Economic Geology Vol. 96, 2001, pp. 535–557

Paragenesis, Elemental Distribution, and Stable Isotopes at the Peña Colorada Iron Skarn, Colima, Mexico

LUKAS ZÜRCHER,† JOAQUÍN RUIZ, AND MARK D. BARTON Department of Geosciences, University of Arizona, Gould-Simpson Bldg. 208, Tucson, AZ 85721

Abstract The Peña Colorada iron skarn is located in the southern part of the Guerrero terrane. It contains 150 mil- lion metric tons (Mt) with a grade of 36 percent magnetite. The deposit occurs at the contact of a 68 Ma equigranular diorite with mid-Cretaceous volcano-sedimentary rocks of the Tepalcatepec Formation. Volcanic units within this formation have tholeiitic affinity and REE patterns that are compatible with a primitive arc setting. The Peña Colorada diorite and associated voluminous aplitic phases are part of a Late Cretaceous calc- alkaline continental arc that subsequently intruded this basin. Northeast-vergent deformation affected the re- gion in the Late Cretaceous. North-south and east-west reverse, and northwest and northeast strike-slip faults localize the mineralization. The alteration halo around the diorite intrusion is 500 m wide. Within an inner 200-m halo, the volcano-sed- imentary section was affected by an early metamorphic event, with discontinuous bands of pyroxene (Di68Hd29Jo3) hornfels and garnet (Gr61Am36Ad3) disseminations. Metasomatic pyroxene, garnet, and plagio- clase cut the early metamorphic event preferentially replacing carbonate and volcanic units up to and includ- ing a 35-m-thick marl located 200 m above the intrusion. Metasomatic pyroxene and garnet have average com- positions of Di77Hd21Jo2 and Ad76Gr20Am2Sp1Py1, respectively. Pyroxene compositions reach Di94Hd5Jo1, whereas garnet exhibits intragrain compositional zoning from Ad49Gr46 in the core to Ad100Gr0 in the rim. Al- bitization of preexisting protoliths modified igneous plagioclase compositions from Ab67An32Or1 up to Ab96An3Or1. Albitization is followed by abundant fracture-controlled Fe-epidote and Cl-bearing chlorite (Cha84Cli14Pen2) that extend from the marl unit outward some 300 m. This association is cut by a later assem- blage of epidote-chlorite-prehnite that affects the entire 500-m width of the alteration halo and part of the in- trusion. Chlorite (Cha54Cli45Pen1) from this later event is richer in Mg than that from the previous association. Igneous hornblende in the intrusion is altered to actinolite. A potassic alteration event related to a hydrother- mal breccia and aplitic dikes overprints the pre-existing calc-silicate associations. Alteration minerals in the breccia and the 200-m potassic halo around it, evolve from early K feldspar-(biotite)-quartz, to late jasperoid- fluorapatite-calcite veins. Mg-rich biotite (Phl77Ann23) and REE-rich apatite contain F/Cl/OH ratios of 1/0/3 and 1/0/0, respectively. Massive magnetite (36%)-specular hematite (7%)-sulfide (<5%) mineralization was deposited contempora- neously with metasomatic plagioclase, forming a shallow-dipping replacement body after the 35-m-thick marl unit located 200 m above the diorite intrusion. Calcite, dolomite and chlorite are the main interstitial con- stituents. The thickest ore section trends north-south through the middle of the orebody, and is rooted in a “blind” fault that is not observed above the marl unit. Ti and Cr in magnetite remain more or less constant throughout the orebody, while Mn and V increase with distance from the diorite intrusion. Co/Ni ratios in pyrite increase away from the diorite. Sulfides contain traces of gold. Ninety million metric tons of dissemi- nated iron oxide was deposited with the retrograde epidote-chlorite-prehnite alteration event. This 20-m-wide magnetite-hematite-(pyrite) endoskarn occurs at depth along the diorite-carbonate contact in the western sec- tor of the mine. High Ti magnetite from this zone contrasts with the Ti-poor massive ore magnetite. The post- mineral potassic alteration event locally dissolved and re-deposited iron and REE from the orebody but is oth- erwise barren. O, C, S, and H stable isotope determinations on minerals from the calc-silicate association are consistent with an igneous source for the initial fluids, which mixed outward with an increasing component of trapped seawater or evaporite. However, water in equilibrium with magnetite exhibits unusually enriched δ18O values relative to other coexisting mineral-derived fluids. A solution that equilibrated with limestone at relatively high temperature is required to explain these heavy values.

Introduction iron occurrences throughout the world, and suggest a model THE ORIGIN of the mineralization at the Peña Colorada iron for its genesis. For this purpose, we summarize the regional skarn, located in southwestern Mexico (Fig. 1), has been the geology and present detailed lithologic, structural, alteration, subject of controversy. Klemic (1970) classified Peña Colorada and mineralization time-space relationships. We also attempt as a Kirunavaara type. Other workers (R. Corona-Esquivel, to estimate bulk major and trace element mass transfer and pers. commun.) have informally attributed the ore to rework- zoning patterns. Composition and sources of ore-forming flu- ing of a volcanogenic system. The objective of this field-based ids are proposed based on geochemical studies. New data study is to describe the characteristics of this important but includes host and ore major, trace, and rare earth element otherwise little known iron deposit, allow comparison to other contents, composition of alteration minerals by electron mi- croprobe analyses, and carbon, oxygen, hydrogen, and sulfur † Corresponding author: e-mail, [email protected] stable isotope determinations.

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Regional Map andesitic flows, rhyolitic pyroclastics, siltstone, sandstone, and Mod. from geology by CMBJ staff Cerro Pelón del Norte 19°25' conglomerate, with an aggregate thickness of 1,200 to 2,000 m Explanation X near Tecalitlán (Pantoja-Alor and Estrada-Barraza, 1986). The Alluvium Ba R ío M Tecalitlán Formation is overlain by the Albian to Cenomanian Leucogranite ina tit lá Tepalcatepec Formation (Corona-Esquivel and Alencaster, Iron Ore n Hornfels 1995), which crops out extensively in the study area (Fig. 1). Gd-Diorite The base of the Tepalcatepec Formation consists of near- Conglomerate 19°24' Andesite Tuffs River shore mudstone, thinly layered carbonaceous shale, siltstone, Limestone Paved Road sandstone, evaporitic horizons, and local reef limestone with Volcaniclastic Rocks Las Pesadas X To Colima abundant bivalves and rudists. The middle section of the for- Fe mation consists of gray medium-bedded, micritic shallow-ma- Minatitlán rine limestone with thin intercalations of limy shale. Carbon- 19°23' ate rocks are followed upward by interlaminations of greenish carbonate-rich shale and andesitic tuff. This middle member Sierra El Mamey of the Tepalcatepec Formation is the main ore host at Peña Ba Colorada. The upper part of the formation consists of subaer- X Peña ial andesite flows that are not preserved at Peña Colorada. Colorada El Salto 19°22' The middle and upper members of the Tepalcatepec Forma- Cerro Pelón del Sur tion are more than 2,500 m thick at their type locality west of Tepalcatepec, Jalisco (Pantoja-Alor and Estrada-Barraza, MN 1986). The Tepalcatepec Formation is unconformably capped Guásimas by a massive, poorly sorted, red conglomerate, which corre- N Company Housing lates with the Cerro de la Vieja Formation of Cenomanian- 19°21' Turonian age. The formation reaches a thickness of up to 2,000 m at Coquimatlán (Razo-Rojas, 1986). 0 km 1 km 2 km A batholith with granodiorite, diorite, quartz diorite, and

04' 07' 06' 05'

° ° ° ° aplitic phases intruded the region in the Late Cretaceous-

104 104 104 To Manzanillo 104 early Tertiary. Roofs of hornfelsed and metasomatized vol-

FIG. 1. Location and geologic setting of the Peña Colorada iron skarn. cano-sedimentary units are preserved at Peña Colorada and vicinity. Several of these nearby areas host important iron prospects. At Peña Colorada, a medium-grained equigranular The earliest work on Peña Colorada was done by Gonzalez- pyroxene hornblende diorite with a K-Ar date of 67.6 ± 3.5 Reyna (1952, 1956), who described the geology of the area at Ma (S. Sanchez-Quiroz and A. Juárez, unpub. report, 1988) a reconnaissance level. In the early 1960s, an aerial survey by produced an extensive contact-metamorphic aureole and as- Pineda et al. (1969) identified a striking magnetic anomaly at sociated iron ore. Voluminous aplitic intrusions occur at the site. Following the discovery, Consorcio Minero Benito Cerro Pelón del Norte and Cerro Pelón del Sur, north and Juárez (CMBJ) was formed (Engineering and Mining Jour- east of Peña Colorada (see Fig. 1). These leucocratic rocks nal, 1965), and a drilling program confirmed the extent of the host small magnetite-barite veins with wide kaolinite alter- iron orebody. Recent attention has been given to paleomag- ation halos. In the mine area, aplitic dikes related to this in- netic, mineralogic, and paleontologic topics by Alva-Valdivia trusive event cut diorite and ore. et al. (1991, 1996), Corona-Esquivel et al. (1991), Corona-Es- Middle Tertiary andesitic to rhyolitic rocks related to the quivel (1993), Alva-Valdivia and Urrutia (1994), and Corona- Sierra Madre Occidental volcanic province occur throughout Esquivel and Alencaster (1995). Other unpublished company the region. At Peña Colorada, only thin aphanitic and por- reports treat aspects of the regional stratigraphy, local geol- phyritic andesite dikes related to this event cut across preex- ogy, and geophysics. isting strata. Regional deformation consists of northeast-vergent broad Regional Geologic Setting symmetric anticlinal and synclinal folds. This deformational The Peña Colorada area is underlain by Middle Cretaceous event is accompanied by syndeformation greenschist-grade volcano-sedimentary rocks of the Guerrero terrane, which ac- metamorphism, inferred to have been imparted in the mid- creted to North America in the Early Cretaceous (Campa- Cretaceous, based on K-Ar dates obtained from the Puerto Uranga and Coney, 1983). Vallarta batholith and the El Encino iron mine areas (Pantoja- The basement is not exposed in the region but is believed to Alor, 1983). Sierra El Mamey, the range that hosts the Peña be Triassic, strongly deformed, low-grade metamorphic ocean Colorada deposit, conforms with a broad N 45° W anticline. floor (Centeno-García et al., 1993). The basement is overlain by Pre-Laramide structures include north-south and east-west the Valanginian to Hauterivian 1,500-m-thick shallow-marine reverse faults. Laramide structures include northwest-southeast Alberca Formation, which consists of black shale, shaly lime- and northeast-southwest faults with strike-slip components. stone, fine-grained sandstone, and andesitic tuff (Corona-Es- quivel, 1993). The Barremian to Aptian volcano-sedimentary Deposit Geology rocks of the Tecalitlán Formation conformably rest on the Al- The volcano-sedimentary section at Peña Colorada is ap- berca Formation. The Tecalitlán Formation includes subaerial proximately 750 m thick. The mine-scale geologic map and

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594600 594800 595000 595200 595400 Geology Alteration Mineralization 15

E Norte Fault E’ E E’

D La Primorosa 65° 2143800 U contact with limestone 15 18 La Encantada 2143600 Chinforinazo PrimorosaChinforinazo Fault Fault 80° Norte D D D D’

Amorfo Fault 2143400 21

2143200 To Minatitlán Poniente Fault D Chinforinazo Espinazo dumps U Centro del Diablo C’ C C 2143000 endoskarn C

Maintenance Encantada Fault Offices 2142800 26 contact with volcanoclastic unit 39 Cerro Los Concentrator Chinforinazo Juanes 85° 2142600 Sur 85° B B B’ Ore Patio Crusher

D 2142400 La Chula FaultU 37 88° M La Chula 2142200 magnetite-quartz veins N A A A A 46 2142000

