Hydrothermal deposition on the East Pacific rise at 21°N

In the spring of 1979, 350°C springs precipitating hydrothermal sulphides and sulphates directly on to the sea-floor were discovered on the crest of the East Pacific Rise (EPR) at 21°N by the astonished scientific party of the RISE submersible expedition. These hot springs are within a linear field of active and inactive hydrothermal vents extending 6 km along the rise axis. Typically the mineral deposits at EPR, 21°N consist of basal sulphide mounds surmounted by mineralized sulphide-sulphate edifices, or “chimneys”, reaching heights up to 13 m above the sea floor. The mounds rest directly on fresh basalt and cover areas up to 450 m². Chimneys atop mounds may be active or dead. The hottest active chimneys (350°C) spew forth fluids blackened by fine-grained sulphide precipitates, dominantly hexagonal pyrrhotite and iron-rich sphalerite. These “black smokers” are distinguished from cooler “white smoker” chimneys which are encrusted by worm tubes and emit milky fluids bearing amorphous silica, barite, and pyrite.     

Talc friction in the temperature range 25°–400 °C: Relevance for Fault-Zone Weakening

Talc is one of the weakest minerals that is associated with fault zones. Triaxial friction experiments conducted on water-saturated talc gouge at room temperature yield values of the coefficient of friction, μ (shear stress, τ/effective normal stress, σ′_N) in the range 0.16–0.23, and μ increases with increasing σ′_N. Talc gouge heated to temperatures of 100°–400 °C is consistently weaker than at room temperature, and μ < 0.1 at slow strain rates in some heated experiments. Talc also is characterized by inherently stable, velocity-strengthening behavior (strength increases with increasing shear rate) at all conditions tested. The low strength of talc is a consequence of its layered crystal structure and, in particular, its very weak interlayer bond. Its hydrophobic character may be responsible for the relatively small increase in μ with increasing σ′_N at room temperature compared to other sheet silicates.Talc has a temperature–pressure range of stability that extends from surficial to eclogite-facies conditions, making it of potential significance in a variety of faulting environments. Talc has been identified in exhumed subduction zone thrusts, in fault gouge collected from oceanic transform and detachment faults associated with rift systems, and recently in serpentinite from the central creeping section of the San Andreas fault. Typically, talc crystallized in the active fault zones as a result of the reaction of ultramafic rocks with silica-saturated hydrothermal fluids. This mode of formation of talc is a prime example of a fault-zone weakening process. Because of its velocity-strengthening behavior, talc may play a role in stabilizing slip at depth in subduction zones and in the creeping faults of central and northern California that are associated with ophiolitic rocks.     

Hydrothermal alteration and fluid inclusion study of the Lower Cretaceous porphyry Cu–Au deposit of Tiámaro, Michoacán, Mexico

The Tiámaro deposit in Michoacán state has been dated as Lower Cretaceous (Valanginian), though most of the porphyry deposits in central Mexico were dated or have an attributed Eocene–Oligocene age. The host rocks belong to a volcanoplutonic complex overlain by red conglomerates. These rocks were intruded by pre-Valanginian plutonic and hypabissal rocks. Propylitic, phyllic, and argillic alteration assemblages developed, and their superimposition draws the evolution of the deposit. Stage I is represented by propylitic assemblages, stage II contains the main ore forming stockworks and both phyllic and argillic assemblages, and stage III contains late carbonatization assemblages. The obtained temperatures and salinities from inclusion fluids are low for a porphyry-type deposit, but we interpret that the known part of the deposit represents the shallow portion of a bigger deposit. The evolution of mineralizing fluids draws a dilution trend of brines from “porphyry-like” to “epithermal-like” stages. The richest ore zone is roughly located between the 300 and 350 °C isotherms, though unnoticed resources may occur at depth.     

Iron isotope fractionation in a buoyant hydrothermal plume, 5°S Mid-Atlantic Ridge

Fe isotopes are a potential tool for tracing the biogeochemical redox cycle of Fe in the ocean. Specifically, it is hypothesized that Fe isotopes could enable estimation of the contributions from multiple Fe sources to the dissolved Fe budget, an issue that has received much attention in recent years. The first priority however, is to understand any Fe isotope fractionation processes that may occur as Fe enters the ocean, resulting in modification of original source compositions. In this study, we have investigated the Fe inputs from a basalt-hosted, deep-sea hydrothermal system and the fractionation processes that occur as the hot, chemically reduced and acidic vent fluids mix with cold, oxygen-rich seawater.The samples collected were both end-member vent fluids taken from hydrothermal chimneys, and rising buoyant plume samples collected directly above the same vents at 5°S, Mid-Atlantic Ridge. Our analyzes of these samples reveal that, for the particulate Fe species within the buoyant plume, 25% of the Fe is precipitated as Fe-sulfides. The isotope fractionation caused by the formation of these Fe-sulfides is δ_Fe(II)–FeS = +0.60 ± 0.12‰.The source isotope composition for the buoyant plume samples collected above the Red Lion vents is calculated to be −0.29 ± 0.05‰. This is identical to the value measured in end-member vent fluids collected from the underlying “Tannenbaum” chimney. The resulting isotope compositions of the Fe-sulfide and Fe-oxyhydroxide species in this buoyant plume are −0.89 ± 0.11‰ and −0.19 ± 0.09‰, respectively. From mass balance calculations, we have been able to calculate the isotope composition of the dissolved Fe fraction, and hypothesize that the isotope composition of any stabilised dissolved Fe species exported to the surrounding ocean may be heavier than the original vent fluid. Such species would be expected to travel some distance from areas of hydrothermal venting and, hence, contribute to not only the dissolved Fe budget of the deep-ocean but also it’s dissolved Fe isotope signature.     

