Limestone Host Rock Research Articles

Diagenetic Implications of Stylolitization In Pelagic Carbonates, Canterbury Basin, Offshore New Zealand

Abstract Stylolites are irregular discontinuity surfaces that are thought to result from localized stress-induced dissolution during burial or tectonic compression. The genesis of stylolites and the controls on stylolitization are still debated, and the interplay between stylolitization, generation of carbonate-rich fluids, diagenetic fluid flow within fractures and matrix, cementation, and porosity modifications is complex. All of these processes have important diagenetic effects potentially altering the intrinsic properties of the host rock, with implications for hydrocarbon exploration and water resources in aquifers. We investigate the process of stylolitization by a macroscopic, petrographic, and geochemical study of pressure-solution features in Eocene to early Oligocene limestones in cores from the Integrated Ocean Drilling Program (IODP) Hole 317-U1352C (Canterbury Basin). The results indicate correlations among stylolite amplitude, stylolite density, and siliciclastic content of the host rock. These relationships are interpreted to suggest that siliciclastic content in carbonate rocks increases heterogeneity, which in turn impacts stylolite nucleation. Moreover, the geochemical data support that clay along the stylolite is not authigenic, but a relict from impurities in the limestone host rock. The statistical approach of stylolite spacing used in this study, which is different than previous studies, reinforces the model of random occurrence of stylolites in (carbonate) lithologic units. This study shows that local stresses in tectonically passive areas may allow the formation of rare oblique stylolites with peaks perpendicular to the stylolite plane. Estimation of the amount of limestone dissolved during stylolitization (minimum 7% to 12% of the depositional limestone) and the volume of sparite in the host rock (up to 1.6% of the compacted limestone) suggests that the pressure-solution fluids cemented the micropores, reducing porosity to about 10% (from a common porosity of about 40% in mechanically compacted chalk). This study thus highlights the importance of microporosity as a sink for burial cements, an observation difficult to make in thin-section. Analysis of stylolite infills at Site U1352 shows no evidence that the stylolites acted as conduits for diagenetic fluids, unless if the fluid was host-rock buffered and thus its chemistry indistinguishable from that of the host rock. The link between siliciclastic content in carbonate rocks and the spacing and amplitude of stylolites improves understanding of burial compaction processes. Predicting the morphology and spacing of stylolites in the subsurface can be used to identify baffles to cross-stylolite fluid flow or cementation (of microporosity) influencing heterogeneity of petrophysical properties in (subsurface reservoir) host rocks.

Provenance and geological significance of red mud and other clastic sediments of the Mugnano cave (Montagnola Senese, Italy)

The Mugnano Cave is characterized by a thick clastic sedimentary fill showing a great variability of sedimentary facies, ranging from clay to coarse-grained sand deposits. This paper deals with combined sedimentological and mineralogical (XRD and SEM) studies of these sediments and bedrock insoluble residues in order to understand the origin and geological significance of cave deposits, with particular attention to red mud sediments, often considered as the residue of host rock dissolution. Three different sedimentary facies were recognized: i) YS, yellow sand with occasionally shell fragments, testifying the arrival of sediments from the surrounding landscape; ii) RS, red laminated mud; iii) GS, grey and red-grey mud and sand, dolomite-rich sediments. Furthermore, the results obtained in the present study allowed the identification of two fingerprint minerals: i) quartz, present only as traces in the limestone host-rock, and ii) dolomite, certainly related to the incomplete bedrock dissolution. Results obtained by this multidisciplinary approach testify that no one of the investigated sediments is representative of a completely autochthonous sedimentation (i.e. accumulation of insoluble residue of limestone in a cave environment). In fact, all the three sedimentary facies show a bulk composition rich in quartz, a mineral indicating an external origin for these sediments. Also the grey sediments, despite of their high content of bedrock-related dolomite, are quite rich in quartz and they testify the mixing of autochthonous and allocthonous sediments. The clay fraction of cave sediments shows strong compositional similarities with bedrock insoluble residue and consequently its analysis cannot be considered as a clear proxy for distinguishing between different parent materials. Therefore, the origin of these cave deposits is dominantly related to external sediments inputs, with terra rossa surface soils as the most probable parent material for red mud sediments.

