Palaeoproterozoic orogenic gold style mineralization at the Southwestern Archaean Tanzanian cratonic margin, Lupa Goldfield, SW Tanzania: Implications from U–Pb titanite geochronology

The Lupa Goldfield, situated at the southwestern Tanzanian cratonic margin, comprises a network of auriferous quartz veins and greenschist facies mylonitic shear zones cutting a suite of Archaean–Palaeoproterozoic granitic–gabbroic intrusions. The existing geochronological database points to a protracted, but episodic 1.96–1.88 Ga magmatic history that is broadly coincident with the 2.1–1.8 Ga Ubendian Orogeny. Molybdenite, pyrite and chalcopyrite samples from mineralized quartz veins and mylonitic shear zones yield Re–Os model ages that range from 1.95 to 1.88 Ga, whereas ca. 1.88 Ga pyrite with gold bearing inclusions and sampled from the host mylonitic shear zone suggest that gold occurred relatively late in this hydrothermal history. The ca. 1.88 Ga gold event is recorded at all five of the studied prospects, whereas the relationship between gold and the disparately older 1.95 and 1.94 Ga Re–Os molybdenite ages is unclear. New U–Pb metamorphic titanite dating of a foliated Archaean granite sample (ca. 2.76 Ga) suggests that the onset of ductile deformation within the Lupa Goldfield occurred at ca. 1.92 Ga, and some ca. 40 Myr prior to auriferous and brittle–ductile mylonitic shear zones at ca. 1.88 Ga. Early ductile deformation is not associated with gold mineralization, but the ductile deformation fabrics and, in particular the development of rheologically weak chloritic folia, may have acted as zones of pre-existing weakness that localized strain and influenced the geometry of later auriferous mylonitic shear zones. The large age difference between U–Pb zircon and titanite ages for the Archaean granite sample is in contrast to new U–Pb titanite ages for the Saza Granodiorite (1930 ± 3 Ma), which are only slightly outside of analytical uncertainty at the 2σ level with a previously reported U–Pb zircon age for the same sample (1935 ± 1 Ma). These new age results, together with previously reported U–Pb and Re–Os ages, highlight the protracted magmatic, hydrothermal and structural evolution of the Lupa Goldfield (1.96–1.88 Ga). They are also consistent with other palaeo-convergent margins where orogenic gold style mineralization occurs relatively late in the orogen's tectono-thermal history

Structural Controls on Orogenic Au Mineralization During Transpression: Lupa Goldfield, Southwestern Tanzania

Au mineralization in the western Lupa goldfield, southwestern Tanzania was associated with transpression and reverse sinistral slip along a network of steeply S dipping shear zones with non-Andersonian geometries. Slip was accommodated by: (1) frictional failure and sliding during emplacement of quartz ± Au-bearing veins; and (2) crystal plasticity and fluid-assisted diffusive mass transfer. The Kenge mineral system is situated along a NW-SE-trending shear zone and is characterized by ≤10-m thick, Au-bearing fault-fill veins hosted by well-developed phyllosilicate-rich mylonites. The broadly contemporaneous Porcupine mineral system is situated along an ENE-WSW– to E–W-trending shear zone, which is characterized by narrow, discontinuous mylonitic shear zone within a silicified and nonfoliated granitoid protolith. Au mineralization at Porcupine occurs within steeply dipping fault-fill and subhorizontal extension/oblique-extension veins. Three-dimensional frictional reactivation theory provides a self-consistent explanation for the different vein styles at Kenge and Porcupine and extends the classic fault valve model to the general case of oblique slip along multiple, arbitrarily oriented shear zones. Analysis of the differential stress required for frictional reactivation suggests the following: (1) the Kenge shear zone was intrinsically weaker than the Porcupine shear zone, consistent with the lack of well-developed mylonites at Porcupine; and (2) frictional reactivation of the Kenge shear zone occurred under suprahydrostatic but sublithostatic pore fluid pressures, whereas frictional reactivation of the Porcupine shear zone occurred under near-lithostatic fluid pressures. We hypothesize that near-lithostatic pore fluid pressures relieved effective normal stresses at grain-grain contacts, helping to preserve intragranular and fracture porosity at the Porcupine orebody. As such, these pore spaces may be important microstructural sites for Au mineralization. Low effective normal stresses can also explain the poorly developed phyllosilicate-rich mylonites and limited degree of shear zone weakening at Porcupine

