Andrea Agangi’s research while affiliated with University of Johannesburg and other places

The Hokuroku district of NE Japan hosts the type locality for Kuroko volcanic‐hosted massive sulphide (VHMS) deposits, which are the product of seafloor hydrothermal venting in a continental rift during the opening of the Sea of Japan in the Miocene. Here we reconstruct the thermal history of the magmas, and find that the shallow emplacement of felsic magmas (< 100 MPa) reached a thermal maximum at the time of ore deposition (ca. 950°C). Post‐ore magmas stalled and crystallised at water‐saturated cotectic conditions at average temperatures of ca. 900°C. We propose an ore‐genetic model that combines the effects of increasing seawater depth (i.e., increasing hydrostatic pressure) on magma behaviour that led to increasing magmatic heat‐flux resulting in the episodic nature of ore deposition. Our results provide an interpretative framework that helps explain the localised occurrence of VHMS deposits that may be used as an indicator for mineral exploration in similar tectonic settings.

The Francevillian Group, deposited during the Lomagundi carbon isotope excursion (LE), recently revealed evidence for complex and diverse Paleoproterozoic biota. This biota is preserved as pyritized and non-pyritized macrofossil structures hosted in black shale deposited in an oxygenated, open-marine environment. However, the timing of the LE, and the time when these macroscopic organisms evolved is still poorly constrained. Here, we present U-Pb ages for zircons separated from coeval volcaniclastic sandstone, 207Pb/206Pb model ages for pyrite preserving the macrofossils, and 40Ar/39Ar dates for K-rich clay minerals from the fossiliferous black shales. The youngest group of zircons yields a weighted average 207Pb/206Pb age of 2132 ± 4 Ma, which is considered as a maximum depositional age for the strata that record the LE and host the earliest known, macroscopic multicellular organisms. By contrast, the 207Pb/206Pb dates of pyritized fossils, scattering between ca. 2085 and 2070 Ma, with a weighted average of 2077 ± 17 Ma, reflect early diagenesis in the Francevillian basin and thus provide a minimum age for fossiliferous strata. This range of ages overlaps with reproducible 40Ar/39Ar dates and the ages for other open-marine sedimentary successions that record the LE in both shallow- and deep-marine environments. Taken together, the data demonstrate that the synchronicity of the record of the Lomagundi carbon isotope excursion in shallow- and deep-marine carbonates worldwide is consistent with a global biogeochemical signature of the Paleoproterozoic oceans, with its latest stage, between ca. 2.13 and 2.08 Ga, being preserved in the fossiliferous Francevillian Group strata. Further, the results also suggest that 40Ar/39Ar dating of K-rich clay minerals extracted from black shale can be used to constrain the depositional and/or early diagenetic age of more than 2-billion-years old sedimentary strata not affected by high-temperature hydrothermal and metamorphic overprint.

Veins intersected by a ~600‐m‐long drill core into the Hokuryu epithermal Au‐Ag deposit in the northeastern part of Hokkaido, Japan, were investigated including mineralogy, texture, quartz composition, and fluid inclusion characteristics, with relation to elevation. Host rocks of these veins are flow‐banded rhyolite, pyroclastic breccia, and andesite and are mainly altered by illite and chlorite. The veins display crustiform and massive macroscopic textures. The crustiform veins generally located at higher elevations, contain Ag‐rich and Au‐Ag‐rich bands. The massive veins are mainly present at lower elevations, and they are generally barren of precious metals. The Ag‐rich bands in the crustiform veins contain mineral assemblage of hessite + sphalerite + pyrite + galena. These minerals, together with anhedral to rhombic adularia, exist within the interstices of quartz that display microspherical texture. The Au‐Ag‐rich bands contain mineral assemblage of electrum ± naumannite‐aguilarite associated with quartz with colloform, ghost‐sphere, and ghost‐bladed textures. Bulk compositions of the veins reflect the ore mineralogy observed in which Ag is strongly correlated with Pb, Te, Zn, Cd, and Bi. The barren massive veins are mainly composed of granular quartz. Textural and mineralogical characteristics of Au‐ and Ag‐bearing bands in the crustiform veins indicate amorphous silica and calcite precursors. Together with the coexistence of adularia, these suggest liquid boiling during vein formation. Quartz associated with Au‐ and Ag‐bearing minerals exhibits blue cathodoluminescence (ca. 400 nm wavelength) triggered by high Al impurity probably due to pH and pressure fluctuations or impurity redistribution during recrystallization from metastable amorphous silica. Fluid inclusions hosted in quartz and adularia, which contain negligible amounts of CO 2 , show a modal homogenization temperature range of 260°C–290°C and a salinity range of 1.2–6.9 wt% NaCl equivalent. The homogenization temperatures follow a boiling point curve and indicate an erosion of up to ~340 m for the deposit. The variable salinities in relation to elevation suggest extreme vapor loss during boiling at depth, episodic influx of high salinity fluids from deeper source, or mixing with high salinity fluids along anastomosing fractures at relatively shallower depths.

