Geochemical mapping in Georgia, USA: a tool for environmental studies, geologic mapping and mineral exploration

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Journal of Geochemical Exploration 67 (1999) 345–360 www.elsevier.com/locate/jgeoexp

Geochemical mapping in Georgia, USA: a tool for environmental studies, geologic mapping and mineral exploration Mark D. Cocker * Georgia Geologic Survey, Atlanta, GA 30334, USA Accepted 29 September 1999

Abstract Because of intensive weathering, poor exposures and thick vegetation, surficial geochemical mapping is a valuable tool in the southeastern United States. Stream sediment and stream water geochemical data collected during the late 1970’s as part of the U.S. Department of Energy’s National Uranium Resource Evaluation (NURE) Program has recently been mapped and analyzed by the Georgia Geologic Survey with the aid of a GIS. Results indicate that bedrock geology and mineralization are the most important variables which influence the stream sediment and stream water geochemistry. Anthropogenic sources influence the geochemistry to a lesser and more localized extent. Geochemical mapping in Georgia has been used to define: (1) the background geochemistry of major river basins for river basin management planning; (2) rock units which have the highest radon potential in the Piedmont and Blue Ridge; (3) geochemical patterns that are related to regional and local geologic units and structures; and (4) geochemical anomalies related to known or previously unidentified mineralization. Geochemical anomalies identified and defined by the NURE data indicate five heavy mineral belts in the Piedmont, Blue Ridge, and Coastal Plain provinces, Mississippi Valley-type mineralization in the Valley and Ridge province, and base-metal (and perhaps Pt-group) mineralization associated with mafic metavolcanic rocks, layered mafic intrusions and a regional magnetic high in the Piedmont province.  1999 Elsevier Science B.V. All rights reserved. Keywords: geochemical mapping; stream sediments; stream; mineral resources; Georgia, USA

1. Introduction Regional mapping of surficial, geochemical data is an important tool in mineral-resource evaluations, geological, agricultural, forestry, and environmental studies (Bolviken et al., 1990; Darnley, 1990; McMillan et al., 1990; Reid, 1993; Simpson et al., 1993; Davenport et al., 1993; Birke and Rauch, 1993; Cocker, 1996a,b, 1998a,b). Because of intensive chemical weathering, poorly exposed geology and thick vegetation, surficial geochemical data can be an important tool in the southeastern United States. Ł Tel.:

C1-404-656-3214; Fax: C1-404-657-8379.

The U.S. Dept. of Energy’s National Uranium Resource Evaluation (NURE) Program in the late 1970’s created the first extensive surficial geochemical database for the United States. Until recently, the NURE database has been largely ignored in the eastern United States. Koch (1988) prepared a set of large-scale (1:1,785,000) geochemical point maps of Georgia, but these were of limited value because of their generalized nature. Increased availability of the data (Hoffman and Buttleman, 1994) and significant advances in computer technology and software (i.e., Geographical Information Systems and spreadsheets) permit a more vigorous spatial analysis of the NURE geochemical data. Recently, Carpenter and

0375-6742/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 5 - 6 7 4 2 ( 9 9 ) 0 0 0 7 9 - 5

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Carpenter (1991) and Grosz and Schruben (1994) used the NURE data to locate and define heavy mineral deposits in the southeastern United States. Mapping of the NURE data was facilitated by Georgia’s need to document the background geochemistry of its major river basins for river basin management planning. Because the NURE data were collected in the 1970’s, NURE geochemical data provide an important geochemical baseline established prior to the rapid growth that Georgia has experienced in the 1980’s and 1990’s. The NURE sampling program covered the northern seventy percent of the state (Fig. 1). In early 1998, the remaining thirty percent of Georgia was sampled as part of a cooperative program with the U.S. Geological Survey to complete NURE-type sampling for the southeastern United States. In addition to the baseline studies, other applications included identifying and defining: (1) regional and local lithogeochemical patterns, (2) regional and local metal anomalies related to known mineral belts and metal deposit occurrences, (3) metal anomalies spatially related to anthropogenic sources, (4) previously unknown mineralization or unidentified contamination, and (5) rock units having the greatest radon-producing potential. This paper examines the distribution of several representative elements that demonstrate the applications in environmental studies, geological mapping, and mineral exploration in Georgia.

