SEDEX vs. MVT DEPOSITS

1. SEDEX (Sedimentary Exhalative): “The Seafloor Volcano”

SEDEX deposits are Syngenetic. This means the ore forms at the exact same time as the rock that hosts it.

• How it forms: Imagine the deep ocean. Mud and clay are slowly accumulating on the seafloor (this will eventually become Shale). Meanwhile, hydrothermal fluids loaded with lead and zinc at 150-300°C erupt into the seawater from deep fault fractures on the seafloor (similar to modern Black Smoker chimneys).
• Deposition: When the hot, metal-rich fluids hit the near-freezing seawater, they undergo thermal shock. The metals instantly precipitate and fall onto the seafloor like a metallic snow. These precipitated metals are then covered by more layers of mud.
• Result: The ore minerals form in thin, page-like (laminated) layers, perfectly parallel to the sedimentary rock itself.

2. MVT (Mississippi Valley-Type): “The Underground Plumber”

MVT deposits are Epigenetic. This means the host rock formed and solidified millions of years ago, and the ore settles into this hard rock much, much later.

• How it forms: Picture a shallow, warm sea on the surface. Corals and shells accumulate over time, eventually turning into massive, solid blocks of Limestone and Dolomite (Carbonates). Millions of years later, distant mountain-building (orogenic) processes act like a giant squeegee, pushing warm, salty, metal-bearing fluids (50-200°C) laterally into these porous limestones.
• Deposition: The slightly acidic fluids dissolve the limestone like a sponge, creating underground caves, vugs (cavities), and collapse zones. Lead and zinc slowly precipitate out of the fluid into these newly opened spaces, growing as massive, beautiful crystals.
• Result: There is no neat, layered structure. Instead, you see cubic Galena and shiny Sphalerite crystals filling the empty spaces within the rock (known as open-space filling or breccia).