The effect of weathering on the grain-size distribution of red soils in south-eastern China and its climatic implications

Particle-size analysis is a useful way to determine the source and deposition of sediments. However, there are inconsistencies when this method is used to constrain the origin of the red soils in south-eastern China. To address this problem, we performed a detailed grain-size analysis of two red soil sequences in Xuancheng and Qiliting located along the lower reaches of the Yangtze River. By comparing their particle-size characteristics with those of the loess on the Loess Plateau in northern China, we found that the aeolian samples plot in a particular zone in the C–M (grain size of the cumulative 1% versus median grain size) plot and cluster nearer the lower left corner of the plot as their degree of weathering increases. The grain-size features suggest that the onset of large-scale aeolian deposition along the lower reaches of the Yangtze River occurred at approximately 0.8 Ma. Although both sedimentary sorting and post-depositional weathering control the grain-size variations in the deposits, the extremely strong weathering due to the humid, warm climate along the lower reaches of the Yangtze River primarily modified the grain-size distributions of the primary red soil deposits. Strong weathering increased the very fine silt (2–5 μm) fraction and decreased the coarse (>63 μm) fraction. We also found that certain grain-size parameters of the red soils varied with the weathering intensity, which can be used as indicators of palaeoclimate variations. The grain size variations in both the Qiliting and Xuancheng sequences suggest that the mid-Pleistocene climate transition (MPT) may have affected the lower reaches of the Yangtze River at 0.9 Ma.     

The relationship between fluid flow and mineral weathering in heterogeneous unsaturated porous media: A physical and geochemical characterization of a waste-rock pile

The relationships between factors that control subsurface flow and the timing, duration, and intensity of acidity generation and leaching of metals from waste-rock dumps are investigated. A 12 m high waste-rock pile that had been constructed in 1994 at Key Lake, Saskatchewan, Canada was disassembled, sampled and characterized in 2000. Physical properties that control water flow were characterized by measuring soil–water suction, volumetric water content, and the grain-size distribution at 60 randomized sites within the pile. Grain-size distribution was also measured at an additional 20 grid locations within the pile. Paste pH, pore-water geochemistry, mineralogy, and water-soluble extractions were used to investigate geochemical processes and sulfide oxidation at each of the 20 grid locations. A field-based soil–water characteristic curve could not be developed from the spatially variable and hysteretic field data; consequently, the grain-size distribution was used as a relative measure of subsurface flow and of the tendency to contain water under unsaturated conditions. The geochemical characterization demonstrated that marcasite underwent preferential weathering relative to pyrite and chalcopyrite, that dolomite was the main buffering carbonate mineral, and that gypsum, jarosite, and Fe oxyhydroxides were the main secondary (supergene) minerals. The pore waters contained up to 78,000 mg L⁻¹ SO₄, 690 mg L⁻¹ Ni and 1400 mg L⁻¹ U (800, 11.7 and 6 mM, respectively), suggesting that significant weathering had occurred. The pore water chemistry varied considerably between sampling sites. However, neither a correlation of pore-water chemistry with grain-size distribution nor a spatial relationship within the sampled grid was discernible.     

Effects of temperature on silicate weathering: Solute fluxes and chemical weathering in a temperate rain forest watershed,Jamieson Creek,British Columbia

Chemical weathering of silicate minerals has long been known as a sink for atmospheric CO₂, and feedbacks between weathering and climate are believed to affect global climate. While warmer temperatures are believed to increase rates of weathering, weathering in cool climates can be accelerated by increased mineral exposure due to mechanical weathering by ice. In this study, chemical weathering of silicate minerals is investigated in a small temperate watershed. The Jamieson Creek watershed is covered by mature coniferous forest and receives high annual precipitation (4000 mm), mostly in the form of rainfall, and is underlain by quartz diorite bedrock and glacial till. Analysis of pore water concentration gradients indicates that weathering in hydraulically unsaturated ablation till is dominated by dissolution of plagioclase and hornblende. However, a watershed scale solute mass balance indicates high relative fluxes of K and Ca, indicating preferential leaching of these solutes possibly from the relatively unweathered lodgement till. Weathering rates for plagioclase and hornblende calculated from a watershed scale solute mass balance are similar in magnitude to rates determined using pore water concentration gradients.When compared to the Rio Icacos basin in Puerto Rico, a pristine tropical watershed with similar annual precipitation and bedrock, but with dissimilar regolith properties, fluxes of weathering products in stream discharge from the warmer site are 1.8 to 16.2-fold higher, respectively, and regolith profile-averaged plagioclase weathering rates are 3.8 to 9.0-fold higher. This suggests that the Arrhenius effect, which predicts a 3.5- to 9-fold increase in the dissolution rate of plagioclase as temperature is increased from 3.4° to 22 °C, may explain the greater weathering fluxes and rates at the Rio Icacos site. However, more modest differences in K and Ca fluxes between the two sites are attributed to accelerated leaching of those solutes from glacial till at Jamieson Creek. Our findings suggest that under conditions of high rainfall and favorable topography, weathering rates of silicate minerals in warm tropical systems will tend to be higher than in cool temperate systems, even if the temperate system is has been perturbed by an episode of glaciation that deposits regolith high in fresh mineral surface area.     

