Immobile Element Ratios Research Articles

Clastic rock geochemistry of the Sirohi Basin, Aravalli Craton, NW India: Implications for paleoweathering, provenance and geodynamics

• Petrographic and geochemical data of clastic rock formations in the Sirohi Basin. • Two wacke horizons with distinct textural and chemical characteristics. • Geological evolution of the Sirohi Basin and relation with geodynamics of the Indian Shield. Sirohi Basin of Trans-Aravalli Region, Rajasthan preserves a thick clastic sequence composed of phyllite and metasandstone. Based on their petrochemical properties, metasandstone is classified as K-feldspar rich wacke (Wacke I) and plagioclase rich wacke (Wacke II). Wacke I is characterised by higher K 2 O, Fe 2 O 3 and lower CaO, Na 2 O, and Na 2 O/K 2 O ratios than Wacke II. Chemical Index of Alteration (CIA) and the A-CN-K systematic prescribe least chemical weathering for the sediments of Wacke II, moderate for Wacke I and moderately intense for phyllite of the Sirohi Basin. Chemical traits perceived from UCC spidergrams, discrimination function diagrams, etc. in conjunction with immobile element ratios (e.g., Al 2 O 3 /TiO 2 , Th/Sc, La/Sc, La/Co, Th/Co, Eu/Eu*, La/Sm N and Gd/Yb N ) establish dual-source terrains for Sirohi clastics, one with preponderance of felsic componets and other mafic plus felsic ingrediatns. The petrochemical signatures of Sirohi clastics, though suggest procurement of detritus form diffrent sources but consanguineous evolution in the Sirohi Basin, in response to extensional stresses. :

Origin, transport, and diagenesis of tuffs in organic-rich lacustrine mudstone: An example from the lower part of the Middle–Late Triassic Chang7 Member, Ordos Basin (NW China)

Tuffs, produced by explosive volcanic eruptions, are widely deposited throughout the Precambrian to Cenozoic stratigraphic record. Due to rapid compaction and erosion, preservation and identification of tuffs in distal environments are challenging. However, in deep lacustrine environments, tuffs may be found well-preserved as interlayers in the organic-rich mudstone. This in turn provides valuable insights into the origin, transport, and diagenesis of volcanic material. In this study, we performed an integrated sedimentological, mineralogical, and geochemical analysis of seven tuff samples collected from the lower part of the Chang7 member of the Maquan section to infer on their provenance, transport mechanisms and post-depositional alterations. Immobile element ratios, tectonic discrimination diagrams, trace element distribution patterns, and negative whole-rock εNd(t) reveal an intermediate to felsic continental arc source of the studied tuff. Supported frameworks and ash aggregates in the tuff are in favor of the transport from pyroclastic density currents and ash plumes during multiple volcanic eruptions with various degrees of eruption volume and water content. Tuff diagenesis is characterized by dissolution of plagioclase, illitization of smectite, cementation of carbonate minerals, and presence of buddingtonite. Relatively ordered illite-smectite and low vitrinite reflectance from the adjacent organic-rich mudstone indicate diagenetic evolution of studied tuff have reached mesogenetic stage.

The Lord Howe Volcanic Complex, Australia: its geochemistry and origins

ABSTRACT The Lord Howe Volcanic Complex (LHVC) is part of a seamount chain associated with the detached continental crust of Zealandia. We present petrographic and geochemical data for the LHVC, including the first reported data for Balls Pyramid and the first trace element data for Lord Howe Island, clarifying the tectonic affiliation and general mantle source characteristics. Classification and discrimination diagrams based on concentrations and ratios of immobile elements, such as rare earth and high field strength elements, Th, Ti, Y, and Mg, show the LHVC comprises transitional to alkali basalts and at least two, and probably three, shield building complexes, consistent with an intraplate tectonic setting. Primitive-mantle normalised trace element patterns, variation in Dy/Yb for a restricted range of Dy/Dy*, Zr/Yb > 90, and variations in Zr/Hf and P2O5/TiO2 through the eruptive sequences indicate the mantle source may be a laterally heterogeneous enriched garnet peridotite that has undergone varying degrees of partial melting and carbonatitic metasomatism, both within the LHVC and along the Lord Howe Seamount Chain. The LHVC is thus an ideal natural laboratory for probing the probability of small intraplate volcanic fields originating via mantle plumes or from previously subducted slabs stagnating at the mantle transition zone.

