bartos 1987

Economic Geology Vol. 82, 1987, pp. 1431-1452

Quiruvilca, Peru' Mineral Zoning and Timing of Wall-Rock Alteration Relative to Cu-Pb-Zn-Ag Vein-Fill Deposition

PAUL J. BARTOS

ASARCO lnc., Rocky Mountain Exploration Division, 9305 West Alameda Parkway, Suite 202, Lakewood, Colorado 80226

Abstract

Quiruvilca district Cu-Pb-Zn-Ag veins have produced over 8 million tons of ore since 1789. The veins occupy tension gashes associated with left-lateral strike-slip faults that cut the central facies of a Miocene andesitic stratovolcano complex. The district has four broadly concentric mineralogic zones from district center to edges which are based on dominant vein fill: enargite, transition, lead-zinc, and stibnite. All veins, regardless of zonal position in the district, share the same four gross paragenetic stages: pyrite, base metal, sulfosalt, and carbonate. Veins closest to the center of the district contain relatively greater proportions of the earlier assemblages. Mineralogic zoning commonly is focused on vein intersections and on a local scale reflects changes in vein width.

Wall-rock alteration types from the vein margin outward and from most intense to least intense are intense sericitic, strong sericitic, moderate sericitic, strong argillic, weak argillic, and propylitic. Petrographic observations indicate that at any given location, propylitic alteration is oldest, followed by argillic, and then sericitic alteration. The zoned alteration halos formed as each inner assemblage advanced, overprinted, and replaced its adjacent outer precedent. The width and intensity of alteration halos enclosing veins increase with depth and toward the district center.

Correlation between paragenetic stages of vein fill and wall-rock alteration is based upon the mineralogy and wall-rock alteration of single-stage veinlets that are zoned about major veins. The veinlet sequence in time correlates with the veinlet distribution in space; this also matches the time-space sequence documented for major veins. Most alteration occurred contemporaneously with the pyrite stage of mineralization. Ore deposition followed in the base metal stage. Thus, ore deposition in the veins in large part postdated formation of the alteration halos associated with those veins. This time sequence is corroborated by breccia dikes which cut alteration halos but precede the base metal stage.

Temperatures of ore deposition, deduced from the stability of various mineral assemblages, decreased through time from •320 ø to <230øC. Fugacity of sulfur (rs2) decreased (from '• 10 -6 to <10 -12'5) both with time and laterally away from the district center. The pH is inferred to have increased laterally away from the center of the district and to have decreased in time at any one spot in the wall rock.

Introduction

THE Quiruvilca district is located in north-central Peru, 80 km east of the city of Trujillo (Fig. 1). Mining at Quiruvilca has been documented as early as 1789 (Santolalla, 1909) and the total recorded production of the district has exceeded 8 million tons. Reserve grades are 2.85 percent Cu, 1.97 percent Pb, 5.78 percent Zn, and 7.25 Troy oz/ short ton Ag (Wippman, 1984).

Veins at Quiruvilca are zoned (Fig. 1) from a copper-rich, enargite-bearing central zone to lead- zinc-silver-manganese outer zones (Lewis, 1956); the district resembles other base metal lode de-

posits such as Julcani, Peru (Petersen et al., 1977; Benavides, 1983), and Butte, Montana (Meyer et al., 1968).

A selected area (Coco-Luz Angelica area) in the southeastern portion of the district (Fig. 2) was chosen as the focal point for studies involving de-

tailed mapping and petrography of vein-fill paragenesis, mineralogic zoning, and temporal relations between veinlets and pervasive alteration halos about large veins.

General Geology

The oldest rocks exposed in the Quiruvilca area are Lower Cretaceous clastic sedimentary rocks that were folded and affected by regional low-grade metamorphism during the early Tertiary, 40 to 65 m.y. ago (Benavides, 1956; Hollister, 1977). Exten- sive Jurassic saline deposits exposed approximately 150 km northeast of Quiruvilca (Benavides, 1968, fig. 4) may underlie the district and have been proposed as the source of sulfur (Petersen, 1970).

The mineralization is hosted by the Calipuy For- mation, an extensive Miocene intrusive-extrusive andesitc pile with an estimated thickness in excess

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QUIRUVILCA, PERU: MINERAL ZONING AND TIMING 1433

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LINE OF SECTION

Legend

80y Vein, showing dip [] Andesitc flows, flow breccias

• Andesire intrusions

?[] Tuffaceous lacustrine sediments

FIG. 2. Surface geology, Coco-Luz Angelica area. Modified after Corporacion Minera Nor Peru maps.

of 2,000 m (Cossio, 1964). At Quiruvilca, the Cali- puy Formation consists of porphyritic andesitc flows and flow breccias interlayered with thin basalt flows and occasional tuffaceous lacustrine sedi-

ments (Lewis, 1956; Bartos, 1984). Zoned, euhedral to subhedral plagioclase (An47 to An54) com- prises 95 to 99 percent of the phenocryst popula- tion of the andesites and averages 20 percent of the rock volume. Mafic phases are hornblende and pyroxene, with pyroxene predominating. The groundmass is orthophyric, composed dominantly of equi- granular hypidiomorphic plagioclase microcrysts with pyroxene and minor altered glass.

Intrusive rocks include andesitc stocks, dikes, and plugs; a premineralization-alteration quartz mon- zonite porphyry stock exposed 2.5 km north of the district (not shown) which is cut by several small Pb-Zn-Ag veins; a series of intensely sericitized da- cite (quartz porphyry) stocks and dikes in the central enargite zone that are believed genetically linked to mineralization (Lewis, 1956); and a zone of unmineralized, slightly propylitically altered da- cite domes that postdate mineralization (Fig. 1).

The geologic environment of the Quiruvilca district is interpreted as the central facies of an andesitic stratovolcano complex based on the presence of andesitic plugs, numerous dikes, and small stocks of andesitic and dacitic (quartz porphyry) composition

and on the radial orientations of flows, flow breccias, and tuffs which dip away from andesitic plugs (Bartos, 1984, fig. 6).

Regional left-lateral strike-slip faults controlled the localization of dikes, domes, and veins at Quiru- viica. Veins are principally contained in two steeply dipping fracture sets, one trending N 60o-70 ø E, the other N 85 ø E-S 85 ø E (Figs. 1 and 2). The N 85 ø E-S 85 ø E fracture set is part of a group of regional strike-slip faults, centered at Quiruvilca and extending for at least 16 km in strike length (Giesecke, 1978). Mineralization is preferentially located in the N 60o-70 ø E fracture set, which is interpreted as a set of tension gashes related to east-west left-lateral shearing on the N 85 ø E-S 85 ø E faults (Lewis, 1956).

Faulting occurred before, during, and after mineralization. Evidence for premineralization faulting consists of: (1) breccia dikes and quartz porphyry stocks and dikes which intruded along fractures that later served as sites f•)r veins, and (2) veins deposited in faults which cut and offset earlier faults

(Lewis, 1956). Local brecciated sulfide grains and vein fragments cemented by late carbonate suggest minor synmineralization faulting. Minor postminer- alization faulting is shown by sheared vein material, fault gouge, and slickensides; the last two occur in veins of all orientations.

1434 PAUL J. BARTOS

Mineralization

General vein characteristics

Mineralization at the Quiruvilca district occurs in over 100 veins, including about 60 that have been developed for mining. Mineable vein widths average 0.3 m and locally may reach widths of 2.5 m (Giesecke, 1978).

Veins at Quiruvilca tend to have extensive lateral and vertical continuity, though with abundant splits, cymoid loops, and pinch and swell structures. In places, a vein may consist of a series of thick ore shoots connected by thinner, subeconomic to non- economic crossings. In general, however. ore-grade mineralization is present throughout the entire vein, a condition similar to Anaconda-age veins at Butte, Montana (Meyer et al., 1968). At depth, veins within the central enargite zone widen, but grades decrease owing to increasing proportions of pyrite (Watson, 1966; W. Eberhart, pers. commun., 1981). The grades at depth in outer zone veins are unknown because those veins have not been suffi-

ciently explored. Veins at Quiruvilea typically are coarse grained

and massive but locally may be vuggy. In many places there is well-developed asymmetrical banding. The bands are usually xenomorphie aggregates of sulfides and gangue, commonly with gangue- filled vugs.

