Geochemical Coherence Research Articles

Significance of radioelements distribution in the Precambrian rocks of Jabel Sayid, western Saudi Arabia, using spectrometric and geochemical data

AbstractIn situ gamma‐ray spectrometry measurements were carried out using RS‐230 BGO spectrometer over an area of 9.4 km2. One hundred and seventy eight spectrometric points were collected along parallel profiles with spacing 325 m in the Easting X direction and Northing Y direction. The results are treated statistically to determine the minimum, maximum, arithmetic mean (X), SD (S), and coefficient of variability (CV %). The spectrometric survey revealed the presence of radioactive zones in the aplite‐pegmatite with maximum eU content of 1,550 ppm and eTh of 7,974 ppm, whereas alkali granite has an average eU content of 12 ppm and eTh of 34 ppm. Felsite has an average eU and eTh close to that of alkali granite, 11 and 32 ppm, respectively. Pegmatite veins cutting alkali granite have high values of eU and eTh, an average of 34 and 101 ppm, respectively. The metamorphosed volcanic rocks of the Mahd Group demonstrate the lowest radioactivity with an average eU content of 0.8 ppm and eTh of 1.6 ppm. These rocks partly show evidences of post‐magmatic alteration processes, for example, silicification, sericitization, and oxidation. Excluding metavolcanics, the strong positive correlation between eU and eTh for the different rock varieties reflects their geochemical coherence during magma crystallization and indicates that both elements are largely accommodated into accessory minerals and were not disturbed by alteration. The CV values of the different rock units show normal distribution, where all the percentages of CV values are less than 100, with the exception of eU of felsite that shows CV % value slightly higher than 100. The aplite‐pegmatite, alkali granite, and the associated pegmatite veins show negative value of uranium outward migration, but are still the most favorable host rocks for uranium and thorium mineralization. A tentative interpretation of such contradiction suggests that uranium is largely accommodated into accessory minerals and only limited labile uranium redistribution has occurred and uranium was precipitated in association with alteration products. This interpretation is supported by the presence of kasolite Pb(UO2)SiO4.(H2O), along the fractured zones and quartz veins in the aplite‐pegmatite. The limited uranium migration in the study area was also revealed by the low weathering rate estimated by the calculations of chemical indices of alteration.

X-ray fluorescence mapping as a first-hand tool in disseminated ore assessment: sandstone-hosted U–Zr mineralization

Analytical capabilities of X-ray fluorescence for 2-dimensional elemental analysis were tested on worldwide-unique sandstone-hosted U–Zr mineralization (Bohemian Cretaceous Basin, Czech Republic), and combined with measured spectral data. The applied approach was aimed at the detection of uranium and zirconium, visualization of their distribution across mineralized lithological units and description of their relationships with associated elements (As, Fe, Nb, Pb, Sr, Ti). The studied samples were low-grade uranium-bearing sediments. Experimental work employed laboratory 2D scanning X-ray fluorescence analyzer with the gold target Mini-X X-ray tube. Maps of U–Zr and other identified elements demonstrated their patchy distribution and a frequent association of the elements with mineral fillings (clay matrix, organic matter or Fe sulfides). A correlation-based analysis was used to verify geochemical coherence between the measured elements. This showed an extremely strong linear dependence (R ≥ 0.90) between U and Zr. In addition, a strong correlation (R ≥ 0.85) between U–Zr and some associated elements (As, Nb ± Fe, Pb, Sr) was confirmed. The efficiency and the limits of X-ray fluorescence mapping were tested by comparison with high-resolution techniques. Non-destructive mapping of large natural samples provided relevant data for further mineralogical study with respect to rapid pre-concentration of the ore material.

Quantitative Mineralogy and Geochemical Coherence Through Siroquant Validation: Implications For a Kaolinite-Gibbsite-Albite Occurrence in Heterogeneous Paleozoic Bedrock of the Iberian Massif (NW Spain)

Quantitative Mineralogy and Geochemical Coherence Through Siroquant Validation: Implications For a Kaolinite-Gibbsite-Albite Occurrence in Heterogeneous Paleozoic Bedrock of the Iberian Massif (NW Spain)

Geochemistry and origin of ferruginous nodules in weathered granodioritic gneisses, Mysore Plateau, Southern India

