Mo Anomalies Research Articles

Geochemical indicators of major, trace, and REE elements for the alteration lithofacies in the Oyu Tolgoi copper-gold deposit in Mongolia

The Oyu Tolgoi world-class porphyry copper-gold deposits in Mongolian are that supergene and epithermal hydrothermal system superimposed in the early stage of porphyry system. Lithofacies of alterations, mineral and geochemical comprehensive methods were employed. Objective of this article are to investigate their formation environment of alteration rocks and mineralization in order to establish REEs and comprehensive prospecting indicators of alteration lithofacies, especially to find ore-forming center for this superimposed mineralization systems. Based on this study, REEs and geochemical prospecting indicators of alteration veins are of evident positive Eu anomaly, and are higher in concentrations of Fe2O3, CaO, S and TiO2, with obvious positive U–K, Pb, P and Ti anomalies. It is very useful to catch the advanced argillic alteration lithofacies to delineate coverage of the later stage epithermal system, in which they are characterized by “U”-shape pattern normalized by chondrite as well as obviously anomalous concentrations of Cu, As, Se, Ba, Pb and Bi, with higher concentrations of K2O, Al2O3 and S. Moreover, it is very valuable to discovery the intermediate argillization lithofacies to outline the mineralized center in which are typified by veinlets and brecciated hematite-magnetite silicification rock, and hematite-ferrocarbonatization rock. The mineralized center is typified by silicification altered rock in structure of veinlets-shape or the lithocap. The hydrothermal breccia zone may be an indicator of structure-lithology for the excrete channel and feeder of metallogenic fluid transportation in the epithermal system. Therefore, it is very more important that they are characterized by noticeable negative Eu anomaly (Eu*＝－0.57) and smooth-shape pattern normalized by chondrite. Copper mineralized center is implied by transitional environment of oxidation-reduction characterized by the ratios of Fe3+/Fe2+ from 0.72 to 2.13, plus Cu、As、Se、Ba、Pb and Bi anomalies, occurred lithofacies of the intermediate argillic alteration are considered to be localization markers whenever associations of Cu、V、Zn、Se and Mo anomalies may be signs for orebody finder.

Initial exploration results of the Collins epithermal Au‐base metal prospect, Aceh, Indonesia

AbstractInterpretation of various exploration data, in particular geochemical prospecting, offers a powerful and rapid assessment of grass‐root projects in a green‐field terrain. Here, we present an example of the Collins epithermal prospect in Aceh Province, Indonesia. In this area, the Au+ base‐metal‐bearing sheeted quartz veins (individually mostly 2–4 cm wide), which are controlled by a 250 m wide by 800 m long NNE‐trending structural corridor within Paleogene sandstone and volcanic rocks, are the product of two main stages of deposition. Stage I formed veins with a sliver of cryptocrystalline quartz wall zone followed by an inner zone of comb quartz with interstitial rhombic adularia that terminates in open space. Stage I or main‐stage sulfide mineralization consisting of early galena + sphalerite and later chalcopyrite occurs with the quartz + adularia. Small amounts of galena also occur in the wall zone. Stage II mineralization brecciated Stage I veins and overprinted them with silicification characterized by vuggy texture. Mineralization associated with this episode consists of earlier chalcopyrite + sphalerite + tennantite–tetrahedrite and later, vug‐filling Au–Ag alloy (Ag0.37–0.41Au0.62–0.59). The above mineralized veins are successively flanked by silicic selvages, an illite + chlorite + pyrite ± kaolinite zone and a chlorite + epidote + carbonate + pyrite zone. Local supergene alteration induced replacement of galena by plumbogummite and anglesite and chalcopyrite by covellite. Data from fluid inclusion microthermometry in quartz indicated that the inner zone of Stage I veins formed from fluids with a 2.3 wt% salinity (0.5–3.3 wt% NaCl equivalent), at 174°C (155–211°C). Combining these physico‐chemical parameters with the mineral assemblage, the mineralization occurred under a reduced environment. Rock and soil assays indicate that elevated Au concentrations (up to 16.5 ppm over 1 m) occur along northeast‐trending zones and show a strong correlation with Pb, while Cu (up to 2.58% over 1 m), Zn, As, Sb, and Mo anomalies lie mostly at the periphery. The high‐grade mineralized veins correlate with moderate to high resistivity and chargeability zones, and the pseudosections of such geophysical signals are interpreted as reflecting coalesced or enlarged veins at depth, or inclined veins in other localities. The intermediate sulfidation affinity for Collins points to potential mineralization at depth as well as preservation of Au‐rich and sulfide‐poor zones in the less eroded areas.

