lueth et al-1990

Economic Geology Vol. 85, 1990, pp. 1462-1472

Encoding the Evolution of an Ore System in Bismuthinite-Stibnite Compositions. Julcani, Peru

VIRGIL W. LUETH, Department of Physical Sciences, Tarleton State University, Stephenville, Texas 76402

PHILIP C. GOODELL, AND NICHOLAS E. PINGITORE, JR. Department of Geological Sciences, University of Texas at El Paso, El Paso, Texas 79968-0555

Abstract

Compositional variations in the bismuthinite-stibnite solid solution, metal ratios, and bulk semimetal ratios provide a record of the chemical evolution of the ore fluids responsible for mineralization of the Julcani mining district in Peru. Bismuthinite-stibnites become more antimony rich along the flow paths (defined by changes in the ratio of lead to copper in the bulk ore) of the veins at the Mimosa and Herminia mines. Rapid changes in the composition of the solid solution coincide with the silver bonanza zone at Herminia and with the appearance of complex sulfosalts of silver and bismuth.

Bismuthinite-stibnites with 5 to 45 mole percent Bi were encountered; such compositions lie within the natural gap proposed by previous workers. Although not common, these samples are consistent with experimental studies which document a complete solid solution.

Comparison of bulk semimetal concentrations in the ores of the two mines reveals differences along the respective flow paths. At Mimosa the ratio of antimony to bismuth increases, consistent with a normal zoning of the semimetals. In contrast, at Herminia antimony nearly disappears from the ore assemblage in the zone of silver bonanzas and rapid changes in bismuthinite- stibnite compositions. Farther along this flow path, antimony concentrations increase in the ores, a return to the normal pattern of semimetal zonation. Volatilization of antimony (semimetal complex boiling) from the ore fluid may be responsible for the anomalous behavior of antimony at Herminia. Antimony trihalides are volatile molecular species and potentially can boil from an ore fluid and later recondense, permitting a resumption in the precipitation of antimony- bearing phases.

Introduction

THE role of semimetals in hydrothermal ore deposits has been underestimated. Recent experimental (Bryndzia and Kleppa, 1987, 1988; Wood et al., 1987; Chang et al., 1988), theoretical (O'Leary and Sack 1987; Sack et al., 1987), and petrologic studies (Johnson et al., 1986, 1987) indicate that these elements may have profound effects on the distribution of some ore metals, notably silver. In addition, Sack and Loucks (1985) suggested that compositional flex- ibility and large energies of reciprocal reactions make tennantite-tetrahedrites ideal petrogenetic indicators in the hydrothermal environment, much as spinel is employed for magmatic systems. Sulfosalt and semimetal sulfide solid solutions thus might provide constraints on models of ore formation.

This study focuses on natural bismuthinite (Bi2Ss) and stibnite (Sb2Ss) in the Mimosa and Herminia mines at Julcani, Huancavelica, Peru. Metal zoning is com- pared with the bismuthinite-stibnite compositions to determine if the solid solution reflects that zoning.

Metal Zoning, Distribution of Bismuthinite and Stibnite, and Paragenetic Relations

The Julcani district is of particular interest because of the high semimetal content of the ores, relatively well established paragenesis, and recognized metal zoning. The Mimosa and Herminia mines occupy op- posite ends of the district within the Julcani volcanic center, as depicted in Figure 1.

Metal zoning was first established at Julcani by Goodell (1970) and further defined by Goodell and Petersen (1974) and Petersen et al. (1977). Their studies utilized mineral paragenesis, metal ratio de- terminations from assay reports, and sulfide normative calculations to derive overall zoning and fluid flow patterns for the district. The log Pb/Cu value proved to be a convenient progress variable for evolution of the fluid along its path. Zoning patterns established at Julcani are presented in Figures 2 and 3. Arrows represent the inferred direction of fluid flow based on metal ratios.

1462

ENCODING ORE SYSTEM, JULCANI, PERU 1463

hi

t o k

FIG. 1. Location map of the Julcani district (inset) and location of the mines investigated in this study. Dotted outline shows the extent of the Julcani volcanic center as defined by Petersen et al. (1977) and dark lines show veins. Boxed areas represent maps of individual mines in Figures 2 and 3.

LOG (Pb/Cu) FIG. 2. Map projection summarizing log Pb/Cu values in the

Mimosa mine. Lines represent the traces of the veins at the indicated levels. Dark arrows represent the generally upward flow directions inferred from metal ratios (Goodell and Petersen, 1974). Dotted lines are log Pb/Cu contours, with values specified in circles.