Fe-nodules occur within saprolites formed from weathering of granodioritic gneisses in the rain-shadow region of the Mysore Plateau adjacent to the Sahyadri Mountains in Southern India. These nodules and their host saprolites were studied for their geochemistry, including chemical speciation, to understand nodule formation and chemical redistribution processes during rock weathering. From their mode of occurrence, and mineralogical and geochemical data, we infer that the nodules originated by a two-stage process in which the initial extensive weathering of gneisses likely facilitated subsequent ferrolysis weathering and nodule formation. Nodules originated by precipitation of goethite, hematite and gibbsite along with several amorphous phases within the matrix of weathered gneisses. This is possible only under hydromorphic conditions, suggesting that parts of the plateau must have gone through a humid phase prior to the present aridity. In the saprolites, Al, Fe, and Ti become enriched because of the removal of Si, Ca, Na, and K. However within the nodule, Fe, Ti, Cr, and Ni are deposited after their chemical transport from the saprolite. Titanium, known for its immobile nature, was also mobilized and concentrated under the conditions of nodule formation. The most important elements in the nodule constitution are Fe, Al, Ti, and Mn, each having both crystalline and amorphous phases. Fe–Ti and Mn oxyhydroxides grain coatings in the saprolites and discrete amorphous Mn and Ti phases in the nodules seem to have scavenged trace elements from the weathering profile. REE were mobilized during weathering and nodule genesis in which Ce and Ti show a strong geochemical coherence. The enrichment of only HREE in saprolite, and both HREE and LREE with significant Ce in the nodule, indicate the control of evolving secondary minerals in the REE redistribution during rock weathering. Strong enrichment of Ce in the weathering profile and in nodules has important implications to the REE chemistry of river waters.

Effects of Hydrothermal Alteration on Geochemical Characteristics of the El Sukkari Granite, Central Eastern Desert, Egypt

The El Sukkari granite is predominantly peraluminous and classified as intrusive in a syn-collisional setting. Geochemical evidence suggests that this granite was generated by partial melting of metagraywackes at ˜800°C as deduced from zircon saturation temperatures. The El Sukkari granite underwent multistage hydrothermal processes accompanying gold mineralization. Intensive hydrothermal alteration of the granite occurred along brittle structures that are mostly filled by dikes and quartz veins. Sodic and potassic metasomatism are the major alteration effects. Hydrothermal alteration is indicated by depletion of both light and heavy REE and Eu-enrichment. Eu-enrichment is due largely to chemical contribution from hydrothermal fluids. Other major and trace elements were enriched or depleted during one of the main alteration stages. Rb, Zn, Nb, Zr, T, Y, and Ga show depletion. Ba and Sr display enrichments, whereas Co, Li, and Cr underwent insignificant changes. Ground gamma-ray spectrometry measurements indicate that radioactive element concentrations are generally low in the altered granite. Correlations between eU, eTh, and eU/eTh of the altered granite and the plot of chemically measured U against corresponding Th values for both fresh and altered granites reflect the geochemical coherence of uranium and thorium during magma crystallization. Both elements are mainly hosted in accessory phases and were not disturbed by the hydrothermal alteration.

Geochemistry of aerosols of northwestern part of India adjoining the Thar desert

The northwestern part of India, which includes the Thar desert on its western side, is a hot and arid region with intense aeolian activity and transport of aerosols by the prevailing SW-W summer winds. Different size fractions of aerosols (free fall = FF, suspended particulate matter = SPM, PM 10 = <10 μm, and >10 μm) from air were sampled simultaneously at four locations along the dominant wind trajectory for ∼600 km. These aerosols and deposited surface sediments were characterized for their texture, mineralogy and geochemistry including REE and Sr isotopes. Within each size fraction, a bimodal distribution of grain size was observed. Quartz is the dominant mineral followed by K-feldspar, mica, calcite, chlorite and plagioclase. Garnet, amphibole, titanite and zircon are some of the identified heavy minerals. All samples, particularly those collected during summer, are compositionally homogeneous, including in their REE geochemistry, and are similar to average upper continental crust (UCC). However, in the winter aerosol samples, large deviations from the UCC composition are observed. This is attributed to meteorological parameters such as low wind velocity and temperature inversions in the winter season. During winter, secondary non-silicate and anthropogenic materials become important sources to Ca, Na, Mg, K, Ba and Ni budget; also reduction in the uptake and transport of heavy minerals lowered the concentration of Ti, Zr, Y, Cr, and REE in the aerosols. Geochemical coherence among aerosols, deposited surface sediments and the Thar sands, and the limited Sr isotopic data indicate that the Thar sediments and certain lithotectonic units of the Himalayan orogen are the proximal and distal crustal sources, respectively, for the aerosols in this region. Prevailing aridity and strong summer winds, and the presence of river alluvium in the Thar act together to transport silt rich dust, the removal of which could be a possible mechanism of ongoing desertification.

