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81.
Gold enrichment mechanism of ore-forming fluid is the essence of gold metallization.This paper summarizes the distinguishing symbols of mantle fluid and effect of crust-mantle structure on fluid movement. Fluid moving processes include osmosis, surge, gas-liquid alterna-tion and mutation of fluid speed. During fluid movement, gold will be enriched gradually. Final-ly, a layered circulatory system is illustrated in this paper. 相似文献
82.
Fluid Inclusions in the Gold—Bearing Quartz Veins at Um Rus Area,Eastern Desert,Egypt 总被引:2,自引:0,他引:2
MOHAMEDELTOKHI ABDALLAELMUSLEM 《中国地球化学学报》2002,21(2):131-139
Fluid inclusions in the gold-bearing quartz veins at the Um Rus area are of three types: H2O, H2O−CO2 and CO2 inclusions. H2O inclusions are the most abundant, they include two phases which exhibit low and high homogenization temperatures ranging
from 150 to 200°C and 175 to 250°C, respectively. The salinity of aqueous inclusions, based on ice melting, varies between
6.1 and 8 equiv. wt% NaCl. On the other hand, H2O−CO2 fluid inclusions include three phases. Their total homogenization temperatures range from 270 to 325°C, and their salinity,
based on clathrate melting, ranges between 0.8 and 3.8 equiv. wt% NaCl. CO2 fluid inclusions homogenize to a liquid phase and exhibit a low density range from 0.52 to 0.66 g/cm3. The partial mixing of H2O−CO2 and salt H2O−NaCl fluid inclusions is the main source of fluids from which the other types of inclusions were derived. The gold-bearing
quartz veins are believed to be of medium temperature hydrothermal convective origin. 相似文献
83.
Keegan E. Alagna Maurizio Petrelli Diego Perugini Giampiero Poli 《Geostandards and Geoanalytical Research》2008,32(1):103-120
In this study we evaluated the capability of a 213 nm laser ablation system coupled to a quadrupole-based ICP-MS in delivering accurate and precise U-Pb ages on zircons and monazites. Four zircon samples ( ca. 50 Ma to ca. 600 Ma) and four monazite samples ( ca. 30 Ma to ca. 1390 Ma) of known ages were analysed utilising laser ablation pits with diameters of 20 μm and 60 μm. Instrument mass bias and laser induced time-dependent elemental fractionation were corrected for by calibration against a matrix-matched reference material. Tera-Wasserburg plots of the calculated U-Pb data were employed to assess, and correct for, common Pb contributions. The results indicated that the LA-ICP-MS technique employed in this study allowed precise and accurate U-Pb isotope dating of zircon and monazite on sample areas 20 μm in diameter. At this spot size, the precisions achieved for single spot 206 Pb/238 U ages, were better than 5% (2s) for monazites and zircons with ages down to 30 Ma and 50 Ma, respectively. The precisions reported are comparable to those generally reported in SIMS and LA-MC-ICP-MS U-Pb isotope determinations. 相似文献
84.
85.
《地学前缘(英文版)》2020,11(5):1593-1608
The Gejiu-Bozushan-Laojunshan W-Sn polymetallic metallogenic belt(GBLB) in southeast Yunnan Province is an important part of the southwestern Yangtze Block in South China.Tin polymetallic mineralization in this belt includes the Niusipo,Malage,Songshujiao,Laochang and Kafang ore fields in the Gejiu area which are spatially and temporally associated with the Kafang-Laochang and Songshujiao granite plutons.These granites are characterized by variable A/CNK values(mostly 1.1,except for two samples with 1.09),high contents of SiO_2(74.38-76.84 wt.%) and Al_2 O_3(12.46-14.05 wt.%) and variable CaO/Na_2 O ratios(0.2-0.65) as well as high zircon δ~(18)O values(7.74‰-9.86‰),indicative of S-type affinities.These rocks are depleted in Rb,Th,U,Ti,LREE[(La/Yb)N=1.4-20.51],Ba,Nb,Sr,and Ti and display strong negative Eu and Ba anomalies.The rocks possess high Rb/Sr and Rb/Ba ratios,relatively low initial ~(87)Sr/~(86)Sr ratios(0.6917-0.7101),and less radiogenic εNd(t)values(-8.0 to-9.1).The zircon grains from these rocks show negative ε_(Hf)(t) values in the range of-3.7 to-9.9 with mean T_(DM2)(Nd) and T_(DM2)(Hf) values of 1.57 Ga and 1.55 Ga.They show initial ~(207)Pb/~(204)Pb ranging from15.69 to 15.71 and ~(206)Pb/~(204)Pb from 18.36 to 18.70.Monazite from Songshujiao granites exhibits higher U and lower Th/U ratios,lower δ~(18)O values and higher ε_(Hf)(t) values than those of the zircon grains in the KafangLaochang granites.The geochemical and isotopic features indicate that the Laochang-Kafang granites originated by partial melting of Mesoproterozoic crustal components including biotite-rich metapelite and metagraywacke,whereas the Songshujiao granites were derived from Mesoproterozoic muscovite-rich metapelite crustal source.Most zircon grains from the Songshujiao,Laochang and Kafang granites have high-U concentrations and their SIMS U-Pb ages show age scatter from 81.6 Ma to 88.6 Ma,80.7 Ma to 86.1 Ma and 82.3 Ma to 87.0 Ma,suggesting formation earlier than the monazite and cassiterite.Monazite SIMS U-Pb ages and Th-Pb ages of three same granite samples are consistent and show yielded 206 Pb/~(238)U ages of 83.7 ± 0.6 Ma,83.7±0.6 Ma,and 83.4±0.6 Ma,and ~(208)Pb/~(232)Th ages of 83.2 ± 0.5 Ma,83.8 ± 0.4 Ma,and 83.5±0.9 Ma,which are within the range of the SIMS zircon U-Pb ages from these rocks.The data constrain the crystallization of the granites at ca.83 Ma.In situ U-Pb dating of two cassiterite samples from the cassiterite-sulfide ore in the Songshujiao ore field and Kafang ore field,and two from the cassiterite-oxide+cassiterite bearing dolomite in the Laochang ore field yielded weighted mean 206 Pb/~(238)U ages of 83.5±0.4 Ma(MSWD=0.6),83.5 ± 0.4 Ma(MSWD=0.5),83.6 ±0.4 Ma(MSWD=0.6) and 83.2 ±0.7 Ma(MSWD=0.6),respectively.Combined with geological characteristics,the new geochronological data indicate that the formation of the granites and Sn polymetallic deposits are coeval.We correlate the magmatic and metallogenic event with lithospheric thinning and asthenosphere upwelling in continental extension setting in relation to the eastward subduction of the Neo-Tethys beneath the Sanjiang tectonic domain during Late Cretaceous. 相似文献
86.
