共查询到20条相似文献，搜索用时 515 毫秒
1.
The Hyde-Macraes Shear Zone (HMSZ) is a regionally continuous, low-angle, NE dipping (~15°) late-metamorphic thrust zone in
the Mesozoic Otago Schist. The shear zone, which is host to large volumes of mineralised schist, consists of foliated fissile
schist with some massive schist pods. Two sets of quartz veins are found within the HMSZ: thrust-related, shallowly dipping
veins that were emplaced parallel or sub-parallel to the shears and swarms of steeply dipping extensional veins, which cut
across the metamorphic foliation. The latter are restricted to the massive schist pods. Mutual cross-cutting relationships
occur between steep extensional veins and shallow-dipping veins, suggesting that they formed contemporaneously. The co-existence
of these two vein types locally implies local rotation of the principal stress axes to produce extensional veins within a
regional thrust setting. The steep extensional veins are spatially related to lateral and oblique ramps within the HMSZ. Three-dimensional
mechanical models show that these lateral or oblique ramps can produce favourable conditions for extensional vein formation
when combined with a high fluid pressure and oblique convergence. Mechanical requirements include a reduced differential stress,
a positive volumetric strain and an increase in the horizontal shear stress. Our models show that under certain conditions,
it is possible for extension-related structures to form during shortening because of local changes in the stress state without
the need for a regional scale switch in the imposed stress field. The convergence direction across the HMSZ during formation
of the steep extensional veins was ~WNW. 相似文献
2.
Dave Craw 《Ore Geology Reviews》2010,37(3-4):224-235
The giant gold placer system on the Otago Schist of southern New Zealand was derived from Mesozoic orogenic gold deposits in the underlying schist basement. The core of the schist basement was exhumed in the middle Cretaceous, coeval with the accumulation of the oldest preserved nonmarine sedimentary rocks in the area (ca 112 Ma). Those sedimentary rocks contain quartz clasts, with distinctive ductile deformation textures, that were derived from structural zones in, or adjacent to, major orogenic gold deposits. Quartz textures in these structural zones are readily distinguishable from the rest of the schist belt, and hence provide a fingerprint for erosion of gold. The earliest sedimentary rocks on the margins of the gold-bearing schist belt are immature, and were derived from unoxidised outcrops in areas of high relief. Gold was not liberated from unoxidised basement rocks during erosion, and was removed from the system without placer concentration. Placer concentration did not begin until about 20 million years later, when oxidative alteration of gold deposits had facilitated gold grain size enhancement from micron scale (primary) to millimetre scale (secondary). Subsequent erosion and recycling of gold in the early Cenozoic, and again in the late Cenozoic, caused additional concentration of gold in progressively younger deposits. The Klondike giant placer goldfield of Canada had a similar geological history to the Otago placer field, and Klondike placer accumulation occurred in the late Cenozoic, at least 70 million years after Mesozoic exhumation of orogenic gold. The giant placer deposit on the western slopes of the Sierra Nevada in California occurs in Eocene and younger sedimentary rocks, at least 40 million years younger than the timing of major exhumation of the source rocks. Circum-Pacific giant gold placers formed under entirely different tectonic regimes from the emplacement of their source orogenic deposits, and these giant placer deposits do not form in foreland basins associated with convergent orogens. Formation of giant placers requires less active erosion and more subdued topography than the collisional orogenic activity that accompanied emplacement of source gold deposits in basement rocks, as well as oxidative alteration of the primary deposits to liberate gold from sulfide minerals and enhance secondary gold grain size. 相似文献
3.
坦桑尼亚马拉-穆索马绿岩带金矿地质特征及成矿规律浅析 总被引：1，自引：0，他引：1
坦桑尼亚太古代马拉-穆索马绿岩带位于坦桑尼亚西北部、维多利亚湖东部地区,是坦桑尼亚重要的金矿聚集区。该地区主要金矿床类型为受陡倾或缓倾剪切构造带控制的构造蚀变岩型和石英脉型金矿床。本文通过对亚斯罗利、瑞万科玛和尼亚布隆渡等3个典型金矿床进行对比研究,系统分析了金矿床的成矿地质条件、矿化蚀变特征及矿石结构构造特征,总结了马拉-穆索马绿岩带金成矿规律,认为金矿床的形成与镁铁质火山-沉积岩、韧-脆性剪切构造带及岩浆活动有密切关系,近东西向韧-脆性剪切带是区内最主要的控矿构造,控制着主要工业矿体的分布。 相似文献
4.
