The geology and petrogenesis of a greenstone belt and related nickel sulfide mineralization at Yakabindie,Western Australia     

A.J. Naldrett  A.R. Turner 《Precambrian Research》1977,5(1):43-103

Two major rock sequences, the Lower and Upper Greenstones, separated by an unconformity and associated conglomerate, are exposed in a narrow (5–30 km) elongate (130 km) greenstone belt in the Lawlers—Mt Keith area of the Eastern Goldfields of Western Australia. The Upper Greenstones, which contain zones of significant nickel sulfide mineralization, appear to have been emplaced in a graben which cuts across folded strata of the Lower Greenstones.Exposed portions of the Lower Greenstones consist of over 90% mafic and less than 10% felsic rocks, whereas the Upper Greenstones consist of approximately one third mafic and two thirds felsic rocks. The mafic rocks of the latter sequence all appear to belong to the komatiitic rock series.⁷ They are divided into two groups of flows and thin sills ranging in composition from peridotite to basalt and a number of strata-bound podiform dunitic bodies which are regarded as feeder chambers for some of the overlying flows. Mafic rocks of the Lower Greenstones include Fe-rich tholeiitic basalts, gabbros, komatiitic⁷ ultramafic rocks and some basalts which appear chemically to be transitional between the two rock series.At Yakabindie in the centre of the Lawlers—Mt Keith belt, Western Australia, the nickel sulfide mineralization occurs mainly within the dunite pods as large (in excess of 70·10⁶ tonnes) low grade (approx. 0.6 wt.% Ni) zones composed largely of disseminated pentlandite. Textures in much of the ore suggest that magmatic sulfides were trapped interstitial to olivine grains, although textures and chemical variations in some of the ore indicate that metals and sulfides have been redistributed during metamorphism or alteration of the host rocks.Chemical variations within the komatiites are consistent with their being the result of low pressure fractional crystallization of a liquid containing in excess of 33 wt. % MgO. In order to produce such a magnesian liquid, it is postulated that the source region of the mantle must have been melted twice, once to give rise to a basaltic liquid and again to give rise to the magnesian, komatiitic liquid. This would require diapiric ascent of mantle material from a depth of perhaps 200–250 km. Consideration of the melting curves of sulfides and mantle periodotite with pressure suggests that sulfides might have become concentrated at these depths, thus accounting for their almost universal association, in Western Australia, with very magnesian komatiites.  相似文献   

A numerical model of the fractionation of olivine and molten sulfide from komatiite magma     

J.M. Duke  A.J. Naldrett 《Earth and Planetary Science Letters》1978,39(2):255-266

A numerical model has been formulated that simulates the differentiation of mafic and ultramafic magmas by the fractionation of olivine and molten sulfide. The model is used to simulate the low-pressure differentiation of a komatiite magma series under both sulfide-undersaturated and sulfide-saturated conditions. Under sulfide-saturated conditions, the molecular ratio of olivine to sulfide removed from the silicate liquid is39 ± 2. Separation of this relatively small proportion of sulfide melt results in significantly different chemical trends in derivative liquids and fractionated material than are produced in the sulfide-undersaturated system, and this observation may be useful in mineral exploration. Comparison of the model results with published analyses of natural rocks indicate that the liquid equivalent members of the komatiite suite at Yakabindie, Western Australia, could be derivative liquids produced by fractional crystallization of olivine from a sulfide-undersaturated parental magma containing about 32 wt.% MgO. Derivation of a komatiitic pyroxenite with 20 wt.% MgO would require fractionation of 43.4 mol.% olivine whereas production of a komatiitic basalt with 12 wt.% MgO would involve removal of 58.5 mol.% olivine. Synvolcanic intrusive dunitic lenses at Yakabindie could have been produced by accumulation of material separated during about 3.8 mol.% fractionation of a similar parental magma, but the concentration of chalcophile elements in these bodies requires that the magma was sulfide-saturated.  相似文献   

