Application of Garnet U-Pb Dating in the Skarn Deposit: A Case Study of Gaoyishan Wo (-Cu) Deposit in Southeast Hubei Province
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摘要: 石榴子石原位U-Pb定年是近年来新发展的低铀矿物同位素定年方法，目前在矿床中成功应用的实例较少，尤其是在非金属矿床中更为罕见.基于详细的岩相学观察，在鄂东南高椅山硅灰石（-铜）矿床中厘定出两期石榴子石，分别为第一期深棕色石榴子石Grt1和第二期浅棕色石榴子石Grt2.电子探针成分分析（EMPA）表明，两期石榴子石均属于钙铁榴石-钙铝榴石固溶体系列，其中Grt1相对富Fe（Adr62.4Gro36.5~Adr94.4Gro0），而Grt2相对富Al（Adr32.6Gro66.4~Adr40.2Gro58.6）.对Grt1和Grt2石榴子石进行LA-ICP-MS U-Pb定年，获得T-W下交点206Pb/238U年龄分别为142.5±2.0 Ma（2σ，MSWD=1.30，n=38）和136.0±14.0 Ma（2σ，MSWD=0.42，n=17），与矿区内广泛出露的石英二长闪长岩锆石206Pb/238U加权平均年龄（139.8±1.5 Ma；2σ，MSWD=0.10，n=22）在误差范围内一致，证明二者之间存在密切的成因联系.高椅山Grt1石榴子石具有较高的U含量和较低的普通铅含量，此为U-Pb同位素测年成功的主要因素.Abstract: In recent years, in situ U-Pb dating of garnet is a newly developed isotopic dating method about the low-U minerals, yet it has been scarcely applied in the ore deposits and especially in the nonmetal deposits. Based on detailed petrographic observations, two generation of garnets have been distinguished from the Gaojiashan Wo(-Cu) deposit in Southeast Hubei Province, namely the first generational garnet (Grt1) with dark-brown color and the second generational garnet (Grt2) with light-brown color. Electron probe composition analysis (EMPA) shows that both of them belong to andradite to grossularite solid solution series, in which Grt1 is relatively rich in Fe (Adr62.4Gro36.5 to Adr94.4Gro0), while Grt2 is relatively rich in Al (Adr32.6Gro66.4 to Adr40.2Gro58.6). LA-ICP-MS U-Pb dating on Grt1 and Grt2 yield Tera-Wasserburg lower intercept 206Pb/238U age of 142.5±2.0 Ma (2σ, MSWD=1.30, n=38) and 136.0±14.0 Ma (2σ, MSWD=0.42, n=17), which is consistent with the weighted mean 206Pb/238U ages (139.8±1.5 Ma; 2σ, MSWD=0.10, n=22) of zircon in the quartz monzodiorite at Gaoyishan within the error range, indicating a close genetic relationship between them. The Grt1 with high U concentration and low common Pb content are the major factors for the success of U-Pb isotopic dating.
