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Economic Geology VoL 68, 1973, pp. 799-815 The Tops and Bottoms f Porphyry Copper Deposits RICHARD H. SILLITOE Abstract Although it is now widely accepted hat porphyry copper deposits consist of zonally arranged shells of alteration and mineralization centered on high-level, calc-alkaline stocks, he 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 nto 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 rom 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 han the stock. Porphyry copper deposits re normally located n the basement beneath a comagmatic volcanic pile, which is transected by a column of hydro- thermal alteration representing he 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 arge native sulfur deposits and small quantities of base metals, particularly copper, in sublimates at high-temperature umaroles n the vicinity of its central vent; these surficial deposits are considered as the effluent products of active porphyry copper systems. The available evidence avors the emplacement f the tops of typical porphyry copper deposits t depths of 1.5-3 km beneath he summits of stratovolcanoes nd suggests hat entire porphyry copper systems possess ertical extensions as great as 8 km. At Chuquicamata, hile, a major high-angle ault may have cut the porphyry copper deposit, and subsequent rosion has removed he portion of the deposit hat 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, n the upthrown block is interpreted as the root zone of the Chuquicamata orphyry copper system. The lowãer most, rMneralized part of a porphyry system s believed o be exposed at Los Loros, Chile. There a zone of molybdenum-rich nd copper-poor otassium ilicate alteration carrying abundant K-feldspar occupies n 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 t to be determined hat porphyry copper emplacement was a late event in the construction of the stratovolcano, succeeded nly by the formation of minor rhyolite intrusives and epithermal veins. Extensive zones of pyritic alteration including widespread silicification, n 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 he vertical interval between porphyry copper deposits and the vent areas of uneroded volcanoes. At Cerro Queva in northwest Argentina, lead- silver mineralization ssociated ith advanced rgillic alteration s located n an altera- tion zone beneath he summit egions of a stratovolcano. It may be concluded hat during the final stages of construction f 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 ormation of alteration and mineralization. The proposed model implies that porphyry copper systems ffectively span he boundary between he plutonic and volcanic environments. 799 800 RiCH/IRD H. SILLiTOE Introduction A ãRã.AT deal of attention has recently been focused on patterns of lateral and vertical zoning of altera- tion and mineralization n porphyry ore deposits (Lowell and Gullbert, 1970; Rose, 1970; James, 1971). This work has ed to the general acceptance of many porphyry deposits s upright cylinders on- sisting of coaxially distributed ones of alteration and mineralization centered on felsic stocks, commonly porphyries. The silicate and sulfide zoning com- prises a core of potassium ilicate alteration envel- oped successively y 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 n the Andes, and is particularly well exemplified y the Los Pelambres eposit Fig. 2) in Chile (Sillitoe, 1973). Knowledge of the character and distribution of alteration-mineralization s, however, largely com- fined to the economically xploitable portions of porphyry ore deposits, whereas he nature of the uneconomic xtensions of these deposits, both up- ward and downward, emains undocumented. When one considers he huge onnage f porphyry eposits, their high level of emplacement n the continental crust, and the large volumes of mineralizing fluids and high temperatures nvolved n their generation, t is evident that entire porphyry copper systems must extend downward and especially upward for very considerable istances beyond the parts which are likely to be of economic nterest. Even at Kalama- zoo, Arizona (Lowell, 1968), where he mineralized body has been ectonically isturbed nd possesses near-horizontal attitude, the complete porphyry copper ystem s not observable. Therefore t seems necessary o combine nformation rom many areas in order o attempt o construct model of a porphyry system. In the first section of this paper, a preliminary speculative model for an idealized porphyry copper system s advanced, lthough t is not claimed hat every porphyry-type evelopment ecessarily