Post-depositional Alteration Research Articles

Complex metasomatism of an Archean placer in the Witwatersrand Basin, South Africa; the Ventersdorp contact reef; a hydrothermal aquifer?

The Ventersdorp contact reef is an auriferous conglomerate horizon and economic orebody on top of the Late Archean Witwatersrand Supergroup in South Africa. It differs from the other conglomerate beds (reefs) in the Witwatersrand basin in having a massive metabasalt sequence as its hanging wall and by its intense post-depositional alteration. A geochemical, petrologic, and fluid inclusion study of the Ventersdorp contact reef reveals a polyphase hydrothermal alteration history. Regional metamorphism under acidic conditions was followed by potassic alteration along the reef. The potassic alteration, which also extended into the hanging-wall and footwall rocks, was subsequently followed by a chloritization episode that was restricted to the reef and its immediate contacts. Mass-balance calculations suggest that the latter alteration required only minor fluid influx from an external source, whereas the potassic alteration reflects the progressive dehydration of the siliciclastic footwall rocks and associated hydration of the basaltic hanging wall. Based on microtextural observations on quartz using SEM-cathodoluminescence imaging, the chloritization event is ascribed to the 2020-Ma Vredefort impact event that created the necessary secondary permeability in the form of microfractures particularly within the conglomerate bed. Considering that the Ventersdorp contact reef is generally regarded as the most intensely altered reef in the Witwatersrand basin, the relatively minor fluid infiltration established in this study suggests that, contrary to prevailing opinion, the other auriferous conglomerate beds might have experienced even less influx of externally derived hydrothermal fluids.

Spatial and temporal variations in carbon and sulfur isotopic compositions of Sinian sedimentary rocks in the Yangtze platform, South China

Eight stratigraphic sections from the Yangtze Platform, South China were selected for a study of spatial and temporal variations in carbon and sulfur isotope compositions of Sinian sedimentary rocks. Carbon isotope compositions of carbonate from lower Sinian strata deposited between pulses of glacial diamictite range from −3.9 to −9.9‰; the more negative values recorded from Mn carbonates suggest that, in part, the carbon was derived from the oxidation of organic matter during early diagenesis. In the upper Sinian strata δ 13C varies with the facies. They are positive for the platform carbonates, whereas they are negative from about −3 to −12‰ for the carbonates formed in or near basin environments. It is evident that post-depositional alteration accounts for the strongly negative δ 13C values. However, some information on primary isotopic compositions can be obtained by evaluating sample quality. The primary δ 13C values of carbonates from the Doushantuo Formation are ca −3 to −4‰, and from the Dengying Formation as low as −6‰. δ 34S values for pyrite from the lower Sinian rocks are highly positive, and range from values near the coeval seawater sulfate up to ca +60‰. In contrast, δ 34S values for pyrite from the upper Sinina rocks are negative down to −27 to −30‰. The feature of sulfur isotopic composition for the lower Sinian rocks is consistent with the late Proterozoic sulfur isotopic record. It might be explained by the ‘supercontinent model’: many basins on the Yangtze block in a supercontinent tectonic background had limited or no access to the open ocean and the basin waters contained sulfate with considerably higher δ 34S values than the coeval seawater for a geologically long period. An alternative explanation is based on the sulfate-minimum zone (SMZ) modal. Because of the slow sinking of organic matter bacterial sulfate reduction would be important in the lower Sinian shallow water environment. Particularly in the SMZ, the residual sulfate might have been significantly enriched in 34S, so that pyrite produced in closed diagenetic system would yield very positive δ 34S values.