Integrating sequential indicator simulation and singularity analysis to analyze uncertainty of geochemical anomaly for exploration targeting of tungsten polymetallic mineralization, Nanling belt, South China

The Nanling belt (South China) is one of the most important metallogenic region in the world in regard to tungsten polymetallic resources. In this belt and surrounding area, the most important elements accompanying tungsten deposits are Sn, Bi, and Mo. Modelling the spatial variability and uncertainty of W, Sn, Bi, and Mo anomalies is critical for tungsten polymetallic exploration risk assessment. However, traditional interpolation methods (e.g., kriging, polynomial trend surface and inverse distance weighted method) for modelling continuous geochemical fields based on sparse sampling data provide smoothed representations of element concentrations, which often result in unreliable decisions. Here, uncertainty analysis of geochemical anomaly for tungsten polymetallic exploration targeting was investigated through combined sequential indicator simulation and local singularity analysis. Anomalous thresholds of E-type singularity indices for the selected elements (W, Sn, Mo and Bi) were determined by singularity-quantile plot analysis. The distribution pattern of probability of not exceeding a threshold singularity index was obtained from a series of equally probable representations of singularity indices of individual elements (W, Sn, Mo and Bi) based on local and spatial uncertainty algorithms. Based on the four probabilistic models, one per element, a synthetic probability map was produced for uncertainty assessment of geochemical anomaly for exploration targeting. The results indicate that zones with very high probabilities are significantly correlated with known tungsten polymetallic deposits and are closely associated with lithostratigraphic contacts. These results provide for important decision-making for risk evaluation of tungsten polymetallic exploration targeting based on spare stream sediment geochemical data in the Nanling belt.

Comparative analysis between concentration- number (C-N) and concentration- area (C-A) fractal models for separating anomaly from background in Siahrood 100,000 sheet, NW Iran

Summary In this paper Concentration-Number (C-N) and Concentration-Area (C-A) fractal models were used for separating anomaly from background for Cu, Mo, Fe2O3 and Sb in Siahrood 1:100,000 sheet, NW Iran. First, 1238 stream sediment samples were taken and analyzed. Then, the threshold values were obtained for anomaly separation from the log- log plots. The log–log plots can be divided into three segments or phases that correlate with lithological formation, faults and alteration in the study area. Using the results of fractal modelling, the anomaly maps were drawn and anomalous areas were identified. Elemental anomaly maps resulted from the C-A and C- N fractal models were compared withthe locations of mineralization/deposits in the study area.Results showed thatthere was a good correlation between the locations of mineralization/deposits and anomaly region for both fractal models. But the concentration-area fractal model could hit more mineralization/deposits. This is more obvious for Cu. Results also showed that most of the Cu, Fe and Mo anomalies located in igneous rocks, altered igneous rocks and faults. Because of activity of hydrothermal solutions in intrusive rocks in the northern part of the study area and volcanic rocks in central parts, intrusive and volcanic rocks, altered and quartzite veins, and mineralization zones occur. Introduction Delineation of anomalies from background is an essential task in geochemical exploration. Geochemical data are usually characterized by their spatial positions, meaning that elemental concentration varies spatially. The concentration–area model (C–A model) has been developed and applied by many geoscientists. Another useful model for separating anomaly from background is the number–size model (N–S model), which has been widely used by many geoscientists. Methodology and Approaches Concentration-number and concentration-area fractal models were used for separating anomaly from background. Then with drawingthe log-log elemental plot the threshold values obtained for anomaly separation. Using the results of fractal modelling the anomaly maps were drawn and anomaly regions were identified. Elemental anomaly maps resulted from the concentration-area and concentration- number fractal models were compared withthe locations of mine indexes in the study area. Results and Conclusions Results showed thatthere was a good correlation between the locations of mine indexes and anomaly region for both fractal models. In other words most of anomalies derived from C-A and N-S models hits mine indexes in the region. But the concentration-area fractal model hit more mine indexes. This is more obvious for Cu. Results also showed that most of the Cu, Fe and Mo anomalies located in igneous rocks, altered igneous rocks and faults.

