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Provenance of beach gray sands from western México
Journal of South American Earth Sciences 14 (2001) 291±305
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Provenance of beach gray sands from western MeÂxico A. Carranza-Edwards a,*, E. Centeno-GarcõÂa b, L. Rosales-Hoz a, R. Lozano-Santa Cruz b a
Instituto de Ciencias del Mar y LimnologõÂa, Universidad Nacional AutonoÂma de MeÂxico, Ciudad Universitaria, Mexico DF 04510, Mexico b Instituto de GeologõÂa, Universidad Nacional AutonoÂma de MeÂxico, Ciudad Universitaria, Mexico DF 04510 Mexico Received 1 October 1999; revised 1 July 2000; accepted 1 January 2001
Abstract Twenty-nine sur®cial beach dark colored sand samples from about 6500 km of the western littoral of Mexico were chemically and petrologically analyzed to search for the relationship between their heavy mineral content and metal and rare earth element variations. Sand samples were dried and grouped according to color in three classes of gray. In order to search for a possible in¯uence on petrological and chemical variations, the sands were also grouped according to grain size, sorting, composition, main sources, wave energy, and climate. It was observed that quartz, feldspars, and rock fragments averaged low values and, as the sand darkened, the heavy mineral content increased. Also, the higher content of heavy minerals in dark gray and very dark gray sands correlated with a humid rather than arid climate. Rare earth element and other trace element contents seemed to be related more to the composition of the source area than to heavy mineral content. However, some elements, such as Zn, Hf, Th, Ni, and Cr, did relate more to heavy mineral content. Geochemical and petrological analyses of these sands suggest that most are derived from juvenile sources. Sedimentary processes such as wave energy appear to play a signi®cant role locally in heavy mineral content, making the energy conditions of the beach environment an important local factor. Concentrations of rare earth and other elements are not of economic interest. q 2001 Published by Elsevier Science Ltd.
Resumen Con la ®nalidad de entender la relacioÂn entre la concentracioÂn de minerales pesados y las variaciones en metales y elementos de tierras raras, se analizaron quõÂmicamente y petroloÂgicamente 29 muestras de arenas recientes de playa, correspondientes al litoral occidental de MeÂxico, a lo largo de 6500 km. Las muestras de arenas fueron secadas y agrupadas por color en tres clases de grises. Para explicar la posible in¯uencia en las variaciones petrologicas y quõÂmicas, las arenas se agruparon de acuerdo con el tamanÄo de grano, composicioÂn, principales fuentes, altura de oleaje y clima. Se observo que el cuarzo, los feldespatos y los fragmentos de roca dan bajos promedios y cuando la arena es maÂs oscura, entonces el contenido de minerales pesados se incrementa. AdemaÂs el contenido maÂs alto de minerales pesados (en arenas grises oscuras y grises muy oscuras) se asocian maÂs con un clima huÂmedo que con un clima aÂrido. Las concentraciones de tierras raras y otros elementos traza parecen relacionarse con sus areas fuente, maÂs que los contenidos de minerales pesados. Sin embargo, la concentracioÂn de algunos elementos, tales como Zn, Hf, Th, Ni, y Cr parecen vincularse maÂs con el contenido de minerales pesados. El anaÂlisis petroloÂgico y geoquõÂmico de las arenas sugiere que la mayorõÂa de ellas se derivaron de fuentes juveniles. Los procesos sedimentarios, tales como energõÂa del oleaje, juegan un papel signi®cante localmente en lo que se re®ere al contenido de minerales pesados, haciendo que las condiciones de energõÂa del ambiente de playa sean un factor importante. Los valores de tierras raras y otros elementos no son lo su®cientemente altos como para ser de interes econoÂmico. q 2001 Published by Elsevier Science Ltd. Keywords: Beach sands; MeÂxico; Metals; Rare earth elements; Gray sands; Paci®c; Black sands
1. Introduction Placer beach sands are considered a source of metals of potential economic interest (Mero, 1965; Wright and Fairbridge, 1977; Cronan, 1980; Kunzendorf, 1986). * Corresponding author. Tel.: 152-5-622-5817; fax: 152-5-616-0748. E-mail address: [email protected] (A. CarranzaEdwards). 0895-9811/01/$ - see front matter q 2001 Published by Elsevier Science Ltd. PII: S 0895-981 1(01)00028-1
Mechanical concentration of resistant heavy minerals takes place in the beach environment, mostly under high energy conditions (Shepard, 1967; Dubois, 1972; Komar, 1976; Davies, 1978; Potter, 1986, 1994; Carranza-Edwards et al., 1992). According to Basu (1976) and Basu and Molinaroli (1989), the petrological composition of recent sediments is mostly controlled by the original source rocks. In the beach environment, metal-rich sands are generally found
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Fig. 1. Simpli®ed geologic and hydrologic map of the study area. The thick lines represent limits of lithologies, medium lines are drainage divides, and the thin lines are main rivers and streams. Dots represent sites of sampled beach locations. Key: bold numbers are sampling stations; small numbers 1±7 are the tectonic±lithologic types shown and discussed in the text; A, B, C, D, E, and F are the main geologic regions sampled.
in the upper beach zone, or backshore (Kunzendorf, 1986), where waves lead to concentrations of high-density minerals that are not easily removed back to the sea by the low energy backwash ¯ow. Mero (1965) was one of the ®rst authors to point out the importance of placer sands from the beach as a potential source of metal-rich marine deposits. The study of these deposits is of interest in that not only does it identify placer sands as an eventual and direct source of metals, it also suggests the presence of the metals in the coastal zone or in the continental shelf (Byrne and Emery, 1960; Van Andel, 1964). Gold distribution anomalies in the Oregon State continental shelf analyzed by Clifton (1968) are an example of such deposits. In metal-enriched beach sands, it is common to ®nd minerals such as magnetite, ilmenite, pyroxenes, zircon, monazite, and others, with concentrations of elements such as gold, silver, and rare earth elements (REE) (Darby, 1984). The economic importance of REE has increased in the last few years. REE of commercial importance are Th, Ce, La, Nd, Pr, Gd, Sm, and Eu (Neary and Highley, 1984). Placer deposits are of major interest, as they constitute one of the main sources
of REE, since they are formed by accumulations of unaltered metal-bearing debris. The possibility of native elements to generate placer deposits is common only among the noble metals: the gold and platinum groups and a few metals that develop a suf®ciently impervious oxide coating to resist total oxidation. In addition, some metal oxides, such as rutile and ilmenite, have accumulated in large amounts in beach sands at many localities around the world. The purpose of this work is to analyze the textural, petrological, and chemical composition of beach gray sands of the western littoral of Mexico, in order to understand the possible relationships between their metal content and different natural conditions. One of the goals of this study was to search for possible placer deposits as indicators for exploration of inland deposits of economic value. In addition, the relationship between sand composition and the source of the sediments is discussed. 2. Study area Twenty-nine beach locations were sampled on the
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Table 1 General data on beach sands from the western littoral of Mexico (source: v volcanic, p plutonic, m metamorphic; wave height: H high (.100 cm), M moderate (.30 cm± , 100 cm), L low (,30 cm)) Sample
Beach locations
Color of dry sample
Mz (phi)
Sorting (phi)
Related climate
Main source
Wave height
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
Playas de Tijuana Primo Tapia San RamoÂn San Quintin Tomatal Guerrero Negro Ensenada Blanca BahõÂa de los Angeles Calamajue Santa Teresa Playa Mexico San Felipe Los Corchos Chalacaltepec Tecuan Apiza San Juan de Lima Las Risas San Telmo Mexiquillo Bonanza Petacalco Papanoa Colotepec Boca del Cielo Palmarcito Zapotal La Barrita Barra del Suchiate
Gray Gray Gray Light gray Dark gray Light gray Light gray Light brownish gray Gray Light brownish gray Light gray Light gray Gray Light gray Gray Dark gray Very dark gray Gray Gray Dark gray Gray Light brownish gray Light gray Dark gray Gray Dark gray Dark gray Gray Dark gray
2.01 2.45 1.51 2.71 2.70 3.06 2.66 1.44 0.42 0.81 2.13 2.69 3.06 2.39 0.87 1.68 3.25 2.99 1.92 2.30 2.86 1.58 2.72 2.40 2.44 1.98 1.31 1.85 1.79
0.47 0.39 0.72 0.30 0.49 0.46 0.51 1.24 0.83 1.52 0.68 0.40 0.42 0.53 0.72 0.60 0.51 0.39 0.73 0.57 0.57 0.83 0.35 0.56 0.51 0.73 0.69 0.83 0.73
Arid Arid Arid Arid Arid Arid Arid Arid Arid Arid Arid Arid Humid Humid Humid Humid Humid Humid Humid Humid Humid Humid Humid Humid Humid Humid Humid Humid Humid
v v p p p p v m p v v p v m,p m,p v v v m,p m,p m p p p p p p v v
H H H H H L L L H H M M H H H M M H L H H M H H H H H H H
Table 2 Mineralogical framework of beach sands (Qm monocrystalline quartz, Qp polycrystalline quartz, Qt quartz, Fk potassic feldspar, Fp plagioclase feldspar, Rf rock fragments, Ch chert, Mi mica, HM heavy minerals, PI provenance index, MI maturity index) Sample
