Subseafloor sulphide deposit formed by pumice replacement mineralisation

Seafloor massive sulphide (SMS) deposits, modern analogues of volcanogenic massive sulphide (VMS) deposits on land, represent future resources of base and precious metals. Studies of VMS deposits have proposed two emplacement mechanisms for SMS deposits: exhalative deposition on the seafloor and mineral and void space replacement beneath the seafloor. The details of the latter mechanism are poorly characterised in detail, despite its potentially significant role in global metal cycling throughout Earth’s history, because in-situ studies require costly drilling campaigns to sample SMS deposits. Here, we interpret petrographic, geochemical and geophysical data from drill holes in a modern SMS deposit and demonstrate that it formed via subseafloor replacement of pumice. Samples from the sulphide body and overlying sediment at the Hakurei Site, Izena Hole, middle Okinawa Trough indicate that sulphides initially formed as aggregates of framboidal pyrite and matured into colloform and euhedral pyrite, which were replaced by chalcopyrite, sphalerite and galena. The initial framboidal pyrite is closely associated with altered material derived from pumice, and alternating layers of pumiceous and hemipelagic sediments functioned as a factory of sulphide mineralisation. We infer that anhydrite-rich layers within the hemipelagic sediment forced hydrothermal fluids to flow laterally, controlling precipitation of a sulphide body extending hundreds of meters.

Gold and gold-bearing volcanogenic massive sulphide deposits of the Central Cuba

Background. Volcanogenic massive sulphide deposits (VMS) are the most important sources of Cu and Zn; they account for a large share of the world production of Pb, Ag, Au, Se, Te, Bi and Sb, as well as small amounts of many other metals. The polymetallic VMS deposits of economic value of varying degrees are known in the rocks of the Los Pasos Cretaceous Formation, Cuba.Aim. To show the potential of the Cretaceous volcanic deposits of Central Cuba for gold, silver, copper, zinc and lead deposit prospecting.Materials and methods. The study characterises the San Fernando, Independencia, Antonio, Los Cerros VMS deposits and the Boca del Toro and El Sol ore occurrences located in the Los Pasos Formation. The similarities and differences in the mineral and elemental composition and structures of the ores of these objects are described, which underlie the assessment of their economic importance.Results. The latitudinal zoning of VMS and noble metal mineralisation of the Central Cuban ore region is outlined. In the west, copper-VMS deposits with accompanying gold ore objects prevail. In the east, copper-zinc VMS deposits with barite and gold-silver objects are widespread.Conclusions. It is necessary to assume the different erosional sections corresponding to the blocks of the Cretaceous volcanic arc of Central Cuba, which is larger in the west and smaller in the east. Proceeding from the presence of veinlet gold ores, their confinement to tectonic zones and the lack of correlation between noble and chalcophile metals at the San Fernando deposit, as well as significantly different gold-silver ratios in the considered ore objects, it could be assumed that some of the gold-silver ores were formed after VMS. The obtained Au/Ag ratios are close to the ores of the high sulphidation type (high sulphide ores) from similar ore regions of Venezuela and the Kur-il island arc. In this regard, one can expect hidden gold deposits in the west and gold-silver deposits in the east of the studied area.

Geology of the recently discovered massive and stockwork sulphide mineralization at Semblana, Rosa Magra and Monte Branco, Neves–Corvo mine region, Iberian Pyrite Belt, Portugal

