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1.
胶东地区是典型的剪切带型金矿集中区,精确厘定该地区控矿剪切带的活动时代,可为探讨剪切带与金矿成因关系提供关键的时限约束,并且对矿床成矿模式的建立具有至关重要的意义。大量矿床地质证据显示胶东地区控矿剪切带中脆性变形活动对金矿体的形成具有直接影响,但其脆性变形时代尚不十分清楚。据此,本文在详细研究金矿体赋存特征的基础上,系统选取胶东焦家、玲珑、邓格庄、乳山这四个金矿区控矿剪切带断层泥中白云母进行40Ar-39Ar年代学研究。定年结果显示,胶东地区焦家剪切带、招平剪切带以及牟乳剪切带的脆性变形时代分别为110.3±1.5Ma、122.8±1.7Ma、119.6±1.2Ma~115.8±1.4Ma。其中焦家剪切带的脆性变形时代明显晚于招平和牟乳剪切带,可能代表了焦家蚀变岩型矿化形成后易遭受后期的构造叠加。综合胶东各金矿区控矿剪切带变形时代、岩体侵位时代、成矿时代及剪切带活动特征和矿体产出特征,本文认为在多期岩体侵位以及控矿剪切带递进变形过程中,剪切带韧性变形区中由高压流体作用产生的同期脆性破裂可形成脉型矿化,如乳山金矿;而在脆-韧性和脆性变形区中发生的大规模脆性变形可导致脉型和蚀变岩型矿化的形成,如玲珑、邓格庄和焦家金矿。但随着剪切带的递进变形和隆升剥蚀,后期多期次的脆性构造变形叠加,可导致多种矿化类型出现在同一构造部位,如焦家金矿中石英脉型和蚀变岩型矿化。 相似文献
2.
华北地台北缘中段是我国重要的金矿化集中区.文中采用石英的(40)Ar-(39)Ar法和单颗粒锆石的U-Pb测年法,结合前人资料,提出本区绿岩带初生型金矿床的成矿时代为古元古代—新太古代,其中辽西排山楼金矿的成矿时代为(2 105.2±10.4)Ma,金厂峪金矿的成矿时代为(2 539±23)Ma,小营盘金矿的成矿时代为1 800Ma左右.构造期后再生型金矿床的成矿时代为燕山晚期—华力西中期,其中东坪金矿床的成矿时代为(350.9±0.9)Ma,冀东头道门子沟金矿的成矿时代为(217.32±2.04)Ma,峪耳崖金矿床的成矿时代为燕山早期,金厂沟梁和二道沟金矿的成矿时代为燕山晚期. 相似文献
3.
一加拿大是世界上重要的产金国之一,以绿岩带金矿著称于世.八十年代以来,又有了新的进展,尤其是绿岩带上部的金矿有了重大突破.在加拿大地盾西北部的斯拉夫构造省,晚太古代花岗-绿岩带耶奈洛夫超群上部与浊流沉积有关的铁建造中的鲁频(Lupin)金矿,八十年代初期正式建矿开采,并成为当今加拿大重要的金矿山之一,它不仅使绿岩带金矿增添了新的类型,而且也提高了黄金的产量.在苏必利尔构造省的瓦瓦(Wawa)花岗-绿岩带上部戈利斯(Goliath)火山-碎屑沉积建造中,发现了黄铁矿化层控浸染型赫姆洛(Hemlo)金矿,它以巨大的规模,轰动了加 相似文献
4.
