Jacques Jedwab

Nankai Trough, Japan Trench and Kuril Trench: geochemistry of Fluids sampled by submersible "Nautile"

Abstract Deep-water samples collected during the Kaiko project are often associated with biological communities located on geological structures favorable to fluid venting. The evidence of fluid venting are the temperature anomalies, the decrease in sulfate concentrations, the content in methane and the lowC 1 (C 2 +C 3 ) ratio of light hydrocarbons. Because of large dilution by ambiant seawater during sampling it is difficult to compute the composition of the advected end-member pore fluid. Part of this fluid should originate in the “petroleum window”, i.e. at temperature about 60°C. Modeling the upward flow of water, taking into account the anomalies of temperature measured on the seafloor and the geochemical anomalies, leads to non-steady-state advection of the pore fluid. The occurrence of a deep component in the fluid has implications for the geological and tectonic models of the subduction zones off Japan.

Verbeekite, monoclinic PdSe2, a new mineral from the Musonoi Cu-Co-Mn-U mine, near Kolwezi, Shaba Province, Democratic Republic of Congo

Abstract Verbeekite, ideally PdSe2, monoclinic with space-group choices C2/m, C2 or Cm; a = 6.659(7), b = 4.124(5), c = 4.438(6) Å, β = 92.76(3)8, V = 121.7(4) Å3; a:b:c = 1.6147:1:1.0761, Z = 2, is a new, very rare, primary mineral, intimately associated with secondary oosterboschite {(Pd,Cu)7Se5}, from the Musonoi Cu-Co-Mn-U mine, near Kolwezi, Shaba Province, Democratic Republic of Congo. Additional associated minerals are Cu- and Pd-bearing trogtalite {(Co,Cu,Pd)Se2}, Se-bearing digenite and Se-bearing covellite. The strongest five lines of the X-ray powder-diffraction pattern {d in Å (I) (hkl)} are: 4.423(30)(001), 3.496 (30)(110), 2.718(100)(111), 1.955(50)(310) and 1.896(50)(1̄12). The mineral has also been identified, as a single anhedral 25 μm-sized grain, from Hope’s Nose, Torquay, Devon, England where it is associated with native gold, chrisstanleyite Ag2Pd3Se4, oosterboschite(?), unnamed Pd2HgSe3 and cerussite. At Musonoi, altered verbeekite grains do not exceed 200 μm in size and are anhedral, black, with a black streak and a metallic lustre. The mineral is opaque, brittle, has an uneven fracture, and lacks discernible cleavage. The VHN5 ranges 490-610, mean 550 kp/mm2 (2 indentations), roughly approximating a Mohs’ hardness of 5Ý. Dcalc. = 7.211 g/cm3 for the ideal formula. Electron-microprobe analyses (mean of 4 spot analyses) yielded Pd 39.6, Cu 0.5, Se 58.8, total 98.9 wt.%. The empirical formula is (Pd0.99Cu0.02)∑1.01Se1.99, based on Pd+Cu+Se = 3. In plane-polarized reflected light, the mineral is a nondescript grey and is neither pleochroic nor perceptibly bireflectant. Anisotropy is moderate with rotation tints in varying shades of brown. Reflectance spectra and colour values are tabulated. The name honours Dr Théodore Verbeek (1927-1991) who was the first geoscientist to study the Musonoi palladium mineralization in the Democratic Republic of Congo (1955-1967) and who co-discovered this new mineral phase.

Trace element (Th, U, Pb, REE) behaviour in a cryptokarstic halloysite and kaolinite deposit from Southern Belgium: Importance of "accessory" mineral formation for radioactive pollutant trapping

