| 其他摘要 | The Qingling orogen belt is the suture of North China plate and Yangtze plate, it can be divided into 4 tectonic units: the southern margin of North China plate (Huashan-Xiong’ershan block), North Qinling orogen belt, South Qinling orogen belt and the northern margin of Yangtze plate in spatially. The XiaoQinling is located in the southern margin of Huashan-Xiong’ershan block, and the XaioQinling gold field is the orogenic gold field that has been agreed with the domestic and foreign scholars. The Dahu Au-Mo deposit, Lingbao County, Henan Province is a typical fault-controlled lode deposit that occurred in the north part of Xiaoqinling gold field. The Dahu Au-Mo deposit is explored for gold deposit at first, its Au reserve reachs to 28t, average grade is 8.7g/t. Part of Au-bearing quartz veins change to molybdenum-quartz veins along with the exploring depth, the ascertained Mo reserve reach to medium size deposit at present.
The author has systematically studied the Dahu Au-Mo deposit in various aspects including regional geological setting, deposit geology, elemental geochemistry, isotope geochemistry, fluid inclusions geochemistry , geochronology and ore-forming mechanism. The main result is obtained as following:
The ore-forming process can be divided into three stages. The early stage is characterized by pyrite-quartz veins, in which minerals were structurally deformed and broken, indicating that they formed in a compression or compressive distortion setting. The middle stage is characterized by molybdenite-pyrite-quartz stockworks (mostly coaxial or conjugated) infilling the cracks of the early-stage minerals such as pyrite and quartz, suggesting that it occurred in a tensional shear setting. The late stage is characterized by extensional open-space filling quartz-carbonate veinlets with comb-like texture.
The ISr of the metal sulfide of the Dahu Au-Mo deposit vary from 0.70470 to 0.71312, its average is 0.70854. Its (143Nd/144Nd)i vary from 0.51143 to 0.51215, its average is 0.51162. Its (206Pb/204Pb)i, (207Pb/204Pb)i, (208Pb/204Pb)i vary from 17.033 to 17.285, from 15.358 to 15.438, from 37.307 to 37.582 respectively. The average ISr, (143Nd/144Nd)i of wall rock(Taihua group) is 0.72294, 0.51107 respectively. Its (206Pb/204Pb)i vary form 17.127-18.392(average 17.547), (207Pb/204Pb)i vary from 15.416 to 15.604 (average 15.470), (208Pb/204Pb)i vary from 37.498 to 37.582(average 37.440) respectively. The ore-forming materials are characteristic in the mixture of Taihua Group and mantle by the comparison of Sr, Nd, Pb isotope of metal sulfide and Taihuo Group.
Various inclusions in sulfide-bearing quartz veins have been identified through microthermometry and Laser Raman microspectroscopy. It can be identified: The early-stage minerals only contain CO2-H2O inclusions, whereas the late-stage minerals only contain H2O-NaCl inclusions. Both the H2O-NaCl and CO2-H2O inclusions, together with pure CO2 and daughter mineral-bearing inclusions can be observed in the middle-stage minerals, implying that strong fluid-boiling occurred in this stage. Homogenization temperatures of fluid inclusions range from 400℃ to 500℃ for the early stage, from 290℃ to 470℃ for the middle stage, and from 220℃ to 260℃ for the late stage. The trapping pressure are estimated at least to be 138-331 MPa for the early stage and 78-237 MPa for the middle stage, respectively, corresponding to the ore-forming depth of 13.8-11.0 km and 7.8-8.0 km, respectively, implying the pressure change from lithostatic to hydrostatic. The ore-forming temperature and pressure is higher than the temperature and pressure of Au-bearing quartz, its ore-forming temperature and pressure is 120-310℃, 100-150MPa respectively. It is consistent with the spatial relation of Au and Mo by the observation.
The molybdenite of Dahu Au-Mo deposit Re-Os isotopic model age vary from 215.4±5.4 to 255.6±9.6, the isochron age is 218±41Ma (2σ, MSWD=38), this shows that the Mo mineralization occured in Indosinian.
At the later Triassic, along with the ancient Qinling ocean closed, the deeper rocks which located in the arc-back transition belt dehydrated under the compression to form the initial mineralization fluid. The mineralization fluid transfered upward alone the fracture(shear zone) formed the Dahu molybdenum ore deposit mineralization system. Along with the transition from compression to extention, regional thermal anomaly reached to maximum, the region metamorphic dehydration was stronger, formed the massive mineralization fluid, the metamorphic fluid provided the heat energy for the shallow fluid circulation, and the shallow tectonic also provided the channel for the fluid circulation because of the reduced pressure, this is advantage for the shallow fluid circulated and mixed in the mineralization system definitely. Simultaneously, increased temperature and the reduced pressure are also advantageous in the mineralization fluid boiling.Therefore, the sufficient fluid, the heat and the intense fluid boiling would cause the most intense mineralization material fast precipitation inevitably. Along with compression weakening, the extension enhancement, region thermal anomaly vanishing, the deeper fluid loss, the shallow source atmosphere fluid is main fluid, the mineralization rapid declined, formed the quartz - carbonate net-vein, it contributed to the mineralization weakly. In a word, the fluid inclusion research indicated that the Dahu gold molybdenum ore deposit forms in the transition from compression to extension, this is consistent with the ore deposit geology characteristic. |
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