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南非主要金矿集区研究现状及存在问题 总被引：1，自引：0，他引：1
南非是世界上金矿石资源量最为丰富的国家,其金矿床主要产于东北部地区,主要有三种类型:1赋存于兰德盆地砾岩型建造中的"兰德"砾岩型金矿床,2产在绿岩带中的绿岩带型金矿床,和3主要与白云岩有关的白云岩型金矿床,以前两种类型为主。"兰德"砾岩型金矿床通常品位高,开采规模最大,目前采矿的深度最大,是南非最重要的矿床类型,其矿床成因长期以来存有争议,主要有两种观点,分别是砂矿模式和热液模式,就目前资料来看笔者等认为其代表了两个不同的矿化阶段;绿岩带型金矿床则以巴伯顿地区为代表,矿床具有规模大、易开采等特征,因此在南非金矿床开采中占有重要位置,其矿床主要赋存于石英脉或剪切带中,矿床成因主要有三种,分别是火山成矿模式、转换滑脱构造模式和构造交叉模式。 相似文献
2.
Microbial remains in some earliest Earth rocks: Comparison with a potential modern analogue 总被引：1，自引：1，他引：0
Miryam Glikson Lawrence J. Duck Suzanne D. Golding Axel Hofmann Robert Bolhar Robyn Webb Justice C.F. Baiano Lindsay I. Sly 《Precambrian Research》2008,164(3-4):187-200
Carbonaceous matter (CM) from ca. 3.5 Ga hydrothermal black cherts of the Pilbara Craton of Western Australia and the Barberton Greenstone Belt of South Africa yielded transmission electron microscopy (TEM) images that are suggestive of microbial remains and possible remnants of microbial cell walls. These are compared to a potential modern analogue, the hyperthermophilic Methanocaldococcus jannaschii, derived from an active seafloor hydrothermal environment and cultured under similar conditions. A striking resemblance to the early Archaean forms was evident in wall structure and thermal degradation mode. Cell disintegration of the cultures occurred at 100 °C marking the limits of life. Complete disintegration, deformation and shrinkage occurred at 132 °C. A multidisciplinary approach to the characterisation of the CM was undertaken using organic petrology, TEM coupled with electron dispersive spectral analysis (EDS), high resolution TEM (HRTEM) to determine molecular ordering, and elemental and carbon isotope geochemistry. Reflectance measurements of the CM to determine thermal stress yielded a range of values corresponding to several populations, and pointing to different sources and processes. The δ13C values of Dresser Formation CM (−36.5 to −32.1‰) are negatively correlated with TOC (0.13–0.75%) and positively correlated with C/N ratio (134–569), which is interpreted to reflect the relative abundance of high Ro/oxidised/recycled CM and preferential loss of 12C and N during thermal maturation. TEM observations, inferred carbon isotopic heterogeneity and isotope fractionations of −27 to −32‰ are consistent with the activity of chemosynthetic microbes in a seafloor hydrothermal system where rapid silicification at relatively low temperature preserved the CM. 相似文献
3.
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
4.
The P–T evolution of amphibolite facies gneisses and associated supracrustal rocks exposed along the northern margin of the Paleo to MesoArchean Barberton greenstone belt, South Africa, has been reconstructed via detailed structural analysis combined with calculated K(Mn)FMASH pseudosections of aluminous felsic schists. The granitoid‐greenstone contact is characterized by a contact‐parallel high‐strain zone that separates the generally low‐grade, greenschist facies greenstone belt from mid‐crustal basement gneisses. The supracrustal rocks in the hangingwall of this contact are metamorphosed to upper greenschist facies conditions. Supracrustal rocks and granitoid gneisses in the footwall of this contact are metamorphosed to sillimanite grade conditions (600–700 °C and 5 ± 1 kbar), corresponding to elevated geothermal gradients of ～30–40 °C km?1. The most likely setting for these conditions was a mid‐ or lower crust that was invaded and advectively heated by syntectonic granitoids at c. 3230 Ma. Combined structural and petrological data indicate the burial of the rocks to mid‐crustal levels, followed by crustal exhumation related to the late‐ to post‐collisional extension of the granitoid‐greenstone terrane during one progressive deformation event. Exhumation and decompression commenced under amphibolite facies conditions, as indicated by the synkinematic growth of peak metamorphic minerals during extensional shearing. Derived P–T paths indicate near‐isothermal decompression to conditions of ～500–650 °C and 1–3 kbar, followed by near‐isobaric cooling to temperatures below ～500 °C. In metabasic rock types, this retrograde P–T evolution resulted in the formation of coronitic Ep‐Qtz and Act‐Qtz symplectites that are interpreted to have replaced peak metamorphic plagioclase and clinopyroxene. The last stages of exhumation are characterized by solid‐state doming of the footwall gneisses and strain localization in contact‐parallel greenschist‐facies mylonites that overprint the decompressed basement rocks. 相似文献
5.
