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1.
The Lega Dembi deposit is the largest gold producer in Ethiopia. It is situated in late-Precambrian metamorphosed sediments
of the N-S trending, volcano-sedimentary Megado belt, which forms part of the late-Proterozoic Adola granite-greenstone terrane
in southern Ethiopia. The lode-gold mineralization occurs in a N-S trending, steep westerly dipping quartz-vein system that
follows the structural contact between underlying feldspathic gneisses and the volcanosedimentary sequence of the Megado belt.
This contact also marks the northernmost extension of the regional-scale, sinistral strike-slip Lega Dembi-Aflata shear zone.
Mineralization and intense quartz-veining is best developed in graphite-rich sediments within an area not more than 80 m away
from this tectonic contact. Hydrothermal wall-rock alteration includes actinolite/tremolite-biotite-calcite-sericite and chlorite-calcite-epidote
assemblages. Gold occurs preferentially in the sericite alteration zone, where it is closely associated and intergrown with
galena. The variable deformation of the gold-quartz veins suggests a syn-kinematic timing for the gold mineralization during
transcurrent shearing in a dilational segment of the shear zone. In addition to the structural control, lithological control
on gold deposition is indicated by the almost exclusive occurrence of the gold mineralization in graphite-rich metasediments.
This close relationship suggests that gold precipitation was the result of chemical reduction of regional ore-bearing fluids.
Temperature conditions of mineralization are constrained by the actinolite-biotite alteration assemblage and by arsenopyrite
chemistry, which indicate that ore deposition occurred at or close to peak metamorphic conditions at upper-greenschist to
lower-amphibolite metamorphic grades. Rb-Sr dating of sericite indicates an age of about 545 Ma. for hydrothermal alteration
and, thus, for gold mineralization. The style of gold mineralization, structural pattern and lithological assemblages at Lega
Dembi are very similar to lode-gold deposits most commonly reported from Archaean granite-greenstone terranes. These similarities
may open new perspectives for the exploration of lode-gold deposits, which has previously primarily focused on Archaean greenstone
belts rather than Proterozoic or even Phanerozoic meta-volcanosedimentary belts.
Received: 26 July 1996 / Accepted: 8 January 1997 相似文献
2.
Archaean gold mineralization in central eastern Brazil: a review 总被引：1，自引：0，他引：1
A. Bernasconi 《Mineralium Deposita》1985,20(4):277-283
3.
R Srinivasan K Naha Y J Bhaskar Rao A B Vrevsky S I Rybakov A I Golubev M Efimov 《Journal of Earth System Science》1993,102(4):567-585
The middle to late Archaean rocks of Kola and Karelia in the eastern Baltic shield consist of the Infracomplex overlain by the Saamian complex, and the Lopian greenstone belts. The Infracomplex which forms the basement is a polymigmatite, parts of which are at least 3100 Ma old. The Saamian in the central Belomorian region comprises granite gneiss, amphibolite, garnet-kyanite gneiss and high alumina gneisses which belong to the Keret, Hetolombina and Chupa suites. The Lopian greenstone belts ranging in age from 3000 to 2700 Ma are composed of peridotitic, pyroxenitic and basaltic komatiites, tholeiitic basalts, andesites, dacites and rhyolites, together with tuffs, graywackes and iron formations. Whereas there is a dominance of volcanic over sedimentary rocks in the greenstone belts of the Baltic shield, a significant proportion of detrital and chemogenic sedimentary rocks characterizes the Dharwar succession of approximately the same time span in the southern Indian shield. Association of mature and immature detrital sedimentary rocks with bimodal volcanic assemblages points to a back-arc setting for the Dharwar belts. This contrasts with the association of immature sediments with calc-alkaline volcanic rocks in the greenstone belts of the eastern Baltic shield, suggesting an island arc environment there. 相似文献
4.
