Volcanogenic Massive Sulfide Occurrences Research Articles

Evidence of a Volcanogenic Massive Sulfide (Zn[sbnd]Pb[sbnd]Cu[sbnd]Ag) district within the Tiébélé Birimian (Paleoproterozoic) Greenstone Belts, Southern Burkina Faso (West – Africa)

Twenty years after the discovery of the Perkoa ZnAg deposit, another type of ZnCuPb ± Ag Volcanogenic Massive Sulfide (VMS) subgroup of occurrences forming a district has been highlighted within the Paleoproterozoic Birimian Greenstone Belts of the West African Craton in Burkina Faso. The geology of the area is characterized by a series of dominantly mafic volcanic rocks with intercalated black shales which increase in proportion upwards in the stratigraphy. This stratigraphic package is overlain by a felsic volcanic sequence comprising reworked tuff and rhyolite. Although mineralization is locally associated with sedimentary rocks, it is more commonly found in rhyolites. The metamorphic grade is dominantly greenschist facies. The main lithologies in the mafic sequence range from basalt to andesite with associated gabbro. The felsic sequence consists of dacite to rhyolite with associated granitoids (granite-granodiorite-tonalite). The volcanic rocks are commonly tholeiitic (Zr/Y = 2–4.5) with relatively high Zr and Y content, although a limited number of samples plot in transitional (Zr/Y = 4.5–7) or calc-alkaline (Zr/Y = 7–25) fields. Rhyolites, which constitute the main mineralized rocks at Tiébélé, have similar key trace element signatures to other rhyolites-related known VMS systems worldwide. Both have low Zr/Y (<7) and low LaN/YbN (<6), which suggests low crustal residence times of magmas in extensional settings. Detailed investigations identified at least four VMS targets notably at Koubongo, Nabenia, Loubel and AVV (Aménagement de la Vallée des Voltas) extending over an area of 332 km2. Typical mineral assemblages defining VMS occurrences are mainly hosted by meta-sedimentary rocks and rhyolite but are also found as veins in tonalite. These assemblages can be grouped into four different styles: (i) Variably banded massive sulfides dominated by sphalerite over galena, pyrite, and chalcopyrite within metasediments; (ii) Pyrite-rich or pyrrhotite banded type associated with sphalerite, chalcopyrite, and galena within silicified and carbonatized units; (iii) Disseminated and finely oriented sulfides, including pyrite – pyrrhotite – magnetite ± sphalerite within quartz-phyric rhyolite; and (iv) Fracture filling pyrrhotite-rich - magnetite - sphalerite mineralization possibly within tonalite. Irrespective of the style of the mineralization, chemistry of altered and least altered rhyolite reveals a significant increase in Mg ± Fe and H2O, and a loss of K within mineralization associated alteration. This may suggest sea water leaching of underlying (?) mafic and sedimentary rocks, with MgFe transfer into the rhyolites concomitant with the loss of K. This paper highlights the features of new occurrences of VMS mineralization type within the Birimian system of West Africa. We think this could help for new discoveries along the belts. Exploration based upon ground mapping, geochemistry and geophysics is highly recommended.

Post-VMS mineralization deformations (1880–1820 Ma) of the Skellefte district (Sweden): insights from the spatial pattern of VMS occurrences

In the last two to three decades or so, the spatial pattern of mineral occurrences of a deposit-type has been studied to derive insights to mineralization controls and assist mineral exploration. In the Skellefte district, Fry plots of volcanogenic massive sulfide (VMS) mines/prospects reveal patterns that are likely due to postmineralization deformation events. The fractal dimensions of the spatial patterns of the present-day VMS mines/prospects and that of the ‘original’ VMS deposits support the concept that spatial patterns of mineral deposits are spatially-invariant. Therefore, analysis of the spatial pattern of mineral deposits is useful not only in research about pre- and syn-mineralization geological settings but also post-mineralization geological settings.