Maarten de Wit Harald Furnes Scott MacLennan Moctar Doucouré Blair Schoene Ute Weckmann Uma Martinez Sam Bowring 《地学前缘(英文版)》2018,9(3):603-665
The Makhonjwa Mountains, traditionally referred to as the Barberton Greenstone Belt, retain an iconic Paleoarchean archive against which numerical models of early earth geodynamics can be tested. We present new geologic and structural maps, geochemical plots, geo- and thermo-chronology, and geophysical data from seven silicic, mafic to ultramafic complexes separated by major shear systems across the southern Makhonjwa Mountains. All reveal signs of modern oceanic back-arc crust and subductionrelated processes. We compare the rates of processes determined from this data and balance these against plate tectonic and plume related models. Robust rates of both horizontal and vertical tectonic processes derived from the Makhonjwa Mountain complexes are similar, well within an order of magnitude, to those encountered across modern oceanic and orogenic terrains flanking Western Pacific-like subduction zones. We conclude that plate tectonics and linked plate-boundary processes were well established by 3.2-3.6 Ga. Our work provides new constraints for modellers with rates of a 'basket' of processes against which to test Paleoarchean geodynamic models over a time period close to the length of the Phanerozoic. 相似文献
6.
Kenneth A. Eriksson 《Australian Journal of Earth Sciences》2013,60(3-4):473-482
Iron‐formation and associated chert and jasper are subordinate to, but commonly intercalated within the upper, dominantly terriginous clastic sequences in the early Archaean Barberton Mountain Land (South Africa) and Pilbara Block (Western Australia). The iron‐formation, jaspillite and chert are exclusively lutitic and occur in a variety of palaeoenvironmental settings, namely: (1) at the base of prograda‐tional submarine fan sequences; (2) within interchannel mudstones enclosing inner and mid‐fan channel deposits; (3) capping Bouma turbidite beds; (4) intercalated within outer‐fan to basin‐plain mudstones; (5) at the base of progradational offshore shelf‐beach sequences; and (6) in lacustrine deposits intercalated within thick, braided, alluvial sequences. These associations suggest that the iron‐formations and associated orthochemical sediments define diastems and that precipitation took place in any environments basinwards of or between events of terrigenous influx. Recognition of such diverse environments of deposition places constraints on existing models for the origin of iron‐formations. Insofar as being removed from terrigenous dilution at the time of accumulation, any of the iron‐formations, cherts or jaspilites are potential precious and base‐metal targets. 相似文献
7.
The two Palaeoarchaean volcano-sedimentary complexes of the Buck Ridge (Barberton Greenstone Belt, South Africa) and Kittys Gap (Coppin Gap Greenstone Belt, East Pilbara, Australia) have a similar geological setting and age (∼3.45 Ga). The predominantly volcaniclastic sediments are concentrated at the top of these complexes, and experienced thorough, (very) early diagenetic silicification. In many places the silicification process has led to excellent preservation of the primary sedimentary structures. Elsewhere it has resulted in their obliteration or replacement by diagenetic structures. The Buck Ridge chert forms a regressive-transgressive succession, deposited around base level, with lacustrine and littoral marine facies. Deposition of the Kittys Gap Chert was also close to base level, almost exclusively subaqueous, with tidal influence and a regressive sequential trend. 相似文献
8.
Valrie Chavagnac 《Lithos》2004,75(3-4):253-281
The Komati Formation of the Barberton greenstone belt (BGB), South Africa, is composed of both Al-undepleted and -depleted komatiites. The Al-undepleted komatiites are characterised by Al2O3/TiO2 and CaO/Al2O3 ratios of 15–18 and 1.1–1.5, respectively, and exhibit chondritic trace element contents and (Gd/Yb)N ratios. In contrast, the Al-depleted komatiites show significantly lower Al2O3/TiO2 ratios of 8–12, highly variable CaO/Al2O3 (0.19–2.81) ratios combined with (Gd/Yb)N ratios varying from 1.08 to 1.56. A Sm–Nd whole rock isochron for komatiites of the Komati Formation gives an age of 3657±170 Ma. 147Sm/144Nd ratios (0.1704 and 0.1964) are all lower than the chondritic value of 0.1967. The komatiite i,Nd(3.45) values cluster at +1.9±0.7.
Trace element distribution indicates that most of the primary geochemical and isotopic features of the komatiites were preserved in line with the conservation of the primary chemical composition of clinopyroxene. High field strength element and rare earth element abundances indicate that crustal contamination and post-crystallisation processes did not disturb the primary features of komatiites.