Rare earth element patterns in a Chinese stalagmite controlled by sources and scavenging from karst groundwater

The rare earth elements and yttrium (REY) in a Chinese stalagmite (SJ3) have been studied to delineate their possible sources and controlling mechanisms. The following results are obtained: (1) The REY in karst groundwater at the study site have very low concentrations, e.g. ranging from 59×10−6 to 614×10−6μmol/L for La, and are transported predominantly in particle/colloidal phases.These elements show a high linear positive correlation with Fe, Mn and Al (r2=0.98, 0.95 and 0.91, respectively), which are also transported largely in particle/colloidal phases, suggesting a close association of REY with Fe, Mn and Al in colloidal/particle phases; (2) REY mobilization from the sources, i.e. the limestone host rock and the overlying soil layer, may not have exerted a significant influence on the REY patterns for SJ3 including anomalies of La, Ce and Y and fractionation between light and heavy REY; (3) The REY patterns of SJ3 display significant variations which are controlled in part by change in contributions from the limestone host rock and the overlying soil layer and can be associated with stadial–interstadial changes in past climate and environment. The REY contribution from the overlying soil layer is higher under a warm–humid climate phase than a cold–dry one and the REY contribution from the limestone host rock is reversed; (4) Some of the variations of the SJ3 REY patterns could not be explained with a simple mixing of different REY sources. Removal of REY from the groundwater in association with precipitation of colloidal and particle materials, an analogue to REY scavenging from seawater, is the most important mechanism influencing the SJ3 REY patterns. Strong REY “scavenging”, which may be caused by both less dynamic hydrology, longer travel-time and increased ionic strength of karst groundwater, is suggested to be responsible for the large positive Y anomalies and negative Ce anomalies and depletion of light REE relative to heavy REE during the cold–dry climate phases (e.g. the periods corresponding to the last glacial maximum and Heinrich event 1).

The influence of igneous intrusions on the peak temperatures of host rocks: Finite-time emplacement, evaporation, dehydration, and decarbonation

Using a 13-m-thick basic sill and its limestone host rocks of the Permian Irati Formation from the Parana Basin, South America, as an example, this paper presents a numerical investigation based on heat conduction models on the effect of the emplacement mechanism of igneous intrusions, pore-water evaporation, and dehydration and decarbonation of host rocks on the peak temperature ( T peak) of host rocks. Our results demonstrate that: (1) the finite-time intrusion mechanism of magma can lower the predicted T peak of host rocks by up to 100 °C relative to the instantaneous intrusion mechanism, and although pore-water evaporation together with dehydration and decarbonation reactions can also depress the thermal effect of the sill on its host rocks, the maximum effect of these mechanisms on T peak only reaches approximately 50 °C. (2) The effect of pore-water evaporation on T peak is obviously greater than that of the dehydration and decarbonation reactions: the former can cause a maximum deviation of 40 °C in the predicted T peak, whereas the deviation due to the latter is less than 20 °C. Further, the effect of the dehydration and decarbonation reactions on T peak is less than 10 °C if pore-water evaporation is allowed simultaneously in the models and can hence be ignored in thermal modeling. (3) The finite-time intrusion mechanism of magma probably represents the natural condition of the sill. Pore-water evaporation and dehydration and decarbonation of host rocks are also likely to play important roles in lowering the thermal effect of the sill.

Radiometric dates of uplifted marine fauna in Greece: Implications for the interpretation of recent earthquake and tectonic histories using lithophagid dates

In AD 365 a great ( M w > 8) earthquake lifted up western Crete, exposing a shoreline encrusted by marine organisms, and up to 10 m of marine substrate beneath it. Radiocarbon ages determined for corals and bryozoans exposed between the paleo-shoreline and present sea level are consistent, within measurement error, with each other and with the date of the earthquake. But radiocarbon ages determined for the boring bivalve Lithophaga lithophaga found on the same substrate are at least 350 years, and up to 2000 years, older than the date of the earthquake that lifted them above sea level. These observations reveal two important effects that limit the use of radiocarbon lithophagid ages in tectonic and paleoseismological studies. The first is that the exceptional preservation potential of lithophagids allows them to remain intact and in situ long after natural death, while the substrate continues to be colonised until eventual uplift. The second, which we confirm with radiocarbon analysis of museum specimens of known age, is the incorporation of old ( 14C-free) carbon into lithophagid shells from the limestone host rock into which the lithophagids bored. The two effects are both significant in Crete and central Greece, and can cause the radiocarbon lithophagid ages to be up to 2000 years older than the uplift event which exposed them. Understanding these effects is important because lithophagids are far more abundantly preserved, and used to date uplift, than most other marine organisms. This study shows that they can rarely be used to distinguish uplift events, or date them to better than 1000 years, or even to distinguish whether observed uplift occurred in a single or in multiple events. After taking account of these uncertainties, the ages of the lithophagids are, however, consistent with the hypothesis that the highest prominent marine notches and exposed lithophagid holes within a few metres of sea level in Greece formed when sea level became relatively stable ~ 6000 years ago, following rapid rise after the last glacial maximum.