Radial mixing and Ru–Mo isotope systematics under different accretion scenarios

The Ru–Mo isotopic compositions of inner Solar System bodies may reflect the provenance of accreted material and how it evolved with time, both of which are controlled by the accretion scenario these bodies experienced. Here we use a total of 116 N-body simulations of terrestrial planet accretion, run in the Eccentric Jupiter and Saturn (EJS), Circular Jupiter and Saturn (CJS), and Grand Tack scenarios, to model the Ru–Mo anomalies of Earth, Mars, and Theia analogues. This model starts by applying an initial step function in Ru–Mo isotopic composition, with compositions reflecting those in meteorites, and traces compositional evolution as planets accrete. The mass-weighted provenance of the resulting planets reveals more radial mixing in Grand Tack simulations than in EJS/CJS simulations, and more efficient mixing among late-accreted material than during the main phase of accretion in EJS/CJS simulations. We find that an extensive homogeneous inner disk region is required to reproduce Earth's observed Ru–Mo composition. EJS/CJS simulations require a homogeneous reservoir in the inner disk extending to ≥3–4 AU (≥74–98% of initial mass) to reproduce Earth's composition, while Grand Tack simulations require a homogeneous reservoir extending to ≥3–10 AU (≥97–99% of initial mass), and likely to ≥6–10 AU. In the Grand Tack model, Jupiter's initial location (the most likely location for a discontinuity in isotopic composition) is ∼3.5 AU; however, this step location has only a 33% likelihood of producing an Earth with the correct Ru–Mo isotopic signature for the most plausible model conditions. Our results give the testable predictions that Mars has zero Ru anomaly and small or zero Mo anomaly, and the Moon has zero Mo anomaly. These predictions are insensitive to wide variations in parameter choices.

Similarities and differences in geochemical distribution patterns in epiphytic lichens and topsoils from kindergarten grounds in Vilnius

Topsoil and lichen Phaeophyscia orbicularis were sampled from the grounds of kindergartens (Vilnius, Lithuania) using a side-by-side design and analysed for the total contents of Al, As, Br, Ca, Cl, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, P, Pb, Rb, S, Si, Sr, Ti, V and Zn. Only Si, Mn and Zn were found to retain their positions in sequences arranged in descending order of their mean contents (>90mgkg−1) in topsoil (Si>Al>Ca>K>Fe>Mg>Na>Ti>P>S>Mn>Cl>Zn) and in lichens (Si>Ca>K>S>Al>Fe>P>Mg>Na>Cl>Mn>Ti>Zn). In lichen thalli, unlike in topsoil, nutrients and lithogenic elements formed separate clusters. Results proved that by origin, the elements captured by lichens were not only from the immediate environs, but also from more distant city districts with different soil lithology. However, both in topsoil and in lichens, As, Cr, Cu, Mo, Ni, Pb, V and Zn form together with S and Br clusters of closely related elements indicating that Phaeophyscia orbicularis is a good urban indicator of polluting elements using which total contamination indices of topsoil ZT and of lichens ZL can be calculated. Higher ZT values were detected in the former industrial-residential areas, while ZL values were higher in new residential-commercial areas. The ratio ZT/ZL was found to be useful in revealing areas where pollution is on the increase.The contents of Cr, Cu, Mn, Mo, Ni, Pb, V and Zn in topsoil from the same sites which had been sampled five years ago significantly (p<0.05) decreased. Locations of Cr, Mn, Ni, V and Zn anomalies were found to have considerably changed, meanwhile Pb, Cu and Mo anomalies were found to be shifting least of all.

