| 其他摘要 | The sediment-hosted lead and zinc ore deposits attract worldwide earth scientists and economic geologists because they are the important metal sources of lead and zinc. In the last fifty years, lots of sediment-hosted lead and zinc ore deposits have been discovered in southwest margin of Tarim, and lots of research works have been carried on. But some key questions are still worthy of consideration, such as the time of mineralization, the source of the metal and sulfur, and mechanism of mineralization.
Tamu lead and zinc ore deposit is one of the most important sediment-hosted lead and zinc ore deposits in the southwest margin of Tarim, the side of Tarim, north of the joint of West Kunlun and Tarim Block, where the regional strata are all deformed and thrusted. The ore deposit locates in the breccias belt of Lower Carboniferous carbonate, has five economic ore bodies, and keeps zinc and lead reserves of ca. 0.2Mt.
Since Tamu lead and zinc ore deposit discovered in 1940s’, many research works have been done, such as fluid inclusions, isotope of lead, sulfur, carbon and oxygen, and rare earth elements (REEs), but the genesis of the ore deposit still confuses economic geologists. Some of them support the ore deposit belong to Mississippi Valley type, but the others think it is one of the sedimentary-exhalative lead and zinc deposits. But in their literatures, it can be pointed that little attention is paid on some basic matters - the types, structures and textures of the breccia and matrixe.
Hence, some new data, investigation results, and views about Tamu lead and zinc ore deposit are listed here, which are distinguished by the principles and methods of mineralogy, petrology, economic geology and geochemistry, aimed at illustrating the genesis and mechanism of ore deposit, supported by the Knowledge Innovation Project, Chinese Academy of Sciences (KZCX2-YW-107-6) and Key Projects in the National Science & Technology Pillar Program, Ministry of Science and Technology of People’s Republic of China (2006BAB07B04-04).
(1). In light of the ore deposits geometric characteristics and contact relationships with wall rock, Tamu lead and zinc ore deposit is controlled by syn-sedimentary growth fault, and the belt of breccias is a part of the fault belt. Furthermore, based on the observation on microscope, breccias, matrixes and ore structures and textures are discussed correctly, the important role of bitumen to the mineralization is pointed out and Ba-K feldspar is distinguished from matrix for the first time. The breccia can be classified under five groups by geometry characteristics and relationships of breccias contact - Intraclast Breccia (IB), Diagenetic Breccia (DB), Epigenetic Breccia (EB), Fall Breccia (FB), and Dislocation Breccia /Fault Breccia (DB), and Dislocation Breccia can be divided into Mineralized Fault Breccia (MFB) and Without Mineralization Fault Breccia (WFB). The matrixe can be classified under three groups by textures, syngenesis-diagenesis matrix (layer texture), dissolving – metasomatic matrix (fine grain with embayed margin), and cavity filling matrix (coarse grain). The three periods of evolution of Tamu lead and zinc ore deposit can be distinguished clearly, they are period of sediment, diagenesis and epigenesis. The mineralization is the results of diagenesis and epigenesis, especially of epigenesis. Two main stages of mineralization are easily distinguished – dissolving–metasomatic stage and cavity filling stage. The dissolving-metasomatic stage is the main stage of mineralization. The pipeline/vein structure is the typical characteristic to dissolving-metasomatic stage, and it always can be seen in matrix of epigenetic breccias.
(2). It can be pointed that the evolution of fluid is controlled by water-rock interaction and TSR, illustrated by carbon and oxygen isotopes, fluid inclusions, trace elements and REEs coming from breccias (dolomite) and matrixes (dolomite). The fluid of cavity filling stage is a low temperature (119~191°C), salinity between 10.9 and 14.4wt% NaCl eq., pressure between 2 and 39 MPa, with small quantity of H2S and absents SO42- fluid. Ca2+ and HCO3- are the major cation and anion in the fluid. The matrix and breccia have a similar REEs distribution model – marine carbonate. If matrix is stained by Ba-K feldspar, the REEs distribution model changes wildly. The isotope characteristics of oxygen and carbon show the matrix is rich of lighter isotopes than the breccia. In light of value of oxygen isotope coming from sphalerites fluid inclusions and breccia, the ore fluid has been stained by meteoric water. Based on oxygen isotope mass balance formula it can be pointed that the ratio of water and rock is 0.16~0.67 (119°C) or 0.12~0.43 (191°C) in closing system or 0.13~0.37 (119°C) or 0.10~0.28 (191°C) in opening system. Thermo-chemical sulfate reduction (TSR) maybe does an important role to induce the bitumen and H2S, and rules the change of carbon isotope values.
(3). It can be pointed the sulfur of Tamu lead and zinc ore deposit should come from seawater or evaporate, stained by bearing sulfur organic compounds and is controlled by TSR and reservoir effect, but for the source of Pb, the partial melting of crystallized basement plays an important role.
(4). By the discussion of the rule and premises of sphalerite Rb-Sr isochron, it can be pointed out that sphalerites must be tested by the methods of mineralogy in detail before to date, and a dating of the Tamu lead and zinc ore deposit is tried.
(5). It can be concluded that the Tamu lead and zinc ore deposit is an epigenetic carbonate-hosted lead and zinc ore deposit controlled by syn-sedimentary growth fault and TSR. |
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