中亚构造域多期叠加斑岩铜矿化:以阿尔泰东南缘哈腊苏铜矿床地质、地球化学和成岩成矿时代研究为例

新疆青河县新近发现哈腊苏铜矿床,正在进行的勘探证实具有大型铜储量前景。它位于阿尔泰东南缘,靠近额尔齐斯构造变形带。这个区域经历了古生代中期的洋-陆俯冲、古生代晚期的陆-陆碰撞以及其后的陆内活化等地质过程。铜成矿与哪种地质地质过程有关受人关注,矿床成因也存在斑岩型、热液脉型和火山岩型等不同认识。哈腊苏铜矿区主要出露中泥盆统基性火山岩(含苦橄岩)及侵入其中的不同时期含铜蚀变斑岩体,包括花岗闪长斑岩、斑状花岗岩、石英二长斑岩和石英闪长斑岩等,斑岩SiO_2质量分数为57.24%～65.45%,其中花岗闪长斑岩δ~(18)O_(V-SMOW)=7.9‰～8.6‰,ε_(Nd)(t)=7.3～8.5(接近于MORB值),(~(87)Sr/~(86)Sr)_t=0.70383～0.70410(接近原始地幔值),暗示岩浆起源于地幔或下地壳。矿区含铜蚀变斑岩全岩矿化(Cu 0.2%),矿体(Cu 0.3%以上)呈透镜状和不规则分枝脉状,产状与斑岩体相仿,95%以上矿体产于斑岩体内。围岩蚀变从矿体到斑岩再到基性火山岩围岩,发育钾长石黑云母化、黑云母绿泥石化、青磐岩化的分带,后期脉状线型钾长石化叠加于早期面状弥散型钾硅酸盐蚀变之上。没有次生硫化物富集现象,原生铜矿石出现细脉浸染型和脉状叠加型两种自然类型,前者以"黄铁矿+黄铜矿+辉钼矿"为典型金属矿物组合,后者呈在前者背景上的"石英+黄铁矿+黄铜矿"脉状叠加矿化。相对于前者,后者Cu、Au品位明显偏高(分别达到Cu 2.21%、Au 0.83 g/t)、微量和稀土元素总量降低,微量元素蛛网图和REE配分曲线更为平缓,Eu正异常更加显著。基性火山喷发、幔源岩浆侵入和多期矿化叠加是哈腊苏铜成矿的关键,早期斑岩型铜成矿基础上的后期构造热液矿化叠加显著。细脉浸染型铜矿石中共生黄铁矿-黄铜矿的硫同位素温度计指示斑岩型铜成矿温度为420～560℃。铜矿石硫化物δ~(34)S_(V-CDT)主体范围为-1‰～-4‰,矿石硫源自幔源斑岩体(有地层硫酸盐还原硫少量混入);黄铁矿~(206)Pb/~(204)Pb=18.052～18.461,~(207)Pb/~(204)Pb=15.501～15.606,~(208)Pb/~(204)Pb=37.813～39.335,与矿床所在区域喀拉通克岩浆Cu-Ni硫化物接近,成矿金属主体来自幔源斑岩;脉状矿化叠加型铜矿石中含铜硫化物石英脉晶出母液(δ~(18)O_(V-SMOW)=6.4‰～10.2‰,δD_(V-SMOW)=-89‰～-80‰)具有岩浆水的O、H同位素组成特点。通过成岩、成矿和热液蚀变的年代学研究获得:(1)含铜蚀变的斑状花岗岩(381.6±2.5)Ma和花岗闪长斑岩(371.8±9.6)Ma的U-Pb谐和年龄、细脉浸染型铜矿石中辉钼矿(376.9±2.2)Ma的Re-Os等时线年龄,是洋-陆俯冲期斑岩成岩成矿的年龄记录;(2)含铜蚀变石英二长斑岩(265.6±3.7)Ma的U-Pb谐和年龄和脉状叠加型铜矿石中钾长石(269.2±3.2)Ma的Ar-Ar坪年龄,是陆-陆碰撞晚期斑岩铜矿化蚀变的年龄记录;(3)含铜蚀变石英闪长斑岩(215.8±4.6)Ma的U-Pb谐和年龄和脉状叠加型铜矿石中钾长石(198.2±2.3)～(206.4±2.7)Ma的Ar-Ar坪年龄,是陆内构造岩浆活化期的年龄记录。多期构造-岩浆-热液矿化叠加作用是哈腊苏铜成矿的显著特征。该研究为认识中亚构造域斑岩铜矿床的多期叠加成矿作用特征积累了新资料。

The multiperiodic superimposed porphyry copper mineralization in Central Asian Tectonic Region: A case study of geology, geochemistry and chronology of Halasu copper deposit, Southeastern Altai, China