Contrasting mineralogical and processing potential of two mineralization types in the platinum group element and Ni-bearing Kapalagulu Intrusion,western Tanzania

The Kapalagulu layered ultramafic and mafic intrusion is emplaced between the Paleoproterozoic Ubendian basement and overlying Neoproterozoic Itiaso Group metasedimentary rocks, located near the western shore of Lake Tanganyika. High-grade platinum group element (PGE) mineralization (1–6 g/t Pt + Pd + Au) is associated with chromitite and sulfide-bearing harzburgite within the southeastern extension of the intrusion, known as the Lubalisi Zone, which is covered by a layer of nickel-rich (0.2–2%Ni) laterite regolith that contains linear areas of PGE mineralization.In the Lubalisi Zone, the mineralization may be divided into several significant geometallurgical domains: (a) high-grade PGE mineralization (1–6 g/t Pt + Pd + Au) associated with stratiform PGE reefs and chromitite seams within a harzburgite unit; (b) high-grade PGE mineralization (up to 12 g/t Pt + Pd + Au) associated with small bodies and veins of nickel massive sulfide within harzburgite below PGE-bearing reefs and chromitite seams; (c) low-grade PGE mineralization (0.1–0.5 g/t Pt + Pd + Au) associated with a sulfide-mineralized harzburgite unit above the PGE-bearing reefs; (d) laterite style residual PGE mineralization (0.2–4 g/t Pt + Pd + Au) associated with chromite concentrations in the saprolite and overlying red clay horizons of the laterite regolith; and (e) supergene Ni associated with the saprock and overlying saprolite clay.Mineralogical study of three samples from the PGE reef consisting of high grade PGE chromitite and harzburgite indicate that this mineralization will give a good metallurgical response to conventional grinding and floatation due to the relatively coarse-grained nature of the PGM (P80 from ∼37 to 52 µm), association with base metal sulfides, and unaltered gangue minerals (Wilhelmij and Cabri, 2016). In contrast, mineralogical and metallurgical study of the Ni and PGE mineralized laterite indicate that it cannot be processed using conventional mineral processing techniques but that a hydrometallurgical route should be used to recover the base and precious metals. Because any process is very much deposit-controlled, significant metallurgical and geometallurgical testing of mineralized samples, as well as pilot plant testing, will be required to arrive at feasibility studies.     

Clay mineral variations in Holocene terrestrial sediments from the Indus Basin

We employed X-ray diffraction methods to quantify clay mineral assemblages in the Indus Delta and flood plains since ~ 14 ka, spanning a period of strong climatic change. Assemblages are dominated by smectite and illite, with minor chlorite and kaolinite. Delta sediments integrate clays from across the basin and show increasing smectite input between 13 and 7.5 ka, indicating stronger chemical weathering as the summer monsoon intensified. Changes in clay mineralogy postdate changes in climate by 5–3 ka, reflecting the time needed for new clay minerals to form and be transported to the delta. Samples from the flood plains in Punjab show evidence for increased chemical weathering towards the top of the sections (6–< 4 ka), counter to the trend in the delta, at a time of monsoon weakening. Clay mineral assemblages within sandy flood-plain sediment have higher smectite/(illite + chlorite) values than interbedded mudstones, suggestive of either stronger weathering or more sediment reworking since the Mid Holocene. We show that marine records are not always good proxies for weathering across the entire flood plain. Nonetheless, the delta record likely represents the most reliable record of basin-wide weathering response to climate change.     

Hydrogeochemistry in the Vouga River basin (central Portugal): Pollution and chemical weathering

To quantify and explain the contributions by pollution and chemical weathering to their composition, we studied the chemistries of springs and surface waters in the mountainous part of the Vouga River basin. Water samples were collected during a number of consecutive summer campaigns. Recharge rates were derived from monitored discharge rates within the basin. Very large contributions by meteoric, agricultural and domestic sources to the water chemistries were found, identified by the chloride, sulfate and nitrate concentrations: on average only 1/4 to 1/3 of the solutes could be attributed to chemical weathering. Two petrologic units characterize the river basin: granites and metasediments. The waters collected within metasediment units are distinct from those in granite terrain by a higher magnesium concentration. On that basis, it could be estimated that the Rio Vouga, when leaving the mountainous part of the basin, has for some 2/5 a signature determined by chemical weathering in the metasediments. The dominant primary minerals subject to chemical weathering are plagioclase (Pl) and biotite (in granite) or Pl and chlorite (in metasediment). Kaolinite, gibbsite and vermiculite are the major weathering products where annual precipitation (P) > 1000 mm y⁻¹, and kaolinite, vermiculite and smectite where P was lower. Using an algorithm based on the ratio of dissolved silica to bicarbonate, the contributions of chemical weathering of primary minerals could be unraveled. The results show that in granite the export rate (as mol ha⁻¹ y⁻¹ wt%mineral⁻¹) of oligoclase (Pl with An_10–30) was 5.0 ± 2.6 and of biotite 3.2 ± 2.6, while in metasediment these rates for albite (Pl with An_0–10) are 16.5 ± 8.9 and for chlorite are 0.5 ± 0.5. The observed decrease of dissolved silica in surface waters relative to springs was ascribed to (summer) uptake by aquatic biota.     