Boron isotope systematics of tourmaline formation in the Sullivan Pb–Zn–Ag deposit, British Columbia, Canada

We report here the results of 54 boron isotope analyses of tourmaline associated with the giant Sullivan Pb–Zn–Ag deposit in southeastern British Columbia, Canada. The δ11B values range from −11.1 to −2.9‰, which is almost as great as the range found worldwide in tourmalines from 33 massive sulfide deposits and tourmalinites in dominantly clastic metasedimentary terranes. The major control on the overall δ11B values of the Sullivan tourmalinites is the boron source. Potential controls over the large range of the data also include: (1) differences in formation temperatures of the tourmalinites, (2) different stages of tourmaline formation, (3) variations in the proportions of dissolved boron incorporated into the tourmaline (Rayleigh fractionation), (4) seawater entrainment, and (5) post-depositional metamorphism. The boron isotope data at Sullivan are consistent with boron derivation from leaching of footwall clastic sediments. However, the great abundance of tourmaline in the Sullivan deposit suggests that the local clastic sediments were not the sole source of boron, and we argue that non-marine evaporites, buried deep below the orebody, are the most viable source of this additional boron. It is likely that some of the variation in tourmaline δ11B values reflect mixing of boron from these two sources. Comparison of the potential effects of these controls with geologic and other geochemical evidence suggests that major causes for the wide range of δ11B values measured at Sullivan are seawater entrainment and Rayleigh fractionation, although in places, post-depositional alteration and thermal metamorphism were important in determining δ11B values of some of the recrystallized tourmalinites.

99/02156 Experimental study on optimal structure of Y-shaped ash returning device: Zhang, S. et al. Huazhong Ligong Daxue Xuebao, 1997, 25, (8), 50–52. (In Chinese)

Hydrothermal petroleums and heavy tars have been analyzed for polycyclic aromatic hydrocarbons (PAH) with molecular weights greater than that of coronene (300 da). Samples from the hydrothermal systems in the Guaymas Basin (Gulf of California) and in the Escanaba Trough and Middle Valley (Northeastern Pacific) were analyzed by gas chromatography-mass spectrometry and high pressure liquid chromatography with diode-array absorbance detection. Mass spectra and fluorescence spectra were used to characterize the compounds. Several large PAHs with six and more rings were identified among the heavy PAH. Production routes via one-ring build-up and Scholl-condensation are proposed to explain the observed structures. The variations in PAH concentrations and distributions between samples from different locales are a consequence of the hydrothermal conditions of generation, migration, and post-depositional alteration.

Peru Basin sediments: Diagenetic implications of a low coercivity overprint of the NRM

Two coexisting populations of magnetite that vary by at least 320 kyr in age characterize the magnetomineralogy in oxic through suboxic Peru Basin sediment (7°04′S / 88°28′W). 1) Primary magnetite barely underwent post‐depositional alteration and yields a straightforward magnetostratigraphy (Brunhes Chron ‐ top of Olduvai Subchron). 2) A stable, low‐coercivity overprint of the NRM resides in secondary, slightly oxidized SD magnetite that recorded the Brunhes Chron direction. The secondary magnetite appears at the modern Fe‐redox boundary (2.6 mbsf) and continues downhole to abruptly disappear at 5.3 mbsf in the Matuyama Chron. It can be discerned by combined analyses of directional data and the Verwey transition in low‐temperature remanence measurements (20–300 K) that appears concomitant with the overprint.

K-Ar dating of white micas from the Ventersdorp Contact Reef of the Witwatersrand Basin, South Africa: timing of post-depositional alteration