Gold deposits occur in greenstone belts world wide, and contribute to anomalously high gold production from Archaean terranes. As in other cratons, Archaean gold mineralization of Western Australia represents a complex array of deposit styles. Despite this, most deposits are clearly epigenetic, and large deposits have a number of features in common, including their strong structural controls, distinctive wallrock alteration (Fe-sulphide, K-mica±albite, Ca---Mg---Fe carbonates), consistent metal associations (Au---Ag---As---Sb---W---B; low base metals), commonly Fe-rich host rocks, great depth extension and lack of appreciable vertical zonation. These shared characteristics, combined with their ubiquitous occurrence, indicate that Archaean gold deposits had a common origin related to the tectonic evolution of greenstone belts.Auriferous hydrothermal systems were broadly synchronous with regional metamorphism and emplacement of synkinematic granitoids and felsic (porphyry) intrusions. Although these gold systems involved low-salinity, lowdensity, reduced, near-neutral H2O---CO2 fluids carrying gold as reduced sulphur complexes, the origin of the fluids is equivocal. Most timing evidence and stable isotope data cannot distinguish metamorphic from magmatic (granitoid or felsic porphyry) orggins, but the lack of consistent spatial relationships between specific, volumetrically significant intrusive phases and large gold deposits in a number of cratons strongly favours metamorphic derivation of fluids.The metamorphic-replacement model for gold mineralization involves devolatilization of the lower portions of the greenstone pile, with high geothermal gradients inhibiting significant melting. CO2 possibly formed by the decarbonation of early alteration, related to mantle degassing along crustal-scale, synbasinal fault zones. Auriferous fluids were channelled along greenstone-scale faults, in part developed during reactivation of crustal-scale faults in a strike-slip regime. Gold deposition occurred largely under greenschist facies conditions (about 300–400°C, 1–2 kb) in response to decreasing gold solubility with declining temperature. However, a major control on gold deposition was fluid/wallrock interaction. Many large deposits formed by sulphidation of Fe-rich host rocks, with synchronous deposition of Fe-sulphides and gold. However, the variable nature of gold-depositing reactions, including lowering of fO2 and pH, allowed a multitude of small, and some large, deposits to form wherever that fluid circulation occurred. In consequence, several of the relatively small deposits currently worked from open pit are hosted by ultramafic and felsic rocks. There are few constraints on the source of components (Au, S, K, CO2) added to gold deposits, but even giant deposits such as the Golden Mile, Kalgoorlie could have formed from a realistic greenstone source volume (ca. 8×8×5 km). Convective circulation of fluids could have contributed to the generation of high fluid-rock ratios.On the regional scale, the markedly heterogeneous distribution of large gold deposits, gold productivity and host rocks to deposits can be accommodated by the metamorphic-replacement model. The most favourable conditions for development of auriferous hydrothermal systems operated in younger (ca. 2.7±0.1 Ga) rift-phase greenstones where greatest extension and crustal thinning produced high geothermal gradients, crustal-scale synbasinal faults, and rapid extrusion and burial of volcanics, including abundant komatiites. Iron-rich tholeiitic basalts and dolerites were preferred host rocks for large gold deposits. The least favourable conditions existed in older (ca. 3.5-3.4 Ga) platformphase greenstones, where gentle sagging on submerged continental crust produced eruption of mainly mafic volcanics with few komatiites, commonly in very shallow-water environments. This allowed intense synvolcanic alteration of both gold source rocks and potential host rocks. The generally smaller gold deposits formed mainly in ultramafic or greywacke hosts. Younger (ca. 3.0 Ga) platform-phase greenstones appear intermediate in nature but, unlike other greenstones, have significant epigenetic gold deposits in originally oxide-facies BIF, which were deposited on relatively deep-water platforms. Similar controls appear to exist on a world scale, with gold mineralization peaking at ca. 2.7±0.1 Ga in response to development of major rift zones in thickened, relatively mature continental crust. Interestingly, the giant Witwatersrand goldfield formed at about the same time. 相似文献
5.
A. A. Kalinin O. V. Kazanov N. M. Kudryashov G. F. Bakaev S. V. Petrov D. V. Elizarov L. V. Lyalina 《Geology of Ore Deposits》2017,59(6):453-481
Data on gold ore objects in the Strelna Greenstone Belt in the southeastern Kola Peninsula are presented in the paper. The studied Vorgovy and Sergozero ore occurrences are localized in the zone of tectonic contact of the Neoarchean complexes making up the greenstone belt and the volcanic–sedimentary sequences of the Paleoproterozoic Imandra–Varzuga Zone. The Vorgovy gold occurrence is related to stockwork of carbonate–quartz veins and veinlets hosted in a biotite gneiss transformed into chlorite–sericite–quartz metasomatic rock with pyrrhotite–arsenopyrite dissemination. The Sergozero occurrence is localized in amphibolites corresponding to komatiitic and tholeiitic basalts hosted in biotite gneiss (metapelite). Mineralization is confined to the zone of tectonized contact between komatiitic and tholeiitic basalts, where it is controlled by a strip of metasomatic biotite–calcite rock with gersdorffite–arsenopyrite dissemination. The native gold grains medium to high in fineness are up to 0.1 mm in size and mainly localized at the contact of arsenopyrite and gersdorffite with gangue minerals. Gold mineralization is of superimposed character, and, as indicated by isotopic geochronology, was formed at the retrograde stage of the Svecofennian regional metamorphism. Comparison of ore occurrences localized in the Strelna Greenstone Belt with gold deposits in greenstone belts of the western Fennoscandian Shield and the Superior Province in Canada allows us to suggest a high perspective of the entire Strelna Belt for gold. 相似文献
6.