Analysis of hyperspectral and lidar data: Remote optical mineralogy and fracture identification

Karst systems are widely recognized as highly complex and often extremely productive reservoirs of water as well as petroleum. They are also often associated with mineralization. The availability of a large (several tens of square kilometers), well-preserved paleokarst outcrop is rare; therefore, maximizing the information that we can extract from examples like the Franklin Mountains is critical to the study of karst-related fluid flow. The mapping process is confounded by the need to map very large areas to find relatively small and somewhat unpredictable zones of extreme deformation. Moreover, the brecciated regions interpreted to be of karst origin are often composed of the same lithology as the surrounding rock and thus make traditional remote sensing data such as multispectral satellite imagery or photographic data inadequate to delineate such systems. The Franklin Mountains in El Paso, Texas, expose lower Paleozoic carbonates deposited over a giant carbonate platform referred to as the Great Ordovician Bank. The limestone-dominated bank was subsequently modified by surface karst and several large, vertically extensive caves that occupy up to 70,000 m2 of outcrop each. The breccia bodies are preferentially dolomitized within the limestone host rock. The size of these features is ideal for testing dolomite-calcite identification with high-elevation hyperspectral imagery at 20-m × 20-m pixel size. Terrestrial-based lidar (light detection and ranging) data were also utilized to identify collapse brecciation highlighted by hyperspectral image analysis. Results of this study delineate the distribution of dolomite and calcite in natural, passive light, well outside the visible spectrum, and combine active (lidar) and passive remote-sensing technologies to conduct remote mineralogical mapping linked to diagenetic alteration of carbonates. Through the combination of hyperspectral image processing and shape/texture analysis of terrestrial lidar data, a quantitative, multiscale facies map was generated in three-dimensional, geographically rectified space.

Origin and fluxes of atmospheric REE entering an ombrotrophic peat bog in Black Forest (SW Germany): Evidence from snow, lichens and mosses

The fate of the Rare Earth Elements (REE) were investigated in different types of archives of atmospheric deposition in the Black Forest, Southern Germany: (1) a 70 cm snow pack collected on the domed part of a raised bog and representing 2 months of snow accumulation, (2) a snow sample collected close to the road about 500 m from the peat bog, (3) two species of lichens and (4) a peat profile representing 400 years of peat accumulation as well as a “preanthropogenic” sample and the living moss layer from the top of the core. REE concentrations in peat are significantly correlated to Ti which is a lithogenic conservative element suggesting that REE are immobile in peat bog environments. Snow, lichens and peat samples show similar PAAS (Post Archean Australian Shale) normalized REE distributions suggesting that the complete atmospheric REE signal is preserved in the peat profile. However, the annual flux of REE accumulated by the peat is ca. 10 times greater than that of the bulk winter flux of REE. This difference probably indicates that the REE concentrations in the snowpack are not representative of the average REE flux over the whole year. Despite the pronounced geological differences between this site (granite host-rock) and a previously studied peat bog in Switzerland (limestone host-rock) similar REE distribution patterns and accumulation rates were found at both sites. Given that both sites confirm an Upper Continental Crust signature, the data suggests both sites are influenced by regional and not local, soil-derived lithogenic aerosols.

Strontium isotope characterization and major ion geochemistry of karst water flow, Shentou, northern China