0 200 400 m Piedras Negras Fault 2141800 80°? Explanation Explanation Explanation Te Andesite dikes 46 Bedding Sericite-clay K-Te Hydrothermal breccia ° K-feldspar-biotite-quartz-fluorapatite Late magnetite veins (with potassic alteration) K-Te Aplite dikes 85 Fault K-Te Pyroxene-garnet hornfels Epidote-chlorite-prehnite Disseminated iron oxide ore (25-33% magnetite) A A’ Section K-Te Pyroxene hornblende diorite Massive iron oxide ore (greater than 33% magnetite) Metasomatic plagioclase K(late) Cerro de la Vieja conglomerate Metasomatic garnet A A’ tuff Section Metasomatic pyroxene Middle marl K(early) Tepalcatepec platform limestone Road UTM coordinates Formation reef limestone 21418002141800 UTM coordinates volcaniclastic unit Building

FIG. 2. a. Peña Colorada mine-scale geology, alteration, and mineralization. sections are shown in Figure 2a and b, respectively. Summa- lized massive limestone. To the east, the lowermost portion rized lithologic descriptions are given next to the idealized consists of metasomatized volcano-sedimentary units. The na- stratigraphic column in Figure 3. Units are described in order ture of the lateral transition between the western massive of deposition or emplacement. limestone and the eastern volcano-sedimentary protolith is not known. It appears to be tectonic, because an abrupt facies Lower Tepalcatepec Member change over such a short distance would be extraordinary. Both the western and eastern units are unconformably over- These rocks are exposed in the western sector of the mine lain by a 35-m-thick, thin- to medium-bedded marl unit com- area. They are in fault contact with Middle Tepalcatepec posed of limestone, shale, and tuff. These beds have been ex- units. The immediate hanging wall of the Poniente fault con- tensively replaced by calc-silicates and iron oxide. Thin sists of highly sheared, dark-colored, massive, pyroxene horn- syndepositional gypsum laminations do occur between these fels, which is overlain by an alternating sequence of relatively layers. The 35-m-thick marl unit is conformably overlain by unmetamorphosed centimeter-scale shale and medium- andesite tuffs interlayered with carbonate-rich lenses. Far- grained sandstone beds that dip 40° SW. The shales are black ther upsection, the sequence consists of thinly bedded pyrox- and fissile. The medium-grained sandstones consist of ap- ene andesite tuffs and fine-grained sandstones, which dip on proximately equal amounts of feldspar and quartz. Upsection, average 20° SW. Sandstone beds become more abundant to- the sandstones are more tuffaceous and interlayered with a ward the unconformable contact with the overlying Cerro de reef limestone lense. This recrystallized pelecypod-rich lens la Vieja Conglomerate. At La Encantada in the northeastern is of limited lateral extent (±300 m). Pale tuffs overlie this car- sector of the mine area, the marl unit is overlain by a 40-m- bonate unit. The Lower Tepalcatepec Member has a thick- thick lithic-vitric andesite tuff, a unit missing from the section ness of approximately 100 m in the mine area. elsewhere. It is a fairly fresh grayish-green unit, composed of plagioclase (15 vol % Ab67), alkali feldspar (3%), and quartz Middle Tepalcatepec Member (2%) fragments in a glassy matrix (65%). Pyroxene (2%), This member is exposed in the central and eastern sectors hornblende (1%), biotite (1%), and magnetite (5%) occur as of the mine. At depth in the western part, the base of this accessory xenocrysts. The maximum thickness of the rock se- member consists of an approximately 200-m-thick recrystal- quence described above is about 400 m.

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E E’ medium-grained, equigranular diorite comprises albitized Cross-sections Geology and 1300 plagioclase (60 vol %), very minor K feldspar (2%) and quartz Mineralization 1200 (2%), augite (En24Fs29Wo47: 5%), actinolite after hornblende ? 1100 ? ? (10%), titanite (1%), apatite (1%), zircon (<1%), and opaques 200 0 200 400 meters (2%). In addition to actinolite and albite, alteration minerals in-

D D’ clude epidote, chlorite, prehnite, magnetite, and sericite. The

1200 rock is cut by quartz veinlets with sulfide traces. Porphyritic 1100 hornblende diorite dikes occur at depth along the Poniente and 1000 Amorfo faults and along the central Encantada horsetail. ? ? 900 ? 600 400 200 0 200 400 600 meters Aplite dikes These dikes cut the diorite pluton. They extend from the C C’ main leucocratic intrusion that conforms Cerro Pelón, 3 km 1000 west of the mine area. This rock is referred to as “syenite” by 900 mine geologists. A fresh aplitic granodiorite phase at El Salto 600 400 200 0 200 400 600 meters B B’ is composed of alkali feldspar (15 vol %), plagioclase (50%),

900 and quartz (30%). Figure 4j is a photomicrograph of an ex- ? 800 ? ceptionally phenocryst rich area. The rock is essentially devoid ? ? of mafic minerals, with the exception of very minor biotite that 700 ? 600 400 200 0 100 meters partially surrounds K feldspar phenocrysts. Alteration miner- A A’ als include epidote, chlorite, and clay. In the mine area, aplitic 800 Vertical 700 ? = 1 dikes crop out along and parallel to the Piedras Negras fault. ? ? Horizontal 600 ? Other aplitic dikes have been intercepted by diamond drilling ? ? 300 100 0 100 meters along the Poniente and Primorosa fault zones. Similar, volu- Explanation metrically important leucocratic units have been described at Te Andesite dikes many other iron skarn localities, such as Daiquiri, Cuba (Lind- K-Te Hydrothermal breccia Fault gren and Ross, 1916), Larap, Philippines (Kihlstedt, 1946), K-Te Aplite dikes and Vancouver Island, British Columbia (Sangster, 1969). Disseminated iron oxide ore (25-33% magnetite) Massive iron oxide ore (greater than 33% magnetite) Hydrothermal breccia K-Te Pyroxene-garnet hornfels K-Te Pyroxene hornblende diorite This unit consists of matrix-supported, cobble-sized, suban- K(late) Cerro de la Vieja conglomerate gular to angular fragments (see Fig. 4k). The breccia gener- tuff Middle marl ally cuts wall rock at a high angle, with a sharp contact. Where K(early) Tepalcatepec platform limestone the breccia cuts through unmineralized rock, the fragments Formation reef limestone volcaniclastic unit are mainly composed of barren calc-silicate associations. Where the breccia cuts through mineralized rock, massive magnetite fragments are also present. On the whole, the frag- FIG. 2. b. Geology and mineralization cross sections. Drill hole locations ments are larger in the central part and become progressively and depths have been left out for clarity. Drill hole control is extensive, ex- cept at the location of question marks. smaller toward the hanging wall and footwall. Upward or downward clast movement appears to have been limited, given that calc-silicate bands are moderately preserved from Cerro de la Vieja Conglomerate the footwall, across the breccia, to the hanging wall. The brec- This competent red conglomerate crops out in the northern cia matrix consists of highly silicified and finely comminuted and southern sectors of the mine area. It is constituted by ma- material of the same composition as the fragments. This hy- trix-supported, poorly sorted, subangular to subrounded drothermal breccia is intimately associated with aplite dikes chert, porphyritic andesite, and subordinate limestone frag- in space and time. ments. The clasts range from millimeter- to pebble-size di- mensions. The silica-cemented, coarse-grained matrix is rich Andesite dikes in hematite and consists of particles with the same composi- Porphyritic basaltic andesite dikes and subordinate sills are tion as the fragments. The abundance of limestone fragments emplaced along open fractures and, to a lesser extent, along and sandstone lenses increases and matrix cement is more lithologic contacts. Their thicknesses vary from centimeters calcareous toward the base. This unit reaches 250 m of thick- up to 10 m, but few exceed 2 m. There are three generations ness in the mine area and dips on average 18° SW. of basaltic andesite dikes. Early-altered apahanitic dikes are cut by unaltered porphyritic dikes and these in turn are inter- Diorite and porphyritic diorite dikes sected by fresh porphyry dikes. At the contact with wall rock, The diorite intrusion crops out in the southern sector of the these dikes commonly exhibit chilled margins. Locally, hornfels mine area. It occupies the La Encantada canyon and the xenoliths are also found. Phenocrysts are plagioclase (30%), hanging wall of the Piedras Negras fault. At depth, this rock alkali feldspar (3%), and hornblende (5%). Glass amounts to underlies the whole study area, as indicated by diamond 50 vol percent of the rock. The presence of relatively abun- drill hole intercepts. Where freshest (Fig. 4a), the greenish, dant calcite veins and fine-grained subhedral magnetite in

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Description Alteration Associations Cerro de la Vieja Conglomerate: red, poorly sorted, Calc-silicate Kspar-Biotite 1-10 cm limestone, andesite porphyry, and chert fragments. Sandstone lenses. Calcareous toward base. Unconformably overlying Tepalcatepec Formation. Lower part epidotized and chloritized. Cl-chorite V V VV (Cha54 V V Cli V V V V 45 V V Lower Tepalcatepec Member (left): felsic tuffs, Pen1) V V V V V thinly laminated sandstones, black fissile shales, V V V and fossiliferous reef limestone. Fault contact with Middle Tepalcatepec Member. Chloritized, V V V cut by quartz and calcite veinlets. V V V V V

Middle Tepalcatepec Member (right): interlayered limestone and carbonate-rich andesitic Fe-epidote- tuffs. Main ore host. Selective replacement by Cl-chorite pyroxene, garnet, plagioclase, epidote, and chlorite. (Cha84 Cli14 Post-mineral aphanitic and porphyritic andesite Pen2) dikes (dark gray): with rare hornfels xenoliths. They use pre-existing NW and N-S structures.

Contact Metamorphism (stippled pattern): pyroxene hornfels and disseminated garnet F-rich chlorite replacing Middle Tepalcatepec sequence. (Cha29Cli71) Chlorite-epidote, and late quartz-calcite-(pyrite) stringers. G G G G G Intramineral Diorite Dike: altered, with grandite diss. mt reaction skarn when intruding limestone. Disseminated magnetite. Cut by late K-feldspar-quartz-pyrite-(chalcopyrite) veinlets.

P P P P P P Hangingwall: plagioclase-(marialite) Cl-chlorite Late Breccia: angular to subrounded hornfels P P F-biotite- P and massive mt (after hm) fragments supported in a P P P K-feldspar-quartz-magnetite-pyrite matrix. Rooted Di77Hd21Jo2- quartz- P P P in aplite. fluorapatite- P Ad76Gr20 jasperoid- P P P P P P Massive Ore Seam: Po (<1%) anhedral; mt (36%) Am2Sp1Py1- calcite P euhedral; hm (7%) specular; py (5%) P P P P P banded & "feather" (after hm). Ab96An3Or1 P P P Footwall (right): pyroxene and albite replacement. P P P P Disseminated magnetite, abundant ep-chl. Fe-epidote Late K-feldspar-biotite-quartz-apatite veinlets. P P P P E Footwall (left): recrystallized, bedded P P P E limestone with very weak disseminated pyrite. Di68Hd29Jo3- E P P E P Gr61Am36Ad3 E P P P Diorite-Tepalcatepec Contact: pyroxene hornfels and diss. garnet (East); garnet exoskarn and P P P albite-mt-epidote/prehnite-chlorite endoskarn P (West); localized sericite-clay alteration. actinolite- sericite- epidote- clay chlorite- Diorite: greenish-brown holocrystalline, medium- prehnite grained, equigranular augite hornblende diorite. Albite-magnetite-prehnite-chlorite stringers, Kspar- cut by K-feldspar-biotite-quartz-mt-pyrite veinlets. F-biotite (Phl77Ann23)- "Syenite": off-white microporphyritic grano- quartz diorite with kspar-magnetite-quartz-apatite veinlets and chlorite stringers.