The Rio Itapicuru greenstone terrain of north-central Bahia State consists of belts of supracrustal rocks surrounding granitic plutons and domes. The basal supracrustal rocks are predominantly massive metabasalts with minor amounts of intercalated chemical sedimentary rocks and mafic tuffs. They are overlain by a middle unit of intermediate to acid pyroclastic rocks, lavas, and volcaniclastic sediments, and an upper unit of greywackes, sandstones and conglomerates.A geochemical study of major and trace elements of the volcanic rocks indicates the existence of a chemical discontinuity between the basaltic and the acid to intermediate members. The basalts are typical tholeiites with Ti, Zr, Sr, Y and Nb contents analogous to those of modern ocean-floor tholeiites or, alternatively, low-K tholeiites of primitive island arcs. In contrast, compositional variations of the hornblende-bearing andesites and dacites fall along indisputably calc-alkaline trends of low FeO and TiO2 contents which decrease with increasing differentiation. The lithostratigraphic and chemical variations within lavas of the Rio Itapicuru greenstone are comparable to those described from the Western Australian greenstone belts. Only in greenstone belts of the Canadian type do thick calc-alkaline sequences containing abudant basaltic andesites overlie conformably and transitionally the underlying tholeiitic basalts. Elsewhere the calc-alkaline sequences, if present, do not contain basaltic andesites and are chemically unrelated to the underlying basalts. 相似文献
7.
8.
T. C. Devaraju R. P. Viljoen R. H. Sawkar T. L. Sudhakara 《Journal of the Geological Society of India》2009,73(1):73-100
Evidence of mafic and ultramafic magmatism exists in many parts of the Dharwar craton which is divided into two blocks, the
West Dharwar Craton (WDC) and the East Dharwar Craton (EDC). The mafic-ultramafic rocks occur in supracrustal/greenstone belts
and in numerous enclaves and slivers in the WDC. The oldest recorded maficultramafic rocks, which are mainly komatiitic in
nature, are preserved in the Sargur Group which is more than 3.3–3.4 Ga old, the youngest being manifested by 63–76 Ma old
mafic dyke magmatism, possibly related to Deccan volcanism.
In the Sargur Group, ultramafics rocks greatly dominate over mafic lithological units. Both extrusive and intrusive varieties,
the latter in the form of differentiated layered complexes, occur. Mafic volcanics exists in all the greenstone belts of the
eastern block and in the Bababudan and Western Ghats belts of the western block. In addition to the Sargur Group where stratigraphic
sequences are unclear, mafic magmatism is recorded in three different formations of the Bababudan Group and two sub-divisions
of the Shimoga and Chitradurga Groups where basaltic flows are conspicuous. In the well studied greenstone belts of Kolar
and Hutti in the EDC, three to four different Formations of mafic volcanic rocks have been mapped. Isotopic dating has indicated
that while mafic magmatism in the greenstone belts of the EDC covers only a short time span of between 2.65 to 2.75 Ga, those
in the Dharwar Supergroup of the WDC cover a much longer time span from 3.35 to 2.5 Ga.
Mafic dyke magmatism has taken place repeatedly from 2.45 Ga to about 1.0 Ga, but, the peak of emplacement was between 1.8
and 1.4 Ga when the densely developed swarms on the western and south western portions of the Cuddapah Basin and in the central
part of Karnataka, were intruded. Emplacement of potassic ultramafic magma in the form of kimberlite-lamproite which is confined
to the EDC, is a later magmatic event that took place between 1.4 Ga and 0.8 Ga.