LOG (Pb/Cu.) FIG. 3. Metal ratio map of the 360 level of the Herminia mine showing location of major veins.

Dotted lines represent log Pb/Cu contours, with values specified in the circles. Dark arrows represent inferred flow directions determined by Goodell and Petersen (1974).

1464 LUETH, GOODELL, AND PINGITORE

J Enrq.

Hbl'o.

=90

A

./ HUMBERTO LEVEL 0 ICOm FIG. 4. Vertical section of the Mimosa mine and a map view projected to the Humberto level.

Locations of bismuthinite-stibnite samples are indicated by sample numbers.

Bismuthinite and stibnite occur in a restricted por- tion of the zoning sequence in the Mimosa and Her- minia mines. These minerals surround and are within the silver bonanza zones, with stibnite also at the outer margin of the zoning sequence. Bismuthinite-stibnites are present where values of log Pb/Cu are between -0.7 and 2.3 (compare Figs. 2 and 3 with Figs. 4 and 5). Both minerals were deposited during the main stage of mineralization, sometimes accompanied by tennantite-tetrahedrite. Bismuthinite occurs early in the paragenetic sequence (Fig. 6) and stibnite is usually with late-stage minerals, those formed after the main period of silver mineralization. Various sulfosalt phases were emplaced after bismuthinite but prior to the deposition of stibnite. Bismuthinite and stibnite most often occur as bladed crystals.

Investigative Methods

Mineral samples were chosen from a collection at the University of Texas at E1 Paso, acquired by P.C.G. during the course of earlier studies. Samples from more recent workings at Julcani were provided by J. Deen (Univ. of Colorado).

Samples for electron microprobe analysis were ei- ther handpicked, pure mineral separates, or polymetallic vein material. These were cold set in 1-in- diam epoxy mounts, polished using 0.05/m alumina, and carbon coated.

A Cameca SX-50 electron microprobe at the Uni- versity of Texas at E1 Paso was used for all analyses. Samples were analyzed for three elements: Bi, Sb, and S. Operating conditions include: 25-kV acceler-


330 LEVEL 0 lOOm

FIG. 5. Vertical section of the Herminia mine with a map view projected to the 330 level. Locations of bismuthinite-stibnite samples are indicated by sample numbers.

ating potential, 20-nA beam monitored current, 12- s count time, 1-/m beam diameter, and a counting precision of 2 percent. Bismuth was standardized on an ASTIMEX bismuth selenide standard; antimony and sulfur were based on an ASTIMEX stibnite standard. Only analyses totaling between 98 and 102 wt percent were included in this report. Data reduction was accomplished by the QUANTI program, part of the Cameca software. Full matrix corrections based on Pouchou and Pichoir (1984) were applied to the data. Rapid identification of coexisting sulfide and

sulfosalt phases was accomplished by energy disper- sive spectroscopy on a KEVEX 8000 unit mounted on the probe.

Results

Antimony and bismuth values in bulk ore Bulk chemical analyses for the ore are presented

in Table 1, along with the calculated Iogarithum of the Pb/Cu and Sb/Bi ratios. These values are from the same areas as the samples of bismuthinite-stibnite


$TIB

GN

MAT

PAV

AIK

BIS

CPY

TT

PY

SEM

AD

STIB

BO

ARA

GN

AIK

BIS

CPY

TT

BA

EN.,

PY

I ,am,

EARLY LATE EARLY LATE

MIMOSA HERMINIA

FIG. 6. Paragenesis diagram for sulfides from Julcani. Note that stibnite follows bismuthinite and Bi-bearing sulfosalt deposition. Abbreviations: AD -- andorite, AIK -- aikenite, ARA -- aramoyoite, BA = barite, BIS = bismuthinite, BO -- bournonite, BL -- boulangerite, CPY = chalcopyrite, EN -- enargite, GN = galena, MAT = matildite, PAV = pavonite, PY = pyrite, SB = stibnite, SEM = semseyite, TT = tennantite-tetrahedrite.

which are characterized in Table 2. The log Pb/Cu values are utilized as a progress variable and the concentrations of antimony and bismuth versus this progress variable are shown in Figure 7. Corresponding log Sb/Bi values versus the progress variable are shown in Figure 8.

