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

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 resulted from filling bubbles of lava. Detail study of it may help us to understand the metallogensis and/thus improve exploration for such kind of deposit. Our exploration recently revealed that the agates were hosted in the 4^th segment trachy lava of the middle Jurassic Tiaojishan Formation, which was covered by the 5^th segment volcanic breccia of the same formation. 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-enriched thermos-fluid likely came from the late-stage explosive volcanisim. Each rhythimical cryptocrystal silicon band represents a pulse of volcanism.