受燕山中晚期地质构造运动和岩浆活动的影响 ,在中酸性侵入岩体与钙镁碳酸盐岩外接触带 ,大银山金矿体呈扁平透镜状产于侵入体的舌状端部。是中低温热液型含金矽卡岩矿床。研究其成矿地质条件和控因素 ,对找矿和成矿预测均有重要意义 相似文献
87.
I. C. W. FITZSIMONS P. D. KINNY S. WETHERLEY D. A. HOLLINGSWORTH 《Journal of Metamorphic Geology》2005,23(4):261-277
Several petrographic studies have linked accessory monazite growth in pelitic schist to metamorphic reactions involving major rock‐forming minerals, but little attention has been paid to the control that bulk composition might have on these reactions. In this study we use chemographic projections and pseudosections to argue that discrepant monazite ages from the Mount Barren Group of the Albany–Fraser Orogen, Western Australia, reflect differing bulk compositions. A new Sensitive High‐mass Resolution Ion Microprobe (SHRIMP) U–Pb monazite age of 1027 ± 8 Ma for pelitic schist from the Mount Barren Group contrasts markedly with previously published SHRIMP U–Pb monazite and xenotime ages of c. 1200 Ma for the same area. All dated samples experienced identical metamorphic conditions, but preserve different mineral assemblages due to variable bulk composition. Monazite grains dated at c. 1200 Ma are from relatively magnesian rocks dominated by biotite, kyanite and/or staurolite, whilst c. 1027 Ma grains are from a ferroan rock dominated by garnet and staurolite. The latter monazite population is likely to have grown when staurolite was produced at the expense of garnet and chlorite, but this reaction was not intersected by more magnesian compositions, which are instead dominated by monazite that grew during an earlier, greenschist facies metamorphic event. These results imply that monazite ages from pelitic schist can vary depending on the bulk composition of the host rock. Samples containing both garnet and staurolite are the most likely to yield monazite ages that approximate the timing of peak metamorphism in amphibolite facies terranes. Samples too magnesian to ever grow garnet, or too iron‐rich to undergo garnet breakdown, are likely to yield older monazite, and the age difference can be significant in terranes with a polymetamorphic history. 相似文献
88.
Whole-rock, Phosphate, and Silicate Compositional Trends across an Amphibolite- to Granulite-facies Transition, Tamil Nadu, India 总被引:2,自引:0,他引:2
Chemical trends from north (amphibolite facies) to south (granulitefacies) along a 95 km traverse in Tamil Nadu, Southern India,include: whole-rock depletion of Rb, Cs, Th and U, enrichmentin Ti and F, and depletion in Fe and Mn in biotite and amphibole;increases in Al and decreases in Mn in orthopyroxene; enrichmentof fluorapatite in F. K-feldspar blebs are widespread alongquartz–plagioclase grain boundaries, and could indicateeither partial melting or metasomatism. In the northernmostportion of the traverse the principal rare earth element (REE)-bearingminerals are allanite and titanite. South of a clinopyroxeneisograd, monazite grains independent of fluorapatite are themajor REE- and Th-bearing phase. Further south independent monaziteis rare but Th-free monazite inclusions are common in fluorapatite.During prograde metamorphism, independent monazite was replacedby REE-rich fluorapatite in which the monazite inclusions laterformed. The loss of independent monazite was accompanied bya loss of whole-rock Th and possibly a small depletion in lightREE. Most mineralogical features along the traverse can be accountedfor by progressive dehydration and oxidation reactions. Trace-elementdepletion is best explained by the action of an externally derivedlow H2O activity brine migrating from a source at greater depth,possibly preceded or accompanied by partial melting. KEY WORDS: granulite facies; charnockite; metasomatism; Archean; Tamil Nadu, India; fluorapatite; monazite; allanite; titanite; biotite 相似文献
89.
90.
TWO EPISODES OF MONAZITE CRYSTALLIZATION DURING METAMORPHISM AND CRUSTAL MELTING IN THE EVEREST REGION OF THE NEPALESE HIMALAYA 总被引:4,自引:2,他引:2
TWO EPISODES OF MONAZITE CRYSTALLIZATION DURING METAMORPHISM AND CRUSTAL MELTING IN THE EVEREST REGION OF THE NEPALESE HIMALAYA 相似文献