Mineralization And Metallogenic Pattern Of Lead-zinc Deposits In Northeast Yunnan

The sedimentary rock-hosted lead-zinc deposit is more in type, huge in reserve, and complicated in origin. To study its characteristics and origin will contribute not only to better understanding of the mineralization so as to enrich the metallogenic theory, but also to revealing of the regional metallogenic pattern to provide scientific evidence for metallogenic prediction and regional exploitation.A large quantity of sedimentary rock-hosted lead-zinc deposits occur in Northeast Yunnan, constituting an important part of the noted lead-zinc metallogenic belt in Sichuan-Yunnan-Guizhou, China. Numerous previous data have been systematically collected, and a detailed study has been carried out on five typical lead-zinc deposits in this region. On the basis of the data and study, the orefield geological and ore deposit geological and geochemical characteristics have been ascertained, leading to propose a classification of the lead-zinc deposits in the region and laying basis for analysis of the ore-controlling factors and mineralization.The following are major achievements obtained in this dissertation.1. The lead-zinc deposits in Northeast Yunnan can be divided into three types based on their host stratohorizon. The first type is hosted in the Dengying Formation of the Upper Sinian to the Yuhucun Formation of the Lower Cambrian (such as lead-zinc deposits located at Maozu in Qiaojia County, Jinshachang in Yongshan County, and Wuxingchang in Huize County, etc.), as well as in the Baizuo Formation of the Lower Carboniferous (such as lead-zinc deposits at Qilinchang and Kuangshanchang in Huize County, etc.).The second type is distributed in the Zaige Formation of the Upper Devonian (such as Maoping deposit in Yiliang County). The deposits of this type are obviously controlled by faults, and some orebodies occur directly in the fault zone (such as Lemachang deposit in Ludian County). The third type occurs in the Maokou Formation of the Lower Permian (such as deposits at Fulechang in Luoping and Dahai in Huize County, etc.). These types of deposits have different lithological assemblages of host strata, ore-controlling structures, lithofacies paleogeography, as well as ore characteristics including type, mineral and texture and structure.2. The lead and zinc deposits in Northeast Yunnan are obviously controlled by the stratohorizons of the strata. The major stratohorizons-hosted include Upper Sinian, Middle to Lower Cambrian, Middle to Upper Devonian, Middle to Lower Carboniferous and Lower Permian. In space, the stratohorizons of the lead and zinc deposits in the region vary from Zbdn at Maozu through D₃zg, C₁b and C₂w at Maoping and Huize to P1m at Fulechang, tending to gradually become newer from NW to SE. Correspondingly, the lithological assemblages of the stratohorizons change from dolomite, limestone, siliceous dolomite, phosphor-bearing siliceous dolomite and silicolite at Maozu through dolomite, limestone, siliceous limestone and siliceous dolomite at Huize to dolomite at Fulechang, indicating a trend from complicated to simple.3. The Middle to Lower Cambrian and underlying strata are a major source bed, providing much material for mineralization, while the Lower Ordovician to the Lower Permian as well as Emeishan Basalt horizon are a second source bed, possibly providing a part of ore-forming materials.4. The formation process of the lead and zinc deposits in the northeast region of Yunnan is mainly controlled by NW fault basins and contemporaneous faults. In contrast, the NE faults not only give modification to lead and zinc deposits in the overburden, but also provide thermo-motive conditions for the formation of hydrothermal deposits and spots.5. The ore-forming fluid is complex in source, derived mainly from meteoric water, and some from deep-source magmatic water and metamorphic water in metamorphic series of basement. This fluid is Na⁺-Ca²⁺-Cl^--SO₄^2-type hydrothermal solution with moderate to low temperature, high to moderate salinity, and moderate to low acidity.6. The study on the geological characteristics, ore-forming conditions, ore-controlling factors and mineralization mechanisms supports classification of the sedimentary rock-hosted lead and zinc deposits in Northeast Yunnan into three types based on mineralization, corresponding to classification of them into the three types based on host stratohorizon. These three types are submarine hot-water deposit-superposition and reformation, hot-water deposit, and hydrothermal types (including non-fault-zone and fault-zone hydrothermal sub-types). Among them, the first type of deposit with large quantity, large scale and high grade is the major one in this region, and the fault-zone hydrothermal sub-type is the one discovered for the first time in this region.