滇西勐满金矿床成矿物质来源分析及矿床成因探讨——来自年代学、矿物学及地球化学的证据

摘要

The Mengman gold deposit in Menghai County of Yunnan Province is geologically located in the south of Lincang-Lancang anticlinorium,Lincang-Jinghong virgation,and had an estimated reserve of 17 million tons gold ore with an average grade of 0.60 × 10-6 Au till the end of 2005.In this paper the authors report the geochronology of the upper Proterozoic Manlai Formation schist,the middle Jurassic Huakaizuo Formation clastic rock,the Lincang granite,and the geochemistry of the silicified rocks and siliceous veins,with the purpose of exploring the origin of the sediments and ore-forming materials.LA-ICP-MS U-Pb dating of detrital zircons from the Manlai Formation schist and Huakaizuo Formation clastic rock yielded weighted average 206pb/238U ages of (957.0 ± 9.8) Ma and (224.2 ± 2.8) Ma,separately.These data reveal that the Manlai Formation belongs to Neo-proterozoic,and the Huakaizuo Formation was coeval with the Lincang biotite monzonitic granite ((228.7 ± 2.8) Ma～ (229.4 ± 2.0) Ma),which suggests that the Huakaizuo Formation probably mainly originated from the Lincang biotite granite.Despite their differences in total content of trace elements,the silicified rocks and siliceous veins in the deposit have similar primitive-mantle-normalized trace element patterns and chondrite-normalized REE patterns that are generally characterized by enrichment in large ion lithophile elements (LILEs) and light REE,and depletion in high field strength elements (HFSEs),with weak to moderate negative Eu anomalies,and no obvious Ce anomalies.These geochemical features of silicified rocks and siliceous veins are similar to those of host rocks and Lincang granite.The total REE content of silicified rocks ranges from 42.3 × 10-6 to 311.0× 10-6(131.8× 10-6 on average),with (La/Yb)N ratios of 2.1～26 (8.5 on average),that of siliceous veins is between 5.0 × 10-6 and 280 × 10-6(56.8 × 10-6 on average),with (La/Yb)N ratios of 4.3 ～133.4 (36.7 on average),indicating that the silicified rocks have higher total REE values and show relatively weaker LREE-HREE differentiation than the siliceous veins.The geochemical characteristics of silicified rocks vary between those of host rocks and siliceous veins,suggesting that the silicified rocks inherited some attributes of the Lincang granite and were modified by the fluid of hot spring.In consideration of the results of zircon dating and the geochemical characteristics of siliceous veins and silicified rocks,the authors hold that the materials of Huakaizuo Formation sandstone were mainly derived from Lincang granite,with a small amount from Manlai Formation.According to the discussion mentioned above,the authors infer that the gold-bearing materials provided by Manlai Formation were insufficient to form the gold orebodies hosted in Huakaizuo Formation,the fluid generated from the hot spring activity is considered to be another source of the gold mineralization and an important factor to result in the late-stage metal enrichment.The authors conclude that Huakaizuo Formation is a orehosted stratum rather than the source of gold mineralization.The geological characteristics and the occurrence of adularia in the deposit reveal that Mengman is a hot spring related low-sulfidation type epithermal gold deposit.%勐满金矿床位于云南省勐海县境内,地处临沧-景洪褶皱束之临沧-澜沧复背斜南部,截至2005年探明金矿资源量17t,主矿体属于地表红土型氧化矿,平均品位为0.60×10-6.文章基于对勐满金矿床矿区内新元古界曼来组片岩和中侏罗统花开左组碎屑岩及矿区外围东部的临沧花岗岩的LA-ICP-MS锆石U-Pb年龄测定,探讨了曼来组片岩和花开左组碎屑岩地层的物质来源,追溯母岩的物质源区,并结合矿区普遍发育的硅化岩和硅质脉体的地球化学特征,分析了金成矿的物质来源.勐满金矿床曼来组中主要碎屑锆石年龄为(957.0±9.8) Ma,属新元古代早青白口纪;中侏罗系花开左组主要碎屑锆石年龄为(224.2±2.8) Ma,与临沧花岗岩基中2件黑云母二长花岗岩的年龄(分别为(228.7±2.8) Ma和(229.4±2.0) Ma)一致,表明花开左组地层的物质来源可能与临沧花岗岩有关.微量元素测试结果显示,矿区硅化岩与硅质脉体具有相似的稀土元素配分模式与微量元素组成,均表现为富集U、Th、Pb等大离子亲石元素,亏损Zr、Hf、Nb、Ta等高场强元素,轻稀土元素明显富集,重稀土元素亏损,中-弱的Eu负异常,Ce异常不明显,与矿区围岩及临沧花岗岩体南部的黑云母花岗岩具有相似的变化趋势.硅化岩稀土元素总量为42.3×10 6～311.0×10-6,平均为131.8×10-6;硅质脉体稀土元素总量为5.0×10 6～280×10-6,平均为56.8×10-6,显示硅化岩相对于硅质脉体具有更高的稀土元素总量.硅质脉体((La/Yb)N为4.3～133.4,平均36.7)相对于硅化岩((La/Yb)N为2.1～26,平均为8.5)具有更高的轻、重稀土元素分馏特征.硅化岩的地球化学特征变化介于围岩与硅质脉之间,表明硅化岩既继承了围岩特征,又有经热泉活动改造的痕迹.综合锆石年代学结果与沉积物源分析,认为矿区花开左组砂岩主要为临沧岩体经受风化搬运沉积的产物,曼来组向花开左组提供金的量不足以形成花开左组中的碎屑岩型矿体,热泉活动产生的流体是金成矿的另一重要物质来源,同时是导致金后期富集的重要因素.据此推断,花开左组是赋矿层位,而并非矿源层.结合低硫型矿床标志性矿物冰长石的出现,提出勐满金矿属于典型的与热泉活动有关的低硫型浅成低温热液金矿.