铜陵地区朝山矽卡岩型金矿床及含矿岩体的成因:^(40)Ar/^(39)Ar年龄、元素地球化学及多元同位素证据  

Genesis of the Chaoshan gold deposit and its host intrusion,Tongling area:Constraints from ^(40)Ar/^(39)Ar ages and elemental and Sr-Nd-O-C-S isotope geochemistry

作　　者：王建中[1,2] 李建威[2,3] 赵新福[4] 钱壮志[1] 马昌前[2] 

机构地区：[1]长安大学地球科学与国土资源学院,西安710054 [2]中国地质大学地质过程与矿产资源国家重点实验室,武汉430074 [3]中国地质大学资源学院,武汉430074 [4]香港大学地球科学系

出　　处：《岩石学报》2008年第8期1875-1888,共14页Acta Petrologica Sinica

基　　金：自然科学基金创新群体基金(40521001);科技部国际合作项目(2007DFA21230);国家自然科学基金重点项目(40334037)联合资助

摘　　要：位于铜陵地区狮子山矿田的朝山金矿是长江中下游成矿带少有的矽卡岩型独立金矿床,成矿与白芒山辉石闪长岩有关,绝大多数金矿体产于白芒山辉石闪长岩与中三叠统南陵湖组灰岩的接触带上。辉石闪长岩中三个角闪石单矿物样品的^(40)Ar/^(39)Ar 激光阶段加热分析给出了很好的坪年龄,年龄值介于139.7±0.3～138.3±0.6Ma(2σ),代表白芒山岩体冷却到500～550℃(氩同位素在角闪石中的封闭温度)的年龄。根据角闪石^(40)Ar/^(39)Ar 年龄和前人获得的 SHRIMP 锆石 U-Pb 年龄(142.9Ma)以及 Ar 同位素和 U-Pb 体系在角闪石和锆石中的封闭温度,计算出白芒山岩体冷却速率大约为100℃/Ma。由于氩同位素在角闪石中的封闭温度与朝山金矿的成矿温度非常接近,因此,上述^(40)Ar/^(39)Ar 年龄可以代表朝山金矿的成矿年龄。白芒山辉石闪长岩具有富碱、高钠、准铝质、富集轻稀土元素和大离子亲石元素、亏损重稀土元素和高场强元素的地球化学特点,其 Sr-Nd 同位素组成((^(87)Sr/^(86)Sr)_1=0.706598～0.707460,ε_(Nd)(t)=-7.78～-8.12)与铜陵及邻区同时代或稍晚时候的辉长岩和玄武岩非常相似。以上地球化学和 Sr-Nd 同位素特征暗示白芒山辉石闪长岩起源于富集岩石圈地幔的部分熔融,并经历了一定程度的分离结晶和地壳混染作用。矿石矿物的硫同位素组成(δ^(34)S=+6.26‰～+9.3‰)呈偏离零值的塔式分布,富集^(34)S,可能是岩浆硫与中石炭统和/或中三叠统膏盐层混合的产物。矿石中脉石矿物方解石的 C-O 同位素与矿区三叠系海相碳酸盐明显不同,而与地幔原始碳酸岩相似,反映成矿流体中碳(主要是 CO_2的形式)的来源以深源为主,与含矿岩体成因研究得出的认识一致。对区域成岩成矿年代学及岩浆活动与成矿作用特点的分析认为,朝山金矿形成于岩石圈伸展的构造背景;强烈的岩石圈伸展及玄武质岩浆底侵The Chaoshan gold deposit, one of few gold-only skaru deposits throughout the Middle-Lower Yangtze metallogenic belt, is loeated in the central of the Shizishan ore-field, Tongling district. The deposit is elosely related to the Baimangshan pyroxene diorite, with a majority of the ore bodies oeeurring within the eontaet between the Baimangshan intrusion and the limestones of the Middle Triassic Nanlinghu Formaion. Three hornblende separates from the Baimangshan pyroxene diorite were dated by the ^40Ar/^39 Ar incremental heating teehnique, yielding well-defined plateau ages of 139. 7 ± 0. 3 - 138.3 ± 0. 6Ma （2σ）. These are interpreted as eooling age of the Baimangshan intrusion at 500 - 550℃. In light of previously published SHRIMP zircon U-Pb age and the present hornblende ^40Ar/^39 Ar age, a cooling rate of ca. 100℃/Ma was estimated for the Baimangshan intrusion based on the elosure temperatures of argon isotope in hornblende and U-Pb isotopes in zircon. Given that the argon closure temperature in hornblende is comparable with the mineralization temperature of the Chaoshan gold deposit, the present ^40Ar/^39 Ar plateau ages can be reliably interpreted as the timing of gold mineralization. Roeks of the Baimangshan pyroxene diorites are alkaline, sodie, metaluminous, and are characterized by enriehment of LREEs and large ion lithophile elements （LILE） but depletion in HREEs and high filed strength elements （HFSE）. The samples exhibit extremely homogeneous, slightly radiogenie Sr-Nd isotopie eompositions, with （^87Sr/^86Sr） i = 0. 706598 - 0. 707460 and εNd （t） = -7. 78 -- 8. 12. In the （^87 Sr/^86 Sr）iversus εNd （t） diagram, the samples lie in the enriched extension of the mantle array defined by mantle-derived rocks. Thus, the petrographieal, geoehemieal, and Sr-Nd isotopic data indieate that the Baimangshan pyroxene diorite was derived from partial melting of enriched lithospherie mantle, with some degree of fractional crystallization of pyroxene, hornblende and plagioelase a

关 键 词：铜陵地区 朝山金矿床 成矿作用 Ar—Ar定年 同位素 岩石圈伸展 

分 类 号：P588.312[天文地球—岩石学] P618.51[天文地球—地质学]