Ken-ichiro Hayashi

Geochemistry of ∼1.9 Ga sedimentary rocks from northeastern Labrador, Canada

Fifty-eight rock chips from fifteen samples of sedimentary rocks from the Ramah Group (approximately 1.9 Ga) in northeastern Labrador, Canada, were analyzed for major and minor elements, including C and S, to elucidate weathering processes on the Earths surface about 1.9 Ga ago. The samples come from the Rowsell Harbour, Reddick Bight, and Nullataktok Formations. Two rock series, graywackes-gray shales of the Rowsell Harbour, Reddick Bight and Nullataktok Formations, and black shales of the Nullataktok Formation, are distinguishable on the basis of lithology, mineralogy, and major and trace element chemistry. The black shales show lower concentrations than the graywackes-gray shales in TiO2 (0.3-0.7 wt% vs. 0.7-1.8 wt%), Al2O3 (9.5-20.1 wt% vs. 13.0-25.0 wt%), and sigma Fe (<1 wt% vs. 3.8-13.9 wt% as FeO). Contents of Zr, Th, U, Nb, Ce, Y, Rb, Y, Co, and Ni are also lower in the black shales. The source rocks for the Ramah Group sediments were probably Archean gneisses with compositions similar to those in Labrador and western Greenland. The major element chemistry of source rocks for the Ramah Group sedimentary rocks was estimated from the Al2O3/TiO2 ratios of the sedimentary rocks and the relationship between the major element contents (e.g., SiO2 wt%) and Al2O3/TiO2 ratios of the Archean gneisses. This approach is justified, because the Al/Ti ratios of shales generally retain their source rock values; however, the Zr/Al, Zr/Ti, and Cr/Ni ratios fractionate during the transport of sediments. The measured SiO2 contents of shales in the Ramah Group are generally higher than the estimated SiO2 contents of source rocks by approximately 5 wt%. This correction may also have to be applied when estimating average crustal compositions from shales. Two provenances were recognized for the Ramah Group sediments. Provenance I was comprised mostly of rocks of bimodal compositions, one with SiO2 contents approximately 45 wt% and the other approximately 65 wt%, and was the source for most sedimentary rocks of the Ramah Group, except for black shales of the Nullataktok Formation. The black shales were apparently derived from Provenance II that was comprised mostly of felsic rocks with SiO2 contents approximately 65 wt%. Comparing the compositions of the Ramah Group sedimentary rocks and their source rocks, we have recognized that several major elements, especially Ca and Mg, were lost almost entirely from the source rocks during weathering and sedimentation. Sodium and potassium were also leached almost entirely during the weathering of the source rocks. However, significant amounts of Na were added to the black shales and K to all the rock types during diagenesis and/or regional metamorphism. The intensity of weathering of source rocks for the Ramah Group sediments was much higher than that of typical Phanerozoic sediments, possibly because of a higher PCO2 in the Proterozoic atmosphere. Compared to the source rock values, the Fe3+/Ti ratios of many of the graywackes and gray shales of the Ramah Group are higher, the Fe2+/Ti ratios are lower, and the sigma Fe/Ti ratios are the same. Such characteristics of the Fe geochemistry indicate that these sedimentary rocks are comprised of soils formed by weathering of source rocks under an oxygen-rich atmosphere. The atmosphere about 1.9 Ga was, therefore, oxygen rich. Typical black shales of Phanerozoic age exhibit positive correlations between the organic C contents and the concentrations of S, U, and Mo, because these elements are enriched in oxygenated seawater and are removed from seawater by organic matter in sediments. However, such correlations are not found in the Ramah Group sediments. Black shales of the Ramah Group contain 1.7-2.8 wt% organic C, but are extremely depleted in sigma Fe (<1 wt% as FeO), S (<0.3 wt%), U (approximately l ppm), Mo (<5 ppm), Ni (<2 ppm), and Co (approximately 0 ppm). This lack of correlation, however, does not imply that the approximately 1.9 Ga atmosphere-ocean system was anoxic. Depletion of these elements from the Ramah Group sediments may have occurred during diagenesis.