Contacts between vein and wall rock are com-

monly sharp and slickensided (Lewis, 1956). The dominant process of deposition was by open-space filling; replacement occurred to a much smaller extent.

Veins in the Coco-Luz Angelica area

In the Coco-Luz Angelica area, there are two major vein systems: Coco and Luz Angelica (Figs. 2 and 3). The Coco vein system consists of Coco, the Coco splits, the Elbas, and the Right veins. The Luz Angelica vein system consists of Luz Angelica, the Luz Angelica splits, the La Merceds, and perhaps the La Compania veins. The Luz Angelica vein fills a major east-west shear zone. The veins in the Luz Angelica vein system appear to coalesce at its western end. In the Coco-Luz Angelica area, the only other vein with significant development is the Ver- dun vein whose western portion strikes N 60 ø E. To the east, the Verdun vein changes orientation to N 15 ø E and intersects the eastern portion of Luz An- gelica.

District-scale zoning of vein fill

Mineral zones of the Quiruvilca district are based on the mineralogy of the vein filling, which varies laterally on a district scale (Lewis, 1956). From the center of the district outward, these zones are the enargite, transition, lead-zinc, and stibnite zones (Fig. 1, Table 1). These mineralogic zones appear to expand with depth (Lewis, 1956, p. 56). Descrip- tion of the transition and lead-zinc zones is based on

relations seen in the Coco-Luz Angelica area; description of the enargite and stibnite zones is based principally on Lewis (1956), supplemented by samples and observations at selected sites outside the Coco-Luz Angelica area.

For simplicity, veins can be divided into three types based on the dominant vein fill: pyrite, sphal-

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FIG. 3. Cross section of the Coco and Luz Angelica vein systems in the Coco-Luz Angelica area. Line of section shown in Figure 2. Levels (except for the 220) correspond to the elevation above sea level in meters. Modified from Corporacion Minera Nor Peru maps.


TABLE l. Definition of Mineralogic Zone Boundaries and Accompanying Vein Types (based in part on Lewis, 1956)

Zone Inner boundary Outer boundary Vein types I

Enargite Transition

Lead-zinc

Stibnite

Absence of enargite

Presence of megascopic galena

Presence of abundant stibnite

Absence of enargite Presence of megascopic galena

Presence of abundant stibnite

Pyritic Pyritic, sphalerite-galena, (carbonate-base metal),

(calcite-clinozoisite) (Pyritic), sphalerite-galena, carbonate-base metal,

(calcite-clinozoisite) Sphalerite-galena, carbonate-base metal, calcite-

clinozoisite?, quartz-stibnite

1 Vein types in parentheses are uncommon

erite-galena, and gangue dominant--the latter are principally filled by carbonate but locally are dominated by quartz. The central enargite zone has only pyrite veins. Within the transition zone, pyrite veins are the most common, but minor sphalerite- galena and carbonate veins are present. These latter two may represent distal portions of pyrite veins (as at Coco). In the lead-zinc zone, all three vein types are found. The loosely defined stibnite zone consists of quartz-dominated veins with stibnite and gangue-dominated base metal veins (Table 1).

Pyrite veins

Pyrite veins are here defined as containing >50 vol percent pyrite, but they typically have a much greater pyrite content. In the central enargite zone, pyrite veins also contain enargite, with minor amounts of sphalerite, tetrahedrite-tennantite, chalcopyrite, and orpiment. Gangue includes quartz, calcite, and barite. In the transition zone, pyrite veins have no enargite, major sphalerite and tetrahedrite-tennantite, and traces of galena and chalcopyrite. Gangue includes quartz, dolomite, and manganocalcite. In the lead-zinc zone, pyritic veins contain major galena and sphalerite and minor tetrahedrite-tennantite, stibnite, arsenopyrite, chalcopyrite, and marcasite. Gangue includes quartz, manganocalcite, dolomite, and limited amounts of rhodochrosite and calcite.

Typically, pyrite veins are composed of several thick pyritic strands separated by intervals of pyrite-rich gouge. The vein filling is coarse grained and locally vuggy. Most other sulfides occur in vugs, interstitial between pyrite grains, or as small clots or stringers cutting through the pyrite; the last is par- ticularly common for tetrahedrite-tennantite. Enar- gite is commonly concentrated into massive mono- mineralic bands which meander through the vein, tending to favor the vein walls (Watson, 1966).

Sphalerite-galena veins

Sphalerite-galena veins are here defined as containing <50 vol percent pyrite and with sulfides > nonsulfides. Sphalerite content is generally >15

vol percent and averages 25 to 30 vol percent. Sphalerite-galena veins show considerable variation in the relative proportion of vein-fill minerals along strike. Along such veins are bands of distinctly different mineralogic abundance separated by fault gouge or highly altered slivers of wall rock. Gener- ally the bands are 15 to 25 cm wide and consist of varying proportions of sphalerite, pyrite, galena, tetrahedrite-tennantite, manganocalcite, rhodochrosite, quartz, dolomite, calcite, and inclusions of wall rock that have been altered to a white seri-

cite + kaolinitc gouge. Pyrite ___ quartz is commonly found at the margin of sphalerite bands.

Gangue-dominant veins

Gangue-dominant veins are here defined as veins in which the sulfide content is less than the nonsul-

fide content. Generally, the nonsulfide is carbonate; quartz-dominant veins are uncommon. Carbonate- dominant veins include carbonate-base metal veins and calcite-clinozoisite veins. Carbonate-base metal

veins occur in all but the central enargite zone. In the transition and lead-zinc zones they contain pyrite, sphalerite, galena, and tetrahedrite-tennantite as the principal sulfides. Arsenopyrite, stibnite, and marcasite occur sparingly in the lead-zinc zone and rarely in the transition zone. Carbonate-base metal veins in the stibnite zone have greater amounts of stibnite and arsenopyrite but still maintain significant concentrations of pyrite, sphalerite, and galena.

Caleite-elinozoisite veins have the same sulfides and textures as carbonate-base metal veins but also

contain elinozoisite and manganaxinite. Caleite- elinozoisite veins are generally thinner than ear- bonate-base metal veins, averaging 8 to 15 cm in width. Alteration immediately adjacent to ealeite- elinozoisite veins is strong argillie rather than seri- erie. Caleite-elinozoisite veins occur in the transi-

tion, the lead-zinc, and probably the stibnite zones.

Paragenetic sequence

The paragenetie sequence at Quiruvilca was derived from four months of underground mapping at

1436 P,4UL J. BARTOS

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FIG. 4. Reflected light photomicrographs showing sulfide mineral textures. A. Pyrite (py, white), breeeiated; replaced by tetrahydrite-tennantite (tt, light gray) and sphalerite (sl, dark gray). Sphal- erite appears to have replaced tetrahedrite-tennantite and pyrite. Calcite (ca, black), veined and replaced all sulfides. Suggested paragenetic sequence: (1) pyrite, (2) tetrahedrite-tennantite, (3) sphaler- itc, (4) calcite. B. Pyrite (py, light gray), surrounded by arsenopyrite (asp, white). Galena (gl, gray with triangular pits) replaced arsenopyrite. Sphalerite (sl, medium gray) with ehalcopyrite inclusions (small white spots) locally replaced galena and arsenopyrite. Quartz (qz, dark gray) replaced galena along cleavage and sphalerite. Suggested paragenetie sequence: (1) pyrite, (2) arsenopyrite, (3) galena, (4) sphalerite, ehalcopyrite, (5) quartz. C. Pyrite (py, light white), eraeke& veined by ehalcopyrite (ep, white). Galena (gl, light gray with triangular pits) replaced pyrite; replaced by sphalerite (sl, dark gray). Tetrahedrite-tennantite (tt, medium gray) veined sphalerite. Chaleopyrite veined sphalerite and replaced (?) tetrahedrite-tennantite. Calcite (ca, very dark gray) breeeiated, veined, and replaced all sulfides. Suggested paragenetie sequence: (1) pyrite, (2) galena, (3) sphalerite, (4) tetrahedrite-ten- nanrite, (5) ehaleopyrite, (6) calcite. D. Pyrite (py, white), replaced by sphalerite (sl, dark gray) and by

PYRITE STAGE

QUIRUVILCA, PERU: MINERAL ZONING AND TIMING

SULFOSALT BASE METAL STAGE STAGE CARBONATE STAGE

PYRITE

ENARGITE/LUZONITE

TETRAHEDRITE/ TENNANTITE

SPHALERITE

GALENA

CHALCOPYRITE

ARSENOPYRITE

STIBNITE

MARCASITE

ORPIMENT/REALGAR

NATIVE ARSENIC

ALABANDITE

STANNITE, CHATKALITE

BOURNONITE

JAMESONITE

HUTCHINSONITE

PB-AS-S GLASS

PB-AS-SB-S SULFOSALTS

BARITE

MANGANAXINITE

CLINOZOISITE

QUARTZ

RHODOCHROSITE

MANGANOCALCITE

DOLOMITE

CALCITE

TIME

FIG. 5. Generalized paragenetic sequence of hypogene vein minerals, Quiruvilca district. Line thicknesses proportional to phase abundances. Phases of economic interest (enargite, tetrahedrite [principally for its silver content], sphalerite, and galena) are almost exclusively deposited in the base metal stage.

