Study Reveals Gold's Ancient Origins in Chinese Mountains
A groundbreaking geological study has provided new insights into the formation and movement of gold within the Sawayaerdun gold deposit, located in northwest China. This large orogenic system was shaped by extensive periods of burial, deformation, and heating over geological timescales.
Early Sedimentary Origins of Gold
Long before any visible gold veins appeared, the host sedimentary rocks contained microscopic pyrite formed during early burial stages. This earliest pyrite, designated as Py0, developed within carbon-rich layers and was unusually rich in gold, even though the gold remained invisible to the naked eye. The gold was locked inside the crystal structure of the mineral rather than forming nuggets or veins.
Along with gold, this pyrite also concentrated elements such as arsenic, cobalt, and nickel. These early conditions demonstrate that gold was present from the very beginning, stored quietly within common sedimentary minerals long before mountain building processes began.
Mountain Building and Gold Mobilization
According to the study titled "Mineralization of the Sawayaerdun gold deposit, South Tianshan, Northwest China: Insights from texture and geochemistry of multistage pyrite and arsenopyrite," as tectonic forces built the South Tianshan mountains, the rocks containing Py0 were compressed and heated during regional metamorphism.
Under these conditions, the original pyrite began to deform and partially recrystallize. This process released gold and other elements into hot fluids moving through the rock mass. New pyrite and arsenopyrite formed during this stage, labeled Py1 and Apy1, but these minerals contained much less gold than the original pyrite.
The rocks existed in a brittle-ductile environment where they were neither fully solid nor fully fractured, allowing fluids to move while interacting strongly with the host rocks. Much of the gold was no longer locked in place but circulated within the system, ready to be redistributed as pressure and temperature continued to change.
Fluid Boiling and Mineral Reshaping
Later in the deposit's evolution, pressure drops caused the gold-bearing fluids to separate and boil. This led to the dissolution of earlier minerals and the formation of new, more stable ones. Zoned pyrite and well-formed arsenopyrite crystallized during this main ore stage.
These minerals, Py2 and Apy2, show chemical patterns that reflect repeated dissolution and reprecipitation rather than simple growth. Gold was present but in lower concentrations than in the earliest pyrite. The process did not create new gold but helped move and concentrate existing gold into specific zones.
Final Stages and Declining Gold Content
In the final stage, late pyrite formed with almost no detectable gold. By this time, most of the gold had already been redistributed into earlier structures or removed from the system. These late minerals mainly record cooling and declining fluid activity rather than active gold concentration.
Isotopic Evidence Points to Sedimentary Sources
Sulfur and lead isotope data show little variation across all mineral stages. This consistency indicates that both sulfur and gold were sourced from the surrounding sedimentary rocks rather than from deep magmatic inputs. Regional metamorphism mobilized these elements, turning solid minerals into gold-carrying fluids that later formed the deposit.
Implications for Gold Exploration
The main conclusion is that gold enrichment began very early, at the sedimentary stage. Later geological processes redistributed this gold but did not generate it. Identifying sedimentary rocks with early gold-rich pyrite is therefore critical for understanding and exploring orogenic gold systems, both in the South Tianshan region and across the wider Central Asian Orogenic Belt.
This research provides valuable insights for geological exploration and mining industries seeking to understand the fundamental processes that create economically significant gold deposits.
