Age and sources of the ore-forming material for the Qibaoshan Cu-polymetallic deposit in Hu'nan Province:Evidence from quartz vein Rb-Sr isotopic dating and S-Pb iso-topes
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摘要:
湘东北七宝山铜多金属矿床位于钦杭成矿带西段，是湘东北规模最大的铜多金属矿床。对七宝山矿床含矿石英脉中的石英矿物进行Rb-Sr同位素年龄测定，获得的Rb-Sr等时线年龄为153.4±2.0Ma（MSWD=1.8），87Sr/86Sr初始值为0.71849±0.00026，与区内石英斑岩形成年龄（153~155Ma）相同，说明七宝山铜多金属矿床成因与石英斑岩体密不可分，成岩成矿年龄均在燕山期。为分析七宝山矿床成矿物质来源，对矿区内的黄铁矿进行了S、Pb同位素分析，矿石δ34S为3.24‰~4.84‰，平均值为4.198‰；岩体δ34S为2.22‰~3.86‰，平均值为2.805‰。δ34S值总体变化较小，岩体δ34S值较矿石小，更趋近于0值，说明岩体中硫极可能源于地幔；而矿床中硫来源于主体地幔硫和少量地壳硫混熔的混合硫源。Pb同位素变化范围也较小，矿石206Pb/204Pb值变化范围为18.315~18.396，平均值为18.359；207Pb/204Pb值变化范围为15.629~15.737，平均值为15.675；208Pb/204Pb值变化范围为38.376~38.856，平均值为38.609。矿化岩体数值与之相似，结果显示，七宝山铜多金属矿床的Pb同位素组成具有下地壳富钍（铅）贫铀（铅）的特点。据此提出，矿床的成矿物质主要来自与含矿斑岩体有联系的深部岩浆分异演化而析出的含矿气-液流体。含矿斑岩体定位-结晶时，通过对周围受热地下水的对流循环作用，可以从围岩中萃取少量成矿物质加入成矿作用。
Abstract:The Qibaoshan Cu-polymetallic deposit, located in the west section of the Qinzhou-Hangzhou metallogenic belt, is the largest Cu-polymetallic deposit in northeastern Hu'nan Province.However, the data of the deposit age are very insufficient.Based on quartz minerals Rb-Sr isotopic dating of the Qibaoshan deposit, the authors tried to obtain the age.The results show that the Rb-Sr isochron age is 153.4±2.0Ma (MSWD=1.8), initial 87Sr/86Sr=0.71849±0.00026, nearly identical with the quartz porphyry (153Ma to 155Ma) in the ore district in age, which suggests that the genesis of the Qibaoshan deposit was closely related to the quartz porphy-ry.The diagenesis and mineralization were both in the Yanshan period.The authors analyzed S, Pb isotope data of the pyrite to iden-tify the mineral sources of the Qibaoshan Cu-polymetallic deposit and obtained the following data:δ34S=3.24‰~4.84‰ for the ore, with an average of 4.198‰; δ34S=2.22‰~3.86‰ for the rock, with an average of 2.805‰.The δ34S values change overall in a small range, and the δ34S values of the rock change even in a smaller range, more close to the value of 0, which suggests that the sul-fur in the rock was most possibly derived from the mantle; and the sulfur in the ore came from the main mantle mixed with a small amount of crustal sulfur.The changes of lead isotopes are also in small ranges:206Pb/204Pb change in the range of 18.315~18.396 for the ore, with an average of 18.359;207Pb/204Pb change in the range of 15.629~15.737, with an average of 15.675;208Pb/204Pb change in the range of 38.376~38.856, with an average of 38.609.The values for the rock change in a similar range.These data show that lead isotope composition of the Qibaoshan Cu-polymetallic deposit has a lower crust (thorium-rich and uranium-depleted) characteris-tics.A comprehensive analysis of the Sr-S-Pb isotopic system of the Qibaoshan deposit indicates that the ore-forming material was derived from the ore gas-liquid fluid, resulting from the deep magma evolution and differentiation in the same evolution process as the quartz porphyry.At the same time, the host rock might have extracted a small amount of minerals from the wall rock to take part in the mineralization process through the circulating convection effect of heating groundwater, resulting in quartz porphyry intrusion and crystallization.
