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Berthierine and Chamosite Hydrothermal: Genetic Guides in the Pena˜ Colorada Magnetite-Bearing Ore Deposit, Mexico”

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Berthierine and Chamosite Hydrothermal: Genetic Guides in the Pena˜ Colorada Magnetite-Bearing Ore Deposit, Mexico” COMMENT Earth Planets Space, 61, 291–295, 2009 Comment on “Berthierine and chamosite hydrothermal: genetic guides in the Pena˜ Colorada magnetite-bearing ore deposit, Mexico” Antoni Camprub´ı1 and Carles Canet2 1Departamento de Geoqu´ımica, Instituto de Geolog´ıa, Universidad Nacional Autonoma´ de Mexico,´ Ciudad Universitaria, 04510 Mexico´ D.F., Mexico 2Departamento de Recursos Naturales, Instituto de Geof´ısica, Universidad Nacional Autonoma´ de Mexico,´ Ciudad Universitaria, 04510 Mexico´ D.F., Mexico (Received February 23, 2007; Revised May 2, 2008; Accepted May 6, 2008; Online published February 18, 2009) 1. Introduction which is ubiquitous in the study area and correlates strati- Rivas-Sanchez et al. (2006) studied the composition and graphically with a regional series that contains peperites structure of berthierine and chamosite in the Pena˜ Col- (Elena Centeno, personal communication). The Tepalcate- orada magnetite deposit (Colima, Southwestern Mexico), pec Fm. hosts most of the magnetite ores in the Pena˜ Col- which they interpreted genetically to represent (1) an early orada deposit. The base of this formation is comprised of SEDEX stage of mineralization and (2) a late “hydrother- marls, carbonaceous pelites, limolites, sandstones, evapor- mal” stage of mineralization. Their mineralogical study has ites, and occasional carbonate reefs. The middle part of the some deficiencies that will be discussed. However, it is the Tepalcatepec Fm. contains micritic marine carbonate rocks deficiencies in their genetic interpretation that are our ma- interbedded with limolites, and the upper part contains in- jor concern. Their paper lacks any description of the re- terbedded calc-alkaline andesitic tuffs, which are charac- gional geologic context and any real discussion on the ori- teristic of a back-arc extensional environment (Centeno- gin of the deposits, which is surprising because there are Garc´ıa et al., 1993; Zurcher¨ et al., 2001). The recrystal- several recent publications on both topics (Klemic, 1970; lized carbonate rocks and interstratified andesitic tuff beds Zurcher¨ et al., 2001; Tritlla et al., 2003; Corona-Esquivel have been dated as Aptian-Albian by Corona-Esquivel and and Henr´ıquez, 2004). Rivas-Sanchez et al. (2006) do not Alencaster (1995). These rocks are capped by reddish con- even consider models for similar deposits in Mexico that are glomerates of the Cerro de la Vieja Fm. (Cenomanian). better undestood than Pena˜ Colorada, like Cerro de Mer- These Cretaceous strata were intruded by the calc- cado (Swanson et al., 1978; Lyons, 1988). alkaline Manzanillo batholith (Schaaf et al., 1995; Calmus In this discussion we describe first the basic published et al., 1999), which contains mostly granodiorites with mi- information that the paper by Rivas-Sanchez et al. (2006) nor plagiogranite and gabbro intrusions. At Pena˜ Colorada, lacks. Then, we discuss what, to our knowledge, are incon- the sedimentary sequence is intruded by andesitic porphyry sistencies and misconceptions in the paper. Finally, we sug- dikes that are probably Tertiary. gest that a “classical” geologic approach to the Pena˜ Col- orada deposit provides a much stronger rationale than the 3. Structure of the Pe˜na Colorada Iron Deposit mineralogical characterization of only two minerals to es- Pena˜ Colorada is a complex polyphase deposit. Sev- tablish the general conditions for mineral deposition that eral magmatic and hydrothermal events produced iron lead to plausible genetic models. mineralization, garnet-rich rocks (granatites) as skarns or skarnoids, and late dikes and faults that crosscut the min- 2. Geological Setting eralized bodies. The main mineralization events are: (a) a The basement of the sedimentary sequence at Pena˜ Col- massive orebody, (b) a disseminated orebody, (c) a layered orada contains highly deformed Triassic rocks (Centeno- barren exoskarn/skarnoid, (d) a polymictic breccia, (e) min- Garc´ıa et al., 1993). The Cretaceous sedimentary suite eralized conglomerates, and (f) late andesitic dikes (Zurcher¨ starts with the Alberca Fm. (Valanginian-Hauterivian), et al., 2001; Tritlla et al., 2003). which comprises biopelites, marls, limestones, fine-grained The massive orebody is tabular and generally massive, sandstones, and andesitic tuffs (Centeno-Garc´ıa, 1994). It up to 40 m thick, over 1000 m long, and nearly parallel is conformably overlain by the Tecalitlan´ Fm. (Barremian- to the local layering of host andesite flows and carbonate Aptian), which is comprised of andesitic flows, rhyolitic rocks. The SW part of this magnetite orebody is in contact pyroclastic flows, sandstones and conglomerates. It is over- with lutites and recrystallized limestones of the Tepalcate- lain in turn by the Tepalcatepec Fm. (Albian-Cenomanian), pec Fm. The contact is usually discordant and sharp, al- Copyright c The Society of Geomagnetism and Earth, Planetary and Space Sci- though it may occasionally