Qm
Qp
Qt
Fk
Fp
Ft
Rf
Ch
Mi
HM
PI
MI
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 Average
9 11 7 16 1 17 1 13 8 8 15 17 3 5 14 3 0 6 2 7 0 6 11 0 6 1 5 6 3 6.9
3 2 1 1 0 1 0 4 7 0 1 0 0 3 3 1 0 0 0 0 1 0 1 1 0 0 0 1 0 1.1
12 13 8 17 1 18 1 17 15 8 16 17 3 8 17 4 0 6 2 7 1 6 12 1 6 1 5 7 3 8.0
2 0 0 0 2 0 7 1 4 12 0 0 0 0 1 0 0 0 0 2 1 0 0 0 0 2 1 0 0 1.2
12 11 12 28 8 26 18 25 7 22 17 24 7 20 10 17 3 13 10 4 5 5 17 2 12 5 9 7 10 12.6
14 11 12 28 10 26 25 26 11 34 17 24 7 20 11 17 3 13 10 6 6 5 17 2 12 7 10 7 10 3.8
8 8 31 17 5 23 41 30 70 29 29 41 17 16 53 49 4 17 10 13 21 26 9 2 37 10 29 32 29 24.2
1 0 0 2 0 4 2 2 1 6 4 6 0 4 7 8 0 1 3 2 3 11 0 3 3 0 1 5 0 2.7
1 0 11 1 0 0 0 5 0 0 0 0 0 0 0 2 0 1 0 1 0 1 0 0 0 0 0 0 0 0.8
64 68 48 35 84 29 31 20 3 23 34 12 73 52 12 20 93 65 75 71 69 51 62 92 42 82 55 49 58 50.8
1.75 1.38 0.39 1.65 2.00 1.13 0.61 0.87 0.16 1.17 0.59 0.59 0.41 1.25 0.21 0.35 0.75 0.93 1.00 0.46 0.29 0.19 1.89 1.00 0.32 0.7 0.34 0.22 0.34 0.57
0.55 0.68 0.19 0.38 0.07 0.37 0.02 0.3 0.19 0.13 0.35 0.26 0.13 0.22 0.27 0.06 0.00 0.22 0.10 0.37 0.04 0.19 0.46 0.25 0.12 0.06 0.13 0.18 0.08 0.21
294
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Table 3 Concentration (%) of major elements in beach sands (LOI loss on ignition; CIA chemical index of alteration: Al2O3/(Al2O3 1 Na2O 1 CaO 1 K2O) £ 100; ND not determined) Sample
SiO2
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
CIA
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 Avg
61.58 66.90 46.00 70.50 13.80 68.30 48.60 57.43 81.27 57.30 69.70 69.20 51.30 75.00 75.20 56.60 32.77 49.04 54.90 54.10 74.73 75.70 55.50 29.81 54.70 43.40 60.90 54.70 50.50 57.22
3.10 2.02 1.10 0.48 23.00 0.49 1.23 0.86 0.25 0.34 2.48 0.66 6.24 1.48 0.75 1.20 9.37 1.53 1.85 1.65 0.47 0.76 4.85 27.79 1.40 4.24 1.30 2.36 2.85 4.00
7.50 7.38 7.54 12.70 2.58 12.50 16.20 12.57 6.03 10.48 10.20 12.80 10.90 10.50 10.30 13.10 5.13 10.34 13.00 14.00 9.54 9.03 8.69 3.80 15.30 8.91 13.50 12.20 12.00 10.30
11.61 9.23 23.10 3.32 45.80 2.77 6.14 6.31 2.02 3.41 4.58 2.72 15.90 2.68 3.37 8.30 33.18 12.01 9.94 9.17 3.83 3.68 16.20 27.45 8.41 24.10 8.07 14.20 17.30 11.68
0.31 0.28 0.75 0.06 1.17 0.08 0.12 0.08 0.09 3.41 0.17 ND 0.40 0.13 0.11 0.26 0.51 0.20 0.26 0.24 0.06 0.07 0.32 2.13 0.24 0.54 0.20 0.34 0.50 0.45
5.84 5.23 3.12 2.11 2.29 1.75 4.43 7.77 1.06 1.14 2.31 2.85 3.60 1.10 0.85 5.71 7.97 7.93 5.32 5.25 1.45 1.85 3.13 3.05 4.97 8.16 4.50 5.05 5.69 3.98
6.69 6.17 5.27 5.48 4.73 5.93 13.60 4.70 6.07 21.53 5.89 4.75 5.91 2.79 2.06 7.70 5.79 11.13 9.11 10.70 2.65 2.70 5.97 2.41 8.13 7.31 5.52 5.62 6.68 6.65
1.53 1.22 2.03 3.15 0.61 3.36 3.25 2.35 1.26 2.41 2.23 2.77 2.16 2.07 2.48 3.22 1.08 2.33 2.90 2.82 2.43 1.91 2.01 0.76 2.96 1.46 2.49 2.71 2.37 2.22
0.82 0.95 0.57 1.21 0.22 1.56 1.91 2.68 1.00 2.62 1.44 2.35 1.66 2.73 3.37 1.37 0.24 0.53 1.31 0.90 1.75 1.87 1.60 0.39 1.52 1.03 1.90 1.59 1.21 1.46
0.45 0.03 0.49 0.09 0.37 1.26 0.43 0.26 0.09 0.00 0.04 0.16 0.25 0.02 0.10 0.15 0.13 0.36 0.16 0.19 0.11 0.10 0.15 0.11 0.14 0.17 0.15 0.23 0.31 0.22
0.15 0.16 0.93 0.64 0.22 1.38 2.63 2.31 0.83 0.66 0.53 0.78 0.38 0.54 0.78 1.81 2.87 2.87 0.86 0.15 1.17 1.17 0.88 2.46 1.10 0.08 0.83 0.23 0.12 1.55
45.34 46.96 48.97 56.34 31.70 53.53 46.34 56.37 41.99 28.29 51.62 56.46 52.84 58.04 56.56 51.60 41.91 42.52 49.39 49.25 58.29 58.22 47.56 51.63 54.82 47.62 57.67 55.15 53.91 50.03
western littoral of Mexico, extending approximately 6500 km along the Paci®c and Gulf of California waters (Fig. 1). The selected localities are beaches with dark colored sand and abundant heavy minerals. All the samples examined in this study are Quaternary in age and variable in grain size. Sample stations 1±12 (Fig. 1) are located in an area which, according to Tamayo (1990), is characterized by an arid climate (25 mm/y). In contrast, sample stations 13±29 are located in areas with a more humid climate (.100 mm/y). High-energy waves characterize the beaches located in open sea (1±6 and 14±29), whereas the beaches studied in the Gulf of California (7±13) are characterized by lowenergy waves. Because of the California Current (Wirtky, 1965), surface ocean currents are stronger on the western side of the Baja California Peninsula, as well as in the southern Mexican Paci®c, than in the Gulf of California. The rock sources and main rivers are summarized in Fig. 1, in which geology was simpli®ed based on the most abundant type of rock per area (modi®ed from OrtegaGutieÂrrez et al., 1992). The present tectonic setting is less active on both margins of the Baja California Peninsula
(passive to trailing margins) than in the southern Paci®c (active subduction margin). The source areas were divided into seven major tectonic±lithologic groups: 1. granitoids and migatites of variable ages; 2. volcanic rocks of intermediate to acid composition, mostly of Early Tertiary age (andesite to rhyolite); 3. volcanic rocks of intermediate to ma®c composition, mostly of Late Tertiary age (andesite to basalt); 4. deformed and/or metamorphosed volcanic-sedimentary complexes (igneous rocks, mostly of basaltic to andesitic composition), with ages ranging from Paleozoic to Mesozoic; 5. high-facies metamorphic complexes, with variable protoliths, mainly of Precambrian age; 6. sedimentary rocks, mostly marine-calcareous and some terrigenous, of Mesozoic±Tertiary ages; and 7. areas with mixed sources, with variable lithologies of igneous, metamorphic and sedimentary composition. The sampled localities are related to six different types of ¯uvial±lithological basins: A. Samples 1±6 were collected from the western part of
Au in ppb.
,2 ,2 ,2 ,2 ,2 ,2 ,2 9 3 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 Average
a
Au a
Sample
,2 4 ,2 6 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2
Ag
99 110 120 99 260 99 120 110 99 99 99 99 390 99 99 170 280 99 180 120 , 100 , 100 99 360 130 340 120 120 250 147
Zn
3.0 2.0 4.0 1.0 5.0 2.0 2.0 3.0 3.0 3.0 7.0 ,1 5.0 2.0 1.0 2.0 3.0 2.0 ,1 ,1 2.0 1.0 2.0 7.0 0.9 1.0 1.0 1.0 1.0 2
Mo
Table 4 Trace elements (ppm) in beach sands
, 10 25 , 10 , 10 79 , 10 , 10 , 10 18 , 10 , 10 , 10 46 , 10 , 10 62 81 38 48 44 , 10 30 29 17 , 10 60 43 , 10 , 10 21
Ni 37 31 39 24 72 10 18 24 23 10 39 9 44 16 12 40 92 41 30 47 19 21 21 39 25 79 45 56 54 35
2.5 3.8 2.9 2.2 8.1 2.9 5.1 2.5 4.3 4 5.7 2.9 22 5.8 4.5 2.2 5.4 4.2 4.1 2.3 5.8 6.3 6.2 1.9 3.8 3.1 3 2.4 2.6 5
Co As
Ba
Cr
W
0.8 230 87 116 1 410 150 140 0.5 280 200 220 0.2 410 34 264 2.3 69 920 82 0.5 520 52 69 0.2 560 48 41 0.3 940 110 110 1 1000 20 284 0.4 880 29 101 0.9 580 100 231 0.3 1100 42 88 4.6 650 290 71 0.7 830 29 110 0.8 700 22 138 0.3 390 190 101 0.3 50 630 39 0.2 260 81 46 0.3 370 120 30 0.2 230 150 87 1.4 450 88 122 1.3 470 81 165 1.1 350 770 34 0.3 150 300 74 0.5 480 27 16 0.6 300 150 19 0.6 480 55 86 0.5 460 82 83 0.6 400 92 95 1 483 171 106
Sb
La
Ce
Sm
, 0.5 29 89 15.8 , 0.5 34 75 13.9 , 0.5 85 170 20.5 , 0.5 9 14 3.2 , 0.5 110 200 19.1 1.4 18 33 4.5 0.8 14 24 3.3 3.6 21 39 4.4 1.6 7 17 1.4 1.3 23 44 5.3 1.6 150 310 32 4.8 12 23 2.3 2.9 53 89 9.1 1.5 18 37 3.8 0.6 16 28 3.5 0.8 11 22 3.6 0.5 13 23 4.1 0.6 19 47 7 0.9 15 29 4.8 0.5 11 24 4.6 2.1 13 29 2.9 2.4 13 24 2.8 0.8 65 110 8.7 0.5 170 300 23.5 1.2 15 27 3.8 0.9 33 57 5.9 0 24 46 4 0.9 27 54 5 1.1 31 60 5.9 1 37 70 8
Cs 2 ,1 3 ,1 3 ,1 2 1 ,1 ,1 4 ,1 ,1 ,1 ,1 1 2 ,1 ,1 1 ,1 ,1 ,1 2 ,1 ,1 ,1 1 1 1
Eu 2 2 3 , 0.5 3 , 0.5 , 0.5 1 , 0.5 1 3 , 0.5 1 , .5 , .5 , .5 1 1 , 0.5 , 0.5 , 0.5 , 0.5 1 2 1 1 , 0.5 1 1 1
Tb 10 9 10 ,2 17 3 ,2 3 ,2 3 5 ,2 7 ,2 3 2 4 3 3 2 ,2 ,2 5 13 ,2 2 ,2 2 ,2 4
Yb 1.1 1.1 1.1 0.3 2.5 0.2 , 0.2 0.3 , 0.2 0.4 0.6 , 0.2 1 , 0.2 0.3 0.2 0.3 0.4 0.3 0.3 0.2 , 0.2 0.7 1.9 0.2 0.3 0.3 , 0.2 0.3 0
Lu
Hf
48 30 41 34 32 70 12 4 40 278 11 6 15 2 15 5 3.9 4 5.8 5 16 8 3.9 3 44 45 11 5 5.9 4 24 2 41 30 38 4 26 3 36 3 10 3 8.4 3 21 40 55 220 22 2 41 5 22 4 26 5 28 8 24 29
Sc 3.7 3.1 5.1 0.9 7.9 1 0.8 1 , 0.5 0.6 6.1 0.6 4.2 0.8 0.9 0.6 1.8 0.6 , 0.5 0.8 1 0.7 3.1 11 , 0.5 1 0.6 0.9 0.7 2
Ta
U
13 5 16 5.2 45 11 1.5 0.8 63 21 3.7 4.7 1.5 1.2 6.6 1.8 2.1 1 6.6 2.9 38 8.5 4.9 1.2 10 5.8 6.9 2 3.6 1.4 2 0.7 1.1 1.1 1.1 0.6 2 0.6 0.9 0.5 3.5 1.5 3.7 1.4 24 6.8 84 16 2.8 1.1 16 3.2 6.1 1.5 13 2.8 11 2.6 14 4
Th
5 13 4 17 36 19 23 23 10 15 12 19 1 12 4 17 1 17 2 5 1 1 21 , 0.5 6 1 1 15 1 10
Br
Zr , 0.5 930 16 1000 , 0.5 2600 28 200 , 0.5 10000 27 200 23 200 87 200 34 200 91 200 63 200 55 200 44 1800 78 200 63 300 30 200 , 0.5 940 , 0.5 200 , 0.5 200 , 0.5 200 50 200 63 320 25 1500 , 0.5 8100 39 200 23 200 39 200 39 380 25 200 32 1085
Rb
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Table 5 Detection limits of analyzed elements Element
Limit
Antimony Arsenic Barium Cadmium Cerium Desium Chromium Cobalt Europium Gold Hafnium Iridium Iron Lanthanum Lutetium Molybdenum Nickel Rubidium Samarium Scandium Selenium Silver Sodium Tantalum Tellurium Terbium Thorium Tin Uranium Yterbium Zinc Zirconium
0.1 ppm 0.5 ppm 50 ppm 5 ppm 5 ppm 0.5 ppm 20 ppm 5 ppm 1 ppm 2 ppb 1 ppm 50 ppb 0.20% 2 ppm 0.2 ppm 1 ppm 10 ppm 5 ppm 0.1 ppm 0.2 ppm 5 ppm 2 ppm 0.02% 0.5 ppm 10 ppm 0.5 ppm 0.2 ppm 100 ppm 0.2 ppm 2 ppm 100 ppm 200 ppm
Fig. 3. Ternary diagram for total quartz (Qt), total feldspar (Ft) and heavy minerals (HM).
