spatial zonation limits magnesite dissolution in porous media abstract we investigate how mineral spatial distribution in porous media affects their dissolution rates specifically we measure the dissolution rate of magnesite interspersed in different patterns in packed columns of quartz sand where the magnesite concentration v v was held constant the largest difference was observed between a mixed column containing uniformly distributed magnesite and a one zone column containing magnesite packed into one cylindrical center zone aligned parallel to the main flow of acidic inlet fluid flow parallel one zone column the columns were flushed with acid water at a ph of 4 0 at flow velocities of 3 6 or 0 36 m d breakthrough data show that the rate of magnesite dissolution is 1 6 2 times slower in the one zone column compared to the mixed column this extent of rate limitation is much larger than what was observed in our previous work 14 for a similar one zone column where the magnesite was packed in a layer aligned perpendicular to flow flow transverse one zone column two dimensional reactive transport modeling with crunchflow revealed that ion activity product iap and local dissolution rates at the grid block scale 0 1 cm vary by orders of magnitude much of the central magnesite zone in the one zone flow parallel column is characterized by close or equal to equilibrium conditions with iap k eq 0 1 two important surface areas are defined to understand the observed rates the effective surface area a e reflects the magnesite that effectively dissolves under far from equilibrium conditions iap k eq a i reflects the effective magnesite surface that lies along the quartz magnesite interface modeling results reveal that the transverse dispersivity at the interface of the quartz and magnesite zones controls mass transport and therefore the values of a e and a i under the conditions examined in this work the value of a e varies from 2 to 67 of the total magnesite bet surface area column scale bulk rates r mgco 3 b in units of mol s vary linearly with a e and a i using a e to normalize rates we calculate a rate constant 10 9 56 mol m 2 s that is very close to the value of 10 10 0 mol m 2 s under well mixed conditions at the grid block scale this implies that the laboratory field rate discrepancy can potentially be caused by differences in the effective surface area if we know the effective surface area of dissolution we will be able to use the rate constant measured in laboratory systems to calculate field rates for some systems in this work approximately 60 70 of the a e is at the magnesite quartz interface this implies that in some field systems where the detailed information that we have for our columns is not available the effective mineral surface area may be approximated by the area of grains residing at the interface of reactive mineral zones although it has long been known that spatial heterogeneities play a significant role in determining physical processes such as flow and solute transport our data are the first that systematically and experimentally quantifies the importance of mineral spatial distribution chemical heterogeneity on dissolution

alteration mineralization and metamorphism in the area of the east south c ore zone 24th level of the dickenson mine red lake northwestern ontario the area of the east south c esc orebody of the dickenson mine red lake consists of variably altered and mineralized basalt basaltic volcaniclastic rocks minor sulphidic iron formation and

syndeformational fluid trapping in quartz determining the pressure temperature conditions of deformation from fluid inclusions and the formation of pure co2fluid inclusions during grain boundary migration observations and microthermometric data on fluid inclusions from a terrane that underwent deformation following peak metamorphic conditions show that grain boundary migration recrystallization favours the entrapment of carbonic inclusions whereas microfracturing during brittle deformation favours the infiltration and eventual entrapment of aqueous fluids our results imply that pure co2 fluid inclusions in metamorphic rocks are likely to be the residue of deformation recrystallization process rather than representing a primary metamorphic fluid where the temperature of deformation can be deduced by other means the densities of fluid inclusions trapped during recrystallization which we call recrystallization primary fluid inclusions can be used to constrain the ambient pressure during deformation using these constraints the data imply that the post metamorphic hercynian exhumation in sardinia brought rocks at 300 c to within 3km of the surface this conclusion is similar to that described for the rapidly uplifted southern alps in new zealand

regolith formation rate from u series nuclides implications from the study of a spheroidal weathering profile in the rio icacos watershed puerto rico abstract a 2 m thick spheroidal weathering profile developed on a quartz diorite in the rio icacos watershed luquillo mountains eastern puerto rico was analyzed for major and trace element concentrations sr and nd isotopic ratios and u series nuclides 238 u 234 u 230 th 226 ra in this profile a 40 cm thick soil horizon is overlying a 150 cm thick saprolite which is separated from the basal corestone by a 40 cm thick rindlet zone the sr and nd isotopic variations along the whole profile imply that in addition to geochemical fractionations associated to water rock interactions the geochemical budget of the profile is influenced by a significant accretion of atmospheric dusts the mineralogical and geochemical variations along the profile also confirm that the weathering front does not progress continuously from the top to the base of the profile the upper part of the profile is probably associated with a different weathering system lateral weathering of upper corestones than the lower part which consists of the basal corestone the associated rindlet system and the saprolite in contact with these rindlets consequently the determination of weathering rates from 238 u 234 u 230 th 226 ra disequilibrium in a series of samples collected along a vertical depth profile can only be attempted for samples collected in the lower part of the profile i e the rindlet zone and the lower saprolite similar propagation rates were derived for the rindlet system and the saprolite by using classical models involving loss and gain processes for all nuclides to interpret the variation of u series nuclides in the rindlet saprolite subsystem the consistency of these weathering rates with average weathering and erosion rates derived via other methods for the whole watershed provides a new and independent argument that in the rio icacos watershed the weathering system has reached a geomorphologic steady state our study also indicates that even in environments with differential weathering such as observed for the puerto rico site the radioactive disequilibrium between the nuclides of a single radioactive series here 238 u 234 u 230 th 226 ra can still be interpreted in terms of a simplified scenario of congruent weathering incidentally the u th ra disequilibrium in the corestone samples confirms that the outermost part of the corestone is already weathered

rates of weathering rind formation on costa rican basalt weathering rind thicknesses were measured on 200 basaltic clasts collected from three regionally extensive alluvial fill terraces qt 1 qt 2 and qt 3 preserved along the pacific coast of costa rica mass balance calculations suggest that conversion of unweathered basaltic core minerals plagioclase and augite to authigenic minerals in the porous rind kaolinite allophane gibbsite fe oxyhydroxides is iso volumetric and ti and zr are relatively immobile the hierarchy of cation mobility ca na k mg si al fe p is similar to other tropical weathering profiles and is indicative of differential rates of mineral weathering anorthite albite hypersthene orthoclase apatite alteration profiles across the cm thick rinds document dissolution of plagioclase and augite and the growth of kaolinite with subsequent dissolution of kaolinite and precipitation of gibbsite as weathering rinds age the rate of weathering rind advance is evaluated using a diffusion limited model which predicts a parabolic rate law for weathering rind thickness rr as a function of time t rr t and an interface limited model which predicts a linear rate law for weathering rind thickness as a function of time rr kappt in these rate laws is a diffusion parameter and kapp is an apparent rate constant the rate of advance is best fit by the interface model terrace exposures are confined to the lower reaches of streams draining the pacific slope near the coast where the stream gradient is less than 3 m km and terrace deposition is influenced by eustatic sea level fluctuations geomorphological evidence is consistent with terrace deposition coincident with sea level maxima when the stream gradient would be lowest assigning the most weathered regionally extensive terrace qt 1 mean rind thickness 6 9 0 6cm to oxygen isotope stage ois 7 ca 240 ka and assuming that at time 0 rind thickness 0 it is inferred that terrace qt 2 rr 2 9 0 1 cm is coincident with stage 5e ca 125 ka and that qt 3 rr 0 9 0 1 cm is consistent with ois 3 ca 37 ka these assignments yield a value of kapp of 8 6 10 13 cm s 1 r 2 0 99 only this value satisfies both the existing age controls and yields ages coincident with sea level maxima using this value elemental weathering release fluxes across a weathering rind from qt 2 range from 6 0 10 9 mol si m 2 s 1 to 2 5 10 11 mol km 2 s 1 the rate of rind advance for the costa rican terraces is 2 8 10 7 my r 1 basalt rind formation rates in lower temperature settings described in the literature are also consistent with interface controlled weathering with an apparent activation energy of about 50 kj mol 1 rates of rind formation in costa rica are an order of magnitude slower than reported for global averages of soil formation rates copyright 2004 elsevier ltd

porosity evolution and crystallization driven fragmentation during weathering of andesite 1 a 10 m thick andesitic sill intrusion from the neuquen basin argentina shows spectacular examples of spheroidal weathering and liesegang banding the liesegang patterns demonstrate how andesite blocks initially cut out by a preweathering joint set are subdivided by fractures formed during the spheroidal weathering process the stresses that cause fracturing originate from the growth of ferrihydrite and calcite in the pore space of the andesite partly at the expense of original ilmenite amphibole and plagioclase the porosity evolution and fracture formation during progressive weathering was characterized by scanning electron microscopy studies x ray computed tomography and he and hg porosimetry fresh andesite has a porosity of approximately 8 and a major fraction 80 of the pore volume is composed of pores less than 10 m in diameter the extent of pore filling during weathering increases with pore size pores more than 100 m are almost completely filled by an intimate intergrowth of calcite and ferrihydrite whereas pores less than 10 m are filled less than 50 the fracturing associated with spheroidal weathering is caused by mineral growth in the largest pores which account for 10 20 of the total porosity the periodic precipitation of the weathering product to form liesegang bands indicates a significant supersaturation threshold before nucleation commences the increase in the weathering product growth rate with increasing size is therefore most likely due to higher nucleation probabilities in larger pores

reactions involving hydration of orthopyroxene in anorthosite gabbro abstract hydration of orthopyroxene en 60 in contact with plagioclase an 36 leads to oronas of the type orthopyroxene cummingtonite edenite quartz edenitic hornblende the compositions of the minerals involved indicate that kyanite needles in the adjacent plagioclases form as part of the reaction orthopyroxene en 60 plagioclase an 36 h 2 o cummingtonite edenite edenitic hornblende plagioclase 36 kyanite quartz the reaction proceedings consumption first of the orthopyroxene and later of the cummingtonite to produce a corona structure consisting of a core of intergrown edenite and quartz surrounded by edenitic hornblende corona evolution of this type is controlled by diffusion

