Economic Geology Vol. 90,199.5, pp. 2-16

Patterns of Mineralization and Alteration below the Porphyry Copper Orebody at EI Salvador, Chile

LEWIS B. GUSTAFSON 5320 Cross Creek Lane, Reno, Nevada 89511

AND JORGE QUIROGA G. o Codelco-Chile, Division EI Teniente, Rancagua, Chile

Abstract Three diamond drill holes, angled below the lowest haulage level at EI Salvador, have doubled the

vertical exposure of the deposit and revealed very different features of alteration and mineralization below this major porphyry copper orebody. Sulfide assemblages persist with depth, but the total sulfide content diminishes. Magnetite becomes a part of all sulfide assemblages, except very late pyritic D veins. Residual traces of pyrrhotite-chalcopyrite found locked in quartz and as abundant and wide-spread inclusions in pyrite apparently represent the remains of an early prograde mineralization oblit-erated by intense sulfidation of subsequent events. Relicts of specularite veinlets may be a similar phe-nomenon. Vein types change. Newly recognized, early biotitic (EB) veinlets, with and without sulfides, quartz, albite, anhydrite, and actinolite, have varied alteration halos containing albite, K feldspar, bio-tite, green sericite, anhydrite, and andalusite. They appear to be deeper equivalents of A quartz veins. Veinlets descriptively similar to both EB and A quartz veins formed as a second generation within the young intramineral L porphyry complex which truncates similar veinlets in older and better mineral-ized rocks. Granular A quartz-K feldspar-sulfide-anhydrite veins diminish in abundance and in content of sulfide and K feldspar with depth, and are hard to distinguish from B quartz-anhydrite veins with characteristic molybdenite. The latter have much better developed K feldspar alteration halos than seen above. Younger C sulfide veins with green sericite, biotite, and anhydrite, and halos with green sericite, alkali feldspar, and andalusite, cut B veins. They are older than relatively sparse D pyrite-quartz veins with sericite-pyrite-calcite-anhydrite halos and occasional tourmaline. Pervasive sericite-chlorite in the pyritic fringe terminates downward and biotitic alteration of andesite diminishes, reveal-ing more restricted and residual actinolite hornfels. Ilmenite and then sphene appear as residual accessory minerals and minor vein constituents. Minor andalusite with alkali feldspar extends to deep-est exposures, mostly within halos of Band C veins. Traces of corundum and cordierite occur with andalusite.

Overall abundance of sulfide, sulfate, and K feldspar diminish with depth whereas albite increases. A sharp downward decrease in copper values below 0.1 percent Cu, within strongly quartz-veined and K feldspar-biotite-altered early feldspar porphyry, represents a barren core below the central chalcopy-rite-bornite zone. It appears to correlate with the bottoming of intense crackling and of boiling during early vein formation, as evidenced by the variation in fluid inclusion abundances in quartz. A deep zone of strong molybdenite with minor tungsten but very low copper contents occurs in one hole. It is associated with Band C veins cutting late L feldspar porphyry. These alteration-mineralization features are somewhat similar to those seen in deep zones at Butte, Montana, and Yerington, Nevada. They emphasize the essential character of porphyry copper formation as dynamic and evolving, in which the resulting spatial patterns are the integrated effect of a sequence of events which includes outward expanding, thermally prograding stages as well as inwardly collapsing, thermally retrograding stages.

Introduction DURING the development and operation of the EI Salvador mine by the Anaconda Company, from 1959 to 1970, a program of detailed mapping and laboratory study was conducted to provide optimum geologic support for the operation as well as geologic understanding of the pro-cesses of porphyry copper formation for use in explora-tion elsewhere. The results of that study were summa-rized by Gustafson and Hunt (1975). As part of that pro-

o Present address: TVX Minerals Chile, Avenida 11 de Septiembre 2353, Santiago, Chile.

0361-0128/95/16.53/0002-1.5$4.00 2

gram, in 1967, two deep diamond drill holes below the bottom of the mine were proposed to the management and approved. These holes would crosscut the mineraliza-tion from the pyritic fringe to the bornite core and essen-tially double the roughly 900 m of vertical exposure of the deposit as it was known, from the top of Cerro Indio Muerto to the Inca adit haulage level at the 2,400-m ele-vation. The purpose was primarily to expand our knowl-edge of patterns and processes at this well-known deposit, in order to enhance our ability to interpret and drill out other porphyry copper exploration targets. The drilling was postponed and never carried out by Anaconda.

In 1978, seven years after the mine was acquired by

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MINERALIZATION BELOW Cu OREBODY, EL SALVADOR, CHILE 3

20500 N

20000 N

18500 N ,/

,/ ,/

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SULFIDE ZONING 1::::1 CHALCOPYRITEBORNITE ZONE I':""': :1 CHALCOPYRITEPYRITE ZONE

~ PYRITE ZONE

ROCK TYPES ~ANDESITE

~XPORPHYRY ~KPORPHYRY ~LPORPHYRY

- - - INCA ADIT WORKINGS - DEEP DRIU HOLE,

HORIZONTAL PROJECTION

zoo ... 1========1

SCALE

FIG. 1. Location of Inca adit workings and deep drill holes relative to major patterns of rock type and sulfide zoning on the 2600 level (after fig. 19A of Gustafson and Hunt, 1975).

CODELCO-Chile, the highest grade portions of the orig-inal enriched orebody were being depleted, and in-creased emphasis was being placed on drilling out other associated centers of mineralization, including portions of the enrichment blanket and high-grade protore below the bottom extraction levels in the mine. At that time, John Hunt was consulting for CODELCO and recommended drilling of Gustafson's old drill recommendations to assist in evaluation of the deep ore potential. This recommen-dation was accepted and the holes, drill holes 946 and 980, were drilled in early 1979. Gustafson, then at the Australian National University, Research School of Earth Sciences, in Canberra, was invited to come back to EI Sal-vador to assist in the interpretation of the results. Copper grades in the holes were disappointingly low, and it be-came evident that these holes had not tested the real cen-ter of the mineralized system. If any real potential existed for high-grade disseminated or breccia mineralization, it was under the complex K porphyry area which was the main conduit for introduction of magma and solutions dur-ing the most intense stages of mineralization. A third deep hole was therefore recommended and drilled, in 1980, by CODELCO (drill hole 1104) beneath the K porphyry area. Figure 1 locates the three deep holes relative to the pattern of rock type and sulfide zoning as defined on the 2600 level. This level is 200 m above the Inca adit but is the lowest level with good definition of the patterns, provided by extensive mine development.

