| The Xiangshan uranium ore field is the largest volcanic rock-hosted hydrothermal uranium ore field in China.In the recent years,deep explorations have revealed some uranium-lead-zinc polymetallic deposits in this region.However,the ore-forming fluid characteristics and metallogenic mechanism of the uranium-lead-zinc polymetallic deposits,especially the study of the relationship between lead-zinc mineralization and uranium deposits in the ore-field,remain controversial.This thesis performed comprehensive studies on the mechanism of lead-zinc and uranium mineralization in the Xiangshan uranium orefield by using the methods of altered petrology,mineral geochemistry,S isotope,comparing and analyzing hydrothermal alteration and ore-forming fluids of the uranium deposits in western and northern parts of Xiangshan,and analyzing sources of ore-forming fluids of the Niutoushan lead-zinc ore body,in order to explore the relationship between lead-zinc and uranium mineralization.Microscopic and electron probe results show that a large number of pyrites in the altered surrounding rock and uranium ore of the Xiangshan uranium deposit are hydrothermal origin.The pyrites mainly formed in the stage before the main uranium mineralization,and played an important role in uranium precipitation from the ore-forming fluid.The characteristics of sulfur isotope compositions of pyrites in the uranium ores from western and northern Xiangshan are obviously different.The?34S values of pyrites(+ 0.1 ‰?8.4 ‰)in the uranium deposits(Ju-long'an,He-yuanbei)in western Xiangshanis lower than the834S values of pyrites(7.5‰ ?16.2‰)in the uranium deposit(Shazhou)in northern Xiangshan.These different?34S values indicate that the sources of hydrothermal fluids and sulfur in these two regions are different.The sulfur forming hydrothermal pyrite in the uranium deposits in western Xiangshan was originated from the basement metamorphic sedimentary rocks and from surrounding rock(volcano-subvolcanic rock)through leaching by fluids of evolved meteoric water,while the sulfur of hydrothermal pyrites in the uranium deposits in northern Xiangshan was likely originated mainly from sulfate in the red strata.This hints the presence of red basin brine in the ore-forming fluids in the uranium mineralization in northern Xiangshan.In addition,the trace and rare earth elements of the ore in the western and northern uranium deposits were also show significant differences relative to their respective fresh rocks and altered rocks,indicating the ore-forming fluids in the western and northern uranium deposits were significantly different.The ore-forming fluids in the western uranium deposits are rich in F,while the ore-forming fluids in the northern uranium deposits are relatively poor in F.The uranium in the western and northern uranium deposits of the Xiangshan ore-field is mainly migrated in the form of fluorine complex.The mineralization scale of the western uranium deposits in the ore field is larger than that of the northern uranium deposit,which may have been related to the amount of F in the ore-forming fluid in the former,suggesting that the western part of Xiangshan has a good prospect for finding large mines.Base on the analysis of S isotope of sulfide ore in the Niutoushan Pb-Zn mineralization in western of the ore-field,we obtained the?34S?S(total sulfur isotope)value of the mineralized fluid,+3.7‰.The temperatures calculated by using the sulfur isotope equilibrium between the paragenetic sulfide mineral(sphalerite-galena)are in the range of 197? to 476?,indicating that the ore-forming fluid is a medium-high temperature magmatic hydrothermal fluid.Furthermore,the trace and rare earth elements of the sulfide minerals show that the magmatic hydrothermal fluid are rich Cl and poor F,which may have come mainly from the latest stage of intrusive rhyolite porphyry magmatic in the ore-field.Base on comparative analysis of uranium deposits and Pb-Zn mineralization,we believe that they were not formed by the same volcano-magmatic hydrothermal ore-forming system.The uranium deposits are medium-low temperature hydrothermal deposits,and the ore-forming fluids are evolved mainly from meteoric waters;the Pb-Zn mineralization is formed by middle and high temperature magmatic hydrothermal fluids.In addition,according to the sulfur isotope characteristics of pyrite in the western uranium deposit and the spatial distribution of uranium ore and Pb-Zn mineralization,it is believed that sulfur of pyrite in the uranium deposits may have partly come from the Pb-Zn mineralized wall rocks below. |
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