Mottled Zone Research Articles

Desert-aeolian origin of late Cenozoic regolith in arid and semi-arid Southwestern Australia

Using a sedimentary geology approach, the late Cenozoic regolith of Southwestern Australia is interpreted as basement saprock and saprolite unconformably overlain by sequences of continental sedimentary deposits. The sedimentary deposits which overlie saprock and saprolite are allochthonous, variously altered, and mainly, partly to wholly, desert-aeolian sediments. They include: aeolian and fluvial sandy clayrock facies (e.g. deposits which resemble the pallid and mottled zones of weathered Precambrian bedrock of the traditional “laterite profile”); altered aeolian sand and sandy dust facies (e.g. “hardpan”, lower parts of “duplex soils”, red sandy silt-claystone and calcrete valley fills); altered sandy duststones (e.g. “bauxite”, “laterite” and “ferricrete” of the traditional “laterite profile”); and bioturbated aeolian sand-sheets (e.g. “sandplain soils”, “lateritic sandplains” of the traditional “laterite profile”, and upper parts of “duplex soils”). The allochthonous and desert-aeolian views proposed here differ fundamentally from the traditional residual, colluvial, and erosional views. Consequently there is a need for both traditional and alternative views to be critically appraised from the perspective of sedimentary geology.

Fluidity and solidification microstructures of flake graphite cast iron in thin sections

Fluidity determinations have been undertaken using a silica tube as the fluidity channel, metal being drawn in to the test tubes by means of a regulated vacuum system. Fe-Si and Ca-Si inoculants were added to a hypoeutectic cast iron melt which was then tested for fluidity. A linear relationship between fluidity length and tube diameter was obtained. Fluidity length increased with increasing suction pressure and with increasing temperature of the molten iron. Fluidity length is proportional to the square root of effective suction pressure. Structures within the fluidity specimens have been classified, from the tip backwards, into three zones, containing ledeburite, mottled and flake graphite structures. The length of each structural zone increases with increasing suction pressure. The total length of the ledeburite zone, plus the mottled zone, is constant for increasing tube diameter. Thus, the flake graphite zone increases linearly with tube diameter. The effects of inoculation on the structure and its fading were evaluated by the length of the flake-graphite zone.

Gold mass balance in lateritic profiles from savanna and rain forest zones

The gold mass balance was studied in two areas gold mineralization; one in a savanna zone in South Mali, the other in a rain forest zone in French Guiana. In the ferricrete profile of the savanna zone, the gold mass balance is strongly negative within the ferricrete itself, where almost all the primary gold stock is leached. Beneath the ferricrete, an absolute accumulation of fine secondary gold is found in the mottled zone. The gold cycle is controlled by the migration of Fe and associated with the neogenesis of iron oxides, particularly hematite. The lateritic profile in the rain forest zone is truncated, and a soil is developing from the underlying saprolite in equilibrium with the equatorial conditions. An absolute accumulation of gold is found in the soil, with gold migration and reprecipitation being controlled by the evolution of the macroporosity. The global mass balance of gold in the Guiana deposit shows a leaching rate of 40 % in the weathered part of the mineralization. This process occurs mostly at the weathering front. In the two examples, gold shows opposite behaviors due to different pedogenic and paleoclimatic conditions. This might influence the interpretation of geochemical soil surveys in various intertropical environments.

A three-stage genetic model for the Igarape Bahia lateritic gold deposit, Carajas, Brazil

