UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY Analytical results for stream sediment and soil samples from the Commonwealth of Puerto Rico, Isla de Culebra, and Isla de Vieques By Sherman P. Marsh Open-File Report 92-353A This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards. Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the USGS. U.S. Geological Survey, DFC, Box 25046, MS 973, Denver, CO 80225 1992 CONTENTS Introduction Geology Methods of Study Sample Media Sample Collection and Preparation Sample Analysis Data Storage System Description of Data Tables Computer Diskette References Cited ILLUSTRATIONS Figure 1. Commonwealth of Puerto Rico, Isla de Culebra, and Isla de Vieques TABLES Table 1. Limits of determination for spectrographic analysis of stream sediment and soil samples Table 2. Limits of determination for atomic absorption and inductively coupled plasma-atomic emissionspectroscopic analysis of stream sediment and soil samples i INTRODUCTION This report presents the results of a geochemical survey of the Commonwealth of Puerto Rico, Isla de Culebra, and Isla de Vieques as digital data files on a 1.2 megabyte floppy disk. A regional stream-sediment geochemical survey of Puerto Rico began in the early 1970's as an outgrowth of the cooperative exploration geochemical studies by the Department of Natural Resources of Puerto Rico (DNR) and the U.S. Geological Survey (USGS) on the Rio Tanama and Rio Vivi copper projects. From the early 1970's through the mid-1980's stream sediment sampling continued over a large part of the island. In 1980 a cooperative project between DNR and the USGS was started to continue the regional stream sediment sampling program and continued for several years. A total of 2560 stream sediment samples were collected during this phase of the project. In 1990 a systematic search of USGS computer records yielded a geochemical data set for the stream sediments which indicated that the geochemical sample net for the island was incomplete. In 1991 two field trips were made to Puerto Rico and an additional 292 stream sediment samples were collected and analyzed in order to complete the regional geochemical survey. The analytical results of a total of 2852 stream sediment samples are included on the floppy disk. In addition to the main island of Puerto Rico, the islands of Culebra and Vieques were sampled. GEOLOGY (from Schellekens, 1991) Puerto Rico, the eastern-most island of the Greater Antilles, is a translational island-arc terrane with a geologic record of 140 million years. The island lies within the seismically active Caribbean-North American Plate boundary zone. The relative motion between the two plates is on the order of 2 cm per year and is mainly taken up by strong oblique underthrusting and left-lateral faulting in the Puerto Rico trench. A well defined southward dipping Benioff zone occurs under the eastern half of the island but is missing under the west side (McCann and Sykes, 1984; Schell and Tarr, 1978). Some plate motion and underthrusting also occurs south of Puerto Rico in the Muertos Trough. Puerto Rico and the Virgin Islands appear to be a separate tectonic block within the plate boundary zone. Puerto Rico is separated from Hispaniola on the west by a zone of active extension, which runs from the Mona canyon through the southwestern quarter of the island. On land extensional faulting has produced the distinctive ridge and valley topography and generally low elevations of southwestern Puerto Rico. Eastern Puerto Rico and the northern Virgin Islands are separated from St. Croix and the Lesser Antilles by another active zone of extension which formed the Whiting Basin (south of Puerto Rico, the Virgin Islands basin, and the Anegada Passage. Puerto Rico consists of volcanic, volcaniclastic, and sedimentary rocks of Late Jurassic to Early Tertiary age, which were intruded by felsic plutonic rocks during the Late Cretaceous and Early Tertiary, and are overlain by slightly tilted Oligocene and younger sedimentary rocks and sediments (Briggs and Akers, 1965). Island-wide lithostratigraphic correlation within the basement rocks is difficult because individual units appear to have limited original lateral extent and the rocks have been subsequently strongly deformed and faulted. To overcome these correlation problems earlier workers divided the island into structural blocks (Cox and Briggs, 1973) or subprovinces (Barabas, 1977). Figure 1. The Commonwealth of Puerto Rico, Isla de Culebra, and Isla de Vieques METHODS OF STUDY Sample Media and Collection During the cooperative project between the USGS and DNR sediment samples were collected from first order streams that drained basins from less than 1 square kilometer to as much as 3 square kilometers. The sediment samples were collected from the main channel of mostly active streams. The sediment samples collected in 1991 were from first and second order streams and represented drainage basins as large as 10 square kilometers. These samples were also collected from the main channel of active streams. The island of Vieques was geochemically sampled for soils in 1972 and a