Comparative study of C–A, C–P, and N–S fractal methods for separating geochemical anomalies from background: A case study of Kamoshgaran region, northwest of Iran

Separation of geochemical anomalies from background is a fundamental topic in the field of geochemical exploration. Fractal and multifractal modeling of geochemical data are among these methods and have been used by many geoscientists for more than two decades. In this study, three fractal methods, consisting of concentration–area (C–A), concentration–perimeter (C–P), and number–size (N–S) methods, were applied to identify geochemical anomalies in 317 stream sediment samples from Kamoshgaran region, Kurdestan province, northwest of Iran. Implementation of C–A, C–P, and N–S methods showed that southwestern, eastern, and central parts of Kamoshgaran region were the most important parts and future detailed exploration including lithogeochemical sampling should be focused on these parts. Also, a good relationship was found between alteration units and anomalous areas using fractal methods. Results also demonstrated a strongly positive relationship between As and Au anomalous areas in the region. The resulted thresholds for C–A and C–P methods were slightly different; consequently, the suggested anomalous areas were slightly different. In addition, distinction of the anomalous areas in terms of Au, Cu, and Mo based on N–S method was much better, which could be attributed to the point that there was no data pre-processing and only the raw data were processed for drawing the N–S log–log plot. Maps of the anomalies revealed that As, Cu, and Mo anomalies were simultaneously located in the eastern part of the studied area and corresponded to Au anomalies. From this point of view, it can be said that the studied area could be very important for Au mineralization and its eastern part can be the target of future exploration.

QUATERNARY INTERGLACIAL SEDIMENTS AS POSSIBLE NATURAL SOURCES OF ARSENIC AND MOLYBDENUM ANOMALIES IN STREAM SEDIMENTS IN LITHUANIA

Geochemical investigations were conducted on Quaternary interglacial sediments in order to reveal if they can be a natural source of 10 potentially harmful chemical elements: As, Ba, Cr, Cu, Mn, Mo, Ni, Pb, V and Zn. Determination of the total contents of elements in 680 samples was performed by EDXRF analysis. The content of Ni, Cu, Pb, Ba in all analyzed samples was lower than maximum permissible concentrations (MPC) for soil, the content of Zn, V, Cr, Mn exceeded MPC values only in several samples (≤4%). The highest percentage of anomalous samples where MPC was exceeded was characteristic of Mo (21%) and As (12.6%). Therefore, interglacial sediments, especially enriched in organic matter and other sorbents, comprise one of the possible natural sources of Mo and As. It is probable that they contribute to As and Mo anomalies in stream sediments in Lithuania.

Coupled W–Os–Pt isotope systematics in IVB iron meteorites: In situ neutron dosimetry for W isotope chronology