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
5.
A.B. Westerhof 《Tectonophysics》1977,39(4):579-591
Large ultramafic masses along the western margin of the Alboran Sea were emplaced in two stages. The first, “hot” stage of emplacement was post-Triassic and pre-Oligocene in age. The second, “cold” stage of emplacement was of Oligo-Miocene age.The first stage caused the development of polymetamorphic aureoles in the surrounding crustal rocks. Metamorphic conditions in the contact zone of the composite aureole series changed from HP-HT to LP-HT. The HP-HT phase of metamorphism created a primary dynamo-thermal aureole. LP-HT metamorphism took place under mainly static conditions. Present contact relations are mainly defined by the amount of secondary dislocation during hot emplacement of mantle off-shoots from the base into the higher levels of the crust. Where the primary contacts between crustal and ultramafic rocks have remained undisturbed by secondary emplacement aureoles with kinzigite series were developed. Although these rocks have partly recrystallized under LP-HT conditions, their original HP-HT characteristics are largely preserved. In other localities, however, secondary dislocation brought mantle rock in contact with lower grade zones of the primary aureole and caused the development of cordierite- and feldspar-rich hornfelses and migmatites of the cordierite-feldspar hornfels series along the new contacts of the ultramafic rocks.Metapelites with composite facies series, very similar to aureole rocks of the Serranía de Ronda but not associated with high-temperature ultramafics, are found in scattered exposures along the Spanish coast east of the Serranía de Ronda over a distance of approximately 300 km.Cold thrusting during the second stage of emplacement obscured the relations between aureole and ultramafic rocks and gave rise to tectonic contacts of younger age (imbrication). In many places broad zones of mylonite and numerous serpentine lenses formed along the younger thrust planes. 相似文献
6.
The Agnew supracrustal belt consists of a greenstone sequence (interlayered metabasalt, differentiated gabbroic sills, ultramafic bodies, and black volcanogenic sediment) unconformably overlain by granitoid-clast conglomerate and meta-arkose. The base of the preserved sequence is intruded by grey tonalite with a crudely concordant upper contact, and by small discordant bodies of leucogranite.An early deformation (D1) produced isoclinal folds and a regional penetrative foliation. These structures were probably gently dipping when formed. D2 produced large-scale NNW-trending upright folds, a regional foliation, and a vertical N-trending ductile fault on the west side of the belt. D2 structures indicate a combination of ENE-WSW shortening, and right-lateral shear along the ductile fault. Both D1 and D2 were accompanied by metamorphism under upper greenschist to lower amphibolite facies conditions.The interpreted sequence of tectonic events is (1) deposition of the greenstone sequence on an unknown basement; (2) intrusion of large volumes of tonalite, separating the supracrustal rocks from their basement; (3) erosion of mafic rocks and tonalite to produce the clastic sedimentary sequence; (4) the first deformation; (5) intrusion of small volumes of leucogranite; (6) the second deformation.The bulk of the granitoid rocks were emplaced before the first recognisable deformation. Thus the granitoid magma cannot have been produced by partial melting of previously downbuckled ‘greenstone belt’ rocks, nor can the large-scale upright folds (D2) be a result of forceful emplacement of the magma — two common postulates for Archaean terrains. The D2 folds are closely related to the ductile fault bounding the zone: these structures, which give the present N-trending tectonic belt its form, are the youngest features in the terrain. 相似文献
7.
