Pure silver is also called white silver in view of its silvery white color. Among all metals silver possesses the highest electrical and thermal conductivity. Besides, it has fairly good ductility and plasticity. Silver can be easily polished and made in arbitrary forms. It can be used with many other metals to produce alloys or pseudo-alloys. Silver is possessed of relatively strong resistance to corrosion, organic acids and alkalis and is hard to oxidize at the normal temperature and humidity. Thanks to so many merits of it, silver not only was used to make coins, adornments and tableware long ago, but also has become an indispensable raw material in construction and national defense and widely applied to modern industries.

3.19.1 Resources

3.19.1.1 Reserves and Resources

By the end of 1997, 572 silver ore districts had been explored, distributed in 27 provinces, municipalities and autonomous regions in China and the retained reserves had reached 117,122 tons, of which 27,811 tons (23.7%) belong to categories A+B+C. The retained reserves doubled in comparison with 1985.     

The silver reserves in China are divided into two parts: ¢Ù silver ore whose grade comes up to relevant industrial standards so that it is of value for exploitation silver alone; and ¢Ú silver ore whose grade is lower than the industrial standards and can be recovered as a by-product during the exploitation of the main elements. Since the 1980s the silver ore reserves have seen a relatively rapid increase. The silver ore reserves accounted for 18.4% of the total ore reserves in China in 1985, but the figure was 42% in 1995. The reserves of by-product silver ore, however, had a rather small increase in the same period (Table 3.19.1).

Table 3.19.1 Retained silver reserves during 1985~1997 (t)

According to Mineral Commodity Summaries, 1996, published by the USA Bureau of Mines, the total silver reserves of the world were 280,000 tons and the reserve base was 420,000 tons, distributed mainly in the USA (reserves: 31,000 tons; reserve base: 72,000 tons), Canada (reserves: 37,000 tons; reserve base: 47,000 tons), Mexico (reserves: 37,000 tons; reserve base: 40,000 tons), Peru (reserves: 25,000 tons; reserve base:37,000 tons) and Australia (reserves: 29,000 tons; reserve base: 33,000 tons). China ranked third in the world after the USA and Canada as comparing the reserves of China's silver ore mined as an ore of silver alone with the reserve bases of the above countries.

3.19.1.2 Characteristics

      In 1996, South-Central China holds the largest portion of the silver reserves of China, amounting to 29.5% of the total retained reserves, and East China comes next, amounting to 15.5%, followed by Southwest China, 15.6%; North China, 13.3%; and Northwest China, 10.2%. Northeast China has the smallest reserves, amounting to 4.7% of the total.

     In terms of the retained ore reserves of provinces (autonomous regions), Jiangxi is the leading producer with retained ore reserves of 18,016 tons, amounting to 15.5% of China's total, followed by Yunnan with 13,190 tons and 11.3%, Guangdong with 10,978 tons and 9.4%, Inner Mongolia with 8,864 tons and 7.6%, Guangxi with 7,708 tons and 6.6%, Hubei with 6,867 tons and 5.9% and Gansu with 5,126 tons and 4.4%. The total retained reserves of the above seven provinces (autonomous regions) make up 60.7% of the total. Table 3.19.2 and Fig. 3.19.1 illustrate the major silver deposits in China and their development status.

     The major silver deposits in China include the Yindonggou Ag deposit, Lengshuikeng Ag deposit, Yindongzi Ag-polymetallic deposit, Poshan Ag deposit, Jiawula deposit and Qagan Bulagen Ag-Pb-Zn deposit.

     The Yindonggou Ag deposit is located in the northern part of Zhushan County, Hubei Province, around the border between Hubei and Shaanxi provinces. It is a large Ag-Au deposit with retained ore reserves of 1,428 tons and a silver grade of 17.3 g/t. Although the deposit was mined during the period from the end of the Ming Dynasty to the beginning of the Qing Dynast (about 350 years ago), essential geological exploration was started in 1972. Reconnaissance and detailed reconnaissance were conducted for polymetallic deposits from 1972 to 1977 and a geological report, Detailed Survey of the Yindonggou Polymetallic Ore District in Hubei Province, was submitted in 1977. Late in the 1970s China placed the exploration of noble metals on an important position, and then silver and gold became major metals in the exploration of the Yindonggou ore district after 1978 and in 1985 Geological and exploration report of the Yindonggou gold-silver ore district in Zhushan County, Hubei Province was submitted. Upon approval of the report, the Hubei Silver Mine was then constructed in 1986 and went into operation in 1990. The designed daily ore mining and dressing capacity was 400 tons, and the annual output of silver was 20 tons and that of gold 200 kg.

    The Lengshuikeng Silver Mine is located in Guixi County, Jiangxi Province.The mine is 7 km long from north to south and 5 km wide from east to west, covering an area of 35km2.

Table 3.19.2 Major silver deposits in China

The retained reserves are 3,336 tons with a silver grade of 146.21 g/t. The discovery and exploration has experienced several partly overlapped stages: exploited as a "vein-like and banded" Pb-Zn deposit, porphyry Pb-Zn deposit, porphyry Ag deposit, and stratabound diplogenic silver and gold deposit. After 20 years of exploration, this large Ag deposit was eventually proved. The first mine, the Yinluling Silver Mine, in the Lengshuikeng ore district was built in 1990 and went into operation in 1992 with a daily ore mining and dressing capacity of 500 tons and an annual silver output of 22.96 tons.

