河北宣化“战国红”玛瑙矿床地质特征及成因分析

Ore-Geology and Metallogenesis of“Zhanguohong”Agate from Xuanhua,Hebei Province

充填熔岩气孔形成的玛瑙并非不常见,但形成具规模矿床的不多。河北宣化"战国红"玛瑙就是这类矿床的典型,解剖该矿床具有一定的理论及勘查意义。通过矿区勘查发现,该类玛瑙产于近水平中侏罗统髫髻山组四段粗面熔岩中,为充填厘米级浑圆气孔之产物,矿体含矿率达0.7~1.2kg/m^3;矿体上覆髫髻山组五段火山集块岩、角砾岩。显微结构观察揭露浑圆状玛瑙矿石由韵律式隐晶燧石环带组成,环带间被细粒显晶石英及少量方解石充填;内含斑点状铁质、锰质杂质。扫描电子显微镜面扫描结合拉曼光谱分析表明,杂质的质量分数及成分变化造就丰富多彩的玛瑙颜色。综合矿区地质、显微结构特点,该类玛瑙为晚期爆发式火山活动形成的富硅流体快速沉淀于先存熔岩气孔中的结果。每一韵律式燧石条带代表一次火山-流体活动,韵律间的粗晶石英或方解石代表火山爆发间隙。

Agates commonly fill the bubbles of lava, but very few agate deposits developed in this way. The “Zhanguohong” agate from Xuanhua, Hebei Province, is a typical one that re-sulted from filling bubbles of lava. Detail study of it may help us to understand the metallo- gensis and/thus improve exploration for such kind of deposit. Our exploration recently re-vealed that the agates were hosted in the 4th segment trachy lava of the middle Jurassic Tiao- jishan Formation, which was covered by the 5th segment volcanic breccia of the same forma-tion. Its ore-bodies contain 0. 7 to 1. 2 kg agate per cubic meter. Microscopic observations demonstrated that a single filled bubble consists commonly of several bands of cryptocrystal SiO2. Among the bands is a belt of fine-grained quartz or calcite. Both the cryptocrystal SiO2-bands and the quartz belts contain numerous Fe-enriched and/or Mn-enriched dots. The change in content of the dot had led to the highly variable colour of the agate. Sythysizing ore-geology and microscopic studies, the authors suggest that the Shanggu agate deposit was resulted from rapid depositing of silicon in the bubbles of lava, and the associated SiO2-en-riched thermos-fluid likely came from the late-stage explosive volcanisim. Each rhythimical cryptocrystal silicon band represents a pulse of volcanism.