Genesis of Precambrian Banded Iron Deposits, Rio Arriba County, New Mexico

The general practice has been to group virtually all Precambrian banded iron de-posits together for purposes of genetic interpretation. The mineralogy and similar general appearance of these deposits have indicated a unique origin. While gross as-pects of each deposit are similar there is great variation in detail. Most hypotheses invoke a sedimentary origin for the deposits. Field and laboratory evaluation of Precambrian banded iron deposits in the Moppin Formation at Cleveland Gulch and Iron Mountain in Rio Arriba County, New Mexico has indicated that these deposits were formed from hydrothermal solutions by selective replacement of a pre-existing banded rock. Some of the criteria for this theory of origin of these banded iron deposits are: occurrence of the iron deposits in meta-morphic rocks which were primarily continent.al volcanic rocks; cross-cutting rela-tionships of the banded iron bodies to the enclosing rocks; proximity of the deposits to intrusive rocks; presence of a pre-existing banded metamorphic rock composed of quartz bands alternating with chlorite-rich, mica-rich, or other mineral-rich bands and in the mineralized sequences the presence of magnetite-rich bands with a much greater percentage of these other minerals than the quartz bands; magnetite bands cross-cutting post metamorphic minerals; near absence of iron-rich metamorphic min-erals associated with metamorphism of iron-rich sediments. All or several of these data considered together support a hydrothermal replacement origin for these deposits as well as for similar Precambrian banded iron formation deposits