More than half of the world’s zinc and lead has come from SEDEX deposits like Mt Isa in Australia, Red Dog in Alaska and the former Sullivan mine in Canada.
SEDEX (SEDimentary Exhalative) deposits are one of several types of sediment-hosted lead-zinc deposits.
Rich accumulations of lead, zinc and silver are found in the ore minerals sphalerite (zinc sulfide) and galena (lead sulfide) deposited between thin layers of marine sand, silt and mud in sedimentary basins.
How do they form?
SEDEX deposits form deep under the ocean where vents in the sea floor allow hydrothermal fluids to mix with seawater. These hot, saline fluids have percolated through several kilometers of sediments and crystalline rocks, picking up precious metals along the way.
As the metal-rich hydrothermal fluids hit the cool sea water, they precipitate material onto the sea floor at and near the vents. Metal-rich minerals are deposited between layers of fine-grained mud, sand and silt.
Temperature and salinity, together with the sulphur content, oxidation state and acidity of these hydrothermal fluids, controls how much zinc and lead they can carry. We know from studying the minerals and the bubbles of fluid trapped inside them, of SEDEX deposits worldwide that their ore-forming fluids contained up to 10-30% dissolved salts and were well over 100oC.
Understanding the chemistry of these fluids is important not only to our understanding of how these giant, rich ore bodies form, but also to understanding how oxygen appeared in our oceans and atmosphere, and when life appeared on Earth.
SEDEX deposits form at or just below the seafloor in thick sediments laid down in rifts – long troughs that form where crust is pulled apart and thinned – or passive margin settings – where continents drift apart. Although they seem similar, they are not associated with volcanic activity like Volcanic Massive Sulphide deposits. Faults that form in these settings become the conduit for hydrothermal fluids.
Modern day examples
Geologists are visiting modern hydrothermal sea floor vents to better understand how SEDEX deposits form and where the rich metals contained within them come from.
The deepest known vents on Earth are nearly 5km (3 miles) below the ocean surface, discovered in the Caribbean Ocean south of Cuba in 2010 and explored in 2013.
As the metal-carrying hydrothermal fluids came in contact with hydrogen sulphide produced by bacteria in the seawater, they precipitated the sulphide minerals: Primary among these minerals are iron sulfides such and pyrite and pyrrhotite, but zinc and lead sulfides are the primary economic minerals associated with these deposits. Silver, copper tin and tungsten may also be deposited in economic quantities. Barite (barium sulfate) is also common in these settings.
Generally, sphalerite, a zinc sulphide and the principal ore of zinc and galena, a lead sulphide and the most important lead ore, dominate the base of a SEDEX deposit, at and around the vent.
Exploration for SEDEX Deposits
As mentioned above, these deposits are found in association with thick sea floor sediments deposited in fault-controlled extensional settings. There also appears to be a time windows associated with their formation. Known deposits occur in Paleozoic and Proterozoic basins, while none are known to occur in Mezozoic or Cenozoic rocks. In other words, none are older than 2.5 Billion years and none are younger than 250 million years. For exploration geologists, the first item on their check list should be finding a region of the right age and right geological setting.
Since the magnetic mineral pyrrhotite is known to occur in SEDEX deposits along with other metal sulfides in massive beds and stringers, airborne EM and mag surveys along with ground magnetics may be useful prospecting tools. The heavy sulfide minerals along with the heavy mineral barite may also contrast against the lighter background sediments in a gravity survey.
Important SEDEX deposits
Mt Isa, Australia
The Mt Isa-McArthur area in northern Australia is host to the biggest accumulations of zinc, lead and silver in the world. The area hosts five supergiant SEDEX deposits – McArthur River, Century, Mt. Isa, Hilton, and George Fisher.
Red Dog, USA
Red Dog in Alaska is the largest mine in the USA and produces 5% of global zinc mine production.
Discovered over 100 years ago, the Sullivan deposit was the largest SEDEX deposit in Canada, producing 116 million tonnes of zinc (5.8 % Zn) and lead (6.7 % Pb). The mine is no longer producing but Santa Fe Metals and Eagle Plains Resources are pursuing SEDEX prospects in the area.
SEDEX Deposits in the Cordillera: Current concepts on their geology, genesis, and exploration (PDF Slide Presentation)
SEDEX- Sedimentary-Exhalative Deposits (PDF, Academic)
Proterozoic sedimentary exhalative (SEDEX) deposits and links to evolving global ocean chemistry (PDF, Academic)