In order to complete laboratory studies on samples from the three holes, Quiroga, then a geologist at EI Salvador, was assigned by CODELCO for a few months in Canberra to work with Gustafson. Petrographic studies, including quantitative counting of fluid inclusions and preliminary heating and freezing stage work, electron microprobe study of alteration assemblages, and chemical analyses of composite samples were undertaken. Nicolas Fuster also studied the deep drill holes in a study focused on molyb-denum mineralization in the mine (Fuster, 1983).

Here, we summarize only the salient descriptive fea-tures of our work, leaving many loose ends for others and the future. We have relied heavily on the published de-scription of the EI Salvador orebody by Gustafson and Hunt (1975) to provide the background and framework for this paper. The reader is referred to this description throughout the present paper, whether a specific refer-ence is made or not. The earlier paper describes the evo-lutionary buildup of the main deposit under Turquoise gulch through several stages of alteration and mineraliza-tion which accompanied a complex sequence of porphy-ritic intrusions. Most of the copper was emplaced as chal-copyrite-bornite, with K silicate alteration, in an early stage dominated by magmatic fluids ("Early stage"). Sub-sequent intrusion of the late intramineral L porphyry complex, punched a large hole in this Early stage pattern. A transitional stage, in which most of the Mo was em-placed in B quartz veins, followed the emplacement of the

4 GUSTAFSON AND QUIROGA G.

L porphyry complex ("Transitional stage"). This pre-ceded the "Late stage," a downward and inward collapse of a meteoric water-dominated hydrothermal system. This was responsible for overprinting of pyritic, feldspar-destructive assemblages of the upper and fringe parts of the resulting pattern. In the deep drill holes reported here is seen the integrated result of this evolutionary process at deeper elevations. Note that the Early, Transitional, and Late stages as defined here were capitalized in the original paper and will be in this paper.

Deep Mineralization A series of changes in mineral assemblage and abun-

dance were encountered in the deep drill holes. Figure 2 shows most of these changes in a cross section through drill holes 980 and 946. This is the special section along the Inca adit which was used in Gustafson and Hunt (197.5) as the front face of the isometric diagrams of fig-ures .5, 20, 21, and 23, except that it continues along the northeast extension of the recta rather than bending east at 199.50N. Rock types are generally continuous with depth and are portrayed with the same symbols as used in Figure 1. Note the dashed blue top of the sulfate line in Figure 2 below which the rock is completely impregnated with anhydrite and minor calcite, and except for local gyp-sum, is completely free of supergene effects. Mineral pat-terns in drill hole 1104 are similar to these illustrated in Figure 2 but are plotted in Figure 3 against elevation in the hole. The K porphyry intrusion complex penetrated by this hole is the main conduit of multiple intrusion ac-tivity and the center of alteration and mineralization dur-ing the most intense period (Early stage) of mineraliza-tion. In Figure 2 this central zone is not seen, having been obliterated by the relatively late intrusion of L porphyry. The L porphyry complex expands with depth and was in-tersected in drill hole 1104 to the southeast of its position on the 2600 level.

Sulfide zoning The pattern of sulfide zoning in the mine extends

steeply to depth, as do most intrusion rock contacts. A central bornite-chalcopyrite zone is surrounded by a chal-copyrite-pyrite zone, with increasing pyrite proportions and a decreasing copper grade to a pyrite fringe with py-rite/chalcopyrite >3: 1. The bornite zone appears to con-tract somewhat with depth rather than expanding, and we did not see a bornite-chalcocite zone at depth as we had expected based on observations at other deposits. This may be partially due to the fact that the bornite zone plunges to the southeast or northeast and is only partially penetrated by drill hole 946. At the edge of the bornite zone in drill hole 946 chalcopyrite-pyrite is partly super-imposed on a low intensity chalcopyrite-bornite mineral-ization. Grades above about 0.2 percent Cu mostly repre-

sent addition of chalcopyrite-pyrite. Superposition of chalcopyrite-pyrite on chalcopyrite-bornite is suggested by the occurrence of both asemblages, but with no pyrite-bornite contacts, within several meters about the zonal boundary. Farther away, however, no evidence of such superposition was seen. Intervals of chalcopyrite-bornite enclosed within chalcopyrite-pyrite are associated with narrow dikes of probable K porphyry (not shown in Fig. 2 due to small scale). There is a general lack of any sequen-tial textural evidence, and thus the sulfide assemblages in veins of several ages reflect the overall zonal pattern, and the pattern appears to represent primary interfingering of contemporaneous assemblages. The abundance of sulfide decreases downward, as illustrated by Cu grades plotted in Figure 2 and by sulfide sulfur analyses discussed below. Primary Cu grades in the X porphyry and andesite, in the bornite zone on this section between the Inca adit and 2,600 m, are among the highest anywhere in the mine, approaching 1 percent Cu. However, as chalcopyrite-bornite they drop to below 0.2 percent in the bottom of drill hole 946 below. The chalcopyrite-pyrite zone in X porphyry averages 0.47 to 0 . .59 percent Cu in the Inca adit compared to less than 0.42 percent Cu 200 to 4.50 m below. Both lateral and vertical variations are involved but apparently grade contours are steep.

An even more dramatic drop in the grade of copper is seen in drill hole 1104 (Fig. 3). This is clearly a vertical rather than a lateral change, because it occurs within the X and K porphyries underlying the central bornite zone of the mine. Supergene enrichment extends to its deepest levels in this area, giving relatively few exposures of pro-tore above the Inca adit, but primary grades average be-tween 0 . .5 and 0.8 percent Cu. Particularly striking is the fact that this drop in copper grades occurs within por-phyries which otherwise are intensely altered to K feld-spar-biotite-quartz-albite and are veined by a variety of quartz veins which comprise .5 to 10 percent of the rock. The veins include many EB and A veins as well as Band younger veins. This drop in copper values is both more abrupt and occurs at a higher elevation than the Inca adit section (Fig. 2). Whereas in the mine the only central bar-ren zone seen is that defined by the late L porphyry intru-sion, this very low grade zone in K silicate-altered and quartz-veined rock is similar to barren core zones seen in many other porphyry copper deposits. Its shape is very poorly constrained, but it is probably a steep-sided domal feature, which is itself partially obliterated by the intru-sion of the downward-expanding L porphyry intrusion. Interpretation of this feature is discussed below.

Molybdenum grades on the Inca adit section reach a maximum of greater than 0.04 percent Mo in an irregular upward-flaring zone, which roughly corresponds with the outer half of the chalcopyrite-pyrite zone at the Inca adit level but encroaches on the chalcopyrite-bornite zone on

FIG. 2. Patterns of alteration and mineralization, Inca adit special section, looking southeast. Elevations give scale. Abbreviations: alk = alkali, andl = andalusite, bn = bornite, cp = chalcopyrite, chI = chlorite, fspar = feldspar, hm = hematite, mg = magnetite, py = pyrite, ru = rutile, ser = sericite.