White micas from metamorphosed and hydrothermally-altered basaltic lavas, conglomerates, quartzites and shales in and around the Ventersdorp Contact Reef of the Witwatersrand Basin, South Africa, were dated by the K-Ar method to constrain post-depositional thermal and mineralization processes. The minerals were separated into various grain sizes between < 0.4 and 10μm, and characterized in terms of composition, paragenetic sequence and texture, by XRD, SEM and TEM techniques. The K-Ar isochron of all white micas in the basaltic lavas suggests an age of 1994 ± 60 Ma, and that of the smaller mica particles (< 2μm) in the quartzites defines a younger age of 1917 ± 66 Ma. This range is considered to reflect the timing of long-lived hydrothermal alteration in the Ventersdorp Contact Reef. The older age is slightly younger than the intrusion of the Bushveld Complex (2060–2054 Ma) and the Vredefort catastrophism (2020 Ma), which are well-documented events that were superimposed onto the Witwatersrand Basin. The younger age may be associated with the Eburnian orogenesis along the western edge of the Kaapvaal Craton resulting in continental-scale fluid migration and hydrothermal activity that extended throughout the Griqualand Basin and even into the Witwatersrand Basin. The K-Ar ages obtained here for the white mica fractions of the Ventersdorp Contact Reef in the Witwatersrand Basin confirm that the period between 2.0 and 1.9 Ga was significant, as far as alteration, and possibly also gold mobilization, was concerned.

Abstract: Climate and Post-depositional Alteration of Paleogene Sandstones of the Silverado Formation, San Joaquin Hills, California.&amp;nbsp;

Abstract: Climate and Post-depositional Alteration of Paleogene Sandstones of the Silverado Formation, San Joaquin Hills, California.&amp;nbsp;

The Permian-Triassic boundary in the Karavanke Mountains (Slovenia): Stable isotope variations in the boundary carbonate rocks of the Košutnik Creek and Brsnina section

The stable isotope composition of the Upper Permian and Lower Triassic beds at two locations (Košutnik Creek and Brsnina) in the Southern Karavanke Mountains has been used to investigate δ13C and δ18O variations of the well exposed undisturbed marine carbonate sequence across the Permian-Triassic boundary. The lithostratigraphic boundary between the Lower Triassic-Scythyan and underlying Upper Permian beds is transitional and no exact line can be drawn between them. The transition from Permian to Triassic is characterized by a major shift in carbonate carbon δ13C and δ18O from heavier to lighter values. The results suggest that the carbon isotope variability at the P/Tr boundary reflects global changes in the carbon cycle and/or climatic changes, probably controlled by the Upper Permian regression and further eustatic oscillations of the Tethys sea level and by tectonic. The corresponding δ18O variability should be regarded as indication of seawater oxygen isotopic composition, salinity and temperature changes, changes in carbonate mineralogy of the rocks, postdepositional alterations or some combinations of all the mentioned possibilities.

Determining the Fuel Constituents of Ancient Hearth Ash Via ICP-AES Analysis

Documenting variation and change in the use of particular plants and plant parts for fuel in ancient households contributes to an understanding of settlement location, local and regional abandonments and resource depletion. Chemical analysis of ash surviving in hearths and other thermal features to determine the kinds and relative amounts of fuels consumed may be less biased by formation processes than the macrofossil record currently employed to document ancient fuel use. Our studies indicate that it is possible to distinguish, chemically, ash of common fuel taxa and tissue types (bark, wood, etc.) in both modern and ancient samples of fuels found in the northern Colorado Plateau region of the American Southwest. However, the chemical signatures of the ancient and modern material of the same taxon differ, indicating possible alteration by post-depositional processes. Although multiple regression performs well in determining the relative contributions of different fuels to modern ash mixtures, possible post-depositional alterations and incomplete characterization of the range of within-taxon variation, currently limit our confidence in applications of this approach to ancient ash from Pueblo III settlements in south-west Colorado.

Chemical and Strontium, Oxygen, and Carbon Isotopic Compositions of Carbonates from the Lesser Himalaya: Implications to the Strontium Isotope Composition of the Source Waters of the Ganga, Ghaghara, and the Indus Rivers