The Halasu copper deposit is recently discovered in Qinghe, Xinjiang, and proved to be a large-scale copper prospect by the exploration in progress. It is located at the southeastern Altai and near the Ertix tectonic deformation zone. The region had undergone the Middle Paleozoic ocean plate subduction, the Later Paleozoic continental collision and the successive intraplate activation processes. That which process was related to the copper ore mineralization has caused much attention, and there are debates on the metallogenesis of the ore, such as, porphyry copper type, hydrothermal copper vein-type and volcanic related copper deposit. The Middle Devonian basic volcanic rock (including picrites) and copper-bearing altered porphyry including granodiorite porphyry, porphyritic granite, quartz monzonite porphyry and quartz diorite porphyry of different periods crop out mainly in Halasu copper ore district. The porphyry's SiO_2 content is from 57.24% to 65.45%; the granodiorite porphyry's δ~(18)O_(V-SMOW) value is from 7.9‰ to 8.6‰, its ε_(Nd)(t) is from 7.3 to 8.5 (near the value of MORB), and (~(87)Sr/~(86)Sr)_t is from 0.70383 to 0.70410 (near the original mantle value); these indicate a mantle or lower crust magma sources. The copper-bearing alteration porphyry is whole mineralized (Cu 0.2%), the orebody (Cu over 0.3%) is lenticular and irregularly dendritic vein-shaped and its occurrence is similar to that of the porphyry, and over 95% of the orebodies are located within the porphyry. The wall-rock alteration developed in a sequence of the zones of K-feldspar-biotite, biotite-chlorite, to propylite from the orebody to the porphyry and then to basic volcanic wall-rock, and the later veinlets of linear K-feldsparization overprinted on the earlier pervasive K-silicate alteration. There is no secondary sulfide enrichment, and the primary copper ores are of two natural types : the veinlet-disseminated and the vein-overprinted, the former is characterized by the typical metallic mineral association of pyrite-chalcopyrite-molybdenite, and the latter is overprinted on the former by quartz-pyrite-chalcopyrite veins. As compared with the former, the latter has the higher grade (Cu to 2.21% and Au to 0.83 g/t), and lower total amount of the trace and rare earth elements with more gentle REE pattern and more evident positive Eu anomaly and flatter spider diagram of the trace elements. The basic volcanic eruption, the mantle-source magma intrusion and the superposition of multiphase metallization are the key factors for the Halasu copper metallization. The porphyry copper mineralizations of early period were overprinted by the later period of structural-hydrothermal metallization. The S-isotopic compositions of associated pyrite-chalcopyrite in the veinlet-disseminated type of copper ores indicate the 420 - 560 ℃ of porphyry copper mineralization temperature. The copper ore sulfide δ~(34)S_(V-CDT) is mainly from -1‰ to -4‰, and the ore sulphur was from the mantle source porphyry intrusion (with minor mixing of reduced sulphur from the sulfates in surrounding strata ). The pyrite in the copper ores shows that ~(206)Pb/~(204)Pb is from 18.052 to 18.461, ~(207)Pb/~(204)Pb from 15.501 to 15.606 and ~(208)Pb/~(204)Pb from 37.813 to 39.335, and these are close to that of Cu-Ni sulfides in Kalatongke ultrabasic intrusion. The metal was mainly from the mantle source porphyry intrusion. The parent hydrothermal solution (δ~(18)O_(V-SMOW)=6.4‰ - 10.2‰,δD_(V-SMOW)=-89‰ - -80‰) from which crystallized the copper-bearing sulfide quartz vein in the vein-overprinted type of copper ores shows the O- and H-isotopic compositions of magma source water. The isotopic chronology research of the porphyry intrusion, the copper ore and the alteration suggest that (1) the U-Pb concordant ages of (381.6±2.5) Ma and (371.8±9.6) Ma by zircons in copper-bearing altered porphritic granite and granodiorite porphyry, respectively, and the Re-Os isochron age of (376.9±2.2) Ma by molybdenites in the veinlet-disseminated copper ores are all the porphyry intrusion and porphyry copper metallization records of the oceanic plate-continent subduction period; that (2) the U-Pb concordant age of (265.6±3.7) Ma by zircon in copper-bearing altered quartz monzonite porphyry and the Ar-Ar plateau age of (269.2±3.2) Ma by K-feldspar in the vein-overprinted type of copper ores are porphyry copper metallization and alteration records of the continent-continent collision period; that (3) the U-Pb concordant age of (215.8±4.6) Ma by zircon in the copper-bearing altered quartz diorite porphyry and Ar-Ar plateau age of (198.2±2.3) - (206.4±2.7) Ma by K-feldspar in vein-overprinted type of copper ores are the copper mineralization and related alteration records of intraplate activization period. The Halasu copper metallization is characterized by multiphase tectonic-magmatic-hydrothermal-overprinting. This study provides some new facts for the understanding of the superposition porphyry copper metallization in the tectonic domain of Central Asia.