Solubility of gold in NaCl-and H2S-bearing aqueous solutions at 250–350°C

A total of 108 silica capsule experiments were performed in order to determine the solubility of gold in aqueous solutions containing both NaCl and H2S at temperatures of 250, 300, and 350°C, at pressures dictated by vapor/liquid coexistence. The starting materials in the capsules were H2O + S° + goldwire + NaCl (0 to 4m) ± Na2SO4. The pH, aH2S(aq), aH2(aq), ƒH2(g),ƒO2(g), and ƒS2(g) values of the solutions were controlled by the sulfur hydrolysis reaction (4S ° + 4H2O(1) = 3H2S(aq) + HSO−4 + H+) and the sulfide/sulfate reaction. The quenched run products were analyzed for Au (0.1 to 66 ppm), ΣH2S, ΣSO2−4, and pH. The calculated solution compositions at 250–350°C fall in the following ranges: pH = 1.9 to 5.0, log aH2S(aq) = −2.0 to −0.7, log aHSO4− = −3.8 to −1.4, and log aH2(aq) = −7-0 to −4.7. The results of our experiments indicate that gold solubility is independent of the activity of Cl− and H+ in the solutions, indicating that chloride complexes are not important. The gold solubility, however, increases with increasing H2S(aq) activity, indicating that gold dissolved largely as a bisulfide complex according to the reaction: Au(s) + 2H2S(aq) = HAu(HS)02 + 12H2(aq) . The equilibrium constant determined from our experimental data for the above reaction is constant over a temperature range of 250 to 350°C at log K = −5.1 ± 0.3. The above equilibrium constant, together with that for a reaction involving Au(HS)−2 obtained by Shenberger and Barnes (1989), determines the dissociation constant of HAu (HS)02: HAu(HS)02 = H+ + Au(HS)−2. The log K value becomes −5.3 ± 0.5 at 250°C, −5.6 ± 0.6 at 300°C, and −6.2 ± 0.6 at 350°C. Therefore, HAu(HS)02 is the dominant gold-sulfide species at pH below about 5.5, while Au(HS)−2 becomes dominant at higher pH conditions. The results from our study suggest that the solubility of gold in ore-forming fluids in equilibrium with pyrite and/or pyrrhotite at 250–350°C is typically between 0.1 ppb to 1 ppm Au, transported mostly as bisulfide complexes; gold-chloride complexes do not become important unless the fluid is H2S-poor (e.g., 0.5m 2Cl), and of low pH (<4.5). Precipitation of gold from ore-forming solutions may occur by increasing aH2(aq), such as by reactions with organic matter or ferrous-bearing minerals, or by decreasing aH2S(aq), such as by precipitation of sulfide minerals or by mixing of H2S-poor fluids. Simple cooling or heating of fluids without changing the H2(aq) and H2S(aq) contents is not an effective mechanism of gold precipitation when gold is transported largely as a bisulfide complex. Effects of oxidation or of boiling on gold precipitation, however, cannot be easily evaluated from the available thermodynamic data alone.

Dissolution of iron hydroxides by marine bacterial siderophore

A series of laboratory experiments was conducted to investigate the dissolution behavior of goethite and poorly crystalline iron hydroxide (PCIH) in the presence of siderophore produced by a marine bacterium, Alteromonas haloplanktis. The amounts of siderophore in the experimental solutions are expressed by the iron complexing capacity (ICC). The experimental data indicate that both the dissolution rates and solubilities of ferric hydroxides increase with increasing contents of siderophore and [H+]. For example, in a solution of pH=8 and ICC=12 μM, the iron content of solution increased from <0.1 to 1.3 μM by reaction with PCIH for 80 h. This Fe content is more than 50 times the solubility value of amorphous Fe(OH)3 in pure water at pH=8. At pH=4, with increasing ICC value from 0 to 12.9 μM, the dissolution rate of goethite increased from 1.5 to 9.5 (nM/h/m2) and that of PCIH increased from 0.4 to 3.4 (nM/h/m2). At ICC=12.9 μM, with increasing pH from 4 to 6.8, the dissolution rate of goethite decreased from 9.5 to 1.9 nM/h/m2, whereas that of PCIH decreased from 3.4 to 0.7 nM/h/m2. Our study suggests that most of the iron utilized by phytoplankton in the oceans may be liberated from ferric hydroxides in aeolian particles by siderophores generated by marine bacteria.