Samples of Precambrian carbonate (mostly dolomite) outcrops collected across the Lesser Himalaya have been analysed for their mineralogy, chemical composition, and isotope ratios of Sr, O, and C to assess the extent of their preservation and their role in contributing to the high radiogenic strontium isotope composition of the source waters of the Ganga, Ghaghara, and the Indus. Their Sr concentrations range from 20 to 363 ppm, δ 18O PDB −1.4 to −12.8‰ and Mn 11–2036 ppm. The petrography of the samples, their low Sr concentrations, and wide range of δ 18O values are suggestive of their postdepositional alteration. The 87Sr/ 86Sr of the bulk samples and their carbonate fractions are similar to one another with values ranging from 0.7064 to 0.8935 and are generally more radiogenic than that of contemporaneous seawater. Comparison of the 87Sr/ 86Sr and Sr/Ca ratios among the carbonates and silicates from the Lesser Himalaya and the source waters of the Ganga, Ghaghara, and the Indus shows that the values for the source waters overlap with those of the silicates but are much higher than those in carbonates. An upper limit of carbonate Sr in the various source waters is calculated to be between 6% and 43%, assuming that all the Ca in the rivers is of carbonate origin. The results show that on the average, weathering of the Precambrian carbonates is unlikely to be a major contributor to the highly radiogenic strontium isotope composition of these source waters; however, they can be a dominant supplier of radiogenic Sr to some rivers on a regional scale. The silicate Sr component in some of the source waters of the Ganga (Bhagirathi, Bhilangna, Alaknanda, and Ganga), Ghaghara (Kali and Sarju), and the Indus (Sutlej) was calculated from the Ca/Na, Sr/Na ratios, and strontium isotope compositions of these rivers and the silicate endmember. These calculations suggest that 33–89% of Sr in the Bhagirathi, Bhilangna, Alaknanda, Ganga, and Sarju rivers is of silicate origin, whereas in the Kali and the Sutlej it is much lower, only ∼8%. The remaining Sr to all these waters has to be supplied from other sources such as weathering of carbonates and evaporites. This study underscores the importance of weathering of silicates, carbonates, and evaporites in contributing to the Sr mass balance and 87Sr/ 86Sr of the source waters of the Ganga, Ghaghara, and the Indus. The present day silicate and carbonate Sr contributions to the Sr budget of the rivers vary considerably, but among the major source waters of the Ganga, silicate Sr exerts a more dominant control on their Sr abundance and 87Sr/ 86Sr.

Azaarenes in sediments, suspended particles and aerosol associated with the River Mersey estuary

Abstract The distributions of a variety of azaarenes in suspended particulate material and sediments in the River Mersey estuary and two tributary canals are described. Azaarene concentrations in the local urban atmosphere are also provided. Total azaarene concentrations (the sum of the concentrations of 48 individual azaarenes) range from 2–364 ng · g −1 for estuarine suspended material and 128–2451 ng · g −1 for canal sediments. Atmospheric aerosol concentrations fell within the range 1 to > 9 ng · m −3 . Whilst sediments and suspended matter showed concentrations similar to those previously reported, the atmospheric concentrations were considerably higher and it is hypothesised that deposition of aerosols is a major source of azaarenes to this estuarine system. Down estuary removal of the lower molecular weight azaarene species from the particulate fraction is reported and attributed to selective solubilisation of these molecules. Seasonal variations in both the total and relative azaarene concentrations occur with the greatest abundance of all species in the colder months when post-depositional alteration is least and inputs from combustion sources are greatest. A clear seasonal variation in atmospheric aerosol azaarenes is reported and this appears to be reflected in the sedimentary record.

Long-range atmospheric transport of mercury to ecosystems, and the importance of anthropogenic emissions—a critical review and evaluation of the published evidence

