THE STRATIGRAPHIC ANO STRUCTURAL SETTING OF THE POTRERILLOS PORPHYRY COPPER OISTRICT, NORTHERN CHILE
STEVEN F. OLSON BHP-Utah Minerals Internationallnc., 550 California SI., San Francisco, California 94104. U.S.A.
ABSTRACT
The Potrerillos district displays major facies boundaries, records changes in the compositions of magmas from Jurassic to Oligocene time, and provides evidence that a porphyry copper deposit and associated skarn (37 Ma) formed between Jurassic-Cretaceous and Paleocene extension and Miocene compressional deformation. Pre-Jurassic granitic and metamorphic basement is overlain by Jurassic-Lower Cretaceous marine sedimentary rocks and basal tic andesite lava flows, and by Paleocene-Eocene bimodal andesite-rhyolite continental volcanic rocks. The Potrerillos district strad dles major changes in facies and thicknesses of strata, between thicker, volcanic-dominant sections on the west and thinner, sedimentary-dominant sections on the east. The changes in thickness and facies are, in part, controlled by down-to-the-west normal/growth faults directly associated with the accumulation of the volcanic rocks. From Ju-assic through Cretaceous time, the district occupied a back-arc setting with respect to the principal magmatic arc. Mineralized and unmineralized porphyry plutons of granodiorite to quartz monzonite composition were emplaced along the Jurassic Cretaceous and Paleocene facies/thickness boundary between 40 and 32 Ma and represent the migration of the mag matic arc through the district. Some of the porphyry plutons were coeval with dacitic flow domes. Principal orientations of dikes ca. 32 Ma old indicate that the state of stress at this time was one of E-W compression (N-S extension). The major movement on reverse faults in Ihe district and surrounding region is constrained to have occurred between 32 and 12 Ma; some Eocene movement cannot be ruled out, however. The thick volcanic-dominant western facies commonly are thrust over the thinner sedimentary-dominant eastern facies along both high-angle basement-involved reverse faults and low-moderate angle thrust laults in the sedimentary-volcanic cover. The Potrerillos porphyry copper pluton was cut and offset by a thrust lault at this time also. Basement and cover rocks record diflerent amounts 01 shortening, which together, could be as high as 50% across the study area. The west-vergent reverse laults in the Potrerillos area are in part responsible lor the uplilt along the western edge 01 the Altiplano. Published rates 01 South America-Nazca plate con vergence suggest that Paleocene bimodal volcanism occurred contemporaneously with ~!ow rates, porphyry intrusion with increased but fluctuating rates, and compressive delormation with greatly increased rates.
Key words: Facies change, Thrust faults, High angle reverse faults, Porphyry copper, 8imodal volcanism, Potrerillos, Chile.
RESUMEN
El distrito de Potrerillos se ubica en una zona de cambios de facies mayor, registrada durante la evolución mesozoica y cenozoica del orógeno andino. Sobre el basamento granítico y metamórfico prejurásico, se disponen, en la parte oriental del distrito, rocas sedimentarias marinas jurásico-cretácicas, con intercalaciones de lavas andesítico-basálti• cas (cubiertas, en discordancia, por una asociación volcánica bimodal paleoceno-eocena), que gradan hacia el oeste hacia potentes secuencias volcánicas. principalmente cretácicas. El cambio de facies está controlado, en parte, por fallas normales ('down the west normal/growth faults'). Durante el Jurásico y Cretácico, el distrito ocupó una posición de tras-arco, con respecto al arco magmático andino principal. Entre 40 y 32 Ma, un conjunto de plutones porfídicos grano dioríticos a cuarzo-monzoníticos (entre los cuales se incluye el pórfido cuprífero de Potrerillos) se emplazó a lo largo de la zona de cambio de facies. Algunos de ellos son contemporáneos con flujos y domos dacíticos. La orientación de los diques indica que el estado de 'stress' durante el período estuvo dominado por compresión en sentido este-oeste (exten sión norte-sur). El período principal de deformación en el distrito, a través de fallas inversas, se produjo entre los 32 y 12 Ma, a pesar de que algunos movimientos podrían haber ocurrido también durante el Eoceno. Las lacies occidentales, predominantemente volcánicas, están, en general, cabalgadas sobre las secuencias sedimentarias orientales, tanto a lo largo de fallas inversas de alto ángulo, que involucran al basamento, como a través de sobrescurrimientos de bajo án gulo, que afectan sólo a la cobertura. Durante la deformación el pórfido de Potrerillos fue cortado y desplazado hacia el este por una de las fallas de bajo ángulo. El acortamiento tectónico, registrado en el distrito, es diferente en el basa-
ReVISta Geológica de Chile. Vol 16. No. 1. p. 3·29. 15 Figs .• 5 Tablas. 1989. 4 THE POTRERILLOS PORPHYRY COPPER DISTRICT
mento y la cobertura, pero la combinación de ambas componentes indica valores cercanos al 50%. Las fallas con ver gencia hacia el oeste, que afectan al distrito son, en parte, responsables del alzamiento del borde occidental del Altiplano. Un examen de las velocidades de convergencia entre la placa de Nazca (y sus precursores) y la placa Sudamericana, señala que el volcanismo bimodal paleoceno tuvo lugar en un perrodo de baja velocidad de convergen cia, la intrusión de los pórfidos, con velocidades de convergencia crecientes, pero fluctuantes, mientras que la etapa de deformación compresiva coincide con un perrodo de elevada velocidad de convergencia de placas.
Palabras claves: Cambio de facies, Fallas inversas, Cobre porffdico, Volcanismo bimodal, Potrerillos, Chile.
INTRODUCTlON
For many years, the belt 01 porphyry copper de been the focus 01 much attention (Jordan et al" posits in northern Chile has been considered a me 1983; Jordan and Allmendinger, 1986; Mon, 1976, tallogenetic province because 01 the remarkable 1979), the styles and timing of compressional de alignment and age similarity 01 the host intrusions formation in the intra-arc areas 01 the Chilean An (Fig. 1; Ruiz et al., 1965; Sillitoe, 1976, 1981; Clark des are only known in a few areas which have et aL, 1976). Major structures in this belt, such as the West Fissure 1ault at Chuquicamata, are thought to have existed prior lo mineralizalion, lo have guided the emplacement 01 the mineralized in 2<)'S trusive rocks, and subsequently to have been re X ,CERRO COLORADO activated as strike-slip or high-angle dip-slip 1aults ") - x QuEBRADA BLANCA 34 Mo (Sillitoe, 1973; 8aker and Guilbert, 1987). Few \. l - :( EL ABR". l5. 4.l1.5Mo district-scale studies have been sufficiently detail X CHUOUICAM ATA 336-3181140 ed, however, to determine the nature, timing, and 1,, ) displacement 01 pre-mineral movement 01 these j ~ Anlo lOQo,IO .J structures. In addition, other 1eatures related to X ES,,-CONClDA, 337-32.8 Me the setting 01 Chilean porphyry copper deposits, such as the composition and nature 01 cogenetic >\ volcanic rocks, their time-space setting in the )( EL 1ALVA,OOR 42-40 Mo ~ le POTRERILLOS, 37 Mo development 01 the Andean continental margin, , r " ~ COPIOPÓ { and the localization, causes and effects 01 post mineral 1aulting have received little attention. (' Although the Jurassic-Miocene landward migra ) 1, tion 01 magmatic arcs has been known to exist in ",. S l o SeruoQ j some segments 01 northern Chile and Argentina 10r (' j many years (Farrar et al., 1970), there are 1ew stud \ X LOS PELMlBRES 10 Me ies 01 how these ares interacted with the Andean !, continental margino Little is known about the styles \ Volporoíso X RIO BLANCO 4,6 Mo o ) DISPUTADA and magnitudes of faulting contemporaneous with 60l\lioQo: the emplacement 01 batholiths and major accumula x' EL TENIENTE, ' .6-43 1140 ",'S tions 01 volcanic rocks in the early stages 01 An dean margins. As the magmatic arc migrates through any given area, the intrusions and asso FIG. 1. Location and K-Ar ages of major porphyry copper ciated volcanic products should reflect the prevail deposits in northern Chile. Ages of porphyry cop per deposits from Quirt et al. (1971; Mocha, Los ing tectonics. This is especially true in areas 01 Perambres, Rro Blanco, and El Teniente), Gustaf shallow porphyr) intrusions where dike patterns son and Hunt (1975; El Salvador), Olson (1984; and associated faulting are expected to be sensi Potrerillos). Internal report, CODELCO-Chile. Divi tive indicators of the stale 01 stress (Heidrick and sión Chuquicamata (no date: Chuquicamata), Titley, 1982; Ode, 1957; Anderson, 1951). Ambrus (1977; El Abra), C. Alpers and G, Brimhall Whereas the styles and timing of compressional (written commun.; Escondida), and Hunt .et al. deformation Of foreland areas in the Andes have (1983; Quebrada Blanca) . S. F. O/son 5
+ + + + + + .,. + + pJ + +
+
+
+ + + + + + + + + + + + + + + + + + + + + + + +
+ + + ... + + + + + ... + + + + + + + +
AfOCOmo Gravels ~ l.Dtw -moderate on918 revene tOylt 30 Feld,par parphyryl -L Fou l , 5OO .. ,ng d'p mlner-oll zed fekispa r por~.,.ry
Conract RhyohtlC In rusions / r hyolltlc lova f IOW$
--ft- ~;~~ ~~ ~ed AndeStfe lova flows ond rh yoh'e osl"l .. flo \lll tu" + Monocli ne oce OU$ manne Antlchne ere + sed lmen ory (OGk$
Foults ond contocts d05hed Juronlc - CretocltOus where opprol.lmate Of lnferreo volcan te rach ond dotted where covered
,JurOSSIC m Oflne 5 ed l rnentor~ rocks
Pre-Jurante bosoment
FIG. 2. Regional geologic map 01 the Potrerillos-Ouebrada El Asiento area. Area north 01 Quebrada El Asiento based on MOller and Perelló (1 982). lellers show foeations 01 stratigraphie columns shown in subsequent ligures. Box en clases area of figure 3. Seetion 2-2' is shown in ligure 13. Rocks to the west 01 the Sierra Castillo lault are shown as Jurassie-Lower Cretaceous marine volcanie roeks , alter Naranjo and Puig (1984). rl r! rl rl M Ofl OCI ne x H IQ ~ ·on g ' l C" @'III'B!'"5efou lt en rf rf rt rnrusr fCl ul rl rl rl rl Faulf snow l~ dlp rf rf rf rf ~... A rf rt rf rf ContoCI " rf rf rf ...r- ,¡;-oults cnd con 'oct!i dostted wnere d opproxtmot@,Quen811 ", t1 e re Inferred r' rf rf rf rf rl rI Pebbfe dllles
rf rl rf rf. u-.mlneroll2:ed teldspor Porph)".-)' oluton! LOSVeQos odit +---'6 H~ Q(l. d dl j(es rl rl ...... rl ,"tI '"T" rl II1tr-u'SIVE' bretclc rl , ," rl~: lo ~ rf rf ::~ MlnerOhit:e d feldspor porphyry plutons rf rf rf i ~ ~ ¡ I\"'al Porp tl'fry eJosr conQlomerote-
~ Rh)loht ~ c IntnJS t'll' @ rock!
