Mobilization and redistribution of major and trace elements in two weathering profiles developed on serpentinites in the Lomié ultramafic complex, South-East Cameroon

The behaviour of major and trace elements have been studied along two serpentinite weathering profiles located in the Kongo-Nkamouna and Mang North sites of the Lomié ultramafic complex.The serpentinites are characterized by high SiO₂ and MgO contents, very low trace, rare earth and platinum-group element contents. Lanthanide and PGE contents are higher in the Nkamouna sample than in Mang North. Normalized REE patterns according to the CI chondrites reveal that: (i) all REE are below chondrites abundances in the Mang North sample; (ii) the (La/Yb)_N ratio value is higher in the Nkamouna sample (23.72) than in the Mang one (1.78), this confirms the slightly more weathered nature of the Nkamouna sample. Normalized PGE patterns according to the same CI chondrites reveal a negative Pt anomaly in the Mang sample. The Nkamouna sample is characterized by a flat normalized PGE pattern.All element contents increase highly from the parent rock to the coarse saprolite.In the weathering profiles, Fe₂O₃ contents decrease from the bottom to the top contrarily to Al₂O₃, SiO₂ and TiO₂. The contents of alkali and alkaline oxides are under detection limit.Concerning trace elements, Cr, Ni, Co, Cu, Zn and Sc decrease considerably from the bottom to the top while Zr, Th, U, Be, Sb, Sn, W, Ta, Sr, Rb, Hf, Y, Li, Ga, Nb and Pb increase towards the clayey surface soil. Chromium, Ni and Co contents are high in the weathered materials in particular in the saprolite zone and in the nodules.REE contents are high in the weathered materials, particularly in Nkamouna. Their concentrations decrease along both profiles. Light REE are more abundant than heavy REE. Normalized REE patterns according to the parent rock reveal positive Ce anomalies in all the weathered materials and negative Eu anomalies only at the bottom of the coarse saprolite (Nkamouna site). Positive Ce anomalies are higher in the nodular horizon of both profiles. An additional calculation method of lanthanide anomalies, using NASC data, confirms positive Ce anomalies ([Ce/Ce^*]_NASC = 1.15 to 60.68) in several weathered materials except in nodules ([Ce/Ce^*]_NASC = 0.76) of the upper nodular horizon (Nkamouna profile). The (La/Yb)_N ratios values are lower in the Nkamouna profile than in Mang site.PGE are more abundant in the weathered materials than in the parent rock. The highest contents are obtained in the coarse saprolite and in the nodules. The elements with high contents along both profiles are Pt (63–70 ppb), Ru (49–52 ppb) and Ir (41 ppb). Normalized PGE patterns show positive Pt anomalies and negative Ru anomalies.The mass balance evaluation, using thorium as immobile element, reveals that:

– major elements have been depleted along the weathering profile, except for Fe, Mn and Ti that have been enriched even only in the coarse saprolite;

– all the trace elements have been depleted along both profiles, except for Cr, Co, Zn, Sc, Cu, Ba, Y, Ga, U and Nb that have been enriched in the coarse saprolite;

– rare earth elements have been abundantly accumulated in the coarse saprolite, before their depletion towards the top of the profiles;

– platinum-group elements have been abundantly accumulated in the coarse saprolite but have been depleted towards the clayey surface soil.

Moreover, from a pedogenetical point of view, this study shows that the weathering profiles are autochtonous, except in the upper part of the soils where some allochtonous materials are revealed by the presence of zircon grains.     

热带地区碳酸盐岩上覆红色风化壳的成因机理及元素演化

开展热带岩溶地区红色风化壳元素地球化学演化规律研究,有助于丰富碳酸盐岩风化成土理论的认识。在越南北部选取典型的碳酸盐岩风化剖面,分析热带气候条件下碳酸盐岩上覆红色风化壳中主量元素的物质来源和演化特征。结果表明:无论Ti/Zr的元素比值分析,Hf-Zr、Nb-Ta及Sm-Nd的元素对协变分析,还是上地壳元素平均值(UCC)标准化分析,两个剖面的上覆风化壳均显示原地残积的特征,即两个剖面是碳酸盐岩的原位风化产物。碳酸盐岩风化成土过程中,CaO、MgO淋失明显,Al_2O_3和Fe_2O_3发生富集,显示两个剖面均经历较强的风化过程,但是白云岩和灰岩剖面有一定的分异特征。越南北部白云岩风化剖面从基岩到上部土层显示出稳定的Ti-Fe元素共富集特征,而灰岩剖面中Fe的增长速率明显超过Ti。白云岩上覆风化壳经历了强烈的风化作用,其脱硅作用弱于灰岩风化剖面,而富集铝的作用强于灰岩。迁移系数的演化规律说明两个剖面中长石成分(钾长石、钠长石等)或次生矿物(伊利石等)在风化成土过程中逐渐分解,同时Al、Fe、Si等稳定元素的在风化剖面中不断富集。     

Lithogenic concentrations of trace metals in soils and saprolites over crystalline basement rocks: A case study from SW Nigeria

