Magmatic Cu-Ni-PGE-Au sulfide mineralisation in alkaline igneous systems: An example from the Sron Garbh intrusion, Tyndrum, Scotland

Magmatic sulfide deposits typically occur in ultramafic-mafic systems, however, mineralisation can occur in more intermediate and alkaline magmas. Sron Garbh is an appinite-diorite intrusion emplaced into Dalradian metasediments in the Tyndrum area of Scotland that hosts magmatic Cu-Ni-PGE-Au sulfide mineralisation in the appinitic portion. It is thus an example of magmatic sulfide mineralisation hosted by alkaline rocks, and is the most significantly mineralised appinitic intrusion known in the British Isles. The intrusion is irregularly shaped, with an appinite rim, comprising amphibole cumulates classed as vogesites. The central portion of the intrusion is comprised of unmineralised, but pyrite-bearing, diorites. Both appinites and diorites have similar trace element geochemistry that suggests the diorite is a more fractionated differentiate of the appinite from a common source that can be classed with the high Ba-Sr intrusions of the Scottish Caledonides. Mineralisation is present as a disseminated, primary chalcopyrite-pyrite-PGM assemblage and a blebby, pyrite-chalcopyrite assemblage with significant Co-As-rich pyrite. Both assemblages contain minor millerite and Ni-Co-As-sulfides. The mineralisation is Cu-, PPGE-, and Au-rich and IPGE-poor and the platinum group mineral assemblage is overwhelmingly dominated by Pd minerals; however, the bulk rock Pt/Pd ratio is around 0.8. Laser ablation analysis of the sulfides reveals that pyrite and the Ni-Co-sulfides are the primary host for Pt, which is present in solid solution in concentrations of up to 22 ppm in pyrite. Good correlations between all base and precious metals indicate very little hydrothermal remobilisation of metals despite some evidence of secondary pyrite and PGM. Sulfur isotope data indicate some crustal S in the magmatic sulfide assemblages. The source of this is unlikely to have been the local quartzites, but S-rich Dalradian sediments present at depth. The generation of magmatic Cu-Ni-PGE-Au mineralisation at Sron Garbh can be attributed to post-collisional slab drop off that allowed hydrous, low-degree partial melting to take place that produced a Cu-PPGE-Au-enriched melt, which ascended through the crust, assimilating crustal S from the Dalradian sediments. The presence of a number of PGE-enriched sulfide occurrences in appinitic intrusions across the Scottish Caledonides indicates that the region contains certain features that make it more prospective than other alkaline provinces worldwide, which may be linked the post-Caledonian slab drop off event. We propose that the incongruent melting of pre-existing magmatic sulfides or ‘refertilised’ mantle in low-degree partial melts can produce characteristically fractionated, Cu-PPGE-Au-semi metal bearing, hydrous, alkali melts, which, if they undergo sulfide saturation, have the potential to produce alkaline-hosted magmatic sulfide deposits

Extension parallel to the rift zone during segmented fault growth: application to the evolution of the NE Atlantic

The mechanical interaction of propagating normal faults is known to influence the linkage geometry of first-order faults, and the development of second-order faults and fractures, which transfer displacement within relay zones. Here we use natural examples of growth faults from two active volcanic rift zones (Koa`e, island of Hawai`i, and Krafla, northern Iceland) to illustrate the importance of horizontal-plane extension (heave) gradients, and associated vertical axis rotations, in evolving continental rift systems. Second-order extension and extensional-shear faults within the relay zones variably resolve components of regional extension, and components of extension and/or shortening parallel to the rift zone, to accommodate the inherently three-dimensional (3-D) strains associated with relay zone development and rotation. Such a configuration involves volume increase, which is accommodated at the surface by open fractures; in the subsurface this may be accommodated by veins or dikes oriented obliquely and normal to the rift axis. To consider the scalability of the effects of relay zone rotations, we compare the geometry and kinematics of fault and fracture sets in the Koa`e and Krafla rift zones with data from exhumed contemporaneous fault and dike systems developed within a > 5×104 km2 relay system that developed during formation of the NE Atlantic margins. Based on the findings presented here we propose a new conceptual model for the evolution of segmented continental rift basins on the NE Atlantic margins

Sulfur isotope variations within the platreef Ni-Cu-PGE deposit: Genetic implications for the origin of sulfide mineralization