fluid inclusion studies of the abawso gold prospect near the ashanti belt ghana the major ghanaian lode gold deposits are preferentially aligned along the western and eastern contacts of the kumasi basin with the ashanti and sefwi belts respectively the investigated area of the abawso small scale concession covering the workings of the old ettadom mine is situated 3 km west of the lithological contact of the birimian metavolcanic rocks of the akropong belt in the east with the birimian metasedimentary rocks of the kumasi basin in the west the rocks of the abawso concession represent a steeply nw dipping limb of a se verging anticline with an axis plunging to the sw quartz veining occurs predominantly in the form of en echelon dilatational veins along nne ssw striking shear zones of a few metres width and shows evidence of brittle and ductile deformation also stockwork style quartz veining occurs in the vicinity of the main shaft of the old ettadom mine hydrothermal alteration includes sericitisation sulphidation and locally carbonatisation the auriferous quartz veins mainly follow the trend of brittle to ductile deformed quartz veins however some occur in stockwork fluid inclusion studies reveal a large number of h2o inclusions along intragranular trails in auriferous quartz vein samples as well as an overall dominance of h2o and h2o co2 inclusions over co2 inclusions textural observations and physico chemical fluid inclusion properties indicate post entrapment modifications for all quartz vein samples due to grain boundary migration recrystallisation this process is interpreted to be responsible for the generation of the co2 inclusions from a h2o co2 parent fluid in comparison with mineralisation at the ashanti and prestea deposits which are characterised by co2 n2 inclusions the observed inclusion assemblage may be due to a shallower crustal level of mineralisation or different degrees and styles of recrystallisation or a less pronounced development of laminated quartz veins due to comparably restricted pressure fluctuations furthermore the microthermometric observations allow the reconstruction of a possible retrograde p t path depicting near isothermal decompression in the p t range of the brittle ductile transition

the effect of curvature on weathering rind formation evidence from uranium series isotopes in basaltic andesite weathering clasts in guadeloupe abstract to quantify rates of rind formation on weathering clasts under tropical and humid climate and to determine factors that control weathering reactions we analyzed uranium series isotope compositions and trace element concentrations in a basaltic andesite weathering clast collected from basse terre island in guadeloupe u th and ti elemental profiles reveal that th and ti behave conservatively during rind formation but that u is added from an external source to the rind in the rind weathering reactions include dissolution of primary minerals such as pyroxene plagioclase and glass matrix as well as formation of fe oxyhydroxides gibbsite and minor kaolinite rare earth element ree profiles reveal a significant eu negative anomaly formed during clast weathering consistent with plagioclase dissolution significant porosity forms in the rind mostly due to plagioclase dissolution the new porosity is inferred to allow influx of soil water carrying externally derived dissolved u due to this influx u precipitates along with newly formed clay minerals and oxyhydroxides in the rind the conservative behavior of th and the continuous addition of u into the rind adequately explain the observed systematic trends of 238 u 232 th and 230 th 232 th activity ratios in the rind rind formation rates determined from the measured u series activity ratios with an open system u addition model increase by a factor of 1 3 0 18 0 24 mm kyr from a low curvature to a high curvature section 0 018 0 12 mm 1 of the core rind boundary revealing that curvature affects rates of rind formation as expected for diffusion limited rind formation u series geochronometry thus provides the first direct evidence that the curvature of the interface controls the rate of regolith formation at the clast scale the weathering rates determined at the clast scale can be reconciled with the weathering rates determined at the watershed or soil profile scale if surface roughness equals values of approximately 1300 2200

fluid evolution and chemical controls in the fazenda maria preta fmp gold deposit rio itapicuru greenstone belt bahia brazil the fazenda maria preta fmp gold deposit is confined to two regional scale sinistral oblique brittle ductile shear zones which are located in a greenschist facies metamorphosed volcano sedimentary sequence in the northern sector of the lower proterozic rio itapicuru greenstone belt ne brazil alteration is pervasive throughout the shear zones and characterized by carbonatization and sericitization of intermediate volcanic volcaniclastic and sub volcanic rocks the orebodies are mainly shear veins hosted by or directly associated with highly deformed carbonaceous volcaniclastic rocks breccia and stockwork systems or disseminations in felsic and mafic sub volcanic bodies native gold appears as free grains finely disseminated in the quartz veins or closely related to arsenopyrite pyrite pyrrhotite sphalerite and fe chlorite in the carbonaceous wallrocks fluid inclusion studies by microthermometry and laser raman microspectroscopy revealed that the vein quartz is largely dominated by primary pseudosecondary and rarely secondary populations of co2 ch4 n2 inclusions type i whereas primary groups of low salinity 6 wt eq nacl h2o co2 ch4 n2 inclusions type ii comprise the dominant inclusion type in only a few veins the thco2 l v l data indicated a variation in the co2 density of 1 05 to 0 60 g cm3 for the type i inclusions and conversely a narrower range of 0 76 0 73 g cm3 for the type ii inclusions the broader variation of the co2 density for the co2 rich fluid could have been the result of trapping of the vein fluid under a variable pressure regime or by its re equilibration during continuous deformation within the shear zone domains and reduction of the overburden pressure during uplift the co2 rich fluid inclusions provide no evidence that they have been the result of h2o loss either during infiltration e g hydration reactions or after trapping of an original h2o co2 fluid accordingly the co2 ch4 n2 and the h2o co2 ch4 n2 fluids are considered as representatives of two distinct fluid regimes which were active during the gold mineralizing events within the shear zones both types of ore fluids are interpreted as part of a deep metamorphic magmatic hydrothermal system in which i a co2 rich fluid probably originated in the mantle was transported to and released in higher structural levels mainly by tonalitic granodioritic and alkaline magmas whereas ii a h2o co2 fluid generated by devolatilization reactions during the regional metamorphism was either directly channeled into favorable structural sites i e metamorphic fluid or absorbed and later exsolved by the crystallization of felsic magmas i e magmatic fluid sensu lato in this context gold deposition occurred between 320 c and 420 c and 2 1 to 4 4 kb and calculated values of fo2 between 10 26 5 and 10 32 4 bar reveal the relatively reducing nature of the mineralizing fluids the ore paragenesis constrains fs2 to the range of 10 10 1 10 6 5 bar for these fluids under such conditions the gold was transported mainly as bisulfide complexes by the h2o co2 fluid but in the case of the co2 rich fluid the role of thiocomplexes or some other types of ligands is obscured by the lack of high temperature experimental data deposition of the metal occurred in response to redox changes which accompanied fluid carbon interaction particularly a decrease in fo2 and fs2 of the fluid on a microscopic scale phisisorption and chemisorption processes at the fluid iron sulfide interface may have further enhanced the deposition of the gold particularly on the surfaces of precipitating pyrite and arsenopyrite

high co2 content of fluid inclusions in gold mineralisations in the ashanti belt ghana a new category of ore forming fluids a reply fluid inclusions were studied in samples from the ashanti konongo southern cross prestea abosso damang and ayanfuri gold deposits in the ashanti belt ghana primary fluid inclusions in quartz from mineralised veins of the ashanti prestea konongo southern cross and abosso damang deposits contain almost exclusively volatile species the primary setting of the gaseous i e the fluid components co2 ch4 and n2 fluid inclusions in clusters and intragranular trails suggests that they represent the mineralising fluids microthermometric and raman spectroscopic analyses of the inclusions revealed a co2 dominated fluid with variable contents of n2 and traces of ch4 water content of most inclusions is below the detection limits of the respective methods used aqueous inclusions are rare in all samples with the exception of those from the granite hosted ayanfuri mineralisation here inclusions associated with the gold mineralisation contain a low salinity co2 and low salinity 6 eq wt nacl however fluid inclusions in quartz from the gold mineralisations in the ashanti belt point to distinctly different fluid compositions specifically the predominance of co2 and co2 h2o have to be emphasized fluid systems with this unique bulk composition were apparently active over more than 200 km along strike of the ashanti belt fluids rich in co2 may present a hitherto unrecognised new category of ore forming fluids

determining mineral weathering rates based on solid and solute weathering gradients and velocities application to biotite weathering in saprolites abstract chemical weathering gradients are defined by the changes in the measured elemental concentrations in solids and pore waters with depth in soils and regoliths an increase in the mineral weathering rate increases the change in these concentrations with depth while increases in the weathering velocity decrease the change the solid state weathering velocity is the rate at which the weathering front propagates through the regolith and the solute weathering velocity is equivalent to the rate of pore water infiltration these relationships provide a unifying approach to calculating both solid and solute weathering rates from the respective ratios of the weathering velocities and gradients contemporary weathering rates based on solute residence times can be directly compared to long term past weathering based on changes in regolith composition both rates incorporate identical parameters describing mineral abundance stoichiometry and surface area weathering gradients were used to calculate biotite weathering rates in saprolitic regoliths in the piedmont of northern georgia usa and in luquillo mountains of puerto rico solid state weathering gradients for mg and k at panola produced reaction rates of 3 to 6 10 17 mol m 2 s 1 for biotite faster weathering rates of 1 8 to 3 6 10 16 mol m 2 s 1 are calculated based on mg and k pore water gradients in the rio icacos regolith the relative rates are in agreement with a warmer and wetter tropical climate in puerto rico both natural rates are three to six orders of magnitude slower than reported experimental rates of biotite weathering

role of inherent geological and climatic characteristics on landscape variability in the tectonically passive western ghat india abstract how does lithological variation affect the landscape and its evolution is a fundamental issue in modern and post orogenic terrains in the absence of active tectonic uplift spatial variation of rock resistance and rainfall distribution are thought to primarily control the fluvial and hillslope processes however these lithologic and climatic controls are difficult to decouple owing to the fact that these post orogenic terrains endure long and complex geomorphic history to examine the relative importance of lithology and rainfall in a post orogenic setting we analyzed west flowing drainage basins in the western ghat wg with a multi proxy analysis we relied on digital topography sediment yield and hydrological data we evaluated topographic relief hypsometry river long profiles and sediment yield to elaborate the importance of spatial variability of rock resistance in the long term evolution of these drainage basins the dominant increase of relief 200 1500 m and decrease of sediment yield 3 32 1 91 ton month km2 from north to south of the escarpment is directly attributed to the rock erodibility and structural characteristics field verification suggests that despite high rainfall condition the channels are mostly detachment limited and therefore lack the abrasive tools we further showed that the harder lithologies sustain higher steepness on the contrary the spatial distribution of satellite derived rainfall showed a negligible effect on the long profile evolution our analysis is also supported by the existing knowledge of chemical denudation rate and implicates that rock hardness and weatheribility defines the topographic characteristics with little or no effect of rainfall in this passive margin setting

