Ore-forming Mechanism And Background Of Continental Dynamics Of The Shuiyindong Carlin-type Gold Deposit, Southwestern Guizhou, China

Southwestern Guizhou is located in the transition zone between Yangzi plateform and Youjiang fold belt. It is one of the most important Carlin-type gold deposits. The Carlin-type gold deposits in the area can be divided into fault-controlled and strata-controlled. The Fault-controlled gold deposits are localized at the high-angle compressive-shear faults, hosted generally in the Middle and Lower Triassic strata and the host rocks are mainly pelitic siltstone and silty mudstone. The Strata-controlled gold deposits mainly are localized at the Upper Permian strata and the host rocks are mainly bioclastic limestones. Although plenty of researchers have made lots of researches and many results have been obtained about the gold deposits , there still is a big difference on the mineralizing mechanism and dynamics background. So the author chooses Shuiyindong to conduct a typical anatomy, especially do a system research about its mineralizing mechanism and the continental dynamics background. The paper is divided into seven chapters to elaborate.1) This paper first summarizes basis of topic selection and the significance of research, and the research status on continental dynamics and metallogenesis, and the Carlin-type gold deposits at home and abroad, then presentes research content, research programs of this thesis, summarizes research results and the principal innovations.2) From the regional geological study, the Shuiyindong gold deposit is not only controlled by the faults but by favorable lithologic associations. The gold deposit is located nearby the core of Huijiabao anticline, the orebodies are mainly stratiform or stratoid ones, hosted mainly in the impurity limestone within the sedimentary series of clayrock—> impurity limestone—> clayrock . Only a small amount of orebodies are controlled by low-angle faults.3) Researches combining the Lithofacies palaeogeography with temporal and spatial distribution of Au, As, Sb, Hg et al. low-temperature deposits in the Southwest Guizhou aera shows that the special lithofacies palaeogeography environment and evolution history of the southwestern Guizhou, not only lay the ore-forming materials foundation for formation of the gold deposits, but also form the favorable series of sedimentary rocks. Therefore, the lithofacies palaeogeography environment and evolution of the district play a controlling role on the mineralization and distribution of the Carlin-type gold deposits.4) The systematic geochemical studies indicate that,the ore-forming materials and fluids of the Shuiyindong gold deposit has multiple sources. The ore-forming minerals mainly is from orogenic belt and sedimentary rocks , originated partly from plutonian minerales brought by mantle magma intrusion and deep crust. The ore-forming fluids mainly originated from? meteoric-hydrothermal water, probably, immingled with certain parts of salt- groundwater and magmatic hydrothermal water, belongs to low-medium temperature (96.7℃～220℃), low sality (l4.282±2.260% wt%NaCl) and medium density (0.910±0.061 g/cm3) fluid. The hydrochemical type of fluid inclusions found in quartz inclusions belongs to Cl--Na+ or SO42-﹣-Cl--Na+ type, while the hydrochemical type of fluid inclusions found in calcite inclusions belongs to SO42--Cl--Ca2+ type. .5. The seismic activity of history and present in the southwesternGuizhou and its adjacent areas is weak, and the shallow focal depth, inferres except the deep faults which controlled Youjiang basin boundary, the other faults formed with Youjiang basin formation and its evolution, while the ore-forming times of?the Carlin-type gold deposits distributed widely in the southwesternGuizhou and its adjacent areas all are the late Yanshan movement ,and shows that it is unlikely that such a huge amount of ore-forming materials are brought to the shallow crust through deep- large faults to reach the mantle to formed large-scale distribution of gold deposits.But crustal extension in late Yanshan period droved large-scale ore-forming fluids formed in the long-peroid evolution of the Youjiang basin to the appropriate parts of strata ro stracture to accumulate and forme wide area gold deposites6) According to structural composition and sedimentary evolution combined with magmatic activity characteristic analysis, the preliminary determination of the evolution series of in continental dynamics in southwesternGuizhou in D-K are: (1) Abstract? ?Hercynian tectonic phase (D-P2), that is stretching - rift stage; (2) Indo-China period (P3-T2), or back-arc basin development stage; (3) early–mid period of Yanshan movement (T3-K), namely, intracontinental orogenic stage; (4) Late Yanshan Movement (roughly from 150-70Ma), which is extension phase of post-orogenic crustal, is also the stage of wide area low temperature deposits in southwestern Guizhou.7) According to study on water sources, mineral sources and heat sources of ore-forming fluids, combined with discussion of regional structure, lithofacies palaeogeography and metallogenic epoch, this thesis puts forward background of continental dynamics of Shuiyindong gold deposit: Late Yanshan Movement (roughly 150 -70Ma), with big conversion of the tectonic regime in eastern China, mantle upwelling, where have a strong mantle magmatism, the southwesternGuizhou and its adjacent areas are converted to vertical stretching uplift from the early shrinkage deformation, mafic dikes and alkaline rocks along the tensile fracture are formed and cause the crust warming. The stretching action and thermal power, drived ore- forming fluid to move to the smaller pressure areas. When the ore-forming fluid flow to deep-large faults at the edge of basin ,it changes flowing direction and upward migrate, while the ore-forming fluid pressure is less than the pressure of underground water, the vertical movement fluids moved into the platform along the ancient karst surface or unconformity interface . The ore-forming fluid acted partly with the surrounding rocks formed structural alteration dodies, or formed Carlin-type gold ore boddies (such as orebodiesⅠa in the Shuiyndong gold deposit,), while most of the remaining ore-forming fluids migrated upward through the sub-faults cutting through the strata and penetrated into favorable strata, formed ore-dodies controlled by ore-hosted strata. sub-faults and favorable strata appear repeatedly, resulted in gold deposits with a multi-floors, such as gold ore dodies in the Shuiyindong gold deposit:Ⅱf,Ⅲa,Ⅲb, etc.