Gary A. Nowlan

Concretionary manganese-iron oxides in streams and their usefulness as a sample medium for geochemical prospecting

Abstract Correlation studies of 400 samples of sieved stream sediments and 325 samples of fluvial, concretionary Mn-Fe oxides from Maine resulted in the separation of elements into the following categories: (1) elements not scavenged by Mn-Fe oxides — B, Cr, K, Mg, Rb, Sc, Ti, V, and Zr; (2) elements probably not scavenged by Mn-Fe oxides — Ag, Be, Ca, Ga, La, Sb, and Y; (3) elements scavenged weakly by Mn-Fe oxides — Cu, Mo, Pb, and Sr; (4) elements scavenged strongly by Mn oxides — Ba, Cd, Co, Ni, Tl, and Zn; and (5) elements scavenged strongly by Fe oxides — As and In. Studies of the scavenged elements showed that the deviation from the mean is characteristically greater in oxide samples as compared to sieved sediments from the same locality. However, a significant increase in contrast between anomalous and background localities, when oxides are the sample medium, more than offsets the disadvantage of data scatter. The use of oxides as a sampling medium clearly and significantly accentuates anomalous localities. In general, non-ratioed data on oxides give very nearly the same results as data consisting of scavenged elements ratioed to Mn and Fe. However, ratioed data expand the geographic area of specific anomalies. Cd and Zn consistently show strong correlations with concretionary Mn-Fe oxides, but their concentrations in the oxides do not generally show as much contrast between anomalous and background localities as do Cu, Mo, and Pb. These latter elements are strongly scavenged where rocks are mineralized.

Origin of concretionary Mn-Fe-oxides in stream sediments of Maine, U.S.A.

Abstract Studies of stream and sediment-pore waters largely explain the genesis of concretionary Mn-Fe-oxides in Maine. Waters of two small streams near Jackman, Maine, were studied in terms of pH, Eh, dissolved oxygen, dissolved organic carbon, dissolved Mn, total dissolved Fe, and ferrous and ferric Fe. Pyrite Creek has profuse concretions and coatings of Mn-Fe-oxides, whereas West Pyrite Creek has only sparse Mn-Fe-oxide stains. Pyrite Creek drains boggy terrain and West Pyrite Creek drains well-drained terrain. In West Pyrite Creek, stream and subjacent pore waters have chemical characteristics that do not differ greatly. However, dissolved Mn, ferrous Fe, dissolved oxygen, and in situ Eh measurements show that a steep Eh gradient exists between stream and subjacent pore waters of Pyrite Creek. The steep Eh gradient is manifested by the common zonation of coatings and stains on rocks in stream sediment. The bottom zone has no deposition of oxides, the middle zone is red and consists mostly of Fe-oxides, and the upper zone is black or dark-brown and consists of Mn-oxides with varying amounts of Fe-oxides. The zonation agrees with theoretical predictions of oxide stability as one moves from a reducing to an oxidizing environment. At locations where concretionary Mn-Fe-oxides form, pore waters are depleted of oxygen because of abundant decaying organic material in the stream sediment. The pore waters are charged with dissolved Mn and Fe because mechanically deposited Mn-Fe-oxides are remobilized due to the low-Eh conditions. Groundwaters also contribute dissolved Mn and Fe. Stream waters, on the other hand, are oxygenated and the high-Eh conditions result in low concentrations of dissolved Mn and Fe in stream waters because of the insolubility of Mn-Fe-oxides in high-Eh environments. Therefore, concretionary Mn-Fe-oxides form at the interface between pore and stream waters because Mn- and Fe-rich pore waters, which are undersaturated with respect to Mn-Fe-oxides, mix with oxygen-rich stream waters, which are saturated with respect to Mn-Fe-oxides.

Guidelines for finding concretionary Mn-Fe oxides in streams

Abstract Concretionary Mn-Fe oxides in streams form at interfaces between oxidizing and reducing environments. A reducing environment produces waters high in dissolved Mn and Fe, and an oxidizing environment causes precipitation. Mineralogical, microprobe, and optical studies of concretionary Mn-Fe oxides may further our understanding of the role of Mn-Fe oxides in determining the trace-element geochemistry of stream sediments.

A probe for sampling interstitial waters of stream sediments and bog soils

Abstract A probe for sampling interstitial waters of stream sediments and bog soils is described. Samples can be obtained within a stratigraphic interval of 2–3 cm, to a depth of 60–80 cm, and with little or no contamination of the samples by sediment or air.

Regional geochemical studies in parts of Maine, New Hampshire and Vermont, U.S.A.

Abstract A geochemical survey of the Sherbrooke and Lewiston 1° × 2° quadrangles was conducted from 1979 to 1982 by the U.S. Geological Survey as part of the Conterminous United States Mineral Assessment Program. The area covers about 23,000 km2. The region is characterized by a temperate climate, abundant rainfall, and dense forests. The topography ranges from rolling and subdued to mountainous. Bedrock is generally covered by glacial deposits, lakes, and bogs. The bedrock includes metamorphic and plutonic rocks ranging in age from Precambrian to Cretaceous. Known mineral deposits within the area include massive-sulfide deposits, Cu-Mo porphyry deposits, minor base-metal vein deposits, minor Sn-bearing vein deposits, and minor Be-bearing skarn deposits. Stream sediments were the primary sample medium because: (1) drainage systems are extensively developed; (2) bedrock is poorly exposed; and (3) previous studies had established stream-sediment sampling as a viable procedure for reconnaissance geochemical studies in this region. The data from about 1,700 stream-sediment samples collected in earlier U.S. Geological Survey programs were included to give a total of 6,935 samples of stream sediment. At 1,420 sites, samples of heavy-mineral concentrate from stream sediment were also collected. Both sample media were analyzed for as many as 32 elements. The distributions for Cr, Cu, Zn, Mo, and U in stream-sediment samples and for Sn and W in nonmagnetic heavy-mineral-concentrate samples are discussed here. Some significant findings are: (1) nonmagnetic heavy-mineral concentrates are a valuable sample medium for the region; (2) certain Jurassic granites are potential hosts to Sn deposits; (3) a prominent linear trend of stream-sediment samples containing high Cr may be caused by glacial transport of mafic or ultramafic rock material for distances as great as 160 km; (4) glacial action must be considered in the interpretation of (a) elements, such as Cr, W, and Sn, that form resistate minerals and (b) elements, such as Cu, that are associated with chromite or other resistate minerals; (5) glacial action seems to be a minor factor in the regional dispersion of elements from sulfide deposits; and (6) the geochemical expression of some known massive-sulfide deposits is subtle; thus, weak anomalies in areas of no known deposits may be significant.