ore geology and fluid inclusions of the hucunnan deposit tongling eastern china implications for the separation of copper and molybdenum in skarn deposits abstract the hucunnan ore deposit is a representative skarn cu mo deposit in the tongling district an important ore district of the renowned middle lower yangtze river metallogenic belt of china the deposit shows distinct zonation of metals with cu mineralization distributed mainly in the exoskarn zone at shallow depths and mo mineralization occurring chiefly in the endoskarn zone located at deeper depths field evidence and petrographic observations indicate that the ore forming processes can be divided into the skarn quartz molybdenite quartz chalcopyrite and carbonate stages five types of fluid inclusions fis are present in the deposit solid bearing type 1 liquid rich type 2 vapor rich type 3 pure vapor type 4 and co 2 bearing type 5 the skarn stage contains mainly type 1 and 2 but also minor type 3 and 4 fis the fis display homogenization temperatures of 434 570 c and salinities of 2 07 66 0 wt nacl equiv the existence of hematite daughter minerals in the type 1 inclusions together with the presence of magnetite in skarn implies that the skarn stage fluid was oxidizing the skarnization of the granodiorite porphyry is commonly accompanied by potassic alteration suggesting that the fluids were rich in alkali similar to the skarn stage the quartz molybdenite stage contains type 1 and 2 and minor type 3 and 4 fis which yield homogenization temperatures of 280 458 c and salinities of 1 40 54 2 wt nacl equiv the presence of sulfide instead of hematite daughter minerals in the type 1 inclusions in quartz molybdenite veins associated with sericitization indicates that the fluids of the quartz molybdenite stage were more reducing and more acidic than the fluids in the skarn stage the decrease in oxygen fugacity and increase in acidity could have resulted from magnetite crystallization and the consumption of alkali cations and oh respectively during the skarn stage the quartz chalcopyrite stage contains all types of fis which show homogenization temperatures of 203 392 c and salinities of 1 22 46 6 wt nacl equiv observations of hematite bearing type 1 fis in quartz chalcopyrite veins containing anhydrite associated with biotitization suggest that the fluids of the quartz chalcopyrite stage were oxidizing and alkali rich probably on account of the inflow of meteoric water and boiling in an open system in the carbonate stage only type 2 fis are present these fis yield the lowest homogenization temperatures of 156 276 c and the lowest salinities of 1 05 12 3 wt nacl equiv microthermometry and h o isotope data indicate that the ore forming fluids were dominated by magmatic water in the early stages skarn and quartz molybdenite stages and that the magmatic water gradually mixed with circulating meteoric water during the late stages quartz chalcopyrite and carbonate stages the coexistence of saline and vapor rich fis as internal trails or clusters within individual crystals with similar homogenization temperatures but contrasting salinities and homogenization modes to the liquid and vapor respectively in the first three stages strongly suggests that three episodes of fluid boiling occurred in these stages as further supported by the hydrogen isotopic compositions of the fluids which are lower than those of magmatic water based on the above data we conclude that temporal changes in redox conditions acidity and temperature in the mineralizing fluids resulted in the temporal separation of cu and mo by selective sulfide precipitation in the hucunnan skarn deposit the competition among metals e g mo and cu for sulfur in magmatic fluids along with vapor brine immiscibility fluid boiling are major factors that contributed to the spatial separation of cu and mo in the deposit

origins of the surface roughness of unweathered alkali feldspar grains abstract the origins of surface roughness of powdered feldspar grains the ratio of the total surface area quantified by gas adsorption to the surface area estimated by assuming an average grain geometry has been investigated by a detailed study of alkali feldspars from a range of igneous rocks and from pegmatites the of 125 63 m powders of ten different feldspar samples ranges from 5 16 22 00 and correlates well with mineral microtexture powders of feldspars which are homogeneous or have straight lamellar exsolution microtextures yield mean values of 5 59 whereas powders of vein macroperthites give a mean of 6 64 and powders of braid macroperthites yield mean values of 12 15 microtexture influences by partly controlling the manner in which feldspar grains break during grinding and this affects the frequency of steps on grain surfaces feldspars which are homogeneous or have straight lamellar exsolution microtextures break to produce a relatively high proportion of grains with smooth step poor external surfaces whereas vein macroperthites and braid microperthites break to produce grains with much rougher step rich surfaces microtexture is able to influence style of breakage because the presence size and crystallographic orientation of exsolution lamellae has a significant influence on the trajectories of fractures propagating through grains as they fragment during grinding we found little correlation between internal surface area due to micropores expressed as either microporosity or mean perimeter area of micropores and indicating that microporosity is a significantly less important contributor to than external surface area due to steps this conclusion is supported by an analysis of the of a number of different size fractions of two of the feldspar powders the origin of is only one part of the more important question of how closely measured surface areas are related to the proportion of grain surfaces which are reactive during experimental dissolution and natural weathering

investigating the complex interface where bedrock transforms to regolith abstract the interface where bedrock transforms to regolith is not planar but rather has a roughness that varies with the scale of observation the complexity of this surface is manifested in both element depth and fragment size depth distributions and may sometimes be related to the longitudinal profiles of watershed streams the fractal nature of the bedrock regolith interface means that the interface has a thickness which is 20 m in two ridgetop examples from pennsylvania and puerto rico such weathering thicknesses modeled as a function of one dimensional fluid flow are affected by the balance between rates of weathering and erosion one dimensional models are consistent with weathering advance rates that vary with equilibrium solubility and porefluid velocities and not reaction kinetics however fluid flow is not strictly downward and one dimensional permeability of regolith changes as particle size and bulk density changes with depth thus both downward and lateral flow occurs especially at reaction fronts where reactions change permeability the rate of weathering advance is therefore affected by the 3 dimensional distribution of reaction zones that affect permeability across the watershed quantitative models of such phenomena over a range of spatial and temporal scales are needed

the early cretaceous yangzhaiyu lode gold deposit north china craton a link between craton reactivation and gold veining the yangzhaiyu gold deposit is one of numerous lode gold deposits in the xiaoqinling district southern margin of the north china craton gold mineralization is hosted in neoarchean to early paleoproterozoic amphibolite facies metamorphic rocks and consists of auriferous quartz veins and subordinate disseminated ores in the vein proximal alteration zone ore related hydrothermal alteration is dominated by sericite quartz sulfide assemblages close to gold veins and biotite quartz pyrite chlorite epidote alteration generally distal from mineralization pyrite is the predominant sulfide mineral locally coexisting with minor amounts of chalcopyrite sphalerite and galena gold occurs mostly as free gold enclosed in or filling microfractures of pyrite and quartz and is also present in equilibrium with au bearing tellurides mainly petzite and calaverite coexisting with hessite tellurobismuthite and altaite fluid inclusion studies suggest that gold veins were deposited at intermediate temperatures 175 313 c from aqueous or aqueous carbonic fluids with moderate salinity 5 14 wt nacl equiv 34s values of sulfide minerals range mainly from 2 0 to 4 4 whereas auriferous quartz vein samples have 18o values of 12 4 to 9 6 with calculated 18oh2o values of 6 0 to 3 2 gold related pyrite grains yield elevated 3he 4he ratios 1 51 0 32 ra relative to crustal reservoirs and mantle like 20ne 22ne and 21ne 22ne ratios 9 90 9 68 and 0 029 respectively the stable and noble gas isotopes thus suggest deep seated most likely magmatic and mantle derived sources for the ore fluids sulfur and by inference other components in the ore system 40ar 39ar dating of ore related sericite and biotite separates indicates two episodes of gold genesis at 134 5 to 132 3 and 124 3 to 123 7 ma the mineralization ages overlap zircon u pb ages of 141 0 1 6 to 125 8 1 4 ma 2 for the wenyu and niangniangshan monzogranite plutons and a number of mafic to intermediate dikes intruding these plutons all being proximal to the yangzhaiyu gold deposit the synchronism of gold genesis and magmatism provides additional weights of evidence for a magmatic derivation of ore fluids and sulfur the geochronologic data also suggest that gold veining took place billions of years after the stabilization of the north china craton and associated metamorphism in the late archean to early paleoproterozoic this contrasts sharply to lode gold deposits in other precambrian cratons that formed predominantly in late archean to paleoproterozoic temporarily and genetically related to regional high grade metamorphism and compressional or transpressional tectonism available data have demonstrated that the north china craton was reactivated in the late mesozoic as marked by voluminous igneous rocks faulted basin formation high crustal heat flow and widespread metamorphic core complexes in the eastern part of the craton it is thus suggested that the yangzhaiyu gold deposit together with other deposits of similar ages in the xiaoqinling district were products of this craton reactivation event lithospheric extension and extensive magmatism related to the craton reactivation may have provided sufficient heat energy fluid and sulfur required for the formation of the gold deposits

new constraints on the genesis and geodynamic setting of the wulong gold deposit liaodong peninsula northeast china evidence from geology geochemistry fluid inclusions and c h o s pb isotopes the wulong lode gold deposit is located in the liaoning province northeast part of north china craton gold ore bodies are mainly hosted in the late jurassic granite and structurally controlled by

fluid characteristics of vein and altered wall rock in archean mesothermal gold deposits certain trends in alteration are commonly observed near archean mesothermal gold deposits this study was designed to monitor the changes in composition of fluid inclusions using solid probe mass spectrometry by 1 establishing and verifying systematic changes in the composition of the fluid during fluid rock interaction 2 characterizing the composition and behavior of fluids in the vein and the wall rock in order to establish a link between them 3 elucidating new insights on the role of fluid immiscibility in gold deposition and 4 scrutinizing the results for potential use in exploration three representative sites of gold mineralization were chosen a large deposit with appreciable amounts of free gold sigma mine in val d9or a second deposit with a smaller amount of free gold norbeau mine in chibougamau and a third area of gold mineralization with altered wall rock and little veining tadd prospect in chibougamau fluid inclusions were carefully selected to sample different alteration zones and the gold mineralization at each site in most of the cases varying degrees of unmixing of co 2 h 2 o were noted mass spectra results show that during alteration all generations of inclusions at all sites contained co 2 and h 2 o for the alteration hosted deposit a decrease in x sub co 2 could be mapped outward from the mineralized zone the most striking result was that fluid inclusions associated with the main mineralizing episode at sigma showed a nearly complete segregation into co 2 and h 2 o end members this can be interpreted as the result of extreme fluid pressure fluctuations this characteristic was absent from the norbeau veins where free gold exists in very limited amounts a test on a high grade ore shoot from the doyon mine resulted in a pattern similar to sigma we suggest extreme pressure fluctuations were limited in vein hosted deposits like norbeau and gold was precipitated essentially from fluid rock mineral interaction unmixing was the most likely cause of free gold deposition mineralizing systems may have undergone major fracturing events at time of free gold deposition this study suggests high co 2 h 2 o ratios are related to gold mineralization in altered rock hosted deposits but that the ratio may be an unreliable guide in vein hosted deposits because of the variable unmixing pattern of co 2 h 2 o fluids decrepitometry may prove a useful and rapid method for identifying co 2 h 2 o unmixing and bonanza gold potential

