Economic Geology VoL 68, 1973, pp. 799-815 The Tops and Bottoms of Porphyry Copper Deposits RICHARD H. SILLITOE Abstract Although it is now widely accepted that porphyry copperdeposits consist of zonally arranged shells of alteration and mineralization centered on high-level, calc-alkaline stocks,the nature of their uneconomic upward and downward extensionsremains un- documented. This paper attempts to characterize these upward and downward ex- tensionsand to integrate the resulting concepts into a hypotheticalmodel for complete porphyry coppersystems. Examples from Chile, Argentina, and elsewhere are used to aid in the substantiation of the model. Programs of exploration for porphyry ore de- posits can clearly benefit from the application of a model of this sort. A typical porphyry copper-bearing stock is inferred to grade downward into stock- work mineralization and potassium silicate alteration in a phaneritic intrusive, which in turn is transitional downward to an essentially unaltered pluton of considerably larger dimensions than the stock. Porphyry copperdeposits are normally locatedin the basement beneatha comagmatic volcanicpile, which is transected by a column of hydro- thermal alteration representing the upper parts of the porphyry copper system. This alteration consists of propylitic and argillic types with localizedpatchesof silicification and advanced argillic alteration. The volcanic pile is thought to constitute a strato- volcano which possesses large native sulfur deposits and small quantitiesof basemetals, particularly copper, in sublimates at high-temperature fumaroles in the vicinity of its central vent; these surficial depositsare consideredas the effluent products of active porphyrycopper systems. The available evidence favors the emplacement of the tops of typical porphyrycopper deposits at depths of 1.5-3 km beneath the summits of stratovolcanoes and suggests that entire porphyry coppersystems possess vertical extensions as great as 8 km. At Chuquicamata, Chile, a major high-anglefault may have cut the porphyry copper deposit, and subsequent erosion has removed the portion of the deposit that was situated in the upthrown block. The hydrothermalalteration pattern in the remaining part of the ore body is incomplete and terminatesabruptly against the fault. The unaltered, phaneriticgranodiorite, containing minor veins and pegmatiticbodies, in the upthrown block is interpreted as the root zone of the Chuquicamata porphyry coppersystem. The low•er most,rMneralized part of a porphyry system is believed to be exposed at Los Loros, Chile. There a zone of molybdenum-rich and copper-poor potassium silicate alteration carrying abundant K-feldspar occupies an area in the interior of a relatively large pluton of phaneritic granite. At Faral16n Negro, northwest Argentina, several small porphyry copper deposits pierce the infrastructure of a temporallyrelated, andesitic stratovolcano. This unusual locus of the deposits above the subvolcanic basement enablesit to be determined that porphyry copper emplacement was a late event in the construction of the stratovolcano, succeeded only by the formation of minor rhyolite intrusivesand "epithermal" veins. Extensive zones of pyritic alteration including widespreadsilicification,in which intrusive rocksare virtually absent, are visible in the centers of erodedstratovolcanoes, as at Cerro Marquez in northern Chile. Such zones are interpreted as the columns of alterationspanning the vertical interval between porphyry copper deposits and the vent areas of uneroded volcanoes. At Cerro Queva in northwest Argentina, lead- silver mineralization associated with advanced argillic alterationis located in an altera- tion zonebeneath the summit regions of a stratovolcano. It may be concluded that during the final stages of construction of stratovolcanoes, fumarolic and hot-spring activity are the surficial manifestations of the effiux of metal- bearing magmatic fluids from magma chambers during retrograde boiling, the inter- action of these fluids with the groundwater system and the consequent formation of alteration and mineralization. The proposed model implies that porphyry copper systems effectively spanthe boundary between the plutonic and volcanic environments. 799
Transcript
Economic Geology VoL 68, 1973, pp. 799-815
The Tops and Bottoms of Porphyry Copper Deposits
RICHARD H. SILLITOE
Abstract
Although it is now widely accepted that porphyry copper deposits consist of zonally arranged shells of alteration and mineralization centered on high-level, calc-alkaline stocks, the nature of their uneconomic upward and downward extensions remains un- documented. This paper attempts to characterize these upward and downward ex- tensions and to integrate the resulting concepts into a hypothetical model for complete porphyry copper systems. Examples from Chile, Argentina, and elsewhere are used to aid in the substantiation of the model. Programs of exploration for porphyry ore de- posits can clearly benefit from the application of a model of this sort.
A typical porphyry copper-bearing stock is inferred to grade downward into stock- work mineralization and potassium silicate alteration in a phaneritic intrusive, which in turn is transitional downward to an essentially unaltered pluton of considerably larger dimensions than the stock. Porphyry copper deposits are normally located in the basement beneath a comagmatic volcanic pile, which is transected by a column of hydro- thermal alteration representing the upper parts of the porphyry copper system. This alteration consists of propylitic and argillic types with localized patches of silicification and advanced argillic alteration. The volcanic pile is thought to constitute a strato- volcano which possesses large native sulfur deposits and small quantities of base metals, particularly copper, in sublimates at high-temperature fumaroles in the vicinity of its central vent; these surficial deposits are considered as the effluent products of active porphyry copper systems.
The available evidence favors the emplacement of the tops of typical porphyry copper deposits at depths of 1.5-3 km beneath the summits of stratovolcanoes and suggests that entire porphyry copper systems possess vertical extensions as great as 8 km.
At Chuquicamata, Chile, a major high-angle fault may have cut the porphyry copper deposit, and subsequent erosion has removed the portion of the deposit that was situated in the upthrown block. The hydrothermal alteration pattern in the remaining part of the ore body is incomplete and terminates abruptly against the fault. The unaltered, phaneritic granodiorite, containing minor veins and pegmatitic bodies, in the upthrown block is interpreted as the root zone of the Chuquicamata porphyry copper system.
The low•er most, rMneralized part of a porphyry system is believed to be exposed at Los Loros, Chile. There a zone of molybdenum-rich and copper-poor potassium silicate alteration carrying abundant K-feldspar occupies an area in the interior of a relatively large pluton of phaneritic granite.
At Faral16n Negro, northwest Argentina, several small porphyry copper deposits pierce the infrastructure of a temporally related, andesitic stratovolcano. This unusual locus of the deposits above the subvolcanic basement enables it to be determined that porphyry copper emplacement was a late event in the construction of the stratovolcano, succeeded only by the formation of minor rhyolite intrusives and "epithermal" veins.
Extensive zones of pyritic alteration including widespread silicification, in which intrusive rocks are virtually absent, are visible in the centers of eroded stratovolcanoes, as at Cerro Marquez in northern Chile. Such zones are interpreted as the columns of alteration spanning the vertical interval between porphyry copper deposits and the vent areas of uneroded volcanoes. At Cerro Queva in northwest Argentina, lead- silver mineralization associated with advanced argillic alteration is located in an altera- tion zone beneath the summit regions of a stratovolcano.