Fig. 2. Correlations among the studied beach sands: (a) sorting and grain size; (b) rock fragments and grain size; (c) grain size and heavy minerals; and (d) rock fragments and heavy minerals.
A. Carranza-Edwards et al. / Journal of South American Earth Sciences 14 (2001) 291±305
Fig. 4. Diagram of whole sediment Fe2O3 and TiO2 contents.
297
the Baja California Peninsula. This region is composed of short rivers (1±2 order) that drain areas mainly of lithology types 1 and 4. Locally, the Vizcaino Peninsula (sample 6) is made up of uplifted subduction complexes (Sedlock et al., 1993). B. Samples 7±12 were collected from the eastern Baja California Peninsula, which is made up of lithologies of types 1, 7, and 2. The drainage is mostly made up of short streams (1±2 order). However, this coastal area might be in¯uenced by the sedimentary discharge of the Colorado River (sample 12). C. Sample 13 was collected from beaches next to long important rivers (.4 order) that drain areas formed
Table 6 Average values of some elements for the studied gray sands (numbers in brackets samples with maximum values) Element
Zn Mn Ni Co As Sn Ba Cr Cs Sc Hf Ta Th U Rb Zr a
Avg. Cont. Crust a
This work Average
Maximum
147.00 2.31 21.40 35.00 5.00 0.78 482.70 171.00 1.15 24.10 28.80 2.05 13.60 3.93 32.50 1085.20
360 [24] 7 [24,11] 81 [17] 92 [17] 2 [13] 2.3 [5] 1100 [11] 770 [23] 4.8 [11] 54.6 [24] 27 [5] 11 [24] 84.2 [24] 21 [5] 91 [10] 10000 [5]
Average Enriched
80 1 105 29 1 0.2 250 185 1 30 3 1 3.5 0.91 32 100
X X X X X
X X X X XX
Average ratios Depleted
X
1.8 2.3 0.2 1.2 5.0 3.9 1.9 0.9 1.2 0.8 9.6 2.1 3.9 4.3 1.0 10.9
Taylor and Mc Lennan (1985).
Fig. 5. Ternary diagram and sediment color: Lg light gray; Lbg light brownish gray; G gray; vdg very dark gray; dg dark gray.
298
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Fig. 6. Rare earth elements normalized to chondrites: (a) REE for Group I are enriched in LREE and are similar to those observed in granites; (b) REE for Group II, with patterns similar to the NASC (North American shale composite), which is represented by the dotted area. This suggests that the sands were derived from evolved old sources.
mostly by Tertiary rhyolites of the Sierra Madre Occidental (type 2), and some of types 3 and 6 (Ortega-GutieÂrrez et al., 1992). D. Samples 14±22 come from beaches related to areas with medium length streams (2±3 order), which are related to areas formed by volcanic rocks of the active Mexican Volcanic Belt (type 3), and types 4 and 1 (Sedlock et al., 1993; Centeno-GarcõÂaetal.,1993;Lapierreetal.,1995).Sample22 was collected next to a long river (.4 order) that carries sediments from variable sources (types 7, 4, 2, 3, and 1). E. Locations 23and 24belong to areas with short streams (1± 2 order), made up of granitoids and migmatites of the Mesozoic Xolapa terrane (Schaaf et al., 1995). F. Samples 25±29 were collected from the southern Paci®c coast; they are related to small ¯uvial basins with mediumsize streams (2±3 order) and mostly sources made up of the Proterozoic to Paleozoic granitoid complex (Chiapas batholith) (Ortega-GutieÂrrez et al., 1992.
3. Material and methods Surface sand samples were collected with a plastic shovel on beaches in western Mexico. These sands are whole samples that were selected according to their colors (gray or dark colors), because they are suitable for high concentration of heavy minerals. The colors of the beach sands was determined in dry sample using the Munsell (1975) color charts; they are listed in Table 1. To determine the mean graphic size (Mz) and the standard graphic deviation (sorting) (Table 1), the sands were quartered and sieved following procedures and formulas suggested by Folk (1974). Petrologic analyses (Table 2) involved counting 300 grains divided into monocrystalline quartz (Qm), polycrystalline quartz (Qp), potassium feldspar (Fk), plagioclase feldspar (Fp), rock fragments (Rf), chert (Ch), mica (Mi), and heavy minerals (HM) according to the procedures and criteria outlined by Franzinelli and Potter (1985).
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299
Fig. 7. Chondrite normalized REE diagrams for Group III, showing more depletion in LREE and patterns similar to volcanic rocks from the accreted Mesozoic terranes. The dotted area represents REE concentrations of volcanic rocks from the Guerrero Terrane (Centeno-GarcõÂa et al., 1993).
Major element analysis (Table 3) of beach sand pressed pellets was performed in a Siemens sequential X-ray spectrometer SRS 3000. Multi-element geochemical analysis (Table 4) of elements was done by Bondar Clegg Inchcape Testing Services by direct irradiation/INAA of vials of prepared sand samples. Analytical detection limits of the trace elements are listed in Table 5. 4. Results and discussion Table 1 shows that the studied beach sands range from coarse sands to very ®ne sands, according to their mean graphic size values. Fine size beach sands are mostly well sorted (Fig. 2a), which seems to be the result of persistent and high wave energy conditions causing an ef®cient reduction of particle grain size. Although there is slight correlation between mean
graphic size (2log2 (mm)) and rock fragment content (Tables 1 and 2, Fig. 2b), this does not follow a speci®c path. There is no clear relationship between grain size and heavy mineral content, suggesting that their abundance might be controlled by other factors such as sand provenance (Fig. 2c). However, heavy minerals are inversely related to rock fragment content (Tables 1 and 2, Fig. 2d). This distribution might be related to abrasion and settling of the particles associated with wave energy and density of the grains. Fig. 3 shows HM-Ft-Qt ternary diagrams for all the samples divided in groups by their geological setting. Samples 1±17 show a wide range in heavy mineral content. In contrast, samples 18±29 are highly enriched in heavy minerals. Fe2O3 and TiO2 coexistence in the same mineral facies is suggested by their statistically signi®cant correlation (Fig. 4, Tables 3 and 6). The association between magnetite and ilmenite has been previously reported for Paci®c beaches
300
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Fig. 8. (a) Chondrite normalized REE diagram for Group III; the dotted area represents REE concentrations of volcanic rocks from the Guerrero Terrane (Centeno-GarcõÂa et al., 1993). (b) Chondrite normalized REE diagram for Group IV, showing variable patterns that suggest more in¯uence of mineral content than of sand provenance.
Fig. 9. La/Yb ratio and heavy mineral variations.
and is related to plutonic and metamorphic sources (Carranza-Edwards, 1980; MartõÂn-Barajas, 1980; Carranza-Edwards et al., 1988; Basu and Molinaroli, 1989). Fig. 4 shows a strong correlation between high Fe2O3 and TiO2 content and type 1 source-rocks. Table 3 shows that the averages of the major elements are more or less similar to those obtained by Potter (1986) for the South American Paci®c association, with the exception of MnO, TiO2, and Fe2O3, which are enriched in the gray sands we studied. These differences can be explained by the fact that, in gray sands, magnetites and ilmenites are concentrated by the energy provided by big waves. The maximum Fe2O3 value was 45.80%, and the maximum for TiO2 was 23.00%, both in the same beach location (site 5). Data from the concentration of major elements (Table 3) were used to construct the ternary diagram of Fig. 5. Samples with very dark gray and dark gray colors are shifted towards the Fe2O3 1 TiO2 pole, whereas the lighter colors
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301
Fig. 10. Element concentrations (ppm) according to sampling site. Th, Sc, Hf, and Ni values are represented by dotted lines. The plain lines are heavy mineral variations.
belong to polygons that are nearest to the SiO2 pole. Apparently the beach sediments with high heavy mineral contents are darker and enriched with resistant minerals with high Fe2O3 and TiO2 values (mostly magnetite and ilmenite). The chemical index of alteration (CIA) was determined (Table 3) according to Nesbitt and Young (1984). Locations from arid climate (sites 1±12) average lower CIA values (46.99) than those from humid climate (sites 13±29), which have a CIA average of 52.18. Although the difference is not signi®cant, it implies that the higher precipitation rates of the southern portion produce a higher CIA. Rare earth element data were chondrite-normalized using Evensen (1978) values. The 29 sand samples were divided into four groups based on their differences in REE patterns. High REE concentrations of economical importance were not found. Most of the sample patterns suggest that sand REE content is controlled by composition of source rocks more than by percentage or type of heavy mineral content.