In the Mimosa mine an enrichment of Sb along the zoning sequence with a corresponding and steady decrease of Bi concentrations in the ores is evident in Figure 7 (with a minor Sb minimum at log Pb/Cu value of-0.25). In Figure 8 the strong relative enrichment of Sb is apparent from the log Sb/Bi values along the zoning sequence, with a minor minimum present at log Pb/Cu = 0. In contrast, the Sb values from the Herminia mine show a more pronounced minimum (Fig. 7). Antimony is responsible for the shape of the log Sb/Bi curve (Fig. 8). Log Sb/Bi values at Herminia display a marked decrease between points in the flow path corresponding to log Pb/Cu values of 0 and 1.0. This log Pb/Cu range coincides with the silver bonanza zone in the mine (Goodell, 1970; Goodell and Petersen, 1974). Antimony concentrations approach zero in the silver bonanza zone. Following this zone with no antimony deposition, the zoning sequence returns to a normal enrichment of antimony. Bismuthinite-stibnite solid solution compositions

For both Herminia and Mimosa the semimetal variation in the bulk ore described before is reflected by a change in the compositions ofbismuthinite-stib-

nite samples along the zoning sequence (Fig. 9). Be- tween log Pb/Cu values of-l.0 to +1.0 the Bi content of bismuthinite-stibnite appears to decrease only slightly. In the log Pb/Cu range of 0.5 to 1.0, which corresponds to the bonanza silver ores, there is a drastic decrease in XBi. At higher log Pb/Cu values the Bi content remains very low.

In the bonanza silver ores bismuthinite-stibnite is replaced by other complex Cu-Ag-Pb-Bi sulfosalt solid solutions, such as pavonite ((Ag, Cu)(Bi,Pb)sSs), aikenite (CuPbBiSs), matildite (AgBiS2), and aramoyoite (Ag2(Sb,Bi)2S4). When present in the bonanza zones, bismuthinite-stibnite solid solution has rare inter- mediate compositions. Farther down the flow path this solid solution is nearly pure stibnite.

Most bismuthinite grains are homogeneous from core to edge as seen in the electron microprobe traverses (Figs. 10 and 11). Pronounced compositional variation is confined to grain edges, with the most bismuth-rich portions of these grains at their base (Fig. 11A). In most bismuthinites, the grain edges are enriched in antimony, with a maximum compositional variation of 12 mole percent (Fig. 11D). Samples with the most variation are proximal to the zones of silver bonanzas, which also correspond to rapid Sb/Bi variations in the bulk ores (samples 8482 and 8485, veins 2 and 2NW, respectively).

Reverse zoning of bismuthinite, with cores rich in antimony and edges or tips enriched in bismuth, was observed in grains in contact with tetrahedrite (Fig.

TABLE 1.


Weight Percentage of Cu, Pb, Sb, and Bi in Bulk Samples from the Herminia and Mimosa Mines and Calculated Ratios

Log Log Level Vein Cu Pb Pb/Cu Sb Bi Sb/Bi

Herminia

420 Doc 9.54 0.32 - 1.47 0.64 0.08 0.90 330 Ano S.46 0.37 - 1.36 1.81 0.06 1.48 420 2 1.94 0,37 - 1.21 0.64 0.20 0.50 420 Doc 2.61 0.40 -0.08 0.79 0.08 0.99 360 Doc 1.88 0.46 -0.06 0.17 0.07 0.38 360 14-271 1.23 1.30 0.02 0.71 0.22 0.50 390 2NW 0.70 1.04 0.17 0.34 0.44 -0.11 330 Doc 0.53 1.07 0.30 0.02 0.08 -0.60 330 Doc 0.29 1.04 0.55 0.05 0.20 -0.60 330 62 0.27 2.20 0.91 0.03 0.14 -0.67 330 323 0.14 1.24 0.95 0.08 0.10 -0.10 390 14-198 0.21 2.02 0.98 0.40 0.10 0.60 390 2 0.31 3.49 1.05 0.69 0.38 0.26 240 14 0.26 3.16 1.08 1.26 0.30 0.62 360 2 0.09 1.32 1.17 1.78 0.10 1.25 420 2 0.23 4.35 1.27 0.94 0.10 0.97 390 14 0.21 4.50 1.33 0.31 0.16 0.28 240 2 0.17 14.80 1.94 1.42 0.19 0.87 240 155 0.13 19.00 2.16 1.13 0.28 0.60