FIG. 3. Idealized stratigraphic column at Peña Colorada, based on core logging and bench mapping by L.Z., 1992. Verti- cal scale approximately 700 m. E = endoskarn, G = reaction skarn, P = albite. For key to other abbreviations see Table 1 footer. early aphanitic dikes may suggest that the emplacement of Negras reverse faults. The Poniente fault juxtaposes the lower this dike generation overlapped the mineralizing event. member of the Tepalcatepec sequence with its middle mem- ber in the western sector. Subsequent intrusion of both dior- Structure ite and aplite dikes obscures field relations along the Piedras Four steeply dipping premineral fault systems divide the Negras fault. The diorite penetrated mainly along a north-south mine area into central, western, eastern, northern, and south- fault or abrupt facies change between limestone and volcano- ern structural blocks (see Fig. 2). Fault and fracture trends, as sedimentary units along the La Encantada Creek in the cen- well as structural controls for diorite, aplite, and breccia are tral block. Since drag folding on either side of this break is not recorded in Figure 5. From oldest to youngest, the attitudes apparent in cross-sectional view, the structure could be a fault are the following: North-south and east-west (Poniente, with lateral rather than vertical displacement. In the eastern Amorfo, and Piedras Negras faults), N 35°–52° W (La Chula part of the mine area, the La Encantada fault does not exhibit and Chinforinazo faults), N 30°–45° E (Encantada fault), and significant displacement but splits into north and northwest N 60°–70° E (Primorosa fault). secondary horsetails, suggesting a structural abutment. The These systems mimic the regional structural fabric. The postconglomerate Chula fault offsets the earlier structures oldest sets include the preconglomerate Poniente and Piedras and displays left-lateral slip. The northeast-striking normal

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FIG. 4. Photomicrographs of selected rocks, alteration, and mineralization associations. White bars = approx 1 mm. Pan- els a, b, c, d, i, j, and l under cross polars; panels e, f, g, and h under reflected light. Pencil magnet for scale in field photo of panel k. a. Diorite with subophitic plagioclase and clinopyroxene and intergranular hornblende and magnetite. b. Contact metamorphic almandine-rich garnet. Partially preserved igneous plagioclase in tuff protolith and late chlorite. c. Composi- tionally zoned metasomatic grandite in calcite and epidote matrix. d. Magnetite pseudomorph after garnet. e. Feather pyrite in magnetite-calcite. f. Granular pyrite partially replacing hematite. g. Chalcopyrite interstitial to pyrite, and pyrite replacing pyrrhotite. Metasomatic pyroxene matrix. h. Probable exsolution. Hematite with pyrite, and magnetite with pyrite and chal- copyrite. i. Diorite alteration. Albite and minor scapolite after igneous plagioclase. Actinolite after hornblende. Epidote after augite(?). Abundant magnetite in groundmass. j. El Salto aplite. Sparse K feldspar, plagioclase, and quartz phenocrysts in mi- crocrystalline anhedral K feldspar, plagioclase, and quartz groundmass. The photo shows an exceptionally phenocryst rich section. k. Hydrothermal breccia boulder with angular to subrounded calc-silicate, tuff, and magnetite (after hematite) frag- ments. Iron oxide-rich matrix. l. Potassic alteration. K feldspar, apatite, and quartz vein cutting granoblastic plagioclase in breccia.

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I) Diorite and intra-mineral diorite dikes II) Main ore control: po-mt-hm-py III) Retrograde: ep-chl-preh IV) Breccia: kspar-bi-qtz-sericite-clay N V) Aphanitic Andesite dikes: late mt and cc

II) Prograde: cc-chl-ep III) Retrograde: ep-chl II) Prograde: cc-chl-ep V) Andesite porphyry dikes: late cc V) Porphyritic andesite dikes: late cc

IV) Aplite dikes and breccia: qtz-fluorapatite-cc

IV) Aplite dikes: qtz-fluorapatite-cc-chl IV) Breccia: cc-jasperoid W E 16 8 4 2 0 2 4 816

FIG. 5. Peña Colorada structural joint and fracture rosette, showing directions and alteration controls. Roman numerals indicate relative time-sequence of events. Concentric circles represent, from the center out, the number of joints and frac- tures per square meter (modified from Instituto Mexicano del Petróleo, 1987).

Primorosa fault cuts all earlier structures and drops down the the diorite intrusion. Granoblastic pyroxene preferentially northern block of the mine area. Postmineral aplite dikes and formed after shaly horizons, whereas garnet developed in cal- hydrothermal breccia utilized preexisting north-south, east- careous tuffs (see Fig. 4b). Pyroxene and garnet compositions west, and N 60° 70° E fractures. average Di68Hd29Jo3 and Gr61Am36Ad3, respectively (see Table Intrusion of thin Paleogene aphanitic and porphyritic 1). The garnet is birefringent and exhibits low totals, suggest- basaltic andesite dikes was the last event in the study area. ing the presence of water in the structure (see Fig. 4b and These dikes cut across all lithologic units and occur mainly Table 1). Pyroxene bands are more abundant toward the in- along northwest and north fractures and faults. North-south trusion but overall do not exceed 10 percent, whereas garnet porphyritic dikes cut northwest-trending aphanitic dikes and zones may account for 20 percent of the rock volume. are in turn offset by northeast porphyry dikes (Mendoza- Díaz, 1991). Minor postore movements along north-south Metasomatic skarn associations and northwest faults are evident from horizontally slicken- Metasomatic pyroxene, garnet, and plagioclase preferen- sided dike surfaces. tially formed after shale-, carbonate-, and tuff-rich beds Peña Colorada shares this lateral-slip structural style with within, and just below, the marl unit located 200 to 250 m other important iron deposits in the world, such as El from the diorite intrusion. Pyroxene (Di62Hd33Jo5 to Romeral, Chile (Bookstrom, 1977), Ginevro, Italy (Dimanche, Di94Hd5Jo1) horizons are subhorizontal, laterally restricted 1971), and Larap, Philippines (Bryner, 1969). (±50 m), and rarely exceed 50 cm in thickness. Garnet reac- tion fronts mostly follow north-south fractures and form small Alteration lenticular bodies. Pyroxene and garnet replace and cut early Alteration and mineralization relationships were examined pyroxene and fine-grained garnet zones. Garnet contains in by bench mapping, core logging, and petrographic analyses. many cases relict clinopyroxene grains. Garnet crystals are The distribution of alteration associations is depicted in Fig- anisotropic and concentrically zoned (see Fig. 4c). Their com- ure 2a. The order of mineral formation is based on replace- position changes from Ad49Gr46 in the core to Ad100Gr0 in the ment and textural criteria proposed by Lamey (1961) for Cal- rim. Einaudi et al. (1981) described similar compositional ifornia iron skarns. Mineral phases and compositions were changes in other skarn systems. Table 1 shows how pyroxene determined using a four-spectrometer Cameca SX 50 elec- and garnet compositions from this event contrast with the tron microprobe at the University of Arizona. Representative previous metamorphic event. Discontinuous zones of meta- analyses are given in Table 1. Abbreviations used throughout somatic plagioclase (Ab97An2Or1) form after tuff or shale in the text and analytical conditions are tabulated as footers to the eastern sector below the marl unit. Isolated granoblastic Table 1. Mineral compositions and the paragenetic succession plagioclase layers are also found within and above the marl are summarized in Figures 3 and 6, respectively. The struc- unit, where it is easier to discern the volcanic protolith. Lo- tural control of alteration associations observed at Peña Col- cally, marialitic scapolite formed along the contact between orada is represented in Figure 5. the marl unit and overlying rocks. Widespread selective al- bitization and rare scapolitization of igneous plagioclase is Early contact metamorphic event also evident in the diorite intrusion (see Table 1). At the de- Discontinuous bands of pyroxene hornfels and disseminated posit scale, the volume of metasomatic plagioclase appears to fine-grained garnet are widespread within a 200-m shell around exceed that of garnet and pyroxene combined. Wollastonite

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TABLE 1. Representative Microprobe Analyses

Calc-silicate event Meta- Meta- Meta- Meta- Scapolite-albite Igneous morphic somatic morphic somatic after igneous Early Early Late Late pyroxene pyroxene pyroxene garnet garnet plagioclase epidote chlorite chlorite prehnite Sample no. 68628-11 68614-21 548231 686231 68617gt51 686142 686282 35705-11 686142 68614-21 Wt percent oxide SiO2 50.70 52.39 51.20 36.92 37.57 74.05 38.37 24.60 26.56 43.24 TiO2 0.12 0.01 0.48 0.00 0.06 0.00 0.01 0.00 0.01 0.00 Al2O3 0.58 0.09 6.37 19.90 8.95 21.14 26.15 18.15 19.31 23.74 FeOT 16.71 10.41 1.38 16.48 17.92 0.20 8.04 39.30 26.69 0.47 MgO 7.91 11.07 14.21 0.17 0.24 0.13 0.01 2.61 12.22 0.00 MnO 0.48 1.68 0.12 0.00 0.40 0.00 0.17 3.46 1.35 0.33 V2O5 0.00 0.00 0.00 0.15 0.08 0.04 0.00 0.00 0.03 0.00 Ce2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.00 0.00 CaO 22.10 23.90 24.88 21.74 33.21 0.59 23.14 0.21 0.42 25.02 Na2O 0.62 0.09 0.17 0.00 0.00 2.96 0.00 0.00 0.03 0.06 K2O 0.00 0.02 0.00 0.00 0.04 0.04 0.02 0.02 0.03 0.00 BaO 0.02 0.01 0.03 P2O5 0.06 0.00 0.00 0.00 0.00 0.03 0.04 0.08 0.00 0.00 S 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.01 0.00 Sc2O3 0.00 0.00 0.00 Cr2O3 0.00 0.02 0.00 CoO 0.00 0.01 0.00 NiO 0.02 0.01 0.00 CuO 0.01 0.00 0.01 ZnO 0.00 0.02 0.03 F 0.00 0.00 0.00 0.00 0.00 0.05 0.00 0.00 0.00 0.00 Cl 0.00 0.00 0.00 0.03 0.00 0.10 0.01 0.04 0.21 0.06 Total-H2Ocalc 99.27 99.66 98.81 95.40 98.47 99.38 96.04 88.46 86.95 92.92 H2Ocalc 0.00 0.00 0.00 –0.07 0.00 –0.49 1.96 10.01 10.60 4.17 O = F, Cl 0.00 0.00 0.00 0.01 0.00 0.04 0.00 0.01 0.05 0.01 Total 99.27 99.66 98.81 95.33 98.47 98.84 97.99 98.46 97.50 97.08 Ion structural positions Si 1.977 1.997 1.888 3.020 3.020 3.498 3.040 5.721 5.796 6.126 Aliv 0.023 0.003 0.112 0.000 0.000 0.502 0.000 2.279 2.204 1.874 Alvi 0.004 0.001 0.164 1.919 0.848 0.675 2.441 2.697 2.764 2.090 Ti 0.003 0.000 0.013 0.000 0.004 0.000 0.001 0.000 0.002 0.000 Cr 0.000 0.000 0.000 0.000 0.000 0.000 0.002 0.000 0.000 0.000 Fe3+ 0.053 0.010 0.000 0.017 1.097 0.000 0.470 0.000 0.000 0.000 Mg 0.460 0.629 0.781 0.021 0.029 0.009 0.001 0.905 3.976 0.000 Fe2+ 0.492 0.322 0.042 1.110 0.108 0.008 0.063 7.644 4.872 0.055 Mn 0.016 0.054 0.004 0.000 0.027 0.000 0.012 0.681 0.250 0.040 V 0.000 0.000 0.000 0.008 0.004 0.001 0.000 0.000 0.004 0.000 Ce 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Ca 0.924 0.976 0.983 1.905 2.860 0.030 1.964 0.051 0.099 3.798 Na 0.047 0.007 0.012 0.000 0.000 0.271 0.000 0.000 0.014 0.017 K 0.000 0.001 0.000 0.000 0.000 0.002 0.002 0.006 0.007 0.000 Ba 0.000 0.002 P 0.002 0.000 0.000 0.000 0.000 0.001 0.002 0.016 0.000 0.000 F 0.000 0.000 0.000 0.000 0.000 0.006 0.000 0.000 0.000 0.000 Cl 0.000 0.000 0.000 0.004 0.000 0.007 0.001 0.018 0.096 0.015 S 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.012 0.000 Oxygen basis 6 6 6 12 12 8 13 36 36 24 End members En 23.85 Di 61.98 Di 94.42 Py 0.68 Py 0.96 Or 0.81 Cli 9.81 Cli 43.71 Fs 28.25 Hd 32.68 Hd 5.13 Am 36.56 Am 3.56 Ab 89.32 Cha 82.82 Cha 53.55 Wo 47.90 Jo 5.33 Jo 0.44 Sp 0.00 Sp 0.90 An 9.87 Pen 7.38 Pen 2.74 Ad 0.90 Ad 56.42 Uv 0.00 Uv 0.00 Gr 61.86 Gr 38.16

Abbreviations used throughout text Pyroxene Garnet Feldspar Chlorite Biotite Enstatite (Mg) En Pyrope (Mg) Py Orthoclase (K) Or Clinochlore (Mg) Cli Phlogopite (Mg) Phl Ferrosalite (Fe) Fs Almandine (Fe) Am Albite (Na) Ab Chamosite (Fe) Cha Annite (Fe) Ann Wollastonite (Ca) Wo Spessartine (Mn) Sp Anorthite (Ca) An Pennantite (Mn) Pen Siderophyllite (Al) Sid Diopside (Mg) Di Andradite (Ca,Fe) Ad Hedenbergite (Fe) Hd Uvarovite (Ca,Cr) Uv Johannsenite (Mn) Jo Grossular (Ca, Al) Gr

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TABLE 1. (Cont.)

