Yuanming Pan

The Lower Changjiang (Yangzi/Yangtze River) metallogenic belt, east central China: intrusion- and wall rock-hosted Cu–Fe–Au, Mo, Zn, Pb, Ag deposits

Abstract The lower valley of Changjiang, from Wuhan of the Hubei Province in the west to Zhenjiang of the Jiangsu Province in the east, contains more than 200 polymetallic (Cu–Fe–Au, Mo, Zn, Pb, Ag) deposits and is one of the most important metallogenic belts in China. This metallogenic belt, situated at the northern margin of the Yangzi craton and bordered by the Dabieshan ultrahigh pressure metamorphic belt to the north, consists mainly of Cambrian–Triassic marine clastic sedimentary rocks and carbonate and evaporite rocks, which overlay a Precambrian basement and are intruded by Yanshanian (205 to 64 Ma) granitoid intrusions and subvolcanic complexes. Repeated tectonism from Late Proterozoic to Triassic resulted in extensively developed networks of faults and folds involving the Cambrian–Triassic sedimentary strata and the Precambrian basement. The Yanshanian granitoid intrusions and subvolcanic complexes in the Lower Changjiang metallogenic belt are characterized by whole-rock δ 18 O of +8‰ to +10‰, initial 87 Sr / 86 Sr of 0.704 to 0.708, and ϵNdt from −10 to −17 and have been interpreted to have originated from mixing between juvenile mantle and old crustal materials. Also, the Yanshanian granitoids exhibit eastward younging and increase in alkalinity (i.e., from older calc–alkaline in the west to younger subalkaline–alkaline in the east), which are related to oblique collision between the Yangzi and Sino-Korean cratons and tectonic evolution from early compressional to late extensional or rifting regimes. Most polymetallic deposits in the Lower Changjiang metallogenic belt are clustered in seven districts where the Yanshanian magmatism is particularly extensive: from west to east, Edong, Jiurui, Anqing–Guichi, Luzhong, Tongling, Ningwu and Ningzhen. Mineralization is characterized by the occurrence of three distinct types of orebodies in individual deposits: orebodies in Yanshanian granitoid intrusions, skarn orebodies at the contact zones between the Yanshanian intrusions and Late Paleozoic–Early Mesozoic sedimentary rocks, and stratabound massive sulfide orebodies in the Late Paleozoic–Early Mesozoic sedimentary strata. The most important host sedimentary strata are the Middle Carboniferous Huanglong Formation, Lower Permian and Lower–Middle Triassic carbonate and evaporite rocks. The intrusion-hosted and skarn orebodies exhibit well-developed zonation in alteration assemblages, metal contents, and isotopic compositions within individual deposits, and apparently formed from hydrothermal activities related to the Yanshanian magmatism. The stratabound massive sulfide orebodies in the Late Paleozoic–Early Mesozoic sedimentary strata have long been suggested to have formed from sedimentary or volcano-sedimentary exhalative processes in shallow marine environments. However, extensive research over the last 40 years failed to produce unequivocal evidence for syngenetic mineralization. On the basis of geological relationships and isotope geochemical characteristics, we propose a carbonate-hosted replacement deposit model for the genesis of these stratabound massive sulfide orebodies and associated skarn orebodies. This model suggests that epigenetic mineralization resulted from interactions between magmatic fluids evolved from the Yanshanian intrusions with carbonate and evaporite wall rocks. Mineralization was an integral but distal part of the larger hydrothermal systems that formed the proximal skarn orebodies at the contact zones and the intrusion-hosted orebodies. The stratabound massive sulfide deposits of the Lower Changjiang metallogenic belt share many features with the well-studied, high-temperature, carbonate-hosted replacement deposits of northern Mexico and western United States, particularly with respect to association with small, shallow granitoid complexes, structural and stratigraphic controls on mineralization, alteration assemblages, geometry of orebodies, metal association, metal zonation and isotopic systematics.

