Evolution of gold mineralization in the Ashanti Gold Belt,Ghana: Evidence from carbonate compositions and parageneses

Summary Ankerite, siderite, calcite and magnesite occur in variable proportions within all host and mineralized rocks of the Bogosu and Prestea mining districts of the Ashanti Gold Belt, Ghana. The compositions of coexisting ankerite-siderite grains establish that complex rhythmically zoned growth banding and replacement textures are present. This compositional variation is attributed to episodic fluctuation in the temperature and composition of fluids in the Bogosu-Prestea mesothermal gold system. Temperatures derived from the ankerite-siderite composition geothermometer are generally consistent with those from calcite-dolomite, arsenopyrite, carbon and oxygen stable isotope, and fluid inclusion geothermometers, and are about 360°C for the metamorphic peak, 400 to 350°C for carbonate alteration of mafic dikes, and 340 to 140°C for gold deposition. The latter range occurs on a thin-section scale and represents separate pulses of fluid in the ore conduit.

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Entstehung der Goldvererzung im Ashanti Gold Belt, Ghana: Rückschlüsse aus Karbonat zusammensetzungen und Paragenesen

Zusammenfassung In allen Wirtsgesteinen und mineralisierten Gesteinen der Bergbaureviere von Bogosu und Prestea im Ashanti Gold Belt, Ghana treten Ankerit, Siderit, Calcit und Magnesit in unterschiedlichen Verhältnissen auf. Die Zusammensetzung von koexistierenden Ankerit-Siderit-Körnern zeigt eine komplexe, rhythmisch zonierte Wachstumsstreifung und Verärdngungsstrukturen. Diese Änderungen in der Zusammensetzung sind auf episodische Fluktuationen der Temperatur und der Zusammensetzung der Fluide im mesothermalen Goldsystem von Bogosu-Prestea zurückzuführen. Temperaturen nach dem Ankerit-Siderit-Geothermometer stimmen im allgerneinen mit jenen aus Geothermometern, die auf Calcit-Dolomit, Arsenopyrit, den stabilen Isotopen von Kohlenstoff and Sauerstoff und auf Flüssigkeitseinschlüssen beruhen, überein. Sie liegen bei rund 360°C für den Höhepunkt der Metamorphose, bei 400 bis 350°C für die Karbonat Alteration der matischen Gänge and bei 340 bis 140°C für die Gold-Fällung. Der letztgenannte Bereich tritt in Dünnschlif Maßstab auf and repräsentiert einzelne Schübe von Fluid in den Erzgängen.

     

Partitioning of platinum-group elements and Au in the Fe−Ni−Cu−S system: experiments on the fractional crystallization of sulfide melt

Partitioning of platinum-group elements (PGE) between sulfide liquid and monosulfide solid solution (mss) has been investigated by crystallizingmss from Fe–Ni–Cu sulfide liquid at 1,000–1,040° C, using bulk compositions and PGE contents typical of magmatic sulfides associated with mafic and ultramafic systems. Products were analyzedin situ for PGE and Au using SIMS. Sulfide liquid compositions were more Ni- and Cu-rich than coexistingmss. Liquid/mss partition coefficients are: Os-0.23±0.04, Ir-0.28±0.11, Ru-0.24±0.05, Rh-0.33±0.06, Pt-4.8±0.7, Pd-4.8±1.9, Au-11.4. Partitioning of PGE is independent of PGE concentration and Ni content in the composition range investigated. Additionally, Henry's law appears to be obeyed up to minor-element contents in the sulfide liquid andmss. Osmium, Ir, Ru, and Rh are compatible elements in the anhydrous Fe–Ni–Cu–S system, whereas Pt, Pd and Au are incompatible elements. These affinities correspond to the partitions of PGE between massive and Cu-rich magmatic sulfides. However, the detailed precious-metal compositions of the Cu-rich sulfides of mafic rock systems, disseminated ores of komatiites and Cu-rich assemblage of droplet ore from the Noril'sk-Talnakh deposits are not consistent with those expected for pristine fractionated sulfide liquids.     

An occurrence of secondary sulfides from Leg 37 basalts

Two types of secondary sulfides occur on the walls of a vesicle in slightly altered basalt recovered from site 332A of DSDP Leg 37. The first consists of euhedral crystals of pure pyrite, with occasional small partial overgrowths of fibrous chalcopyrite, embedded within smectite (altered glass) which lines the vesicle. The second type occurs as small composite sulfide spherules lightly attached to the surface of the smectite. The spherules are composed of pyrite crystallites arranged in approximate pyritohedral symmetry, with marcasite crystallites decorating the pyritohedron edges. Precession camera photographs and optical studies suggest the marcasite and pyrite grew simultaneously from a pyrite nucleus, with marcasite adopting the pyrite growth habit. These spherules differ sharply from the magmatic sulfide spherules previously described.     