1000 If!

I

MINERALIZATION BELOW Cu OREBODY, EL SAL VADOR, CHILE

I

LEG END ROCK TYPES

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" .... tTOS" RKY'OL I n: fI"tItOO\.MT I CS

- "HORNITOS" tNXIHI'tIItMlTY

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WITH AYERAK I CU

SULfIDE ZONING P'tIItln;..eQItN IT!-OtW.~ IT %OHr P'YIt I T! ZCNE ClfW.CIP'tR I T&-P'I'IIt I lilt ZCIHE c::tN,.ccrMl~fU ZONE ~ ~"DCZOMr

..... TCP (# SUL,. DC :::..... TOP 01" SULIATE ZOM!

ALTIRATJON ZONING -- TOPOI"~ - TOP OP' ......uL,.D VE .... ~ - TOP OI"ADTIIIOlITE _ IOTTOIII OP' PDlVASI'4 SOt-CHI. - TOP OP' I LMENtT! - T\'JP 01" e: 01' o\fI)AUlSIT!

5

6 GUSTAFSON AND QUIROGA G.

w w 0 z z w w w 0 W l- I- 0.. N ...J ::J w i= ~ W III 0 Z W 0 Z Z w z ~ :::J 0 u:: a: i= ::c Cl 0 0 ~ ...J VEIN TYPE 0 0 0.. c( 0 :J ::c c( (/) ~ a: t!. t!. (/) IAI BlclD

2400

I Andesite 0.04 percent crossing into the low sul-fide zone of the L porphyry as well. In drill hole 1104, one of the highest grade continuous molybdenum intervals seen anywhere in the deposit occurs, in both K and L por-phyries between the elevations of 2,206 and 2,012 m (Fig. 3), extending well below the bottom of the 0.1 per-cent Cu grades. This zone could be the downward exten-sion of an irregular separate high but is similar to that ob-served on the Inca adit section. Mo data are too incom-plete to define the shape of this high, but it clearly extends well into both the chalcopyrite-bornite zone and into the L porphyry, rather than being confined within the chalco-pyrite-pyrite zone. Clearly, the bulk of the molybdenum

was emplaced during the Transitional B vein stage and un-der the strong influence of the thermal and fluid flow re-gime imposed by the L porphyry intrusion (Gustafson and Hunt, 1975; Fuster, 1983).

A striking feature of the deep mineralization is the oc-currence of magnetite, both disseminated and in sulfide and quartz veinlets, in all assemblages (except D veins) from the outer pyritic fringe to the bornite zone. In Figure 2, the top of magnetite sulfide veining is shown with a black line. Although on review, a few intervals of andesite with previously overlooked magnetite in sulfide veinlets were seen in the Inca adit, the rule in upper elevations of the orebody is for Fe-Ti oxides to be destroyed both by sulfidation and by removal of Fe, leaving only rutile with or without sulfide. The exception occurs within late L por-phyry, where accessory magnetite is preserved and ilmen-ite is altered to hematite-rutile. The pale blue line marks the top of disseminated magnetite and hematite-rutile (Gustafson and Hunt, 1975, fig. 23). At greater depth be-

MINERALIZA nON BELOW Cu OREBODY, EL SAL VADOR, CHILE 7

low the purple line, ilmenite occurs as a residual dissemi-nated accessory mineral and less commonly as a vein con-stituent. Above the Inca adit, ilmenite had been seen only in portions of L porphyry and always in association with sphene. The deep ilmenite in andesite and older porphyr-ies may occur with sphene, below the brown top of the sphene. However, the two minerals have independent patterns of distribution and their destruction is not linked by a coupled reaction as it is in L porphyry at a higher elevation.

Based on the tendency for zonal changes to be charac-terized by increasing CufFe inward and decreasing S/Cu + Fe downward, a downward-expanding pyrrhotite-chal-copyrite zone had been predicted in 1967, flanking the deep bornite zone. No such zone was encountered, but three occurrences of tiny pyrrhotite-chalcopyrite grains locked in quartz were seen, in the inner pyrite zone be-tween elevations of 1,630 and 2,275 m. Such grains are very common as inclusions, or blebs, within pyrite throughout the deposit, but their origin has been enig-matic and they have never before been seen outside of pyrite crystals. Their occurrence within quartz precludes an origin by exsolution from pyrite or some kind of solid state modification by a pyrite host, though they were probably derived from an earlier intermediate solid solu-tion (Yund and Kullerud, 1966). The pyrrhotite-chalco-pyrite grains appear to be replacement residuals of an as-semblage formed early during the evolutionary growth of the mineralizing system and over a broad area, as dis-cussed below.

Vein types

A systematic evolution of quartz vein types has been doc-umented within the main EI Salvador orebody and de-scribed by Gustafson and Hunt (1975, figs. 15 and 16, table 2) as characterizing Early, Transitional, and Late stages of mineralization. At depth, we see new vein types and the dis-tinction between the established vein types becomes less certain. Very limited deep exposure, variations within each vein type, and inherent difficulty of fixing age relations of intrusions and veins in drill core alone provide much less certainty than was possible within the overlying mine.

Probably the earliest of all vein types is represented by a unique occurrence of a specular hematite veinlet, seen in andesite at 739 m in drill hole 946. Residual, euhedral specular hematite is partially replaced by magnetite in a magnetite-pyrite-quartz veinlet. The veinlet has a seri-cite-anhydrite alteration halo with a weak outer halo of chlorite-calcite. Previously the only specular hematite recognized as part of the primary assemblage occurs as veinlets peripheral to the orebody. Specularite is seen in high peripheral sericitic parts of the mineralization pat-tern, and at lower elevations in the propylitic fringe asso-ciated with epidote. As discussed below, it seems proba-ble that this deep specular hematite, like the pyrrhotite-chalcopyrite mentioned above, is a relict of the very ear-liest phase of mineralization, formed during initial ex-panding development of the mineralizing system.