In this study, an assessment of the lithogenic concentrations of trace metals in soils and saprolite over basement rock units in Ibadan, SW-Nigeria is presented in respect of bedrock types and geochemical controls on the weathering-associated release of trace metals. Consequently, soil, weathered and fresh rock samples from the Precambrian Basement of SW Nigeria were collected from three different bedrock units within Ibadan metropolis and subjected to mineralogical and geochemical analyses. The analytical results revealed major proportions of oxides in the range of 18–20% Al₂O₃, 2–6% Na₂O and 1–6% K₂O for weathered profiles over granite-gneiss and pegmatite units, compared to 2–3% Al₂O₃, <0.5% Na₂O and <1.0% K₂O over schist-quartzite. For the trace elements, weathered profiles on granite-gneiss and schist-quartzite settings exhibit similar enrichment trends (enrichment factor, EF l) for most of the trace elements, unlike the pegmatite bedrock. However, enrichments are relatively greater in the top soil unit compared to the intermediate saprolite unit, especially for Pb, Ni, Zn, Cr, Co, Rb, Sr and Ba, a situation attributed to leaching and redistribution within the weathered profiles through pedogenetic process and percolating groundwater.Furthermore, the estimated weathering indices using Ruxton Ratio (RR = {SiO₂/Al₂O₃}) and Chemical Index of Alteration (CIA = 100{Al₂O₃/[Al₂O₃ + CaO + Na₂O + K₂O]}) revealed RR of 2.9–3.7 and CIA of 54–73% for granite-gneiss and pegmatite units, implying medium levels of weathering, compared to RR of 30.8–35.5 and CIA of >60% for schist-quartzite units, which suggest weak chemical weathering. Also, the estimated high percentage loss, especially for Pb, Rb, Sr, Ba relative to the bedrocks, shows that the trace elements can be mobilized within the weathering profiles even at a low degree of chemical weathering. Such weathering-induced release of trace metals is of environmental significance as natural lithogenic input sources and as background reference for future monitoring of possible human/anthropogenic impacts.     

Gold dispersion in a tropical rainforest weathering profile at Dondo Mobi, Gabon

At Dondo Mobi in the gold district of Eteke, south Gabon, gold mineralization was studied in a tropical forest setting. The mineralization occurs in quartz veins within amphibolites of an Archaean gneiss-amphibolite series. Gold distribution patterns were studied in the different horizons of the weathering profile and in the different grain-size fractions of the materials sampled from three pits sunk in the weathered zone: upslope (pit P1), directly overlying (pit P2) and downslope (pit P3) of the mineralization.The weathering profile consists of an upper, thick, loose sandy argillaceous horizon (H1), an irregular nodular horizon with laterite nodules or blocks (H2) and a saprolite layer (H3) up to 70 m thick. In each pit, the specific geochemical signature of the bedrock is recognized in the three horizons of the weathering profile. Some groups of elements, e.g., Cr-Ni or Ba-V-P₂O₃, characterize amphibolite or black shale, respectively. Signal attenuations in the upper horizon can be explained by a homogenization effect. That can be related to a mushroom supergene dispersion, simultaneously inducing vertical decrease and lateral enrichment in element contents. Gold also mushrooms extensively in the different horizons of the weathering profile. The dispersion pattern is somewhat anisotropic, a strong enlargement being observed in the upper (H1) and intermediate nodular (H2) horizons alike. However, the evolution of Au distribution is not the same for all grain-size fractions: (a) in the finest fraction, Au is regularly distributed in the weathering profile at the anomalous top and tends to preserve an equivalent level of concentration laterally; (b) in the coarsest fraction, the maximum Au content is found in the upper horizon just above the mineralized structure; it is rapidly decreases laterally, in the surface halo.The Au geochemical dispersion halo consists mainly of fine-grained gold developed from rather coarse-grained mineralization. Its characteristics suggest that Au dispersion was produced as a result of Au dissolution together with collapse of the weathering profile causing gold migration over short distances.The minimal volumetric reduction of saprolite in the upper horizon can be estimated to be in the range of 60%, on the basis of the content of the different grain-size fractions and the density variations in the various horizons.In exploration, geochemical techniques, even using wide sample spacing, are proving quite adequate to prospect for Au in tropical rainforest weathering environment. Indeed, significant, well-contrasted dispersion haloes facilitate detection of rather confined mineralization.     

滇东地区碳酸盐岩上覆风化层的元素地球化学特征及其成因讨论

碳酸盐岩风化形成的红土保存着喀斯特发展演化历史证据,同时也是喀斯特地区土壤研究的重要对象。文章选取云 南石林地区的两处典型碳酸盐岩剖面为研究对象,对主量元素,微量元素及稀土元素在风化层的迁移特征及分布规律进行 研究,为探究风化层的成因提供依据。结果显示：（1） 以Ti为参比元素的剖面迁移特征表明,两剖面的主量元素在成土过 程中有相似的迁移规律,多数表现为淋失;微量元素略有差异,富集淋失程度不一。（2） UCC 标准化蜘蛛图显示,相对于 基岩,风化层中的Ca和Sr均出现亏损;与UCC相比,Fe、Ti等元素轻微富集,Mg、Ca、Na、K、P等元素显示了强烈的亏 损特征。（3） 基岩与风化层的REE分布模式相似,但风化层的稀土相对富集,轻稀土元素间的分异较大而重稀土元素间的 分异较小,且SJC剖面的轻、重稀土元素比值大于QST剖面;稀土元素球粒陨石标准化后,SJC剖面的Eu为负异常,剖面 上部和下部出现Ce负异常;QST剖面Ce负异常,Eu明显负异常。（4） 元素含量变化和元素对Al-Ti、Al-Fe及Zr-Hf相关性 说明剖面上覆红土是下伏基岩风化的结果。研究结果显示,两个剖面的元素地球化学特征与基岩存在很好的继承性,风化 层是基岩原位风化的产物。     