Tungsten isotope compositions of magmatic iron meteorites yield ages of differentiation that are within ±2Ma of the formation of CAIs, with the exception of IVB irons that plot to systematically less radiogenic compositions yielding erroneously old ages. Secondary neutron capture due to galactic cosmic ray (GCR) irradiation is known to lower the ε182W of iron meteorites, adequate correction of which requires a measure of neutron dosage which has not been available, thus far. The W, Os and Pt isotope systematics of 12 of the 13 known IVB iron meteorites were determined by MC-ICP-MS (W, Os, Pt) and TIMS (Os). On the same dissolutions that yield precise ε182W, stable Os and Pt isotopes were determined as in situ neutron dosimeters for empirical correction of the ubiquitous cosmic-ray induced burn-out of 182W in iron meteorites. The W isotope data reveal a main cluster with ε182W of ∼−3.6, but a much larger range than observed in previous studies including irons (Weaver Mountains and Warburton Range) that show essentially no cosmogenic effect on their ε182W. The IVB data exhibits resolvable negative anomalies in ε189Os (−0.6ε) and complementary ε190Os anomalies (+0.4ε) in Tlacotepec due to neutron capture on 189Os which has approximately the same neutron capture cross section as 182W, and captures neutrons to produce 190Os. The least irradiated IVB iron, Warburton Range, has ε189Os and ε190Os identical to terrestrial values. Similarly, Pt isotopes, which are presented as ε192Pt, ε194Pt and ε196Pt range from +4.4ε to +53ε, +1.54ε to −0.32ε and +0.73ε to −0.20ε, respectively, also identify Tlacotepec and Dumont as the most GCR-damaged samples. In W–Os and W–Pt isotope space, the correlated isotope data back-project toward a 0-epsilon value of ε192Pt, ε189Os and ε190Os from which a pre-GCR irradiation ε182W of −3.42±0.09 (2σ) is derived. This pre-GCR irradiation ε182W is within uncertainty of the currently accepted CAI initial ε182W. The Pt and Os isotope correlations in the IVB irons are in good agreement with a nuclear model for spherical irons undergoing GCR spallation, although this model over-predicts the change of ε182W by ∼2×, indicating a need for better W neutron capture cross section determinations. A nucleosynthetic effect in ε184W in these irons of −0.14±0.08 is confirmed, consistent with the presence of Mo and Ru isotope anomalies in IVB irons. The lack of a non-GCR Os isotope anomaly in these irons requires more complex explanations for the production of W, Ru and Mo anomalies than nebular heterogeneity in the distribution of s-process to r-process nuclides.

Chalcophile element systematics in volcanic glasses from the northwestern Lau Basin

The Lau Backarc Basin (S.W. Pacific) hosts numerous spreading centers and rifts, including the Rochambeau Rifts (RR), Northwest Lau Spreading Center (NWLSC), and Central Lau Spreading Center (CLSC). Samples from the NWLSC, RR and CLSC show no evidence for a subduction‐derived component in their mantle source regions or evidence for S loss during eruption. The contents of S in glasses from the NWLSC and many from the CLSC and the RR are lower than MORB at a given FeOTOT, indicating melts were initially sulfide‐undersaturated. During differentiation, the decrease in Cu and Ag contents at ∼7 wt% MgO and the concomitant change in chalcophile element ratios marks the onset of sulfide saturation. The initially sulfide‐undersaturated compositions of samples from the NWLSC are attributed to partial melting at pressures higher than parental MORB. The NWLSC and some of the CLSC and RR samples are strikingly enriched in Cu and Ag compared with MORB. This is a characteristic shared by basalts generated in many plume‐related tectonic settings. The only plume‐related samples that appear to be sulfide‐saturated during differentiation and plot within the MORB array are alkaline basalts from the nearby Samoan islands. RR and CLSC basalts have a range in Cu contents, which can be explained by variable mixing between a high‐Cu NWLSC‐type melt with low‐Cu sources from the Samoan plume (RR) and MORB‐type mantle (CLSC). The RR alone of these three suites have markedly positive Pb, As, Tl and subtle Mo anomalies, possibly related to assimilation of old, hydrothermally altered, Vitiaz Arc crust.