1437

a 1:250 or 1:500 scale coupled with detailed study of more than 180 polished blocks, polished thin sections, and thin sections. Vug sequences and obvious replacement relationships such as relict islands, cleavage veining, overgrowth rims, etc. (Fig. 4) formed the basis of the paragenetic sequence. Ad- ditional information can be found in Bartos (1984).

All veins in the Quiruvilca district, regardless of zonal position, display a paragenesis in which four gross stages are present: a pyrite stage, a base metal stage, a sulfosalt stage, and a carbonate stage (Fig. 5).

Uncommon crosscutting vein relations mimic the paragenesis. At the 3800 level of the Luz Angelica drift, several splits can be seen diverging from the main Luz Angelica vein to form the Verdun vein. At this point, Luz Angelica is a sphalerite-galena vein with the sulfides contained within a quartz matrix. Yet the Verdun vein is composed principally of manganocalcite with minor quartz at the outer mar- gins; this cuts the quartz-base metal sulfides of Luz Angelica. The inference is that the carbonate-dominant Verdun vein postdates the sphalerite-galena Luz Angelica vein.

galena (gl, light gray, left-hand side of picture). Tetrahedrite-tennantite (tt, medium gray) surrounded pyrite. Bournonite (b, slightly lighter), intergrown with tetrahedrite-tennantite. Sphalerite (sl, dark gray) replaced tetrahedrite-tennantite along cleavage. Chalcopyrite (cp, very light gray) veined sphalerite and replaced tetrahedrite-tennantite. No obvious relations between galena and tetrahedrite-tennantite. Suggested paragenetic sequence: (1) pyrite, (2) galena, tetrahedrite-tennantite, bournonite, (3) sphalerite, (4) chalcopyrite. E. Pyrite (py, white), replaced by sphalerite (sl, gray), and locally, stibnite (sb, light gray). Two varieties of pyrite are present: "wormy" and euhedral. Stibnite, replaced (?) by manganocalcite (mc, dark gray). No obvious relations between sphalerite and stibnite. Suggested paragenetic sequence: (1) pyrite, (2) sphalerite, stibnite, (3) manganocalcite. F. Pyrite (py, white), replaced by stibnite (sb, gray). Uncertain relations between stibnite and manganocalcite (mc, dark gray). Quartz (qz, slightly darker gray bleb), surrounded and replaced (?) by manganocalcite and stibnite. No relations between quartz and pyrite. Suggested paragenetic sequence: (1) pyrite, quartz, (2) manganocalcite, stibnite.

1438 PAUL J. BARTOS

Another locality where apparent crosscutting relations between different vein types occur is along strike of the Coco vein on the 3800 level, west of the Coco split. There the vein changes from pyrite to sphalerite-galena. At the point of change, splits principally composed of sphalerite with manganocalcite, dolomite vug fill cut the pyritic main vein segment.

These field relations suggest a time sequence of early pyrite veins, intermediate sphalerite-galena veins, and late carbonate veins, a time sequence that is consistent with the paragenetic relations of vein fill (Fig. 5) and crosscutting veinlet relations (discussed below).

Zonal distribution of vein-forming minerals

The district-scale lateral zoning in the vein fill involves both changes in the proportions of the minerals and changes in the mineralogic species (Fig. 6). Because the different species are from different paragenetic stages, this zoning also involves changes in proportions of the different stages from zone to zone.

Changes in mineralogic proportions are striking. The abundance of pyrite strongly decreases outward; copper sulfosalts are the most abundant in the inner enargite and transition zones; the abundance

of sphalerite and galena reaches a maximum in the intermediate lead-zinc zone; and the abundance of arsenopyrite and stibnite reaches a maximum in the outer stibnite zone (Fig. 6). The proportion of nonsulfide gangue minerals also changes laterally from the center of the district. Barite is most abundant in

the enargite zone; dolomite reaches a maximum in the transition zone; manganocalcite is most abundant in the lead-zinc zone; and rhodochrosite and quartz are most abundant in the stibnite zone (Fig. 6).

Changes in mineral species also occur outward from the district center. Of the copper sulfosalts, enargite is restricted to the central enargite zone, whereas tetrahedrite-tennantite occurs in the outer

zones. Of the arsenic minerals, orpiment and realgar only occur within the enargite zone; native arsenic and arsenopyrite are principally found in the stibnite zone; Pb-As-S glass (Milton and Ingham, 1959; Burkart-Baumann and Otteman, 1972) and sulfosalts are found in the intervening transition and lead-zinc zones.

In short, all paragenetic stages are present in major veins of any zone, but the relative abundance of late-stage minerals increases toward the periph- ery of the district (Bartos, 1983).

The district-scale zoning is also marked by tex-

ENARGITE ___,,,,.•.__ TRANSITION LEAD-ZINC STIBNITE •*-- ZDN • ZON • ZON I• ZON

ORPIMENT/REALGAR

.... >o TO 1% ENARGITE / / / / / / /•:•"--- ß

TETRAHEDRITE/ ------ >1 TO 5% TENNANTITE

SP.LER,TE ....... •.__••///////

o

CHALCOPYRITE

•S8•.O•Y•IT• .............. • ....

NATWE ARSENIC

HOTCHINSONITE •-•S-S GL•SS '

PB-SB-•S-S ........................... SULFOSALTS

BARITE MANA•AX•NTE/ .................. ? • ? ? ?

CLINOZOISITE '

• • • • T • -- •/ / / / / , / / •/////•

MANGANOCALCITE

DOLOMITE --

CALCITE

FIG. 6. Zonal distribution of vein-forming minerals, Quiruvilca district. Certain minor phases not shown owing to insufficient data. The relative proportion of vein fill in each respective zone is semi- quantitative and was derived from the average of visual estimates of vein fill taken when mapping and from samples collected by R. Lewis and the author. Note similarity in appearance to district paragenesis diagram (Fig. 5). At a given point, early phases that characterize more central zones are replaced by later phases that characterize more distal zones.


tural changes. Typically, the average grain size of ore minerals decreases outward from the center of the district. In addition, vein banding becomes finer and better developed away from the center of the district, becoming most evident in the stibnite and lead-zinc zones.

Vein-fill zoning in individual veins Individual veins commonly show considerable

variation in mineralogic abundance along strike and with depth. An excellent example is the transition zone Coco vein, which exhibits roughly bilaterally symmetric zoning away from a pyritic core. Later- ally from the pyritic core, the pyrite and tetrahedrite contents decrease, whereas the sphalerite-galena, total carbonate, and perhaps quartz contents increase (Fig. 7). Vein intersections are clearly the foci of the local zoning at Coco; this is also reflected by metal ratios which in addition indicate near-vertical fluid flow (Bartos, 1984; and in prep.).

The lateral variation in Coco mineralogy in part corresponds to changes in the vein width. An abrupt change from pyrite to sphalerite-galena dominance in the Coco vein on the 3870 level (Fig. 7) occurs where the vein width constricts eastward from 1.5 m to 10 to 17 cm. Farther east, the Coco vein locally reaches widths up to 35 cm and is still sphalerite dominant; local areas with widths greater than this are pyrite dominant. Similar structural correla- tions to vein mineralogy are observed elsewhere on the Coco vein (Fig. 8) and on the Right and Elba 3 veins (Bartos, 1984).

