Emplacement of magmatic Cu-Au-Te(-Ni-PGE) sulfide blebs in alkaline mafic rocks of the Mordor Complex, Northern Territory, Australia

Magmatic Ni-Cu-PGE sulfide mineralisation is mostly confined to tholeiiticto komatiiticmafic-ultramafic intrusions, yet there have been an increasing number of occurrences recorded in alkaline-ultramafic, post-collisional magmatic systems, particularly in the lower and middle crust that generally display a characteristic Cu-Au-Te enrichment over more conventional Ni-Cu(-PGE)-rich mineralisation. The Mordor Alkaline Igneous Complex, Australia, is a mid-crustal, zoned alkaline complex comprised of a syenite body with an alkaline mafic-ultramafic subcomplex containing dunites, wehrlites and shonkinites. Sulfide mineralisation is present either in thin, PGE-enriched stratiform ‘reefs’ within layered ultramaficsin the centre of the subcomplexor thicker zones of Cu(-Au-PGE-Te)-rich sulfide hosted by phlogopite-rich shonkinites towards the intrusion margins. This latter stylecomprises blebs of pyrite, chalcopyrite and millerite with PGE tellurides formed from the cooling of a Cu-dominant sulfide liquid. Primary igneous calcite is present in intimate association with the sulfide. We note that the circular nature of the complex, with a dunite core and shonkinite rim with chalcophile element mineralisation, are all comparable to the pipe-like, intracratonic, alkaline-ultramafic Aldan Shield intrusions in Russia. As such, Mordor may have an intracratonic rather than post-collisional affinity. Nevertheless, sulfide mineralisation is typical of other alkaline-hosted occurrences, with a Cu-Au-Te-rich signature, low Ni-contents and textural association with calcite;supporting models of chalcophile metal and S fluxing alongside carbonatein alkaline systems derived from ow degrees of partial melting of hydrous and carbonated mantle sources. Mordor illustrates that alkaline igneous rocks are prospective for magmatic Cu-Au-PGE-Ni sulfide mineralisation, and the classic ‘marginal, base metal-and sulfide-rich’ and ‘stratiform PGE-rich and sulfide-poor’ mineralisation settings may both be found in such intrusions

Mineralogical constraints on the genesis of an alkalic-type epithermal Au-Te deposit: Tuvatu, Fiji

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.</p

Lithospheric hydrous pyroxenites control localisation and Ni endowment of magmatic sulfide deposits

Magmatic Ni–sulfide ore deposits are generally associated with basaltic to komatiitic igneous rocks that originate by partial melting of the mantle, which is usually modelled as a uniform four-phase peridotite. Existing models accept that the key metal contributors to mantle melts are olivine (Ni) and sulfide (Cu, platinum group elements (PGEs) and minor Ni). However, melting in the mantle commonly begins in volumetrically minor mantle assemblages such as hydrous pyroxenites that occur as veins in the peridotite mantle, which are rich in the hydrous minerals phlogopite, amphibole and apatite. The contribution of hydrous pyroxenites to the metal endowment of mantle melts may have been underestimated or overlooked in the past, partly because evidence of their input is partially erased as melting intensifies to involve peridotite. Here, we compile new results from experiments and natural rocks which demonstrate that the hydrous minerals such as phlogopite, amphiboles and apatite all have high partition coefficients for Ni (3–20) and may be important repositories for Ni in mantle sources of igneous rocks. This implies that hydrous minerals hosted in metasomatic mantle lithologies such as hydrous pyroxenites may be important contributors to some magmatic Ni–sulfide ore systems. Hydrous pyroxenites contain hydrous minerals in large modal abundances up to 30–40 vol% in addition to clinopyroxene and a few vol% of oxide phases, such as rutile and ilmenite. These mantle lithologies are commonly associated with cratonic and continental regions, where low-temperature, low-degree volatile-rich melts commonly modify lithospheric peridotite mantle, depositing variable hydrous pyroxenites. The lower melting temperatures of hydrous minerals in hydrous pyroxenite lithologies also means that the generation of magmatic ore deposits may not require a major thermal perturbation such as a plume, as the melting temperatures of hydrous pyroxenites lie around 300–350 °C lower than dry peridotites. Partial melts of hydrous pyroxenite are more voluminous at low temperatures than melts of peridotite would be. Furthermore, it is argued in the following that they would contain similar or even higher concentrations of Ni. Thus, predictive exploration models should consider domains of the lithospheric mantle where hydrous pyroxenites may be localised and concentrated, as they may have been episodically melted throughout the long-lived geological evolution of cratonic blocks, yielding Ni-rich melts that may be hosted in conduits of varying size and geometry at various crustal levels.</p

