Sequential extraction techniques applied to a porphyry copper deposit in the basin and range...

Journal of Geochemical Exploration, 14 (1981) 155--174 155 Elsevier Scientific Publishing Company, Amsterdam --Pr inted in The Netherlands

SEQUENTIAL EXTRACTION TECHNIQUES APPLIED TO A PORPHYRY COPPER DEPOSIT IN THE BASIN AND RANGE PROVINCE

LORRAINE H. FILIPEK and PAUL K. THEOBALD, JR.

U.S. Geological Survey, Denver, Colo. 80225 (U.S.A.)

(Received July 16, 1980; revised and accepted October 10, 1980)

ABSTRACT

Filipek, L.H. and Theobald, P.K., Jr., 1981. Sequential extraction techniques applied to a porphyry copper deposit in the Basin and Range Province. J. Geochem. Explor., 14: 155--174.

Samples of minus-80-mesh (< 180 ~m) stream sediment, rock containing exposed fracture coatings, and jarosite and chrysocolla were collected from an area surrounding the North Silver Bell porphyry Cu deposit near Tucson, Arizona. The samples were subjected to a series of extractions in a scheme originally designed for use on samples from humid or sub-humid environments, in which the following fractions can effectively be separated: (1) carbonates and exchangeable metals; (2) Mn oxides; (3) organic compounds and sulfides; (4) hydrous Fe oxides; and (5) residual crystalline minerals. Jarosite and chrysocolla, two major minerals of the North Silver Bell area, were found to dissolve over two or more steps of the extraction scheme. The results represent only a limited number of samples from one copper deposit. Nevertheless, they do suggest that in a semi- arid to arid environment, where mechanical dispersion of such minerals predominates, uncritical assignment of unique phases, such as Mn oxides or organics to a given extraction would lead to false interpretations of weathering processes. However, the relative proportions of elements dissolved in each step of the jarosite and chrysocolla extractions could be used as a "fingerprint" for recognition of the presence of these two minerals in the stream-sediment and rock samples. The relative abundance of hydrous Fe oxide and jarosite and the alteration zoning could be mapped using data from jarosite and chrysocolla extractions. Manganese oxides were also found to have a greater influence on Zn than on Cu or Pb during supergene alteration.

The rapid change in relative importance of the first (1M-acetic acid) extraction for Cu, Zn, and Pb near the mineralized zone suggested the occurrence of minor hydromorphic processes within the stream sediments. Thus, the acetic acid extraction proved the most effective for pinpointing mineralization in sediments. In contrast, the residual fraction had the longest dispersion train, suggesting that total metal concentrations are most effective in arid environments for reconnaissance surveys.

INTRODUCTION

As geochemical exploration becomes a more mature science, an increas- ing emphasis is being placed on the chemical and physical processes involved in the formation and weathering of ore deposits. Partial solution techniques

0375-3839/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company

156

have proven to be effective tools in understanding these processes in many environments (e.g. Gatehouse et al., 1977; Carpenter et al., 1978; L.H. Filipek, unpubl, data, 1980). Selective extractions have been used to remove individual phases, such as Mn oxides (Chao, 1972) and sulfides (Olade and Fletcher, 1974; Peachey and Allen, 1977) from soil, stream-sediment, and rock samples.

Sequential extraction schemes (e.g. McLaren and Crawford, 1973; Chao and Theobald, 1976; Gatehouse et al., 1977), which strip off successively more resistant components, have also been developed to determine the relative effectiveness of various hydromorphic and biogenic processes in competing for metals weathered from mineralized zones. The partitioning of metals among hydromorphic and detrital components is highly dependent on the geochemical environment. For example, Bradshaw et al. (1974) reported that the strength of metal bonding varied significantly among different temperate and tropical environments. However, they could not give any reasons for this variability. Detailed studies are thus required in a vari- ety of geochemical environments in order to understand the chemical and physical factors influencing metal partitioning. Only when these processes are known, can an intelligent choice be made about the proper extraction technique for any given environment.

The present study is an investigation Of the partitioning of Cu, Pb, Zn, Mn, and Fe in samples of stream bed sediment and of rock with exposed fracture coatings from a zone of porphyry Cu mineralization in an arid environment -- the North Silver Bell area near Tucson, Arizona. The sequential extraction scheme used has proven effective in determining hydromorphic and biogenic processes in a humid sub-tropical environment in the south- eastern United States (L.H. Filipek, unpubl, data, 1980). The present study was undertaken to determine the effectiveness of techniques designed to investigate hydromorphic processes when applied to an environment where mechanical weathering presently predominates. The two sample types were chosen to provide maximum contrast. The fracture coatings on rocks should reflect highly localized relict supergene weathering, whereas the stream- sediment samples should tend to average variations within a drainage area mainly by mechanical dispersion of abraded rock particles. Results of the stream-sediment partitioning at North Silver Bell were also compared with partitioning results from stream sediments in a neighboring nonmineralized area to determine the effectiveness of the various extractants in accentuating anomalies.

GEOLOGICAL SETTING

The North Silver Bell deposit is located about 60 km northwest of Tucson, Arizona, in the Silver Bell Mountains, a small, rugged range rising above extensive alluvial plains (Fig. 1). The deposit is recognized as a northwesterly extension of the Silver Bell porphyry Cu system and is jointly controlled by

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Minerals Exploration, Inc. (Union Oil) and ASARCO, Inc. The deposit has not been mined and therefore, presents a relatively undisturbed environment for study.