Vertical changes in vein-scale zoning are less dra- matic but still significant: at the Coco vein, the abundance of pyrite, tetrahedrite-tennantite, and

perhaps dolomite increases with depth whereas the abundance of spalerite, quartz, and manganocalcite decreases (Fig. 7). In other words, those phases more characteristic of veins from inner district-scale

horizontal zones are more abundant with depth. Further, the phases which increase in abundance with depth typically tend to be early in the paragenetic sequence (Fig. 5).

The consistent zoning patterns seen at all scales, when coupled with the consistent paragenetic sequence seen in vein fill throughout the district, suggest that the Quiruvilca veins formed during a single mineralization event.

Alteration

Pervasive alteration associated with major veins

Six distinct types of alteration, based on major mineral assemblages and textures, can be discerned. Each alteration type grades into the next so that sharp boundaries between them cannot be mapped. From the vein margin outward, these alteration types are intense sericitic, strong sericitic, moderate sericitic, strong argillic, weak argillic, and propylitic (Fig. 9, Table 2). The first three types, com- prising sericitic alteration, are the following: intense, where wall-rock texture is destroyed and the assemblage is sericite + quartz + kaolinite + pyrite; strong, where texture is preserved and the assemblage is quartz + sericite + pyrite; and moderate, where texture is preserved and the assemblage is sericite + quartz + brown kaolinire + pyrite + calcite. The next two types make up argillic alteration: strong, consisting of sericite + brown kaolinite + calcite + pyrite _ quartz, and weak, which con-

4000m

700W I OOW

PYRITIC CORE• •' 3900rn

3755 •Y •,• dl• •m%• •,' L•U---'• " -- • sl-•-gl • PY •s• I •,•o --• •-• • ; t• c3 •

JUNCTION WITH JUNCTION WITH ELBA COCO SPLIT

0•0m

FIG. 7. Longitudinal section, looking north of the Coco vein showing lateral and vertical variations in vein fill. The Coco vein is a transition zone vein, markedly zoned along strike. Pyritic core shown with strong hachures. Horizontal axis is true scale distance; vertical axis is volume percent vein-fill minerals from 0 to 100 percent estimated at different sites along strike of vein. No vertical exaggeration. Abbreviations: ca = calcite, clz = clinozoisite, dl -- dolomite, gl -- galena, ma = manganaxinite, mc = manganocalcite, py = pyrite, qz = quartz, sl = sphalerite, tt = tetrahedrite-tennantite.

1440 PAUL J. BARTOS

LINE OF SECTION sl dl

I

// //

Ii PY•

tt• II sl , I az IqzJ I sl •-py •---•-g

•o•aaSSoA/ N 3720 LEVEL ø 50 m

VEIN TYPES CROSS-CUTS & LATERALS,,? ß 0 PYRITIC DRIFTS// ß ß SPHALERITE-GALENA

ß ß CARBONATE-BASE METAL

FIG. 8. Variations in relative abundance of vein fill for different vein types along strike, Coco-Luz Angelica area, 3720 level. Each box represents a visual estimation of total vein fill at that site with vertical scale representing 0 to 100 vol percent. See Figure 7 caption for mineral abbreviations. The change in vein type from pyritic to sphalerite-galena at the Coco vein accompanies a change in vein width (from 35 cm to a series of 3-4-cm-wide stringers). At point A, several stringers have coalesced into a pod 80 cm wide, which explains the pyrite-dominant mineralogy. At B, the pyritic Coco vein has narrowed down to a thin gougy zone owing to a strike change; the mineralogy locally changes to sphalerite-galena.

sists of brown kaolinite + calcite + pyrite. The sixth type, propylitic alteration, consists of chlorite + calcite + pyrite _ illite, brown kaolinite, and epidote. Relict plagioclase phenocrysts exist in both argillic and propylitic alteration; relict chlorite (from propylitic alteration) exists in weak argillic alteration (Fig. 9, Table 2). In addition to the three main kinds of alteration, chloritic alteration, consisting of chlorite + quartz + calcite, occurs locally as a distal alteration effect or in areas of abrupt decrease in vein width.

Tourmaline is fairly common in the central part of the district (Entwistle and Hollister, 1957) but is absent from the Coco-Luz Angelica area. The closest known occurrence of tourmaline to the Coco-

Luz Angelica area is a tourmaline-bearing breccia dike intersected in drill core 75 m north of the Coco vein.

Montmorillonites are conspicuous by their absence in argillically altered rocks at Quiruvilca. In- stead, there is a translucent brown, low-relief clay mineral here termed "brown kaolinite." X-ray dif-


DISTANCE FROM VEIN

MINERALS/ ARGILLIC ,fiERICITIC & ROCK •PROPYLITIC TEXTURE LxAvx•v• WEAK STRONG •IODERATE STRONG INTENSE RELICT •LAGIOCLAS•

CHLORITE • •::;•= .....

O VARTZ CA LCITE /

ILLITE

KAOLINITE •:• // /,/./

BROWN KAOLINITE • /r'////• '//•'•"•> --

PYRITE ..........

.EUCOXENE ..... • ..................................

RUTILE .....................................................

CLINOZOISITE -- • ..........

EPIDOTE ..........................

RELICT .......... ,,., .... , ...................... TEXTURE '•'''''' '''''''' ''''•'''' •'''',, ''',•'•

RC• WHITE ', • ',;',',',I', DO)LT. GRA*• +•- ; : : ', ; ',', '1'+ C L•MED-DARK KO• GRAY I '' ' .............. i i i i i i i i i iiii iIii J•-'{ -

LEGEND

ß ß ß ß 0 TO 1% 5o• I I I I PRESENT 25 •10% I TO 5% + + •- MAY BE PRESENT

lo

• 5 T010%

FIC. 9. Generalized ranges and amounts of the most common alteration minerals in wall rock, Coco-Luz Angelica area.

fraction suggests that brown kaolinite is a mixture of chlorite and kaolinite.

Fresh pyrite at or within a few meters of the surface throughout the area of the deposit indicates that alteration minerals associated with the pyrite are primary and not related to supergene processes.

Alteration types are distributed symmetrically about major fractures (Fig. 11A). Within the alteration halo about a major vein the entire rock is pervasively altered. All wall rock in the Coco-Luz An- gelica area is altered to some degree but was origi- nally chemically and physically homogeneous. Differences in mineralization and alteration style cannot be attributed to differences in wall-rock composition.

Width and intensity of alteration halos are related to vein type (Figs. 10 and 11A), lateral distance from the center of the district, and depth (Fig. 11A). Pyritic veins have large sericitic halos including the intense and strong subtypes. Sphalerite-galena veins have thinner sericitic halos commonly with the strong and moderate subtypes but locally with intense sericitic alteration. Carbonate-base metal veins have sericitic halos of the moderate

subtype. Calcite-clinozoisite veins have strong argillic alteration halos.

Petrographic textures suggest that at any one area, propylitic alteration was earliest, followed by argillic and then sericitic alteration. In weak argillic alteration, mafic sites are almost entirely composed of chlorite (from propylitic alteration) with cloudy patches of brown kaolinite intermixed with and ap- parently replacing the chlorite. Plagioclase is replaced by calcite, brown kaolinite, and very minor amounts of sericite, illite, and chlorite along cracks and cleavage planes. In strong argillic alteration, sericite and pyrite replace the chlorite along microfractures and in patchy zones. Sericite, calcite, and brown kaolinite occur within certain zones of the

oscillatory zoned plagioclase or fill microfractures. In moderate sericitic alteration, plagioclase sites

commonly contain a center of coarse calcite plates with outer sericite and brown kaolinite. Near the

strong to moderate sericitic boundary, the calcite plates have ragged edges and are being replaced by sericite and brown kaolinite along these edges and also along cleavage cracks. Sericite and brown kaolinite appear to coexist stably in moderate sericitic alteration. However, where relations can be discerned in strong sericitic alteration, sericite appears to replace brown kaolinite by rimming it or by mi- croveining it. Sericite and kaolinite occur together in the groundmass of strong sericitic alteration and within intense sericitic alteration. Typically, these minerals are too fine grained to determine whether a reaction relationship is present. However, locally in intense sericitic alteration, microveinlets of kaolinite cut and may possibly replace sericite.