Geology, Geochemistry, and Geochronology of the Giant Rio Tinto VMS Deposit, Iberian Pyrite Belt, Spain

Abstract The Rio Tinto deposit is a giant volcanogenic massive sulfide deposit (VMS) that contains more than 500 Mt of pyrite-rich massive sulfides and more than 2 Gt of mineralized stockwork. Three broad lithostratigraphic groups occur in the regional stratigraphy: the phyllite-quartzite group, the volcano-sedimentary complex, and the Baixo Alentejo Flysch Group. These three major packages reflect the evolution of a depositional environment from a stable platform to deposition in pull-apart continental basins during oblique subduction and collision and coeval synorogenic flysch sequence. The volcano-sedimentary complex, which hosts massive sulfide mineralization at Rio Tinto, can be divided into four major units: (1) the Mafic Siliciclastic Unit, (2) the Lower Sedimentary Unit, (3) the Felsic Unit, and (4) the Upper Sedimentary Unit. The Felsic Unit is further subdivided based on new U-Pb zircon geochronology into three distinct subunits. Felsic Unit I (ca. 356 Ma) includes dome complexes dominated by rhyodacite and reflects the onset of felsic magmatism in the region. Felsic Unit II (ca. 352–348 Ma) represents the main interval of volcanic activity, also dominated by rhyodacite domes and related aprons, and is associated with widespread VMS mineralization. Felsic Unit III (ca. 340 Ma) reflects a late pulse of rhyolitic volcanism. Massive sulfides occur as two different styles of mineralization: (1) replacive ores as discordant pipes hosted by glass-rich felsic rocks and enclosed by a large zone of stockwork-like mineralization and (2) overlying shale-hosted exhalative mineralization in small anoxic basins, probably formed during the collapse of the volcanic domes of Felsic Unit II in the Middle-Late Tournaisian. New lithogeochemical data illustrate two types of mafic rocks in the Mafic Siliciclastic Unit: a basaltic andesite and a high–Ti-Zr basalt, both of tholeiitic affinity. Using immobile element ratios (heavy rare earth elements [HREEs], Al, Y, Zr, and Ti) of the Felsic Unit, fundamental differences have been recognized between the subunits. The unmineralized Felsic Unit I is characterized by high Zr content (225–300 ppm) and a pronounced Eu negative anomaly, and probably represents the most fractionated rocks. Felsic Unit II is characterized by Zr values between 50 and 200 ppm. The low Zr values of the mineralized unit contrast with the typically high Zr values of the felsic rocks related to volcanogenic massive sulfides elsewhere and, at a regional scale, can help to discriminate potentially fertile domes from barren volcanism.

Petrogenesis of Early Carboniferous Ultramafic–Mafic Volcanic Rocks in the Southern Changning–Menglian Belt, Southeastern Tibetan Plateau: Implications for the Evolution of the Paleo‐Tethyan Ocean

AbstractThe Changning–Menglian Belt represents the main Paleo‐Tethyan Suture in the southeastern Tibetan Plateau, which divides Gondwana‐ and Eurasia‐derived continental fragments from each other. The belt contains ultramafic–mafic volcanic rocks that provide evidence of the tectonic processes which operated during the evolution of the Paleo‐Tethyan Ocean. New geochemical data for Early Carboniferous volcanics in the southern Changning–Menglian Belt show that wehrlites have cumulate and poikilitic textures, and contain low‐Fo (84.2–87.2) olivine, clinopyroxene with low Mg# values (79.4–85.6), and spinel with high Cr# values (67.8–72.4). Estimated equilibrium temperatures obtained using olivine‐spinel Fe‐Mg exchange geothermometry range from 978°C to 1373°C (mean = 1205°C; n = 3). These observations combined with a lack of reaction or melt impregnation textures indicate that these units represent magmatic cumulates rather than having formed as a result of mantle‐melt reactions. Both wehrlites and basalts in the belt have subparallel rare earth element (REE)‐and primitive‐mantle‐normalized multi‐element patterns with slightly positive Nb‐Ta anomalies, but negligible Eu and Zr‐Hf anomalies. The volcanics have similar Sr‐Nd‐Pb isotopic compositions with εNd(t) values of 4.2–4.5 (mean = 4.3; n = 3) and 4.0–4.4 (mean 4.2; n = 4), respectively, and also have similar immobile element ratios, such as Nb/La, Nb/U, Th/La, Zr/Nb, Th/Ta, La/Yb, Nb/Th, Nb/Y, and Zr/Y. These characteristics indicate both units have ocean island basalt (OIB)‐like geochemical affinities, consistent with the fact that the clinopyroxene in the wehrlites is compositionally similar to OIB‐related cumulus clinopyroxene. This suggests that both the wehrlites and basalts were derived from similar parental magmas that underwent generally closed‐system magmatic differentiation dominated by fractionation of the olivine and clinopyroxene. This parental magma was likely generated in an oceanic seamount setting from an OIB‐type mantle source (i.e., asthenospheric mantle) containing garnet‐spinel lherzolite material. Combing this new data with that from oceanic seamount volcano‐sedimentary suites derived from previous research enables the identification of a mature late Paleozoic ocean basin between the passive northeastern Gondwanan margin and the northward‐migrating microcontinent of Lanping–Simao.