Literature on the long-range atmospheric transport of both anthropogenic and naturally occurring mercury (Hg) to terrestrial and aquatic ecosystems was reviewed for the purpose of assessing the quantitative importance and environmental significance of the anthropogenic emissions. The weight of evidence, comprising many different kinds of data that corroborate each other independently, supports the following conclusions. (i) Approximately 5000 t of anthropogenic Hg are introduced into the atmosphere every year, both by direct emission from sources of pollution and by reemission of previously deposited Hg from diffuse secondary sources in the environment. The primary emissions ( 4000 t) result from various human activities, especially the combustion of fossil fuels (notably coal) and solid wastes. Natural emissions amount to about 2000 t/year. (ii) Although some of the annual anthropogenic Hg output (about half the quantity emitted by primary sources) is deposited near its point sources, the rest (a total of 3000 t) is subject to transport over great distances by atmospheric circulation, resulting in measurable contamination of terrestrial and aquatic environments and organisms up to several thousand kilometers from the points of origin. Indeed, a number of remote ecosystems receive most of their Hg input from the atmosphere. (iii) The available evidence supports the generally accepted conclusion that the Hg enrichment commonly seen in the uppermost horizons of sediment cores from remote lakes is due primarily to contamination by airborne anthropogenic Hg in the recent past, rather than postdepositional redistribution of Hg. Although postdepositional alteration may result in detectable remobilization of sedimentary Hg, its effects on total Hg profiles in lake sediment cores have been found, thus far, to be negligible. (iv) Atmospheric transport of anthropogenic Hg to aquatic and terrestrial ecosystems is a cause for concern, as the Hg is accumulating, to a greater or lesser extent, in organisms (e.g., fish in remote lakes). Moreover, the Hg is associated with, and interacts with, other by-products of fossil fuel combustion, including the strong acids responsible for acid precipitation. The acids aggravate the effects of Hg pollution by furthering the accumulation of methyl Hg in fish inhabiting ill-buffered lakes. Contamination of the atmosphere with Hg and associated pollutants is a serious international problem that calls for reduction or elimination of emissions.

Origin and Classification of Coastal Plain Kaolins, Southeastern USA, and the Role of Groundwater and Microbial Action

AbstractAlong the inner Coastal Plain, kaolinite-metahalloysite-rich, neritic muds of Cretaceous-Eocene age have undergone intense postdepositional alteration in the recharge area of the regional groundwater system. Weathering processes have had the following profound effects on the original sediments: 1) strong compositional and textural modification of both clay and non-clay minerals; 2) whitening of the originally darker sediments by partial removal of organic matter, Fe and Mn; and 3) recrystallization of kaolinite and metahalloysite, most conspicuous where there are coarse stacks and vermiforms. Where the combination of initial sediment composition and alteration intensity was most favorable, these changes have produced important deposits of commercial quality, which now sustain the world's largest kaolin production district. The earliest change was partial sequestration of iron as sulfide and concurrent destruction of some organic matter, mediated by sulfate-reducing bacteria. Subsequent weathering resulted in gradual leaching of alkalies, alkaline earths, iron and silica, and attendant nucleation and growth of minerals compatible with the compositional changes. The existence of several closely spaced erosional unconformities, separated by neritic sediments, is proof that weathering conditions commonly changed at a given site, in response to changes in thickness or lithology of the overlying rocks. Dsyoxic → ← oxic reversals modified both the rate and kind of alteration. (“Dysoxic” refers to molecular oxygen concentration too low to be toxic to anaerobes or cause abiotic oxidation; less extreme than “anoxic”.) Kaolins were produced partly by slower dysoxic weathering in saturated groundwater zones but mainly by more rapid oxic weathering in unsaturated zones, where bauxites also locally formed. Gradual transformation of some sediments to kaolin rarely began and ended in the same epoch. At several places most of the kaolinization (see “Definitions”) took place during Recent time, tens of millions of years after deposition of the sediments. Since the kaolins resulted from postdepositional alteration rather than sedimentary processes, they are better referred to as “Coastal Plain” rather than “sedimentary” kaolins.

