Magmas transicionales del tipo adakítico a calcialcalino en un ambiente continental extensivo en los depósitos de Au-Cu de tipo skarn de La Paz, Mesa Central, México: implicaciones metalogenéticas

Porfirio J. Pinto–Linares¹, Gilles Levresse^2,*, Jordi Tritlla², Víctor A. Valencia³, José M. Torres–Aguilera⁴, Manuel González⁵, and David Estrada⁵

¹ Instituto Potosino de Investigación Científica y Tecnológica, División Geociencias Aplicadas, Camino a la Presa San José 2055, Col. Lomas 4^a Sección, 78216, San Luis Potosí, S.L.P., México.

² Programa de Geofluidos, Centro de Geociencias Universidad Nacional Autónoma de México, Campus Junquilla, Blvd. Villas del Mesón s/n, 76230 Querétaro, Qro., Mexico.* glevresse@geociencias.unam.mx

³ Department of Geosciences, University of Arizona, 1040 East Fourth Street, Room 510, Tucson, Arizona 85721–0077, USA.

⁴ Universidad Autónoma de San Luis Potosí, Depto. Ciencias de la Tierra, Dr. Manuel Nava 8, 78290 San Luis Potosí, S.L.P., México.

Manuscript received: June 11, 2007
Corrected manuscript received: October 9, 2007
Manuscript accepted: October 12, 2007

ABSTRACT

The granodiorite intrusions with associated Cu–Au skarn mineralization of La Paz district are located in the east part of the Mesa Central of Mexico. The skarn developed at the contact between a middle Cretaceous calc–argillaceous sedimentary sequence and the magmatic intrusions. AAg–Pb–Zn vein system postdates the intrusive–skarn assemblage. Two well defined fault systems (N–S and E–W) divide the La Paz district. The N–S Dolores fault, with a normal vertical displacement estimated between 500 to 1000 m, separates the western Au–Cu skarn zone from the eastern hydrothermal Ag–Pb–Zn vein system. This fault is considered to be part of the Taxco–San Miguel de Allende fault system. The U–Pb dating of the intrusives at the La Paz district clearly indicates a single emplacement event dated at ca. 37 Ma (monocrystal zircon age). This age probably represents the last post–Laramide orogenic mineralizing event known to occur in the Sierra de Catorce district. Also, four calculated discordant ages suggest the presence of greenvilian basement underneath a a thick crust (35–45 km).

The chemistry of the intrusive show a certain variability in composition, but they mostly belong to the high–K calc–alkaline magmatic series. Major and trace elements relationships for the intrusives show a chemical evolution from the adakite to the island arc fields, and from mineralized to barren intrusives, repectively. They also suggest the importance of crustal delamination processes, and the necessity of deep cortical drains to transfer oxidized magmas and metals to surface.

Key words: Adakite–like, Au–Cu skarn, U–Pb, geochronology, geochemistry, La Paz, Mesa Central, Mexico.

RESUMEN

Las intrusiones granodioríticas que dieron origen a un depósito de Au–Cu tipo skarn en el distrito minero de La Paz, S.L.P., se localizan en la parte oriental de la Mesa Central. El skarn se desarrolló en el contacto entre una secuencia sedimentaria calco–argílica del Cretácico medio y los intrusivos. Un sistema de vetas mineralizadas en Ag–Pb–Zn post–datan el Skarn. El distrito de La Paz está dividido por dos sistemas de fallas muy bien definidas (N–S y E–W). La falla Dolores, de dirección N–S, muestra un desplazamiento normal vertical estimado entre 500 a 1000 m y separa la zona occidental de skarn de Au–Cu de la zona oriental que contiene al sistema hidrotermal de vetas de Ag–Pb–Zn. Esta falla se considera como parte del sistema de fallas Taxco–San Miguel de Allende. Elfechamiento de los intrusivos mediante el método U–Pb en circones indica claramente un único evento de emplazamiento alrededor de 3 7Ma. Esta fecha representa el último de los pulsos mineralizantes, posteriores a la orogenia Laramide, reconocido en el distrito de la Sierra de Catorce. Asimismo se reportan cuatro edades discordantes que sugieren la presencia de rocas greenvilianas en la base de una corteza gruesa (35–45 km).