It may be concluded that during the final stages of construction of stratovolcanoes, fumarolic and hot-spring activity are the surficial manifestations of the effiux of metal- bearing magmatic fluids from magma chambers during retrograde boiling, the inter- action of these fluids with the groundwater system and the consequent formation of alteration and mineralization. The proposed model implies that porphyry copper systems effectively span the boundary between the plutonic and volcanic environments.
799
800 RiCH/IRD H. SILLiTOE
Introduction
A •R•.AT deal of attention has recently been focused on patterns of lateral and vertical zoning of altera- tion and mineralization in porphyry ore deposits (Lowell and Gullbert, 1970; Rose, 1970; James, 1971). This work has led to the general acceptance of many porphyry deposits as upright cylinders con- sisting of coaxially distributed zones of alteration and mineralization centered on felsic stocks, commonly porphyries. The silicate and sulfide zoning com- prises a core of potassium silicate alteration envel- oped successively by zones of sericitic, argillic, and propylitic alteration. This typical pattern of altera- tion and mineralization has been widely recognized during studies of porphyry copper deposits in the Andes, and is particularly well exemplified by the Los Pelambres deposit (Fig. 2) in Chile (Sillitoe, 1973).
Knowledge of the character and distribution of alteration-mineralization is, however, largely com- fined to the economically exploitable portions of porphyry ore deposits, whereas the nature of the uneconomic extensions of these deposits, both up- ward and downward, remains undocumented. When one considers the huge tonnage of porphyry deposits,
their high level of emplacement in the continental crust, and the large volumes of mineralizing fluids and high temperatures involved in their generation, it is evident that entire porphyry copper systems must extend downward and especially upward for very considerable distances beyond the parts which are likely to be of economic interest. Even at Kalama- zoo, Arizona (Lowell, 1968), where the mineralized body has been tectonically disturbed and possesses a near-horizontal attitude, the complete porphyry copper system is not observable. Therefore it seems necessary to combine information from many areas in order to attempt to construct a model of a porphyry system.
In the first section of this paper, a preliminary speculative model for an idealized porphyry copper system is advanced, although it is not claimed that every porphyry-type development necessarily com- plies with all its features. Future studies of porphyry deposits in various parts of the world will un- doubtedly be able to clarify or modify some of the more enigmatic aspects of the model. In the second section of the paper descriptions are given of localities in Chile, Argentina and elsewhere that are thought to be typical of various levels in the upward and downward extensions of porphyry copper deposits.
Votc6n ,hlJCOi'lq,ui[chlJ. $ KH. (IN R•LATION TO SEA-LEVEL)
Fro. 1. Idealized cross section of a typical, simple porphyry copper deposit showing its position at the boundary between plutonie and volcanic environments. Vertical and horizontal dimensions are meant to be only approximate.
TOPS AND BOTTOMS OF PORPHYRY COPPER DEPOSITS 801
This section does not purport, however, to vindicate every aspect of the proposed model.
It is dear that a hypothetical model of this sort will be of considerable value for the discrimination of zones of hydrothermal alteration in the search for porphyry ore deposits.
Proposed Model for a Porphyry Copper System
The proposed model accepts the premise that economic concentrations of copper and molybdenum in a typical porphyry copper system occur in a sub- volcanic environment associated with small, high- level stocks, and emphasizes the close association with subaerial calc-alkaline volcanism. It is pro- posed that commonly a porphyry copper-bearing stock grades downward into a pluton of larger dimensions that may possess stockwork mineraliza- tion in its upper parts, but that is essentially un- mineralized, except perhaps for veins or pegmatite bodies, at deeper levels. It is further proposed that a porphyry copper deposit is overlain by a column of pyritic alteration which transects a calc-alkaline vol- canic pile, commonly surmounted by an andesitic stratovolcano with native sulfur deposits. This model is schematized in Figure 1.
A survey of the literature on porphyry copper de- posits in the circum-Pacific and Alpide orogenic belts reveals that most of the deposits were emplaced within much older and genetically unrelated forma- tions. In Chile, for example, deposits are normally much younger than their host rocks (except per- haps for Ticnamar and Mocha; Fig. 2), which are commonly andesitic volcanics of Cretaceous or Juras- sic age. It seems, therefore, that the normal location for the economic portions of porphyry ore deposits is in rock formations that underlie the coeval volcanic pile. Hence, overlying volcanic formations would be expected normally to have been completely eroded from the vicinity of porphyry copper systems by the time that Cu-Mo mineralization is exposed. The instructive examples cited below of economic deposits with which calc-alkaline volcanics are still spatially associated may therefore be considered as somewhat atypical.
Many porphyry copper deposits are genetically re- lated to porphyritic-textured stocks that are com- monly the hosts for a significant proportion of the Cu-Mo mineralization. In depth a transition is considered to occur from a porphyritic to a phaneritic rock of similar composition. Such a transition, from dacite porphyry to quartz diorite, has been observed in drill holes which transect the El Teniente deposit (Fig. 2) and has been documented by Portigliati (1971). The position of this textural change is also envisaged as the approximate point at which the small stock starts to increase in diameter downward
FIG. 2. Positions of localities in Chile and Argentina that are referred to in the text.
(Fig. 1). In many instances, therefore, porphyry stocks may be likened to cupola-like projections on the upper surfaces of larger plutons, a concept ex- pounded long ago by Emmons (1927). • Stockwork and disseminated mineralized may extend downward from the stock into the upper parts of the subjacent pluton, but it is thought probable that it soon dies out with depth. Pegmatitic bodies in the underlying plutons may result from trapping of aqueous fluid- rich magma fractions, genetically related to fluids which give rise to porphyry-type mineralization at higher levels.
Despite known exceptions, a typical porphyry
•A variation of this model (Fig. 1) is the case where porphyry copper-bearing stocks do not represent the apical portions of extensive plutons but were intruded into the already solid upper parts of probably genetically related, but slightly earlier, plutons. This case is included in the sub- hood cupola model of Sales (1954). Examples include El Abra, Chile (Fig. 2) (Sillitoe and Neumann, unpub.) and the Highland• yalley del•o•sit• in British Columbia (White, 1957). ' '
802 RICHARD H. SILLITOE
copper deposit possesses a central economic section characterized by concentric shells of potassium sili- cate, sericitic, argillic, and propylitic alteration, as noted previously (Fig. 1). In the deeper parts of deposits potassium silicate alteration tends to be the preponderant alteration type, and in the basal parts of deposits may grade into a modified deep potassium silicate alteration type in which biotite is less common and which consists of the assemblage quartz-K- feldspar-sericite-chlorite (Lowell and Guilbert, 1970).
In an upward direction in a typical deposit sericitic and argillic alteration take on an increasing im- portance at the expense of potassium silicate altera- tion. At this point, near to the upper limit of economic hypogene mineralization, intrusive bodies are likely to be smaller and less regular, and large areas are likely to be occupied by hydrothermal breccias ('Fig. 1).