The Sm/Nd and Nb/U ratios of komatiites indicate that the Barberton greenstone belt mantle source has undergone melt extraction prior to komatiite formation. Variations of Al2O3/TiO2, (Gd/Yb)N, Zr/Sm and Sm/Nd ratios of komatiites indicate that a batch melting of slightly depleted mantle source during with garnet and/or clinopyroxene remained in the residue can produce the geochemical isotopic feature of the Barberton greenstone belt komatiites. Typical geochemical fingerprints of subduction-related processes (LILE enrichment, HFSE depletion compared to REE), as known from modern subduction zones, are not observed. Komatiites exhibit Ti/Zr, La/Nb, Nb/U, Sr/Nd and Ba/La ratios comparable to those of oceanic island basalt and mid-ocean ridge basalt. (La/Nb)PMN, (Sm/Yb)PMN, positive δNb values and flat or slightly enriched REE patterns suggest that BGB komatiites are part of an oceanic plateau rather than an oceanic island such as Iceland. Therefore, an oceanic plateau or mid-ocean ridge, in connection with an oceanic plateau, such as Ontong Java plateau or Caribbean–Colombian oceanic plateau, is a suitable tectonic setting for the formation of the BGB komatiites. 相似文献
9.
In the Kaapvaal craton of southern Africa, as well as other Archaean cratons worldwide, the progression from dominant tonalite-trondhjemite-granodiorite(TTG) to granite-monzogranite-syenogranite(GMS)rock types is interpreted to reflect progressive reworking and differentiation of the continental crust.Here we re-evaluate the early Archaean evolution of the Kaapvaal craton and propose a unified view of the plutonic and volcanic records based on elemental and isotopic(Nd, Hf) data and zircon U-Pb ages.We also report new whole-rock major and trace element analyses, zircon U-Pb ages and Hf-in-zircon analyses of igneous clasts from a conglomerate of the 3.2 Ga Moodies Group of the Barberton Greenstone Belt. Many of these clasts are derived from shallow intrusive rocks of granitic composition, which are scarcely represented in outcrop. Despite alteration, the volcanic rocks can be classified based on their trace element contents into two main groups by comparison with plutonic rocks. One group has characteristics resembling TTGs: relatively low and fractionated rare earth element concentrations with no Eu anomaly and relatively low concentrations of high field strength elements(Nb mostly ≤12 ppm). The second group has GMS-like characteristics: less fractionated REE, marked negative Eu anomalies and HFSE-increasing trends with progressing fractionation(Nb ≤ 50 ppm or more, Th up to 30-40 ppm). In addition, igneous clasts of Moodies Group conglomerate have chemical, mineralogical and isotopic characteristics that link them to GMS. New analyses of some of these clasts indicate elevated high field strength elements(Nb up to 20 ppm) and_(εHf)(t)of zircon down to -3.5. These rocks imply the presence of an already differentiated felsic crust at 3.5 Ga, which has Nd and Hf model ages indicating mantle extraction ages extending back to the Eoarchaean. The combined record of plutonic and volcanic rocks of the Kaapvaal craton provides a more complex scenario than previously suggested and indicates that TTG and GMS-like felsic magmas were emplaced broadly coevally in multiple pulses between ~3.5 Ga and 3.2 Ga. 相似文献
10.
S.W. Parman J.C. Dann T.L. Grove M.J. de Wit 《Earth and Planetary Science Letters》1997,150(3-4):303-323
This paper provides new constraints on the crystallization conditions of the 3.49 Ga Barberton komatiites. The compositional evidence from igneous pyroxene in the olivine spinifex komatiite units indicates that the magma contained significant quantities of dissolved H2O. Estimates are made from comparisons of the compositions of pyroxene preserved in Barberton komatiites with pyroxene produced in laboratory experiments at 0.1 MPa (1 bar) under anhydrous conditions and at 100 and 200 MPa (1 and 2 kbar) under H2O-saturated conditions on an analog Barberton composition. Pyroxene thermobarometry on high-Ca clinopyroxene compositions from ten samples requires a range of minimum magmatic water contents of 6 wt.% or greater at the time of pyroxene crystallization and minimum emplacement pressures of 190 MPa (6 km depth). Since high-Ca pyroxene appears after 30% crystallization of olivine and spinel, the liquidus H2O contents could be 4 to 6 wt.% H2O. The liquidus temperature of the Barberton komatiite composition studied is between 1370 and 1400°C at 200 MPa under H2O-saturated conditions. When compared to the temperature-depth regime of modern melt generation environments, the komatiite mantle source temperatures are 200°C higher than the hydrous mantle melting temperatures inferred in modern subduction zone environments and 100°C higher than mean mantle melting temperatures estimated at mid-ocean ridges. When compared to previous estimates of komatiite liquidus temperatures, melting under hydrous conditions occurs at temperatures that are 250°C lower than previous estimates for anhydrous komatiite. Mantle melting by near-fractional, adiabatic decompression takes place in a melting column that spans 38 km depth range under hydrous conditions. This depth interval for melting is only slightly greater than that observed in modern mid-ocean ridge environments. In contrast, anhydrous fractional melting models of komatiite occur over a larger depth range ( 130 km) and place the base of the melting column into the transition zone. 相似文献
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