Provenance of Lower Carboniferous Bauxite Deposits in Northern Guizhou, China: Constraints from Geochemistry and Detrital Zircon U-Pb Ages

The Lower Carboniferous Jiujialu Formation bauxite deposits of northern Guizhou Province, China, are a Kazakhstan subtype of karst bauxite deposits. The provenance of the Jiujialu Formation has long been debated, with uncertainty about the formation of the bauxite deposits. Here we report new geochemical data that indicate the affinity between the Lower-Middle Ordovician clastic rocks and argillaceous dolostones and the overlying Carboniferous bauxite deposits, all of which are characterized by high Al2O3, K2O, and ΣREE contents, flat post-Archean Australian shale (PAAS)-normalized REE patterns, and uniform immobile element ratios (TiO2/Al2O3, Nb/TiO2, and Zr/TiO2). Their similar detrital zircon age distributions further indicate the link between the bauxite deposits and the clastic rocks and argillaceous dolostones. Zircon age spectra of clastic rocks of the Lower Silurian Hanchiatien Formation in northern Guizhou match those of the bauxite deposits, with a maximum age peak at ∼980 Ma and other secondary age peaks, suggesting these clastic rocks may represent the provenance of the bauxite deposits. The youngest detrital zircons (∼445 Ma) occur only in the bauxite deposits and are probably sourced from K-bentonite beds of the Ordovician-Silurian transition. Our analyses indicate that the source materials of the bauxite deposits in the Jiujialu Formation are of mixed provenance: Lower-Middle Ordovician aluminosilicate rocks and argillaceous dolostones of the underlying strata, and Lower Silurian clastic rocks and K-bentonite from adjacent areas. A comparison of Early Carboniferous bauxitic provenances in northern and central Guizhou indicates that paleotopography was the major factor controlling the provenance of these bauxite deposits.

Hikurangi Plateau subduction a trigger for Vitiaz arc splitting and Havre Trough opening (southwestern Pacific)

AbstractSplitting of the Vitiaz arc formed the Tonga-Kermadec and Lau-Colville Ridges (southwestern Pacific Ocean), separated by the Lau Basin in the north and Havre Trough in the south. We present new trace element and Sr-Nd-Hf-Pb isotope geochemistry for the Kermadec and Colville Ridges extending ∼900 km north of New Zealand (36°S–28°S) in order to (1) compare the composition of the arc remnants with Quaternary Kermadec arc volcanism, (2) constrain spatial geochemical variations in the arc remnants, (3) evaluate the effect of Hikurangi igneous plateau subduction on the geochemistry of the older arc lavas, and (4) elucidate what may have caused arc splitting. Compared to the Kermadec Ridge, the Colville Ridge has higher more-incompatible to less-incompatible immobile element ratios and largely overlapping isotope ratios, consistent with an origin through lower degrees of melting of more enriched upper mantle in the Vitiaz rear arc. Between ca. 8 and 3 Ma, both halves of the arc (∼36°S–29°S) included a more enriched (EM1-type) composition (with lower 206Pb/204Pb and 207Pb/204Pb and higher Δ8/4 Pb [deviation of the measured 208Pb/204Pb ratio from a Northern Hemisphere basalt regression line] and 87Sr/86Sr) compared to older and younger arc lavas. High-Ti basalts from the Manihiki Plateau, once joined to the Hikurangi Plateau, could serve as the enriched Vitiaz arc end member. We propose that the enriched plateau signature, seen only in the isotope ratios of mobile elements, was transported by hydrous fluids from the western margin of the subducting Hikurangi Plateau or a Hikurangi Plateau fragment into the overlying mantle wedge. Our results are consistent with plateau subduction triggering arc splitting and backarc opening.

Geochemistry of the Yudong bauxite deposit, south-eastern Guizhou, China: Implications for conditions of formation and parental affinity

The Yudong bauxite deposit is located in the Kaili–Huangping bauxite-concentrated area, in south-eastern Guizhou Province, China. The ore-bearing rock series consists of the lower part (Fe-bearing member) and middle part (Al-bearing member) of the Middle Permian Liangshan Formation, covering the Upper Devonian Gaopochang Formation. The elemental content in the ore-bearing profile is variable. In general, Al, Ti, V, Cr, Zr, Nb, Hf, Th, and U increase from the Fe- to the Al-bearing member, whereas Si, Fe, Rb, Cs, Ba, and total rare earth elements (REE) decrease. These observations indicate the occurrence of bauxitisation, which corresponds to a process of elemental variation during which Si and Fe were dissolved and migrated, and Al was enriched. Compared with Ordovician–Silurian (O–S) mudrock, the ore-bearing rock series are enriched in Al, Ti, V, Zr, Hf, Nb, Ta, Th, and U, but depleted in Si, Fe, Rb, Cs, and Ba as a result of weathering. The REEs show negative correlations with Si, Fe, K, Na, Ca, Mg, and Y, indicating that the REEs may be associated with clay, Fe-bearing, and Y-bearing minerals.The La/Y ratio, (La/Yb)N ratio, and Ce anomalies show that the Fe-bearing member formed in an alkaline reducing environment, whereas the Al-bearing member formed in an acidic oxic environment. The immobile element ratios, Eu anomalies, REE distribution patterns, and Cr–Ni diagram, as well as the comparison with worldwide bauxite deposits, show that the ore-bearing rock series may have originated in the O–S mudrock, with contributions from the dolomite of the Gaopochang Formation. Furthermore, the position of the samples in the Al2O3–Fe2O3–SiO2 ternary plot, the chemical index of alteration, and the strong positive correlations of some stable elements (such as Al, Ti, Zr, Hf, Nb, and Ta) indicates that the degree of weathering gradually increases from the O–S mudrock to the bauxitic mudrock, and further increases into the bauxite; the bauxitic mudrock and bauxite therefore originated from the O–S mudrock. The laterisation products of the O–S mudrock from the northern part of the ore collection area were transported to the karst depression in the early stages of the Middle Permian. The weathering products of the O–S mudrock and a small amount of the Gaopochang Formation dolomite accumulated on the karst interface, forming the original bauxite layers through cycles of compaction and concretion. These bauxite layers were then uplifted to or near the surface during the Late Cretaceous to Paleogene, eventually, forming karstic bauxite.

Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source

Thin fallout tuffs are common in the terrestrial deposits of the Eocene Duchesne River Formation on the flanks of the Uinta Mountains of eastern Utah. Their ages and compositions provide new insight into the tectonic events and magmatic history of the western Cordillera and provide important constraints on the Cenozoic land mammal chronology. Whole-rock compositions of the volcanic ash show that they underwent post-emplacement argillic alteration, typical of a wetland/floodplain depositional setting. However, immobile element ratios and abundances, such as Zr/Ti, La/Nb, and Y are typical of rhyolites formed in a subduction-related setting. Glass shards preserved in one sample all had SiO2 values >75%, typical of high-silica rhyolite. Preserved phenocrysts in the ash beds include quartz, sanidine, plagioclase, and biotite with variable amounts of accessory zircon, apatite, titanite, and allanite. Biotite compositions have Fe/(Fe+Mg) ratios typical of calc-alkaline igneous rocks and clusters of chemical compositions suggest a genetic relationship to three or four separate eruptions. Sanidine compositions from five samples range from Or73 and Or79. Only one sample had preserved plagioclase with compositions ranging between An22 – An49. Allanite from the ash beds has lower total rare earth elements (REE) concentrations than allanite from other well-studied rhyolites. Titanite in one sample has lower concentrations of REE, Fe, and Al than expected of rhyolites and is probably detrital.Plagioclase and sanidine from two different tuff beds near the middle of the Duchesne River Formation yielded analytically indistinguishable 40Ar/39Ar ages of 39.47 ± 0.16 Ma and 39.36 ± 0.15 Ma, respectively. These dates, along with the compositional data seem to limit the eruptive source for these fallout tuffs to the northeast Nevada volcanic field, one of the few volcanically active regions of western North America at the time. These new radiometric ages, along with stratigraphic relations and previously published ages for tuffs in the Bishop Conglomerate (which unconformably overlies the Duchesne River Formation), constrain the timing of late Laramide uplift in the region from 39 to about 37 Ma and post-Laramide epeirogenic uplift from 34 Ma to 30 Ma. Finally, the ages also provide additional evidence that the Duchesnean North American Land Mammal Age ended in the Eocene, which was originally named and defined from the Duchesne River Formation.

Evaluation of zircon U-Pb geochronology as a tool to determine soil provenance in a limestone terrane, Middle TN, USA

Soil can form by bedrock weathering or by deposition of alluvial or aeolian material. To test the utility of detrital zircon UPb geochronology for determining soil provenance, and to determine the provenance of soils in Middle Tennessee, samples of soil and underlying bedrock were collected from two sites, along with alluvium from a nearby river and loess from West Tennessee. Multi-dimensional scaling plots were used to compare complex detrital zircon LA-ICP-MS UPb sample age distributions. Concentration ratios of immobile elements and Nd and Hf isotope compositions were also used as provenance indicators for comparison. At one site zircon UPb ages and bulk sample trace element and isotopic compositions of soil were similar to the underlying limestone bedrock, indicating soil formation by bedrock weathering. In contrast, at a second site provenance tests indicate that soil from a late Pleistocene river terrace was sourced from alluvium rather than the underlying bedrock. Provenance inferences based on zircon UPb ages were generally consistent with evidence from Zr/Hf ratios and Hf isotope compositions, which are all dominated by zircon, and Nd isotope compositions, which are not, although the best matches for different provenance indicators did not always agree. Zircon geochronology should be used with other soil provenance indicators to reduce effects of bias introduced by sedimentary sorting and sample processing.

Geochemical study of the Neoproterozoic clastic sedimentary rocks of the Khambal Formation (Sindreth Basin), Aravalli Craton, NW Indian Shield: Implications for paleoweathering, provenance, and geodynamic evolution

Petrographic and geochemical studies characterize lithologies of the Khambal Formation deposited in the Sindreth Basin as arkosic, subarkosic, and quartzarenite. Weathering indices, such as CIA and CIW in conjunction with the ACNK diagram, prescribe mostly moderate chemical weathering with intermittent pulses of extreme weathering in the source area. The discrimination diagrams suggest that these Neoproterozoic clastics were deposited in an active rift basin. Provenance indicators of the detritus components point to a terrane possessing subordinate mafic material in conjunction with large felsic components. A comparison of immobile element ratios with probable source rocks suggest that the Mesoproterozoic Delhi arc situated to the east of the Sindreth Basin could be the possible source. Our mixing calculation defines the proportion of end member components in the Delhi arc. Geodynamic considerations relate the origin of the Sindreth Basin to the processes of disintegration and reassembly of supercontinents.