A significant gold deposit in laterite is under development in the Carajas region of Brazil. The lateritic profile of the Igarape Bahia deposit, Carajas, can be divided into six horizons: basal unweathered rocks followed by saprolite, a pallid zone, a mottled zone, a ferruginous zone, and topsoil. Native gold and electrum are found within iron oxide-hydroxide nodules in the ferruginous zone where gold orebodies have been delineated. Most of the gold grains are less than 10 mu in size. There are three morphologicial types: (1) rounded grains in voids, associated with colloform goethite and needlelike goethite; (2) irregularly shaped grains filling shrinkage cracks in amorphous goethite; and (3) subidiomorphic crystals associated with amorphous goethite or needlelike goethite. The morphological features and occurrences of the gold suggest that it has been mobilized and reprecipitated during lateritization.In the ferruginous zone, gold grains from the upper part contain more silver than those from the lower part, i.e., the fineness of gold increases downward in the profile. Quartz veinlets and sulfides in bedrock suggest that the primary rocks were enriched in gold. In the first weathering stage thiosulfate ions from weathering of sulfides are hypothesized to transport both gold and silver into the saprolite and mottled zones where secondary electrum and silver-rich gold were formed. In the second stage, gold and silver were remobilized and incorporated into the ferruginous zone as the weathering proceeded and the erosion surface was lowered. In the third stage, after uplift and incision of the landscape, chloride leaching separated silver from gold grains to purify the electrum. The survival of electrum in the upper part of the ferruginous zone can be attributed to lower chloride concentrations in near-surface ground water and to enrichment of organic matter which can lower the redox potential and increase the stability of native silver. Some silver was then adsorbed by manganese oxide minerals in the lower part of the ferruginous zone.

Neogene oceanographic variations recorded in manganese nodules from the Somali Basin

A detailed study of a nodule from the Somali Basin dated by 230Th excess was correlated with the paleoceanographic events recorded in Site 236 (Leg 24) Deep Sea Drilling Project (DSDP) cores. Tentative indications are that the phase of nodule accretion starting with the development of pillar structure at a depth of 20 mm in the nodule around 13 Ma coincides with increased Antarctic Bottom Water (AABW) flow and an elevated calciumcarbonate compensation depth (CCD). The Late Miocene lowering of the CCD is represented by the mottled zones between 8 and 18 mm in the nodule is characterised by an abundant silicate component (>20%) of aeolian origin. The Miocene/Pliocene boundary (5 Ma) occurs at a depth of about 8 mm and is represented by the development of pillar structure and a minimum of aeolian dust (10.3%). The increased biological productivity of the Somali surface water since the Middle Miocene is demonstrated by the increasing C org content of the nodule (from 0.11 to 0.19%) towards its surface.

Anconichnus horizontalis: A Pervasive Ichnofabric-Forming Trace Fossil in Post-Paleozoic Offshore Siliciclastic Facies

The fabric of often intensely mottled, upper offshore to lower shoreface siliciclastic sediments of post-Paleozoic age is analyzed and the principal trace described from material collected from outcrop (U.K.) and the North Sea Basin (core). The characteristic trace of these mottled zones is Anconichnus horizontalis, which is a narrow, discontinuous, twisting, muddy fecal string within a poorly defined burrow fill depleted in mud and inertinite. This trace formed endogenically, principally in association with small-scale cross-stratification and thin (cm) event beds. Seven taphonomic-sediment associations (ichnofabrics) are recognized: (1) in siltstone to very fine-grained sandstone in thin (cm) event beds, often as only one trace; (2) in association with small-scale cross-stratification; (3) in dense concentrations in fine-grained sediment where the primary structures have been obscured; (4) similar to (3) but with a patchy distribution of the trace; (5) siltstone to very fine-grained sandstone, often associated with heterolithic stratification, with a higher ichnodiversity, including Phoebichnus, Palaeophycus, Thalassinoides, Rhizocorallium, Cylindrichnus and Diplocraterion; (6) as (3) but with muddy patches and sand-filled Chondrites sp.; (7) mud dominated, with silty horizons and associated with Terebellina. Associations 1 to 4 were produced by opportunistic shallow tier burrowers penecontemporaneous with deposition of mud-depleted event beds in an offshoremore » environment. Association 5, with diverse, later mid-tier burrowers, suggests a more equilibrium endobenthic community in aggrading sedimentation conditions, under fair weather, in the offshore transition zone to lower shoreface, while associations 6 and 7 indicated muddier offshore situations.« less

Morphology and geochemistry of gold grains in lateritic profiles of southern Mali