Experimental study of the behavior of copper and zinc in a boiling hydrothermal system

Vapor-liquid partitioning coefficients for Cu and Zn in aqueous fluid were determined under boiling conditions at 500–650 °C and 35–100 MPa in sulfur-bearing and sulfur-free systems. A synthetic fluid inclusion technique was used to sample the experimental system of coexisting vapor-rich and liquid-rich fluid inclusions; the Cu and Zn concentrations in individual fluid inclusions were then analyzed by synchrotron radiation X-ray fluorescence. The vapor-liquid distribution constant of Cu [K D m Cu(vapor) / m Cu(liquid) ], where = m denotes the molality of metal, is found to be strongly dependent on the sulfur content in the experimental solutions. In sulfur-bearing systems, Cu preferentially partitions into the vapor phase, whereas Zn preferentially fractionates into the hypersaline liquid. The K D values for Cu and Zn obtained in this study correspond well with those obtained from natural fluid inclusions in hydrothermal ore deposits. The results suggest that differential volatility of metals is an important factor affecting the enrichment of certain metals in different hydrothermal ore deposits.

Solubility of sphalerite in aqueous sulfide solutions at temperatures between 25 and 240°c

Abstract In order to assess the role of zinc sulfide complexes in ore-forming solutions, the solubility of sphalerite was measured in NaOHH 2 S aqueous solutions of 0.0 to 3.0 m NaHS concentration at temperatures of 25 to 240°C. Solubilities vary with temperature, activities of H 2 S(aq) and HS − , total reduced sulfur concentration (∑S), and pH. From the solubility data, the main reactions that form zinc sulfide complexes were determined as follows: ZnS ( s ) + H 2 S ( aq ) = Zn ( HS ) 2 0 , ZnS ( s ) + H 2 S ( aq ) + HS − = Zn ( HS ) − 3 , ZnS ( s ) + H 2 S ( aq ) + 2 HS − = Zn ( HS ) 4 2− , ZnS ( s ) + H 2 O (1) + HS − = Zn ( OH )( HS ) − 2 , and ZnS ( s ) + H 2 O (1) + 2 HS − = Zn ( OH )( HS ) 2− 3 . Their equilibrium constants (log K ) are 25°C: −5.3, −3.3, −3.4, −4.4, −4.9; 100°C: −5.2, −3.5, −3.2, −4.1, −5.0; 150°C: −4.7, −3.8, −3.1, −4.7, −5.2; 200°C: −5.1, −3.4, −3.1, −4.6; 240°C: −4.9, −3.3, −3.1, −4.9, respectively. Zn(OHXHS) 2− 3 is not stable at temperatures higher than 200°C. Zinc sulfide complexes predominate over chloride complexes in relatively low temperature hydrothermal solutions which have high ∑S, low ∑C1 − , and high pH values. In these solutions, ZnS is precipitated in response to changes of temperature, pH, and ∑S. Among them, decrease of ∑S is more effective than that of temperature and pH. Zinc sulfide complexes do not transport significant zinc in those ore-forming solutions responsible for economic zinc sulfide deposits. However, they become predominant zinc species in certain geothermal solutions and ore-forming solutions responsible for some epithermal precious metal deposits.

Experimental study of the solubilities of pyrite in NaCl-bearing aqueous solutions at 250–350°C☆

