The Study Of Mineralization Process Of Xiangquan Independent Thallium Deposits In He County, Anhui Province, China

The conclusion "Disperse element can highly be enriched to form independent deposit" has been accepted and the study of mineralization process of disperse element has aroused great interesting of many researchers. As the geology and geochemistry characteristics of Xiangquan thallium deposit are quite different from other thallium deposits even found in the world, the study of this deposit will improve the research of minerlization process of disperse element.Typical sample are selected after careful detail field study. Microscope observation, chemical analysis and high precise electron probe are adopted to analyze occurring forms of thallium in different minerals;fluid inclusion study in fluorite of different stages are used to calculate the physical-chemical parameter;the hydrogen, oxygen, carbon, silicon, sulfur and lead isotope of samples of deferent stages are measured to determine the origin, composition and character of ore forming fluid;and the age of deposit is determined by precise Sm-Nd isotope isochrone. At last, we put forward evolution model of fluid metallogenic system, set up the metallogenic model of thallium deposit, and provide the important information and indexes of prediction, exploration and estimation of regional thallium deposit.The host stratum of Xiangquan thallium deposit is Ordovician sub-series of Lunshan Group and the main ore-control structure is Xiaoshan-Xiaolongshan-Dalongshan anticline and two reversed faults (F₁ and F₂). The alteration is weak as a whole and the main alteration types are fluoritization, baritization, quartzification, carbonation and limonitization, etc. The thallium deposit are mainly consisted by one ore body, which is bedded-like, lens-shaped, extends paralleling the strata and is about 1000 m long, 50m wide and between 10 to 15 m thick. The total reverse of thallium is about 150t. The main ore mineral is pyrite and more than 95 percent thallium occurs in pyrites. The main structures and textures of ores are disseminated texture, breccia structure, massive structure, striped structure, microlayere-shaped structure, oolitic(pisolitic) texture, colloid concentric ring texture and organic texture, etc. All thallium ores are composed of framboidal pyrite aggregates and clay silicate minerals. Although the framboidal pyrite aggregates appear different, the framboidal pyrites are quite same. The deposit formed during submarine hydrothermal sediment metallogenic period and low temperature fluid alteration metallogenic period, and the former is central. Low temperature fluid alteration metallogenic period can be further divided into two ore-forming stages and the early stage is primary.No independent thallium mineral bigger than one micron has been found in the deposit so far. Framboidal pyrite is the main thallium-containing mineral. The content of thallium in framboidal pyrite is positive correlation with arsenic and negative correlation with iron. The content of trace element such as copper, lead, zinc, cobalt and nickel has no obvious correlation with thallium. Thallium occurs in two forms;most of thallium enters pyrite crystal lattice in the form of isomorph and the rest occur as thallium mineral particle. Very fine grained (n×10 to n×lOOnm) thallium minerals have been observed in thallium-rich framboidal pyrite, but because the thallium minerals are too small that its exact content can not been determined by electric probe directly. Due to the semi-quantitative analysis, we speculate molecular formula of thallium minerals are TIL (Tl,Fe)(As,I^ Tl(As,l) and (Tl,Fe)l. Meanwhile, sub Nano-thallium mineral has also been found in framboidal pyrite. Those thallium minerals mainly distribute along the micro-fracture in pyrite, reflecting the alteration of low temperature fluid which enrich the thallium in pyrite to form thallium minerals.The study of major element, trace element, REE and sulfur, hydrogen, oxygen, lead and silicon isotope of framboidal pyrite, fluorite and different alteration rocks show that the framboidal pyrites were the product of thallium rich hot water in the seafloor;the fluid of low temperature alteration metallogenic period did not carry in thallium and the thallium disperse during the decomposing of pyrite. The tow stages fluid are all meteoric source and has no direct relationship with magma, but evolution of the two stages fluid are different. The study of fluid inclusion in fluorite show that the two stage fluid are all Ca2+-Na+-K+-F type, the early stage fluid's temperature is 220°C²80°C, gathering between 240²60°C, the salinity is between 0.5 to 6.0%.NaCleq gathering between 2.8^<sup>5.0wt%.NaCleq, and the mineralization degree, deoxidize parameter, fO2 and Eh value are 81, 0.220, -51.36 and -0.12, respectively. The late stage fluid's temperature is 130°C²10°C, gathering between 160¹80°C, the salinity is between 0.8⁶.5%.NaCleq gathering between 2.2^<sup>3.7wt%.NaCleq, and the mineralization degree, deoxidize parameter, fo and Eh value are 65,0.211, -41.50 and -0.22, respectively.Fluorite samples of two stages have been collected for Sm-Nd isotope analyses. The early and late stage fluorites isochrone ages are 131.7±2.7Ma and 79.3±9.4Ma, respectively. This indicates that the Xiangquan thallium early stage hydrothermal mineralizing event occurred during the late Yanshanian period(131.7±2.7Ma). It is well known that the mineral deposits of the Middle and Lower reaches of the Yangtze River metallogenic belt mostly formed during the Mesozoic Yanshanian period. The discovery of the Xiangquan thallium deposit may indicate the epithermal metallogenic event may have relationship with hydrothermal metallogenic even. The age of late stage fluorites indicates that there is another fluid activity at the end of late Yanshanian period(79.3±9.4Ma).According to the geological and geochemical characteristics of Xiangquan thallium deposit, the metallogenic model has been set up and the concept of independent thallium deposits has been re-defined. We define indepentdent thallium deposit as "the deposit scale is big in which thallium is highly enriched, while the economic value of other element are minor or less important, thallium occur as thallium minerals or in the sulfur minerals".