Upward in many systems, but perhaps not in all, a comagrnatic volcanic superstructure is encountered, in which alteration tends to possess a less regular distribution and to consist of propylitic and argillic alteration, with areas of intense silicification and advanced argillic alteration; these two alteration types perhaps reflect areas preferred by ascending hydrothermal fluids. This proposal conforms with that of Hemley et al. (1969) who showed that sericitic alteration is likely to pass upward into advanced argillic alteration. Pyrite is ubiquitous and other sulfides, with the possible exception of mamasite, are uncommon. Intrusive rocks are rare in these overlying volcanics and are restricted to dikelike bodies, although hydrothermal breccias may still be widespread.
"Epithermal" copper, lead, zinc, and precious metal veins and replacements are considered not only to represent the fringe products of mineralization in the propylitic-altered parts of an economic deposit (Lowell and Gullbert, 1970), but also to accomvany advanced argillic alteration, silicification, or propylitic alteration in the supradjacent volcanic ediface (Fig. 1).
It is inferred that the tops of porphyry copper systems are characterized by deposits of native sul- fur, perhaps accompanied by pyrite or marcasite, and while the system is still active, by high-temperature fumaroles; these may be considered as the effluent products of porphyry copper formation in depth. The stratovolcano above a pomhyry copper deposit need not be a simple cone (Fig. 1), but may be multiple in character and include the development of domes and collapse calderas, perhaps resurgent.
In Figure 1 an attempt has been made to quantify the vertical dimensions of a porphyry copper sys- tem, although the depths given should be treated only as approximations based on currently available
data, some of which are included in the succeeding section. The column of potentially economic por- phyry-type Cu-Mo mineralization may extend down- ward from its apex for some 3 km if the situation at San Manuel-Kalamazoo, Arizona (Lowell and Gullbert, 1970), is typical. Further support for a vertical extent of this magnitude comes from obser- vations at Los Pelambres (Sillitoe, 1973) and E1 Teniente (Howell and Molloy, 1960) (Fig. 2) which show that alteration and mineralization have
vertical extents of at least 1.45 and 1.3 km, re- spectively. At Santa Rita, New Mexico, Nielsen (1968) suggested that the porphyry copper deposit was emplaced beneath a cover of not greater than 0.5 km of older rocks. It is here contended, how- ever, that this figure merely represents the depth beneath the top of the subvolcanic basement, and in this context it has been used in the construction of
Figure 1.2 Since stratovolcanoes in the Andes have heights of 1,000-2,500 m above the underlying base- ment, the apices of the columns of Cu-Mo mineraliza- tion, commonly associated with sericitic alteration, were most probably formed at approximately 1.5-3 km beneath the summits of stratovolcanoes. This
figure might be somewhat reduced if a deposit was eraplaced eccentrically with respect to the principal volcanic cone, or if the magma from which the cone was constructed was less viscous than the andesire
considered here, so reducing the height of the cone. This estimate is in accord with evidence derived
from studies of fluid inclusions that is interpreted to show that potassium silicate alteration at Bingham, Utah, took place beneath a cover 4.3 km thick (Roedder, 1971). Therefore, from the available evidence, it would seem reasonable to estimate that a complete, uneroded porphyry copper system could have a vertical extent of 4-6 km, and perhaps nearer 8 km if the upper part of the underlying phaneritic intrusive is included (Fig. 1).
Evidence Bearing on the Tops and Bottoms of Porphyry Copper Deposits
Regional generalizations
In Chile and adjacent Argentina, longitudinal belts of post-Paleozoic batholiths, stocks, and porphyry copper deposits young eastward from the Pacific coast and are geometrically comparable with belts of Pliocene and more recent andesitic volcanoes in the
high Andes (Sillitoe, 1972a). Porphyry copper de- posits and prospects are uncommon in the batholithic
2 Failure to recognize that many porphyry devosits were originally overlain by penecontemporaneous volcanic piles may provide an explanation for the very shallow depths of formation that have been proposed for some porphyry copver deposits located in regions, such as the southwestern United States, where the volcanics have been largely lost by erosion.
TOPS .,'IND BOTTOMS OF PORPHYRY COPPER DEPOSITS 803
Fro. 3. The West Fissure and hydrothermal alteration pattern at Chuquicamata, Chile, taken mainly from Taylor (1935). The unshaded area between the sericitic and propylitic alteration zones probably cor.responds largely to argillic alteration: it was termed "normal rock" by Taylor and is described as having less intense sericitic alteration and retaining its magmatic texture.
belt adjacent to the coast where most deposits are assumed to have been removed by erosion. Economic deposits are considered to have been exhumed only locally from beneath the belt of recent volcanoes. North-south belts of stocks of intermediate age, situated between the batholiths in the west and the volcanoes in the east, where erosion levels may generally be said to be intermediate between the eastern and western extremes, host the majority of the porphyry copper deposits, including all the major ore bodies.
'Furthermore, the distributions and aerial extents of Chilean porphyry copper deposits and prospects in the belts of stocks and of alteration zones associated with stratovolcanoes in the recent volcanic belt bear
a strong mutual resemblance.
Bottom of a porphyry copper system
Chuquicamata, Chile: At Chuquicamata (Fig. 2), the bottom part of a porphyry copper system is be- lieved to have been exposed as a result of high-angle faulting, the present ore body representing the
804 RICHARD H. SILLITOE
/•",11ii, ,.. I Fresh granodiorite I+'+1 •;; I I ', 11 ',.• • / ß
surviving portion of the original porphyry copper deposit. Despite the fact that only a part of the original ore body now remains, Chuquicamata is one of the largest of the world's porphyry copper deposits.
An elongate stock consisting of several distinct, but closely related, porphyries of probable grano- dioritic composition is host to the Chuquicamata ore body, which occupies an area some 3 x 1.5 km (Perry, 1952). To the east, the porphyries appear to grade into an equigranular granodiorite (Elena granodiorite), although the precise significance of the contact relations is uncertain. To the west, the ore body is abruptly terminated by a major fault, the West Fissure, beyond which outcrops the Fortuna granodiorite, in which porphyry copper-type min- eralization is absent (Taylor, 1935; 'Figs. 3 and 4).
Recent K-Ar dating (Quirt et al., 1971) a has resulted in apparent ages of 36.9--+ 0.6 m.y. for the Fortuna granodiorite and 29.2-----0.5 m.y. for hydrothermal sericite from the ore body. Ruiz et al. (1965) gave a slightly younger K-Ar age of 35 m.y. for the Fortuna granodiorite. A much older age is indicated by geologic characteristics for the Elena granodiorite.