Mimosa

490 Por 3.70 0.40 -0.97 1.36 0.69 0.29 430 Mim 1.57 0.54 -0.46 0.34 0.11 0.49 430 Mim 1.17 0.58 -0.30 0.18 0.08 0.35 460 Por 2.44 1.25 -0.29 0.66 0.28 0.37 490 Mim 1.11 0.63 -0.24 0.53 0.23 0.36 430 Por 2.87 1.71 -0.22 0.70 0.12 0.76 JE SD 3.02 2.20 -0.13 0.65 0.10 0.81

460 A 1.66 1.51 -0.04 0.34 0.24 0.15 490 Por 1.21 1.71 0.15 0.28 0.18 0.19

6 A 2.50 11.80 0.67 1.52 0.06 1.40 6 SD 1.86 14.80 0.90 1.30 0.03 1.60 9 Gaby 0.36 30.60 1.93 0.84 0.02 1.62

Data from propriety reports and Goodell (1970) Vein abbreviations: Doc = Docenita, Mim = Mimosa, Por = Porvenir, SD -- San Demetrio

1 lB). Bismuthinite consistently displays bismuth enrichment adjacent to tetrahedrite and this behavior is even more pronounced at contacts with tennantite. Antimony, unlike bismuth, partitioned preferentially into tennantite-tetrahedrite, and this phase, by locally sequestering Sb, had an effect on the composition of the coeval bismuthinite-stibnite solid solution. Stib- nites which contained a measurable amount of bismuth usually had bases slightly enriched in bismuth (Fig. 11C).

Discussion

In Figure 9 two different populations of compositions are shown, each becoming somewhat more Bi poor as the progress-variable log Pb/Cu becomes more positive. The separation of these two bismuthinite-stibnite populations is consistent with the com-

position gap first described by Springer (1969). By electron microprobe analysis, Springer (1969) doc- umented compositions ranging from 100 to 45 mole percent Bi and from 5 to 0 mole percent Bi (as seen in Fig. 9). On this empirical basis he proposed a compositional gap from 45 to 5 mole percent bismuth. Subsequently, Springer and LaFlamme (1971) reex- amined the phase relations in the system Bi2Sa-Sb2S3 to determine whether the gap in natural compositions was manifest in the experimental system. Their lab- oratory syntheses revealed no gap in compositions at temperatures between 200 and 800C. This is not surprising because bismuthinite and stibnite are iso- structural, with only a 3.5 percent maximum differ- ence in the lengths of the cell edges (Wuensch, 1974). Springer and LaFlamme (1971, p.) then attributed the gap to "completely dissimilar conditions" of de-


TABLE 2. Electron Microprobe Analyses of the Bismuthinite-Stibnite Solid Solution Series at Julcani, Peru

Avg. Sample composition Composition range

no. n Level (m) Vein (mole % Bi) (mole % Bi) Zoning 1 Herminia

8102 19 300 0 None None B26 24 330 54 84 86-76 N/tip B28 11 330 54 86 87-85 None 8693 3 270 Doc 8 9-8 None 3 11 270 82 89 94-82 N-R@ 6029 27 580 Doc 86 94-75 N 8618 25 330 Doc 96 97-95 wR 7636 4 300 14 84 84-83 None 8168 7 420 17 0 None None 8476 25 420 2NW 92 93-81 Tip 8086 23 420 2NW 90 100-79 Tip 8084 15 390 2NW 83 90-82 Tip 8485 34 390 2NW 90 100-79 wN-R@ 8482 21 390 2 85 89-69 Tip 8050 6 360 2 2 3-2 wN 8025 10 240 2 0 None None 8145 2 360 14-198 77 None None 8163 10 420 14-198 88 92-82 R 8103 17 390 4NW 90 97-77 R 8604 2 330 144 64 None None B27 19 240 12 2 5-0 N 8671 12 270 1 9-0 Irr 8486 27 330 8 (10) 27 42-16 N

Mimosa

8289 8 Humberto A 32 45-27 N 8291 4 Humberto A 95 97-94 None B20 29 490 P 93 100-92 Irr

Data from Lueth (1988) 1 Abbreviations: irr -- irregular, N = normal zoning, N/tip -- normal zoning at tip of the crystal, R -- reverse zoning, R@ = portions

of crystal have reverse zoning, tip -- Sb-rich tip of crystal, wN -- normal zoning across width, wR = reverse zoning across width

position. Although our microprobe compositions sta- tistically comprise two groups, the stoichiometries of several samples from both Mimosa and Herminia fall within the previously inferred compositional gap.

