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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.

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 widespread inclusions in pyrite apparently represent the remains of an early prograde mineralization obliterated by intense sulfidation of subsequent events. Relicts of specularite veinlets may be a similar phenomenon. 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, biotite, 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 mineralized 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 pyritequartz veins with sericite-pyrite-calcite-anhydrite halos and occasional tourmaline. Pervasive sericitechlorite in the pyritic fringe terminates downward and biotitic alteration of andesite diminishes, revealing 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 deepest 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 chalcopyrite-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.

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 mineralization from the pyritic fringe to the bornite core and essentially 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 elevation. The purpose was primarily to expand our knowledge 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

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 processes of porphyry copper formation for use in exploration elsewhere. The results of that study were summarized by Gustafson and Hunt (1975). As part of that proo Present address: TVX Minerals Chile, Avenida 11 de Septiembre
2353, Santiago, Chile.

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

MINERALIZATION BELOW Cu OREBODY, EL SALVADOR, CHILE

20500 N

SULFIDE ZONING

1::::1 CHALCOPYRITEBORNITE ZONE

I':""': :1 CHALCOPYRITEPYRITE ZONE
~ PYRITE ZONE

ROCK TYPES
~ANDESITE
~XPORPHYRY
~KPORPHYRY
~LPORPHYRY

20000 N

- - -

INCA ADIT WORKINGS

DEEP DRIU HOLE,
HORIZONTAL PROJECTION

18500 N
,/

,/
,/

./

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 original enriched orebody were being depleted, and increased 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 recommendation 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 Salvador to assist in the interpretation of the results. Copper
grades in the holes were disappointingly low, and it became evident that these holes had not tested the real center 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 during 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 molybdenum mineralization in the mine (Fuster, 1983).
Here, we summarize only the salient descriptive features of our work, leaving many loose ends for others and
the future. We have relied heavily on the published description 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 reference is made or not. The earlier paper describes the evolutionary buildup of the main deposit under Turquoise
gulch through several stages of alteration and mineralization which accompanied a complex sequence of porphyritic intrusions. Most of the copper was emplaced as chalcopyrite-bornite, with K silicate alteration, in an early
stage dominated by magmatic fluids ("Early stage"). Subsequent 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 emplaced in B quartz veins, followed the emplacement of the

GUSTAFSON AND QUIROGA G.

L porphyry complex ("Transitional stage"). This preceded the "Late stage," a downward and inward collapse
of a meteoric water-dominated hydrothermal system.
This was responsible for overprinting of pyritic, feldspardestructive 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 abundance 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 figures .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 gypsum, is completely free of supergene effects. Mineral patterns 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 activity and the center of alteration and mineralization during the most intense period (Early stage) of mineralization. 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 intersected 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 chalcopyrite-pyrite zone, with increasing pyrite proportions
and a decreasing copper grade to a pyrite fringe with pyrite/chalcopyrite >3: 1. The bornite zone appears to contract 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 superimposed on a low intensity chalcopyrite-bornite mineralization. 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 pyritebornite 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 sequential 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 chalcopyritebornite 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 protore above the Inca adit, but primary grades average between 0 ..5 and 0.8 percent Cu. Particularly striking is the
fact that this drop in copper grades occurs within porphyries which otherwise are intensely altered to K feldspar-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 barren zone seen is that defined by the late L porphyry intrusion, 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 intrusion 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.

MINERALIZATION BELOW Cu OREBODY, EL SAL VADOR, CHILE

LEG END

ROCK TYPES
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GUSTAFSON AND QUIROGA G.

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FIG. 3. Changes of alteration and mineralization in drill hole 1104, below the central bornite-chalcopyrite zone.
Width of bars denotes qualitative abundance. Elevations of rock type and grade boundaries are shown (meters).

