Depositional Modes of Manganese Oxides at Artillery Peak, Mohave County, Arizona.

摘要

Economically significant manganese oxides deposits within Southwestern North America share many commonalities regarding alteration, depositional environment/mode, relative age, and mineralogy. These deposits can be broadly divided into two categories; terrestrial vein -- type deposits (volcanogenic or hydrothermal) and terrestrial sedimentary rock -- hosted stratiform -- type deposits. Original depositional environments for stratiform deposits are interpreted as soils and bogs with hydrothermal enrichment or as a detrital component of a hot springs system redeposited into lacustrian basins. Vein -- type manganese is believed to be a product of mineralizing aqueous fluids undergoing Eh -- pH changes during ascent through near surface host rock or sediments (Roy, 1992). Manganese ions are naturally mobile at the earth's surface due to weathering, and manganese is the second most abundant transition metal at the earth surface. Reworking of manganese readily occurs due to late fluid influences within a wide range of temperature and chemical settings. As a result, some economically viable manganese oxide deposits are a combination of the aforementioned deposit styles. The mineral resource under development near Artillery Peak, AZ can be classified as a syngenetic stratiform manganese oxide deposit (Lasky and Webber, 1949). Recent evaluations of mineral deposits in western Arizona have led to the proposal of a new overall model for the Artillery Peak Manganese district as being a Detachment Fault Related Deposit (Long, 2004). This deposit type is distinguished by a unique suite of precious and base metals, alteration gangue mineralogy, salinity, temperature, and structural setting. Resources at Artillery Peak exist in large stratabound manganese -- rich sediments ranging from clays to coarse sands. Apparent vein - type manganese deposits are also present at the locality and are known to have formed approximately 5 Ma after the emplacement of the stratiform manganese (Spencer, 1989). Fissures associated with local parallel faults striking northwest through the area are known to contain dense manganese chemically similar to stratiform manganese, but with highly elevated manganese and potassium. Manganese oxide mineralogy present at the site is dominantly hydrated manganese oxides (wad); ancillary minerals include romanechite -- psilomelane ((BaH2O) 2(Mn,Mn)5O10), pryrolusite (MnO2), ramsdellite (polymorph MnO2), coronadite ((Pb (Mn,Mn)8O 16)), hollandite ((Ba(Mn,Mn)8O16)), cryptomelane ((K(Mn,Mn)8O16)), and other high oxide manganese minerals. Anomalous amounts of strontium, barium, and arsenic are associated with both vein -- type and stratiform -- type manganese. Presently there has not been any in -- depth investigation of so called vein -- type of manganese origin. Current commercial technical reports and past authors from various federal and state agencies propose conflicting theories regarding origin of so called vein -- type manganese. The current Artillery Peak 43-101 argues for the deposition of hydrothermal vein type manganese associated with mid -- Miocene volcanism (Tribe, 2010). Other authors have proposed migration and concentration of stratiform - type manganese via chemical interaction with alkalic fluids as a source for so called vein -- type manganese (Spencer, 1991). Other evidence indicating that veins may have been derived from stratiform deposits includes; close spatial relationships of modal types and alkalic basalt flows and a high degree of similarity between low - temperature hydrous minerals, textures, and geochemistry in altered sections of stratiform -- types and vein -- type. Such remobilization via alkalic fluids has been noted in other Miocene manganese deposits of the southwestern US and Mexico.