Geochemistry of mineralization with exchangeable REY in the weathering crusts of granitic rocks in South China

Mineralization with exchangeable rare earth element (REE) and yttrium (MEX-REY) has been recognized in the weathering profiles in South China since the early 1970's. This type of REY mineralization occurs in weathering profiles of parent rocks ranging in composition from granite to acidic volcanic rocks and lamprophyre. The majority of the known resources occurring in the weathering profiles of granitic rocks. Total resources of this type of REY amount to millions tons of rare earth oxides, and therefore represent one of the most important types of rare earth resources in China, particularly for heavy rare earth elements (HREE) and yttrium.Accumulation of REY in the weathering profiles of granitic rocks is strongly controlled by the resistance to weathering of the principal REY-bearing accessory minerals in the parent rocks; only a limited proportion of total REY (< 30%) is incorporated in the rock-forming minerals. MEX-REY more commonly occur in weathering profiles developed on granitic rocks within which most of the REY are incorporated in accessory minerals weakly resistant to weathering (doverite, parisite, etc.). For the well-developed weathering profiles, three horizons can be distinguished from surface downwards: the lateritic horizon (A), the weathered horizon (B), and the weathering front (C). Continuous leaching, coupled with low rate of denudation, results in the accumulation of REY in the subsurface horizons (the B and C horizons), and thus results in REE differentiation within the well-developed, layered, and mature weathering profiles. Exchangeable REY, which can be replaced by cations like NH⁴⁺ and Na⁺ etc. in electrolyte solutions and can be removed by complexing agents such as EDTA, are commonly the major form of REE occurrence in the B horizon. Cerium is enriched in the top layer (A horizon) and depleted in the subsurface horizons of the weathering profiles, most likely due to the oxidation of Ce(III) to Ce(IV) followed by cerianite formation or absorption onto clays and/or Fe and Al oxyhydroxides.     

Regolith-landform processes and geochemical exploration for base metal deposits in regolith-dominated terrains of the Mt Isa region,northwest Queensland,Australia

The regolith in the Mt Isa region of Queensland consists of a variety of saprolites and duricrusts developed on Proterozoic basement rocks and fresh to weathered Mesozoic, Tertiary and Quaternary cover, all of which has impeded base metals exploration. This paper presents an overview of some of the regolith-geochemical work conducted in the Mt Isa region as part of an industry-supported three year CRC LEME/AMIRA Project. A complex weathering and landscape history has produced a landscape of (a) continuously exposed and exhumed basement rocks that have undergone varying intensities of weathering and partial stripping; (b) weathered and locally eroded Mesozoic cover sequences and (c) areas with younger transported cover concealing basement and Mesozoic cover. Various regolith sample media have been evaluated at a number of prospects and deposits which represent different regolith-landform terrains and landscape history. Geochemical dispersion processes and models are presented and false anomalies explained.Where ferruginous duricrust or ferruginous nodular gravel are preserved on weathered bedrock on an eroded plateau, they exhibit large (> 500 m) multi-element (As, Pb, Sb) dispersion haloes and are useful sampling media. Dispersion haloes in truncated profiles on weathered bedrock covered with colluvium are restricted, are limited to tens of metres from subcrop of the source, and contrast to the extensive anomalies in ferruginous duricrust and nodules. Geochemical exploration in covered areas depends on the possible presence of dispersion through the sediments or leakage along faults or fractures, but may be complicated by high metal backgrounds in the sediments themselves. Some of the most prominent anomalies occur in ferruginous materials and soils representing emergent residual terrain developed on Mesozoic sediments. These are largely due to weathering of sulfide mineralization that continued during submergence in a marine environment, with hydromorphic dispersion into the sediments as they accumulated. Multi-element (Cu, As, Zn, Sb, Au) anomalies occur in basal sediments and at the unconformity, due to a combination of clastic and hydromorphic dispersion and represent a useful sample target. Metal-rich horizons in weathered sediments, higher in the sequence, can also be targeted, particularly by specifically sampling ferruginous units and fragments. However, these are less certainly related to mineralization. Zinc and Cu, concentrated in Fe (and Mn) oxides at redox fronts, may be derived by leaching from the sediments with concentration in the sesquioxides, and be unrelated to any proximal basement mineralization. In all these regolith-dominated terrains, a clear understanding of local geomorphology, regolith framework, topography of unconformities and the origins of ferruginous materials is essential to sample medium selection and data interpretation.     