Convergence structures of the Peru trench between 10°S and 14°S

High resolution seafloor studies of the Peru Trench between 10°S and 14°S with the GLORIA long-range side-scan sonar system show that the Nazca plate is broken by numerous normal faults as it bends into the trench. These bending-induced faults strike subparallel to the trench axis and overprint and cut across spreading fabric structures of the plate. They commonly form grabens having widths and spacings of 3–5 km and extend for as much as 100 km along strike. Vertical displacements are generally 200 m or more by the time they reach the trench axis. Turbidite deposits are found in the trench north of 11.5°S. Both turbidite and pelagic sediments are folded and temporarily accreted to the base of the overriding plate along the length of the trench axis. They are apparently subsequently implaced in the grabens by slumping and subducted with the Nazca plate. The Mendaña Fracture Zone, which intersects the trench between 9°40′S and 10°35′S, appears to be the locus of a seaward propagating rift that is forming in response to subduction-induced extensional stresses in the Nazca plate.     

Genetic significance of fluid inclusions in the CSA Cu–Pb–Zn deposit, Cobar, Australia

CSA mine exploits a ‘Cobar-type’ Cu–Pb–Zn±Au±Ag deposit within a cleaved and metamorphosed portion of the Cobar Supergroup, central New South Wales. The deposit comprises systems of ‘lenses’ that encompass veins, disseminations and semi-massive to massive Cu–Pb–Zn ores. The systems and contained lenses truncate bedding, are approximately coplanar with regional cleavage and similarly oriented shear zones and plunge parallel to the elongation lineation. Systems have extreme vertical continuity (>1000 m), short strike length (400 m) and narrow width (100 m), exhibit vertical and lateral ore-type variation and have alteration haloes. Models of ore formation include classical hydrothermalism, structurally controlled remobilisation and polymodal concepts; syntectonic emplacement now holds sway.Fluid inclusions were examined from quartz±sulphide veins adjacent to now-extracted ore, from coexisting quartz–sulphide within ore, and from vughs in barren quartz veins. Lack of early primary inclusions precluded direct determination of fluids associated with D₂–D₃ ore and vein emplacement. Similarly, decrepitation (by near-isobaric heating) of the two oldest secondary populations precluded direct determination of fluid phases immediately following D₂–D₃ ore and vein emplacement. Post-decrepitation outflow (late D₃ to early post-D₃) is recorded by monophase CH₄ inclusions. Entrained outflow of deeply circulated meteoric fluid modified the CH₄ system; modification is recorded by H₂O+CH₄ and H₂O+(trace CH₄) secondary populations and by an H₂O+(trace CH₄) primary population. The contractional tectonics (D₂–D₃) of ore emplacement was superseded by relaxational tectonics (D_4P) that facilitated meteoric water penetration and return flow.Under D₂ prograde metamorphism, entrapment temperatures (Tt) and pressures (Pt) for pre-decrepitation secondary inclusions are estimated as Tt300–330 °C and Pt1.5–2 kbar≈Plith (the lithostatic pressure). Decrepitation accompanied peak metamorphism (T350–380 °C) in mid- to late-D₃, while in late-D₃ to early post-D₃, essentially monophase CH₄ secondary inclusions were entrapped at Tt350 °C and Pt=1.5–2 kbar≈Plith. Subsequently, abundant CH₄ and entrained meteoric water were entrapped as H₂O+CH₄ secondaries under slowly decreasing temperature (Tt330–350 °C) and constant pressure (Pt1.5–2 kbar). Finally, with increasingly dominant meteoric outflow, H₂O+(trace CH₄) populations record decreasing temperatures (Tt>300 to <350 down to 275–300 °C) at pressures of Phydrostatic<Pt (1 kbar) <Plith (1.5 kbar).The populations of inclusions provide insight into fluid types, flow regimes and P–T conditions during parts of the deposit's evolution. They indirectly support the role of basin-derived CH₄ fluids in ore formation, but provide no insight into a basement-sourced ore-forming fluid. They fully support post-ore involvement of meteoric water. The poorly constrained entrapment history is believed to span 10 Ma from 395 to 385 Ma.     

Global 1° × 1° thermal model TC1 for the continental lithosphere: Implications for lithosphere secular evolution

