内蒙古黄岗锡铁矿床流体包裹体特征及成矿机制研究

Fluid inclusion characteristics and metallogenic mechanism of Huanggang Sn-Fe deposit in Inner Mongolia

黄岗锡铁矿床是大兴安岭南段多金属成矿带内的一个重要的矽卡岩型矿床。下二叠统大石寨组及黄岗梁组是矿区的主体含矿层位,矿体总体顺层分布,空间上与矽卡岩密切相关。矿床的形成经历了矽卡盐阶段、退化蚀变阶段、石英硫化物阶段和碳酸盐阶段。对不同成矿阶段的石榴子石、绿帘石、角闪石、石英、方解石和萤石中流体包裹体所进行的岩相学和显微测温研究表明,与成矿有关的包裹体类型主要有硅酸盐熔融包裹体、H2O-NaCl型包裹体、CO2-H2O±CH4型包裹体和CO2-H2O-NaCl型包裹体。成矿早阶段以硅酸盐熔融包裹体和H2O-NaCl型包裹体为主,晚阶段出现少量CO2-H2O±CH4型包裹体和CO2-H2O-NaCl型包裹体。从早期到晚期4个阶段的均一温度分别为(257~432℃、>550℃)、322~403℃、202~304℃、153~221℃;盐度w(NaCleq)为(12.13%~19.88%、>66.8%)、16.43%~22.34%、1.74%~14.77%、1.74%~11.9%。成矿流体的均一温度和盐度w(NaCleq)主要为220~432℃和1.74%~22.34%,属于高-中温、中-低盐度流体。流体包裹体的气相成分以CO2和H2O为主,其次为N2、O2和CH4,有少量C2H2、C2H4和C2H6;液相成分中,阳离子以Na+、K+为主,其次为Ca2+、Mg2+,阴离子以Cl-、SO24-为主,其次为F-,还含有少量Br-、NO-3。氢、氧同位素特征显示,前3个成矿阶段成矿流体的δ18OH2O值为-6.0‰~9.8‰,δDV-SMOW值为-116‰^-73‰,表明成矿流体主要为岩浆水;碳酸盐阶段成矿流体的δ18OH2O值为-10.9‰^-1.6‰,δDV-SMOW值为-104‰^-101‰,表明在成矿后期混入有部分大气降水。方解石的δ13CPDB值为-11.706‰~1.1‰,表明流体中的碳主要由岩浆-地幔经溶解作用所提供,且受到低温蚀变作用和大气降水的影响。硫化物的δ34SV-CDT值为-4.3‰~2.7‰,表明成矿物质主要来源于深部,可能受到后期成矿作用的叠加和改造。流体的降压沸腾和相分离,以及流体混合作用可能是成矿的主要原因。

The Huanggang Sn-Fe deposit in Inner Mongolia is one of the important skarn deposits in Southern Da Hinggan Ling polymetallic belt.Lower Permian Dashizhai Formation and Huanggangliang Formation are the main ore-hosting horizons.Ore bodies are largely concordant with the bedding of the strata,and the distribution of the ore bodies is spatially related to skarn.The formation of the Huanggang ore deposit experienced skarn phase,retrograde eroding phase,quartz sulfide phase and carbonate phase.Petrographic and microthermometric studies of fluid inclusions in garnet,epidote,hornblende,quartz,calcite and fluorite from different phases show that the inclusions related to mineralization include mainly silicate melt inclusions,H2O-NaCl type inclusions,CO2-H2O±CH4 type inclusions and CO2-H2O-NaCl type inclusions.Inclusions of the early mineralization phase mostly contain silicate melt inclusions and H2O-NaCl type inclusions,whereas small quantities of CO2-H2O±CH4 type inclusions and CO2-H2ONaCl type inclusions occur in the late mineralization phase.Homogenization temperatures of the four phases are from early to late（257～432℃,550℃）,322～403℃,202～304℃ and 153～221℃ respectively,whereas salinities are（12.13%～19.88%,66.8%）,16.43%～22.34%,1.74%～14.77% and 1.74%～11.9% respectively.Homogeneous temperature and salinity of the ore-forming fluid are concentrated in 220～432℃ and 1.74%～22.34%,suggesting the deposit type of medium-high temperature and medium-low salinity.Gases of fluid inclusions mostly contain CO2 and H2O,followed by N2,O2,and CH4,with a small amount of C2H2,C2H4 and C2H6.Cations of liquid composition mostly contain Na＋,K＋,followed by Ca2＋,Mg2＋,while anions are mainly Cl-,SO2-4,followed by F-,with a small amount of Br-,NO-3.Hydrogen-oxygen isotope characteristics show that the δ18OH2O values of the ore-forming fluids in the first three stages are in the range of-6.0‰～9.8‰,and δDV-SMOW values vary between-116‰ and-73‰,which implies that the ore-forming fluids were mainly derived from magmatic fluids.δ18OH2O values of the ore-forming fluids in the carbonate phase range from-10.9‰ to-1.6‰,and δDV-SMOW values vary between104‰ and-101 ‰,indicating the mixture of some metamorphic water in the late mineralization phase.The δ13CPDB values of calcite from the Huanggang ore deposit range from-11.706‰ to 1.1‰,indicating that the carbon in the fluids was mainly derived from the mantle and affected by low temperature alteration and metamorphic water.The δ34SV-CDT values of sulfides range from-4.3‰ to 2.7‰,suggesting that the ore-forming material mainly came from deep sources and might have been altered by the late metamorphic water and magmatic hydrothermal solution.Buck boiling and phase separation of fluids as well as fluid mixing might have been the main factors for mineralization.