The area lies within the extensive porphyry Cu province of the semi-arid southwestern United States. Formations ranging in age from Precambrian to Recent are exposed in the vicinity. The peaks and ridges of the Silver Bell Mountains are composed of predominantly the more erosion-resistant Paleozoic sedimentary unit and Tertiary(?) volcanic rocks. The porphyry Cu mineralization occurs in hydrothermally altered intrusive igneous rocks of Laramide age along the southwestern flank of the mountains (Richard

158

and Courtright, 1966). Host rocks include dacite porphyry alaskite, and the Paleozoic sedimentary rocks. The primary sulfide minerals in the Silver Bell system, pyrite and chalcopyrite, are spatially associated with quartz monzon- ite porphyries (Soc. Econ. Geol., 1979).

The hydrothermal alteration is fracture-controlled within systems of close- ly spaced, parallel, nearly vertical fissures (Richard and Courtright, 1966). Early alteration, during which chalcopyrite mineralization took place, consists of typical potassium silicate (potassic) alteration zoning that grades out- ward to epidote-chlorite (propylitic) alteration. Late-stage quartz-sericite (phyllic) alteration, which added considerable pyrite, masks much of the potassic and propylitic alteration (Soc. Econ. Geol., 1979).

Supergene enrichment

Ore metals in the Silver Bell area have been enriched through several cycles of oxidation and leaching, with present chalcocite ore deposits lying beneath about 30 m of leached capping (Richard and Courtright, 1966). Detailed mapping at Silver Bell has demonstrated that the pattern of relatively strong chalcocite at depth is reflected by the distribution and abundance of "diagnostic" limonites (Blanchard, 1939; 1968) present in outcrops (Richard and Courtright, 1966). Much of the North Silver Bell capping shows a relatively poor correlation with the known hypogene Cu minerals as compared with the major Silver Bell deposits to the south (Soc. Econ. Geol., 1979).

SAMPLING AND ANALYSIS

Sampling methods

In April and October, 1979, minus-80-mesh (<180 ~m) stream-sediment samples were collected along two major dry stream beds (Fig. 1), here designated as "stream A" (the northeasternmost stream) and "stream B" (the southwesternmost stream). These streams drain the northern portions of the Silver Bell Mountains. Stream A begins in the zone of propylitic alteration and cuts through the phyUic zone. Its closest approach to the potassic zone, and the buried chalcocite blanket, is at site 21. Stream B drains the zone of potassic alteration and runs over and parallel to the chalcocite blanket for about 1 km (Soc. Econ. Geol., 1979). The sediments of Stream B may also contain some contamination from a tailings dump located just above the head of the stream. Samples were also collected from two tributaries (sites 52 and 54), above their intersection with stream A. These two tributaries are northeast of the major zones of alteration.

Chipped samples of rock containing fracture coatings were collected from the banks of many of the stream sites and on the hillside separating streams A and B. These were ground to minus-80-mesh for analysis. In addition, sam-

1 5 9

ples of jarosite and pale-green Cu mineral, tentatively identified as chrysocolla, were scraped from outcrops at sites 48 and 62 (Fig. 1).

Samples numbered 1 through 13 (rocks) and 21 through 33 (sediments) were collected in April; those numbered 41 through 62 were collected in October. All samples collected in October were stream sediments except 48 and 62, which were jarosite and chrysocolla. Samples labelled 26 and 44 were collected from the same site in different months to determine reproducibility over time.

In addition to the samples collected at North Silver Bell, seven stream- sediment samples (minus-80-mesh) were also collected from the Papago Indian Reservation about 60 km south of Silver Bell, in an area of no known mineralization. These samples were used as background data in order to determine anomaly contrasts for the various extractions.

Analytical methods

The samples of sediment and rock were subjected to a 5-step sequential extraction scheme based on procedures developed by Chao (1972), Chester and Hughes (1967), Gupta and Chen (1975), and Filipek and Owen (1978). This scheme was designed to separate individual hydromorphic and biogenic fractions from resistant crystalline minerals in the following order: (1) carbonates and exchangeable metals; (2) Mn oxides; (3) organic compounds and sulfides; (4) hydrous Fe oxides; and (5) residual crystalline minerals. The scheme has been discussed in detail by Filipek and Owen (1978), so only a flow-chart is presented here (Fig. 2). For ease in discussion, the following labels will be used to identify the fractions, based generally on the type of chemical extraction: (1) HOAc (acetic acid); (2) C-Hxl (cold hydroxylamine hydrochloride); (3) H202 (hydrogen peroxide); (4) H-Hxl (hot hydroxylamine hydrochloride); and (5) residual. The sum of the first four extractions will be referred to as the non-residual fraction. Total concentrations, as reported here, refer to the sum of all five extractions. Separate analyses of total metal concentrations were not performed for these samples because it had been determined previously on several sample types that the sum of metal concentrations in the five extractions generally equals within + 10 percent of "total" metal, as determined using the final extraction on a separate sample.