This paper reports a new 1° × 1° global thermal model for the continental lithosphere (TC1). Geotherms for continental terranes of different ages (> 3.6 Ga to present) constrained by reliable data on borehole heat flow measurements (Artemieva, I.M., Mooney, W.D. 2001. Thermal structure and evolution of Precambrian lithosphere: a global study. J. Geophys. Res 106, 16387–16414.), are statistically analyzed as a function of age and are used to estimate lithospheric temperatures in continental regions with no or low-quality heat flow data (ca. 60% of the continents). These data are supplemented by cratonic geotherms based on electromagnetic and xenolith data; the latter indicate the existence of Archean cratons with two characteristic thicknesses, ca. 200 and > 250 km. A map of tectono-thermal ages of lithospheric terranes complied for the continents on a 1° × 1° grid and combined with the statistical age relationship of continental geotherms (z = 0.04  t + 93.6, where z is lithospheric thermal thickness in km and t is age in Ma) formed the basis for a new global thermal model of the continental lithosphere (TC1). The TC1 model is presented by a set of maps, which show significant thermal heterogeneity within continental upper mantle, with the strongest lateral temperature variations (as large as 800 °C) in the shallow mantle. A map of the depth to a 550 °C isotherm (Curie isotherm for magnetite) in continental upper mantle is presented as a proxy to the thickness of the magnetic crust; the same map provides a rough estimate of elastic thickness of old (> 200 Ma) continental lithosphere, in which flexural rigidity is dominated by olivine rheology of the mantle.Statistical analysis of continental geotherms reveals that thick (> 250 km) lithosphere is restricted solely to young Archean terranes (3.0–2.6 Ga), while in old Archean cratons (3.6–3.0 Ga) lithospheric roots do not extend deeper than 200–220 km. It is proposed that the former were formed by tectonic stacking and underplating during paleocollision of continental nuclei; it is likely that such exceptionally thick lithospheric roots have a limited lateral extent and are restricted to paleoterrane boundaries. This conclusion is supported by an analysis of the growth rate of the lithosphere since the Archean, which does not reveal a peak in lithospheric volume at 2.7–2.6 Ga as expected from growth curves for juvenile crust.A pronounced peak in the rate of lithospheric growth (10–18 km³/year) at 2.1–1.7 Ga (as compared to 5–8 km³/year in the Archean) well correlates with a peak in the growth of juvenile crust and with a consequent global extraction of massif-type anorthosites. It is proposed that large-scale variations in lithospheric thickness at cratonic margins and at paleoterrane boundaries controlled anorogenic magmatism. In particular, mid-Proterozoic anorogenic magmatism at the cratonic margins was caused by edge-driven convection triggered by a fast growth of the lithospheric mantle at 2.1–1.7 Ga. Belts of anorogenic magmatism within cratonic interiors can be caused by a deflection of mantle heat by a locally thickened lithosphere at paleosutures and, thus, can be surface manifestations of exceptionally thick lithospheric roots. The present volume of continental lithosphere as estimated from the new global map of lithospheric thermal thickness is 27.8 (± 7.0) × 10⁹ km³ (excluding submerged terranes with continental crust); preserved continental crust comprises ca. 7.7 × 10⁹ km³. About 50% of the present continental lithosphere existed by 1.8 Ga.     

Geochemical and temperature controls on ore mineralization at the Emperor gold mine, Vatukoula, Fiji

Emperor is a large epithermal gold-tellurium deposit which occurs at the margin of the Tavua caldera and is associated with high-level mineralized tuffs, breccias and silicious sinters. The deposit consists of mainly narrow, steep “shear” and shallow dipping “flatmake” mineralized vein structures. Mineralization spanned a period during which different veins and dyke intrusions occurred so that some early structures are offset by later ones. High Au values and high Au/Ag ratios, corresponding to the intersection of shear and flatmake structures, generally correlate with high Ag, Te and Ba values in some structures (Crown Shear) and high Hg in others (166 Flatmake). Other elements commonly anomalous in the high-grade ore zones are As, Mo and Cu. Thallium, Se and Sb are commonly high below ore zones while Hg may be high above (e.g., Crown Shear), in (166 Flatmake) or even below (e.g., Prince of Wales Shear) ore zones.Gold precipitated in a temperature interval of 180–210°C; higher temperatures commonly occur at depth in some flatmakes (166 N, 608) and lower temperatures nearer the surface. In the two steep shears analyzed, higher temperatures correspond to where they are intersected by flatmakes. Temperatures may decrease below such intersections in which case Hg, Sb, etc., commonly increase with depth.Evidence of boiling, characterized by coexisting gas- and liquid-rich fluid inclusions, is minor and spatially sporadic. Quartz and carbonate found in ore containing bonanza concentrations of Au contain few to no gas-rich fluid inclusions. The intensity of wall-rock alteration is unrelated to gold grades.Ore deposition is inferred to have been caused mainly in response to fluid mixing rather than extensive boiling and/or wall-rock alteration, although these processes occurred. The source of the Au, Ag, Te, etc. is inferred to be near a neutral, relatively reduced, bisulphide-rich, ore solution at near 300°C and derived in some way from the shoshonitic volcanics or associated monzonitic intrusions. The second non-ore solution necessary for mixing, is inferred to have been an acid, oxidized solution at 150°C and having a large meteoric component. The result of fluid mixing was ore genesis, and a telluride, minor sulphide and very minor sulphate (barite) and anhydrite ore assemblage.     

The polymetallic Au–Ag-bearing veins of Bou Madine (Jbel Ougnat, eastern Anti-Atlas, Morocco): tectonic control and evolution of a Neoproterozoic epithermal deposit

The Bou Madine ore deposit is located SW of Jbel Ougnat, the easternmost inlier of the Anti-Atlas Pan-African belt in Morocco. The host rocks are high-K calc-alkaline volcanic rocks, that are part of the Neoproterozoic Tamerzaga-Timrachine Formation (TTF, lower PIII). The TTF consists of ignimbrites of rhyolitic to dacitic compositions, andesite flows and hypovolcanic bodies (andesite dykes and rhyolite chonoliths) emplaced along N160°E tension gashes associated with a regional N30°E sinistral fault system. The mineralization is related to a high enthalpy geothermal system, eventually evolving into a low temperature epithermal system. A regional propylitisation (T around 260 °C) overprinted the TTF rocks prior to the emplacement of the mineralization. There were two main hydrothermal stages. During the first stage, massive veins with pyrite, arsenopyrite and minor pyrrhotite and cassiterite were formed. The veins were emplaced along N160°E-trending en echelon joints related to N120°E dextral arrays. A quartz-sericite-pyrite alteration overprinted the propylites around the veins (“bleached haloes”), at temperatures up to 300–310 °C. The second stage of mineralization was coeval with dextral re-activation of the N160°E veins, in relation with a NE-ward shift of the shortening direction. First, polymetallic sulphides (sphalerite, chalcopyrite, stannite, galena) were deposited at temperatures 260 °C. Younger quartz veinlets contain arsenopyrite and minor micrometre-size sulphides and sulpho-salts, hosting the precious metals. This was the low temperature epithermal stage (≈150 °C), in relation with invading meteoric water.     