Molybdenum isotope anomalies in meteorites: Constraints on solar nebula evolution and origin of the Earth

The early evolution of the solar nebula involved substantial transport of mass, resulting in mixing and homogenization of isotopically diverse materials that were contributed to the solar system from multiple stellar nucleosynthetic sources. The efficiency of this mixing, as well as its timescale can be quantified by determining nucleosynthetic isotope variations among meteorites and terrestrial planets. Here we present Mo isotopic data for a wide range of samples, including Ca–Al-rich inclusions, chondrites and differentiated meteorites, as well as martian and terrestrial samples. Most meteorites are depleted in s-process Mo relative to the Earth, and only the IAB–IIICD irons, angrites and martian meteorites have terrestrial Mo isotopic compositions. In contrast, most Ca–Al-rich inclusions are enriched in r-process Mo, but one inclusion is characterized by a large s-process deficit. Molybdenum isotopic anomalies in the bulk meteorites correlate with those in Ru exactly as predicted from nucleosynthetic theory, but no obvious correlation is apparent between Mo and Ni anomalies. Therefore, s-process Mo and Ru seem to be hosted in the same carrier, which must be distinct from the carrier responsible for isotopic anomalies in the Fe-group elements (Ni, Cr, Ti). Furthermore, the isotopic heterogeneity in Mo (and other elements) contrasts with the isotopic homogeneity for Hf and Os, indicating that different s-process carriers once existed in the early solar nebula and that only some of these were heterogeneously distributed. The Mo isotopic anomalies of meteorites and their components decrease over time and with increasing size of the parent bodies, providing evidence for a progressive homogenization of the solar nebula. However, the carbonaceous chondrites exhibit larger Mo anomalies than expected for their age, indicating that they received a greater portion of material from the outer solar system (where homogenization was slow) than other meteorite parent bodies and terrestrial planets. Compared to the meteorites, Earth is enriched in s-process Mo and must have accreted from material distinct from the meteorites. Combined Mo and O isotopic data show that the composition of the Earth cannot be reconstructed by any known combination of meteorites, implying that meteorites may be inappropriate proxies for the isotopic composition of the bulk Earth. This is exemplified by the covariation of 92Mo and 142Nd anomalies in chondrites, showing that the 142Nd deficit of chondrites compared to the accessible Earth may not unequivocally be interpreted as a signature of an early differentiation of the Earth. However, further high precision isotopic data are needed to evaluate the role of chondrites in defining the isotopic composition of the Earth.

Butalón igneous rocks, Neuquén, Argentina: Age, stratigraphic relationships and geochemical features

Three main groups of igneous rocks of different age are recognized in the Butalón area, including volcanic rocks of the Choiyoi Group (Permian–Triassic), stocks and dikes of the Granodiorita Varvarcó (Upper Cretaceous–Paleocene) and stocks and dikes of the Tonalita Butalón (Paleocene–Eocene?). The Choiyoi Group consists of ignimbrites, breccias, tuffs and silicic lavas. Most are subalkaline and four groups can be distinguished on the basis of their trace element patterns. Most samples are enriched in Ba, but depleted in Nb, Sr, P, Ti and Eu. The subvolcanic rocks of the Granodiorita Varvarcó and Tonalita Butalón are mainly calc-alkaline, metaluminous to peraluminous, with low to medium potassium contents. They have similar subparallel REE and multielement patterns with low La/Yb and Eu/Eu ∗ ratios. These patterns mimic those observed in Pleistocene–Holocene volcanic rocks, of similar SiO 2 content, from the CSVZ of the Andes. They are depleted in Nb and Ti, have normal to low P, and are enriched in Sr and Zr. Hornfels-skarn type hydrothermal alteration is produced by Tonalita Butalón in the adjacent igneous rocks. Magmatic-hydrothermal breccias are developed on top of some of the stocks. While some of the breccias exhibit Mo anomalies, Au and Ag anomalies are present in the Choiyoi Group.