Iron, Mn, Cu, Zn, and Pb were determined in each extract by flame atom- ic absorption spectrophotometry (AAS). A deuterium lamp background correction and standards containing Fe in concentrations similar to those of the samples were used to correct for matrix effects when measuring Pb. Standards for calibration curves were prepared in dilute HC1 rather than in the various extracting solutions because it had been determined previously that matrix effects of the extractants were insignificant for the elements measured.

160

HOAc

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80°C, 4 hr.

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EXTRACT IN I' I0% HCI

CARBONATES, I EXCHANGEABLE METALS

~ HYDROUS MN OXIDES (EASILY REDUCIBLE)

~ORGANIC COIIPOUNDS, SULFIDES l

~'J HYDROUS FE OXIDES [ "~ (MODERATELY REDUCIBLE) l

RESIDUAL CRYSTALLINE I FRACTION

Fig. 2. Flow-chart for the chemical fractionation of the sediments and outcrop samples.

Total C and S were determined using a Leco Carbon-Sulfur Analyzer 1, in which the samples are combusted and the resulting C and S gases are oxi- dized to CO2 and SO2. The CO2 concentration is measured by thermal con- ductivity. The SOs is collected in dilute HC1 containing KI and starch. The solution is titrated with standard KIO3 solution and the liberated Is is measured colorimetrically. The SOs concentration is proportional to the liberated Is (Walsh, 1971). Carbonate C was determined using the Leco Analyzer on the residue after the samples had been heated in a muffle fur- nace at 450°(] overnight to destroy organics. Corrections were made for

IThe use of brand names is for descriptive purposes only and does not necessarily con- stitute endorsement by the U.S. Geological Survey.

161

weight loss upon heating. Organic C was taken as the difference between total C and carbonate C.

Several of the rock samples were subjected to X-ray diffraction analysis to determine their mineralogy. In addition, the mineralogy of the heavy- mineral fraction of several of the stream-sediment samples was identified using a binocular microscope.

R E S U L T S

The total concentration of each metal in the rock samples is plotted against distance across the zone of alteration in Fig. 3. The partitioning results and total metal concentrations are listed in Table I, in which the residual fraction is expressed as a percentage of the total metal, and the first

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Fig. 3. The tota l c o n c e n t r a t i o n o f Fe, Mn, Cu, and Zn in rock samples as a func t ion o f d is tance across the z o n e s o f a l terat ion, North Silver Bell depos i t , Arizona. (Lead is n o t inc luded because its c o n c e n t r a t i o n was b e l o w the l imits o f d e t e c t i o n for several fract ions . )

162

T A B L E I

C o n c e n t r a t i o n s of me ta l s in rock samples , N o r t h Silver Bell depos i t , Ar izona . Tota l m e t a l c o n c e n t r a t i o n s are the s u m of m e t a l s f r o m all par t ia l ex t rac t ions . C o n c e n t r a t i o n s of individual f rac t ions are expressed as per- cen t of non-res idua l m e t a l ( for H O A c , C-Hxl, H202, a nd H-Hxl f rac t ions) or p e r cen t of to ta l me t a l ( for res idual f r ac t ion)

Sites a long s t r eam A

10 8 7 6 5 4 3 2 1

Propyl i t ic Phyllic Potassic

F e H O A c (% non-res . ) 3 7 2 7 0.5 4 6 3 2.5 C-Hxl (% non-res . ) 2 4 2 2 1 2 2 1.5 2 H202 (% non-res . ) 2 5 4 10 1.5 2 2 2.5 2.5 H-Hxl (% non-res . ) 93 84 92 81 97 92 90 93 93

Res idual (% to ta l ) 78 67 85 78 88 85 86 74 48

To ta l Fe (%) 2 .94 1 .20 6 .25 2.27 1.18 3.18 2.30 1.73 1 .00

Mn H O A c (% non-res . ) 69 9 15 53 24 13 77 41 59 C-Hxl (% non-res . ) 17 70 71 20 8 63 12 27 7 H202 (% non-res . ) 7 16 10 16 9 21 6 22 6 H-Hxl (% non-res . ) 7 5 4 11 59 3 5 10 28

Res idual (% to ta l ) 37 3 3 18 62 4 28 14 59

To ta l Mn ( p p m ) 324 2 2 5 0 1090 296 311 507 110 181 212

Cu H O A c (% non-res . ) 11 12 41 19 12 11 26 36 37 C-Hxl (% non-res . ) 3 22 12 5 3 12 7 6 10 H202 (% non-res . ) 23 15 19 30 11 19 17 17 28 H-Hxl (% non-res . ) 63 51 28 46 74 58 50 41 25

Res idual (% to ta l ) 50 46 34 42 68 72 68 34 7

To ta l Cu ( p p m ) 657 295 6 6 4 229 3440 219 235 448 3 5 2 0

Zn H O A c (% non-res . ) 16 11 39 24 19 28.5 36 45 32 C-Hxl (% non-res . ) 3 49 24 4 6 20 7 7 5 H202 (% non-res . ) 32 15 18.5 20 19 23 28.5 21 28 H-Hxl (% non-res . ) 49 25 18.5 52 ' 5 6 28.5 28 .5 27 35