2. Overview of geology Georgia’s five major geologic provinces include the Appalachian Plateau, Valley and Ridge, Blue Ridge, Piedmont, and Coastal Plain. The Valley and Ridge and Appalachian Plateau provinces (Fig. 2) contain Cambrian to Pennsylvanian carbonate and clastic sedimentary rocks that have been folded and thrust-faulted to the northwest. Precambrian to Paleozoic igneous and metamorphic rocks of the Blue Ridge and Piedmont provinces are thrust over the Paleozoic sedimentary rocks. Cretaceous to Holocene sediments of the Coastal Plain are in unconformable contact with the Piedmont along the Fall Line (Fig. 2). The Blue Ridge and Piedmont provinces may be subdivided into six tectonostratigraphic terranes

— Blue Ridge, Inner Piedmont, Pine Mountain, Kings Mountain, Uchee and Carolina (Fig. 2). Fault zones that separate these terranes are commonly not exposed, thereby resulting in some ambiguity regarding the placement of the boundaries between the terranes. Blue Ridge rocks consist generally of intermediate- to locally high-grade metavolcanic, metasedimentary and metaplutonic rocks that range from ultramafic to felsic in composition. The Inner Piedmont terrane consists mainly of a Paleozoic-age, high-grade migmatitic belt of gneisses, schists, amphibolites, and ultramafic to felsic plutons (Horton and Zullo, 1991). Mesoproterozoic metasedimentary rocks overlying granulite facies granitic gneiss and charnockite are found in the Pine Mountain terrane. The Kings Mountain terrane may be a Neoproterozoic or Cambrian clastic and calcareous metasedimentary package metamorphosed from middle greenschist to upper amphibolite grade (Butler and Secor, 1991). Greenschist to upper amphibolite grade metavolcanic and metaplutonic rocks characterize the Carolina and Uchee terranes. Geological and geochemical similarities of the Carolina and Uchee terranes suggest that they may be part of the same terrane. Late Paleozoic granitic plutons and Jurassic–Triassic diabase dikes are most abundant in the Inner Piedmont and Carolina terranes. In the Coastal Plain, Late Cretaceous through Eocene-age fluvial, deltaic, lagoonal, marginal marine, and near-shore clastic sediments are found near the Fall Line (Fig. 2). Eocene, Oligocene and Miocene shelf carbonate sediments are found down dip and in the central and southern parts of the Coastal Plain. Younger Miocene clastic fluvial and near-shore marginal marine sediments cover much of the central and southeastern Coastal Plain. Pliocene, Pleistocene and Holocene marginal marine and near-shore clastic sediments in southeastern Georgia record repeated marine transgressions and regressions.

3. Mineral deposits Mineral deposits in Georgia contain Au, Cu, Zn, Fe, Mn, pyrite, barite, bauxite, ceramic and structural clays, dolomite, feldspar, fuller’s earth, gran-

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Fig. 1. Location of stream sediment samples from NURE Program (1976–1978) and 1998 sampling program.

ite, heavy minerals, kaolin, limestone, marble, mica, ochre, and talc. Many of these deposits were formed as a result of or have been affected by intensive

chemical weathering particularly from the Cretaceous through the Eocene. The following section will focus mainly on the metallic deposits.

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Fig. 2. Major tectonic terranes and fault structures in Georgia (modified after Williams, 1978).

