FORMATION OF MANGANESE OXIDE DEPOSITS IN THE NAVAJO SANDSTONE BY OXIDATION OF MANGANESE CARBONATE MINERALIZATION

The Navajo Sandstone of southern Utah is host to a number of small manganese deposits. The rocks in these deposits contain as much as 20% Mn in the form of Mn-oxides -notably pyrolusite and cryptomelane (Chan et al. 2000. AAPG Bull. 84: 1281-1310)- that occur as intergranular cement. At Cottonwood Canyon, these manganese oxide cements occur as three-dimensional dendrites. These dendrites occur in oxide-carbonate botryoidal clusters that have radii as long as 10 cm. In hand sample, the Mn-oxide dendrites grade outward into carbonate cement that has the same three dimensional dendritic form. The sandstone pore space between these dendrites is also filled with carbonate cement that apparently postdates the dendrites. The rock cemented with the later (non-dendritic) carbonate cement is red and contrasts with the dark gray dendrites. The Mn-oxides are associated with a luminescent carbonate phase. The oxide cement will penetrate into the carbonate cement along cleavage planes locally. On the other hand, the carbonate cement that occurs between the dendrites is largely non luminescent. We interpret these deposits to have formed in a manner similar to many of the iron-oxide deposits on the Colorado Plateau. Waters charged with carbon dioxide and methane descended through the aquifer stripping oxidized iron and manganese from sand grains. Siderite was deposited in the more proximal portions of the flow system whereas manganiferous and ferroan carbonates were deposited in the distal regions. More oxidized ground waters invaded the system during uplift. Rhodochrosite or Mn-rich carbonate would have been converted to Mn-oxide, apparently retaining the dendritic form of the original Mn-carbonate. The non luminescent (ferroan) carbonate weathers red on exposure.