The recently discovered massive and stockwork sulphide mineralization of Semblana-Rosa Magra and Monte Branco, situated ESE of the Neves–Corvo volcanogenic massive sulphide (VMS) deposit in the Iberian Pyrite Belt (IPB) is presented. Geological setting and tectonic model is discussed based on proxies such as palynostratigraphy and U–Pb zircon geochronology. The mineralization is found within the IPB Volcano-Sedimentary Complex (VSC) Lower sequence, which includes felsic volcanic rocks (rhyolites) with U–Pb ages in zircons of 359.6 ± 1.6 Ma, and black shales of the Neves Formation of late Strunian age. Massive sulphides are enveloped by these shales, implying that felsic volcanism, mineralization and shale sedimentation are essentially coeval. This circumstance is considered highly prospective, as it represents an important exploration vector to target VMS mineralization across the IPB, in areas where the Lower VSC sequence is present. The Upper VSC sequence, with siliciclastic and volcanogenic sedimentary rocks of middle–late Visean age, shows no massive mineralization but a late Tournaisian (350.9 ± 2.3 Ma) volcanism with disseminated sulphides was also identified. Nevertheless, stratigraphic palynological gaps were found within the Strunian and in the Tournaisian sediments, between the Lower and Upper VSC sequences, reflecting probable erosion and uplift mechanisms linked with extensional tectonics. The Semblana and Monte Branco deposits and the Rosa Magra stockwork are enclosed by tectonic sheets that dismembered the VSC sequence in a fold-and-thrust tectonic complex, characteristic of the NE Neves–Corvo region. The methodologies used allow a geological comparison between Neves–Corvo and other IPB mine regions such as Lousal–Caveira, Herrerias, Tharsis and Aznalcollar.

Whole-rock lithogeochemistry, Nd-Hf isotopes, and in situ zircon geochemistry of VMS-related felsic rocks, Finlayson Lake VMS district, Yukon

The Finlayson Lake district in southeastern Yukon is composed of a Late Paleozoic arc-backarc system that consists of metamorphosed volcanic, plutonic, and sedimentary rocks of the Yukon-Tanana and Slide Mountain terranes. These rocks host >40 Mt of polymetallic resources in numerous occurrences and styles of volcanogenic massive sulphide (VMS) mineralization. Geochemical and isotopic data from these rocks support previous interpretations that volcanism and plutonism occurred in arc-marginal arc (e.g., Fire Lake formation) and continental back-arc basin environments (e.g., Kudz Ze Kayah formation, Wind Lake formation, and Wolverine Lake group) where felsic magmatism formed from varying mixtures of crust- and mantle-derived material. The rocks have elevated high field strength element (HFSE) and rare earth element (REE) concentrations, and evolved to chondritic isotopic signatures, in VMS-proximal stratigraphy relative to VMS-barren assemblages. These geochemical features reflect the petrogenetic conditions that generated felsic rocks and likely played a role in the localization of VMS mineralization in the district. Preliminary in situ zircon chemistry supports these arguments with Th/U and Hf isotopic fingerprinting, where it is interpreted that the VMS-bearing lithofacies formed via crustal melting and mixing with increased juvenile, mafic magmatism; rocks that were less prospective have predominantly crustal signatures. These observations are consistent with the formation of VMS-related felsic rocks by basaltic underplating, crustal melting, and basalt-crustal melt mixing within an extensional setting. This work offers a unique perspective on magmatic petrogenesis that underscores the importance of integrating whole-rock with mineral-scale geochemistry in the characterization of VMS-related stratigraphy.

Tectonics of Volcanogenic Massive Sulphide (VMS) Deposits at Flores Back Arc Basin: A Review

The bathymetry, petrology, marine magnetic, and seismic-SBP data have identified the northwest-southeast direction submarine ridge that shows hydrothermal activity. This activity occurred through Mount Baruna Komba, Abang Komba, and Ibu Komba. The volcanic rocks are andesite basaltic lava flows, tuff, and pumice. The andesite basaltic lava shows porphyritic, intergranular, intersertal to glomeroporphyritic textures. The rock composes anhedral minerals of k-feldspar, plagioclase, and pyroxene. These minerals present in small-sized, short prismatic dispersed in very fine groundmass minerals or glasses. Most of the volcanic rocks have experienced various degrees of alteration. The k-feldspar and plagioclase are most dominantly transformed into sericite, clay mineral, carbonate, epidote and oxide mineral, opaque mineral, and secondary plagioclase through the albitization process, while pyroxene replaced by chlorite. Other minerals are biotite and quartz, and base metals are present Cu, Zn, Ag, As, Pb, and gold. Mineralization categorizes as the phyllic zone, sub-prophylithic zone, and phyllic-potassic zone that formed at a temperature range of 250-400oC. The submarine hydrothermal alteration in the Komba Ridge is associated with a volcanogenic sulphide deposit controlled by crust thinning due to the crust rifts in the back-arc tectonic setting.