Residual (% to ta l ) " 58 32 39 79 86 68 75 48 44

To ta l Zn ( p p m ) 701 490 97 136 3 3 4 28 17 17 39

Pb H O A c (% non-res . ) n .d . 1 n .d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. C-Hxl (% non-res . ) n.d. n.d. n.d. n .d . n .d . n.d. n.d. n.d. n.d. H~O 2 (% non-res . ) n .d . n.d. n .d . n.d. 37 n.d. n.d. n.d. n.d. H-Hxl (% n o n c e s . ) 100 100 100 100 63 100 100 100 100

Res idual (% to ta l ) 2 82 51 n .m . 3 68 80 64 91 n .m. 61

To t a l Pb ( p p m ) 2 27.2 12 .0 n .m . 19.9 46 .5 11 .2 32.8 n .m . 12.9

]n.d. = n o t d e t e c t e d a t a l owe r l imi t of d e t e c t i o n of 2 p p m Pb. 2Total Pb is the sum of Pb c o n c e n t r a t i o n for those f rac t ions in wh ich Pb was p r e sen t in de t ec t ab le a m o u n t s . 3n.m. = no t m e a s u r e d .

four extractions are listed as percentages of the non-residual metals; i.e. each of the first four fractions is given as a percentage of the s u m of the first four fractions.

The yel low Fe mineral scraped from an outcrop at site 48 was identified by X-ray analysis as jarosite. The pale-green Cu mineral from site 62 was

1 6 3

Si tes o n hil ls ide Site a long Average Jaros i te Chrysoco l la b e t w e e n s t reams A and B s t r eam B Sites 48 62 10A 11 12 13 1- -13 Phyll ic

Phyll ic Potass ic

3 6 2 3 4 1 0.5 1 1 1 2 2 1 1.5 3 3 1 3 3 2 3

93 90 96 92 91 96 95

87 81 71 25 73 79 61

3.66 1 .72 2.52 0 .54 3.16 19 .2 1.16

60 77 63 51 47 12 5 20 14 16 37 29 74 83 10 5 8 9 11 10 11 10 4 13 3 12 4 1

19 28 15 6 23 4 4

119 52 171 154 444 334 585

11 12 25 26 21 12 20 2 2 5 5 7 6 9

16 18 19 21 19 14 43 71 68 51 48 52 68 28

75 64 27 7 46 45 1

311 294 1990 2260 1120 2 8 8 5 4 2 . 3 0 0

31 42 17 32 29 17 27 6 7 5 5 11 8 9

31 27 17 19 23 30 27 32 24 61 44 37 45 37

56 80 68 20 58 60 48

16 21 35 36 151 100 84

n.d. n .d . n.d. n ~ . n .d . n.d. n .d . n .d . n .d . n .d . n .d . n .d . n.d. 20 n .d . n.d. n.d. n .d . n .d . n.d. 20 100 100 100 100 100 100 60

53 87 64 n .m . 76 83 50

11.7 17.1 12 .4 n .m . 20 48 40

tentatively identified as chrysocolla, but was X-ray amorphous. The partitioning results for these two minerals are listed in Table I, under the headings "jarosite" and "chrysocolla."

The results of the sequential extractions on the samples of minus-80-mesh sediments are listed in Table II, using the same format as for Table I. The last

164

T A B L E II

C o n c e n t r a t i o n s of me ta l s in s t r eam s e d ime n t , Nor th Silver Bell (NSB) depos i t , Ar izona . To ta l m e t a l c o n c e n t r a t i o n s are the s um of me ta l s f r o m all par t ia l ex t rac t ions . Concen t r a t i ons , of individual f rac t ions are expressed as p e r c e n t of non-res idual me t a l ( for H O A c , C-Hxl, H202, and H-Hxl f rac t ions) or pe r c e n t of to ta l me t a l ( for res idual f rac t ion) , as in Table I

S t r eam A

41 29 28 27 26 44 24 21

Propylitic Phyllic

Fe H O A c <0 .2 < 0 . 2 < 0 . 2 < 0 . 2 < 0 . 2 < 0 . 2 < 0 . 2 < 0 . 2 C-Hx1 2 1 1 2 1 2 1 1 H202 7 13 17 11 12 8 13 16 H-Hxl 91 86 82 87 87 90 86 83

Residual 90 83 86 85 88 80 92 88

To ta l Fe (%) 3 .98 1.96 2.36 1.79 2.27 2.00 4.96 3.22

Mn H O A c 13 13 20 13 15 14 10 11 C-Hxl 61 47 54 59 56 61 55 58 H202 20 34 21 22 23 18 29 25 H-Hxl 6 6 5 6 6 7 6 6

Res idual 22 15 18 16 16 18 21 20

To ta l Mn ( p p m ) 1095 949 976 903 893 882 981 861

Cu H O A c 7 12 9.5 12 13 9 12 12 C-Hxl 9 9 9 12 11 11 11 9 H202 21 25 27.5 24 29 28 27 31 H-Hxl 63 54 54 52 47 52 50 47

Residual 42 33 39 44 47 36 47 43

To ta l Cu ( p p m ) 59 91 91 83 135 85 167 211

Zn H O A c 17 31 35 36 34 33 32 37 C-Hxl 12 17 19 16 21 15 23 17 H202 24 26 24 25 24 23 24 24 H-Hxl 47 26 22 23 21 29 21 22

