Characteristics Comparison Of Gold Deposits In Different Ore-bearing Layers In Daduhe Basin

Daduhe gold mineralization belt is located in the west margin of Yangtze Block and on the both sides of the Daduhe in Kangding district. According to its tectonic position, Daduhe gold mineralization belt lies in the zone of transition among the Songpan-Garze orogenic belt, the Sichuan-Yunnan NS direction tectonic belt and Southwest Sanjiang orogenic belt. In recent years, the geological exploration work shows that gold ore is born in three different ore-bearing layers which consist of cap rock, basement and interface. The ore-host surrounding rocks are metamorphic rocks and the granite in the basement, while the ore-host country rocks are the carbonatite and carbonaceous slates in the cap rock. Gold deposits in this district are divided into quartz-vein type and altered rock-quartz vein type. The selection of this master’s thesis is based on the item(12120113095000) from CGS managed by the author’s supervisor. In order to figure out the similarities and differences in different ore-bearing layers in Daduhe, author carried out field geological investigation and indoor geochemical experiment. Meanwhile, author compare deposit geological features among different ore-bearing layers and study the genesis of mineral deposit with the research of the deposits geological feature, the source of ore-forming fluids, the metallogenic epoch and the time of dike formation, etc. After a series of related researches, the master’s thesis achieves the following main results and understanding:(1)The gold deposits metallogenic types are the same in different ore-bearing layers. The main types are quartz vein types and the rest are altered rock-quartz vein types.(2)The ore mineral compositions are quite simple in the basement, the cap rock and the interface. The main metal mineral is pyrite and the main gangue mineral is quartz. The important normal gold-bearing mineral is pyrite and quartz, yet fluoride is also a major gold-bearing ore mineral in the cap rock gold deposits. Maybe the lithologic difference of the ore-host surrounding rocks result in the difference of mineral species among different ore-bearing layers. According to the character of mineral symbiosis combinations, elements vertical zone phenomenon is widespread in different ore-bearing layers. There are pyrite-quartz combination in the upper layers; pyrite-chalcocite-ferrodolomite-quartz combination in the middle layers; galena-sphalerite-tetrahedrite-quartz-pyrite combination in the deep layers.(3)The REE distribution patterns of pyrites and quartzs which are closely related to gold mineralization are quite similar to each other in different ore-bearing layers. The REE distribution patterns showing gentle right curves with negative Eu anomaly and very weak negative Ce anomaly characteristics indicate that the physic-chemical conditions of ore-forming fluid formation may be relatively reducing environment in Daduhe gold deposits. Meanwhile, the homogenization temperature ranges of the quartz inclusion in the period of gold metallogenic are similar among different ore-bearing layers and are concentrated in 140℃ to 180℃. It’s obvious that the ore-forming fluid belongs to middle-low temperature hydrothermal fluid. Wide homogenization temperature implies a complicated ore-forming process matching the analysis consequence of the rare earth elements.(4)Based on the trace elements characteristics of pyrites both from basement and cap rock, ore-forming fluid mainly belongs to metamorphic hydrothermal origin with the influence of magmatic hydrothermal movement at the same time. Ore-forming fluid is almost metamorphic hydrothermal fluid in the early mineralization, inheriting some volcanic hydrothermal components as well, and almost deep hydrothermal fluid in the late mineralization, with the influences of shallow hydrothermal fluid and mixed genesis. Because the metamorphic-hydrothermal type gold deposit belong to the orogenic type gold deposits, the emergence of As and Te element indicates that Daduhe gold deposits lie in the middle mineralization belt of the orogenic type gold deposits.(5)The peak content distribution of S-isotope shows the same characters in different ore-bearing layers. To all the pyrites in the basement and most of those in cap rocks, the range of δ34S contents is small which is around 0 and peak content is relatively concentrated which approximates the features of sulfur from crust and mantle. Above conclusions indicate that the nature of ore-forming fluid is close to the fluid from the mantle. A minority of pyrites δ34S contents in cap rocks are between mantle δ34S contents and seawater δ34S contents in cap rock and it implies that the sulfur may partly come from upper Proterozoic-Paleozoic Erathem neritic facies carbonatite strata. Another pyrites δ34S contents in cap rocks are between mantle δ34S contents and reduzate δ34S contents and it implies that sectional sulfur may come from Devonian carbonaceous slate strata. All of the S-isotope features manifest that the sulfur in the gold ore have the characteristics of deep sources in different ore-bearing layers.(6)Experimental data by testing the quartz with the method of ESR shows the approximate mineralogenetic epoch among the three different ore-bearing layers. Generally, metallogenic ages center on 20~10Ma,while individual metallogenic ages are concentrated in 40~35Ma and 50~48Ma. Obviously, it is the characteristics of the multiphase and multi-stage. Because of the coincidence that 20~10Ma equals to Tibet Plateau post-collision(25~0Ma) periods and 40~35Ma equals to late-collision(40~26Ma) periods and 50~48Ma equals to syn-collision(65~41Ma) periods, Daduhe gold deposits are verified to be formed during the post-collision periods with the shear action and belongs to orogenic-type gold deposit.(7)After a series of experiments on mafic dikes with the method of dating Zircon U-Pb ages by LA-MC-ICP-MS, diagenesis ages of mafic dikes, outcropped both in cap rocks and basement gold deposits, are similar and mainly divided into two stages, around 210 Ma and 425 Ma. It manifests two periods of basic magma from mantle or deep crust emplacement in different ore-bearing layers in this district. Because both diagenesis ages are far earlier than metallogenetic epochs, material sources are not the same between widespreaded mafic dikes and ore-forming fluids in gold deposits in different ore-bearing layers in Daduhe basin. However, contributions of the movement of mafic dikes followed as two aspects, providing fluid migration channel and offering ore-forming geochemical interface. On the other hand, study results on main element features of mafic dikes demonstrate the same consequence that the mafic dikes formed in the structure of the orogenic belt environment.(8)By comprehensive researching of ore petrology, geochemical and chronology, different ore-bearing layers, orogenic-type gold deposit, have the coincident geological features and belong to the same metallogenic series in Daduhe basin.