Hydrothermal alteration mapping using ASTER data in the Infiernillo porphyry deposit, Argentina

The Infiernillo Cu–(Mo) porphyry deposit is located in the San Rafael Massif, in southern Mendoza Province, Argentina. It is hosted by and genetically related to Lower Permian Gondwanan volcanic rocks, which developed in a magmatic arc tectonic setting. The alteration zone has an oval shape about 3 km × 2 km in size, with a NNE–SSW strike. It consists of a small central quartz neck with appreciable hematite surrounded by an intense quartz-injected zone with local pervasive potassic alteration. Outwards there is a well-developed phyllic halo with intense bleaching which consists of pervasive and vein-type silicification, sericitization and pyritization. Cu and Mo anomalies are approximately coincident and are located between the silicified zone and the phyllic halo. In the outer part of the alteration zone, small polymetallic veins with pyrite, arsenopyrite, galena and minor chalcopyrite, sphalerite and electrum in quartz gangue crop out. An Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), with three visible and near infrared (VNIR) bands, six shortwave infrared (SWIR) bands and five thermal infrared (TIR) bands, was used to characterize the Infiernillo porphyry deposit alteration area. A circular zoning pattern was clearly observed in the VNIR + SWIR band combination. Spectral analysis of the surface reflectance SWIR detected 2.20 and 2.26 μm absorptions. Field data and sample spectroscopic analyses obtained using a GER 3700 spectral radiometer and a SWIR reflectance spectrometer (Portable Infrared Mineral Analyzer PIMA) showed a good correlation with the ASTER data. The 2.20 μm absorptions, due to Al-hydroxyl, reveal the presence of clay minerals (Al-illite, kaolinite) and sericite, whereas the 2.26 μm absorption due to Fe-hydroxyl, resulted from jarosite. The field samples were also analyzed with a TIR portable spectrometer Micro-FTIR model 102. The TIR was useful for detecting surface silica and potassic alteration through analysis of five-band surface emissivity data. ASTER images provided preliminary mineralogic information and geo-referenced alteration maps at low cost and with high accuracy. In this way ASTER has been proven to be a powerful tool in the initial steps of ore deposit exploration.

Source and redox controls on metallogenic variations in intrusion-related ore systems, Tombstone-Tungsten Belt, Yukon Territory, Canada

ABSTRACTThe Tombstone, Mayo and Tungsten plutonic suites of granitic intrusions, collectively termed the Tombstone-Tungsten Belt, form three geographically, mineralogically, geochemically and metallogenically distinct plutonic suites. The granites (sensu lato) intruded the ancient North American continental margin of the northern Canadian Cordillera as part of a single magmatic episode in the mid-Cretaceous (96–90 Ma). The Tombstone Suite is alkalic, variably fractionated, slightly oxidised, contains magnetite and titanite, and has primary, but no xenocrystic, zircon. The Mayo Suite is sub-alkalic, metaluminous to weakly peraluminous, fractionated, but with early felsic and late mafic phases, moderately reduced with titanite dominant, and has xenocrystic zircon. The Tungsten Suite is peraluminous, entirely felsic, more highly fractionated, reduced with ilmenite dominant, and has abundant xenocrystic zircon. Each suite has a distinctive petrogenesis. The Tombstone Suite was derived from an enriched, previously depleted lithospheric mantle, the Tungsten Suite is from the continental crust including, but not dominated by, carbonaceous pelitic rocks, and the Mayo Suite is from a similar sedimentary crustal source, but is mixed with a distinct mafic component from an enriched mantle source.Each suite has a distinctive metallogeny that is related to the source and redox characteristics of the magma. The Tombstone Suite has a Au-Cu-Bi association that is characteristic of most oxidised and alkalic magmas, but also has associated, and enigmatic, U-Th-F mineralisation. The reduced Tungsten Suite intrusions are characterised by world-class tungsten skarn deposits with less significant Cu, Zn, Sn and Mo anomalies. The Mayo Suite intrusions are characteristically gold-enriched, with associated As, Bi, Te and W associations. All suites also have associated, but distal and lower temperature Ag-Pb- and Sb-rich mineral occurrences. Although processes such as fractionation, volatile enrichment and phase separation are ultimately required to produce economic concentrations of ore elements from crystallising magmas, the nature of the source materials and their redox state play an important role in determining which elements are effectively concentrated by magmatic processes