Summary Karst water is the most important source of water supply for Shanxi province, northern China. The Shentou springs are representative of the 19 major karst springs at Shanxi. The total area of the Shentou karst water system is 5316 km 2 , the Middle Ordovician limestone being its major karst aquifer. In this study, data about the strontium isotope geochemistry and major ion hydrochemistry were analyzed to understand the flow patterns and hydrogeochemical processes of karst water at Shentou. The contour map of TDS value of karst water and that of Sr concentration are similar, showing the general tendency of increase from the northern, western and southern boundary to the discharge area. The average values of 87 Sr/ 86 Sr ratios of karst water decrease from recharge (0.7107) to discharge area (0.7102), evolving towards those of limestone hostrocks. Comparison of 87 Sr/ 86 Sr ratios with Sr content suggests that isotopic compositions of some karst water samples from the recharge and flow through area should be the result of interaction between aquifer limestone matrix and strontium-poor recharge waters of meteoric origin. However, for samples from the discharge area that are plotted above the mixing line, mixing with groundwater in the Quaternary aquifers with high 87 Sr/ 86 Sr ratios may be another factor controlling Sr isotope chemistry. Two major groundwater flow paths were discerned from hydrogeological and geochemical data. Along both flow paths, the 87 Sr/ 86 Sr ratios of karst water show a general tendency of decrease. Geochemical modeling of the major ion geochemistry of karst water using PHREEQC also indicates that the chemistry of springs should be affected by the incorporation of groundwater in Quaternary aquifer. The effect of the mixing action on the spring hydrochemistry in flow path 1 is more remarkable than that in flow path 2, according to different mixing ratios in both paths (30% in flow path 1 and 5% in flow path 2).

HYDROTHERMALLY FLUORITIZED ORDOVICIAN CARBONATES AS RESERVOIR ROCKS IN THE TAZHONG AREA, CENTRALTARIM BASIN, NW CHINA

Reservoir rocks at the Tazhong 45 oil pool, central Tarim Basin, consist of fluoritized carbonate strata of Middle - Late Ordovician age. Petrological observations indicate that the fluorite replaces calcite. Several other hydrothermal minerals including pyrite, quartz, sphalerite and chlorite accompany the fluorite. Two generations of fluid inclusions are present in the fluorite. Homogenization temperatures (Th) for primary inclusions are mostly between 260°C and 310°C and represent the temperature of the hydrothermal fluid responsible for fluorite precipitation. Th for secondary inclusions range from 100°C to 130°C, and represent the hydrocarbon charging temperature as shown by the presence of hydrocarbons trapped in some secondary inclusions. The mineral assemblage and the homogenization temperatures of the primary fluid inclusions indicate that the precipitation of fluorite is related to hydrothermal activity in the Tazhong area. Strontium isotope analyses imply that the hydrothermal fluids responsible for fluorite precipitation are related to late-stage magmatic activity, and felsic magmas were generated by mixing of mafic magma and crustal materials during the Permian. Theoretical calculations show that the molecular volume of a carbonate rock decreases by 33.5% when calcite is replaced by fluorite, and the volume shrinkage can greatly enhance reservoir porosity by the formation of abundant intercrystalline pores. Fluoritization has thus greatly enhanced the reservoir quality of Ordovician carbonates in the Tazhong 45 area, so that the fluorite and limestone host rocks have become an efficient hydrocarbon reservoir. According to the modelled burial and thermal history of the Tazhong 45 well, and the homogenization temperatures of secondary fluid inclusions in the fluorite, hydrocarbon charging at the Tazhong 45 reservoir took place in the Tertiary.

Tracing element sources of hydrothermal mineral deposits: REE and Y distribution and Sr-Nd-Pb isotopes in fluorite from MVT deposits in the Pennine Orefield, England

Fluorite from Mississippi Valley Type (MVT) deposits in the South Pennine Orefield, England, displays significantly different distributions of rare earths and yttrium (REY) compared to fluorite from similar MVT deposits in the North Pennine Orefield. Samples from the South Pennine Orefield display negative Ce and positive Gd and Y anomalies but lack any Eu anomaly, indicating that the REY were mobilized from relatively pure marine sedimentary carbonates. In marked contrast, fluorite from the North Pennine Orefield lacks any Ce and Gd anomalies but shows a pronounced positive Eu anomaly, suggesting that the REY were provided by different source rock(s), that the mineralizing hydrothermal fluid had experienced higher temperatures prior to fluorite precipitation, and that it was derived from deeper crustal levels in the north compared to the south. The isotopic composition of Sr in Blue John fluorite from the South Pennine Orefield suggests that Sr was mobilized from Lower Carboniferous (Tournaisian) limestones, whereas Pb isotopes suggest that in contrast to REY and Sr, Pb was derived from aluminosilicate rocks. Neither Nd nor Sr or Pb isotopes can be used to radiometrically date the formation of Blue John fluorite. All isotope systems studied indicate that the limestone host rock of this fluorite mineralization did not contribute to the trace element budget of the hydrothermal fluid. Our results show that different solutes in a natural water (hydrothermal fluid, groundwater, etc.) may be derived from different sources, and that the study of a small set of elements or isotope ratios may not provide full insight into the genesis or history of a mineralization or a hydrothermal fluid. Our data provide evidence for the uncoupling of Sr, Nd and Pb during fluid-rock interaction and fluid migration, and show that the use of plots such as 87Sr/86Sr vs. eNd. to learn about mixing relationships (as is commonly done in igneous geochemistry) is unreliable when applied to natural waters and their precipitates.