A total of sixty-three silica capsule experiments were performed to determine the solubilities of pyrite in NaCl-bearing aqueous solutions (0, 0.1, 0.5, 1, 2, 3, and 4 m) at 250, 300, and 350°C at pressures of vapor/liquid coexistence. The starting materials in the capsules were H2O(1) + FeS2(s) + S ° (s) ± NaCl (s). After reaction times up to ~ 60 days, the quenched solutions were analyzed for ΣFe, σH2S, ΣSO42−, and pH; the ΣFe content, ranging 5–1,300 ppm, generally increased with increasing temperature and ΣCl content of solution. The calculated solution compositions at the experimental P-T conditions fall mostly in the following ranges: pH = 2.0 to 3.2, logaH2s = −1.9 to −1.0, logaHSO4− = −3.8 to −2.0, and logaH2(aq) = −7.0 to −5.0. Evaluation of the experimental data suggests that the various redox equilibria between solution and mineral were attained in most of the experimental solutions. The pH, aH2S(aq), and aH2(aq) of the solutions were controlled by the sulfur hydrolysis reaction (48° + 4H2O(l) = 3H2S(aq) + HSO4− + H+) and the sulfide/sulfate reaction (H2S(aq) + 4H2O(l) = 4H2(aq) + H+ + HSO4−). The pyrite solubility is controlled by a general reaction: FeS2(s) + nCl− + 2H+ + H2(aq) = FeCln2−n + 2H2S(aq). The equilibrium constants for this reaction, as well as those for association of ferrous chloride complexes (Fe2+ + nCl− = FeCln2−n), were obtained at 250, 300, and 350°C; they were used also to compute the equilibrium constants for the reactions controlling the solubilities of pyrrhotite, magnetite, and hematite: FeS(s) + 2H+ + nCl− = FeCln2−n + H2S(aq); Fe3O4(s) + 6H+ + 3nCl− + H2(aq) = 3 FeCln2−n + H2O(aq); Fe2O3(s) + 4H+ + 2nCl− + H2(aq) = 2FeCln2−n + 3H2O(aq). Our experimental data suggest that the dominant Fe-Cl complex is FeCl+ in solutions of ΣCl ≤ 0.5m at 250°C and ΣCl ≤ 0.1 m at 300 and 350°C; FeCl20 is dominant in solutions of the higher ΣCl contents at each temperature. The association constants for FeCl+ and FeCl2 estimated from this study are in good agreement with those estimated recently by Heinrich and Seward (1990), Ding and Seyfried (1992), Fein et al. (1992), and Palmer and Hyde (1992). Our solubility constants for pyrite are in good agreement with those obtained by Crerar et al. (1978) and Wood et al. (1987) for 3 m ΣCl solution at 350°C, but are 0.5–2 orders of magnitude higher than those obtained by them at lower temperatures and/or at lower ΣCl values. Our data suggest that natural hydrothermal fluids that are in equilibrium with pyrite, the most abundant sulfide mineral in the upper crust, are able to transport sufficient amounts (> 10−m) of both Fe and H2S to produce pyrite-rich ore deposits at temperatures above 250°C, and possibly at lower temperatures. The solubility of pyrite (and of other Fe-bearing minerals) is affected very little by a change of temperature, provided the pH, aH2(aq), aH2S(aq), and ΣCl values remain constant.

Sulphide minerals in Early Archean chemical sedimentary rocks of the eastern Pilbara district, Western Australia

SummaryThe occurrence and paragenesis of sulphide minerals in chemical sedimentary rocks from the McPhee and the Towers Formations of the Warrawoona Group, eastern Pilbara Craton were examined, in order to evaluate the Archean sedimentary environment. The chemical sedimentary facies of both formations are comprised of chert or chertcarbonate units, which are highly depleted in detrital materials. The cherty rocks are mostly composed of microcrystalline quartz, containing significant types of syndepositional (or diagenetic) sulphide minerals. In particular, the cherty rocks in the Towers Formation (North Pole Chert, Marble Bar Chert) include primary sulphide minerals, such as pyrite, chalcopyrite, sphalerite, monoclinic pyrrhotite, pentlandite, gersdorffite and millerite. This assemblage and the measured FeS content (8.4–10.4 mol%) of sphalerite associated with the Fe-sulphide minerals suggest that the cherty rocks were formed under reducing conditions at temperatures below 200°C (about 150°C), and also that the metamorphic temperature of the rocks was less than 325 °C. Furthermore, the virtual absence of detrital materials and the minor element compositions imply that the cherty rocks of the eastern Pilbara Block were formed by rapid precipitation from reducing hydrothermal solutions.ZusammenfassungDas Auftreten und die Paragenese von Sulfiden in chemischen Sedimentgesteinen der McPhee und der Towers Formation der Warrawoona Gruppe, östlicher Pilbara Block, wurden untersucht, um das sedimentäre Milieu im Archaikum besser abschätzen zu können. Die chemisch-sedimentäre Fazies beider Formationen besteht aus Chert- oder Chert-Karbonat-Einheiten, die hochgradig an detritärem Material verarmt sind. Die Cherts bestehen aus mikrokristallinem Quartz, der beträchtliche Mengen an syngenetischen bzw. syndiagenetischen Sulfiden enthält. Vor allem die Cherts der Towers Formation (North Pole Chert, Marble Bar Chert) führen Pyrit, Kupferkies, Zinkblende, monoklinen Magnetkies, Pentlandit, Gersdorffit und Millerit als primäre Sulfide. Diese Vergesellschaftung und die gemessenen FeS-Gehalte der mit den Fe-Sulfiden assoziierten Zinkblende (8.4–10.4 Mol%), weisen darauf hin, daß die Cherts unter reduzierenden Bedingungen bei Temperaturen unter 200°C entstanden sind und daß die Matamorphosetemperatur 325 °C nicht überschritten hat. Das Fehlen detritärer Sedimentkomponenten und die Spurenelementzusammensetzungen lassen darauf schließen, daß die Cherts im östlichen Pilbara Block durch rasche Ausfällung aus reduzierenden hydrothermalen Lösungen entstanden sind.