Lithogeochemistry in exploration for Proterozoic porphyry-type molybdenum and copper deposits, southern Finland

Over 450 samples were collected around four Proterozoic porphyry-type occurrences and analyzed for 30 elements. Relatively broad primary halos are associated with all four occurrences in spite of their moderate size and grade. The halos are characteristic of each particular occurrence depending on the geological and geochemical conditions. Copper forms extensive and highly contrasted anomalies, whereas Mo anomalies are of limited size. Gold, Zn, Pb, As, and Sb seem to be useful pathfinders for slightly eroded or blind occurrences. Pyritization and wall-rock alteration are weaker than in Phanerozoic porphyry deposits, consequently, S halos are small and often erratic. K 2O, Rb, and SiO 2 produce weak anomalies near the core of the occurrences. A density of a few tens of samples per km 2 is required for detailed follow-up exploration. The host granitoids have normal chemical compositions outside the mineralized areas with low and constant background contents of ore elements and are thus very suitable for lithogeochemical exploration. On a regional scale a density of one to five samples per km 2 is sufficient to identify mineralized intrusive phases and areas, because the porphyrytype occurrences were formed by extensive hydrothermal processes, which produced scattered anomalies over large areas.

A Geochemical Drainage Survey and its Implications for Metallogenesis, Central Coast Mountains, British Columbia

Two thousand stream sediment samples from 3,300 square miles of the central Coast Mountains of British Columbia were analyzed for Mo, Cu, Zn, and cold-extractible copper (cxCu). The data were grouped according to the lithologies underlying the drainage basins, and the distributions and interrelationships of the trace metals were examined using cumulative frequency plots and relatively simple statistical methods. Multimodal metal populations may be caused by analytical bias, heterogeneity of lithologic units, and sulfide mineralization. Positive correlations between mean Cu and Zn contents of stream sediment samples and mean specific gravity of associated plutonic rocks are interpreted as resulting principally from variations in the mafic mineral content of the plutonic rocks.Samples containing anomalous metal values were obtained from streams draining most known skarn, massive sulfide, and pegmatitic deposits in the areas sampled. Sulfide mineralization associated with skarn, and many Mo anomalies, are spatially associated with major faults and are interpreted as having resulted from fault-controlled metallization that may be genetically unrelated to nearby plutonic rocks. Massive sulfide deposits and Cu, cxCu, and Zn anomalies from the Ecstall Belt of metasedimentary rocks are thought to be the results of metallization that was restricted to the immediate vicinity of the anomalies. Hypothermal quartz veins of the Surf Point Mine and many of the Mo anomalies on Porcher Island are related genetically to the forcible intrusion of a nearby pluton.Relatively simple statistical techniques are useful in interpreting geochemical prospecting data in some cases where more sophisticated techniques cannot usefully be applied. Reconnaissance stream sediment surveys in the central Coast Mountains are useful both in prospecting for massive sulfide deposits and in metallogenic studies.

CERTAIN RESULTS OF LANDSCAPE-GEOCHEMICAL INVESTIGATIONS IN THE VICINITY OF A CHALCOPYRITIC ORE DEPOSIT IN THE SOUTHERN URALS

The paper, a local study, is an integral part of the regional symposium on landscape-geochemical prospecting for copper in the Southern Urals. Examination of normal and anomalous concentrations of copper and of other metals, typornorphic of chalcopyritic ores of the test area and of the region, .shows that, in contrast with the greenstone areas, the secondary dispersion halos of the ores are best expressed in the illuvial horizon of gray forest soils and that such halos are found also in ground Waters and in hydromorphic soils. Characteristically, Mo anomalies in gray forest soils may be taken as indications of the sulfide mineralization in the underlying rocks, but Ba anomalies are obscure against the high Ba background of the test area. V. P. Sokoloff