The lithogeochemical signatures of hydrothermal alteration in the Waihi epithermal district, New Zealand

ABSTRACTWhole rock lithogeochemical data from the andesite rocks which host low-sulfidation epithermal gold veins in the Waihi district, New Zealand have been used to interpret hydrothermal alteration processes that variably altered the andesite rocks. The andesite host rocks are interpreted to have similar primary chemical compositions throughout the Waihi area, as reflected in similar ratios of immobile elements (eg Zr, Ti and other high field strength elements). The geochemical data reveal that elevated K/Al ratios occur in rocks proximal to low-Au–Ag deposits, reflecting adularia alteration in the host rocks associated with the upflowing hydrothermal fluids. Silicification of host rocks is achieved both by the addition of SiO2, as well as the destruction of plagioclase (reflected in the loss of CaO and Na2O). The rocks have been variably altered, with calcite and pyrite abundance reflecting the addition of carbon and sulphur. Pathfinder element (eg As, Sb) concentrations are variable across the district, with high pathfinder element concentrations interpreted to reflect proximity to hydrothermal fluid flow. Together, the results provide a district-scale overview of hydrothermal alteration at Waihi, and emphasise the utility of lithogeochemistry for classifying and quantifying hydrothermal alteration that can then be used to interpret hydrothermal processes and assist mineral exploration.

Microtopography-mediated hydrologic environment controls elemental migration and mineral weathering in subalpine surface soils of subtropical monsoonal China

Local topography and elevation gradients can exert important influences on soil formation processes such as elemental migration, mineral weathering, and soil organic matter (SOM) accumulation, yet these influences remain insufficiently investigated to date, particularly in surface soils of subtropical monsoonal regions. Here, we report on an investigation of a series of surface soils collected from four different topographic locations across the subalpine Dajiuhu Critical Zone Observatory (CZO), representing hillslope (planar), swale and river channel (convergent), and bulge (divergent) microtopographic sites. Evidence provided by rare-earth element (REE) patterns, immobile element ratios, clay-mineral compositions, and particle-size distributions suggests that these soils have rather uniform parent materials. X-ray diffraction (XRD) analysis revealed that secondary clay minerals in these soils are complex, being dominated by various interstratified clays. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy shows similar patterns among the swale, bulge, and river-channel soils that differ from those of the hillslope soils. Most soils at convergent sites with poor drainage contain more smectitic clays (interstratified illite/smectite and chlorite/smectite) and less vermiculitic clays (hydroxyl-interlayered vermiculite and interstratified illite/hydroxyl-interlayered vermiculite), and exhibit weaker chemical weathering and fewer elemental losses than those from non-convergent sites. The diversity of clay types can be ascribed to the complexity and heterogeneity, in particular of pH and hydrology, in these soil environments. Across the range of microtopographic sites investigated here, elemental migration and chemical weathering are generally coupled, with greater elemental losses associated with more intense chemical weathering. Soil organic carbon (SOC) content generally increases at higher elevations, which is attributable to lower temperatures and a consequent reduction of microbial remineralization, and under more reducing soil water conditions. Though influenced by eolian dust, variations in Fe/Mn, Ce anomaly, and Corg/P can reflect redox conditions of different soils. SOC retention is also closely associated with soil redox status, with more reducing conditions being more conducive to SOC preservation.

Pervasively anoxic surface conditions at the onset of the Great Oxidation Event: New multi-proxy constraints from the Cooper Lake paleosol

