Study On The Coupling Relationship Between Dolomite And Metallic Mineralization In The Pb - Zn Deposit, Mao Ping, Northeastern Yunnan, China

Maoping lead and zinc deposit located in the southwestern margin of the Yangtze Block. The proven lead and zinc metal resources are more than 2.4 million tons. Maoping lead-zinc deposit is another Pb-Zn polymetallic deposit with superlarge-sized potential in Sichuan-Yunnan-Guizhou metallogenic region. Ⅰ, Ⅱ, Ⅲ orebodies occured in the Upper Devonian coarse crystalline dolomite and Carboniferous dolomitization dolomite. The orebodies showing bedded and veins occurred in NW side of the Shimenkan revised anticline. Alteration-mineralization is characterized by significant horizontal zoning and non-significant vertical zoning. Host rocks have undergone varying degrees dolomitization. Based on detailed studies on dolomitization, dolomite crystal structure and isotopic tracer, the following views are obtained.(1)The dolomitization showed different zoning characteristics with the different rock lithology in Maoping lead-zinc deposit. Three types dolomitization developed in the study area. They are planar dolomitization in limestone, planar dolomitization in dolomite and vein like dolomitization. The planar dolomitizaion enveloped the orebody with a halo and showed the significant symmetric zoning. The alteration zoning are leached limestone(LL), light gray withe dolomite(LGD), gray withe dolomite(GD) and saddle dolomite(SD) from outside to inside of the planar dolomitization in limestone. While, the alteration zoning are fine-microtek crystalline dolomite, microtek-coarse crystalline dolomite, very coarse-coarse crystalline dolomite from outside to inside of the planar dolomitization in dolomite. Five stage hydrothermal veins developed in the Maoping deposite, and three of theese veins have dolomite.(2)The dolomite in different alternation zone shows five Cathode-luminescence zones, totally. They are Zone1, Zone2, Zone3, Zone4 and Zone5. Zone 1 is Primary dolomite core, and Zone 2-5 are hydrothermal dolomite zone. The characteristic of hydothermal dolomite zones can indicate the features of corresponding hydothermal fliuid. Zone1-2 occurred in dolomite in LL, Zone 1-4 occurred in dolomite in LGD, Zone3-5 occurred in dolomite in GD, and Zone3-4 occurred in dolomite in SD. Those evidences indicate that the hydorthermal fluid evolution history constricted from outside to inside. The Cathode-luminescence zones of dolomite in different alternation zone of dolomite are different. For example, Zone 1-5 occurred in dolomite in fine-mi crotek crystalline dolomite, Zone 1-4 occurred in dolomite in microtek-coarse crystalline dolomite, and Zonel-3 occurred in dolomite in very coarse-coarse crystalline dolomite. Those evidences indicate that the hydorthermal fluid evolution history spreaded from outside to inside. The Cathode-luminescence zones of dolomite in different dolomite veins are different. Zone 1-4 occurred in dolomite in second vein, which indicate the second vein keep open for the fisrt to thid stages hydrothermal fluid flowing. Zone3 occurred in dolomite in third vein, which indicate the third vein keep open just for the thid stage hydrothermal fluid flowing. Zone3-5 occurred in dolomite in forth vein, which indicate the second vein keep open for the second to forth stages hydrothermal fluid flowing.(3)Based on infrared spectrum study, We is found that saturated hydrocarbons in the limestone still occurred in their alteration zones. It is indicate that the material released from reaction between limestone and hydothermal fluid didn’t be migrated to other place. Thus, the place where the orebody emplaced is closed. The dolomite in very coarse-coarse crystalline dolomite have no saturated hydrocarbons, while, the dolomite crystallines in microtek-coarse crystalline dolomite and fine-microtek crystalline dolomite have saturated hydrocarbons. Those evidences indicate that the material released from reaction between dolomite and hydothermal fluid were migrated to other place. The dolomite crystallines in third sage vein located in the middle-low part of the orebody have no saturated hydrocarbons. However, saturated hydrocarbons occurred in dolomite crystallines in third sage vein located in the top of the orebody. Those evidences indicate the hydrothermal fluid migrate upward. Calcite contain the same kinds of saturated hydrocarbons with limestone, which indicate that saturated hydrocarbons didn’t paticipate in the Sulfate reduction process. Thus, reducing sulfur may be provided by host rock.(4)Diagenesis of hydrothermal dolomite and metal mineralization process can be divided into five stages. First stage fluid form the basic pattern of host rock in limestone. The altered range of first stage fluid flow is the maximum extent of later mineralization-alteration. The fluid opens channel in dolomite, dissolves channel dolomite and re-grow to form zone2. Second stage fluid migrate along channels formed in this stage and first stage, grow up comb dolomite in new fissure and continue to grow out dolomite zone in first stage channel. A large number of metal sulfide precipitation.At the same time, there is a greater range of dissolution makes vugs formation. Eventually, alternated dolomite, hydrothermal dolomite formed. Fractures remain open, third stage fluid continue migrate, then, dolomite zone form. Accompany with the large number of metal sulfide precipitation, fractures open keeping on and forth stage fluid migrate continuely, but fluid has almost exhausted, after that, calcite precipitated, fissures close, and dedolomitization occurred locally. The new tensile fissures form and be filled with the fifth stage fluid riching for calcium. In the end of fluid, large calcite vein has been formed. Ore-forming fluid is a substantially continuous process. Fluid exhausting marks the end of fluid activity. Many fissures remain open, and no later fluid activity any more, which indicates that Maoping lead-zinc mineralization process is a product of hydrotherm event, PH value of the fluid plays an important role in the whole process.(5) The dolomite crystalline are near the orebody, and have the lower degree of order and the lower pyrolysis temperature.(6)C-O isotopic alteration models suggest that stratum in mine district does not contain pore water, the isotopic composition of fluid itself (C-O-Sr-Ca-Mg) determine the differences between itself and hostrock. The water-rock interaction can occurred between dolomitization fluid and hostrock. Hydrothermal carbonate minerals result from the water-rock interaction between host rock and fluid or and precipitation, or directly precipitated from the dolomitization fluid.(7)There is a time relationship between the formation of dolomite growth zoning and the precipitation of sulfide. Saddle dolomite formation is not before sulfide, but cross each other. We have modified the original model of HTD lithological sequences.(8)Dolomitization and metallogenic precipitation are two separate processes in the same fluid system. Metal precipitates constrain the growth of dolomite, In turn, Dolomite owe growth indicator metal precipitation, Dolomite dissolution edges are signs.(9)In the case of a single orebody, orebody and the development of hydrothermal dolomite restrict each other. The greater range of hyfrothermal dolomite with smaller orebody, and vice versa. The large area of dolomitization is the product of the mutual joint of the multiple orebody’s dolomitization zone. The larger dolomitization area indicate the more disper of sulfide precipitation, and it is not conducive to forming large and high grade orebody.(10)The above views provide new evidences for understanding the mineralization of MVT Pb-Zn deposits, and can also be used to prospecting work.