Abstract. 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.
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.
To study the characteristics and genetic constraints on alkalic-type epithermal Au mineralisation, here we use the example of the Tuvatu Au-Ag deposit in Fiji, with an emphasis on detailed, quantitative mineralogy. Tuvatu mineralisation is hosted in a weakly altered potassic monzonite in parallel sided-veins of K-feldspar, biotite, sericite, calcite, and quartz, with epidote-bearing propylitic or sericite-rich selvages. Petrographic study of core and automated SEM-based mineralogical mapping of thin sections have been utilised to update previous parageneses of the deposit. Automated SEM techniques enable identification of small amounts of obscure minerals that form minuscule grains, which would otherwise be very difficult to identify and measure. As a result, our data show that gold fineness is extremely high, with the mean and median Au content of native-Au and Au-Ag alloy being 96.7% and 100% respectively, yet precious-metal tellurides make up the majority of the Au deportment. Tellurides show evidence of multiple phases and zoning with depth. For the first time at Tuvatu, Pt- and Pd-tellurides have been identified. Tuvatu has a number of features in common with alkalic systems elsewhere, including quartz-poor, carbonate-rich veins and alteration, abundant and varied telluride minerals, high gold grades, and Pt-Pd occurrences. We suggest these characteristics are a result of relatively high temperature (250–300 °C) fluids and immiscible semi-metal melts fluxing into the shallow epithermal environment. High pH fluids lead to quartz-poor alteration, but mildly acidic conditions dominate in areas of high fluid flux, where the lower pH causes precipitation of tellurides with quartz. Boiling of the fluids produces zonation of tellurides with depth but leaves relatively subtle textural evidence compared to boiling in most epithermal systems, in common with other quartz poor, carbonate-rich alkalic epithermal deposits around the world.
Magmatic sulfide deposits are the most significant source of platinum-group elements (PGE) in the world. Key to understanding their genesis is determining the processes and timing of sulfide saturation, metal enrichment and crustal contamination. In this study, we have identified droplets of magmatic sulfide from the Platreef, South Africa, where droplets of sulfide have been trapped in the earliest crystallising phase, chromite. Due to their early entrapment at high temperatures, metal concentrations and ratios that they display are indicative of a very early-stage sulfide liquid in the system, as they will have cooled and fractionated within an essentially closed system, unlike interstitial blebs that crystallise in an open system as the magma cools. Analysis of these droplets in an opaque mineral like chromite by LA-ICP-MS is problematic as some of the fractionated inclusion is necessarily lost during cutting and polishing to initially identify the inclusion. This particularly affects the ability to representatively sample the most fractionated phases such as gold and platinum minerals. Here, using a novel technique whereby the inclusions are homogenized and quickly quenched, so that any cutting, polishing and subsequent LA-ICP-MS analysis samples a truly representative portion of the droplet. This has been used to show that early sulfide liquids in the Platreef were highly PGE-rich and had Pt/Pd ratios of close to unity that supports genetic models invoking sulfide saturation and metal enrichment prior to intrusion, with pre-enriched sulfides entrained within the Platreef magma.
Research Article| April 01, 2017 Magmatic Sulfide Ore Deposits Stephen J. Barnes; Stephen J. Barnes 1CSIRO Mineral Resources26 Dick Perry Avenue, Kensington WA 6151, AustraliaE-mail Steve.Barnes@csiro.auMargaux.Levaillant@csiro.au Search for other works by this author on: GSW Google Scholar David A. Holwell; David A. Holwell 2Department of Geology, University of LeicesterLeicester, LE1 7RH, United KingdomE-mail: dah29@le.ac.uk Search for other works by this author on: GSW Google Scholar Margaux Le Vaillant Margaux Le Vaillant 1CSIRO Mineral Resources26 Dick Perry Avenue, Kensington WA 6151, AustraliaE-mail Steve.Barnes@csiro.auMargaux.Levaillant@csiro.au Search for other works by this author on: GSW Google Scholar Elements (2017) 13 (2): 89–95. https://doi.org/10.2113/gselements.13.2.89 Article history first online: 13 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Stephen J. Barnes, David A. Holwell, Margaux Le Vaillant; Magmatic Sulfide Ore Deposits. Elements 2017;; 13 (2): 89–95. doi: https://doi.org/10.2113/gselements.13.2.89 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyElements Search Advanced Search Abstract Magmatic sulfide ore deposits are products of natural smelting: concentration of immiscible sulfide liquid ('matte'), enriched in chalcophile elements, derived from silicate magmas ('slags'). Sulfide ore deposits occupy a spectrum from accumulated pools of matte within small igneous intrusions or lava flows, mined primarily for Ni and Cu, to stratiform layers of weakly disseminated sulfides within large mafic–ultramafic intrusions, mined for platinum-group elements. One of the world's most valuable deposits, the Platreef in the Bushveld Complex (South Africa) has aspects of both of these end members. Natural matte compositions vary widely between and within deposits, and these compositions are controlled largely by the relative volumes of matte and slag that interact with one another. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Magmatic Ni-Cu-PGE sulfide assemblages are almost ubiquitously comprised of pyrrhotite-pentlandite-chalcopyrite(-pyrite). Sulfide alteration is common during syn- or post-magmatic fluid interaction, usually replacing sulfides with amphiboles or serpentine. However, some are altered to a low temperature (<200 °C) hydrothermal assemblage of pyrite-millerite-chalcopyrite (PMC). An example is the Ni-Cu-PGE mineralisation in the Grasvally-Norite-Pyroxenite-Anorthosite (GNPA) Member, northern Bushveld Complex, which displays a continuum of mineralogical styles formed through progressive alteration: Style 1 primary pyrrhotite-pentlandite-chalcopyrite; which is altered to Style 2 pyrrhotite-pyrite-pentlandite-chalcopyrite; Style 3 pyrite-pentlandite-chalcopyrite; Style 4 pyrite-pentlandite-millerite-chalcopyrite; and Style 5 pyrite-millerite-chalcopyrite-cubanite. Modelling using CHILLER confirms this mineralogical sequence is thermodynamically possible at ∼200 °C. Quantitative characterisation using automated Energy-Dispersive X-ray spectroscopy mapping alongside in situ laser ablation analyses determined mineral proportions, major and trace element concentrations and deportments in each style. The early loss of pyrrhotite removes over half of the bulk Fe and S during the initial stages of PMC alteration, increasing Cu, Ni and PGE tenors of the remaining sulfides significantly. As water–rock interaction progresses, pyrrhotite is replaced by pyrite and pentlandite by millerite, with concurrent losses in Fe, S and Ni. Copper is lost throughout the alteration, and is most pronounced in the more advanced stages. The fluids responsible were most likely acidic and oxidised, with metals mobilised as chloride complexes. Using Rh as an immobile normalising element, the overall mass loss in the most altered samples is calculated to be up to 90%, consistent with textural relationships that indicate 40–90% volume loss from Styles 2–5, with sulfides replaced by secondary silicates, including phlogopite, quartz, chlorite, pyroxenes and minor amphiboles. Magnetite is not a significant alteration product and thus Fe is mobilised, or incorporated into silicates. Most trace elements present in the magmatic sulfide (the IPGE, Rh and Bi) remain in the sulfide phases, and are effectively transferred to pyrite during PMC alteration, except Pd, which remains in pentlandite, and is liberated from the sulfide assemblage when pentlandite disappears. Selenium tenors increase slightly with alteration, demonstrating that alteration decreases S/Se ratios. The significant mobilisation of Ni, Cu and Pd during PMC alteration produces fluids enriched in these elements that may represent a metal source for a number of enigmatic hydrothermal Ni deposits such as Avebury, Enterprise and Talvivaara, whose metal sources remain speculative. The PMC alteration of the GNPA Member may be specifically a source for the nearby Waterberg hydrothermal Pt deposit. Furthermore, this study has implications not only for magmatic ore deposits, but also for the general implications of sulfide transformation and metal transfer in ore systems in general.
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.
AbstractAbstractThe Platreef of the northern limb of the Bushveld Complex is one of the world's most significant deposits of platinum-group elements (PGE) with associated Ni and Cu. The origin of the Platreef is controversial. Some workers suggest that it is a northern facies of the Merensky Reef or part of the Upper Critical Zone, while others have suggested that the Platreef formed by processes entirely contained within the northern limb, unrelated to mineralisation events elsewhere in the complex. The northern limb is separated from the rest of the complex by the Thabazimbi–Murchison Lineament (TML) and the effect that this structure had on the intrusion of Bushveld magmas is debated. The presence of chilled rocks and cross-cutting relationships between the Platreef and its hangingwall gabbronorites would seem to preclude the magma that formed the hangingwall also acting as a source of PGE to the Platreef. The base metal sulphides in the Platreef carry very high PGE tenors (comparable with the Merensky Reef) indicating that the PGE must have been concentrated from a large volume of magma, but the source of that magma has not been established. In order to solve this PGE mass balance paradox, McDonald and Holwell have suggested that the magmas that formed the (pre-Platreef) Lower Zone may have been the source of PGE. At present, other models do not involve any significant role for the Lower Zone magmas in forming the Platreef. The data presented in this pilot study of the Lower Zone intrusion at Zwartfontein test some of the predictions arising from the McDonald and Holwell model. They highlight some important first order differences between Lower Zone intrusions in the northern limb compared with the rest of the Bushveld Complex. The enrichment in Th and LREE that characterizes the Lower Zone rocks in the eastern and western Bushveld appears to be missing in the northern limb. The lithophile element signatures of the different types of Lower Zone are suggested to result from mafic magmas intruding north and south of the TML and being contaminated by these different types of crust. Most significantly, the study has also revealed strong depletion of chalcophile elements (Ni, Cu and PGE) in the Lower Zone intrusion at Zwartfontein. The results are consistent with the depleted products expected from the processing of pre-Platreef magma(s) by interactions with sulphides at a deeper level within the magmatic plumbing system. The results provide a positive first test of one of the predictions arising from the McDonald and Holwell Platreef model. The existence of a system capable of removing PGE and producing a large volume of depleted ultramafic cumulates, in close proximity to the most highly mineralised sector of the Platreef, is suggested to be highly significant.Keywords: BUSHVELD COMPLEXPLATREEFLOWER ZONEZWARTFONTEINSULPHIDESPGESTAGING CHAMBER
Abstract Magmatic arcs are terrestrial environments where lithospheric cycling and recycling of metals and volatiles is enhanced. However, the first-order mechanism permitting the episodic fluxing of these elements from the mantle through to the outer Earth’s spheres has been elusive. To address this knowledge gap, we focus on the textural and minero-chemical characteristics of metal-rich magmatic sulfides hosted in amphibole-olivine-pyroxene cumulates in the lowermost crust. We show that in cumulates that were subject to increasing temperature due to prolonged mafic magmatism, which only occurs episodically during the complex evolution of any magmatic arc, Cu-Au-rich sulfide can exist as liquid while Ni-Fe rich sulfide occurs as a solid phase. This scenario occurs within a ‘Goldilocks’ temperature zone at ~1100–1200 °C, typical of the base of the crust in arcs, which permits episodic fractionation and mobilisation of Cu-Au-rich sulfide liquid into permeable melt networks that may ascend through the lithosphere providing metals for porphyry and epithermal ore deposits.
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