Rhyollte IOlJo/rhyolll.c sondst one ond gri t
rtt7I ModefCltely · welded tuff /de n$ e l ~ ~ ~ ~ !.J ".Ided luff !':"'A...:?l /:r¡ ndesl hc t lo ..,.... bre<:CHJ I 0l1d851te ~ lovo ~ W .., ~• 'o.o . Andes lhe cont;¡lomerate ~ Ton brec:.cIO neOf Ihe Los Ve~ O$ adl t
u pper CretcC"eous --t 'VV\/V' I unconform1ty m -c ~ 11 • AQIJO I-tetcdo FormOhon ~ {.. :D m ~~ :D ~Phe~ ~~~~ ~~n;~~~~\ U~:-eom¡ an ¡= ~ m r '-' Marl n.e Olldes. lte lavo In t .r ~ ddMl wrt h O fhe Pe¡j!9foO lls on(J A Silen to$ tormotions. en -c Asumt os FormC 'lon upper membe r O j :D ~ 0090er -c ~El{- I ASl enros FOr"m otlOfl I low81' member t -< ªs r::z:~ DoQQe r ~ () Monton.dOn Formction, LIQS O ;~ { r.:;::¡ -c 5: ~ -c .Ji¡ m :D o 2k ~ O ~ :D ~ FIG. 3. Geologic map 01 lhe POlrerillos district. Location 01 map afea is shown in figure 2. Note Ihat tha Potrerillos mine fault has cut and offset the Cobre porphyry and !he porphyry copper and skam deposit. Mapping immediately west 01 Ihe Cobre Porphyry PIulo n from Raed (1 932). S.F. O/son 7 clear, crosscutting relations (Reutter, 1974; Lah Total relief within the Potrerillos district (Fig. 3) sen, 1982; Mpodozis and Davidson, 1979; Mak including underground exposures and drill core is saev et al., 1984). Shortening within the arc(s) is about 2 km, providing exposure to the wide range usually not considered in most models of Andean of geologic environments encountered in areas of uplift (Cobbing, 1985; Allmendinger, 1986). shallow porphyry intrusions. During 1980 and This paper first describes the stratigraphic and 1981, the author spent approximately 20 months structural evidence for an important zone of facies mapping in the Potrerillos district at 1 :5,000 (78 and thickness changes and normal faulting that km 2) and 1:2,000 (6 km 2) scales and in the sur formed in the Potrerillos area (Fig. 2) in Jurassic rounding area at 1:25,000 scale (200 km 2). Several Cretaceous and Paleocene time, and documents kilometers of underground exposures were map the bimodal nature of volcanism up to the time of ped at 1 :500 and 1:1,000 scale, and several thou the emplacement of porphyry intrusions in Eocene sand meters of drill core were inspected primarily Oligocene time. It then describes the intrusion his for structures and lithologic contacts. K-Ar dates tory of the district, the nature of post-mineral com were obtained in K-Ar laboratories at Stanford Uni pressional deformation and the setting of the dis versity and the United States Geological Survey in trict in the Andean continental margino Menlo Park, California.
BACKGROUND
HISTORY IIos porphyry copper deposit is one of several ma jor porphyry copper deposits of Eocene-Oligocene The richer seams and veins of the Potrerillos dis age in northern Chile (Fig. 1); this intrusive event trict were mined by selective methods as far back appears to represent a single period of magmatism as 1894. William Braden acquired most of the in the Andes. Farrar et al. (1970) demonstrated existing claims in 1913, and sold the property to that in the Copiapó-EI Salvador area, the magmatic the Andes Copper Mining Company, a subsidiary arc migrated from Jurassic to Recent time in a step of The Anaconda Copper Company, in 1916. Be wise fashion from the coast of Chile to the present tween 1926 and 1959, the mine produced 169 mil volcanic arc along the Chilean-Argentina border, a lion metric tons of sulphide ore averaging 1.11% distance of 200 to 300 km. In general, deep levels Cu and 39.2 million metric tons of oxide ore aver of erosion are found in the pre-Tertiary intrusions aging 1.07% Cu (Reyes, M.: Mina Potrerillos. Inter of the coast batholith (Tilling, 1976; Aguirre et al., nal reporto CODELCO-Chile, División Salvador, 1974), subvolcanic porpbyry plutons crop out in 1981) by block caving methods. The mine, which the Eocene-Oligocene porphyry copper belt (nor was nationalized in 1971, is now the property of thern Chile only; Gustafson and Hunt, 1975; Am the Corporación Nacional del Cobre de Chile (Ce brus, 1977; Hunt et al., 1983), and only volcanic DELCO-Chile), the state-owned mining company. rocks and minor subvolcanic bodies are exposed Gold and silver mineralization has recently been in the Miocene-Recent arc(s) (Zeil, 1979; J. David discovered in the district, but grades and ton son, personal commun., 1987). nages are not available at the time of this writing. Most of the porphyry copper deposits in northern Chile lie within or near the Cordillera Do GEOLOGIC SETIING IN NORTHERN CHILE meyko. This discontinuous range of hills, which consists of basement blocks uplifted along high The Jurassic-Recent magmatic ares and their angle reverse faults (Godoy and Davidson, 1976; associated sedimentary basins of the Andes of Chong, 1977), appears to have been structurally northern Chile developed in and on a pre-Middle active since Jurassic time (Frutos, 1974). Many of Triassic basement that consists of Paleozoic the changes in facies and stratigraphic thickness Triassic intrusive, volcanic, sedimentary, and met discussed in this paper coincide with the western amorphic rocks (Coira et al., 1982). The Potreri- edge of the Cordillera Domeyko. 8 THE POTAEAILLOS POAPHYAY COPPEA DISTAICT
GEOLOGIC HISTORV PRIOR TO FORMA TlON OF THE PORPHVRV COPPER SVSTEM
JURASSIC-CRETACEOUS MARINE Montand6n Formation (Pérez, 1982). The Montan SEOIMENTATION ANO VOLCANISM d6n Formation is unconformably overlain by lime stone, sandstone, gypsum and plagioclase-phyric Jurassic marine sedimentary rocks in the Potre basaltic andesite lava flows (Table 1, Fig. 5) of the rillos-Quebrada El Asiento area (Fig. 4) unconfor Callovian Asientos Formation and the upper Tithon mably overlie a basement consisting of metavol ian lower Neocomian Pedernales Formation (Pé canie rocks and granitie-syenitic intrusions of prob rez, 1982). Red sandstone of the Lower Creta able Permo-Triassic age (Pérez, 1982). The oldest ceous Agua Helada Formation (García, 1967) caps marine rocks in the area eonsist of thin-bedded car the Mesozoic marine sequence. bonaceous blaek marl and limestone of the Lias From Jurassic to Lower Cretaceous time, the fu-
~ Paleoeene voleanie racks w z Gypsum m ~ 1000 fil Andesite ..J ct Limestone, sandy Ilmestone
Fossiliferous limestone oolitic limestone
Siltstone, eherty slltstone
Sondstone I conglomercte
Morl, Corbonaceous limestone
500 <.l .: Vi .2 (/)
e <.l ¡¡¡ .C? (/) (;
o m A B e D E West East
FIG. 4. Jurassic-Lower Cretaceous marine stratigraphy of the Potrerillos-Quebrada El Asiento area. Column 8 IS from Pérez (1982). Locations of columns are shown in figure 2. S.F. O/son 9
FIG. 5. Jurassic-Lower Cretaceous marine volcanic rocks A. Plagioclase-phyric basal tic ande site; B. Amygdaloidal plagioclase-phyric basaltic andesite; quartz-clorite amigdules.