From a mineralization perspective, mafic magmatism of the supracrustal groups of the WDC and the greenstone belts of the EDC
are the most important. V-Ti-magnetite bands constitute the most common deposit type recorded in the mafic-ultramafic complexes of the Sargur Group with commercially
exploitable chromite deposits occurring in a number of belts. PGE mineralization of possible commercial value has so far been recorded in a single mafic-ultramafic complex, while copper-nickel mineralization occurs at certain localities in the Sargur and Chitradurga Groups. Gold mineralization hosted by mafic (occasionally ultramafic) rocks has been noted in many of the old workings located in supracrustal
groups of rocks in the WDC and in the greenstone belts of EDC. Economically exploitable mineralization, however, occurs mainly
in the greenstone belts of the Kolar, Ramagiri-Penkacherla and Hutti-Maski and along the eastern margin of the Chitradurga
belt, where it is associated with a major N-S striking thrust zone separating the WDC from the EDC. Gold deposits of the eastern
greenstone belts are comparable to those of the younger greenstone belts of Canada, Zimbabwe and Australia where the mineralization
is associated with quartz carbonate veins often in iron-rich metabasic rocks. The gold was emplaced as hydrothermal fluids,
derived from early komatiitic and tholeiitic magmas, and injected into suitable dilatent structures.
The other common type of mineralization associated with the ultramafic rocks of the Sargur Group and supracrustal belts, particularly
of the WDC, are asbestos and soapstone, related to autometamorphism/metasomatism. Ruby/sapphire deposits occur in places at the contacts of ultramafic rocks with the Peninsular Gneiss, and are related to contact metamorphism
and metasomatism. Mineable magnesite deposits related to low-temperature hydrothermal/lateritic alteration exist in the zone of weathering, particularly in the
more olivine-rich rocks. Recent spurt in diamond exploration is offering promise of discovering economically workable diamondiferous
kimberlite/lamproite intrusions in the EDC. 相似文献
9.
津巴布韦太古宙花岗-绿岩型金矿床受构造的控制,金矿可分为褶皱控矿型、剪切带控矿型、层控型、深成花岗岩控矿型和构造-蚀变岩型等5类。津巴布韦花岗-绿岩型金矿多产于花岗质片麻岩与绿岩带的接触带上,金以自然金或金的硫化物形式产出。金矿成矿时代大致分为(2660±50)Ma和(2410±70)Ma(米德兰德绿岩带北部白钨矿Sm-Nd测年)2个时段,分别与新太古代TTG事件和大岩墙侵入相关。津巴布韦花岗-绿岩型金矿较多,但发现的中-大型绿岩型金矿较少,具有较好的金矿找矿前景。 相似文献
10.
Thomas Skulski Andrew Hynes Don Francis 《Contributions to Mineralogy and Petrology》1988,100(2):236-245
Despite the fact that some greenstone belts preserve the record of contemporaneous komatiitic and tholeiitic volcanism, a genetic link between the two is not widely accepted. The significance of a compositional gap seperating these magma types and differences in their respective degree of light rare earth element (LREE) enrichment, cited as evidence against a derivative relationship, are complicated by the possibility of crustal assimilation by magmas of komatiitic affinity. In the Archean La Grande Greenstone belt of northern Quebec a succession of metamorphosed tholeiitic basalts and younger, high-Mg, LREE-enriched andesites are preserved. The tholeiites are differentiated basaltic rocks whose chemical compositions appear to have been controlled by low pressure, gabbroic fractional crystallization and are similar to Type 1 MORB. Parental magmas were probably high-Mg liquids of compositions similar to komatiitic basalts which also occur in the greenstone belt. These high-Mg liquids are believed to be themselves the product of high pressure, OLIV+OPX fractional crystallization of more magnesian primary liquids of komatiitic composition. The higher La/Sm ratios of komatiitic basalts and tholeiites relative to komatiites in this belt, can be explained by small degrees of crustal assimilation. In the central part of the belt, late-stage, mafic igneous rocks have chemical compositions similar to Archean examples of contaminated volcanic rocks (e.g., Kambalda, Australia). These late-stage lavas consist of basalts and andesites with high-Mg, Ni and Cr abundances, LREE-enriched profiles and low Ti abundances. They are believed to be the products of crustal assimilation and crystallization of OPX-PLAG-CPX from high-Mg liquids of komatiitic affinity. The volcanic stratigraphy records the progressive effects of crustal contamination through time. A light sialic crust may have initially acted as a density barrier, preventing the eruption of primary high-Mg liquids and forcing fractionation at depth which produced more buoyant compositions. With subsequent thinning of the crust, the density barrier presumably failed, and primary liquids migrated directly toward the surface. Reaction of these liquids with tonalitic crust produced contaminated differentiates. 相似文献
11.