A geochemical study of two peraluminous granites from south-central Iberia: the Nisa-Albuquerque and Jalama batholiths

Abstract In this paper we present new petrological and geochemical data for two peraluminous granite batholiths (Nisa Alburquerque and Jalama batholiths) representative of the ‘Araya-type’ granites of the Central-Iberian Zone. Both granites are composite with several facies (monzogranites and leucogranites) that can be grouped into two main granite units: the external units and central units. Intrusive relationships and lack of geochemical coherence between the central and external units indicate that they are not comagmatic but represent different pulses. The central units of both batholiths are petrologically and geochemically different. On the other hand, external units show a lot of similarities and are the main object of this study. The main characteristics of the external granites can be interpreted in terms of an incomplete fractional crystallization process of early mineral phases (plg + Kf + bt) which probably took place at the level of emplacement. Other possible mechanisms of magmatic differentiation (magma mixing, restite unmixing, sequential melting) can be discarded based on field, petrography and geochemical data. We propose that the ‘Araya-type’ granites are formed by the intrusion of distinct magma pulses (central and external). Further evolution within each pulse can be due to incomplete fractional crystallization possibly taking place at the emplacement level.

A Geochemical Study of Two Peraluminous Granites from South-Central Iberia: The Nisa-Albuquerque and Jalama Batholiths

In this paper we present new petrological and geochemical data for two peraluminous granite batholiths (Nisa Alburquerque and Jalama batholiths) representative of the 'Araya-type' granites of the CentralIberian Zone. Both granites are composite with several facies (monzogranites and leucogranites) that can be grouped into two main granite units: the external units and central units. Intrusive relationships and lack of geochemical coherence between the central and external milts indicate that they are not comagmatic but represent different pulses. The central units of both batholiths are petrologically and geochemically different. On the other hand, external units show a lot of similarities and are the main object of this study. The main characteristics of the external granites can be interpreted in terms of an incomplete fractional crystallization process of early mineral phases (plg + Kf + bt) which probably took place at the level of emplacement. Other possible mechanisms of magmatic differentiation (magma mixing, restite unmixing, sequential melting) can be discarded based on field, petrography and geochemical data. We propose that the 'Araya-type' granites are formed by the intrusion of distinct magma pulses (central and external). Further evolution within each pulse can be due to incomplete fractional crystallization possibly taking place at the emplacement level.

Partitioning of platinum-group elements (Os, Ir, Ru, Pt, Pd) and gold between sulfide liquid and basalt melt

New measurements on partitioning of platinum-group elements (PGE) Os, Ir, Ru, Pt, Pd, and Au between (Fe,Ni)-sulfide liquid and basaltic melt using low pressure, sealed silica glass tube, and controlled atmosphere methods at 1200–1250°C show an extremely wide variation, which spans much of the range reported in the literature. In spite of their geochemical coherence in natural high-temperature assemblages, PGE-Au apparently behave as seven different metals in individual laboratory experiments with silicate melts. Sulfide/silicate partition coefficients, D, generally increase with run time, in a manner consistent with either serial change in chemical state of the metals in the silicate melt or passivation of alloy phases. Ruthenium is the only metal presently investigated that exhibits reversible behaviour. Summary partition coefficients for the lowest PGE contents investigated under reducing conditions and low Ni contents remain unchanged, except for the addition of a value of (4.4 ± 2.4) × 10 3 for Ru. Absence of interference from the nugget effect and homogeneity and equilibration of silicate melt in the earlier experiments is demonstrated by new measurements on Ir in different grain-size fractions of glass. An additional set of partition coefficients for 37 mol% NiS and 100–1000 ppm PGE in the sulfide is: D(Os) = (30 ± 6) × 10 3, D(Ir) = (26 ± 11) × 10 3, D(Ru) = (6.4 ± 2.1) × 10 3, D(Pt) = (10 ± 4) × 10 3, and D(Pd) = (17 ± 7) × 10 3.