Abstract Hectometer wide cryptokarsts in Paleozoic limestone from Southern Belgium have been studied, to determine to what extent U, Th, Pb and rare earth elements (REE) have been mobilized in the karst sedimentary filling, during a Miocene weathering event. The weathering process resulted in the massive halloysite/kaolinite formation at the karst wall. As with most fossil systems, data on weathering fluid chemistry are lacking, hence it is difficult to quantify relevant parameters such as pH, Eh, and to address solution chemistry. However, on the basis of both field studies of more recent systems, and of geochemical modeling, it is proposed that moderately acid fluids percolated through a multi-layer sedimentary filling, in near-surface conditions and in a temperate/warm climate. Special attention is paid to the trace element immobilization/trapping processes, in newly crystallized REE phosphates, at the karst wall. Analytical methods used include major/trace element geochemistry (emission ICP, ICP–MS) and mineralogy (XRD, SEM, TEM, microprobe). The results suggest that both the sandy sediments that are in contact with the karst carbonate wall, and the carbonate wall itself acted as a kind of geochemical “barrier”. Mineralization cells settled there, at the decimeter to meter scale. This results in sequential trace element (Pb, Th, REE, U) trapping, according to the affinity of these elements for the aqueous solution. At the end of the sequence, minute U-rich automorphic (Ce, Nd) monazite crystals (from 3 nm upwards) formed on kaolinite flakes. Though the analogy between the studied cryptokarst and planned surface-based repositories for low-level radioactive waste (LLW) in argillaceous context is far from complete, the results outlined here are relevant because they show that even in natural—i.e. intrinsically uncontrolled and unmonitored—systems, “pollutant” radionuclide (U, Th, REE, Pb) migration paths are often limited in space. Various processes converge towards trapping of these elements, that are present in the radioactive waste.

Copper, zinc and lead minerals suspended in ocean waters

Abstract Individual particles of malachite, tenorite, paratenorite, laurionite and sphalerite, as well as several intermediate compounds of Cu, Zn, Pb, Cl, S, OH and CO 3 have been identified among paniculate matter filtered from Atlantic and Pacific deep water samples. These particulate matter samples were provided mainly by the GEOSECS 1972 and 1974 cruises. The detection and the identification of the individual heavy metal compounds were made by a combination of light microscopy, SEM, EMP and XRD. The highest frequency of heavy metal compounds has been found at several North Atlantic, and at one Antarctic stations. Their origin can, at least partly, be ascribed to technological influences, since metallic copper and brass particles are sometimes found associated with, or included in the heavy metal compounds. Others, however, result clearly from growth in the water.

On a correlation between the common lead model age and the trace-element content of galenas

Abstract It is now well known that the isotopic composition of common lead varies in a significant way with the age of the lead occurrence. Providing certain assumptions are made, this isotopic composition can be used for an estimate of the age of the mineral. This “model” or “conventional” age is often in accordance with other evidence. However there exist a certain number of leads which do not conform to the systematic pattern generally observed. The purpose of this study is to investigate certain aspects of the geology and geochemistry of those leads, the model ages of which are older than the geological age. Their isotopic composition usually appears to be perfectly normal and if the age of the host formation or of the emplacement of the lead occurrence is unknown it may be impossible to detect which leads belong to this type, so that the search for criteria enabling to recognize them is of some importance. Twenty leads from the Alps and twenty-four from North Africa have been studied. In each case the model age, derived from the isotopic composition, has been compared to the age of the host formation or that of the emplacement of the galena. The silver content (and that of some trace elements) of each of these galenas has been measured and compared to the abovementioned ages. Practically all galenas we have investigated that have model ages comparable to the age of the host formation are found in veins, often with more or less obvious magmatic connexions and they are silver rich. On the other hand the leads of those occurrences, located in sedimentary rocks, and apparently independent of igneous activity, are poor in silver and have model ages which are older than the host formation. The origin of the latter, “telethermal”, type of occurrence has often been debated and it has been suggested that these occurrences are “rejuvenated” from an older source of ore. This seems to be borne out by the fact that it is precisely occurrences of this type which contain leads with model ages older than the age of the host formation. It thus seems that when the age of the host formation is known, isotope analyses of lead minerals and the model age derived therefrom, constitute objective criteria enabling us to distinguish which occurrences are emplaced for the first time and which are derived or rejuvenated. In formations of unknown age, a similar criterion which might furnish the same information appears to be the silver content of the galena. In each of the areas studied, we have found that, as regards silver content, there is a sharp distinction, with little or no overlap, between leads with model ages in agreement with the age of the host formation and those with model ages older than that of the host formation.

