Paleozoic and Mesozoic chert-greenstone complexes in the western United States host stratiform Mn ore deposits, which may be analogous to Fe-Mn accumulations recently discovered at spreading centers in the modern oceans. The stratiform Mn deposits were deposited by submarine hot springs. At the exhalative vents, the Mn-Fe-SiO2-bearing fluids deposited lenticular massive Mn orebodies, massive chert lenses, and jasperized the host radiolarian chert. Some problems arise with the analogy between these ancient deposits and similarly mineralized rocks found in the modern oceans. Hot spring systems and Mn deposits may have formed in marginal ocean basins, at the base of island arcs, and at the base of oceanic islands, in addition to the mid-ocean rises. The mineralogy of the ancient and modern deposits is not the same.

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