A major gold province of the world exists in the Proterozoic Birimian and Tarkwaian supracrustal rocks of West Africa. The bulk of the gold comes from the primary lode occurrences of the Birimian rocks of Ghana (formerly The Gold Coast). Birimian lithofacies is characterised by subaqueous fine-grained sediments with bimodal volcanic material. Metasedimentary rocks include phyllites and metawackes. Metavolcanic rocks are predominantly tholeiitic basalts. Komatiites and banded iron formations (BIF) are absent.Gold is in 5 parallel, evenly spaced, more than 300 km long, northeast-trending volcanic belts separated by basins containing pyroclastic and meta-sedimentary units. The most prominent is the Ashanti volcanic “greenstone” belt, which hosts the Ashanti Goldfields Corporation mines at Obuasi (more than 800,000 kg Au since 1896), the Billiton Bogosu Gold mine at Bogosu, and the State Gold Mining Corporation mines at Prestea, Bibiani and Konongo.Gold, ranging from 2 to 30 ppm, is in quartz veins of laterally extensive major orebodies which deeply penetrate fissures and shear zones at contacts between metasedimentary and metavolcanic rocks. The veins consists mainly of quartz with carbonate minerals, green sericite, carbonaceous partings and metallic sulfides and arsenides of Fe, As, Zn, Au, Cu, Sb, and Pb. Gold occurs in carbonate fillings in fractured quartz veins. Country rocks, which contain rutile, anatase and granular masses of leucoxene, along ore channels, have been hydrothermally altered to carbonates, sericite, silica and sulfide minerals. Fluid inclusion evidences suggest that mineral deposition took place at about 350°C and 140 bar from dilute aqueous solutions. Timing deduced from ore textures, however, show complex multi-stage mineralization events, with higher temperature minerals commonly having formed later than lower temperature ones. Geochemical studies of materials produced by tropical processes, especially soils, are essential in prospecting poorly exposed terranes of west Africa. Trace and major element distributions at mines and mineral occurrences can indicate mineralization otherwise difficult to detect.This paper highlights the features of the Ghanaian gold deposits that may aid the current search for new deposits along the gold belts. Exploration based on geochemistry is highly important, but should be integrated with data from accompanying geological, lithologic, mineralogical, and structural studies. 相似文献
12.
Many Archaean mesothermal gold deposits are spatially associated with felsic to lamprophyric minor intrusions and it has been suggested that magmatic processes related to such intrusions may be important in the genesis of these deposits. A comparison of the Pb-isotopic signature of gold-related galenas from Kambalda and Norseman with that of spatially associated minor intrusions (at the time of mineralization) indicates that the ore-fluid Pb cannot have been derived solely from the intrusions or their source regions. For both study areas, the galena Pb-isotopic compositions are bracketed by those of local volcanic (mafic) and intrusive (largely felsic) rock types. This is consistent with the ore fluid having derived metallic components from the crust (or crustally derived granitic rocks) and the mantle (or mantle-derived rocks of the greenstone succession) via metamorphic dewatering or mantle/crustal degassing. Interaction of granite-derived magmatic fluids with greenstone lithologies could plausibly produce a similar array of Pb-isotopic signatures. The Norseman data, as a whole, are more radiogenic than the Kambalda data for broadly synchronous mineralization, reflecting the greater abundance of older granitic rocks with respect to mafic/ultramafic rocks in the Norseman district. The provinciality exhibited by the Pb-isotopic composition of the ore fluid indicates that the gold-mineralizing process formed galena whose Pb-isotopic composition was very sensitive to local variations in crustal Pb-isotopic composition, either within the source region of the fluid or along fluid conduits. 相似文献
13.