Geometric and kinematic analysis was performed in an area located in the central part of the Seridó Belt (NE Brazil), where supracrustal rocks affected by polyphase deformation are well exposed. The first event recognized in this area (and regionally known as the D2 deformation) is characterized by top to the south thrust tectonics while a second one (D3 deformation) is marked by upright folds, strike-slip or transpressive shear zones and the development of flower structures. Major pegmatite swarms were emplaced during and late as regards the second event (dated ca. 580 Ma), being part of the Brasiliano orogeny; similar dyke swarms are known from the Nigerian Shield. These pegmatite swarms provide reliable kinematic markers of the late evolutionary stage of the Neoproterozoic Trans-Sahara-Borborema collisional belt. Mineralogical, geometric and kinematic features support two stages of pegmatite emplacement during the strike-slip event: (i) older, syn-D3 homogeneous pegmatites intruded mostly along lithological and structural discontinuities, such as foliation surfaces; (ii) late, D3 heterogeneous pegmatites were emplaced along tension gashes and other dilation structures. The heterogeneous pegmatites are economically more important, being exploited for precious metals and stones, as well as industrial minerals. 相似文献
8.
金川铜镍硫化物矿床是世界第三大镍矿床,但其成岩成矿过程及侵位机制一直存在较大争论。根据金川含矿超镁铁岩岩石学特征、穿插关系、矿物成分及地球化学特征,提出了金川含矿岩体5阶段的成岩、成矿侵位序列,它们分别是:(1)超镁铁质岩浆侵位;(2)浸染状硫化物矿浆侵位;(3)网状硫化物矿浆侵位;(4)块状硫化物矿浆侵位;(5)铂钯富集体侵位。金川铜镍(铂)矿床中Ni,Cu,Pt,Pd,Rh,Ir,Ru,及Co与S呈正相关关系;当ω(S)=5%～15%时,铂族元素发生明显的分离作用,铂族金属主要富集在铂钯富集体中。铂钯富集体是硫化物矿浆经单硫化物固溶体结晶后的残余熔浆;块状矿石是单硫化物固溶体堆积而成的产物。金川铜镍硫化物矿床的侵位机制为岩墙型岩浆通道。 相似文献
9.
Geological and structural controls on gold mineralization in the Tanami District, Northern Territory
Gold mineralization in the Tanami district is hosted within moderately northwest dipping turbiditic sedimentary and basaltic
volcanic rocks of the Paleoproterozoic Mt. Charles Formation. The gold occurs within a complex sinistral wrench-fault array
and associated veins and alteration haloes. The main mineralized faults have a northerly trend and dip steeply east. Subsidiary
structures trend at 030° and 070° and dip towards the southeast. Paleostress calculations based on fault striation populations
and geometry (strike and dip) of faults indicate that at the time of the mineralizing event, σ
1 was sub-horizontal and SE–NW directed with σ
2 subvertical. Structural studies indicate that the mineralization occurred after the regional folding event and synchronous
with the emplacement of felsic dykes into the mine sequence. Gold veins in the Tanami district are interpreted to be part
of an outer thermal aureole gold system that formed during the emplacement of granitoids in the nearby ∼1,815 to ∼1,799 Ma
Frankenia and/or Coomarie domes. Economic gold mineralization occurred late in the paragenetic history of the district. Gold
is hosted by quartz-carbonate veins within shear zones, and also in the surrounding sericite- quartz- pyrite ± carbonate-altered
wallrocks. Gold-mineralized veins precipitated at depths of 3 to 6 km from high temperature (∼300°C), low salinity (∼5 wt%
NaCl equivalent) fluids with low CO2 contents. Barren quartz, dolomite and calcite veins that occur in pre- and post-mineralization thrust faults formed from
high salinity (∼20 wt% NaCl equivalent), low temperature (∼120–150°C) basinal brines. Pyrite in the gold mineralized veins
and alteration halos has lower δ
34S values (6.8 to 12.5‰) than local diagenetic pyrite (17.8 to 19.2‰) or pyrite in pre-mineralization thrust faults (31.7 to
37.1‰). The mineralizing fluids are inferred to have contained a well-homogenized mixture of magmatic and sedimentary-derived
sulfur.
Editorial handling: D. Huston 相似文献
10.