The earliest veinlets common in the deep zone can be grouped in an early biotitic (EB) type, not previously de-scribed at EI Salvador. Several varieties are illustrated in Figure 4B, C, D, E, F, and G. They can occur with or with-out magnetite and sulfides that are characteristic of the sul-fide zone in which they occur, and with or without quartz. They contain biotite, with varying proportions of albite, K feldspar, green sericite, anhydrite, actinolite, and more rarely, apatite, andalusite, corundum, cordierite, ilmenite, and sphene. The biotite ranges from brown to green, with Mg/Mg + Fe from 53 to 70 mole percent as measured by electron microprobe in several representative samples. Green sericite has high Mg and Fe contents, seemingly gra-dational to phlogopite, and a wide range of Mg/Mg + Fe from 35 to 88 mole percent. The texture of the biotite var-ies widely, from very fine grained and disseminated within alkali feldspar in poorly defined streaks (Fig. 4C), to rela-tively coarse grained and confined by clean fracture walls. Crosscutting relationships suggest that coarse-grained bio-tite veinlets with little sulfide are formed earlier than finer grained biotite with quartz and sulfide. Many EB veins have no alteration halo, but one common type has a pale albitic halo (Fig. 4D and E). Rare biotite matrix breccias have been seen in more recent drilling and are probably related to this EB stage of veining. Some granular quartz-alkali fel-dspar-anhydrite-sulfide veins with biotitic halos (Fig. 4B) seem to be transitional between EB and A quartz vein types. EB veins are invariably truncated by younger B veins. Their age relationship to A veins is less clear, be-cause typical A veins are rather rare in the deep holes. Where both occur, quartz veins with features typical of rel-atively late A veins cut biotitic veins, but it is possible that EB and early A veins are largely contemporaneous in deep and shallow levels, respectively. A few biotitic veins with chalcopyrite and alkali feldspar and some with actinolite cut L porphyry in drill hole 1104, as do some quartz veins most easily identified as A veins. Actinolite, most common in deep andesite and L porphyry host rock, is a component of many EB veins. The actinolite is commonly replaced by biotite or chlorite, but it also occurs in late biotitic veins cutting EB veins with albitic halos. Actinolite is also seen in veins with biotite-anhydrite and halos containing K feld-spar-sphene-molybdenite cutting L porphyry. Some actinolite veinlets in both andesite and L porphyry do not fit well into any of the established vein classifications (Fig.4G).

These age relationships are not consistent with a classi-fication of biotitic and quartz-K feldspar veinlets as EB or A veins which are parts of a single Early stage of mineral-ization preceding intrusion of the L porphyry complex. Some of the uncertainty is due to the impossibility of a positive identification of feldspar porphyry, seen only in isolated drill holes, as L porphyry; some could well be older. However, recent mine developments in the L-K porphyry contact area, on the 2445 level, clarify the situ-ation. Several veinlets of quartz-K feldspar and of biotite, with and without actinolite, which fit well within the A and EB vein criteria, are seen cutting L porphyry. Both occupy the same structures and are apparently contempo-

8 GUSTAFSON AND QUIROGA G.

FIc. 4. Vein types in the deep drill holes below the present operations at EI Salvador. All photos are of polished slabs except for G, which is a thin section. A. X porphyry with K silicate alteration, disseminated chalcopyrite-bornite-magnetite-rutile: cut by three A veins (1), granular quartz-(K feldspar-anhydrite) with chalcopyrite-bornite and thin K feldspar halo; and truncated by B quartz vein (2) with chalcopyrite-molybdenite, irregular halo of K feldspar-biotite-sericite-andalusite-corundum. Drill hole 1104, 44.80 m. B. X porphyry as in A: cut by A or EB? quartz-(K feldspar-anhydrite) vein (I) with chalcopyrite-bornite, halo ofbiotite-K feldspar-albite-sericite with chal-copyrite-bornite-magnetite, truncated by two B quartz veins (2) with chalcopyrite-bornite-molybdenite and very thin K feldspar halos, and truncated by coarse granular B quartz-anhydrite veinlet (3) with chalcopyrite-molybdenite and relatively wide halo of K feldspar-albite-(biotite). Drill hole 1104, 9.5.70 m. C. X porphyry with minor residual actinolite with biotite-alkali feldspar, disseminated magnetite, ilmenite, and hematite-rutile with trace chalcopyrite-pyrite: cut by EB streak (1) of biotite-green sericite-chlorite-anhydrite with residual alkali feldspar, quartz, and strong disseminated chalcopyrite-pyrite, cut by B quartz-anhydrite vein (2) with chalcopyrite-molybdenite and very thin K feldspar halo. Drill hole 946,489.9 m. D. Biotized andesite: cut by EB vein (1) of biotite-albite-green sericite-anhydrite and trace actinolite-halo is albite-anhydrite-green sericite-biotite with relatively abundant magnetite-chalcopyrite-(bornite)-and cut by coarse-grained A(?) quartz-biotite-anhydrite vein (2) with K feldspar halo; only trace chalcopyrite-bornite. Drill hole 946, 661.7 m. E. X porphyry, weakly biotized with disseminated magnetite, hematite-rutile, and trace pyrite-chalcopyrite: cut by two EB chlorite-(residual) biotite-green sericite-anhydrite-pyrite-(chalcopyrite) veins (1) with halo of albite-anhydrite; cut by C(?) quartz-pyrite-magnetite vein (2) with green sericite-chlorite-anhydrite and halo of green sericite-chlorite-alkali feldspar-andalusite-anhydrite-sphene; and cut by anhydrite-filled fault (3). Drill hole 980, 661.3.5 m. F. Biotized andesite: cut by EB biotite veinlets (1) with no sulfide or halo; and cut by a B quartz-anhydrite vein (2) with chalcopyrite-molybdenite-bornite and thin K feldspar halo; both cut by C veins (3) of biotite-green sericite-anhydrite-chalcopyrite-(bornite) with alkali feldspar-green sericite halos. Drill hole 1104, 6.80 m. G. Biotite-(actinolite) altered andesite with disseminated magnetite-(chalco-pyrite-bornite): cut by barren chlorite-(residual) actinolite-sphene-anhydrite vein (1) with albite-chlorite-anhydrite-sphene halo; and cut by actinolite-anhydrite-chalcopyrite-bornite-magnetite veinlets (2). These veinlets do not eas-ily fit any of the established vein types. Drill hole 946, 730.80 m. H. X porphyry with disseminated magnetite, hematite-rutile, and chalcopyrite-pyrite: cut by D pyrite-anhydrite-(quartz) vein with sericite-pyrite-anhydrite-ru-tile halo. Drill hole 946, 492.3 m.

raneous. It is fairly clear that here the intrusion of L por-phyry reimposed near-magmatic conditions characteristic of Early stage mineralization at a time later than the for-mation of EB and A veins in the older rocks. This is also

consistent with the development within the southeast lobe of the L porphyry of Early stage mineralization and alteration, with increasing intensity at much higher eleva-tion. It emphasizes an important point: mineralization

MINERALIZATION BELOW CII OREBODY, EL SALVADOR, CHILE 9

FIG. 4. (COllt.)

types are not necessarily time lines but rather parts of an evolving sequence which may be repeated.