Samples from Group I (1, 2, 3, 5, 13, 23, and 24; Fig. 6) show a pattern that is approximately ¯at to enriched in light rare earth elements (LREE) and ¯at in heavy rare earth elements (HREE), with average normalized values of 100 or higher but with a strong Eu anomaly. Similar patterns are shown by monzogranites and syenogranites (Wilson, 1989), as well as by muscovite-rich rocks. The variable mica content (0±11%) and the proportions between HM (48± 92%) and Q 1 F (10±28%) suggest that the REE composition of sands from Group I might be controlled by their provenance. Their geographic location (samples 1±5), near the insular range batholith of Baja California and next to the Xolapa Terrane (samples 23 and 24), supports this interpretation. Samples from Group II (25±29) are less enriched (Fig. 6) in LREE (LaN 100±40), with Eu anomaly, and down-slope HREE pattern (HREE . 10) (Fig. 6). Samples from Group II have the same modal variability of HM, Q and F as Group
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Table 7 Signi®cant correlation coef®cients (99%) (Qm monocrystalline quartz, Qp polycrystalline quartz, Fk potassic feldspar, Fp plagioclase feldspar, Rf rock fragments, Ch chert, Mi mica, Mz mean graphic size, HM heavy minerals, PI provenance index, MI maturity index.) SiO2 (1) SiO2 (2) Al2O3 (1) Al2O3 (2) Fe2O3 (1) Fe2O3 (2) TiO2 (1) TiO2 (2) HM (1) HM (2) Qm (1) Qm (2) Mz (1) Mz (2) Rf (1) Rf (2) Fp (1) Fp (2)
Ba (0.61), K (0.59), Qm (0.59), CIA (0.56), Rb (0.53), Rf (0.50), Qp (0.46) Fe (0.90), Ti (0.77), Hf (0.71), Zr (0.70), Sc (0.69), Co (0.69), Zn (0.67), Cr (0.67), HM (0.61), V (0.60), Yb (0.59), Ni (0.57), Th (0.57), Ta (0.53), La (0.46) Na (0.93), K (0.51), CIA (0.49) Yb (0.71), Hf (0.68), U (0.66), Ta (0.65), Th (0.63), Fe (0.62), Tb (0.62), CR (0.58), M0 (0.57), La (0.52), Ce (0.51), Sm (0.50) Ti (0.83), Cr (0.80), Co (0.80), Hf (0.77), Sc (0.73), U (0.70), Zn (0.70), HM (0.70), Yb (0.69), Th (0.64), Ni (0.63), Ta (0.61), Tb (0.54), La (0.51), Ce (0.47) Si (0.90), Ba (0.69), K (0.67), Na (0.63), Al (0.62), Rb (0.58), Qm (0.56), Rf (0.53), Chert (0.51), Fp (0.48) Hf (0.95), Zr (0.95), Th (0.90),, Ta (0.88), U (0.87), Yb (0.83), Fe (0.83) La (0.77), Ce (0.72), M0 (0.67), Cr (0.65), Tb (0.65), Zn (0.63), Sc (0.62), Sm (0.61), HM (0.57), Eu (0.52), Al (0.68), Na (0.65), K (0.56), Ba (0.53), Rf (0.46) Sc (0.79), Fe (0.70), Co (0.66), Zn (0.62), Ti (0.57), Ni (0.55), Cr (0.54), Mz (0.49), Hf (0.48), Zr (0.47), Yb (0.47) Rf (0.87), Ba (0.76), Qm (0.66), Si (0.61), K (0.61), Fp (0.60), Rb (0.65), Na (0.52), Ch (0.52) MI (0.66), Si (0.56), Rf (0.58), Ba (0.47) Zn (0.62), Co (0.58), Fe (0.56), Ni (0.53), Hf (0.51) HM (0.49) s (0.74), Rf (0.63) Ba (0.67), Si (0.50), K (0.48), Qp (0.45) Sc (0.66), PI (0.64), Mz (0.63), Fe (0.53), Cr (0.48) Qm (0.58), Na (0.55), Al (0.47), Ba (0.47) Co (0.59), Sc (0.53), Fe (0.48), Zn (0.48)
I. Therefore; the more suitable factor controlling the REE content is the geology of the source areas. The REE patterns of sands from Group II (Fig. 6) are similar to those proposed for post-Archean shale composites (NASC: the North American shale composite) (Taylor and Mc Lennan, 1985). This similarity suggests that the sands of Group II have either been homogenized by the sedimentary processes or derived from homogenized evolved rock. Samples from Group II are close to mountain ranges formed by Proterozoic to Paleozoic granitoids (localities 25±29). Samples of Group III (Figs. 7 and 8) (4, 6, 8, 10, 12, and 14±22) are more depleted in REE. Normalized LREE values are smaller than NASC (LaN , 50), with ¯at HREE patterns. These samples have mostly a small or no Eu anomaly. Heavy mineral percentages vary from 12±93% and, overall, they have low quartz percentages (,20%). Source rocks for the sands of Group III are mainly formed by accreted Mesozoic oceanic island-arc sequences. Volcanic rocks from these source areas have similar juvenile REE patterns (Figs. 7 and 8) (Centeno-GarcõÂa et al., 1993), suggesting that the source rock is controlling the sand geochemistry. However, samples 8, 10, and 12, are located in regions where granitic and volcanic rocks with high REE values crop out. Sand samples from Group IV (7, 9, 11) show irregular REE patterns, with a positive Eu anomaly (Fig. 8). Their pattern might be related to an abundance of Eu enriched minerals, such as feldspar and magnetite (Henderson, 1984). The nature of the source rock of the studied sands seems, generally, more important in the control of REE composition than the concentration of heavy minerals. This is suggested by the distribution of La/Yb vs. HM%, as shown in Fig. 9. Samples 1±6 from the western coast of
the Baja California Peninsula show low La/Yb ratios, although HM% has a wide range. Samples 7±12 from the eastern coast of the Baja California Peninsula show an increment in La/Yb with increment in HM%. Samples 13±23, which are associated with juvenile volcanics, have low La/Yb values that do not undergo a major change with increments in HM%. In contrast, samples 24±29 have much higher La/Yb values that do not change with increments in HM%. The high La/Yb ratios might be related to the `evolved' nature of the source rock (Precambrian basement and sedimentary cover) that forms the southern part of the Paci®c Coast. The same interpretation can be inferred from other trace elements, such as Th, Sc, and Hf. Fig. 10 shows that Th concentrations are independent of HM%, since Th behaves in diverse ways. However, there is an approximate relationship between Th and the nature of the source sample (Fig. 10). Samples with Th concentrations similar to or higher than NASC (.10 ppm) are related to areas with granitoids and Precambrian or sedimentary rocks. Similar concentrations have been described for deep-marine turbidites associated with collision/strike slip, continental arc, and trailing edge (Mc Lennan et al., 1990). In contrast, samples with low Th content (,10) belong to areas associated with accreted island-arc or recent volcanic rocks and are similar to those from sediments associated with recent fore-arc areas (Mc Lennan et al., 1990). Sc content seems to have a direct relationship with heavy mineral content, since its concentration increases with increments of HM% (Fig. 10). Samples with high HM% (.60%) show Sc concentrations (.30) much higher than those reported by Mc Lennan and Taylor (1991) for oceanic sediments and upper crust (4±30) (Taylor and Mc Lennan 1985) as shown in Table 6.
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303
Fig. 11. Element concentrations (ppm) according to sampling site. Ta, Zr, Cr, U, and Zn values are represented by dotted lines. The plain lines are variations in heavy mineral percentages.
However, there is some relationship with provenance since, in samples with the same HM%, Sc concentrations are higher on the central Paci®c coast than in the southern Paci®c (Fig. 10). Although Hf is common in heavy minerals, the studied sand samples show Hf content associated with both percentage of heavy minerals and nature of the source (Fig. 10, Table 7). Samples enriched with heavy minerals from areas with granitic or sedimentary sources are enriched in Hf. However, samples with high HM% collected from areas with volcanic or accreted
oceanic-arc sources do not have Hf enrichment (Fig. 11d). Concentrations of Ni are lower, overall, than NASC (Ni 55±95) and upper crust (30) in most samples (.30 ppm) (Fig. 10). Nine samples show Ni concentrations between 30 and 90 ppm, mostly those with high heavy mineral content (Fig. 10). Low Ni contents are common in volcanic rocks (25 ppm in island arcs) (Taylor and Mc Lennan, 1985); thus, low concentrations in beach sands might be related to depletion in their source. The abundance of other elements, such as Zn and Cr,
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Table 8 Variations of petrological data and CIA (Qt quartz, Fk potassic feldspar, Fp plagioclase feldspar, Rf rock fragments, Ch chert, Mi mica, H.M. heavy minerals, PI provenance index, MI maturity index, CIA chemical index of alteration: Al2O3/(Al2O3 1 Na2O 1 CaO 1 K2O) £ 100, vdg very dark gray, dg dark gray, lbg light brownish gray, lg light gray)
Color of dry sand Grain size
Sorting Main source Wave energy Climate
vdg and dg Gray lbg and lg Coarse Medium Fine Very ®ne , 0.71 . 0.71 Metamorphic Plutonic Volcanic High Moderate Low Arid Humid
Qt
Ft
Rf
Ch
Mi
HM
PI
MI
CIA
2.8 8.2 12.0 13.3 5.9 8.4 7.0 7.8 8.4 7.0 8.9 7.5 7.5 8.6 7.8 11.9 5.2
8.0 10.4 22.2 18.7 11.6 14.6 12.0 14.1 13.3 10.6 13.6 15.3 9.0 13.2 22.4 19.8 9.6
17.6 27.4 26.1 50.7 27.3 18.6 14.7 20.2 32.0 22.6 25.0 24.2 22.0 29.8 29.0 27.7 21.8
1.8 2.2 4.1 4.7 3.3 2.2 1.3 2.3 3.5 3.8 2.6 2.4 1.0 5.8 2.6 2.3 3.0
0.4 1.2 0.7 0.0 2.1 0.3 0.0 0.3 1.7 0.2 1.1 0.8 6.0 0.6 1.0 1.5 0.3
69.4 51.6 34.9 12.7 50.9 55.8 65.0 55.3 42.1 55.8 49.6 49.8 59.5 42.0 37.2 37.6 60.1
0.72 0.64 1.01 0.51 0.49 1.04 0.78 0.92 0.53 0.64 0.85 0.78 0.42 0.49 0.78 10.02 63.00
0.13 0.19 0.26 0.19 0.14 0.29 0.16 0.25 0.17 0.20 0.22 0.22 0.28 0.17 0.13 0.28 0.17
48.16 50.53 51.28 42.28 53.21 49.78 49.43 50.23 49.65 54.31 50.54 47.74 49.11 51.96 50.39 46.99 52.35
seems to be more related to heavy mineral content. The samples with the highest content of Zr, Ta, and U are related to samples with more than 60% heavy mineral content (Fig. 11). Only samples 5, 13, 23, and 24 show high concentrations of such elements. They seem to be related to coastal areas with granitoids. In contrast, sands collected near areas with volcanic rocks have lower concentrations of Zr, although their heavy mineral content is high (Fig. 11). Only one sample (site 8) shows a gold content three times higher than the average continental crust (Table 4). Compared with values reported by Wedepohl (1995), two samples (sites 2 and 4) are enriched 40±60 times in silver, respectively. They are probably elements disseminated in rocks from the Alisitos Terrane described by Campa and Coney (1983). Table 8 summarizes variations in petrological data and CIA. The higher concentration of heavy minerals is associated with beach sands that are very dark gray and dark gray in color, whereas the provenance index (PI F/ Rf) and maturity index (MI Q/F 1 Rf) show higher values in association with light brownish gray and light gray colors.
5. Conclusions The content of heavy minerals is not related to grain size, sorting, or sand provenance. Its variation is probably related to settling conditions of particles and local sedimentologic processes. The main source does not seem to play an important role in the distribution of heavy minerals or in the observed CIA values. In contrast, high wave energy and a
humid climate seem to be responsible for the heavy mineral content and CIA values, respectively. There is a higher content of heavy minerals in humid zones than in arid regions, particularly in the well sorted and ®ne beach sands that are dark gray in color, which contain higher concentrations of iron, titanium, and zirconium. These sands are rich in REE. The metal content was more abundant in regions from beach sands affected by high-energy waves. REE and other trace element concentrations, such as Th, Sc, and Hf, seem to be more associated with source composition than to heavy mineral content. Concentrations of other elements, such as Ni, Ta, Zr, Cr, and major elements, are more dependent on heavy mineral content. The results suggest that the studied beach sands have little economic interest. However, they are good indicators of the nature of the source rock and, as such, can be used indirectly for large-scale mineral exploration. Acknowledgements This work was partially supported by UNAM-CONACYT Project 3477T. We thank the Instituto de Ciencias del Mar y LimnologõÂa for support of the project entitled `Sedimentology of Mexican Beaches.' We are grateful to Susana Santiago and Eduardo Morales for their collaboration in laboratory activities. References Basu, A., 1976. Petrology of Holocene ¯uvial sand derived from plutonic source rocks: implications for paleoclimatic interpretation. Journal of Sedimentary Petrology 46 (3), 694±709.