tectonics fracturing of rock and erosion 1 we argue that by fracturing rock not by raising it relative to base level tectonics plays its most important role in causing rapid incision of valleys and rapid erosion of hillslopes tectonic deformation riddles the upper crust with fractures which not only provide avenues for water flow and thus promote weathering and further disintegration of rock but also fragment bedrock into debris that is readily extracted and transported by surface processes bends in active faults require straining of adjacent rock masses aftershocks that occur subsequent to slip on primary faults reflect penetrative brittle deformation of the upper crust at least some aftershocks must nucleate or lengthen cracks which contribute to the comminution of these rock masses scaling rules suggest that dimensions of ruptures for very small m 2 earthquakes can be meters or less the gutenberg richter recurrence relationship implies that such earthquakes are common as high magnification seismographs in low noise environments confirm moreover large differences among fault plane solutions for aftershocks show that the small faults on which they occur are not parallel to one another some faults must intersect thus the upper crust in tectonically active regions should be fragmented into blocks down to the scale of boulders or smaller dismembered rock arrives at the earth s surface already prepared to be transported away as a corollary both deeply exhumed lower crust and posttectonic igneous rock never deformed under brittle conditions and not deformed recently should be less susceptible to detachment and subsequent transport than fractured rock

hydrothermal transport and depositional processes in archean lode gold systems a review abstract compelling evidence now exists for the formation of archean lode gold deposits over a substantial range in metamorphic conditions and crustal depth the commonality of ore fluid isotopic and geochemical characteristics the extreme fluid rock ratios within actual ore zones and the observed depth continuity within individual orebodies all suggest that these deposits formed within vertically extensive crustal scale hydrothermal systems calculations based on new thermodynamic data and estimated ore fluid conditions suggest that gold would have been transported as au hs 2 over most of this range gold transport in the ore fluids that formed some diopside and diopside k feldspar class deposits t 550 c would have been as aucl2 whereas the au hs complex may have been significant in some of the lower temperature 270 c less alkaline ore fluids the high solubility of gold in lode gold ore fluids at amphibolite facies conditions 10 s to 1000 s ppb suggests that fluids may have been undersaturated in gold within their source regions promoting the ore fluid s capacity to retain gold in solution over the large transport distances involved ore depositional mechanisms were also likely to vary between crustal levels fluid rock interactions and wallrock sulphidation in particular appear to be the most important type of precipitation mechanism however with decreasing depth of formation the scope of other depositional mechanisms notably phase separation and fluid mixing becomes greater

geology geochemistry and geochronology of the east bay gold trend red lake ontario canada the red lake greenstone belt is situated in northwestern ontario within the uchi subprovince superior province most gold deposits therein are associated with major deformation corridors the east west oriented mine trend hosts most of the large deposits and the northeast southwest east bay trend hosts several small deposits and showings gold along the east bay trend typically occurs in quartz replacement veins that were emplaced into pre existing quartz carbonate veins gold can occur as free gold or along vein margins associated with pyrite and pyrrhotite most primary fluid inclusions preserved in relatively undeformed portions of veins are carbonaceous with lesser quantities of aqueous inclusions the average homogenization temperature of aqueous fluids is 250 c however the abundance of three phase inclusions variation in liquid vapor ratios and a wide range in homogenization temperatures indicate that immiscibility effervescence and fluid mixing are mechanisms associated with gold deposition the age 2550 ma of alteration minerals in the abino area is considerably younger by 100 myr than alteration minerals in other deposits in the red lake district indicating that the mineralizing fluid history was more protracted than previously thought along the east bay trend barren veins generally have lower 18ovsmow values 0 0 to 8 5 relative to auriferous veins 9 6 and 13 1 consequently the oxygen isotopic composition of quartz could be used as a vector for gold mineralization the genetic model for the east bay trend involves several stages of vein formation auriferous veins formed near the upper boundary of the mesozonal regime depth of 5 6 km

iron transport and deposition mechanisms in the taochong iron rich skarn deposit middle lower yangtze valley eastern china abstract the taochong skarn iron deposit is one of 38 iron prospects in the fanchang iron ore district of the middle lower yangtze river valley one of the most important economic mineral districts in china orebodies are hosted mainly within the middle upper carboniferous huanglong dolomitic limestone in this deposit two types of ore types 1 and 2 are recognized based on the mineral assemblage the type 1 ore is dominant throughout the deposit and consists mainly of hematite magnetite quartz and calcite this ore is characterized by euhedral and skeletal textures and massive brecciated comb like and vein structures the type 2 ore is composed of hematite garnet pyroxene actinolite chlorite quartz and calcite and has a relict metasomatic texture and disseminated structure field evidence and petrographic observations indicate that three stages of hydrothermal activity i e the skarn iron oxide and carbonate stages contributed to the formation of the taochong iron deposit the iron oxide stage can be further divided into early and late oxide stages which saw the formation of the type 2 and type 1 ores respectively fluid inclusion and stable isotope data indicate that the skarn formed at depths of 2 km under lithostatic pressure of 500 bars from high temperature 500 c acidic saline up to 50 wt nacl equiv and iron rich fluids that had chemical and isotopic compositions 18owater 4 5 11 7 similar to those of magmatic fluid replacing a certain amount of the huanglong dolomitic limestone the iron ores formed under hydrostatic conditions after fracturing of the early skarn fluid inclusions within quartz in equilibrium with hematite in the type 2 ores yields homogenization temperatures of 372 390 c and salinities of 31 39 wt nacl equiv whereas fluid inclusions within quartz in equilibrium with hematite in the type 1 ores has lower homogenization temperatures 280 320 c and variable salinities 0 40 wt nacl equiv during the formation of the type 1 ores fluids contained moderate concentrations of ions variable 13c values 5 3 to 5 8 and relatively convergent 18o values 1 7 4 5 the temporal decrease in the concentration of ions and 18o values in the fluid inclusions is matched by marked decreases in fluid salinity and temperature suggesting that the fluids of the oxide stage were mixtures of magmatic brine and meteoric water the coexistence of saline and vapor rich fluid inclusions as internal trails or clusters within individual quartz grains of the late oxide stage with contrasting homogenization characteristics for the liquid and vapor at similar temperatures strongly suggests that fluid boiling occurred during the late oxide stage our estimates of the physico chemical conditions during ore formation together with fluid inclusion data and the results of previous experimental studies indicate that iron in the taochong deposit was transported mainly as fecl4 along with minor amounts of other chloride complexes e g fecl2 and fecl2 h2o 4 the precipitation of hematite followed the breakdown of the iron chloride complexes probably induced by a decrease in temperature and ligand chloride ion activity and an increase in ph based on the results we infer that deposition of the type 2 ore was caused by the replacement of garnet and pyroxene by a retrograde alteration assemblage which was likely induced by fluid rock interaction and the mixing of magmatic brine and meteoric water the type 1 ore was deposited in open spaces probably induced by fluid boiling

a reactive transport model for weathering rind formation on basalt abstract saprolite formation rates influence many important geological and environmental issues ranging from agricultural productivity to landscape evolution here we investigate the chemical and physical transformations that occur during weathering by studying small scale saprolites in the form of weathering rinds which form on rock in soil or saprolite and grow in thickness without physical disturbance with time we compare detailed observations of weathered basalt clasts from a chronosequence of alluvial terraces in costa rica to diffusion reaction simulations of rind formation using the fully coupled reactive transport model crunchflow the four characteristic features of the weathered basalts which were specifically used as criteria for model comparisons include 1 the mineralogy of weathering products 2 weathering rind thickness 3 the coincidence of plagioclase and augite reaction fronts and 4 the thickness of the zones of mineral reaction i e reaction fronts four model scenarios were completed with varying levels of complexity and degrees of success in matching the observations to fit the model to all four criteria however it was necessary to 1 treat diffusivity using a threshold in which it increased once porosity exceeded a critical value of 9 and 2 treat mineral surface area as a fitting factor this latter approach was presumably necessary because the mineral water surface area of the connected accessible porosity in the costa rica samples is much less than the total porosity navarre sitchler et al 2009 the model fit surface area here termed reacting surface area was much smaller than the bet measured surface area determined for powdered basaltic material in the parent basalt reacting surface area and diffusivity are low due to low pore connectivity and early weathering is therefore transport controlled however as pore connectivity increases as a result of weathering the reacting surface area and diffusivity also increase and weathering becomes controlled by mineral reaction kinetics the transition point between transport and kinetic control appears to be related to a critical porosity 9 at which pore connectivity is high enough to allow rapid transport based on these simulations we argue that the rate of weathering front advance is controlled by the rate at which porosity is created in the weathering interface and that this porosity increases because of mineral dissolution following a rate that is largely surface reaction controlled

metal fractionation between magmatic brine and vapor determined by microanalysis of fluid inclusions the major and trace element compositions of individual fluid inclusions from a range of magmatic hydrothermal ore deposits were analyzed by laser ablation inductively coupled plasma mass spectrometry to explore the behavior of ore forming components during fluid phase separation boiling in high temperature saline fluid systems data from 13 samples showing unambiguous evidence for coeval trapping of a liquid brine and a coexisting vapor phase identify two groups of elements with drastically different geochemical behavior na k fe mn zn rb cs ag sn pb and tl preferentially partition into the brine probably as cl complexes whereas cu as au probably as hs complexes and b selectively partition into the vapor fluid phase separation is probably a major previously underestimated process in the chemical differentiation that contributes to the extreme range of selective element enrichments in magmatic hydrothermal systems from deep plutons through porphyry style and greisen deposits to epithermal mineralization and volcanic fumaroles