Typical A quartz veins, characterized by granular quartz-alkali feldspar-anhydrite-sulfides, occur only in the uppermost parts of drill holes 946 and 1104. At depth, most quartz veins earlier than B veins and younger than EB veins have relatively coarse quartz and contain rela-tively little alkali feldspar and little or no disseminated sulfide. These features make them similar to relatively late A veins higher up. There are, however, many veins which have gradational characteristics between A and B quartz veins (Fig. 4A, B, C, and F). Because they contain molyb-denite, which is characteristic of B veins above, most of these have been logged as B veins. They contain minor magnetite and alkali feldspar as well as anhydrite with the quartz. They almost never have drusy centerlines, which are common in B veins above, and have more or less well developed alteration halos of K feldspar with occasional albite, biotite, sericite, andalusite, or corundum. More-over, there are commonly several ages of B veins cutting one another, something rarely seen above. This could be evidence of an earlier introduction ofMo, encroaching on the late A vein period at deep levels. In a few instances, deep B veins are truncated by dikes of feldspar porphyry.

In the mine workings above, B veins cut all intrusions, ex-cept a few aplites and postminerallatite. This again may indicate that some of these B veins are older than the B veins above, or that there are deep injections of feldspar porphyry younger than the L and A porphyry seen above. Rare B veins have associated tourmaline, typically at the margin.

A new type of dark micaceous veins which are younger than B veins but older than pyritic D veins with sericitic halos has been termed "c" (a letter fortuitously avail-able). C veins are characterized by abundant sulfide with green sericite and biotite, anhydrite, and usually minor quartz within the vein. Sulfides are those of the surround-ing zone, pyrite, chalcopyrite-pyrite, or chalcopyrite-bornite, with or without relatively rare molybdenite and/ or magnetite. Halos contain alkali feldspar, green sericite, biotite or chlorite, anhydrite, andalusite, and locally sphene; they may be zoned. Biotite is commonly green and has a range of Mg/Mg + Fe of 54 to 88 mole percent. Green sericite has abundant Mg and Fe, but less than does the green sericite in EB veins; it also has a more limited Mg/Mg + Fe of 45 to 79 mole percent. Megascopically, these veins are easily confused with EB veins and they probably extend an unknown distance above the Inca adit,

10 GUSTAFSON AND QUIROGA G.

Early Transitional Late

EB A B C D

BIOTITE QUARTZ

ANHYDRITE

K-SPARIALBITE

SERICITE ~-- ~-- !---CHLORITE ~-- ~-- 1--- ---ANDALUSITE ~-- ~--ACTINOLITE ---~--1---APATITE ~. --TOURMALINE ---

------

MAGNETITE --- --

MONTMORILLONITE ~

SPHENE ,. -

BORNITE ---CHALCOPYRITE

---

PYRITE --~--MOLYBDENITE --- 1--- --SPHALERITE &

---TENNANTITE

FIG .. 5. Changes in mineral abundance with evolution of vein types in the deep zone at EI Salvador. Width of bars denote qualitative abun-dance in veins and halos.

where they may have been lumped previously with D veinlets. Two variants of C veinlets are illustrated in Fig-ure 4E and F.

D veins, pyritic quartz veins with conspicuous sericite-pyrite alteration halos, are less abundant than at higher elevations but persist to the bottom of drilling. As in Inca adit exposures, they contain pyrite with practically no other sulfide, relatively little quartz, and no magnetite. Rutile is the only oxide mineral occurring with pyrite in the halos. Calcite as well as anhydrite is abundant in these veins and halos. The sericite is low in Mg and Fe. Tourma-line is common in deep D veins but is rare in D veins above. Tourmaline veins and breccias at higher elevations are most commonly separate features, with little associ-ated mineralization and alteration, and predate D veins. At the surface, however, disseminated tourmaline ro-settes are locally abundant in sericitic rock.

Figure .5 presents a graphic summary of the changes in mineral abundance with evolution of vein types in the deep central zone at EI Salvador.

Deep Alteration Patterns Mineral boundaries

Pervasive sericite-chlorite alteration within the pyritic fringe bottoms out at about the olive green line in Figure 2. Below this boundary, residual areas of biotized andes-

ite, biotite-sodic plagioclase-anhydrite-quartz assem-blages formed during the Early stage of alteration-miner-alization, are increasingly abundant. Alteration of biotite to chlorite and plagioclase to sericite and anhydrite is re-stricted to halos of individual pyrite veinlets with or with-out magnetite, quartz, and chalcopyrite. These vein lets are typically small discontinuous structures which are difficult to classify, but this background pyrite veining with sericite-chlorite alteration is apparently related to the stage of C or D veining and decreases in intensity downward. Not all pyrite-magnetite veinlets have alter-ation halos in biotized andesite.

Below the pale green line in Figure 2, increasingly abundant residuals of actinolite are seen, both within veins and as part of the background alteration assemblage (Le., not within veins or halos). Actinolite has been seen as part of the background alteration assemblage only in biotized andesite. Here it takes the place of biotite, in-creasing irregularly with depth. It is suggested that at greater depth, the andesite in the contact zone around the porphyries becomes an actinolite hornfels rather than be-ing biotized. Usually it is difficult to discern a replacement relationship between background actinolite and biotite. Actinolite is also a constituent of veinlets, both in the por-phyries and andesite. These commonly do not clearly fit into any of the vein types described above but may be part of the EB and A vein suites. They usually contain some magnetite and sulfides which are appropriate to their sul-fide zonal position. The actinolite in veinlets is coarser than background actinolite and is locally clearly replaced by pseudomorphic biotite. A few veins contain actinolite-albite-sphene-anhydrite with or without quartz. One such vein with abundant quartz, cutting K porphyry in drill hole 1104, has a strong K feldspar alteration halo. Based on sparse electron microprobe data, actinolites appear to be compositionally identical to fine-grained hornblende in the groundmass of the deep X and K porphyries and overlap the low Al and high MgjFe end of the range of hornblende phenocrysts in all porphyries.

The purple line in Figure 2 represents the top of ilmen-ite. Ilmenite occurs primarily as a disseminated accessory mineral but also rarely in deep veins. Everywhere above this line, and commonly below, the ilmenite is altered to hematite-rutile intergrowths whereas magnetite remains unaltered or, rarely, is rimmed by hematite. Deep veinlets containing ilmenite are rare but varied. They include EB veins, probable A veins, and veinlets with sulfide but no quartz or alteration halos. Ilmenite in drill hole 1104 (Fig. 3) is seen only in L porphyry.