Fluid evolution in and around the Rosh Pinah massive sulphide deposit in the external Pan-African Gariep Belt, Namibia

A regional fluid inclusion and stable isotope (H, O, C) study of the external part of the Pan-African Gariep Belt, with emphasis on the Rosh Pinah Zn-Pb sulphide deposit, demonstrated that the bulk of those fluids present at the time of syn-tectonic peak metamorphism and subsequent uplift and exhumation are of aqueous composition with low salinity. Exceptions are few moderately saline inclusions that are limited to the tectonic contact with the pre-Gariep basement near the volcano-sedimentary Rosh Pinah Formation. These are explained by derivation from a nearby, locally developed lagoonal, possibly evaporitic, facies. The infiltrated host rocks largely controlled the composition of the syn-orogenic fluids and no evidence was found for a regional pervasive flux of orogenic brines through the external parts of the belt. Very rare pre-orogenic fluids discovered in Rosh Pinah mine samples display markedly different composition and density that are in line with a derivation from a volcanic source. These fluid inclusions are interpreted as remnants of the original ore fluid that infiltrated the host sediments and pyroclastic rocks at a temperature of around 400°C and a hydrostatic pressure of a few hundred bars. This infiltration led not only to intense brecciation and silicification of arenitic footwall and sulphide precipitation but also to the dolomitisation of original limestone host rock. The good preservation of the syn-rift Rosh Pinah ore bodies during the Gariep orogeny is explained by the lack of regional orogenic brine flow.

Alteration products of uraninite from the Colorado Plateau

Uraninite and associated alteration products from the Colorado Plateau were studied by optical microscopy, electron microprobe analysis (EMPA), scanning electron microscopy (SEM) and backscattered electron (BSE) imaging in order to determine the behavior and fate of trace elements, such as Pb, Ca, Si, Th, Zr, and REE, during corrosion under oxidizing conditions. The long-term alteration products and processes of uraninite may provide insight into the corrosion of the UO2in spent nuclear fuel. Uraninite, schoepite, calciouranoite, uranophane, fourmarierite, a Fe-rich uranyl phase, and coffinite were identified. The primary uraninites and alteration phases generally have low trace element contents, except for coffinite from Caribou Mine, Colorado that has Y2O3as high as 0.88 wt%. The highest Zr, Ti, Th and REE values of the uraninite are ThO20.17, Y2O30.28, La2O30.03, Ce2O30.10, Pr2O30.02, Nd2O30.14, Sm2O30.12, Eu2O30.04, Gd2O30.08, ZrO20.93, and TiO20.54 wt%, which are in general lower than the corresponding components in secondary uranyl phases (the highest values are ThO20.21, Y2O30.88, La2O30.05, Ce2O30.15, Pr2O30.05, Nd2O30.18, Sm2O30.14, Eu2O30.11, Gd2O30.08, ZrO22.11, and TiO22.74 wt%), suggesting that trace elements preferentially enter the structures of these secondary uranyl phases (Table 2). A compositional profile of a concentric structure in schoepite shows that, with increasing alteration, UO2, PbO and ZrO2decrease, and SiO2, TiO2, CaO and P2O5increase. Alteration causes loss of U, Pb and Zr and incorporation of Si, Ti, Ca and P into uranyl phases. The Ca-rich calciouranoite from Grants, New Mexico, indicates an interaction between the limestone host rock and uranium minerals. Texturally, concentric structures and micro-fractures are common. Concentric structures are usually composed of both uraninite and uranyl phases, while micro-fractures are common in the secondary phases.