In the Chuquicamata ore body there is evidence of an early phase of potassium silicate alteration repre- sented by veinlets of quartz and K-feldspar with minor chalcopyrite (Jarrell, 1944; Perry, 1952). This alteration type has been largely obliterated by the superimposition of later feldspar-destructive alteration. Adjoining and roughly parallel to the West 'Fissure pervasive sericitic alteration and in- cluded patches of silicification, in which sericite is present in small amounts, occupy a belt some 3.5 km long and averaging 0.5 km in width (Taylor, 1935; L6pez, 1939) (Figs. 3 and 4). Abundant pyrite accompanied by enargite, lesser chalcopyrite, chalco- cite, and molybdenite are hypogene components of the sericitic alteration and provide the highest grade hypogene ore, although a band of lower grade altered rock is in contact with the West Fissure. The serici-
tic alteration grades eastward into propylitic altera- tion (Taylor, 1935; L6pez, 1939; Perry, 1952) (Figs. 3 and 4), characterized by albite, chlorite, and specularire, which gives way further eastward in the Elena granodiorite to a zone carrying abundant epidote (L6pez, 1939). In a drainage tunnel beneath
a Radiometric dating of samples collected by the writer, with the assistance of H. Neumann, from Chilean porphyry copper deposits was carried out by S. Quirt and E. Farrar of Queen's University, Canada.
the ore body, Perry (1952) noted a zone of argillic alteration separating the sericitic and propylitic alteration types, and at surface the day content of the rock increases as the propylitic zone is approached.
The West Fissure is a major fault, marked by crush and gouge zones, which attains a width of 150 m, is near-vertical, but dips 80 ø to the west at the southern end of the ore body and dips steeply east- ward at its northern end (Perry, 1952). The fault and its extensions can be traced for tens of kilometers
both north and south of the ore body. It has been repeatedly but erroneously stated
(Taylor, 1935; L6pez, 1939; Jarrell, 1944) that the mineralizing fluids ascended the West Fissure and subsequently spread out eastward along a stockwork of fractures to give rise to the observed zonation of alteration. However, drag sulfides in the West Fissure testify to important movement along it sub- sequent to the emplacement of the Chuquicamata ore body (Perry, 1952). Although the fault was prob- ably active prior to mineralization, and may even have helped to control the general location of the ore body in the upper crust, it is most unlikely that it acted as a conduit for hydrothermal fluids at the presently observed level, an interpretation that would be at odds with current ideas on the genesis of porphyry ore deposits.
To the west of the West Fissure the 'Fortuna
granodiorite is part of a normal epizonal pluton which extends at least 8 km south and 12 km north
of Chuquicamata. Pegmatitic patches, several meters across, composed of quartz, K-feldspar, biotite, and pyrite, were observed locally in the Fortuna grano- diorite near the ore body. In addition to these pegmatitic patches, there are a few copper-bearing quartz-tourmaline veins and veinlets and a small stockwork of oxidized copper minerals and limonite which contains a little postmagmatic biotite.
It is apparent from Figure 3 that a complete, bi- laterally symmetrical pattern of hydrothermal altera- tion does not exist at Chuquicamata, the normal se- quence of alteration types occurring only as N10 ø E-trending belts to the east of the West Fissure and terminating against it. This clearly demonstrates that the ore body has been bisected by displacement along the West Fissure and that the western portion of the ore body was moved either laterally or vertically and subsequently eroded away.
The available evidence favors essentially vertical displacement on the West Fissure during the Ceno-
Fro. 4. A generalized east-west section across the middle of the Chuquicamata deposit to show the geologic relations and alteration pattern. A hypothetical reconstruction of the ore body is at- tempted above the present ground sur. face to show the characteristics at the point in time when faulting had terminated but erosion had not yet commenced. (This makes the assumption that faulting occurred as a single displacement, which was almost certainly not the case). It is assumed that the superimposition of sericitic on potassiumsilicate alteration at Chuquicamata has resulted in a predominance of the former.
806 RICHARD H. SILLITOE
zoic, with upthrow of the western block. Hence, the western part of the ore body has been destroyed by erosion to reveal the underlying phaneritic intrusive --the Fortuna granodiorite ('Fig. 4). With refer- ence to Figure 1, it can be estimated that such a process necessitates a fault displacement of some 3- 3.5 km since the ore body was formed some 30 m.y. ago. A displacement of this magnitude is consistent with both the width and regional extent of the West Fissure and its extensions to the north and south.
Miocene alluvial gravels immediately south of the Chuquicamata ore body and east of the West Fissure in the vicinity of the Exotica exogene copper deposit consist of a large proportion of fragments of Fortuna granodiorite and very few porphyry or mineralized fragments. This observation can be interpreted to show vigorous erosion of the upthrown western block (at a time when the suprajacent portion of the ore body had already been removed) and little erosion of the present Chuquicamata ore body occupying the downthrown block.
Although the sense and amount of displacement on the West Fissure have not been precisely calculated, no evidence for extensive transcurrent movement
exists. Independent mapping in this part of northern Chile by Thomas (1970) revealed no evidence for transcurrent fault movements, and the Cenozoic tectonics of the western Andes, east of the Coastal Cordillera, are known to be characterized exclusively by vertical displacements accompanying the uplift of the Andes. Nevertheless, the fact that the Fortuna granodiorite extends both north and south of Chuqui- camata means that a component of several km of transcurrent movement on the West 'Fissure could
be accommodated by the above interpretation. The radiometric age determinations clearly show
that the ore body and Fortuna granodiorite are tem- porally related, but are somewhat at odds with the above interpretation as it would be expected that the deeper level Fortuna granodiorite would be equiva- lent, or youngei', in age than the mineralization, and not older as indicated. However, since fine- grained sericite has a lower retentive capacity for argon than that of coarse magmatic biotite, a reduc- tion in the apparent sericite age by 6-8 m.y. could easily have occurred, so accounting for the results.
Lower mineralized part of a porphyry copper system
Los Loros, Chile: Bearing in mind the generaliza~ tion that the level of erosion decreases westward in
Chile and that the age of intrusive events increases in the same direction, one might expect an old age and deep erosion to be characteristics of the Los Loros alteration-mineralization area, which is situated only 25 km from the Pacific littoral (Fig. 2).
Porphyry-type mineralization at Los Loros is
located within a composite pluton that intruded the downfaulted axial zone of a NNW-trending syncline composed, in the immediate vicinity of the pluton, of Lower Cretaceous sedimentary and volcanic rocks (Aguirre and Egert, 1970). The composite pluton is part of the coastal batholith of Chile. Alteration and mineralization are genetically related to the innermost and youngest part of the composite pluton --a body of medium-grained, phaneritic-textured granite (sensu stricto), which is made up of quartz, K-feldspar, oligoclase, biotite, and small quantities of hornblende. The granite is bordered by a dis- continuous zone of granodiorite, diorite, and some gabbro. Magmatic biotite from the granite, some 2.5 km south-southeast of the altered area, yielded a K-Ar age of 89+0.6 m.y. 8 This early Upper Cretaceous age accords well with the local strati- graphic evidence (Aguirre and Egert, 1970), and is the oldest age yet obtained for a porphyry-type development in the Andes. Most Chilean deposits are markedly younger and range in age from Lower to Upper Tertiary (Quirt et al., 1971). Very small and restricted dikes and bosses of rhyolite porphyry, andesitc, and aplite cut the granite in the altered area and are only sparsely mineralized. Contact metasomatic copper and iron deposits have been worked in the aureole of the composite pluton (Aguirre and Egert, 1970).
