西藏甲玛铜多金属矿辉钼矿Re-Os定年及其成矿意义

Re-Os Dating of Molybdenite from the Jiama Copper Polymetallic Deposit in Tibet and Its Metallogenic Significance

西藏墨竹工卡县甲玛铜多金属矿中,辉钼矿普遍发育,产于各类矿石中,尤其在矽卡岩型和角岩型矿石中最常见,其次是斑岩型矿石,极少量产于大理岩和结晶灰岩型矿石中。本文采集了甲玛铜多金属矿矽卡岩、角岩和斑岩中不同产状、不同形态的辉钼矿,进行Re-Os同位素定年,获得了27件样品的模式年龄为14.2～17.5Ma,等时线年龄为15.22±0.59Ma。其中,斑岩型辉钼矿等时线年龄为14.78±0.33Ma,角岩型辉钼矿等时线年龄为14.67±0.19Ma,结果一致。辉钼矿中187Re含量变化于38.75～387.4μg/g,其中,角岩中辉钼矿187Re含量为121.5～387.4μg/g,矽卡岩中为123.7～304.7μg/g,含量较高,而斑岩中辉钼矿的187Re含量相对较低,38.75～130.5μg/g,平均69.0μg/g;辉钼矿187Os含量变化情况基本与187Re相同。甲玛辉钼矿187Re值与冈底斯其他矿体的值对比,显示冈底斯成矿带斑岩-矽卡岩型矿床成矿原岩具有相对较高187Re值的特点。本文研究成果表明,甲玛大型铜多金属矿床形成于中新世Langhian期,辉钼矿为主成矿期的产物之一。鉴于辉钼矿的产出状态,以及其与黄铜矿等的共生组合关系,辉钼矿的成矿时代可代表矿区内主要矿石矿物的成矿时代,且与冈底斯成矿带上一大批大中型斑岩型铜矿的成矿时代一致,成矿集中在20～10Ma之间,形成于印度大陆与亚洲大陆碰撞之后,从而否定了前人海底喷流沉积的成因观点,为矿区内及其外围进一步的找矿指明了方向。

Molybdenite is very common in the Jiama copper polymetallic deposit and occurs in the different types of ores, especially in skarn-type and hornfel-type ores, minor in porphyry-type ores, and extremely less in mable and limestone ores. Molybdenite samples with different occurrences were collected from skarn, hornfel and porphyry and tested by Re-Os isotopic dating. Mode age of 27 samples ranges between 14.2 and 17.5 Ma and the isochron age is 15.22±0.59 Ma. The isochron ages of porphyry and hornfel are 14.78±0.33 Ma and 14.67±0.19 Ma, which are consistent to metallogenic period of other porphyry copper deposits in the Gangdese metallogenic belt. The 187Re content in molybdenite is correspondingly higher in the range of 38.75-387.4 μg/g. Molybdenite in the Jiama ore deposit was supposed to form in one period. It was in the Langhian Stage of Miocene with the feature of postcollision. The metallogenic period of the Jiama ore deposit is close to that of molybdenite, therefore the previous overview of effusive sedimentary metallogeny was not scientific.