Strongly deformed volcaniclastic metasediments and ophiolitic slices hosting the Sukari gold mineralization display evidence of a complex structural evolution involving three main ductile deformational events (D1–D3). D1 produced ENE-trending folds associated with NNW-propagating thrust slices and intrusion of the Sukari granite (689 ± 3 Ma). D2 formed a moderately to steeply dipping, NNW-trending S2 foliation curved to NE and developed arcuate structure constituting the Kurdeman shear zone (≤ 595 Ma) and East Sukari imbricate thrust belt. Major NE-trending F2 folds, NW-dipping high-angle thrusts, shallow and steeply plunging mineral lineation and shear indicators recorded both subhorizontal and subvertical transport direction during D2. D3 (560–540 Ma) formed NNE-trending S3 crenulation cleavage, tight F3 folds, Sukari Thrust and West Sukari imbricate thrust. The system of NW-trending sinistral Kurdeman shear zone (lateral ramps and tear faults) and imbricate thrusts (frontal ramps) forming the actuate structure developed during SE-directed thrusting, whereas the prevailing pattern of NNE-trending dextral Sukari shear zone and imbricate thrusts forming Sukari thrust duplex developed during NE-directed tectonic shearing. Sukari granite intruded in different pluses between 689 and 540 Ma and associated with at least four phases of quartz veins with different geometry and orientation. Structural analysis of the shear fabrics indicates that the geometry of the mineralized quartz veins and alteration patterns are controlled by the regional NNW- and NE-trending conjugate zones of transpression. Gold-bearing quartz veins are located within NNW-oriented sinistral shear zones in Kurdeman gold mine area, within steeply dipping NW- and SE dipping thrusts and NE- and NS-oriented dextral and sinistral shear zones around Sukari mine area, and along E-dipping backthrusts and NW-SE and N-S fractures in Sukari granite. The high grade of gold mineralization in Sukari is mainly controlled by SE-dipping back-thrusts branched from the major NW-dipping Sukari Thrust. The gold mineralization in Sukari gold mine and neighboring areas in the Central Eastern Desert of Egypt is mainly controlled by the conjugate shear zones of the Najd Fault System and related to E-W directed shortening associated with oblique convergence between East and West Gondwana. 相似文献
11.
华北南缘古元古代末岩墙群侵位的磁组构证据 总被引：7，自引：3，他引：4
华北克拉通南缘的中条山及邻区广泛发育元古宙放射状基性岩墙群,与五台山-恒山和大同地区的北北西向基性岩墙群以及熊耳中条拗拉谷的火山岩在时空分布和地球化学方面均具有密切的相关性。中条山及邻区放射状基性岩墙群的宏观和微观流动构造(包括捕虏体、冲痕构造、矿物线理和定向斑晶)指示岩墙群以一定的仰角向北西侵位。通过该区岩墙群磁化率各向异性(AMS)测量得到磁组构的最大磁化率长轴优势方位分布图和磁组构各向异性特征分析进一步指示华北南缘古元古代末岩墙群从熊耳中条拗拉谷的底部向北西侵位。岩墙群的流动构造和磁组构的统计成果夯实了华北克拉通古元古代末基性岩墙群与熊耳中条拗拉谷的成生联系。 相似文献
12.
13.
木纳布拉克蛇绿岩是新疆境内最大的蛇绿混杂岩带,由超镁铁质岩、镁铁质岩、浅色分异物和变玄武岩组成,形成时代为中元古代晚蓟县世,侵位时代为新元古代早青白口世,可能代表了阿尔金山地区中元古代的板块俯冲带.蛇绿岩可能形成于孤后或孤间有限洋盆小扩张脊;最后侵位到深海沉积物中,形成了造山带中的蛇绿岩. 相似文献
14.