Use of Sr isotopes as a tool to decipher the soil weathering processes in a tropical river catchment,southwestern India

River water composition (major ion and ⁸⁷Sr/⁸⁶Sr ratio) was monitored on a monthly basis over a period of three years from a mountainous river (Nethravati River) of southwestern India. The total dissolved solid (TDS) concentration is relatively low (46 mg L⁻¹) with silica being the dominant contributor. The basin is characterised by lower dissolved Sr concentration (avg. 150 nmol L⁻¹), with radiogenic ⁸⁷Sr/⁸⁶Sr isotopic ratios (avg. 0.72041 at outlet). The composition of Sr and ⁸⁷Sr/⁸⁶Sr and their correlation with silicate derived cations in the river basin reveal that their dominant source is from the radiogenic silicate rock minerals. Their composition in the stream is controlled by a combination of physical and chemical weathering occurring in the basin. The molar ratio of SiO₂/Ca and ⁸⁷Sr/⁸⁶Sr isotopic ratio show strong seasonal variation in the river water, i.e., low SiO₂/Ca ratio with radiogenic isotopes during non-monsoon and higher SiO₂/Ca with less radiogenic isotopes during monsoon season. Whereas, the seasonal variation of Rb/Sr ratio in the stream water is not significant suggesting that change in the mineral phase being involved in the weathering reaction could be unlikely for the observed molar SiO₂/Ca and ⁸⁷Sr/⁸⁶Sr isotope variation in river water. Therefore, the shift in the stream water chemical composition could be attributed to contribution of ground water which is in contact with the bedrock (weathering front) during non-monsoon and weathering of secondary soil minerals in the regolith layer during monsoon. The secondary soil mineral weathering leads to limited silicate cation and enhanced silica fluxes in the Nethravati river basin.     

Significance of weathering and regolith/landscape evolution for mineral exploration in the NE Albany-Fraser Orogen,Western Australia

The Albany-Fraser Orogen (AFO), southeast Western Australia, is an underexplored, deeply weathered regolith-dominated terrain that has undergone complex weathering associated with various superimposed climatic events. For effective geochemical exploration in the AFO, integrating landscape evolution with mineralogical and geochemical variations of regolith and bedrock provides fundamental understanding of mechanical and hydromorphic dispersion of ore and pathfinder elements associated with the different weathering processes.In the Neale tenement, northeast of the AFO, a residual weathering profile that is 20-55 m thick was developed under warm and humid climatic conditions over undulating Proterozoic sheared granitoids, gneisses, schists and Au-bearing mafic rocks. From the base, the typical weathering profile consists of saprock, lower ferruginous saprolite, upper kaolinitic saprolite and discontinuous silcrete duricrust or its laterally coeval lateritic residuum. These types of duricrusts change laterally into areas of poorly-cemented kaolinitic grits or loose lateritic pisoliths and nodules.Lateritic residuum probably formed on remnant plateaus and was transported mechanically under arid climatic conditions over short distances, filling valleys to the southeast. Erosion of lateritic residuum exposes the underlying saprolite and, together with dilution by aeolian sands, constitutes the transported overburden (2-25 m thick). The reworked lateritic materials cover the preserved silcrete duricrusts in valleys. The lower ferruginous saprolite and lateritic residuum are well developed over mafic and sulphide-bearing bedrocks, where weathering of ferromagnesian minerals and sulphides led to enrichment of Fe, Cu, Ni, Cr, Co, V and Zn in these units. Kaolinitic saprolite and the overlying pedogenic silcrete are best developed over alkali granites and quartzofeldspathic gneisses, which are barren in Au and transition elements, and enriched in silica, alumina, rare earth and high field strength elements.A residual Au anomaly is formed in the lower ferruginous saprolite above a Au -bearing mafic intrusion at the Hercules prospect, south of the Neale tenement, without any expression in the overlying soil (< 20 cm). Conversely, a Au anomaly is recorded in the transported cover, particularly in the uppermost 3 m at the Atlantis prospect, 5 km southwest of the Hercules prospect. No anomalies have been detected in soils using five different size fractions (> 2,000 μm, 2,000-250 μm, 250-53 μm, 53-2 μm and < 2 μm). Therefore, soil cannot be efficiently applied as a reliable sampling medium to target mineralization at the Neale tenement. This is because mechanical weathering was interrupted by seasonal periods of intensive leaching under the present-day surface conditions and/or dilution by recently deposited aeolian sediments which obscure any signature of a potential Au anomaly in soils. Therefore, surface soil sampling should extend deeper than 20 cm to avoid dilution by aeolian sands and seasonal leaching processes. Regolith mapping and the distinction between the residual and transported weathering products are extremely significant to follow the distal or proximal mineralization.     

Petrological and geochemical characteristics of Zhaibei granites in Nanling region,Southeast China: Implications for REE mineralization

Mineralization with ion adsorption rare earth elements (REEs) in the weathering profile of granitoid rocks from Nanling region of Southeast China is an important REE resource, especially for heavy REE (HREE) and Y. However, the Jurassic granites in Zhaibei which host the ion adsorption light REE (LREE) ores are rare. It is of peraluminous and high K calc-alkaline composition, which has similar geochemical features of high K₂O + Na₂O and Zr + Nb + Ce + Y contents and Ga/Al ratio to A-type granite. Based on the chemical discrimination criteria of Eby [Geology 20 (1992) 641], the Zhaibei granite belongs to A1-type and has similar source to ocean island basalts. The rock is enriched in LREE and contains abundant REE minerals including LREE-phosphates and halides. Minor LREE was also determined in the feldspar and biotite, which shows negligible and negative Eu anomalies, respectively. This indicates that the Zhaibei granite was generated by extreme differentiation of basaltic parent magmas. In contrast, granites associated with ion adsorption HREE ores contain amounts of HREE minerals, and show similar geochemical characteristics with fractionated felsic granites. Note that most Jurassic granitoids in the Nanling region contain no REE minerals and cannot produce REE mineralization. They belong to unfractionated M-, I- and S-type granites. Therefore, accumulation of REE in the weathering profile is controlled by primary REE mineral compositions in the granitoids. Intense fractional crystallization plays a role on REE enrichment in the Nanling granitoid rocks.     