Geology, geochronology, and fluid inclusion studies of the Xiaorequanzi volcanogenic massive sulphide Cu–Zn deposit in the East Tianshan Terrane, China

The Xiaorequanzi Cu–Zn deposit is in the westernmost part of East Tianshan Terrane in northwestern China. The deposit is unique in the region being a volcanogenic massive sulphide (VMS) deposit located near a zone (or belt) containing giant late Paleozoic porphyry Cu deposits. Aiming to better understand the genesis of the mineral deposits in the terrane and their tectonic setting, we report our findings of detailed studies on fluid inclusion microthermometry, Re–Os dating of chalcopyrite from the massive ore, and U–Pb dating of zircons from the host volcanic rocks. There are two sulphide stages with early pyrite succeeded by chalcopyrite–sphalerite, which are hydrothermally overprinted and supergene enriched. The hydrothermal overprinting is characterised by quartz–sulphide veins crossed by carbonate-rich quartz veins. Quartz from the chalcopyrite–sphalerite stage is characterised by primary fluid inclusions containing H2O–NaCl(–CO2) and homogenise at 228–392 °C with a salinity of 2.2–13.3 wt.% NaCl equiv. Secondary fluid inclusions related to the hydrothermal overprinting homogenise at 170–205 °C with a salinity of 2.7–12.1 wt.% NaCl equiv. Fluid inclusions in the quartz–sulphide stage of the hydrothermal overprinting contain H2O–NaCl with homogenisation temperatures of 164–281 °C and salinities in ranging from 2.9 to 12.4 wt.% NaCl equiv. Fluid inclusion in the quartz–calcite stage contain H2O–NaCl with homogenisation temperatures of 122–204 °C with salinities of 1.4–12.4 wt.% NaCl equiv. These characteristics are like those of the secondary fluid inclusions in the VMS mineralisation. Combining these findings with H–O isotopic data from previous studies, we propose that the primary mineralising fluid is magmatic in origin. Tuff hosting the mineralisation yields a SHRIMP U–Pb zircon age of 352 ± 5 Ma, which is interpreted as the age of the tuff, and a porphyritic felsite dyke intruding the tuff yields a SHRIMP U–Pb zircon date of 345 ± 6 Ma, interpreted as the emplacement age of the dyke. Chalcopyrite from the main orebody at Xiaorequanzi yields a Re–Os isochron age of 336 ± 13 Ma with an initial 187Os/188Os ratio of 0.25 ± 0.55 (MSWD = 12). Given that the VMS deposit is a syngenetic deposit, we regard the upper ca. 349 Ma limit of the Re–Os date as the approximate age of the chalcopyrite. The three dates are the same within error, and the upper limit of the Re–Os date of ca. 349 is taken as the age of the volcanic, dyke, and mineralisation. The volcanic rocks around the Xiaorequanzi deposit have been previously classified as calc–alkaline to high-K calc–alkaline enriched in large-ion lithophile elements and depleted in high-field-strength elements, which are characteristics indicative of a forearc setting. It is suggested that VMS mineralisation formed in a forearc setting related to the north-directed subduction of the Palaeo-Kangguer or North Tianshan oceanic plates.

Hylite: a hyperspectral toolbox for open pit mapping

<p>Hyperspectral imaging is a powerful tool for mapping mineralogy and lithology in core and outcrops, as many minerals show distinct spectral features in the commonly analysed visible, near, short-wave and long-wave infrared regions of the electromagnetic spectrum. High resolution ground and UAS (unmanned aerial system)-based sensors thus have significant potential as a tool for rapid and non-invasive geological mapping in mining operations, exploration campaigns and scientific research. However, the geometrical complexity of many outcrops (e.g. cliffs, open-pit mines) can result in significant technical challenges when acquiring and processing hyperspectral data. In this contribution we present updates to the previously published MEPHySTo python toolbox for correcting, georeferencing, projecting and analysing geometrically complex hyperspectral scenes. We showcase these methods using datasets covering volcanogenic massive sulphide (VMS) mineralisation exposed within open pit mines in Rio Tinto (Spain), and interpret possible structural and lithological controls on mineralization. Potential applications of hyperspectral mapping for grade control, outcrop mapping and the characterisation of different mineral deposit styles are also discussed.</p>