Lithospheric hydrous pyroxenites control localisation and Ni endowment of magmatic sulfide deposits

Magmatic Ni–sulfide ore deposits are generally associated with basaltic to komatiitic igneous rocks that originate by partial melting of the mantle, which is usually modelled as a uniform four-phase peridotite. Existing models accept that the key metal contributors to mantle melts are olivine (Ni) and sulfide (Cu, platinum group elements (PGEs) and minor Ni). However, melting in the mantle commonly begins in volumetrically minor mantle assemblages such as hydrous pyroxenites that occur as veins in the peridotite mantle, which are rich in the hydrous minerals phlogopite, amphibole and apatite. The contribution of hydrous pyroxenites to the metal endowment of mantle melts may have been underestimated or overlooked in the past, partly because evidence of their input is partially erased as melting intensifies to involve peridotite. Here, we compile new results from experiments and natural rocks which demonstrate that the hydrous minerals such as phlogopite, amphiboles and apatite all have high partition coefficients for Ni (3–20) and may be important repositories for Ni in mantle sources of igneous rocks. This implies that hydrous minerals hosted in metasomatic mantle lithologies such as hydrous pyroxenites may be important contributors to some magmatic Ni–sulfide ore systems. Hydrous pyroxenites contain hydrous minerals in large modal abundances up to 30–40 vol% in addition to clinopyroxene and a few vol% of oxide phases, such as rutile and ilmenite. These mantle lithologies are commonly associated with cratonic and continental regions, where low-temperature, low-degree volatile-rich melts commonly modify lithospheric peridotite mantle, depositing variable hydrous pyroxenites. The lower melting temperatures of hydrous minerals in hydrous pyroxenite lithologies also means that the generation of magmatic ore deposits may not require a major thermal perturbation such as a plume, as the melting temperatures of hydrous pyroxenites lie around 300–350 °C lower than dry peridotites. Partial melts of hydrous pyroxenite are more voluminous at low temperatures than melts of peridotite would be. Furthermore, it is argued in the following that they would contain similar or even higher concentrations of Ni. Thus, predictive exploration models should consider domains of the lithospheric mantle where hydrous pyroxenites may be localised and concentrated, as they may have been episodically melted throughout the long-lived geological evolution of cratonic blocks, yielding Ni-rich melts that may be hosted in conduits of varying size and geometry at various crustal levels.</p

A chancellorid-like metazoan from the early Cambrian Chengjiang Lagersttäte, China

Nidelric pugio gen. et sp. nov. from the Cambrian Series 2 Heilinpu Formation, Chengjiang Lagerstatte, Yunnan Province, China, is an ovoid, sac-like metazoan that bears single-element spines on its surface. N. pugio shows no trace of a gut, coelom, anterior differentiation, appendages, or internal organs that would suggest a bilateral body plan. Instead, the sac-like morphology invites comparison with the radially symmetrical chancelloriids. However, the single-element spines of N. pugio are atypical of the complex multi-element spine rosettes borne by most chancelloriids and N. pugio may signal the ancestral chancelloriid state, in which the spines had not yet fused. Alternatively, N. pugio may represent a group of radial metazoans that are discrete from chancelloriids. Whatever its precise phylogenetic position, N. pugio expands the known disparity of Cambrian scleritome-bearing animals, and provides a new model for reconstructing scleritomes from isolated microfossils

Magmatic cannibalisation of a Permo-Triassic Ni-Cu-PGE-(Au-Te) system during the breakup of Pangea – Implications for craton margin metal and volatile transfer in the lower crust

