Bernd Lehmann

Hydrogen sulphide release to surface waters at the Precambrian/Cambrian boundary.

Animal-like multicellular fossils appeared towards the end of the Precambrian, followed by a rapid increase in the abundance and diversity of fossils during the Early Cambrian period, an event also known as the ‘Cambrian explosion’. Changes in the environmental conditions at the Precambrian/Cambrian transition (about 542 Myr ago) have been suggested as a possible explanation for this event, but are still a matter of debate. Here we report molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of around 542 Myr. We find a transient molybdenum isotope signal immediately after the Precambrian/Cambrian transition. Using a box model of the oceanic molybdenum cycle, we find that intense upwelling of hydrogen sulphide-rich deep ocean water best explains the observed Early Cambrian molybdenum isotope signal. Our findings suggest that the Early Cambrian animal radiation may have been triggered by a major change in ocean circulation, terminating a long period during which the Proterozoic ocean was stratified, with sulphidic deep water.

Highly metalliferous carbonaceous shale and Early Cambrian seawater

We report evidence for the seawater origin of an extremely metal-enriched sulfide- and organic carbon–rich marker bed in a transgressive Early Cambrian black shale sequence along the passive margin of the Yangtze platform. The element concentration pattern in this marker bed suggests that it formed in a sediment-starved, stratified basin with a euxinic water column below an oxic surface layer. Biological activity was high in the surface layer, which was resupplied by communication with oxic oceans. The extremely low terrigenous input and the sulfate-reducing environment in the deeper part of the basin led to exceptionally high metal enrichments by factors of ∼107 with respect to modern seawater. The composition of the sulfidic rocks reflects the composition of the Early Cambrian oceans. The molybdenum isotope ratio suggests that during this time <35% of marine Mo was deposited in oxic sediments, and that suboxicanoxic marine environments were more widespread during the Early Cambrian than today.

Magnetite-apatite deposits of the Bafq district, Central Iran: apatite geochemistry and monazite geochronology

Abstract The Bafq mining district is in the Early Cambrian Kashmar-Kerman volcano-plutonic arc in Central Iran and hosts important ‘Kiruna-type’ magnetite-apatite deposits. The hydrothermal magnetite-apatite mineralization occurs mostly as massive orebodies, metasomatic replacements, veins and stockworks. Apatite (low-Sr fluorapatite containing small amounts of hydroxyl) has undergone a partial hydrothermal overprint which involved leaching of Na, Cl and REE. The REE were remobilized into monazite (and minor allanite, parisite and xenotime) which nucleated as inclusions within apatite or as individual crystals. The monazites have very small ThO2 contents (usually <1 wt.%), but they occasionally show an inner core of high-Th monazite, with low-Th overgrowth rims. The chemical Th-U-total Pb dating of the high-Th monazites by electron microprobe analysis yields an isochron age of 515±21 Ma (initial PbO intercept = 68 ppm), or 529±21 Ma (forced initial PbO = 0), which is contemporaneous with the emplacement of the volcano-plutonic host rocks of the magnetite-apatite mineralization, as well as with widespread sedimentation of Late Proterozoic to Cambrian evaporitic rocks in Central Iran. The monazite age and the mineralogical and geochemical data suggest that the magnetite-apatite deposits are probably related to large-scale brine circulation induced by felsic magmatism during the Cambrian.

Metallogeny of tin in central Thailand: A genetic concept

Tin mineralization in central Thailand is associated with granitic rocks of the Thai-Burmese border range (western group). Granitic intrusions east of Bangkok and near the Kampuchea border (eastern group) have no tin. Fractional crystallization is the fundamental petrogenetic process that controls the evolution of both groups of granites. The tin-bearing alkali-feldspar aplogranites of the western group display an extreme degree of differentiation that has no petrological equivalent in the eastern group. These aplogranites are the product of a combination of magmatic fractionation (primary tin enrichment trend) modified by fluid interaction. The latter process is responsible for a secondary geochemical tin deficiency that is balanced by redistribution of tin in fracture systems—i.e., tin mineralization. The pattern of tin depletion in the aplogranites provides an indication of the tin potential in a given ore-forming system.