Res idual 66 28 32 25 25 28 38 34

To ta l Zn ( p p m ) 177 322 366 331 345 327 440 3 4 0

Pb H O A c 12 8 11 5 6 5 9 5 C-Hxl 35 23 29 33 31 34 27 29 H 20 ~ 37 39 35 33 32 36 36 33 H-Hxl 16 30 25 29 31 25 27 33

Res idual 16 8 13 18 15 18 21 32

To ta l Pb ( p p m ) 330 332 348 295 279 290 424 258

IAverage of 7 sites with no known mineralization in the PaPago Indian Reservation, about 60 km south of North Silver Bell. 2n.d. = not detected at a lower limit of detection of 2 ppm Pb.

two columns give the average total concentrations and partitioning for the sediments from North Silver Bell and from a nearby nonmineralized area (Papago). Table III gives the mean metal concentration and standard devia- tion for each extraction of the sediments at North Silver Bell and of the nonmineralized area for comparison. The maximum anomaly/background ratios for Cu and Zn were determined for each extraction by dividing the

165

S t r e a m B Average Average

50 52 54 56 60 61 33 49 59 NSB non-min , l s ites s i tes Una l t ered (?) PhyUic Potassic Unal tered (7)

~ 0 . 2 ~ 0 . 2 ~ 0 . 2 ~ 0 . 2 ~ 0 . 2 ~ 0 . 2 ~ 0 . 2 ~ 0 . 2 ~ 0 . 2 ~ 0 . 2 0.4 1 1 1 1 2 1 1 1 1 1 2 7 7 6 6 6 10 10 6 5 9 10

92 92 93 93 92 89 89 93 94 89 87

87 82 85 83 91 84 87 80 84 86 96

3 ,65 2 .40 3.01 1 .74 3 .15 3.46 3 .93 2.44 1.81 2.83 4.6

11 17 12 15 17 28 23 11 8 15 23 61 54 58 56 57 48 41 61 63 56 43 20 19 22 19 18 16 28 19 20 22 18

8 1 0 8 1 0 8 8 7 9 9 7 16

29 30 29 26 36 22 23 27 25 23 58

787 452 629 548 578 716 667 466 4 4 4 755 6 6 0

9 5 5 4 6 20 22 41 28 13 7 9 7 7 5 9 11 9 36 33 12 7

26 19 14 25 20 41 36 29 48 28 16 56 69 74 65 65 28 33 26 56 52 71

45 55 57 47 65 29 32 37 53 44 66

150 176 143 64 66 856 947 519 222 239 26

35 16 11 14 13 22 22 20 19 25 16 18 I 0 I 0 I I 14 12 29 12 I I 16 8 22 21 22 25 28 40 30 29 28 26 20 25 53 57 50 45 26 19 39 42 33 55

37 57 63 40 58 43 35 53 59 42 83

323 119 127 129 144 306 301 144 102 255 59

3 2 2 5 6 5 8 4 6 6 n.d. 2 33 18 20 25 34 22 18 21 22 27 n.d. 37 35 36 45 33 30 21 27 34 34 n.d. 27 45 42 25 27 43 53 48 38 33 n.d.

29 49 42 28 34 37 32 52 53 29 100

279 99 86 87 128 196 134 100 68 220 < 2 4

m a x i m u m concentration in the North Silver Bell sediments by the average concentration in the nonmineralized sediments.

As mentioned above, samples were collected at site 26 of North Silver Bell in both months to determine reproducibility over time (the October sample is labelled 44). In most cases, the results were comparable to within + 10 percent, which is the reproducibility of analyses done in duplicate

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167

from splits of a single sample. Therefore, it is assumed that there were no significant changes in partitioning over the six months between sample col- lections.

Organic C, which is low (<0.8 percent) in the sediments, comes mainly from cactus and other common desert plants; thus, organic compounds are not effective scavengers of metals in this arid environment.

In the following sections, each of the metals will be discussed individually.

I r o n

About 20 percent of the jarosite from site 48 dissolved in the H-Hxl extraction that was designed to remove hydrous Fe oxides. The remainder required the final harsh extraction for dissolution (Table I), indicating the resistant nature of jarosite to weak extractions. Similarly in the rock samples, almost all of the Fe is divided between the H-Hxl and the residual fractions. However, in samples 1 and 13 from the potassic zone, the H-Hxl extraction accounts for 50 to 75 percent of the total Fe, suggesting that the Fe in this zone is probably a hydrous Fe oxide, which is soluble in the H-Hxl (Chester and Hughes, 1967), rather than the more resistant jarosite. These results agree with theoretical models of Fe oxide and jarosite occurrences in porphyry Cu deposits. Jarosite is expected to form in the leached capping of zones with high sulfur/metal ratio (usually in the phyllic zone, which has a high pyri te/chalcopyri te ratio) because it is stable under fairly acidic conditions. Oxides are expected to form at progressively lower pyrite/chalcopyrite ratios as one approaches the Cu-rich potassic zone (Blanchard, 1968; Saegart et al., 1974). Jarosite was actually identified by X-ray diffraction in rock samples over the phyllic zone, where the maximum total Fe concentration (Fig. 3) and the maximum S/Fe ratios (Table IV, Sites 2, 3, 4, 6, and 7) also occurred.