Sulphide mineralization and wall-rock alteration in ophiolites and modern oceanic spreading centres

Massive and stockwork Fe-Cu-Zn (Cyprus type) sulphide deposits in the upper parts of ophiolite complexes represent hydrothermal mineralization at ancient accretionary plate boundaries. These deposits are probable metallogenic analogues of the polymetallic sulphide deposits recently discovered along modern oceanic spreading centres. Genetic models for these deposits suggest that mineralization results from large-scale circulation of sea-water through basaltic basement along the tectonically active axis of spreading, a zone of high heat flow. The high geothermal gradient above 1 to 2 km deep magma chambers emplaced below the ridge axis drives the convective circulation cell. Cold oxidizing sea-water penetrating the crust on the ridge flanks becomes heated and evolves into a highly reduced somewhat acidic hydrothermal solvent during interaction with basaltic wall-rock. Depending on the temperature and water/rock ratio, this fluid is capable of leaching and transporting iron, manganese, and base metals; dissolved sea-water sulphate is reduced to sulphide. At the ridge axis, the buoyant hydrothermal fluid rises through permeable wall-rocks, and fluid flow may be focussed along deep-seated fractures related to extensional tectonic processes. Metal sulphides are precipitated along channelways as the ascending fluid undergoes adiabatic expansion and then further cooling during mixing with ambient sub-sea-floor water. Vigorous fluid flow results in venting of reduced fluid at the sea-floor/sea-water interface and deposition of massive sulphide. A comparison of sulphide mineralization and wall-rock alteration in ancient and modern spreading centre environments supports this genetic concept.Massive sulphide deposits in ophiolites generally occur in clusters of closely spaced (< 1–5 km) deposits. Individual deposits are a composite of syngenetic massive sulphide and underlying epigenetic stockwork-vein mineralization. The massive sulphide occurs as concordant tabular, lenticular, or saucer-shaped bodies in pillow lavas and pillow-lava breccia; massive lava flows, hyalcoclastite, tuff, and bedded radolarian chert are less commonly associated rock types. These massive sulphide zones are as much as 700 m long, 200 m wide, and 50 m thick. The pipe-, funnel-, or keel-shaped stockwork zone may extend to a dehpth of 1 km in the sheeted-dike complex. Several deposits in Cyprus are confined to grabens or the hanging wall of premineralization normal faults.Polymetallic massive sulphide deposits and active hydrothermal vents at medium- to fast-rate spreading centres (the East Pacific Rise at lat. 21°N, the Galapagos Spreading Centre at long. 86°W, the Juan de Fuca Ridge at lat. 45°N., and the Southern Trough of Guaymas Basin, Gulf of California) have interdeposit spacings on a scale of tens or hundreds of metres, and are spatially associated with structural ridges or grabens within the narrow (< 5 km) axial valleys of the rift zones. Although the most common substrate for massive sulphide accumulations is stacked sequences of pillow basalt and sheet flows, the sea-floor underlying numerous deposits in Guaymas Basin consists of diatomaceous ooze and terrigenous clastic sediment that is intruded by diabase sills. Mound-like massive sulphide deposits, as much as 30 m wide and 5m high, occur over actively discharging vents on the East Pacific Rise, and many of these deposits serve as the base for narrow chimneys and spires of equal or greater height. Sulphides on the Juan de Fuca Ridge appear to form more widespread blanket deposits in the shallow axial-valley depression. The largest deposit found to date, along the axial ridge of the Galapagos Spreading Centre, has a tabular form and a length of 1000 m, a width of 200 m, and a height of 30 m.The sulphide assemblage in both massive and vein mineralization in Cyprus type deposits is characteristically simple: abundant pyrite or, less commonly, pyrrhotite accompanied by minor marcasite, chalcopyrite, and sphalerite. With few exceptions, the composition of massive sulphide ranges from 0.3 to 5 wt. % Cu, from 0.1 to 3 wt. % Zn, from 0.5 to 30 ppm Au, and from 1 to 50 ppm Ag. The only common gangue minerals — quartz, chlorite, calcite, and gypsum generally make up less than 10 percent of the massive zone.Sulphide assemblages in massive sulphide samples recovered from the Juan de Fuca Ridge (abundant sphalerite, wurtzite, and pyrite; minor marcasite, chalcopyrite, and galena), East Pacific Rise (abundant sphalerite, pyrite, and chalcopyrite; minor wurtzite, marcasite, and pyrrhotite), and Guaymas Basin (abundant pyrrhotite and sphalerite; minor chalcopyrite) contrast with ophiolitic deposits. Bulk analyses of two zinc-rich sulphide samples from the Juan de Fuca Ridge yield the following average values: Zn, 56.6 wt. %; Cu, 0.2 wt. %; Pb, 0.15 wt. %; Fe, 4.9 wt. %; Ag, 260 ppm; and Cd, 775 ppm. Other minerals precipitated with sulphides at hydrothermal-vent sites include anhydrite, barite, gypsum, Mg-hydroxysulphate-hydrate, talc, sulphur, and amorphous silica.Massive sulphide lenses in some Cyprus-type deposits are underlain by a silica-rich zone consisting of massive quartz, opaline silica, red jasper, or chert mixed with disseminated and veinlet Fe-Cu-Zn sulphides. Some deposits are overlain by ochre, a gossanous Mn-poor Fe-rich bedded deposit composed of goethite, maghemite, quartz, and finely disseminated sulphide. In the Solomon Islands, ochre is overlain by siliceous sinter containing anhydrite, barite, and sulphide; the sinter contains anomalous Ag, Au, Cu, Zn, and Hg, and grades upward into Fe-rich chert and manganiferous wad. Amorphous Fe-Mn deposits (umber) and Mn-bearing chert enriched in Ba, Co, Cu, Ni, Cr, Pb, and Zn are common features near the top of ophiolite sequences. Although their genetic relation to sulphide mineralization is uncertian, they probably formed during off-axis hydrothermal discharge.At modern, medium-rate spreading centres, thin blankets of unconsolidated hydrothermal sediment have been observed near hydrothermal sulphide deposits. Basalt fragments recovered with massive sulphide from the Juan de Fuca Ridge have surfaces coated with smectite, magnetite, hematite, opaline silica, and Fe---Mn-oxyhydroxides. Sediment mounds composed largely of nontronitic clay and hydrated Fe and Mn oxides, and more distal metalliferous (Fe, Mn, Cu, Ni, Pb, Zn) sediment on the flanks of ceanridges, are also products of off-axis hydrothermal processes.Pillow lavas, diabase dikes, and gabbro in ophiolite sequences, and deeper, layer 2 basalt and diabase recovered from oceanic ridges, are altered to greenschist-facies assemblages (albite + chlorite + actinolite ± sphene ± quartz ± pyrite) during high-temperature sub-sea-floor hydro-thermal metamorphism near the axis of spreading. Chemical changes in the wall-rock during this large-scale sea-water/rock interactive episode depend on the water/rock ratio and temperature but generally include gains in Mg, Na and H₂O and losses of Ca. Subsequent low temperature sea-water/rock interaction away from the axis of spreading results in fracture-controlled zeolitefacies alteration, characterized by smectite, caledonite, zeolite, calcite, prehnite, hematite, marcasite, and pyrite. This retrograde alteration involves increases in total Fe, K, and H₂O and decreases in Mg and Si in the wallrock; Ca may be lost or gained.Wall-rock alteration in Cyprus type stockwork zones is more striking, in that the basalt and diabase between veins of Fe---Cu-Zn sulphides, quartz, and chlorite have undergone partial to complete conversion to fine-grained aggregates of quartz + chlorite + illite + pyrite; kaolinite and palygorskite may be present in minor amounts. Calcium and Na are strongly depleted; K, Al, Ti, Mn, and Ni are leached to a lesser extent; and Fe, S, Cu, Zn, and Co are strongly enriched in the wall-rock underlying massive sulphide. Mafic rocks at depth in the volcanic pile may be enriched in K, Rb, and Li, and depleted in Cu, Co, and Zn. Lavas lateral to and overlying massive sulphide mineralization may have low concentrations of Cu and high concentrations of Zn and Co relative to background levels.Mutual consideration of hydrothermal sulphide deposits and associated wall-rock alteration in ophiolites and at modern oceanic spreading centres can provide useful criteria for the development of regional exploration models for ophiolitic terrains.     