Veinlets and their associated alteration

Surrounding the major veins are myriad minor veinlets. Like major veins, the dominant process of deposition in these was open-space filling with little evidence for wall-rock replacement. Veinlets may be anastomosing or through-going; larger veinlets are generally through-going and have orientations subparallel to the nearest major vein. As a result, crosscutting relations between different veinlet types are uncommon. Veinlets are zoned about major pyrite veins with quartz ___ pyrite, pyrite, and sphalerite +__ galena veinlets in a proximal position, followed by carbonate-base metal veinlets in an intermediate position, with calcite-clinozoisite and calcite veinlets most distal (Figs. 11 and 12). Veinlet zoning has not been recognized in the walls of sphalerite-galena-dominant or carbonate-dominant veins.

Veinlet types are described in Table 3. Each veinlet type may have several different alteration selvages, but in a local area of constant background alteration, each veinlet type has only one type of alteration selvage (Table 4). Crosscutting relations between veinlets of different types yield a consis-

1442 P.4UL J. BARTOS

TABLE 2. Mineralogy of Pervasive Alteration

Pervasive Magnetite- alteration ilmenite Important reaction

type Plagioclase sites Mafic sites sites Groundmass Typical assemblage or texture

Intense No relict sericitic texture

Strong Sericite Sericite, Sericite, sericitic pyrite, pyrite,

(cloudy cloudy rutile) rutile

Moderate Sericite, brown Sericite, sericitic kaolinitc, calcite, brown

___illitc kaolinRe, pyrite, (quartz)

Strong Brown kaolinitc, Brown argillic sericite, calcite, kaolinitc,

plagioclase*, sericite, +_clinozoisite, pyrite, +_illitc calcite,

(cloudy rutile), +_chlorite*, _+clinozoisite

Weak Plagioclase*, Chlorite*, argillic calcite, brown calcite,

kaolinitc, pyrite, (sericite), (illitc), brown (chlorite) kaolinitc,

sericite

Propylitic Plagioclase*, Chlorite, calcite, chlorite, leucoxene, _+illitc, +-epidote, +_pyrite, +brown kaolinitc +_epidote

Sericite, quartz, Sericite, quartz, Sericite -• kaolinitc, pyrite, kaolinitc, pyrite, kaolinitc; no (clear rutile) (rutile) relict texture

Quartz, sericite, Quartz, sericite, pyrite Brown pyrite, kaolinitc kaolinitc -•

sericite

Leucoxene, Quartz, sericite, Quartz, sericite, Calcite-• sericite iron pyrite, brown brown kaolinitc, and brown oxides, kaolinitc pyrite, calcite, kaolinitc; no cloudy (rutlie), (leucoxene) relict plagioclase rutile, pyrite

Leucoxene, Quartz, pyrite, iron sericite, brown oxides, kaolinitc or cloudy plagioclase*, rutile brown kaolinitc,

pyrite, calcite

Brown kaolinitc, sericite, pyrite, calcite +_ quartz, (rutile), (leucoxene)

Leucoxene, Plagioclase*, calcite, Brown kaolinitc, iron pyrite, brown calcite, pyrite, oxides, kaolinitc, (rutile), (leucoxene) rutile chlorite*

No visible chlorite

(hand sample)

Leucoxene, Plagioclase*, calcite, Chlorite, calcite, iron chlorite, leu- pyrite, (rutlie), oxides, coxene, pyrite, (leucoxene), +-illitc, rutile +_brown kaolinitc, +_brown kaolinitc,

+_illitc +-epidote

Chlorite -• brown kaolinitc

Chloritic Chlorite, sericite or Chlorite, Leucoxene, Chlorite, quartz, Chlorite, quartz chlorite, brown calcite, iron sericite or kaolinitc, calcite brown oxides, chlorite, brown

kaolinitc, rutile kaolinitc, calcite pyrite

Sericite -• chlorite

Key: ( ) = minor, * = relicit phase, -• = is replaced by; phases listed in order of relative abundance

tent age sequence (from oldest to youngest): quartz +_ pyrite, pyrite, sphalerite _ galena, carbonate- base metal, calcite-clinozoisite, and calcite. Thus the veinlet sequence in time correlates with the veinlet-type distribution in space away from a major pyrite vein; it also matches the time-space sequence documented for major veins.

The similarity in time relationships and mineralogy between veins and veinlets suggests that deposition of both occurred during the same period in the history of the hydrothermal system and that specific stages of vein fill can be correlated with specific veinlet types. If so, then the alteration sel-

vages associated with specific veinlet types can be correlated to specific paragenetic stages of vein fill.

Figures 11 and 12 compare distribution of pervasive alteration types about the Coco vein system to the alteration selvages associated with individual veinlets. Sericitic selvages on pyrite, sphalerite _ galena, and carbonate-base metal veinlets are first visible near the outer boundary of strong sericitic alteration on major veins (Figs. 11A and 12A, B, C). Closer to the vein, the rock is pervasively sericitized and no sericitic selvages on the veinlets are visible. This is the typical case for quartz ___ pyr- itc and sphalerite veinlets (Table 4). Beyond the


N45W S45E

LOOKIN• NORTHEASTERLY •___: __ __•d•___ • __

_7

= • / •-- • ............ o // I- • .•qz 3755 LEVEL

.' / /

sl dl 3720 LEVEL

.

220 LEVEL

NO VERTICAL EXAGGERATION

• 2'5 51Ore

FIG. 10. Cross section of Coco vein showing vertical variations in vein fill. Vertical axis in block diagrams represents volume percent vein fill from 0 to 100 percent. Line of section shown in Figure 8. See Figure 7 for mineral abbreviations. Differences between pyritic veins such as Coco and sphalerite-galena veins such as Elba 3 are readily apparent. Compare vein-fiji assemblage to width and type of alteration halo (Fig. 11A). The Elba 3 vein on the 3870 level has a mineralogy corresponding to a calcite-clinozoisite vein; this is accompanied by strong argillic alteration. Else- where the vein has a sphalerite-galena mineralogy with accompanying strong sericitic alteration.

front of sericitic selvages, pyrite veinlet abundance docreases (Fig. l lB); these veinlets typically contain no discernible alteration selvages within argillically or propylitically altered rocks (Table 4). Car- bonate-base metal veinlets contain chlorite and/or brown kaolinitc selvages in strongly argillized rock or in moderately sericitized rock near the boundary of the strong argillic alteration type. The absence of alteration selvages associated with carbonate-base metal veinlets in argillized rock is also common (Table 4).

Calcite and calcite-clinozoisite veinlets only occur in argillically or propylitically altered rocks (Fig. 12D and E). Calcite-clinozoisite veinlets are found in argillized rocks and have a characteristic brown kaolinitc selvage. Calcite veinlets are the most distal in the Coco-Luz Angelica area (Fig. 12); these typically do not contain a discernible alteration selvage even in propylitically altered rocks (Table 4).

Table 5 summarizes the observations which allow

the correlation of the alteration style seen as selvages on veinlets to specific paragenetic stages of vein fill. The pyrite stage of vein fill can be correlated with sericitic alteration. The base metal and

sulfosalt stages can be correlated with both sericitic and argillic alteration, but sericitic alteration is interpreted to have remained dominant during those stages. The carbonate stage can be correlated with sericitic, argillic, and propylitic alteration (Table 5).

Breccia dikes and their associated alteration

Breccia dikes are thin tabular bodies containing angular to rounded clasts of sericitically altered andesitc and minor quartzite, in a matrix of pulverized rock, clays, sericite, quartz, and pyrite (Fig. 13A). Tourmaline occurs in the matrix of breccia dikes in

the central portion of the district. Many small breccia dikes occur within the Coco-Luz Angelica area, mostly associated with the Luz Angelica fault system. The dominant strike of Coco-Luz Angelica breccia dikes is east-west (_10 ø) and the dips are usually steep. Widths average 3 to 30 cm and vertical dimensions are unknown. In rare cases, the smaller breccia dikes split and feather out toward the surface. Variation among breccia dikes is seen in the amount, size, and degree of rounding of the clasts. Relative timing between different types is ambiguous (Fig. 13B). Among the types are fine- grained breccia dikes consisting of a sericite-illite matrix with subordinate amounts of quartz, fine- grained pyrite (up to 15%), brown kaolinitc, and small (0.75-2.5 mm) sericitically altered andesitc (?) rock fragments. Generally, the contact of the breccia with its walls is sharp but somewhat sheared, and the wall rock is sericitically altered. Fine-grained breccia dikes commonly display flow layering oriented parallel to the wall-rock contact. Another variety contains lathlike pieces of sericitically altered andesitc in a matrix similar to the fine- grained breccia dikes. These fragments comprise around 50 percent of the dike volume and have dimensions of 0.3 to 2.5 cm, with the long axis subparallel to the dike contact. Shape, angularity, and orientation of the clasts suggest that they were spalied off from adjacent wall rock and underwent little transport. A third variety is true pebble dikes.