Oceanic element inventories derived from marine sedimentary rocks place important constraints on oxidative continental weathering in deep time, but there remains a scarcity in complementary observations directly from continental sedimentary reservoirs. This study focuses on better defining continental weathering conditions near the Archean-Proterozoic boundary through the multi-proxy (major and ultra-trace element, Fe and Cr stable isotopes, µ-XRF elemental mapping, and detrital zircon U-Pb geochronology) investigation of the ca. 2.45 billion year old (giga annum, Ga) Cooper Lake paleosol (saprolith) developed on a sediment-hosted mafic dike within the Huronian Supergroup (Ontario, Canada).Throughout the variably altered Cooper Lake saprolith, ratios of immobile elements (Nb, Ta, Zr, Hf, Th, Al, Ti) are constant, indicating a uniform pre-alteration dike composition, lack of extreme pH weathering conditions, and no major influence from ligand-rich fluids during weathering or burial metasomatism/metamorphism. The loss of Mg, Fe, Na, Sr, and Li, a signature of albite and ferromagnesian silicate weathering, increases towards the top of the preserved profile (unconformity) and dike margins. Coupled bulk rock behaviour of Fe-Mg-Mn and co-localization of Fe-Mn in clay minerals (predominantly chlorite) indicates these elements were solubilized primarily in their divalent state without Fe/Mn-oxide formation. A lack of a Ce anomaly and immobility of Mo, V, and Cr further support pervasively anoxic weathering conditions. Subtle U enrichment, if primary, is the only geochemical evidence that could be consistent with oxidative element mobilization. The leaching of ferromagnesian silicates was accompanied by variable mobility and depletion of transition metals with a relative depletion order of Fe ≈ Mg ≈ Zn > Ni > Co > Cu (Cu being significantly influenced by secondary sulfide formation). Mild enrichment of heavy Fe isotopes (δ56/54Fe from 0.169 to 0.492‰) correlating with Fe depletion in the saprolith indicates open-system loss of isotopically light aqueous Fe(II). Minor REE + Y fractionation with increasing alteration intensity, including a decreasing Eu anomaly and Y/Ho ratio, is attributed to albite breakdown and preferential scavenging of HREE > Y by clay minerals, respectively. Younger metasomatism resulted in the addition of several elements (K, Rb, Cs, Be, Tl, Ba, Sn, In, W), partly or wholly obscuring their earlier paleo-weathering trends.The behavior of Cr at Cooper Lake can help test previous hypotheses of an enhanced, low pH-driven continental weathering flux of Cr(III) to marine reservoirs between ca. 2.48–2.32 Ga and the utility of the stable Cr isotope proxy of Mn-oxide induced Cr(III) oxidation. Synchrotron µ-XRF maps and invariant Cr/Nb ratios reveal complete immobility of Cr despite its distribution amongst both clay-rich groundmass and Fe-Ti oxides. Assuming a pH-dependent, continental source of Cr(III) to marine basins, the Cr immobility at Cooper Lake indicates either that signatures of acidic surface waters were localized to uppermost and typically unpreserved regolith horizons or were geographically restricted to acid-generating point sources. However, given detrital pyrite preservation in overlying fluvial sequences, it is probable that the oxidative sulfide corrosion required to drive surface pH < 4 lagged behind in this region relative to other early Proterozoic sequences. The entire saprolith exhibits a consistently light stable Cr isotope composition (δ53/52Cr: −0.321 ± 0.038‰, 2sd, n = 34) that cannot be linked to Cr(III) oxidation and is instead interpreted to have a magmatic origin.The combined paleosol chemical signatures and preservation of detrital pyrite/uraninite indicate low atmospheric O2 during weathering at ca. 2.45 Ga in the rift-related environment of the Lower Huronian. The aqueous flux from the reduced weathering of mafic rocks was presumably characterized by a greater transition metal (Fe, Mn, Zn, Co, Ni) load with isotopically light Fe(II) compared to modern environments, as well as higher Eu/Eu* and Y/Ho than the source rock. In most models of Precambrian ocean element inventories, hydrothermal fluids are viewed as the main supplier of several metals (e.g., Fe, Zn), although the results herein suggest that a riverine metal supply may have been substantial and that using Eu-excess as a strict proxy for hydrothermal flux may be misleading in near-shore marine sedimentary deposits.

Interpreting element addition and depletion at the Ann Mason porphyry-Cu deposit, Nevada, using mapped mass balance patterns

Elemental mass changes were calculated for 154 samples from four rock types around the Ann Mason porphyry-Cu-(MoAu) deposit, Yerington district, Nevada. The purpose of these calculations was to investigate addition and depletion patterns in two-dimensional (2-D) map space. Mass changes in Ca, Mg, Na, P, Cu, Mo, As, Sb, Sr, and Pb from samples in the pre- to syn-mineralisation Yerington batholith vary systematically with lateral and vertical distance from the Ann Mason deposit. At the core of the Ann Mason deposit, Cu and Mo were strongly added (>10,000% relative to average protolith values), with restricted lateral mass addition halos around the deposit. Copper is depleted in wall rocks surrounding the deposit. Calcium and Sr were removed (up to 98% depletion) from parts of the deposit centre due to the destruction of calcic-plagioclase and other calcium-bearing minerals during potassic alteration. Calcium (up to 1570%) and Sr (up to 315%) were added outside the deposit and were concentrated around 4 km from the deposit centre where epidote alteration is most intense. Arsenic and Sb were removed from the deposit centre (up to 99% depletion) where silicate-mineral alteration assemblages dominate. Antimony shows an addition halo (ranging from 25% up to 1900%) that extends from the deposit centre 500 m laterally away, and correlates spatially with the increased abundance of pyrite that typically surrounds porphyry deposits. Epidote-altered samples of Shamrock monzonite from outside the Casting Copper skarn show addition of Ca (up to 196%), Sb (up to 1100%), and depletion of Na (up to −98%). Samples from the central part of the post-mineralisation Shamrock monzonite record lower mass changes in Mg, Ca, P, and Mo, compared to other parts of the batholith. Some samples from the northern contact of the Shamrock batholith exhibit addition of Ca (up to 554%), Sr (up to 504%), and depletion in Na (up to −98%). These samples are coincident with strong epidote alteration.Our results provide a rock-normalised geochemical map that reflects quantified element addition and depletion patterns, which is useful for interrogating geological processes at a regional scale. Although interpreting relative element addition and depletion in map space is possible using conventional immobile element ratio approaches, mass balance calculations provide the benefit of quantified results.