TABLE 1. COMPOSITION OF JURASSIC-LOWER CRETACEOUS LAVA FLOWS
DC-1 DC-27 DC-28 DC-29
Si02 50.8 53.0 48.2 55.6 Ti02 1.89 1.83 1.78 1.73 AI20a 15.12 16.31 16.78 16.53 Fe2Üa 13.30 11.72 11.15 11.72 MnO 0.164 0.263 0.114 0.147 MgO 4.64 5.04 6.00 4.41 CaO 9.51 6.10 9.07 6.60 Na2Ü 3.18 3.40 2.48 3.99 K2Ü 1.06 1.64 0.63 1.78 P2ÜS 0.355 0.314 0.376 0.357 S 0.05 0.16 0.05 0.02 Cu 0.031 0.21 0.020 0.022 Total 100.1 100.01 96.65 102.91
DC-1 Basalt lava Ilow, 6 km ESE 01 Cerro El Hueso; DC-27 Basaltic andesite, Las Vegas adit, 2,051.5 m lrom portal; DC-28 Basaltic andesite, Las Vegas adit, 1,918 m Irom portal; DC-29 Basalticandesite, Las Vegas adit, 1,850 m Irom portal;
Analyses by the Superintendencia de Geologra, CODELCO-Chile, División Salvador. SiÜ2 and P20S by wet chemicaf methods; remainder by atomic absorption. AII iron reported as F920a. 10 THE POTRERILLOS PORPHYRY COPPER DISTRICT
w E rlOfI 01 5""0 Costillo Foul!
• ~o~soo~:~... :nd.Sil. FIG. 6. Cross section through the Potrerillos
UsorusalOl'lt area during Lower Cretaceous time showing the relations between the J o ; Asilntos FortnOTlon ~U"''' I Ont '000 m thick, western sequences 01 marine Vertical 'l.oQglro!ion Jm' Monlondón Formolion 8 GypsurPI opprOllmot.ly Zl basal tic andesite lava Ilows, and the .------+0 ~ Cherlysllt,tone thinner, eastern sequen ces 01 ma • km ~ =;~:~II~;'OM rine sedimentary rocks. rez, 1982). Red sandstone of the Lower Creta (1.5-2 km) sequences of basaltic-andesite lava ceous Agua Helada Formation (Garcfa, 1967) caps flows interbedded with sandstone and minor lime the Mesozoic marine sequence. stone of the Asientos and Pedernales Formations From Jurassic to Lower Cretaceous time, the fu crop out on the west side of the district whereas ture location of the Potrerillos district coincided much thinner sequences (1 km) of limestone, sand with a majar facies and thickness change in marine stone, gypsum and marl crop out on the east side sedimentary and volcanic rocks (Fig. 2-4). Thick of the district. These changes in facies and
West East
G H o J L K m
_ Plag,oclase-phyroc andes,le lava¡ 0-20 0/oplag,oclase often pi liowed
500 ~ Limestone ; bioclasllc and oolll,c
Gypsum and interbedded red sondstone 400 -E2J Sandstone ~ Mari and carbonaceous t..=.:::=J limestone
300 ~ Andeslte , contlnentol
1°0'1>01 Cong lome rote
200 ~ Andesite flow brecclo
ILE -_ ' o" - _ 100 'O - o-" - eo -- ::< -- -:: --d!' o m ~<~------15km ------~»
FIG. 7. Stratigraphic relations below the Upper Cretaceous unconlormity. Locations 01 columns are shown in figure 2. S. F. O/son 11
thickness can be mapped for at least 20 km to the tary-volcanic section thickens from approximately north-northeast of the district (Müller and Perell6, 1 km on the east to 1.5-2.0 km on the west, over a 1982; Naranjo and Puig, 1984) and 50 km to the pre-thurst horizontal distance of 2-4 km. The ab south-southeast of the district (S. F. Olson, ruptness 01 the changes in lacies and thickness unpubl. data). and their persistence along strike suggest that In most cases, the changes in facies and thick this zone was occupied by down-to-the-west nor ness occur across high-angle reverse and thrust mallgrowth laults active during sedimentation and 1aults 01 Oligocene-Miocene age. The Río Sal fault volcanism. A reconstructed cross section 01 the (after Tobar, 1978) and Bailey Willis fault (after region at the end 01 Lower Cretaceous time (Fig. 6) Reed, E.F.: Unpublished map of the Potrerillos dis illustrates down-to-the-west normal/growth laults trict. Andes Copper Mining Co., Potrerillos, Chile. bounding the thick western sequences of lava 1932; Figs. 2, 3) are two of the principal reverse Ilows. Extremely thick sequences (>10 km?) 01 1aults over which the most abrupt changes in fa Jurassic-Cretaceous lava lie to the west 01 the Sie cies and thickness take place. Reconstructing rra Castillo fault (Naranjo and Puig, 1984; Mpodo these faults (this study) shows that the sedimen- zis and Davidson, 1979; Muzzio, 1980) and indi-
N
o 2 3 4 5 Mm CI==I==II==~I==~I==~1
Pedernales Formation
Marine be.eltie andesite lava
Asientos Formation, uppcr membo!r
Asientos formotion. lower member I ond Mon1ondón Formotion undlvided
Pro -Jurossie besement
+ +
FIG . 8. Regional relations at the Upper Cretaceous unconformity. This map iIIustrates !he distribution 01 lormations directly below the Upper Cretaceous unconformity. Strikes and dips indicate the angle between the unconformity and stratilication in the underlying rocks. Contacts indicate where a lormation pinches out beneath the uncon formity . The Manto Oliva fault was active during the development 01 the unconlormity. Effects 01 Oligocene Miocene reverse laulting were removed lor this figure. 12 THE POTRERILLOS PORPHYRY COPPER DISTRICT cate an additional westward increase in thickness between the underlying marine rocks and the un of these rocks. conformity indicate that 200-300 m of erosion took place in Late Cretaceous time (Fig. 7) and was di UPPER CRETACEOUS UNCONFORMITIES rectly associated with movement on normal faults. NORMAL FAULTING ANO EROSION Regional stratigraphic relations, however, suggest that the faulting and erosion were confined mainly A major unconformity, hereafter called the Up to the immediate Potrerillos area (Fig. 2; Mercado, per Cretaceous unconformity for its approximate 1978, 1983). stratigraphic position, separates the Jurassic The Manto Oliva Fault, located only 100 m west Cretaceous marine sedimentary-volcanic se of the Potreritlos porphyry copper deposit, is one quence from overlying Paleocene continental vol of the faults active during the development of the canic rocks. Stratigraphic and structural relations Upper Cretaceous unconformity (Fig. 3). Before
West East m 2200 CH3- 6 52.9.:t 0.4 Ma (biOl) CH3- 5 54.7.! 0. 9 M a ( biol )
2000 0 o o A ndesilic conglomerate 0o o
57.3:!: 0 .5 Ma (biol.' ~~vv Andesile lava
A 1500 It: tAl Lithic-rich ash-flow tuff
Rhyalite ash-flow luff
~ Sandstone and gri t (interflow) •~ 1000 ~~ Rhyodacile
Porphyry clasl eonglomerate
rrrrl~ Rhyolite lavo Porphyry elo.I eonglomerole ~oo 39. 2:!:O.3Ma (blol'
DC-17
« m
Las Vegas Vegas FaulI F G H J
FIG. 9. Paleocene-Eocene volcanic sequen ces in Ihe Potrerillos area. Locations 01 stratigraphic sections are shown in figure 2. Sample numbers indicate approximate locations 01 rocks wilh chemical analyses (Table 2), K-Ar ages (Table 3) and photographs (Figure 10). S. F. O/son 13
TABLE 2. CHEMICAL ANALYSES OF PALEOCENE-EOCENE VOLCANIC ROCKS
CH3-1 DC-17 DC-19 DC-14 DC-33 DC-36 CH3-5
Si02 51.4 51 .6 66.1 74.0 74.5 73.5 70.8 Ti02 1.12 1.45 0.87 0.15 0.16 0.12 0.33 A12Ü3 17.4 16.4 14.5 12.8 12.7 13.0 13.7 Fe2Ü3 7.55 8.29 4.26 1.14 1.02 0.97 2.24 MnO 0.19 0.12 0.07 0.10 0.04 0.03 0.03 MgO 4.71 2.39 0.36 0.30 0.19 0.20 0.60 CaO 7.49 9.27 1.78 1.27 0.58 0.35 0.89 Na2Ü 2.81 3.22 4.01 1.21 3.58 3.88 3.19 K20 3.73 1.91 4.76 5.21 4.52 3.91 5.40 P2ÜS 0.49 0.50 0.22 0.05 0.05 0.05 0.09 S· N.O. N.O. 0.06 0.01 0.08 0.07 0.06 Cu· 0.004 0.004 0.001 0.004 0.002 0.002 0.002 LOI 2.71 4.28 2.06 3.53 0.98 1.30 1.63 Total 99.6 99.43 96.99 99.77 98.67 98.38 98.97
Location of samples in Appendix CH3· 1 Plagioclase-hornblende-pyroxene-phyric andesite; DC-17 Plagioclase-phyroxene-phyric andesite; DC-19 Plagioclase phyric dacite; DC-14 Crystal-poor rhyolite ash-Ilow tull, eastern sequence; DC-33 Crystal-poor rhyolite ash-flow tuf!, western sequence; DC-36 Flow banded rhyolite; CH3-S Rhyodacite.