Partition of Mg and Fe2+ in coexisting pyroxenes

A general theory for the partition of elements between coexisting, multicomponent phases is outlined and applied to data for Ca-rich pyroxene (Cap) — Ca-poor pyroxene (Op) assemblages from the Skaergaard and Bushveld intrusions and from charnockites. The intercrystalline partition of Mg and Fe²⁺ are studied separately rather than through the exchange reaction, MgSiO ₃ ^Cap +FeSiO ₃ ^Op FeSiO ₃ ^Cap +MgSiO ₃ ^Op .The separate distributions for x_MgSiO3> and x_FeSiO3> are quite distinct and demonstrate directly that solutions of both Mg and Fe²⁺ in the two pyroxenes are nonideal.     

Partition of major and minor elements and equilibration in coexisting pyroxenes

The partition of Si, Al, Ti, Fe³⁺, Mg, Fe²⁺, Mn, Ca and Na between coexisting Ca-rich and Ca-poor pyroxenes from a wide variety of igneous and metamorphic rocks have been investigated systematically. Many of the distributions, and especially those for the partition of Ti, Mg, Fe²⁺, Mn and Na, indicate characteristic trends for pyroxenes from the various petrologic groups identified. The partition of Mg, Fe²⁺ and Mn correlate with inferred cooling rates, the partition co-efficients of pyroxenes from extruded and other quickly cooled rocks most nearly approaching unity. In contrast, the partition of Si and Ti and the absolute amounts of Al may be related to the physicochemical conditions prevailing during original crystallisation; Ti being particularly relatively enriched in Ca-rich pyroxenes of ultramafic associations. The trends of the compositions of the Ca-rich pyroxenes plotted in the pyroxene quadrilateral also correlate with cooling rates and comparison with the limited data available on the phase relations of coexisting pyroxenes suggests that sub-solidus chemical readjustments have occurred in both phases.     

Light-rare earth element-rich minerals associated with platinum-group element mineralization in the Archean Boston Creek Flow,Ontario

Summary A variety of LREE-rich minerals are associated with late magmatic-stage platinum-group element (PGE) mineralization [(PGE + Au) = 300 ppb) in unsheared clinopyroxenite and gabbro proximal to sheared amphibolite in the Boston Creek Flow (BCF) Al-depleted komatiitic basalt, Archean Abitibi greenstone belt, Ontario. The LREE-rich minerals are LREE-rich apatite (La₂O₃ + Ce₂O₃ 1.5 wt%), LREE-rich epidote (Ce, La: 12 wt% REE), and bastnaesite [(Ce,La)(CO₃)(F,OH)]. The LREE-rich apatite forms rare zones in altered apatite grains and discrete, multifaceted micrometric-sized grains. LREE-rich epidote forms large (up to 100 ,m), compositionally zones grains in amphibolitized plagioclase. Bastnaesite forms areas marginal to and veinlets within the LREE-rich epidote and analyses are characterized by up to 0.4 wt% Cl. Compared to other unsheared rocks from the flow, the REE-rich mineral host rocks contain: intermediate REE contents (REE = 38 to 71 ppm), Ba contents (up to 240 ppm), and U/Th values (0.3 to 7.2); variable Cl contents (21 to 60 ppm); and slightly elevated ³⁴S values (up to 3.3). In contrast, the sheared amphibolite is characterized by low contents of REE (REE = 25 ppm), Cl (15 ppm), Ba (20 ppm), U (0.5 ppm), and Th (0.4 ppm), and a distinctive chondrite-normalized whole-rock REE pattern profile [(La/Sm)_n = <2 and (Tb/Yb)_n = < 1).The restricted occurrence, textures and chemical compositions of the LREE-rich minerals are interpreted as the result of mobilization and localized concentration of the LREE by hydrothermal fluids during greenschist facies contact metamorphism. LREE-rich epidote represents LREE redistribution accompanying the breakdown of plagioclase during abnormally intense amphibolitization and shear deformation within the flow at the peak of greenschist facies contact metamorphism. LREE-rich apatite and bastnaesite represent LREE mobilization and very localized reprecipitation during later stage, retrogressive amphibolitization. The low chlorine contents of the LREE-rich minerals and their host rocks suggest that complexing with Cl was only of minor importance in the concentration of LREE.The spatial association of the LREE-rich minerals with the PGE mineralization reflects concentration of shear deformation and amphibolitization at this stratigraphic level within the flow. The attendant hydrothermal fluid activity induced limited mobilization and reconcentration of PGE in veinlets and fractures within the mineralization. The low Cl suggests that complexing with Cl was not of importance in the PGE mobilization, nor in the late magmatic-stage mineralization process.