1444 PAUL ]. BARTOS

A

ALTE• SERICITIC "•:il

:......[] '•OOERATE • ARGILLIC

'?[] STRONG •[] WEAK

PROPYLITIC

•..'::'!.:,

',4 ':.*:;.• ;' ;v.:::.:•:: ./.:..':o

:.• ::.:.::. :: .:

B N4SW

0 25 50m NO VERTICAL EXAGGERATION

PYRITE VEINLET DENSITY

FI•. 11. A. Cross section of alteration about the Coco vein system. Enclosed ovoids are drifts; double dashed lines are crosscuts. Line of section shown in Figure 8; crosscuts projected to this line. The width of the intense sericitic alteration halo is too thin to be depicted at this scale. The small area of anomalous propylitic alteration near the Coco vein on the 3870 level represents a late andesitc dike. Blank areas at eastern end of workings represent area affected by alteration halo of the Luz Angelica vein system; this alteration has not been shown for purposes of clarity. B. Pyrite veinlet density about the Coco vein system. Note close correspondence of pyrite veinlet density to alteration type (A).

These contain 40 to 65 percent subangular to rounded, poorly sorted clasts of quartzite and seri- tically altered andesitc in a matrix of sand to silt- sized pulverized rock fragments, along with quartz, pyrite, sericite, and brown kaolinitc. The sedimentary rock clasts indicate upward transport of at least 250 m.

Base and precious metal deposition with accompanying alteration occurred after the emplacement of the breccia dikes. The Probable vein (Fig. 1) cuts a pebble dike and its wall-rock contact. Fragments of brecciated vein fill are absent from breccia dikes, suggesting that deposition of vein fill occurred after emplacement of breccia dikes. Veinlets of all types except quartz _ pyrite and rare pyrite veinlets cut breccia dikes. Pyrite cubes in breccia dike matrices typically surround clasts and are euhedral (una- braded) indicating that pyrite formed after the matrix was in place.

Breccia dike clasts, matrix, and adjacent wall rock are consistently pervasively sericitically altered. However, there are rare breccia dikes at Quiruvilca in which the clasts are differentially altered, from strong argillic with 15 percent relict feldspar to strongly sericitic alteration.

Interpretation of the Space-Time Evolution of Alteration and Mineralization

Petrographic relations indicate that, for any given location, propylitic alteration developed first, followed by the argillic, and then the sericitic assemblages (Table 2). This sequence suggests that the zoned alteration halos formed by the advance of the outer edge of a particular alteration zone while the inner edge was being contemporaneously replaced by the next inner zone (Graton and Bowditch, 1936; Sales and Meyer, 1948). However, deposition of veinlet and vein fill largely postdated the development of the zoned pervasive wall-rock alteration adjacent to major veins. If the deposition of veinlet fill had occurred precisely contemporaneously with the development of pervasive wall-rock alteration, then, for example, younger pyrite veinlets associated with sericitic alteration would necessarily have cut older calcite-clinozoisite veinlets asso-

ciated with earlier argillic alteration. However, mapped field relations indicate that, instead, calcite-clinozoisite veinlets consistently cut and displace pyrite veinlets. Additionally, some veinlets and their associated alteration selvages contradict


B

0 25 50m

SPHALERITE +/- GALENA

O 25 50m

CARBONATE - BASE METAL

ALTERATION HALOS:

D

'• .![] REGION OF • / i !

OCCURRENCE •},/ I I /

/ ,d

ß I

:' ' t

; ß I

•? f:---:: 0 25 50m

CALCITE + CLINOZOISITE

:[] NONE '.::[• SERICITE

] CHLORITE ] BROWN KAOLINITE

E

;;•[] REGION OF • / OCCURRENCE • /

,p- -- ----/b

! i i

I I In z• ......

I /, : I ;

... :

CALCITE

FIG. 12. A. Distribution of alteration selvages on pyrite veinlets about the Coco vein system. Pyrite veinlets lacking alteration selvages occur throughout the figure (Fig. 1lB). Line of section shown in Figure 8. B. Distribution of sphalerite-galena veinlets and accompanying alteration selvages about the Coco vein system. C. Distribution of carbonate-base metal veinlets and accompanying alteration selvages about the Coco vein system. D. Distribution of calcite-clinozoisite veinlets about the Coco veiu system. Note absence of calcite-clinozoisite veinlets in region of overlap with high density of pyrite veinlets southeast of Elba vein on the 220 level (Fig. 1 lB). E. Distribution of calcite veinlets about the Coco vein system. Note restriction of calcite veinlets to weakly altered rocks far from major veins (Fig. 11A).

the simple model of outward-expanding pervasive alteration halos; such contradictory features include the sphalerite _+ galena and carbonate-base metal veinlets with chlorite or brown kaolinitc selvages which were observed within sericitically altered wall rock (Fig. 14).

The preferred interpretation of these field relations is that veinlet filling somewhat postdated the bulk of pervasive wall-rock alteration. If the deposition of veinlet fill was synchronous with the corresponding paragenetie stages of major vein fill (Table 5), then deposition in the major veins must have

1446 PAUL J. BARTOS

TABLE 3. Characteristics of Individual Veinlet Types

Veinlet type Description Key observations

Quartz _ pyrite Thin (0.005-0.5 mm), wispy to throughgoing Strongly sericitized fragments containing cutoff quartz _ pyrite veinlets found in breccia dikes

Most common veinlet type; wide variety of morphologies from Density of pyrite veinlets corresponds to hairline thin to 5 cm thick, wispy to through-going distribution of different alteration types

Dominantly composed of zoned, light brown to red-brown Typically found in sericitically altered sphalerite plus minor pyrite; galena, if present, found at rock, in some cases, beyond an outer veinlet margin intervening interval of pyrite veinlets

Sericite is a minor veinlet filling associated with sphalerite even in veinlets with argillic selvages

Pyrite

Sphalerite _ galena

Carbonate-base metal Carbonate mineralogy varies with depth--upper levels: manganocalcite; lower levels: dolomite or calcite; quartz occurs as singly terminated crystal growing outward from vein margin; sulfides (sphalerite, pyrite _ galena, rare tetrahedrite-tennantite) inner to quartz may be replaced by late quartz; carbonate latest, filling inner portion of veinlet

Calcite-clinozoisite Commonly anastomosing and thin (0.5-1.0 mm); clinozoisite typically occurs at outer veinlet margin separating brown kaolinitc selvage from calcite veinlet filling

Calcite Veinlets composed dominantly of calcite with rare chlorite veinlet filling

Selvage of brown kaolinitc ___ pyrite, rare chlorite characteristic

Typically contain no alteration selvage, even in propylitically altered rocks

postdated the bulk of pervasive alteration as well. Ore deposition which postdated pervasive alteration has been documented in other base metal lode

deposits (Farmin, 1934; Lovering, 1949; Tooker, 1963).

At Quiruvilea, there is strong evidence that a portion (exact amount unknown) of the alteration preceded vein fill ore deposition. Serieitie alteration was interrupted by breeeia dike eraplacement, (Fig. 13C), whereas ore mineralization dearly postdated breeeia dike formation. Mineralization which

appears to have postdated breeeia dike formation is common at other base metal and Pb-Zn-Ag lode deposits such as East Tintie (Farmin, 1934; Morris and Lovering, 1979), Juleani (J. D. Benavides, 1983, pers. eommun.), Bisbee (Bryant and Metz, 1966; Bryant, 1968), Cerro de Pasco (Einaudi, 1982, pers. eommun., 1983), Camp Bird (Spurt, 1925), Smug- gler-Union (A. Colima in Spurt, 1925), and Bache- lor-Syracuse (King and Allsman, 1950; Bartos, pers. observation, 1983).

