A recipe for ancient gold
The gold deposits in the Barberton region are not buried treasure; they are a direct result of the planet's early geological processes. They exist within the Barberton Greenstone Belt, a 15-kilometer-thick sequence of some of the oldest and best-preserved volcanic and sedimentary rocks on Earth. These rocks range in age from 3.6 to 3.25 billion years old, a time known as the Archean Eon.
The formation process began when seawater seeped into fractures on the ocean floor. Magma deep in the crust heated this water, creating a superheated, mineral-rich fluid. This hydrothermal fluid circulated through the volcanic rocks of the seafloor, dissolving silica, gold, and other elements. As this fluid moved into cooler areas or experienced pressure changes, the dissolved minerals precipitated out of the solution. Gold and quartz crystallized together, forming the veins found today within the ancient rock formations. The host rocks are often komatiites, an unusual type of ultramafic volcanic rock named after the local Komati River.
Trapped seawater tells a story
Analysis of the Barberton rocks shows the conditions of early Earth. Within the quartz crystals of the gold-bearing veins, geologists find microscopic fluid inclusions. These are tiny, sealed pockets of the actual 3.2-billion-year-old seawater that formed the deposits. The chemistry of this trapped water shows that the Archean ocean was significantly different from today's—it was much saltier, perhaps twice as salty, and had a different chemical composition.
The Barberton Greenstone Belt is divided into three main units. The oldest, at the bottom, is the Onverwacht Group, composed largely of the mafic and ultramafic volcanic rocks that the hydrothermal fluids circulated through. Above this lie the Fig Tree Group and the Moodies Group, which contain sedimentary rocks that record evidence of ancient beaches, rivers, and tidal zones. The entire region is so well-preserved that scientists also study it for some of the earliest evidence of life on Earth, including fossilized microbial mats dating back 3.5 billion years. The presence of ancient hydrothermal systems, similar to modern deep-sea vents, suggests a possible environment for life's origins.