赞比亚希富玛铜矿床含铜碳酸岩岩石学和地球化学及其成岩成矿意义

Petrology and geochemistry of the Cu-bearing carbonatite from the Shivuma copper deposit,Zambia:Implications to petrogenesis and metallogeny

新发现的赞比亚希富玛IOCG碳酸岩铜矿床位于泛非造山带卢菲里安弧复向斜带中部南缘,处于NW向深断裂带与NEE-EW向断裂带的交会部位。铜矿体主要赋存于含铜碳酸岩内。含铜碳酸岩主要侵入于新元古界上部孔德龙古群上部火山碎屑岩中,与矿区及其外围的岩浆岩构成含铜碳酸岩的杂岩组合。含铜碳酸岩灰白色块状构造,常见气孔构造和流动构造;含围岩捕掳体、熔融包体和流体包体;半自形细粒不等粒结构为主,镶嵌结构和包含结构很普遍;方解石和白云石双晶发育;矿物成分达40余种,主要非金属矿物为方解石,其次是硬石膏、白云石和萤石等;主要金属矿物为磁铁矿、黄铜矿和黄铁矿,其次有磁黄铁矿、闪锌矿、赤铁矿、辉钼矿、斑铜矿等。含铜碳酸岩全岩矿化,矿石构造主要为浸染状构造和斑杂状构造,其次为块状构造及稠密浸染状构造;矿石结构有自形-半自形细粒结构,他形细粒结构,交代熔蚀结构,固溶体分解结构,海绵陨铁结构等。含铜碳酸岩富CaO、FeO和Fe_2O_3,贫MgO,属铁质方解石碳酸岩;REE含量高,ΣREE=57.75×10^(-6)~1076×10^(-6),轻重稀土明显分馏,LREE/HREE=6.3~83.8,强正铕异常,弱负铈异常;富集Ba、Sr、Pb、U、Nb、P和LREE,亏损Ta、Zr、Hf和Ti;Zr/Hf、Y/Ho值和Y含量反映含铜碳酸岩出现了高度演化的熔体-流体过渡的岩浆体系;(87Sr/86Sr)i=0.705315~0.706708,在世界主要碳酸岩范围内;Sr-Nd同位素示踪显示岩浆可能源自EMⅠ;方解石的δ13CV-PDB为-17.8‰^-2.6‰,δ18OV-SMOW值变化于14.5‰~21.9‰;白云石的δ13CV-PDB为-18.8‰,δ18OV-SMOW值为13.5‰,均在世界碳酸岩的范围内;2个磁铁矿样品的δ18OV-SMOW值分别为4.3‰和4.6‰,接近地幔的氧同位素组成范围;金属硫化物的δ34SV-CDT(‰)值变化范围为-4.1^+10.5,在岩浆硫的范围内。此矿床为铁氧化物-铜-金(IOCG)型碳酸岩铜矿床。成岩成矿发生于泛非造山运动后造山伸展阶段拉张应力场构造环境。含铜碳酸岩和成矿物质可能主要源自受到富CO2地幔流体交代形成的EMⅠ富集地幔端元。成岩成矿机制可能是:从地幔源区部分熔融出的初始熔浆随着上侵和演化,液态不混溶出碱性硅酸盐岩浆和碳酸盐+硫酸盐岩浆;铜等成矿元素因亲硫而在碳酸盐+硫酸盐岩浆中富集。此岩浆随着上侵和演化发生液态不混溶作用,形成富集成矿物质和挥发分的含铜碳酸岩浆-热液过渡态流体。随着温度下降,其中碳酸盐矿物、硫酸盐矿物和磁铁矿大量晶出,氧被大量消耗掉,致使氧逸度降低,硫逸度增高,还原硫产生并快速增加,与Cu、Fe、Zn、Co、Mo等金属离子化合形成金属硫化物而成矿。

An Iron Oxide-Copper-Gold （IOCG） carbonatite-type copper deposit was recently discovered near a village named Shivuma of Kasempa City of Northwest Province, Zambia. The deposit occurs at the intersection of a NW-striking deep fault and a NEE- to EW-striking fault, which are on the southern edge of the middle section of the synclinorium belt of the Lufilian Arc of the Pan- African orogen. The major strata are volcaniclastic rocks of the Kundelungn Group, which are on upper part of the Neo-Proterozoic Katanga Supergroup in the Cu deposit district, and igneous rocks are abundant, such as gabbro, diabase, gabbroic diorite, diorite, pyroxenite, lamprophyre, quartz-albitite and andesite. The major Cu-bearing rock is carbonatite, which varies in thickness from less than one centimeter to tens of meters and mainly intruded in the voleaniclastics. It has been observed that the carbonatite shows vesicular and flow structures, penetrates into and branches out in the country rock and contains xenoliths of volcaniclasfics and andesite. The Cu-bearing carbonatite is greyish white, non-bedded, and dominantly of subhedral and fine-grained textures but mosaic and inclusion textures are also common. Calcite and dolomite are often twined. The mineralogy of the Cu-bearing earbonatite is complex with more than 40 minerals, with the major non-metal minerals being calcite followed by anhydrite, dolomite and fluorite, the major metal minerals being magnetite, ehaleopyrite and pyrite, and subordinately pyrrhotite, sphalerite, molybdenite and bornite, and other silicates and rare earth minerals being also seen. The Cu-bearing earbonatite is wholly and unevenly mineralized with major ore structure being disseminated and taxitic, and subordinately massive and densely disseminated, and common ore textures being euhedral- to subhedral-fine-grained, anhedral-fine-grained, replacement-resorption, exsolution, sideronitic, mosaic, inclusion and border-sharing. The Shivuma copper deposit is featured with IOCG deposits and can thus be called IOCG carbonatite-type Cu deposit. The Cu-bearing carbonatite is characterized by higher contents of CaO （28.48 % - 41.72% ）, FeO （3.91% - 20. 