Sphene is seen at greatest depth below the brown top of the sphene line in Figure 2. It occurs in veinlets with actinolite-albite-anhydrite and in more typical EB veinlets with or without sulfides. It is also seen within Band C veinlets and their halos, and in coarse-grained quartz veins probably related to A veins. Sphene also occurs as an alteration product of hornblende, with biotite, anhydrite, and calcite, whereas above the top of the sphene, rutile is seen in this position. At the 2,400-m elevation and above, sphene is recognized only as an accessory mineral in por-

MINERALIZA TION BELOW Cu OREBODY, EL SAL VADOR, CHILE 11

phyries, pseudomorphically altered to rutile plus anhy-drite or calcite. Here, fresh sphene is only seen in Land younger porphyries, and strongly correlates with the also rare occurrence of residual ilmenite and hornblende in the same thin sections. Although concomitant reactions of sphene, hornblende, and ilmenite have apparently oper-ated in these porphyries above 2,400 m, no such linked reactions are apparent in the other rock types. Fresh re-sidual hornblende phenocrysts are very rare in porphyry older than the L porphyry, though they are seen locally in K porphyry in drill hole 1104.

Andalusite is very abundant and widespread at upper elevations of the deposit, where it occurs with sericite and in other advanced argillic assemblages. Gustafson and Hunt (1975, p. 894 and fig. 20B) reported that andalusite seemed to pinch downward into confined root zones be-low about the 2,700-m elevation. In these zones it occurs in veinlets and halos associated with alkali feldspar, bio-tite, and green sericite. Andalusite was interpreted as be-ing part of the Transitional stage of mineralization-alter-ation and related to the intrusion ofL porphyry. Although this probably is valid for much of the andalusite, some of it was apparently formed earlier than the intrusion of L porphyry. Figure 2 shows andalusite extending (dark blue lines) at least 900 m below the Inca adit as a broad band generally parallel to and 150 to 450 m outside the L por-phyry contact. Andalusite is less abundant in drill hole 1104, apparently confined to the X porphyry between the 2,320-and 2,360-m elevations. Andalusite is a constituent of a variety of veins and veinlets. The earliest, along with albite-K feldspar-biotite-anhydrite-quartz, appears to be contained in a probable EB vein which is truncated by an aplite dike. However, most EB veinlets have no andalu-site. The earliest veins with common andalusite are vari-eties of A quartz veins of the type described above as be-ing similar to relatively late veins of the A family recog-nized in the mine. One such vein is cut by an aplite dike and has traces of ilmenite as well as magnetite with gran-ular quartz-albite. It has a broad halo of granular albite-biotite-quartz-andalusite-anhydrite. Molybdenite occurs with chalcopyrite in the vein and halo. Deep B veins com-monly have some andalusite in their halos (Fig. 4A), as do C veins (Fig. 4E). Andalusite is typically in contact with albite, anhydrite, and quartz within altered plagioclase sites and is close to but seldom in contact with K feldspar. On the Inca adit section, andalusite occurs in the pyrite and chalcopyrite zones. In drill hole 1104 it occurs asso-ciated with chalcopyrite-bornite. Minor corundum is a common associate of andalusite.

Cordierite was discovered during electron microprobe studies in four thin sections from drill holes 946 and 980 below the 2,000-m elevation. It is associated with biotite, K feldspar, albite, green sericite, quartz, and anhydrite in halos about dark micaceous veins which could be either EB or C veins. In two of these it is in contact with corun-dum. Because it is virtually impossible to make a sure identification of very fine grained cordierite in thin sec-tion, its abundance and range of occurrence are not known.

Chemical patterns

In order to document chemical patterns within the deep zone, composites of assay pulps were prepared from the three deep holes and from channel samples in the Inca adit. Roughly 30 meter-long composites were prepared, with boundaries adjusted to conform with certain signifi-cant rock or Cu assay changes. The CODELCO laboratory at EI Salvador provided analyses of Cu, Mo, Au, Ag, Fe, Mg, Na, K, Co, Ni, Cr, Ba, Li, Mn, Pb, Zn, Sr, and Y. Lab-oratories at the Research School of Earth Sciences, Aus-tralian National University in Canberra, provided analy-ses of total and sulfate S, CO2, CI, F, Fe2+, Fe3+, andP20 s. The Australian mineral Development Laboratories in Ad-elaide provided analyses of W, Sn, and As. Even though we are looking at only part of the overall deposit, and comparable data on the upper half are not available for most elements, these chemical data serve to confirm and quantify trends which are visually apparent.

The pattern of copper and molybdenum mineralization at deep levels is discussed above and is partially illustrated in Figures 2 and 3. The drop in grade with depth is marked by a decrease in visual abundance of sulfides and of sulfide sulfur (Stotal-Ssulfate) analyses. In these composite samples, vein sulfides as well as the background assemblage are in-cluded. As the grade drops from 0.52-0.79 to 0.16-0.29 percent Cu, within the bornite zone below the Inca adit, there is an accompanying decrease in sulfide sulfur from 0.63 to 0.24-0.36 percent S. In drill hole 1104, also within the bornite zone, a grade drop from 0.36 to 0.06 percent Cu is accompanied by a decrease from 0.63 to 0.24-0.36 percent Ssulfide.

The sulfate sulfur content of the rocks is strongly de-pendent on how much Ca was liberated from plagioclase, hornblende, sphene, and apatite during alteration and subsequently fixed as anhydrite. Within individual rock types, there is a clear gradual decrease in Ssulfate with depth below the Inca adit. For example, in the X porphyry there is a decrease from 2.01 to 0.72 percent in drill hole 946, and from l.91 to l.36 percent in drill hole 1104. This decrease in sulfate sulfur correlates with the obser-vation of less conspicuous anhydrite in thin sections from deeper in the holes. On the other hand, calcite and prob-ably also dolomite are increasingly conspicuous in these same thin sections. The carbonate is rather irregularly dis-tributed, being strongly associated with late D veins, late faults, and particularly latite and pebble dikes. Chemical analyses for CO2 range from 0.25 to 0.50 percent CO2 throughout the three holes and on the Inca adit, with no systematic vertical gradients. S04/S04 + CO2 decreases with depth as S04 decreases. Laterally, both S04 and CO2 decrease weakly toward the zonal center, but the ratio S04/S04 + CO2 remains essentially unchanged.

No systematic gradients in MgjFe or K/Na are seen. Ap-parently, any subtle metasomatic effects, accompanying mineral patterns reported here, are hidden by larger orig-inal bulk chemical variations within each of the mappable volcanic and intrusion units. Whole-rock CI ranges from 125 to 400 ppm, F ranges from 300 to 800 ppm, and F /

12 GUSTAFSON AND QUIROGA G.

CI ranges from 1.2 to 3.9, but with no consistent spatial patterns. One trace element which does show marked and very interesting variation is tungsten. Values range from 0.17 percent.

