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Digital Subscriptions > Rock&Gem Magazine > November 2018 > MINERALS THROUGH GEOLOGIC TIME


Part 2: Paleozoic, Mesozoic, and Cenozoic Eras

In Part 1 of this mini-series on time-exclusive rocks and minerals, the vast span known as the Precambrian was explored. The Earth underwent many changes ranging from the atmosphere, to first establishing plate tectonics, and being much hotter than it is today; all of which influenced an exclusive array of natural treasures.

As with Part 1, there are rare exceptions to what is presented (it is the nature of science After all), and this is not meant to be an exhaustive scientific treatise.

Throughout much of the Paleozoic Era, there is not terribly much in the way of making minerals that would make them timeexclusive, as compared with the Precambrian.

One notable exception, however, comes with a group of deposits starting around the Devonian-Permian Periods in the way of Mississippi Valley Type (MVT) deposits. These are carbonate-hosted lead-zinc deposits that chiefly host galena, sphalerite, calcite, dolomite, and potentially chalcopyrite, fluorite, and barite. The United States Geological Survey (USGS) reports that roughly 90% of MVT ore comes from Paleozoic rocks, with a few scattered occurrences hosted in those from the Precambrian. The prevailing view on the genesis of MVT deposits is a series of continental collisions (known as orogenies) that served as an appetizer of sorts for the grand construction of the Pangaea supercontinent, roughly 250 million years ago. In the United States, MVT deposits were formed in several present-day midwestern states.

Some deposits are now defunct (i.e. the southern Illinois fluorite district and the “Tri-State” zinc district), whereas others still thrive (such as the southeastern Missouri “new” lead belt). The formation of those MVT deposits relates to an event known as the Ouachita Orogeny, which involved the collision of South America (part of the supercontinent Gondwana) with the southern portion of North America (which was part of the supercontinent Laurentia) around 400 Ma. This resulted in a series of uplifts and basins that served as the important topographic structures that controlled ore fluid flow. Some theories believe this to be heated groundwater used to transport dissolved sulfur and metals to the basins, where fluid-mixing commenced to precipitate ore minerals.

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Rock & Gem November 2018, GEOLOGICAL GENESIS: Dating Mineral Deposits, A MODERN discovery in Silver valley, And More.....