Analyses by U.S. Geolog ical Survey, Branch 01 Analytical Chemistry, Lakewood, Colorado; by x-ray Iluorescence . • Analyses 01 sulfur and copper by the Superintendencia de Geologra, CODELCO-Chile, División Salvador; by atomic absorption. N.o. = not detected. AII iron reported as Fe20a the deposition of the Paleocene continental volcan litic, and minor dacitic volcanic rocks (Table 2) ic rocks, the Lower Cretaceous Agua Helada For accumulated in and around the Potrerillos district. mation was eroded from the upth(own, eastern, Like the Jurassic-Cretaceous volcanic rocks, the footwall-side of the fault, and preserved in the Paleocene rocks are much thicker on the west downdropped, western, hanging-wall side of the side of the district, 2-3 km, than the east, about fault, indicating about 150 m of down-to-the-west 0.5 km (Fig. 9). Some, if not all, of the volcanic movement between Early Cretaceous and Tertiary rocks accumulated on the west side of the district time. The dip of the Manto Oliva fault after removal in fault-bounded depressions. of Oligocene-Miocene deformation, approximately Radiometric age-dating of the continenta vol 50-60oW, indicates normal movement. canic sequences indicates that the majority of the Figure 8 illustrates the distribution of formations andesite and rhyolite lavas and tuff accumulated directly below the Upper Cretaceous unconformity. in Paleocene time (Table 3). Hornblende andesite The map shows that stratification in the underlying lava near the base of the sequence yielded a K-Ar rocks is subparallel to the unconform ity, even age of 61 .2 ± 0.8 Ma. A biotite-bearing ash-flow though erosion at the unconformity had, in some tuff yielded a K-Ar age of 57.3 ± 0.5 Ma and a bio places, removed several hundred meters of the tite-bearing rhyodacite lava higher in the seq uence Jurassic-Lower Cretaceous section. The Manto Oli yielded K-Ar ages of 52.9 ± 0.4 and 54.7 ± 0.9 Ma. va fault forms the contact between the down Uthic-rich, crystal-poor rhyolite ash-flow tuff, dropped Agua Helada Formation and the Pederna crystal-poor rhyolite lava flows (Fig . 10, A), and in les Formation. terflow sandstone and conglomerate crop out be tween Potrerillos and the Potrerillos mine (column PALEOCENE CONTINENTAL BIMOOAL F, Fig. 9). This sequence is bounded on the south ANOESITE-RHYOLlTE VOLCANISM ANO east by a major down-to·the-west normal fault with NORMAL FAULTING about 1 km stratigraphic offset (the Quebrada Las Vegas Fault, Fig. 3) and to the north and west by During Paleocene time, basaltic, andesitic, rhyo- faults of unknown displacement (Fig. 2). The res- 14 THE POTRERILLOS PORPHYRY COPPER DISTRICT
TABLE 3. K-Ar DATA FOR VOLCANIC AND INTRUSIVE ROCKS
Sample No. Sample description and rock unit Sample weight K(wt%) Radiogenic«lAr «lAr Age ± 10 (gms) (10-1Omol/gm) atm(%) (Ma)
Continental volcanic rocks
CH3-1 Hornblende, from andesite lava 0.33359 0.961 ± 0.004 1.056 33.2 61.2± 0.6 DC-43 Biotite, Irom rhyolite ash-flow tull 0.2666 5.21 ± 0.2 1.939 23.6 57.3± 0.5 CH3-5 Biotite, rhyodacite lava 0.15609 6.22 ± 0.04 5.967 20.7 54.7 ± 0.4 CH3-6 Biotite, rhyodacite lava 0.34610 6.36 ± 0.04 5.941 36.9 52.9± 0.4 CH3-10 Biotite, from clast 01 porphyry- 0.26669 3.51 ± 0.2 2.270 51.2 39.2± 0.3 clast conglomerate
Intrusions
DC-2 Whole rock; biotite-pyroxene- 2.13924 2.57± 0.007 1.099 34.9 45.2± 1.0 plagioclase lamprophyre, 4 km east 01 Cerro El Hueso DC-7 Hornblende, Irom plagioclase- 0.16711 0.615 ± 0.001 1.757 60.1 40.5±4.2 biotite-hornblende porphyry nearest to outcrop 01 porphyry- clast conglomerate. SNA 8iotite associated with strong 0.20251 7.36 ± 0.02 1.646 33.2 37.0± 0.5 potassic alteration, Irom line- grained phase 01 Cobre porphyry. OC6 8iotite, lrom Cerro El Hueso Por- 0.3918 6.67 ± 0.03 3.709 29.3 31.8±0.4 phyry DC-9 8iotite, from Cerro El Hueso Porphyry 0.12266 6.9 ± 0.01 3.05 43.5 32.6±0.5
Errors are estimated standard deviations (Cox and Dalrymple, 1967. Potassium analyses by U.S. Geological Survey, Menlo Park, California Mass Speclroscopy by J. Metz and B. Turrin. Argon extractions by S.F. Ofson and 8 . Turrin. Sample locations in Appendix 1). i-,: = 0.581 x 10-1J/year; "13 = 4.962 x 1O-10/year; «lK!K = 1.167 x 10.4.
tricted occurrence of this sequence in the region, Plagioclase-phyric basaltic andesite lava flows its roughly circular outcrop pattern, and its strati predate and post-date the rhyolite lava flows. The graphic and structural relations with the surround andesite lava flows (Figs. 10 B-D), over 300 m ing rocks suggest that the rhyolite flows and asso thick to the west of the district (column G, Fig. 9), ciated tuffs accum ulated in a caldera 5-10 km in are only 50-100 m thick to the east and are repre diameter located at the western edge of the dis sented by andesitic conglomerate, breccia, and trict. A block-breccia (clasts up to 2 m in diameter) flow breccia (columns H, I and J, Fig. 9). North that lies immediately northwest of the intersection northeast trending andesitic dikes, which cut of the Bailey Willis and Quebrada Las Vegas faults across the rhyolite lava flows, may have been may represent talus deposits formed adjacent to feeders for some of the andesite lavas and the caldera margin o indicate WNW-ESE directed extension. Tobar Abundant rhyolitic dikes, sills and irregular intru (1978) obtained a 57 Ma K-Ar age from a lithologi sions in the immediately surrounding country cally similar dike in Quebrada El Asiento (Fig. 2). rocks appear to be cogenetic with the rhyolite Thick sequences (0.5-1 .0 km) of crystal- and flows, and are the oldest intrusive rocks in the lithic-poor densely welded rhyolite ash-flow tuff to area based on cross·cutting relationships. In the west of the district (columns F and G, Fig. 9; many cases, the rhyolite intrudes along faults Fig. 10, E) appear to correlate with one or two thin bounding the sequence of rhyolite lava flows des (50 m) persistent and compositionally similar units cribed above. The rhyolite lavas and intrusions on the east (columns H, I and J, Fig . 9; Fig. 10, F). crop out only in the western part of the district and The thick western sequences of tuff may repre form part of a larger north-northeast trending belt sent intracaldera accumulation, whereas the thin of rhyolite and granrte intrusions (Fig. 2). eastern sheets may represent associated outflow s. F. O/son 15
FIG. 10. Textures 01 Paleocene volcanic rocks in the Potrerillos district. Sample numbers indicate samples with K-Ar age date and/or chemical analyses. A. Flow-banded rhyolite, 2 km south 01 Potrerillos; B. Plagioclase-hornblende pyroxene basaltic andesite; sample CH3-1; Irom base 01 Paleocene volcanic section near !he Cobre porphyry; C. Phenocrysts-poor plagioclase-pyroxene andesite; sample DC-19; this is a dominant rock type in the Paleo cene volcanic sequence; D. Phenocryst-rich plagioclase-pyroxene andesite, sample DC-17; this is a minor rock type in the Potrerillos area; E. Typical densely welded ash-Ilow tuf! Irom the thickest accumulation 01 ash-Ilow tull in the western volcanic sections; sample DC-33; streaks indicate rheomorphic Ilowage along flaltenad pumice lapilli, some 01 which show small lolds; F. Typical (altered) rhyolitic ash-flow tuf! Irom 'outflow' sheets from the eastern par! 01 the district; sampla DC-14. facies. The thickest sequences of densely-welded and H of figure 9. tuff partially overlap the rhyolite lava flows sug gesting that they accumulated in an overlapping SUMMARV OF THE POTRERILLOS DISTRICT caldera or volcano-tectonic depression. Bounding HISTORY PRIOR TO INTRUSION OF THE faults were probably obscured and/or remobilized PORPHYRY COPPER SVSTEM by later reserve faults. One such fault, the Bailey Willis Fault, (Figs. 2, 3), lies between columns G The first major porphyry plutons in the Potreri- 16 THE POTRERILLOS PORPHYRY COPPER DISTRICT
IIos district were emp/aced in Eocene-O/igocene The composition and structures associated with time; prior to this time, the loei 01 igneous intrusion the Jurassic-Pa/eocene vo/canic rocks suggest were to the west and had been s/ow/y migrating to an extensiona/ tectonic environment. Basa/tic an the east (Farrar et al., 1970; Zentil/i, 1974). The Jur desite and rhyo/ite constitute the dominant rock assic-Pa/eocene vo/canic rocks in the district are types, a bimoda/ assemb/age indicative of exten asymmetrical/y distributed and ref/ect the east sion in many environments (Hildreth, 1981). Pa/eo ward migration of magmatism; the Pa/eocene con cene dike orientations indicate approximate east tinenta/ vo/canic rocks over/ap the pinch out of the west extension and fau/ts recognizab/e as being Jurassic-Cretaceous vo/canic rocks. During this direct/y re/ated to vo/canism (Quebrada Las Vegas time, the western side of the Potreril/os district sub fau/t and associated structures) are normal faults. sided more than 1.0 km re/ative to the east by both Re/ations at the Upper Cretaceous unconformity accumu/ation of vo/canic rocks on the west side of indicate broad up/ift, norma/ fau/ting (Manto Oliva the district and erosion of sedimentary rocks on fault), and erosiono the east.