Mississippi Valley-type mineralization is found in fractured Cambrian quartzites, and fractures and solution cavities in Cambrian and Ordovician dolomite and limestone in the Valley and Ridge terrane (Fig. 2). The Cartersville mining district and other areas of the Valley and Ridge contain important residual deposits of ochre and umber, Mn-oxides, and barite (Reade et al., 1980). Brecciated Cambro– Ordovician dolomite and limestone host unusual Mo mineralization (Foss et al., 1983). Residual Mnoxide deposits in fractured Cambrian quartzite and dolomite, Cambro–Ordovician dolomite, and Carboniferous chert may contain 0.2–1.6% Co (Pierce,

1944). Residual bauxite deposits were mined from solution cavities in Cambro–Ordovician dolomite. Numerous Zn, Cu, Pb, Au, pyrite, and pyrrhotitebearing Kuroko and Besshi-type massive sulfide deposits in the Blue Ridge terrane are principally strata-bound and lie within a sequence of metamorphosed mafic to felsic volcanic rocks interlayered with subordinate metasedimentary rocks (Neathery and Hollister, 1984). Besshi-type mineralization represented by the ore-bearing horizon in the Ducktown District of southern Tennessee continues into northern Georgia (Abrams, 1985). Gold deposits occur mainly in the Dahlonega,

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Hall County, Carroll County, and McDuffie County Au belts. An estimated 400,000 to 500,000 ounces of Au (Pardee and Park, 1948) in the Dahlonega belt were produced from alluvial placer deposits, saprolite deposits, and primary veins or lodes. The primary deposits are spatially related to pre- and syn-kinematic felsic intrusions within dominantly metavolcanic rocks (Albino, 1990). In the Hall County belt, Au occurs in dilatant fractures in metasedimentary rocks within the Brevard fault zone. Gold mineralization in the Carroll County belt is associated with volcanogenic massive sulfide deposits. Deposits in the McDuffie County belt are associated with basemetals, pyrite and barite mineralization in the Carolina terrane (German, 1993). These latter deposits are spatially and probably genetically related to intermediate to felsic intrusive–extrusive complexes and are in a similar stratigraphic position to that of the Brewer, Haile, Barite Hill and Ridgeway Au mines in South Carolina. Other, generally small, base- and precious-metal deposits in the Carolina terrane appear related to a number of subvolcanic intrusions. A large, layered mafic complex, the Gladesville Norite, may contain Pt, Pd and base-metal sulfides as suggested from drill core data (Cocker, 1995). Several regionally elongate heavy mineral belts extend through the Blue Ridge, Piedmont and Coastal Plain. Three heavy mineral belts are associated with predominantly metasedimentary aluminous and micaceous schists and felsic biotite schists in the Blue Ridge and Piedmont terranes (Cocker, 1998c) and may represent potential for paleoplacer deposits. Two heavy mineral belts in Cretaceous to Eocene fluvial to shallow marine elastic sediments of the Upper Coastal Plain host placer concentrations. Pleistocene and Holocene heavy mineral deposits along the Atlantic coast are associated with barrier island sediments and are important sources for Ti-oxides, zircon, and monazite (Cocker, 1993).

4. Nature of samples, sampling methods, sample preparation and analysis Stream sediment and stream geochemical data collected as part of the U.S. Department of Energy’s National Uranium Resource Evaluation (NURE) Program are available on CD-ROM from the U.S.

