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261.
The color of pumice: case study on a trachytic fall deposit, Meidob volcanic field, Sudan 总被引:1,自引:0,他引:1
This paper examines the cause of color variations of trachytic pumices which are essentially uniform in chemical composition
and proposes a geological model for their formation. A pyroclastic sequence of distinct subunits with brown, buff, and black
pumices was deposited during the 5000-B.P. eruption of a tuff ring in the central Meidob volcanic field (Sudan). Subunits
of buff pumices locally contain minor amounts of streaky pumice with pale-gray and dark-gray domains. The combined evidence
of petrographic studies, chemical analyses of whole pumices and groundmass separates, electron microprobe analyses, optical
spectroscopy, transmission electron microscopy, and magnetic susceptibility measurements show that color variations of the
pumice clasts are related to the size and distribution of Fe3+-rich oxide microcrysts. Buff pumice and light-gray domains of streaky pumice have a colorless, transparent groundmass with
very few microcrysts. Dark-gray domains of streaky pumice contain abundant hematite and/or magnetite microcrysts visible in
thin section within a transparent, colorless glass groundmass. The groundmass of the black pumice clasts is brown in thin
section which is most likely caused by submicroscopic magnetite microcrysts. Brown pumice clasts have a mixed groundmass consisting
of brown domains and domains with opaque microcrysts in transparent glass. Variations in the eruption dynamics have been inferred
from lithological observations. Subunits of black pumices are related to eruption pulses with low magma discharge and high
water/magma mass ratio, whereas subunits of buff pumice were deposited during eruption pulses with high magma discharge and
low water/magma mass ratio. Brown pumices represent the top part of the magma body, and the initial stage of the eruption
probably had a low magma discharge. Streaky pumices are interpreted as the product of syn-eruptive mixing of Fe3+-rich oxide microcryst-bearing magma and microcryst-free magma.
Received: 3 February 1997 / Accepted: 28 July 1997 相似文献
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267.
Spatial-temporal Frame,Evolution and Mineralization of the Northern Qilian Metallogenic Province 总被引:1,自引:0,他引:1
Sun Haitian Wu Jieren Li Jinping Institute of Mineral Deposits Chinese Academy of Geological Sciences Beijing Xi''an Institute of Geology Mineral Resources Chinese Academy of Geological Sciences Xi''an Shaanxi Institute of Mineral Deposits Chinese Academy of Geological Sciences Beijing 《《地质学报》英文版》1997,71(2):204-216
Four metallogenic epochs occurred in different tectonic environments during theevolution of the Northern Qilian metallogenic province through the geological time. The Mid-dle Proterozoic metallogenic epoch witnessed the tectonic environment of crustal breakupcaused by mantle diapirism, in which ultramafic-mafic rocks were intruded along beep faultbelts and the superlarge Jinchuan magmatic Cu-Ni sulphide deposit was formed. In theMiddle-Late Proterozoic metallogenic epoch the crust was further broken to form anintracontinental rift, in which the Chenjiamiao style massive Cu-Fe sulphide deposits hosted bybasic volcanic tuff were formed in the lower volcano-sedimentary sequence, while the largesedex type Jingtieshan style Fe-Cu deposits were formed within the upper abyssal carbon-richargillaceous sedimentary sequence. The Early Palaeozoic saw the aulacogen environment, with-in which the Baiyinchang style superlarge massive base and precious metal sulphide depositshosted by quartz keratophyric tuff were formed in the Middle-Late Cambrian rifted island arcand the massive Cu-Zn sulphide deposits and magmatic chromite deposits associated with theophiolite suite were formed in the Early-Middle Ordovician, and the Honggou style massiveCu-Fe sulphide deposits hosted by spilite were formed in the Late Ordovician back-arc basinenvironment. In the Late Palaeozoic-Meso-Cenozoic, the metallogenic province went into anintracontinental orogenic stage characterized by compressive tectonic environment, in whichthere occurred carbonate-quartz vein type and tectono-alteration gold deposits associated withductile-shear structures. 相似文献
268.
Zhang Sheng Li Tongjin Wang LiankuiGuangzhou Institute of Geochemistry Chinese Academy of SciencesWushan Guangzhou 《《地质学报》英文版》1997,71(4):433-445
The Changkeng gold-silver deposits consist of a sediment-hosted, disseminated gold deposit and a replacement-type silver deposit. The mineralizations of gold and silver are zoned and closely related to the silicification of carbonate and clastic rocks, so that siliceous ores dominate in the deposit. The mineralizing temperature ranges mainly from 300 to 170℃, and K+, Na+, Ca2+, Mg2+, and Cl- are the major ions in the ore-forming fluid. Calculations of distribution of metal complexes show that gold is mainly transported by hydrosulphide complexes, but chloride complexes of silver, iron, lead, and zinc, which are transformed into hydroxyl and hydrosulphide complexes under neutral to weak-alkaline circumstances in the late stage, predominate in the ore-forming solutions. Water-rock interaction is confirmed to be the effective mechanism for the formation of silver ores by computer modelling of reaction of hydrothermal solution with carbonate rocks. The solubility analyses demonstrate that the precipitation 相似文献
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赵文浩 《华北地质矿产杂志》1997,(2)
近年来“矿源层”这一概念已被广大地质学家所接受,他们认为矿床中的金是从围岩中萃取的,本文认为矿床中的金来源于岩浆残余热液。在地壳深部,当丰度值高的基性岩或其他岩石,在深熔作用下产生花岗质岩浆时,同时带出许多金,当岩浆侵入到上部地壳冷凝结晶时,其中的大部金进入残余热液,最后富集成矿。 相似文献