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表 1 七宝山铜多金属矿床含矿石英脉中的石英矿物Rb-Sr同位素测定结果
Table 1. Rb-Sr isotopic data of fluid inclusions in quartz mineral of the Qibaoshan Cu-polymetallic deposit
样号 样品名称 Rb/10-6 Sr/10-6 87Rb/86Sr 87Sr/86Sr 2σ (87Sr/86Sr)i QB22-1 石英 1.335 0.4996 7.724 0.73554 0.00020 0.71863 QB22-2 石英 0.622 0.3968 4.524 0.72829 0.00006 0.71839 QB22-3 石英 1.004 0.4501 6.450 0.73254 0.00018 0.71842 QB22-4 石英 1.143 0.3416 9.674 0.73945 0.00010 0.71827 QB22-5 石英 2.408 0.4854 14.360 0.74981 0.00016 0.71837 
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表 2 七宝山铜多金属矿床硫化物的S同位素组成
Table 2. Surfur isotopic composition of pyrite from the Qibaoshan Cu-polymetallic deposit
编号 样品描述 采样位置 分析矿物 δ34SCDT/‰ QB2 块状黄铁矿矿石 老虎口矿段40m中段之33m分段13线 黄铁矿 4.50 QB4 块状黄铁矿磁铁矿矿石 老虎口矿段40m中段之33m分段93线 黄铁矿 4.11 QB6 块状黄铁矿矿石 老虎口矿段40m中段之33m分段7线 黄铁矿 4.45 QB6 块状黄铁矿矿石 老虎口矿段40m中段之33m分段7线 黄铁矿 4.68 QB8 块状桁粒黄铁矿矿石 老虎口矿段40m中段5线 黄铁矿 4.29 QB9 块状细粒黄铁矿矿石 老虎口矿段40m中段5线 黄铁矿 4.84 QB12 含铜磁铁矿矿石 鸡公湾矿段80m中段12线 黄铁矿 3.24 QB19 含铜黄铁矿矿石 大七宝山矿段146m矿段28线 黄铁矿 3.47 QB1 黄铁矿化石英斑岩 老虎口矿段40m中段之33m分段13线 黄铁矿 2.84 QB17 细脉侵染状黄铁矿化石英斑岩 大七宝山矿段80m矿段26线 黄铁矿 3.86 QB20 闪锌矿化石英斑岩 大七宝山矿段146m矿段28线 黄铁矿 2.22 QB20 闪锌矿化石英斑岩 大七宝山矿段146m矿段28线 黄铁矿 2.30 注：主量元素含量单位为%，微量、稀土元素为10-6
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表 3 七宝山铜多金属矿床硫化物的Pb同位素组成
Table 3. Lead isotopic composition of pyrite from the Qibaoshan Cu-polymetallic deposit
样品类型 分析矿物 分析结果 样号 同位素比值 表面年龄/Ma φ值 μ值 Th/U 208Pb/(206Pb+207Pb) 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb QB2 黄铁矿 18.384±0.007 15.689±0.007 38.509±0.015 295 0.594 9.64 3.79 1.1302 QB4 黄铁矿 18.315±0.002 15.661±0.002 38.376±0.005 310 0.596 9.59 3.77 1.1295 QB6 矿石 黄铁矿 18.384±0.002 15.737±0.003 38.856±0.007 351 0.600 9.74 3.95 1.1388 QB8 黄铁矿 18.326±0.006 15.665±0.004 38.632±0.015 307 0.596 9.6 3.87 1.1365 QB12 黄铁矿 18.350±0.001 15.629±0.001 38.577±0.004 247 0.590 9.53 3.83 1.1353 QB19 黄铁矿 18.396±0.003 15.666±0.003 38.705±0.008 258 0.591 9.6 3.87 1.1363 QB1 石 黄铁矿 18.390±0.003 15.711±0.001 38.571±0.006 316 0.596 9.69 3.82 1.1311 QB17 英 黄铁矿 18.318±0.004 15.652±0.003 38.569±0.01 297 0.595 9.58 3.85 1.1354 QB20 斑 黄铁矿 18.412±0.006 15.717±0.006 38.734±0.015 308 0.596 9.69 3.88 1.1349 QB20 岩 黄铁矿 18.373±0.007 15.707±0.006 38.728±0.016 324 0.597 9.68 3.89 1.1364
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表 4 钦杭结合带主要斑岩相关铜矿床及其成岩成矿年龄
Table 4. Porphyry-related copper deposits and their ages of diagenesis and mineralization in QHSZ, southern China
序号 矿床名称 矿石种类 侵人体类型 侵人体年龄 成矿年龄 资料来源 测试方法 年龄/Ma 测试方法 年龄/Ma 1 德兴 Cu-Au-Mo 花岗闪长斑岩 锆石U-Pb 171±3 辉钼矿Re-Os 170.4±1.8 [30-31] 2 永平 Cu-W-Mo 石英斑岩 锆石U-Pb 160±2.3
135±7.4辉钼矿Re-Os 156.7±2.8
155.7±3.6[32] [33] 3 村前 Cu多金属 花岗斑岩 锆石U-Pb 169±1.1 [34] 4 铜山岭 Cu多金属 花岗闪长斑岩 SHRIMP锆石U-Pb 149±4 [35] 5 宝山 Cu-Mo-W 花岗闪长斑岩 锆石U-Pb 173±1.9 黄铁矿Rb-Sr 174±7 [4, 36] 6 水口山 Cu多金属 花岗闪长斑岩 锆石U-Pb 172.3±1.6 [4] 7 七宝山 Cu多金属 石英斑岩 锆石U-Pb 155~153 石英Rb-Sr 153.4±2.0 [23], 本文 8 圆珠顶 Cu-Mo 花岗斑岩 SHRIMP锆石U-Pb 154±2
153.4±1.6辉钼矿Re-Os 155±5 [37-38]
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