A study of the chemistry of pore fluids and authigenic carbonates in methane seep environments: Kodiak Trench, Hydrate Ridge, Monterey Bay, and Eel River Basin   总被引：3，自引：0，他引：3  

Joris Gieskes  Chris Mahn  Shelley Day  Jonathan B. Martin  Jens Greinert  Tony Rathburn  Brian McAdoo 《Chemical Geology》2005,220(3-4):329-345

Analyses of the chemical and isotopic composition of carbonates rocks recovered from methane seepage areas of the Kodiak Trench, Hydrate Ridge, Monterey Bay Clam Flats, and the Eel River Basin, coupled with the studies of the chemistry of the pore fluids, have shown that these carbonates have grown within the sediment column. Geochemical profiles of pore fluids show that, in deep water seeps (Kodiak Trench—4450 m; Monterey Bay—1000 m; Hydrate Ridge—650 m), δ¹³C (DIC) values are low (isotopically light), whereas in the Eel River area ( 350–500 m), δ¹³C (DIC) values are much higher (isotopically heavier). In all cases, the δ¹³C values indicate that processes of methane oxidation, associated with sulfate reduction, are dominant in the shallow sediments. Data on the isotopic composition of authigenic carbonates found at sites in Kodiak Trench, Eel River Basin South, and Eel River Basin North indicate a variable composition and origin in different geochemical environments. Some of the authigenic carbonates from the study sites show a trend in their δ¹³C values similar to those of the pore fluids obtained in their vicinity, suggesting formation at relatively shallow depths, but others indicate formation at greater sediment depths. The latter usually consist of high magnesium calcite or dolomite, which, from their high values of δ¹³C (up to 23‰;) and δ¹⁸O (up to 7.5‰), suggest formation in the deeper horizons of the sediments, in the zone of methanogenesis. These observations are in agreement with observations by other workers at Hydrate Ridge, in Monterey Bay, and in the Eel River Basin.  相似文献   

中国以外的世界天坑概览     

Tony WALTHAM  张远海 《中国岩溶》2006,25(Z1):25-34

就中国以外已发现的26个天坑的特征和成因进行了一般性的描述和讨论.此外,对其它类型的一些特大型塌陷漏斗也与天坑进行了比较.  相似文献   

Quaternary Listserver Newsgroup     

Dana L. Naldrett 《Journal of Paleolimnology》1995,13(3):291-292

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Peralkaline nephelinites. I. Comparative petrology of Shombole and Oldoinyo L'engai,East Africa     

Tony D. Peterson 《Contributions to Mineralogy and Petrology》1989,101(4):458-478

Shombole, a nephelinite-carbonatite volcano in south Kenya, erupted silicate lavas, carbonatite dikes and tuffs, and pyroclastic rocks similar to those at other East African alkaline centres. Shombole lavas containing cpx + nepheline + accessory minerals range from perovskite-bearing nephelinites (43% SiO₂, volatile-free) to sphene-bearing and phonolitic nephelinites (46–49% SiO₂) and phonolites (49–56% SiO₂) and have low peralkalinity ([Na+K]/Al 1.15) which does not correlate with SiO₂. Early fractionation of olivine and clinopyroxene strongly depleted Ni and Cr concentrations (10 ppm); fractionation of perovskite, melanite, sphene, and apatite produced negative correlations of all REE with SiO₂. Many lavas contain cognate intrusive xenoliths and xenocrysts and oscillatory zoning is a common feature of clinopyroxene, nepheline, and melanite crystals, indicating recycling of intrusive material. Irregular calcite-rich bodies in many samples are interpreted as quenched immiscible Ca-carbonatite liquid, and [Ca-carbonate]-silicate liquid immiscibility is observed in experiments with one nephelinite. Chemical variation in the Shombole suite can be modeled as a combination of crystal fractionation (clinopyroxene and heavy minor phases) and retention of neutral density nepheline derived from disaggregated xenoliths entrained during emplacement of dike swarms. Six newly analyzed lavas from Oldoinyo L'engai, northern Tanzania, are geochemically similar to Shombole nephelinites except that they have relatively high Na₂O+K₂O (average 18% vs 12%) and Zr (average 680 ppm vs 400 ppm). They are extremely peralkaline and are not typical of nephelinites from other centres. Three with [Na+K]/Al1.5 contain euhedral wollastonite phenocrysts; three with [Na+K]/Al2.0 contain combeite (Na₂Ca₂Si₃O₉) phenocrysts and pseudomorphs after wollastonite. Both types contain abundant sodalite phenocrysts (+nepheline+clinopyroxene+melanite+sphene). Seven other wollastonite nephelinite samples from L'engai have been described, but it is a lava type rarely seen in other centres. Combeite has been described from only two other locations (Mt. Shaheru, Zaire; Mayener Feld, Eifel). The hyperalkaline L'engai nephelinites have compositions similar to those of experimental silicate liquids immiscible with natrocarbonatite. Textural evidence for both carbonate-silicate (as carbonate globules) and silicate-silicate (as two optically discrete glasses with distinct compositions) liquid immiscibility is observed in the samples.  相似文献   

