A Seafloor Metal Factory
The Karatau mountain range holds one of the world's major concentrations of lead and zinc, but these are no ordinary mineral veins. The deposits are classified as Sedimentary Exhalative, or SEDEX. This means they formed during the Late Devonian period, roughly 380 million years ago, when metal-rich, heated brines seeped out from vents onto the floor of an ancient, shallow sea.
This sea was not a tropical sea. It was a restricted basin, where water circulation was poor, leading to anoxic (low-oxygen) conditions at the bottom. Metal-bearing fluids, with temperatures estimated between 100-200°C, rose along faults in the Earth's crust. As these hot, saline fluids mixed with the cold, anoxic seawater, they rapidly precipitated their dissolved metals. The result was the deposition of fine-grained layers of sulfide minerals directly onto the seafloor, which became interbedded with the accumulating sediments.
The primary ore minerals are sphalerite (zinc sulfide, ZnS) and galena (lead sulfide, PbS), which are still the main sources for these metals globally. These layers of metal sulfides built up over thousands of years, eventually forming the massive, stratiform ore bodies mined today, such as the major Shalkiya deposit. The entire mineralized sequence is found within carbonate rocks like dolomite and limestone from the Famennian age of the Devonian period.
The Bacterial Fingerprint
The formation of these vast metal deposits was not a purely geological process. It required a biological catalyst. The hot brines carried the lead and zinc, but the sulfur source was hydrogen sulfide (H₂S), which provided the sulfur needed to turn dissolved metals into solid sulfide minerals. Thermophilic ("heat-loving") microbes are part of the process.
Scientists use sulfur isotopes to trace the origin of the sulfur in the ore. Sulfur has two common stable isotopes, a lighter ³²S and a heavier ³⁴S. During a metabolic process called bacterial sulfate reduction (BSR), microbes consume sulfate (SO₄²⁻) from the seawater and excrete hydrogen sulfide. These bacteria show a strong preference for the lighter ³²S isotope, making the H₂S they produce isotopically very light. The lead and zinc sulfides in the Karatau deposits are exceptionally enriched in ³²S, a chemical signature of this bacterial process.
The specific isotopic composition indicates the BSR process occurred at around 80°C. This points to the activity of thermophilic bacteria thriving in the warm sediments near the seafloor vents. The ore also contains abundant pyrite (FeS₂) in the form of microscopic spherical clusters called framboids. The formation of framboidal pyrite is strongly associated with rapid sulfide precipitation in environments influenced by microbial activity. This combination of isotopic and microscopic evidence proves that ancient, heat-loving bacteria were essential to this seafloor process, creating the conditions necessary to precipitate one of the planet's great metal resources.