contain granatites up to 10 cm ences (SGEPSS); The Seismological Society of Japan; The Volcanological Society thick within. The mineralogy of this orebody is dominated of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sci- > ences; TERRAPUB. by subhedral to euhedral magnetite ( 85% in volume), partially martitized, with subordinate pyrite, chalcopyrite, 291 292 A. CAMPRUBI´ AND C. CANET: COMMENT pyrrhotite, K-feldspars, pyroxenes (basically hedenbergite), only in a few drill core samples in the deepest parts of the chlorite, apatite, and carbonates. Fragments of feldspathic deposit; it is unclear whether it was the first mineraliza- rocks are found sporadically in this orebody that were ap- tion event; (2) barren stratabound skarn or skarnoid, not in parently dragged from a pre-existing mineralization (Tritlla contact with the microgranite; (3) disseminated orebody; et al., 2003). Ferroan chlorite is intimately associated with the rock rich in K-feldspar and magnetite in fault contact K-feldspar and the distribution of crystals in this particular with this body may be older than it; (4) massive orebody association suggests that these minerals are pseudomorphs (may be coeval with the disseminated body); it contains gra- of pyroxenes and, notably, garnet. Potassic metasomatism natite fragments replaced by K-feldspar that were probably has been reported in other allegedly Iron Oxide-Copper- dragged from the barren stratabound skarnoid; (5) polymic- Gold (IOCG) deposits such Candelaria in Chile (Marschik tic breccia and associated veins and veinlets; (6) intrusion and Fontbote,´ 2001). The K-feldspar gives a K-Ar date of of andesitic dikes with associated small barren skarn-like 57.3±2.1 Ma (Tritlla et al., 2003). associations of garnets and pyroxenes. The disseminated orebody, which is found ∼40 m below The time span between the ages obtained by Tritlla et al. the massive ore body, is over 10 m thick, and its hanging- (2003) is, accounting for the standard deviations, at least wall is subconcordant to the local layering of the host an- 4.4 M.y. This duration is too long for the whole deposit desitic rocks. This orebody has reserves of >90 Mt grading to be a typical skarn, as proposed by Zurcher¨ et al. (2001). 26% magnetite. It shows a laminated internal arrangement Consequently, Tritlla et al. (2003) proposed that this deposit due to alternating K-feldspar, chlorite, and pyroxene lay- is a Phanerozoic equivalent of IOCG-type deposits. ering. The groups of layers are a few mm to over 30 cm thick with coarse-grained magnetite-rich layers and euhe- 5. Inconsistencies, Misconceptions and Misleads dral pyrite crystals at their base. The pyrite content and in Rivas-Sanchez et al. (2006) size of magnetite crystals decrease upwards, as the content 5.1 Why SEDEX? of fine-grained pyroxene increases. Laterally to these lay- Rivas-Sanchez et al. (2006) propose a sedimentary- ered deposits are accumulations of poikilitic euhedral to an- exhalative (SEDEX) model for the formation of the Pena˜ hedral non-orientated K-feldspar crystals (similar to a syen- Colorada magnetite deposits, but offer no evidence to sup- ite) with subordinate euhedral magnetite crystals that form port such a model. The high-grade diagenetic origin (sic) of irregular bodies that extend for several meters. Such ac- some mineral associations is equally dogmatic and unsus- cumulations may represent portions of deeper bodies that tained. These authors assume the SEDEX option but they were upthrown by faulting. Such a “syenite” was dated at support it with neither solid consistent geological data nor 65.3±1.5 Ma (K-Ar in K-feldspar, Tritlla et al., 2003). citations of other authors. To date this deposit has been ex- The polymictic breccia is a subvertical zone of breccia- plained as a skarn (Zurcher¨ et al., 2001) or as an orthomag- tion that cuts the massive and disseminated bodies (Tritlla matic or IOCG-like deposit (Klemic, 1970; Tritlla et al., et al., 2003). The structure, morphology and mineralogy of 2003; Corona-Esquivel and Henr´ıquez, 2004). Conversely, the breccia are highly variable through its vertical extent of SEDEX models have been disregarded as inconsistent with >300 m. This breccia is ∼5 m wide in its lower part and the vast majority of the geologic evidence. contains sharp-edged andesite blocks in a fine-grained mag- Rivas-Sanchez et al. (2006) do not describe the local and netite matrix that resemble a breccia formed by hydraulic regional geology, although it is a key factor for understand- fracturing. Other fragments up to 1 m in diameter are made ing the genesis of any deposit. Accordingly, they fail to up of aggregates of cm-sized crystals of magnetite, apatite describe sedimentary sequences that are characteristic of and hedenbergite. Tritlla et al. (2003) interpreted these frag- SEDEX environments such as turbidites and marine clastic ments to be xenoliths that were dragged from a deeper mag- sequences deposited in intracontinental rift basins or in pas- netite body. There are similar mineral associations in other sive continental margins (e.g., Russell et al., 1981; Large, similar deposits in the region and in the Cerro de Mercado 1983), which, by the way, are virtually absent in the region.
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