Coupled molybdenum and niobium depletion in continental basalts

Molybdenum concentrations have been determined for a suite of Late Cenozoic basalt lavas from the Basin and Range province of the western United States. Lavas older than about 5 Ma, which are thought to have had a lithospheric mantle source, are strongly depleted in Nb and Mo relative to La and Nd, respectively. Lavas younger than 5 Ma, which are chemically and isotopically similar to ocean island basalt, show no such depletion in either Nb or Mo. A close geochemical coherence between Nb and Mo is consistent with their similar ionic radii (Nb 5+, 0.64; Mo 4+, 0.65). Both elements are probably retained in residual rutile in a dehydrating subducted slab, while the overlying lithospheric mantle is enriched in large-ion lithophile elements (including light REE), contained in silica-rich hydrous fluids expelled from the slab.

Intra-Ordovician deformation in southeast Ireland: evidence from the geological setting, geochemical affinities and U—Pb zircon age of the Croghan Kinshelagh granite

AbstractThe Croghan Kinshelagh alkali granite intrudes a cleaved volcano-sedimentary sequenceon the border of counties Wicklow and Wexford in southeast Ireland. U-Pb dating of zircons fromthe granite indicate a mid-Caradoc emplacement age of 454 ± 1 Ma. The Duncannon Group hostrocks form the southwestern end of the Avoca Volcanic Belt, a Mid-Ordovician (Caradoc) sequenceof acid and intermediate lavas and volcaniclastics. Dolerite dykes intrude the granite; elsewhere in theregion dolerites are generally associated with volcanic rocks. The main, Dl deformation within theDuncannon Group rocks is manifest as a steep Dl cleavage generally regarded as a product of LateCaledonian regional deformation in southeast Ireland. The Croghan Kinshelagh granite showsstrong geochemical coherence with subalkaline varieties of the Caradoc volcanic rocks; relativelyhigh Th, Y, Nb and REE contents set it apart from any other known granite type in southeastIreland. Together with the geochemical evidence, the age determination of 454 Ma indicates that theCroghan Kinshelagh granite was generated and emplaced during Ordovician volcanism in southeastIreland. Volcanism was closely followed by penetrative deformation and emplacement of the granite.The intra-Ordovician deformation may have been a consequence of closure of the Iapetus Ocean ormore localized events such as accretion on the hanging wall of the subduction zone. The age of theCroghan Kinshelagh granite provides an important datum for Ordovician volcanism and subductionin southeast Ireland.

Late proterozoic fractionated granitoids of the mineralized Telfer area, Paterson province, Western Australia

Abstract The Proterozoic Paterson Province is a newly defined, polymetallic province which has been interpreted to represent a collisional orogenic belt between the geologically contrasting Western Australian Craton and Northern Australian Province. It contrasts with the Proterozoic terrains of Northern Australia which are interpreted to be intracratonic basins. Within the Telfer area of the Paterson Province, late- to post-tectonic monzogranites to alkali-feldspar granites intrude the upper Yeneena Group (> 750, The granitoids are highly fractionated (average SiO2 content > 71 wt.%, Rb/Sr ranges up to 20.5), metaluminous and I-type in nature, and some suites have high contents of heat-producing elements (average pre-weathering U and Th contents of 15 ppm and 66 ppm, respectively, and average K2O content of 4.9 wt.%). On a global scale these features are common to granitoids associated with mineralization. The granitoids generally show geochemical coherence on variation diagrams, but the contrasting Pb-isotope initial ratios for two of the complexes indicate that they are not cogenetic. This limited geochemical variation is typical for fractionated granitoids, due to a minimal restite component, and the behaviour of trace elements is controlled by the fractional crystallization of accessory minerals. Similarly, the granitoids plot close to boundaries between fields for collisional, volcanic-arc and within-plate granitoids because of their highly fractionated nature (i.e. high Rb): this is probably a general problem for fractionated granitoids on such discriminant diagrams. However, the Telfer granitoids can be broadly discriminated as syn- to post-collisional with calc-alkaline compositions. The granitoids of the Telfer area are thus late- to post-tectonic granites whose age and geochemistry are both significantly different to other Proterozoic granites in Northern Australia. Their young age, calc-alkaline I-type association, the short crustal residence time of their source and their interpreted syn- to post-collisional environment all support the recently proposed model that the Paterson Province represents a unique tectonic setting in the Proterozoic, involving the welding of two continental plates at ∼690 Ma.