report describing the results are discussed in Learned and Boissen (1972). A soil sample survey for Isla de Vieques was conducted, rather than a stream sediment survey, because stream drainages were poorly developed and commonly filled with colluvium and, when near populated areas, highly contaminated. A total of 421 soil samples were taken of the C horizon (weathered bedrock) on 0.5 kilometer centers on northwest trending traverses spaced approximately 1 kilometer apart. The small island of Culebra was sampled geoochemically in late 1970 as part of a study to determine the islands natural resources, development potential, and socio-economic aspects (Commonwealth of Puerto Rico, 1970). Because of the lack of active streams on the island geochemical samples of dry stream bed material were collected. This material included pebbles and cobbles showing the most intense iron staining and any material showing traces of mineralization. A total of 46 samples were collected. Sample Preparation All samples were sieved to minus 80 mesh (0.18 mm) and then pulverized to approximately minus-100 mesh (minus-0.15 mm) with a grinder using ceramic plates. Sample Analysis All samples were analyzed for 35 elements using a semiquantitative, direct-current arc emission spectrographic method (Grimes and Marranzino, 1968). The elements analyzed and their limits of determination are listed in table 2. Values determined for the major elements (iron, magnesium, calcium, phosphorus, sodium, sulfur and titanium) are given in weight percent; all others are given in parts per million (micrograms/gram). Spectrographic results were obtained by visual comparison of spectra derived from the sample against spectra obtained from standards made from pure oxides and carbonates. Standard concentrations are geometrically spaced over any given order of magnitude of concentration as follows: 100, 50, 20, 10, and so forth. Samples whose concentrations are estimated to fall between those values are assigned values of 70, 30, 15, and so forth. The precision of the analytical method is approximately plus or minus one reporting interval at the 83 percent confidence level and plus or minus two reporting intervals at the 96 percent confidence level (Motooka and Grimes, 1976). Atomic absorption results for gold were obtained after preparing the samples for analysis using the method of Thompson and others (1968) for samples collected between 1970 and 1985 and by the method of O'Leary and Meier (1986) for samples collected in 1991. Atomic absorption results for zinc were obtained by using a nitric acid digestion method described by Ward and others, (1969) for the samples collected between 1970 and 1985. for samples collected in 1991 zinc, and 9 other elements, were analyzed using ICP-AES by the method of Motooka (1988) (Table 2). DATA STORAGE SYSTEM Upon completion of the analytical work, the results were entered into a U.S. Geological Survey computer data base called RASS. This data base contains both descriptive geological information and analytical data. Any or all of this information may be retrieved and converted to a binary form (STATPAC) for computerized statistical analysis or publication (VanTrump and Miesch, 1977). UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY Analytical results of stream sediment samples from the Commonwealth of Puerto Rico By Sherman P. Marsh Open-File Report 92-353B This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards. Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the USGS. U.S. Geological Survey, DFC, Box 25046, MS 973, Denver, CO 80225 1992 ______________________________________________________________________________ FILES ON THIS DISK [ online at http://pubs.usgs.gov/of/1992/of92-353/ ] CULEBRA.STP is a binary file of chemical data for Isla de Culebra VIEQUES.STP is a binary file of chemical data for Isla de Vieques PRNEW.STP is a binary file of 1991 chemical data for Puerto Rico PROLD.STP is a binary file of 1975-1985 chemical data for Puerto Rico STP2DAT.EXE is a conversion program from "STP" data file to other commonly used data files (DBF, CMN, PST, DIF, and others). The data tables (*.STP) can be converted into several formats, including DBF, DIF, and ASCII by using the program STP2DAT, authored by W.D. Grundy of the USGS. This program is included on this disk. This disk contains geochemical data from 2560 stream sediment samples collected in Puerto Rico, 421 soil samples collected on Isla de Vieques, and 35 soiL samples were collected on Isla de Culebra. Each rock sample was analyzed for 35 elements by a semiquantitative spectrographic method and for gold and zinc by other chemical methods. Requirements: IBM PC or compatible, 1.2 megabyte disk drive, and a minimum 512K RAM. To order a paper or microfiche copy of this report, order OF92-353A, p. and to order an executable diskette, order OF92-353B. Disclaimer: Although the program STP2DAT.EXE has been used by the U.S. Geological Survey, no warranty, expressed or implied is made by the USGS as to the accuracy and functioning of the program and related material, nor shall the fact of distribution constitute any such