The zone of hydrothermal alteration at Los Loros has dimensions of 4 X 1.2 km and is therefore com-
parable in size with most porphyry deposits. The alteration zone is located in the interior of the south-
eastern part of the granite pluton, which possesses an area of the order of 100 km 2. As illustrated in
Figure 1, the restriction of alteration-mineralization to an area within, and away from the contacts of, a homogeneous intrusive is expected in the root zone of a porphyry deposit, where the intrusive host has begun to expand in size.
Potassium silicate alteration, locally weak, is the principal alteration type at Los Loros; it grades laterally into propylitic alteration, which in turn gives way outward to fresh granite. The potassium silicate alteration is characterized by quartz-K-feld- spar-sericite-chlorite, with subordinate biotite and calcite. Stockwork veinlets are normally composed of quartz and K-feldspar, biotite is largely replaced by chlorite, and sericite peppers the feldspars. This alteration assemblage is similar to the deep potassium silicate alteration described from the lower parts of the Kalamazoo deposit, Arizona (Lowell and Guil- bert, 1970). The accompanying metallic minerals are chiefly pyrite and magnetite with lesser amounts of molybdenite and chalcopyrite. The Cu and Mo occur in veinlets with quartz, quartz-K-feldspar, or alone. Some alteration minerals, especially K-feld-
TOPS AND BOTTOMS OF PORPHYRY COPPER DEPOSITS 807
spar, quartz, and magnetite, commonly occur as irregular, somewhat pegmatitic, masses in the rock. A limited amount of drilling has shown that potas- sium silicate alteration and mineralization extend to
a depth of at least 250 m below surface, and, at the site of drilling, the molybdenum grade of hypogene ore averages 0.058% Mo, although copper assays are consistently low (0.02% Cu) (United Nations, 1971). Such a molybdenum grade is higher than those in most productive porphyry copper deposits and, taken in conjunction with the fact that the host intrusive is a granite, might indicate that Los Loros belongs to the class of molybdenum-rich porphyry deposits. It should be remembered, however, that molybdenum mineralization is known to persist to greater depths than copper mineralization at the Bingham, Utah, porphyry copper deposit (James, 1971).
Two small areas of pyrite-rich sericitic alteration, one of which is partially brecciated, are superim- posed on the potassium silicate alteration. As sug- gested above, a predominance of potassium silicate alteration is expected in the lower parts of a porphyry deposit.
The above characteristics are considered to be
consistent with the interpretation of Los Loros as the root zone of an upright cylinder of porphyry- type mineralization. The column of mineralization which is believed to have occupied higher levels in the intrusive, perhaps a cupola-like stock, has been removed by the extensive erosion suffered by the coastal zone since early Upper Cretaceous times.
Other areas: In British Columbia, the Brenda porphyry copper and the Endako porphyry molyb- denum both are deposits related to phaneritic in- trusives and they have been interpreted as deep-level deposits by Brown (1969). The possibility exists that they were at one time overlain by more typical porphyry-type mineralization and coeval volcanics.
Porphyry copper deposits spatially related to coeval volcanic rocks
Faralldn Ne#ro, northwest Argentina: Although a general contemporaneity of porphyry ore deposits and calc-alkaline volcanism seems to be indicated
(Sillitoe, 1972a), the detailed inter-relationships be- tween the two phenomena are rarely conducive to preservation. However, since the porphyry copper deposits at Faral16n Negro in Catamarca Province, northwest Argentina ('Fig. 2), were emplaced within the comagmatic volcanic pile, instead of in the sub- volcanic basement as normally seems to be the case, their position in space and time with respect to volcanic events can be elucidated.
In the Faral16n Negro district, the basal wreck of a late Tertiary, calc-alkaline, composite stratovolcano,
some 16 km in diameter, which cuts and overlies a pre-Mesozoic basement can still be recognized. Six small porphyry copper-type developments are ex- posed in the central, most deeply eroded, parts of the volcanic edifice, and two other developments cut the basement further south (Fig. 5). The basement consists of granites, gneisses, slates, and phyllites, covered in parts by Miocene or older continental sediments (Garcia, 1969; Llambias, 1970). Fault- ing prior to, during and subsequent to Cenozoic magmatism delimited the southern and eastern mar- gins of the Faral16n Negro volcanic complex, which occupies a tectonic depression (Fig. 5). The late Tertiary age of the complex (Gonz/dez Bonorino, 1950) has been continned recently by K-Ar dating (Caelles et al., 1971 ).
An extensive study of the Faral16n Negro volcanic complex has recently been undertaken by Llambias (1970, 1972), whose conclusions are summarized here. Magmatism in the Faral16n Negro district commenced in the late Tertiary with production of extrusive, and some intrusive, igneous breccias and tuffs of largely andesitic composition. These were followed by or are contemporaneous with the em- placement of andesite domes around the periphery of the complex, perhaps on the margin of a caldera, along with dikes, sills, and flows of andesite and basalt. The next event was the passive emplacement of a monzonite intrusive. Subsequent porphyry copper-type mineralization ('Fig. 5) accompanied a stock of granodiorite porphyry associated with ring dikes and a radial dike swarm of andesitic to dacitic
composition in the Bajo del Durazno area (Fig. 5), and a NW-SE-trending belt of small stocks and dikes ranging in composition from quartz andesite to dacite and rhyodacite. The final stages in the development of the complex include the intrusion of scarce domes and dikes of rhyolite and the emplace- ment of rhodochrosite-Au-Ag veins in which a little Cu, Pb, and Zn are present locally. Within the complex a strong northwest regional trend is em- phasized by the alignment of dikes, fractures, veins, intrusive bodies and alteration zones.
Some of the zones of porphyry copper-type min- eralization exhibit characteristic and well-developed zonal patterns of hydrothermal alteration-mineraliza- tion, even though they occupy areas of less than 1.5 km 2 (Garcia, 1969, 1971; Sillitoe, unpub.). A potas- sium silicate-altered core centered on a porphyry stock, surrounded by a halo of sericitic and argillic alteration, and passing outward into propylitic altera- tion in the enclosing volcanic rocks is well shown at Bajo de la A!umbrera and Bajo del Durazno (Fig. 5). Evidence from drill core shows that most of the copper occurs in the potassium silicate core as chalco- pyrite and is accompanied by pyrite, magnetite, and
808 RICH•IRD H. SILLITOE
minor molybdenite and bornite. A pyrite halo coin- cides with feldspar-destructive alteration types. Else- where, as at Bajo de Agua Tapada (Fig. 5), potas- sium silicate alteration is poorly developed in the core of the stock and is accompanied by only weak min- eralizafion.
The results of radiometric dating by Caelles et al. (1971) further emphasize that the porphyry copper- type mineralization at Faral16n Negro is intimately related to late intrusive stages in the development of a complex stratovolcano. They determined an age of 10.7 m.y. for an andesire flow from the volcanic superstructure, and clearly younger ages of 7.9 and
7.1 m.y. for mineralized stocks in the Bajo del Durazno and Bejo de San Lucas areas (Fig. 5).
