西藏昌都赵发勇溶洞控矿MVT铅锌矿床地质特征与矿床成因

Characteristics and Genesis of the Zhaofayong Karst-controlled MVT Deposit in the Changdu Region,Tibet

赵发勇铅锌矿床位于三江成矿带中段昌都地区,铅锌矿化发育在新生代区域逆冲推覆构造中,以逆冲断层相关的古溶洞构造为主要控矿构造,代表了逆冲褶皱系MVT铅锌矿床中一种新的成矿模式——溶洞控矿成矿模式。因此,笔者在对赵发勇矿区详细地质填图基础上,对其成矿流体特征、成矿期方解石和硫化物的同位素组成进行了系统研究,以期对逆冲褶皱系MVT铅锌矿床中这一新成矿模式的成因机制进行探讨。赵发勇铅锌矿床矿体呈漏斗状-筒状发育在逆(冲)断层上、下盘的下二叠统和上三叠统灰岩古溶洞中,以角砾状、块状和皮壳状为主要矿石构造,以方铅矿、闪锌矿为主要矿石矿物,经历了硫化物期(I)和硫化物-碳酸盐期(II)两期成矿过程。I期成矿流体总体具低温度(130~140℃)、高盐度(23%~24%NaCl eq.)特征,部分呈现中高温度(约高达400℃)、中低盐度(约低达8%NaCl eq.)特征;δDV-SMOW值介于–147‰~–94‰,δ18O流体值介于1.25‰~13.62‰;同成矿期方解石δ13CV-PDB值为–2.4‰~5.1‰,δ18OV-SMOW值为15.1‰~27.4‰。两期硫化物δ34S均为负值(–15.1‰~–1.6‰),206Pb/204Pb、207Pb/204Pb、208Pb/204Pb组成分别为18.703 0~18.966 2、15.638 9~15.687 2、38.554 8~38.924 0。研究表明,I期铅锌矿化由①封存在地层中的蒸发浓缩的海水或/和区域古近纪—新近纪盆地下渗的盆地卤水形成的低温度、高盐度的本地流体和②由区域或矿区地层释放的蒸发浓缩海水和变质基底释放的变质水混合而成的中低温、中高盐度的区域流体两种流体组成;两期矿化的成矿金属来自造山带中从基底变质岩到盖层灰岩或/和碎屑岩等在内的多套地层,还原硫来自古近纪—新近纪盆地卤水中的硫酸盐±灰岩地层封存水中的硫酸盐的细菌还原作用;富还原硫的本地流体和富金属物质的区域流体的混合是铅锌硫化物沉淀的主要机制。结合对三江带区域溶洞控矿MVT铅锌矿床研究认识,笔者初步建立了逆冲褶皱系溶洞控矿MVT铅锌矿床成矿模型。

The Sanjiang metallogenic belt on the Tibetan Plateau is an important polymetallic metallogenic belt in China.The sediment-hosted Pb-Zn deposits in this belt not only have huge reserves of resources,but also have a great potential for scientific research,because they represent a new type of MVT Pb-Zn deposits developed in the thrust-fold system of the collisional orogenic belt.The Zhaofayong lead-zinc deposit is located in the Changdu area in the middle part of Sanjiang metallogenic belt.Its Pb-Zn mineralization is developed in the Cenozoic regional thrust nappes and is controlled by the paleokarst cave structures related to the thrust faults.This deposit presents a new deposit style among the thrust-fold hosted MVT Pb-Zn deposits and can be called karst-controlled style.Therefore,systematical geological mapping and detailed genetic study of the Zhaofayong deposit have been performed and a new karst-controlled model for the MVT deposits in the thrust-fold system has been built.The Pb-Zn mineralization in the Zhaofayong deposit occurs in the hanging wall and footwall of a thrust fault,and orebodies,mainly produced in tubular shape,are distributed in the paleokarst caves in limestones of Permian Licha Formation and Triassic Jiapeila Formation.Breccia,massive and crust are main ore structures,and galena and sphalerite are main ore minerals.The main ore-forming process in the Zhaofayong deposit can be divided into Period I(sulfide period)and Period II(sulfide-carbonate period),and between them a compressing deformation existed.A kind of LV inclusions rich in liquid occurred in Period I.Microthermal testing work suggests that the main hydrothermal fluids in this period had low temperature(130~140℃)and high salinity(23%~24%NaCl eq.)characteristics.However,some data indicate that some of the fluids had different characteristics in that they had higher temperature(as high as^400℃)but lower salinity(as low as^8%NaCl eq.).the liquid parts of the hydrothermal fluids in Period I belonged to a complex Mg2+-Na+-K+-SO2–4-Cl–-F–-NO–3 system.Combined with the H-O isotope composition(–147‰~–94‰ for δDV-SMOW and 1.25‰~13.62‰forδ18Ofluid)and the ratios of the ion content(14~287 for Cl/Br and 4~148 for Na/Br)for the hydrothermal fluids and the C-O isotope composition of calcite(–2.4‰~5.1‰ for δ13CV-PDB and 15.1‰~27.4‰ for δ18OV-SMOW),two resources of hydrothermal fluids were recognized,i.e.,①low-temperature,high salinity basin brine from the regional paleogene basins and/or trapped evaporated seawater in the strata and②low to middle-temperature,middle to high-salinity regional fluid composed of evaporated-concentrated seawater cocooned in strata and metamorphic water released from metamorphic rock.The δ34S values of sulfide for the two periods are negative(–15.1‰~–1.6‰),and those of barite are positive(10.8‰).Compared with theδ34S values of the regional gypsum,the sulfur isotopes suggest that reducing S came from biological sulfate reduction derived from the basin brine descending from overlying Paleogene basin and/or evaporated-concentrated seawater cocooned in strata,and mixing of two kinds of fluids,which were local fluid rich in reducing sulfur and regional fluid rich in metal,caused the deposition of mineralized minerals.The 206Pb and 207Pb compositions of the sulfides(18.703 0~18.966 2 for 206Pb/204Pb and 15.638 9~15.687 2 for 207Pb/204Pb)in the two periods are similar to those of the regional limestone;however,the 208Pb compositions of the sulfides(38.554 8~38.924 0 for 208Pb/204Pb)are different from those of the regional limestone.This information indicates that the metals came from metamorphic basement and limestone(and clastic rock)caprock distributed in the orogenic belt.Based on the geological characteristics and genesis of the Zhaofayong deposit,combined with the information from the regional MVT lead-zinc deposits controlled by karst caves,the authors propose a metallogenic model of karst-controlled MVT lead-zinc deposit in the thrust-fold system:①Regional thrusting and nappe activities led to the uplift of local limestone strata in the deposit and paleokarst caves were formed with shallowing strata,forming the most favorable trap space in the thrust fault.The brine and/or evaporative concentrated seawater in the infiltrating Paleogene basin converge in the trap space and formed a local fluid reservoir rich in reducing sulfur under the action of bacterial reducing sulfur.②Regional extrusion continued,metamorphic water and sealed water were released to become regional fluids,and regional fluids migrated over a long distance in deep detachment systems to form exotic fluids rich in metals and halogens;③After thrusting and stress relaxes,extensional faults were developed into open space,and exotic fluids migrated upward along the open space.Metal-rich and sulfur-rich fluids met,forming cylindrical lead-zinc orebodies in the most advantageous ancient karst caves.