Geochemical behavior under tropical weathering of the Barama–Mazaruni greenstone belt at Omai gold mine,Guiana Shield

Mineralogical, petrographical, and geochemical studies of the weathering profile have been carried out at Omai Au mine, Guyana. The area is underlain by felsic to mafic volcanic and sedimentary rocks of the Barama-Mazaruni Supergroup, part of the Paleoproterozoic greenstone belts of the Guiana Shield. Tropical rainy climate has favoured extensive lateritization processes and formation of a deeply weathered regolith. The top of the weathering profile consists of lateritic gravel or is masked by the Pleistocene continental-deltaic Berbice Formation. Mineralogical composition of regolith consists mainly of kaolinite, goethite and quartz, and subordinately sericite, feldspar, hematite, pyrite, smectite, heavy minerals, and uncommon mineral phases (nacrite, ephesite, corrensite, guyanaite). A specific feature of the weathering profile at Omai is the preservation of fresh hydrothermal pyrite in the saprolith horizon. Chemical changes during the weathering processes depend on various physicochemical and structural parameters. Consequently, the depth should not be the principal criterion for comparison purposes of the geochemical behavior within the weathering profile, but rather an index that measures the degree of supergene alteration that has affected each analyzed sample, independently of the depth of sampling. Thus, the mineralogical index of alteration (MIA) can provide more accurate information about the behavior of major and trace elements in regolith as opposed to unweathered bedrock. It can also aid in establishing a quantitative relationship between intensity of weathering and mobility (leaching or accumulation) of each element in each analyzed sample. At Omai, some major and trace elements that are commonly considered as immobile (ex: TiO₂, Zr, etc.) during weathering could become mobile in several rock types and cannot be used to calculate the mass and volume balance. In addition, due to higher “immobile element” ratios, the weathered felsic volcanic rocks plotted in identification diagrams are shifted towards more mafic rock types and a negative adjustment of ∼20 units is necessary for correct classification. In contrast, these elements could aid in defining the material source in sedimentary rocks affected by weathering. Generally, the rare-earth element (REE) patterns of the bedrock are preserved in the saprolith horizon. This can represent a potentially useful tool for geochemical exploration in tropical terrains. Strong negative Ce and Tb anomalies are displayed by weathered pillowed andesites, which are explained by the influence of the water/rock ratio.     

Surface geochemical and biogeochemical expression of base‐metal mineralization at Woodlawn,New South Wales,Australia

The presence of base‐metal mineralization at Woodlawn was first recognised early in 1968 when a roadside reconnaissance geochemical sampling survey, conducted over felsic volcanic rocks in the Goulburn‐Tarago area, encountered anomalous B horizon soils containing up to 200 ppm Cu, 800 ppm Pb and 300 ppm Zn. Regional soil thresholds have been determined at 50 ppm Cu, 90 ppm Pb and 50 ppm Zn. Chip samples from the subsequently located gossan revealed up to 2000 ppm Cu, 8000 ppm Pb and 2000 ppm Zn, 500 ppm Sn, 25 ppm Ag and 3000 ppm As.

The first grid B horizon soil geochemical survey was conducted in 1968 over the gossan and surrounding area, and repeated with closer spaced sampling in the first half of 1970. The first survey delineated strong Cu (to 1000 ppm) and Pb (to 2500 ppm) anomalies coincident with the gossan zone, and intense hydromorphic zinc anomalies (to 3000 ppm) located down slope from the gossan in residual clay‐soils derived from dolerite bedrock. Threshold values have been determined at 140 ppm Cu, 700 ppm Pb and 580 ppm Zn. Ag and Sn in B horizon soils show pronounced anomalies coincident with the gossan and are suitable metals for geochemical target definition. Of fourteen trace elements determined in 1974 from B and C horizon soils on two lines across the ore zone Cu, Pb, Zn, Se, Ba, Sn and Ag show direct correlation with the mineralization, whereas Cd and Mn show moderate hydromorphic dispersion, having accumulated principally in clay soils derived from dolerite weathering. As, Sb and Bi, whilst responding over the ore zone, show elevated values in soils over hanging‐wall units; Ni and Co show maximum levels in soils over dolerite bedrock.

Bark and leaves of Acacia mearnsii, collected from a line across the gossan, contain anomalous levels of Cu, Pb, Zn, Sn and Ti near the ore zone, and weaker, but clearly anomalous Mn and Ni levels over dolerite bedrock. Both bark and leaves of Acacia mearnsii reflect the presence of concealed mineralization. The shrub Solanum linearifolium grows preferentially over and close to the Woodlawn ore zone, where it contains up to 840 ppm Cu, 250 ppm Pb, 7300 ppm Zn, 6 ppm Sn and 250 ppm Ti in leaf ash compared with levels of 200 ppm Cu, 2 ppm Pb, 400 ppm Zn, 0.8 ppm Sn and 60 ppm Ti in plants growing 1.5 km from the ore zone. This shrub has potential as an indicator of base‐metal mineralization.     

Palaeoweathering characteristics of an intrabasaltic red bole of the Deccan Flood Basalts near Shrivardhan of western coast of India

An intrabasaltic red bole horizon is studied for its weathering characteristics with respect to the underlying and overlying basalts. The study indicates that all the three units have been considerably weathered; the red bole unit, however shows some distinctive characteristics. The red boles show a higher cation exchange capacity (CEC) and lower sodium adsorption ratio (SAR) and organic carbon (OC) as compared to the weathered basalts. The lower values of Al₂O₃, TiO₂ and Fe₂O₃(T) in red boles indicate their lesser weathering than the underlying and overlying basalts, which is further corroborated by the weathering intensity measured by the indices like chemical index of alteration (CIA) and statistical empirical index of chemical weathering (W). It is also evident that the red bole samples show more retention of original mafic and felsic components. While K₂O exhibits an erratic behaviour, the MgO and CaO do not show much leaching in red boles. Lesser leaching and salinity in the red boles is indicated by the higher values of calcification and lower values of salinization. The SiO₂–Al₂O₃–Fe₂O₃ plot indicates that red bole samples are close to the basalt field, while the weathered upper basalt is more kaolinized than the weathered lower basalt. These observations reveal that the post-formational weathering processes have least affected the original palaeoweathering characters of the red bole horizon and hence the intrabasaltic palaeosols (weathering horizons) can effectively be used to constrain the palaeoweathering and palaeoclimates during the continental flood basalt episodes in the geologic past.     