A comparative study on the dissolution and solubility of hydroxylapatite and fluorapatite at 25 °C and 45 °C

Dissolution of the synthetic hydroxylapatite (HAP) and fluorapatite (FAP) in pure water was studied at 25 °C and 45 °C in a series of batch experiments. The XRD, FT-IR and SEM analyses indicated that the synthetic, microcrystalline HAP and FAP with apatite structure used in the experiments were found to have no obvious variation after dissolution except that the existence of OH groups in FT-IR spectra for FAP after 2880 h dissolution was observed. During the HAP dissolution (0–4320 h), the aqueous calcium and phosphate concentrations reached the maxima after 120 h and then decreased slowly with time. For the FAP dissolution in pure water, after a transient time of 1440 h (< 60 d), element concentrations and pH became constant suggesting attainment of a steady-state between the solution and solid. During early stages of the FAP dissolution reaction (< 72–120 h), mineral components were released in non-stoichiometric ratios with reacted solution ratios of dissolved Ca:P, Ca:F and P:F being lower than mineral stoichiometric ratios of Ca₅(PO₄)₃F, i.e., 1.67, 5.0 and 3.0, respectively. This indicated that F were preferentially released compared to Ca from the mineral structure. The mean K_sp values were calculated by using PHREEQC for HAP of 10^− 53.28 (10^− 53.02–10^− 53.51) and for FAP of 10^− 55.71 (10^− 55.18–10^− 56.13) at 25 °C, the free energies of formation ΔG_f^o[HAP] and ΔG_f^o[FAP] were calculated to be − 6282.82 kJ/mol and − 6415.87 kJ/mol, respectively.     