Interpretation: Evolution of the Mineralizing System

Barren core and onset of boiling hydrothermalfluids An intriguing question is the cause of the barren core

seen in drill hole 1104 in the X and K porphyries above (southeast) of the L porphyry contact. Obviously, abun-dant hydrothermal fluids flowed through this barren core to produce the K silicate alteration and quartz veins.

In the overlying levels of the mine, the coexistence of halite-bearing with low-density fluid inclusions was inter-preted by Gustafson and Hunt (1975) as representing boiling of dominantly magmatic fluid. The great abun-dance of fluid inclusions, most in minute healed fractures within quartz, was interpreted as reflecting the intensity of pervasive crackling of the recently solidified rock dur-ing the ongoing intrusion process when the great bulk of the copper was emplaced. Near the end of B vein forma-tion, the intensity of shattering diminished greatly, as ev-idenced by both megascopic and microscopic features, and fluid inclusion evidence of boiling ceased. This was seen as a Transitional stage between the Early stage dom-inated by fluids near magmatic temperature and pressure and a Late stage dominated by incursion of cooler mete-oric water at hydrostatic pressure.

It has long been recognized that the pressure increase accompanying separation of a probably single-phase aqueous fluid from a melt can cause massive shattering of the top of porphyry intrusions (Burnham, 1979). Early stage crackling at EI Salvador was probably the result of separation of mineralizing fluid somewhat deeper within the crystallizing K porphyry complex. Separation of a va-por phase from that fluid, rising through the increasingly shattered cupola within the dynamic transition zone from ductile to brittle fracture, was probably triggered by the drop in pressure to below lithostatic within the crackled rock. The multiple ages of fractures filled by A quartz veins indicate that shattering was repeated many times during the intrusion of the porphyry complex. The very irregular, discontinuous shapes and deformation of the

c U/ f-Z ::::I 0 0 z Q 1/1 ::::I oJ 0

~ ~ ::::I oJ u.

Z oJ

14 GUSTAFSON AND QUIROGA G.

earliest veins indicate that the porphyry was initially sub-ject to brittle fracture by very short term stress, but to ductile deformation by longer term stress, analogous to the "Silly Putty" children play with. Drop of pressure in fractures was, therefore, probably transient initially but became permanent and approached hydrostatic by the end of A quartz vein time.

An explanation for the patterns we see must be sought in the solubility behavior of the elements in the fluids which flowed upward through this barren core. The work of Hem ley et al. (1992) on the solubility behavior of base metals along various hydrothermal P-T paths indicates that along the probable quasi-adiabatic path of expansion (a path somewhere between geothermal and adiabatic) metal solubilities would decrease very little in the homo-geneous fluid region. In fact, they would probably initially even increase somewhat, and leaching would occur if metal were present to be leached. Copper is less affected by pressure changes than are lead and zinc, and therefore its solubility would more likely approach saturation caused by cooling and attendant expansion. When fluid phase separation or boiling finally develops, with atten-dant changes in volatile content, pH, etc., copper precip-itation might therefore also occur. Thus, in a given depth or pressure region characterized by steady-state phase separation, continuous precipitation of copper could oc-cur, resulting in a continuous increase in copper grade as time progresses. This agrees with the correlation, in the deep drill holes, of higher copper grades with increased crackling of quartz and abundant evidence of boiling. Leaching of earlier precipitated copper in the deepest zones is also a possibility, given the wide variations in tem-perature and pressure in the vicinity of an intruding magma. However, evidence of such leaching would be very difficult to see, especially if accompanied by deposi-tion and recrystallization of quartz and silicates. In con-trast to higher elevations, where extraction of Fe from he-matite-rutile after ilmenite leaves a rutile sponge (Gustaf-son and Hunt, 1975, fig. 18B), there is also no evidence of leaching of Fe from this deep zone. Magnetite is present in veins, indicating that the solutions were saturated with iron at some stage but could have moved toward under-saturation without leaving obvious traces.

The change in pattern of fluid inclusions attending or following Transitional stage veining appears to represent a drop in the elevation of boiling of the hydrothermal flu-ids. At this stage, however, abundant molybdenite but practically no copper was deposited. Does the lack of cop-per deposition with molybdenum, despite boiling, reflect a depletion of copper in the continuing magmatic source of fluids? The similarity of Wand Mo patterns described above with those at Climax (Wallace et aI., 1968) is strik-ing' even though the values of Wand Mo are very much lower and of no commercial value. Do they suggest intru-sion of a new Cu-poor but Mo-rich melt at depth?

Comparisons with other porphyry copper districts Gustafson and Hunt (1975) interpreted evolution of al-

teration and mineralization at EI Salvador as the result of a

unidirectional change of conditions, from near-magmatic pressure and temperature dominated by magmatic fluids in early stages to hydrostatic pressure dominated by low-temperature meteoric water in late stages. The less well defined and less consistent zonal and temporal patterns in many other deposits may be ascribed to more widely timed intrusions, causing reversals of the evolutionary trend by resumption of magmatic conditions, following the initial incursion of ground water on earlier mineral-ized intrusions (Gustafson, 1978). Still, there are very few clear descriptions of such relations. This may be because intrusions accompanied by mineralizing fluids tend to obliterate earlier features, whereas barren late intrusions have only very subtle effects. Part of the problem is the very limited exposure in most mines, confined to the im-mediate vicinity of the orebody.

Interpretation at EI Salvador is seriously restricted by the limited exposure of the deep zone. Analogies with other districts must be called upon. Most useful is Yering-ton, Nevada, where rotation of about 90 by basin and range faulting has exposed an original cross section 6 km deep through the top of a batholith, which produced three separate porphyry copper deposits (Proffett and Dilles, 1984). Carten (1986) gives a description of deep alteration features in the main Yerington mine, and the recent paper by Dilles and Einaudi (1992) on the adjacent Ann-Mason deposit offers by far the most complete look to date at a complete cross section through a porphyry copper system.