Geological and isotopic geochemical constraints on the evolution of the Fuping Complex, North China Craton

Abstract The Neoarchean to Paleoproterozoic Fuping Complex together with the adjacent Hengshan and Wutaishan complexes form the middle part of the Central Zone that separates the North China Craton into the Eastern and Western Archean continental blocks. The Fuping Complex comprises three major lithological assemblages: the ∼2.50 Ga Fuping trondhjemite–tonalite–granodiorite (TTG) gneisses, the Wanzi paragneisses, and the 2.08 and 2.02 Ga Nanying granitic gneisses. The TTG gneisses contain abundant enclaves of highly-metamorphosed supracrustal rocks, including ∼2.70 Ga mafic granulites and amphibolites with eNdt of +3.29 to +3.96 and TDM of 2.72–2.82 Ga, and are characterized by TDM of 2.76–3.04 Ga and a linear correlation between eNdt (−1.64 to +0.96) and 1/Nd, indicative of mixing of melts derived from the ∼2.70 Ga metabasic rocks and ∼3.0 Ga crustal materials. Mafic dikes along the Chengnanzhuang shear zone were most likely emplaced at ∼2.31 Ga and have eNdt of +2.24 to +4.78 and TDM of 2.34–2.59 Ga. The Nanying granitic gneisses with eNdt of −4.04 to −5.24 were most likely related to partial melting of the ∼2.70 Ga supracrustal rocks and/or the ∼2.50 Ga TTG gneisses. These geological relationships and isotopic data suggest that the Fuping Complex probably formed from the following five major tectonomagmatic events: (1) mafic magmatism and early crustal growth at ∼2.7 Ga, with the possible existence of older (∼3.0 Ga) crust(s); (2) collision-related crustal thickening, underplating of basaltic magma, high-grade metamorphism, crustal anatexis and TTG magmatism at ∼2.5 Ga; (3) extensional deformation and emplacement of the mafic dikes and the Nanying granitic plutons along the Chengnanzhuang shear zone (∼2.5 to ∼2.0 Ga); (4) S-type granitic magmatism and associated amphibolite-facies metamorphism (∼2.0 to ∼1.80 Ga); and (5) emplacement of granitic pegmatite dikes at ∼1.80 Ga.

Al K-edge XANES spectra of aluminosilicate minerals

This is the first of a two-part molecular dynamics (MD) study that examines the effects of temperature, pressure, and composition on the structure and properties of ten compositions in the system NaAISiO.-Si02. Results were obtained for collections of at least 1300 atoms at temperatures between 2500 and 4500 K, pressures of 2-5 GPa, and simulation durations on the order of 0.1 ns. Durations and numbers of particles are both about three times larger than for previous MD simulations on molten aluminosilicates. This study addresses experimental matters, including aspects of simulation methodology that affect the accuracy of atom trajectories (hence computed properties). These include Nt (total number of atoms in the MD box) and spatial resolution achieved in the interparticle force calculation. Static melt structures and their systematic variation with temperature and composition are also explored. In the second part (Stein and Spera, in preparation), the mechanism of diffusion is studied in detail, and MD-computed results for a variety of thermodynamic and transport properties are reported and related explicitly to melt structure. The present study employs a pairwise-additive form of the interatomic potential energy function, using the parameters of Dempsey and Kawamura (1984) with electrostatic interactions computed by the Ewald sum technique. Greater than 90% of Si and AI are fourfold-coordinated by 0 and >90% of 0 is in twofold coordination by the tetrahedral (T) cations, showing little change with composition and temperature. T -T coordination is found to have a more distinct dependence on composition than does T -0 coordination. Structures and properties determined with this set of parameters differ from those computed using the parameters published by Angell et al. (1987), which give a less strongly ordered tetrahedral network and rates of diffusion for network-forming ions that are larger by about an order of magnitude. Results indicate that transport properties computed from time-correlation functions (e.g., diffusivity and ionic conductivity) apparently become asymptotic for system sizes (Nt) greater than about 1000 particles. Relative to these values, properties computed with Nt less than about 400-600 particles are generally overestimated by 10-100% and show increased variance. Truncation of the Ewald sum (limiting the number of reciprocal space vectors) introduces additional variance in computed properties. Melt structure (e.g., nearest-neighbor coordination statistics, static pair-correlation functions, and intertetrahedral bridging angle distributions) are less dependent upon system size than transport properties. The particular form and parameterization ofinterionic potential influence computed properties much more than does system size or spatial resolution in the force calculation. Substitution of (Na + AI) for Si is accompanied by a decrease in the mean intertetrahedral bridging (T -0T) angle related to increasing numbers of AI-O-AI bridges and produces broader and less sharply peaked angle distributions. Radial density functions computed from the MD configurations reveal muted structure beyond the T -T 1 coordination shell in comparison with published XRD analyses, although there is evidence of structure due to T-02 and T-T2 correlation near 5 A, especially in the simulation of molten NaAlSiO.. Structures produced by the MD model reflect the effects of using simplistic potential energy functions, as well as characteristically high experiment temperatures and rapid quench rates required by the practical limitations of the MD method. 0003-004X/95/0506-0417