Pt-Os and Re-Os systematics of the Sudbury Igneous Complex, Ontario     

John W. Morgan  Richard J. WalkerMary F. Horan  Ellyn S. BearyAnthony J. Naldrett 《Geochimica et cosmochimica acta》2002,66(2):273-290

We measured by negative thermal ionization mass spectrometry (NTIMS) Re, Os and ¹⁸⁶Os/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os in 26 samples of 18 Ni-Cu sulfide ores from the Falconbridge, McCreedy West, and Strathcona mines at Sudbury, Ontario. At McCreedy West and Falconbridge, the isochron Re-Os ages are 1835 ± 70 Ma and 1827 ± 340 Ma, and the initial ¹⁸⁷Os/¹⁸⁸Os ratios 0.514 ± 0.019 and 0.550 ± 0.024, respectively. The ages agree with the canonical value of 1850 ± 1 Ma for the Sudbury Igneous Complex (SIC). For Hangingwall and Deep Zone ores at Strathcona, the age of 1780 ± 7 Ma may reflect resetting by dyke activity. The high initial ¹⁸⁷Os/¹⁸⁸Os of 0.934 ± 0.005 in these ores is distinct from those at McCreedy West and Falconbridge. Strathcona Deep Copper Zone ores have highly radiogenic Os giving a mean model age of 1883 ± 54 Ma that is similar to ages at McCreedy West and Falconbridge, but distinct from other Strathcona sulfides. Initial ¹⁸⁶Os/¹⁸⁸Os in two Strathcona ores with low ¹⁹⁰Pt/¹⁸⁸Os average 0.119 826 ± 0.000 009 (n = 3) and 0.119 827 ± 0.000 004 (n = 3), respectively, with a grand mean of 0.119 827 ± 0.000 003. This ratio may be slightly lower than the chondritic value at that time. Similar ores at Falconbridge and McCreedy West show more scatter, averaging 0.119 855 ± 0.000 008 (n = 6) and 0.119 867 ± 0.000 020 (n = 3), respectively. These values are substantially suprachondritic. The Re-Os isotope systematics of Sudbury ores are clearly of crustal origin and may be derived from a binary mixture of Superior Province and Huronian metasedimentary rocks, with Strathcona, Falconbridge, and McCreedy West ores containing, respectively, 55%, 16%, and 12% of Os from Superior sediments. The suprachondritic ¹⁸⁶Os/¹⁸⁸Os at McCreedy West and Falconbridge may be due to admixture of Archean or Paleozoic mafic rocks with ¹⁹⁰Pt/¹⁸⁸Os ≈ 0.1. No trace of the asteroid that produced the Sudbury Structure has been reported. At the Whistle mine S-poor olivine melanorite inclusions with high Ni and Os and low ¹⁸⁷Os/¹⁸⁸Os may contain the signature of a magmatically fractionated asteroidal core contributing 1 to 2.5 % metal. The S-poor melanorite Ni and Os data are equally well explained by admixture of ≈40% mantle peridotite, however.  相似文献   