REE mineralization at Gakara, Burundi: Evidence for anomalous upper mantle in the western Rift Valley

Abstract The 587 Ma-old hydrothermal bastnaesite mineralization of Gakara in the western branch of the East African Rift in Burundi has geological and chemical features typical of carbonatite-related REE ore deposits, but Nd, Ce, and Sr isotope data on the bastnaesite indicate a time-integrated LREE-enriched source with ϵNd(T)−11.9, ϵCe(T) 1.6, and 87 Sr 86 Sr ≤0.7075 . The mid-Proterozoic Nd model age of this source coincides with the formation of the intracratonic Kibaran basin of central Africa and may indicate a mid-Proterozoic rift-related event of small-scale, but locally very effective, mantle metasomatism in this region. The formation of REE ore deposits in carbonatite environments is possibly linked to enriched mantle domains.

Palladseite and its oxidation: evidence from Au-Pd vein-type mineralization (jacutinga), Cauê iron-ore mine, Quadrilátero Ferrífero, Minas Gerais, Brazil

Abstract Palladseite (palladium selenide) and palladian gold occur as euhedral inclusions in specularite and as infill of microfractures and interstices in magnetite aggregates (host rock fragments) within speculariterich auriferous veins (jacutinga). Palladseite has minor amounts of Pt (0.6−3.5 wt.%), Cu (2.9−3.5 wt.%), Hg (0.9− 1.9 wt.%), and Ag (0.3− 0.5 wt.%). The palladian gold contains up to 6 wt.% Pd, and minor Cu and Ag. Isomertieite (Pd11Sb2As2) and sudovikovite (PtSe2) are also recorded. The veins cross-cut the main tectonic foliation of the wallrock (itabirite). The feature of fracture infill and the Se-Sb-As-Hg-Cu-Ag geochemical/mineralogical signature of the Au-Pd mineralization point to a post-ductile deformation and possibly low-temperature (epithermal) origin of the jacutinga mineralization style. Palladseite is replaced locally by a Pd-rich oxidation phase. Electron microprobe analysis of alteration halos around palladseite yields 76−80 wt.% Pd. Oxygen calculated by stoichiometry accords with a monoxide compound of the type PdO. However, the measured oxygen content is lower than expected for the PdO stoichiometry, suggesting loss of volatile species, and a more ‘hydrated’ compound, such as an oxyhydroxide.

A two-stage process of native palladium formation at low temperatures: evidence from a palladian gold nugget (Gongo Soco iron ore mine, Minas Gerais, Brazil)

Abstract Native palladium is rare and occurs in oxidizing environments (hematite stability field) and consistently has very low contents of other metals. On the other hand, gold may contain significant amounts of alloyed palladium. A nugget of palladian gold, obtained from a specularite-rich auriferous vein (‘jacutinga’) of the Gongo Soco iron ore mine, Quadrilátero Ferrífero, Minas Gerais, allows the definition of a paragenetic sequence of native palladium formation. The cm-sized nugget is brecciated and consists of fragments of palladian gold [Au0.79Pd0.09Cu0.07Ag0.04Hg0.01] with inclusions of isomertieite [Pd11Sb2As2], cemented by kaolinite-hematite-palladian gold infill. The isomertieite inclusions are preferentially fractured and display various degrees of alteration to a Pd-O phase. Native palladium occurs as a crystallized component within the Pd-O phase, with microstructural and chemical features suggesting a desiccation process. It is proposed that native palladium forms by (1) oxidation of a pre-existent palladium mineral to a Pd-O phase, and (2) subsequent crystallization of native palladium from the Pd-O phase, by oxygen loss and dehydration. This process operates in a low-temperature hydrothermal or weathering environment and leads to strong fractionation of palladium and gold.