orogenic gold formation in an evolving decompressing hydrothermal system genesis of the samut gold deposit eastern desert egypt abstract samut is a vein type gold mineralization in the eastern desert of egypt which shares many geologic features with other orogenic deposits in this region the deposit has a long history of mining activity dating back to ancient times but so far has received little attention from the standpoint of geologic study here we present new field petrographic microthermometric and microanalytical data from the samut gold deposit to evaluate its genesis and fluid evolution vein quartz at samut shows abundant evidence of precipitation coinciding with early phases of ductile deformation that show transitions to increasing brittle fracturing and fragmentation through time in some samples brecciation of the vein quartz is intense and shows evidence of repeated episodes of fragmentation this brittle style of deformation culminates with latest stage brittle opening and loss of confining pressure resulting in open space filling textures backscattered electron images and electron microprobe analysis reveal three varieties of pyrite in the mineralized quartz veins py i py ii and py iii laser ablation icpms analysis of pyrites from mineralized quartz veins reveals that the earlier py i is low in arsenic and gold whereas py ii and py iii are enriched in both elements anhedral py ii is particularly gold rich and gives way to blocky euhedral py iii that is rich in arsenic but not as rich in gold compared to py ii fluid inclusions in the least brittle deformed vein quartz indicate that the veins formed at minimum temperatures of 300 c early in the hydrothermal system the calculated fluid pressures at homogenization of the aqueous carbonic inclusions furnish a wide range throughout the paragenesis perhaps on the order of 1 to 3 kbar the 34s values of pyrites imply sulfur isotopic composition of the mineralizing fluid on the order of 0 6 to 4 9 overlapping with the broad range of 34s values calculated for ore fluids in orogenic gold deposits worldwide together these results support a general model in which gold and sulfide deposition was coinciding with an ongoing syn deformational vein formation and tending towards the late stages of vein growth prior to loss of confinement free milling gold appears to be mostly related to the later stages of brittle fracturing and re sealing of the veins during repeated fluid release as the background fluid pressure evolved from earlier dominantly lithostatic to later fully hydrostatic conditions

controls on rind thickness on basaltic andesite clasts weathering in guadeloupe abstract a clast of low porosity basaltic andesite collected from the b horizon of a soil developed on a late quaternary volcaniclastic debris flow in the bras david watershed on basse terre island guadeloupe exhibits weathering like that observed in many weathered clasts of similar composition in other tropical locations specifically elemental profiles measured across the core rind interface document that primary minerals and glass weather to fe oxyhydroxides gibbsite and minor kaolinite in the rind the earliest reaction identified in the core is oxidation of fe in pyroxene but the earliest reaction that creates significant porosity is plagioclase dissolution elemental loss varies in the order ca na k mg si al fe p ti consistent with the relative reactivity of phases in the clast from plagioclase pyroxene glass apatite ilmenite the rind surrounds a core of unaltered material that is more spherical than the original clast the distance from the core rind boundary to a visually prominent rind layer l was measured as a proxy for the rind thickness at 36 locations on a slab cut vertically through the nominal center of the clast this distance averaged 24 4 3 1 mm maximum and minimum values for l 35 8 and 20 6 mm were observed where curvature of the core rind boundary is greatest 0 12 mm 1 and smallest 0 018 mm 1 respectively extrapolating from other rinds in other locations the rate of rind formation is estimated to vary by a factor of about 2 from 4 to 7 10 14 m s 1 from low to high curvature the observation of a higher rate of rind formation for a higher curvature interface is consistent with a diffusion limited model for weathering rind formation the diffusion limited model predicts that like rind thickness values of the thickness of the reaction front h for a given reaction defined as the zone over which a parent mineral such as plagioclase completely weathers to rind material should also increase with curvature values of h were quantified as a function of interface curvature using bulk chemical analysis 500

carbonic metamorphism of charnockites in the southwestern indian shield a fluid inclusion study abstract forming the southwestern segment of the precambrian granulite facies terrain of the indian shield the kerala region largely comprises charnockites khondalites and migmatitic gneisses fluid inclusions in quartz from the charnockites show distinct distribution patterns consistent with three generations of inclusions the early monophase type records entrapment of high density co2 rich fluid 0 95 1 0 g cm 3 a subsequent monophase type with lower density co2 rich fluid 0 65 0 75 g cm 3 coexists with co2 h2o inclusions having an average degree of filling of 0 2 h2o 20 co2 80 late aqueous biphase inclusions show coexistence with a second category of co2 h2o inclusions showing a degree of filling of 0 6 h2o 60 co2 40 the co2 isochores for early carbonic inclusions yield a pressure range of 4 6 6 1 kbar at granulite facies temperatures of 650 800 c depicting the entrapment of fluids present during or close to the peak metamorphic stage a definite sequence of fluid evolution is traceable for the subsequent stages thus the coexisting co2 and co2 h2o inclusions were entrapped at 510 c and 2 2 kbar marking the waning of carbonic regime and the beginning of aqueous regime at 330 c and 0 4 kbar fluid unmixing occurred leading to the simultaneous entrapment of mixed co2 h2o and h2o inclusions along rehealed microfractures the data presented indicate that the metamorphic fluids evolved from early high density carbonic through mixed carbonic aqueous to late aqueous types the dry granulite mineral assemblage of charnockites is a result of metamorphic equilibration under water deficient and high pco2 conditions

comment on the paper by schmidt mumm et al high co2 content of fluid inclusions in gold mineralisations in the ashanti belt ghana a new category of ore forming fluids mineralium deposita 32 107 118 1997

models of transport and reaction describing weathering of fractured rock with mobile and immobile water we propose models for calculating the rate of chemical weathering of minerals as a function of depth in the weathering zone a macropore network is assumed to be responsible for the transport of mobile water which removes soluble weathering products at the interface of that network and the matrix conditions of infrequent rainfall a and of very frequent rainfall b are separately modeled but both lead to a volumetric weathering rate with the general form 0exp z where the amplitude 0 and the equilibration length depend on the pore geometry of regolith and on parameters describing transport across the macropore matrix interface a or mineral dissolution b this is obtained with no assumption of steady state for regolith evolution extrapolating these end members into intermediate conditions the model is consistent with the exponential decay of regolith production rate versus depth reported by several authors and yields in a variety of regolith types in the range from centimeters to meters the velocity of the regolith bedrock interface also shows an exponential decay with weathering zone thickness and is enhanced by bedrock fractures when compared to models of unfractured bedrock when the residence time of fluid in the weathering zone f is large the bulk weathering rate rb is inversely proportional to this time and for small f rb reverts to the laboratory rate application to compiled data from several sites leads to estimates of a dissolution rate constant and specific surface area consistent with those of albite

the origin and significance of non aqueous co2 fluid inclusions in the auriferous veins of bin yauri northwestern nigeria non aqueous co2 and co2 rich fluid inclusions are found in the vein quartz hosting mesothermal gold sulphide mineralization at bin yauri northwestern nigeria although mineralizing fluids responsible for gold mineralization are thought to be co2 rich the occurrence of predominantly pure to nearly pure co2 inclusions is nevertheless unusual for a hydrothermal fluid system many studies of similar co2 rich fluid inclusions mainly in metamorphic rocks proposed preferential loss leakage of h2o from h2o co2 inclusions after entrapment in this study however it is proposed that phase separation fluid immiscibility of low salinity co2 rich hydrothermal fluids during deposition of the gold mineralization led to the loss of the h2o phase and selective entrapment of the co2 the loss of h2o to the wallrocks resulted in increasing oxidizing effects there is evidence to suggest that the original co2 rich fluid was intrinsically oxidized or perhaps in equilibrium with oxidizing conditions in the source rocks the source of the implicated fluid is thought to be subducted metasediments subjected to dehydration and devolatilization reactions along a transcurrent anka fault shear system which has been described as a pan african 450 750 ma crustal suture

geochronological constraints on the timing of magmatism deformation and gold mineralization in the red lake greenstone belt northwestern ontario the paper presents u pb ages on zircon baddeleyite titanite rutile and monazite from volcanic and plutonic rocks of the red lake greenstone belt in the northern superior province the purpose of the study was to refine a previously outlined time frame for the volcanic evolution of the belt define the time of major plutonism and deformation and constrain the age of formation of gold deposits volcanism spanned at least 270 ma between 3000 and 2730 ma new ages of 2989 3 and 2894 2 ma for units of a lower volcanic sequence and and 2744 1 ma for units of an upper volcanic sequence corroborate and refine the previously established relationships local gabbroic and felsic intrusions cutting through the lower sequence yield ages of 2870 15 and respectively volcanism was succeeded by major plutonism deformation alteration and metamorphism between about 2730 and 2700 ma these processes were interrelated and progressed as a relatively continuous sequence of events accentuated by major activity d

fluids in metamorphic rocks abstract basic principles for the study of fluid inclusions in metamorphic rocks are reviewed and illustrated a major problem relates to the number of inclusions possibly formed on a wide range of p t conditions having also suffered in most cases extensive changes after initial trapping the interpretation of fluid inclusion data can only be done by comparison with independent p t estimates derived from coexisting minerals but this requires a precise knowledge of the chronology of inclusion formation in respect to their mineral host the three essential steps in any fluid inclusion investigation are described observation measurements and interpretation observation with a conventional petrographic microscope leads to the identification and relative chronology of a limited number of fluid types same overall composition eventually changes in fluid density for the chronology the notion of gis group of synchronous inclusions is introduced it should serve as a systematic basis for the rest of the study microthermometry measurements completed by nondestructive analyses mostly micro raman specify the composition and density of the different fluid types the major problem of density variability can be significantly reduced by simple considerations of the shape of density histograms allowing elimination of a great number of inclusions having suffered late perturbations finally the interpretation is based on the comparison between few isochores representative of the whole inclusion population and p t mineral data essential is a clear perception of the relative chronology between the different isochores when this is possible as illustrated by the complicated case of the granulites from central kola peninsula a good interpretation of the fluid inclusion data can be done if not fluid inclusions will not tell much about the metamorphic evolution of the rocks in which they occur

on the origin of co2 rich fluid inclusions in migmatites nearly pure co2 fluid inclusions are abundant in migmatites although h2o rich fluids are predicted from the phase equilibria processes which may play a role in this observation include 1 the effects of decompression on melt 2 generation of a co2 bearing volatile phase by the reaction graphite quartz biotite plagioclase melt orthopyroxene co2 rich vapour 3 selective leakage of h2o from co2 h2o inclusions when the pressure in the inclusion exceeds the confining pressure during decompression and 4 enrichment of grain boundary vapour in co2 by subsolidus retrograde hydration reactions