Feature     

Tony Waltham 《Geology Today》2001,17(3):101-104

The last decade has seen a complete revision of the sedimentary environment, stratigraphic sequence and structural history of the early Ordovician Manx 'Slate' Group of the Isle of Man .  相似文献   

贵州锦丰(烂泥沟)金矿床含砷黄铁矿和脉石英及其包裹体的稀土元素特征   总被引：3，自引：0，他引：3  

陈懋弘  吴六灵Phillip J.Uttley　Tony Norman郑建民  秦运忠 《岩石学报》2007,23(10):2423-2433

贵州锦丰(烂泥沟)金矿是滇黔桂“金三角“目前已探明的最大的卡林型金矿床.含砷黄铁矿是该矿床最主要的载金矿物,脉石英是最显著的热液蚀变作用产物.含砷黄铁矿、脉石英及其包裹体具有相同的稀土元素特征,表现为轻重稀土分馏明显(LREE/HREE:7.95～31.77,(La/Yb)N:8.97～40.49);轻稀土有一定的分异((La/Sm)n:3.20～5.29),曲线右倾程度大;重稀土分异不明显((Gd/Yb)n:1.41～3.35),曲线平坦;负铕异常明显(δEu 0.59～0.71);微弱的铈负异常(δCe 0.97～0.98).代表成矿流体特征的脉石英包裹体稀土配分型式及特征值与区域上不同时代的幔源基性-超基性岩差别较大,而与矿床围岩、矿石、含砷黄铁矿及其包裹体稀土配分型式及特征值十分相似,具上部地壳普通沉积岩的特点,反映成矿流体主要不是来源于地幔,而是以壳源为主.Eu负异常反映了偏酸性(富CO2)、还原性的成矿环境;微弱的Ce负异常表明成矿流体来源于以沉积水和热脱水为主的盆地流体.  相似文献   

贵州锦丰（烂泥沟）超大型金矿床构造解析及构造成矿作用   总被引：3，自引：4，他引：3       下载免费PDF全文

陈懋弘  毛景文  Phillip J. Uttley  Tony Norman  吴六灵  郑建民  秦运忠 《矿床地质》2007,26(4):380-396

在矿区大比例尺填图的基础上,运用构造解析的理论和方法,对贵州锦丰(烂泥沟)金矿的构造特征、构造演化和构造控矿作用进行了系统的阐述。矿区构造线主要分为近NS向、NW向和NE向3组,其中NW向构造线控制了矿区的构造格架。构造变形主要经历了同生期裂陷、造山期挤压、后碰撞造山侧向挤压和岩石圈伸展等4个阶段。盆地裂陷期间(D2-T2)形成的同生断层为后续的构造活动提供了最初的构造薄弱面,并成为主要的热液通道;造山期挤压期间(T3)形成了矿区最为明显的近NS向和NW向大型倒转褶皱和逆冲断层以及配套的NE向右旋走滑断层;后碰撞造山侧向挤压期间(J1)形成走向NE的叠加褶皱,并在F2-F3"X"型断裂系上形成近EW向的拉张区,矿质沿着拉张的空间沉淀就位;燕山期(J2-K)岩石圈伸展,形成小规模的近水平逆断层。上述4个主要构造变形阶段与右江盆地的裂陷-闭合-坍塌的发展演化历史相吻合,成矿发生在由挤压向拉张过渡的构造体制转换阶段。造山期逆冲作用形成的构造闭圈和后碰撞造山期间挤压向伸展转变的过程中形成的局部张性构造环境是矿质得以大量聚集、沉淀的主要构造控制因素。构造成矿作用是锦丰(烂泥沟)金矿单一矿体垂向延深1000m以上的主要制约因素。构造成矿的观点将为在滇黔桂"金三角"卡林型金矿带的深部找矿提供重要的理论依据。  相似文献