the 2600 level and above. Below this zone, values of 0.02
to 0.03 percent Mo extend roughly through the chalcopyrite-pyrite zone in drill holes 980 and 946. Mo values
drop to 0.01 to 0.005 percent Mo in the bornite zone both
on the Inca adit level and drill hole 946 below. In plan
view above the Inca adit, the Mo pattern shows rather irregular highs of >0.04 percent crossing into the low sulfide 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 porphyries between the elevations of 2,206 and 2,012 m
(Fig. 3), extending well below the bottom of the 0.1 percent Cu grades. This zone could be the downward extension of an irregular separate high but is similar to that observed on the Inca adit section. Mo data are too incomplete 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 chalcopyrite-pyrite zone. Clearly, the bulk of the molybdenum

was emplaced during the Transitional B vein stage and under the strong influence of the thermal and fluid flow regime imposed by the L porphyry intrusion (Gustafson and
Hunt, 1975; Fuster, 1983).
A striking feature of the deep mineralization is the occurrence 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 porphyry, where accessory magnetite is preserved and ilmenite 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

low the purple line, ilmenite occurs as a residual disseminated accessory mineral and less commonly as a vein constituent. 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 porphyries 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 characterized by increasing CufFe inward and decreasing S/Cu
+ Fe downward, a downward-expanding pyrrhotite-chalcopyrite 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 between 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 enigmatic 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 solution (Yund and Kullerud, 1966). The pyrrhotite-chalcopyrite grains appear to be replacement residuals of an assemblage formed early during the evolutionary growth of
the mineralizing system and over a broad area, as discussed below.
Vein types
A systematic evolution of quartz vein types has been documented within the main EI Salvador orebody and described 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 distinction 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 sericite-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 pattern, and at lower elevations in the propylitic fringe associated with epidote. As discussed below, it seems probable that this deep specular hematite, like the pyrrhotitechalcopyrite mentioned above, is a relict of the very earliest phase of mineralization, formed during initial expanding development of the mineralizing system.

The earliest veinlets common in the deep zone can be

grouped in an early biotitic (EB) type, not previously described at EI Salvador. Several varieties are illustrated in
Figure 4B, C, D, E, F, and G. They can occur with or without magnetite and sulfides that are characteristic of the sulfide 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 gradational to phlogopite, and a wide range of Mg/Mg + Fe
from 35 to 88 mole percent. The texture of the biotite varies widely, from very fine grained and disseminated within
alkali feldspar in poorly defined streaks (Fig. 4C), to relatively coarse grained and confined by clean fracture walls.
Crosscutting relationships suggest that coarse-grained biotite 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 feldspar-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, because typical A veins are rather rare in the deep holes.
Where both occur, quartz veins with features typical of relatively 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 feldspar-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 classification of biotitic and quartz-K feldspar veinlets as EB or
A veins which are parts of a single Early stage of mineralization 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 situation. 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-

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 chalcopyritebornite-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 chalcopyrite-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 chalcopyritepyrite: 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 sericiteanhydrite and trace actinolite-halo is albite-anhydrite-green sericite-biotite with relatively abundant magnetitechalcopyrite-(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-anhydritepyrite-(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-(chalcopyrite-bornite): cut by barren chlorite-(residual) actinolite-sphene-anhydrite vein (1) with albite-chlorite-anhydritesphene halo; and cut by actinolite-anhydrite-chalcopyrite-bornite-magnetite veinlets (2). These veinlets do not easily 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-rutile halo. Drill hole 946, 492.3 m.

raneous. It is fairly clear that here the intrusion of L porphyry reimposed near-magmatic conditions characteristic
of Early stage mineralization at a time later than the formation 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 elevation. It emphasizes an important point: mineralization

MINERALIZATION BELOW CII OREBODY, EL SALVADOR, CHILE

FIG. 4.

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 relatively 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 molybdenite, 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. Moreover, 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.

(COllt.)

In the mine workings above, B veins cut all intrusions, except 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 available). 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 surrounding zone, pyrite, chalcopyrite-pyrite, or chalcopyritebornite, 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.

Transitional

Early
EB

Late

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 abundance in veins and halos.

where they may have been lumped previously with D

veinlets. Two variants of C veinlets are illustrated in Figure 4E and F.
D veins, pyritic quartz veins with conspicuous sericitepyrite 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. Tourmaline 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 associated mineralization and alteration, and predate D veins.
At the surface, however, disseminated tourmaline rosettes 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 assemblages formed during the Early stage of alteration-mineralization, are increasingly abundant. Alteration of biotite
to chlorite and plagioclase to sericite and anhydrite is restricted to halos of individual pyrite veinlets with or without 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 alteration 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, increasing 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 being 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 porphyries 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 sulfide zonal position. The actinolite in veinlets is coarser
than background actinolite and is locally clearly replaced
by pseudomorphic biotite. A few veins contain actinolitealbite-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 ilmenite. 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