Mineralogy of high-field-strength elements (Y,Nb, REE) in the world-class Vergenoeg fluorite deposit,South Africa

The Vergenoeg fluorite deposit in the Bushveld Complex in South Africa is hosted by a volcanic pipe-like body. The distribution characteristics, composition and formation conditions of high-field-strength element (HFSE)-rich minerals in different lithological units of the deposit were investigated by optical and cathodoluminescence microscopy, scanning electron microscopy, X-ray fluorescence, inductively-coupled plasma mass-spectrometry and electron-probe microanalysis. The Vergenoeg host rocks comprise a diverse silica-undersaturated assemblage of fayalite–magnetite–fluorite with variably subordinate apatite and mineral phases enriched in rare-earth elements (REEs). The Sm–Nd isotope systematics of the fluorite from the various lithological units of the pipe support the model that the HFSE budget of the Vergenoeg pipe was likely derived from a Lebowa-type granitic magma. Isotopically, there is no evidence for other REE sources. Formation of the pipe, including development of the fluorite mineralization, occurred within the same time frame as the emplacement of other magmatic rock units of the Bushveld Complex (Sm–Nd isochron age for fluorite separates: 2040 ± 46 Ma). Hydrothermal alteration is manifested in strongly disturbed Rb–Sr isotope systematics of the Vergenoeg deposit, but did not affect its HFSE and REE budget. Whole-rock chondrite-normalized REE + Y distribution patterns of two types were observed: (i) flat patterns characteristic of magnetite–fluorite unit, gossan, metallurgical-grade fluorite (“metspar”) plugs and siderite lenses, and (ii) U-shaped patterns showing enrichment towards the heaviest REE (Tm–Lu) observed in the fayalite-rich units. Common HFSE minerals are complex Nb-rich oxides (samarskite, fergusonite), REE phosphates and fluorocarbonates. Additionally, fluocerite and REE silicates, whose identification requires further work, were found. Most of the HFSE-rich minerals are spatially associated with Fe-rich phases (e.g., pyrite, magnetite, greenalite and hematite). To a smaller extent, they are found finely disseminated or healing micro-fractures in fluorite. The whole-rock REE + Y distribution patterns of the individual lithological units are mainly controlled by the distribution of Yb-rich and Y-rich xenotime in these rocks. The common occurrence of bastnäsite-(Ce) in the gossan, “metspar” plugs and especially in the rhyolitic carapace at the pipe–wall-rock contact, controls the REE + Y distribution patterns of these rocks. HFSE minerals in the Vergenoeg pipe rocks have formed in several stages. Samarskite and coarse fluorapatite belong to the primary mineral assemblage. Fergusonite and Yb-rich xenotime formed during high- to moderate-temperature hydrothermal activity. Significant remobilization of the HFSE from the early-crystallized minerals (breakdown of fluorapatite and possibly allanite with release of REE + Y) and subsequent partial redistribution of these elements into near surface rocks are inferred. The late-stage assemblages are characterized by the presence of fine-grained REE fluorocarbonates, monazite-(Ce), monazite-(La) and xenotime-(Y).     

Phosphorus and iron cycling in deep saprolite,Luquillo Mountains,Puerto Rico

Rapid weathering and erosion rates in mountainous tropical watersheds lead to highly variable soil and saprolite thicknesses which in turn impact nutrient fluxes and biological populations. In the Luquillo Mountains of Puerto Rico, a 5-m thick saprolite contains high microorganism densities at the surface and at depth overlying bedrock. We test the hypotheses that the organisms at depth are limited by the availability of two nutrients, P and Fe. Many tropical soils are P-limited, rather than N-limited, and dissolution of apatite is the dominant source of P. We document patterns of apatite weathering and of bioavailable Fe derived from the weathering of primary minerals hornblende and biotite in cores augered to 7.5 m on a ridgetop as compared to spheroidally weathering bedrock sampled in a nearby roadcut.Iron isotopic compositions of 0.5 N HCl extracts of soil and saprolite range from about δ⁵⁶Fe = 0 to ? 0.1‰ throughout the saprolite except at the surface and at 5 m depth where δ⁵⁶Fe = ? 0.26 to ? 0.64‰. The enrichment of light isotopes in HCl-extractable Fe in the soil and at the saprolite–bedrock interface is consistent with active Fe cycling and consistent with the locations of high cell densities and Fe(II)-oxidizing bacteria, identified previously. To evaluate the potential P-limitation of Fe-cycling bacteria in the profile, solid-state concentrations of P were measured as a function of depth in the soil, saprolite, and weathering bedrock. Weathering apatite crystals were examined in thin sections and an apatite dissolution rate of 6.8 × 10^? 14 mol m^? 2 s^? 1 was calculated. While surface communities depend on recycled nutrients and atmospheric inputs, deep communities survive primarily on nutrients released by the weathering bedrock and thus are tightly coupled to processes related to saprolite formation including mineral weathering. While low available P may limit microbial activity within the middle saprolite, fluxes of P from apatite weathering should be sufficient to support robust growth of microorganisms in the deep saprolite.     