The geologic setting, mineralogy, geochemistry, and genesis of gold deposits of the Archean Kolar schist belt, India

The Kolar schist belt, one of the auriferous Archean schist belts in the Dharwar craton, includes two types of gold deposits: (1) a stratiform sulfide type, associated with amphibolites and banded ferruginous quartzite, and (2) a gold quartz-carbonate vein type associated with light rare earth element (LREE)-enriched komatiitic and tholeiitic amphibolites on the eastern part of the belt. Many lodes of the former are banded and show deformation similar to that of their host rocks. The latter type consists of veins localized within shear zones. Both types have only a few inches of altered border rock characterized dominantly by biotite with minor diopside and garnet.The sulfide lodes contain 5 to 30 vol percent sulfides which include dominantly pyrrhotite and arsenopyrite. The lodes also contain variable amounts of magnetite and ilmenite. No systematic variation in ore minerals is discernible among the lodes of the entire belt. However, in the Kolar gold fields area, a set of four parallel sulfide lodes exhibit certain textural and mineralogical zoning. Ore minerals show metamorphic equilibrium textures and the temperature of final equilibration of sulfides is 500 degrees C or higher.Sulfide lodes have low but variable abundances of base metals. No correlation among total sulfide contents, sulfide minerals, and abundances of base metals and gold is observed. Nor is there any geochemical coherence between gold and arsenopyrite in the several lodes studied. Major oxides, particularly Al 2 O 3 , TiO 2 , and K 2 O also show a large variation. The REE abundances of the sulfide lodes vary widely, but their patterns are similar, with enrichment in both light REE and heavy REE. Normalized Eu shows strong to negligible enrichment. The REE patterns are similar to those of the banded ferruginous quartzite or iron-formation. Samples with high REE abundances also have high base metals, Al 2 O 3 , TiO 2 , and K 2 O abundances and low arsenopyrite contents. Based on these observations and limited Nd and O isotope data it is suggested that the sulfide lodes were originally auriferous, complex interflow sediments formed from submarine hydrothermal exhalations, seawater, and detrital sediments.The vein-type mineralization has only trace amounts of sulfides with galena as the dominant phase (Narayanaswami et al., 1960). The veins consist dominantly of quartz with minor amounts of calcite and alkali feldspar and trace amounts of scheelite and tourmaline. The gold content of the veins is generally > 10 ppm. The veins are also unusually enriched in Cr and Ni. These observations and the fluid inclusion studies on vein quartz (Santosh, 1986) suggest the presence of CO 2 -bearing ore fluids.One sample of the ore vein has a strongly fractionated, light REE-enriched pattern with an epsilon Nd value of +2.8 at 2,600 Ma. Galena from the ore samples have Pb isotope ratios, reported by Venkatasubramanian et al. (1977) and Chernyshev et al. (1980), that are similar to those of the 2,600-Ma intrusive granodioritic gneisses present on the west side of the schist belt (Krogstad et al., 1989). The delta 18 O of vein quartz is 11.6 + or - 0.7 per mil (Golding, 1982). These geochemical data suggest that the gold quartz veins were formed dominantly from magmatic fluids derived from the crystallization of granitic rocks. A contribution from metamorphic fluids is also conceivable as indicated by the abundance of Ni and Cr in the veins.

Plutonium, uranium and rare earths in the phosphates of ordinary chondrites—the quest for a chronometer

The distributions of Pu, U and the REE (in particular Nd) in single crystals of the calcium phosphates, merrillite and apatite, of ordinary chondrites were studied to establish whether 244Pu can be used to determine time differences in meteorite formation. Uranium was measured by an induced-fission track technique and the REE by secondary ion mass spectrometry. Limits on Pu concentrations (at the time of track retention) were inferred from fossil track measurements. It is argued here that most previous estimates of Pu concentrations, obtained from fossil tracks in phosphates, failed to correct adequately for spallation-induced tracks and were probably not correct. Plutonium is preferentially enriched in merrillite relative to apatite. In contrast, U concentrations are on the average 15 times higher in apatite than in merrillite. There are grain to grain variations, in the same phosphate phase from a given meteorite, for both elements. On the other hand, in the merrillite of a given meteorite, the REE concentrations are remarkably constant (typical grain to grain variations for type 5 and 6 chondrites are in most cases less than 10%). Average Nd concentrations in the merrillite of 15 different chondrites range from 96 to 164 ppm. REE abundances in apatite show more variations. Average Nd concentrations in the apatite of 13 chondrites range from 8 to 54 ppm and the ratios of Nd concentrations in merrillite relative to apatite range from 2.7 to 11. Despite the higher affinity of both Pu and the REE for merrillite than for apatite, there is no quantitative correlation between the abundances of these elements in merrillite grains from a given ordinary chondrite. Because of this lack of geochemical coherence between Pu and any of the REE, it is concluded that 244Pu cannot be used to determine the relative formation times of chondrites. Bulk measurements that avoid the problems associated with fossil track measurements are discussed, however they have so far proven to be of little utility.

