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The New Consort Gold Mine in the Palaeo- to Mesoarchaean Barberton greenstone belt, South Africa is one of the oldest recognized
orogenic gold deposits on Earth. The gold mineralization is hosted by discrete mylonitic units that occur at, or close to,
the contact between the mafic and ultramafic volcanic rocks of the c. 3,280 Ma Onverwacht Group and the mainly metasedimentary
rocks of the overlying c. 3,260–3,230 Ma Fig Tree Group. This contact, locally referred to as the Consort Bar, formed during
ductile D1 imbrication of the metavolcanosedimentary sequence and predates the main stage of the gold mineralization. The imbricate
stack is situated in the immediate hanging wall of the basal granitoid–greenstone contact along the northern margin of the
greenstone belt. It is characterized by a condensed metamorphic profile in which the metamorphic grade increases from upper
greenschist facies conditions (510–530°C, 4 kbar) in rocks of the Fig Tree Group to upper amphibolite facies grades (600–700°C,
6–8 kbar) in the basal Onverwacht Group. Detailed structural and petrological investigations indicate that the Consort Bar
represents a major structural break, which is largely responsible for the telescoping of metamorphic isograds within the structural
sequence. Two stages of mineralization can be distinguished. Loellingite, pyrrhotite, and a calc–silicate alteration assemblage
characterize an early high-T mineralization event, which is restricted to upper amphibolite facies rocks of the Onverwacht
Group. This early mineralization may correlate with the local D1 deformation. The second and main stage of gold mineralization was associated with renewed ductile shearing during D2. The D2 deformation resulted in the reactivation of earlier structures, and the formation of a NNW trending, steeply dipping shear
zone system, the Shires Shear Zone, which separates two regional SE plunging D1 synclines. The mineralized shear zones are intruded by abundant syn-kinematic pegmatite dykes that have previously been dated
at c. 3040 Ma. Petrological and geothermobarometric data on ore and alteration assemblages indicate that the main stage of
gold mineralization, which affected a crustal profile of ca. 1.5 km, was characterized by increasing temperatures (c. 520
to 600°C) with increasing structural depth. Sulfide assemblages in the ore bodies change progressively with metamorphic grade,
ranging from arsenopyrite + pyrite + pyrrhotite in the structurally highest to arsenopyrite + pyrrhotite + chalcopyrite +
loellingite in the structurally deepest part of the mine. The main stage of gold mineralization was broadly syn-peak metamorphic
with respect to the Fig Tree Group, but postdates the peak of metamorphism in upper amphibolite facies rocks of the structurally
underlying Onverwacht Group. This indicates that the mineralization coincided with the juxtaposition of the two units. As
the footwall rocks were already on their retrograde path, metamorphic devolatilisation reactions within the greenstone sequence
can be ruled out as the source of the mineralizing fluids. 相似文献
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Otto Reithofer 《Mineralogy and Petrology》1954,4(1-4):90-100
Ohne ZusammenfassungMit 1 Textabbildung und 12 Diagrammen. 相似文献
127.
Most public decisions ultimately have economic content. Decisions that deal with externalities, such as pollution, carry costs as well as benefits for society. Actions that mitigate nutrient over-enrichment in the Gulf of Mexico would require actions throughout the Mississippi River Basin resulting in both direct and indirect economic impacts. This paper describes and explains the economic linkages and trade-offs involved in actions that could be cost effective and meet a public goal of reducing nitrogen over-enrichment within the Mississippi River Basin. The impacts of different approaches that reduce the major source of nitrogen flows to the Gulf, nonpoint pollution from agriculture, are simulated for both source reduction the major source of nitrogen flows to the Gulf, nonpoint pollution from agriculture, are simulated for both source reduction and interception of nitrogen. Lessons learned include the fact that any one approach by itself has increasing marginal cost. The approaches considered have modest direct and indirect costs when only required to mitigate 20% to 25% of the nitrogen losses. Simultaneous multiple approaches appear even more attractive to induce only moderate negative impact. The impacts of mitigation are not just confined to the Mississippi River Basin but spread beyond the basin and are themselves influenced by external factors such as commodity prices and import and export markets for agricultural commodities. Success in reducing excess nitrogen flows will depend on institutional factors as well as technical efficacy. Finally, the nature of soil system sinks and the resulting long lead time likely before results might be apparent present a special obstacle to enlisting cooperation, assessing efficacy, and designing adaptive behavior. 相似文献
128.
Otto EUGSTER Ernst POLNAU Emma SALERNO Dario TERRIBILINI 《Meteoritics & planetary science》2000,35(6):1177-1181
Abstract— We derived the cosmic‐ray and solar particle exposure history for the two lunar meteorites Elephant Moraine (EET) 96008 and Dar al Gani (DaG) 262 on the basis of the noble gas isotopic abundances including the radionuclide 81Kr. For EET 96008, we propose a model for the exposure to cosmic rays and solar particles in three stages on the Moon: an early stage ~500 Ma ago, lasting less than 9 Ma at a shallow shielding depth of 20 g/cm2, followed by a stage when the material was buried, without exposure, until it was exposed in a recent stage. This recent stage, at a shielding depth in a range of 200–600 g/cm2, lasted for ~26 Ma until ejection. This model is essentially the same as that previously found for lunar meteorite EET 87521; thus, pairing of the two Elephant Moraine lunar meteorites that were recovered on the same icefield in Antarctica is confirmed by our data. The cosmic‐ray‐produced isotopes, the trapped solar and lunar atmospheric noble gases, as well as the radionuclide 81Kr observed for the DaG 262 lunar meteorite are consistent with a one‐stage lunar exposure history. The average burial depth of the Dar al Gani material before ejection was within a range of 50–80 g/cm2. The exposure to cosmic rays at this depth lasted 500–1000 Ma. This long residence time for Dar al Gani at relatively shallow depth explains the high concentrations of implanted solar noble gases. 相似文献
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