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Geological Survey (Hoffman and Buttleman, 1994). That CD-ROM contains technical information concerning types of data collected in the field and obtained by laboratory analysis. Sampling in Georgia occurred during the period 1976–1978. A minimum of five sediment sub-samples was composited from each stream site (Ferguson, 1978). Approximately 400 g of sediment passing a 420-µm (U.S. Std. 40-mesh) screen were collected. Sample locations were marked on compilation maps and digitized. Latitude and longitude data were entered for each site into the SRL-NURE data base. The U.S. Department of Energy’s Savannah River Laboratory (SRL) oversaw the collection and analyses of samples for 30 eastern states that included Georgia. All analyses in the NURE study were done by automated neutron activation (NAA) techniques (Ferguson, 1978). In Georgia, analyzed elements include Al, As, Ba, Be, Ce, Cr, Co, Cu, Dy, Eu, Fe, and La, Pb, Li, Mg, Mn, Mo, Nb, P, Sm, Sc, Ag, Na, Sr, Ta, Th, Sn, Ti, W, U, V, Yb, Y, and Zn. After analysis of all samples for an initial suite of elements, a second suite consisting of As, Ba, Be, Cr, Cu, Co, Pb, Li, Mo, Ag, Sn, and Zn were analyzed from the original samples. Analyses for the second suite of elements is incomplete, as some samples could not be relocated before the program ended. Gaps in the data are evident in Figs. 6 and 8. NURE data are organized into data sets that are arranged by quadrangles of 1 ð 2 degrees. Geochemical data used in this study are from the Greenville, Rome, Macon, Waycross, Rome, Atlanta, Phenix City and Dothan quadrangles. A total of 7682 data points cover the northern two thirds of Georgia and adjacent parts of Alabama and South Carolina with 5658 data points in Georgia (Fig. 1). Average sample density in Georgia was one per 17 km2 . NURE data on the CD-ROM (Hoffman and Buttleman, 1994) required a variety of data processing to transform them into a usable format. Concentrations given in ppb were converted to ppm. Concentrations shown as below detection limit were converted to half the detection limit. Data from certain data sets were incomplete in part due to deterioration of the original computer tapes from which the CD-ROM was derived. Some of these data were captured from a hard copy of data for Georgia (Koch, 1988) and manually added to the data sets.

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GIS point coverages for each data set were created from the latitudes and longitudes supplied for each NURE sample point. The NURE data from the 1 ð 2 degree quadrangles were then joined to the point coverages created by the GIS from latitude and longitude data for each sample point. Contoured geochemical maps for the stream sediment and stream data were produced for all the available NURE stream sediment data in Georgia and adjacent parts of South Carolina and Alabama using a triangulated integrated network (TIN) program within the GIS. These maps are useful for defining regional trends and local anomalies and providing a quick visual check of the data processing. The GIS used during the course of this study was ArcInfo. The GIS was used to identify each sample point that was geographically within rock unit polygons on the digital Geologic Map of Georgia. Samples were grouped by rock unit, and the geochemical data were analyzed by statistical tools within a QuattroPro spreadsheet. This technique helped define the geochemical signature of each rock unit polygon, a group of polygons of the same rock unit, and groups of rock units.

Table 1 Summary geochemistry of samples from the Valley and Ridge and Appalachian Plateau, Blue Ridge and Piedmont, and Coastal Plain (alkalinity is in meq=l, conductivity is in µmhos=cm, elements are in ppm) Element

Valley and Ridge, Appalachian Plateau

Blue Ridge and Piedmont

Coastal Plain

pH Alkalinity Conductivity Al Ce Dy Fe Hf K La Mn Na Pb Sc Sm Th Ti U V Y Yb Zn

7.5 0.95 117.1 25,630 62.7 4.8 22,660 23.6 17,270 46.5 913.5 1,160 13.4 4.3 5.1 9.5 4,437 3.5 41.3 26.6 4.5 43.5

7.1 0.3 60.8 43,220 235.9 16.6 44,520 60.3 11,200 130.7 1,160.1 6,497 9.0 11.0 22.4 46.9 11,904 10.1 101.2 27.6 10.8 23.4

6.2 0.22 46.1 20,138 216.2 11.8 14,281 70.1 121.3 290.0 736 4.0 17.6 41.7 6,594 9.4 35.3 52.1 8.7

5. Interpretation of the results The regional distribution of alkalinity, Na, Fe, Zn, Ce, and Cr are discussed in regards to their relation to regional geology, and their use in environmental studies, geologic mapping and mineral investigations (Figs. 2–8). These elements and physicochemical parameter were selected because of their strong geochemical contrast on the geochemical maps and their representation of the distribution of related elements. Major terranes and the Brevard and Towaliga fault zones are shown for reference. 5.1. Alkalinity Overall, stream alkalinities (Fig. 3) are highest (mean D 0.95 meq=l, high of 3.3 meq=l) in the Valley and Ridge (Table 1) and are locally high (up to 3.0 meq=l) in the Coastal Plain. Dissolution of carbonate rocks such as those in the Valley and Ridge and the Coastal Plain contribute significantly to the high stream alkalinities in these areas (Fig. 3).