Re-Os Molybdenite, Pyrite, and Chalcopyrite Geochronology, Lupa Goldfield, Southwestern Tanzania: Tracing Metallogenic Time Scales at Midcrustal Shear Zones Hosting Orogenic Au Deposits

A detailed Re-Os molybdenite, pyrite, and chalcopyrite geochronology at five shear-hosted Au occurrences (Kenge, Mbenge, Porcupine, Konokono, and Dubwana) in the Lupa goldfield, southwestern Tanzania, is reported in this paper. Au occurrences within the Lupa goldfield share many geologic similarities with the orogenic Au deposit type and are situated within a Paleoproterozoic magmatic arc that intruded the Archean Tanzanian cratonic margin. Pyrite ± chalcopyrite ± molybdenite-bearing fault-fill veins and mylonitic shear zones crosscut granitic host rocks and are associated with the highest Au grades. Re-Os sulfide ages are deemed a suitable proxy to constrain the timing of Au based on the occurrence of Au-bearing minerals as inclusions within pyrite and chalcopyrite, whereas Au-bearing minerals filling pyrite fractures may represent a younger and undated metallogenic event. Molybdenite at Kenge occurs as ultrafine disseminations within fault-fill veins (1953 ± 6 Ma; n = 3) that possess nominally older weighted average Re-Os ages than molybdenite hosted by stylolite-like veins (1937 ± 8 Ma; n = 7). Both sample sets are ca. 70 m.y. older than a weighted average Re-Os pyrite age from the mylonitic shear zones at Kenge and Mbenge (1876 ± 10 Ma; n = 13), which contain fault-fill veins and record the timing of mylonitization. Molybdenite at Porcupine occurs as ultrafine disseminations within quartz veins and mylonitized granite samples (1886 ± 6 Ma; n = 4) that are broadly equivalent in age to weighted average Re-Os ages of molybdenite occurring as stylolite-like veins (1873 ± 5 Ma; n = 6) and pyrite within oblique-extension veins (1894 ± 45 Ma; n = 2). Weighted average Re-Os pyrite model ages at Konokono (1880 ± 14 Ma; n = 9) and Dubwana (1905 ± 25 Ma; n = 2) are also consistent with the ca. 1.88 Ga event observed at Kenge, Mbenge, and Porcupine. Gold occurrences in the Lupa goldfield therefore record a protracted hydrothermal history (1.95–1.87 Ga) comprising at least three temporally distinct hydrothermal events (ca. 1.95, 1.94, and 1.88 Ga), which are each represented in detail by a complex vein history that occurred at a time scale less than the resolution of the Re-Os method. The sampling of broadly contemporaneous sulfides from five shear zones suggest that mylonitic shear zones represented an interconnected network of midcrustal permeable fluid conduits at ca. 1.88 Ga that permitted the transportation and deposition of gold. Comparison between Re-Os sulfide and high-precision U-Pb zircon ages for the granitic host rocks provides unequivocal evidence for sulfidation concomitant with magmatism. However, the range of Re-Os ages argues against an intrusion-related deposit model whereby metallogenic fluids are solely derived from an individual intrusion. The regional ca. 1.88 Ga metallogenic event identified as part of this study occurred concurrently with eclogite facies metamorphism during the Ubendian orogenic cycle and provides one of Earth’s earliest temporal links between subduction zone processes and orogenic Au deposit formation during the Paleoproterozoic

Lithogeochemistry, Geochronology and Geodynamic Setting of the Lupa Terrane, Tanzania: Implications for the Extent of the Archean Tanzanian Craton

Herein we provide new zircon U–Pb ID-TIMS, U–Pb LA-MC-ICP-MS and Lu–Hf LA-MC-ICP-MS results from the Lupa Terrane, SW Tanzania, and demonstrate that previously considered Paleoproterozoic granites comprising the Paleoproterozoic Ubendian Belt are in fact Archean (ca. 2.74 Ga). Disparately older inherited zircons (ca. 2.85 Ga) and Lu–Hf zircon model ages (≥3.1 Ga) suggest that these Archean granites likely interacted with substantially older crust underlying the Lupa Terrane at the Tanzanian cratonic margin. Foliated Archean granites are in turn cut by non-foliated, voluminous and greenschist facies granitic-gabbroic intrusions (1.96–1.88 Ga), which are broadly concomitant with granulite- to amphibolites-facies rocks that characterize the other lithotectonic terranes comprising the Ubendian Belt. New geochemical results suggest that these Paleoproterozoic intrusive phases possess trace element compositions that are typical of continental magmatic arcs, which is consistent with the crust-magma interaction inferred from field relationships and inherited Archean zircons within Paleoproterozoic granitic intrusions. Together the available field and geochemical evidence suggest that the Lupa Terrane was a continental cratonic margin during the Paleoproterozoic onto which the other Ubendian lithotectonic terranes were accreted and implies at least a 150 km southwest extension of the currently accepted position of the Tanzanian craton margin. These new results are consistent with re-worked Archean crust reported in other metamorphic belts enveloping the Tanzanian Craton and seismic tomography studies that suggest significant portions of the Ubendian Belt represent re-worked Archean lithosphere