Identification of rock weathering and environmental control in arid catchments (northern Xinjiang) of Central Asia

Chemical weathering is an integral part of the earth surface processes, whose spatial patterns and controlling factors on continental scale are still not fully understood. Highlands of the Asian continent have been shown having some of the highest observed rates of chemical weathering yet reported. However, the paucity of river gauge data in many of these terrains has limited determination of chemical weathering budget in a continental scale. A dataset of three large watersheds throughout northern Xinjiang in Central Asia is used to empirically identify chemical weathering regimes and interpret the underlying controlling factors. Detailed analysis of major ion ratios and a forward model of mass budget procedure are presented to distinguish the relative significances and contributions of silicate, carbonate weathering and evaporite dissolution. The analytical results show that carbonic acid is the most important weathering agent to the studied watersheds. Silicate weathering contributes, on average, ∼17.8% (molar basis) of total cations on a basin wide scale with an order of Zhungarer > Erlqis > Yili, indicating that silicate weathering, however, does not seem to be intense in the study basins. Evaporite dissolution, carbonate weathering and precipitation input contribute 43.6%, 29.7% and 8.9% of the total dissolved cations on average for the whole catchment, respectively. The three main morphological and hydrological units are reflected in water chemistry. Rivers from the montane areas (recharge area) of the three watersheds are very dilute, dominated by carbonate and silicate weathering, whereas the rivers of piedmont areas as well as the rivers of the sedimentary platform (runoff area) are dominated by carbonate weathering, and rivers of desert plain in the central Zhungarer basin (discharge area) are dominated by evaporite dissolution and are SO₄ rich. This spatial pattern indicates that, beside lithology, runoff conditions have significant role on the regional chemical weathering regimes. Chemical weathering processes in the areas appear to be significantly climate controlled, displaying a tight correlation with runoff and aridity. Carbonate weathering are mostly influenced by runoff, which is higher in the mountainous part of the studied basins. The identification of chemical weathering regimes from our study confirmed the weathering potential and complexity of temperate watersheds in arid environment and that additional studies of these terrains are warranted. However, because the dominant weathering reactions in the sedimentary platform of northern Xinjiang are of carbonates and evaporites rather than silicate minerals, and the climatic factors have important role on the rock weathering regimes, we think that weathering at the arid temperate drainage system (Central Asia) is maybe not an important long-term sink for atmospheric CO₂, if the future climate has no great change.     

Genesis of REE minerals in the karstic bauxite in western Guangxi,China, and its constraints on the deposit formation conditions

Karstic bauxites in western Guangxi, China, comprise two subtypes: Permian bauxite and Quaternary bauxite. The Quaternary bauxite originated from the breaking up, rolling, and accumulating of Permian bauxite in karstic depressions in Quaternary. Various types of rare earth element (REE) minerals were discovered during the formation of the Permian and Quaternary bauxites from the Xinxu, Longhe, and Tianyang bauxite deposits in this study. Five types of REE minerals, including bastnäsite, parisite, cerianite, rhabdophane, and churchite, were identified. Bastnäsite and parisite are the most abundant, and they are widely developed in the Permian ore and also present in the Quaternary ore. Obvious variations in bastnäsite and parisite REE compositions were observed, which is ascribed to distinctions in the source materials in the primary weathering profile from different areas. The mode of occurrence of bastnäsite and parisite suggests they were mainly precipitated under alkaline and reducing conditions during the Permian bauxite-forming stage and underwent intensive corrosion in the Quaternary. Churchite was formed during the Permian weathering stage under acidic condition. Both cerianite and rhabdophane occur in fractures within the Permian bauxite ore, indicating that both formed during the Quaternary weathering stage. It is considered that the rhabdophane enriched in Ce have formed locally, in the process of that the Ce^3 +, released from bastnäsite rapidly, entered the rhabdophane lattice before being oxidized to Ce^4 +. Cerianite was mainly found in association with Mn–Al hydroxides, suggesting that the released Ce^3 + was oxidized into Ce^4 + and precipitated cerianite in fractures within the Permian bauxite ore. Mass balance equations reveal a depletion in nearly all REEs during the transformation from the Permian to the Quaternary bauxite ore, mainly caused by the dissolution of bastnäsite and parisite. The genesis of the REE minerals, together with the occurrence of other minerals, indicates that intensively acidic and oxidizing conditions developed before the formation of the Permian bauxite ore. Towards the end of the Permian, the conditions became reducing and alkaline, favorable for the large-scale bauxitization. The Quaternary bauxite-forming stage was characterized by variable pH and Eh conditions, with acidic (pH = 4–6) and oxidizing (Eh > 2) conditions at the surface of the exposed Permian bauxite ore.     