om- plies with all its features. uture studies f porphyry deposits n various parts of the world will un- doubtedly e able to clarify or modify some of the more enigmatic spects f the model. In the second section f the paper descriptions re given of localities in Chile, Argentina and elsewhere hat are thought to be typical of various evels n the upward and downward extensions f porphyry copper deposits. Votc6n ,hlJCOi'lq,ui[chlJ. KH. IN ãLATION O EA-LEVEL) Et Queva& Cerro Marctã Mocna Los el. rrã. $J-. Los Loros Fortuna Grano- diorite, hua. icamata PORPHYRY TOCK PHANERITIC ORANODIORITE HYDROTHER 4AL INTRUSION BRECCIA LIMESTONE: ORIZON ROCK TYPES ã ã- tg4SEMENT L- 0 2 HORIZONTAL CALE (same as vertical) = , , Kilometers ALTERATIO_N SlLICIFICATION ADVANCED ARGILLIC PRO Yt. TIC SE:RICITIC .ot^sãu. ã.c^te /....'ã Fro. 1. Idealized ross ection f a typical, imple orphyry opper eposit howing ts position t the boundary etween plutonie nd olcanic nvironments. ertical nd orizontal imensions re meant o be only pproximate. TOPS AND BOTTOMS OF PORPHYRY COPPER DEPOSITS 801 This section oes not purport, however, o 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 lteration n the search or porphyry ore deposits. Proposed Model for a Porphyry Copper System The proposed model accepts he premise hat economic oncentrations f copper nd molybdenum in a typical porphyry copper ystem ccur n a sub- volcanic environment ssociated ith small, high- level stocks, and emphasizes he close association with subaerial alc-alkaline olcanism. t is pro- posed hat commonly porphyry copper-bearing stock grades downward nto a pluton of larger dimensions hat may possess tockwork mineraliza- tion in its upper parts, but that is essentially n- mineralized, xcept perhaps or veins or pegmatite bodies, t deeper evels. It is further proposed hat a porphyry opper eposit s overlain y a column f pyritic alteration which transects a calc-alkaline ol- canic pile, commonly urmounted y an andesitic stratovolcano ith native sulfur deposits. This model s schematized n Figure 1. A survey f the iterature n porphyry opper e- posits n the circum-Pacific nd Alpide orogenic elts reveals hat most of the deposits ere emplaced within much older and genetically nrelated orma- tions. In Chile, or example, eposits re normally much younger han their host rocks except per- haps or Ticnamar nd Mocha; Fig. 2), which are commonly ndesitic olcanics f Cretaceous r Juras- sic age. It seems, herefore, hat the normal ocation for the economic ortions f porphyry re deposits s in rock formations that underlie the coeval volcanic pile. Hence, overlying volcanic ormations would be expected ormally o have been completely eroded rom he vicinity f porphyry opper ystems by the time hat Cu-Mo mineralization s exposed. The instructive xamples ited below of economic deposits with which calc-alkaline olcanics are still spatially ssociated ay therefore be considered s somewhat typical. Many porphyry opper eposits re genetically e- lated to porphyritic-textured tocks hat are com- monly he hosts or a significant roportion f the Cu-Mo mineralization. n depth a transition s considered o occur rom a porphyritic o a phaneritic rock of similar composition. Such a transition, rom dacite porphyry o quartz diorite, has been observed in drill holes hich ransect he El Teniente eposit (Fig. 2) and has been documented y Portigliati (1971). The position f this extural hange s also envisaged s the approximate oint at which he small stock starts o increase n diameter downward ã5 o -20 ø ã.[ ãoCHOqUE'LIMPIE' TICNAMARãj . xã ã 'CEãRRO ARQUEZ 'L VOLCAN ã ß'ãAUCãQUãLCãA J & HU(ã .ã' / i ./ 'ã' ©CERRO . EVA EL'ã, Lu S,ãL.V,ãOOR / FARALLON EGRO ; J CERRO ICOã /.OROS - ã> -- © LOS PEL,4MBRES 0 I00 I I I /C> i L TENIENTE I' i 500 I ß LOCALITIES DESCRIBED IN THE TEXT 0 LOCALI TIES MENTIONED IN THE TEXT 70 ø FIG. 2. Positions of localities in Chile and Argentina that are referred to in the text. (Fig. 1). In many instances, herefore, porphyry stocks may be likened to cupola-like projections on the upper surfaces f larger plutons, a concept x- pounded ong ago by Emmons 1927). ã Stockwork and disseminated ineralized may extend downward from the stock nto the upper parts of the subjacent pluton, but it is thought probable hat it soon dies out with depth. Pegmatitic bodies n the underlying plutons may result from trapping of aqueous luid- rich magma fractions, genetically elated to fluids which give rise to porphyry-type mineralization at higher evels. Despite known exceptions, a typical porphyry ãA 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 n the sub- hood cupola model of Sales (1954). Examples include El Abra, Chile (Fig. 2) (Sillitoe and Neumann, unpub.) and the Highlandã alley delãoãsitã n British Columbia (White, 1957). ' ' 802 RICHARD H. SILLITOE copper deposit possesses central economic ection characterized y concentric hells of potassium ili- cate, sericitic, argillic, and propylitic alteration, as noted previously Fig. 1). In the deeper parts of deposits otassium ilicate alteration ends o be the preponderant lteration ype, and in the basal parts of deposits may grade nto a modified deep potassium silicate lteration ype in which biotite s less common and which consists of the assemblage quartz-K- feldspar-sericite-chlorite Lowell and Guilbert, 1970). In an upward direction n a typical deposit sericitic and argillic alteration take on an increasing m- portance t the expense f potassium ilicate altera- tion. At this point, near to the upper limit of economic ypogene mineralization, ntrusive 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, ut perhaps ot in all, a comagrnatic olcanic superstructure s encountered, in which alteration tends to possess less regular distribution nd to consist of propylitic and argillic alteration, with areas of intense silicification and advanced argillic alteration; these two alteration types perhaps eflect areas preferred by ascending hydrothermal luids. This proposal onforms with that of Hemley et al. (1969) who showed hat 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 opper, ead, zinc, and precious metal veins and replacements re considered ot only to represent he fringe products f mineralization n the propylitic-altered arts of an economic deposit (Lowell and Gullbert, 1970), but also o accomvany advanced rgillic alteration, ilicification, r propylitic alteration in the supradjacent volcanic ediface (Fig. 1). It is inferred that the tops of porphyry copper systems re characterized y deposits f native sul- fur, perhaps ccompanied y pyrite or marcasite, nd while the system s still active, by high-temperature fumaroles; hese may be considered s the effluent products of porphyry copper ormation n depth. The stratovolcano bove a pomhyry copper deposit need not be a simple cone (Fig. 1), but may be multiple n character nd include he development f domes and collapse alderas, perhaps esurgent. In Figure 1 an attempt has been made o quantify the vertical dimensions f a porphyry copper sys- tem, although he depths given should be treated only as approximations ased n currently vailable data, some of which are included n the succeeding section. The column of potentially economic or- 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 or a vertical extent of this magnitude comes rom 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 hat 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 he depth beneath he top of the subvolcanic asement, nd in this context it has been used in the construction of Figure 1.2 Since stratovolcanoes n the Andes have heights of 1,000-2,500 m above he underlying base- ment, he apices f the columns f Cu-Mo mineraliza- tion, commonly associated with sericitic alteration, were most probably ormed at approximately 1.5-3 km beneath the summits of stratovolcanoes. This figure might be somewhat educed f a deposit was eraplaced ccentrically with respect o the principal volcanic cone, or if the magma from which the cone was constructed was less viscous than the andesire considered ere, so reducing he height of the cone. This estimate is in accord with evidence derived from studies of fluid inclusions hat is interpreted to show hat potassium ilicate alteration at Bingham, Utah, took place beneath a cover 4.3 km thick (Roedder, 1971). Therefore, from the available evidence, t would seem reasonable o estimate that a complete, neroded 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 s included Fig. 1). Evidence Bearing on the Tops and Bottoms of Porphyry Copper Deposits Regional generalizations In Chile and adjacent Argentina, ongitudinal belts of post-Paleozoic atholiths, stocks, and porphyry copper deposits young eastward from the Pacific coast and are geometrically omparable ith belts of Pliocene and more recent andesitic volcanoes in the high Andes (Sillitoe, 1972a). Porphyry copper de- posits nd prospects re uncommon n the batholithic 2 Failure to recognize that many porphyry devosits were srcinally overlain by penecontemporaneous olcanic piles may provide an explanation for the very shallow depths of formation that have been proposed for some porphyry copver deposits ocated in regions, such as the southwestern United States, where the volcanics have been largely lost by erosion.