Mineralization Ages of the Jiapigou Gold Deposits,Jilin 总被引：1，自引：0，他引：1
Li Junjian Shen Baofeng Mao Debao Li Shuangbao Zhou Huifang Cheng Yuming Tianjin Institute of Geology Mineral Resources Chinese Academy of Geological Sciences Tianjin Nonferrous Metal Geological Exploration Bureau Changchun Jilin 《《地质学报》英文版》1997,71(2):180-188
The Jiapigou gold deposits are typical vein type deposits associated withArchaean greenstone belts in China. According to the crosscutting relationships between dykesand auriferous veins, single hydrothermal zircon U-Pb dating and quartz K-Ar,~(40)Ar-~(39)Ar andRb-Sr datings, the main mineralization stage of the Jiapigou deposit has been determined to be2469-2475 Ma, while mineralization superimposition on the gold deposit occurred in1800-2000 Ma and 130-272 Ma. They form a mineralization framework of one oldermetallogenic epoch (Late Archaean-Early Proterozoic) and one younger metallogenic epoch(Mesozoic) of gold deposits in Archaean greenstone belts in China. 相似文献
14.
Field studies in the Eucalyptus area, northeastern Yilgarn Block have shown intrusive and extrusive rocks in an Archaean greenstone sequence to be comagma‐tic, and have suggested the sequence of subsequent granitoid intrusion and gold mineralisation. Andesitic volcanic rocks and related subvolcanic granodiorite porphyry and epiclastic sediments were followed by tholeiitic basalt with gabbro/dolerite sills and dykes, which were in turn succeeded by high‐Mg basalt with associated peridotite intrusions. Large, irregular gabbro and peridotite intrusions, which are inferred to represent subvolcanic magma chambers, occur in lower stratigraphic levels, whereas comformable subvolcanic sills occur in higher stratigraphic levels. Granodiorite plutons were followed by adamellite plutons; at least some gold mineralisation was contemporaneous with granitoid emplacement. 相似文献
15.
Gravity anomaly patterns in the south-central Zimbabwe Archaean craton and their geological interpretation 总被引：1，自引：0，他引：1
The granite-greenstone terrain of south-central Zimbabwe, encompassing the Belingwe (Mberengwa) greenstone belt and sections of the Great Dyke, provides important constraints on models for the evolution of the Zimbabwe craton and the Archaean crust in general. In this paper we enhance and model existing and recently acquired gravity data from the region and correlate the anomalies and their derivatives with the known basement geology to evaluate models for greenstone belt development. We also study the spatial gneiss-granite-greenstone association in general, and the geologic implications of models of the anomaly patterns in particular. Although the Belingwe greenstone belt has been mapped, its subsurface geometry is poorly known. Similarly, the Great Dyke is well studied, but no systematic study of the extent and cross-cutting relations of other mafic dykes in the Archaean crust has been undertaken.The regional gravity field shows no evidence for crustal thickness variations in the area and the gravity anomalies can be explained by lateral density variations of the supracrustal rocks. Prominent gravity highs are observed over the high density ( 3000 kg/m3) volcano-sedimentary piles (greenstone belts) and ultramafic complexes. Well-defined elongate, sub-oval/elliptical gravity lows are associated with intrusive granitic plutons. The granite-greenstone contacts are marked by steep gravity gradients of up to 5 mGal/km that imply steeply dipping or near-vertical contacts for the anomalous bodies. This is tested and confirmed by 21/2D modelling of gravity profiles across the Belingwe and Fort Rixon greenstone belts, constrained by measured densities and observed geological data. The modelling also indicates that these belts, and possibly all the belts in the study area (based on comparable densities and anomaly amplitudes), have limited depth extents in the range of 3–5 km. This is comparable to thicknesses obtained elsewhere from deep seismic reflection data and geoelectrical studies, but mapped stratigraphic thicknesses give a maximum depth extent of about 9.5 km. Present studies and previous work support the idea that the volcanics were extruded within rift zones and laid on older granitic crust, followed by subsidence and rapid deposition of sediments that were sourced from the adjacent basement terrains. The volcano-sedimentary sequences were subsequently deformed by intruding younger plutons and affected by late-stage strike-slip activity producing cross-cutting structures. 相似文献
16.
L.M. Lobato J.O.S. Santos N.J. McNaughton I.R. Fletcher C.M. Noce 《Ore Geology Reviews》2007,32(3-4):674
U–Pb SHRIMP results of 2672 ± 14 Ma obtained on hydrothermal monazite crystals, from ore samples of the giant Morro Velho and Cuiabá Archean orogenic deposits, represent the first reliable and precise age of gold mineralization associated with the Rio das Velhas greenstone belt evolution, in the Quadrilátero Ferrífero, Brazil. In the basal Nova Lima Group, of the Rio das Velhas greenstone belt, felsic volcanic and volcaniclastic rocks have been dated between 2792 ± 11 and 2751 ± 9 Ma, coeval with the intrusion of syn-tectonic tonalite and granodiorite plutons, and also with the metamorphic overprint of older tonalite–trondhjemite–granodiorite crust. Since cratonization and stable-shelf sedimentation followed intrusion of Neoarchean granites at 2612 + 3/− 2 Ma, it is clear that like other granite–greenstone terranes in the world, gold mineralization is constrained to the latest stages of greenstone evolution. 相似文献
17.
