Black And Red Alterations Associated With The Baimadong Uranium Deposit (Guizhou,China):Genetic Relationship With Uranium Mineralization

The Baimadong uranium deposit is located in the Yantze Platform,south part of central Guizhou uplift?Qianzhong uplift?.This deposit has great uranium metallogenic potential and prospecting prospects,which is one of the most famous carbonaceous-siliceous-pelitic rock-type uranium deposits in China.The Baimadong uranium deposit is medium-size,rich-grade and accompanied by multi-metal,such as Hg,Mo.The multi-metal have meet the requirments of comprehensive industrial utilization.This PHD thesis takes the Baimadong uranium deposit as the research object,based on the detailed field observation,indoor microscopic petrographic studies,contemporary analytical methods,modern uranium metallogenic theory,systematic research and summary of previous research results and regional geological events to discuss the genesis of black-red alteration and their relationship with uranium mineralization.Finally,a comprehensive model of carbonaceous-siliceous-pelitic rock-type uranium deposit is established to provid the new mark for exploring this type uranium deposit.The main results of this research are as follows:?1?It is confirmed that the uranium mineralization in the Baimadong uranium deposit has close relationship with silicification,argilization,black alteration,red alteration,and well development of sulfide.?2?The black material in the black alteration is mainly composed with organic carbon such as graphite and bitumen.These organic matters are considered to be external brought into by hydrothermal activitives rather than syn-diagenesis organic matter.The red material in the red alteration is mainly composed with iron oxides such as hematite and goethite.?3?A large number of main and trace elements,especially the materials beneficial to uranium mineraliation,such as organic matter,U and other multi-metal elements are brought in through black alteration.?4?The black material in the black-altered rocks of the Baimadong uranium deposit has similar geochemical characteristics with that of the Niutitang Formation.Based on the research of organic geochemical characteristics,it is confirmed that the organic matter in black-altered rocks is marine organic matter,derived from the Niutitang Formation.In addition,the altered rocks have a similar dirtribution pattern of the metal element with the black rock series of the Niutitang Formation.It is believed that U,multy-metal elements and ore-forming materials are derieved from the black rock series of the Niutitang Formation.What's more,U also derived from the Cambrian U-rich sedimentary rocks,pathed through by meteoric water.?5?The ore-forming fluid is characterized by medium-low temperature,medium-low salinity and medium density.It is a reductive H₂O-NaCI±CH₄ fluid system in the early stage of the mineralization and oxidizing H₂O-NaCI fluid system in the late stage of the mineralization.There are deep fluid participate during the black alteration period through the research of carbon,oxygen and sulfur isotope.The fluid in the red alteration period is meteoric water.Meteoric water plays a vital role in uranium mineralization of the Baimadong uranium deposit.The Baimadong uranium deposit is a type of medium-low temperature hydrothermal uranium deposit.?6?The metallogenic model of the Baimadong uranium deposit is initially established.According to the model,the marine organic matter moved up along the Baimadong fault under the driving of deep heat.They were trapped in the interformational fracture zone and the black alteration formed.At the same time,Si-rich hydrothermal fluid carried U,Hg and Mo moving along the deep fault.The black alteration plays a crucial role in the formation f the Baimadong uranium deposit:?1?creating a reducing environment for the uranium mineralization;?2?preliminarily enriches uranium.Although a lot of organic matter and multi-metal elements such as U were brought into during the black alteration period,it can not produce economical ore bodies with industrial value only through black alteration.A hydrothermal event after black alteration—red alteration brought more uranium,further activate the migration of uranium,and finally precipitate to make the formation of the Baimadong uranium deposit.During the red alteration period,the fluid flowed through various types of rocks?including Cambrian U-rich sedimentary rocks?,causing U⁴⁺in the rocks to be oxidized to U⁶⁺.When this type of oxidizing carrying U⁶⁺fluid encountered the reducing environment created by the black alteration,U⁶⁺was reduced to U⁴⁺and U precipitated.At the same time,Fe²⁺was oxidized to Fe³⁺,causing the precipitation of hematite and goethite and the red alteration occurred.?7?The formation of the Baimadong uranium deposit is related to the black rock series,as the organic matter in the black alteration is external,so it is significantly different with the black shale-type uranium deposit defined by the International Atomic Energy Organization.The Baimadong uranium deposit is classified as carbonaceous-siliceous-pelitic rock-type uranium deposit by Chinese scholars.Combined with the latest classification of the International Atomic Energy Organization and the determined geological characteristics and the formation mechanism of carbonate-type uranium,it is believed that the Baimadong uranium deposit belongs to the carbonate-type uranium deposit,and can be further classified into cataclastic carbonate-type uranium deposit.This type uranium deposits are controlled by tectonics.The regional tectonic activities lead the well developed of the structural fault system and interformational fracture zone and the rock permeability increased in the study area.At the same time,the uranium-rich and organic-rich black rock series,which formed in the Cambrian,provided uranium and organic matter for the formation of the Baimadong uranium deposit.These two points are the most important factors for the formation of the Baimadong uranium deposit.?8?It is confirmed that the Baimadong uranium deposit has much similar geological and geochemical characterisitics with the carbonaceous-siliceous-pelitic rock-type uranium deposit in South China.Therefore,this study provides important theoretical support for the future prospecting and exploration of this type of uranium deposit in South China.