Geochemical and mineralogical characteristics of weathered ore in the Dalucao REE deposit,Mianning–Dechang REE Belt,western Sichuan Province,southwestern China

The Dalucao deposit in western Sichuan Province, southwest China, is one of the largest and most extensive rare earth element (REE) deposits in the Himalayan Mianning–Dechang REE Belt. Moreover, this is the only deposit identified in the southern part of the belt. The deposit contains the No. 1, 2, and 3 orebodies. The No. 1 and 3 orebodies are hosted in two breccia pipes within syenite–carbonatite rocks that intrude a Proterozoic quartz–diorite pluton. Both breccia pipes have elliptical horizontal cross-sections at the surface, being 200–400 m long, 180–200 m wide, and extending to > 450 m depth. The No. 1 and No. 3 orebodies have total thicknesses of 55–175 m and 14–58 m, respectively. REE mineralization is associated with four brecciation events that are recorded in both pipes. The ore grades in the No. 1 and 3 orebodies are similar, with the rocks containing 1.0–4.5% rare earth oxides (REOs). The No. 1 orebody is characterized by a mineral assemblage comprising fluorite + barite + celestite + bastnäsite (i.e., Type I), whereas the No. 3 orebody is characterized by an assemblage comprising fluorite + celestite + pyrite + muscovite + bastnäsite + strontianite (i.e., Type II). Significant amounts of weathered high-grade REE ore (up to 60 wt.% of the rock mass) is mainly present in the No. 1 orebody. This is the main ore-type targeted for exploration within the Dalucao deposit, but is rarely present in other deposits in the Mianning–Dechang REE Belt.Faulting and cryptoexplosive breccia events, possibly linked to movement on the Panxi Fault, were more common in the No. 1 orebody than in the No. 3 orebody. This facilitated the introduction of ore-forming hydrothermal fluids and provided space for the precipitation of REE minerals. Based on the present results, we infer that the Dalucao deposit was the product of multiple stages of ore formation. REE minerals formed in envelopes around, or fractures within, quartz, fluorite, calcite, barite, and celestite in the brecciated ores. The main REE minerals were deposited from hydrothermal fluids within cryptoexplosive breccia, followed by weathering that increased the ore grade. Petrographic studies and X-ray powder diffraction (XRD) analyses indicate that the weathered ore contains 5–60% REE minerals (including bastnäsite, parisite, and monazite), together with gangue (quartz, barite, celestite, and fluorite), large amount of clay minerals (smectite, illite, kaolinite, and sepiolite), and relict igneous minerals (quartz, albite, and K-feldspar). The weathered samples are strongly enriched in La (up to 92,390 ppm), Ce (up to 103,500 ppm), Pr (up to 8006 ppm), and Nd (up to 16,690 ppm) compared with the unweathered brecciated ores. Conversely, Sr concentrations are significantly more enriched in the brecciated ores (up to 256,500 ppm) than in the weathered ores (generally less than 2671 ppm with one exception of 37,850 ppm) due to less celestite. Calcite is largely absent from the weathered ores (except one sample with up to 30% mode), which contrasts with the brecciated ores that contain up to 75% calcite. The effects of weathering, oxidation, loss of ions, and hydration on the brecciated ores led to the refertilization of the REEs and an increase in the grade of the ore deposit.     

Development of unusual rock weathering features in the Cordillera Blanca,Peru

Mylonite textures in granodiorite boulders are responsible for higher rates of surface denudation of host rocks and the progressive development of unusual rock weathering features, termed weathering posts. These textures are characterized by smaller grain sizes, higher biotite content, and a higher biotite axial ratio in host rocks relative to weathering posts. Elemental concentrations do not show a significant difference between weathering posts and the host rocks in which they are found, and this reflects the absence of a weathering residue on the rock surfaces. Chemical weathering loosens the bonds between mineral grains through the expansion of biotite, and the loosened grains fall off or are blown off the boulder surface and continue their chemical alteration in the surrounding soil. The height of weathering posts on late Quaternary moraines increases at a linear rate of ~ 1.45 ± 0.45 cm (1000 yr)^? 1 until post heights reach the diameter of host rocks. Such a rate of boulder denudation, if unrecognized, would generate significant errors (> 20%) in cosmogenic exposure ages for Pleistocene moraines. Given the paucity of boulders with diameters that significantly exceed 1.5 m, the maximum age of utility of weathering posts as a numeric age indicator is ~ 100 ka.     

The effects of climate and landscape position on chemical denudation and mineral transformation in the Santa Catalina mountain critical zone observatory

Understanding the interactions of climate, physical erosion, chemical weathering and pedogenic processes is essential when considering the evolution of critical zone systems. Interactions among these components are particularly important to predicting how semiarid landscapes will respond to forecasted changes in precipitation and temperature under future climate change. The primary goal of this study was to understand how climate and landscape structure interact to control chemical denudation and mineral transformation across a range of semiarid ecosystems in southern Arizona. The research was conducted along the steep environmental gradient encompassed by the Santa Catalina Mountains Critical Zone Observatory (SCM-CZO). The gradient is dominated by granitic parent materials and spans significant range in both mean annual temperature (>10 °C) and precipitation (>50 cm a^?1), with concomitant shift in vegetation communities from desert scrub to mixed conifer forest. Regolith profiles were sampled from divergent and convergent landscape positions in five different ecosystems to quantify how climate-landscape position interactions control regolith development. Regolith development was quantified as depth to paralithic contact and degree of chemical weathering and mineral transformation using a combination of quantitative and semi-quantitative X-ray diffraction (XRD) analyses of bulk soils and specific particle size classes. Depth to paralithic contact was found to increase systematically with elevation for divergent positions at approximately 28 cm per 1000 m elevation, but varied inconsistently for convergent positions. The relative differences in depth between convergent and divergent landscape positions was greatest at the low and high elevation sites and is hypothesized to be a product of changes in physical erosion rates across the gradient. Quartz/Plagioclase (Q/P) ratios were used as a general proxy for bulk regolith chemical denudation. Q/P was generally higher in divergent landscape positions compared to the adjacent convergent hollows. Convergent landscape positions appear to be collecting solute-rich soil–waters from divergent positions thereby inhibiting chemical denudation. Clay mineral assemblage of the low elevation sites was dominated by smectite and partially dehydrated halloysite whereas vermiculite and kaolinite were predominant in the high elevation sites. The increased depth to paralithic contact, chemical denudation and mineral transformation are likely functions of greater water availability and increased primary productivity. Landscape position within a given ecosystem exerts strong control on chemical denudation as a result of the redistribution of water and solutes across the landscape surface. The combined data from this research demonstrates a strong interactive control of climate, landscape position and erosion on the development of soil and regolith.     