using a mathematical model of a weathering clast to explore the effects of curvature on weathering abstract for the first time we show that rind thicknesses developed on surfaces of a clast with different values of curvature can be used to estimate the duration of clast weathering to obtain an analytical expression for the velocity of the curvilinear weathering front on a clast of arbitrary shape we approximate our previous multi mineral reactive diffusion model and explore a simplified 2 d model numerically and analytically our analysis documents that with increasing curvature of the weathering front the mathematical description of weathering advance is equivalent to that derived for advection as the dominant solute transport mechanism even for the case where transport is occurring by diffusion only specifically for a curvilinear weathering front with constant curvature k

enrichment of co2 in fluid inclusions in quartz by removal of h2o during crystal plastic deformation abstract ductile strain induced leakage of h 2 o from mixed co 2 h 2 o fluid inclusions in quartz is proposed as a mechanism for producing occurrences of pure co 2 fluid inclusions in metamorphic rocks the h 2 o needed for hydrolytic weakening of quartz under stress may be provided by the h 2 o in fluid inclusions with dislocation creep as dislocations nucleate on the walls of fluid inclusions or intersect them h 2 o can be transported with the dislocations from the inclusion to the grain boundaries the process should continue so long as there is stress on the host quartz and the inclusion contains h 2 o the h 2 o taken to the grain boundaries would due to its wetting properties be wicked out along the grain boundaries if the h 2 o is totally removed from an inclusion by a flux of dislocations through the quartz during the crystal plastic flow then a residual inclusion bearing components other than water should remain a mixed co 2 h 2 o inclusion would as a consequence of the process become a pure co 2 fluid inclusion during dynamic recrystallization the co 2 would collect at grain boundary triple junctions and following grain boundary migration would become incorporated into the recrystallized quartz as fluid inclusions this mechanism of generating pure co 2 inclusions could result in co 2 fluid inclusions having densities appropriate for the pressure temperature conditions during deformation using estimates of temperature of deformation the total confining pressure during deformation could be determined from the density of the co 2 fluid in these inclusions

characteristics and origin of archean premetamorphic hydrothermal alteration at the campbell gold mine northwestern ontario canada the campbell mine located in northwestern ontario canada is developed within a metamorphosed and deformed fault controlled archean gold deposit hosted predominantly within basalt and ultramafic rocks with lesser quantities of rhyolite and diorite the sequence of hydrothermal alteration has been subdivided into three partially overlapping phases consisting of an early alteration phase followed by the main stage vein phase and culminating in the mineralization phase early alteration consists of two substages represented by carbonatization and pervasive biotite potassic alteration and early silicification and aluminosilicate bearing alteration carbonate alteration is relatively wide spread whereas biotitic alteration occurs proximal to alteration controlling structures in highly carbonatized and or silicified rocks carbonate biotite alteration grades into carbonate chlorite alteration laterally outward

euclidean and fractal models for the description of rock surface roughness accurate description of the topography of rock surfaces is essential because surface roughness affects frictional strength the flow of fluids in joints and fractures the seismic behavior of faults and the formation of gouge and breccia in fault zones real rock surfaces can be described using self similar and self affine fractal models of surface roughness if a surface is self similar a small portion of the surface when magnified isotropically will appear statistically identical to the entire surface if a surface is self affine a magnified portion of the surface will only appear statistically identical to the entire surface if different magnifications are used for the directions parallel and perpendicular to the surface at least two parameters are required to describe a fractal model one parameter typically describes how roughness changes with scale while the other specifies the variance or surface slope at a reference scale the divider method and the spectral method are in common use to determine the best fit fractal model from surface profile data power spectra from self similar surfaces have slopes of 3 on log log plots of power spectral density versus spatial frequency while spectra from self affine surfaces have slopes other than 3 power spectra can be interpreted with greater facility if dimensionless amplitude to wavelength ratios are contoured on plots of power spectral density versus frequency the topography of many natural rock surfaces including both fractures and faults is approximately self similar within the 6 5 order of magnitude wide wavelength band of 10 m to 40 m within smaller wavelength bands natural rock surfaces may exhibit self affine behavior

timing of gold mineralization at red lake northwestern ontario canada new constraints from u pb geochronology at the goldcorp high grade zone red lake mine and the madsen mine the goldcorp high grade zone at the red lake mine has reserves proven and probable established at 1 775 million tons mt at an average grade of 80 6 g t au new u pb geochronologic data combined with detailed mapping and crosscutting relationships provide timing constraints and new insights into the formation of the exceptionally rich goldcorp high grade zone the results show that the main stage of the high grade mineralization in the high grade zone formed before 2712 ma and that a second stage of gold mineralization much smaller in terms of total gold content but spectacular in terms of grade formed after 2702 ma this second stage is attributed to gold remobilization caused by the enhanced thermal gradient and deformation associated with the emplacement of the cat island pluton 2699 ma approximately 7 km east of the deposit or by the ca 2 63 to 2 66 ga postorogenic regional thermal event indicated by hornblende muscovite and biotite ar ar cooling ages from the uchi subprovince it is proposed that the main stage of high grade mineralization formed between ca 2723 to 2712 ma possibly synchronous with emplacement of the dome and mckenzie island stocks the abino granodiorite and hammell lake batholith as well as with penetrative d2 main stage regional deformation lamprophyre dikes spatially associated with the deposit postdate main stage mineralization by at least 10 m y a postore granodiorite dike from the madsen mine the second largest deposit in the district was dated at 2698 1 ma this age is similar to a postalteration and late deformation diorite dike at the creek zone showing near the former starratt olsen mine which was dated at 2696 2 ma the crosscutting relationships confirm that the main stage of mineralization at the madsen mine is pre 2698 ma a folded and metamorphosed polymictic conglomerate located stratigraphically above the campbell red lake deposit and deposited after ca 2747 but before 2712 ma is correlated with the newly recognized huston assemblage crosscutting and overprinting relationships between the conglomerate and the alteration indicate protracted multistage aluminous and iron carbonate quartz veining and or alteration event s before and after deposition of the conglomerate the age of the conglomerate confirms that the campbell red lake deposit is proximal to a folded regional unconformity consistent with the empirical relationship between large gold deposits and unconformities elsewhere in archean greenstone belts this paleosurface represents a key first order exploration target and it is proposed that areas of high potential for gold exploration in red lake occur in the balmer assemblage rocks within 500 m to 1 km of the unconformity

silica gel in a fault slip surface field evidence for palaeo earthquakes high velocity friction experiments have shown an almost complete loss of strength associated with silica gel formation on slip surfaces the identi cation of frictional silica gel products in palaeo seismic faults is however problematic because there are multiple natural sources of silica gel and recrystallization of gel to quartz complicates preservation the importance of gel formation on natural faults is therefore unknown here we report a structurally distinct and semi continuous 0 5 10 mm thick layer of microcrystalline quartz along a major carbonate hosted fault the olive fault in the naukluft nappe complex namibia the quartz layer is distinguished by ow banding like textures and unusual cathodoluminescence characteristics the layer consists of 2 20 m hexagonal quartz crystals which include distinct crystalline pore bearing micro to nano spheres separated by pore geometries indicative of volumetric contraction and with grain boundaries enriched in aluminium we interpret these features to indicate that the quartz crystals formed from recrystallization and dehydration of a silica gel because it is found in a carbonate hosted fault and crosscuts lithological layering the silica source is not from comminution of local wall rocks rather the gel likely formed from reshear of a quartz coated fault surface or incremental shear slip associated with precipitation of silica driven by co seismic pressure drops this example of fault related silica gel may have formed by a different mechanism than the gels produced in high velocity friction experiments but once formed may have comparable rheological effects

slow advance of the weathering front during deep supply limited saprolite formation in the tropical highlands of sri lanka abstract silicate weathering initiated by major mineralogical transformations at the base of ten meters of clay rich saprolite generates the exceptionally low weathering flux found in streams draining the crystalline rocks of the mountainous and humid tropical highlands of sri lanka this conclusion is reached from a thorough investigation of the mineralogical chemical and sr isotope compositions of samples within a regolith profile extending 10 m from surface soil through the weathering front in charnockite bedrock a high grade metamorphic rock corestones formed at the weathering front as well as from the chemical composition of the dissolved loads in nearby streams weatherable minerals and soluble elements are fully depleted at the top of the profile showing that the system is supply limited such that weathering fluxes are controlled directly by the supply of fresh minerals we determine the weathering rates using two independent means 1 in situ produced cosmogenic nuclides in surface soil and creek sediments in the close vicinity of the regolith combined with immobile element mass balance across the regolith and 2 river dissolved loads silicate weathering rates determined from both approaches range from 16 to 36 t km 2 y 1 corresponding to a weathering front advance rate of 6 14 mm ky 1 these rates agree across the 10 1 to 10 4 y time scales over which our rate metrics integrate suggesting that the weathering system operates at steady state within error these rates are furthermore compatible with those obtained by modeling the advance rate of the weathering front from chemical gradients and mineral dissolution rates the silicate weathering flux out of the weathering profile measured on small creeks amounts to 84 of the profile s export flux the remaining 16 is contributed by non silicate atmospheric derived input the silicate weathering flux as measured by dissolved loads in large catchments amounts to ca 50 of the total dissolved flux the remainder being contributed by dust rain and weathering of local marble bands spheroidal weathering is the key processes of converting the fresh bedrock into saprolite at the weathering front the mineralogical composition of weathering rinds shows that the sequence of mineral decomposition is pyroxene plagioclase biotite k feldspar observable biotite alteration does not appear to initiate spheroidal weathering within corestones therefore we infer that other processes than biotite oxidation like pyroxene oxidation clay formation from pyroxene and plagioclase decomposition the development of secondary porosity by plagioclase dissolution or even microbiologic processes at depth enable the coupling between slow advance of the weathering front and slow erosion at the surface the comparison to tectonically more active tropical landscapes lets us conclude that the combination of hard rock with tightly interlocked mineral grains and slow erosion in the absence of tectonically induced landscape rejuvenation lead to these exceptionally low weathering rates