广东大宝山多金属矿床花岗岩锆石LA-ICP-MSU-Pb定年及其地质意义 总被引：8，自引：2，他引：6
广东大宝山矿床位于南岭花岗岩带中带。它是我国著名的大型多金属矿床,开采历史久远。近年来的研究表明大宝山矿床与成矿作用有关的斑岩体为燕山早期岩浆活动的产物,因而人们较多地关注中生代的岩浆活动,而忽视了对其他时代岩浆活动的研究。本文在前人研究的基础上,利用锆石LA-ICP-MS U-Pb定年方法系统地测试了大宝山多金属矿床多个花岗质岩体和辉绿岩脉的形成时代,研究表明徐屋片理化流纹斑岩年龄为426.9±2.2Ma、九曲岭黑云母花岗闪长斑岩、船肚花岗闪长岩和大宝山花岗闪长斑岩形成时代分别为162.2±0.7Ma、160.2±0.9Ma和161.0±0.9Ma。矿区内两条辉绿岩脉的年龄分别为210.4±1.4Ma和163.9±1.8Ma。这些结果证实大宝山矿区内存在加里东期、印支期和燕山期等多个旋回的岩浆活动,中晚侏罗世铁镁质的岩浆活动可能存在对成矿的贡献。 相似文献
5.
津巴布韦CHAMPION金矿位于津巴布韦Odzi成矿带上,绿岩呈残留体存在太古代结晶基底中。矿体严格受太古代绿岩带控制,变质片岩内的剪切构造是成矿最有利部位;其次在片岩和含铁建造金、与绿岩带接触的花岗岩体内的石英脉也有金矿产出;在绿岩带与花岗岩接触面的金矿经济价值不大。 相似文献
6.
霍吉河钼矿是小兴安岭地区于近年发现的大型斑岩型钼矿床。本文在对矿化特征分析的基础上,采用LA-ICP-MS锆石U-Pb技术对霍吉河成矿花岗质杂岩测年,获得中细粒花岗闪长岩和斑状花岗岩成岩年龄分别为181.0±1.9 Ma(n=15, MSWD=4.7) 和193.6±1.4 Ma(n=25, MSWD=2.3)。通过辉钼矿Re-Os同位素分析,获得等值线年龄176.3±5.1 Ma(n=5, MSWD=1.3)和加权平均年龄为181.2±1.8 Ma (n=5, MSWD=1.3)。两种方法获得的年龄大致相近,表明成岩与成矿于同一系统形成,成岩大致始于193.6±1.4 Ma,成矿于岩浆期后181.2±1.8 Ma。霍吉河钼矿成岩成矿年龄与乌奴格吐山铜钼矿、兰家沟钼矿和杨家杖子钼矿等矿床辉钼矿Re-Os同位素年龄相近,表明中国东北地区广泛存在早侏罗世岩浆成矿作用。微量元素和同位素显示,霍吉河花岗闪长岩和斑状花岗岩属A型花岗岩,形成于地壳挤压向拉伸转换的构造环境,成矿物质主要来自岩浆。 相似文献
7.
塞拉利昂中部马卡利地区出露有大片的太古界花岗-绿岩带地质体,其金矿资源丰富。通过矿产勘查和成矿规律研究表明,金的来源与太古界花岗-绿岩带地体密切相关。岩金矿床类型为含铁建造金矿床,主要赋存于绿岩带的绿片岩中及含铁建造的片岩中,受褶皱和断裂构造控制;砂金矿的类型为河流冲积型砂金矿,主要分布在花岗-绿岩带内及其周边河流中的全新统冲积层中下部砂砾石层中。重砂异常和地球化学异常区内具备良好的岩金、砂金成矿地质背景和找矿前景,实施砂金-岩金综合找矿方法,找矿潜力巨大。 相似文献
8.
五台山绿岩带中 ,与变质火山 沉积岩系直接相关的金矿化分布广泛 ,具有多种成矿和控矿机制。但就两种主要类型金矿———剪切带型金矿和铁建造型金矿的时空分布、地质特征和赋存的构造部位来看 ,它们都明显地与剪切变形 (韧性的或脆性的 )所产生的构造 热液活化作用有关 ,具显著的构造控矿意义。区域构造分析表明 ,五台山绿岩带的构造格架是一个多级和多期褶皱与断裂的组合 ,总的构造样式是一个紧闭程度向中心增强的复式倒转向形。它们是在北西 南东向挤压作用下递进变形的结果。在这一区域应力场作用下 ,处于不同构造部位的岩石发生变形分解作用 ,从而在紧闭褶皱的翼部产生一系列剪切变形带 ,构成了五台山绿岩带构造控矿机制。本文将以两类绿岩金矿的典型矿床为例 ,对这一控矿构造机制进行全面分析。 相似文献
9.