Streams draining sand-rich, Cretaceous to Mioceneage sediment such as those near the Fall Line have very low alkalinities (0.02–0.2 meq=l). These sandy sediments are highly permeable, and the quartz sands do little to buffer the stream water or contribute to the dissolved load of ions. Stream alkalinities in the Carolina terrane are commonly above 0.6 meq=l and are significantly higher than streams in the remainder of the Piedmont and Blue Ridge (Fig. 3) which have alkalinities generally below 0.3 meq=l. Mean alkalinities of streams that are spatially associated with greenschist to amphibolite grade metadacites, felsic metavolcanic rocks, amphibolites, gabbros and sericitic argillites in the Carolina terrane range from 0.61 to 1.59 meq=l. Higher alkalinities of the surface and ground waters are attributed to greater permeability and reactivity of the Carolina terrane rocks than the less permeable high-grade metamorphic rocks in the Inner Piedmont, Pine Mountain and Blue Ridge terranes.

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351

Fig. 3. Alkalinity of streams in eastern and central Georgia.

Regionally, the distribution of stream alkalinity shows a good correlation with that of pH and conductivity. Areas of high alkalinity in the Valley and Ridge, Carolina terrane and Coastal Plain have pH’s of 7.0–10.9 and conductivities of 50–790 µmhos=cm. Areas of low alkalinity in the Valley and Ridge, Inner Piedmont, Blue Ridge and Pine Mountain terranes and Coastal Plain have pH’s of 4.0–6.9 and conductivities of 5–70 µmhos=cm.

5.2. Sodium Regional distribution of Na (Fig. 4) is controlled, in part, by the differing compositions of the underlying rocks. Stream sediments spatially associated with amphibolites, metadacites, gabbros, and biotite gneiss in the Carolina terrane contain mean concentrations of Na that range from 6430 to 34,800 ppm. Believed to represent an island arc, the submarine volcanic rocks in the Carolina terrane are relatively

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Fig. 4. Regional geochemical map of Na in stream sediments in eastern and central Georgia.

enriched in Na (Whitney et al., 1978). The regionally high Na in the Carolina terrane extends along the Uchee terrane, and along with other geochemical and geologic data, suggests that rocks of the Uchee terrane should be referred to as Carolina terrane. High Na values in the Carolina terrane contrast with Na values in the adjacent Inner Piedmont and Pine Mountain terranes and with Na values in the Coastal Plain. In general, Na is considerably more abundant in the Blue Ridge and Piedmont crystalline rocks than in the sedimentary rocks of the Valley

and Ridge and Coastal Plain (Table 1). A large Na anomaly in the northern part of the Inner Piedmont terrane is underlain by a poorly known mixture of felsic biotite gneiss, granitic gneiss and amphibolite, but has not been linked to any particular rock type. 5.3. Iron Regional distribution of Fe (Fig. 5) is generally similar to that of Na and alkalinity. Stream sediments in the Carolina terrane contain more Fe

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353

Fig. 5. Regional geochemical map of Fe in stream sediments in eastern and central Georgia.

(generally 30,000–459,000) than stream sediments in the adjacent terranes (generally 15,000–75,000) and the Coastal Plain (generally less than 20,000). Stream sediments spatially associated with amphibolites, gabbros, and biotite gneiss in the Carolina terrane contain mean concentrations of Fe that range from 106,500 to 188,000 ppm. A large anomaly (approximately 50 km wide) in the center of Fig. 5 contains up to 459,000 ppm Fe. The Gladesville

Norite, several additional gabbroic intrusions and several large amphibolitic rock units lie within this Fe anomaly. Geochemical data from a drill hole in the Gladesville Norite suggest enrichment trends in base metals, Pt, Pd and sulfides. Anomalous concentrations of Zn, Mg, Ti, and V and a regional magnetic high coincident with this Fe anomaly may indicate important mineralization associated with these mafic rocks.