an advection dispersion reaction model of bauxite formation abstract the formation of bauxite from a granitic protolith at 25 c has been simulated using a modified version of the 1dreact software for one dimensional reactive transport which uses a kinetic approach to describe the mineral dissolution and precipitation reactions the rate laws describing these reactions incorporate the important effects of mineral surface area ph and solution saturation state and are based on experimentally determined rates published in the literature the original program has been modified to allow for a different treatment of the mineral surface areas and reaction rate laws the results of long term simulations 1 ma show the development of gibbsite rich weathering profiles bauxites which are underlaid by muscovite rich zones these muscovite rich zones are not observed in the field the typical bauxitic profile consists of an upper gibbsite rich and a lower kaolinite rich zone a strong nucleation barrier for muscovite could explain the absence of muscovite from the weathering profiles the results also show that the infiltrating solutions get closer to saturation with respect to microcline than with respect to albite in agreement with numerous field observations the simulations also predict the formation of a 5 10 m thick gibbsite kaolinite transition zone which is in agreement with field observations and argues for the applicability of the experimentally determined rates or at least their ratios to field conditions regarding the chemistry of the solutions leaving the weathering profiles the si na k ratio of the waters increases to values higher than 3 with time intense weathering in agreement with some of the river chemistry data from the guayana shield reported by edmond et al 1995

basalt weathering across scales abstract weathering of silicate minerals impacts many geological and ecological processes for example the weathering of basalt contributes significantly to consumption of atmospheric carbon dioxide co2 and must be included in global calculations of such consumption over geological timeframes here we compare weathering advance rates for basalt wd where d and indicate the scale at which the rate is determined and surface area measured respectively from the laboratory to the watershed scales data collected at the laboratory weathering rind soil profile and watershed scales show that weathering advance rate of basalt is a fractal property that can be described by a fractal dimension dr 2 3 by combining the fractal description of rates with an arrhenius relationship for basalt weathering we derive the following equation to predict weathering advance rates at any spatial scale from weathering advance rates measured at the bet scale w d k 0 a d r 2 e e a r t here k0 is the pre exponential factor 1 29 107 mm3 mm 2 yr 1 ea is the activation energy 70 kj mol 1 and a is a spatial constant related to the scale of measurement of bet surface area 10 7 mm the term a d r 2 is the roughness the roughness fractal dimension can be conceptualized as a factor related to both the thickness of the reaction front and the specific surface area within the reaction front however the above equation can also be written in terms of a surface fractal dimension and the hypothetical average grain radius these fractal dimensions provide insight into reaction front geometry and should vary with lithology once the surface area discrepancy has been accounted for using this method we find a one to two order of magnitude range in weathering advance rates measured at any scale or temperature that can be attributed to factors such as changes in erosional regime parent lithology mechanism climate composition of reacting fluid and biological activity our scaled equation when used to predict global basalt co2 consumption based upon global lithologic maps yields an uptake flux 1 75 1013 mol co2 yr 1 within the predicted error of fluxes estimated based upon riverine measurements

formation of the campbell red lake gold deposit by h 2 o poor co 2 dominated fluids the campbell red lake gold deposit in the red lake greenstone belt with a total of approximately 840 t of gold past production reserves and an average grade of 21 g t au is one of the largest and richest archean gold deposits in canada gold mineralization is mainly associated with silicification and arsenopyrite that replace carbonate veins breccias and wallrock selvages the carbonate veins and breccias which are composed of ankerite quartz and characterized by crustiform cockade textures were formed before and or in the early stage of penetrative ductile deformation whereas silicification arsenopyrite replacement and gold mineralization were coeval with deformation microthermometry and laser raman spectroscopy indicate that fluid inclusions in ankerite and associated quartz q1 and main ore stage quartz q2 are predominantly carbonic composed mainly of co2 with minor ch4 and n2 aqueous and aqueous carbonic inclusions are extremely rare in both ankerite and quartz h2o was not detected by laser raman spectroscopic analyses of individual carbonic inclusions and by gas chromatographic analyses of bulk samples of ankerite and main ore stage quartz q2 fluid inclusions in post mineralization quartz q3 are also mainly carbonic but proportions of aqueous and aqueous carbonic inclusions are present trace amounts of h2s were detected by laser raman spectroscopy in some carbonic inclusions in q2 and q3 and by gas chromatographic analyses of bulk samples of ankerite and q2 3he 4he ratios of bulk fluid inclusions range from 0 008 to 0 016 ra in samples of arsenopyrite and gold homogenization temperatures th co2 of carbonic inclusions are highly variable from 4 1 to 30 4 c mostly to liquid some to vapor but the spreads within individual fluid inclusion assemblages fias are relatively small within 0 5 to 10 3 c carbonic inclusions occur both in fias with narrow th co2 ranges and in those with relatively large th co2 variations the predominance of carbonic fluid inclusions has been previously reported in a few other gold deposits and its significance for gold metallogeny has been debated some authors have proposed that formation of the carbonic fluid inclusions and their predominance is due to post trapping leakage of water from aqueous carbonic inclusions h2o leakage model whereas others have proposed that they reflect preferential trapping of the co2 dominated vapor in an immiscible aqueous carbonic mixture fluid unmixing model or represent an unusually h2o poor co2 dominated fluid single carbonic fluid model based on the fia analysis reported in this study we argue that although post trapping modifications and host mineral deformation may have altered the fluid inclusions in varying degrees these processes were not solely responsible for the formation of the carbonic inclusions the single carbonic fluid model best explains the extreme rarity of aqueous inclusions but lacks the support of experimental data that might indicate the viability of significant transport of silica and gold in a carbonic fluid in contrast the weakness of the unmixing model is that it lacks unequivocal petrographic evidence of phase separation if the unmixing model were to be applied the fluid prior to unmixing would have to be much more enriched in carbonic species and poorer in water than in most orogenic gold deposits in order to explain the predominance of carbonic inclusions the h2o poor co2 dominated fluid may have been the product of high grade metamorphism or early degassing of magmatic intrusions or could have resulted from the accumulation of vapor produced by phase separation external to the site of mineralization

ore forming fluids in archean gold bearing quartz veins at the sigma mine abitibi greenstone belt quebec canada archean gold bearing quartz veins at the sigma mine occur in deformed metavolcanics intruded by two generations of preore porphyries the veins were emplaced during a late compressional tectonic event and they consist primarily of quartz tourmaline carbonates pyrite and scheelite vein minerals have been deformed and are also cut by abundant healed fractures outlined by fluid inclusions in the veins gold typically occurs in late fractures and in recrystallized quartz microthermometric sem eda leachate analysis and gas chromatographic techniques were used to study fluid inclusions in quartz most fluid inclusions at sigma occur in late fractures and are secondary the fluid inclusions are commonly found in fractures parallel to or less commonly within gold bearing fractures and are genetically related to the gold three types of fluid inclusions are identified 1 high salinity na ca cl aqueous inclusions 25 34 wt salt with small amounts of co 2 some containing halite calcite and nahcolite as daughter minerals 2 co 2 rich inclusions containing small amounts of h 2 o and ch 4 and 3 low salinity h 2 o co 2 inclusions 10 equiv wt nacl with co 2 contents which are variable but commonly in the range of 15 to 30 mole percent co 2 leachate analyses show that in addition to na and ca the aqueous fluids contain minor amounts of k fe mg and li homogenization temperatures of h 2 o co 2 inclusions with constant phase ratios range from 285 degrees to 395 degrees c those of aqueous inclusions range from 60 degrees to 295 degrees c the aqueous and co 2 rich inclusions are interpreted to have trapped two coexisting fluids resulting from the unmixing of a homogeneous h 2 o co 2 parent fluid itself occasionally trapped in late fractures the ore bearing fluid is interpreted to be a low salinity 10 equiv wt nacl fluid containing 15 to 30 mole percent co 2 at minimum temperatures in the range of 285 degrees to 395 degrees c the relatively low homogenization temperatures of aqueous inclusions are interpreted to be the result of co 2 effervescence from a co 2 saturated aqueous fluid due to pressure fluctuations at the time of entrapment it is also proposed that the onset of gold deposition is related to the onset of fluid unmixing

very high density co2 associated with ultrahigh temperature metamorphism in the eastern ghats granulite belt india spinel bearing high mg al granulites of the vizianagram area in the eastern ghats granulite belt show textural features clearly establishing that the association spinel ss quartz fe ti oxide solid solution sillimanite porphyroblastic orthopyroxene was stable during peak metamorphic conditions pressure temperature p t conditions estimated from both mineralogical thermobarometry and phase equilibrium limitations indicate that the peak metamorphism occurred under ultrahigh t conditions 1000 c at 8 9 kbar pressure retrograde p t conditions of 750 800 c 6 7 kbar are deduced from the compositions of coronal garnet and orthopyroxene which have rims of spinel against quartz indicating significant cooling with slight lowering of pressure quartz associated with the ultrahigh t assemblage at vizianagram contains ubiquitous single phase carbonic inclusions as isolated clusters that belong to two categories group i shows extremely high density homogenization temperature 51 1 8 c density 1 15 g cm 3 and group ii trapped relatively lower density fluids homogenization temperature 18 4 2 4 c density 1 05 g cm 3 the iso chores for group i inclusions pass through the peak metamorphic p t conditions whereas those for group ii coincide with the p t conditions of the formation of coronal garnet and orthopyroxene our study is the first report of very high density co 2 associated with the eastern ghats granulite belt rocks and provides a strong case for the presence of co 2 rich fluids during ultrahigh t metamorphism at lower crustal levels

a mathematical model for steady state regolith production at constant erosion rate it has been hypothesized that many soil profiles reach a steady state thickness in this work such profiles were simulated using a one dimensional model of reaction with advective and diffusive solute transport a model rock is considered consisting of albite that weathers to kaolinite in the presence of chemically inert quartz the model yields three different steady state regimes of weathering at the lowest erosion rates a local equilibrium regime is established where albite is completely depleted in the weathering zone this regime is equivalent to the transport limited regime described in the literature with an increase in erosion rate transition and kinetic regimes are established in the transition regime both albite and kaolinite are present in the weathering zone but albite does not persist to the soil air interface in the weathering limited regime here called the kinetic regime albite persists to the soil air interface the steady state thickness of regolith decreases with increasing erosion rate in the local equilibrium and transition regimes but in the kinetic regime this thickness is independent of erosion rate analytical expressions derived from the model are used to show that regolith production rates decrease exponentially with regolith thickness the steady state regolith thickness increases with the darcy velocity of the pore fluid and in the local equilibrium regime may vary markedly with small variations in this velocity and erosion rate in the weathering limited regime the temperature dependences for chemical weathering rates are related to the activation energy for the rate constant for mineral reaction and to the h of dissolution while for local equilibrium regimes they are related to the h only the model illustrates how geochemical and geomorphological observations are related for a simple compositional system the insights provided will be useful in interpreting natural regolith profiles copyright 2010 john wiley sons ltd