Reconstruction of the hydrothermal history of the Cu–Ag vein-type mineralisation at Dikulushi, Kundelungu foreland, Katanga, D.R. Congo

The Kundelungu foreland, north of the Lufilian arc in the Democratic Republic of Congo, contains a number of various vein-type and stratiform copper mineralisations. The geodynamic context and metallogenesis of these mineral occurrences remain enigmatic. Currently, the vein-type Cu–Ag ore deposit at Dikulushi is the most significant deposit in the region. Mineralisation at Dikulushi comprises two major styles: 1) a polysulphide assemblage (Zn–Pb–Fe–Cu–As) within brecciated rocks along an anticlinal closure; and 2) a vein-hosted Cu–Ag assemblage. Petrographic and fluid inclusion studies indicate that the early Zn–Pb–Fe–Cu–As assemblage formed from a high-salinity Ca–Na–Cl fluid of modest temperature (135–172 °C). The later, economically more significant vein-related Cu–Ag mineralisation formed from intermediate salinity, lower temperature (46–82 °C) Na–Cl fluids. Weathering of the sulphide minerals resulted in a supergene enrichment with the formation of secondary Cu-minerals.     

Reappraisal of paleomagnetic data from Gargano (South Italy)

The platform limestones of Apulia (Italy) outcropping in the Gargano peninsula have been restudied. Paleomagnetic research has been carried out on Upper Cretaceous, Lower Cretaceous and Jurassic rocks. Despite the low intensities of the NRM (10–100 μA/m), all samples (268) could be cleaned by stepwise A.F. and/or thermal demagnetization treatments. NRM directions could be determined accurately and reproducibly for 85% of the samples, using a ScT cryogenic magnetometer and double precision measuring procedures. NRM of the Jurassic limestone is carried by secondary haematite and the results are therefore rejected from further consideration. The Upper and Lower Cretaceous limestones have an NRM carried by magnetite. Minor bedding tilt corrections improve the grouping of the site-mean results. The Upper Cretaceous “Scaglia” limestone (Turonian-Senonian) reveals a characteristic mean direction of decl. = 327.7°, incl. = 38.2°, α₉₅ = 4.3° (21 sites), while the Lower Cretaceous “Maiolica” limestone (Neocomian-Aptian/Albian) reveals a characteristic mean direction of decl. = 303.1°, incl. = 35.1°, α₉₅ = 8.7° (8 sites). The Cretaceous results show a post-Aptian/Albian counterclockwise rotation of about 25°, which is expressed by the smeared distribution of the Late Cretaceous site-mean results and a post-Senonian (i.e. Tertiary) counterclockwise rotation of the same amount with respect to the pole. These results are in excellent agreement with contemporaneous paleomagnetic results from other peri-Adriatic regions. A Tertiary counterclockwise rotation of all the stable Adriatic block is strongly supported by the new results.     

Quaternary stratigraphy and geochemistry at the Owl Creek Gold Mine, Timmins, Ontario, Canada

The Owl Creek Gold Mine is located in Hoyle Township, approximately 18 km northeast of Timmins, Ontario, Canada. The open-pit mine exposes a sequence of altered and mineralized mafic tholeiitic volcanics bounded to the north and south by greywacke and argillite. Gold occurs in the free state in quartz veins, often with graphite, and as fine gold on surfaces of, and within fractures in, pyrite.The study was designed to determine the distribution and distance of transport of Au in overburden down-ice from subcropping Au mineralization. This required an understanding of the glacial history of the area.The Quaternary stratigraphy at Owl Creek was studied and sampled by means of 17 sonic and 15 reverse-circulation overburden drill holes near the open pit, and several overburden exposures in the open-pit walls. Nonmagnetic heavy-mineral concentrates (specific gravity >3.3) were made from the <2000 μm (−10 mesh) fraction of all overburden samples from the drill hole and section sampling. The heavy-mineral concentrates were analyzed for Au by neutron activation. A till pebble lithology study was done on the >2000 μm (−10 mesh) fraction of the sonic drill core.Our stratigraphic studies indicate that there were three major Wisconsinan (Weichselian) ice advances and one minor, late readvance in the Timmins area. The transport and deposition of sediments comprising the “Oldest”, “Older”, Matheson and Cochrane stratigraphic “packages” (oldest to youngest) are related to three ice advances and one readvance which moved towards 240° ± 10°, 150° ± 5°, 170° ± 5° and 130° ± 5°, respectively.Geochemically anomalous levels of Au in the overburden define two dispersal trains down-ice of the Owl Creek Gold Mine. One, in the “Older” lodgement till, is 400–500 m long. The other in Matheson ablation and waterlain tills, is approximately 700 m long.The till pebble lithology study showed that pebble counting can be used to approximate bedrock contacts, but may not necessarily identify the source rock type of the matrix.     

Compressional and shear wave velocities of igneous rocks and volcanic glasses to 900° C and 20 kbar

Simultaneous measurements of compressional and shear wave velocities, V_p and V_s, in acidic and basic igneous rocks and volcanic glasses, were made up to 900°C and at 10–20 kbar.The effects of pressure and temperature on V_p and V_s in glasses and glassy rocks change at about 600°C, presumably the glass transition temperature. These effects are directly related to the silica content in the samples. and for obsidian are negative at room temperature and 245°C, but are positive at 655°C. The velocity—pressure relations for obsidian display an obvious hysteresis phenomena. for basalt glass is slightly negative, but is positive for usual substances at room temperature, and for obsidian and glassy andesite are positive up to about 600°C but are negative above that temperature. However, for basalt glass as well as other crystalline rocks, and are negative at all temperatures. Glass once heated above the glass transition temperature T_g under pressure P₁ retains the memory of pressure P₁ after it is cooled down below T_g and while subjected to another pressure P₂. An abrupt shift of the velocities correlating to pressure P₂ occurs when the glass is again heated to T_g. V_p−T and V_s−T relations for obsidian, glassy andesite, and basalt glass clearly exhibit this pressure memory.     