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5.4. Zinc The distribution of Zn in stream sediments differs from the previously discussed elements in that Zn is not strictly controlled by terrane geology. A large cluster of Zn anomalies (up to 253 ppm) in the center of Fig. 6 extends from the Inner Piedmont through the Carolina terrane and into the Coastal Plain. Part of this large cluster of Zn anomalies in the center of the map (Fig. 6) is coincident with and may be related to the large Fe anomaly and amphibolitic metavolcanic rocks and gabbroic intrusions in the Carolina terrane. The cluster of the Zn anomalies extending into the Inner Piedmont is not presently understood. Several Zn anomalies (up to 450 ppm) near the northern edge of the Coastal Plain are in or near the city of Macon and may be anthropogenic. Isolated Zn anomalies scattered throughout the Piedmont and Blue Ridge (Fig. 6) are spatially related to base- and precious-metal mines in the Carolina terrane and Blue Ridge terrane and, in some instances, to anthropogenic sources such as small cities and dumps of various types. Large Zn anomalies (up to 193 ppm) found in the Valley and Ridge to the west of the area shown in Fig. 6 are spatially associated with anomalous Ba, Co, Cr, Pb and Ni (high values of 160, 35, 63, 143, and 43 ppm, respectively) and Cambro–Ordovician carbonate rocks and probably represent Mississippi Valley-type base-metal mineralization. 5.5. Cerium High concentrations of Ce and other rare-earth elements (REE’s) reflect the presence of monazite and define the extent of heavy mineral belts in the

Blue Ridge terrane, the Inner Piedmont terrane and the Upper Coastal Plain of Georgia (Fig. 7). High concentrations of Ce that extend principally along the southeastern half of the Inner Piedmont terrane and mainly north of the Towaliga fault zone define the principal heavy mineral belt in Georgia. Two additional, discontinuous heavy mineral belts are located northwest of the Brevard fault zone. In the eastern half of the Upper Coastal Plain, a less well defined heavy mineral belt is found in Cretaceous to Eocene clastic sediments. Further to the west (beyond the extent of Fig. 7), this belt splits where Eocene-age sediments strike to the southwest. These heavy mineral belts extend through Georgia parallel to the principal structural trends in the Piedmont and Blue Ridge and along strike of clastic sediments in the Coastal Plain. The large number of high Ce values in the Piedmont and Blue Ridge (Fig. 7) skew the mean Ce value in these provinces up to 235.9 ppm (Table 1). In addition, stream sediments with high Ce generally contain high concentrations of REE’s, Hf, Th, Ti, and U (Table 2). In the Georgia Piedmont, stream sediments with high Ce concentrations are associated with rock units that include mainly sericite schists, mica schists, sillimanite schists, felsic and biotite gneisses (Table 2). These rock units are dominantly metasedimentary suggesting metamorphosed paleoplacer deposits. Rock units in the Coastal Plain with the highest mean Ce concentrations (Table 3) also generally have high mean concentrations of REE’s, Hf, Th, Ti, and U. Heavy minerals found in the Upper Coastal Plain include anatase, ilmenite, leucoxene, monazite, xenotime, zircon, and a few labile heavy minerals. The degree of weathering of ilmenite or the presence of chemically higher-grade Ti-oxides can be

Table 2 Geochemistry (arithmetic means) of selected rare-earth elements, Th, Ti and U in stream sediments spatially associated with Piedmont and Blue Ridge rock units containing the highest mean Ce concentrations (concentrations are in ppm) Element

Sericite schist

Amphibolite

Biotitic gneiss

Garnet mica schist

Sillimanite schist

Sillimanite schist

Biotite gneiss

Ce Hf La Sm Th Ti U

1,011.5 154.8 482.6 143.4 216.7 15,745.4 35.7

780.1 111.0 369.5 43.4 120.4 15,184.2 19.4

754.5 79.6 401.9 91.8 134.2 14,247.4 22.8

727.0 60.0 381.0 43.0 216.0 8,700.0 44.7

718.9 76.1 404.7 71.8 133.9 15,407.0 27.7

688.0 56.0 420.3 83.8 143.2 13,573.3 31.5

684.6 79.8 486.9 72.9 157.9 11,755.2 30.2

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Fig. 6. Regional geochemical map of Zn in stream sediments in eastern and central Georgia.