PORPHVRV COPPER ENVIRONMENT
Numerous bodies of granodiorite to quartz mon severa/ different fe/dspar porphyries (Reyes, M.: zonite porphyry (Table 4) were emplaced in the Po Mina Potreril/os. Internal Report. CODELCO-Chile, treril/os district between 40 and 32 Ma (Table 3) División Salvador, 1981; Parra, 1983; Reed, and represent the migration of the magmatic are 1932): the 'fine-grained' porphyry, the 'interme through the district. Their intermediate composi diate' porphyry, the Secreto Porphyry, and several tion and crysta/-rich textures represent a dramatic breccia bodies (Figs 11, A, C, D) . The intrusion is shift from the bimodal crystal-poor Paleocene vol e/ongate north-south and numerous northeast canic rocks. Three of the porphyry intrusions con trending dikes emanate from the eastern contact. tain copper mineralization: the Cobre porphyry and At the present levels of exposure, the Cobre por two satel/ite bodies, the North porphyry and the phyry intrudes the Jurassic-Lower Cretaceous ma González porphyry (Fig. 3). The remainder of the in rine sedimentary-vo/canic sequence in which ex trusions, here col/ectively cal/ed the Cerro El Hue tensive metamorphic and metasomatic aureoles so porphyries because of lithologic similarity with and minor skarn developed (Olson, 1984). the porphyry of Cerro El Hueso, are generally Hypogene copper grades in the Cobre porphyry unaltered, but host intense alteration local/y. pluton in the Las Vegas /eve/ (2,983 m above sea level), range up to 1.17% Cu with significant areas Lamprophyre over 0.8% Cu (Reyes, op. cit.). Information on the extent of supergene enrichment above the Las An irregular intrusion (dike and sil/) of fine Vegas level is not available. grained biotite-pyraxene-plagioclase lamprophyre crops out 3 km east of Cerro El Hueso. The dike North Porphyry cut s acrass the lower-most conglomerate and ash flow tuff of the Paleocene volcanic sequence. The The North porphyry (Fig . 11, B) is a NE-SW elon rack yielded a whole rack K-Ar age of 45.2 + 1.0 gate leldspar porphyry intrusion about 300 m long Ma. With the exception of the rhyolite dikes and and 100 m across. Old mine plans indicate that at sil/s to the west of the district, it is the oldest intru depth the intrusion consists 01 northeast-trending sive rock in the district. anastamosing dikes. At the surface, the North por phyry intrudes the uppermost Asientos and lower MINERALlZED PORPHYRY INTRUSIONS most Pedernales formations, which have been metamorphosed and metasomatized up to 500 m Cobre Porphyry from the contacts. Isolated zones of quartz veins and weak copper mineralization occur in the North The Cobre porphyry is a composite intrusion of porphyry. TABLE 4.INTRUSIVE ROCKS IN THE POTRERILLOS DISTRICT
~ :ti Intrusion Phenoeryst Groundmass Mineralization/Alteration Dimensions at Age Comments O ¡¡; Content Composition Surfaee g North of Cerro Silíea Plagioclase 10-20% (2- Quartz and potas- Biotite and hornblende altered to 800 m diameter, 40.5 ± 4.2 Ma; K- Contemporaneous with 5 mm); biotite 2-5% (3- sium leldspar (apha- aggregates 01 ehlorite, sericite, circular Ar; hornblende porphyry - clast eonglomerate 5 mm); homblende 1- nitie) and limonite: locally Iresh 2% (4-5 mm) hornblende
Cobre Porphyry Composite Pluton 'Fineijrained' Plagioclase 30-40% (1- Quartz and potas- Eany: intense quartz veining, K- 1500 m long; 500 37.0 ± 0.5 Ma; K- Carries bulk 01 hypogene porphyry 3 mm); biotite 1-2% (2- sium leldspar (0.1- silicate alteration, and cp-bn m wide, elongate Ar; hydrotermal copper mineralization in district; 4 mm); homblende 1- 0.5 mm) mineraliza~on N-S biotite associated data on supergene enrichment 2% (4-6 mm); (often late: pervasive sericitic alteration; with early altera- unavailable; ineludes abundant replaeed by py-en mineralization (Parra, 1983) tion and mineraliza- igneous breccias aggregates 01 biotite) tion
'Intermediate' Plagioclase 40-60% (1- Quartz and potas- Eany: weak-moderate quartz Cuts across quartz veins in the porphyry 6 mm); biotite 1-2% (2- sium leldspar (0.05- veins, K-silicate alteration, and cp- line-grained porphyry 4 mm); homblende 1- 0.2 mm) bn mineralization (Parra, 1983) 2% (4-6 mm) Intermediate: quartz-py-mo veins Aphanitic, light near contacts green late: pervasive sericitic alteration; py-en mineralization (Parra, 1983) Secreto Porphyry Plagioclase 5-10% (3- Quartz, potassium Fresh to weak pyrite-sericite Occupies south- Probably associated wilh Ihe 6 mm); biotite 1-2% (1- leldspar, and mag- alteraban ern part 01 Cobre unmineralized Cerro El Hueso 2%) netite (0.1-0.5 mm) Porhyry compos- porphyries; cuts line-grained ite pluton and interrnediate porphyries
Satellite Mineralized Porphyry Intrusions North Porphyry Plagioclase 40-50% (2- Quartz and sericite Homblende partially to totally 500 m long, 200 Anastamosing dike swarm at 4 mm); homblende 1- (altered) chloritized, quartz veins in central m wide; elongate depth 2% (3-6 mm) portion 01 stock ENE-WSW
González Plagioclase 30% (2-4 Quartzand Eany: quartz veins, weak K- mm); quartz 5% (1 -3 potassium leldspar silicate alterabon 500 m long, 200 Chalcocite enrichment at depth mm), rounded; biotite (0.05-0.2 mm) late: steeply dipping limonite m wide; elongate 2-3% (3-6 mm), (alter pyrite) veins with sericite N-S sericitized selvages; pervasive sericitization
Unmineralized Porphyry Plulons Minar areas 01 disseminated pyrite Up lo 4 km long, 31 .8±0.4and32.6 Found in southem par! 01 Cerro El Hueso Plagioclase 10-20% (2- (0.5%); associated with a 2 km wide; elon- ± 0.5 Ma; K-Ar; district; probably associated 5 mm); biotite 1-2% (3- phenocryst-poor phase with a gate N-S; biotite with pebble dikes; miarolitic 5 mm) quartz trace; dark, aphanatic matrix; strong associated with cavities or vesicles are lound at homblende trace sericitization and silicihcatlon E-W dike swarm the top 01 Cerro El Hueso
locally elevation 4,397 m ~ --J 18 THE POTRERILLOS PHOPHYRY COPPER DISTRICT
FIG. 11. Texture of Eocene-Oligocene porphyry intrusions in the Potrerillos district. Sample numbers indicate samples with chemical analyses and/or K-Ar age dates. A. 'Fine-grained' porphyry, from underground exposure on the Las Vegas level (2 .983 m); B. North Porphyry; sample DC-21 ; C. 'Intermediate porphyry', from underground exposure on the Las Vegas level; D. 'Fine-grained' porphyry with intense K-silicate alteration and quartz veins, from underground exposure on the Las Vegas level ; E. Cerro El Hueso Porphyry with miarolitic cavities ; sample OC-S; F. Cerro El Hueso Porphyry dike, from about 1 km east ofthe Cobre Porphyry.