Diffuse magnetic anomalies in marginal basins: Their possible tectonic and petrologic significance

Marginal basins, areas of oceanic lithosphere peripheral to large ocean basins, may be formed by several processes, but the young active marginal basins have the geophysical and geochemical characteristics of young normal oceanic lithosphere. We recognize two distinct tectonic settings in which new oceanic lithosphere may be formed in areas which would be termed marginal basins:

1. (1) Upwelling of fractional melts of mantle material from the region above subducted lithospheric slabs leads to the generation of new oceanic lithosphere behind island arcs. The general case for this tectonic setting involves random location of magma leaks and does not produce correlatable magnetic anomalies. In special cases, an orthogonal ridge—transform system may duplicate the magnetic patterns found on ocean-basin crust.

2. (2) The second tectonic setting develops on very long “leaky” transform faults separating spreading ridges. In areas where the transform has dislocated a block of continental crust, or an island arc, the map view of the resulting marginal basin may resemble the setting of a basin behind an active island arc. However, the “leaky” transform setting is unrelated to active plate convergence or to Benioff zones.

At “normal” ridge-crests, and possibly in some marginal basins, basalt is erupted on long linear magma leaks and rapid cooling forms thick lithosphere with correlatable linear magnetic anomalies. Some marginal basins have high thermal flux, spread slowly and may have thick sediment cover. Slow cooling, numerous point-source magma leaks and extensive hydrothermal alteration diminish magnetic intensities and cause diffuse magnetic patterns. The correlation problems caused by diffuse magnetic anomalies make interpretations of spreading rates and directions in young marginal basins a difficult, if not futile, task.It is likely that fragments of marginal-basin lithosphere form some of the ophiolite complexes; their recognition is critical to paleo-tectonic interpretations. The geochemical characteristics of marginal-basin basalts do not appear to be useful criteria for distinguishing them from ocean-ridge basalts. However, the abundance of short ridges and seamounts in many young marginal basins suggests that an abundance of seamount material, as well as differentiated volcanic and plutonic rocks, in ophiolites may be an indication of derivation from marginal-basin lithosphere.     

Geochemistry of red residua underlying dolomites in karst terrains of Yunnan-Guizhou Plateau: II. The mobility of rare earth elements during weathering

The aim of this study is to characterize the evolution of the rare earth elements (REE) in the Pingba red residua on karst terrain of Yunnan-Guizhou Plateau. The in-situ weathering and the two-stage development of the profile had been inferred from REE criterions. The REE were significantly fractionated, and Ce was less mobilized and separated from the other REEs at the highly enriched top of the profile. This is consistent with the increase of oxidation degree in the regolith. And it is also suggested that the wet/dry climate change during chemical weathering caused Ce alternative change between enrichment and invariance in the upper regolith. Chondrite-normalized REE distribution patterns for samples from dolomites and the lower regolith are characteristic of MREE enrichment and remarkable negative Ce-anomalies patterns (similar to the convex-up REE patterns). The following processes are interpreted for the patterns in this study: (1) the accumulation of MRRE-rich minerals in dolomite dissolution, (2) water–rock interaction in the weathering front, and (3) more leaching MREE from the upper part of the profile. The latter two explanations are considered as the dominant process for the formation of the REE patterns. Samples from the soil horizon exhibit typical REE distribution patterns of the upper crust, i.e., La_N/Yb_N=10 and Eu/Eu*=0.65. All data indicate that the leaching process is very important for pedogenesis in this region. The experiments demonstrating that abnormal enrichment of REE at the upper regolith–bedrock interface is caused by a combination of volume change, accumulation of REE-bearing minerals, leaching of REE from the upper regolith, and water–rock interaction during rock–soil alteration processes. Our results support the conclusion that the weathering profile represents a large, continental elemental storage reservoir, whereas REE enrichment occurs under favorable conditions in terms of stable tectonics, low erosion and rapid weathering over sufficiently long time.     

Seasonality of bedrock weathering chemistry and CO2 consumption in a small watershed, the White River, Vermont

The draw down of CO₂ from the atmosphere during mineral weathering plays a major role in the global budget of this greenhouse gas. Silicate minerals remove twice the CO₂ of carbonate minerals per mole of calcium in runoff during weathering. Bedrock weathering chemistry was investigated in the White River watershed of northeastern USA to investigate whether there are seasonal differences in carbonate and silicate weathering chemistry. Geographic Information Systems analyses of bedrock geology were combined with major element concentrations in river waters to gain an understanding of the consistency of mineral weathering during three seasons. The percent of carbonate mineralogy comprising the bedrock in tributaries of the White River varied from less than 5% to 45% by area. A mass balance calculation using major element concentrations in waters was applied to estimate the seasonal relationships between bedrock geology and bicarbonate flux. In all tributaries and the main stem of the White River the highest calculated percent of bicarbonate from carbonate mineral weathering was measured in the late fall. The results suggest that carbonate and silicate bedrock weathering processes are seasonally controlled. Thus single season sampling could not accurately represent an entire year's geochemical budget. In the White River, water samples obtained solely during the summer would consistently underestimate the total yearly source of bicarbonate from carbonate bedrock weathering. The same sample set would also provide data that would lead to an underestimation of the yearly atmospheric CO₂ draw down by bedrock weathering in the watershed. For example at four of the seven locations studied there was an almost two-fold difference between summer and spring calculated atmospheric CO₂ consumption rates.     