It can be appreciated that the older, marginal parts of the complex are composed of outward-dipping agglomerates, tuffs, and flows that represent the lower slopes of the volcaffo. The generally younger, inner parts possess a greater number of intrusive bodies together with the altered and mineralized areas and were formed in the root zone of the volcano.
In Figure 6 an attempt has been made to recon- struct the volcanic edifice. The distribution of dips within the complex and the locations of the stocks suggest that its eastern and southern margins have
0 5 I •
KILOMETERS
Bajo de Agua Tapada- ka Josefa
Bajo de(
Durazno
+*+?+*•jo e las
ALLUVIUM & GRAVEL
rARALLO" .EGRO VO'CA.,C- ,.RUS,VE CO..LEX
.O.O.TE ,.RUS, VE ZO.S or .DROT.ERMAL ALTERAT, O. CONTINENTAL SEDIMENTS
GRANITE. GNEISS, SLATE & PHYLLITE
FAULT• SHOWING DOWNTHROWN SIDE
Bajo dot Espanto
Bai• d• tos
MIOCENE
MIOCENE
PRE- MESOZOIC
A-A I LINE OF SECT!ON
Fro. 5. Zones of hydrothermal alteration at Faral16n Negro, Argentina. Geology mainly after Llambias (1970). The Bajo de las Juntas zone is located beyond the limits of the figure, some 8 km southeast of Bajo de San Lucas.
Fro. 6. A generalized section across the Faral16n Negro volcanic-intrusive complex along line A-A• in Figure 5, taken from Llamb;as (1970). The reconstruction of the volcanic cone is largely hypothetical.
been eroded away as a result of faulting (Fig. 5). If this simplified reconstruction approximates the truth, then the uneroded stratovolcano had an altitude of about 6,000 m, thus implying the removal of more than 3,000 m of volcanic rocks by subsequent erosion. It can be appreciated that the dimensions of the reconstructed edifice are closely similar to those of the Quaternary andesitic cones in the high Andes of Chile and Argentina. 'From this reconstruction it can be inferred that the apex of the Bajo de la Alumbrera porphyry copper deposit was eraplaced at a maximum depth of about 3,000 m beneath the summit of the volcano, ignoring the effects of possible caldera development. If the volcanic superstructure consisted of several mutually interfering cones, and not of a single cone as depicted, like many Quater- nary Andean volcanoes, then the cover over the de- posit could have been somewhat less. (The dips in the volcanic complex could also be interpreted in terms of two or perhaps more cones.) Figure 6 also indicates that the monzonite intrusive was eraplaced beneath only some 2,000 m of cover. Columns of silicification and advanced argillic alteration that are inferred to have once overlain the porphyry copper ore bodies have been eroded away along with the top 3 km of the edifice.
El Salvador, Chile: At E1 Salvador (Fig. 2), some of the highest parts of the alteration zone are capped by rhyolites that seem, at least in part, to have had an extrusive origin, and rhyolitic and andesitic wall rocks are also present at lower elevations (Swayne and Trask, 1960). The results of radiometric dating have shown that rhyolitic and andesitic volcanism pre-dated by some 4-9 m.y. the eraplacement of
several underlying porphyry stocks with which the potassium silicate alteration and mineralization are intimately associated (Gustafson and Hunt, 1971). Although morphological evidence has been destroyed, it is here considered that these volcanic rocks, in particular the upper rhyolites, represent the basal portion of a comagmatic volcanic pile that overlay the ore body at its time of formation about 41 m.y. ago. The mineralized stocks were eraplaced close to the contact between basement and volcanic cover.
The occurrence in the upper parts of the E1 Salvador alteration zone of advanced argillic altera- tion, characterized by pyrophyllite, diaspore, and alunite (Gustafson and Hunt, 1971), supports the contention that the overlying volcanic environment is being approached. Available evidence does not, however, permit an estimation of the thickness of volcanic covers that has been eroded since the ore body was eraplaced.
Other areas: At Faral16n Negro and E1 Salvador the contemporaneous volcanic rocks possess an inti- mate and fairly clear spatial relationship to intrusives, alteration, and mineralization. At some other locali- ties, however, contemporaneous volcanic rocks have been recognized but are not in such direct contact with the ore bodies.
At Bingham, Utah, stock intrusion and porphyry copper-type mineralization overlap in time with the nearby extrusion of a series of coma#matic latitic and quartz latitic volcanic rocks (Moore et al., 1968). There, as at Faral16n Negro, magmatic and hydro- thermal events were terminated by the intrusion of a rhyolite plug. The Sar Cheshmeh porphyry copper deposit in Iran is located 3 km south of an outcrop of
810 RICHARD H. SILLITOE
dacitic volcanic rocks thought to be the extrusive equivalent of the mineralized granodiorite porphyry stock (Bazin and Hfibner, 1969). To the south and west of the Mount Fubilan (Ok Tedi) porphyry copper deposit, Territory of Papua and New Guinea, pyrodastic rocks, perhaps cogemetic with the min- eralized quartz latite porphyry stock and other nearby stocks, have been identified (Barnford, 1972). At Stafford, Arizona, a pyrodastic-filled pipe, inter- preted by Robinson and Cook (1966) as a volcanic vent, formed pemecontemporaneously with the ore body.
Alteration and mineralixation overlying porphyry copper deposits
Cerro Marqu•x, Chile: In the high Andes of Chile and northwest Argentina many Plioceme and more recent andesitic stratovolcanoes have undergone dis- section to reveal extensive zones of hydrothermal alteration. Much of the erosion can be attributed
to the effects of the Pleistocene glaciation. Like most of the volcanic cones in northern Chile, these eroded
examples were constructed on a platform of Middle to Upper Tertiary ignimbrite flows (Katsui and Gonzalez, 1968; Guest, 1969). Cerro Marquez in Tarapac/t Province, northern Chile (Fig. 2), has been selected as an example.
The eroded Cerro Marqu6z stratovolcano of prob- able Quaternary age (Salas et al., 1966) has a present maximum height of 4,960 m above sea level about 1,000 m above the surrounding ignimbrite plateau. The flanks of the edifice, extending upward to its highest point, are composed of unaltered andesitic lavas, tuffs and agglomerates which in the central, eroded portion of the structure (4,350- 4,700 m) are conspicuously altered (Fig. 7). The edifice occupies an area of 12 x 9 km and the altera- tion covers an inner area approximating 8 x 6 km. Reconstruction of the probable initial form of the volcano, based on the observed outward dip of the andesitic volcanics of 20-25 ø, suggests that some 1,250-1,500 m have been removed by erosion from above the alteration zone.
The hydrothermal alteration is largely of the
Fro. 7. The zone of propylitic and ar. gillic alteration and silicification in the central part of Cerro Marquez, Chile, a deeply eroded andesitic stratovolcano. The white alteration zone bounded by the uneroded lower slopes of the volcano is dearly visible on the horizon. The volcano overlies a plateau constructed of mid-to-late Tertiary ignimbrites which is seen in the foreground.