95% ） and Fe2 03 （3.84% N31.56% ） , and lower contents of MgO （0. 68%-2. 51% ） with CaO/MgO ratios ranging 10. 5 - 60. 0 and can thus be classified as ferrous calcite carbonatite. The REE concentrations of the Cu-bearing carbonatite are high with EREE = 57. 75 × 10^-6 -1076 × 10^-6 and LREE being richer than HREE with LREE/HREE ratios ranging 6. 3 - 83. 8 that results in the normalized distribution patterns tilting to the HREE end. High positive Eu anomalies （SEu= 2. 92 - 11.1 ） and low negative Ce anomalies （SCe = 0. 68 - 0. 83 ） are present. Metal sulfides and magnetite show similar REE patterns as the Cu-bearing carbonatite, an indication that they may have derived from the same source. The Cu-bearing carbonatite is enriched in Ba, Sr, Pb, U, Nb, P and REE but depleted in Ta, Zr, Hf and Ti, with Sr concentration varying from 147 ×10^-6 to 6360 × 10^ -6 and an average of 3209 × 10 ^-6, and that of Ba varying from 13.2×10^-6 to 10600 × 10 ^-6 and an average of 3640 × 10 ^-6. The trace element geochemistry of the metal minerals is similar to that of the Cu-bearing carbonatite. Zr/Hf and Y/Ho rations and Y contents of the Cu-bearing carbonatite suggest that the rock belongs to ahighly evolved magmatic system, transitional between pure melts and hydrothermal fluids. The initial （87 Sr/S6Sr）i（t = 530Ma） ratios of the Cu-bearing carbonatite vary from 0. 705315 to 0. 706708, being within the range of those for the major mantle-derived carbonatites across the world. The （143 Nd/144Nd） 0 ratios of the Cu-bearing carbonatite and the massive copper ore occurred within the carbonatite are 5.11815 and 5. 11718, and their 6Na （t） values are -4. 6 and -2. 7, respectively. These Sr-Nd isotopic ratios probably indicate that the magma of the Cu-bearing carbonatite was derived from type I enriched mantle end-member （EM Ⅰ ）. The 206pb/204pb and 20TPb/204Pb ratios are unusually high, indicating that they belong to radiogenic lead. The δ13 CV-PDB values of the calcite from the Cu- bearing carbonatite range from - 17.8%o to - 2. 6%v, and that of the dolomite is - 18.8%v, similar to those of carbonatite and diamond. The δ180v values of the calcite vary from 14. 5‰ to 21.9‰, and that of the dolomite is 13.5%v, all in the range of the carbonatites across the world. The 6TM Ov values measured for two magnetite samples are 4. 3‰ and 4. 6%o, close to the oxygen isotopic composition of the mantle. The δ34 SV-CDT values of the metal sulfides vary from - 4. 1 to ＋ 10. 5, falling into the range of magmatic sulfur. The range of δ34Sv-cm. values for the anhydrite is from ＋ 14. 