10.
为了确定五龙金矿围岩的侵位和变形时代,进一步探讨辽东地区晚中生代构造体制转折时限,采用岩相学观察和锆石SHRIMP U-Pb定年技术,对辽东五龙大型金矿的围岩五龙岩体、侵入五龙岩体中的花岗斑岩脉和三股流岩体进行了研究。结果表明,五龙岩体具弱片麻状、片麻状构造,花岗斑岩脉也发生了变形作用,三股流岩体未发生变形作用。五龙岩体SHRIMP U-Pb年龄为（159.2±1.9） Ma和（160.0±1.4） Ma,花岗斑岩SHRIMP U-Pb年龄为（132.14±0.85） Ma,三股流岩体SHRIMP U-Pb年龄为122~124 Ma。辽东五龙岩体经历的推覆、挤压事件止于132~124 Ma。辽东地区晚侏罗世的推覆、挤压事件与古太平洋板块向欧亚大陆俯冲作用关系密切,五龙金矿的成矿大地构造背景亦与此次俯冲有关。 相似文献
11.
In the Archaean Murchison Province of Western Australia, granitoid batholiths and plutons that intruded into the ca. 2.7–2.8
Ga and ca. 3.0 Ga greenstone belts can be divided into three major suites. Suite I is a ca. 2.69 Ga monzogranite-granodiorite
suite, which was derived from anatexis of old continental crust and occurs as syn-tectonic composite batholiths over the entire
province. Suite II is a trondhjemite-tonalite suite (termed I-type) derived from partial melting of subducted basaltic crust,
which intruded as syn- to late-tectonic plutons into the greenstone belts in the northeastern part of the province where most
of the major gold deposits are situated. One of the Suite II trondhjemite plutons has a Pb−Pb isochron age of 2641±36 Ma,
and one of the structurally youngest tonalite plutons has a minimum Pb−Pb isochron age of 2630.1±4.3 Ma. Suite III is a ca.
2.65–2.62 Ga A-type monzogranite-syenogranite suite which is most abundant in the largely unmineralised southwestern part
of the province. Gold deposits in the province are mostly hosted in brittle-ductile shear zones, and were formed at a late
stage in the history of metamorphism, deformation and granitoid emplacement. At one locality, mineralisation has been dated
at 2636.8±4.2 Ma through a pyritetitanite Pb−Pb isochron. Lead and Sr isotope studies of granitoids and gold deposits indicate
that, although most gold deposits have initial Pb isotope compositions most closely similar to those of Suite II intrusions,
both Suite I and Suite II intrusions or their source regions could have contributed solutes to the ore fluids. These preliminary
data suggest that gold mineralisation in the Murchison Province was temporally and spatially associated with Suite II I-type
granitoids in the northeastern part of the province. This association is consistent with the concept that Archaean gold mineralisation
was related to convergent-style tectonic settings, as generation of both Suite II I-type granitoids and hydrothermal ore fluids
could have been linked to the dehydration and partial fusion of subducted oceanic crust, and old sialic crust or its anatectic
products may also contribute solutes to the ore fluids. Integration of data from this study with other geological and radiogenic
isotope constraints in the Yilgarn Block argue against direct derivation of gold ore fluids from specific I-type granitoid
plutons, but favour a broad association with convergent tectonics and granitoid magmatism in the late Archaean. 相似文献
12.