TOPS AND BOTTOMS OF PORPHYRY COPPER DEPOSITS 811
argillic and propylitic types with abundant pyrite (mainly oxidized to limonite) and gypsum; some argillic alteration may be of supergene origin. Silicification, resulting in the replacement of vol- canics by dense, aphanitic silica, commonly occurs as patches, perhaps representing the principal conduits for hydrothermal fluids; some outcrops are brec- ciated. A little native sulfur and alunite are present, minerals that are more common in other areas of this
type. A small body, probably a dike, of quartz porphyry in which the feldspars are altered to sericite was observed within the altered zone.
The size and character of alteration zones like
that at Cerro Marqudz are consistent with the sug- gestion that they represent the near-surface effects produced by hydrothermal fluids emanating from porphyry copper deposits at deeper levels.
Cerro Queva, northwest .dr#entina: Cerro Queva ('Fig. 2) and adjoining peaks comprise a complex Pliocene or Quaternary stratovolcano (Vilela, 1969) built on Paleozoic basement that has suffered intense
erosion of its upper parts during the Pleistocene glaciation. The erosion has removed fresh andesites and dacites to reveal large alteration areas in which patches of intense silicification are interspersed with argillic and propylitic alteration; hydrothermal breccias occur locally. Pyrite and gypsum are wide- spread, and native sulfur has been worked at the highest elevations. In some of the alteration areas, a few hundred meters lower than the native sulfur
concentration, fine-grained lead-silver mineralization occurs and is exploited at the E1 Queva mine. The mineralization, accompanied by pyrite, chalcedony, alunite, barite, and clay minerals (not yet identified), formed as a replacement of flows along a pronounced east-west structural trend.
This locality 4 is an excellent example of "epi- thermal" mineralization associated with widespread hydrothermal alteration within a volcanic super- structure, above the level at which porphyry copper- type mineralization might be expected (Fig. 1).
Other areas: An "epithermal," polymetallic de- posit consisting of veins and breccia zones was once worked for silver at Choquelimpie, Tarapacfi. Prov- ince, northern Chile (Fig. 2). The mineralization is located in an area of alteration in an eroded andesitic stratovolcano of Pliocene or more recent
age. Silicification, chalcedony, and alunite ac- company the ore minerals.
Precious metal and Pb-Zn-Cu mineralization in
young, propylitic-altered volcanic rocks in other parts of the world, such as the San Juan Mountains of Colorado (Burbank and Luedke, 1969), is corn-
4 A more detailed account of the Cerro Queva district will be presented elsewhere when work is completed.
monly accompanied by advanced argillic alteration and represents a similar high-level situation, perhaps above the porphyry environment.
Additional evidence: In this section, the presence of native sulfur has been referred to at both Cerro
Marquez and Cerro Queva, localities cited as typical of the extensions upward into the truly volcanic en- vironment of porphyry copper deposits. The tops of volcanoes possess even greater concentrations of native sulfur, as noted below.
Moreover, in this regard, it may be significant that several localities possessing the characteristics of the upper parts of porphyry copper deposits, albeit with some copper mineralization, also contain native sul- fur. At Ticnamar, Chile (Fig. 2), a large zone of alteration characterized by irregular dikelike in- trusives, pyritic feldspar-destructive alteration, and widespread hydrothermal intrusion breccias, the as- semblage native sulfur.-pyrite is fairly abundant. The hypogene assemblage native sulfur-pyrite-covellite occurs commonly in an extensive body of hydrother- mal intrusion breccia that has undergone feldspar- destructive alteration at the Cerro Rico porphyry copper deposit, Argentina (Fig. 2). Furthermore, a minor occurrence of native sulfur was described
from the marginal propylitic alteration zone at the Los Pelambres porphyry copper deposit (Sillitoe, 1973).
Tops of porphyry copper systems
.ducanquilcha, Chile: The Aucanquilcha volcano in Antofagasta Province, northern Chile (Fig. 2), is a Quaternary andesitic stratovolcano that attains an altitude of 6,186 m above sea level. The volcanic edifice measures approximately 10 km in diameter and towers some 1,500 m above the surrounding ignimbrite plateau. The volcano is inactive and no fumaroles were observed (in 1971), although patches of hot ground exist. The summit region, composed of andesitic lavas and pyroclastics, is characterized by large deposits of native sulfur in which small pockets of friable, deep blue covellite are admixed with the elemental sulfur (Ruiz et al., 1965). In a micro- scopic study of samples from this locality, Clark (1970) found the phase Cua..•FeS0..5 intergrown with the normal covellite.
Aucanquilcha is typical of over 1,000 strato- volcanoes in northern Chile, of which only 13 are active (Casertano, 1963), though many are in the fumarolic stage. The edifices have heights of 1,000- 2,500 m above the surrounding volcanic plateau. Many of the volcanoes possess summit accumulations of native sulfur and beneath the surface pyrite is commonly abundant (A. Thomas, pers. commun., 1972). No evidence is available, however, on the occurrence of metals in the fumarolic deposits, o[
812 RICHARD H. SILLITOE
which no detailed studies have yet been undertaken. Additional evidence: The occurrence of large ac-
cumulations of native sulfur near the vents of andesitic stratovolcanoes is not restricted to the
Andes but is a widespread phenomenon in the cir- cum-Pacific and Alpide orogenic belts. Large ton- nages of pyrite and marcasite have been described as associates of native sulfur at andesitic volcanoes in
Japan (Takeuchi et al., 1966) and Taiwan (Kinkel, 1966). Despite this abundance of native sulfur and, in places, of iron disulfides in the summit regions of subaerial volcanoes, significant accumulations of base metals seem to be absent. However, it is well known that some high-temperature fumarolic vapors carry minor values (White and Waring, 1963), and, in addition to the copper showing at Aucanquilcha, several instances of the occurrence of base metals, particularly copper, in volcanic sublimates have been recorded.
In the volcanoes of Kamchatka and the Kurile
Islands, copper is concentrated up to 6,000-fold in some sublimates compared with the amounts in the associated lavas, and sublimates containing 60% Cu and a variety of other base metals also occur (Naboko, 1959). Copper, chiefly as tenorite and copper-bearing aphthitalite, accompanied by iron, lead, and zinc, are abundant in fumarolic products both in the summit crater and on the surface of lava
flows at Cerro Negro volcano, Nicaragua (Stoiber and Rose, 1971), and copper and zinc are present in gases from high-temperature fumaroles at Showa- shinzan volcano, Japan (Mizutani, 1970). It would seem that base metals, particularly copper, are rela- tively widespread as minor constituents of sublimates near the central vents of volcanoes, and it seems probable that large quantities of these metals are lost to the atmosphere as gases or as components of con- centrated brines in aerosols.
In addition to noting the presence of base metals in high-temperatute fumarolic products, it is worth emphasizing that the compositions of their emissions (Iranov, 1959; White and Waring, 1963) and the accumulations of alkali chlorides and sulfates around
fumarolic orifices would seem to correspond in a gross way with the nature of mineralizing fluids in the potassium silicate alteration zone, as inferred from fluid inclusion (Roedder, 1971 ) and mineralogic (e.g., abundance of anhydrite) evidence.