The Metallogeny of Late Triassic Rifting of the Alexander Terrane in Southeastern Alaska and Northwestern British Columbia

A belt of unusual volcanogenic massive sulfide (VMS) occurrences is located along the eastern margin of the Alexander terrane throughout southeastern Alaska and northwestern British Columbia and exhibits a range of characteristics consistent with a variety of syngenetic to epigenetic deposit types. Deposits within this belt include Greens Creek and Windy Craggy, the economically most significant VMS deposit in Alaska and the largest in North America, respectively. The occurrences are hosted by a discontinuously exposed, 800-km-long belt of rocks that consist of a 200- to 800-m-thick sequence of conglomerate, limestone, marine clastic sedimentary rocks, and tuff intercalated with and overlain by a distinctive unit of mafic pyroclastic rocks and pillowed flows. Faunal data bracket the age of the host rocks between Anisian (Middle Triassic) and late Norian (late Late Triassic). This metallogenic belt is herein referred to as the Alexander Triassic metallogenic belt. The VMS occurrences show systematic differences in degree of structural control, chemistry, and stratigraphic setting along the Alexander Triassic metallogenic belt that suggest important spatial or temporal changes in the tectonic environment of formation. At the southern end of the belt, felsic volcanic rocks overlain by shallow-water limestones characterize the lower part of the sequence. In the southern and middle portion of the belt, a distinctive pebble conglomerate marks the base of the section and is indicative of high-energy deposition in a near slope or basin margin setting. At the northern end of the belt the conglomerates, limestones, and felsic volcanic rocks are absent and the belt is composed of deep-water sedimentary and mafic volcanic rocks. This northward change in depositional environment and lithofacies is accompanied by a northward transition from epithermal-like structurally controlled, discontinuous, vein- and pod-shaped, Pb-Zn-AgBa-(Cu) occurrences with relatively simple mineralogy, to sulfosalt-enriched VMS occurrences exhibiting characteristics of vein, diagenetic replacement, and exhalative styles of mineralization, and finally to Cu-Zn-(Co-Au) occurrences with larger and more clearly stratiform orebody morphologies. Occurrences in the middle of the belt are transitional in nature between structurally controlled types of mineralization that formed in a shallow-water, near-arc setting, to those having a more stratiform appearance, formed in a deeper water, rift-basin setting. The geologic setting in the south is consistent with shallow subaqueous emplacement on the flanks of the Alexander terrane. Northward, the setting changes to an increasingly deeper back- or intra-arc rift basin. Igneous activity in the Alexander Triassic metallogenic belt is characterized by a bimodal suite of volcanic rocks and a previously unrecognized association with mafic-ultramafic hypabyssal intrusions. Immobile trace and rare earth element (REE) geochemical data indicate that felsic rocks in the southern portion of the belt are typical calc-alkaline rhyolites, which give way in the middle of the belt to peralkaline rhyolites. Rhyolites are largely absent in the northern part of the belt. Throughout the belt, the capping basaltic rocks have transitional geochemical signatures. Radiogenic isotope data for these rocks are also transitional (basalts and gabbros: eNd = 4–9 and 87Sr/86Sr initial at 215 Ma = 0.7037–0.7074). Together these data are interpreted to reflect variable assimilation of mature island-arc crust by more primitive melts having the characteristics of either mid-ocean ridge (MORB) or intraplate (within-plate) basalts (WPB). The ore and host-rock geochemistry and the sulfosalt-rich mineralogy of the deposits are strikingly similar to recent descriptions of active sea-floor hydrothermal (white smoker) systems in back arcs of the southwest Pacific Ocean. These data, in concert with existing faunal ages, record the formation of a belt of VMS deposits and occurrences in a propagating intra- to back-arc rift tectonic setting during the Late Triassic. A modern analogue having similar tectonic and metallogenic features is the southward projection of the Lau basin, from the active sea-floor hydrothermal vents of the Valu Fa Ridge to the Taupo volcanic zone of the North Island, New Zealand.

