at least the yellowhead ores was moderately acid geochemical speciation calculations indicate that relatively high temperatures are needed to dissolve enough iron to make iron rich mississippi valley type deposits furthermore moderately acid brines with relatively low sulfur contents are enriched in iron whereas those with high sulfur contents lack iron suggesting that the variations in iron content of these deposits reflect variations in the dissolved sulfur content of the parent brines