Abstract:The Dawan porphyry molybdenum deposit is located in a rather small thyolite porphyry body that contains extremely high K2O and has dolomite as its wall rock. Three types of alteration are well developed within the porphyry, namely potassic alteration, silicification and green alteration. The chief product of potassic alteration is hydrothermal biotite; silicification is of multiple phases; green alteration mainly occurring at the contact zone is noted for the assemblage of Fe-rich phlogopite, maguesiohornblende, epidote and/or albite. Being characteristic of the deposit, the green alteration is related to dolomite breccias in the porphyry and is accompanied by rich molybdenite precipitation. In view of the very tiny size of the minerals, and with a purpose of depicting quantitatively the relationship between the alteration and the mineralization, the author applies X-ray diffraction analysis to whole rock samples with known molybdenum contents. The intensities of the characteristic peaks of quartz, orthoclase, plagioclase, mica and hornblende as well as the height of the background have been determined systematically, which leads to the establishment of an alteration profile. From the bottom upward and from the middle outward, the potash alteration changes into silicification and then into green alteration. It is revealed that molybdenum is enriched in green alteration zone, and high extremes of molybdenum contents are always in consistency with high abundance of mica, hornblende and Fe-content as well as low contents of quartz and orthoclase. This suggests that green alteration was favorable to molybdenum enrichment, for there must have been obvious changes in the physical-chemical condition of 'the hydrorthermal solution in green alteration zone, and the highly permeable breccias and their rims could provide space for mineralization. At the earlier stage, the quartz inclusions of ore-bearing veinlets had homogenization temperature ranging from 460-260℃, salinity 50-16 NaCl wt%, δ18O 10.7-8.7‰, and δ18OH2O 5.7-4.6‰, indicating that molybdenite had been precipitated from hydrothermal solution mainly composed of magmatic water. At the late stage, homogenization temperature was 260-120℃, salinity 11.8-0.3%, δ18O 0.8-0.5‰ and δ18OH2O under-10.9‰, suggesting that by this stage, meteoric water had probably played a predominant role in the hydrothermal solutions and δ18O of whole rock had also been more or less reduced.
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涂勘.1986.河北涞源大湾钼矿床的蚀变矿化研究[J].矿床地质,5(4):49~61.1986.Hydrothermal Alteration And Molybdenum Mineralization Of The Dawan Molybdenum Deposit,Laiyuan County,Hebei Province[J].Mineral Deposits5(4):49~61
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