Petrography, mineralogy, geochemistry and genesis of the Balaya bauxite deposits in Kindia region, Maritime Guinea, West Africa

The Balaya bauxite plateau is located in the Kindia bauxite province; where the bauxite ores are found above Ordovician and Silurian formations in the southern part of the Guinean Paleozoic Bove Basin, West African Craton. In this study, petrographical, mineralogical and geochemical investigations were carried out on the different type of bauxites (sedimentary and lateritic) of this plateau. Mineralogical analysis and petrological study revealed that the mineralogy is almost identical in both bauxite types of the Balaya plateau. Gibbsite (ave. 83.90%), goethite and aluminous goethite are the major mineral components; anatase, rutile, diaspore and kaolinite are minor minerals, with zircon and clastic quartz as accessory minerals in the bauxites. Whole rock and trace elements analysis show that the main components of the Balaya bauxites ore are Al2O3 (49.70–61.00 wt%), SiO2 (0.30–5.00 wt%), Fe2O3 (1.60–19.00 wt%), and TiO2 (1.71–3.70 wt%). Trace elements investigation indicates that HFSEs, Ti, Cr, Ga, Sn, HREE were immobile and specifically enriched; whereas alkalis and alkaline earth metals were depleted, in bauxites during lateritization. Geochemical paths of dismantlement, immobile element ratios (Zr vs Hf, Nb vs Ta and Yb vs Lu; and Cr/Ni) and REE patterns of the bauxites indicate that they were all originated from the destruction of the kaolinite by a deep of lateritization, through an indirect bauxitization of the Silurian argillites and siltstones of the Télimélé Suite. Thus, Balaya sedimentary bauxites were further formed by redeposition from these lateritic bauxites, by different mechanical transportation. Pearson correlation and mass change calculations revealed that Ti-bearing and Fe-bearing minerals are the carrier for HREE, HFSE, Sn, U, Ga, W and Y; and Mo, Cu, Pb, As, Sb and Bi respectively. P-bearing minerals and kaolinites could have played signifiquent roles in LREE concentration, but the possible presence of authigenic minerals such as strontianite, celestite or florencite bearing the LREE are expected.

Lower Palar River Sediments, Southern Peninsular, India: Geochemistry, Source-Area Weathering, Provenance and Tectonic Setting

Abstract This present study describes the geochemistry of fluvial sediments of the Palar river (lower reaches), Southern Peninsular India, with an aim to evaluate their provenance, weathering and tectonic setting. The bulk sediment chemistry is influenced by grain size. The river sediments are enriched with SiO2 and depleted in Al2O3, K2O, CaO, Na2O, MgO, P2O5, MnO, Fe2O3 as compared with UCC values. Geochemical classification indicate that the sediments are mainly arkose, wacke and shale in composition. Discriminant diagrams together with immobile element ratio plots reveal that, the Palar river sediments are mostly derived from rocks formed in an active continental margin. Additionally, the rare earth element ratios as well as chondrite-normalized REE patterns with flat HREE, LREE enrichment, and negative Eu anomalies indicate felsic rock sources. The chemical indices of alteration suggest that Palar river sediments are chemically immature and have experienced low chemical weathering effects. This is further supported by the Th/U Rb/Sr ratio and A-CN-K ternary diagram, with most of the sample data points falling close to the plagioclase-smectite line. The bivariate plot of Th/Sc versus Zr/Sc suggest a moderate recycled origin of the sediments.

Geochemistry, petrogenesis and tectonic significance of the volcanic rocks of the Las Tortolas Formation, Coastal Cordillera, northern Chile

The Las Tórtolas Formation extends from 29° to 26° S along the northern chilean coast and has been interpreted as a Carboniferous accretionary prism. This essentially metasedimentary turbiditic formation constituted the late Paleozoic basement of the Upper Triassic-Jurassic magmatism during the inception of the Andean cycle in northern Chile. The geochemical characteristics and field relations studied in the volcanic rocks of the Las Tórtolas Formation, described with detail in this work, mean a key fact to assess the notable paleogeographic changes occurred during the pre-Andean cycle in the western margin of South America.The volcanic domain included in the Las Tórtolas Formation is mainly comprised by basaltic rocks, with minor tuffs and rhyolites, and presents thrust-fault contacts with the mélange facies to the west and the metasedimentary Eastern Series to the east. Together, the Las Tórtolas Formation was conformed by the stacking of the basal highly deformed units of this accretionary complex, represented by the mélange facies, and the volcanic domain that became overlain by the turbiditic sequences at their top.The geochemical characteristics of the Las Tórtolas basaltic rocks involve an enriched mantle source, as a mantle plume. In spite of the N-MORB signature observed in the basal hyaloclastitic basalts, the lava flows, as the metabasites sampled in the mélange facies, yield E-MORB characteristics according to the multi-element and tectonic discrimination diagrams and several immobile element ratios as the Th/Yb, TiO2/Yb and Nb/Yb, among others. Therefore, the origin of the volcanic domain of the Las Tórtolas Formation may be related to a hot mantle plume impinged on the base of the oceanic lithosphere under the tip of the overriding plate, which firstly promoted the partial melting of the depleted surrounding lithosphere (N-MORB hyaloclastites) and continued with the extrusion of lava flows and pyroclastic rocks derived from the enriched source, interlayered with the distal turbiditic succession. The development of the magmatic system along the oceanic crust enabled the formation of magma chambers where small volumes of rhyolitic melts were fractionated. The geochemical evolution of this basaltic magmatism was correlated with the building of volcanic edifices during a first submarine stage (hyaloclastites, pillows and lava flows) and a subsequent sub-aerial explosive phase depicted by sedimentary and rhyolitic breccias.Finally, we explore a tectonic setting defined by a plume overridden by the lithosphere during the relative convergence with the continental margin to explain the formation of the volcanic domain and its accretion to the sedimentary units. Besides, this tectonic configuration may be taken into account as the driving force of the tectonic and magmatic events that characterize the pre-Andean cycle.