emplacement depth and carbon dioxide rich fluid inclusions in intrusion related gold deposits phanerozoic intrusion related gold deposits have many consistent characteristics such as a spatial temporal and geochemical association with moderately reduced predominantly ilmenite series i type intrusions the deposits exhibit a range of characteristics that vary over a wide range of emplacement depths 7 km deposits in shallow crustal settings 5 km have characteristics of mesothermal environments and are hosted by plutons containing sheeted veins greissen and disseminated gold fluid characteristics also vary with depth deposits in shallow environments contain high temperature 350 c immiscible brine 30 wt nacl equiv and low salinity 5 wt nacl equiv vapor that commonly contains carbon dioxide deposits in deeper environments contain abundant low salinity carbon dioxide rich aqueous fluids 10 wt nacl equiv that in some deposits are postdated by moderate to high salinity brines 10 40 wt nacl equiv these contrasting fluid types are interpreted to be magmatic in origin and are the result of the complex interplay between exsolution of different volatiles carbon dioxide water and chlorine from felsic magmas emplaced at different crustal levels experimental studies have shown that carbon dioxide will exsolve from felsic magmas at much higher pressures than water and chlorine due to its lower solubility in the melt thus deposits formed from felsic magmas in deeper environments will contain abundant carbon dioxide rich fluids subsequent fluids released from the magma are likely to be water rich and more saline felsic magmas in shallow environments will contain lower concentrations of carbon dioxide that will partition into the vapor phase during phase separation between brine and vapor

weathering and erosion of fractured bedrock systems we explore the contribution of fractures joints in controlling the rate of weathering advance for a low porosity rock by using methods of homogenization to create averaged weathering equations the rate of advance of the weathering front can be expressed as the same rate observed in non fractured media or in an individual block divided by the volume fraction of non fractured blocks in the fractured parent material in the model the parent has fractures that are filled with a more porous material that contains only inert or completely weathered material the low porosity rock weathers by reaction transport processes as observed in field systems the model shows that the weathering advance rate is greater for the fractured as compared to the analogous non fractured system because the volume fraction of blocks is 1 the increase in advance rate is attributed both to the increase in weathered material that accompanies higher fracture density and to the increase in exposure of surface of low porosity rock to reaction transport for constant fracture aperture the weathering advance rate increases when the fracture spacing decreases equations describing weathering advance rate are summarized in the list of selected equations if erosion is imposed at a constant rate the weathering systems with fracture bounded bedrock blocks attain a steady state in the erosional transport limited regime bedrock blocks no longer emerge at the air regolith boundary because they weather away in the weathering limited or kinetic regime blocks of various size become exhumed at the surface and the average size of these exposed blocks increases with the erosion rate for convex hillslopes the block size exposed at the surface increases downslope this model can explain observations of exhumed rocks weathering in the luquillo mountains of puerto rico published 2017 this article is a u s government work and is in the public domain in the usa

reactive transport modeling in fractured rock a state of the science review abstract the field of reactive transport modeling has expanded significantly in the past two decades and has assisted in resolving many issues in earth sciences numerical models allow for detailed examination of coupled transport and reactions or more general investigation of controlling processes over geologic time scales reactive transport models serve to provide guidance in field data collection and in particular enable researchers to link modeling and hydrogeochemical studies in this state of science review the key objectives were to examine the applicability of reactive transport codes for exploring issues of redox stability to depths of several hundreds of meters in sparsely fractured crystalline rock with a focus on the canadian shield setting a conceptual model of oxygen ingress and redox buffering within a shield environment at time and space scales relevant to nuclear waste repository performance is developed through a review of previous research this conceptual model describes geochemical and biological processes and mechanisms materially important to understanding redox buffering capacity and radionuclide mobility in the far field consistent with this model reactive transport codes should ideally be capable of simulating the effects of changing recharge water compositions as a result of long term climate change and fracture matrix interactions that may govern water rock interaction other aspects influencing the suitability of reactive transport codes include the treatment of various reaction and transport time scales the ability to apply equilibrium or kinetic formulations simultaneously the need to capture feedback between water rock interactions and porosity permeability changes and the representation of fractured crystalline rock environments as discrete fracture or dual continuum media a review of modern multicomponent reactive transport codes indicates a relatively high level of maturity within the yucca mountain nuclear waste disposal program reactive transport codes of varying complexity have been applied to investigate the migration of radionuclides and the geochemical evolution of host rock around the planned disposal facility through appropriate near and far field application of dual continuum codes this example demonstrates how reactive transport models have been applied to assist in constraining historic water infiltration rates interpreting the sealing of flow paths due to mineral precipitation and investigating post closure geochemical monitoring strategies natural analogue modeling studies although few in number are also of key importance as they allow the comparison of model results with hydrogeochemical and paleohydrogeological data over geologic time scales

a mechanism for preferential h 2 o leakage from fluid inclusions in quartz based on tem observations preferential leakage of h2o from fluid inclusions containing multiple gas components has been suspected in natural metamorphic rocks and has been demonstrated experimentally for synthetic h2o co2 rich inclusions in natural quartz knowledge of the physical and chemical characteristics of the leakage mechanism which may be very complex increases the value of natural fluid inclusions to metamorphic geology it is proposed that crystal defects play a major role in nondecrepitative preferential h2o leakage through quartz and remain effective during metamorphism inclusions with either an internal overpressure or underpressure produce strain in the adjacent quartz crystal via the nucleation of many dislocations and planar defects like dauphine twin boundaries these defects allow preferential loss of h2o from h2o co2 rich inclusions at supercritical conditions the transport capacity of this leakage mechanism is enhanced by nucleation of small bubbles on defect structures the nucleation of these bubbles seems to be a recovery process in strained crystals solubility gradients of quartz in water in a crystal with internally underpressurized inclusions may result in optical visible implosion halos in a three dimensional spatial arrangement caused by the growth of small bubbles at the expense of the larger original fluid inclusion natural fluid inclusions from naxos greece are always associated with numerous interlinked dislocations these dislocations may have been produced by plastic derormation or by crystal growth related processes e g crack healing the presence of small bubbles on these dislocations indicates that a similar leakage mechanism for h2o must have occurred in these rocks

modelling chemical depletion profiles in regolith abstract chemical or mineralogical profiles in regolith display reaction fronts that document depletion of leachable elements or minerals a generalized equation employing lumped parameters was derived to model such ubiquitously observed patterns c c 0 c 0 c x 0 c x 0 exp ini k x 1 here c cx 0 and co are the concentrations of an element at a given depth x at the top of the reaction front or in parent respectively ini is the roughness of the dissolving mineral in the parent and k is a lumped kinetic parameter this kinetic parameter is an inverse function of the porefluid advective velocity and a direct function of the dissolution rate constant times mineral surface area per unit volume regolith this model equation fits profiles of concentration versus depth for albite in seven weathering systems and is consistent with the interpretation that the surface area m2 mineral m 3 bulk regolith varies linearly with the concentration of the dissolving mineral across the front dissolution rate constants can be calculated from the lumped fit parameters for these profiles using observed values of weathering advance rate the proton driving force the geometric surface area per unit volume regolith and parent concentration of albite these calculated values of the dissolution rate constant compare favorably to literature values the model equation useful for reaction fronts in both steady state erosional and quasi stationary non erosional systems incorporates the variation of reaction affinity using ph as a master variable use of this model equation to fit depletion fronts for soils highlights the importance of buffering of ph in the soil system furthermore the equation should allow better understanding of the effects of important environmental variables on weathering rates

hillslope derived blocks retard river incision the most common detachment limited river incision models ignore the effects of sediment on fluvial erosion yet steep reaches of mountain rivers often host clusters of large 1 m blocks we argue that this distribution of blocks is a manifestation of an autogenic negative feedback in which fast vertical river incision steepens adjacent hillslopes which deliver blocks to the channel blocks inhibit incision by shielding the bed and enhancing form drag we explore this feedback with a 1 d channel reach model in which block delivery by hillslopes depends on the river incision rate results indicate that incision dependent block delivery can explain the block distribution in boulder creek colorado the proposed negative feedback may significantly slow knickpoint retreat channel adjustment and landscape response compared to rates predicted by current theory the influence of hillslope derived blocks may complicate efforts to extract base level histories from river profiles

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

investigation of in situ weathering of quartz diorite bedrock in the rio icacos basin luquillo experimental forest puerto rico abstract the rio icacos basin in puerto rico is the site of the highest measured chemical solute fluxes for a catchment on granodiorite this is partly attributable to high annual rainfall 4300 mm high average temperature 23 c and moderate relief the bulk of these fluxes is contributed by dissolution of plagioclase and amphiboles in zones of partially weathered rock 0 5 1 5 m thick underlying saprolite these zones are characterized by systems of onion skin rindlets each 3 10 cm thick in which porosity development is dominated by weathering of plagioclase to kaolinite fe bearing aluminosilicate minerals hornblende augite and biotite persist in the weathered rock after plagioclase has weathered to completion but hornblende and augite disappear between the porous rindlet systems and saprolite a new watershed scale solute mass balance corroborates earlier studies and yields an overall reaction stoichiometry that closely resembles complete weathering of the rock except quartz and biotite to kaolinite and fe3 oxides our findings indicate that steady state assumptions are reasonable and useful in studying weathering at this site estimated steady state sediment yields based on net solute fluxes and average bedrock composition of 3 24 10 9 to 3 60 10 9 kg m 2 s 1 are smaller than measured values indicating that sediment generation is currently accelerated with respect to steady state however sediment generation has recently been closer to steady state since weathering within the rindlet systems occurs on multiple fronts moving upward and downward two new steady state conceptual models are developed to account for 1 multiple parallel weathering fronts and 2 differential rates of advance of weathering fronts due to porosity development based on these models in situ weathering rates for plagioclase at two sample locations of 2 7 10 15 and 5 0 10 15 mol na g 1 s 1 are calculated values comparable to bet surface area normalized rates of 2 7 10 16 to 5 0 10 15 mol na m 2 s 1