A. Carranza-Edwards et al. / Journal of South American Earth Sciences 14 (2001) 291±305 Basu, A., Molinaroli, E., 1989. Provenance characteristics of detrital opaque Fe±Ti oxide minerals. Journal of Sedimentary Petrology 59 (6), 922±934. Byrne, J.V., Emery, K.O., 1960. Sediments of the Gulf of California. Bulletin of the Geological Society of America 71, 983±1010. Campa, M.F., Coney, P.J., 1983. Tectono-stratigraphic terranes and mineral resource distribution in Mexico. Canadian Journal of Earth Sciences 20, 1040±1051. Carranza-Edwards, A., 1980. Ambientes sedimentarios recientes de la llanura costera sur del Istmo de Tehuantepec. Anales de la Centro Ciencias del Mar y LimnologõÂa, Universidad Nacional AutonoÂma de MeÂxico 7 (2), 13±66. Carranza-Edwards, A., Rosales-Hoz, L., Lozano-Santa Cruz, R., 1988. Estudio sedimentoloÂgico de playas del Estado de Oaxaca, MeÂxico. Anales de la Centro Ciencias del Mar y LimnologõÂa, Universidad Nacional AutonoÂma de MeÂxico 15 (2), 23±38. Carranza-Edwards, A., Reinoso, H., Rosales-Hoz, L., 1992. Sedimentological study of beaches from Central Ecuador. Revista de la Sociedad Mexicana Historia Natural 43, 15±24. Centeno-GarcõÂa, E., RuõÂz, J., Coney, P., Patchett, J.P., Ortega-GutieÂrrez, F., 1993. Guerrero Terrane of Mexico: its role in the Southern Cordillera from new geochemical data. Geology 21 (5), 419±422. Clifton, H.E., 1968. Gold Distribution in Surface Sediments on the Continental Shelf off Southern Oregon: A Preliminary Report. US Geological Survey, Circular C-0587, 6 p. Cronan, D.S., 1980. Underwater Minerals (Ocean Science, Resources and Technology: An International Series). Academic Press, London (362 p.). Darby, D.A., 1984. Trace elements in ilmenite: a way to discriminate provenance or age in coastal sands. Bulletin of the Geological Society of America 95, 1208±1218. Davies Jr., R.A. (Ed.), 1978. Coastal Sedimentary Environments Springer, New York. Dubois, R.N., 1972. Inverse relation between foreshore slope and mean grain size as a function of the heavy mineral content. Bulletin of the Geological Society of America 83, 871±875. Evensen, M.N., 1978. Rare earth abundances in chondritic meteorites. Geochimica et Cosmochimica Acta 42, 1203. Folk, R.L., 1974. Petrology of Sedimentary Rocks. Hemphill's, Austin, TX (159 p.). Franzinelli, E., Potter, P.E., 1985. Petrology, chemistry and texture of modern river sands, Amazon River system. Journal of Geology 91, 23±39. Henderson, P. (Ed.), 1984. Rare Earth Element Geochemistry (Developments in Geochemistry 2) Elsevier, Amsterdam (510 p.). Komar, P.D., 1976. Beach Processes and Sedimentation. Prentice-Hall, Englewood Cliffs, NJ (429 p.). Kunzendorf, H. (Ed.), 1986. Marine Mineral Exploration (Elsevier Oceanography Series) Elsevier, Amsterdam (300 p.). Lapierre, H., Ortiz, L.E., Abouchami, W., Monod, O., Coulon, C., Zimmermann, J.L., 1995. A crustal section of an intra-oceanic island arc: The Late Jurassic-Early Cretaceous Guanajuato magmatic sequence, central Mexico. Earth and Planetary Science Letters 108, 1±77. MartõÂn-Barajas, A., 1980. DistribucioÂn de minerales pesados en placeres de playa en una porcioÂn del litoral de los estados de Guerrero y Oaxaca. VIII Seminario Interno del Consejo de Recursos Minerales. Unpublished Report, Mineral Resources Council (Mexico), pp. 183±211.
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www.elsevier.com/locate/jsames
Provenance of beach gray sands from western MeÂxico A. Carranza-Edwards a,*, E. Centeno-GarcõÂa b, L. Rosales-Hoz a, R. Lozano-Santa Cruz b a
Instituto de Ciencias del Mar y LimnologõÂa, Universidad Nacional AutonoÂma de MeÂxico, Ciudad Universitaria, Mexico DF 04510, Mexico b Instituto de GeologõÂa, Universidad Nacional AutonoÂma de MeÂxico, Ciudad Universitaria, Mexico DF 04510 Mexico Received 1 October 1999; revised 1 July 2000; accepted 1 January 2001
Abstract Twenty-nine sur®cial beach dark colored sand samples from about 6500 km of the western littoral of Mexico were chemically and petrologically analyzed to search for the relationship between their heavy mineral content and metal and rare earth element variations. Sand samples were dried and grouped according to color in three classes of gray. In order to search for a possible in¯uence on petrological and chemical variations, the sands were also grouped according to grain size, sorting, composition, main sources, wave energy, and climate. It was observed that quartz, feldspars, and rock fragments averaged low values and, as the sand darkened, the heavy mineral content increased. Also, the higher content of heavy minerals in dark gray and very dark gray sands correlated with a humid rather than arid climate. Rare earth element and other trace element contents seemed to be related more to the composition of the source area than to heavy mineral content. However, some elements, such as Zn, Hf, Th, Ni, and Cr, did relate more to heavy mineral content. Geochemical and petrological analyses of these sands suggest that most are derived from juvenile sources. Sedimentary processes such as wave energy appear to play a signi®cant role locally in heavy mineral content, making the energy conditions of the beach environment an important local factor. Concentrations of rare earth and other elements are not of economic interest. q 2001 Published by Elsevier Science Ltd.
Resumen Con la ®nalidad de entender la relacioÂn entre la concentracioÂn de minerales pesados y las variaciones en metales y elementos de tierras raras, se analizaron quõÂmicamente y petroloÂgicamente 29 muestras de arenas recientes de playa, correspondientes al litoral occidental de MeÂxico, a lo largo de 6500 km. Las muestras de arenas fueron secadas y agrupadas por color en tres clases de grises. Para explicar la posible in¯uencia en las variaciones petrologicas y quõÂmicas, las arenas se agruparon de acuerdo con el tamanÄo de grano, composicioÂn, principales fuentes, altura de oleaje y clima. Se observo que el cuarzo, los feldespatos y los fragmentos de roca dan bajos promedios y cuando la arena es maÂs oscura, entonces el contenido de minerales pesados se incrementa. AdemaÂs el contenido maÂs alto de minerales pesados (en arenas grises oscuras y grises muy oscuras) se asocian maÂs con un clima huÂmedo que con un clima aÂrido. Las concentraciones de tierras raras y otros elementos traza parecen relacionarse con sus areas fuente, maÂs que los contenidos de minerales pesados. Sin embargo, la concentracioÂn de algunos elementos, tales como Zn, Hf, Th, Ni, y Cr parecen vincularse maÂs con el contenido de minerales pesados. El anaÂlisis petroloÂgico y geoquõÂmico de las arenas sugiere que la mayorõÂa de ellas se derivaron de fuentes juveniles. Los procesos sedimentarios, tales como energõÂa del oleaje, juegan un papel signi®cante localmente en lo que se re®ere al contenido de minerales pesados, haciendo que las condiciones de energõÂa del ambiente de playa sean un factor importante. Los valores de tierras raras y otros elementos no son lo su®cientemente altos como para ser de interes econoÂmico. q 2001 Published by Elsevier Science Ltd. Keywords: Beach sands; MeÂxico; Metals; Rare earth elements; Gray sands; Paci®c; Black sands
1. Introduction Placer beach sands are considered a source of metals of potential economic interest (Mero, 1965; Wright and Fairbridge, 1977; Cronan, 1980; Kunzendorf, 1986). * Corresponding author. Tel.: 152-5-622-5817; fax: 152-5-616-0748. E-mail address: [email protected] (A. CarranzaEdwards). 0895-9811/01/$ - see front matter q 2001 Published by Elsevier Science Ltd. PII: S 0895-981 1(01)00028-1
Mechanical concentration of resistant heavy minerals takes place in the beach environment, mostly under high energy conditions (Shepard, 1967; Dubois, 1972; Komar, 1976; Davies, 1978; Potter, 1986, 1994; Carranza-Edwards et al., 1992). According to Basu (1976) and Basu and Molinaroli (1989), the petrological composition of recent sediments is mostly controlled by the original source rocks. In the beach environment, metal-rich sands are generally found
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Fig. 1. Simpli®ed geologic and hydrologic map of the study area. The thick lines represent limits of lithologies, medium lines are drainage divides, and the thin lines are main rivers and streams. Dots represent sites of sampled beach locations. Key: bold numbers are sampling stations; small numbers 1±7 are the tectonic±lithologic types shown and discussed in the text; A, B, C, D, E, and F are the main geologic regions sampled.
in the upper beach zone, or backshore (Kunzendorf, 1986), where waves lead to concentrations of high-density minerals that are not easily removed back to the sea by the low energy backwash ¯ow. Mero (1965) was one of the ®rst authors to point out the importance of placer sands from the beach as a potential source of metal-rich marine deposits. The study of these deposits is of interest in that not only does it identify placer sands as an eventual and direct source of metals, it also suggests the presence of the metals in the coastal zone or in the continental shelf (Byrne and Emery, 1960; Van Andel, 1964). Gold distribution anomalies in the Oregon State continental shelf analyzed by Clifton (1968) are an example of such deposits. In metal-enriched beach sands, it is common to ®nd minerals such as magnetite, ilmenite, pyroxenes, zircon, monazite, and others, with concentrations of elements such as gold, silver, and rare earth elements (REE) (Darby, 1984). The economic importance of REE has increased in the last few years. REE of commercial importance are Th, Ce, La, Nd, Pr, Gd, Sm, and Eu (Neary and Highley, 1984). Placer deposits are of major interest, as they constitute one of the main sources
of REE, since they are formed by accumulations of unaltered metal-bearing debris. The possibility of native elements to generate placer deposits is common only among the noble metals: the gold and platinum groups and a few metals that develop a suf®ciently impervious oxide coating to resist total oxidation. In addition, some metal oxides, such as rutile and ilmenite, have accumulated in large amounts in beach sands at many localities around the world. The purpose of this work is to analyze the textural, petrological, and chemical composition of beach gray sands of the western littoral of Mexico, in order to understand the possible relationships between their metal content and different natural conditions. One of the goals of this study was to search for possible placer deposits as indicators for exploration of inland deposits of economic value. In addition, the relationship between sand composition and the source of the sediments is discussed. 2. Study area Twenty-nine beach locations were sampled on the
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Table 1 General data on beach sands from the western littoral of Mexico (source: v volcanic, p plutonic, m metamorphic; wave height: H high (.100 cm), M moderate (.30 cm± , 100 cm), L low (,30 cm)) Sample
