ALBERT

Notes: Abstract The sulfur isotopic composition of sulfides and barite from hydrothermal deposits at the Valu Fa Ridge back-arc spreading center in the southern Lau Basin has been investigated. Sulfide samples from the White Church area at the northern Valu Fa Ridge have δ34S values averaging +3.8‰ (n= 10) for bulk sphalerite-chalcopyrite mineralization and +4.8‰ for pyrite (n= 10). Barite associated with the massive sulfides exhibits an average of +20.7‰ (n= 10). Massive sulfides from the active Vai Lili hydrothermal field at the central Valu Fa Ridge have much higher δ34S ratios averaging +8.0‰ for bulk sphalerite-chalcopyrite mineralization (n= 5), +9.3‰ for pyrite samples (n= 5), and +8.0‰ and +10.9‰ for a chalcopyrite and a sphalerite separate, respectively. The isotopic composition of barite from the Vai Lili field is similar to that of barite from the White Church area and averages +21.0‰ (n= 8). Sulfide and barite samples from the Hine Hina area at the southern Valu Fa Ridge have δ34S values that are considerably lighter than those observed for samples from the other areas and average −4.9‰ for pyrite (n= 9), −4.0 and −5.7‰ for two samples of sphalerite-chalcopyrite intergrowth, and −3.4‰ for a single chalcopyrite separate. The total spread in the isotopic composition of sulfides from Vai Lili and Hine Hina is more than 20‰ over a distance of less than 30 km. The δ34S values of sulfides at Hine Hina are the lowest values so far reported for volcanic-hosted polymetallic massive sulfides from the modern seafloor. Barite from the Hine Hina field also has unusually light sulfur with δ34S values of +16.1 to +16.7‰ (n= 5). Isotopic compositions of the sulfides at Hine Hina indicate a dramatic decrease in δ34S from ordinary magmatic values and, in the absence of biogenic sulfur and/or boiling, imply a unique 34S-depleted source of probable magmatic origin. Sulfide-barite mineralization in the Hine Hina area is associated with a distinctive alteration assemblage consisting of cristobalite, pyrophyllite, kaolinite, opal-CT, talc, pyrite, native sulfur, and alunite. Similar styles of alteration are typically known from high-sulfidation epithermal systems on land. Alunite-bearing, advanced argillic alteration in the Hine Hina field confirms the role of acidic, volatile-rich fluids, and a δ34S value of +10.4‰ for the sulfur in the alunite is consistent with established kinetic isotope effects which accompany the disproportionation of magmatic SO2 into H2S and H2SO4. The Hine Hina field occurs near the propagating tip of the Valu Fa back-arc spreading center (i.e., dominated by dike injections and seafloor eruptions) and therefore may have experienced the largest contribution of magmatic volatiles of the three fields. The sulfur isotopic ratios of the hydrothermal precipitates and the presence of a distinctive epithermal-like argillic alteration in the Hine Hina field suggest a direct contribution of magmatic vapor to the hydrothermal system and support the concept that magmatic volatiles may be an important component of some volcanogenic massive sulfide-forming hydrothermal systems.

Notes: Abstract In 1992, the Ford gold deposit was rediscovered during field work in the Kwekwe district near the Indarama mine, approximately 200 km southwest of Harare, Zimbabwe. Based on diamond drilling and open pit operations, estimated ore reserves are at least 3 Mt with an average gold content of 2.5 g/t. The gold deposit is located within a porphyritic granite dike with a thickness of 20–50 m, striking 800 m NNW-SSE. It dips 60–70° to the NE and intrudes a volcano-sedimentary sequence of tholeiitic basalts, acid volcanics, and banded iron formations of the Bulawayan Group (2900–2700 Ma). The intrusion of the dike occurred at 2541 ± 17 Ma (Pb/Pb step leaching technique) within a second order structure and is related to displacement along transcrustal deformation zones such as the Sherwood- and Taba-Mali deformation zones. Gold mineralization is confined to the s-shaped part of the dike intrusion. At the present stage of mining, the deposit is characterized by the absence of major veins, the occurrence of disseminated pyrite throughout the orebody, and a distinct alteration pattern comparable to that of porphyry copper deposits. The central zone of the dike shows a typical K-feldspar-albite-sericite-pyrite (±biotite?) alteration, followed by a narrow external propylitic zone. Native gold with an average Ag content of 5 wt.% and a grain size of 5–100 μm is rare and occurs within pyrite and secondary K-feldspar. Sulphide mineral separates of pyrite and minor arsenopyrite probably contain invisible gold (up to 120 ppm) amenable to cyanidation. Anomalously high gold values of ∼7 ppm have been found in the transition between the K-feldspar-albite-sericite-pyrite alteration and the propylitic zone, indicating that the mineralizing fluids have experienced major physico-chemical changes in the transition zone. The regional tectonic position of the orebody suggests that the emplacement of the granite and the gold mineralization are structurally controlled. The Pb isotope composition of several leachates of pyrite indicate isotope disequilibrium with magmatic minerals and point to a contamination of the mineralizing fluid by Pb from older (sedimentary?) sources. Stable isotope geochemistry of sulphides and carbonates as well as the metallogeny of the deposit compare to shear-zone hosted gold mineralization in the Kwekwe district, for which a deep crustal origin has been discussed. Although this study documents contrasting evidence for a porphyry-gold versus a shear-zone type of mineralization, it is suggested that gold-bearing fluids were syntectonically introduced into a ductile shear zone within the granite dike either during cooling of the intrusion or later in Archaean or early Proterozoic times.