In the North Silver Bell sediments, about 86 percent of the Fe is in the residual fraction (Table II). This percentage is probably a combination of detrital magnetite, hematite, and altered biotite -- all of which were found in the heavy mineral fraction -- and jarosite. A relatively greater percentage of H202~xtractable Fe is found in the sediments than in the coated rock samples, especially throughout the phyllic zone in stream A (Table II, Sites 21 through 29; Table I). This suggests that the sediments are relatively pyrite-rich. In the nonmineralized area, the concentration of residual Fe in the sediments is higher than at North Silver Bell (Tables II and III), while the concentration in the non-residual fractions are lower, reflecting the higher mobil i ty of Fe in hydrothermal systems.

M a n g a n e s e '

In the samples of rock with coatings (Table I), the Mn concentrations are generally very low, except near the phyUic-propylitic boundary (Sites 7 and

168

8). In this vicinity, a considerable Mn-oxide (C-Hxl) component exists (Fig. 3; Table I, Sites 7 and 8). Depletion of Mn in zones of intense phyUic alteration and(or) enr ichment in the peripheral zone of a porphyry Cu deposit have been reported elsewhere (Jerome, 1966; Olade and Fletcher, 1976; Chaffee, 1976) and have been attr ibuted to redistribution of Mn during hydrothermal breakdown of ferromagnesian minerals to sericite, clay, and quartz. In sericite-rich phyUic zones, Mn (and Zn) tend to be depleted, whereas in chlorite- and epidote-rich propylitic zones, Mn and Zn are enriched (Olade and Fletcher, 1976). Manganese may also be lost from the high-pyrite phyllic zone during the supergene weathering, because the low pH conditions would tend to keep Mn in solution. In either case, Mn oxides are unlikely to be as important as Fe species in scavenging heavy metals on most of the outcrop coatings, due to the generally low Mn concentrations.

Most of the Mn in the North Silver Bell sediments is concentrated in the C-Hxl, or Mn oxide, fraction (Table II). In both streams, Mn concentrations in all of the non-residual fractions decrease downstream. This decrease may be caused by dissolution of Mn during periods of high rainfall. Compared with the non-mineralized area to the south, the North Silver Bell sediments have relatively higher concentrations of non-residual Mn and lower concentrations of residual Mn (Table III). This suggests that Mn, like Fe, is more mobile in the weathering environment associated with hydrothermal systems than in neighboring nonmineralized areas.

Copper

Based on its composition, chrysocoUa should not be greatly affected by the oxidant or mild reductants used in the H202, C-Hxl, and H-Hxl extractions. However, chrysocolla does decompose in dilute mineral acids (Dolezal et al., 1968). Therefore, chrysocoUa might be expected to dissolve, at least partially, in any of the extractions because all extractants have a pH less than 2.5. Indeed, the partitioning results on the sample believed to be chrysocolla (Table I) indicate that some Cu is released in each extraction. In fact, the amount of Cu released is proportional to the duration of the extraction (Fig. 4). In other words, it appears that there is a constant slow dissolution of the Cu mineral throughout each of the extractions due to the mildly acidic conditions. By the end of the H-Hxl extraction, essentially all of the Cu is dissolved. If, then, all of the Cu in the samples of sediment and coated rock were in the form of this one mineral (chrysocolla?), the Cu would appear to be partit ioned among several hydromorphic fractions, as defined by the sequential extraction scheme, rather than as a single mineral. However, it is possible to use the relative proportions of Cu in each step in the extraction of the chrysocolla as a "fingerprint" of this mineral. Similarly, the Cu in the jarosite sample (Table I) leaves a different characteristic fingerprint: In jarosite, Cu occurs mainly in the H-Hxl and residual fractions.

From the above discussion, one can then see that chrysocolla is found

169

I00[

F

F o 6oF

5o L L,J >: ~ 40

d 30 . , ~ i H x [

+ - / o ~ 20 Ac

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1 2 3 4

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s

H 2 Q 2

I 5 6 7 8 9 10

T I M E IN A C I D M E D I U M

~ '+ H - H x l /

12 (hr)

Fig. 4. Cumulative dissolution of the Cu mineral believed to be chrysocoUa as a function of time in the first four extractions (duration of each extraction: HOAc, 2 hrs; C-Hxl, 30 rain; H202, approximately 5 hrs; H-Hxl, 4 hrs).

mainly in stream B, especially at sites 61 and 33, which overlie the chalcocite blanket and have the highest total Cu values (Fig. 1, Table II). The relative amount of chrysocolla decreases downstream, where the majority of the Cu is in the H-Hxl and residual fractions, suggesting an association of Cu with jarosite and other Fe minerals. In stream A, most of the Cu appears to be associated with Fe minerals in the vicinity of site 21 (Table II), where the stream makes its closest approach to the chalcocite blanket (Fig. 1). In general, stream B, which is closest to the chalcocite blanket, has much higher total Cu concentrations than stream A, which flows mainly through the propylitic and phyllic zones (Fig. 1). This could also be due to contamination from the tailings dump located upslope from stream B. It is im- possible to separate contamination from natural processes because the same Cu minerals appear in both the railings dump and in rock outcrops along the bank of stream B.

