Structure Of Metallogenic Systems And Deep Mineralization Prospect Of The Ag, Mo Deposits In The Middle Taihang Mountains

As the most important ore-cluster region in the North China Craton, the Fupingore-cluster region in the middle Taihang Mountains contains tens of Au, Ag, Mo andPb-Zn deposits. Up to now, except for the large quartz-vein type gold deposit, i.e.Shihu gold deposit in this part, others are all middle to small scale deposits. So it issignificant to research middle to small scale deposits deeply, and to prospect thedeep mineralization for regional mining prospection.In this thesis, we mainly study the Qiubudong Ag deposit and the Yanjiagou Modeposit. Through the methods of modern metallogeny, genetic mineralogy, andzircon U-Pb, sulfide Rb-Sr, Re-Os isotope geochronology and fluid inclusions, weanalyse the structures, factors, formation ages and characteristics of metallogenicsystems of the Qiubudong Ag deposit and the Yanjiagou Mo deposit. Then wediscuss their deep mineralization prospection.The Qiubudong Ag deposit is a porphyry-cryptoexplosive breccia type deposit.The breccia pipe is cylinder-shaped and the ore bodies exist in the sericitization zonewith layered or lenticular shape. The ore formation process can be divided into fivestages: magnetite-quartz stage, biotite-quartz stage, quartz-pyrite stage, polymetallicsulfide stage and quartz-carbonate stage. Trace, REE and Pb isotope characteristicsof pyrite suggest a lower crustal origin of the ore-forming materials, and alsocontrolled by the Yanshanian intrusions. The S、He、Ar isotope values consistent witha mantle input. Calculated average mantle helium proportions of the fluid inclusionstrapped in pyrite is12.32%. Quartz H-O isotope data and carbonate C-O data suggestthat the hydrothermal fluids mainly come from the magmatic water, and mixed withair water during the evolution. The hydrothermal fluids rise along the fractures in thebreccia body. Accompanied with the decrease of pressure and temperature, theenvironment changed from alkaline into acid, and from oxidation into reduction，resulted with the enrichment of ore minerals in the forms of hydrothermal cements inthe breccia bodies.The Yanjiagou Mo deposit is a porphyry type deposit, located at the periphery of the southwest Mapeng batholith. The ore bodies exist in the transitional contacts ofthe inner and external alteration zones with veinlet or lenticular shape. We identifyfour stages of ore formation in this deposit: pyrite phyllic stage, quartz-pyrite stage,quartz-pyrite-molybdenite stage, and quartz-carbonate stage. Pb isotopecompositions of pyrite infer that it is origined from the lower crust and FupingComplex. S isotope of pyrite and Re content of molybdenite reflect the mantle originof ore-forming materials. Quartz H-O isotope data suggest that the hydrothermalfluids mainly come from the magmatic water. The hydrothermal fluids rise alongwith porphyry magmas, deposited at the fractured zone between the Mapengbatholith and the Fuping Complex as network mineralization.The zircon U-Pb age of quartz porphyry from Qiubudong Ag deposit is107.95±0.87Ma. The mineralization time obtained through pyrite Rb-Sr isochron ageis102.04±0.61Ma, consistent with the zircon U-Pb age of associated quartz porphyry,which suggests that the formation of Qiubudong Ag deposit is closely related withthe intermediate dikes in this region. We obtained the mineralization time of theYanjiagou Mo deposit as126.7±1.1Ma (molybdenite Re-Os isochron age). Thereare two important tectonic-diagenetic-metallogenic episodes during127Ma and102~108Ma in the middle Taihang Mountains.Based on the study of structures of metallogenic systems, we research thegenetic of metallogenic systems and enrichment regularity of minerology, thenproposed the genetic model, and evaluated the metallogenic potential in the deep partof the deposits. The mineralization depth of Qiubudong Ag deposit is2.40km, andthe breccia were mainly cemented by clastic cement at the earth’s surface, whichindicate the erosion is very weak. The conduction type of pyrite from Qiubudong Agdeposit is dominanted by P-type (88.78%), which also suggests the weakly erosionand there are superior metallogenic potential in the deep part of Ag deposit. Whilethe metallogenic potential of Mo deposit needs further discussion.