西秦岭大水金矿床方解石Sm-Nd等时线年龄及其地质意义

doi:10.19657/j.geoscience.1000-8527.2019.03.01

摘要/Abstract

摘要：

大水金矿床位于西秦岭造山带南缘,矿床中金矿化与硅化、去碳酸盐化和方解石化密切相关。硅化主要表现为石英或玉髓交代(白云质)灰岩,在石英中常见灰岩或白云质灰岩的交代残余碎屑,在石英颗粒内或裂隙中见有自然金颗粒,说明硅化、去碳酸盐化和金矿化是同步的;成矿期的脉状-网脉状载金方解石主要分布于硅化-去碳酸盐化蚀变带的边缘或附近裂隙中,说明方解石的结晶与金矿化同步或稍晚。对该矿床成矿期的8件载金方解石样品进行了Sm-Nd同位素定年体系研究,方解石的¹⁴⁷Sm/¹⁴⁴Nd值变化于0.069 489~2.243 873,¹⁴³Nd/¹⁴⁴Nd值变化于0.511 673~0.514 369,获得Sm-Nd等时线年龄为(189.4±1.4) Ma,代表了大水金矿床的成矿年龄。大水金矿床的成矿时限与秦岭造山运动相关的碰撞型花岗闪长岩岩浆活动时限一致;此外,前人的稀土元素地球化学、黄铁矿Co/Ni比值及H-O同位素研究亦表明成矿物质来源与花岗闪长岩岩浆活动关系密切,因而本次获得的大水金矿床成矿年龄,在一定程度上为“大水金矿床的金成矿与花岗闪长岩岩浆活动存在成因联系”提供了年代学证据。

关键词: 西秦岭, 大水金矿床, 方解石, Sm-Nd等时线年龄

Abstract:

The Dashui gold deposit is located at the southern margin of West Qinling orogenic belt. Gold mine-ralization has close relationship with silicification, decarbonation and calcitization. Silicification is mainly chara-cterized by quartz or chalcedony replacing (dolomitic) limestone; residual debris of limestone or dolomitic limestone is commonly observed in quartz; natural gold exists in quartz grains or fractures, which indicate that the silicification, carbonation, and gold mineralization are synchronous. The veinlet-mesh-vein gold-bearing calcites of the ore-forming stage are mainly distributed at the edge of the silicification-decarbonation alteration zone or near the fractures, indicating the crystallization of calcite is coeval with the gold mineralization or a little later. The Sm-Nd isotopic system of eight gold-bearing calcite samples from this deposit was studied. The ¹⁴⁷Sm/¹⁴⁴Nd values and ¹⁴³Nd/¹⁴⁴Nd values of these calcites range from 0.069 489 to 2.243 873, and 0.511 673 to 0.514 369, respectively. All calcite samples fell on one line and yielded a precise Sm-Nd isochron age of (189.4±1.4) Ma with a MSWD=1.8. This age can represent the time of gold mineralization of the Dashui gold deposit. In addition, the mineralization age of the Dashui gold deposit is coincident with the intrusion of granodiorite magma, which is related to the Qinling Orogeny. What's more, rare earth elements geochemistry, Co/Ni ratios of pyrite and H-O isotope of calcite from previous studies also suggest that the source of ore-forming materials has close relationship with this granodiorite magmatism. Thus, the calcite Sm-Nd isochron age presented in this study provides chronological evidence to the argument that the gold mineralization of the Dashui gold deposit has a genetic relationship with the granodiorite magmatism.

Key words: West Qinling, Dashui gold deposit, calcite, Sm-Nd isochron

中图分类号:

P618.51
P597

李林积, 李堂积, 王丹. 西秦岭大水金矿床方解石Sm-Nd等时线年龄及其地质意义[J]. 现代地质, 2019, 33(03): 469-475.

LI Linji, LI Tangji, WANG Dan. Calcite Sm-Nd Isochron Age of the Dashui Gold Deposit in West Qinling Orogenic Belt and Its Implications[J]. Geoscience, 2019, 33(03): 469-475.

图/表 4

图1 大水金矿床地质简图

Fig.1 Simplified geological map of the Dashui gold deposit

图2 大水金矿床蚀变及矿化特征的典型野外及镜下照片 A.平硐中I号矿体的硅化、赤铁矿化、黄钾铁矾化及方解石化蚀变特征;B.钻孔中I号矿体岩心照片,(a)区域为矿化段(强烈的硅化、方解石化及赤铁矿化,方解石呈脉状-网脉状分布于裂隙中),(b)区域为近矿围岩蚀变段(方解石化、弱硅化),(c)区域为有后期方解石脉穿插的围岩;C.图B中(a)区域强硅化蚀变(BSE照片),热液石英中包裹蚀变残余的围岩(灰岩)碎屑,同时见有赤铁矿;D.图C中红色方框放大示意(BSE照片),不规则的自然金颗粒分布于石英内及裂隙中;E.图B中(a)区域方解石脉(反射光,10×50),方解石中见有自然金;F.与图E为同一视域的BSE照片。Au.金; Cal.方解石; Hem.赤铁矿; Jar.黄钾铁矾; Lim.灰岩; Qtz.石英; Sil.硅化

Fig.2 Typical field and microscopic photographs of alteration and mineralization from the Dashui gold deposit

表1 大水金矿床方解石Sm-Nd同位素组成

Table 1 Sm-Nd isotopic composition of calcite from the Dashui gold deposit

样品编号	产地	产状	w(Sm)/10^-6	w(Nd)/10^-6	¹⁴⁷Sm/¹⁴⁴Nd	¹⁴³Nd/¹⁴⁴Nd(2σ)
GEK-01	Ⅰ号矿体	网脉状	0.323	2.799	0.069 489	0.511 673(18)
GEK-02	Ⅰ号矿体	网脉状	0.363	0.799	0.283 527	0.511 918(20)
GEK-03	Ⅰ号矿体	脉状	0.588	0.882	0.401 483	0.512 074(16)
GEK-04	Ⅰ号矿体	脉状	0.584	0.777	0.452 645	0.512 147(12)
GEK-05	Ⅰ号矿体	网脉状	0.450	0.453	0.598 271	0.512 326(22)
GEK-06	Ⅰ号矿体	脉状	0.749	0.295	1.529 545	0.513 482(16)
GEK-07	Ⅰ号矿体	网脉状	0.715	0.192	2.243 873	0.514 369(20)
GEK-08	Ⅰ号矿体	网脉状	0.227	0.900	0.151 884	0.511 795(18)

图3 大水金矿床方解石的Sm-Nd同位素等时线图解

Fig.3 Sm-Nd isochrone for 8 calcite samples from No.1 orebody in the Dashui gold deposit

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