González Porphyry
The González porphyry is the southern-most min stone of the Asientos Formation at the surface, in eralized porphyry intrusion in the Potrerillos dis which a hornfels aureole developed. The González triet. It is about 500 m long, 150 m wide and elong Porphyry hosts strong sericitic alteration. Secon ate north-south. Numerous east-west trending dary eopper mineralization is found in a fault zone dike-like digitations oceur along its eastern con that truneates the porphyry at depth (J . Ouiroga, taet. The porphyry intrudes limestones and sand- personal commun., 1982). S.F. O/son 19
UNMINERALlZEO PORPHYRY INTRUSIONS VOLCANIC ANO TECTONIC SETIING OF THE PORPHYRY INTRUSIONS Cerro El Hueso Porphyries Porphyry-Clast Cong lomerate-Evldence The Cerro El Hueso porphyries are the largest tor Cogenetlc Volcanic Rocks feldspar porphyry intrusions in the Potrerillos dis trict. These intrusions are found in the southern A conglomerate with clasts up to 1 m in diameter part of the district, intrude the Jurassic-Creta of plagioclase-biotite-hornblende porphyry overlies ceous sedimentary-volcanic sequences and are Paleocene andesite and ash-flow tuff about 5 km elongate north-south. Narrow hornfels aureoles, in northeast of the Potrerillos mine. Parts of this unit comparison to the extensive aureoles that sur are almost monolithologic and consist of angular round the mineralized porphyries, developed clasts of feldspar-biotite porphyry in a matrix of around the Cerro El Hueso porphyries. similar composition; this may represent a block and An east-west striking dike swarm originates ash-fall deposit derived from an extrusive dome from the northern end of the porphyry of Cerro El complex. A fresh biotite mineral separate from one Hueso. The dikes, which are most abundant next to clast yielded a K-Ar date of 39.2 ± 0.3 Ma. The the pluton, extend up to 7 km from the pluton on coarse clast size, intermediate composition, either side. The Cerro El Hueso porphyry dikes crystal-rich porphyry texture, and the proximity to (Fig. 11, F) cut the mineralized Cobre and González the Eocene-Oligocene porphyry stocks of the Po porphyry stocks, breccia bodies related to the trerillos district suggest that the conglomerate was Cobre Porphyry composite pluton, and Paleo derived from an unroofed stock or an extrusive cene(?) andesitic dikes to the west of the district. equivalent of one of the stocks. A hornblende Miarolitic cavities or vesicles formed in the up separate from the nearest feldspar porphyry stock, permost 50 m of the porphyry of Cerro El Hueso 3 km to the south-southwest, yielded a K-Ar age of (elevation 4,397 m; Fig. 11, E) . The cavities are 2-4 40.5 ± 4.2 Ma, analytically identical to the K-Ar age mm in diameter, and in some cases contain calcite. of the clasts. The presence of miarolitic cavities or vesicles in the highest esposure of the porphyry of Cerro El Compression Versus Extension at Times Hueso porphyries suggests that they were emplac ot Porphyry Intrusion ed close to the surface, or may have been in part extrusive. Dike orientations indicate that the directions of relative extension and compression changed be Pebble Dikes tween Paleocene and Oligocene times. As stated earlier, north-northeast trending andesitic dikes Pebble dikes are sparse but widespread in the ca. 57 Ma old (Tobar, 1978) indicate that the area Potrerillos district. Within the mine area, the pebble was subjected to approximate east-west extension dikes post-date copper mineralization trend, ESE during Paleocene time. East-west trending dikes WNW, and show close spatial and temporal re related to the Cerro El Hueso porphyry pluton, ca. lations with the Cerro El Hueso porphyry dikes.The 32 Ma, and related pebble dike orientations (Fig. pebble dikes contain angular to rounded clasts of 12) indicate that the area was under east-west com the surrounding wall rocks and sometimes clasts of pression or north-south extension) during emplace granitic pre-Jurassic basement in a sandy, rock ment of the younger intrusions of the district flour matrix. Alteration associated with the pebble (Anderson, 1951; Ode, 1957). The general north dikes is notably absent; clasts and the immediate south trend of the major porphyry intrusions in the wall rock are generally fresh and unaltered. The district probably reflects deeper structural control. width of the dikes varies from almost hairline cracks to parallel-walled dikes over 2 m thick. 20 THE POTRERILLOS PORPHYRY COPPER DISTRICT
,,': ;,í' North Porphyry ,,.,=--IJ~ { '-.:::'-~" NE 10 ENE trendlng dlkes at depth N I Cobre Porphyry 37 Mo , .. __ ... / ___ ./:_ "." ~ I Pebble dikes ossocloted wlth ~ \.' --.... Cerro El Hueso Porphyry dikes \~~=":'~ .. _,;/:~ \" Z" ___ / . /,\'/,: ,.. E-W dl ke 5worm OssoclOted wlth (= /,'\ 1''1'--- Cerro El Hueso Porphyry \'" ~." \~~ ",,~) ,.l.!!, '\ -- ,. ).,,-;;¡\'.:::r~. .-" ~_i:r--- --J.' - -- ___ ---- \-1\, ~ E -W dike sworm ossocioted v... th - __ CerroA/ El Hueso Porphyry - 111/ /V _:"k-- ~ :ll_~ ~ -- _ .... -, 'I\'~I ~ ,,--,------./,- {, ¡itP-::------_/ .... 1- 1 < l___ (i~ -,- .... "< '~ \- '_ < .. ~_~:,,"""'~~ Gonzalez Porphyry ( v · ~ _ \ ';... "... ,_,./
V.. \... ,/ -/ '... I \ ... : -:;:;:-=;;:-- \ "';,' (\ .... ,.... ,_'" /-" :: :;, ~ " _ Porphyry of \ - \ ' C~ El Hueso \ \ ( \,. 32 Me - ~ \ : '_ ~ .- 1... \ ..... 1.... , ... \ I ~ , í " \ \-" ,- , I \... ,_ 1_ .... <. J... \ , I <.. \ ... - ..... \ : " _ .. \ _ \ ... <: '_ / I _ \ -,J \ \ ' ,,,, I .; ... \ I~ \ .. ,. 1 ('\ Co E I Hueso '( / : '- 8', '... ¡- t.. \ ... ; '- /
FIG. 12. East-west trending Cerro El Hueso dike swarm about 32 Ma old indicates east-west directed compres sion (north-soulh extension).
GEOLOGICAL HISTORV AFTER FORMATION OF THE PORPHVRV COPPER SVSTEM OLlGOCENE-MIOCENE COMPRESSIONAL DEFORMATION
After the emplacement of the mineralized and STYLES OF FAULTING IN POTRERILLOS unmineralized porphyry intrusions, the Potrerillos AREA district and the surrounding area experienced a pe riod of intense compressional deformation. Major Three major types of faults formed at this time: reverse faults juxtapose the thick, western, vol 1. High-angle reverse faults; 2, Thrust faults; and canic-dominant sections against the thin, eastern, 3. E-W to ENE-WSW trending transverse faults. Fold sedimentary-dominant sections (Figs, 2, 3 and 13, ing in the area in minor and usually developed ad A). jacent to high-angle reverse faults. Timing of compresive deformation is bracketed The Potrerillos district Iies at the boundary by stocks and dikes ca, 32 Ma old (this study) that between major north-south changes in the style are cut and offset by faults associated with the de and trend of compressional structures. To the formation (Fig. 3) and by ash-flow tuff and gravel south, structures are dominated by NNW-SSE 10-12 Ma old (Clark et al" 1967) that mantle re trending high-angle reverse fau~s; to the north, verse faults on strike to the north of the Potrerillos structures are dominated by NNE-SSW trending district (Fig. 2). s. F. O/son 21 z m z· Bend A R,'o Sal Faul! ./
-r + + + + + + + + + + + + + + pJb + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
B ..