High-Throughput Analysis of Gene Essentiality and Sporulation in Clostridium difficile

Clostridium difficile is the most common cause of antibiotic-associated intestinal infections and a significant cause of morbidity and mortality. Infection with C. difficile requires disruption of the intestinal microbiota, most commonly by antibiotic usage. Therapeutic intervention largely relies on a small number of broad-spectrum antibiotics, which further exacerbate intestinal dysbiosis and leave the patient acutely sensitive to reinfection. Development of novel targeted therapeutic interventions will require a detailed knowledge of essential cellular processes, which represent attractive targets, and species-specific processes, such as bacterial sporulation. Our knowledge of the genetic basis of C. difficile infection has been hampered by a lack of genetic tools, although recent developments have made some headway in addressing this limitation. Here we describe the development of a method for rapidly generating large numbers of transposon mutants in clinically important strains of C. difficile. We validated our transposon mutagenesis approach in a model strain of C. difficile and then generated a comprehensive transposon library in the highly virulent epidemic strain R20291 (027/BI/NAP1) containing more than 70,000 unique mutants. Using transposon-directed insertion site sequencing (TraDIS), we have identified a core set of 404 essential genes, required for growth in vitro. We then applied this technique to the process of sporulation, an absolute requirement for C. difficile transmission and pathogenesis, identifying 798 genes that are likely to impact spore production. The data generated in this study will form a valuable resource for the community and inform future research on this important human pathogen. IMPORTANCE Clostridium difficile is a common cause of potentially fatal intestinal infections in hospital patients, particularly those who have been treated with antibiotics. Our knowledge of this bacterium has been hampered by a lack of tools for dissecting the organism. We have developed a method to study the function of every gene in the bacterium simultaneously. Using this tool, we have identified a set of 404 genes that are required for growth of the bacteria in the laboratory. C. difficile also produces a highly resistant spore that can survive in the environment for a long time and is a requirement for transmission of the bacteria between patients. We have applied our genetic tool to identify all of the genes required for production of a spore. All of these genes represent attractive targets for new drugs to treat infection. FOOTNOTE

Bacterioplankton community diversity in a maritime Antarctic lake, determined by culture-dependent and culture-independent techniques

The biodiversity of the pelagic bacterioplankton community of a maritime Antarctic freshwater lake was examined by cultivation-dependent and cultivation-independent techniques to determine predominant bacterioplankton populations present. The culture-dependent techniques used were direct culture and observation, polymerase chain reaction amplification of 16S rRNA gene fragments, restriction fragment length polymorphism (RFLP) analysis followed by selective sequencing and fatty acid methyl ester analysis. The culture-independent techniques used were 16S ribosomal DNA gene cloning, RFLP analysis and sequencing, in situ hybridisation with group-specific, fluorescently labelled oligonucleotide probes and cloning and sequencing of dominant denaturing gradient gel electrophoresis products. Significant differences occurred between the results obtained with each method. However, sufficient overlap existed between the different methods to identify potentially significant groups. At least six different bacterial divisions including 24 genera were identified using culture-dependent techniques, and eight different bacterial divisions, including 23 genera, were identified using culture-independent techniques. Only five genera, Corynebacterium, Cytophaga, Flavobacterium, Janthinobacterium and Pseudomonas, could be identified using both sets of techniques, which represented four different bacterial divisions. Significantly for Antarctic freshwater lakes, pigment production is found within members of each of these genera. This work illustrates the importance of a comprehensive polyphasic approach in the analysis of lake bacterioplankton, and supports the ecological relevance of results obtained in earlier entirely culture-based studies. (C) 2003 Federation of European Microbiological Societies