2 to ＋ 15.5, similar to that of sulfur isotope of the sulfate from the porphyry copper deposits. In post-Pan-African orogeny, the region where the Shivuma copper ore deposit was discovered was under extension, and the lithosphere was subjected to tensional stress, and the deep faults were developed, resulting in partial melting of the upper mantle due to decompression. The partially melted upper mantle was an enriched EM I type mantle metasomatized by fluid rich in CO2. The initial meh was rich in volatiles, metallogenetic components and incompatible elements, but was relatively homogeneous at high temperature and pressure. It rose up into the crust along the deep faults and was separated into Si-unsaturated alkali silicate magma and carbonate-sulfate magma due to liquid immiscibility as the temperature and pressure deceased. Sulphophile elements such as Cu, Zn, Mo, Co, Ni, Au, Ag and Cd as well as Fe, which is both oxyphile and sulphophile, became enriched in the carbonate-sulfate magma. In addition, such magma contained lots of volatiles and incompatible elements. With further decease in temperature and pressure when the magma reached the shallow part of the crust, silicates, carbonates, sulfates and magnetite started crystallizing. At this time, because the oxygen fugacity was relatively high, copper and other heavy metal elements did not enter the crystal lattice but instead remained in the magma and became gradually enriched. Crystal differentiation enriched the volatiles and metallogenetic components more and more. When the volatiles got saturated or even oversaturated, liquid immiscibility took place, resulting in a transition-state fluid that contained solidified phase （ minerals）, isolated volatile phase and residual carbonate-sulfate melt phase. This liquid may be called Cu-bearing carbonatite magma-hydrothermal transitional fluid. Part of such fluid that contained large amounts of metallogenetic components and volatiles intruded the weak and porous volcaniclastic rocks and decreasing temperatures allowed lots of carbonates, sulfates and magnetite to crystallize. When large amounts of oxygen was consumed, oxygen fugacity deceased and sulfur fugacity increased, resulting in reduced sulfur being combined with Cu, Fe, Zn, Co, Mo and other metal ions to form metal sulfides. At the beginning, metal sulfides were suspended in the Cu-bearing carbonatite magma-hydrothermal transitional fluid in the form of small drops and/or minerals. When more and more metal sulfides were crystallized, they might flow with the magma-fluid and created banded flow structure, and they might also replace early-crystallized magnetite. The Cu-bearing carbonatite magma-hydrothermal transitional fluid may form medium and fine veins of Cu-bearing carbonatite in the country rock and make the latter carbonatized and mineralized. At the final stage, the Cu-bearing carbonatite magma-hydrothermal transitional fluid completely solidified and formed whole-rock mineralized Cu-bearing carbonatites. When the contents of copper are up to the industrial grade, these carbonatites become orebodies. The copper orebodies are primarily reside in the Cu-bearing carbonatite; dense medium and fine veins of Cu-bearing earbonatite as well as carbonatized volcaniclastic rock can also form coooer orebodies.