Although only a small amount of Cu from the sediments is removed in the HOAc fraction, its relative proport ion increases several fold near the chalcocite source (Table II; Sites 21--29 of stream A, all of stream B). In addition, the maximum anomaly/background contrast for this fraction is 240 (Table III) -- higher than for any other fraction. The HOAc fraction is thus+an excellent indicator of secondary Cu mineralization. The forms of Cu removed in the HOAc extraction probably include exchangeable Cu and Cu carbonates, as well as chrysocolla, because all are extracted in varying degrees. Trace amounts of malachite were identified optically in the heavy- mineral fraction of the sediment at sites 28 and 29, in the propylitic zone.

In contrast to the short dispersion train given by HOAc-soluble Cu, the H-Hxl and residual fractions of the North Silver Bell sediments exhibit not

170

only high concentrations, bu t also somewhat smaller deviations relative to their mean concentrations, suggesting a longer dispersion train (Tables II and III). These patterns suggest that although total Cu gives a much lower anomaly/background contrast than HOAc Cu (Table III), the longer dispersion train makes total Cu more useful in the recognition of anomalous areas during reconnaissance surveys.

The amount of Cu in the samples of rock coatings is highly variable (Table I). However, a distinct trend is present in the relative partitioning of Cu into the residual fraction that relates to the alteration type of the surrounding rocks: The potassic zone (Sites 1 and 13) has the highest total Cu concentrations and the lowest proport ion of residual Cu, suggesting that most of the Cu here is chrysocolla. The maximum percentage of residual Cu is found in the phyllic zone, where jarosite dominates the coatings (Table I).

To decrease the effect of variable proportions of rock and fracture-coating material in the crushed rock samples, we determined the ratio of total Cu/Fe (Table IV). The Cu/Fe ratios in the coatings show maximum values within and near the potassic zone and minimum values in the phyllic zone; thus the pattern of alteration could actually be inferred from the Cu/Fe pattern of the coatings. This effect results from a combination of (a) the decrease in intensity of the Cu mineralization away from the potassic zone and (b) the high Fe in the pyrite of the phyllic zone.

Zinc

The samples of rock indicate a zone of high Zn content at the propylitic- phyllic boundary in the vicinity of sites 8 and 10 (Fig. 3). The zoning of Zn is also reflected in the Zn/Fe ratios of the rock samples (Table IV), which increase by one to two orders of magnitude in this same vicinity. The location of Zn mineralization peripheral to a porphyry Cu system is a common occurrence (e.g. Lowell and Guilbert, 1970; Olade and Fletcher, 1976; Fleming and Neale, 1979). It is believed due to redistribution of Zn during hydrothermal breakdown of ferromagnesian minerals, as mentioned above for Mn (Olade and Fletcher, 1976).

The Zn partitioning in the rock samples is similar to that of Zn in the jarosite and chrysocoUa samples, except that HOAc Zn is higher in the rock samples from the potassic zone and much of the phyllic zone (Table I; Sites 13, 1--4, 6, 7, 10, and 11). The large relative percentage of Zn in the H-Hxl and residual fractions and the increase of Zn in the residual fraction from the potassic to the phyllic zone (Table I) suggest coprecipitation of Zn with the Fe in jarosite and hydrous Fe oxides. In addition, a shift can be seen in the relative Zn partitioning from the H-Hxl to the C-Hxl fraction at the locations where Mn oxides are dominant (Table I; Sites 4, 7, and 8), suggesting some compet i t ion by Mn oxides for Zn. This increase in the C-Hxl Zn is much more pronounced than is the case for Cu at the same locations (Table I), indicating that Mn oxides are more efficient scavengers of Zn than of Cu, in

171

agreement with the results of Nowlan (1976) and L.H. Filipek (unpubl. data, 1980) for other geochemical environments.

In the sediments of stream A, the Zn concentrations in all of the non- residual fractions increase several fold at site 29 over their concentrations upstream at site 41 (Table II), reflecting Zn mineralization at the propylitic- phyllic boundary. The concentrations remain high throughout the altered zones and then decrease through dilution downstream from the mineralized system. In stream B, non-residual Zn concentrations are generally low and decrease downstream more rapidly than in stream A. In both streams, the partitioning of Zn is similar. Away from the zone, the majority of the Zn is in the residual and H-Hxl fractions (Table II). Over the altered zone, the more soluble (non-residual) forms, especially the HOAc fraction in stream A and the H20: fraction in stream B, increase in importance (Table II). These increases may be a result of the presence of willemite and(or) zincian calcite, which were identified in the heavy mineral fraction. As with Cu, the HOAc extraction gives an excellent contrast between mineralized and nonmineralized zones for Zn (Table III). However, the HOAc extraction removes a larger proport ion of the more hydromorphically mobile Zn than it does of the less mobile Cu (Table II). Also, due to the association of Zn with Mn oxides discussed above, C-Hxl Zn shows high anomaly contrast (Table III).

Lead

In the rock samples, Pb concentrations are low (Table I). The majority of the Pb is in the residual and H-Hxl fraction; the Pb concentration in the first three fractions is below the detection limit of 2 ppm. These results suggest that Pb in the supergene environment associates primarily with Fe phases. The affinity of Pb for Fe oxides and the rejection of Pb by Mn oxides have been reported previously (c.f. Levinson, 1980).