Ore-fluid evolution at the Getchell Carlin-type gold deposit, Nevada, USA

Minerals and fluid-inclusion populations were examined using petrography, microthermometry, quadrupole mass-spectrometer gas analyses and stable-isotope studies to characterize fluids responsible for gold mineralization at the Getchell Carlin-type gold deposit. The gold-ore assemblage at Getchell is superimposed on quartz-pyrite vein mineralization associated with a Late-Cretaceous granodiorite stock that intruded Lower-Paleozoic sedimentary rocks. The ore assemblage, of mid-Tertiary age, consists of disseminated arsenian pyrite that contains submicrometer gold, jasperoid quartz, and later fluorite and orpiment that fill fractures and vugs. Late ore-stage realgar and calcite enclose ore-stage minerals. Pre-ore quartz trapped fluids with a wide range of salinities (1 to 21 wt.% NaCl equivalent), gas compositions (H2O, CO2, and CH4), and temperatures (120 to > 360°C). Oxygen- and hydrogen-isotope ratios indicate that pre-ore fluids likely had a magmatic source, and were associated with intrusion of the granodiorite stock and related dikes. Ore-stage jasperoid contains moderate salinity, aqueous fluid inclusions trapped at 180 to 220°C. Ore fluids contain minor CO2 and trace H2S that allowed the fluid to react with limestone host rocks and transport gold, respectively. Aqueous inclusions in fluorite indicate that fluid temperatures declined to ∼ 175°C by the end of ore-stage mineralization. As the hydrothermal system collapsed, fluid temperatures declined to 155 to 115°C and realgar and calcite precipitated. Inclusion fluids in ore-stage minerals have high δDH2O and δ18OH2O values that indicate that the fluid had a deep source, and had a metamorphic or magmatic origin, or both. Late ore-stage fluids extend to lower δH2O values, and have a wider range of δ18OH2O values suggesting dilution by variably exchanged meteoric waters. Results show that deeply sourced ore fluids rose along the Getchell fault system, where they dissolved carbonate wall rocks and deposited gold-enriched pyrite and jasperoid quartz. Gold and pyrite precipitated together as H2S in the ore fluids reacted with iron in the host rocks. As ore fluids mixed with local aquifer fluids, ore fluids became cooler and more dilute. Cooling caused precipitation of ore-stage fluorite and orpiment, and late ore-stage realgar. Phase separation and/or neutralization of the ore fluid during the waning stages of the hydrothermal ore system led to deposition of late ore-stage calcite.

Ultrastructure and geochemistry of endolithic microorganisms in limestone of the Niagara Escarpment

Endolithic microbial communities dominated by photosynthetic cyanobacteria occur at a depth of 0.5 – 3.0 mm inside dolomitic limestone that forms the vertical cliffs of the Niagara Escarpment in Ontario, Canada. Coccoid Gloeocapsa was the most prominent cyanobacterial species in colonized rock samples examined by differential interference contrast and episcopic fluorescent microscopy. Filamentous varieties of cyanobacteria were less common than coccoid forms but could be easily distinguished in all slide mount preparations. Scanning electron microscopy showed that the endoliths occurred in communal populations situated within the open pore spaces of the limestone. Heterotrophic bacteria were also observed growing as epiphytes on dead and living cyanobacteria, and in epilithic biofilms on pore space walls. In thin-sectioned specimens examined by transmission electron microscopy, these adherent bacteria commonly exhibited extensive accumulations of fibrous extracellular polymeric materials external to their cell walls. Biomass concentrations, estimated from the organic phosphorus content of the samples, generally paralleled rock porosity and ranged from 3.3 to 17.1% dry weight. Carbon isotope (14C) measurements indicated that atmospheric carbon dioxide is used by the endolithic cyanobacteria, rather than dissolved inorganic carbon from the weathering of carbonate minerals in the limestone host rock. In addition, whole rock multielement analyses revealed an enrichment of some elements (e.g., phosphorus, barium, lead, and zinc) in the endolith zone over the host rock, while other elements (e.g., magnesium, calcium, iron, and copper) were depleted. The implication is that the endolithic microorganisms play an active role in the biogeochemical cycling of nutrient and trace elements in cliff face ecosystems of the Niagara Escarpment.Key words: limestone, endolith, biofilm, cyanobacteria, geochemistry.

Mapping of jasperoid in the Cedar Mountains, Utah, U.S.A., using imaging spectrometer data

Abstract Jasperoid, an important indicator of disseminated gold deposits, is mapped in the Cedar Mountains, Utah U.S.A. using data gathered by the Airborne Visible Infra-Red Imaging Spectrometer (AVIRIS). Ratio analysis is employed to detect the sharp rise in reflectance between 1000 nm and 1700 nm in the jasperoid spectrum due to iron absorption at about 900 nm in association with quartz and bound water. A pre-requisite is to remove the effects of dead and senescent vegetation from the ratio image. This is achieved using a mask of dead vegetation created by absorption band-depth analysis of ligno-cellulose absorptions in the Short-Wave Infra-red (SWIR). The absorption feature at about 2080 nm was used, but this choice is partly dictated by the limestone host rocks which exhibit a strong absorption feature which overlaps with the longer wavelength lingo-cellulose absorptions. Validation of the ratio map is accomplished by overlaying a geologic boundary which defined an area of jasperoid in the field.