After a century of virtual neglect, exploration in the Yandal greenstone belt of the Yilgarn Craton of Western Australia
has yielded resources of 12 Moz Au during the 1990s. Success has come from a combination of conceptual geological models,
surface prospecting, understanding the weathering environment, and systematic drilling. The Archaean Yandal greenstone belt
comprises a lowermost banded iron formation, extensive basalt and dolerite sills, ultramafic rocks, intermediate to felsic
volcanic rocks, and variable clastic sedimentary rocks. Early shear zones trend NNW and form the greenstone belt margins,
or trend N–S within the belt. Later brittle cross-faults are critical in gold localization. Gold resources and past production
at major deposits include Bronzewing (4 Moz Au), Jundee (5 Moz) Mt.␣McClure (1 Moz) and Darlot (3␣Moz, some of which was produced
before 1990). All major deposits are hosted by Fe-rich mafic rocks, and mineralization displays a combination of different
orientations and morphologies. Quartz veins are surrounded by broad carbonate alteration with proximal K-mica and Fe-sulphides.
The recognition of a critical role for the late brittle structures in localizing gold implicates mid-crustal processes within
the greenstone belt for fluid generation, and with the host rock control, supports the model in which fluid was derived by
metamorphic devolatilization.
Received: 19 September 1997 / Accepted: 7 January 1998 相似文献
18.
The Mangalur greenstone belt of Dharwar Craton, South India, is an Archaean schist belt dominated by metavolcanic rocks. The
gold mineralization occurs within the metavolcanics and the fabric, mineralogy and geochemistry of these host rocks indicate
that they were tholeiitic basalts regionally metamorphosed under medium to low-grade greenschist facies. The basic metavolcanic
rocks occur as tholeiitic metabasalts and amphibolites. The rocks have undergone some fractionation and appear to be derived
from melts generated by 10 to 25% melting of the mantle at depths 30 to 35 km around temperature 1200°C and pressure 12 kb.
The source of gold is mainly in the basalts and not in the surrounding granites. 相似文献
19.
The 43 t (1.4 Moz) of gold in the Woodcutters goldfield 50 km north of Kalgoorlie has wide geological significance in terms of gold in Archaean granite, as well as its local commercial and exploration significance. Woodcutters is already one of the largest Archaean gold systems in granite, and is unusual in being so far laterally from the nearest greenstone belt. Gold in the Federal zone, one of the deposits making up the Woodcutters goldfield, is hosted in hornblende‐biotite granodiorite,6 km from the mapped contact with greenstone. In Federal open pit, the granodiorite is coarse‐grained in the northern half, and a fine‐grained granodiorite in the south, with both hosting gold. These two types of granodiorite are rather similar in both mineralogy and geochemistry. There is also a subordinate fine‐grained monzodiorite. The Federal gold mineralisation is in a northwest‐striking, northeast‐dipping (315° strike/60°E dip) shear zone in the Scotia granite. Variation in grainsize of the host rocks might have affected the style of deformation with more brittle fabrics in the coarse‐grained phase and more ductile fabrics prominent in the fine‐grained granodiorite. Hydrothermal alteration is extensively developed around the Federal deposit and is a useful vector towards gold mineralisation. Distal epidote alteration surrounds a proximal muscovite‐biotite alteration zone that contains quartz‐sulfide veins. The alteration shares some of the common alteration characteristics of Archaean greenstone‐hosted gold, but differs in that carbonate‐chlorite alteration is only weakly developed. This difference is readily explained in terms of host‐rock composition and lower concentrations of Fe, Mg and Ca in the granite compared with greenstone. Fluid‐inclusion studies demonstrate that the fluids associated with the hydrothermal alteration at Woodcutters shared the common characteristics of fluids in Archaean greenstone gold, namely low‐salinity and dominant H2O–CO2. Fluid inclusions with moderate salinity were found in one fresh sample away from mineralisation, and are inferred to represent possible magmatic fluid. There is no evidence of a granite‐derived fluid being responsible for gold mineralisation. The granodiorite host rock had cooled, crystallised and had at least started to undergo deformation prior to gold introduction. The distribution of gold mineralisation in the Woodcutters goldfield has the style, shape and orientation comparable with greenstone‐hosted gold deposits in the same region. The northwest trend, the quartz veining and simple pyrite mineralogy are all features common to other