~------1Bkm ------~
shortenlng in bosemen! ~ lB -t8.5 = -2.7% lo 18.5
Amoun! may vory depending on degre. 01 RI'o Sal Faul! ottenuotion in the over!urned limb 01 the syncl Ine "'-. e /
18 km ------~.I Shor!ening in Cover = J..:::l.2. = 19- 35.5 = -460/0 lo 35.5
To'al shor!ening 18 .0 - 35.5 =_ 490/0 35.5
TO · Mlocene Atacomo gravels Jo . As ientos Formoflon
Tv Poleocene- Eocene volconlc rocks Jm : Montando'n formotlon
Kp Pedernole: Formo,t ian PJb . pre· JurosslC bosement
block , Jurasslc - Lower Cretoceous morlne volconlc rocks
FIG. 13. A. Cross section Ihrough the Potrerillos area; see ligure 2 lor location 01 section. B. Calculation 01 per cent shortening in basement rocks ; extreme attenuation 01 cover rocks and apparent lolding 01 granitic basement takes place immediately adjacent to the Río Sal laults; these phenomena are not taken into account in the calculation 01 shortening. C. Calculation 01 percent shortening in cover rocks; the bend in the cross section was made to avoid the structural complications 01 majar transverse laults and does not appreciably change the calculated values 01 percent shortening (Iess Ihan 5% 01 the given values) . 22 THE POTRERILLOS PORPHYRY COPPER DISTRICT
A Recons!ruc!ion ------,,------;,,------,
pJb Reconstructed present surlace
B Movemen! an !hrus! faulls
~..-=:.-,:::::=::- =----'"---¡ I I ~' pJb I ,, e Movement on sudace high-angle reverse faulls
S ie rro Coshllo Fa ull I I No ver tical e xouuerotion Río Sol fou lt
FIG. 14. Structural evolution 01 the thrust and high-angle reverse laults in the Potrerillos ares. Symbols as in ligure 13. A. Reconstruction 01 the rocks now lound between the Sierra Castillo lault and the eastern edge 01 the map area a long section Z-Z'. An attempt has been made to show the position 01 the thrust laults relative to the cover rocks. Contacts above the reconstructed present surface are segmented and Jurassic-Lower Cretaceous marine volcanic rocks (black) are stippled; B. Movement on thrust laults that transported about 10 km 01 cover rocks into the map area and may have been responsible lor the initial westward tilting of the basement. C. Movement on high-angle reverse faults .
high-angle reverse faults, thrust faults, and trans ing the Potrerillos district (Fig. 2). Most of the verse faults. This bend in strike of the high-angle faults are east-vergent and involve both the pre reverse faults coincides with the bend in strike of Jurassic basement and the Jurassic-Paleocene the changes in facies and thicknesses of the sedimentary and volcanic cover. These faults dip Jurassic-Cretaceous marine-volcanic sequence. steeply to the west, juxtapose older rocks on the west against younger rocks on the east, and often Hlgh-Angle Reverse Faults display more than 1,000 m of apparent dip-slip dis placement. The Sierra Castillo fault (after Tobar, High-angle reverse faults are the most impor 1978), found to the northwest of the district (Fig. tant through-going structures in the area surround- 2), is an important exception, however; it is a west- S.F. O/son 23 vergent high-angle reverse fault with about 2 km ciated tilting is probably not more than 3%. To apparent dip-slip displacement (Perelló and Müller, gether, the amount of shortening in the cover and 1984). The Sierra Castillo fault forms the western basement rocks is probably about 50% (Fig. 13, boundary of the Cordillera Domeyko in the Potreri- C). 1I0s area. Where exposed, the pre-Jurassic basement Sequence of Deformation contact dips consistently 10-20 degrees to the west and has apparently been tilted between major The disparity between these various amounts 01 high-angle reverse faults. The amount of shorten shortening calculated for basement and cover ing associated with this tilting and faulting is pro rocks suggests that the different styles of reverse bably not more than 3% (Fig. 13, B). faulting acted at different times during compres sive deformation and that a significant amount of Thrust Faults cover rocks were transported into the area by thrust faults. The geometry 01 the thrust faults with Thrust faults are best developed Irom the Potre respect to the basal unconlormity and the high rillos district northward (Figs. 2, 3). At the present angle reverse faults requires that thrusting pre levels of exposure, the thrust faults involve only ceded movement on the high angle reverse laults. rocks of the Jurassic-Paleocene sedimentary-vol Figure 14 illustrates one possible scenario where canic cover. The major thrust faults are al! east by thrust faults carry additional cover rocks into ve'rgent, with displacements estimated between 1 the area prior to movement on the high-angle and 2 km . The faults are subparallel (5-10 degrees) reverse faults. Westward tilting of the basement at to bedding in marls and cut moderately to steeply this time may have been caused by stacking of (25-30 degrees) up-section in limestones and sand thrust plates. In the final stages of thrusting, per stone units. In areas of intense thrust faulting, haps when the thrust faults became 'Iocked', major dips 01 the cover rocks vary between 20 and 70 movement on the high-angle reverse faults occur degrees west because: 1. The basement has been red. tilted an additional 20 degrees west beneath the thrust plates; and 2. Ramp anticlines in the upPElr Transverse Faults plates of thrust faults increased the westward dip in some places (J. Skarmeta, personal commun., Transverse laults in the Potrerillos area mark 1985). the inception of high-angle reverse and thrust The Cobre porphyry pluton and associated faults and bound changes in the structural style skarn were cut and offset by a major thrust fault, and dips in the upper plates of thrust laults. They the Potrerillos mine lault (Fig. 3). Drillhole data indi trend east-northeast to east-west and have cates that the stock was displaced about 1.5 km apparent right-Iateral and both normal and reverse to the east-southeast, consistent with the dis offset, ranging Irom several meters to up to 500 m. placement of sedimentary horizons on the surface The intense zone of transverse faults in the and with the orientations of mullions and striations Potrerillos district marks the change in structural on fault surfaces on the Las Vegas level. style from NNW-SSE trending high-angle reverse The amount of shortening in the cover by thrust faults to the south of the district, to NNE-SSW laults and folding in the Potrerillos-Quebrada El trending high-angle reverse, thrust, and trans Asiento area is about 46% (Fig. 13). As stated ear verse faults to the north. Many of the thrust faults lier, the amount 01 shortening due only to the base to the north of the district begin as transverse or ment-involved high-angle reverse faults and asso- tear fautts.
DISCUSSION
The stratigraphic, structural, and intrusive evo cumulation of Jurassic-Cretaceous and Paleocene lution of the Potrerillos district is shown in figure volcanic rocks to the west is shown as the prin 15. Normal laulting contemporaneous with the ac- cipal means 01 down-to the-west subsidence. 24 THE POTRERILLOS PORPHYRY COPPER DISTRICT
Rhyolite intruded along some of these faults in Pal gard, 1978) and central Chile (Charrier, 1981), eocene time. The porphyry intrusions of Eocene suggesting that most of the Andes underwent a Oligocene age also intruded into this zone. Some similar stratigraphic and structural evolution from of the intrusions vented, providing the clasts for at least Jurassic through Cretaceous time. the Eocene porphyry-clast conglomerate and The facies changes described aboye do not co block breccia now found in the northeast part of incide with the porphyry copper belt everywhere the district. Compressional deformation between and may not have been significant in the localiza Late Oligocene and Miocene time reactivated the tion of intrusions except at Potrerillos. The geolo zone of earlier normal faults; the thick, western gy of the Potrerillos district suggests that: 1. The volcanic-dominant sequences were thrust over the porphyry copper plutons intruded between Paleo thin, eastern, sedimentary-dominant sequences at cene extension and Oligocene-Miocene compres this time. One of these thrust faults cut and offset sion; 2. The state of stress was one of east-west the Cobre Porphyry pluton. compression during the intrusion of some of the Although evidence in the Potrerillos are a indi porphyries; and 3. The host intrusions were of inter cates a Late Oligocene-Miocene age for the defor mediate compositions, in contrast to the preceding mation, it does not exclude the possibility for some basaltic andesite-rhyolite bimodal magmatism. Eocene deformation. Data from the surrounding Plate tectonic reconstructions of convergence area indicate that some compressive deformation rates between the South American and Nazca may have taken place in Eocene or perhaps earlier plates (Pardo-Casas and Molnar, 1987) suggest time. The Eocene porphyry plutons at El Salvador that convergence was slow during Paleocene time, intrude folded sequences of Cretaceous-Paleo increased in Early Eocene time, decreased in Late cene(?) andesite lava and ash-flow tuff ('Cerrillos' Eocene-Oligocene time, and greatly increased in and 'Hornitos' formations, Gustafson and Hunt, Miocene time in response to reorganization and 1975; personal observation, 1981). Here the plu faster spreading at the East Pacific Rise (Hand tons themselves do not appear to be offset by any schumacher, 1976). In the Potrerilos are a, slow majar faults. Also, in the Quebrada Paipote-Carre convergence rates in Paleocene time were contem ra Pinto area, Mpodozis and Davidson (1979) and poraneous with extension and bimodal volcanism, Sepúlveda and Naranjo (1982) found evidence for the fluctuating convergence rates between Early compressive deformation befare the deposition of Eocene and Oligocene time were contempora Lower Tertiary pyroclastics. neous with porphyry intrusion, and high conver gence rates during Miocene time were contempora TIME, SPACE ANO MAGMATIC SETIING OF neous with intense compressional deformation. THE POTRERILLOS PORPHYRY COPPER OISTRICT IN THE ANOEAN CONTINENTAL MAGNITUOE ANO SETIING OF MARGIN OF NORTHERN CHILE COMPRESSIONAL OEFORMATION
The mineralized and unmineralized intrusions in Most of the majar high-angle reverse and thrust the Potrerillos district intruded into a long-lived faults which formed during late Oligocene-Miocene zone of facies and thickness changes and asso compressional deformation in the Potrerillos area ciated normal faults. This zone formed contempora have at least 1 km displacement. Collectively, neously with majar accumulations of volcanic they could be responsible for as much as 50% rocks and batholitic emplacement to the west of shortening across the map area (presently about the district. This zone of faulting and changes in 20 km; Fig . 2). The majar through-going structures facies and thicknesses also marks important in the area are high-angle reverse faults which fol changes in the regional stratigraphy of the Andes low the western edge of the Cordillera de Domey of northern Chile, separating Jurassic-Cretaceous ka. These faults occur throughout northern and volcanic accumulations along the coast and in the central Ch ile (Reutter, 1974; Chong, 1977; Mak central valley regions from thinner sedimentary saev et al., 1984; Mpodozis and Ramos, in press) dominant sequences in the Cordillera de Domeyko and roughly coincide with the western edge of the (García, 1967; Chong, 1977). A similar facies Altiplano. The majar west-vergent faults in the change is also seen in Perú (Cobbing, 1985; Me- Potrerillos area (the Sierra Castillo fault) as well as S.F. O/son 25
A JURASSIC- PALEOCENE Accumulotion of volcanie ond sedimentary rocks down-to -the- west normol foulflng
------~--~~~~~~~~------vvvVv Vv V v v v v v v~-v--v- v
dt
B EOCENE - OLlGOCENE Porphyry intruslon Setting of Cerro El Hue~o Pluton obout 4 km south 01 section (32 M o )
v v V
dt
Cobre pnrDhyry 37Mo (hydrothermal biotit.)