Specific surface area and ultramicroporosity in polymorphs of silica

A detailed study of the specific surface area (SSA) and the volume of pores between 10 A and 400 A (ultramicroporosity -UMP-) in different types of silica phases is carried out in this paper. The samples were chosen from different locations and geological settings, with an emphasis on high silica content (above 80%) and mineralogical simplicity (one silica phase predominates in every sample). Some samples belong to continental siliceous rocks and silcretes of the Miocene age, and others to radiolarites transformed by thermal metamorphism. The results show that small proportions of other minerals in the siliceous rocks can alter the SSA and ultramicroporosity of the silica rock in a substantial way. The specific surface and pore volume are closely related to the crystalline structure of the silica polymorphs. Opal-A (diatomite) has 23.87 m 2 /g of SSA and 9.90% of UMP, opal-CT 7.76-14.05 m 2 /g of SSA and 6.68-8.50% of UMP, α-cristobalite 1.93-2.43 m 2 /g of SSA and 1.27-1.52% de UMP and α-trydimite 1.93-2.56 m 2 /g of SSA and 1.05-1.62% de UMP. There exists a loss of SSA and ultramicroporosity in the silica phases involved in the diagenetic transformation opal-A → opal-CT → opal-C (α-cristobalite). The host-rock in which the silica phase is formed can play an important role in the amount of the SSA. Thus, opal-CT formed in a sepiolite host-rock exhibits a greater SSA than opal-CT formed in a limestone host-rock. The pore volume distribution is characteristic of the silica phase although this distribution can vary in relation to the texture in the opal-CT

Using volatile constituents of soils and soil gases to determine the presence of copper‐zinc ore bodies at Johnson Camp, Arizona

Soils and soil gas samples were collected along three traverses across the copper‐zinc ore bodies at Johnson Camp, in southeastern Arizona. The copper‐zinc ore occurs as isolated sulfide and oxidized sulfide replacement bodies in limestone host rocks covered by 10–225 m of alluvium. Samples were analyzed for helium using mass spectrometry and for carbon dioxide and sulfur compounds by gas chromatography. Above average concentrations of carbon dioxide and helium in soil gases, as well as above average concentrations of carbon dioxide degassed from soils, occurred over and adjacent to all three ore bodies. Carbonyl sulfide and carbon disulfide degassed from soils occurred in the vicinity of ore bodies as deep as 150 m, but not to 225 m depth. Volatile constituents in soil gas samples collected by hollow probe defined the ore bodies better than volatile constituents degassed from surficial soil samples.

Drainage‐basin evolution and aquifer development in a karstic limestone terrain South‐Central Texas, U.S.A.

AbstractThe Edwards artesian aquifer occurs in cavernous limestones of Cretaceous (Albian) age within the Balcones fault zone in south‐central Texas. The major recharge and discharge zones of the aquifer are contained within the upper reaches of three river systems: the Nueces, the San Antonio, and the Guadalupe. Within these watersheds, recharge dominates in the semiarid Nueces basin to the west while most discharge occurs farther east from wells in the subhumid San Antonio basin and from springs in the subhumid Guadalupe basin. This long‐distance transfer of ground water (up to 240 km) is a result of several factors: depositional and early diagenetic history of the limestone host rock, geometry and magnitudes of fault displacement, and physiographic responses to faulting.The loci of greatest discharge from the aquifer occur in an area that was exposed subaerially with concomitant porosity enhancement due to dissolution of limestone during late Early Cretaceous time. This area also was subjected to the greatest fault displacement during Miocene time. Thus, faults and associated joints superimposed additional avenues for porosity and permeability development onto an area that already had considerable secondary porosity. Further determinants on aquifer properties resulted from late Tertiary and Quaternary drainage evolution in response to faulting along the Balcones trend. The strike of the fault zone lay at acute angles to the courses of the main trunk streams in the ancestral Guadalupe and San Antonio River systems, whereas in the Nueces basin the trend of the fault zone was normal to the courses of the main streams. Thus, as a fault‐line scarp began to form in the eatern basins, scarp‐normal streams were incised rapidly into northwest‐trending canyons. These steep‐gradient streams captured the eastward‐flowing major streams in the easten watersheds. These pirate streams incised into the aquifer at the lowest topographic levels within the region because of: 1. The sudden acquisition of extensive catchment areas in a subhumid area; and 2. Steep stream gradients that reflected the larger fault displacement in the east. The low topographic points of discharge became the loci of major springs.Recharge is dominant in the Nueces basin mainly because streams cross permeable limestone units at higher topographic levels than in the San Antonio and Guadalupe basins. The topographic characteristics of the Nueces watershed resulted from a combination of diverse factors: lesser fault displacement, no major stream piracy, and less vigorous erosion because of a semiarid climate.

