An abstract is not available for this content. As you have access to this content, full HTML content is provided on this page. A PDF of this content is also available in through the ‘Save PDF’ action button.
The addition of crustal sulphur to magma can trigger sulphide saturation, a process fundamental to the development of some Ni–Cu–PGE deposits. In the British Palaeogene Igneous Province, mafic and ultramafic magmas intrude a thick sedimentary sequence offering opportunities to elucidate mechanisms of magma–crust interaction in a setting with heterogeneous S isotope signatures. We present S-isotopic data from sills and dykes on the Isle of Skye. Sharp contrasts exist between variably light δ34S in Jurassic sedimentary sulphide (−35‰ to −10‰) and a local pristine magmatic δ34S signature of −2.3 ± 1.5‰. Flat-lying sills have restricted δ34S (−5‰ to 0‰) whereas steeply dipping dykes are more variable (−0‰ to −2‰). We suggest that the mechanism by which magma is intruded exerts a fundamental control on the degree of crustal contamination by volatile elements. Turbulent flow within narrow, steep magma conduits, discordant to sediments, and developed by brittle extension or dilation have maximum contamination potential. In contrast, sill-like conduits emplaced concordantly to sediments show little contamination by crustal S. The province is prospective for Ni–Cu–PGE mineralization analogous to the sill-hosted Noril’sk deposit, and Cu/Pd ratios of sills and dykes on Skye indicate that magmas had already reached S-saturation before reaching the present exposure level.
The ∼60 m thick Magilligan Sill is part of the British Palaeogene Igneous Province in the North Atlantic. The sill comprises layers of dolerite and olivine gabbro, and it intrudes a thick sequence of Mesozoic mudstones and marls, which are locally baked at the sill margins. Since 2014, the sill has been an exploration target for orthomagmatic Ni – Cu – platinum group element (PGE) sulfide mineralisation analogous to the Noril’sk-Talnakh intrusion in Russia. We present new petrological, geochemical, and S isotope data to assess the prospectivity of the sill and the underlying magmatic plumbing system. Most sulfides in the dolerite portions of the sill are <50 μm in size and comprise only pyrite with PGE abundances below the detection limit. In the olivine gabbros, >150 μm size pentlandite, chalcopyrite, and pyrrhotite grains contain <4 ppm total PGE, 1460 ppm Co, and 88 ppm Ag. Pyrite from the dolerites have δ 34 S ranging from −10.0‰ to +3.4‰ and olivine gabbro sulfides range from −2.5‰ to −1.1‰, suggesting widespread crustal contamination. The S/Se ratios of sulfides in the dolerites and olivine gabbros range from 3500 to 19 500 and from 1970 to 3710, respectively, indicating that the latter may have come from upstream in the magma plumbing system. The Magilligan Sill records multiple injections of mafic magma into an inflating sill package, each with distinct mechanisms towards S saturation. Whilst the sulfide minerals in the sill do not constitute significant mineralisation themselves, detailed in situ studies highlight a divergence in S saturation histories and suggest that a larger volume of olivine gabbro sulfides at depth may be prospective.
<p>Porphyry magmatic systems emplaced within carbonate host rocks constitute a major source of the world&#8217;s Cu, Mo, Pb, Zn and Au [1]. Mineralisation is generally either porphyry-style or endoskarn-style within, or porphyry-, exoskarn- or manto-style outside the porphyry intrusion(s) [1,2]. Genetic models for porphyry and skarn mineralisation are well established, however questions remain as to why endoskarn- rather than porphyry-style mineralisation predominates within certain systems and regions. This is the case in Japan, for example, where there are very few signs of porphyry mineralisation despite generally favourable geological conditions, but there are large endoskarn and exoskarn deposits [3]. Recent studies show that magmas can assimilate large volumes of crustal carbonates, potentially providing a significant amount of CO<sub>2</sub> to late and post-magmatic hydrothermal fluids [4]. High levels of CO<sub>2</sub> in magmatic-hydrothermal systems may favour endoskarn formation and affect metal fractionation and solubility of ore minerals [5]. In this contribution, we test the hypothesis that endoskarn alteration may eliminate porphyry-style Cu mineralisation and mobilise Cu into other parts of the pluton and surrounding carbonate wall-rocks (exoskarns). &#160;</p><p>To address this hypothesis, the Daye ore district in the Middle-Lower Yangtze River metallogenic belt was selected for study as it hosts porphyry-, exoskarn- and endoskarn-styles of mineralisation [6]. The porphyry and skarn deposits lie within Late Mesozoic intrusions or along their contacts with Late Triassic carbonates. From among the many porphyry-related systems, the Tonglushan Fe-Cu-(Au) endoskarn-bearing system was selected for detailed field-, light microscopy-, cathodoluminescence-, SEM- and QEMSCAN&#174;-based genetic studies. The current study is mainly based on a comparison of samples from a single core through altered granite, endoskarn and exoskarn. From preliminary data for the Tonglushan system, the granites distal to the endoskarn were affected by Na-Ca alteration (replacement of intermediate composition plagioclase with albite, calcite and chlorite, and hornblende with calcite and chlorite), potassic alteration (replacement of plagioclase with K-feldspar), and later quartz-calcite veining. The endoskarn, which shows relict minerals and textures from the granite, underwent: 1) sericitic alteration, 2) prograde endoskarn formation, 3) retrograde endoskarn formation, 4) potassic alteration and 5) late carbonate veining stage. The textural relationships of oxide minerals in exoskarn and endoskarn indicate that magnetite and hematite likely formed during Stage 3, whereas Cu-(Au) mineralisation in the exoskarn is considered to be genetically associated with the potassic alteration phase, with precipitation of sulphides caused by acid neutralisation within the carbonates.</p><p>References:</p><p>[1] Sillitoe R (2010) Econ Geol 105:3-41</p><p>[2] Meinert L D et al. (2005) Econ Geol 100:299-336</p><p>[3] Ishihara S (1980) Mining Geol 30:59-62</p><p>[4] Carter L B and Dasgupta R (2016) Geochem Geophys Geosyst 17:3893-3916</p><p>[5] Lowenstern J B (2001) Mineral Deposita 36:490-502</p><p>[6] Zhai Y S et al. (1996) Ore Geol Rev 11:229-248</p>