greenstone‐hosted gold deposits near Kalgoorlie such as Mt Pleasant. The alteration fluid appears to have penetrated the granite on the scale of many hundreds of metres, causing large‐scale alteration. Woodcutters gold mineralisation resulted from the same metamorphic fluid processes that led to formation of greenstone gold deposits. In this metamorphic model, granitic rocks are predicted to be less‐favourable gold hosts than mafic rocks for two reasons. Granitic rocks do not generally fracture during regional deformation in such a way as to create large‐scale dilation. Furthermore, with less iron and no carbon, granitic rocks have lower potential to precipitate gold from solution by wall‐rock reaction. The metamorphic model predicts that those granite types with higher Fe should host better gold deposits, all other factors being equal. Accordingly, tonalite‐trondhjemite and hornblende‐bearing granodiorite should provide better environments for major gold deposits compared with monzogranite, and granite sensu stricto, as borne out by Woodcutters, but mafic rocks should be better hosts than any of these felsic to intermediate rocks. 相似文献
20.
Structural geometry of orogenic gold deposits: Implications for exploration of world-class and giant deposits 总被引：3，自引：0，他引：3
With very few exceptions, orogenic gold deposits formed in subduction-related tectonic settings in accretionary to collisional orogenic belts from Archean to Tertiary times. Their genesis, including metal and fluid source, fluid pathways, depositional mechanisms, and timing relative to regional structural and metamorphic events, continues to be controversial. However, there is now general agreement that these deposits formed from metamorphic fluids, either from metamorphism of intra-basinal rock sequences or de-volatilization of a subducted sediment wedge, during a change from a compressional to transpressional, less commonly transtensional, stress regime, prior to orogenic collapse. In the case of Archean and Paleoproterozoic deposits, the formation of orogenic gold deposits was one of the last events prior to cratonization. The late timing of orogenic gold deposits within the structural evolution of the host orogen implies that any earlier structures may be mineralized and that the current structural geometry of the gold deposits is equivalent to that at the time of their formation provided that there has been no significant post-gold orogenic overprint. Within the host volcano-sedimentary sequences at the province scale, world-class orogenic gold deposits are most commonly located in second-order structures adjacent to crustal scale faults and shear zones, representing the first-order ore-forming fluid pathways, and whose deep lithospheric connection is marked by lamprophyre intrusions which, however, have no direct genetic association with gold deposition. More specifically, the gold deposits are located adjacent to ～10°-25° district-scale jogs in these crustal-scale faults. These jogs are commonly the site of arrays of ～70° cross faults that accommodate the bending of the more rigid components, for example volcanic rocks and intrusive sills, of the host belts. Rotation of blocks between these accommodation faults causes failure of more competent units and/or reactivation and dilation of pre-existing structures, leading to deposit-scale focussing of ore-fluid and gold deposition.Anticlinal or antiformal fold hinges, particularly those of 'locked-up' folds with ～30° apical angles and overturned back limbs, represent sites of brittle-ductile rock failure and provide one of the more robust parameters for location of orogenic gold deposits.In orogenic belts with abundant pre-gold granitic intrusions, particularly Precambrian granitegreenstone terranes, the boundaries between the rigid granitic bodies and more ductile greenstone sequences are commonly sites of heterogeneous stress and inhomogeneous strain. Thus, contacts between granitic intrusions and volcano-sedimentary sequences are common sites of ore-fluid infiltration and gold deposition. For orogenic gold deposits at deeper crustal levels, ore-forming fluids are commonly focused along strain gradients between more compressional zones where volcano-sedimentary sequences are thinned and relatively more extensional zones where they are thickened. World-class orogenic gold deposits are commonly located in the deformed volcano-sedimentary sequences in such strain gradients adjacent to triple-point junctions defined by the granitic intrusions, or along the zones of assembly of micro-blocks on a regional scale. These repetitive province to district-scale geometrical patterns of structures within the orogenic belts are clearly critical parameters in geology-based exploration targeting for orogenic gold deposits. 相似文献