An experimental model approach of biologically-assisted silicate dissolution with olivine and Escherichia coli – Impact on chemical weathering of mafic rocks and atmospheric CO2 drawdown

Chemical weathering of Mg, Ca-silicates and alumino-silicates contributes significantly to the drawdown of atmospheric CO₂ over long time scales. The present work focuses on how this mode of weathering may change in the presence of free-living bacteria in oligotrophic waters, which compose most of the surface freshwaters of the Earth. Forsterite (Fo90) was reacted for 1 week with a stable Escherichia coli population in water maintained at 37 °C and neutral pH in a batch reactor. Control samples with suspensions of pure olivine powders and E. coli cells in pure water were also used for reference. Olivine controls reproduce the Mg, Si and Fe release in solutions predicted from rates published in the literature with pH shifts of less than 0.5 unit. After 1 week, under abiotic conditions, weathered surfaces are enriched in Fe and Fe³⁺ relative to the initial composition of the mineral. Bacterial controls (without minerals) show decreasing Eh with increasing cell concentrations (−50 mV with 7 × 10⁷ cells/mL and −160 mV with 8 × 10⁸ cells/mL). Magnesium concentrations in bacterial control solutions are in the μg/L range and can be accounted for by the release of Mg from dead cells. More than 80% of the cells were still alive after 1 week. The solutions obtained in the experiments in which olivine reacts in the presence of cells show Mg and Si concentrations a few tens of percent lower than in the mineral control samples, with a prominent depletion of Fe(III) content of the mineral surfaces. Magnesium mass balance discounts both significant bacterial uptake and inhibition of the Mg dissolution rates as a consequence of changing pH and Eh. Coating by bacterial cell layers is also negligible. E. coli reduces the chemical weathering of olivine. This study infers that the presence of free-living Proteobacteria, a prevalent group of subsurface bacteria, should decrease the amount of riverine Mg released by chemical weathering of mafic rocks.     

Geochemistry of S,Cu, Ni,Cr and Au-PGE in the garnet amphibolites from the Akom II area in the Archaean Congo Craton,Southern Cameroon

The fresh and weathered garnet amphibolites, from the Akom II area in the Archaean Congo Craton, were investigated to determine the S, Cu, Ni, Cr, and Au-PGE values. The garnet amphibolites are composed of amphibole, plagioclase, garnet, quartz, and accessory apatite, spinel, sericite, pyrite, chalcopyrite and non-identified opaque minerals. The presence of apatite, sericite, and two generations of opaque minerals suggests that they might be affected by hydrothermal alteration. They are characterized by moderate Al₂O₃, Fe₂O₃, CaO, V, Zn, and Co contents with negative Eu- and Ce-anomalies. The sulfur concentrations are variable (380–1710 ppm). According to the sulfur contents, amphibolites can be grouped into two: amphibolites with low contents, ranging between 380 and 520 ppm (av. = 457 ppm); and amphibolites with elevated contents, varying from 1140 to 1710 ppm (av. = 1370 ppm). Amphibolites contain contrast amounts of Cu (∼ 1800 to 5350 ppm) while nickel contents attain 121 ppm. Chromium contents vary from 43 to 194 ppm. Sulfur correlates positively with Cu and Cr, but negatively with Ni and Ni/Cr ratio. The total Au-PGE contents attain 59 ppb.The presence of amphibole and feldspars confirms the low degree of amphibolite weathering. The secondary minerals are constituted of kaolinite, gibbsite, goethite and hematite. Despite the accumulation of some elements, the major and trace element distribution is quite similar to that of fresh amphibolites. Nevertheless, the weathering processes lead to the depletion of several elements such as S (239–902 ppm), Cu (520–2082 ppm), and Ni (20–114 ppm). Chromium and Au-PGE show an opposite trend marked by a slight enrichment in the weathered amphibolites. Amidst the Au-PGE, Pd (60 ppb) and Pt (23 ppb) have elevated contents in the fresh rocks as well as in the weathered materials. The PPGE contents are much higher than IPGE contents in both types of materials. The Pd/Pt, Pd/Rh, Pd/Ru, Pd/Ir, Pd/Os, and Pd/Au values indicate that Pt, Rh, Ru, Ir, Os and Au are more mobile than Pd. Chondrite-normalized base metal patterns confirm the abundance of Pd and the slight enrichment of Au-PGE in weathered rocks. Palladium, Rh and Ir are positively correlated with S. Conversely Pt and Ru are negatively correlated with S and Au is not correlated with S. Despite the high and variable S and Cu contents, the garnet amphibolites possess low Au-PGE and other base metals contents.     