This paper concerns the study of shape and chemical composition of Au nuggets sampled at various depths in a lateritic profile in the proximity of a site where Au mineralization has occurred. In southern Mali, weathering profiles are well developed and generally very thick (≈50–100 m). Samples were collected in pits dug in the Au-bearing laterite and Au grains were recovered by panning. A statistical analysis was made of the size and shape of the Au grains. The average size of the Au particles gradually decreases upwards in the lateritic profile from 600 μm in the saprolite, to 320 μm in the mottled zone and 240 μm in the ferricrete. S.E.M. studies show that Au grains become etched and rounded upwards to the top of the profile. Dissolution of Au is suggested by corrosion features at the surface of Au grains. The importance of chemical etching increases particularly at the top of the profile. Gold particles show also an increased fineness associated with weathering intensity, reflecting leaching of Ag from Au grains during lateritization. Secondary Au is sometimes visible. It is characterized by a high fineness and a size generally smaller than 1 μm. It has particular shapes like microspherulites and dendrites, and it can form also secondary Au cortexes around strongly etched Au grains.

Early diagenetic processes affecting nutrients in the pore waters of Central Indian Ocean cores

Pore-water nutrients, nitrite, nitrate, phosphate, silicate, pH and solid-phase organic carbon were analysed for one core from the Arabian Sea and three cores from the manganese nodule area in the Central Indian Ocean Basin. Possible denitrification and unoxidized organic carbon are observed in the Arabian Sea core, while nitrification with intermediate low nitrate and high organic carbon are found in the mottled zones of the pelagic cores. Mobilization of iron-host oxyhydroxides and peaked profiles of organic carbon are related to a gradual increase in the phosphate profiles. The asymptotic silica values are less than the 100% saturation levels of amorphous silica and they fall outside the stability field of sepiolite.

Gold exploration in the callion area, eastern goldfields, western Australia

Abstract The Callion gold-quartz veins are hosted by metabasalts, interflow sediments and acid metatuffs within the Archaean Norseman-Wiluna greenstone belt. The area has been subjected to deep lateritic weathering. Gold prospecting using soil geochemistry led to the discovery of a laterite Au deposit (geological reserves of 103,000 t with 1.86 g/t Au at 0.7 g/t cut-off) and subsequently to the discovery of the underlying quartz vein mineralization (geological reserves of 104,000 t with 7.9 g/t Au at 2 g/t cut-off). The quartz vein mineralization is blind, beneath 2–10 m of laterite cover. The vein, which can be traced along strike for 1 km, is mineralized over a strike length of 300 m. Weathering around the vein extends to a depth of 70 m below surface. Ore microscopy shows that the Au in the weathered vein was partially mobilized and deposited contemporaneously with Fe-hydroxides. Mining of the laterite and the top of the quartz vein enabled a detailed reconstruction to be made of the Au-grade distribution from the top of the laterite down to the saprolite. The grade distribution is mushroom-shaped, widening at the top of the mottled zone or base of the pisolithic zone. From the vein in the saprolite, the “mushroom”, defined by the 2.5 ppm Au contour, widens to 40 m, at least 5 times the original width. This zone represents the transition to the overlying wide soil anomaly which has peak Au values of 2 ppm, which are about 50% of those in the laterite in the vicinity of the vein. Magnetic methods were also used during the course of exploration and led to the discovery of another small mineralized vein.