In contrast to the rock samples, the sediments have high concentrations of Pb over the entire altered zone (Table II). Stream A, which drains the propylitic and phyllic zone, has higher concentrations of Pb than stream B, which drains the potassic zone, in all fractions bu t the residual. Combined with the information from the rock samples, these facts point to a source of Pb in the propylitic zone upstream from site 10, where the first rock sample was collected.

The residual fraction increases in importance away from the mineralized zone, indicating dilution and(or) dissolution of the non-residual fractions. As with Cu and Zn, the HOAc-soluble Pb shows the greatest contrast between the mineralized zone and downstream locations (Table II). This fraction may consist mainly of anglesite, which was found in the heavy-mineral concentrates of the sediments a t sites 28 and 29, or possibly cerussite, which was not seen, but which may be present. In the nonmineralized area, non- residual Pb was undetectable at a lower detection limit of 2 ppm Pb.

172

SUMMARY AND CONCLUSIONS

The data presented here represent samples from one Cu deposit. There- fore, the conclusions reached here cannot be projected to all porphyry Cu deposits. The study does point out, however, that caution must be used when applying partial solution techniques to investigate geochemical partitioning in arid and semi-arid environments, such as' the Basin and Range Province.

In the Basin and Range Province, the water table is usually quite deep -- commonly 60 m or more below the surface. Most of the oxidation of sulfide ore minerals above the water table occurs locally in rock fractures and capillary pores within softs (Blanchard, 1968). Oxidizing sulfides produce supergene ore minerals and Fe oxides which, when exposed in outcrops, be- come mechanically abraded during the violent action of flash floods. The subsequent exposure of abraded particles to f lood waters is generally too brief for significant dissolution and reprecipitation. Thus, anomaly trains in stream sediments in arid environments consist mainly of particles of gossans and ore minerals, rather than of hydromorphic or biogenic precipitates (Lovering and McCarthy, 1978). For this reason, extraction schemes designed to separate hydromorphic components do not produce the same results in arid environments as they do in humid and sub-humid areas. Assignment of unique phases, such as Mn oxides or organic compounds, to a given extraction can lead to incorrect interpretations, as illustrated by the following example: The dissolution of the light-green Cu mineral tentatively identified as chrysocolla, was observed to increase linearly with time as it was subjected to a sequence of extractions. In the C-Hxl, or "Mn oxide," extraction of this mineral (Table I), almost 10 times as much Cu was dissolved as Mn. In this case, it is obvious that the Cu is not associated with Mn oxides. However, if detrital chrysocolla were in the stream sediments, it could be incorrectly interpreted as being associated with Mn oxides (or some other hydromorphic component) based on an uncritical interpreta- tion of the extraction scheme results.

The present results also indicate that the H-Hxl extraction is relatively ineffective in dissolving jarosite and the non-hydrated Fe oxides, such as hematite and magnetite. A hot three-percent oxalic-acid leach was also used on several samples (not reported here) and found to be only slightly more effective than the H-Hxl extraction in dissolving Fe from the rock samples. The relative insolubility of jarosite in the H-Hxl extraction proved valuable because it allowed the separation of jarosite and hydrous Fe oxides, which are soluble in H-Hxl. Thus, ratios of H-Hxl Fe to residual Fe, as well as ratios of Cu, Zn, and Pb, respectively, to Fe, in either of these extracts, and similar ratios of total concentrations of these four elements, delineate effectively the various zones of hydrothermal alteration and the location of buried ore bodies.

Although the partitioning results cannot be interpreted quantitatively,

173

certain minor hydromorphic trends can be distinguished in the stream sediments. For Cu, Zn, and Pb, the HOAc-soluble fraction is most effective in pinpointing mineralization, having the shortest dispersion train and a high anomaly/background ratio. In contrast, the residual, or most resistant, fraction shows the longest dispersion train. The HOAc fraction probably consists of exchangeable metals and carbonates, such as malachite, zincian calcite, and cerussite, along with some anglesite and amorphous silicates, such as chrysocolla. The fact that the HOAc fraction is diluted in the sediments more rapidly than are the more resistant fractions suggests that the minerals in the HOAc fraction undergo dissolution during floods. As the flood waters recede, the dissolved elements either are lost to the underground water table or precipitate on the rock surfaces. The surface precipitates will probably not be the original mineral phases because the concentrations would be too dilute. Therefore, relative to the HOAc fraction, the association of Cu, Zn, and Pb with such phases as Fe oxides should increase downstream from the ore zone, as demonstrated in this study.

ACKNOWLEDGMENTS

This study was made possible through the cooperation and assistance of ASARCO, Inc., and Minerals Exploration, Inc. (Union Oil). We especially wish to acknowledge S. Anzalone and F.T. Graybeal of ASARCO.

We would like to thank J.C. Negri for carbon and sulfur analyses and T. Botinelly for X-ray and optical identification of the heavy minerals, jarosite and chrysocolla. The critical comments of M.A. Chaffee and M.E. Hinkle greatly improved the manuscript.

This work was completed while the first author was a National Research Council Postdoctoral Research Associate with the U.S. Geological Survey.

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