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Key words:
- garnet /
- U-Pb dating /
- wollastonite skarn /
- ore-forming age /
- Gaoyishan /
- Southeast Hubei Province /
- geochemistry
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图 1 鄂东南地区岩浆岩及多金属矿床分布
Fig. 1. The distribution of magmatic rocks and polymetallic deposits in Southeast Hubei Province
图 2 鄂东南高椅山矽卡岩型硅灰石（-铜）矿床地质图
Fig. 2. Geological map of the Gaoyishan Wo (-Cu) skarn deposit
图 3 高椅山硅灰石（-铜）矿床石英二长闪长岩手标本及显微特征
a.石英二长闪长岩（手标本照片）；b.石英二长闪长岩具二长结构，主要由斜长石、钾长石、角闪石和石英组成（正交偏光显微照片）；c.石英二长闪长岩中的榍石，主要呈半自形-他形粒状与角闪石、斜长石、钾长石等矿物共生（单偏光显微照片）；d.石英二长闪长岩中的磷灰石，主要呈自形粒状被包裹在角闪石和钾长石中（单偏光显微照片）. Pl.斜长石；Kfs.钾长石；Hb.角闪石；Qtz.石英；Ttn.榍石；Ap.磷灰石；Mt.磁铁矿
Fig. 3. Hand specimen and photomicrograph of the quartz diorite in the Gaoyishan Wo (-Cu) deposit
图 4 高椅山硅灰石（-铜）矿床蚀变矿化期次
Fig. 4. Alteration and mineral paragenesis of the Gaoyishan Wo (-Cu) deposit
图 5 高椅山硅灰石（-铜）矿床典型蚀变类型及矿物组合特征
a.矽卡岩阶段的硅灰石和石榴子石，石榴子石可以分为第一期深棕色石榴子石（Grt1）和第二期浅棕色石榴子石（Grt2）（手标本照片）；b.矽卡岩阶段的两期石榴子石（背散射电子图像）；c.矽卡岩阶段的石榴子和透辉石，透辉石主要呈细粒状被包裹在石榴子石中（单偏光显微照片）；d.矽卡岩阶段的透辉石呈细粒状分布在大理岩中方解石颗粒的间隙（正交偏光显微照片）；e.退化蚀变阶段的绿帘石交代Grt2浅棕色石榴子石（手标本照片）；f.退化蚀变阶段的绿帘石交代Grt2石榴子石及透辉石，又被石英-硫化物阶段的石英交代（单偏光显微照片）；g.退化蚀变阶段的绿帘石和阳起石（单偏光显微照片）；h.退化蚀变阶段的透闪石、蛇纹石和赤铁矿组合（正交偏光显微照片）；i.（大理岩中）石英-硫化物阶段的绿泥石和石英交代绿帘石，又可见磷灰石被绿帘石交代（单偏光显微照片）；j.石英-硫化物阶段的斑铜矿呈弥散状、不规则状等交代硅灰石-石榴子石-透辉石矽卡岩，石榴子石可分为核部的深棕色石榴子石（Grt1）和边部的浅棕色石榴子石（Grt2）（手标本照片）；k.石英-硫化物阶段的斑铜矿-蓝辉铜矿-黄铁矿呈不规则状交代硅灰石矽卡岩（反射光照片）；l、m.石榴子石（Grt2）局部发生绿帘石化蚀变，然后被黄铜矿交代，晚期有方解石交代Grt2石榴子石和黄铜矿（l为单偏光显微照片，m为反射光照片）. Wo.硅灰石；Di.透辉石；Cal.方解石；Ep.绿帘石；Qtz.石英；Act.阳起石；Hm.赤铁矿；Tr.透闪石；Srp.蛇纹石；Ap.磷灰石；Chl.绿泥石；Bn.斑铜矿；Dg.蓝辉铜矿；Py.黄铁矿；Ccp.黄铜矿
Fig. 5. Typical alteration and mineral assemblages in the Gaoyishan Wo (-Cu) deposit
图 6 高椅山硅灰石（-铜）矿床石榴子石三角分类图解
底图据Meinert et al.（2005）. Gro.钙铝榴石；Adr.钙铁榴石；Alm.铁铝榴石；Py.镁铝榴石；Spe.锰铝榴石；Uv.钙铬榴石
Fig. 6. Triangular classification diagram of garnet in the Gaoyishan Wo (-Cu) deposit
图 7 高椅山硅灰石（-铜）矿床石英二长闪长岩锆石和矽卡岩石榴子石LA-ICP-MS U-Pb年龄谐和图及稀土元素球粒陨石标准化配分曲线
球粒陨石标准化数据引自Boynton et al. (1984)
Fig. 7. LA-ICP-MS U-Pb concordia diagrams and chondrite-normalized REE patterns of zircon in the quartz diorites and garnet in the skarns from the Gaoyishan Wo (-Cu) deposit
图 8 高椅山硅灰石（-铜）矿床石榴子石代表性LA-ICP-MS信号图
Fig. 8. Representative time-resolved signals obtained by depth profile analyses of garnet from the Gaoyishan Wo (-Cu) deposit
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