Clay mapping using electromagnetic methods

Conductive water-saturated clays occur in weathered layers, fracture zones, and unconsolidated sediments. Generally, clays are not as conductive as massive sulphides or graphitic sediments, but their resistivity is less than that of fresh igneous and metamorphic rocks. Clay conductors can be detected by modern ground and airborne electromagnetic (EM) systems. A fully developed weathering layer consists of several horizons (from bottom to top): zone of fractured rock, saprolite, leached zone, mottled zone, and duricrust. Electrical properties of the weathered layer depend on the type of clay present and its water content. The most conductive horizon is saprolite; duricrust is the most resistive. Saprolite developed over mafic and ultramafic rocks is more conductive than that formed over felsic rocks, hence the results of resistivity or EM surveys can be used for pseudogeological mapping. Examples from the Itapicurú greenstone belt in Bahia, Brazil, a crystalline area near Nantes, France, and the Rice Lake greenstone belt in Manitoba, Canada, demonstrate that mafic and ultramafic rocks can be identified by airborne EM surveys. Often, as in the three areas shown, mafic rocks are not magnetic. In tropical areas, outcrops are scant and mapping of soil colour differences often results in some formations being missed or incorrectly outlined. In regions of temperate elimate (e.g. France), a thin cover of Quaternary sediments makes mapping of the underlying lithological units difficult. In areas covered by glacial sediments (e.g. Canada), the continuity of formations may not be obvious from existing outcrops. The extent of weathered layers in countries of temperate and cold climate is larger than previously thought. Conductive saprolite may also be preserved in a sedimentary sequence below basal sediments. Such a situation is demonstrated in borehole resistivity data from Bells Corners, Ontario, Canada. Resistivity and EM measurements can be used to map the extent and thickness of Quaternary clays. Examples of such measurements are given from the Sennecey test site, near Dijon, France.

The generation of magnetic anomalies by combustion metamorphism of sedimentary rock, and its significance to hydrocarbon exploration

Low-temperature exothermic oxidation of fine pyrite in carbonaceous marine rocks can lead to spontaneous combustion. As a result, the organic material in the rocks is consumed as fuel, but unlike burnt coal seams and lignite beds, the host rock remains after combustion. Thermal metamorphism, and in some cases partial to complete melting of the rock, will result. Zones of combustion metamorphism are frequently associated with oil fields. These occurrences can be related to either surface outcrops of source rock or leakage from reservoirs. A study of the magnetic properties of a suite of combustion-metamorphosed specimens of the Monterey and Sisquoc Formations in southern California shows orders of magnitude increases in magnetic intensity and susceptibility for both thermally metamorphosed and fused samples. These changes reflect both the production of new magnetic minerals, derived mainly from oxidized iron sulfide, and their acquisition of thermal remanence. Ground magnetic surveys across combustion metamorphism zones in three southern California oil fields reveal anomalies of up to 2,000 gammas associated with the metamorphosed and melted rock. Large-scale aero-magnetic anomalies are also associated with extensive combustion metamorphism zones in the Marcelina Formation of Venezuela and the Ghareb and Taquiya Formations (the “Mottled Zone”) in Israel. The recognition of magnetic anomalies caused by combustion metamorphism in areas of oil production suggests that closely spaced low-altitude aeromagnetic surveying can be a useful petroleum-exploration tool.

Expandable layer silicates associated with hydrothermal talc deposits of Murphy, North Carolina

Corrensite and other mixed-layer chlorites occur in the insoluble residues from the Murphy Marble. They are found in an ordered arrangement around one of the eight zones in the Murphy Marble. The envelope, 60 m wide, is centered on the mottled zone in the upper part of the marble formation. The progression of the layer silicates is: vermiculite --> vermiculite-smectite --> smectite --> smectite-chlorite --> corrensite + chlorite --> chlorite. In addition, quartz, plagioclase, and muscovite may occur. The mottled zone appears to have been a conduit of the hydrothermal fluids which have altered the silicates in the marbles; isotopic data supports this hypothesis. The altered carbonate rock is depleted in heavy isotopes. A genetic connection, between the talc bodies and the hydrothermal plumbing system, is indicated by an increase in the amount of expanding and contracting layers in the mixed-layer minerals of the mottled zone near the orebody.--Modified journal abstract.