      The Yindongzi Ag and polymetallic deposit is located in Zhashui County on the southern slope of the Qinling Mountains in southern Shaanxi. The ore district is 3 km long from east to west and 1.5~2 km wide from north to south. Its retained reserves are 2,257 tons with a silver grade of 107 g/t. This deposit was mined intensively in ancient times. In the 1950s geological teams under the Ministry of Geology and Ministry of Metallurgical Industry conducted geological surveys and mineral prospecting. A geological and detailed exploration report was completed in 1981. At present the Shaanxi Silver Mine has been established to carry out further prospecting work. As designed, 99,000 tons of ores are mined and dressed annually, and 140,000 tons of ore were produced in 1991.

      The Poshan silver deposit is located 24 km to the north of Tongbo County. As early as Southern Qi (A.D.479~502) of the Northern and Southern Dynasties, the mining and smelting of silver was administered by Weishan County and the mine was abandoned at the end of the Ming Dynasty (about 350 years ago). Before 1949 when the People's Republic of China was founded, limited geological route surveys were done; whereas a great deal of regional geological surveys and ore prospecting were carried out afterwards. In 1975, the No.8 Geological Team of the Shaanxi Bureau of Geology conducted exploration for the Poshan silver deposit and submitted a relevant exploration report, Detailed exploration report of the Poshan silver ore district in Tongbo County, Henan Province, yielding silver reserves of 2,662 tons with a silver grade of 278 g/t. Altogether 13 orebodies were discovered, which are all of industrial value except No.2 orebody. Of these orebodies, No.1 is the largest one, 1,900 m long and 5.38 m thick with a maximum downward extension of 530 m. In 1985, the China National Non-ferrous Metals Industry Corporation set up the Tongbo Silver Mine to deal with the exploration and utilization of this deposit. The designed daily mining and dressing capacity was 800 tons and the annual output of silver was 50 tons.

      The Jiawula and Qagan Bulagen Ag-Pb-Zn deposit is located in Xin Barag Right Banner, Inner Mongolia. The two deposits lie in the same NW-trending structural belt and are only 6 km apart, which were discovered in 1985 and 1986 respectively. Exploration proved that the Jiawula deposit has Ag ore reserves of more than 1,000 tons, Pb-Zn ore reserves of 800,000 tons and Cu ore reserves of 45,000 tons; while the Qagan Bulagen deposit has Ag ore reserves of more then 500 tons, and Pb-Zn ore reserves of 80,000 tons. The average grade of silver of the Jiawula deposit is higher than 50 g/t. Silver orebodies with grades of 170~200 g/t provide about 50% of the reserves of this deposit.      In China, silver mineralization had been extensively developed during a long geological history from the Proterozoic to Mesozoic, especially in the Yanshanian, though silver deposits of industrial value formed in the Archean and Cenozoic have not been discovered so far. Both the number and scale of the silver deposits formed in the Yanshanian are larger than those of other periods.

      Spatial distribution shows that all silver deposits in China are widespread in geosynclinal fold belts and platform depression basins as well as volcanic-sedimentary fault depressions in activated platforms. For instance, the Ag deposits in Zhushan, Hubei Province, and Maqiong, Sichuan Province, occur respectively in Caledonian and Indosinian eugeosynclinal fold belts; the Ag-Pb-Zn deposit in Xingshan, Hubei Province, and Bajiazi, Liaoning Province, occur respectively in a depression in the Yangtze platform and the Yanshan basin of the North China platform; the polymetallic deposits in Tiantaishan, Zhejiang Province, and Dexing, Jiangxi Province, occur in fault volcanic basins of activated platforms. A clear tendency has been seen in regard to the distribution of silver deposits: a great number of Ag-rich polymetallic deposits are found to occur in fault volcanic basins of activated platforms and, moreover, these deposits are usually large in size.

     The syngenetic ore-forming processes are essential to the formation of silver deposits. Ag and Ag-polymetallic ore beds were formed when volcanism and sedimentation produced their wall rocks independently or jointly. These ore beds and wall rocks have the same formation environment or macroscopic geological conditions and they were formed almost in the same period. The ore beds originating from such syngenetic ore-forming processes were subjected to superposition of certain geological events to become industrial deposits. Such deposits are usually characterized by high contents of metallic elements, persistent horizons and large scales. Although some deposits related to intermediate-acid intrusive rocks have been found, they are only of secondary importance and ore deposits of industrial value related to basic intrusive rocks have not been discovered thus far. In other countries, however, silver deposits due to the above two geological processes are fairly noticeable.

      According to the statistics in 1992, among the silver deposits with silver as the main metal, there are 12 large and 40 medium-sized ones, whose reserves account for 95% of the total reserves of the deposits of its kind, and 29 small ore deposits account for merely 5% of the total.

      In regard to by-product silver deposits, there are 14 large and 73 medium-sized deposits, amounting to 79% of the total reserves of by-product silver; and 271 small ones, amounting to 21% of the total.

      There are rich resources of by-product silver in China. The retained ore reserves were 66,146 tons in 1995, making up 58% of the total in that year. However, this estimation is actually less than the reality since the analysis and estimation were not complete. By-product silver has been found in all provinces, autonomous regions and municipalities except Ningxia. Most reserves and ore districts of by-product silver have been proved in Jiangxi, Hubei, Guangdong, Guangxi and Yunnan. In China rich by-product silver deposits are fewer than poor ones and those with a grade higher than 50 g/t have reserves accounting for only 1/4 of the total, while the proportion of poor ore deposits with a grade lower than 50 g/t is as high as 3/4 of the total number.

      In China most associated silver ore occurs in Ag-Pb-Zn deposits, amounting to 64.3% of the total retained silver reserves; whereas by-product silver ore occurs often in Pb-Zn deposits, amounting to 44% of the total by-product silver reserves, and in Cu deposits, amounting to 31.6%. In addition to Pb, Zn and Cu deposits, silver can occur also in tin, gold and polymetallic deposits as am associated or by product mineral.