Noble metals segregation and fractionation in magmatic ores from Ronda and Beni Bousera Lherzolite Massifs (Spain, Morocco)

SummaryThree types of mineralization are found in high-temperature lherzolite massifs of Southern Spain and Northern Morocco: (Cr) chromite, (Cr-Ni) chromite-nickel arsenide, (S-G) sulphide-graphite. The ore veins are distributed in this order from the plagioclase-lherzolite core to the garnet-lherzolite border of the massifs. These hightemperature ore assemblages (1200-600°C) have cumulate textures including orthopyroxene and/or cordierite as main silicate minerals.High average PGE concentrations are present in the Cr-Ni ores (2000 ppb) in relation to the Ni-arsenide abundance. The Cr ores have only 900 ppb PGE, and the S-G ores are PGE-poor (350 ppb). Gold roughly follows the PGE distribution: 13,000 ppb in Cr-Ni ores, 570 ppb in Cr ores, and only 88 ppb in S-G ores. The chondrite normalized PGE patterns of the Cr-Ni ores are chondritic, whereas those of the Cr and S-G ores have respectively negative and positive slopes. The Pd/Ir ratio strongly increases from the Cr ores (0.39) to the Cr-Ni and the S-G ores (2.7 and 3.4)). There are some (Os, Ru)S2 inclusions in the chromite of the Cr ores. In the Cr-Ni ores, some minute Au, Au-Cu, and Au-Bi-Te grains are observed. No PGM have been found, except in a weathered Cr-Ni ore sample where abundant PGM (PtAs2, IrAsS) are present., suggesting that PGE may be hidden as solid solution in the Ni-arsenide.The ore-forming magma probably has a mantle source-rock. The earliest chromites (Cr ores) contain Os-Ir-Ru mineral inclusions, whereas most of the gold and the remaining PGE with higher Pd/Ir ratio were partitioned into an immiscible As-S-liquid, which fractionated later into an earliest PGE-Au-rich NiAs-phase (Cr-Ni ores) and then a PGE-Au-poor MSS-phase (S-G ores).ZusammenfassungIn den Hochtemperatur-Lherzolit Massiven von Süd-Spanien und Nord-Marokko kommen drei Typen von Vererzung vor: (Cr) Chromit, (Cr-Ni) Chromit-Nickelarsenid, (S-G) Sulfid-Graphit. Die Erzgänge sind in dieser Abfolge vom Plagioklas-Lherzolit Kern zum Granat-Lherzolit Rand der Massive angeordnet. Diese Hochtemperaturparagenesen (1200°-600° C) haben Kumulattexturen mit Orthopyroxen und/oder Cordierit als Hauptsilikatminerale.Hohe Durchschnittsgehalte an PGE kommen in den Cr-Ni Erzen (2000 ppb) vor, und diese stehen in Beziehung zur Häufigkeit der Nickel-Arsenide. Die Cr-Erze führen nur 900 ppb PGE und die S-G Erze sind PGE-arm (350 ppb). Gold folgt in ungefähr der PGE-Verteilung: 13000 ppb in Cr-Ni Erzen, 570 ppb in Cr Erzen, und nur 88 ppb in S-G Erzen. Die Chondrit-normalisierten PGE Verteilungen der Chrom-Nickel Erze sind chondritisch, während jene der Cr- und S-G Erze negative, bzw. positive Neigungen zeigen. Das Pd/Ir Verhältnis nimmt von den Cr-Erzen (0, 39) zu den Cr-Ni und den S-G Erzen (2,7 und 3,4) deutlich zu. Es gibt einige (Os, Ru)S2 Einschlüsse in den Chromiten der Cr Erze. In den Cr-Ni Erzen, kommen winzige Einschlüsse von Au, Au-Cu und AuBi-Te Körnern vor. Keine PGM konnten nachgewiesen werden, mit Ausnahme eines verwitterten Cr-Ni Erzes wo reichlich PGM (PtAs2,1rAsS) vorliegen. Dies weist darauf hin, daß PGE in fester Lösung in den Nickel-Arseniden gebunden sein könnten.Das erzbildende Magma dürfte dem Mantel entstammen. Die am frühesten gebildeten Chromite (Cr-Erze) enthalten Einschlüsse von Os-Ir-Ru Mineralen, während ein Großteil des Goldes und der verbleibenden PGE mit höheren Pd/Ir Verhältnissen in eine nicht mischbare As-S fluide Phase gingen; die letztere fraktionierte später in eine frühe PGE-Au-reiche NiAs-Phase (Cr-Ni Erze) und dann in eine PGE-Au-arme MSS-Phase (S-G Erze).