Rare-earth elements in chlorapatite [Ca10(PO4)6Cl2]: Uptake, site preference, and degradation of monoclinic structure

02.00 417

Fractional crystallization of anhydrous sulfide liquid in the system Fe-Ni-Cu-S, with application to magmatic sulfide deposits

Abstract Differences in the inter- and intracrystalline partitioning behavior of rare earth elements (REE) between chlorapatite (ClAp), fluorapatite (FAp), and hydroxylapatite (OHAp) are directly or indirectly related to substitution mechanism and spatial accommodation. The substitution of REE for Ca is charge compensated by Na in ClAp, Na and Si in FAp, and Si in OHAp. Twinned crystals of REEsubstituted ClAp [La-ClAp, Nd-ClAp, Sm-ClAp, Dy-ClAp; Ca10-2yNayREEy(PO4)6Cl2, with y = 0.05- 0.09; space group P21/b] have been grown from SiO2-bearing, H2O- and Na-rich phosphate-chloride melts, and their hexagonal (P63/m) subcell structures refined at room temperature with single-crystal X-ray intensities to R = 0.020-0.023. The crystal/melt partition coefficients for La, Nd, Sm, and Dy are 0.073, 0.128, 0.122, and 0.101. Thus, uptake of REE is up to two orders of magnitude lower in ClAp than in REE-substituted FAp and OHAp crystallized under equivalent conditions, but remains peaked at Nd. REE site occupancy ratios [(REE-Ca2)/(REE-Ca1)] obtained from direct refinement of electron densities are 0.71, 1.11, 0.21, and 0.09 for La-, Nd-, Sm-, and Dy-ClAp, respectively, and are consistent with relative change in Ca-O bond distances and sizes of Ca polyhedra. Thus, La, Sm, and Dy favor the Ca1 position of ClAp, not Ca2, as in FAp and OHAp; this unusual site preference is attributed to the large increase in size (6-8%) and distortion of the Ca2O6X polyhedron on substitution of Cl for (F,OH). The slight preference of Nd for Ca2 and the peaking of REE uptake at Nd are attributed to a 4f crystal-field contribution, which is revealed by an anomalous decrease in the Ca2-O1 bond length and volume of the Ca2O6X polyhedron in Nd-ClAp, Nd-Fap, and Nd-OHAp. The P21/b structure of La-ClAp has been refined by detwinning the diffraction pattern, confirming that this monoclinic superstructure largely represents ordering of Cl atoms displaced along [001]. The P21/b structure of Dy-OHAp has been refined similarly. The intensity of superstructure reflections decreases abruptly with increasing substitution of REE for Ca, showing that REE cations interfere with ordering of Cl atoms(and OH- groups) during the P63/m → P21/b transition.

Site preference of rare earth elements in fluorapatite; binary (LREE+HREE)-substituted crystals