Geochemical interpretation of the Precambrian basement and overlying Cambrian sandstone on Bornholm, Denmark: Implications for the weathering history

A geochemical study of the Precambrian basement granites from the Borggård borehole on Bornholm, Denmark, suggests that the granites were moderately weathered (Chemical Index of Alteration-CIA=66–71) during subaerial exposure in a humid climate. The microcline is well preserved, whereas plagioclase was thoroughly altered to clay minerals (Plagioclase Index of Alteration-PIA=93–99) which is likely due to its original Ca-rich composition. The primary Fe–Ti accessory minerals were oxidized to hematite and anatase. Evidence from REE distribution patterns and immobile element ratios, e.g. Zr/Hf and Nb/Ta, between the weathered basement granite from the Borggård borehole and regional granitoids on Bornholm, constrains the Svaneke Granite as the original basement lithology. A tau (τ) mass transport model (assuming immobile Ti) was applied to quantify the mass transfer during weathering of the basement granite. The results show a depletion of major elements in the following order: Na>Ca>Mg>Si; Al and Ti are immobile and stay constant; K shows sample dependent enrichment or depletion; Fe is slightly enriched. The Cambrian sandstone overlying the basement in the Borggård borehole, assigned to the Gadeby Member of the Nexø Formation, is feldspathic litharenite–litharenite in composition. Provenance indicators including (Gd/Yb)N, Zr/Hf and Nb/Ta ratios and petrological features indicate that source material was derived from both weathered and fresh basement granite of intermediate composition. The Gadeby Member equivalents in Germany, the basal lower Cambrian Adlergrund Konglomerat Member (AKM) in the offshore G-14 well north of Rügen, and the approximately coeval Lubmin Sandstein Formation (LSF) from the Loissin-1 borehole, mainland Germany, must have been sourced from a basement with compositions comparable to the intermediate group of the regional granitoids on Bornholm. The source materials for the AKM (CIA=71–72, PIA=94–96), the Gadeby Member in the Borggård borehole (CIA=52–69, PIA=56–99) and an outcrop in Nexø, eastern Bornholm (CIA=52–66, PIA=61–96), have endured similar degrees of weak to moderate weathering but lost most of the plagioclase. The LSF has a comparable weathering history (CIA=63–73), but the plagioclase is better preserved (PIA=65–78). The significant variation of weathering rates of plagioclase and K-feldspar in the basement granite and the provenance of sandstone from the Borggård borehole are likely due to the different permeability developed within the internal crystal structures, a Ca- rich plagioclase original composition of the plagioclase, and the occurrence of weathering in a very humid climate. ​K metasomatism occurred in the basement granite and sandstone in both the Borggård and the G14–1 boreholes, mainly through the conversion of aluminous clay minerals (e.g. kaolinite) to illite, with transformation of plagioclase to K-feldspar occurring locally. This my have taken place during deep burial in the Caledonian foreland basin.

High-Resolution Physical Properties, Geochemistry, and Alteration Mineralogy for the Host Rocks of the Archean Lemoine Auriferous Volcanogenic Massive Sulfide Deposit, Canada

Multiparameter data on drill cores have a range of useful applications for ore deposit modeling and mineral exploration. Density, magnetic susceptibility, eight major elements, seven trace elements (portable X-ray fluorescence [pXRF]), five mineral groups (near-infrared spectrometry), and average visible light reflectance were determined on rock cores from several drill holes. These are located near the Archean Lemoine auriferous volcanogenic massive sulfide (VMS) deposit in the Chibougamau mining district, Abitibi subprovince, Quebec, Canada. This case study focuses on one particular drill hole that has been recently characterized in detail by conventional methods (laboratory geochemistry, petrography, and stable isotopes); this provides validation for the multisensor dataset. The best variables and immobile element ratios to distinguish between different lithological units in the multiparameter data are density, magnetic susceptibility, Ti/Zr, Al/Zr, and Zr/Y. Downhole profiles of these parameters allow the four different felsic units in LEM-37, many of which are visually similar, to be distinguished, and geologic contacts to be positioned precisely. Such methods would be even more useful in environments where recognition of primary characteristics is strongly hampered by intense hydrothermal alteration and superimposed deformation or metamorphism. Volcanic and intrusive rocks can be placed on classic diagrams to assign them names and magmatic affinities based on the corrected pXRF data. Downhole profiles of major and trace elements, combined with infrared mineralogy, allow hydrothermal alteration to be characterized, yielding results comparable to those of conventional techniques for the same drill hole, but with a much tighter spacing and within potentially shorter time frames.