The World's Most Spectacular Carbonate Mud Mounds (Middle Devonian, Algerian Sahara)

ABSTRACT In the southern Ahnet Basin of Algeria a unique Middle Devonian underwater scenery is revealed, which consists of 191 mud mounds, 15 mud ridges, and 2 mud atolls distributed over nine discrete areas of a shallow-pelagic carbonate shelf. All the buildups are totally exhumed, thus perfectly exhibiting their original morphologies, slope angles, elevations, and the pattern of the onlapping strata as well as their relationship to the surrounding off-buildup sequences. Types of buildups include (1) small mud mounds (1-5 m high, 10-20 m diameter), (2) large mud mounds (up to 40 m high, up to 200 m diameter), (3) lens-shaped mounds(1-30 m thick, 10-15° slopes), (4) mud ridges (up to 100 m high, 150-8500 m long) and (5) semicircular mud atolls (2500-2800 m long, 20-30 m high). The buil ups were constructed during a short time span in the early Givetian (part of the Lower varcus zone), probably not exceeding a few hundred thousand years. Sedimentation rates in coeval off-buildup areas were at least 3-10 times lower. The lithology of the buildups is a massive boundstone with a high proportion of irregular, spar-filled cavities (stromatactis) but only very scattered potential frame builders. The carbonate mud is considered to be an in situ, cyanobacterial precipitate, although evidence of such is poor. Organic components of the buildups are not significantly different from those of the off-buildup lithologies, and include numerous crinoid ossicles, some tabulate and solitary rugose corals, brachiopods, mollusc debris, trilobites, and very rare sponge spicule and bryozoans. The total absence of stromatoporoids, large tabulate and colonial rugose corals, green algae, and micritic envelopes around grains suggests growth of the buildups in the lower part of the photic zone (100-200 m). They are onlapped by a discontinuous sequence, a few meters thick, of skeletal packstones and grainstones, and locally, orthoceratid floatstones that were deposited later in the Givetian. A widespread rise of sea level in the early Frasnian buried both buildups and adjacent areas under shales and siltstones. Postdepositional alteration of the mud buildups includes: (1) formation of stromatactis cavities, probably by dewatering and internal erosion, (2) cementation of stromatactis cavities and interparticle pore spaces by early high-Mg calcite and late, blocky low Mg calcite, (3) neomorphic transformation of Mg-calcitic mud and aragonitic skeletal debris, (4) occasional formation of neptunian dikes, (5) slight erosion of buildup surfaces, (6) compaction and slumping of onlapping capping beds, and (7) local dolomitization. The formation and growth of the mud build-ups is envisaged as a sequence of stages starting with patchy coral thickets evolving into small mud mounds that coalesced into larger mounds. By continuous vertical and lateral growth, closely spaced mounds amalgamated into mud ridges and mud atolls. Regular NW-SE and N-S patterns of some mound rows and ridges show a relationship of buildup formation to rejuvenated late Precambrian tectonic lineaments, which reappear in the Hercynian orogeny. The isotopic signature of matrix and early ements of mound rocks, however, does not support the idea of fluid migration and hydrothermal CO2 supply during buildup growth.

Biologic and climatic signals in the oxygen isotopic composition of Eocene-Oligocene equid enamel phosphate

Abstract Oxygen isotope results for tooth enamel-phosphate (δP) from late Eocene-early Oligocene fossil horses are presented to determine if paleobiologic and paleoclimatologic information is recorded in fossil tooth enamel chemistry. Teeth from jaws of Mesohippus and Miohippus from the White River Formation (or Group) in the western Great Plains are well preserved and have excellent geochronologic control. Although there is clear evidence for post-depositional alteration of the enamel, a hydroxylapatite mineralogy is preserved and isotopic exchange of oxygen does not appear to be significant. There are distinctive patterns of δP variation among individual teeth from the same jaw. These patterns reflect season of birth and timing of enamel mineralization. Most of the horses were born in the spring, and mineralization of the enamel is complete after 1–1.5 years. These results show that paleoclimate reconstruction from enamel δP must account for tooth position and timing of mineralization. The observation that there are different intrajaw patterns of δP variation among Mesohippus and Miohippus may provide a basis to reconstruct changes in climate seasonality in ancient environments.