Leon Bagas David L. Huston James Anderson Terrence P. Mernagh 《Mineralium Deposita》2007,42(1-2):127-144
Significant gold deposits in the western Tanami region of Western Australia include deposits in the Bald Hill and Coyote areas. The ca. 1,864 Ma Bald Hill sequence of turbiditic and mafic volcanic rocks hosts the Kookaburra and Sandpiper deposits and a number of smaller prospects. The ca. 1,835 Ma turbiditic Killi Killi Formation hosts the Coyote deposit and several nearby prospects. The Kookaburra deposit forms as a saddle reef within a syncline, and the Sandpiper deposit is localized within graphitic metasedimentary rocks along a limb of an anticline. Gold in these deposits is hosted by anastomosing quartz–(–pyrite–arsenopyrite) veins within quartz–sericite schist with disseminated arsenopyrite, pyrite, and marcasite (after pyrrhotite). Based on relative timing relationships with structural elements, the auriferous veins are interpreted to have been emplaced before or during the ca. 1,835–1,825 Ma Tanami Orogeny (regional D1). Gold deposition is thought to have been caused by pressure drops associated with saddle reef formation (Kookaburra) and chemical reactions with graphitic rocks (Sandpiper). The Coyote deposit, the largest in the western Tanami region, consists of a number of ore lenses localized along the limbs of the Coyote Anticline, which formed during the Tanami Orogeny. The largest lenses are associated with the Gonzalez Fault, which is located along the steeply dipping southern limb of this fold. Gold was introduced at ca. 1,790 Ma into dilatant zones that formed in local perturbations along this fault during later reactivation (regional D5) towards the end of a period of granite emplacement. Gold is associated with quartz–chlorite–pyrite–(arsenopyrite–galena–sphalerite) veins with narrow (<?5 mm) chloritic selvages. A quartz–muscovite–biotite–K–feldspar–(tourmaline–actinolite–arsenopyrite) assemblage, which is interpreted to relate to granite emplacement, overprints the regional greenschist facies metamorphic assemblage. The mineralogical similarity between this overprinting assemblage and the vein assemblage suggests that the auriferous veins at the Coyote deposit are associated with the granite-related metamorphic–metasomatic assemblage. Gold deposition is thought to have been caused by pressure drops within dilatant zones. 相似文献
15.
The Macraes gold-tungsten deposit occurs in a low-angle thrust system in biotite grade Otago Schist. Native gold, scheelite, pyrite and arsenopyrite are found in and adjacent to quartz veins and silicified schist of lenticular reef zones, where the thrust system cuts through graphitic pelitic schist. Mineralization is confined to a shear zone, up to 80 m thick, which is closely sub-parallel to the regional schistosity. Chemical alteration is dominated by silicification, with some addition of Cr and depletion of Sr and Ba. Alteration extends only about 5 m from major veins. Oxygen becomes isotopically heavier away from veins due to temperature decrease as hot fluids penetrated into cooler (250°C?) rock. Graphite within the shear zone rocks has reflectance of 6–7% (in oil), similar to graphite in medium-high grade Otago Schist, and is presumed to be metamorphic in origin. This graphite has acted as a reducing agent to cause precipitation of gold where the thrust system, acting as a conduit for metamorphic fluids, intersects the graphitic schist. The metals were derived from the underlying schist pile which may include an over-thrust oceanic assemblage containing metal-enriched horizons. 相似文献
16.
《Journal of Structural Geology》1988,10(3):283-296
Metasomatic tremolite-rich mylonites are widespread in imbricate thrust slices of ultramafic rocks of the ophiolitic Ingalls Complex in Washington State. Protoliths for these amphibolite-facies mylonites were peridotite and serpentinite. Abundant syntectonic tremolite veins in the ultramafites record narrowly channelized flow of infiltrating fluids, whereas metasomatic mylonite zones record more pervasive flow. Fluids were probably released mainly by prograde devolatization reactions within serpentinite and mafic ophiolitic rocks that experienced earlier hydrothermal metamorphism.Olivine apparently deformed by dislocation creep in the mylonites. In the tremolite-rich rocks, locally preserved amphibole porphyroclasts deformed mainly by microfracturing. Acicular tremolites, which dominate the mylonites, form syntectonic overgrowths on porphyroclasts and probably record diffusive mass transfer which may have accompanied cataclasis. Acicular tremolites subsequently were folded and define both post-crystalline crenulations and polygonal arcs.Fluid flow, deformation and metamorphism were apparently complexly interrelated in the imbricate zone. Thrusts juxtaposed contrasting rock types that were sources and sinks for fluids, and shear zones focused fluid flow. Metamorphism probably facilitated deformation through the release of fluids during dehydration reactions. High fluid pressure may have led to hydraulic fracturing and may have controlled strain softening in the tremolitic mylonite zones as it favored microcracking and diffusive mass transfer over dislocation creep. Infiltrating metasomatic fluids probably play an important role in the evolution of shear zones in many ultramafic bodies during medium-grade metamorphism. 相似文献
17.