Platinum-group element mineralization in an As-rich magmatic sulphide system, Talnotry, southwest Scotland

Abstract Arsenic-rich magmatic sulphide mineralization is hosted by a diorite intrusion at Talnotry, southwest Scotland. A relatively abundant and diverse platinum-group mineral assemblage is present and is dominated by sperrylite, irarsite and electrum with subordinate merenskyite, michenerite and froodite. Early euhedral gersdorffite is enriched with respect to Rh, Ir and Pt and in some cases contains exsolved blebs of irarsite or euhedral grains of sperrylite. Sperrylite is also enclosed within silicates and sulphides indicating that it crystallized directly from an As-rich sulphide liquid. Pyrrhotite-chalcopyrite mineral assemblages are consistent with the fractional crystallization of monosulphide solid solution and are overlain by PGE-, Ni- and As-rich mineral assemblages indicative of crystallization from a NiAs liquid. Late-stage, cross-cutting, electrum-bearing chalcopyrite veins are consistent with the crystallization of Cu- and Au-rich intermediate solid solution. The chemistry, mineralogy and lithological relationships of the diorite suggest that it may be an appinite and as such is potentially analogous to the Au-rich lamprophyre dykes present within southwest Scotland.

'Irite' (Hermann, 1836/1841) from the Urals

Abstract A reappraisal of Hermann’s ‘irite’, described in 1836 and 1841, has been attempted by examining and analysing three museum samples, accessioned before 1865. The three samples were determined to be mixtures of chromite, native Os and Ir, rutheniridosmine, platinum, laurite, hongshiite, irarsite, cuprorhodsite, iridsite, prassoite, hollingworthite, erlichmannite, several Ru-Ir-Pt-Ti-Cr-Fe oxide/ hydroxide compounds (sometimes well crystallized), Pt-Ir-K chlorocompounds, and Pt-Ir-Ru-K-Clcontaining opal. The latter two are considered as artifacts generated by the hydrometallurgical processes applied to the crude platinum ores. Individual particles of native iridium and osmium display baroque crystal habits (filaments, leafs, blades, hollow crystals) that could be their natural, although rarely observed, states. There is still doubt about the origin (either natural or artificial) of all Ru-Ir-Pt- Ti-Cr-Fe oxides, but they can be compared with those found in several pristine placer and hard-rock deposits worldwide and in the Urals.

Search for Magnetite in Lunar Rocks and Fines

Magnetite crystals larger than 2 micrometers are absent from rocks and fines. Smaller opaque spheres in the fines can tentatively be identified as magnetite. Their concentration is not higher than 1 x 10-6 particle per particle smaller than 1 millimeter. In the fines from the sampling site, the contribution of material similar to type 1 carbonaceous meteorites is insignificant, either because it never existed, or because it was evaporated or comminuted by impact or was diluted by indigenous material. Other magnetite habits typical of carbonaceous meteorites or possibly of cosmic dust or comets were also sought without success-such as rods, platelets, framboids, spherulites, and idiomorphic crystals.

Empirical Pt 7Cu from an alluvial platinum concentrate and its significance for platiniferous quartz lodes in the lubero region, dr congo

An alluvial platinum grain from Lubero, DRC, is compositionally analogous to Pt 7Cu. This phase is so far unknown in nature, but is known from the binary platinum-copper system to have an ordered structure and formed below 500 °C, and as low as 100 °C. Such low temperatures rule out magmatic platinum mineralisation as source rock. The presence of previously reported inclusions of vein quartz and hematite in some alluvial grains of platinum, together with the relatively low formation temperatures of the Pt 7Cu compound, suggests that hydrothermal quartz lodes were the source of the platinum found in the Lubero alluvia.