phyries, pseudomorphically altered to rutile plus anhydrite 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 operated in these porphyries above 2,400 m, no such linked
reactions are apparent in the other rock types. Fresh residual 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 below about the 2,700-m elevation. In these zones it occurs
in veinlets and halos associated with alkali feldspar, biotite, and green sericite. Andalusite was interpreted as being part of the Transitional stage of mineralization-alteration 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 porphyry 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 andalusite. The earliest veins with common andalusite are varieties of A quartz veins of the type described above as being similar to relatively late veins of the A family recognized in the mine. One such vein is cut by an aplite dike
and has traces of ilmenite as well as magnetite with granular quartz-albite. It has a broad halo of granular albitebiotite-quartz-andalusite-anhydrite. Molybdenite occurs
with chalcopyrite in the vein and halo. Deep B veins commonly 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 associated 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 corundum. Because it is virtually impossible to make a sure
identification of very fine grained cordierite in thin section, its abundance and range of occurrence are not
known.

11

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 significant 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. Laboratories at the Research School of Earth Sciences, Australian National University in Canberra, provided analyses of total and sulfate S, CO 2 , CI, F, Fe 2 +, Fe 3 +, andP 2 0 s .
The Australian mineral Development Laboratories in Adelaide 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 included. 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 dependent 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 observation of less conspicuous anhydrite in thin sections from
deeper in the holes. On the other hand, calcite and probably also dolomite are increasingly conspicuous in these
same thin sections. The carbonate is rather irregularly distributed, being strongly associated with late D veins, late
faults, and particularly latite and pebble dikes. Chemical
analyses for CO 2 range from 0.25 to 0.50 percent CO 2
throughout the three holes and on the Inca adit, with no
systematic vertical gradients. S04/S04 + CO 2 decreases
with depth as S04 decreases. Laterally, both S04 and CO 2
decrease weakly toward the zonal center, but the ratio
S04/S04 + CO 2 remains essentially unchanged.
No systematic gradients in MgjFe or K/Na are seen. Apparently, any subtle metasomatic effects, accompanying
mineral patterns reported here, are hidden by larger original 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
<10 to 3.5 ppm W throughout drill holes 980 and 946,
and above 100 to 200 m in drill hole 1104. Below 300 m
(2, 1.50-m elev) in drill hole 1104, W values range from 4.5
to 170 ppm. This high W zone occurs within both the K
and L porphyries, overlaps and extends below the high
Mo zone, and is below the bottom of 0.1 percent Cu. Only
local spikes of anomalous Zn, Pb, and As, related to D
veins, are seen at deep levels. These values increase markedly higher in the deposit, but are very poorly documented. None of the other elements show either marked
anomalies or significant patterns of variation. This includes P 2 0 5 , which averages a nonanomalous 1,600 to
2,200 ppm despite the common occurrence of apatite in
deep veins. This apparent remobilization of phosphate
has not been noted as being common at higher elevations.

to be ubiquitous but characteristic oflate D veins and late

overgrowths on B vein quartz. A brief reconnaissance
study revealed that these type III inclusions had low salinities (freezing point depression) and homogenization temperatures below 350C (Gustafson and Hunt, 1975). As
yet there are no data on freezing and homoginization temperatures for fluid inclusions from drill hole 1104, but
geologic reasoning suggests that most probably formed at
a higher temperature and have moderate salinity.
Figure 6E presents the abundance of total fluid inclusions enclosed within quartz of B veins and other quartzmolybdenite-apatite veins of the Transitional stage. There
is not nearly as marked a decline in the total number of
inclusions with depth as seen in the deep Early stage
veins, nor a clear decline in proportion of very saline inclusions (Fig. 6F). Note that the downward extension of
abundant fluid inclusions extends at least to the 1, 900-m
elevation, as do Mo grades> 0.17 percent.