Southern Cross was one of the earliest gold mining centres in Western Australia. Over 142 tonnes of gold have been produced
from the district, and, on a gold per hectare basis, the Southern Cross greenstone belt in the southwestern Yilgarn Craton
is the most productive of Western Australia's Archaean greenstone belts. The SW Yilgarn Craton is characterised by high-grade
(amphibolite- to granulite-facies) metamorphism, extensive granitoid magmatism and older greenstone volcanism ages, compared
to the well-known greenschist-facies metamorphism and younger (2.7 Ga) eruption ages which dominate in the Eastern Goldfields
Province. The Pb-isotope compositions of deep-seated granitoids in the SW Archaean Yilgarn Craton, which were emplaced coeval
with a craton-wide major orogenic lode-gold mineralization event at about 2.64–2.63 Ga, have been determined for 96 whole-rock
and 24 K-feldspar samples. The Pb isotope data of the granitoids are consistent with a crustal origin for their genesis, probably
by reworking (partial melting) of older continental crust. The Pb isotope composition of greenstones, which are the main host
rocks for gold mineralisation, and pyrites from the komatiite-hosted syngenetic Ni deposits in the amphibolite-facies Forrestania
greenstone belt, have also been determined, with initial Pb-isotope ratios higher than that for the Eastern Goldfields Province.
The Pb isotopic character of the orogenic lode-gold deposits in the region is intermediate between coeval granitoid and greenstone
Pb, indicating that the ore fluids contained metals from both reservoirs. The Pb in the ore fluid of the most deeply formed
deposit, Griffin's Find, overlaps the isotopic composition of coeval granitoids, indicating the deep-seated granitoid magmatism
was the primary source for Pb in the ore fluids.
Received: 8 October 1998 / Accepted 22 December 1998 相似文献
13.
The Archaean lode gold deposits in the Mt. York District, Pilbara Block, Western Australia are hosted in banded iron formation (Main Hill/Breccia Hill prospect) of the ca. 3.33 Ga Gorge Creek Group and in amphibolites (Zakanaka prospect) of the ca. 3.46 Ga Warrawoona Group. Gold mineralisation at the Main Hill/Breccia Hill prospect is associated with breccias comprising quartz clasts in a quartz-pyrrhotite matrix, and quartz-amphibole veins, with löllingite being the major host for gold. Minimum temperatures for gold mineralisation at the prospect are constrained as 455°C to 550°C by arsenopyrite thermometry. Gold mineralisation at the Zakanaka prospect is spatially associated with quartzclinopyroxene-calcite-microcline-calcic-amphibole veins and biotite altered wallrock adjacent to the veins. Temperatures for vein emplacement are estimated as 480°C to 570°C using both plagioclase-amphibole thermometry and mineral equilibria with respect to T and XCO2. The timing of gold mineralisation relative to the peak of metamorphism is constrained by mineral textures and the relative temperatures of hydrothermal alteration and metamorphism. Gold mineralisation at both deposits was broadly synchronous with the peak of regional amphibolite facies metamorphism, which reached temperatures of 520°C to 640°C based on amphibole-plagioclase and garnet-biotite thermometry. In this respect, the deposits are similar to other well documented syn-amphibolite facies lode gold deposits from the Archaean Southern Cross greenstone belt in the Yilgarn Block, and represent the deeper section of a crustal continuum of lode gold deposits that includes mesothermal deposits such as those at Kalgoorlie at higher crustal levels. 相似文献
14.
D. Srinivasa Sarma Ian R. Fletcher Birger Rasmussen Neal J. McNaughton M. Ram Mohan David I. Groves 《Mineralium Deposita》2011,46(3):273-288
New zircon U–Pb ages for a felsic volcanic rock (2,588 ± 10 Ma) and an intrusive granite (≥2,555 ± 6 Ma) in the Gadag greenstone
belt in the Western Dharwar Craton, southern India, are similar to dates for equivalent rocks in the Eastern Dharwar Craton
and indicates docking of the two cratons prior to this time. The zircons in the intrusive granite are strongly overprinted,
and coexisting titanites yielded two different age populations: the dominant group gives an age of 2,566 ± 7 Ma, interpreted
as the emplacement age, whereas the minor group gives an age of 2,516 ± 10 Ma, reflecting a hydrothermal overprint. In situ
U–Pb dating of monazite and xenotime in gold reefs of the Gadag (2,522 ± 6 Ma) and Ajjanahalli (2,520 ± 9 Ma) gold deposits
reveal a previously undated episode of gold mineralization at 2.52 Ga, substantially younger than the 2.55 Ga Hutti deposit
in the eastern Dharwar Craton. The new dates confirm that both the younger greenstone belts and lode gold mineralization in
the Dharwar Craton are about 100–120 My, younger than in other well-dated Archaean cratons. Although gold mineralization across
the craton postdates most of the magmatic activity and metamorphism at upper crustal levels, widespread thermal reworking
of the lower-middle crust, involving partial melting, metamorphism, and lower crustal granitoid intrusion, occurred concurrently
with gold mineralization. It is likely that the large-scale hydrothermal fluid flow that produced widespread gold deposition
was also part of this tectono-thermal event during the final stages of cratonization of the Dharwar Craton in southern India. 相似文献
15.