There are several similarities between primary features at Yerington and EI Salvador. With the exception of abun-dant magnetite, zonal and temporal patterns in the Yerin-gton mine are very similar to those at EI Salvador. Domi-nantly potassic alteration in the ore zone gives way at depth to assemblages with albite and actinolite, and the grade of Cu diminishes markedly. Early high-level pat-terns at Ann-Mason also are similar, giving way to assem-blages with albite, actinolite, sphene, magnetite, and il-menite below and outside of the strong K feldspar-biotite altered ore zone. The differences between features at Yer-ington and EI Salvador are more fundamental. Some, such as the abundance of epidote at Yerington, compared to its absence at EI Salvador except in the propylitic fringe, may be due partly to epidote proxying for anhydrite in a rela-tively low sulfate environment. Most of the actinolite-, albite-, epidote-, and sphene-bearing assemblages at Yer-ington' however, are plausibly ascribed to the major in-fluence of a sodic-calcic metasomatism accomplished by inward-flowing, heating (thermally prograding), saline fluids of nonmagmatic origin (Dilles and Einaudi, 1992). These fluids leach magnetite and Cu from deep and pe-ripheral zones and result in a significant copper enrich-ment at Ann-Mason in veins which postdate Early stage A and B quartz veins. Fluid inclusions give little evidence of boiling. Early sodic-calcic assemblages extend at Ann-Mason along structural zones, through and around K sili-cate assemblages to the Tertiary surface, probably within 1 km of the original Jurassic surface. Chlorite, commonly with albite, is much more abundant at Yerington, both at

MINERALIZA TION BELOW ell OREBODY, EL SAL VADOR, CHILE 15

depth and shallow. The major difference in the shallow patterns is partly due to the late superposition of very strong advanced argillic alteration at El Salvador, which was formed at lesser depth. Nothing at El Salvador sug-gests the influence of the laterally flowing, saline fluids responsible for sodic alteration at Yerington.

At least in descriptive detail, the Pre-Main Stage veins at Butte, Montana, described by Brimhall (1977), offer close similarities with deep veining at El Salvador. The Butte biotitic veinlets, green mica veins, and some early dark micaceous (EDM) veins all have close analogues within EB veins at El Salvador. Quartz and quartz-molyb-denite veins with and without alkali feldspar halos at Butte have close analogues among deep A and B quartz veins. C veins at El Salvador are indistinguishable from many early dark micaceous veins at Butte, although the timing is different. EDM veins at Butte are invariably older than the quartz veins. Finally, the Main Stage veins at Butte are quite analogous to D veins. Gross geometric and timing aspects, however, are rather different in the two districts. Pre-Main Stage mineralization at Butte is apparently re-lated to quartz porphyry dikes which are volumetrically much less important than intrusions at El Salvador and most porphyry copper deposits. Pre-Main Stage and Main Stage events may be separated by a 5-m.y. time gap. Butte is a unique district and one which, despite over 2,200 ver-tical meters of workings and drill holes, still has vertical exposure less than half its lateral extent of mineralization. The significance of the analogies between Butte and this more typical porphyry copper is still not clear. Maybe the Pre-Main and Main Stages at Butte were not produced by totally separate events.

Carten (1986) and Dilles and Einaudi (1992) review briefly a number of other porphyry copper districts which contain reported sodic alteration. They are relatively few, and we are not familiar enough with any of these to know how they may illuminate the deep mineralization under discussion.

Prograde features The evolution of mineralization and alteration at El Sal-

vador and most other porphyry copper deposits is primar-ily one of retrograde, collapsing inward and downward of lower temperature hydrothermal regimes on early high-temperature features. Evidence of the earliest, outwardly expanding and thermally prograding growth stage is very rare. In this deep drilling, the first evidence of this period at El Salvador seems to be observed. The rare occurrences of pyrrhotite-chalcopyrite and specularite veining, re-ported above, appear to be relicts of such early growth stages of mineralization. Both features may have been widespread throughout the deposit, but the only evidence is widespread pyrrhotite-chalcopyrite in the tiny blebs locked in pyrite. Hemley and Hunt (1992, p. 31) com-ment that pyrrhotite should be a much more common early phase in porphyry copper mineralization, but its rar-ity "presumably results because f0 2 does not typically fall to low values in the porphyry copper environment." Pyr-

rhotite-chalcopyrite could indeed be a common early phase in many deposits, not just in rare deposits in reduc-ing carbonate and ultramafic host rock; however, this ear-liest phase is almost entirely obliterated by the pervasive sulfidation of successive events. Preservation of pyrrho-tite, outside of blebs in pyrite, only at depth may well be due to somewhat less pervasive shattering of the rock pro-viding less access to subsequent fluids. The two very different assemblages represent rather different chemical conditions and probably different stages of prograde evo-lution. The specularite veinlets presumably represent an outer, more oxidized and lower temperature, i.e., earlier, stage than the pyrrhotite-chalcopyrite. Unfortunately, there will probably never be enough new deep exposures at El Salvador to resolve the questions.

One bit of evidence reported previously for thermally prograding Late stage solutions is a zone of corundum on surface at the outer edge of the andalusite zone (Gustaf-son and Hunt, 1975, fig. 20B). This was interpreted as forming, even in the presence of abundant quartz, through local leaching of silica by inward-moving mete-oric water as its temperature increased. Hemley et al. (1980) have interpreted the deep-level andalusite with al-kali feldspar at El Salvador as the result of inward-moving, prograding hydrothermal water. The general symmetry of andalusite around the late L porphyry body and the post-L porphyry timing of formation of much of the andalusite make this a very plausible interpretation. There is an al-ternative, however. Andalusite also occurs in halos of some EB and A veins prior to L porphyry intrusion, and the deep andalusite zone occupies a position at the inner edge of pyrite-bearing assemblages which is probably in part contemporaneous with Early stage K silicate-bornite. Deep-level andalusite-alkali feldspar is probably also the deep manifestation of retrograde hydrolytic alteration, a high-temperature sericite analogue formed above the thermal limit of sericite stability.

Conclusions

The deep exposures at El Salvador add a new dimension to the understanding of the evolution of this major por-phyry copper deposit. They reemphasize the essential character of the formation of these deposits as dynamic and evolving; the resulting spatial patterns are the inte-grated effect of a sequence of events which include out-ward expanding, thermally prograding stages as well as inwardly collapsing, thermally retrograding stages. The symmetry or asymmetry of these developments is deter-mined by the geometry and timing of successive intrusive events and of fracture development during this evolution.

Acknowledgments

Although the authors accept full responsibility for in-terpretations presented here, we gratefully acknowledge the following important contributions. The geological management of CODELCO-Chile generously supported the drilling of the deep holes, the subsequent study of the core, including QUiroga's travel to and stay in Canberra,

16 GUSTAFSON AND QUIROGA G.

and the preparation of this report. Pedro Carrasco, Walter Orquera, and Mario Castro have been particularly helpful in evaluation of our early interpretations in the light of ongoing developments in the mine. John P. Hunt, Marco T. Einaudi, and J. Julian Hemley reviewed early versions of the manuscript and made suggestions which resulted in significant improvements to both the interpretation and presentation of the work. Helpful reviews were also pro-vided by two Economic Geology reviewers. The Society of Economic Geologists and CODELCO-Chile funded the color illustration.

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