High molecular weight polycyclic aromatic hydrocarbons in hydrothermal petroleums from the Gulf of California and Northeast Pacific Ocean

Hydrothermal petroleums and heavy tars have been analyzed for polycyclic aromatic hydrocarbons (PAH) with molecular weights greater than that of coronene (300 da). Samples from the hydrothermal systems in the Guaymas Basin (Gulf of California) and in the Escanaba Trough and Middle Valley (Northeastern Pacific) were analyzed by gas chromatography-mass spectrometry and high pressure liquid chromatography with diode-array absorbance detection. Mass spectra and fluorescence spectra were used to characterize the compounds. Several large PAHs with six and more rings were identified among the heavy PAH. Production routes via one-ring build-up and Scholl-condensation are proposed to explain the observed structures. The variations in PAH concentrations and distributions between samples from different locales are a consequence of the hydrothermal conditions of generation, migration, and post-depositional alteration.

Trace and minor element ratios inHalimeda aragonite from the Great Barrier Reef

The calcareous green algaHalimeda can be a substantial contributor to aragonite sediment in reef ecosystems. In contrast to coral aragonite, little is known about the trace and minor element composition ofHalimeda aragonite, so it is difficult to test oceanographic hypotheses about factors controlling its past growth. We investigated adapting trace element cleaning protocols for modern and HoloceneHalimeda aragonite, modern and HoloceneHalimeda trace and minor element compositions, and the potential utility ofHalimeda aragonite for paleoceanographic investigations. We successfully adapted and applied sample treatment protocols developed for measuring trace elements in coral aragonite (generally less than 500 y old) toHalimeda aragonite (modern to approximately 5000 y old in this study). ModernHalimeda aragonite from John Brewer Reef in the Central GBR had mean Cd/Ca ratios of 5.19 ± 1.68 nmol/mol\(( \pm 2\sigma /\sqrt n )\) forHalimeda micronesica and 2.35 ± 0.38 nmol/mol for three closely related species important in bioherm accumulationHalimeda copiosa, Halimeda hederacea, andHalimeda opuntia. Mn/Ca ratios, with means from 89–239 nmol/mol for these four species, showed both intra-and inter-specific variability. Sr/Ca ratios (10.9 ± O.1 mmol/mol) and Mg/Ca ratios (1.35 ± 0.26 mmol/mol) were similar for all samples. HoloceneHalimeda aragonite samples from cores of two bioherms in the northern GBR seemed well preserved on the basis of mineralogy and Sr/Ca and Mg/Ca ratios similar to those in modernHalimeda aragonite. Cd/Ca ratios (overall mean 0.96 ± 0.15 nmol/mol) were lower than those measured in the modernHalimeda from the central GBR location. However, Mn/Ca ratios in both cores were substantially higher than in modernHalimeda aragonite. While it may be possible to extract paleoceanographic information fromHalimeda aragonite, substantial care is needed to evaluate and avoid the effects of post-depositional alteration.

Geochemistry of Precambrian carbonates: VII. Belt supergroup, Montana and Idaho, USA