Notes: Abstract ODP drilling of the active TAG hydrothermal mound at 26°N on the Mid-Atlantic Ridge provided the first insights into the third dimension of a volcanic-hosted massive sulfide (VMS) deposit on a sediment-free mid-ocean ridge. Sulfide precipitation at this site started at least 20,000 years ago and resulted in the formation of a distinctly circular, 200-m diameter, 50-m-high pyritic mound and a silicified stockwork complex containing approximately 3.9 million tonnes of sulfide-bearing material with an average of 2.1 wt% Cu and 0.6 wt% Zn in 95 samples collected from 1–125 m below the seafloor. The periodic release of high-temperature hydrothermal fluids at the same location for several thousand years with intermittent periods of hydrothermal quiesence is the dominating process in the formation of the TAG hydrothermal mound. Distinct geochemical, mineralogical and isotopic zonation as well as a complex assemblage of sulfide-anhydrite-quartz bearing breccias can be related to this process. Geochemical depth profiles indicate extremely low base and trace element concentrations for the interior of the mound, which clearly contrasts with published analyses of samples collected from the surface of the TAG mound. This is explained by continued zone refining during which metals were mobilized from the interior of the mound by upwelling, hot (〉350 °C) hydrothermal fluids. Mixing of these fluids with infiltrating ambient seawater subsequently caused redeposition of metals close to the mound-seawater interface. The sulfur isotopic composition of bulk sulfides (+4.4 to +8.2‰δ34S; average +6.5‰) is unusually heavy when compared to other sediment-free mid-ocean ridge deposits and implies the introduction of heavy seawater sulfur to the hydrothermal fluid. The slight increase in sulfur isotope ratios with depth and distinct variations between early, disseminated sulfides related to wallrock alteration, and massive as well as late vein sulfides indicates widespread entrainment of seawater deep into the system. Fluid inclusion measurements in quartz and anhydrite reveal high formation temperatures throughout the TAG mound (up to 390 °C) at one time with an overall increase in trapping temperatures with depth. Lower formation temperatures close to the paleo-seafloor indicate local entrainment of seawater into the mound. Formation temperatures for a central anhydrite-bearing zone range from 340–360 °C and are slightly lower than the exit temperature of hydrothermal fluids presently venting at the Black Smoker Complex (360–369 °C). Fluid inclusions in quartz and anhydrite from the stockwork zone are characterized by formation temperatures higher than 375 °C, indicating conductive cooling of the hydrothermal fluids or mixing with ambient seawater prior to venting. Formation temperatures for quartz from an area of extremely low heat flow at the western side of the mound reach up to 390 °C, implying that this area was once part of a high-temperature hydrothermal upflow zone. The low heat flow and the absence of anhydrite within this part of the mound are strong indications that the recent pulse of high-temperature hydrothermal activity is not affecting this area and provides evidence for significant changes in the fluid flow regime underneath the deposit between hydrothermal cycles.

Notes: A self-consistent APW band structure calculation for insulating Be2C is reported. From the APW results local partial densities of states have been obtained showing a considerable beryllium admixture in the carbon 2p band. An LCAO tight binding fit and a subsequent Mulliken analysis of the eigenvectors reveals that the carbon carries a charge of approximately 3.5 e. The electrons remaining on the beryllium atom are predominantly of p character giving rise to a covalent pp interaction which is responsible for pronounced features in the valence bands not present in a completely ionic description. A further analysis of the ionicity is made on the basis of a fit of the structure factors derived from the APW valence electron density using a superposition of atomic or ionic form factors. To obtain an optimum fit the electronic charge on the carbon has to be decreased to approximately 3.0. This reduction of charge, compared with the LCAO fit, is traced back to the fact that the atomic form factors have been calculated from a nonorthogonal basis, whereas an orthogonal one underlies the LCAO fit.

Notes: A new method is described to express the Madelung parameter of a structure with a non-cubic Bravais lattice as a function of a lattice parameter by means of a power series expansion. The method is derived for the case of a cubic lattice which is tetragonally distorted. The generalization to other crystal structures is shown. For illustration the method is applied to the rutile and cadmium iodide structures.