Calc-silicate event K feldspar-biotite event Actinolite Late Early after igneous endoskarn magnetite Pyrrhotite Pyrite amphibole magnetite Biotite Apatite Chlorite Magnetite Sample no. 17505-11 357051 357051 686142 686191 1010151 175091 101015-21 1200031

Wt percent oxide SiO2 51.88 40.93 0.39 33.37 0.00 TiO2 0.06 0.15 0.37 2.74 0.00 2.26 0.54 Al2O3 0.21 0.79 0.32 11.56 0.03 12.27 0.21 FeOT 31.36FeO 12.41 30.63FeO 7.77 0.73 14.24 31.39FeO 68.42Fe O 59.40Fe 46.14Fe 65.24Fe O 67.05Fe O 2 3 2 3 2 3 MgO 9.91 21.38 0.00 19.59 0.06 MnO 0.00 0.73 0.14 0.00 0.00 0.22 0.28 V2O5 0.11 0.00 0.19 0.00 0.00 0.00 0.18 Ce2O3 0.00 0.00 0.14 0.00 0.00 CaO 22.93 0.05 53.64 1.10 0.00 Na2O 0.82 0.15 0.17 0.03 0.00 K2O 0.00 9.50 0.00 0.36 0.00 BaO 0.02 P2O5 0.00 0.00 40.62 0.00 0.00 S 38.37 53.03 0.00 0.00 0.17 0.00 0.00 Sc2O3 0.00 Cr2O3 0.04 0.00 0.00 CoO 0.00 NiO 0.00 0.26 0.19 0.01 0.00 CuO 0.00 0.00 0.00 0.00 0.00 ZnO 0.00 0.00 0.00 0.04 0.00 F 0.00 2.09 3.91 1.07 0.00 Cl 0.02 0.15 0.14 0.20 0.00 Total-H2Ocalc 100.20 98.03 99.36 99.71 96.89 96.32 99.94 84.72 99.71 H2Ocalc 0.00 0.00 0.00 2.04 0.00 2.22 -0.09 10.89 0.00 O = F, Cl 0.00 0.00 0.00 0.00 0.00 0.92 1.68 0.50 0.00 Total 100.20 98.03 99.36 101.74 96.89 97.63 98.17 95.11 99.71 Ion structural positions Si 7.621 6.095 0.034 7.213 0.000 Aliv 0.010 0.137 0.015 1.905 0.000 0.787 0.010 Alvi 0.000 0.000 0.000 0.124 0.003 2.339 0.000 Ti 0.002 0.017 0.011 0.307 0.000 0.368 0.016 Cr 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Fe3+ 1.978 0.114 0.000 0.000 1.948 0.000 0.000 0.000 1.945 Mg 2.170 4.746 0.000 6.311 0.004 Fe2+ 1.007 0.825 0.998 1.525 1.016 0.967 0.053 2.574 1.012 Mn 0.000 0.090 0.005 0.000 0.000 0.041 0.009 V 0.003 0.000 0.005 0.000 0.000 0.000 0.005 Ce 0.000 0.000 0.004 0.000 0.000 Ca 3.609 0.008 4.912 0.254 0.000 Na 0.235 0.043 0.028 0.014 0.000 K 0.000 1.804 0.000 0.099 0.000 Ba 0.001 P 0.000 0.000 2.939 0.000 0.000 F 0.000 1.024 1.047 0.893 0.000 Cl 0.005 0.040 0.020 0.091 0.000 S 1.057 1.999 0.000 0.000 0.080 0.000 0.000 Oxygen basis 4 1 sulfur 2 sulfur 23 4 24 13 4 24 End members Phl 77.00 Cli 70.70 Ann 23.00 Cha 28.84 Sid 0.00 Pen 0.46

Analytical conditions Package Elements (counting time in seconds) Current (nA) Voltage (kV) 1Microprobe analyses in 1993 Silicate condition 1 Si, Al, Fe, Mg, Mn, Ca, K (10), and Na (20) 20 15 Silicate condition 2 V, P, S (10); Ti, F, Cl (20); and Ce (30) 60 15 Oxides condition 1 Ti, Al, Fe, Mn, V, Cr, Ni, Cu, Zn (10) 60 15 Sulfide condition 1 Fe, S, Ni, Cu, Zn (10) 60 15 2Microprobe analyses in 1999 Silicate condition 1 Si, Al, Fe, Mg, Ca, K (15); Ti, Mn, Cl, (20); Na, Cr (25); Ba (30) 30 15 Silicate condition 2 S (10); Zr, Y, Ce, P, F, (15); Zn, V, Cu (30); Sc, Co, Ni (40) 100 15

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Mineral Phase Thermal Calc-silicate Association Potassic Association Supergene Metamorphism Prograde Retrograde

Pyroxene Di68Hd29Jo3 Di94Hd5Jo1 Garnet Gr61Am36Ad3 Ad49Gr46 Ad100Gr0 Calcite

Plagioclase Ab67An32Or1 Ab96An3Or1 Scapolite Sphene Actinolite Fe-epidote

Chlorite Cha84Cli14Pen2 Cl-rich Cha54Cli45Pen1 F-rich Prehnite

K-feldspar

F-Biotite Phl77Ann23 Quartz Sericite Fluorapatite Jasperoid Siderite

Pyrrhotite Magnetite Ti-poor Ti-rich Specularite Pyrite (Ni-Co) Chalcopyrite Sphalerite ?

Martite Covellite Chalcocite Delafossite Manganese Oxide

FIG. 6. Alteration and mineralization paragenetic sequence at Peña Colorada. was not observed at Peña Colorada. In the marl unit, plagio- the hydrothermal breccia form a 200-m-wide halo around it. clase bands contain epidote-filled vugs, giving the rock a K feldspar is cloudy, and biotite (Phl77Ann23Sid0) is fluorine pseudo-orbicular texture. Viewed in thin section, sparse titan- rich but apparently chlorine free (see Table 1 for biotite com- ite, moderate calcite, and dolomite fill open spaces. position). Biotites with high Mg contents have been described from several calcic iron skarn localities, such as Leglier, Rus- Retrograde associations sia (Zubkov, 1986), and Kroumovo, Bulgaria (Vassileff, 1971). Between 200 and 400 m from the intrusion, chlorite veins These veins preferentially occupy north-south, east-west, and and vug fillings occur within the marl unit and the hanging N 70° E fractures and locally occur along aplite-wall-rock wall above it. Veins of Fe epidote and Cl-bearing Fe-rich contacts. Younger north-south-striking (biotite)-K feldspar- chlorite (Cha84Cli14Pen2) from this event are emplaced along quartz-fluorapatite and calcite veinlets cut older veins and N 30°–45° E and N 35°–52° W directions. This event is cut both breccia matrix and clasts (see Fig 4l). Table 1 shows a by more widespread chlorite-prehnite veins that occur from representative fluorapatite analysis. Fluorapatite associated the base of the conglomerate to the diorite intrusion 500 m with postmineral potassic alteration at Larap, Philippines, is below. The composition of this chlorite (Cha54Cli45Pen1) con- also described by Frost (1965). Late sericite-clay formed at trasts with that of the earlier association (see Table 1). It is on the contact with the diorite and breccia, while epidote par- average richer in Mg. These later veins preferentially occupy tially developed after K feldspar. Fluorine- and Mg-rich chlo- north-south and N 35°–52° E subvertical fractures. Amphi- rite (Cha29Cli71/F3.6Cl0.4OH12) stringers are widespread and bole occurs only very sparingly throughout the mine area. cut breccia as well as aplite dikes. The last hydrothermal as- Epidote and chlorite replacement may have obscured its for- sociation consists of east-west, REE-rich jasperoid-apatite- mer presence. Late chlorite is most abundant along the hang- calcite-(siderite) open fracture fillings, mainly observed at La ing-wall contact of the marl unit. Late calcite veins are most Primorosa. Calcite stringers also related to this pulse cut the common in and around the marl unit. first generation of andesite dikes. At Chinforinazo Centro, a highly fractured endoskarn zone projects to the west along the limestone-diorite contact. Mineralization Relict intrusive textures are locally well-preserved (see Fig. 4i). Much of the igneous hornblende was transformed to acti- Mineralization associated with the prograde skarn event nolite, and the rock is cut by late Mg-rich chlorite stringers The massive iron oxide replacement body is relatively and prehnite veins after epidote. competent. Its geometry and structural control is depicted in Figure 7a. The orebody dips shallowly to the southwest in K feldspar-biotite association the northern part of the mine and more steeply in the south. Breccia clasts are cemented by a K feldspar-(biotite)-quartz The thickest ore section trends north-south through the mid- association. Locally, breccia borders grade outward to K dle of the orebody, corresponding to the projection of the feldspar-quartz stockworks and local solution horizons. The underlying diorite nose, which penetrated along the struc- ≥50-cm-wide K feldspar-(biotite)-quartz veins that extend from tural break between the western limestone and the eastern

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594200 594400 594600 594800 595000 595200 595400 Deposited with Deposited with 2144000 early calc-silicate early calc-silicate alteration alteration

2143800 Pinch-Out against Limestone (Mined-Out) Primorosa Fault 2143600

2143400

2143200

Poniente Fault 2143000 Encantada Fault

2142800

2142600

2142400 La Chula Fault 80% Mt >10 Piedras Negras Fault >than 70 m 2141800 7a: Ore Zone Thickness 7b: Magnetite Distribution 7c: Mt/Hm Ratio

Introduced Deposited with Introduced with with early early calc-silicate early calc- calc-silicate alteration silicate alteration alteration as in plagioclase and plagioclase with late potassic alteration as quartz and K-feldspar

0.9 to 1.5% 1.3 to 2.0% 12 to 15% 1.5 to 2.0% 2.0 to 3.0% 15 to 20% >2.0% >3.0% >20% 7d: Na2O Distribution 7e: Sulfur Distribution 7f: SiO2 Distribution

Introduced Introduced by early Explanation with late potassic calc-silicate Projected Geology: alteration as alteration, and orthoclase re-precipitated Breccia by late potassic alteration as Diorite fluorapatite Volcanoclastic Limestone Marl

Foowall Fault Fault

Geologic Contact Trace Iso-Value Contour DDH Location

0.7 to 1.5% UTM 2141800 Coord. 1.5 to 2.0% 0.1 to 0.2% >2.0% 0.2 to 0.3% >0.3% Major Oxide Distribution 7g: K2O Distribution 7h: Phosphorus Distribution in Massive Ore

FIG. 7. Isopach map, magnetite, hematite, and fluid channels in massive ore. Lower right panel shows projected geologic controls.