The relative depletion of high field strength elements (HFSE), such as Nb, Ta and Ti, on normalised trace-element plots is a geochemical proxy routinely used to fingerprint magmatic processes linked to Phanerozoic subduction. This proxy has increasingly been applied to ultramafic-mafic units in Archaean cratons, but as these assemblages have commonly been affected by high-grade metamorphism and hydrothermal alteration/metasomatism, the likelihood of element mobility is high relative to Phanerozoic examples. To assess the validity of HFSE anomalies as a reliable proxy for Archaean subduction, we here investigate their origin in ultramafic rocks from the Ben Strome Complex, which is a 7 km2 ultramafic-mafic complex in the Lewisian Gneiss Complex of NW Scotland. Recently interpreted as a deformed layered intrusion, the Ben Strome Complex has been subject to multiple phases of high-grade metamorphism, including separate granulite- and amphibolite-facies deformation events. Additional to bulk-rock geochemistry, we present detailed petrography, and major- and trace-element mineral chemistry for 35 ultramafic samples, of which 15 display negative HFSE anomalies. Our data indicate that the magnitude of HFSE anomalies in the Ben Strome Complex are correlated with light rare earth-element (LREE) enrichment likely generated during interaction with H2O and CO2-rich hydrothermal fluids associated with amphibolitisation, rather than primary magmatic (subduction-related) processes. Consequently, we consider bulk-rock HFSE anomalies alone to be an unreliable proxy for Archaean subduction in Archaean terranes that have experienced multiple phases of high-grade metamorphism, with a comprehensive assessment of element mobility and petrography a minimum requirement prior to assigning geodynamic interpretations to bulk-rock geochemical data.
Abstract The Platreef, northern limb of the Bushveld Complex, South Africa, forms one of the world’s largest resources of platinum group elements (PGEs), with additional Ni-Cu-Co mineralization. It is widely considered that the Platreef formed via the emplacement of a series of discrete magmatic units; however, the relationship between this magmatic stratigraphy and the distribution of Ni-Cu-Co-PGE mineralization remains poorly constrained. This study constitutes the first in-depth examination of the Platreef magmatic stratigraphy at Tweefontein 238 KR, located directly north of the Flatreef extension at Turfspruit. Petrology and whole-rock and mineral chemistry define three magmatic units: the Upper Platreef, Main zone finger, and Lower zone transition, each displaying distinct pyroxene Mg# contents (79.6, 71.2, and 88.6 respectively), mineral assemblages, and bulk geochemistries. Updip the sequence thins considerably from >600 to <350 m, and contamination signatures of elevated CaO and FeO increase. However, local contamination is seldom evident in the PGE-bearing Upper Platreef. The intrusion of the overlying Main zone is proposed to have eroded the Upper Platreef considerably in some locations, locally reducing the economic viability of this mineralized horizon. The presented stratigraphy indicates that at Tweefontein (1) the Lower and Critical zone magmas are not necessarily separate and evolve from Lower to Critical over a distinct transitional zone, (2) there is only one main Critical zone unit that is host to the PGE mineralization, and (3) the Main zone not only forms a magmatic uniformity at the top of the Critical zone but also intrudes the Critical zone.
Abstract Element mobility and chemical mass transfer are evaluated in the formation of Cu—Fe exoskarn deposits and endoskarn and minor porphyry‐style alteration in the Tonglushan quartz monzodiorite (QMD) system, eastern China. Endoskarn formation involved the migration of Ca into the QMD from the exoskarnification of carbonates (now marble) xenoliths and wall rocks, addition of Fe and Mn by magmatic‐hydrothermal fluids emanating from the interior of the QMD, and removal of alkali elements due to the replacement of feldspars and mica by prograde skarn minerals. Zirconium, Hf, U, and rare earth elements (REE) were added by hydrothermal fluids which were able to carry these often poorly mobile high field strength elements (HFSE) due to elevated F activity. Additions of Al were likely from Na‐rich fluids that also caused sodic alteration. Several factors favored mineralization within the exoskarns rather than endoskarns and QMD. The endoskarns were relatively oxidizing, as evidenced by a significant addition of Fe 3+ , which caused Cu to remain in magmatic‐hydrothermal fluids until they entered and precipitated sulphides in the more reducing environment of the exoskarns. Fluid migration from the QMD through the endoskarns and into the exoskarns was favored due to decarbonation of wall rock carbonates and related upwards migration of CO 2 to produce a self‐sustaining chimney effect, which drew further fluids towards the carbonates to form, alter and mineralize the exoskarns. The higher porosity and permeability of the endoskarns compared with the QMD further promoted the lateral flow of Cu‐bearing fluids towards the exoskarns and limited porphyry‐style alteration and mineralization within the QMD. This proposed mechanism is only likely to be relevant for porphyry‐type systems developed predominantly within carbonate host rocks. Its significance for exploration models is that relatively poorly mineralized porphyry stocks in this setting may be associated with more substantive exoskarn deposits on their margins.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
