甘肃北山老金厂金矿床载金矿物特征、原位硫同位素组成及其对成矿的指示意义

Characteristics of Gold-Bearing Minerals and Compositions of In-Situ Sulfur of Laojinchang Gold Deposit in Beishan,Gansu Province and Its Ore-Forming Implications

老金厂金矿床是北山成矿南带最具代表性的中低温岩浆热液型金矿床之一,其规模为中型。依据脉体穿插、矿物共生组合和矿石结构构造等特征,将矿床矿化作用过程划分为石英-黄铁矿阶段(Ⅰ)、石英-含砷黄铁矿-毒砂阶段(Ⅱ)、石英-黄铁矿-多金属硫化物阶段(Ⅲ)和石英-方解石阶段(Ⅳ)。利用电子探针研究了不同成矿阶段载金矿物的元素组成及其分布规律。Ⅰ阶段:黄铁矿以粗粒自形立方体为主,粒度为0.50~1.50 mm,贫As、Au;毒砂含量极少,呈细粒他形。Ⅱ阶段:含砷黄铁矿周围常有大量毒砂产出,含砷黄铁矿多为立方体、五角十二面体,粒度为0.30~1.00 mm,富As、Au;该阶段矿化最为强烈,毒砂主要形成于此时期,多呈棱柱状、柱状、放射状集合体,显示富S亏As特征。Ⅲ阶段:多以黄铁矿-黄铜矿-闪锌矿共生组合脉的形式产出,黄铁矿多呈长条状,以富S、Cu、Zn、Au和贫Fe、As为特征。Ⅳ阶段:矿化作用极弱,毒砂、黄铁矿含量极少,为细粒他形。原位硫同位素组成显示:Ⅰ阶段黄铁矿δ^34SV-CDT值为-3.8‰^-2.9‰,均值为-3.3‰;Ⅱ阶段黄铁矿和毒砂δ^34SV-CDT值为-4.7‰~2.6‰,均值为-3.3‰;Ⅲ阶段黄铁矿和闪锌矿δ^34SV-CDT值主要分布于-1.9‰~1.0‰之间,均值为0.1‰。此3个阶段硫同位素组成反映了成矿期硫主要来源于幔源岩浆,混入了部分地层硫。综合前人研究成果,认为成矿早期至晚期,成矿流体总体上由富S贫As向富As贫S演化。Ⅰ阶段体系处于中性稳定的环境,硫源充足;Ⅱ阶段为贫S富As的高氧逸度环境,由于大气降水对地层的淋滤渗透,混入富As流体,Au可能与As结合形成Au-As络合物,在成矿有利部位富集沉淀;Ⅲ阶段成矿元素种类丰富,体系为富S贫As的弱还原环境,Au很可能与HS-、S-形成络合物进入黄铁矿晶格。

Laojinchang gold deposit is one of the most representative medium-sized magmatic hydrothermal gold deposits formed at medium-low temperature in the southern Beishan metallogenic belt.Based on the cross cutting relationships of the different auriferous veins,mineral paragenesis,ore texture and structure,its mineralization stages can be divided into quartz-pyrite stage(Ⅰ),quartz-arsenian pyrite-arsenopyrite stage(Ⅱ),quartz-pyrite-polysulfide stage(Ⅲ),and quartz-calcite stage(Ⅳ).In this paper,the element concentration and composition in the gold-bearing minerals of different ore-forming stages were analyzed by using the electron microprobe analyzer(EMPA).The pyrite of stage I is primarily coarse-grained(0.50-1.50 mm)euhedral cube,with low content of As,Au,and a small amount of fine-grained anhedral asenopyrite.The arsenian pyrites of stage II are surrounded by a large number of arsenopyrites,and the arsenian pyrites are dominantly cubic and pentagonal dodecahedron,rich in As and Au with particle size of 0.30-1.00 mm.Stage II is the most intensive period of mineralization,and the main formation period of arsenopyrites.The arsenopyrites in this stage are primarily appeared as rhombic-columnar,columnar,and radiate-columnar aggregates,rich in S but depleted of As.The pyrites in stageⅢcommonly occur as veins of pyrite-chalcopyrite-sphalerite mineral paragenesis,appeared as long strip,with high content of S,Cu,Zn,Au but low content of Fe and As.The mineralization in stageⅣis relatively weak with a small amount of fine-grained anhedral pyrites and asenopyrites.Theδ^34SV-CDT values of pyrite or and asenopyrite range from-3.8‰to-2.9‰(average-3.3‰)in stageⅠ,theδ^34SV-CDT values of pyrite and asenopyrite range from-4.7‰to 2.6‰(average-3.3‰)in stageⅡ,and theδ^34SV-CDT values of pyrite and sphalerite range from-1.9‰to 1.0‰(average 0.1‰)in stageⅢ;which suggests a mantle-derived magmatic sulfur provenance,and contaminated by sedimentary sulfur at the late stage,as indicated by in-situ sulfur isotope compositions.Based on the previous study,it is proposed that the ore-forming fluid was evolved from S-rich and As-poor fluids to As-rich and S-poor fluids during the mineralization.In stageⅠ,the ore-forming system was in neutral and stable environment with abundant sulfur.In the main stage(II),the ore forming fluid was rich in As,poor in S,and high in oxygen fugacity,and the As-rich fluids was injected into hydrothermal system due to the leaching-infiltration of meteoric water,which led to the formation of the Au-As complexes,and the possible precipitation and accumulation in appropriate place.The ore forming system was characterized by variety of metallogenic elements,rich in S,poor in As,and weak reduction in stageⅢ,and Au might enter the pyrite lattice in the form of[Au(HS)2]-or[AuS]-complexes.