Metal liberation by magma fracturing

摘要: Demand for metals is increasing with the advancement of technology. Silicic magmas contain metals of interest in trace quantities which need to be released from the magma and concentrated by many orders of magnitude to form economically viable mineral deposits. Mechanisms that can act on such large volumes of magma and allow metal-rich fluids to move through the magma are keenly debated. Here we present evidence from the Sandfell laccolith, an exposed rhyolitic magmatic intrusion in Eastern Iceland. Sandfell hosts a rarely-preserved magmatic fracture network that contains extensive mineralization of hydrous Fe-silicates, hydrous Mn-oxides, and La-oxides. The bands of tensile fractures are pervasive throughout the exposed intrusion and have, remarkably, survived 11 Ma of weathering processes post-emplacement. Their excellent preservation and exposure have allowed the first thorough documentation of what role magmatic fractures play in a metal-partitioning process. We propose that small-scale yet extensive fracturing of silicic magma during emplacement provides essential pathways for vapor-rich volatiles to decouple fluid-mobile elements from the melt, and sufficiently localize them for later transport into high-grade veins. Understanding the processes behind metal partitioning and fluid transport in the magmatic environment will increase the efficiency of mineral exploration, contribute to geothermal applications, and shed light on degassing mechanisms occurring at depth under active and hazardous volcanoes today.