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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. 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 ,/
,/ ,/
./
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
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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.
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1000 If!
I
MINERALIZATION BELOW Cu OREBODY, EL SAL VADOR, CHILE
I
LEG END ROCK TYPES
n:::c LATin I2i:!3) "L "-1YP r!t.DSf'AIt P'(W'H't'R'( I!!:a "X-
fIOIUIH'r"In' CE:IJ QUMTt .f:Yt PCJNI'N'IRY I%::JD IIHtOlI TI!:
" .... tTOS" RKY'OL I n: fI"tItOO\.MT I CS
- "HORNITOS" tNXIHI'tIItMlTY
Ii:3J =~~ All) OIICES ~ TIWX M DRII.L tl3t.E
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
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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-
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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-
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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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