Abstract Phase relations for coexisting monosulfide solid solution [(Fe,Ni)]1−xS; mss] and sulfide liquid have been determined in the Fe-rich portion of the anhydrous quaternary System Fe-Ni-Cu-S at 52.5 and 50.0 at% S, 850–1050°C and low pressure. Equations for the complex dependence of the mss solidus, sulfide liquidus and distribution coefficients for exchange of Ni/Cu and Fe/Cu on temperature, S content, and proportion of metals have been derived by multiple regression analysis. These equations may be used for geothermometry of pristine magmatic sulfides and prediction of liquid evolution paths, but a limiting factor is uncertainty in the precise bulk composition of natural assemblages, and particularly of S content. Intermediate solid solution [(Cu,Fe)S1−x; iss] is stable up to 900–950°C in Ni-poor compositions, and a large field of a new ternary (Fe,Cu,Ni)S solid solution intermediate between iss and mss exists at and below 850°C. Disseminated sulfides in komatiites are markedly depleted in Cu, relative to an origin by sulfide liquid immiscibility. The compositions of magmatic sulfide ores from Sudbury, Canada and the Insizwa complex, Transkei are not consistent with derivation of the associated late Cu-rich ores by fractional crystallization of sulfide magma, although the Cu-rich ores of the Norilsk-Talnakh district, Russia could have evolved through a complex sequence of fractional crystallization events.

Cerium anomaly and Th/U fractionation in the 1.85 Ga Flin Flon Paleosol: Clues from REE- and U-rich accessory minerals and implications for paleoatmospheric reconstruction

Abstract Crystals of binary (LREE+HREE)-bearing fluorapatite [La,Gd-FAp, Ce,Dy-FAp, Pr,Er- FAp, Eu,Lu-FAp; Ca10 - x - 2yNayREEx+ y(P1 - xSixO4)6A with x = 0.12-0.20, y = 0.26-0.42; P63/m] have been grown from H2O-rich phosphate-fluoride melts, and their structures refined at room temperature with single-crystal X-ray intensities to R = 0.017-0.022. These binary-REE-substituted fluorapatite samples have REE site-occupancy ratios (REE- Ca2/REE-Cal) of 2.32, and 2.32, 2.03, 1.71, respectively, which are 0.47-0.16 smaller than corresponding ratios calculated using data for reference single-REE-substituted fluorapatite. Discrepancies in intracrystalline partitioning between multiple-REE-substituted apatites and single-REE-substituted fluorapatite decrease with a decrease in REE concentration, becoming negligible at 0.2-0.3 total REE cations pfu in synthetic binary-REE-substituted fluorapatite and at trace abundances of REE in natural apatites. However, quantitative transference of laboratory REE site preferences to natural apatites is frustrated by the compositional complexity in nature. In the synthetic fluorapatite, there is a profound change in the spatial accommodation of REE in the apatite structure at about the position of Nd in the 4f transition-metal series, corresponding to the peak in the experimental uptake curve. Discrepancies in intracrystalline partitioning between binary- REE- and single-REE-substituted fluorapatite are attributed to non-ideal mixing of LREE and HREE that results in contraction of the Cal coordination sphere.

Late alteration in titanite (CaTiSiO5): Redistribution and remobilization of rare earth elements and implications for U/Pb and Th/Pb geochronology and nuclear waste disposal

Abstract The 1.85 Ga paleosol at Flin Flon, Manitoba, Canada, was one of the first paleoweathering profiles taken as evidence for a dramatic rise in the oxygen content of the Paleoproterozoic atmosphere. Diagenesis and greenschist-facies metamorphism have modified the abundances of some major elements (e.g., K and Fe) in the Flin Flon paleosol. Inductively coupled plasma-mass spectrometry (ICP-MS) analyses reveal a positive Ce anomaly on both chondrite- and Amisk basalt-normalized REE patterns and marked Th/U fractionation in the uppermost maroon paleosol. The Ce anomaly is confirmed by the occurrence of cerianite and by the compositions of other REE- and U-rich accessory minerals (i.e., secondary monazite and uraninite) in the uppermost maroon paleosol. Th/U fractionation is supported by the presence of uraninite in both the paleosol and overlying Missi sedimentary rocks. Chemical ages of uraninite (1.85 to 1.0 Ga) suggest that this mineral might have formed during weathering or diagenesis but was susceptible to disturbance. Although igneous monazite and zircon are well preserved in even the uppermost maroon paleosol, primary fluorapatite exhibits variable degrees of weathering in the basal green paleosol and has been obliterated completely in the upper maroon paleosol. Published paleomagnetic data suggest a tropical paleolatitude for the Flin Flon region at the time of formation of the paleosol. The positive Ce anomaly in the Flin Flon paleosol confirms previous studies for an oxic atmosphere (PO2 ≥ 10-1.5 to 10-2 PAL) at 1.85 Ga. However, extreme caution must be exercised in the interpretation of Th/U fractionation in paleosols. Moreover, this study demonstrates the advantage of a detailed characterization of REE- and U-rich accessory minerals in the application of trace-element geochemistry of paleosols for paleoatmospheric reconstruction. Eh-pH diagrams for the stabilities of cerianite and uraninite in their respective systems Ce-P-C-H-O and UP- C-H-O at 25 °C and 1 bar reveal the importance of phosphate complexes in the formation of Ce anomalies and Th/U fractionation in both modern and ancient weathering environments.