warranty, and no responsibility is assumed by the USGS in connection therewith. REFERENCES CITED Barabas, A.H., 1977, Petrologic and geochemical investigation of porphyry copper mineralization in west-central Puerto Rico: New Haven, Conn., Yale University, unpublished Ph.D. thesis, 466 p. Briggs, R.P., and Akers, J.P., 1965, Hydrogeologic map of Puerto Rico and adjacent islands: U.S. Geological Survey Hydrologic Investigations Atlas HA-197, scale 1:240,000. Cox, D.P., and Briggs, R.P., 1973, Metallogenic map of Puerto Rico: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-721. Environmental Quality Board, 1971, An island in transition: a staff report on the environment to theGovernor's special committee on Culebra: 106 p. Grimes, D.J., and Marranzino, A.P., 1968, Direct-current arc and alternating-current spark emission spectrographic field methods for the semiquantitative analysis of geologic materials: U.S. Geological Survey Circular 591, 6 p. Learned, R.E., Grove, G.R., and Boissen, R, 1973, A geochemical reconnaissance of the island ofVieques, Puerto Rico: U.S. Geological Survey Open-File Report, 6 p. McCann, W.R., and Sykes, L.R., 1984, Subduction of a seismic ridges beneath the Caribbean Plate--Implications for the tectonics and seismic potential of the northeastern Caribbean: Journal of Geophysics Research, v. 89, p. 4493-4519. Motooka, J.M., 1988, A exploration geochemical technique for the determination of preconcentratedorganometallic halides by ICP-AES: Applied Spectroscopy, 42, p. 1293-1296. Motooka, J.M., and Grimes, D.J., 1976, Analytical precision of one-sixth order semiquantitative spectrographic analyses: U.S. Geological Survey Circular 738, 25 p. O'Leary, R.M. and Meier, A.L., 1986, Analytical methods used in geochemical exploration, 1984: U.S.Geological Survey Circular 948, p. 23-27. Schell, B.A., and Tarr, A.C., 1978, Plate tectonics of the northeastern Caribbean Sea, in MacGillavry, H.J., and Beets, D.J., eds., Transactions 8th Caribbean Geological Conference (Willemstad, 1977): Geol. en Mijnbouw 57, p. 319-324. Schellekens, J.H., Joyce, J., Smith, A.A., and Larue, D.K., 1991, Tectonics and mineral deposits of the Caribbean, in Schellekens, J.H., ed., 10th Annual Symposium on Caribbean Geology: Mayaguez, University of Puerto Rico, Department of Geology, 37 p. Thompson, C.E., Nakagawa, H.M., and Van Sickle, G.H., 1968, Rapid analysis for gold in geologicmaterials, in Geological Survey Research 1968: U.S. Geological Survey Professional Paper 600-B, p. B130-B132. VanTrump, George, Jr., and Miesch, A.T., 1977, The U.S. Geological Survey RASS-STATPAC system for management and statistical reduction of geochemical data: Computers and Geosciences, v. 3, p. 475-488. Ward, F. N., Nakagawa, H.M., Harms, T.F., and Van Sickle, 1969, Atomic-absorption methods useful in geochemical exploration: U. S. Geological Survey Bulletin 1289, 45 p. TABLE 1. Limits of determination for spectrographic analysis of stream sediment and soil samples --------------------------------------------------------------------------------------------------------- Elements Lower determination limits upper determination limits -------------------------------------------------------------------------------------------------------------------- Percent --------------------------------------------------------------------------------------------------------------------- Iron (Fe) 0.05 20 Magnesium (Mg) 0.02 10 Calcium (Ca) 0.05 20 Titanium (Ti) 0.002 1 -------------------------------------------------------------------------------------------------------------------- Parts per million (ppm) -------------------------------------------------------------------------------------------------------------------- Manganese (Mn) 10 5,000 Silver (Ag) 0.5 5,000 Arsenic (As) 200 10,000 Gold (Au 10 500 Boron (B) 10 20,000 Barium (Ba) 20 5,000 Beryllium (Be) 1 1,000 Bismuth (Bi) 10 1,000 Cadmium (Cd) 20 500 Cobalt (Co) 5 2,000 Chromium (Cr) 10 5,000 Copper (Cu) 5 20,000 Lanthanum (La) 20 1,000 Molybdenum (Mo) 5 2,000 Niobium (Nb) 20 2,000 Nickel (Ni) 5 5,000 Lead (Pb) 10 20,000 Antimony (Sb) 100 10,000 Scandium (Sc) 5 100 Tin (Sn) 10 1,000 Strontium (Sr) 100 5,000 Vanadium (V) 10 10,000 Tungsten (W) 50 10,000 Yttrium (Y) 10 2,000 Zinc (Zn) 200 10,000 Zirconium (Zr) 10 1,000 Thorium (Th) 100 2,000 ----------------------------------------------------------------------------------------- Table 2. Limits of determination for atomic absorption (aa) and inductively coupled plasma-atomic emission spectroscopic analysis of stream sediment and soil samples ________________________________________________________________________ Element Method Lower Limit of References determined Determination (ppm) ________________________________________________________________________ Gold (Au) aa 0.05 Thompson and others, 1968 Gold (Au) aa 0.002 O'Leary and Viets, 1986 Zinc (Zn) aa 5 Ward and others, 1969 Zinc (Zn) ICP 0.05 Motooka, 1988 Copper (Cu) " 0.05 " " Lead (Pb) " 0.60 " " Silver (Ag) " 0.10 " " Gold (Au) " 0.15 " " Bismuth (Bi) " 1.0 " " Cadmium (Cd) " 0.05 " " Molybdinum (Mo) " 0.09 " " Antimony (Sb) " 1.0 " " Arsenic (As) " 1.0 " "