Fluid Inclusions

Interpretation: Evolution of the Mineralizing System

In order to quantify the marked decrease in abundance

of fluid inclusions within quartz in deep drill holes, compared with the overlying mine, a scheme of classification
and counting was devised. To the basic types of inclusions
established by Gustafson and Hunt (I 97.5, p. 901 and fig.
26) were added a type IV, which contains a bright pleochroic elongate daughter phase (with or without halite),
and a type V, which has a large bubble like the previous
type II but which contains no opaque phase. The number
of each type of inclusion was counted within one-quarter
of a 500-power field of view, in four different but close
and genetically related quartz grains. Counts were then
averaged. Results from drill hole 1104, the only hole in
which counting was completed, are presented in Figure 6,
with individual counts averaged over .50-m intervals down
the hole.
In Figure 6A the total number of fluid inclusions
counted within Early stage vein quartz is plotted. The ordinate is the average number of inclusions counted in the
microscope field, and the abscissa the elevation of the
sample. The solid bar represents the number of classified
inclusions, and the hachured bar represents numbers of
fluid inclusions too small to classify with certainty. In Figure 6B, the proportion of classified inclusions which contain a halite daughter crystal, type I, is plotted. Figure 6C
and D show similar data for inclusions within groundmass
quartz. There is a clear decrease in the abundance of fluid
inclusions below about 2,300- to 2,200-m elevation. Note
that this is the same interval in which the copper content
of the core drops below 0.2 and then to 0.1 percent Cu.
The proportion of fluid inclusions containing halite is
markedly lower below 2,100 m in the L porphyry, and
there is a suggestion of a decrease within the K porphyry.
The proportion of fluid inclusions with a large bubble,
types II + V, decreases in a parallel fashion. Below this
elevation, most fluid inclusions have simply a moderate to
small size bubble and commonly no solid phase. This type
of inclusion, type III, within the mine workings was seen

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, abundant 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 interpreted by Gustafson and Hunt (1975) as representing
boiling of dominantly magmatic fluid. The great abundance of fluid inclusions, most in minute healed fractures
within quartz, was interpreted as reflecting the intensity
of pervasive crackling of the recently solidified rock during the ongoing intrusion process when the great bulk of
the copper was emplaced. Near the end of B vein formation, the intensity of shattering diminished greatly, as evidenced by both megascopic and microscopic features,
and fluid inclusion evidence of boiling ceased. This was
seen as a Transitional stage between the Early stage dominated by fluids near magmatic temperature and pressure
and a Late stage dominated by incursion of cooler meteoric 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 vapor 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

13

MINERALIZATION BELOWCIl OREBODY, EL SALVADOR, CHILE

EARLY VEINLETS
B.

A. TOTAL N" FLUID INCLUSION COUNTED

EARL Y MINERALIZATION

~~~~~~~---r----~-------------------'
.~. jANDEq X PORPH'IR'I
IK PORPH
l PORPH'IR'I

60Jr--~~~~-r----~---r~------G-.G,-------------;

~5~--~--~--------~----~------------------~

U/

fZ
::::I

0
0

~o

35

Q 30
1/1

::::I

oJ

25

0
~

20

~
::::I
oJ

15

10

u.

oJ

'?'

......... P'

./

CPP'I
501r~~~--~~~------~~----------------~

E2I

./

......... V
V

'.011

.------~

........ V V
V V......... V VV V-

.... ./

..................................
'T
./

~o

III

'"

./ .......... .

...............:;

=====-=.,

........ .

30

........ .

20

........ .

10

........ .

f-

~::::i=

<C

ff-

...................................

% TYPE 1 FLUID INCLUSION

EARL Y MINERALIZATION

2~00

2~OO

2319
2238 2157 2076 1995
1915
18:14
2360 2279 2198 2117 203G 1955
1874 ElE'tIATION

2319
2238 2157
2076 1995
1915
111'14
2360 2279
2198 2117 2036
1955
1874 ElE 'tIATION

G ROUN DMASS QUARTZ

C.

TOTAL N" FLUID INCLUSION COUNTED

GROUNDMASS QUARTZ

D.

l PORPHVR'I

60

III
f-

1AND. I

70

I 0.3& I 0.25 I

~Mo

1 0.0.'1

50

::::I

JC PORPHVR'I

.6Cu

lONINGI

60

% TYPE I FLUID INCLUSION

GROUNDMASS QUARTZ

G.018
CpBR

IKPORPHI

0.11
~.068\

L POnPH'IR'I

0.0'

0.081
1

0.011

cpp,

0
0

z 50

iii

::::I

oJ

0
~

40

III

30

............. .

20

........ ..

10

........ ..

oJ

oJ
0(
f-

............ ..

f-

30

::::I
II.

.olD

20
10

.0

i""'9'
2076
1995
2400
2319
2238 2157
1915
2360
2279 2198
2117 2036
1955
187.ol

=====--;0

2400 2319 2238 2157 2076 1995

1915
1R'I'"
2360 2279 2198 2117 2036 1955
1874 ElE'tIATJON

ElE'tIATION

TRANSITIONAL VEINLETS
E.

50
C

III
f-

::::I

45
40

TOTAL N" FLUID INCLUSION COUNTED

TRANSITIONAL MINERALIZATION

~~~~--~~__--~~~r---__~~~~-----,
T~' IANDESI

X PORPHYRY

\KPORPHI

l PORPH'IR'I

::::I

25

.........
~ 20

::::I

oJ

u.