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
16.
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
17.
Gold occurs in a number of different ore types in the Fennoscandian Shield ranging in age from Late Archean to Late Proterozoic. Until recently, the metal was exploited primarily as a byproduct in volcanogenic massive sulphide deposits but during the 1980s more gold mines have been opened than during any other episode in the mining history of northern Europe. The occurrence of gold in the Fennoscandian Shield is reviewed in the context of the major tectonostratigraphic units:
These metallogenic features, deposit modelling and economic properties of the known occurrences suggest that the potential for new gold discoveries is highest in Late Archean to Early Proterozoic greenstone belts and in Early Svecofennian tonalite plutons. The gold potential of the Sveconorwegian Domain is also worth further consideration. 相似文献
| 1. | In the Karelian Province, gold is hosted by greenstone belts of the Archean basement complex e.g. at Ilomantsi, eastern Finland. Greenstone belts of the Nordkalott Province, which are interpreted as part of an Early Proterozoic cover sequence, contain gold deposits associated with copper (Bidjovagge, Saattopora and Pahtohavare). Gold is also associated with cobalt in the metasomatically altered Early Proterozoic cover in north-eastern Finland (Meurastuksenaho and Juomasuo). |
| 2. | In the Svecofennian Domain, the major gold deposits were generated during the emplacement of 1.92–1.87 Ga old accretional magmatism. These deposits occur in the northeastern part of the Svecofennian Domain, close to the Archean-Proterozoic boundary. They comprise two major types: (a) the porphyry-type and shear-zone gold hosted by tonalite at Tallberg, Laivakangas, Kopsa and Osikonmäki; (b) as a component of volcanogenic massive sulphide deposits (e.g. Holmtjärn, Boliden and Pyhäsalmi). Other types are: (c) gold-bearing quartz-alumina alteration zones formed during the 1.92–1.87 Ga magmatic period (Enåsen); (d) gold in massive sulphide and iron ore deposits in Bergslagen. |
| 3. | Gold associated with 1.84–1.54 Ga granites has been reported from several sites in the Shield, including quartz veins and contact-metasomatic deposits. In addition, shear-zone-related gold deposits post-dating these granites have been identified in southeastern Sweden (Ädelfors). |
| 4. | In the Sveconorwegian Domain, the gold deposits at Bleka, Eidsvoll, Glava and Hamas are associated with shear zones which developed penecontemporaneously with the intrusion of late (1.0–0.9 Ga) granites. |
18.
华北陆块北缘中段金矿床分布较多 ,产出较为集中 ,也是我国重要的黄金生产基地。本区是我国地质历史演化最久 ,又经历了多期、多阶段的构造 -岩浆活动的叠加 ,因而金矿床的形成具有多期性、多样性、继承性和新生性等特点。本区金矿床主要产在绿岩带构造 -岩浆活动的有利部位 ,统称绿岩带型金矿床 ,可分为同构造晚期初生型金矿床和构造期后再生型热液金矿床两类 ,又可细分为若干亚类 ,其中再生型热液金矿床分布较广 ,规模较大 ,具有较大找矿潜力。由于区内发育多期、多阶段的成矿构造 -岩浆活动 ,因而在一个金矿床密集区内 ,有时在不同的构造部位 ,分别产出初生型和再生型热液金矿床 ,这是指导找矿的重要地质依据 相似文献
19.