Beach locations
Color of dry sample
Mz (phi)
Sorting (phi)
Related climate
Main source
Wave height
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
Playas de Tijuana Primo Tapia San RamoÂn San Quintin Tomatal Guerrero Negro Ensenada Blanca BahõÂa de los Angeles Calamajue Santa Teresa Playa Mexico San Felipe Los Corchos Chalacaltepec Tecuan Apiza San Juan de Lima Las Risas San Telmo Mexiquillo Bonanza Petacalco Papanoa Colotepec Boca del Cielo Palmarcito Zapotal La Barrita Barra del Suchiate
Gray Gray Gray Light gray Dark gray Light gray Light gray Light brownish gray Gray Light brownish gray Light gray Light gray Gray Light gray Gray Dark gray Very dark gray Gray Gray Dark gray Gray Light brownish gray Light gray Dark gray Gray Dark gray Dark gray Gray Dark gray
2.01 2.45 1.51 2.71 2.70 3.06 2.66 1.44 0.42 0.81 2.13 2.69 3.06 2.39 0.87 1.68 3.25 2.99 1.92 2.30 2.86 1.58 2.72 2.40 2.44 1.98 1.31 1.85 1.79
0.47 0.39 0.72 0.30 0.49 0.46 0.51 1.24 0.83 1.52 0.68 0.40 0.42 0.53 0.72 0.60 0.51 0.39 0.73 0.57 0.57 0.83 0.35 0.56 0.51 0.73 0.69 0.83 0.73
Arid Arid Arid Arid Arid Arid Arid Arid Arid Arid Arid Arid Humid Humid Humid Humid Humid Humid Humid Humid Humid Humid Humid Humid Humid Humid Humid Humid Humid
v v p p p p v m p v v p v m,p m,p v v v m,p m,p m p p p p p p v v
H H H H H L L L H H M M H H H M M H L H H M H H H H H H H
Table 2 Mineralogical framework of beach sands (Qm monocrystalline quartz, Qp polycrystalline quartz, Qt quartz, Fk potassic feldspar, Fp plagioclase feldspar, Rf rock fragments, Ch chert, Mi mica, HM heavy minerals, PI provenance index, MI maturity index) Sample
Qm
Qp
Qt
Fk
Fp
Ft
Rf
Ch
Mi
HM
PI
MI
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 Average
9 11 7 16 1 17 1 13 8 8 15 17 3 5 14 3 0 6 2 7 0 6 11 0 6 1 5 6 3 6.9
3 2 1 1 0 1 0 4 7 0 1 0 0 3 3 1 0 0 0 0 1 0 1 1 0 0 0 1 0 1.1
12 13 8 17 1 18 1 17 15 8 16 17 3 8 17 4 0 6 2 7 1 6 12 1 6 1 5 7 3 8.0
2 0 0 0 2 0 7 1 4 12 0 0 0 0 1 0 0 0 0 2 1 0 0 0 0 2 1 0 0 1.2
12 11 12 28 8 26 18 25 7 22 17 24 7 20 10 17 3 13 10 4 5 5 17 2 12 5 9 7 10 12.6
14 11 12 28 10 26 25 26 11 34 17 24 7 20 11 17 3 13 10 6 6 5 17 2 12 7 10 7 10 3.8
8 8 31 17 5 23 41 30 70 29 29 41 17 16 53 49 4 17 10 13 21 26 9 2 37 10 29 32 29 24.2
1 0 0 2 0 4 2 2 1 6 4 6 0 4 7 8 0 1 3 2 3 11 0 3 3 0 1 5 0 2.7
1 0 11 1 0 0 0 5 0 0 0 0 0 0 0 2 0 1 0 1 0 1 0 0 0 0 0 0 0 0.8
64 68 48 35 84 29 31 20 3 23 34 12 73 52 12 20 93 65 75 71 69 51 62 92 42 82 55 49 58 50.8
1.75 1.38 0.39 1.65 2.00 1.13 0.61 0.87 0.16 1.17 0.59 0.59 0.41 1.25 0.21 0.35 0.75 0.93 1.00 0.46 0.29 0.19 1.89 1.00 0.32 0.7 0.34 0.22 0.34 0.57
0.55 0.68 0.19 0.38 0.07 0.37 0.02 0.3 0.19 0.13 0.35 0.26 0.13 0.22 0.27 0.06 0.00 0.22 0.10 0.37 0.04 0.19 0.46 0.25 0.12 0.06 0.13 0.18 0.08 0.21
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Table 3 Concentration (%) of major elements in beach sands (LOI loss on ignition; CIA chemical index of alteration: Al2O3/(Al2O3 1 Na2O 1 CaO 1 K2O) £ 100; ND not determined) Sample
SiO2
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
CIA
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 Avg
61.58 66.90 46.00 70.50 13.80 68.30 48.60 57.43 81.27 57.30 69.70 69.20 51.30 75.00 75.20 56.60 32.77 49.04 54.90 54.10 74.73 75.70 55.50 29.81 54.70 43.40 60.90 54.70 50.50 57.22
3.10 2.02 1.10 0.48 23.00 0.49 1.23 0.86 0.25 0.34 2.48 0.66 6.24 1.48 0.75 1.20 9.37 1.53 1.85 1.65 0.47 0.76 4.85 27.79 1.40 4.24 1.30 2.36 2.85 4.00
7.50 7.38 7.54 12.70 2.58 12.50 16.20 12.57 6.03 10.48 10.20 12.80 10.90 10.50 10.30 13.10 5.13 10.34 13.00 14.00 9.54 9.03 8.69 3.80 15.30 8.91 13.50 12.20 12.00 10.30
11.61 9.23 23.10 3.32 45.80 2.77 6.14 6.31 2.02 3.41 4.58 2.72 15.90 2.68 3.37 8.30 33.18 12.01 9.94 9.17 3.83 3.68 16.20 27.45 8.41 24.10 8.07 14.20 17.30 11.68
0.31 0.28 0.75 0.06 1.17 0.08 0.12 0.08 0.09 3.41 0.17 ND 0.40 0.13 0.11 0.26 0.51 0.20 0.26 0.24 0.06 0.07 0.32 2.13 0.24 0.54 0.20 0.34 0.50 0.45
5.84 5.23 3.12 2.11 2.29 1.75 4.43 7.77 1.06 1.14 2.31 2.85 3.60 1.10 0.85 5.71 7.97 7.93 5.32 5.25 1.45 1.85 3.13 3.05 4.97 8.16 4.50 5.05 5.69 3.98
6.69 6.17 5.27 5.48 4.73 5.93 13.60 4.70 6.07 21.53 5.89 4.75 5.91 2.79 2.06 7.70 5.79 11.13 9.11 10.70 2.65 2.70 5.97 2.41 8.13 7.31 5.52 5.62 6.68 6.65
1.53 1.22 2.03 3.15 0.61 3.36 3.25 2.35 1.26 2.41 2.23 2.77 2.16 2.07 2.48 3.22 1.08 2.33 2.90 2.82 2.43 1.91 2.01 0.76 2.96 1.46 2.49 2.71 2.37 2.22
0.82 0.95 0.57 1.21 0.22 1.56 1.91 2.68 1.00 2.62 1.44 2.35 1.66 2.73 3.37 1.37 0.24 0.53 1.31 0.90 1.75 1.87 1.60 0.39 1.52 1.03 1.90 1.59 1.21 1.46
0.45 0.03 0.49 0.09 0.37 1.26 0.43 0.26 0.09 0.00 0.04 0.16 0.25 0.02 0.10 0.15 0.13 0.36 0.16 0.19 0.11 0.10 0.15 0.11 0.14 0.17 0.15 0.23 0.31 0.22
0.15 0.16 0.93 0.64 0.22 1.38 2.63 2.31 0.83 0.66 0.53 0.78 0.38 0.54 0.78 1.81 2.87 2.87 0.86 0.15 1.17 1.17 0.88 2.46 1.10 0.08 0.83 0.23 0.12 1.55
45.34 46.96 48.97 56.34 31.70 53.53 46.34 56.37 41.99 28.29 51.62 56.46 52.84 58.04 56.56 51.60 41.91 42.52 49.39 49.25 58.29 58.22 47.56 51.63 54.82 47.62 57.67 55.15 53.91 50.03
western littoral of Mexico, extending approximately 6500 km along the Paci®c and Gulf of California waters (Fig. 1). The selected localities are beaches with dark colored sand and abundant heavy minerals. All the samples examined in this study are Quaternary in age and variable in grain size. Sample stations 1±12 (Fig. 1) are located in an area which, according to Tamayo (1990), is characterized by an arid climate (25 mm/y). In contrast, sample stations 13±29 are located in areas with a more humid climate (.100 mm/y). High-energy waves characterize the beaches located in open sea (1±6 and 14±29), whereas the beaches studied in the Gulf of California (7±13) are characterized by lowenergy waves. Because of the California Current (Wirtky, 1965), surface ocean currents are stronger on the western side of the Baja California Peninsula, as well as in the southern Mexican Paci®c, than in the Gulf of California. The rock sources and main rivers are summarized in Fig. 1, in which geology was simpli®ed based on the most abundant type of rock per area (modi®ed from OrtegaGutieÂrrez et al., 1992). The present tectonic setting is less active on both margins of the Baja California Peninsula
(passive to trailing margins) than in the southern Paci®c (active subduction margin). The source areas were divided into seven major tectonic±lithologic groups: 1. granitoids and migatites of variable ages; 2. volcanic rocks of intermediate to acid composition, mostly of Early Tertiary age (andesite to rhyolite); 3. volcanic rocks of intermediate to ma®c composition, mostly of Late Tertiary age (andesite to basalt); 4. deformed and/or metamorphosed volcanic-sedimentary complexes (igneous rocks, mostly of basaltic to andesitic composition), with ages ranging from Paleozoic to Mesozoic; 5. high-facies metamorphic complexes, with variable protoliths, mainly of Precambrian age; 6. sedimentary rocks, mostly marine-calcareous and some terrigenous, of Mesozoic±Tertiary ages; and 7. areas with mixed sources, with variable lithologies of igneous, metamorphic and sedimentary composition. The sampled localities are related to six different types of ¯uvial±lithological basins: A. Samples 1±6 were collected from the western part of
Au in ppb.
,2 ,2 ,2 ,2 ,2 ,2 ,2 9 3 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 Average
a
Au a
Sample
,2 4 ,2 6 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2 ,2
Ag
99 110 120 99 260 99 120 110 99 99 99 99 390 99 99 170 280 99 180 120 , 100 , 100 99 360 130 340 120 120 250 147
Zn
3.0 2.0 4.0 1.0 5.0 2.0 2.0 3.0 3.0 3.0 7.0 ,1 5.0 2.0 1.0 2.0 3.0 2.0 ,1 ,1 2.0 1.0 2.0 7.0 0.9 1.0 1.0 1.0 1.0 2
Mo
Table 4 Trace elements (ppm) in beach sands
, 10 25 , 10 , 10 79 , 10 , 10 , 10 18 , 10 , 10 , 10 46 , 10 , 10 62 81 38 48 44 , 10 30 29 17 , 10 60 43 , 10 , 10 21
Ni 37 31 39 24 72 10 18 24 23 10 39 9 44 16 12 40 92 41 30 47 19 21 21 39 25 79 45 56 54 35
2.5 3.8 2.9 2.2 8.1 2.9 5.1 2.5 4.3 4 5.7 2.9 22 5.8 4.5 2.2 5.4 4.2 4.1 2.3 5.8 6.3 6.2 1.9 3.8 3.1 3 2.4 2.6 5
Co As
Ba
Cr
W
0.8 230 87 116 1 410 150 140 0.5 280 200 220 0.2 410 34 264 2.3 69 920 82 0.5 520 52 69 0.2 560 48 41 0.3 940 110 110 1 1000 20 284 0.4 880 29 101 0.9 580 100 231 0.3 1100 42 88 4.6 650 290 71 0.7 830 29 110 0.8 700 22 138 0.3 390 190 101 0.3 50 630 39 0.2 260 81 46 0.3 370 120 30 0.2 230 150 87 1.4 450 88 122 1.3 470 81 165 1.1 350 770 34 0.3 150 300 74 0.5 480 27 16 0.6 300 150 19 0.6 480 55 86 0.5 460 82 83 0.6 400 92 95 1 483 171 106
Sb
La
Ce
Sm
, 0.5 29 89 15.8 , 0.5 34 75 13.9 , 0.5 85 170 20.5 , 0.5 9 14 3.2 , 0.5 110 200 19.1 1.4 18 33 4.5 0.8 14 24 3.3 3.6 21 39 4.4 1.6 7 17 1.4 1.3 23 44 5.3 1.6 150 310 32 4.8 12 23 2.3 2.9 53 89 9.1 1.5 18 37 3.8 0.6 16 28 3.5 0.8 11 22 3.6 0.5 13 23 4.1 0.6 19 47 7 0.9 15 29 4.8 0.5 11 24 4.6 2.1 13 29 2.9 2.4 13 24 2.8 0.8 65 110 8.7 0.5 170 300 23.5 1.2 15 27 3.8 0.9 33 57 5.9 0 24 46 4 0.9 27 54 5 1.1 31 60 5.9 1 37 70 8
Cs 2 ,1 3 ,1 3 ,1 2 1 ,1 ,1 4 ,1 ,1 ,1 ,1 1 2 ,1 ,1 1 ,1 ,1 ,1 2 ,1 ,1 ,1 1 1 1
Eu 2 2 3 , 0.5 3 , 0.5 , 0.5 1 , 0.5 1 3 , 0.5 1 , .5 , .5 , .5 1 1 , 0.5 , 0.5 , 0.5 , 0.5 1 2 1 1 , 0.5 1 1 1
Tb 10 9 10 ,2 17 3 ,2 3 ,2 3 5 ,2 7 ,2 3 2 4 3 3 2 ,2 ,2 5 13 ,2 2 ,2 2 ,2 4
Yb 1.1 1.1 1.1 0.3 2.5 0.2 , 0.2 0.3 , 0.2 0.4 0.6 , 0.2 1 , 0.2 0.3 0.2 0.3 0.4 0.3 0.3 0.2 , 0.2 0.7 1.9 0.2 0.3 0.3 , 0.2 0.3 0
Lu
Hf
48 30 41 34 32 70 12 4 40 278 11 6 15 2 15 5 3.9 4 5.8 5 16 8 3.9 3 44 45 11 5 5.9 4 24 2 41 30 38 4 26 3 36 3 10 3 8.4 3 21 40 55 220 22 2 41 5 22 4 26 5 28 8 24 29
Sc 3.7 3.1 5.1 0.9 7.9 1 0.8 1 , 0.5 0.6 6.1 0.6 4.2 0.8 0.9 0.6 1.8 0.6 , 0.5 0.8 1 0.7 3.1 11 , 0.5 1 0.6 0.9 0.7 2
Ta
U
13 5 16 5.2 45 11 1.5 0.8 63 21 3.7 4.7 1.5 1.2 6.6 1.8 2.1 1 6.6 2.9 38 8.5 4.9 1.2 10 5.8 6.9 2 3.6 1.4 2 0.7 1.1 1.1 1.1 0.6 2 0.6 0.9 0.5 3.5 1.5 3.7 1.4 24 6.8 84 16 2.8 1.1 16 3.2 6.1 1.5 13 2.8 11 2.6 14 4
Th
5 13 4 17 36 19 23 23 10 15 12 19 1 12 4 17 1 17 2 5 1 1 21 , 0.5 6 1 1 15 1 10
Br
Zr , 0.5 930 16 1000 , 0.5 2600 28 200 , 0.5 10000 27 200 23 200 87 200 34 200 91 200 63 200 55 200 44 1800 78 200 63 300 30 200 , 0.5 940 , 0.5 200 , 0.5 200 , 0.5 200 50 200 63 320 25 1500 , 0.5 8100 39 200 23 200 39 200 39 380 25 200 32 1085
Rb
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Table 5 Detection limits of analyzed elements Element
Limit
Antimony Arsenic Barium Cadmium Cerium Desium Chromium Cobalt Europium Gold Hafnium Iridium Iron Lanthanum Lutetium Molybdenum Nickel Rubidium Samarium Scandium Selenium Silver Sodium Tantalum Tellurium Terbium Thorium Tin Uranium Yterbium Zinc Zirconium
0.1 ppm 0.5 ppm 50 ppm 5 ppm 5 ppm 0.5 ppm 20 ppm 5 ppm 1 ppm 2 ppb 1 ppm 50 ppb 0.20% 2 ppm 0.2 ppm 1 ppm 10 ppm 5 ppm 0.1 ppm 0.2 ppm 5 ppm 2 ppm 0.02% 0.5 ppm 10 ppm 0.5 ppm 0.2 ppm 100 ppm 0.2 ppm 2 ppm 100 ppm 200 ppm
Fig. 3. Ternary diagram for total quartz (Qt), total feldspar (Ft) and heavy minerals (HM).
