Metallogenic Processing Of Post-batholith Gold Deposits

Located in northeast Hebei Province, and east sector of Yanshan mountain ridge, Jidong area is one of the most important gold deposits zones in China. In Yanshan terrain, a great number of gold deposits (or occurrences) have been found around Dushan complex.The metallogenic epoch of these gold deposits is different from not only that of the Dushan complex, but also other intrusions and deposits, suggesting their ore-forming particularity. As a batholith, Dushan complex occurred with the uplift of the crust and the forming of the lower mafic crust which is favorable for the delamination of the lithosphere. The detachment provided advantageous conditions for the large scale ore-forming event. Based on the previous work, field investigation, modern testing methods and a great deal of synthesized studies, new ideas and achievement were acquired as followed:1. SHRIMP U-Pb dating showed that the age of the granitic porphyry in Tangzhangzi gold ore district was 173±2Ma. This result also can be regarded as the lower age limit of cryptoexplosion breccia and the gold ore deposit. Both Yuerya (174±3 Ma) and Niuxinshan complex (173±2Ma) have the similar age, showing they were the outcome the magmatic activities in the early-middle Yanshania period in Cenozoic. A main metallogenic epoch can be inferred in Jidong area, the lower limit of which being the early-middle Yanshania period.2. The idea that gold deposits occurred in one main metallogenic epoch, one main metallogenic event, encircled in one ore-forming center, was put forward. Based on the theory of good mineralization occurred in small intrusions, different ore-forming systems, including orthomagmatic metallogenic system, contact belt system and long-distance hydrothermal system, have been identified. They were regarded as the outcome of similar ore-forming fluid in different geological conditions.3. The"laccolith effect"of batholith was brought forward firstly. The thick plank-like batholith with good integrality is helpful to shied the ore-bearing fluid bleeding from small intrusions. Combined with the alteration of the lithosphere structure during the batholith emplacement and the later geological process, the"post-batholith metallogenic model"have been put forward to make clear the distribution of the gold deposits around Dushan complex reasonably. it may have universal implication.4 The small Yueryan intrusion, which can bring on big type gold deposit, may implicate its close origin relation with Dushan complex.5. The next step ore prospecting should be focused on Dushan complex, and the study on the deep channel between the complex and gold deposits should be strengthened, so that a great breakthrough for ore exploration can be achieved in Jidong area.