The copper-bearing breccia pipes of the Messina district South Africa

The Messina Copper Mines comprise a group of breccia pipes, disseminated replacement and fissure deposits within an area 20 km long by 1 km wide, emplaced within high-grade metamorphic rocks of the Limpopo mobile belt. Breccia pipes as well as associated hydrothermal copper deposits are aligned along a northeast trend which forms a southwestern projection of the Nuanetsi Igneous Province. The "L" and West lode breccia columns are known over a vertical extent of 1 250 metres, and are not exposed on surface; they are circular to polygonal in plan outline, as a result of joint controls. Peripheral microbreccia grades downwards and inwards to macrobreccia. Downward displacement of country rock marker zones indicate a maximum 1 – 3 per cent volume increase, while observed interfragmental fill constitutes 15 – 25 per cent. Hydrothermal alteration of wallrock gneiss mantles the breccia columns and does not extend to surface; it is comprised of distinct zones, the outermost characterized by sericitized gneiss, grading to an albitite, and finally to zoisite-quartz rock around the immediate brecciated contacts, and within the pipes. The interfragmental fill consists largely of quartz and sulphides, and grades downwards to albite + sulphides in the West Lode pipe. Sulphide mineral distribution is zoned, pyrite dominating in the apex of the pipes, grading downwards to chalcopyrite-bornite and then bornite-chalcocite at deeper levels. The nature of the fragmentation precludes normal stoping collapse mechanisms for brecciation and suggests a single implosive event. Chemical reaction induced shrinkage of fragments probably created the bulk of the interfragmental volume.     

Rapakivi texture from the O’Leary Porphyry,Arizona (U.S.A.)

The rhyodactic O’Leary Porphyry which forms the Pleistocene (0.233±0.37 m.y.) volcanic domes of O’Leary Peak and Darton Dome in the San Francisco Volcanic Field (northern Arizona, U.S.A.) contains sanidine phenocrysts with oligoclase mantles (rapakivi texture). Rapakivi texture occurs worldwide in silicic rocks of many ages and has been attributed to various igneous and metamorphic processes. The O’Leary Porphyry contains both mantled and unmantled sanidine (both are Or_63–69 Ab_30–36An₁), oligoclase and quartz phenocrysts, labradorite (An₅₃Ab₄₅Or₂) and kaersutite xenocrysts and andesite xenoliths. The compositional range of oligoclase is the same (An_11–26Ab_70–80Or_r–10) for the rapakivi mantles, the oligoclase phenocrysts, and the oligoclase crystals poikilitic within sanidines. Most mantles are discontinuous. The sanidine appears to have been resorbed prior to mantling. Experimental melting studies on the O’Leary Prophyry show that, for a 15 wgt.% water system, plagioclase crystallized prior to sanidine and quartz crystallized last. The O’Leary Porphyry, although inhomogeneous, plots on a Q-Or-Ab-An diagram well within the plagioclase stability field. Poikilitic plagioclases within sanidines further support crystallization of plagioclase prior to sanidine in the O’Leary Porphyry. Exsolution of a ternary feldspar to form a plagioclase mantle is the most commonly accepted igneous theory of rapakivi texture formation but has been eliminated as the origin of the O’Leary Porphyry rapakivi. Petrologic models by Tuttle and Bowen and by Stewart are rejected for the O’Leary rapakivi because of inconsistencies with the O’Leary occurrences. Two theories are viable for the O’Leary rapakivi texture. First, is a decrease in water vapor pressure which would enlarge the plagioclase stability field possibility causing mantling of metastable sanidines. The second and preferred theory is that of an addition of sodium and calcium by basification (chemical assimilation without melting) of the xenoliths within the O’Leary Porphyry. This would move the bulk composition of the melt into the plagioclase field possibly resulting in crystallization of plagioclase on sanidine crystals. Diffusion of sodium and calcium from the xenoliths to sanidine would result in mantling only those crystals near to the xenoliths. Later, convection would result in distribution throughout the melt of rapakivi, unmantled sanidines, and xenolithic kaersutite as is seen in the porphyry. Basic xenoliths are extremely common in rapakivi-bearing rocks. Those within the O’Leary Porphyry are andesitic and show resorption, and in some areas of O’Leary Peak itself, have been drawn out into schlieren.     

Late Holocene environmental changes inferred from diatoms in a lake on the western Taimyr Peninsula, northern Russia

Changes in the diatom assemblages preserved in a sediment core taken from a small lake located north of arctic treeline on the western Taimyr Peninsula, Russia, were examined in order to investigate late Holocene (i.e., ca 5000 cal yr BP to present) climatic and environmental changes within the region. Early diatom assemblages were dominated by benthic Fragilaria taxa and indicate a transitional phase in the lake history, most likely reflecting lake development and environmental change associated with treeline retreat to the south of the study site. Concurrent with pollen and macrofossil evidence of a vegetation shift to shrub tundra in the catchment basin at ca 4200 cal yr BP, an increase in cold-water taxa, followed by little change in diatom assemblages until ca 2800 cal yr BP, suggests that conditions were relatively cool and stable at this time. The last 2000 years of the Middendorf Lake record have been marked by fluctuating limnological conditions, characterized by striking successional shifts between Fragilaria pinnata and Aulacoseira distans var. humilis. Recent conditions in Middendorf Lake indicate an increase in diatom taxa previously rare in the record, possibly associated with twentieth-century climatic warming. The Middendorf Lake record indicates that significant limnological change may occur in the absence of catchment vegetation shifts, suggesting late-Holocene decoupling of aquatic and terrestrial responses to climatic and hydrological change. Our study results represent one of the few paleoecological records currently available from northern Russia, and highlight the need for further development of calibration data sets from this region.     

Dynamics of ammonia-oxidizing archaea and bacteria in relation to nitrification along simulated dissolved oxygen gradient in sediment–water interface of the Jiulong river estuarine wetland,China

Dissolved oxygen (DO) is a very important factor controlling the nitrogen cycle in wetlands. However, it is still unclear to what extent the presence of ammonia-oxidizing archaea (AOA) and bacteria (AOB), and related nitrification, are influenced by DO in estuarine wetlands. The aims of this study were to determine changes of nitrification at the sediment–water interface, to examine the abundance and diversity of archaeal and bacterial ammonia oxidizers in estuarine sediments, and to identify the correlation between nitrification and ammonia-oxidizing microorganisms along a simulated dissolved oxygen gradient in a Chinese estuarine wetland. The results showed that the nitrification rate was positively correlated with the diversity and abundance of AOA but not AOB, and the abundance and diversity of AOA can explain 87 % of the total variance of the first axes in the redundancy analysis. This indicates that AOA were primarily involved in ammonia oxidation in this study. Additionally, AOB were much more influenced by DO than AOA inferred from the assessment of dominant species and principal coordinates analysis of AOA and AOB. Higher diversity and abundance of AOA occurred in the mangrove sediments, which explain the higher nitrification rates in the mangrove sediments compared to the bare mudflat sediments. Notably, the trend of nitrification rate in the bare mudflat sediments was different from that in the mangrove sediments, suggesting that the extent of nitrification as impacted by DO depends largely on the sediment biotic and nutrient properties, and its environmental conditions including DO levels.