Fig. 2. Correlations among the studied beach sands: (a) sorting and grain size; (b) rock fragments and grain size; (c) grain size and heavy minerals; and (d) rock fragments and heavy minerals.
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Fig. 4. Diagram of whole sediment Fe2O3 and TiO2 contents.
297
the Baja California Peninsula. This region is composed of short rivers (1±2 order) that drain areas mainly of lithology types 1 and 4. Locally, the Vizcaino Peninsula (sample 6) is made up of uplifted subduction complexes (Sedlock et al., 1993). B. Samples 7±12 were collected from the eastern Baja California Peninsula, which is made up of lithologies of types 1, 7, and 2. The drainage is mostly made up of short streams (1±2 order). However, this coastal area might be in¯uenced by the sedimentary discharge of the Colorado River (sample 12). C. Sample 13 was collected from beaches next to long important rivers (.4 order) that drain areas formed
Table 6 Average values of some elements for the studied gray sands (numbers in brackets samples with maximum values) Element
Zn Mn Ni Co As Sn Ba Cr Cs Sc Hf Ta Th U Rb Zr a
Avg. Cont. Crust a
This work Average
Maximum
147.00 2.31 21.40 35.00 5.00 0.78 482.70 171.00 1.15 24.10 28.80 2.05 13.60 3.93 32.50 1085.20
360 [24] 7 [24,11] 81 [17] 92 [17] 2 [13] 2.3 [5] 1100 [11] 770 [23] 4.8 [11] 54.6 [24] 27 [5] 11 [24] 84.2 [24] 21 [5] 91 [10] 10000 [5]
Average Enriched
80 1 105 29 1 0.2 250 185 1 30 3 1 3.5 0.91 32 100
X X X X X
X X X X XX
Average ratios Depleted
X
1.8 2.3 0.2 1.2 5.0 3.9 1.9 0.9 1.2 0.8 9.6 2.1 3.9 4.3 1.0 10.9
Taylor and Mc Lennan (1985).
Fig. 5. Ternary diagram and sediment color: Lg light gray; Lbg light brownish gray; G gray; vdg very dark gray; dg dark gray.
298
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Fig. 6. Rare earth elements normalized to chondrites: (a) REE for Group I are enriched in LREE and are similar to those observed in granites; (b) REE for Group II, with patterns similar to the NASC (North American shale composite), which is represented by the dotted area. This suggests that the sands were derived from evolved old sources.
mostly by Tertiary rhyolites of the Sierra Madre Occidental (type 2), and some of types 3 and 6 (Ortega-GutieÂrrez et al., 1992). D. Samples 14±22 come from beaches related to areas with medium length streams (2±3 order), which are related to areas formed by volcanic rocks of the active Mexican Volcanic Belt (type 3), and types 4 and 1 (Sedlock et al., 1993; Centeno-GarcõÂaetal.,1993;Lapierreetal.,1995).Sample22 was collected next to a long river (.4 order) that carries sediments from variable sources (types 7, 4, 2, 3, and 1). E. Locations 23and 24belong to areas with short streams (1± 2 order), made up of granitoids and migmatites of the Mesozoic Xolapa terrane (Schaaf et al., 1995). F. Samples 25±29 were collected from the southern Paci®c coast; they are related to small ¯uvial basins with mediumsize streams (2±3 order) and mostly sources made up of the Proterozoic to Paleozoic granitoid complex (Chiapas batholith) (Ortega-GutieÂrrez et al., 1992.
3. Material and methods Surface sand samples were collected with a plastic shovel on beaches in western Mexico. These sands are whole samples that were selected according to their colors (gray or dark colors), because they are suitable for high concentration of heavy minerals. The colors of the beach sands was determined in dry sample using the Munsell (1975) color charts; they are listed in Table 1. To determine the mean graphic size (Mz) and the standard graphic deviation (sorting) (Table 1), the sands were quartered and sieved following procedures and formulas suggested by Folk (1974). Petrologic analyses (Table 2) involved counting 300 grains divided into monocrystalline quartz (Qm), polycrystalline quartz (Qp), potassium feldspar (Fk), plagioclase feldspar (Fp), rock fragments (Rf), chert (Ch), mica (Mi), and heavy minerals (HM) according to the procedures and criteria outlined by Franzinelli and Potter (1985).
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299
Fig. 7. Chondrite normalized REE diagrams for Group III, showing more depletion in LREE and patterns similar to volcanic rocks from the accreted Mesozoic terranes. The dotted area represents REE concentrations of volcanic rocks from the Guerrero Terrane (Centeno-GarcõÂa et al., 1993).
Major element analysis (Table 3) of beach sand pressed pellets was performed in a Siemens sequential X-ray spectrometer SRS 3000. Multi-element geochemical analysis (Table 4) of elements was done by Bondar Clegg Inchcape Testing Services by direct irradiation/INAA of vials of prepared sand samples. Analytical detection limits of the trace elements are listed in Table 5. 4. Results and discussion Table 1 shows that the studied beach sands range from coarse sands to very ®ne sands, according to their mean graphic size values. Fine size beach sands are mostly well sorted (Fig. 2a), which seems to be the result of persistent and high wave energy conditions causing an ef®cient reduction of particle grain size. Although there is slight correlation between mean
graphic size (2log2 (mm)) and rock fragment content (Tables 1 and 2, Fig. 2b), this does not follow a speci®c path. There is no clear relationship between grain size and heavy mineral content, suggesting that their abundance might be controlled by other factors such as sand provenance (Fig. 2c). However, heavy minerals are inversely related to rock fragment content (Tables 1 and 2, Fig. 2d). This distribution might be related to abrasion and settling of the particles associated with wave energy and density of the grains. Fig. 3 shows HM-Ft-Qt ternary diagrams for all the samples divided in groups by their geological setting. Samples 1±17 show a wide range in heavy mineral content. In contrast, samples 18±29 are highly enriched in heavy minerals. Fe2O3 and TiO2 coexistence in the same mineral facies is suggested by their statistically signi®cant correlation (Fig. 4, Tables 3 and 6). The association between magnetite and ilmenite has been previously reported for Paci®c beaches
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Fig. 8. (a) Chondrite normalized REE diagram for Group III; the dotted area represents REE concentrations of volcanic rocks from the Guerrero Terrane (Centeno-GarcõÂa et al., 1993). (b) Chondrite normalized REE diagram for Group IV, showing variable patterns that suggest more in¯uence of mineral content than of sand provenance.
Fig. 9. La/Yb ratio and heavy mineral variations.
and is related to plutonic and metamorphic sources (Carranza-Edwards, 1980; MartõÂn-Barajas, 1980; Carranza-Edwards et al., 1988; Basu and Molinaroli, 1989). Fig. 4 shows a strong correlation between high Fe2O3 and TiO2 content and type 1 source-rocks. Table 3 shows that the averages of the major elements are more or less similar to those obtained by Potter (1986) for the South American Paci®c association, with the exception of MnO, TiO2, and Fe2O3, which are enriched in the gray sands we studied. These differences can be explained by the fact that, in gray sands, magnetites and ilmenites are concentrated by the energy provided by big waves. The maximum Fe2O3 value was 45.80%, and the maximum for TiO2 was 23.00%, both in the same beach location (site 5). Data from the concentration of major elements (Table 3) were used to construct the ternary diagram of Fig. 5. Samples with very dark gray and dark gray colors are shifted towards the Fe2O3 1 TiO2 pole, whereas the lighter colors
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301
Fig. 10. Element concentrations (ppm) according to sampling site. Th, Sc, Hf, and Ni values are represented by dotted lines. The plain lines are heavy mineral variations.
belong to polygons that are nearest to the SiO2 pole. Apparently the beach sediments with high heavy mineral contents are darker and enriched with resistant minerals with high Fe2O3 and TiO2 values (mostly magnetite and ilmenite). The chemical index of alteration (CIA) was determined (Table 3) according to Nesbitt and Young (1984). Locations from arid climate (sites 1±12) average lower CIA values (46.99) than those from humid climate (sites 13±29), which have a CIA average of 52.18. Although the difference is not signi®cant, it implies that the higher precipitation rates of the southern portion produce a higher CIA. Rare earth element data were chondrite-normalized using Evensen (1978) values. The 29 sand samples were divided into four groups based on their differences in REE patterns. High REE concentrations of economical importance were not found. Most of the sample patterns suggest that sand REE content is controlled by composition of source rocks more than by percentage or type of heavy mineral content.