The Ivrea Zone of NW Italy records the polyphased evolution of a magmatic sulfide mineral system, which occurred at multiple stages over a >80 Ma time interval. Between 290 and 250 Ma, a series of hydrated and carbonated ultramafic alkaline pipes containing Ni-Cu-PGE-(Te-Au) mineralisation was emplaced in the lower continental crust. At 200 Ma, a subsequent mineralising event occurred in association with the emplacement of the La Balma-Monte Capio (LBMC) intrusion. Modelling of the LBMC parental magma shows derivation from up to 60% partial melting of an anhydrous depleted juvenile mantle outside of garnet stability. The inferred composition of the parental melt is consistent with magmatism associated with the Central Atlantic Magmatic Province (CAMP). However, its tellurium-enriched composition together with the S-C-O isotope signature of the associated magmatic sulfide mineralisation (δ S = +0.53 to +1.00‰; δ C = −10.41 to −4.07‰; δ O = +6.57 to +13.74‰) cannot be reconciled with the CAMP source. It is argued that the geochemical and isotopic signature of the LBMC intrusion reflects interaction and mixing of a primitive magma sourced from a juvenile source with localised domains enriched in carbonate and metal-rich sulfides located in the lower crust, consistent with the composition of the Permo-Triassic pipes. We propose that this interaction resulted in sulfide-supersaturation and enrichment in volatiles and metals of the LBMC magma. Upwards magma transport may have been facilitated by devolatisation of magmatic carbonate from the pipes and the generation of a CO supercritical fluid that acted as buoyancy aid for the dense sulfide liquid. Evidence of this magmatic interaction informs on the first-order processes that control enhanced metallogenic fertility along the margins of lithospheric blocks. The scenario depicted here is consistent with reactivation and enrichment of a Gondwana margin Ni-Cu-PGE-(Te-Au) mineral system during the breakup of Pangea. 34 13 18

A chancelloriid-like metazoan from the early Cambrian Chengjiang Lagerstätte, China

Nidelric pugio gen. et sp. nov. from the Cambrian Series 2 Heilinpu Formation, Chengjiang Lagerstätte, Yunnan Province, China, is an ovoid, sac-like metazoan that bears single-element spines on its surface. N. pugio shows no trace of a gut, coelom, anterior differentiation, appendages, or internal organs that would suggest a bilateral body plan. Instead, the sac-like morphology invites comparison with the radially symmetrical chancelloriids. However, the single-element spines of N. pugio are atypical of the complex multi-element spine rosettes borne by most chancelloriids and N. pugio may signal the ancestral chancelloriid state, in which the spines had not yet fused. Alternatively, N. pugio may represent a group of radial metazoans that are discrete from chancelloriids. Whatever its precise phylogenetic position, N. pugio expands the known disparity of Cambrian scleritome-bearing animals, and provides a new model for reconstructing scleritomes from isolated microfossils

Sulfide-silicate textures in magmatic Ni-Cu-PGE sulfide ore deposits: Disseminated and net-textured ores

A large proportion of ores in magmatic sulfide deposits consist of mixtures of cumulus silicate minerals, sulfide liquid and silicate melt, with characteristic textural relationships that provide essential clues to their origin. Within silicate-sulfide cumulates, there is a range of sulfide abundance in magmatic-textured silicate-sulfide ores between ores with up to about five modal percent sulfides, called “disseminated ores”, and “net-textured” (or “matrix”) ores containing about 30 to 70 modal percent sulfide forming continuous networks enclosing cumulus silicates. Disseminated ores in cumulates have a variety of textural types relating to the presence or absence of trapped interstitial silicate melt and (rarely) vapour bubbles. Spherical or oblate spherical globules with smooth menisci, as in the Black Swan disseminated ores, are associated with silicate-filled cavities interpreted as amygdales or segregation vesicles. More irregular globules lacking internal differentiation and having partially facetted margins are interpreted as entrainment of previously segregated, partially solidified sulfide. There is a textural continuum between various types of disseminated and net-textured ores, intermediate types commonly taking the form of “patchy net-textured ores” containing sulfide-rich and sulfide-poor domains at cm to dm scale. These textures are ascribed primarily to the process of sulfide percolation, itself triggered by the process of competitive wetting whereby the silicate melt preferentially wets silicate crystal surfaces. The process is self-reinforcing as sulfide migration causes sulfide networks to grow by coalescence, with a larger rise height and hence a greater gravitational driving force for percolation and silicate melt displacement. Many of the textural variants catalogued here, including poikilitic or leopard-textured ores, can be explained in these terms. Additional complexity is added by factors such as the presence of oikocrysts and segregation of sulfide liquid during strain-rate dependent thixotropic behaviour of partially consolidated cumulates. Integrated textural and geochemical studies are critical to full understanding of ore-forming systems