Heavy metal analyses in the marine environment - approaches to quality control

The validity of analyses of elements in the femto- and picomolar ranges in waters and sediments can be assessed on the basis of several criteria: an evaluation of the chemical technique; the use of primary standards and intercalibration exercises; and chemical and geochemical coherence of the results. Herein are reported analyses of ruthenium, platinum, palladium, rhenium, iridium and gold in oceanic samples. The gold concentrations in surface seawaters, at least 25 times less than those previously reported, are in accord with the concentrations of their periodic table neighbors as well as with crustal and sedimentary contents. The remarkable concentration of rhenium in seawater, in relation to its very low crustal abundance, can be explained by the inertness of the perrhenate ion.

Experimental determination of crystal/melt partitioning of Ga and Ge in the system forsterite-anorthite-diopside

The crystal/liquid partitioning of Ga and Ge has been measured experimentally between forsterite, diopside, anorthite and spinel and melts in the pseudoternary system forsterite-anorthite-diopside at one atmosphere pressure and 1300°C. Gallium is incompatible with forsterite and diopside [ D ( Ga) = 0.024 and 0.19, respectively], is only slightly incompatible in anorthite [ D ( Ga) = 0.86] and is highly compatible in spinel [ D ( Ga) = 4.6]. The partition coefficient for Ge is within a factor of two of unity for forsterite, diopside and anorthite [ D ( Ge) = 0.62, 1.4 and 0.51, respectively], but Ge is incompatible in spinel [ D ( Ge) = 0.1]. The coefficients for the exchange of Ga and Al and the exchange of Ge and Si between minerals and melts generally are within a factor of two of unity, as is expected from the geochemical coherence of these element pairs in natural samples. The application of our results to the interpretation of natural basaltic and mantle samples from the Earth and basalts from the Moon and the Shergottite Parent Body demonstrates that it is possible to discriminate between different mantle source compositions using Ga/Al and Ge/Si ratios. The Ge variation among lunar mare basalts may be indicative of a heterogeneous lunar mantle. The substantial depletion of Ge in Chassigny relative to the other SNC meteorites may be evidence of either a heterogeneous Shergottite Parent Body (SPB) mantle, or of different geochemical behavior for Ge in the SPB.

Experimental geochemistry of Pu and Sm and the thermodynamics of trace element partitioning

The partitioning of Pu and Sm between diopside/liquid and whitlockite/liquid has been investigated experimentally to evaluate the geochemical coherence of Pu and the light REEs. PuD SmD is 2̃ for both diopsidic pyroxene and whitlockite. This small amount of fractionation would be decreased further if Pu were compared to Ce or Nd. Our experimental results thus validate the suggestion that Pu behaves as a LREE during igneous processes in reducing environments. Our data and the data of Ray et al. (1983) indicate that temperature rather than melt composition is the most important control on elemental partitioning. This is true even though we demonstrate that additions of only 1–2 wt.% of P 2O 5 to the diopside-anorthite-albite system change Pu D cpx by a factor of two. Our data suggest that P 2O 5 in aluminosilicate melts serves as a complexing agent for the actinides and lanthanides.