Bismuthinite-stibnite compositions become more Bi poor and Sb rich along the zoning sequence defined by the log Pb/Cu progress variable. Such an increase in antimony with evolution of the ore fluid has been suggested for other mineral solid solutions, including tetrahedrite (Wu and Petersen, 1977; Hackbarth and Petersen, 1984) and geocronite (Birnie and Petersen, 1977). Those studies examined arsenic and antimony distributions; arsenic was concentrated in the earlier formed minerals of the solid solutions. The observation that bismuth-bearing minerals precipitate prior to antimony-bearing phases has been noted in a num- ber of deposits (Barnes, 1975) and demonstrated in experimental studies (Wood et al., 1987).

Bulk semimetal contents, illustrated in Figure 8, indicate that the initial log Sb/Bi value at Herminia was higher than at Mimosa. At Herminia a low Sb/Bi value coincides with the silver bonanza zone first defined by Goodell (1970) in an assemblage almost de- void of antimony (Fig. 7). Eventually the compositions at Herminia return to the normal Sb/Bi increase as the ore fluid evolves along the zoning sequence.

The silver bonanza at Herminia coincides with the onset of rapid bismuthinite compositional variation and the appearance of numerous bismuth-bearing sulfosalt phases (Fig. 12). Antimony does not reappear in the bulk compositions until log Pb/Cu rises to 1.0, at the first appearance of pure stibnite. Observation ofsemimetal concentrations (Fig. 7) reveals the cause of the variation in log Sb/Bi. At Herminia the bismuth concentrations in the bulk ore samples increase slightly along the zoning sequence, whereas in the


MIMOSA

2-

HERMINIA

" -I.0 -0.5 0 0.5 1.0 1.5 2.0

LOG Pb/Cn

FIG. 7. Graph of metal concentration against the progress- variable log Pb/Cu. Triangles represent wt percent bismuth; dots represent wt percent antimony in bulk ore samples.

range of log Pb/Cu from 0 to 1.0 antimony is not present. At Mimosa the presence of a minor minimum in the antimony curve suggests that the same situation may apply. The location of the antimony minimum in Figure 7 also correlates with a minor perturbation in the Ag/Cu values at Mimosa, represented by figure 16 of Goodell and Petersen (1974). Nevertheless, the normal pattern of decreasing Bi with a simultaneous increase in antimony in the ores is predominant over the zoning sequence at Mimosa.

The coincidence at Herminia of an absence of antimony with the silver bonanza ores implies a signif- icant perturbation of the chemistry of the ore fluid. Fluid mixing has been suggested as a depositional mechanism for the Julcani ores by Deen et al. (1988). This might terminate antimony deposition in the ore but would fail to explain the reappearance of antimony farther along the flow path.

In our model of semimetal complex boiling, antimony complexes would enter the vapor phase, pre- venting the precipitation of antimony-bearing minerals. When this vapor recondensed farther along the flow path renewed precipitation of antimony-bearing minerals would be possible. During the period of Sb boiling, Ag-Bi sulfosalts would precipitate due to the loss of solvents (antimony complexes?) caused by such a process. Greenwood and Earnshaw (1984) note that antimony trihalides (a potential complex in hydrothermal fluids) behave as volatile molecular species

2.0-

[,5-

1.0-

0.5-

-0.5

-I.0

-I.5

MIMOSA

.....-- HERMINIA

I LOG Pb/C u, 2.0

HERMINIA MIMOSA

FIG. 8. Graph of log Sb/Bi against the progress-variable log Pb/Cu for ore blocks containing bismuthinite or stibnite samples.


XBi

ioo

80

6o

zo

e

i I " - 1.0 - 0.5 0 0.5 hO 1.5 2.0

MIMOSA

HERMINIA

LOG Pb/Cu

FIG. 9. Graph of bismuthinite-stibnite compositions in mole percent against the progress-variable log Pb/Cu.

whereas the bismuth trihalides behave differently. Antimony-bearing sublimates are noted from volca- noes (Krauskopf, 1979), indicating a vapor transport for antimony. Other potential antimony complexes, hydroxy (Wood et al., 1987), thio, and hydroxothio (Krupp, 1988), may behave similarly.