J. R. Vearncombe 《Mineralium Deposita》1992,27(3):182-191
Although many Archaean greenstone-hosted mesothermal gold deposits are in steep, reverse-motion fault zones, other fault geometries
are prospective for mineralization. Harbour Lights is one of a number of deposits at Leonora hosted in a normal-motion shear
zone, probably related to movement off the adjacent domal granitoid. The deposit is also atypical in that mineralization predates
the last deformation to affect the mine sequence, but formed by similar processes to other mesothermal deposits in all respects
other than the detail of shear zone geometry, kinematics and timing.
Gold mineralization at Harbour Lights is related to D1 quartz veins parallel to a well-developed gently NE- to E-dipping D1
cleavage, both of which are deformed in steeply dipping and later extensional shear bands (D2). Gently dipping quartz veins,
as at Harbour Lights, must have formed at extremely high fluid pressures, capable of holding the weight of the overlying crust.
In the gently dipping normal-motion shear zone continued reactivation and veining was possible only with extremely high fluid
pressures, and steeply dipping structures, such as the D2 extensional shear bands, were initiated as the fluid pressure dropped
after the mineralizing event.
The rarity of gold mineralization hosted in normal-motion shear zones is due to their being linked to steep structures which
pump fluid upwards and prevents the build-up of extremely high fluid pressures. At Harbour Lights it appears that these links
were (atypically) absent, probably because deformation was a result of granitoid doming, and was subparallel to strata. 相似文献
18.
Gold mineralisation in the White River area, 80 km south of the highly productive Klondike alluvial goldfield, is hosted in
amphibolite facies gneisses in the same Permian metamorphic pile as the basement for the Klondike goldfield. Hydrothermal
fluid which introduced the gold was controlled by fracture systems associated with middle Cretaceous to early Tertiary extensional
faults. Gold deposition occurred where highly fractured and chemically reactive rocks allowed intense water–rock interaction
and hydrothermal alteration, with only minor development of quartz veins. Felsic gneisses were sericitised with recrystallisation
of hematite and minor arsenic mobility, and extensively pyritised zones contain gold and minor arsenic (ca 10 ppm). Graphitic
quartzites (up to 5 wt.% carbon) caused chemical reduction of mineralising fluids, with associated recrystallisation of metamorphic
minerals (graphite, pyrrhotite, pyrite, chalcopyrite) in host rocks and veins, and introduction of arsenic (up to 1 wt.%)
to form arsenopyrite in veins and disseminated through host rock. Veins have little or no hydrothermal quartz, and up to 19 wt.%
carbon as graphite. Late-stage oxidation of arsenopyrite in some graphitic veins has formed pharmacosiderite. Gold is closely
associated with disseminated and vein sulphides in these two rock types, with grades of up to 3 ppm on the metre scale. Other
rock types in the White River basement rocks, including biotite gneiss, hornblende gneiss, pyroxenite, and serpentinite, have
not developed through-going fracture systems because of their individual mineralogical and rheological characteristics, and
hence have been little hydrothermally altered themselves, have little hydrothermal gold, and have restricted flow of fluids
through the rock mass. Some small post-metamorphic quartz veins (metre scale) have been intensely fractured and contain abundant
gold on fractures (up to 40 ppm), but these are volumetrically minor. The style of gold mineralisation in the White River
area is younger than, and distinctly different from, that of the Klondike area. Some of the mineralised zones in the White
River area resemble, mineralogically and geochemically, nearby coeval igneous-hosted gold deposits, but this resemblance is
superficial only. The White River mineralisation is an entirely new style of Yukon gold deposit, in which host rocks control
the mineralogy and geochemistry of disseminated gold, without quartz veins. 相似文献
19.