La geoquímica de los intrusivos muestra algunas diferencias en su composición, pero pertenecen a la serie magmática calco–alcalina con alto contenido de K. Los estudios de elementos mayores y traza muestran una evolución desde el campo adakítico hasta el campo de arco de islas, desde los intrusivos mineralizados a los estériles, respectivamente. Estos datos también sugieren la importancia del proceso de delaminación cortical y la necesidad de fallas profundas para transferir dicho magma y metales hacia la superficie.

Palabras clave: Adakite, Au–Cu skarn, U–Pb, geocronología, geoquímica, La Paz, Mesa Central, México.

INTRODUCTION

Intrusion–related hydrothermal systems obtain their thermal energy and variable amounts of volátiles, metals and other components largely from subduction–related magmas emplaced at shallow levels of the Earth's crust (Cathles 1981; Sawkins, 1990). Most of the Au–Cu–Ag–Pb–Zn profitable skarn deposits in the world are spatially related to porphyry copper deposits and alike. In western Mexico, this relationship has been repeatedly outlined by several authors (Clarke/ al, 1982; Campa and Coney, 1984; Sillitoe and Gappe, 1984; Megaw et al, 1988; Sawkins, 1990; Albinson and Nelson, 2001; Valencia–Moreno et al, 2006). The "Copper Cluster", located between northwestern Mexico, Arizona and New Mexico (USA), is one of the most important copper accumulation on Earth, which may compete in size with the famous deposits of the Andes Cordillera of South America (Clark, 1993; Camus, 2003).

Most of the Mexican porphyry copper deposits (Cu–Au–Mo) are located in the eastern part of the Laramide magmatic belt (90^0 Ma). The largest and best preserved deposits outcrop in northeastern Sonora, where Cananea (–30 Mt Cu) and La Caridad (~8 Mt Cu) stand out as world–class ore deposits.

For the Andes deposits, Skewes and Stern(1994) suggested that exsolution of copper–bearing magmatic fluids were responsible forbrecciation, alteration, and mineralization due to a rapid decrease of lithostatic pressure. In northern Mexico (Valencia–Moreno et al, 2006), the association of Laramide deformation and magmatism is a consequence of the subduction regime, due to the radical change of the dip angle of the Farallón oceanic plate underneath the North America continental crust during Cretaceous–Tertiary ages. At the end of the Cretaceous, the angle of the subducted plate considerably diminished as result of the increase in the velocity of the converging plate, inducing the accelerated migration of the magmatic arc axis towards the east (Dickinson and Snyder, 1978; Clark et al, 1982;Bird, 1988; Meschede et al, 1997; Bunge and Grand, 2000).

GEOLOGICAL SETTING

The La Paz district is located in the eastern border of the Mesa Central, in central Mexico (Figure 1). The Mesa Central is an elevated plateau mainly covered by Cenozoic volcanic sequences, affected by the Eocene and Oligocene east–west extension (Nieto–Samaniego et al, 2005) that created a series of deep continental basins filled with alluvial and lacustrine sediments. The eastern boundary of the Mesa Central is the Oligocene Taxco–San Miguel de Allende deep fault system. The major structure that separates the northern and southern regions of the Mesa Central is the San Luis–Tepehuanes fault system, that was active mostly between the Eocene and the Oligocene, but also during Pliocene–Quaternary times in its northwestern segment (Nieto–Samaniego et al, 2005).