The distribution of pervasive alteration strongly mimics the density of pyrite veinlets (Fig. 11). However, many of these pyrite veinlets lack alteration selvages regardless of the background pervasive wall-rock alteration in which they occur (Table 4). Thus, the formation of pervasive alteration is probably unrelated to the deposition of pyrite veinlets. Both phenomena reflect the distribution of fracturing and permeability about major structures. The front of serieitie selvages on pyrite, sphalerite- galena, and carbonate-base metal veinlets in seriei-

tized wall rock probably represents the last minor outward expansion of the pervasive sericitic halo which was principally developed before these veinlets and their alteration selvages were deposited.

In summary, field and petrographic relations demonstrate that pervasive alteration formed as a series of contemporaneous zones that grew outward with time; inner zone minerals replaced outer zone minerals. Veinlet fillings and selvages were super- imposed on this early pervasive alteration event. In contrast with the development of pervasive alteration zones, veinlet zones did not grow contemporaneously; rather, inner zones are oldest and outer zones are youngest.

A generalized sequence of alteration and mineralization can be summarized as follows:

1. Initial development of the hydrothermal system utilizing a preexisting fracture set. The hydrothermal system was probably triggered by the igneous event represented by the emplacement of quartz porphyry dikes.

2. Initial development of zoned alteration halos around major fractures. Accompanying initial development of strongly sericitic alteration halos were quartz +_ pyrite veinlets with sericitic selvages. No significant deposition of vein fill occurred, as evi- denced by the lack of vein fragments in breccia dikes.

3. Emplacement of breccia dikes. 4. Major development of zoned alteration halos

by intergranular infiltration and diffusion, probably

Q UIRUVILCA, PER U: MINERAL ZONING AND TIMING 14 4 7

TABLE 4. Location of Veinlet Types and Associated Alteration Selvages in Pervasive Alteration

Veinlets and their associated

alteration selvages Background (pervasive) alteration

Alteration Strongly Moderately Strongly Weakly Veinlet type selvage sericitic sericitic argillic argillic Propylitic

Quartz None _+ pyrite Sericitic

Pyrite

Sphalerite _+ galena

Carbonate- base metal

Calcite- clinozoisite

Calcite

None

Sericitic

Chloritic

None

Sericitic

Chloritic Brown kaolinite

(argillic)

None

Sericitic

Chloritic Brown kaolinite

(argillic)

Brown kaolinite

(argillic)

None

Chloritic

m --

M M

c M

r r

r --

r --

c c c

c c r

r m m

-- m c

M c

m

c M M -- r r

M = major c -- common

m = minor

r = rare

- absent

synchronous with the early portion of the pyrite stage of vein fill.

5. Advance of pervasive alteration halos by co- alescence of sericitic alteration selvages about some

pyrite veinlets, synchronous with the pyrite stage of vein fill.

6. Continued minor advance of pervasive alteration halos and formation of sphalerite _+ galena

TABLE 5. Correlation of Paragenetic Stage to Alteration Type

Paragenetic stage Alteration type Evidence

Pyrite Sericitic

Base metal Sericitic

Sulfosalt

Carbonate

Argillic

Sericitic

Argillic

Sericitic

Argillic

Propylitic

Presence of sericitic selvages on pyrite veinlets in strongly argillized rocks; association of sericitic alteration with veins which contain the early pyrite stage; strong correlation of high pyrite veinlet density to areal extent of pervasive strong sericitic alteration; absence of brown kaolinite alteration selvages on pyrite veinlets

Absence of alteration selvages on sphalerite _+ galena veinlets in pervasively sericitized rocks (implying stability with sericitic alteration); presence of sericitic selvages on sphalerite _+ galena veinlets in sericitized rocks, outboard of sphalerite _+ galena veinlets without visible sericitic selvages; direct association of sericite with sphalerite in vein fill of carbonate-base metal veinlets regardless of alteration selvage

Brown kaolinite selvages on sphalerite _+ galena veinlets in strongly sericitized rock

Sericitic selvages associated with quartz-stibnite veins Brown kaolinite selvages on calcite-clinozoisite _ pyrite, sphalerite, galena veins and

veinlets in argillically altered rocks

Sericitic halos on carbonate-base metal veins (i.e., Verdun, La Compafiia) and veinlets in sericitized and argillized rocks

Carbonate and brown kaolinite selvages associated with some carbonate-base metal veinlets which lack vein-filling sericite; brown kaolinite alteration selvages associated with calcite _+ pyrite veinlets in argillically altered rocks

Absence of alteration halos on calcite veinlets in propylitically altered rocks implying stability with propylitic alteration

1448 PAUL J. BARTOS

A ! I

, !

FIG. 13. Breccia dikes. A. Typical breccia dike. The white, "spoRed," subround clasts are strongly sericitized andesitc fragments. The light gray, subangular clasts are quartzite. The matrix consists of sand-sized, pulverized, sericitically altered rock fragments, quartz, sericite, pyrite, and brown kaolinitc. Scale bar is 5 cm in length. B. Contact showing a pebble dike (upper left), which contains rounded clasts and relatively little matrix, curing a breccia dike with subangular clasts with relatively more matrix. Consistent crosscutting relations between the various types of breccia dikes were not observed; however, vein mineralization consistenfiy cuts all breccia dikes. Length of pencil is approxi-

s!

FIG. 14. Sphalerite (sl, dark black) veinlet with brown kaolinitc alteration selvage (bk, black) in strong sericitically altered wall rock (s, speckled). Carbonate (c, white) fills vugs and separates the sphalerite from the brown kaolinitc. Veinlets such as this constitute evidence against a simple outward expanding pervasive alteration halo model.

veinlets with sericitic selvages, simultaneous with the base metal stage of vein fill mineralization.

7. Formation of carbonate-base metal veinlets

with differing alteration selvages dependent upon spatial position within the pervasive alteration halo about a major vein. This correlates in time with the later part of the base metal stage, the sulfosalt stage, and the initial portion of the carbonate stage of vein filling.

8. Formation of calcite-clinozoisite veinlets with

brown kaolinitc selvages, distal from major veins in argillically altered rocks. This correlates to the sulfosalt and early to mid-carbonate stage of vein filling.

9. Formation of late, distal calcite veinlets with no alteration halos in propylitically altered rocks, correlated to the late carbonate siage of vein filling.

Geochemical Environment of Ore Deposition

Temperature constraints

Temperature constraints for the environment of ore deposition at Quiruvilca are based on sulfide and silicate phase equilibria. In the central enargite zone, abundant enargite-luzonite intergrowths of the early base metal stage (C. R. Patri, 1961, unpub. data) indicate a temperature of formation in the region of 300 ø to 320øC, which is the lower stability temperature range of enargite (Maske and Skinner, 1971; Feiss, 1974). The presence of enargite and

mately 16 cm. C. Drill core fragment of a breccia dike containing a sericitized clast with truncated quartz + pyrite veinlets. Since . breccia dikes are cut by vein mineralization, some sericitic alteration must be early relative to vein filling. Scale bar is 5 cm in length.


the absence of luzonite suggest that at some point temperatures were higher than 300 ø to 320øC, assuming end-member Cu-As-S composition (Lewis, 1956, p. 54). Temperatures at the end of the base metal stage in the central enargite zone were less than 307øC, i.e., the maximum thermal stability of realgar (Barton and Skinner, 1979) which occurs late in the paragenesis.

In the transition and lead-zinc zones, the presence of late Pb-As-S glass, (As,Cu)-rich pyrite and other sulfosalts indicate temperatures of less than 300øC during the sulfosalt stage (Burkart-Baumann and Otteman, 1972). Within the transition zone, the presence of jordanitc (Maier, 1981) indicates that temperatures were greater than 250øC (Barton and Skinner, 1979). The possible presence of gra- tonitc (suspected by Lewis, 1956) in the lead-zinc zone suggests temperatures at one point may have been below 250øC (Roland, 1968).