Electrogeochemical patterns in surface soils - Detection of blind mineralization beneath exotic cover, Thalanga, Queensland, Australia

The problem of using surface geochemical exploration techniques in areas of very thick and electrically conductive weathering residuum is common to much of Australia. At the Elura deposit (New South Wales) a distinct electrogeochemical H⁺ anomaly can be detected in the top few cm of residual soil above about 100 m of conductive residual overburden. In the present paper the results of an investigation of the much more difficult problem of detecting sulfide mineralization beneath thick conductive transported overburden are described.The objective of the study was to demonstrate that sulfide mineralization beneath thick transported overburden can be detected by geochemical patterns in surface soils in the context of an electrogeochemical model of dispersion.The Thalanga massive sulfide deposit in northeast Queensland has at least 4 million tonnes of 15% combined Zn, Pb and Cu. The mineralized horizon lies at the contact between rhyolitic and dacitic rocks of the Cambro-Ordovician Mt. Windsor Volcanics. The deposit is covered by transported cemented Tertiary terrigenous clayey sandstones and grits; these are electrically conductive and vary in thickness from 0 to 70 m.Near-surface soil samples were collected along five traverses normal to the strike of mineralization. The traverses were located to give 0 m, 1 m, 30 m, 50 m, and 70 m overburden thicknesses; there is no known significant mineralization along the last traverse which is assumed to be background, and there is a small gossan where the overburden is absent.Dispersion patterns influenced by electrogeochemical processes should result in relatively low values for ions over massive sulfides with lateral peaks; this has been termed a “rabbit-ear” anomaly. “Rabbit-ear” anomalies in surface soils for H⁺, Cu, and Zn occur over the sulfide zone. The H⁺ pattern is better defined where there is a significant depth of overburden (where the anomaly is about 500 m wide). The Cu anomaly is 300–600 m wide, and the Zn anomaly is 450–675 m wide.Even where the overburden is 50 m thick, anomalous “rabbit-ear” anomalies for H⁺ and Zn are clearly identifiable, but the anomaly for Cu is a single peak of 20 ppm over the hanging wall. It is suggested that the results of this work convincingly demonstrate that at Thalanga surface soil samples may reliably be used to detect massive sulfide deposits - even where they are effectively blind beneath a considerable thickness of transported and conductive overburden. The processes of dispersion are speculated to be diffusion, and it is argued that the pattern-controlling mechanism is electrochemical.     

Geochemistry and mineralogy of saprolite in Finnish Lapland

An ancient saprolite has developed on the Palaeoproterozoic granulite, granite gneiss and amphibolite bedrock of the Vuotso–Tankavaara area of central Finnish Lapland. The present day climatic regime in Finnish Lapland lies within the northern boreal zone and so the saprolite there can be regarded as fossil. Cores of saprolite were collected from 4 sections (42 samples) and analyzed chemically and mineralogically. In the study area, progressive weathering of the rocks has been marked by gradual enrichment in Al, Fe and Ti; and depletion of Na, K and Ca. The higher concentration of Fe(III) and water and reduced Na and Ca in weathered bedrock in the 4 sections are indicative of oxidation, hydration and leaching processes involved during weathering. The primary minerals in the saprolite are plagioclase feldspar, K-feldspar, quartz, garnet (almandine) and hornblende; the common secondary minerals are kaolinite, halloysite, and vermiculite in addition to minor amounts of sericite. Intense weathering is indicated by: (1) the presence of kaolinite and halloysite in 4 sections of different bedrock types, and (2) the comparatively lower SiO₂/Al₂O₃ (wt.%) ratio (2.30) of weathered granulites (3 sections) as compared to fresh granulite (4.33) and that of weathered amphibolite (2.68) as compared to fresh amphibolite (3.56). In general, kaolinite and halloysite have formed through the weathering of feldspars, garnet, and biotite. Vermiculite is the most probable alteration product of biotite. The formation of kaolinite and halloysite in Finnish Lapland indicates wetter and warmer climatic conditions during the time of their formation than at present. The possible time for formation of the saprolite is early Cretaceous–early Tertiary into Middle Miocene.     

