A three-stage metallogenic model for gold deposits in metamorphosed microclastic rocks

Abstract

Gold deposits occurring in metamorphosed microclastic rocks are distributed extensively at home and abroad. Some deposits of this type are of superlarge tonnage. The formation of gold deposits in metamorphosed microclastic rocks involves three stages : the sedimentary stage, the regionally metamorphic stage, and the ore-forming stage. At the first stage, microclastic sedimentary source rocks were developed in a relatively semi-enclosed reducing sea basin and were enriched in carbon, sulfur and gold. At the second stage, the gold adsorbed on organic matter and clay minerals was released and poorly concentrated during the destruction of organic matter and the depletion of clay minerals by regional metamorphism with increase temperature and pressure. At the third stage, a tectono-hydrothermal event took place. As a result, gold was leached from metamorphosed microclastic rocks, transported to ore depositional locus and/or mixed with gold of other sources in the course of migration, and finally precipitated as ores. Gold deposits of this type were eventually formed at the third stage, and they also can be classified as the orogenic belt type and the activation zone type. The gold deposits occurring in metamorphosed microclastic rocks are the products of reworking processes and the influence of magmatism should be taken into consideration in some cases.