5.0 Geochemical Groups and Associations

5.1 Stream Sediment Major Elements

5.2 Stream Sediment Trace Elements

5.3 Factor Analysis

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5.1 Stream Sediment Major elements

 (expressed as oxides Al₂O₃, CaO, Fe₂O₃, K₂O, MgO, Na₂O, SiO₂, TiO₂):

The depositional setting of stream sediments strongly influences the mineralogy of the sediments and their concentrations of major elements. High values of SiO₂ are associated with areas of coarser silt and sand-rich sediments in areas of glacial outwash deposits and quartz-rich and feldspar-mica-poor source rocks. High Al₂O₃ is associated with areas of clay and silt-rich sediments in areas dominated by glacial till and aluminous rocks such as schist and phyllite. SiO₂ and Al₂O₃ tend to show antithetic relationships to one another, reflecting the variation between quartz silt and clay in the sediment. Na₂O and K₂O are high over areas of granites, reflecting the influence of feldspar silt and detrital micas. CaO and MgO are high over areas of carbonate rocks (carbonate rock and calcpelite rock group categories), reflecting the influence of soluble carbonate and calcsilicate minerals.   Fe₂O₃ and TiO₂ are high in some areas of metamorphic rocks that weather to sediments containing magnetite and ilmenite in the silt-size heavy mineral suite.

5.2 Stream Sediment Trace Elements:

5.2.1 Alkali Earths (Ba, Sr):

Elevated concentrations generally occur in stream sediments associated with clastic metamorphic rocks and intermediate to mafic igneous rocks.

5.2.2 First-Row Transition elements (Sc, V):

Elevated concentrations of Sc and V occur in stream sediments associated with mafic rocks and carbonate-bearing metamorphic rocks.  Chemical variation in Sc and V is similar to variation in Fe₂O₃.

5.2.3 Heavy Mineral suite (REE, Zr, Th, U, Y, Ce/Yb(n)):

Elevated concentrations of REE (Ce, La, Nd), Zr, Th, Y, and U occur in silt- and sand-rich stream sediments enriched in the heavy-mineral suite of monazite (light REE, Th, U), zircon (Zr, U, heavy REE), and related minerals, such as xenotime (heavy REE). Monazite and zircon are resistant to weathering and are concentrated in the silt-sized heavy mineral fraction of stream sediments. Contributing sources of monazite to stream sediments include high-metamorphic-grade clastic metasedimentary rocks and REE-rich granites. Xenotime and zircon is often abundant in the heavy mineral suite derived from weathered alkali granites. Chondrite-normalized Ce/Yb ratios (Ce/Yb(n)) shows high values over areas dominated by monazite in the heavy-mineral suite and low values over areas dominated by zircon and xenotime in the heavy mineral suite. Chondrite rare earth element normalizing factors are from Evensen and others, (1978).

5.2.4 Base and toxic metals (Cu, Zn, Pb, As, Hg):

Arsenic and the base metals Cu, Pb, Zn tend to be associated with Fe_Mn-hydroxoxide minerals in stream sediments. Natural sources of base metals to stream sediments include weathered sulfide minerals (As, Cu, Pb, Zn), oxide and silicate minerals (Zn), and mafic rocks (Cu). Anthropogenic sources of base metals (Cu, Pb, Zn) are related to industrial sites and automobile traffic. Anthropogenic sources of As to stream sediments include the past agricultural use of arsenical pesticides. Elevated Hg concentrations in stream sediments tend to occur in urban areas and areas affected by acid deposition.

5.3 Factor Analysis:

         Factor analyses for groups of major elements (Al₂O₃, CaO, Fe₂O₃, K₂O, MgO, Na₂O) and trace metals (Fe [in Fe₂O₃], Cu, Zn, Pb, As) were performed to detect possible relationships between elements in the data set. The presence of one particular element in sediment may be associated with the presence of another element, due to natural concurrence or a particular land use characteristic in the area (such as an industrial site).  The associations found between elements using factor analysis provide a means for predicting the presence of a particular element in sediment based on another. Calculations were performed using the Factor Analysis function in Statview 5.0.1, and factor names were determined by the two or more elements within the factor with the highest orthogonal solutions, which have the main association(s). For major elements, three factors were calculated, termed “Fe₂O₃/MgO/CaO”, “K2O/Al₂O₃”, and “Na₂O/CaO”. Two metal factors were determined, termed “Fe₂O₃/Cu/Zn” and “As/Pb”. 

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