Biogeochemical weathering of serpentinites: An examination of incipient dissolution affecting serpentine soil formation

Serpentinite rocks, high in Mg and trace elements including Ni, Cr, Cd, Co, Cu, and Mn and low in nutrients such as Ca, K, and P, form serpentine soils with similar chemical properties resulting in chemically extreme environments for the biota that grow upon them. The impact of parent material on soil characteristics is most important in young soils, and therefore the incipient weathering of serpentinite rock likely has a strong effect on the development of serpentine soils and ecosystems. Additionally, porosity generation is a crucial process in converting rock into a soil that can support vegetation. Here, the important factors affecting the incipient weathering of serpentinite rock are examined at two sites in the Klamath Mountains, California. Serpentinite-derived soils and serpentinite rock cores were collected in depth profiles from each sampling location. Mineral dissolution in weathered serpentinite samples, determined by scanning electron microscopy, energy dispersive spectrometry, electron microprobe analyses, and synchrotron microXRD, is consistent with the order, from most weathered to least weathered: Fe-rich pyroxene > antigorite > Mg-rich lizardite > Al-rich lizardite. These results suggest that the initial porosity formation within serpentinite rock, impacting the formation of serpentine soil on which vegetation can exist, is strongly affected both by the presence of non-serpentine primary minerals as well as the composition of the serpentine minerals. In particular, the presence of ferrous Fe appears to contribute to greater dissolution, whereas the presence of Al within the parent rock appears to contribute to greater stability. Iron-oxidizing bacteria present at the soil–rock interface have been shown in previous studies to contribute to the transition from rock to soil, and soils and rock cores in this study were therefore tested for iron-oxidizing bacteria. The detection of biological iron oxidation in this study indicates that the early alteration of these Fe-rich minerals may be mediated by iron-oxidizing bacteria. These findings help provide insight into the incipient processes affecting serpentinite rock weathering, important to the development of extreme serpentine soils and the biota that grow on them.     

Trace-element composition and zoning in clinopyroxene- and amphibole-group minerals: Implications for element partitioning and evolution of carbonatites

The present work is a first comprehensive study of the trace-element composition and zoning in clinopyroxene- and amphibole-group minerals from carbonatites, incorporating samples from 14 localities worldwide (Afrikanda, Aley, Alnö, Blue River, Eden Lake, Huayangchuan, Murun, Oka, Ozernaya Varaka, Ozernyi, Paint Lake, Pinghe, Prairie Lake, Turiy Mys). The new electron-microprobe data presented here significantly extend the known compositional range of clinopyroxenes and amphiboles from carbonatites. These data confirm that calcic and sodic clinopyroxenes from carbonatites are not separated by a compositional gap, instead forming an arcuate trend from nearly pure diopside through intermediate aegirine–augite compositions confined to a limited range of CaFeSi₂O₆ contents (15–45 mol%) to aegirine with < 25 mol% of CaMgSi₂O₆ and a negligible proportion of CaFeSi₂O₆. A large set of LA-ICPMS data shows that the clinopyroxenes of different composition are characterized by relatively low levels of Cr, Co and Ni (≤ 40 ppm) and manifold variations in the concentration of trivalent lithophile and some incompatible elements (1–150 ppm Sc, 26–6870 ppm V, 5–550 ppm Sr, 90–2360 ppm Zr, and nil to 150 ppm REE), recorded in some cases within a single crystal. The relative contribution of clinopyroxenes to the whole-rock Rb, Nb, Ta, Th and U budget is negligible. The major-element compositional range of amphiboles spans from alkali- and Al-poor members (tremolite) to Na–Al-rich Mg- or, less commonly, Fe-dominant members (magnesiohastingsite, hastingsite and pargasite), to calcic–sodic, sodic and potassic–sodic compositions intermediate between magnesio-ferrikatophorite, richterite, magnesioriebeckite, ferri-nyböite and (potassic-)magnesio-arfvedsonite. In comparison with the clinopyroxenes, the amphiboles contain similar levels of tetravalent high-field-strength elements (Ti, Zr and Hf) and compatible transition elements (Cr, Co and Ni), but are capable of incorporating much higher concentrations of Sc and incompatible elements (up to 500 ppm Sc, 43 ppm Rb, 1470 ppm Sr, 1230 ppm Ba, 80 ppm Pb, 1070 ppm REE, 140 ppm Y, and 180 ppm Nb). In some carbonatites, amphiboles contribute as much as 25% of the Zr + Hf, 15% of the Sr and 35% of the Rb + Ba whole-rock budget. Both clinopyroxenes and amphiboles may also host a significant share (~ 10%) of the bulk heavy-REE content. Our trace-element data show that the partitioning of REE between clinopyroxene (and, in some samples, amphibole) and the melt is clearly bimodal and requires a revision of the existing models assuming single-site REE partitioning. Clinopyroxenes and amphiboles from carbonatites exhibit a diversity of zoning patterns that cannot be explained exclusively on the basis of crystal chemistry and relative compatibility of different trace-element in these minerals. Paragenetic analysis indicates that in most cases, the observed zoning patterns develop in response to removal of selected trace elements by phases co-precipitating with clinopyroxene and amphibole (especially magnetite, fluorapatite, phlogopite and pyrochlore). With the exception of magnesiohastingsite–richterite sample from Afrikanda, the invariability of trace-element ratios in the majority of zoned clinopyroxene and amphibole crystals implies that fluids are not involved in the development of zoning in these minerals. The implications of the new trace-element data for mineral exploration targeting REE, Nb and other types of carbonatite-hosted rare-metal mineralization are discussed.