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Seltene Erd-Minerale in Assoziation mit Platinvererzung im Archaischen Boston Creek Flow, Ontario

Zusammenfassung Verschiedene LSEE-reiche Minerale kommen zusammen mit spätmagmatischer Platinvererzung (PGE + Au = 300 ppb) in Klinopyroxeniten und Gabbro, in engster Nachbarschaft mit zerschertem Amphibolit im Al-verarmten, komatiitischen Basalt des Boston Creek Flow (BCF), im archaischen Abitibi Grünstein-Gürtel, Ontario, vor. Die LSEE- reichen Minerale sind LSEE-reicher Apatit (La₂O₃ + Ce₂O₃ > 1.5 Gew.%), LSEE-reicher Epidot (Ce, La: 12 Gew.% SEE), und Bastnaesit ((Ce, La) (CO₃)(F, OH)). Der LSEE-reiche Apatit bildet Zonen in umgewandelten Apatitkörnern und auch individuelle, flächenreiche Körner, die einige Mikron groß sind. LSEE-reicher Epidot bildet große (bis zu 100 m) Körner mit zonierter Zusammensetzung in amphibolitisierten Plagioklasen. Bastnaesit bildet randliche Bereiche von, und Gängchen in LSEE-reichem Epidot. Analysen zeigen bis zu 0.4 Gew.%. Cl. Verglichen mit anderen unzerscherten Gesteinen aus dem BCF, enthalten die Wirtsgesteine der SEE-reichen Minerale: intermediäre SEE-Gehalte (SEE = 38 bis 71 ppm), Ba-Gehalte von bis zu 240 ppm und U/Th-Werte von 0.3 bis 7.2. Wechselnde Cl-Gehalte (21 bis 60 ppm) und etwas erhöhte ³⁴S-Werte (bis zu 3.3). Im Gegensatz dazu zeigt der zerscherte Amphibolit niedrige Gehalte von SEE (SSE = 25 ppm), Cl (15 ppm), Ba (20 ppm), U(0.5 ppm), und Th (0.4 ppm), sowie charakteristische Chondrit-normalisierte SEE-Verteilungsmuster ((La/Sm)_n = <2 und (Tb/Yb)_n = < 1).Das beschränkte Vorkommen, die Texturen und die chemische Zusammensetzung der LSEE-reichen Minerale werden als Ergebnis der Mobilisierung und örtlichen Konzentration von LSEE durch hydrothermale Fluide während einer Kontaktmetamorphose unter Bedingungen der Grünschiefer-Fazies interpretiert. LSEE-reicher Epidot ist das Ergebnis von LSEE Umverteilung im Zusammenhang mit dem Zerfall von Plagioklas während besonders intensiver Amphibolitisierung und Scherungsdeformation innerhalb des Basalt-Ergusses zum Höhepunkt der Kontaktmetamorphose. LSEE-reicher Apatit und Bastnaesit gehen auf LSEE-Mobilisierung und sehr lokale Wiederausfällung während retrograder Amphibolitisierung während eines späteren Entwicklungsstadiums zurück. Die niedrigen Chlorgehalte der LSEE-reichen Minerale und ihrer Wirtsgesteine weisen darauf hin, daß die Bildung von Chlorid-Komplexen bei der Konzentration von LSEE nur eine geringe Rolle gespielt hat.Die räumliche Verbindung von LSEE-reichen Mineralen mit der PGE-Vererzung dürfte auf die Verbindung von Scher-Deformationen und Amphibolitisierung in diesem stratigraphischen Niveau innerhalb des Basaltes zurückgehen. Die begleitende hydrothermale Fluid-Aktivität führt zu beschränkter Mobilisierung und Anreicherung von PGE in Gängen und Sprüngen innerhalb der Vererzung. Die niedrigen Chlorgehalte weisen darauf hin, daß Chlorid-Komplexe weder bei der PGE-Mobilisierung, noch bei der spätmagmatischen Vererzung von Bedeutung gewesen sind.

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With 4 Figures     

南大西洋风场和海浪场时空特征分析

本文根据1950～1995年共46a的南大西洋船舶气象报资料,按1*1和5*5网格统计的风、浪要素进行分析研究。通过分析每月风、浪各要素的等值线分布图,得出南大西洋风场与海浪场季节变化特点不如北半球各大洋显著,但仍有较明显的季节变化,只是季节性差异较小,冬季风比夏季风强盛,相应的平均波高、大浪大涌频率也较大;盛行风向、风浪传播方向、涌浪传播方向基本一致,低纬地区常年盛行东南浪,高纬地区则盛行偏西向浪。本文为船舶远洋交通运输、远洋出访和科学试验等活动,提供了较为翔实的风场和浪场资料及变化规律。     

Silicon K-edge XANES spectra of silicate minerals

Silicon K-edge x-ray absorption near-edge structure (XANES) spectra of a selection of silicate and aluminosilicate minerals have been measured using synchrotron radiation (SR). The spectra are qualitatively interpreted based on MO calculation of the tetrahedral SiO ₄ ^4? cluster. The Si K-edge generally shifts to higher energy with increased polymerization of silicates by about 1.3 eV, but with considerable overlap for silicates of different polymerization types. The substitution of Al for Si shifts the Si K-edge to lower energy. The chemical shift of Si K-edge is also sensitive to cations in more distant atom shells; for example, the Si K-edge shifts to lower energy with the substitution of Al for Mg in octahedral sites. The shifts of the Si K-edge show weak correlation with average Si-O bond distance (d_Si-O), Si-O bond valence (s_Si-O) and distortion of SiO₄ tetrahedra, due to the crystal structure complexity of silicate minerals and multiple factors effecting the x-ray absorption processes.