The Omatapati copper and silver prospect within the Kaoko Belt is located in the Opuwo district, Kunene region, Namibia. The prospect is hosted by dolomite and interbedded argillites of the Neoproterozoic Devede Formation, the Ombombo Subgroup. An ore body is exposed in shallow artisanal mining pits and drill‐cores of the prospect have grades of 0.4 to 5.2 wt% Cu, and 23 to 312 g/t Ag. The prospect was formed by both hypogene and supergene mineralization processes. The hypogene mineralization occurred in three stages. Stage 1 is represented by calcite veins containing chalcopyrite, bornite, sphalerite and galena. Stage 2 consists of quartz and calcite veins with chalcopyrite and bornite. Stage 3 consists of quartz‐calcite‐barite veins with chalcopyrite. The veins of stages 1 and 2 are subparallel or discordant to the foliation of argillites, and those of Stage 3 are NE‐striking and steeply dipping in the brecciated dolostone and argillite. The mineralization stages 1, 2, and 3 are overprinted by supergene chalcocite, digenite, annite, covellite, malachite, delafossite, hematite, and goethite. The supergene process forms a semi‐massive chalcocite‐covellite zone, which extends from the surface to ~ 50 m depth. The veins show average concentrations of 2.1 wt% Cu, 80 ppm Ag, and 532 ppm Pb for the stage 1, 3.1 wt% Cu, 118 ppm Ag, and 66 ppm Pb for the stage 2, and 18.6 wt% Cu, 675 ppm Ag, and 565 ppm Pb for the Stage 3 with supergene overprinting. Silver occurs as impurity in supergene chalcocite (6303 ppm Ag), digenite (4425 ppm Ag), and covellite (3060 ppm Ag). Fluid inclusions in quartz and calcite revealed that the hypogene mineralization occurred under a pressure >7–8 MPa. Temperatures of ore formation of the stages 1, 2, and 3 were >145–155 °C, >290–300 °C, and >190–230 °C, respectively. Large variation of salinities in each fluid inclusion assemblage suggests fluid mixing and dilution processes. Fluid inclusion gas compositions in the Stage 3 have weighted means of 98.98 mol% H 2 O, 0.75 mol% N 2 , 0.52 mol% CO 2 , 0.02 mol% CH 4 , 0.0021 mol% Ar, 0.0009 mol% H 2 S, and 0.00006 mol% He and indicate a signature of magmatic fluids mixed with meteoric waters or seawater. δ ³⁴ S CDT values of sulfides in the stages 1, 2, and 3 in the prospect have three data clusters, (1) near 0 ‰ of chalcopyrite in the stages 1 and 2, (2) near −11 ‰ of bornite in the stage 2, and (3) +5 to +11 ‰ of the supergene chalcocite and covellite. Modes of occurrence of ores, fluid inclusions and sulfur isotope data show that the prospect is likely the same type as sediment‐hosted copper‐silver deposits with veining found in the Central African Copperbelt.

The ca. 2.96 Ga Dominion Group (DG) preserves the first subaerial volcano-sedimentary succession on the Kaapvaal Craton. Based on field and core observations, this study provides revised stratigraphic logs and geological maps from the area around Ottosdal that refine the understanding of the stratigraphic and tectonic evolution of these ancient rocks. Fluvial sandstone and conglomerate of the Rhenosterspruit Formation (RsF) were deposited on granite basement, followed by andesitic to basaltic volcanic rocks of the Rhenosterhoek Formation (RhF). These amygdaloidal andesitic to basaltic lava units show fragmentation around lava flows that may represent an indication of prevalent subaerial environment. However, local hyaloclastite and pillow lava units indicate periodic aqueous conditions. The Syferfontein Formation (SF) has the most extensive exposures in the Ottosdal area and represents the youngest volcanic unit of the DG. The porphyritic and spherulitic volcanic rocks tell a story of subaerial volcanism interspersed with periods of lacustrine deposition of sandstone and shale. The Witwatersrand Supergroup (WSG) overlies the DG along an angular unconformity. Folding affected the succession of the DG, WSG and the Ventersdorp Supergroup (Rietgat Formation). This event is reflected in small-scale folds and mullion structures in the central part of the study area and by larger scale north-northwest–south-southeast-striking anticlines and synclines. Folding was accompanied by northwest-southeast-striking thrust faulting, either during or shortly after the formation of the Ventersdorp Supergroup. In the study area, post-Ventersdorp deformation is restricted to east-northeast–west-southwest-striking faults.