Mineralogy and Iron Characterization of Plinthitic Soils on Alluvial Landforms in Venezuela

AbstractThe iron oxides and clay mineralogy were characterized for several soils of Venezuela which contain plinthite and pseudo‐plinthite. Molar ratios of SiO2/Al2O3 (Ki ratios) of the plinthite and pseudo‐plinthite zones range from below 2 to above 4 suggesting a wide range in mineral composition. Cation exchange capacity for these zones ranged from 14.6 to 65.6 meq/100 g of clay suggesting that these mottled zones are not always associated with low activity clays. Kaolinite dominated the clay minerals. Montmorillonite was identified in a few pedons.Goethite was the dominant iron oxide present in the plinthite. The percentage of iron was determined by extracting the soil with citrate‐bicarbonate‐dithionite and by extracting with ammonium oxalate. These two extractions were ratioed (OX/CBD) yielding values of >0.05 for pseudo‐plinthite and values of <0.05 for plinthite in most cases. Soils with plinthite contained most of the iron in the sand‐ and silt‐size fraction following one drying cycle while soils with pseudo‐plinthite contained most of the iron in the clay‐size fraction.

Tertiary weathering profiles in the plateau deposits of East Devon

A complex of superposed weathering profiles forms the dominant feature of the geology of the East Devon tableland. In the Palaeocene, a lateritic weathering profile, the Combpyne Soil, formed as Chalk deposits underwent dissolution beneath their Tertiary overburden. The profile is well differentiated and polygenetic. The Peak Hill Gravels represent reworked pallid and mottled zones and occur extensively over the tableland. Red earths survive in isolated pockets in the subsequent land surface and traces of laterites occur. In the Sidmouth area. following earth movements in the early Eocene, the Seven Stones Soil was formed. Chemical weathering was less intense than in the older profile. Silcrete formed in both profiles in the Sidmouth area. During the later Tertiary but especially in the Pleistocene these earlier profiles suffered erosion and local redistribution to form the Mutters Moor Gravels. Quaternary soils are developed on fresher Permo-Triassic rocks where the tableland is deeply dissected and are also superposed at various levels of erosion in the older profiles on the plateau surface.

Thermal effects in “oil shales” — Naturally occurring kaolinite and metakaolinite organic associations

Hydrogen bonding between kaolinite and organic material in “oil shales” of the Campanian-Maastrichtian Ghareb Formation was inferred from perturbation of the hydroxyl stretching vibrations, and was confirmed by oxidation and pyrolysis experiments. The kaolinite is not detectable by X-ray diffraction analysis, due to poor crystallinity. Pyrite, although associated with bituminous samples, was probably not formed by reduction or extraction of kaolinite iron (Spiro and Rozenson, in prep.), hence the poor crystallinity of kaolinite is not caused by these processes. Geochemical indicators suggest that this hydrogen bonding occurred at elevated temperatures. This is supported by the geological setting and is related to the thermal “Mottled Zone” event in the area. At the border of the “Mottled Zone” organic associations with metakaolinite were identified in the thermally affected “oil shales”. The preservation of kaolinite and metakaolinite may be due to a shielding effect of the enveloping organic material.

The nature and possible origins of soluble salts in deeply weathered landscapes of eastern Australia

Extensive areas of salt-affected soils in eastern Australia occur on the exposed lower zones of lateritic profiles or in derived materials. Analyses of 96 samples of mottled and pallid zones collected in this study show that they generally contain appreciable soluble salts, predominantly sodium chloride, particularly where they developed in argillaceous sedimentary rocks which underlie about two-thirds of the area, and in some granitic rocks. Analyses of 96 samples of unweathered rocks indicate that some contain moderate to appreciable quantities of sodium and chloride, and it is suggested that these and other soluble ions accumulated in the lower zones as a result of deep weathering in the Mid-Tertiary. Atmospheric accessions of sea salts in rain or dry fallout at present rates account partly for the occurrence of salt-affected soil landscapes in some coastal areas but are unlikely sources of salts far inland. Soils formed in situ on weathered basalt, argillaceous sedimentary and granitic rocks which have been exposed at the surface since the Late Tertiary are not salt-affected, because climatic, topographic and hydrologic conditions are different from those of the Mid-Tertiary and only small volumes of rock are involved.