The Platreef, the world’s third largest platinum group element (PGE) deposit, is a 10–400-m thick pyroxenitic unit at the base of the northern limb of the mafic Bushveld Complex, hosting PGE mineralization in association with base metal sulfides. The sulfide mineralization is thought to have been either largely magmatic in origin, with contamination by assimilation of local floor rocks considered an ore-modifying process, or a product of assimilation of country-rock S and silicic contamination which caused S saturation in the magma, with the contamination being fundamental to the ore-forming process. We have performed an extensive and detailed sulfur isotope study of the Platreef, which indicates that magmatic signatures (δ34S = 0 to +2‰) are preserved in early-formed sulfide droplets within the Platreef pyroxenites in the area from Sandsloot to Witrivier. These values are comparable to sulfide inclusions in diamonds in the nearby Klipspringer kimberlite and are considered to have a primary mantle origin. There is no indication of any significant external S in any of the primary sulfides in the Sandsloot-Witrivier area, however, later sulfides found in calc-silicate floor rocks at Sandsloot and Zwartfontein and in xenoliths of calc-silicate throughout the section indicate an input of country rock S. Anhydrite-bearing horizons in the Malmani dolomites may have exchanged S with magmatic Platreef sulfide during Platreef intrusion, imparting a heavier S isotope signature in the fluid-affected rocks than those in the early sulfides. The Archean basement, although containing minor amounts of sulfide with a negative δ34S value, is not a significant contributor to the S budget of the Platreef. Previous studies have indicated that in areas where the sedimentary floor rocks contain appreciable sulfides, rather than sulfates, such as at Turfspruit, the Platreef sulfides are extensively contaminated with country-rock S. Assimilation of pyrite-bearing shales in the Turfspruit area has locally upgraded the S content of the Platreef and given the basal Platreef sulfides a heavier isotopic signature. Sulfur saturation in the Platreef magma took place before contamination, probably in a staging chamber prior to intrusion. A major pulse of magma entrained the preformed PGE-rich sulfides and was injected to form the Platreef, where assimilation of country-rock sulfides upgraded the S content on a strictly local scale, and hydrothermal leaching introduced S from country-rock sulfates into later stage sulfides, again on a local scale. South of Zwartfontein, it is proposed that the Platreef was intruded into sediments of the Transvaal Supergroup, which formed the floor and the roof of the Platreef. North of Zwartfontein, the Platreef intruded the boundary between the Malmani Supergroup sediments and the Archean basement. This could have potentially mineralized the roof calc-silicates north of Zwartfontein in a manner similar to the floor in the Sandsloot area. The later intrusion of the Main zone magma formed a magmatic unconformity on top of the cold Platreef and entrained large rafts of mineralized calc-silicate for tens of kilometers north of the last footwall outcrop of calc-silicate

The nature and genesis of marginal Cu-PGE-Au sulphide mineralisation in Paleogene Macrodykes of the Kangerlussuaq region, East Greenland

The Kangerlussuaq region of East Greenland hosts a variety of early Tertiary extrusive and intrusive igneous rocks related to continental break up and the passage of the ancestral Iceland plume. These intrusive bodies include a number of gabbroic macrodykes, two of which—the Miki Fjord Macrodyke, and the newly discovered Togeda Macrodyke—contain Cu–PGE–Au sulphide mineralisation along their margins. Sulphides occur as disseminated interstitial blebs and rounded globules of chalcopyrite and pyrrhotite with some Fe–Ti oxides and platinum-group minerals, comprising largely Pd bismuthides and tellurides. The globules are interpreted to have formed from fractionation of trapped droplets of an immiscible Cu- and Pd-rich sulphide melt and show geopetal indicators. Sulphur isotopes imply a local crustal source of S in these from pyritic sediments of the Kangerlussuaq Basin. Thus, generation of these sulphide occurrences was controlled by local country rock type. Low Ni/Cu and Pt/Pd ratios, also present in the Platinova reefs in the Skaergaard Intrusion, indicate that early fractionation of olivine may have depleted the magma of Ni and suggest the likely presence of a large magma chamber at depth. Xenoliths of Ni-rich olivine cumulates in the Miki Fjord Macrodyke may have been sourced from such a body. The location of thus far unidentified conduit or feeder zones to the macrodykes beneath the present day surface may represent potential targets for more massive sulphide orebodies

Announced inspection of Park Manor Nursing Home on 8 April 1999

SIGLEAvailable from British Library Document Supply Centre-DSC:f99/2469 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

A Re-Os date for Molybdenite-bearing quartz vein mineralisation within the Kangerlussuaq Alkaline Complex, east Greenland: implications for the timing of regional metallogenesis

The Kangerlussuaq Alkaline Complex, East Greenland, is one of the largest alkaline complexes in the world. It is known to host a number of occurrences of base and precious metal mineralization, including the Flammefjeld porphyry Mo deposits and a suite of epithermal style base and precious metal veins in an area around the Søndre Syenitgletscher. Here we describe and date a previously unknown occurrence of molybdenite-pyrite-scheelite–bearing quartz veins associated with alkali basalt dikes that intrude the main syenites of the complex. The veins appear to be cogenetic with the dikes, which crosscut and brecciate granodiorites of the Cirque 1320 complex. Sulfur isotope signatures of molybdenite and pyrite in the veins give a tight range with mean δ34S of 2.2 ± 0.7‰, consistent with a magmatic S source related to the intrusion of the dikes. Molybdenite from the veins gave an Re-Os age of 52.74 ± 0.26 Ma, some 13 m.y. older than molybdenite at the Flammefjeld porphyry deposit, thus distinguishing this metallogenic episode as a distinct event, temporally unrelated to the known Mo mineralization at Flammefjeld. Significantly, in terms of the timing of regional metallogenesis and magmatism, our date suggests that the age of the Kangerlussuaq Alkaline Complex itself may be slightly older than the 50 Ma currently accepted, based on 40Ar-39Ar dating from biotites in the main syenites. Given the common discrepancy in ore systems of 40Ar-39Ar, which produce slightly younger dates than those by Re-Os and U-Pb, we suggest the actual age of the Kangerlussuaq Alkaline Complex may be closer to 53 to 52 Ma

The Genesis of Precious Metal Sulfide Deposits in the Skaergasrd and Other Related Intrusions, Kangerlussuaq Area, East Greenland

No abstract available