Discussion
In this final section some further implications of the proposed model for porphyry copper systems are considered, with particular emphasis on certain genetic aspects. This discussion accepts the ortho- magmatic model for porphyry copper formation, as supported by Nielsen (1968) and Lowell and Guil-
bert (1970), whereby bodies of calc-alkaline magma lose their fluid content, at times violently, during retrograde boiling after the cessation of ascent at high crustal levels. Alteration, mineralization, and brecciation in the early-consolidated hood of the intrusive and its immediate wall rocks are ac-
complished by the escaping fluids.
Relation to volcanic cycle
It follows from the proposed model that fumarolic, solfataric, and hot-spring activity are the surficial manifestations of retrograde boiling in one or more subjacent magma chambers, and of the interaction between forcefully released magmatic fluids and convectively circulating ground waters. Therefore, porphyry copper formation, as well as fumarolic activity, is the terminal stage in the development of a stratovolcano, postdating the main explosive activity and lava effusion conducive to cone construction, and probably also postdating caldera development when it is present (e.g., 'Faral16n Negro). This suggests that the magma bodies that now host porphyry copper deposits may perhaps represent magma cham- bers that were not emptied by eruption but were permitted to evolve under relatively quiescent condi- tions. Relatively nilnor magmatism may sometimes occur subsequent to porphyry copper'formation, however, and is represented by postmineralization intrusives and breccias (e.g., Carter, 1970), such as the rhyolites at Faral16n Negro.
Bodies of hydrothermal breccia are concentrated in the upper parts of porphyry copper systems and, although some are undoubtedly blind, major masses such as the Braden pipe at E1 Teniente (with a diameter of 1,200 m and a vertical extent of at least 1,600 m; Howell and Molloy, 1960), may have possessed connection with the surface (Fig. 1). Therefore the operation of fluidized systems in depth may in certain cases be represented by surficial explosive or hydrothermal phenomena.
Hydrothermal fluids in porphyry copper systems
The idea (Rose, 1970; Meyer and I-Iemley, 1967) that the potassium silicate/sericitic alteration inter- face might represent the inner boundary of ground water participation in mineralization has gained con- siderable support from oxygen and hydrogen isotope data obtained from these two alteration types at several porphyry ore deposits (Sheppard et al., 1971). Thus much of the magmatic fluid involved in potassium silicate alteration tended to be grossly diluted by interaction with ground water as it moved upward and, to some extent, outward from a con- solidating stock. I-lot springs and low-temperature fumaroles, generally metal-deficient (Naboko, 1959), probably represent points where these heated ground
TOPS •IND BOTTOMS OF PORPHYRY COPPER DEPOSITS 813
waters debouch. It is postulated, however, that a small portion of the magmatic fluid is occasionally released upward in the vicinity of the central vents of volcanoes to produce high-temperature fumaroles; separation of volatile phases during ascent gives rise to the source materials for at least a part of the sublimates and their contained metals. This is in
agreement with the conclusions reached by Stoiber and Rose (1970) who confirm the importance of ground water contamination but stress that certain components of sublimates (e.g., C1, SO4, and some Na and K) have a direct magmatic source.
The hypothetical model advanced here for a porphyry copper system is preferred to that sug- gested by White et al. (1971) who claimed that porphyry copper systems are generated in vapor- dominated geothermal systems involving the con- vective circulation of meteoric (and cormate) fluids above a magmatic heat source, with little direct con- tribution of fluids or metals from the magma. While an association of convectively circulating meteoric fluids with the upper parts of active porphyry copper systems seems inevitable, the highly saline, metal- bearing brines involved in potassium silicate altera- tion seem better derived as a normal facet of the late
stages of magmatic evolution (e.g., Kilinc and Burnham, 1972), in view of the stable isotope data of Sheppard et al. (1971). Furthermore, deep drilling of vapor-dominated geothermal systems has so far not encountered alteration and mineralization of the
type found in porphyry copper deposits.
Implications for super#ene alteration
If it is correct to assume that porphyry copper systems outcrop at surface, then supergene alteration of their upper parts might be expected to commence immediately upon cessation of hypogene mineraliza- tion. Moreover, the interaction of magmatic and meteoric fluids above the potassium silicate altered core during mineralization suggests that hypogene effects may well be transitional to supergene ones. Erosion of a recently formed stratovolcano is likely to be rapid, especially under tropical conditions, en- abling the effects of supergene alteration to progress downward to the upper part of the column of Cu-Mo mineralization in a relatively short time. Gustarson and Hunt (1971) stated that superserie enrichment in the E1 Salvador deposit commenced within 6 m.y. of intrusion and mineralization.
A further possibility during the initial stages of superserie alteration of a porphyry copper system is the leaching of copper from unconsolidated ashes by acid solutions and its dissolution from sublimates by rain water with subsequent precipitation of the copper at deeper levels, perhaps aided by hydrogen sulfide in late-stage volcanic gases.
Regional considerations
If the model proposed in this report is correct, then porphyry copper systems effectively span the boundary between the plutonic and volcanic environ- merits. This conclusion lends further credence to
Hamilton's (1969) thesis that batholithic belts repre- sent the roots of eruptive chains.
Furthermore, the intimate genetic relation between porphyry copper deposits and calc-alkaline intrusive and extrusive activity is a cogent argument in favor of the restriction of this type of mineralization to belts above active, or once-active, subduction zones (Sillitoe, 1972a,b; Mitchell and Gatson, 1972), since this class of magmatism is the hallmark of convergent plate margins.
Acknowledgments
Most of the information in this paper was gathered while the writer was engaged by the Instituto de Investigaciones Geo16gicas of Chile, the Empresa Nacional de Mineria-Junta de Adelanto de Africa of Chile, and the United Nations, New York (as- signed to the Chile-28 Project: United Nations- ENAMI and Plan NOA-1 Geo16gico-Minero, Repfiblica Argentina). Personnel from all these organizations are thanked for their assistance and for the provision of useful data; in particular I should like to thank Sres. Harry Neumann, Carlos Porti- gliati, Arthur Thomas, David Pacci and Eugenio Rodrigudz from Chile, Jorge Mancini and Carlos Lurgo from Argentina, and Messrs. Donald Robert- son, Franco Maranzana and Gerald Moorhead from the United Nations. Permission to include data on
Los Loros was granted by Messrs. John Carman and Robertson of United Nations, New York.
Drs. John Angus and George P. L. Walker provided useful discussions, and Professors David Williams, Angus, and the reviewers, Drs. L. B. Gustafson and J. D. Lowell, made numerous useful comments on the manuscript.
The paper was prepared at the Royal School of Mines, Imperial College, London, under tenure of a Shell Postdoctoral Research Fellowship.
iDEPARTMENT OF 3,/lINING GEOLOGY RoY^L School oF MINES
IMPERIAL COLLEGE OF SCIENCE AND TECHNOLOGY
LoNr•oN SW7 2BP, ,4,tgust 15, 1972; February 28, 1973
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814 RICH.4RD H. SILLITOE
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