The hydromagnesite playas of Atlin, British Columbia, Canada: A biogeochemical model for CO2 sequestration

Anthropogenic greenhouse gas emissions may be offset by sequestering carbon dioxide (CO₂) through the carbonation of magnesium silicate minerals to form magnesium carbonate minerals. The hydromagnesite [Mg₅(CO₃)₄(OH)₂·4H₂O] playas of Atlin, British Columbia, Canada provide a natural model to examine mineral carbonation on a watershed scale. At near surface conditions, CO₂ is biogeochemically sequestered by microorganisms that are involved in weathering of bedrock and precipitation of carbonate minerals. The purpose of this study was to characterize the weathering regime in a groundwater recharge zone and the depositional environments in the playas in the context of a biogeochemical model for CO₂ sequestration with emphasis on microbial processes that accelerate mineral carbonation.Regions with ultramafic bedrock, such as Atlin, represent the best potential sources of feedstocks for mineral carbonation. Elemental compositions of a soil profile show significant depletion of MgO and enrichment of SiO₂ in comparison to underlying ultramafic parent material. Polished serpentinite cubes were placed in the organic horizon of a coniferous forest soil in a groundwater recharge zone for three years. Upon retrieval, the cube surfaces, as seen using scanning electron microscopy, had been colonized by bacteria that were associated with surface pitting. Degradation of organic matter in the soil produced chelating agents and acids that contributed to the chemical weathering of the serpentinite and would be expected to have a similar effect on the magnesium-rich bedrock at Atlin. Stable carbon isotopes of groundwater from a well, situated near a wetland in the southeastern playa, indicate that  12% of the dissolved inorganic carbon has a modern origin from soil CO₂.The mineralogy and isotope geochemistry of the hydromagnesite playas suggest that there are three distinct depositional environments: (1) the wetland, characterized by biologically-aided precipitation of carbonate minerals from waters concentrated by evaporation, (2) isolated wetland sections that lead to the formation of consolidated aragonite sediments, and (3) the emerged grassland environment where evaporation produces mounds of hydromagnesite. Examination of sediments within the southeastern playa–wetland suggests that cyanobacteria, sulphate reducing bacteria, and diatoms aid in producing favourable geochemical conditions for precipitation of carbonate minerals.The Atlin site, as a biogeochemical model, has implications for creating carbon sinks that utilize passive microbial, geochemical and physical processes that aid in mineral carbonation of magnesium silicates. These processes could be exploited for the purposes of CO₂ sequestration by creating conditions similar to those of the Atlin site in environments, artificial or natural, where the precipitation of magnesium carbonates would be suitable. Given the vast quantities of Mg-rich bedrock that exist throughout the world, this study has significant implications for reducing atmospheric CO₂ concentrations and combating global climate change.     

Geochemistry of trace and rare earth elements during weathering of black shale profiles in Northeast Chongqing,Southwestern China: Their mobilization,redistribution, and fractionation

In this study, the mobilization, redistribution, and fractionation of trace and rare earth elements (REE) during chemical weathering in mid-ridge (A), near mountaintop (B), and valley (C) profiles (weak, weak to moderate, and moderate to intense chemical weathering stage, respectively), are characterized. Among the trace elements, U and V were depleted in the regolith in all three profiles, Sr, Nb, Ta, Zr, and Hf displayed slight gains or losses, and Th, Rb, Cs, and Sc remained immobile. Mn, Ba, Zn, Cu, and Cr were enriched at the regolith in profiles A and B, but depleted in profile C. Mn, Pb, and Co were also depleted in the saprock and fractured shale zones in profiles A and B and enriched in profile C. REEs were enriched in the regolith and depleted at the saprock zone in profiles A and B and depleted along profile C. Mobility of trace and REEs increased with increasing weathering intensity. Normalized REE patterns based on the parent shale revealed light REE (LREE) enrichment, middle REE (MREE), and heavy REE (HREE) depletion patterns. LREEs were less mobile compared with MREEs and HREEs, and this differentiation increased with increasing weathering degree. Positive Ce anomalies were higher in profile C than in profiles A and B. The Ce fractionated from other REE showed that Ce changed from trivalent to tetravalent (as CeO₂) under oxidizing conditions. Minimal REE fractionation was observed in the saprock zone in profiles A and B. In contrast, more intense weathering in profile C resulted in preferential retention of LREE (especially Ce), leading to considerable LREE/MREE and LREE/HREE fractionation. (La/Yb)_N and (La/Sm)_N ratios displayed maximum values in the saprock zone within low pH values. Findings demonstrate that acidic solutions can mobilize REEs and result in leaching of REEs out of the highly acidic portions of the saprock material and transport downward into fractured shale. The overall behavior of elements in the three profiles suggests that solution pH, as well as the presence of primary and secondary minerals, play important roles in the mobilization and redistribution of trace elements and REEs during black shale chemical weathering.     

Laboratory and field evidence in support of the electrogeochemically enhanced migration of ions through glaciolacustrine sediment