Bismuthinite-stibnite compositional variations reflect the changes in bulk semimetal variation in the ores along the flow path. Compositional variations along the flow path at Herminia and Mimosa are sim- ilar in terms ofbismuthinite compositions. Therefore, variation in the bismuthinite-stibnite solid solution, coupled with bulk semimetal variations in the ores, provides a measure of fluid evolution and records geochemical perturbations in the vein system at Jul- cani.

Conclusions

1. The compositions of individual samples of bismuthinite-stibnite reflect the metal zoning at the Mi-

TIP

PROBE TRAVERSES

BASE

FIG. 10. Sketch of stibnite crystal illustrating traverse paths during microprobe analysis.

mosa and Herminia mines and thus encode the evolution of solution chemistry in these ore systems. Where present, the composition of the solid solution mimics ore zoning; bismuthinite compositions occur near the source of the fluids and the stibnite compositions are distal.

2. The discontinuity or "natural gap" in the bismuthinite-stibnite solid solution (Springer, 1969; Springer and LaFlamme, 1971) is a depositional ar- tifact. At Julcani these minerals have compositions which span the gap between Bi45 and Bis, a finding consistent with experimental studies (Springer and Flamme, 1971).

3. Semimetal complex boiling and the consequent temporary transfer of antimony into the vapor phase appears responsible for antimony depletion in the silver bonanza zone at Herminia, which is marked by Ag-Bi sulfosalt precipitation and rapid changes in the bismuthinite-stibnite compositions. Transfer of antimony to the vapor phase while bismuth continues to precipitate with other sulfosalts may account for the scarcity ofbismuthinite-stibnite with compositions in the so-called natural gap. Semimetal complex boiling may be an important but unrecognized process in other hydrothermal mineral deposits.

4. Detailed semimetal sulfide petrography and microanalysis, coupled with bulk compositional data, is a powerful tool for tracing the precipitation history and fluid evolution at Julcani and may be useful in other deposits. Semimetal ratios appear to be more sensitive than metal ratios to changing fluid chemistry and may be useful in the study of minor variations in the fluid chemistry of ore systems.


BIS

i MM I

FIG. 11. Compositional zoning determined by electron probe microanalysis of individual grains of bismuthinite and stibnite. Vertical axis gives wt percent Sb except C, which is reported in wt percent Bi. BW = base width traverse, L = length traverse, TW -- tip width traverse. In length traverses, the base of the crystal is to the left and the tip to the right. For base and tip width traverses the abscissa is the distance completely across the crystal. A. Sample 6029 illustrates normal zoning. B. Sample 3, a width traverse, shows reverse zoning in bismuthinite included in a tennantite grain. C. Sample 8693 illustrates normal zoning in stibnite. D. Sample 8482, a length traverse, displays an antimony-rich overgrowth on the tip of a bismuthinite crystal.

AIK/PAV WT% MOLE % Acj BONANZA Acj/p b

i

2..,oo &?.-. '80 OMPOSITION STI/ylTE Apb

-ioo

Sb Go .75

.40 -50

.......

"' X, h....__._ 1.0 0.5 0 0.5 1.0 1.5

LOG (Pb/Cu) FIC. 12. Summary diagram [or the I-Ierminia mine. Lines

be]ed Sb and Bi represent wt percent concentrations o these elements in the ores. The Ag-Pb line is calculated rom Ag and Pb values given in figure 15 of Ooodell and Petersen (1974) and shows the close association of the silver bonanza zones with changes in semimetal concentrations in the ores and variation in the compositions of the bismuthinite-stibnite solid solution reported in mole percent Bi. The AIK-PAV bar and arrow represent the approximate range in log Pb/Cu over which aikenite and pavonite series minerals precipitated.

'Acknowledgments

This study is an outgrowth of part of a dissertation presented at the University of Texas at E1 Paso by V.W.L. Partial funding and release time to V.W.L. was provided by the Organized Research Grant program at Tarleton State University. We would like to thank J. Deen at the University of Colorado for the samples from recent mine workings he provided for this study. The Cameca SX-50 electron microprobe used in this study was purchased with funds provided by NSF-RIMI award RII-8504371 and by the Per- manent University Fund of the University of Texas system. C. Podpora provided assistance with sample preparation. Support for N.E.P. was provided in part by a faculty development grant from the University of Texas at E1 Paso. Sulfosalt studies have been supported by the University Research Institute of the University of Texas at E1 Paso to P.C.G. Initial sample collecting was supported by the Buenaventura Mining Company of Lima, Peru.

We would also like to thank two Economic Geology reviewers for their constructive review of the manu- script.


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