Origin and evolution of the calcic and magnesian skarns hosting the El Valle-Boinás copper–gold deposit, Asturias (Spain)

The El Valle-Boinás copper–gold deposit is located in the southern part of the Rio Narcea Gold Belt 65 km west of Oviedo (NW Spain), within the Cantabrian Zone (Iberian Hercynian Massif). The deposit is related to the Boinás stock, which ranges from quartz-monzonite to monzogranite and intruded (303 Ma) the carbonated Láncara Formation (early Cambrian) and the siliciclastic Oville Formation (middle-late Cambrian).A copper–gold skarn was developed along the contact between the igneous rock and the carbonated sedimentary rocks. The skarn distribution and mineralogy reflects both structural and lithologic controls. Two types of skarn exists: a calcic skarn mainly developed in the upper calcic member of the Láncara Formation, and a magnesian skarn developed in the lower dolomitic and organic-rich member. The former mainly consists of garnet, pyroxene and wollastonite. Retrograde alteration consists of K-feldspar, epidote, quartz, calcite, magnetite, ferroactinolite, titanite, apatite, chlorite and sulfides. Magnesian skarn mainly consists of diopside with interbedded forsterite zones. Pyroxene skarn is mainly altered to tremolite, with minor phlogopite and serpentine. Olivine skarn is pervasively altered to serpentine and magnetite, and is commonly accompanied by high sulfide and gold concentrations. This altered skarn results in a very dark rock, referred to as “black skarn”, which has great importance in gold reserves. Sulfide mineralization mainly consists of chalcopyrite, bornite, arsenopyrite, pyrrhotite and pyrite, while wittichenite, sphalerite, digenite, bismuthinite, native bismuth and electrum occur as accessory minerals.After extensive erosion, reactivation of the northeast-trending fracture zone provided conduits for the subsequent emplacement of porphyritic dikes (285±4 Ma) and diabasic dikes (255±5 Ma). Alteration, characterized by sericitization, silicification, carbonatization and hypogene oxidation took place, as did sulfide mineralization (pyrite, arsenopyrite, sphalerite, chalcopyrite, galena, bournonite, and Fe–Pb–Sb sulfosalts). Veins with quartz, carbonate, adularia and sulfide minerals crosscut all previous lithologies. Jasper and jasperoid breccias developed at the upper parts of the deposits.The fluid inclusion and stable isotope studies suggest a predominantly magmatic prograde-skarn fluid characterized by high-salinity (26–28 wt.% KCl and 32–36 wt.% NaCl) and high temperature, above 580°C. This fluid evolved into two immiscible fluids: a CO₂- and/or CH₄-rich, high-salinity aqueous fluid. Temperatures for the first retrograde-stage are between 350 and 425°C. A second stage is related to a more diluted aqueous fluid (3–6.2 wt.% NaCl eq.) and temperatures from 280 to 325°C. The fluid inclusion study performed on quartz from low-temperature mineralization indicates a very low salinity (0.2–6.2 wt.% NaCl eq.), low-temperature aqueous fluid (from 150 to 250°C), and trapping pressure conditions less than 0.2 kbar. In addition, the stable isotope study suggests that an influx of metamorphic waters derived from the country rocks produced these lower temperature fluids. The last control for the Au mineralization is the Alpine tectonism, which developed fault breccias (cataclasites to, locally, protomylonites) and gold remobilization from previous mineralization.     

Regional soil-gas helium distribution of the Ely and Delta 1° × 2° quadrangles, Basin and Range Province

A reconnaissance soil-gas helium survey was made in the Ely, Nevada and Delta, Utah 1° × 2° quadrangles in the Basin and Range Province. Helium concentrations in 510 samples ranged from −147 to 441 ppb He with respect to ambient air. The median helium value for the study area was 36 ppb. Concentrations of more than 100 ppb He and less than −20 ppb He occur more commonly in the Ely quadrangle and are especially numerous in the western one-half of this quadrangle. Interpretation of the data reveals that the helium concentrations reflect the rock type, particularly the silicic volcanic occurrences, and the geological structure of the area created by crustal extension. The regional soil-gas helium distribution is important information to consider when interpreting anomalies from detailed surveys.     

Draa Sfar, Morocco: A Visean (331 Ma) pyrrhotite-rich, polymetallic volcanogenic massive sulphide deposit in a Hercynian sediment-dominant terrane