Early Neoproterozoic rare metal (Sn, Ta, W) and gold metallogeny of the Central Africa Region: a review

Abstract The four metals of economic significance in the Central Africa or Great Lakes region, i.e. gold, tin, tantalum and tungsten, are part of one composite metallogenic system that operated about 980±20 Ma. The main driving agent was peraluminous ilmenite-series granite magmatism, synchronous with intracratonic compression and associated with the final amalgamation of the supercontinent Rodinia. The granitic melts were emplaced at intrusive levels of ≧2 kbar (≧8 km); the intrusions display a variable and often advanced degree of fractionation, including abundant Sn–Ta–Li–Be–Rb–Cs pegmatites, and are associated with hydrothermal systems enriched in tin, tungsten and/or gold. Based on cumulative past production and present metal prices, gold in hydrothermal quartz veins is the major commodity, followed by tin either in rare metal pegmatites or in sheeted, hydrothermal quartz veins. Many deposits in the province occur in siliciclastic metasedimentary, or metabasaltic roof rocks above parental granites; mainly in its western part, the zone of mineralisation retracts into the granite roof. Typically in the first case, antiformal sites acted as fluid escape zones, with carbonaceous or metabasaltic rocks as chemical traps for tungsten and gold. Examples of pegmatitic and magmatic–hydrothermal deposits are presented in some detail in order to illustrate characteristics and genetic controls, and to support the metallogenic hypothesis here advanced. Impeding strategic exploration, published elements of understanding the evolution and mineralisation of the Kibara belt are contradictory and essential links are missing, foremost an understanding of the 1 Ga flare up of fertile granites. Towards solving this conundrum we suggest that the key is delamination of the mantle lithosphere and dense mafic lower crust, residual after extraction of voluminous 1·38 Ga granitic melts. During pan-Rodinian orogenic events, the Tanganyika spur of the Tanzania craton acted as an indenter whose impact caused foundering of the early Kibaran lithosphere. Consequent influx of asthenospheric heat triggered large-scale crustal melting that resulted in the tin granites. The stress state was largely compressive but possibly punctuated by short or local extensional events. The correlation of geological evolution and mineralisation substantiates the formal recognition of a Kibara Metallogenic Domain, which is composed of two units: The Mesoproterozoic (1·4 Ga) Kabanga-Musongati nickel (±copper, cobalt, platinum) province; and the early Neoproterozoic (1 Ga) Kibara rare metal and gold province that is the main subject of this paper. The present understanding of the operating metallogenic systems remains limited. Regarding the application of modern concepts and technologies, this province is drastically underexplored.

Palladium-oxygenated compounds of the Gongo Soco mine, Quadrilátero Ferrífero, central Minas Gerais, Brazil

Abstract New data on the Gongo Soco palladium-oxygenated compounds are presented for the first time since the work of Johnson and Lampadius (1837). A variety of PdO species is documented, always in association with Cu, Fe and Mn, and includes Pd(OH)2 or PdO.H2O, and phases with metal excess in relation to oxygen, interpreted as metastable. Their remarkable occurrence in ‘ouro preto’ (black gold) and jacutinga (auriferous itabirite/iron ore) is the result of mertieite-II alteration. It involves leaching of Sb and As, limited removal of Pd, and incorporation of Cu, Mn and Hg into PdO species. The locus of Hg-bearing PdO species in microfractures parallel to the enclosing gold-mineralized shear band suggests at least one low-temperature hydrothermal generation. The presence of a halite crystal attached to gold testifies to the involvement of Cl-saturated hydrothermal fluids in the oxidizing environment of jacutinga formation.

Contribution to a New Genetical Concept on the Bolivian Tin Province

The Bolivian tin province is composed of four different groups of tin deposits distinguished by geotectonic setting, paragenesis, and geometric shape. There are Precambrian tin-bearing granites of the Brazilian Shield, Silurian meta-sedimentary strata-bound “manto” deposits, and two cycles of magmatic regeneration in Early Mesozoic (vein deposits) and Cenozoic time (tin porphyries). Heredity of tin derived from the Precambrian metallotect can be traced by exogene processes (paleo-placers) in Silurian, deep-seated magmatism in Mesozoic, and subvolcanic intrusions in Tertiary. The new evidence of Silurian synsedimentary cassiterite enrichment provides the missing link for a conclusive model of the metallogenetic development in the Bolivian crustal segment, thus proving the formation of different types of time- and strata-bound tin deposits in accordance with the geotectonic development of the continent.