A Study of Features, Modes of Occurrence and Enrichment Mechanism of Silver-rich Ore Bodies in the Jiama Copper-Polymetallic Deposit of Tibet
西藏甲玛铜多金属矿富银矿体地质特征、银赋存状态及富集机理研究

Located in Maizhokunggar County of Tibet, the Jiama copper-polymetallic ore deposit discovered in recent years has silver reserves of over 6000t, constituting a superlarge ore deposit in the eastern part of central Gangdise. According to modes of occurrence, the silver ore bodies in the Jiama copper-polymetallic ore deposit can be classified into skarn-type and marble-type. Mineragraphic studies and electron probe microanalysis reveal that silver exists in two types, i.e., visible silver and invisible silver, with visible silver being dominant. Invisible silver exists in copper minerals in the form of lattice silver besides existing in galena in the form of sub-microinclusions. Visible silver generally replaces sulfides such as bornite and chalcopyrite, or fills in the microsection of sulfides and lodestones in the form of four independent minerals, i.e., hessite, electrum, silver-lead ore and native silver. In the premetallogenic and high temperature phases, the metal ions in the Jiama ore deposit such as Cu, Mo, Au, Pb, Zn, Ag were transported in the form of complex compound. With the lowering of the temperature of metallogenic hydrothermal solution and the rising of the oxygen fugacity and pH value, the complex compound disintegrated due to the falling of its stability. S- and H- complex compounds constituted the main transporting form of Cu, Mo, Au, Pb, Zn, Ag. With the lowering of the temperature, the metal S- and H- complex compounds, such as Cu, Mo complex compounds, started to disintegrate and formed sulfides like molybdenum, chalcopyrite and bornite. And at this time, part of silver was distributed in these sulfides in the form of micro and sub-microinclusions and lattice silver. With the continuous lowering of the temperature, the precipitation of large quantities of Pb and Zn sulfides caused remarkable alteration of the components and properties of metallogenic hydrothermal solution, leading to thorough silver disintegration from S- and H- complex compounds and combination with Cu2+ to form large quantities of independent silver minerals, while the oversaturated silver in the solution was settled in the form of native silver.