Carbonates from the ~1100–1450 Ma old Proterozoic Belt Supergroup were collected from stratigraphic sections throughout Montana and Idaho, USA. The sampled sequences, in ascending stratigraphic order, include the Newland, Altyn, Spokane/Greyson transition, Empire, Wallace, Helena, Siyeh, Snowslip, Shepard, and Libby formations. An increase in the degree of postdepositional alteration of Belt limestones is reflected in a diminution of Sr and Mg contents, an increase in Mn, and depletion in 13C and 18O. Two diagenetic trends can be resolved for the limestones. One, affecting the presumed originally aragonite-rich sediments, includes carbonates from the Lower Belt Newland Formation. In contrast, the Middle Belt Carbonate (Wallace, Helena, Siyeh formations) may have been originally of high-Mg calcitic mineralogy. Projection of the alteration trends for the Lower and Middle Belt limestones suggest ~21‰ SMOW and ~ +2.5‰ to +1.0‰ PDB as the best preserved values for the δ 18O and δ 13C of seawater, respectively; both comparable to results from other Mesoproterozoic carbonate sequences. The oxygen isotope data for limestones show a regional westward depletion of ~8‰ in 18O, possibly reflecting a higher temperature of postdepositional alteration in the western Belt basin. This depletion in 18O is accompanied by a comparable decrease in δ 13C values, most likely because a higher proportion of carbon was incorporated from CO 2 generated by thermal cracking of hydrocarbons at depth. Dolostones in the Belt basin are dominantly micritic, with good preservation of depositional textures. Chemically and isotopically, their alteration trends mimic those of limestones, leading to comparable projected “best” values for δ 13C. 87Sr/ 86Sr values of Belt carbonates range between 0.70484 and 0.74991. Progressive diagenesis, as indexed by decreasing concentrations of Sr and depletions in 18O and 13C, results in an increase in 87Sr/ 86Sr values. The least radiogenic measurement, from a Lower Belt Newland limestone sample, appears to fit reasonably well into the general trend for Proterozoic seawater, as discussed in Mirota and Veizer (1994). This and the consistency of δ 18O and δ 13C in limestones with other coeval sequences suggests that the (Lower and Middle) Belt carbonate sections are marine, deposited in an environment that was not continuously separated from the open ocean.

Postdepositional chemical alteration of Ouachita shales

Shales belonging to the Carboniferous flysch exposed in the Ouachita Mountains of west-central Arkansas and southeastern Oklahoma have undergone postdepositional alteration that has significantly affected their whole-rock chemistry. Alteration is particularly pronounced in the northeastern Ouachitas and along the Benton-Broken Bow uplift. Altered rocks have lost Ca, Mg, K, Na, Fe, and Si relative to the “conservative” elements Al, Ti, Zr, Cr, and Ni. Some evidence exists for slight loss of Al and Ti relative to Cr, Ni, and Zr. Pyrophyllite- and chloritoid-bearing samples are enriched in 18 O relative to less altered samples, and this enrichment is not due to changes in the δ 18 O of quartz. Mineralogical and textural evidence support a postdepositional alteration model rather than mixing of sediment from different sources, or sedimentary sorting, to account for the variations in whole-rock chemistry. Development of microscopic foliation-parallel zones enriched in phyllosilicates and in Ti and Al is related to slaty cleavage development and suggests that chemical alteration is coeval with deformation and very low grade metamorphism. Volume-loss calculations based on conservation of “immobile” elements, and on deformation of detrital mica grains, give minimum estimates of ≈30%–50% volume loss in the most altered shales. The postdepositional changes recorded in whole-rock chemistry of these shales may be unique to the Ouachita flysch, but suggest a need for greater caution in the uncritical use of shale chemistry in studies of crustal evolution and provenance.

Witwatersrand iron-formations and their significance for gold genesis and the composition limits of orthoamphibole

In the West Rand Group of the 3.07–2.71 Ga old Witwatersrand Supergroup, South Africa, a series of banded iron-formations occur. They are of chemical origin and were deposited in an offshore shelf environment. The coarser-grained, in places pyrite-bearing, and partly auriferous metasedimentary rocks forming the bulk of the Witwatersrand Supergroup are regressive. The iron-formations, however, were deposited during transgression. The presence of allogenic pyrite in the fluviatile metaconglomerates and that of magnetite and, in places, haematite in the marine iron-formations suggests a lower pH and higher sulfur activity for the Archaean meteoric environment than for recent hydrothermal fluids on the ocean floor. Post-depositional alteration of the Witwatersrand rocks includes burial metamorphism at temperatures between 300 and 350 °C and pressures around 2.5 kbar, and multiple hydrothermal inflitration events at slightly lower temperatures, coeval with the brittle deformation of the basin fill during the deposition of the Transvaal Supergroup and the Bushveld Vredefort events.