扬子北缘勉略宁地区山坪-东皇沟铅锌矿地质地球化学特征与成矿模式

    Geological, geochemical characteristics and metallogenic model of Shanping-Donghuanggou lead-zinc deposit in Mian-Lue-Ning Area, northern margin of the Yangtze Block

    • 摘要: 扬子板块北缘碧口东段勉(县)-略(阳)-宁(强)地区新元古代发生大规模的海相火山活动,在区内形成沿北东向展布的火山机构群与广泛分布的碧口岩群(Pt3BK)双峰式火山岩,并伴随大规模的火山沉积与气液成矿作用,在区内形成一系列海相火山岩型铁、铜、铅锌多金属矿床。位于宁强县代家坝镇的山坪-东皇沟铅锌矿即是上述矿床的典型代表。为揭示矿床成因、建立成矿模式,本次开展了系统的矿床地质、岩石地球化学与矿床地球化学研究。研究结果表明:山坪-东皇沟铅锌矿赋存于受北西向、北北西向和北东向断裂共同夹持的近似穹隆状古火山机构,主要赋矿岩石为碧口岩群大安岩组(Pt3d)爆发相细碧角斑岩,岩石属富Na低Ti的钙碱性细碧角斑岩系;矿化特征上,矿区具有上“黑矿(方铅矿+闪锌矿)”、下“黄矿(黄铁矿+黄铜矿)”、上块状矿(条带状、浸染状矿)、下脉状矿的分带特征,且发育赤铁碧玉岩、重晶石岩等喷流沉积岩。S、Pb同位素特征显示,硫、铅等成矿物质主要源于下伏富钠质细碧角斑岩。硫化物-石英脉的H-O同位素特征表明成矿流体主要来自于深源流体,包裹体成分显示成矿流体属K+-Ca2+-Cl-SO4 2−型。获得赋矿细碧角斑岩的锆石U-Pb年龄为857.2 Ma,推测成矿年龄与之相近,为新元古代中晚期。综上,认为山坪-东皇沟铅锌矿属“黑矿”VMS(Volcanic Massive Sulfide)型Zn-Pb-(Cu)矿床,形成于新元古代中晚期扬子北缘裂解背景下,山坪-东皇沟地区双峰式火山活动第一旋回晚期的海相火山热液成矿作用,含大量H2S、CO2、CH4、Cl、SO4 2−与Pb、Zn、Cu等物质的成矿流体,沿火山管道上涌与火山岩及部分海水发生混合形成含矿流体,在火山附近的低洼地段,沿爆发相细碧角斑岩与喷发相细碧质凝灰岩岩相界面附近富集沉淀成矿,形成似层状多金属块状Zn-Pb-(Cu)硫化物矿床。

       

      Abstract: During the Neoproterozoic Era, the large-scale marine volcanic activity, which occurred in the Mian (xian)-Lue (yang)-Ning (qiang) Area, eastern section of Bikou on the northern edge of the Yangtze Block, had formed a NE directional distributed volcanic apparatus group and the widely distributed Bikou rock-group (Pt3BK) bimodal volcanic rocks. Along with large-scale volcanic sedimentation and gas-liquid mineralization, a series of marine volcanic rock-type iron, copper, lead-zinc polymetallic deposits are formed, which includes the Shanping-Donghuanggou lead-zinc deposit, located in Daijiaba Town, Ningqiang County. In order to reveal the genesis of the deposit and establish the metallogenic model, a systematic geological and geochemical study of this deposit has been carried out in this paper. It indicates that the Shanping-Donghuanggou lead-zinc deposit is located in an approximately dome-shaped ancient volcanic apparatus, sandwiched by NW, NNW, and NE trending faults. The main ore bearing rock is the eruptive phase spilite keartophyry of the Da’an Formation (Pt3d), which belongs to the Na-rich and low Ti-calcium alkaline spilite keartophyry series. In terms of mineralization characteristics, the mining area has a zoning feature of upper “black ore (galena+sphalerite)” and lower “yellow ore (pyrite+chalcopyrite)”, upper blocky (banded, disseminated ores) and lower vein-like ores, and develops exhalation sedimentary rocks such as hematite and barite. The S-Pb isotope characteristics indicate that the ore-forming materials mainly originated from the underlying sodium-rich spilite keartophyry. The H-O isotope characteristics indicate that the ore-forming fluid mainly came from deep-seated fluids. The inclusion composition shows that the ore-forming fluid belongs to the K+- Ca2+- Cl - SO4 2− type. The zircon U-Pb age obtained from the ore-bearing spilite keartophyry is 857.2 Ma, indicating a similar mineralization age in the middle to late Neoproterozoic. In summary, it is believed that the Shanping Donghuanggou lead-zinc deposit belongs to the “black ore” VMS-type Zn Pb - (Cu) deposit, which was formed in the northern margin of Yangtze Block cracking background in the late stage of the Neoproterozoic Era. The deposit was formed by marine volcanic hydrothermal mineralization during the late period of the first cycle of bimodal volcanic activity, containing a large amount of H2S, CO2, CH4, Cl, SO4 2− and Pb, Zn, Cu and other substances. The ore-forming fluid surged up the volcanic pipeline, mixed with volcanic rocks and seawater, forming a mineral-bearing fluid. In the low-lying areas near the volcano, it enriched and precipitated mineralization near the interface between explosive spilite keartophyry and eruptive spilite tuff facies, forming layered polymetallic Zn Pb-(Cu) sulfide deposit.

       

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