B. Lacroix M. Buatier P. Labaume A. Travé M. Dubois D. Charpentier S. Ventalon D. Convert-Gaubier 《Journal of Structural Geology》2011,33(9):1359-1377
In orogenic systems, thrust faults play a major role in stacking different tectonic units and may act as conduits for the expulsion of large amounts of fluid of different origins (metamorphic, diagenetic, meteoric). This study focuses on the Monte Perdido thrust unit emplaced in the Paleogene Jaca thrust-sheet-top basin, in the SW-central Pyrenees. We aim to decipher the mechanisms and P-T conditions of deformation in fault zones and characterize the related fluid involvement, through combined microstructural, geochemical and microthermometry analyses. Two thrust faults cutting platform limestones, marls and siliciclastic turbidites of the lower part of the basin-fill (Paleocene–lower Eocene) have been studied. The fault zones are characterized by metre-thick shear zones with highly deformed, foliated clay-rich sediments. Foliation is underlined by preferentially oriented phyllosilicates. Several generations of shear and extension calcite, quartz and chlorite-bearing veins attest to fluid-rock interactions during a multi-stage deformation. Microstructural observations and stable isotope analyses on calcite from veins and host sediments suggest that deformation was aseismic and dominated by diffusive mass transfer from pressure solution sites along cleavage and stylolites to the precipitation sites in veins, with mineralizing fluids in equilibrium with the host sediments. Our results suggest an essentially closed hydrologic system, and imply the absence of significant fluid flow along the studied fault zones. Microthermometric study on fluid inclusions present in calcite and quartz veins, and calcite-quartz oxygen isotopic fractionation determined for the first generation shear veins, allow a geothermal gradient of 34 °C/km to be estimated. Analytical results demonstrate an evolution of the fault zones in three stages. The first stage was related to the emplacement of the Monte Perdido thrust unit during the middle Eocene at a temperature of ～208 °C and a burial depth of ～5.7 km. The second stage corresponds to a fault reactivation at a temperature of ～240 °C and a burial depth of ～6.5 km. The latter deformation may have been related to folding of the Monte Perdido thrust unit during the emplacement of the underlying Gavarnie thrust unit during the late Eocene–early Oligocene, with deeper burial resulting from aggradation of the thrust-sheet-top basin-fill. The last event corresponds to the formation of a dilatant vein system likely related to the exhumation of the massif. 相似文献
20.
J. Trofimovs B. K. Davis R. A. F. Cas M. E. Barley G. I. Tripp 《Australian Journal of Earth Sciences》2013,60(2):303-327
Two main deformational phases are recognised in the Archaean Boorara Domain of the Kalgoorlie Terrane, Eastern Goldfields Superterrane, Yilgarn Craton, Western Australia, primarily involving south-over-north thrust faulting that repeated and thickened the stratigraphy, followed by east-northeast – west-southwest shortening that resulted in macroscale folding of the greenstone lithologies. The domain preserves mid-greenschist facies metamorphic grade, with an increase to lower amphibolite metamorphic grade towards the north of the region. As a result of the deformation and metamorphism, individual stratigraphic horizons are difficult to trace continuously throughout the entire domain. Volcanological and sedimentological textures and structures, primary lithological contacts, petrography and geochemistry have been used to correlate lithofacies between fault-bounded structural blocks. The correlated stratigraphic sequence for the Boorara Domain comprises quartzo-feldspathic turbidite packages, overlain by high-Mg tholeiitic basalt (lower basalt), coherent and clastic dacite facies, intrusive and extrusive komatiite units, an overlying komatiitic basalt unit (upper basalt), and at the stratigraphic top of the sequence, volcaniclastic quartz-rich turbidites. Reconstruction of the stratigraphy and consideration of emplacement dynamics has allowed reconstruction of the emplacement history and setting of the preserved sequence. This involves a felsic, mafic and ultramafic magmatic system emplaced as high-level intrusions, with localised emergent volcanic centres, into a submarine basin in which active sedimentation was occurring. 相似文献