云南中甸岛弧烂泥塘斑岩铜矿床岩体氧逸度特征及成矿意义

OXYGEN FUGACITY OF INTRUSIONS FROM LANNITANG PORPHYRY COPPER DEPOSIT IN ZHONGDIAN ISLAND ARC,YUNAN:IMPLICATIONS FOR MINERALIZATION

烂泥塘斑岩铜矿床是义敦岛弧带南段(中甸岛弧)西斑岩成矿带新发现的中型铜矿床。针对烂泥塘斑岩铜矿床含矿石英二长斑岩和不含矿石英闪长玢岩、闪长玢岩的锆石Ce^(4+)/Ce^(3+)比值、δEu值以及氧逸度等特征进行研究,并通过计算得到2个含矿石英二长斑岩体Ce^(4+)/Ce^(3+)比值分别为294.85和283.31,δEu分别为0.72和0.77;不含矿的石英闪长玢岩和闪长玢岩Ce^(4+)/Ce^(3+)比值分别为105.63和87.07,δEu分别为0.73和0.36。表明与成矿有关的岩体具有相对较高的氧化状态。对比西藏冈底斯成矿带、云南金沙江-红河成矿带中段马厂箐铜钼矿和北段玉龙成矿带铜钼矿床岩体Ce^(4+)/Ce^(3+)比值,发现含矿岩体锆石的Ce^(4+)/Ce^(3+)比值通常都大于240,而不含矿岩体锆石的Ce^(4+)/Ce^(3+)比值都小于120(邦铺矿床除外)。并且可将锆石Ce^(4+)/Ce^(3+)>260、δEu>0.65参数作为烂泥塘斑岩型铜矿床成矿的地球化学标志。此外,烂泥塘矿区内含矿石英二长斑岩体的氧逸度(lg(fo_2))基本位于MH缓冲对之上,平均值分别为MH+2.1和MH+1.2,含矿斑岩体具有较高的氧逸度条件。锆石U-Pb年龄与锆石Ce^(4+)/Ce^(3+)比值以及氧逸度(lg(fo_2))存在负相关关系,表明复式岩体中晚阶段侵位的岩体更有利于成矿。

The zircon Ce4＋/Ce3＋ ratios,6Eu and oxygen fugacity of ore-bearing quartz monzonitic porphyry, barren quartz dioritic porphyrite and dioritic porphyrite of Lannitang porphyry copper deposit in south Yidun island arc （Zhongdian island arc） are investigated. It shows that the Ce4＋ / Ce3＋ ratios of two ore-bearing quartz monzonitie porphyry samples are 294.85 and 283.31 with the averages of 289.08 and the 6Eu in ores are 0. 72 and 0.77 with the averages of 0. 75. Meanwhile,the Ce4＋/Ce3＋ ratios of barren quartz dioritic porphyrite and dioritic porphyrite sample are 105.63 and 87.07 respectively with average of 96.36 and the δEu is 0.73 and 0.36 respectively with average of 0. 54. It indicates that the mineralized intrusions have relatively higher oxidation state. Comparison of the Ce4＋/Ce3＋ ratios of ore-bearing intrusions and barren intrusions with the Gangdese metallogenic belt of Tibet and with the Machangqing porphyry Cu-Mo deposit in central area of the Jinsha-Red River metallogenie belt and with the Yulong porphyry Cu-Mo belt in northern segment shows that the Ce4＋/Ce3＋ ratios of ore-bearing intrusions are usually greater than 240,but the barren intrusions usually less than 120 （Bangpu deposit exception）. The value of Ce4＋/Ce3＋ 〉260 and δEu〉0.65 from zircon can be used as the geochemical indicators for Lannitang porphyry copper deposits mineralization. In addition, the oxygen fugacity of ore-bearing quartz monzonitic porphyry （lg（fo2）） obtained by calculation of absolute oxygen from zircon Ce ratios and zircon Ti thermometer is basically located on the MH buffer with averages of MH＋2. 1 and MH＋1. 2, indicating that the ore-bearing porphyry has higher oxidation state. The zircon U- Pb ages which have a negative correlation with the Ce4＋/Ce3＋ ratios and the oxygen fugacity （lg （fo2）） demonstrate that in the late stage of emplacement of granite intrusions is more conducive to mineralization.