0361-0128/98/000/000-00 $6.00 545 546 ZÜRCHER ET AL. volcano-sedimentary footwall units. This structure appears to Atomic absorption analysis and a fire assay on a pyrite sep- have been the most important mineralizing channel in that arate by Jacobs Lab in Tucson, AZ (Cu = 0.17%, Ni = 0.15%, the orebody reaches a thickness in excess of 50 m in this area. Co = 0.12%, Zn = 0.01%, and Au traces) confirmed that Other +30-m-thick ore sections are located along the north- pyrite is rich in cobalt and nickel, and that it may contain gold. striking Poniente and Norte faults. Overall, gold is not anomalous in the deposit. The only ap- The sections in Figure 2 illustrate the lithologic control on preciable in situ gold (up to 1 g/t) occurs in a small magnetite- the orebody. The orebody is confined to the 35-m-thick marl pyrite vein adjacent to the eastern flank of the breccia at La unit located 200 m above the intrusion. It follows stratigraphy Chula. However, drilling in the pyrite-rich tailings has indi- closely and pinches out within this unit to the west, north, and cated sections of up to 0.25 g/t Au (Colín, pers. commun.). east. Iron oxides replaced limestone beds and to a lesser ex- The Zn and Cu values suggest that pyrite contains chalcopy- tent, pyroxene, garnet, and plagioclase bands within the marl rite and sphalerite inclusions. Sphalerite, galena, and marca- unit. West of the Poniente fault, limited replacement of the site were not observed in this study but were reported by M. Lower Tepalcatepec sequence is evident in two places along Rivas-Sanchez (unpub. repts., 1988, 1989, 1991) in trace the contact with the reef limestone lenses. amounts. At the Shinyama orebody in Japan, Uchida and The contact of the ore with the eastern volcano-sedimen- Iiyama (1982) detected small amounts of sphalerite included tary footwall consists of a metasomatic plagioclase band, in pyrrhotite and chalcopyrite. At Peña Colorada, the con- which contains disseminated magnetite and sparse pyrite. centration of zinc in sulfides does not appear to account for Small garnet-magnetite-calcite masses are present below this the zinc budget found in massive ore (109 ppm). Microprobe zone. The contact with the western limestone footwall is analyses of late epidote and chlorite from the calc-silicate sharp. The hanging-wall contact zone is more diffuse. It con- event (see Table 1) suggest that Zn may be hosted by these tains abundant chlorite and plagioclase, cut by fine-grained minerals, but this is difficult to assess because zinc concen- magnetite-(pyrite)-calcite veins. Small isolated magnetite trations are below detection limit (<300 ppm). bands and veins do occur in the hanging wall above the main manto. Rare magnetite rims on limestone clasts occur as far Mineralization associated with the retrograde event upsection as in the Cerro de la Vieja Conglomerate. A 20-m-wide but laterally extensive (±90 Mt) endoskarn Massive ore is dominated by euhedral magnetite (36 wt % zone with an average of 26 wt percent disseminated iron avg) and lesser bladed specular hematite (7 wt % avg). Sulfides oxide occurs along the diorite-limestone contact. The diorite constitute less than 5 vol percent of the massive ore zone. Cal- hosts coarse-grained euhedral magnetite disseminations and cite, dolomite, and chlorite are the main interstitial con- magnetite-hematite-(pyrite) stockworks. Microprobe analyses stituents. The most frequently seen replacement textures are on magnetite from this zone indicate higher contents of Ti, magnetite after calc-silicates (see Fig. 4d) and hematite after Al, and Mn than magnetite from the prograde event (see magnetite. Pyrite commonly replaces hematite and more Table 1), consistent with the composition of the intrusive host rarely magnetite. Discrete subhorizontal bands of minor mag- it replaced. Kiseleva and Matreyev (1967) report similar com- netite-abundant specular hematite-lesser pyrite occur mainly positions from endoskarn magnetite at the eastern Sayan in the central and upper parts of the orebody. Here, sporadic Mountains in Siberia. inclusions (exsolution?) of chalcopyrite occur in magnetite grains, and pyrite exhibits a “feathery” habit that consists of Mineralization associated with the pseudomorphs after specular hematite (see Fig. 4e and h). K feldspar-biotite-quartz association However, some pyrite pseudomorphs are barrel shaped, sug- At Peña Colorada, this alteration event is essentially barren. gesting replacement of preexisting pyrrhotite. Pyrrhotite may However, pyrite does occur sporadically in K feldspar-(bi- have formed after reduced, shaly horizons as argued by Mein- otite)-quartz veins outside the hydrothermal breccia. Here, ert (1983). The interlocking nature of feather pyrite causes outermost vein selvages locally consist of finely disseminated problems in magnetite recovery. Similar textures have been magnetite. In contrast, where the breccia cuts through min- reported by Kelly and Turneaure (1970) and Imai et al. (1978) eralized rock, abundant massive magnetite (commonly after from other replacement deposits. Outside these hematite-rich hematite) fragments occur in addition to calc-silicate clasts. bands, abundant magnetite-lesser hematite occurs throughout Ubiquitous fine-grained magnetite occurs in the silica- the entire thickness of the manto, but crystal size changes flooded breccia matrix. Because of the hardness and abra- from medium grained in the lower and central parts, to fine siveness of the silicated and silicified material, mineralized grained toward the top of the orebody. Rare concordant mil- sections of the breccia are usually refractory. Within the limeter- to centimeter-wide granular pyrite bands occur close breccia, late (biotite)-K feldspar-magnetite-(hematite)-quartz- to the base and top of the orebody (see Fig. 4f). Magnetite also pyrite-(chalcopyrite) veins cut both fragments and matrix. occurs as disseminations in isolated calc-silicate bands within The southern part of the mine hosts massive iron oxide veins the manto. In these zones, it locally developed along cleavages along the Piedras Negras fault and breccia. These veins are of pyroxene and garnet crystals. Two such pyroxene-magnetite adjacent to aplite dikes and associated K feldspar-quartz al- bodies occur at Chinforinazo Sur and La Primorosa. Here, py- teration and could be the product of local solution and repre- roxene and magnetite are cut by pyrrhotite-pyrite-chalcopyrite cipitation of a preexisting iron oxide body. A small en- veins (see Fig. 4g). Pyrrhotite shows a granular habit and chal- doskarn(?) body does occur at depth (see section A-A', Fig. copyrite fills pyrite and pyrrhotite interstices. West of the 2). Compared to magnetite deposited with the calc-silicate Poniente fault, abundant chlorite surrounds very fine grained event, magnetite from the potassic association appears to magnetite and renders the mineralization uneconomic. have higher Ti and Mn contents (see Table 1). No trace

0361-0128/98/000/000-00 $6.00 546 PEÑA COLORADA IRON SKARN, COLIMA, MEXICO 547 metals were detected in pyrite or chalcopyrite from the potas- the north-south footwall structural break where the ore zone sic association. is the thickest. The Poniente fault also exerts control on min- Two magnetite-barite prospects located at Cerro Pelón del eralization, but the north-south structural break appears to Norte and Cerro Pelón del Sur deserve mention (see Fig 1). have been the most important mineralizing channel. High Up to 2-m-thick veins consist of moderate magnetite, abun- hematite to magnetite ratios along the central and Poniente dant barite, and quartz. At Cerro Pelón del Sur, a few thou- faults could be documenting the introduction of relatively sand tons of barite were recovered commercially. These veins more oxidized fluids in these areas. exhibit up to 50-m-wide clay-sulfate alteration halos. XRD Good linear correlations between Na2O, SiO2 (r = 0.84), analyses on six samples at the University of Arizona indicate a and Al2O3 (r = 0.95) imply that the bulk of silica and alumina complex association of kaolinite, natrolite, psilomelane, na- are constituents of plagioclase. Metasomatic plagioclase (Fig. troalunite, and gypsum, with subordinate amounts of mont- 7d) appears to be strongly controlled by the same north-south morillonite, barite, and alunite. structure. Figure 7e illustrates the sulfur distribution. Most sulfur is contained in pyrite, and its distribution was probably Supergene minerals controlled more by favorable lithologic units than structure Supergene minerals include very sparse delafossite (Fe2CuO4) within the marl unit. K2O (i.e., K feldspar) is also enriched in and magnetite that has been partially transformed to martite the central part of the orebody (Fig. 7g). Nevertheless, its along cleavages. Very rarely, chalcopyrite grain rims are re- highest concentrations are somewhat shifted to the west with placed by covellite or chalcocite. Dendritic manganese oxides respect to sodium and occur over the main breccia body. Fig- are present in the outermost parts of the alteration halo. Sec- ure 7f shows that at Chinforinazo Centro and La Primorosa, ondary earthy hematite and limonite occur only in the Pri- the breccia appears to exercise further control on K2O and morosa pit, which is the portion of the orebody that was SiO2, (i.e., K feldspar and quartz). Figure 7h depicts the phos- mined first and has been exposed to weathering the longest. phorus distribution. In general, phosphorus highs correspond to areas where late fluorapatite-bearing jasperoid-calcite Massive Orebody Bulk Chemistry veins are most abundant; namely, in the northern sector of Discrimination between original and added or removed the orebody (La Primorosa) and along the Poniente fault. components was difficult because of the high variability and lack of fresh protolith. Alteration mineral abundances such as Zoning of trace elements plagioclase, K feldspar, and quartz (estimated from wt per- Ti contents in iron oxide related to the calc-silicate event cent oxides) could not be discriminated with confidence. are more or less constant throughout the orebody, while Mn However, bulk chemistry provided information on the loca- and V concentrations increase with distance from the diorite tion of the most important mineralizing channels about the intrusion (Fig. 8a, b, and c). Like titanium, chromium (Fig. causative diorite intrusion. The major element bulk distribu- 8d) is more or less evenly distributed, although some fraction tion maps in Figure 7 represent subhorizontal plan sections appears to have been added along north-south faults. Zn (in derived from weighted average percent concentrations across sphalerite and epidote?) was also introduced via north-south the entire thickness of the massive iron seam. Major element channels (Fig. 8e). Cu (as chalcopyrite) was introduced along distribution maps of the massive orebody are based on 816 north-south and northwest faults (Fig. 8f) but also appears to core sample analyses supplied by the Peña Colorada staff. be lithologically controlled. The Co/Ni ratio (in pyrite) (Fig. The trace element distribution contour maps of Figure 8 8g) shows that Co concentrations increase away from the were constructed in the same fashion to investigate overall diorite, with respect to Ni. metal zoning about the diorite. Threshold values between Assessment of the mineralization associated with the K-Mg anomalous and background populations are derived from the event (Mo, Bi, Ag, Sb, and Hg) was not possible because of frequency distribution of log-transformed concentrations (Rose the limited number of anomalous samples. Nevertheless, et al., 1979). Trace element maps are based on ICP analyses these elements do appear to be related to the metal-poor of 56 14-m ore composites, done at Chemex Labs in Vancou- potassic event. Figure 8h illustrates the possible north-south ver, British Columbia. Trace element host minerals were control on molybdenum. Discrete Mo mineral phases were identified with the aid of the microprobe. Of the 30 elements not identified. analyzed, 18 were significantly anomalous to test zoning pat- terns. Geochemistry of Igneous Rocks and the Hydrothermal System Distribution of major elements Figure 7b shows the distribution of magnetite in the mas- Tectonic affinity of igneous rocks sive orebody. Magnetite and hematite proportions were esti- Table 2 lists the major oxide composition, CIPW norm, and mated from magnetic Fe measurements and wet chemistry trace element concentrations of representative andesite tuff, iron analyses of the 816 core samples provided by the mine tuffaceous sandstone, and sandstone from the Tepalcatepec staff. We did not take into account the contribution of total Formation. Compositions suggest that andesite tuffs are of iron from other Fe-bearing minerals, but it is comparatively tholeiitic affinity, typical of a primitive island-arc environ- small within the massive iron oxide body. From Figure 7b and ment. These tuffs are also alkali-calcic, characteristic of a the isopach map of Figure 7a, it is evident that the magnetite- back-arc extensional regime. The tectonic affinity of these rich zones correspond to the thinner parts of the manto. Sim- rocks is in agreement with other geochemical studies carried ilarly, Figure 7c shows low magnetite to hematite ratios along out on rocks of this back-arc basin (Lapierre et al., 1992;

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Introduced with In iron oxide, Titanium (in iron iron oxide inherited from oxide) proximal volcaniclastic to intrusion with host respect to Vanadium

1200-1500 ppm 280-400 ppm 5-10 1500-1800 ppm 400-600 ppm 10-15 >1800 ppm >600 ppm >15 8a: Manganese Distribution 8b: Vanadium Distribution 8c: Titanium/Vanadium Ratio

In iron oxide, In epidote (?), Copper inherited from introduced mainly (in chalcopyrite) volcaniclastic host along N-S faults controlled mainly and introduced by N-S and NW along N-S faults faults

20-55 ppm 120-160 ppm 100-180 ppm 55-150 ppm 160-200 ppm 180-320 ppm >150 ppm >200 ppm >320 ppm 8d: Chromium Distribution 8e: Zinc Distribution 8f: Copper Distribution

Cobalt distal to Introduced as sulfide (?) Explanation intrusion along N-S faults Projected Geology: with respect with late potassic to Nickel alteration Breccia (both elements in pyrite) Diorite Volcaniclastic Limestone Marl

Foowall Fault Fault

Geologic Contact Trace Iso-Value Contour DDH Location UTM 4-8 2-5 ppm 2141800 Coord. 8-12 5-8 ppm >12 >8 ppm Trace Element Distribution 8g: Cobalt/Nickel Ratio 8h: Molybdenum Distribution in Massive Ore

FIG. 8. Bulk trace element zoning in Peña Colorada massive ore. Lower right panel shows projected geologic controls.

