The Characteristics Of Fluid Inclusions And Mineralization Of The SaiShitang Cu-polymetallic Deposit, Qinghai Province

The SaiShitang Cu-polymetal deposit is located in the sourtheastern rifed margic of the Qaidam crustal block in Neopaleozoic. The min-eralization was related to Late Indosinian magmatic intrusions, emplaced in Low-Middle Triassic epimetamorphic sandstone, sericite-phyllite and marble, and is divided into two episodes. The skarn episode is further divided into three substages (A1～A3), and the sulfide episode into six (B1～B6). The ore bodies are lenticular, vein-like and stratiform-like, formed at the contact zone between the rock mass and stratum.The gas component of fluid inclusions are dominantly H2O and CO2, with minor H2 and CH4. The cation ions in fluid inclusions are mainly Ca2+,Na+and K+, and the main anion ions are SO42- and Cl-.Fluid inclusions were studied in garnet and quartz from stages A1, B1, B3, B4 and B5. Three types and four subtypes of fluid inclusions were distinguished based on fluid composition and phase assemblages. Type I-a fluid inclusions are aqueous inclusions and homogenize into the liquid phase, whereas type I-b inclusions homogenize into the vapor phase. Type II fluid inclusions are aqueous ones containing one or more salt daughter minerals and homogenize into the liquid phase. And typeⅢinclusions are CO2-containing, homogenize to the aqueous phase.The homogenization temperature and the salinity of the inclusions of stage A-1 is respectively 228～463℃and 15.47～24.61 wt% NaCl equiv; B-1 is respectively 313-351℃and 4.23-7.25 wt% NaCl equiv; B-3 is respectively 112～420℃and 1.98℃10.98 wt% NaCl equiv; B-4 is respectively 124～343℃and 0.80～15.37 wt% NaCl equiv; and B-5 is respectively 326～360℃and 5.47～6.37 wt% NaCl equiv. In addition, the inclusions of B-3 also have high salinity groups of 29.36～46.80 wt% NaCl equiv. The lithostaic pressure of fluid estimated by the mineralization depth is 400 bar, and the mineralization temperatures corrected by the mineralization depth is 134～503℃. The mineralization temperatures of the skarn episode and the sulfide episode are 266～503℃and 134～460℃.And the salinity of the mineralization fluids from the two episodes are 29.36～46.80 and 0.80～15.37 wt% NaCl equiv,respectively. There is a trend of decreasing temperatures and salinities from early episode to late episode. According to the temperature, the salinity, and the pressure estimated by fluid inclusions type III indicated that there was an environment of ultrahigh pressure. The ore-forming hydrothermal solution consisted of magmatic water mainly and a part of grund water. The effect of ultrahigh pressure fluid lead to rupture of wallrock, pressure decrease, fluid boiling, and the mixture of large amount of grund water, that caused the altering of characteristics of ore-forming fluids and mineralization. Based on the above research, metallogenic model of SaiShitang is established.