Garnet secondary ion mass spectrometry oxygen isotopes reveal crucial roles of pulsed magmatic fluid and its mixing with meteoric water in lode gold genesis

Abstract

Lode gold deposits, which are currently the world’s major gold supply, have been shown to be generated mostly by phase separation of metamorphic fluids and/or interaction between these fluids and wall rocks. Here we use garnet oxygen isotopes by secondary ion mass spectrometry to document the crucial role of magmatic hydrothermal fluids and their mixing with meteoric water in the formation of the world-class Dongping gold deposit in the North China Craton. Garnet grains from quartz veins of various paragenetic stages and the mineralized alteration envelope at Dongping have dynamic δ18O variations of 3.8 to 211.0%, with large intragrain fluctuations up to 5.3%. These values correspond to calculated δ18O values of 6.1 to 29.1% for the hydrothermal fluids from which the garnet formed. The isotope data, notably the cyclic alternation in δ18O within individual garnet grains, are best interpreted to reflect multiple pulses of magmatically derived fluids and subsequent mixing of each pulse with variable amounts of meteoric water. The results presented here allow us to quantify the significant interplay between magmatic hydrothermal fluids and meteoric water that spanned the entire mineralization history and triggered gold deposition of a lode gold deposit. This study highlights the potential use of in situ oxygen isotope analysis of garnet in tracing the origin and evolution of hydrothermal fluids in the Earth’s crust that may have formed other giant ore deposits.