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. 0361-0128/01/3146/535-23 $6.00 535 536 ZÜRCHER ET AL. 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-