Z
oJ

0.011
50~~---L--~~~--------~----__~C~p.~P~'__________~

V .. ...... .. .... .. _
.. .. V

z 35 .. V ....................................
V
Q
.............. / ................. '/
1/1
30
oJ

60~--~~~r-~--~-L--.-~-----O-.O-,----------~

r::21

..... ;a;......................................
V
UNIDENTIFIED

0
~

F.
% TYPE I FLUID INCLUSION
TRANSITIONAL MINERALIZATION

./

IDENTIFIED

................. ./ ................. 9'

........."'" .....
./

./

'T

./

.... :7 ./ ..... ./
./

III
~

40

........................................................................... .

30

..................................... ~ .... ..

20

............ ..

f-

15
10

<C

f-

f-

2.ol00 2319 2238 2157 2076 1995

1915 1834
2360
2279 2198 2117 2036
1955
1874 ElE 'tIATION

2400 2319 2238

2157 2076
1995
1915
1834
2360 2279 2198
2117
2036
1955
1874 ElE'tIATION

FIG. 6. Fluid inclusion counting, drill hole 1104. Data of individual counts are averaged over .50-m intervals
down the hole. Unidentified fluid inclusions are either too small or optically poor to permit classification among the
inclusion types.

14

GUSTAFSON AND QUIROGA G.

earliest veins indicate that the porphyry was initially subject 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 homogeneous 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 attendant changes in volatile content, pH, etc., copper precipitation might therefore also occur. Thus, in a given depth
or pressure region characterized by steady-state phase
separation, continuous precipitation of copper could occur, 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 temperature and pressure in the vicinity of an intruding
magma. However, evidence of such leaching would be
very difficult to see, especially if accompanied by deposition and recrystallization of quartz and silicates. In contrast to higher elevations, where extraction of Fe from hematite-rutile after ilmenite leaves a rutile sponge (Gustafson 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 undersaturation 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 fluids. At this stage, however, abundant molybdenite but
practically no copper was deposited. Does the lack of copper 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 striking' even though the values of Wand Mo are very much
lower and of no commercial value. Do they suggest intrusion of a new Cu-poor but Mo-rich melt at depth?

Comparisons with other porphyry copper districts

Gustafson and Hunt (1975) interpreted evolution of alteration 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 lowtemperature 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 mineralized 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 immediate 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 Yerington, 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 abundant magnetite, zonal and temporal patterns in the Yerington mine are very similar to those at EI Salvador. Dominantly 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 patterns at Ann-Mason also are similar, giving way to assemblages with albite, actinolite, sphene, magnetite, and ilmenite below and outside of the strong K feldspar-biotite
altered ore zone. The differences between features at Yerington 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 relatively low sulfate environment. Most of the actinolite-,
albite-, epidote-, and sphene-bearing assemblages at Yerington' however, are plausibly ascribed to the major influence 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 peripheral zones and result in a significant copper enrichment 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 AnnMason along structural zones, through and around K silicate 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 suggests 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-molybdenite 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 related 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 vertical 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.

rhotite-chalcopyrite could indeed be a common early

phase in many deposits, not just in rare deposits in reducing carbonate and ultramafic host rock; however, this earliest phase is almost entirely obliterated by the pervasive
sulfidation of successive events. Preservation of pyrrhotite, outside of blebs in pyrite, only at depth may well be
due to somewhat less pervasive shattering of the rock providing less access to subsequent fluids. The two very
different assemblages represent rather different chemical
conditions and probably different stages of prograde evolution. 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 (Gustafson 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 meteoric water as its temperature increased. Hemley et al.
(1980) have interpreted the deep-level andalusite with alkali 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 postL porphyry timing of formation of much of the andalusite
make this a very plausible interpretation. There is an alternative, 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.