Carbon, Hydrogen, Oxygen, and Sulfur Isotope Study of the Darwin Lead-Silver-Zinc Deposit, Southern California

The ores at Darwin occur as massive replacement bodies in silicated limestones of Pennsylvanian and Permian age adjacent to a Jurassic quartz monzonite stock. Three types of ore have a definite spatial relationship to the quartz monzonite: (1) pyrite-sphalerite-galena ores, (2) pyrite-pyrrhotite-magnetite-sphalerite-galena ores, and (3) galena-Ag-Bi-Se ores.The delta 34 S values of all sulfide minerals range from +4.4 to --5.7 permil. The delta 34 S values for individual minerals tend to decrease with respect to both space and paragenetic time. The delta (sub sl-gn) values range from 1.5 to 2.5 permil and correspond to a temperature range of 325 degrees + or - 55 degrees C. The sphalerite-galena sulfur isotope temperatures at a given locality are reproducible to + or -30 degrees C and are consistent with temperatures determined by other means.Analyses of water in three samples of fluid inclusions in sphalerite indicate that the ore fluids had delta D values of --66 + or - permil, a total salinity that reached at least 20 percent, and K/Na atomic ratios of 0.23 + or - 0.03. Isotopic and thermochemical data indicate that the ore fluids had average values of delta 34 S (sub Sigma S) = 3 permil, delta 13 C (sub Sigma O) = -- 3.5 permil, Sigma S = 0.01 mole/Kg H 2 O, f (sub CO 2 ) = 12 + or - 8 bars, and Sigma C = 0.15 + or - 0.06 mole/Kg H 2 O.The delta 18 O values of postsulfide calcites have a range of 12.8 + or - 1.5 permil in the pyrite ores and 17 + or - 2 permil in the pyrrhotite ores. Similar values for relict limestone host rock in these areas indicate that oxygen isotopic equilibrium was established between the calcite precipitating fluids and the wall rock. The delta 13 C values of the calcites, however, range from --5.8 to --3.6 permil and are considerably more negative than the limestone in the host rock, indicating that carbon isotopic equilibrium was not established between the fluids and the wall rock.Thermochemical data indicate that the pH of the ore fluids was about 4.8 as they traveled through the quartz monzonite at T nearly equal 350 degrees C. When the fluids completely equilibrated with the relict limestone and calc-silicate host rock, the pH increased to 6.7. This increase in pH as the fluids traveled upward and away from the quartz monzonite was responsible for the spatial distribution of iron sulfide assemblages, for variations in delta 34 S values of sulfides, and probably caused precipitation of the ore.

Chert in Lisburne Limestone of Alaska: ABSTRACT

Cherts from the Mississippian/Pennsylvanian Lisburne Limestone of northern Alaska can be divided into 2 distinct fabric types. These are (1) intragranular cherts in which the silica has started nucleation within the skeletons of crinoids and pelecypods, and (2) matrix chert which occupies the intergranular spaces and shows replacement of contained bioclastic debris only from the exterior. The matrix chert is considered to be of direct biogenic origin, but preservation of the original skeletons is very rare because of their progressive recrystallization since deposition. The intragranular chert began nucleation early in the diagenetic process, probably as a result of the reaction of the organic tissue in the crinoid pore-spaces with the silica of the interstitial waters. T e organo-silicic acid produced by this mechanism then could react with the calcium carbonate of the crinoid plates and deposit silica on a piecemeal basis. Dolomite is a common accessory of chert even when dolomite is absent from the adjacent limestone host-rock. The dolomite usually is distributed homogeneously through the chert either as euhedral or corroded rhombs. In places iron-rich dolomite forms a reaction-rim at the chert-calcite interface. A process of progressive solution of dolomite in the interior of the chert and reprecipitation at its edge is necessary to explain this distribution. Dolomite probably is only locally soluble in organo-silicic acids and the source of its magnesium is almost certainly the high-Mg calcite skeleton of the crinoid. End_of_Article - Last_Page 663------------