Site preference of U and Th in Cl, F, and Sr apatites

Abstract Translucent, dark-coloured titanites from a wide variety of geological environments are commonly partly replaced by very fine-grained aggregates of REE-rich minerals (including allanite, monazite, and bastnasite), rutile, quartz, calcite, apatite, and chlorite along grain fractures and boundaries. The aggregates of REE-rich minerals are bordered by an irregular zone of altered titanite, which, by secondary ion mass spectrometry (SIMS), is considerably lower in REEs, U, and Th than the adjacent unaltered matrix. Light REEs and Th are differentially depleted in the altered zone and correspondingly enriched in secondary minerals. Additionally, HREEs and U have been lost to the external system, the loss of HREEs being in proportion to the increase in atomic number. Natural radiation damage was not an important factor in promoting the late alteration of titanite. Titanite which has been subjected to late hydrothermal activity would not remain a closed system in respect to either U or Th decay series and therefore cannot be used for U/Pb and Th/Pb geochronology. The observed leaching and partial dissolution of titanite under hydrothermal conditions are generally consistent with laboratory experimentation, indicating that proper isolation from any communication with hydrothermal fluids is essential in the application of titanite-based ceramics for the immobilization of radioactive waste.

Geochemistry and origin of cordierite-orthoamphibole gneiss and associated rocks at an Archaean volcanogenic massive sulphide camp: Manitouwadge, Ontario, Canada

Abstract Crystals of U- and Th-doped fluor-, chlor-, and strontium-apatite have been synthesized from phosphate-halide-rich melts, and their structures were refined at room temperature with single-crystal X-ray diffraction intensities to R = 0.0167-0.0255. Structure refinements of U-doped fluorapatites indicate that U substitutes almost exclusively into the Ca2 site with site occupancy ratios UCa2/UCa1 that range from 5.00 to 9.33. Similarly, structure refinements of Th-doped fluorapatites indicate that Th substitutes dominantly into the Ca2 site with ThCa2/ThCa1 values that range from 4.33 to 8.67. Structure refinements of U-doped chlorapatites show that U is essentially equally distributed between the two Ca sites with UCa2/UCa1 values that range from 0.89 to 1.17. Results for Th-doped chlorapatites show that Th substitutes into both Ca1 and Ca2 sites with ThCa2/ThCa1 values that range from 0.61 to 0.67. In the Th-doped strontium-apatites with F and Cl end-members, Th is incorporated into both the Ca1 and Ca2 sites. The range of ThCa2/ThCa1 values is 0.56 to 1.00 for the F end-member, and 0.39 to 0.94 for the Cl end-member. XANES measurements of the U-doped samples indicate that U in fluorapatite is tetravalent, whereas in chlorapatite it is heterovalent but dominantly hexavalent. According to our calculation, the volume of the Ca2 polyhedron increases by about 5.8% from fluorapatite to chlorapatite, but that of Ca1 polyhedron increases by only 0.59%. We speculate that the much greater size of the Ca2 polyhedron in chlorapatite may diminish the selectivity of this position for U and Th. The incorporation of U and Th into fluorapatite results in a decrease in the size of both Ca polyhedra, but the incorporation of U and Th into chlorapatite results in an increase in the volume of both Ca polyhedra. We suggest that the preference of U and Th for both Ca sites in chlorapatite is attributable to the large increase in size and distortion of the Ca2 polyhedron upon substitution of Cl for F.