东秦岭钼矿带内碳酸岩脉型钼(铅)矿床地质-地球化学特征、成矿机制及成矿构造背景

Geological and Geochemical Characteristics,Metallogenetic Mechanism and Tectonic Setting of Carbonatite Vein-Type Mo(Pb) Deposits in the East Qinling Molybdenum Ore Belt

东秦岭钼矿带位于华北板块南缘,NW—NWW向的固始?栾川深断裂带控制着钼矿床的空间分布。黄水庵碳酸岩脉型钼(铅)矿床的确定,为本矿带内已有碳酸岩脉型钼(铅)矿床(黄龙铺地区的大石沟、石家湾和桃园等)增添了又一新成员。本矿带不仅钼金属储量居世界已知单个钼矿带之首,而且碳酸岩脉和花岗斑岩两个成矿体系并存,亦是本区钼矿带的一大特色。业已查明,黄水庵和黄龙铺(大石沟)等碳酸岩脉型钼(铅)矿床的δ13C=-5.3‰～-7.0‰,87Sr/86Sr=0.7049～0.7065。同时,方解石富含轻稀土(LREE/HREE=1.8～2.9)。辉钼矿以富含Re(平均为110×10-6～244×10-6)为特征。基于含矿碳酸岩脉方解石的Sr、Nd、Pb同位素比值(87Sr/86Sr对206Pb/204Pb、207Pb/204Pb对206Pb/204Pb和143Nd/144Nd对87Sr/86Sr)的关系图,我们初步判断本矿带区域陆壳之下可能存在有EMI(富集地幔Ⅰ),这些含矿碳酸岩脉是源于EMI的碱性硅酸盐-碳酸盐熔体-溶液结晶分异的产物,成矿金属Mo、Pb主要来自EMI。根据黄水庵和黄龙铺(大石沟)钼(铅)矿床的成矿年龄(Re-Os年龄分别为209.5Ma和221Ma),我们推断,碳酸岩脉型钼(铅)矿床形成于华北和扬子两大板块三叠纪碰撞造山后伸展阶段的晚三叠世时期,而在侏罗纪陆内造山晚期的伸展阶段,形成了晚侏罗-早白垩世的斑岩型和斑岩-矽卡岩型钼矿床(Re-Os年龄介于147～116Ma)。

The East Qinling molybdenum ore belt is located on the southern margin of North China plate,and the distribution of molybdenum ore deposits are controlled by the NW—NWW trending Gushi-Luanchuan deep fault zone.The Huangshuian Mo（Pb） deposit is defined to one of the carbonatite vein-type by the authors in this paper,thus we have one more example of this type in addition to the existing diposits as Dashigou,Shijawan and Taoyuan etc.in the Huanglongpu area.The molybdenum metal reserves of the ore belt occupies the first place for a singular molybdenum ore belt known in the world. Moreover,the coexistence of the ore-forming system for carbonatite veins and that for granite-porphyry is an important characteristic different from other molybdenum ore belts. The results of the study show that the carbonatite vein-type Mo（Pb） deposits of Huangshuian and Huanglongpu have δ13C values ranging from-5.3‰ to-7.0‰ and 87Sr/86Sr ratios ranging from 0.7049 to 0.7065. At the same time,the calcites are enriched in light REE（LREE/HREE ratios vary from 1.8∶1 to 2.9∶1） . Moybdenit is characterized by high Re content（average value ranging from 110 to 244 ppm） . Based on the diagrams showing 87Sr/86Sr vs. 206Pb/204Pb,207Pb/204Pb vs. 206Pb/204Pb and 143Nd/144Nd vs. 87Sr/86Sr of calcites from the ore-bearing carbonatite veins,the authors consider that EMI might exist below the continental crust in the region of the ore belt,and these ore-bearing carbonatite veins are crystal-fractionational products of alkaline silicate-carbonate melt-solution originated from the EMI,and molybdenum and lead as ore-forming metals should come mainly from EMI. In consideration of the ore-forming ages of Huangshuian and Huanglongpu（Dashigou） Mo（Pb） deposits（Re-Os ages are 209.5Ma and 221Ma,respectively ） ,the carbonatite vein-type Mo（Pb） deposits should be formed in the extension stage of late Triassic which is post-collisional orogenesis of the North China and Yangzi plates,while the porphyry-and porphyry-skarn-type Mo deposits should be formed in late Jurassic-early Cretacous（Re-Os ages ranging from 147Ma to 116Ma） which is also an extension stage after Jurassic intra-continental orogenesis.