A re-examination of herzenbergite–teallite solid solution at temperatures between 300 and 700°C

Abstract In order to investigate the range of the solid solution series in herzenbergite-teallite minerals, samples of different composition were synthesized. Herzenbergite-teallite minerals were synthesized by an evacuated silica glass tube method at 700°C. A linear relationship between cell dimensions, a, b and c and composition is established. Extension of solid solution to the Pb-rich portion of the system PbS-SnS is limited; the solid solution area is between Pb1.060Sn0.940S2 and SnS at 700°C. Teallite coexisting with galena was also synthesized by hydrothermal recrystallization at 300, 400 and 450°C. The compositions of teallite are Pb1.140Sn0.860S2 at 300°C, Pb1.114Sn0.886S2 at 400°C, and Pb1.124Sn0.876S2 at 450°C, respectively. Their compositions shift towards the PbS end-member from stoichiometric teallite. The cell dimensions of teallite, which was synthesized hydrothermally, follow the linear relationship between cell dimensions and composition established at 700°C.

Quantitative analysis of fluid inclusions by synchrotron X-ray fluorescence: calibration of Cu and Zn in synthetic quartz inclusions

A scheme for achieving accurate, quantitative analysis of fluid inclusions by synchrotron radiation X-ray fluorescence (SXRF) spectroscopy is proposed. Equations accounting for the inclusion depth and thickness are derived and verified through analysis of Cu and Zn in synthetic fluid inclusions. The method involves the construction of a two-dimensional distribution map of XRF for each element (Cu and Zn), followed by the measurement of XRF over a longer irradiation period to obtain a value of “relative intensity”. The relative intensity is shown to be more accurate than the integrated intensity obtained for the entire inclusion area in the map. The equations derived for inclusion fluid concentrations in terms of the relative intensity produce results that are consistent with the values expected by calculation using the mass absorption coefficients and density of quartz. The proposed scheme is also applicable for the analysis of other elements with similar atomic number in a range of host minerals.

Origin and Formational Environment of Noda-Tamagawa Manganese Ore, Northeast Japan: Constraints from Isotopic Studies

Abstract The Noda-Tamagawa mine, one of the biggest bedded manganese deposits in Japan, is located in the north of the Kitakami mountain range. The manganese ore (1–5 m in thickness) is conformably embedded in the cherty and shaly wall rocks, and is mineralogically zoned from the rhodonite ore of the wall rock side to the central ore constituted by pyrochroite-hausmannite passing by the intermediate tephroite ore. This zonal distribution of the manganese ore is the result of thermal metamorphism by the Cretaceous Tanohata granodiorite body. The presence of manganosite and pyrochroite in the metamorphosed ore suggests that the primary manganese mineral, before the metamorphism, was rhodochrosite. The sulfur and carbon content in the wall rocks and manganese ore were quantified, and the results display a pattern similar to that of Black Sea sediments. A series of sulfur isotopic data obtained from a ∼200 m stratigraphic traverse including the manganese ore horizon shows that the sulfur isotopic composition (δ34S) varies between –42 and +10‰. The majority of values is around –25‰ suggesting that the sulfides in the rocks of the Noda-Tamagawa mine were formed during diagenesis by the intervention of sulfur reducing bacteria. The rarely found manganese carbonate in hausmannite ore has light δ13C values ranging between –21‰ and –3‰, confirming the sedimentary origin of this primary manganese ore. Thus, rhodochrosite (the most probable precursor of the manganese minerals of the Noda-Tamagawa deposits) was metamorphosed by the intrusion of the Tanohata granodiorite body to form the several manganese silicates and oxide minerals currently found in the Noda-Tamagawa site.