Back in January I made a trip to Sharps Chapel, TN to visit the New Prospect Mine and found some beautiful lead and zinc sulfides concentrated in the dolostone country rock. After sawing these rocks open and getting a clean face, it’s apparent that these deposits are hydrothermal in origin. Specifically, these rocks are a classic example of Mississippi Valley Type ore deposits (also called carbonate-hosted lead-zinc ore deposits) which occur worldwide. These deposits form by the emplacement of briny fluids from which the galena, zinc and other minerals precipitate. MVT deposits can be identified by low temperature formation as well as the occurrence of minerals like fluorite and barite (gangue minerals).
Mississippi Valley Type deposits supply thousands of tons of lead and zinc ore and are a highly sought-after economic deposit. They are named after the first of these types of deposits that were discovered along the Mississippi River.The leading hypothesis for their formation is associated with the formation of Pangea along rift zones. Metals such as zinc can be absorbed in the crstal lattice of carbonates such as aragonite (CaCO3), and during meamorphism, these metals can be ejected and leeched out by saline brine solutions such as highly evaporated sea waters and saline meteoric water. The saline brines with dissolved metals are often very sulfurous and organic-rich and are transported to higher strata where the sulfide minerals crystallize. In this case, the galena and sphalerite formed when the ore fluid ate away at the carbonate country rock, where the sulfides replaced the limestone and filled in cracks in the dolostone.
These deposits have no association with igneous processes and are found only in sedimentary carbonate rocks. The formation temperature is estimated to be around 50-200°C, and the ore minerals usually crystallize in disequilibrium. However, it is still possible to find high quality fluorite, calcite, barite, and galena crystals.
I have to give a huge thanks to my good friend Bryan for helping me research this and providing key insight to economic ore deposits. Thanks, Bryan!
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