The oldest rocks exposed in the Mesa Central are represented by Triassic marine facies which are overlain, all along the Mesa Central, by lower and middle Jurassic continental rocks, mainly volcanics, conglomerates and sandstones, and an upper Jurassic to late Cretaceous marine sedimentary sequence. Cenozoic materials are mostly represented by conglomerates and volcanic rocks of andesitic to rhyolitic composition. The last Cenozoic magmatic felsic event is characterized by the presence of F–rich rhyolites with normative topaz (Orozco–Esquivel et al, 2002). Locally, very small alkaline basalt flows of Miocene to Quaternary age also appear. The Laramide orogeny affected all the Mesozoic sedimentary column and caused folding and reverse faulting of the whole sedimentary sequence. Locally, one of the mid–scale related structures, the Dolores fault (Figure 2), with an estimated vertical displacement of 500 to 1000 m controls the outcropping of the mineralized system (Spurr et al., 1912). The basin of the La Paz district is covered by alluvial sediments with ages spanning from the Pleistocene to present.

In La Paz district, the oldest outcropping sediments are limestones and shales that belong to the Albian–Cenomanian Cuesta del Cura Formation, with up to 200 m in thickness (García–Gutiérrez, 1967; Machado, 1970;Barboza–Gudiño et al., 2004). This unit is overlain by the Turonian–Coniacian Indidura Formation (locally also known as Agua Nueva Formation) composed by alternating limestones and shales (Barboza–Gudiño et al., 2004). The Caracol Formation (locally known as San Felipe and Méndez formations), of Santonian–Maastrichtian age (Barboza–Gudiño et al., 2004) is composed by alternating limestones and shales with up to 100 m thick. All this Cretaceous sedimentary column is crosscuted by a granodiorite intrusion, that developed a metasomatic aureole with an associated Au–Cu skarn mineralization.

An ENE–WSW branching fault system crosscuts the skarn, and acted as a channelway for both dyke emplacement and the formation of a hydro thermal vein system. The San Acacio and San Augustin dykes are spatially related to the Ag–Pb–Zn–bearing vein system (Castro–Larragoitia, 1990). These veins display a mineralogical zonation, from Cu–Ag–Pb–Zn–Au–bearing veins near the intrusive stocks to Ag–Pb–Zn–Cu–bearing veins enclosed within either dykes or limestones, and to Ag–Pb–Zn–rich veins exclusively enclosed in limestones, the latter representing the apical part of the vein system towards the east end of the mineralized structures (Figure 2). The N–S and E–W fault systems present a complex polyphase movement history, both pre and post mineralization (Gunnesch et al., 1994).

ANALYTICAL TECHNIQUES

Zircon recovery

U–Pb age dating

Zircons recovered from the La Paz intrusions were carefully selected for dating. The selected grains display well–preserved prismatic shapes and euhedral growth zones, suggesting that they are of magmatic origin (Pupin, 1992), and no significant subsequent resorption and/or recrystallization.

Analyses were carried out at the University of Arizona (V.A. Valencia, analyst) with a LA–ICPMS Micromass system (Dickinson and Gehrels, 2003). 50–35 micron spots were analyzed with an output energy of 32 mJ and a repetition rate of 10 Hz. Each analysis consisted on a background measurement (20 second integrations on peaks with no laser firing) and twenty second integrations on peaks with the laser firing on. Any ²⁰⁴Hg contribution to the ²⁰⁴Pb mass is accordingly removed by subtracting the background values. The collectors were configured for simultaneous measurement of ²⁰⁴Pb in an ion–counting channel and ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³²Th, and ²³⁸U in Faraday detectors. All analyses were conducted in static mode. Inter–element fractionation was monitored using a standard (SL–1, natural zircon, 564±4 Ma; G.E. Gehrels, unpublished data). The reported ages for zircon grains are based entirely on ²⁰⁶Pb/²³⁸U ratios because errors of the ²⁰⁷Pb/²³⁵U and ²⁰⁶Pb/²⁰⁷Pb ratios are significantly greater. This is due primarily to the low intensity (commonly <0.5 meV) of the ²⁰⁷Pb signal from these young, low–U grains. ²⁰⁷Pb/²³⁵U and ²⁰⁶Pb/²⁰⁷Pb ratios and ages are accordingly not reported. The ²⁰⁶Pb/²³⁸U ratios were corrected for common Pb by using the measured ²⁰⁶Pb/²⁰⁴Pb, a common Pb composition from Stacey and Kramers (1975), and an uncertainty of 1.0 on the common ²⁰⁶Pb/²⁰⁴Pb. The weighted mean of 20 individual analyses was calculated according to Ludwig (2003) for each sample. This measurement error is added quadratically to the systematic errors, which include contributions from the calibration correction, decay constant, age of the calibration standard, and composition of common Pb. The systematic errors are 1–2% for these samples. Isotopic ratios and ages are reported in Table 1.

