大兴安岭北段富林矽卡岩铜矿床成因：印支期含矿岩浆源区特征、蚀变矿物学及勘查意义

Indosinian ore-bearing magma source,alteration mineralogy and prospecting signification of the Fulin skarn copper deposit in the northern Great Xing'an Range,NE China

新近发现的印支期富林矽卡岩铜矿床位于大兴安岭北段新林区新林镇东约90km。矿区发育与成矿直接相关的花岗岩和侵入花岗岩中的花岗斑岩,矿体主要赋存于花岗岩与古元古代兴安桥组大理岩和早奥陶世黄斑脊山组钙质粉砂岩接触带内。富林矿床矿化与矽卡岩密切相关,整个矿化过程可分为两期:矽卡岩期和石英-硫化物期,5个阶段:早期矽卡岩阶段、退化蚀变阶段、氧化物阶段、早期硫化物阶段和晚期硫化物阶段,其中铜矿化主要发生在早期硫化物阶段。硫化物主要包括黄铜矿和黄铁矿,并含少量闪锌矿、方铅矿、斑铜矿等。蚀变类型包括石榴子石-透辉石矽卡岩化、阳起石-透闪石矽卡岩化、绿帘石矽卡岩化、绿泥石化、绢云母-粘土化、钾长石化和局部角岩化。电子探针分析(EMPA)结果表明:矿区内的石榴子石属钙铝-钙铁榴石系列,主要为钙铁榴石,辉石为透辉石-钙铁辉石系列,以透辉石为主,闪石主要为透闪石、阳起石以及少量镁绿钙闪石和铁浅闪石,帘石为黝帘石-绿帘石系列。黑云母以镁铁黑云母和铁叶云母为主,绿泥石主要为密绿泥石和铁叶绿泥石,长石以正长石和钠长石为主。石榴子石成分剖面显示从核部到边部,石榴子石呈现出钙铝榴石和钙铁榴石交替变化的环带特征,且Fe^(3+)含量的逐渐升高暗示后期成矿流体氧逸度升高,结合黑云母Mg-Fe^(3+)-Fe^(2+)图解,说明富林矽卡岩型铜矿床形成于较强的氧化环境。锆石LA-ICP-MS U-Pb测年结果显示花岗岩形成于~253Ma,花岗斑岩形成于~244Ma,为大兴安岭地区一期新的成矿事件。锆石原位Hf同位素测试结果表明,花岗岩εHf(t)介于-1.60~2.23,花岗斑岩εHf(t)介于-3.53~1.90,二者均具有古老的两阶段模式年龄,结合前人对区域构造背景的研究,认为富林花岗质岩石可能来自于松辽地块和兴安地块后碰撞背景下俯冲板片断离软溜圈上涌导致古老下地壳的部分熔融并卷入少量地幔物质而形成的岩浆。花岗斑岩及与斑岩有关的脉状矿化和蚀变的出现暗示富林地区可能存在印支期的斑岩型矿床,此外结合石榴子石、辉石成分特征以及辉石Mn/Fe比值与世界矽卡岩矿床对比,指出富林矿区外围找矿应综合考虑铜、钼、铁、金、铅、锌等矿化组合。

Newly discovered Indosinian Fulin skarn copper deposit is located in the northern Great Xing＇an Range and is at a distance of 90 km from the east of Xinlin Town. Granite which is responsible for the skarn copper mineralization and granite porphyry which crosscut the granite are the two intrusion rocks in the Fulin deposit. The ore body mainly occurs within the contact zone between Late Permain granite and Paleoproterozoic Xing＇anqiao Formation marble and Early Ordovician Huangbanjishan Formation calcareous siltstone. The mineralization is closely related to the skarn. The mineralization process can be divided into two periods： Skarn period and quartz-sulfide period. The two periods can be further divided into five stages： Early skarn stage,regressive alteration stage,oxide stage,early sulfide stage and late sulfide stage. The early sulfide stage is the major stage for Cu mineralization. Chalcopyrite and pyrite are main sulfides,and there are also few sphalerite,galenite and bornite. The alteration types in Fulin ore deposit contain garnetdiopside skarnization,actinolite-tremolite skarnization,epidote skarnization,chloritization,sericitization-clayzation,potassic and partly biotite hornfels. The electron microprobe analyses（ EMPA） show that the garnet comprises mainly grossular-andradite series,and mainly belongs to andradite. The end member of pyroxene is dominated by diopside and hedenbergite,and mainly belongs to diopside.The amphibole is mainly tremolite,actinolite and few magnesiohastinsite and ferroedenite. The epidote group comprises mainly zoisite and epidote series. The biotite is mainly Mg-rich lepidomelane and siderophyllite. The chlorite is mainly pycnochlorite and euralite.The feldspar is mainly orthoclase and albite. The EMPA components of garnet from core to rim have an alternation changing between grossular and andradite,and the content of Fe^（3＋）increasing,which suggests the oxygen fugacity of later ore-forming fluid is high.Combined with the Mg-Fe^（3＋）-Fe^（2＋）diagram of biotite we think the Fulin skarn deposit is formed in a relatively high oxidation environment. Zircon LA-ICP-MS U-Pb analysis for granite and granite porphyry yield ~ 253 Ma and ~ 244 Ma respectively,which represents a new metallogenic stage in the Great Xing＇an Range. Zircon in-situ Hf isotope analysis reveal the εHf（ t） of granite and granite porphyry is-1. 6 ~ 2. 23 and-3. 35 ~ 1. 9 respectively,both of them show a very old two-stage model age. So,combined the geological setting we think the granitoid may derive from the old lower crust which was heated by the underlying asthenosphere during the slab break-off after the collision of Songliao massif and Xing ＇an massif and may contaminated some mantle materials. The appearance of granite porphyry and associated sulfide veins and alterations suggests the possibility of Indosinian porphyry-type mineralization in the Fulin ore deposit field. In addition,the comparison analysis on the components of garnet and pyroxene and the Mn/Fe ratio of pyroxene with the skarn deposits all over the world,we suggest that copper,molybdenum,iron,gold,lead and zinc mineralizations should be comprehensively considered for the further exploration in the Fulin ore district and adjacent areas.