Porphyr~ elost LATE OLlGOCENE - MIOCENE Potrenllos Mine Feult CompresslVe deformatlon /
o-- Hm
,W ~~ IIG0~1 Porphyry elost conglomera te 'VVV' Upper CretaceOU5 Unconformlty Feldspor Porptlyry: mlnerollzed / uJ~ o ,_..! 10 ..• : ..• Agua Helado Formatlon O"'lm unmlnerollzed Vl« Rh)'olitlc Intruslve rocks ':?~-Vlt-~fCJ Jurossic - Lower Cretoceous manne onde'5lte flows W E2l" «w z uJ Rhyollte lavo flows ~5.., ~ JurosslC - Lower Cretaceous marme sedimentar y racks u .J o 0 W Ash-flow tuft , den ..ly-weldedl 'VVV' Bo~ol unconformlly .J moderotely~ poorly ~ 'Welded «a. ~ Andesltlc con910merate /andeslre Pre -Jurosslc bosement rn flo\MS ~
FIG. 15. Evolution of!he Potrerillos district from Paleocene through Miocene time. Explanation in texto 26 THE POTRERILLOS PORPHYRY COPPER DISTRICT the other faults bounding the western edge of the area (Segerstrom, 1967; Mercado, 1978, 1983; Altiplano could thus be viewed as part of the sys Servicio Nacional de Geología y Minería, 1982) sug tem of "aults responsible for the uplift along the gest that most reverse faulting is confined to relati western edge of the Altiplano. vely narrow zones separated by large areas of Jt is doubtfuJ, however, that the shortening pre gently dipping strata. The deformation in the Potre sent in the Potrerillos area is representative of the rillos area appears to represent a zone of un usual Andes in general. Geologic maps of the PotrerilJos Iy high strain .
TABLE 5. CHEMICAL ANALVSES OF EOCENE-OLlGOCENE INTRUSIVE ROCKS IN THE POTRERILLOS DISTRICT
DC-21 DC-9 DC-3 Fine* Int*
SiÜ2 64.0 67.2 60.8 65.83 61.48 TiÜ2 0.59 0.34 0.49 0.51 0.65 AI203 17.1 16.2 16.5 16.07 20.24 Fe203 3,51 2.07 4.53 1.64 2.34 MnO 0.06 0.02 0.09 0.018 0.013 MgO 1.14 0.80 2.02 0.92 1.15 CaO 3.76 2.61 4.54 2.67 3.95 Na20 4.67 4.72 4.08 4.21 5.30 K2Ü 2.18 2.16 1.73 4.15 2.30 P2ÜS 0.19 0.16 0.19 0.13 0.20 S 0.19 0.24 0.06 Cu 0.043 0.002 0.004 0.79 0.24 LOI 1.19 1.18 4.28 Total 98.89 98.78 99.31 97.03 97.86
DC-21 PI3.gioclase-hornblende granodiorite porphyry, from North Porphyry stock; DC-9 Pagioclase-biotite-hornblende-quartz monzonite porphyry, from the Cerro El Hueso Pluton; DC-3 PlagiocJase-biotite-hornblende granodiorite porphy'v, from porphyry stock 2 km south of Cerro El Hueso; Fine' Average of four samples of 'fine-grained' porphyry; Int' Average 01 tour samples of 'intermediate' porphyry ('pórfido grano-grueso-feldespático' of Parra, 1983).
Analyses by U.S. Geological Survey, Branch of Analytical Chemistry, Lakewood, Colorado; by major element x-ray fluorescence. AII iron reportad é.S Fe203.
Analyses:Jf sulfur and copper by the Superintendencia de Geología, CODELCO-Chile, División El Salvador: by atomic absorption .
, Fro'Tl Perra (1983)
CONCLUSIONS
The Eocene-Oligocene porphyry intrusions of time to intermediate composition crystal-rich por the PotrerilJos district were emplaced in a long phyry intrusion in Eocene-Oligocene time. Major lived zone of facies sand thickness changes and dike orientations suggest that the state of stress associated normal faults contemporaneous with changed from approximate east-west extension in major accumulations of Jurassic-Paleocene volca Paleocene time to east-west compression (north nic rocKs to the west. This zone was subsequently south extension) at 32 Ma. The changes from Pa reactivated in Late Oligocene-Miocene time as a leocene bimodal volcanism to Eocene-Oligocene zone 01 intense reverse faulting. The rocks of the porphyry intrusion and finally to Oligocene Potreriflos district al so record the change from Miocene compressional deformation are coinci basaltic andesite-rhyolite volcanism in Paleocene dent with slow, fluctuating but increased, and S.F. O/son 27
greatly increased convergence rates at the Nazca study area. This value is probably not representa South American plate boundary, respectively. tive of the shortening across the Andes, but more Both thrust and high-angle reverse faults in the likely indicates a zone of especially high strain are a account for up to 50% shortening across the along the western edge of the Altiplano.
ACKNOWLEDGEMENTS
This paper is the outgrowth of a Ph.D. study at Jenny Metz (Stanford University) helped with the K Stanford University supervised by Marco Einaudi Ar dating. Estela Cubillos (Minera Utah de Chile) (Professor of Applied Earth Science at Stanford helped redraft many of the original figures for this University). John Hunt (consulting geologist of paper. A special thanks is due to Eric Seedorff Hunt, Ware, and Proffet) and CODELCO-Chile, Divi (Chevron Resources Company), who greatly im sión Salvador initiated the author's involvement in proved the original manuscript and encouraged the present study. Funding during the study was publication of this paper. Other helpful comments provided by CODELCO-Chile, División El Salvador were given by Charles Alpers (University of Califor and a National Science Foundation Graduate Fel nia at Berkeley), James Bratt (BHP-Utah Minerals), lowship. The geological and technical sta!! at El Mary Gilzean (Minera Utah de Las Américas),John Salvador, particularly Enrique Tidy, Jorge Quiroga, Davidson and Constantino Mpodozis (Servicio Na and Manuel Reyes, provided excellent geologic, cional de Geología y Minería) and Richard Allmen logistic and technical support. Brent Turrin and dinger (Cornell University).
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APPENDlX 1. SAMPLE LOCATlONS
Sample UTM Coordinates Comments Number North East
Table 1 DC-1 7062.560 462.580 6 km ESE 01 Cerro El Hueso DC-27 7071 .260 458.440 Underground, Las Vegas adit DC-28 7071 ,290 458.355 Underground, Las Vegas adit DC-29 7071 .330 458.250 Underground, Las Vegas adit Table 2 CH3-1 7070.700 456.100 Ridge north 01 Cerro San Antonio DC-17 7068.840 461 .630 1 km al ENE 01 Silíca mine DC-19 7068.840 461 .870 2.5 km N25°E 01 Silíca mine DC-14 7070.200 456.230 Ridge north 01 Cerro San Antonio DC-33 7069.800 452.560 Quebrada Cajoncito DC-36 7070.860 452.420 Quebrada Cajoncito CH3-5 7062.850 448.360 Vega Primera, near Cerro Vicuña Table3 DC-2 7066-440 461 .400 4 km west 01 Cerro El Hueso CH3-1 7070.700 456.100 Ridge north 01 Cerro San Antonio DC-43 7063.550 449.500 East 01 Cerro Vicuña CH3-5 7062.850 448.360 Vega Primera, near Cerro Vicuña CH3-10 7072.000 463.600 5.5 km ENE 01 Cobre Porphyry DC-7 7070.190 468.780 2.5 km E 01 Cobre Porphyry SNA 7070.600 457.530 Underground, Las Vegas level DC-8 7065.380 457.190 Cerro El Hueso DC-9 7065.180 457.100 Cerro El Hueso Table 5 DC-21 7071 .750 457.810 North Porphyry DC-9 7065.180 457.100 Cerro El Hueso DC-3 7064.100 459.200 2 km ESE 01 Cerro El Hueso