Samples from Group I (1, 2, 3, 5, 13, 23, and 24; Fig. 6) show a pattern that is approximately ¯at to enriched in light rare earth elements (LREE) and ¯at in heavy rare earth elements (HREE), with average normalized values of 100 or higher but with a strong Eu anomaly. Similar patterns are shown by monzogranites and syenogranites (Wilson, 1989), as well as by muscovite-rich rocks. The variable mica content (0±11%) and the proportions between HM (48± 92%) and Q 1 F (10±28%) suggest that the REE composition of sands from Group I might be controlled by their provenance. Their geographic location (samples 1±5), near the insular range batholith of Baja California and next to the Xolapa Terrane (samples 23 and 24), supports this interpretation. Samples from Group II (25±29) are less enriched (Fig. 6) in LREE (LaN 100±40), with Eu anomaly, and down-slope HREE pattern (HREE . 10) (Fig. 6). Samples from Group II have the same modal variability of HM, Q and F as Group
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Table 7 Signi®cant correlation coef®cients (99%) (Qm monocrystalline quartz, Qp polycrystalline quartz, Fk potassic feldspar, Fp plagioclase feldspar, Rf rock fragments, Ch chert, Mi mica, Mz mean graphic size, HM heavy minerals, PI provenance index, MI maturity index.) SiO2 (1) SiO2 (2) Al2O3 (1) Al2O3 (2) Fe2O3 (1) Fe2O3 (2) TiO2 (1) TiO2 (2) HM (1) HM (2) Qm (1) Qm (2) Mz (1) Mz (2) Rf (1) Rf (2) Fp (1) Fp (2)
Ba (0.61), K (0.59), Qm (0.59), CIA (0.56), Rb (0.53), Rf (0.50), Qp (0.46) Fe (0.90), Ti (0.77), Hf (0.71), Zr (0.70), Sc (0.69), Co (0.69), Zn (0.67), Cr (0.67), HM (0.61), V (0.60), Yb (0.59), Ni (0.57), Th (0.57), Ta (0.53), La (0.46) Na (0.93), K (0.51), CIA (0.49) Yb (0.71), Hf (0.68), U (0.66), Ta (0.65), Th (0.63), Fe (0.62), Tb (0.62), CR (0.58), M0 (0.57), La (0.52), Ce (0.51), Sm (0.50) Ti (0.83), Cr (0.80), Co (0.80), Hf (0.77), Sc (0.73), U (0.70), Zn (0.70), HM (0.70), Yb (0.69), Th (0.64), Ni (0.63), Ta (0.61), Tb (0.54), La (0.51), Ce (0.47) Si (0.90), Ba (0.69), K (0.67), Na (0.63), Al (0.62), Rb (0.58), Qm (0.56), Rf (0.53), Chert (0.51), Fp (0.48) Hf (0.95), Zr (0.95), Th (0.90),, Ta (0.88), U (0.87), Yb (0.83), Fe (0.83) La (0.77), Ce (0.72), M0 (0.67), Cr (0.65), Tb (0.65), Zn (0.63), Sc (0.62), Sm (0.61), HM (0.57), Eu (0.52), Al (0.68), Na (0.65), K (0.56), Ba (0.53), Rf (0.46) Sc (0.79), Fe (0.70), Co (0.66), Zn (0.62), Ti (0.57), Ni (0.55), Cr (0.54), Mz (0.49), Hf (0.48), Zr (0.47), Yb (0.47) Rf (0.87), Ba (0.76), Qm (0.66), Si (0.61), K (0.61), Fp (0.60), Rb (0.65), Na (0.52), Ch (0.52) MI (0.66), Si (0.56), Rf (0.58), Ba (0.47) Zn (0.62), Co (0.58), Fe (0.56), Ni (0.53), Hf (0.51) HM (0.49) s (0.74), Rf (0.63) Ba (0.67), Si (0.50), K (0.48), Qp (0.45) Sc (0.66), PI (0.64), Mz (0.63), Fe (0.53), Cr (0.48) Qm (0.58), Na (0.55), Al (0.47), Ba (0.47) Co (0.59), Sc (0.53), Fe (0.48), Zn (0.48)
I. Therefore; the more suitable factor controlling the REE content is the geology of the source areas. The REE patterns of sands from Group II (Fig. 6) are similar to those proposed for post-Archean shale composites (NASC: the North American shale composite) (Taylor and Mc Lennan, 1985). This similarity suggests that the sands of Group II have either been homogenized by the sedimentary processes or derived from homogenized evolved rock. Samples from Group II are close to mountain ranges formed by Proterozoic to Paleozoic granitoids (localities 25±29). Samples of Group III (Figs. 7 and 8) (4, 6, 8, 10, 12, and 14±22) are more depleted in REE. Normalized LREE values are smaller than NASC (LaN , 50), with ¯at HREE patterns. These samples have mostly a small or no Eu anomaly. Heavy mineral percentages vary from 12±93% and, overall, they have low quartz percentages (,20%). Source rocks for the sands of Group III are mainly formed by accreted Mesozoic oceanic island-arc sequences. Volcanic rocks from these source areas have similar juvenile REE patterns (Figs. 7 and 8) (Centeno-GarcõÂa et al., 1993), suggesting that the source rock is controlling the sand geochemistry. However, samples 8, 10, and 12, are located in regions where granitic and volcanic rocks with high REE values crop out. Sand samples from Group IV (7, 9, 11) show irregular REE patterns, with a positive Eu anomaly (Fig. 8). Their pattern might be related to an abundance of Eu enriched minerals, such as feldspar and magnetite (Henderson, 1984). The nature of the source rock of the studied sands seems, generally, more important in the control of REE composition than the concentration of heavy minerals. This is suggested by the distribution of La/Yb vs. HM%, as shown in Fig. 9. Samples 1±6 from the western coast of
the Baja California Peninsula show low La/Yb ratios, although HM% has a wide range. Samples 7±12 from the eastern coast of the Baja California Peninsula show an increment in La/Yb with increment in HM%. Samples 13±23, which are associated with juvenile volcanics, have low La/Yb values that do not undergo a major change with increments in HM%. In contrast, samples 24±29 have much higher La/Yb values that do not change with increments in HM%. The high La/Yb ratios might be related to the `evolved' nature of the source rock (Precambrian basement and sedimentary cover) that forms the southern part of the Paci®c Coast. The same interpretation can be inferred from other trace elements, such as Th, Sc, and Hf. Fig. 10 shows that Th concentrations are independent of HM%, since Th behaves in diverse ways. However, there is an approximate relationship between Th and the nature of the source sample (Fig. 10). Samples with Th concentrations similar to or higher than NASC (.10 ppm) are related to areas with granitoids and Precambrian or sedimentary rocks. Similar concentrations have been described for deep-marine turbidites associated with collision/strike slip, continental arc, and trailing edge (Mc Lennan et al., 1990). In contrast, samples with low Th content (,10) belong to areas associated with accreted island-arc or recent volcanic rocks and are similar to those from sediments associated with recent fore-arc areas (Mc Lennan et al., 1990). Sc content seems to have a direct relationship with heavy mineral content, since its concentration increases with increments of HM% (Fig. 10). Samples with high HM% (.60%) show Sc concentrations (.30) much higher than those reported by Mc Lennan and Taylor (1991) for oceanic sediments and upper crust (4±30) (Taylor and Mc Lennan 1985) as shown in Table 6.
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303
Fig. 11. Element concentrations (ppm) according to sampling site. Ta, Zr, Cr, U, and Zn values are represented by dotted lines. The plain lines are variations in heavy mineral percentages.
However, there is some relationship with provenance since, in samples with the same HM%, Sc concentrations are higher on the central Paci®c coast than in the southern Paci®c (Fig. 10). Although Hf is common in heavy minerals, the studied sand samples show Hf content associated with both percentage of heavy minerals and nature of the source (Fig. 10, Table 7). Samples enriched with heavy minerals from areas with granitic or sedimentary sources are enriched in Hf. However, samples with high HM% collected from areas with volcanic or accreted
oceanic-arc sources do not have Hf enrichment (Fig. 11d). Concentrations of Ni are lower, overall, than NASC (Ni 55±95) and upper crust (30) in most samples (.30 ppm) (Fig. 10). Nine samples show Ni concentrations between 30 and 90 ppm, mostly those with high heavy mineral content (Fig. 10). Low Ni contents are common in volcanic rocks (25 ppm in island arcs) (Taylor and Mc Lennan, 1985); thus, low concentrations in beach sands might be related to depletion in their source. The abundance of other elements, such as Zn and Cr,
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Table 8 Variations of petrological data and CIA (Qt quartz, Fk potassic feldspar, Fp plagioclase feldspar, Rf rock fragments, Ch chert, Mi mica, H.M. heavy minerals, PI provenance index, MI maturity index, CIA chemical index of alteration: Al2O3/(Al2O3 1 Na2O 1 CaO 1 K2O) £ 100, vdg very dark gray, dg dark gray, lbg light brownish gray, lg light gray)
Color of dry sand Grain size
Sorting Main source Wave energy Climate
vdg and dg Gray lbg and lg Coarse Medium Fine Very ®ne , 0.71 . 0.71 Metamorphic Plutonic Volcanic High Moderate Low Arid Humid
Qt
Ft
Rf
Ch
Mi
HM
PI
MI
CIA
2.8 8.2 12.0 13.3 5.9 8.4 7.0 7.8 8.4 7.0 8.9 7.5 7.5 8.6 7.8 11.9 5.2
8.0 10.4 22.2 18.7 11.6 14.6 12.0 14.1 13.3 10.6 13.6 15.3 9.0 13.2 22.4 19.8 9.6
17.6 27.4 26.1 50.7 27.3 18.6 14.7 20.2 32.0 22.6 25.0 24.2 22.0 29.8 29.0 27.7 21.8
1.8 2.2 4.1 4.7 3.3 2.2 1.3 2.3 3.5 3.8 2.6 2.4 1.0 5.8 2.6 2.3 3.0
0.4 1.2 0.7 0.0 2.1 0.3 0.0 0.3 1.7 0.2 1.1 0.8 6.0 0.6 1.0 1.5 0.3
69.4 51.6 34.9 12.7 50.9 55.8 65.0 55.3 42.1 55.8 49.6 49.8 59.5 42.0 37.2 37.6 60.1
0.72 0.64 1.01 0.51 0.49 1.04 0.78 0.92 0.53 0.64 0.85 0.78 0.42 0.49 0.78 10.02 63.00
0.13 0.19 0.26 0.19 0.14 0.29 0.16 0.25 0.17 0.20 0.22 0.22 0.28 0.17 0.13 0.28 0.17
48.16 50.53 51.28 42.28 53.21 49.78 49.43 50.23 49.65 54.31 50.54 47.74 49.11 51.96 50.39 46.99 52.35
seems to be more related to heavy mineral content. The samples with the highest content of Zr, Ta, and U are related to samples with more than 60% heavy mineral content (Fig. 11). Only samples 5, 13, 23, and 24 show high concentrations of such elements. They seem to be related to coastal areas with granitoids. In contrast, sands collected near areas with volcanic rocks have lower concentrations of Zr, although their heavy mineral content is high (Fig. 11). Only one sample (site 8) shows a gold content three times higher than the average continental crust (Table 4). Compared with values reported by Wedepohl (1995), two samples (sites 2 and 4) are enriched 40±60 times in silver, respectively. They are probably elements disseminated in rocks from the Alisitos Terrane described by Campa and Coney (1983). Table 8 summarizes variations in petrological data and CIA. The higher concentration of heavy minerals is associated with beach sands that are very dark gray and dark gray in color, whereas the provenance index (PI F/ Rf) and maturity index (MI Q/F 1 Rf) show higher values in association with light brownish gray and light gray colors.
5. Conclusions The content of heavy minerals is not related to grain size, sorting, or sand provenance. Its variation is probably related to settling conditions of particles and local sedimentologic processes. The main source does not seem to play an important role in the distribution of heavy minerals or in the observed CIA values. In contrast, high wave energy and a
humid climate seem to be responsible for the heavy mineral content and CIA values, respectively. There is a higher content of heavy minerals in humid zones than in arid regions, particularly in the well sorted and ®ne beach sands that are dark gray in color, which contain higher concentrations of iron, titanium, and zirconium. These sands are rich in REE. The metal content was more abundant in regions from beach sands affected by high-energy waves. REE and other trace element concentrations, such as Th, Sc, and Hf, seem to be more associated with source composition than to heavy mineral content. Concentrations of other elements, such as Ni, Ta, Zr, Cr, and major elements, are more dependent on heavy mineral content. The results suggest that the studied beach sands have little economic interest. However, they are good indicators of the nature of the source rock and, as such, can be used indirectly for large-scale mineral exploration. Acknowledgements This work was partially supported by UNAM-CONACYT Project 3477T. We thank the Instituto de Ciencias del Mar y LimnologõÂa for support of the project entitled `Sedimentology of Mexican Beaches.' We are grateful to Susana Santiago and Eduardo Morales for their collaboration in laboratory activities. References Basu, A., 1976. Petrology of Holocene ¯uvial sand derived from plutonic source rocks: implications for paleoclimatic interpretation. Journal of Sedimentary Petrology 46 (3), 694±709.
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