ALBERT

Notes: Abstract Mineralisation at the Zarshuran, NW Iran, occurs on the flank of an inlier of Precambrian rocks hosted in black silty calcareous and carbonaceous shale with interbedded dolomite and limestone varying in thickness from 5 to 60 m and extending along strike for approximately 5–6 km. Two major, steeply dipping sets of faults with distinct trends occur in the Zarshuran: (1) northwest (310–325) and (2) southwest (255–265). The main arsenic mineralisation occurs at the intersection of these faults. The mineral assemblage includes micron to angstrom-size gold, orpiment, realgar, stibnite, getchellite, cinnabar, thallium minerals, barite, Au-As-bearing pyrite, base metal sulphides and sulphosalts. Hydrothermal alteration features are developed in black shale and limestone around the mineralisation Types of alteration include: (1) decalcification, (2) silicification, (3) argillisation, (4) dolomitisation, (5) oxidation and acid leaching and (6) supergene alteration. The early stage of mineralisation involved removal of carbonates from the host rocks, followed by quartz precipitation. The main stage includes massive silicification associated with argillic alteration. In the late stage veining became more dominant and the main arsenic ore was deposited along fault cross cuts and gouge. These characteristics are typical of Carlin-type sediment-hosted disseminated gold deposits. The early stage of mineralisation contains only two-phase aqueous fluid inclusions. The main stage has two groups of three-phase CO2-bearing inclusions with minor CH4 ± N2, associated with high temperature, two-phase aqueous inclusions. During the late stage, fluids exhibit a wide range in composition, salinity and temperature, and CH4 becomes the dominant carbonic fluid with minor CO2 associated with a variety of two-phase aqueous fluid inclusions. The characteristics of fluids at the Zarshuran imply the presence of at least two separate fluids during mineralisation. The intersections of coexisting carbonic and aqueous inclusion isochores, together with stratigraphic and mineral stability evidence, indicate that mineralisation occurred at 945 ± 445 bar and 243 ± 59 °C, implying a depth for mineralisation of at least 3.8 ± 1.8 km (assuming a lithostatic pressure gradient). Fluid density fluctuations and the inferred depth of formation suggest that the mineralisation occurred at the transition between overpressured and normally pressured regimes. Geochronologic studies utilising K/Ar and Ar/Ar techniques on hydrothermal argillic alteration (whole rock and separated clay size fractions) and on volcanic rocks, indicates that mineralisation at Zarshuran formed at 14.2 ± 0.4 Ma, and was contemporaneous with nearby Miocene volcanic activity, 13.7 ± 2.9 Ma. It is proposed that mineralisation was the result of the infiltration of hydrothermal fluids containing a magmatic gas component, and that it was localised in the Zarshuran Unit because of the redox boundary that it provided and/or because it lay between an overpressured region at depth and a zone of circulating, hydrostatically pressured fluids above.

Notes: [Auszug] Palmer Ridge, part of the Kings Trough feature, N.E. Atlantic, consists of metamorphic and igneous basement with a cover of Tertiary sediments uplifted and intruded by a mass of serpentinite. The basement was formed about 60 million years ago at the mid-ocean ridge crest, and the uplift of the ...

Notes: [Auszug] Sea-floor morphology in the vicinity of the Atlantis transform fault (Fig. 1) is typical of that near many large transform faults that offset the Mid-Atlantic Ridge. Two strips of anomalous crust - 15 km wide run on either side of the fault trace from the inside corners of the ridge-transform ...

Notes: A model is derived which allows a more general approach to the interpretation of bromine profiles in halite rocks than is possible with the models of Kuhn or Holser. Allowing for an open system, with influx and reflux, reduces the depth of brine necessary to generate a given salt deposit. Even very small and regular bromine gradients, which historically have been interpreted as the result of deposition from deep brine bodies, can be generated in an open system from brine no more than a few tens of metres deep. The bromine gradient produced from a given depth of brine is strongly dependent on the composition and amount of influx, but less so on the composition of brine already in the basin.A Zechstein profile is analysed which in a closed basin would require brine 2665 m deep. It can equally be modelled by influx of normal seawater which fully replenishes a basin no more than 140 m deep. A somewhat irregular profile from the Paradox Basin, instead of requiring a basin 100–400 m deep, can be modelled as being produced by deposition from brines only 10 m deep. Reflux in both cases can only be a few per cent of influx at a maximum.

Notes: [Auszug] The black smokers described from the crest of the East Pacific Rise at 21N 1-4 are very hot springs discharging water blackened by finely divided iron, zinc and copper sulphides. They occur very close to the spreading axis and are the discharge points of single pass thermosyphon hydrothermal ...

Notes: [Auszug] Studies of magnetic anomalies1,2 and earthquake fault plane solutions3'4 have shown that the mid-ocean ridges are lines along which plates of the lithosphere are moving apart. During this movement, upper mantle material rises as a hot diapiric intrusion from the mobile asthenosphere to fill the ...

Notes: [Auszug] THE study of hydrothermal vents on the ocean floor has been pursued vigorously for only 10 years, so it is not surprising that new problems constantly arise. Some of our admittedly shaky ideas about hydro-thermal venting have recently been challenged. Vents were not supposed to occur on ...

Notes: [Auszug] Instead of tubeworms, clams and mussels that are characteris-tic of vent sites on the East Pacific Rise, the fauna of Atlantic vents is dominated by shrimp. Several species coexist, but Rimicaris exoculata is by far the most abundant. As indicated by its specific name, R. exoculata lacks eyestalks ...

Notes: [Auszug] IN Nature of 12 March 1987 Wallace Broecker described how he set about desk-top publishing, from the Lamont-Doherty Geological Observatory (Eldigio Press - got it?), to try to avoid what he sees as the stifling and unhelpful miasma of mainstream publishers. Here is the book of the article, an ...

Notes: Abstract Gabbros have been dredged from Gettysburg Bank, 110 km west of Portugal on the Azores/Gibraltar fracture zone. Primary minerals in olivine, pyroxene and brown hornblende gabbros are partially replaced by metamorphic minerals. Igneous textures are inhomogeneously overprinted by a granular polyhedral deformation and a cataclastic deformation. Amphiboles show characteristics which indicate a transition from crystallisation in a magma chamber to formation of amphibole in solid gabbro under metasomatic conditions. Of the amphiboles analysed, chlorine was present in the green amphiboles but below 0.05% in the brown suggesting the penetration of sea water after the formation of the brown amphibole but during the formation of the green.