Microbial involvement in the formation of Cambrian sea-floor silica-iron oxide deposits, Australia

The Cambrian-Ordovician Mount Windsor volcanic belt in northern Australia is host to stratiform lenses of massive ferruginous chert that are spatially associated with volcanogenic massive sulfide occurrences, in particular the Thalanga zinc-lead-copper-silver deposit. The rocks are composed principally of Fe2O3 and SiO2, with very low concentrations of alkalic elements, and lithogenous elements such as Al, Zr, and Ti; they are interpreted as nearly pure chemical sediments. Textural evidence is documented of the integral role of filamentous bacteria (and/or fungi) in depositing iron from hydrothermal fluids, and of the inorganic precipitation of silica-iron-oxyhydroxide gels that subsequently matured to subcrystalline and crystalline silica forms. At least three distinct iron-accumulating microbial forms are distinguished: networks of septate filaments, nonseptate filament networks, and extremely coarse branching filaments that do not reconnect. Values for δ34S in disseminated pyrite are up to 50‰ lighter than those of contemporaneous Cambrian seawater, suggesting postdepositional colonization of some ironstones by sulfur-reducing bacteria. The site not only preserves the textural interplay of biological and inorganic depositional processes in exhalites, but also extends the oldest known instance of microbial mediation in vent-proximal hydrothermal iron precipitation to at least 500 Ma.

Variations in style of volcanism and massive sulfide deposition in Early to Middle Ordovician island-arc sequences of the Newfoundland Central Mobile Belt

The Newfoundland Central mobile belt records the Early Paleozoic birth, development, and destruction of the Iapetus ocean. Following opening of the ocean in the Cambrian, a compressional plate margin developed in the Early Ordovician and an island arc was formed above an east-dipping subduction zone. Volcanism was active at this plate margin from late Tremadocian to early Caradocian, at which time subduction ceased and the products of the volcanism were blanketed by argillite and flysch.The pre-Caradocian island-arc volcanic sequences of the Central mobile belt are host to numerous volcanogenic massive sulfide occurrences. Both the volcanic rocks and the associated massive sulfide occurrences show distinct contrasts across the volcanic belt. In the northern part of the belt (Notre Dame Bay area), submarine, dominantly mafic, volcanic rocks host pyritic Cu-Zn deposits associated with local felsic domes. These sequences pass southward into the Victoria River area, where volcanic rocks are characterized by laterally extensive felsic pyroclastic sheets which host polymetallic Zn-Pb-Cu deposits. In southern Newfoundland (the Hermitage flexure area), partially emergent, preponderantly felsic volcanic rocks host Pb-Zn-Ag massive sulfide mineralization formed during the waning stages of volcanism.Lead isotope ratios in massive sulfide deposits in the Hermitage flexure area are consistently more radiogenic than in deposits hosted by more mafic volcanic sequences to the north. This, coupled with geologic evidence, suggests that these sequences formed under the influence of sialic crustal material. It is inferred that the Early-Middle Ordovician island-arc volcanism in the Newfoundland Central mobile belt occurred in both oceanic and continental margin settings. The compositions and geologic environments of the associated massive sulfide deposits reflect these variations in tectonostratigraphic settings.Time and space relationships within the volcanic arc during the interval of active volcanism are not entirely clear. All Early to Middle Ordovician volcanic rocks in central Newfoundland may have formed from contemporaneous volcanic activity. If this is the case, simultaneous volcanism must have occurred both in oceanic environments and at or near the continental margin. Alternatively, the volcanism may have been slightly diachronous with an early episode of ensimatic mafic volcanism followed by migration of the volcanic centers toward the continental margin.The massive sulfide deposits of the pre-Caradocian island arc exhibit lead isotope ratios which are distinctly more radiogenic than those in the Buchans, Roberts Arm, and post-Caradocian Cutwell Groups. This appears to support geologically based inferences that the latter sequences are not part of the main phase of island-arc activity in central Newfoundland.