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TABLE 2. Composition of Peña Colorada Rocks (the hypothetical marl and average ore used for gains and losses calculations are listed as well)

Host rocks Intrusions Postmineral intrusions

Andesite Tuffaceous Sandstone Hypothetical Diorite Diorite Average Aplite Andesite tuff sandstone marl3 dike ore4 dike

Wt percent oxide SiO2 51.90 51.70 57.10 10.39 58.20 50.00 11.81 73.80 55.40 TiO2 1.16 1.07 1.08 0.23 1.23 1.63 0.19 0.17 1.09 Al2O3 16.40 16.70 18.00 3.28 16.60 14.80 3.59 14.80 17.80 1 Fe2O3 12.60 11.20 3.36 2.52 3.57 11.50 40.23 0.78 7.85 MgO 3.98 4.49 2.84 0.80 4.02 3.88 1.25 0.19 2.83 MnO 0.14 0.13 0.16 0.03 0.13 0.24 0.16 0.04 0.27 CaO 1.47 1.55 1.61 45.13 6.15 10.40 23.44 0.33 2.58 Na2O 4.90 3.58 4.77 0.98 7.56 4.66 0.91 6.07 7.49 K2O 3.80 5.51 5.25 0.76 0.36 0.62 0.72 2.57 1.26 P2O5 0.25 0.27 0.32 0.05 0.30 0.40 0.24 0.08 0.59 S 1.27 LOI 2.70 3.20 4.50 35.77 2.30 2.30 1.30 1.70 Sum 99.58 99.61 99.22 100.00 100.50 100.50 100.00 100.20 99.01

CIPW norm Quartz 1.31 27.86 Corundum 1.60 0.76 Zircon 0.05 0.03 0.02 0.06 Orthoclase 2.13 3.67 15.20 7.45 Albite 63.97 39.43 51.36 63.38 Anorthite 10.29 17.63 1.16 9.14 Diopside 8.44 17.28 Enstatite 3.56 1.65 0.47 3.10 Forsterite 1.78 2.76 Ilmenite 0.29 0.55 0.09 0.60 Sphene 2.65 3.29 Rutile 0.12 0.77 Apatite 0.69 0.93 0.19 1.37 Sum 93.85 85.77 98.08 89.41

Trace elements (ppm)2 Y <10.0 10 30 30 20 20 20 Zr 160 150 170 32 240 150 100 310 Nb 30 30 20 6 20 20 20 30 U 0.50 0.64 1.16 0.10 1.22 0.39 0.55 0.99 Rb 120 170 160 24 <10.0 20 30 20 Sr 490 380 710 98 300 500 32 60 530 Ba 900 1100 860 180 40 70 17 140 140 La 15.49 11.81 14.59 3.10 21.18 14.36 10.74 8.31 24.28 Ce 28.16 21.95 40.64 5.64 51.95 37.11 20.68 32.73 59.07 Nd 12.06 9.16 20.72 2.41 22.63 21.19 8.45 11.09 25.76 Sm 3.58 3.51 6.13 0.72 4.90 5.41 1.78 2.26 5.42 Eu 1.30 1.59 1.98 0.26 1.71 1.82 0.39 0.65 1.91 Gd 4.19 2.45 6.48 0.84 7.51 7.93 3.32 2.63 8.17 Tb 0.41 0.35 0.75 0.08 0.86 0.97 0.33 0.25 0.85 Dy 1.58 1.93 3.12 0.32 3.85 4.21 1.07 3.48 Ho 0.33 0.41 0.63 0.07 0.66 0.85 0.27 0.23 0.69 Er 1.01 1.22 1.87 0.20 2.45 2.54 0.87 0.72 2.17 Tm 0.13 0.17 0.25 0.03 0.34 0.32 0.10 0.11 0.28 Yb 0.79 1.04 1.53 0.16 2.11 2.01 0.61 0.70 1.66 Lu 0.12 0.17 0.22 0.02 0.34 0.33 0.09 0.12 0.26 V 74.47 77.92 66.04 14.91 66.42 175.41 279.32 1.80 35.25 Cr 42.14 41.78 37.99 8.44 3.62 2.93 57.07 0.78 1.88 Co 10.15 12.62 2.44 2.03 7.99 21.06 126.84 0.45 4.60 Ni 150.17 149.23 38.98 30.06 26.91 150.52 57.45 8.30 98.08 Cu 13.70 34.47 40.68 2.74 20.79 61.26 240.80 71.41 27.23 Zn 35.84 95.31 198.45 7.17 39.88 81.60 168.85 19.40 204.90 Mo 0.43 2.46 0.53 0.09 3.48 0.93 2.06 2.60 5.57 Sn 0.08 0.15 0.02 0.02 0.04 0.03 0.05 Hg 0.09 0.11 0.17 0.02 0.09 0.28 1.28 0.15 0.19 Pb 2.24 2.35 3.69 0.45 3.14 3.04 2.14 9.04 17.52 Bi 0.06 0.05 0.07 0.01 0.05 0.25 2.21 0.23 0.11

1 Total Fe reported as Fe2O3 2 Trace elements reported to two significant figures analyzed by ICP at the University of Arizona; major and HFS elements analyzed by XRF at XRAL Activation Services in Ann Arbor, Michigan 3 Hypothetical marl calculated from one part andesite tuff and four parts calcite 4 Average ore corrected for carbon dioxide loss during analytical procedure

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Ortiz-Hernandez, 1992; Centeno-García et al., 1993). Table 2 fresh andesite dike exhibits a small Eu anomaly and is more also lists the composition, CIPW norm, and trace element fractionated than the diorite (Ce/Ybcn = 9.5). concentrations of the most unaltered diorite, diorite dike, and aplite samples. Their compositions indicate that these intru- Rare earth elements in the hydrothermal system sive phases have calc-alkaline, metaluminous, continental arc Individual REE patterns for nine ore samples and a late affinity. The composition of the diorite is consistent with jasper-calcite vein of the potassic event are shown in Figure rocks found elsewhere along this continental arc (Koehler et 9b. Results are listed in Table 3. For comparison, the shaded al., 1988). Similarly, the tectonic affinity of Paleogene basaltic area of Figure 9a reproduces the ore range. Microprobe re- andesite dikes is calc-alkaline. sults and good REE versus phosphorus correlations indicate that much of the REE are in apatite. The patterns show a Rare earth elements in igneous rocks Ce/Ybcn ratio of around 10, which corresponds with the ap- Rare earth elements were determined on seven represen- atite signature of Taylor and McLennan (1985). With the ex- tative wall-rock and intrusive samples at the W. M. Keck Lab- ception of a more prominent Eu anomaly, REE contents of oratory, University of Arizona. Results are listed in Table 2. ore samples resemble those of the andesitic tuffs of the vol- Chondrite-normalized REE patterns (Anders and Ebihara, cano-sedimentary host. This is consistent with an igneous 1982) of igneous rocks are graphically illustrated in Figure 9a. rock source. Average ore plots very close to the aplite signa- Andesite tuff samples are considered representative of the ture and most enriched ore mimics the intramineral diorite wall-rock signature at Peña Colorada. Their REE patterns dike pattern. This behavior suggests local remobilization of plot within the wide-dotted range. They exhibit slight light preexisting REE contents. Solution and reprecipitation by REE enrichment (Ce/Ybcn = 7.4) and no Eu anomaly. Diorite postmineral F-dominated fluids of the potassic alteration and an intramineral diorite dike show a restricted composi- event is supported by the substantial enrichment in the light tional range, no Eu anomaly, and Ce/Ybcn ratios of 4.9, and REE exhibited by fluorapatite-bearing jasper-calcite veins. 6.6, respectively. In comparison to the wall-rock signature, the diorite and diorite dike exhibit slight enrichment in the heavy Stable Isotopes and the light REE, respectively. Compared to the diorite, the Stable isotope compositions were determined on 97 min- aplite exhibits a negative La anomaly, a small negative Eu eral separates using conventional techniques (Brown, 1983; anomaly, and a larger Ce/Ybcn ratio of 12.6. Furthermore, the Sharp, 1990). Isotopic ratios were measured with a Finnigan aplite is comparatively depleted in REE and exhibits a slightly Mat Delta S mass spectrometer at the University of Arizona. concave-upward heavy rare earth pattern. The postmineral Analytical data are presented in Table 4. One to three extrac- tions per sample were carried out. The reported results indi- cate average values, and the attached uncertainties express

1000 the maximum spread between spectrometer standard errors. a) REE Patterns of Peña Colorada Rocks Carbon and oxygen isotope compositions of calcite were ob- tained from unaltered limestone, metamorphic event, iron 100 oxide ore, and postmineral potassic veins (Fig. 10a). Calcite Explanation isotope signatures form two groups. The first one clusters δ18 δ13 Andesite dike around O and C values of 11.3 and –2.5 per mil, and the 10 Aplite second one averages δ18O and δ13C values of 19.1 and –11.5 Ore range Diorite dike per mil, respectively. Oxygen isotope compositions were de- Sample/Chondrite Diorite termined on pyroxene, plagioclase, magnetite, hematite, and Andesite tuff range 1 epidote from the calc-silicate mineral association and on K feldspar, magnetite, quartz, and epidote from the postmineral potassic association. These mineral separates are representa- 0.1 tive of intrusion, endoskarn, footwall, ore, and hanging-wall Tb Ho Ce Yb Sm Tm La Lu Eu Er Dy Nd Gd zones. Calc-silicates show an average δ18O value of 6.0, with a 1000 δ18 b) REE Patterns of Peña Colorada Ore total range of 4.7 per mil. The O values of minerals from Explanation the potassic association are overall somewhat heavier. They Ore samples: range between 5.0 and 9.3 and average 7.3 per mil. The re- 100 Late jasper-calcite 13G1 sults are illustrated in Figure 10b. We calculated oxygen iso- 512 tope temperatures for several mineral pairs based on the 678 809 work of Bottinga and Javoy (1975), Matthews et al. (1983a), 10 761 528 and Clayton and Kieffer (1991). Most intermineral fractiona- 403

Sample/Chondrite tions, however, indicated that equilibrium between the min-

1464 increasing conc. 293 1 eral pairs was not attained. In order to constrain oxygen iso- Igneous rock range tope compositions of coexisting water (see Table 4) we relied on temperatures obtained with the pyroxene-garnet ther- 0.1 mometer of Powell (1985) and results reported in the litera- Tb Ho Ce Yb Tm Sm La Eu Lu Er Nd Gd ture from other skarn systems (Einaudi et al., 1981). Plausi- FIG. 9. Peña Colorada rare earth element patterns. a. Igneous rocks. b. ble ranges and selected temperatures are shown in Figure Selected ore samples. 10c and given in Table 4. With the exception of epidote-H2O,

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Table 3. Rare Earth Element Contents of Ore Samples

Iron oxide ore samples Postmineral Sample no. 293 1464 403 528 761 809 678 512 13G1 jasper-calcite vein