indicated by low Fe=Ti ratios. Rock units with the highest Ce concentrations generally have the lowest Fe=Ti ratios (i.e. below 2.00) and should be prospective for economic heavy mineral deposits. 5.6. Chromium Most of the anomalous Cr in the Piedmont and Blue Ridge (Fig. 8) are represented by one or several

sample points. A number of these anomalies appear to be roughly aligned parallel to the regional structural trend such as between A–A0 , B–B0 and C–C0 in Fig. 8. Stream sediments that are spatially associated with occurrences of ultramafic or mafic rocks contain an average of 10 ppm Cr, 10 ppm Ni, 2900 ppm Mg, 12 ppm Co, 50,800 ppm Fe, and 18,100 ppm Ti. Concentrations of ultramafic rock units occur to the west of A0 , B0 and C0 and are spatially associated

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Table 3 Arithmetic mean concentrations of selected rare-earth elements, Th, Ti and U in stream sediments spatially associated with Coastal Plain rock units containing the highest mean Ce concentrations (concentrations are in ppm) Element

Tuscaloosa Formation

Undifferentiated Cretaceous and Tertiary

Blufftown Formation

Irwintown Formation

Providence Sand

Twiggs Clay

Ce Hf La Sm Th Ti U

708.1 106.6 153.8 62.4 138.3 7,454 21.0

484.2 93.9 245.9 33.2 90.0 11,051 16.4

368.1 120.4 186.4 27.8 75.9 7,063 10.3

364.1 86.4 175.0 27.9 54.2 8,569 37.2

311.0 83.4 181.6 26.0 62.1 8,473 49.1

300.5 123.7 221.4 30.5 77.4 10,272 15.2

with somewhat larger Cr anomalies. Other mapped ultramafic rock units are found along the trends suggested by the Cr anomalies, and some of these rock units are coincident with the Cr anomalies. Unmapped occurrences of other ultramafic rocks may be related to some of the other Cr anomalies that lie along strike. The common occurrence of ultramafic or mafic rocks as tectonic slices in or adjacent to sutures between tectonic terranes, suggests that mapping of Cr anomalies may help define such structures in poorly exposed areas in the southeastern United States.

6. Applications 6.1. Environmental studies NURE data were used to document the background geochemistry of the several major river basins in Georgia (Cocker, 1996b, 1998a,b). With approximately 800 samples per river basin and location of the sample sites on first- and second-order streams, NURE data provided far greater geochemical detail for these river basins than the 10 to 30 state and federally maintained monitoring stations located on the major tributaries in each basin. NURE data provided a unique geochemical baseline with which new geochemical data may be compared. Most of the geochemical patterns such as those discussed in this paper could be directly related to the underlying geology. A small number of isolated geochemical anomalies could not be related to natural sources and appear to be anthropogenic. Samples collected near

