| Shanyang-Zhashui ore concentration area in the northern fore-arc basin system of Qinling orogeny belt experienced an evolution history including subduction-accretion orogeny of oceanic crust and collision orogeny, with a complex tectonic setting, extensive development of magmatism, and a excellent geological setting for mining. This ore concentration area involved a series of medium-large scale gold deposits, and the Longtougou Gold deposit is the largest of these. This paper choosed Longtougou gold deposit as the research object, combined with field investigation, laboratory study and microscopic observation to identified the different mineralizing periods, used electronic probes for composition analysis on different stages, different types of metal mineral, at the same time, we estimated physicochemical conditions of ore fluids through the temperature measurement of the fluid inclusions, and analysis carbon, hydrogen and oxygen isotope on quartz and calcite, analysis sulfur isotope on sulfides of different mineralization stage, combine with the study of tectonic evolution of this region, to study the genesis of Longtougou gold deposit and its relationship with the geodynamic background of this area. Through the above study we reached the following conclusions:(1).The deposit is hosted in sandstone of Dafenggou formation, its ore-controlling structure is a series of reverse faults which strikes in a nearly E-W direction, its distribution of the orebody is controlled by cataclastic alteration zone. The main types of mineralization is crashed-eroded rock type, and also has small amounts of quartz vein type. the main gold-bearing mineral is pyrite, followed by tetrahedrite. Mineralization can be divided into three phases: quartz-pyrite stage, quartz-gold-polymetallic sulfide stage, quartz-calcite stage.(2).By microthermometry fluid inclusions of different mineralization stages, It found that quartz-pyrite stage formed in 160-200℃, and its mineralization salinity is 0.18-1wt% Na Cl, Quartz-gold-polymetallic sulfide stage is formed at 200-260℃, and its mineralization salinity is 1.5-4wt% Na Cl, Quartz-calcite stage formed at 120-180℃, and its mineralization salinity is 0.5-2wt% Na Cl. Through the above comparison can be found, from the early stage to the late stage of mineralization, ore-forming fluid temperature decreases, the salinity decreases. At the same time through a fluid microthermometry estimated pressure of the deposit mineralization is 140-200 MPa, a depth 6.3-15.5km.(3).Sulfur isotopic characteristics indicate that the Longtougou gold deposit’s sulfur source from deep magma, mantle-derived magma and hydrothermal mineralization is closely related, carbon-oxygen isotope characteristics indicates that the deposit’s carbon isotope source from mantle and contamination contaminated by stratigraphy, indicating mineralization fluid from the mantle and are subject to contaminated by stratigraphy; hydrogen-oxygen isotopic characteristics indicate the source of water is mainly meteoric water, and a weak trend close to magmatic water;(4).Integrated Regional Information and geological characteristics of the Longtougou gold deposit, we inferred that the continued reduction of ancient Qinling ocean beneath the North China Plate and the collision orogeny between North China plate and the Yangtze plate are closely related. Shanyang-Fengzhen major fracture and the major folds of the Longtougou gold deposit are formed at 287.8±2.6Ma, and then under this stress, at 200.9±1.9Ma, formed a thrust faults which cutting the major fold’s shaft position on the basis of fold and its recoil faults, with the activity of mantle-derived magma and in the above process formed the Longtougou gold deposit.(5).By contrast with the characteristics of typical orogenic gold deposit of world, we can see the features of metallogenic characteristics between Longtougou gold deposit and abroad typical of orogenic gold are similar in many ways: tectonic setting, ore-hosting geologic body, nature of ore-controlling structure, alteration mineral assemblages, ore type, the main mineralization temperature, pressure, salinity, etc. So we think Loutougou gold deposit belongs to orogenic gold deposit. |
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