Ionic aureoles, overlying or contiguous to massive sulphide occurrences, are postulated to have been emplaced as a result of natural galvanic forces. The occurrences of these galvanic forces, most commonly referred to as the self-potential or spontaneous polarization phenomena, are well documented, but ionic migration in response to these electrical forces remained to be proven conclusively.The purpose of this study was to attempt to document ionic migration attributable solely to electrogeochemical transport. Pleistocene glaciolacustrine sediments rich in clay-size particles was the geological substrate chosen to minimize migration processes other than those of electrogeochemical transport.Laboratory experiments which simulated a field situation and utilized the radioisotope ⁶⁵Zn as a tracer, revealed a readily detectable diffusion of Zn²⁺ into glaciolacustrine clay; the distance of penetration of the Zn²⁺ being significantly increased by the application of a DC electrical potential, similar in magnitude to a natural self-potential. The rate of diffusion was greatest around the margins of the cathodic electrode located at the base of the clay layers; the areas shown to have the greatest current density. The resulting pattern of Zn penetration into the clay, when viewed in a vertical section, showed a double peaked increase in Zn concentration on either side, and a distinct decrease in Zn penetration directly above this cathode. Extrapolation of the average diffusion coefficient found for Zn²⁺ (D₀ = 6.3 × 10⁻⁹cm s⁻¹) to other ions commonly associated with base metal deposits, revealed that only H⁺ could be expected to move through appreciable thicknesses (>20 m) of clay in the 8000 years since sediment deposition.The hypothesis that H⁺ would diffuse through varved clay, and that the resulting pattern would be predictably modified by a self-potential field generated by an oxidizing sulphide body, was tested over the Magusi River volcanogenic massive sulphide deposit near Noranda, Quebec. The organic soil horizon (H) and top of the clay soil layer (C) were sampled and analyzed for conductivity, pH, and other major and minor elements. The mineral horizon revealed distinct changes in conductivity, pH, Ca, Mg, and Fe above the contacts of the hanging wall and footwall of the sulphide horizon, thus producing a double-peaked “rabbit-ear” anomalous pattern. The H soil horizon, when corrections for variable “total” element and carbon concentrations were applied, showed clear anomalies in Fe and other pH sensitive elements. A ratio of EDTA Fe/TOTAL Fe to total organic carbon revealed the best “rabbit-ear” anomaly above the sulphide horizoon. The use of organic horizon geochemistry as a method of evaluating EM conductors overlain by water-saturated lacustrine sediments is recommended.     

“Lag” — a geochemical sampling medium for arid regions

Desert land surfaces are commonly characterized by a veneer or pavement of siliceous and/or ferruginous stony material. This material can be shown in most cases to be predominantly bedrock-derived, despite often severe modification by prolonged weathering, and can therefore be selectively sampled and analyzed as an indicator of bedrock geochemistry in geochemical surveys.Most pavements probably owe their existence to the interaction of several dispersion mechanisms. However, a common factor in all cases is the concentration of coarse particles at the surface as a result of the selective removal by erosion of fine dilutant material. Hence, the preferred name when used in the geochemical context is “lag”.In lag sampling, particles in the range 2.0–6.0 mm are screened on site from the unconsolidated surface material. Material in this size range has been found to be quite uniformly distributed over a wide range of arid region environments, including areas where residual soils are severely diluted by transported alluvial and aeolian materials.Data from exploration programmes for Au, Cu-Pb-Zn-Ba, and Ni allow comparison of results for lag sampling with those for alternative sample media in a variety of arid region environments. Analysis of lag samples for Au, Cu and As clearly indicates the presence of bedrock Au mineralization in the Paterson and Eastern Goldfields Provinces of Western Australia. In these areas both lags and soils exhibit good anomaly contrast, but lags show more extensive lateral dispersion, leading to advantages in reconnaissance exploration.Strong anomalies for Ni and Cu are developed in lags, compared with subdued response in fine-fraction soils over a Ni sulphide occurrence in the Eastern Goldfields which has been subjected to deep lateritic weathering.Lag geochemistry also clearly reflects sub-economic base metal and barite occurrences in the McArthur Basin, N.T., in spite of the dilution of surface soils by sands probably related to a Mesozoic marine incursion. Orientation sampling over a Pb-Zn prospect in the Pine Creek Geosyncline has demonstrated optimum response in lag samples compared with various size fractions of the associated lithosols.Variable dilution of lag samples by coarse quartz sand can be a problem in areas with substantial transported overburden. A simple procedure to ‘correct’ trace-element values using regression analysis based on the Fe content of samples is described as a means of reducing ‘noise’ resulting from such matrix variations.     

Oxygen and carbon isotope composition of structural carbonate in weathering apatites from laterites, southern Brazil and western Senegal

The oxygen (δ¹⁸O_c) and carbon (δ¹³C_c) isotope compositions of the structural carbonate group (CO₃) in apatites from lateritic profiles were investigated. The weathering profiles, located in southern Brazil and in western Senegal, are developed on three different types of apatite-rich parent rock: carbonatite, metamorphosed marine phosphorite and sedimentary marine phosphorite. The parent rock apatites are of magmatic, hydrothermal, metamorphic and sedimentary origins. The in situ formation of apatite of weathering origin in the profiles is well documented petrographically and geochemically.The overall range of measured δ¹⁸O_c and δ¹³C_c values of apatites of weathering origin (22 to 27 SMOW for δ¹⁸O_c and −15 to −10 PDB for δ¹³C_c) is much smaller than the range of measured and/or published isotope compositions of parent rock apatites (4–35 for δ¹⁸O_c and −11 to +1 for δ¹³C_c). In any profile, the apatites of weathering origin can exhibit lower, similar or higher δ¹⁸O_c values than parent rock apatites. In contrast, their δ¹³C_c values are systematically and significantly lower than those of the parent rock apatites. Apatites formed as a result of weathering in laterites can therefore be readily distinguished from apatites of other origin on the basis of their isotope composition.Assuming that apatite CO₃ fractionates O in a way similar to calcite CO₃, the structural carbonate group of the apatites of weathering origin appears to form in approximate isotopic equilibrium with the weathering solutions. The very low δ¹³C_c values exhibited by these apatites indicate that the dominant sources of dissolved CO₂ in the soil water are organic. The isotope composition of structural carbonate in apatite of weathering origin in lateritic profiles may provide useful information for paleoenvironmental studies.