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
20.
Hielke A. Jelsma Michael L. Vinyu Jan R. Wijbrans E. A. T. Verdurmen P. J. Valbracht G. R. Davies P. J. Valbracht 《Contributions to Mineralogy and Petrology》1996,124(1):55-70
The U-Pb ages of zircons from seven felsic volcanic and plutonic rocks from northern Zimbabwe combined with field data and
Pb-Pb and Sm-Nd whole-rock isotope data, constrain the timespan of development of the Harare-Shamva granite-greenstone terrain
and establish the relative involvement of juvenile mantle-derived and reworked crustal material. Basement-cover field relationships
and isotope and geochemical data demonstrate that the greenstones were deposited onto 3.2–2.8 Ga basement gneisses, in ensialic,
continental basins. Geodynamic models for the generation of the areally extensive bimodal magmatic products and growth of
the pre-existing crustal nucleus consistent with our interpretations are rift-related: (1) intracontinental rifting related
to mantle plume activity or; (2) rifting in a back-arc environment related to a marginal volcanic arc. The data, in conjunction
with field evidence, do not indicate the presence and accretion of an older (ca. 2.70 Ga) and a younger (ca. 2.65 Ga) greenstone
sequence in the Harare part of the greenstone belt, as was recently postulated on the basis of SHRIMP zircon ages. Zircon
ages for basal felsic volcanics (2715±15 Ma) and a subvolcanic porphyry (2672±12 Ma) constrain the initiation and termination
of deposition of the greenstone sequence. The timespan of deposition of the Upper Bulawayan part of the greenstone sequence
corresponds well with radiometric ages for Upper Bulawayan greenstones in the central and southern part of the craton and
supports the concept of craton-wide lithostratigraphic correlations for the late Archaean in Zimbabwe. Zircon ages for a syn-tectonic
gneiss (2667±4 Ma) and a late syn-tectonic intrusive granodiorite (2664±15 Ma) pinpoint the age of deformation of the greenstone
sequence and compare well with a Pb-Pb age of shear zone related gold mineralization (2659±13 Ma) associated with the latter
intrusive phase. The intimate timing relation of greenstone deformation and granitoid emplacement, but also the metamorphic
evidence for a thermal effect of the batholiths on the surrounding greenstone belts, and the structural and strain patterns
in the greenstone sequence around and adjacent to the batholiths, imply that the intrusion of the granitoids had a significant
influence on the tectono-thermal evolution of the greenstone belt. Prolonged magmatic activity is indicated by the zircon
ages of small, post-tectonic plutons intrusive into the greenstone belt, with a mineralized granodiorite dated at 2649±6 Ma
and an unmineralized tonalite at 2618± 6 Ma. The 2601±14 Ma crystallization age of an “external” Chilimanzi-type granite agrees
well with existing radiometric ages for similar granites within the southern part of the craton, demonstrating a craton-wide
event and heralding cratonization. The similarity between U-Pb zircon ages and TDM model ages (2.65–2.62 Ga) and the positive ɛNdT values (+3 to +2) for the late syn-tectonic and post-tectonic intrusive plutons within the greenstone belt indicate magmatism
was derived directly from the mantle or by anatexis of lower crustal sources, with very short crustal residence times, and
minor contamination with older crust. The rather high model μ1 values (8.2–8.6) are unlikely to indicate the involvement of significant amounts of older crust and may be inherited from
a high U/Pb mantle source, as was suggested by previous workers for the Archaean mantle beneath Southern Africa. The older
TDM ages for the felsic volcanics (3.0–2.8 Ga) and the porphyries (2.8–2.7 Ga) suggest that these felsic magmas were derived
by partial melting of a source that was extracted from the mantle ca. 200 Ma prior to volcanism or may indicate interaction
between depleted mantle-derived melts and older crustal material.
Received: 15 August 1995 / Accepted: 12 January 1996 相似文献