| There is a special complex mineral composition and special faulted basin sedimentary tectonic setting of the mud-sized crystalline dolostone in Xiagou Formation, Low Cretaceous(Qingxi Sag, Jiuquan Basin). Which is completely different from the general hydrothermal dolomite(rock) filling in the hydrothermal dolomitization or enclosed environments. It is a very rare kind of primary iron-containing mud-sized crystalline dolostone with precipitate directly of the hydrothermal cascade in lake of Mesozoic, where this set of hydrothermal sedimentary rock developed, has few pyrite, exceptly some of the medium-high temperature minerals. In veins or brecciated distributed between hydrothermal breccias, or else, layered or veined in hot sedimentary rock laminae. The pyrite in this set of hydrothermal sedimentary rock shows fine crystal grains and lower ordering degree. It appears of high aggregate, and located in mud veins of argillaceous dolomite beds or brecciated and veined breccia located between framework of spray breccia with high heterogeneity characteristics.Our team mad a comprehensive study in petrology, mineralogy and geochemistry to the pyrite and surrounding rocks, deeply and systematically, including texture and structure of the pyrite, and major elements, trace elements and sulfur isotopes of pyrite, and structures, carbon-oxygen-strontium isotopes, trace elements and rare-earth elements of surrounding rocks. Studied on pyrite from white smoker hydrothermal sedimentary rock of lake systematically, first in China, and got following conclusions and results:1. There are mainly two types of pyrites in hydrothermal sedimentary rock of Xiagou Formation(Qingxi Sag, Jiuquan Basin), they are exhalation sedimentary pyrite and gas-water hydrothermal pyrite. Among them, the standard structures of exhalation sedimentary pyrite are: lumpy, brecciated, oolitic, invaded, tuberculosis, spot-like, erosion structure, margin structure, layered, lamellar and lenticular, of which, tiny structures included: clastic texture, idiomorphic granular, hypautomorphic granular and xenomorphic texture. And, structures of gas-water hydrothermal pyrite included: banded and veined, while tiny structure are: invaded structure.2. The main source of pyrite ore-forming fluid is hydrothermal in spout and hot brine in sublacustrine.The characteristics of sublacustrine hot brine are: rich in hydrothermal sedimentation standard elements:Sb, Ba, Sr, Mn and V; relative rich in LREE, A typical negative Eu anomaly of REE patterns, The values of oxygen isotope is significant negative bias; compositions of strontium isotopic strontiumes have affected mainly by the shell source and mixed with a small amount of mantle source strontium, which reflect depositional environment of lamellar containing-iron micritic dolomite is controlled by basement faults, and have a stable, closed and hypoxic conditions in the hot brine pool from deep lake bottom.The characteristics of hydrothermal in spout are: sulfur-rich, poor nickel(Ni), mixed more mantle Sr(low 87Sr/86 Sr ratio), and have a higher temperature than that of pyrite.3. Micron berry spherical pyrite is small and uniform, the layered structure pyrite consists of very small grains, which reflect the pyrite formed in a stagnant- anoxic lake.4. Different types of pyrite developed in their special positions, for their special fomation types, in which, exhalation sedimentary pyrite I which associated with hydrothermal spout mainly developed in the inner hydrothermal spout and chimney, and exhalation sedimentary pyrite II developed in the place they could flow and would not be melt.5. Established a metallogenic model for hydrothermal sedimentary pyrite from white smoker sedimentary rocks in lake. |