In the Coco-Luz Angelica area of the transition and lead-zinc zones, calcite +__ pyrite, sphalerite, and galena veinlets cut and displace earlier calcite- clinozoisite +__ pyrite, sphalerite, and galena veinlets. This relation suggests that the temperature had decreased to below 230øC during the deposition of sphalerite and galena in the late sulfosalt stage, based on the lower thermal stability of clinozoisite in modern geothermal systems (Seki, 1972; Cavar- retta et al., 1982; Bird et al., 1984; Schiffman et al., 1984). Late calcite occurring at the end of the carbonate stage may have formed at still lower temperatures.

In summary, mineralogy of vein filling and alteration points to a temperature of formation of approximately 350 ø (?) to 300øC for the bulk of ore deposition (pyrite and base metal stages). Tempera- ture is believed to have decreased in time throughout ore deposition as the sulfosalt and carbonate stages probably formed in the vicinity of 230 ø to 300øC, whereas the carbonate stage continued to form at temperatures lower than approximately 230øC. This temperature range is comparable to those of other base metal lode deposits as revealed by fluid inclusions (Sims and Barton, 1962; Meyer et al., 1968; Imai et al., 1985) or by sulfur isotope fractionation (Einaudi, 1977).

The very coarse grained vein material at Quiru- viica suggests that boiling did not occur; declining temperatures probably account for ore deposition.

Sul fidation

Sulfide assemblages document decreasing sulfidation laterally away from the center of the district (Fig. 15), a common characteristic of base metal lode deposits (Petersen, 1970). The upper sulfidation limit for the enargite zone is provided by the absence of the assemblage bornitc 4- pyrite and the

presence of chalcopyrite. The enargite-tennantite sulfidation curve in Figure 15 represents a mini- mum sulfidation boundary for the enargite zone, assuming no significant antimony in enargite (Lewis, 1956, p. 54).

Constraints on the sulfidation boundary of the transition zone are provided by the presence of tennantite rather than enargite, and the presence of trace amounts of native arsenic with apparent inclusions of pyrite (Haagensen et al., 1979) rather than arsenopyrite. The lower sulfidation boundary for the lead-zinc zone is the presence of pyrite rather than pyrrhotite. The presence of arsenopyrite rather than pyrite + native arsenic provides the upper sulfidation limit.

Arsenopyrite is common in the stibnite zone; its presence rather than pyrite and native arsenic de- fines the upper sulfidation limit of the stibnite zone. This is confirmed by the absence of gratonite or jordanitc and the presence of the assemblage galena + native arsenic (suggested by Lewis, 1956, p. 59). The lower sulfidation boundary for the stibnite zone is provided by the absence of pyrrhotite and the presence of pyrite.

A decrease in sulfidation state with time at Qui- ruvilca is compatible with the replacement of enargite by tennantite-tetrahedrite, the progressive darkening of sphalerite color with time, and the abundance of late arsenopyrite (Fig. 15).

Alteration chemistry

Alteration assemblages are shown in Figure 16. For this diagram, constructed for conditions of 300øC, the concentration of total sulfur was chosen as 0.01 m, within the range of values thought to be boundary conditions for most sulfur-bearing hydrothermal systems (Barnes and Czamanske, 1967; Ohmoto, 1972). Potassium content was chosen as 0.05 m; this falls within the range 40 to 4,000 ppm which encompasses fluid inclusion concentrations from other vein-type deposits (Roedder, 1972; Ka- milli and Ohmoto, 1977; Slack, 1980). Total carbonate and calcium both were chosen at 0.1 m; these values are bracketed by those determined at Sunnyside, Colorado (Casadevall and Ohmoto, 1977). The calcium value of 0.1 m is also comparable to those determined for lodes at Colqui, Peru; Lake City, Colorado; and Rochester, Nevada (Ka- milli and Ohmoto, 1977; Slack, 1980; Vikre, 1981).

In Figure 16, region i represents the immediately adjacent wall rocks from the enargite zone and corresponds to the assemblage pyrite 4- kaolinitc 4- quartz, with anhydrite absent.

Region 2 encompasses the environment of wall- rock alteration for the transition, lead-zinc, and stibnite zones. Region 2 has been divided into three subregions. Subregion 2a, representing intense ser-

1450 PAUL J. BARTOS

O-

-2

-4

-6

-12

-14

-16

/ /

/ /

• ENARGITE ZONE TRANSITION ZONE

LEAD-ZINC ZONE

STIBNITE ZONE

T(*C)

200 250 300 350 400 450 5O0 550 600

2 2 2.0 I 8 1.6 1.4 i.2

103/T(K)

FIG. 15. Univariant sulfidation curves (from Einaudi, 1977, fig. 13) applicable to Quiruvilca ores. Shaded areas represent the stability fields of the four mineralogic zones. Arrow represents the evolu- tionary trend of the Quiruvilca system.

icitic alteration, is characterized by an assemblage of pyrite + quartz + kaolinitc + sericite with calcite and anhydrite absent. Strong sericitic (2b) alter-

pH

o

2

4

6

8-

io

12 -:58

KAOL

MUSC

MUSC +QTZ• KSPAR

.

H SO;

ANHYDR tTE -- - - •C•'-- CALCITE

-

H2CO •

I i i I I• i f f

-:36 -34 -32 -:30 -28

log fo? '-

I I i

-26 -24

FIG. 16. Isothermal pH-logfo• diagram, at 300øC, showing proposed chemical environments for alteration at Quiruvilca. In- dividual assemblage regions (1-3) are discussed in text. Con- straints: ES = 0.01 m, Y.C = 0.1 m, K + = 0.05 m, Ca +2 = 0.1 m. Oxide, sulfide, sulfate, and carbonate boundaries based on data in Crerar and Barnes (1976). The K silicate mineral boundaries are derived from Montoya and Hemley (1975). Abbreviations: Hm = hematite, KAOL = kaolinitc, Mt = magnetite, MUSC -- mu- scovite, Po = pyrrhotite, Py = pyrite; QTZ -- quartz.

ation is represented by the assemblage pyrite + sericite + quartz, with calcite and anhydrite absent. Moderate sericitic alteration (2c) is depicted by the assemblage pyrite + sericite + quartz + calcite.

Argillic and propylitic alteration is represented by region 3; calcite and pyrite are stable and sericite + quartz is absent. Sericite without quartz is present in argillic alteration but absent in propylitic alteration. At an assumed molality of Na + of 0.1 m, albite is stable as part of the propylitic assemblage at pH values greater than 5.6 (Montoya and Hem- ley, 1975).

The sequence of regions 1 to 3 represents the zonal pattern outward within the wall rock from a major vein in the enargite zone; the sequence of regions 2 to 3 is representative of alteration selvages in the rest of the district.

A lateral decrease in H + activity (increase in pH) away from the center of the district is implied assuming that the activities of potassium, sodium, and calcium ions were constant and that alteration en-

velopes developed isothermally about fluid chan- nelways.

At any one spot within the wall rock, pH decreased with time as propylitic alteration was replaced by argillic and then sericitic alteration.

Summary

Overall the Quiruvilca base metal lode hydrothermal system exhibits: (1) a temporal separation


between the formation of pervasive alteration halos about major veins and ore deposition; (2) alteration relations which suggest decreasing pH with time at any one spot within the wall rocks; (3) vein-filling relations which suggest decreasing temperatures and sulfidation state through time; and (4) mineral zoning which suggests decreasing sulfidation and increasing pH laterally away from the center of the district.

Acknowledgments

This paper represents much of a Master's thesis at Stanford University under Marco Einaudi. I wish to thank Douglas M. Smith, Jr., manager of the Rocky Mountain Exploration Division of ASARCO, Inc., who arranged the project and provided funding. Sal Anzalone, ASARCO's chief mine geologist, arranged room and board at Quiruvilca. The generous cooperation of the Corporacion Minera Nor Peru, in particular, Walter Eberhart and Dave Des- Rosiers, insured a successful field season. Discus- sions with ASARCO personnel, including Darby Fletcher, Fred Graybeal, and Bill Kurtz, were help- ful. Drafts of this manuscript were reviewed and considerably improved upon by Marco Einaudi, Horacio Ferriz, Tim Hayes, and Eric Seedorff. Diana Scheidle Bartos provided much love and sup- port. Lois Bormolini prepared the manuscript. Per- mission to publish was provided by ASARCO Inc.

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bartos 1987

Documents

o o o o o

o o o o w o o quiruvilca

sericitic alteration

propylitic alteration

zoned alteration halos

major veins

o la merced

rock alteration relative