Draa Sfar is a Visean, stratabound, volcanogenic massive sulphide ore deposit hosted by a Hercynian carbonaceous, black shale-rich succession of the Jebilet terrane, Morocco. The ore deposit contains 10 Mt grading 5.3 wt.% Zn, 2 wt.% Pb, and 0.3 wt.% Cu within two main massive sulphides orebodies, Tazakourt (Zn-rich) and Sidi M'Barek (Zn–Cu rich). Pyrrhotite is by far the dominant sulphide (70 to 95% of total sulphides), sphalerite is fairly abundant, chalcopyrite and galena are accessory, pyrite, arsenopyrite and bismuth minerals are rare. Pyrrhotite is monoclinic and mineralogical criteria indicate that it is of primary origin and not formed during metamorphism. Its composition is very homogeneous, close to Fe₇S₈, and its absolute magnetic susceptibility is 2.10^− 3 SI/g. Ar–Ar dating of hydrothermal sericites from a coherent rhyolite flow or dome within the immediate deposit footwall indicates an age of 331.7 ± 7.9 Ma for the Draa Sfar deposit and rhyolite volcanism.The Draa Sfar deposit has undergone a low-grade regional metamorphic event that caused pervasive recrystallization, followed by a ductile–brittle deformation event that has locally imparted a mylonitic texture to the sulphides and, in part, is responsible for the elongated and sheet-like morphology of the sulphide orebodies. Lead isotope data fall into two compositional end-members. The least radiogenic end-member, (²⁰⁶Pb/²⁰⁴Pb = 18.28), is characteristic of the Tazakourt orebody, whereas the more radiogenic end-member (²⁰⁶Pb/²⁰⁴Pb  18.80) is associated with the Sidi M'Barek orebody, giving a mixing trend between the two end-members. Lead isotope compositions at Draa Sfar testify to a significant continental crust source for the base metals, but are different than those of the Hajar and South Iberian Pyrite Belt VMS deposits.The abundance of pyrrhotite versus pyrite in the orebodies is attributed to low fO₂ conditions and neither a high temperature nor a low aH₂S (below 10^− 3) is required. The highly anoxic conditions required to stabilize pyrrhotite over pyrite are consistent with formation of the deposit within a restricted, sediment-starved, anoxic basin characterized by the deposition of carbonaceous, pelagic sediments along the flank of a rhyolitic flow-dome complex that was buried by pelitic sediments. Deposition of sulphides likely occurred at and below the seafloor within anoxic and carbonaceous muds.Draa Sfar and other Moroccan volcanogenic massive sulphide deposits occur in an epicontinental volcanic domain within the outer zone of the Hercynian belt and formed within a sedimentary environment that has a high pelagic component. In spite of the diachronous emplacement between the IPB deposits (late Devonian to Visean) and Moroccan deposits (Dinantian), all were formed around 340 ± 10 Ma following a major phase of the Devonian compression.     

Linear volcanoes along the Pacific-Cocos plate boundary, 9°N to the Galapagos triple junction

A Seabeam-based reconnaissance of the 500 km of the East Pacific Rise crest between 7°N and 2°40′N shows that the axial ridge is segmented by four 4–13 km non-transform offsets into an en echelon string of distinctively different linear volcanoes. These axial volcanoes are oriented orthogonal to relative plate motion, except where their overlapping ends veer 15° toward each other and where small intra-volcano offsets of their crestal rift zones create abrupt kinks. Longitudinal gradients of the crestlines are less than 5 m/km, except where they plunge at rift-zones' overlapped ends and where they rise locally to small axial peaks. Transverse profiles vary from trapezoidal to triangular, with a steep shield-shaped cross-section being most common. Conventional sounding data indicate that this pattern continues to the 140 km-offset Siqueiros transform fault system at 8.2°N. Within this fault system is a short spreadingcenter volcano contained in a rift valley that links two strike-slip fault zones. Immediately to the north is the shallow 9.0°–8.3°N axial volcano, with unusual relief mapped by a deeply towed instrument package. At the southern end of the plate boundary, as the rise crest enters the region of the Pacific-Cocos-Nazca triple junction, the axial ridge narrows, deepens, and acquires a more irregular long profile. South of 2°30′N the rise crest has a 15 km-wide rift valley that contains multiple volcanic ridges with north-south strikes. Structural hypotheses suggested or supported by these morphologic observations include a point-source magma supply to the spreading center from mantle diapirs, the along-strike continuity of axial magma chambers on fast-spreading rises, even across small rift-zone offsets, and the importance of magma intrusion as well as eruption for building the axial ridge. Hypotheses inconsistent with the new data include magma supply and long-distance dispersal from a few widely spaced plumes, primary control of the topographic, volcanic, and tectonic characteristics of the rise crest by distance from transform faults, and localization of triple junctions over major mantle upwellings.     

Initial volatilization temperatures and average volatilization rates of coal — Their relationship to coal rank and other characteristics

Two thermal parameters, initial volatilization temperature (IVT) and average volatilization rate (AVR), have been determined by thermogravimetric analysis in argon for 38 coal samples ranging in rank from lignite to low-volatile bituminous. Both IVT and AVR are correlated with percent volatile matter and vitrinite reflectance.The IVT values increase gradually from about 250 to 445°C with increasing rank; however, a change in slope is observed in the region of high-volatile bituminous coals (from about 340°C to about 380°C) when IVT's are plotted against percent volatile matter or percent fixed carbon. The changes in slope near 340°C and near 380°C occur at “coalification jumps” recognized on the basis of changes in the optical and chemical character of the macerals. In general, AVR values decrease gradually with increasing rank for the lignite and sub-bituminous coals and for the medium- and low-volatile bituminous coals; however, a sharp increase in AVR occurs in high-volatile bituminous coals. The change in slope of the IVT curves and sharp increase in the AVR values for high-volatile bituminous coals reflect the development of new, higher vapor pressure organic compounds produced during this stage of the coalification process.A plot of AVR vs IVT reveals three regions which correspond to: (1) lignite and sub-bituminous coals; (2) high-volatile bituminous coals; and (3) medium- to low-volatile bituminous coals.