REE (ppm)1 La 2.22 1.37 3.45 11.26 8.93 18.83 7.50 13.59 27.65 95.47 Ce 4.34 3.17 6.93 15.56 17.32 32.89 16.61 34.85 51.06 123.83 Nd 1.89 1.86 2.64 4.46 6.44 10.22 9.18 18.03 20.44 31.84 Sm 0.26 0.36 0.43 0.84 1.13 1.40 2.35 4.65 4.49 4.07 Eu 0.08 0.07 0.08 0.20 0.27 0.21 0.58 1.27 0.77 1.13 Gd 0.80 0.74 1.13 2.06 2.40 3.81 3.29 6.66 8.85 12.10 Tb 0.07 0.08 0.09 0.16 0.22 0.30 0.36 0.72 0.93 0.70 Dy 2.17 Ho 0.05 0.06 0.06 0.13 0.17 0.21 0.30 0.62 0.86 20.43 Er 0.16 0.18 0.22 0.47 0.53 0.66 1.03 1.99 2.57 1.46 Tm 0.02 0.03 0.03 0.06 0.06 0.06 0.12 0.23 0.30 0.16 Yb 0.12 0.12 0.15 0.38 0.30 0.37 0.81 1.41 1.86 0.92 Lu 0.03 0.02 0.03 0.06 0.04 0.05 0.12 0.21 0.27 0.14

1 Analyzed by ICP at the University of Arizona we calculated mineral-water fractionations based on the work subeconomic iron oxide endoskarn. Aplitic intrusions pro- of Bottinga and Javoy (1973), Friedman and O’Neil (1977), duced postmineral brecciation and related potassic alteration. and Matsuhisa et al. (1979). For epidote-H2O, we used the The thickness of the volcano-sedimentary rock pile above the zoisite-water oxygen fractionation expression of Matthews et diorite is estimated at 5,300 m from type localities in the re- al. (1983b) and corrected its compositional influence on oxy- gion, suggesting a pressure of emplacement of about 1,500 gen fractionation based on grandite behavior (Taylor and bars. Late-stage aplite injection and brecciation in the mine O’Neil, 1977). Hydrogen isotope compositions were obtained area suggests that fluid pressure eventually exceeded lithosta- on epidote from both the calc-silicate and postmineral potas- tic pressure. sic events. It was the only hydrous mineral that could be sep- Northeast-vergent deformation affected the region in the arated effectively. Epidote from the calc-silicate association Late Cretaceous. Late Cretaceous reverse faults exhibit shows δD values that range from –31 to –75 per mil. Results trends consistent with the northeast-directed subduction of are listed in Table 4. Epidote-H2O hydrogen isotope fraction- the Farallon plate. This compression evolved into a trans- ations were calculated after Graham et al. (1980). Figure 10d pressional system, along which the Chortis block translated displays the results. A few sulfur isotope compositions were southeastward (Sedlock et al., 1993). Mineralization at Peña determined on pyrite (and one pyrrhotite) from various areas Colorada appears to be related to this transition from com- in the ore zone, veinlets in wall rock, and the hydrothermal pressional to strike-slip stress regimes, as reflected by a shift breccia. Pyrites of the calc-silicate association exhibit δ34S val- in fracture orientations that control the alteration associations ues that average at 5.8, with a range of 2.1 per mil. Two (see Fig. 5). pyrites of the potassic association appear lighter at around 4.5 per mil. Table 4 and Figure 11 show the sulfur isotope results. Bulk gains and losses in the massive iron oxide body We attempted to model the relative gains and losses that Discussion occurred during replacement of the marl that hosts the mas- sive orebody. Such an estimated composition is crude given Geologic history the known heterogeneity and alteration that affected the host The middle Cretaceous Tepalcatepec Formation defines a protolith. Nevertheless, it provides an additional basis for transgression-regression cycle, beginning with generation of mass transfer comparison with other iron systems. The com- evaporites, followed by formation of marine volcanoclastic- positions of average ore and a hypothetical marl used for the carbonate sequences, and concluding with deposition of sub- estimation of gains and losses are shown in Table 2. The ore aerial volcanic rocks. The tholeiitic affinity and the REE pat- composition was corrected for CO2 lost during analysis. The terns are compatible with a primitive arc setting (Fig 9a). This marl composition was generated by adding four parts lime- basin was subsequently intruded by calc-alkaline phases of a stone to one part tuff. These proportions are roughly consis- continental arc that developed in the Late Cretaceous. The tent with the Al2O3 and TiO2 contents of the ore, assuming Peña Colorada diorite is part of this event. Rare earth geo- that these elements were approximately conserved. Figure 12 chemistry suggests that diorite fractionation produced a shows the mass changes. The model suggests that among the restitic fluid, which is represented by aplite in and around the more abundant elements, iron, manganese, and phosphorous mine area. The diorite intrusion produced an early pyroxene- are enriched by a factor of four or more, whereas over half of garnet metamorphic halo, followed by pyroxene-garnet-pla- the calcium and volatiles are lost. Other major elements, no- gioclase metasomatism and associated iron oxide exoskarn. tably silica, show changes of less than a few tens of a percent. This early alteration association was overprinted by late-stage This is consistent with iron oxide (and apatite) replacement of epidote-chlorite-prehnite fracture-controlled alteration and a the carbonate component of the host rock and approximate

0361-0128/98/000/000-00 $6.00 551 552 ZÜRCHER ET AL. 1 S 34 δ [4.45±0.06] +5.99±0.05 +4.64±0.06 +5.99±0.07 +5.10±0.05 +6.78±0.05 +5.56±0.04 +5.39±0.02 +3.61±0.05 +5.87±0.04 +6.13±0.05 1 C 13 δ 1 D Ep Cc Py Ep Cc Py/[Po] δ 2 O 2 H (400°C) (400°C) D δ Ep Ep (500°C) Qtz (300°C) (300°C) ham et al (1980), and Matthews (1983b) Cc Cc 2 ) °° (°/ O (400°C) (400°C) 2 H Ep Ep O 18 δ (450°C) (450°C) Mt Mt Calculated (500°C) (500°C) 4. Stable Isotope Results 7.00±0.08 4.72 7.19±0.217.25±0.177.24±0.10 4.91 4.97 4.96 ABLE T 7.64±0.13 6.21 –49.04 –84.94±0.20 1 ) °° (°/ 11.23±0.09 5.65 –1.96±0.05 21.35±0.05 15.77 –13.48±0.03 13.12±0.07 7.54 –2.82±0.03 17.07±0.07 11.49 –8.24±0.04 22.29±0.06 16.71 +2.96±0.03 mineral O 18 δ 4.69±0.29 11.38±0.034.92±0.16 8.51±0.03 11.20 11.43 5.80 2.93 –3.15±0.03 –3.36±0.07 +6.15±0.07 4.98±0.09 12.69±0.09 11.49 7.11 –0.38±0.15 5.18±0.08 11.16±0.05 11.69 5.58 –3.13±0.05 +6.02±0.07 4.95±0.54 11.50±0.05 11.46 5.92 5.61±0.04 7.13±0.27 12.12 5.70 –29.10 –65.00±0.12 6.35±0.03 7.94±0.25 12.86 6.51 –17.80 –53.70±0.56 [7.02±0.08] 8.40±0.10 4.95±0.04 5.61±0.03 7.28 11.46 4.18 4.58 -31.32±0.39 7.94±0.03 4.35±0.078.52±0.31 3.87±0.10 5.89±0.06 6.15±0.09 6.82 7.40 10.86 10.38 4.46 4.72 4.39 –7.14 –31.51±0.38 –43.04±0.28 7.78±0.04 5.55±0.26 6.20±0.09 6.66 12.06 4.77 –38.71 –74.61±0.40 9.34±0.07 8.00±0.16 8.93 6.57 –39.86 –75.76±0.91 5.32±0.09 8.37±0.26 4.59±0.27 5.55±0.10 7.25 11.10 4.12 –11.92 –47.82±0.16 6.57±0.02 5.62±0.19 12.13 5.82±0.04 3.91±0.05 10.42 7.15±0.02 5.17±0.04 7.47±0.09 11.68 6.04 –31.38 –67.28±0.44 Ore H wall 6.39±0.21 4.11 H wall 8.21±0.08 4.44±0.14 6.46±0.18 7.09 10.95 5.03 –12.78 –48.68±0.85 Breccia Footwall 19.02±0.07 13.44 –12.67±0.06 Footwall 8.49±0.09 6.21 Ore level 7.29±0.11 5.01 Intrusion 7.59±0.23 3.88±0.12 5.34±0.09 6.47 10.39 3.91 –31.86 –67.76±0.56 Endoskarn [8.97±0.22]                                              Analytical procedures for O after Sharp (1990), and H, C, S Brown (1983) Mineral-water fractionations calculated from Bottinga and Javoy (1973), Friedman O’Neil (1977), Matsuhisa et al (1979), Gra 1 2 54801 54802 120005 SampleCa-Na association54801 Location 54802 Px Plag Mt/[Ht] Cc110807 Ep35705b 101005 68625 Plag K-Mg association Ksp Mt Cc Ep Qtz Ksp 120003 101003 35705a 98201a 68624 120001 17503a 120008 88202 54807 54808 54809 110803 92601 92606 92610 101015 101009 54817a 54817b 17503b 54819 98201b 54807 54808 54817 68628 54823 68613 35702 35707 68628 54823a 54823b 17509 68614

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Explanation Late breccia pyrite Pyrite vein (late) Central ore zone pyrite

(PDB) South ore zone pyrrhotite South ore zone pyrite North ore zone pyrite

C Calcite -1 0 +1 +2 +3 +4 +5 +6 +7 +8

13 δ34 δ S

FIG. 11. Peña Colorada δ34S sulfide values.

conservation of the other components in calc-silicate phases. δ18 a) O Calcite(SMOW) Although calculated trace element concentrations in the hy- pothetical marl are less dependable, most trace elements show positive changes. The rare earth elements are uniformly enriched by a factor of about three, with the exception of Eu, which shows little change (perhaps due to differential redox control). Most metals, notably Co, Cu, Zn, and Mo, are en- riched by a factor of ten or more. Of the trace elements ana- lyzed, only Sr and Ba are depleted. Sr loss is comparable to that of Ca, whereas Ba loss may be due to selective recon- centration elsewhere (for instance as barite-magnetite veins). Rare earth elements and aplite petrogenesis The more fractionated but relatively depleted aplite REE pattern (Fig. 9a) suggests that the aplite formed from an aque- δ18 b) O Mineral(SMOW) ous solution that produced a restitic fluid. A REE pattern that

Mass Change Loss Gain

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

SiO2 TiO2 Al2O3 Fe2O3 MgO MnO

CaO

Na2O K2O P2O5 Sr δ18 c) O H2O(calculated) Ba La Ce Nd

Sm Eu

Gd Tb Ho Er

(calculated) Tm

O Yb 2 Hypothetical Marl vs. Average Ore Average Hypothetical Marl vs. Lu V D H

δ Cr Co Ni Cu Zn Mo Hg δ18 Pb d) O H2O(calculated) Bi

FIG. 10. Stable isotopes at Peña Colorada. a. δ13C vs. δ18O for calcites. b. -97% -90% -70% 0% 200% 1,000% 3,000% 10,000%30,000% δ18O values of alteration minerals. c. δ18O values of water in equilibrium with alteration minerals. Bars indicate how much the oxygen isotope signatures FIG. 12. Relative gains and losses between hypothetical marl and average 18 o a o a o o would shift with a change in temperature. d. δD vs. δ O for water in equi- Peña Colorada ore. Mass change = log[X -X (Al2O3 /Al2O3 )]/X , where X = librium with epidote. concentration in marl, and Xa = concentration in average ore.