Oil shales, evaporites and ore deposits

The relationships between oil shales, evaporites and sedimentary ore deposits can be classified in terms of stratigraphic and geochemical coherence. Oil shale and black shale deposition commonly follows continental red beds and is in turn followed by evaporite deposition. This transgressive-regressive sequence represents an orderly succession of depositional environments in space and time and results in stratigraphic coherence. The amount of organic carbon of a sediment depends on productivity and preservation, both of which are enhanced by saline environments. Work on Great Salt Lake. Utah, allows us to estimate that only 5% of TOC originally deposited is preserved. Inorganic carbonate production is similar to TOC production, but preservation is much higher. Oil shales and black shales commonly are enriched in heavy metals through scavenging by biogenic particles and complexation by organic matter. Ore deposits are formed from such rocks through secondary enrichment processes, establishing a geochemical coherence between oil shales and ore deposits. The Permian Kupferschiefer of N. Europe is used as an example to define a Kupferschiefer type (KST) deposit. Here oxygenated brines in contact with red beds become acidified through mineral precipitation and acquire metals by dissolving oxide coatings. Oxidation of the black shale leads to further acid production and metal acquisition and eventually to sulfide deposition along a reducing front. In order to form ore bodies, the stratigraphic coherence of the red bed-black shale-evaporite succession must be joined by the geochemical coherence of the ore body-evaporite-black shale association. The Cretaceous Cu-Zn deposits of Angola, the Zambian Copperbelt as well as the Creta, Oklahoma, deposits are other KST examples. In the Zambian Copperbelt, evaporites are indicated by the carbonate lenticles thought to be pseudomorphs after gypsum-anhydrite nodules. MVT deposits are also deposited by acid brines, but at more elevated temperatures and with carbonates as principal host rocks. The Pine Point deposits are cited for their close association with evaporites. Alkaline, metal-rich brines are postulated for the HYC deposit of McArthur River, Australia. Such brines are known from the Green River Formation and deposits formed from such brines constitute the GRT class. They can be recognized by the presence of Magadi-type cherts and zeolite-analcime-K-spar tuffs. The Cu-Co ore bodies of Outokumpu, Finland, might also belong to this type. A new classification of sedimentary ore deposits is proposed, based on their geochemical environment. KST and MVT are formed from acid ore fluids, while GRT and CT (Creede type) are derived from basic ore fluids. pH of the fluids is best evaluated not from the ores themselves, but from their effect on the host-rocks.

The behavior of thallium during alteration of the K-alkaline rocks from the Roccamonfina Volcano (Campania, southern Italy)

Tl behavior in the alteration of leucite and related K-alkaline rocks from the Quaternary Roccamonfina Volcano has been outlined. The dominant weathering product is analcite, that shows a moderate loss of Tl relative to the coexisting leucite (R̄ = 0.74 ± 0.21). Weathering of leucite to halloysite is subordinate and characterized by a very strong depletion in Tl (R̄ = 0.14 ± 0.15).The geochemical coherence between Tl and K observed in the Roccamonfina igneous rocks breaks down when leucite weathers to analcite, as, unlike Tl, K is almost totally lost. In contrast, in the alteration of leucite to halloysite both elements still behave coherently, being almost totally mobilized to groundwater.In spite of the significant amount of Tl made available by weathering of Tl-rich country rocks, groundwaters from the study area are not Tl-enriched, their contents matching the average for natural waters. On the other hand at Roccamonfina widespread sedimentary Fe-rich Mn-bearing occurrences are richer in Tl than any other rock. These findings lead one to infer that Tl mobility as well as its concentration level in groundwater are strongly attenuated due to the scavenging properties of such Fe- and Mn-bearing sediments.

Trace element determination in lunar rocks — quantitative pixe analyses on thick samples

Information about the formation processes and geochemical coherence can be obtained by determination of the partitioning of trace elements in coexisting minerals. Systematic trace element analysis among coexisting mineral phases requires the measurement of the elemental concentrations in a small volume, which can be obtained by applying a proton microprobe. Different opaque minerals, like armalcolite (Fe, Mg) Ti 2O 5, ilmenite FeTiO 3, ulvöspinel Fe 2TiO 4 and chromite FeCr 2O 4 (of the spinel solid solution series Fe 2TiO 4FeCr 2O 4) in samples of three Apollo missions were investigated. Comparative analyses of the mineral pair armalcolite/ilmenite in two Apollo 17 basalts (sample 74242,19), which differ in their crystallization sequences, show that the partitioning of Zr among the two mineral phases is different in the two basalt types. Coexisting ilmenite/ulvöspinel were studied in the Apollo 15 basalts (samples 15065,93; 15555,42). The trace elements Zr and Nb were found to partition in favour of ilmenite. The investigated mineral pairs of chromite/ulvöspinel in Apollo 12 (samples 12051,60; 12051,62) and Apollo 15 basalts are zoned in their major element contents. The trace element Zr detected by proton microprobe analyses also shows zoning behaviour. Primarily, crystallized chromite does not incorporate Zr, whereas the Zr content in the coexisting ulvöspinel rises with increasing Ti content.