waste disposal sites (noted as dumps in the NURE database) had higher mean Co, Cu, Pb and Zn (8, 7, 24, and 28 ppm, respectively) than non-contaminated sample sites. Single point Pb anomalies of 30 to 525 ppm that showed little correlation with Co, Cu, Ni, or Zn, were spatially related to small cities and could not be related to a known geological source area suspected of being anthropogenic. In the Coastal Plain, a stream sample with a specific conductivity of l73 µmhos=cm is spatially related to a major kaolin processing center. Several isolated anomalous concentrations of Na (26,000 to 52,000) in the Coastal Plain (Fig. 4) are unusual and may also be related to kaolin-processing centers. Heavy mineral belts which are defined by the Ce anomalies in Fig. 6, as well as high REE’s, Th, Ti and U, are also potential sources of radon generated during the decay of U. A map of equivalent U derived from an aeroradiometric survey (Duval et al., 1989) indicates that the highest equivalent U is coincident with the Ce anomalies (Fig. 6). Rock units with the highest mean U concentrations were identified with the GIS as a means of defining which rock units had the highest radon potential (Cocker, 1999). Ten rock units that contained 20 ppm or higher average U were mainly felsic biotitic gneisses, aluminous mica schists, sericite schists, and charnockite, the latter from the Pine Mountain terrane. Rock units with the lowest U concentrations (1.5 to 5 ppm) and lowest radon potential include ultramafic, amphibolitic, gabbroic, metavolcanic, graphitic, metapelitic, metacalcareous, and metaquartzitic rocks, many of which are located in the Carolina terrane.

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357

Fig. 7. Regional geochemical map of Ce in stream sediments in eastern and central Georgia.

6.2. Geologic mapping Recognition of surficial geochemical patterns that may characterize rock units, geologic terranes or structural features is a valuable tool that can aid geologic mapping particularly in the southeastern United States. Stream sediments derived from rocks of the Carolina terrane are geochemically distinguishable from adjacent parts of the Inner Piedmont and Pine

Mountain terranes. In addition to generally higher Fe, Na, alkalinity, conductivity, and pH noted earlier, the Carolina terrane is characterized by generally higher Al, Mg, Sc, and V and by lower K, U, Th, and rare-earth elements. Placement of the Carolina–Inner Piedmont boundary (Figs. 2–7) could be improved through careful examination of the NURE data and field evidence. Geochemical data suggest that the relatively unknown Uchee terrane is geochemically

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Fig. 8. Regional geochemical map of Cr in stream sediments in eastern and central Georgia.

similar to the Carolina terrane and is likely an extension of it. Similarly, interpretation of surficial geochemical data may aid in delimiting rock units or suites of rock units. Boundary structures such as the Towaliga fault zone could be better defined by closer investigation of the geochemical data such as the REE’s, the Carolina terrane suite of elements and perhaps the Cr–Ni–Mg–Fe–Ti suite which represents ultramafic rocks.

6.3. Mineral exploration Regional and more localized multi-element metal anomalies are spatially related to known mineralization. Other multi-element metal anomalies may indicate previously undetected mineralization. Basemetal anomalies coincident with lower Paleozoic carbonates in the Valley and Ridge province suggest Mississippi Valley-type mineralization. The large,

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multi-county sized Zn–Fe–Mg–Ti–V anomaly centered in the Carolina terrane is coincident with a regional magnetic high and a poorly exposed amphibolitic sequence that is intruded by numerous gabbroic plutons. At least one of these gabbroic plutons is a layered mafic complex with enrichment trends in base-metals, Pt, Pd and sulfides. NURE data helped define heavy mineral belts in the Blue Ridge, Piedmont and in the Upper Coastal Plain (Cocker, 1997, 1998c).

7. Conclusions Mapping of NURE stream sediment and stream geochemical data defines regional and local geochemical patterns related primarily to the underlying geology and secondarily to anthropogenic sources. Geochemical mapping has been successfully used in Georgia for environmental studies, defining potential economic mineralization and to facilitate geologic mapping. Large (multi-county sized), multi-element anomalies for base-metals, REE’s–Th–Ti–U, and Zn–Fe–Mg–Ti–V and the underlying geology suggest the presence of Mississippi Valley-type mineralization, heavy mineral paleoplacers, and mafic volcanic-layered mafic pluton-hosted base and Ptgroup mineralization. Stream sediment geochemical data for REE’s, Th, Ti and U that define heavy mineral belts in the Piedmont and Blue Ridge, coincide with high equivalent U from radiometric data and delimit areas and rock units that have the greatest radon potential. The NURE data provide a geochemical baseline for river basin management planning and for monitoring changes in a river basin’s composition. Geochemical mapping can be used to focus mapping in critical areas and aid in the geologic mapping.

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