0361-0128/98/000/000-00 $6.00 553 554 ZÜRCHER ET AL. closely matches that of aplite can be constructed for a differ- 435°C for samples 120003 and 98201, respectively. Even entiate derived by 90 percent Rayleigh fractionation of the though these temperatures appear reasonable, textural rela- diorite, based on fluid-silicate melt partition coefficients from tionships question deposition of these two minerals under Flynn and Burnham (1978). equilibrium conditions. Furthermore, the thermometer is calibrated for temperatures above 500°C. Similarly, the ther- Rare earth elements and hydrothermal fluid chemistry mometer of Matthews et al (1983a) gives plagioclase-epidote Rare earth element abundances provided information on temperatures of 467° and 408°C for samples 54817 and the chemistry of the mineralizing fluids at Peña Colorada. 68628, respectively. The thermometer of Clayton and Kieffer REE patterns of ore samples exhibit a more prominent Eu (1991) suggests a magnetite-calcite temperature of 564°C. anomaly than that of their igneous host. This may be due to Even though textural relationships suggest that the two min- preferential partitioning of Eu2+ into hydrothermal plagio- erals are in equilibrium, the temperature estimate seems too clase (Whitford et al., 1988) or Ba2+ interference related to high when compared to the temperature of 495°C obtained the formation of late quartz-magnetite-barite veins. The in- for this event with the pyroxene-garnet exchange thermome- tramineral diorite dike shows a small degree of enrichment in ter of Powell (1985). Carbon isotope results for calcite inter- the light REE with respect to the diorite (see Fig. 9a). After stitial to ore similarly point to a magmatic source. The δ13C Taylor and Fryer (1983), this could be due to alteration by versus δ18O values of calcite are shown in Figure 10a. Fresh Cl–-bearing fluids, consistent with microprobe and isotopic limestone below the ore has δ13C and δ18O values that are results obtained from the calc-silicate association. Further, very similar to other Cretaceous carbonates (Keith and both the diorite and diorite dike exhibit slight enrichment in Weber, 1964). Significant changes in limestone grain size the heavy rare earths. Taylor and Fryer (1982) have proposed occur only within a short distance of the contact with ore. Cal- that enrichment in the heavy rare earths may be a result of al- cite associated with early disseminated garnet, and calcite in- teration by F—bearing fluids. This enrichment coincides with terstitial to ore, form two distinct clusters below and to the the F-rich composition of the postmineral potassic associa- left of fresh limestone. The first of these is interpreted as the tion. Fluorapatite contained in jasper-calcite veins generates product of local decarbonation reactions during thermal the pronounced light REE enrichment. metamorphism, which produce extensive 13C loss but limited 18O depletion. The second probably reflects metasomatic in- Stable isotopes and fluid sources for the calc-silicate event δ13 filtration of CO2- or CH4-bearing fluids with external C The stable isotope data are consistent with several possible and δ18O signatures. The same two processes have been pro- sources of components, including the volcano-sedimentary posed by many workers for other skarn systems (see Valley, host rocks, the cogenetic intrusive rocks plus their derived 1986). In the high oxidation environment of Peña Colorada, fluids, and fluids equilibrated at moderate to high tempera- the carbon species introduced by the mineralizing fluids was 13 12 18 16 ture with sediment-dominated sources. most likely CO2, making the C/ C and O/ O ratios com- Temperature effects: In general, δ18O values (Fig. 10b) of patible with fluids derived from a magmatic source (Ohmoto minerals from the calc-silicate alteration association follow and Rye, 1979). Therefore, the average δ18O value (6.2‰) of the same fractionation order observed in many igneous and the water in equilibrium with ore calcite is believed to have metamorphic processes (Epstein and Taylor, 1967). Most been produced by the same magmatic fluid that generated minerals show gradual depletion in 18O from footwall to hang- the calc-silicate minerals of the metasomatic event. Figure 11 δ34 ing-wall rocks. This zoning is interpreted to be the result of a illustrates a narrow Spyrite range, with an average of 5.8 per temperature decrease away from the intrusion and/or mixing mil. The slight departures from the mean value are probably with external fluids. due to higher and lower temperatures of deposition. The δ34S Volcaniclastic source: The metamorphic pyroxene δ18O values are somewhat heavier than expected from an igneous value of 7.2 per mil most likely reflects the isotopic composi- source alone, since subaqueous extrusive rocks such as the tion of the volcaniclastic wall rock it replaced, since early py- ones found in the Tepalcatepec Formation commonly have roxene was formed during the rock-buffered thermal event δ34S values around 5 per mil (Ohmoto and Rye, 1979). There- (see Fig. 10b). Pyroxene-garnet thermometry point to tem- fore, identification of possible sources requires consideration peratures around 590°C for this contact metamorphic event. of the effects of 34S fractionation in the fluid from which it Magmatic water source: Field relationships, petrography, precipitated. The relatively lower temperatures of deposition and bulk composition changes indicate that Mg-rich pyrox- of pyrite (relatively late in the paragenesis) and the high oxi- ene, plagioclase, magnetite, calcite, and epidote are the prod- dation state of the ore (around the magnetite-hematite buffer) uct of a fluid-dominated system. With the exception of waters suggest that much of the sulfur was introduced in the form of 2– 2– in equilibrium with magnetite, oxygen isotope results suggest SO4 (Barnes and Kullerud, 1961). Such an SO4 -dominated 34 that the fluids responsible for this metasomatic association solution must have been substantially enriched in Stotal sulfur were close to equilibrium with the diorite and/or the intruded (Ohmoto, 1972) and probably originated from sources such as wall rock, at an average δ18O value of 6.0 per mil (see Fig. trapped seawater sulfate or evaporitic material similar to the 10c). This also coincides with oxygen signatures expected one contained in the lower part of the Tepalcatepec Forma- from intermediate to mafic igneous rocks (Ohmoto, 1986). tion. Sulfides deposited from such fluids generally exhibit Temperatures calculated from oxygen fractionations indicate, large variations in δ34S values but at Peña Colorada the range in most instances, that equilibrium between mineral pairs was in 34S/32S ratios is narrow. The uniform isotopic signature of not attained. The thermometer of Bottinga and Javoy (1975) the sulfides could be explained if the causative magma was suggests pyroxene-plagioclase temperatures of 402° and allowed to homogenize well after assimilation of trapped

0361-0128/98/000/000-00 $6.00 554 PEÑA COLORADA IRON SKARN, COLIMA, MEXICO 555 seawater or evaporite. A magmatic source has been suggested source is also implied from the two sulfur isotope results de- for other iron skarns, such as the Aldan-Shield deposits that termined on pyrite from the potassic association. The rela- exhibit uniform δ34S values around zero (Shepel and Goly- tively more 34S depleted breccia pyrite (see Fig 10) could shev, 1979). However, δ34S values around zero have also been have precipitated from a magmatic fluid produced again, dur- obtained from the basinal brines in the Salton Sea (McKibben ing degassing (Taylor, 1986). and Hardie, 1997). Preexisting iron oxide source: The similarities in 18O/16O ra- Carbonate-rich wall-rock source: The magnetite isotope tios of calcite and magnetite from the calc-silicate and the signature suggests that it formed from fluids more enriched in postmineral potassic alteration associations suggest a com- 18O than the solutions in equilibrium with other coexisting mon origin and most likely solution-reprecipitation of these calc-silicate minerals (including interstitial calcite). The un- two mineral phases by the latter event. This possibility is fur- usually heavy values for the magnetite require a sedimentary ther supported because δ13C values of calcite from the potas- source. Some of this might be achieved by reaction with the sic event coincide with calcite of the calc-silicate event. carbonate-dominated protolith (Fig. 10a, c) but that only pro- External source: The quartz signature is problematic. At vides a partial explanation because magnetite was deposited 500°C, the quartz-derived water is excessively depleted in in a fluid-dominated regime. Therefore, a fluid that equili- δ18O (5.0‰). Unrealistically higher temperatures of deposition brated with sedimentary or altered volcanic rocks at relatively would be required to shift the signature up to the level of en- high temperature is required to produce these heavy values. richment of water in equilibrium with other minerals of the Trapped seawater or evaporitic source: The outward deple- potassic association. As a consequence, the quartz signature tion of 18O is best explained in conjunction with hydrogen iso- could have been derived from an external source. A good candi- tope data. Figure 10d shows the calculated δD versus δ18O of date would be quartz-rich tuffs in the Tepalcatepec Formation. water in equilibrium with epidote. The departure of δ18O epi- dote values from the numerically calculated mixing line is be- Conclusions lieved to be mainly due to temperature effects. None of the In conventional classification schemes (Burt, 1972; Einaudi epidote samples shows the low δ18O and δD values charac- et al., 1981; Einaudi and Burt, 1982; Meinert, 1983, 1884), teristic of rocks that have exchanged O and H isotopes with Peña Colorada is a calcic iron skarn. The island-arc setting, isotopically lighter meteoric water. Consequently, and high-temperature calc-silicate alteration mineralogy, metal notwithstanding that the use of epidote for hydrogen isotope zoning about a cogenetic diorite, deposit morphology and studies has been the subject of controversy (Kyser and Ker- control, and sources of the mineralizing fluids support this rich, 1991), the internal consistency of the hydrogen signa- conclusion. The original Peña Colorada iron resource may ture of Peña Colorada epidote suggests that the original δD have well been on the order of 500 Mt, the same order of content is fairly well preserved. The δD and δ18O values of magnitude as other world-class iron skarn occurrences. water in equilibrium with epidote are compatible with mixing Peña Colorada shares some of the characteristics exhibited between a proximal-early magmatic fluid and a progressively by Fe oxide-(REE)-Cu-Au-U class deposits (Barton and John- more important distal-late component of trapped seawater, a son, 1996). Evaporitic and other external fluid sources appear closed basinal brine, and/or low-latitude–low-elevation mete- to have played a role in the formation of the deposit. An ad- oric water (Taylor, 1974). Given the paleogeographic and de- ditional similarity is the occurrence of volumetrically impor- positional environment of the intruded Tepalcatepec wall tant “syenitic phases,” which are the product of widespread rocks, we favor trapped seawater or an evaporite-derived fluid albitization at other iron deposit localities. However, petrog- as a likely end member. raphy of “syenite” (i.e., aplite) from samples collected at Peña Colorada shows no evidence of albitization (see Fig. 4j). REE Stable isotopes and fluid sources for the potassic event geochemistry suggests that it is a restitic fluid produced dur- Quartz, K feldspar, and calcite from the potassic alteration ing differentiation of the diorite. The relative timing of potas- event do not follow the normal fractionation order. The sic versus sodic alteration events has economic implications in change from a lithostatic to a fluid-dominated system during Fe oxide-(REE)-Cu-Au-U deposits, where the introduction emplacement of the hydrothermal breccia may explain the of potassic alteration during early stages is believed to largely disequilibrium. O and H isotope ratios from the potassic control the presence of copper and gold (Theodore et al, event are consistent with magmatic degassing related to aplite 1990; Leveille and Marschik, 2000). Aplite injection and as- injection and brecciation. sociated potassic alteration are late at Peña Colorada. Magmatic water source: The δD versus δ18O values of fluid More work is necessary to verify the significance of the pe- in equilibrium with epidote in the postmineral breccia are culiar oxygen isotope signatures obtained from Peña Colorada shown in Figure 10d. The values lie high within the magmatic magnetite and quartz. In addition, fluid inclusion studies water field, suggesting a source that could be the product of should further constrain the temperatures and compositions magma degassing (Nabelek et al., 1983). This interpretation is of the mineralizing solutions. consistent with field relations, because the potassic alteration is intimately associated with the intrusion of aplitic dikes. Acknowledgments These magmatic waters exhibit somewhat heavier δ18O aver- We are grateful to CMBJ-Peña Colorada for allowing access age values (6.8‰) than the calc-silicate association fluids to the mine, company data, and providing living quarters and (6.0‰). The more felsic composition of the aplitic phase meals during fieldwork. Special thanks are extended to Othón and/or temperature effects could be responsible for this Colín, Eduardo Espinoza, and their geologic staff for the gen- difference. Although less certain, evidence for a magmatic erous cooperation and helpful discussions. The manuscript

0361-0128/98/000/000-00 $6.00 555 556 ZÜRCHER ET AL. benefited substantially from the thoughtful and critical re- Engineering and Mining Journal, 1965, Four-company consortium to de- views of Marco Einaudi, Alan Clark, and Robert Linnen. This velop iron ore in Mexico [abs.]: v. 166, p. 118. Epstein, S., and Taylor, H.P., 1967, Variation of O18/O16 in minerals and rocks: research was supported by funds from the University of Ari- Researches in Geochemistry, v. 2, p. 29–62. zona-U.S. Geological Survey-Industry Mexico Consortium Flynn, R.T., and Burnham, C.W., 1978, An experimental determination of and National Science Foundation grant EAR 91-17372. rare earth partition coefficients between a chloride containing vapor phase and silicate melts: Geochimica et Cosmochimica Acta, v. 42, p. 682–701. 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