Corrigenda - The nature and possible origins of soluble salts in deeply weathered landscapes of eastern Australia

Extensive areas of salt-affected soils in eastern Australia occur on the exposed lower zones of lateritic profiles or in derived materials. Analyses of 96 samples of mottled and pallid zones collected in this study show that they generally contain appreciable soluble salts, predominantly sodium chloride, particularly where they developed in argillaceous sedimentary rocks which underlie about two-thirds of the area, and in some granitic rocks. Analyses of 96 samples of unweathered rocks indicate that some contain moderate to appreciable quantities of sodium and chloride, and it is suggested that these and other soluble ions accumulated in the lower zones as a result of deep weathering in the Mid-Tertiary. Atmospheric accessions of sea salts in rain or dry fallout at present rates account partly for the occurrence of salt-affected soil landscapes in some coastal areas but are unlikely sources of salts far inland. Soils formed in situ on weathered basalt, argillaceous sedimentary and granitic rocks which have been exposed at the surface since the Late Tertiary are not salt-affected, because climatic, topographic and hydrologic conditions are different from those of the Mid-Tertiary and only small volumes of rock are involved.

Morphology of Discontinuous Phase Plinthite and Criteria for its Field Identification in the Southeastern United States

AbstractPlinthite is defined in Soil Taxonomy as an iron‐rich material that hardens on repeated wetting and drying, especially when exposed to the sun. Many morphologically similar materials rich in iron oxides do not harden on repeated wetting and drying and are misidentified as plinthite. We propose that the following field‐tested criteria be used to distinguish plinthite from similar materials that will not harden and from material that has already irreversibly hardened. Plinthite has a color range from 10R to the 7.5 YR hues. It occurs as discrete bodies > 2 mm that can be separated from the matrix. It is firm when moist and hard to very hard when dry, yet it can be broken in the hands. A moist body of plinthite withstands moderate rolling between the thumb and forefinger, and either moist or air dry it will not slake when submerged in water even with periodic gentle agitation.Plinthite has platy and nodular forms. Platy plinthite bodies are red to yellowish red or strong brown about 1 cm thick and 2 to 4 cm long. They commmonly have a horizontal orientation. Nodular plinthite bodies have a similar color range but have an irregular to spherical shape. A horizon with about 10% platy plinthite will perch water. Horizons with nodular plinthite do not perch water, but the underlying reticulately mottled zone is a restrictive horizon that perches water. It is suggested that platy plinthite forms on level landscapes with a freely fluctuating water table. Nodular plinthite apparently forms on more sloping landscapes where lateral movement of water above a restrictive horizon is a contributing factor.

Oxygen isotope fractionation in decarbonation metamorphism: the Mottled Zone event

Calculated univariant equilibria and oxygen isotope compositions of silicates and carbonates support the proposal that the “Mottled Zone Event” is a low-pressure (1–25 atm), high-temperature (200° < T < 1300°C) metamorphism of calcareous siliceous sediments in which the thermal energy is provided by combustion of organic matter. δ 18O of silicates decreases systematically with increasing metamorphic grade from averages of 18.1‰ in protolith shales, to 16.6‰ in grossular-diopside-zeolite rocks, 15.6‰ in wollastonite and anorthite-diopside-gehlenite-grossular fels, 14.1‰ in spurrite-brownmillerite marbles and 11.7‰ in the highest-grade larnite-gehlenite-brownmillerite assemblages. Decarbonation is the principal mechanism influencing the oxygen isotope compositions. The progressive decrease of δ 18O in silicates can be modelled as a Rayleigh distillation of CO 2 approximately 16‰ enriched in 18O relative to whole rock assemblages i.e., of initial isotopic composition 8.5‰ heavier than the parent carbonates. The mineral assemblage of one sample with an unusual granoblastic texture is in apparent isotopic equilibrium at a temperature of 540°C.

Thermal Metamorphism by Combustion of Organic Matter: Isotopic and Petrological Evidence

New evidence is presented in support of the previously suggested hypothesis that combustion of organic matter in a Cretaceous bituminous sedimentary sequence supplied the heat for the high-temperature, low-pressure metamorphism of the Mottled Zone in Israel. Such a model is compatible with stable isotope data on carbonates from the Mottled Zone which are strongly enriched in and .