Prograde features

The deep exposures at El Salvador add a new dimension

to the understanding of the evolution of this major porphyry copper deposit. They reemphasize the essential
character of the formation of these deposits as dynamic
and evolving; the resulting spatial patterns are the integrated effect of a sequence of events which include outward expanding, thermally prograding stages as well as
inwardly collapsing, thermally retrograding stages. The
symmetry or asymmetry of these developments is determined by the geometry and timing of successive intrusive
events and of fracture development during this evolution.

The evolution of mineralization and alteration at El Salvador and most other porphyry copper deposits is primarily one of retrograde, collapsing inward and downward of
lower temperature hydrothermal regimes on early hightemperature 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, reported 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) comment that pyrrhotite should be a much more common
early phase in porphyry copper mineralization, but its rarity "presumably results because f0 2 does not typically fall
to low values in the porphyry copper environment." Pyr-

Conclusions

Acknowledgments
Although the authors accept full responsibility for interpretations 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 provided by two Economic Geology reviewers. The Society
of Economic Geologists and CODELCO-Chile funded the
color illustration.

March 2, August 26,1994

REFERENCES
Brimhall, G.H, J r., 1977, Early fracture-controlled disseminated mineralization at Butte, Montana: ECONOMIC GEOLOGY, v. 72, p. 37 -.59.
Burnham, C.W., 1979, Magmas and hydrothermal fluids, in Barnes,
H.L., ed., Geochemistry of hydrothermal ore deposits: New York,
John Wiley Interscience, p. 71-136.
Carten, RB., 1986, Sodium-calcium metasomatism: Chemical, temporal and spatial relationships at the Yerington, Nevada, porphyry copper deposit: ECONOMIC GEOLOGY, v. 81, p. 149.5-1.519.
Dilles, J.H., and Einaudi, M.T., 1992, Wall-rock alteration and hydrothermal flow paths about the Ann-Mason porphyry copper deposita 6-km vertical reconstruction: ECONOMIC GEOLOGY, v. 87, p. 19632001.

Fuster, N., 1983, EI molibdeno en el porfido cuprifero de EI Salvador:

Unpublished memoria, Santiago, University de Chile, 80 p.
Gustafson, L.B., 1978, Some major factors of porphyry copper genesis:
ECONOMIC GEOLOGY, v. 73, p. 600-607.
Gustafson, L.B., and Hunt, J.P., 197.5, The porphyry copper deposit at
El Salvador, Chile: ECONOMIC GEOLOGY, v. 70, p. 87.5-912.
Hemley, J.J., and Hunt, J.P., 1992, Hydrothermal ore-forming processes in the light of studies in rock-buffered systems: II. Some general
geologic applications: ECONOMIC GEOLOGY, v. 87, p. 23-43.
Hemley, J.J., Montoya, J.W., Marinenko, J.W., and Luce, RW., 1980,
Equilibria in the system AI 2 0 3 -Si0 2 -H 2 0 and some general implications for alteration/mineralization processs: ECONOMIC GEOLOGY, v.
7.5, p. 210-228.
Hemley, J.J., Cygan, G.L., Fein, J.B., Robinson, G.R, and d'Angelo,
W.M., 1992, Hydrothermal ore-forming processes in the light of studies in rock-buffered systems: I. Iron-copper-zinc-Iead sulfide solubility relations: ECONOMIC GEOLOGY, v. 87, p. 1-22.
Proffett, J.M., and Dilles, J.H., 1984, Geologic map of the Yerington
district, Nevada: Nevada Bureau of Mines and Geology Map 77.
Meyer, C., and Hemley, J.J., 1967, Wall rock alteration, in Barnes, H.L.,
ed., Geochemistry of hydrothermal ore deposits: New York, Holt,
Reinhart and Winston, p. 166-232.
Wallace, S.R, Muncaster, N.K., Jonson, D.C., MacKenzie, W., B., Bookstrom, A.A., and Surface, V.E., 1968, Multiple intrusion and mineralization at Climax, Colorado, in Ridge, J.D., ed., Ore deposits of the
United States, 1933-1967 (Graton-Sales Volume): New York, American Institute of Mining, Metallurgical and Petroleum Engineers, v. 1,
p.60.5-640.
Yund, RA., and Kullerud, G., 1966, Thermal stability of assemblages in
the Cu-Fe-S system: Journal of Petrology, v. 7, p. 4.54-488.