Geochemical analyses

Rock samples were analyzed for major and trace elements at the SARM of the CRPG–CNRS (Nancy France). Whole–rock samples were crushed and powdered in an agate mortar and pestle, and divided into two equivalent portions. For major and trace element analyses, sample powders (300 mg) were decomposed by fusion with lithium metaborate and subsequently diluted using a HC1 solution.

Major elements analyses were performed with a Jobin–Yvon JY70 Inductively Coupled Plasma–Atomic Emission Spectrometer (ICP–AES). Analytical precision was estimated at ±0.2% for SiO₂, and ±1% for the other major elements. The relative deviations of the standard analyses to the reference values are typically well below 1%.

Trace elements were analyzed with a Perkin Elmer ELAN 5000 Inductively Coupled Plasma–Mass Spectrometrer (ICP–MS). The analytical procedure was validated by repeated independent sample preparation, blanks, and analysis of international reference standards. The relative deviations of the standard analyses to the reference values are typically below 1%. Major and trace–element analysis are reported in Tables 2 and 3.

Petrography

Mineralization and alteration relationships were examined by field and underground reconnaissance mapping, optical petrography and electron microprobe analyses. Mineral phases and compositions were determined with a Cameca SX 100 electron microprobe (voltage: 15 kV; intensity: 10 nA; raster length, 25 micron) at the University of Nancy, France.

RESULTS

Four principal stocks and a dyke trend are recognized within the district crosscutting the whole sedimentary sequence. The horst and graben configuration controls the outcropping structural level. So, at the horst, the granodiorite stocks with associated Cu–Au skarn mineralization outcrop in Cobriza and Dolores; at the graben, part of the former stock as well as some blind dykes can be found in the underground mining works, including the western Au–Cu mineralizing stocks (El Membrillo and San Agustín mines) and, in the eastern part, the Ag–Pb–Zn vein mine of San Acacio (Figure 2).

Petrography

The studied stocks present an overall porphyritic texture, where the phenocrysts are essentially composed by zoned plagioclase (oligoclase–andesine; 35–40%), K–feld–spar(orthoclase–microcline, 30–25%), quartz (20–25%), and mafic minerals (biotite, hornblende, 8–10%), with accessory zircon, apatite and titanite, all included in a groundmass composed by quartz and feldspar microliths (Figure 3). As a local observation, the size of the phenocrysts progressively diminishes from Dolores to Cobriza (south to north), and from Dolores to Membrillo and San Agustín (west to east). The San Acacio dykes present a clear porphyritic texture, with phenocrysts of millimetric size. The phenocrysts are mainly composed by zoned plagioclase (andesine; 32–50%), quartz (30%), K–feldspar (orthoclase–microcline, 15%), and rare mafic minerals (biotite, hornblende, 5%), with accessory zircon, apatite and titanite, all included in a groundmass composed by quartz and feldspar microlits.

The metasomatism caused by the intrusion of the granodioritic stocks, as well as by some dykes (San Acacio dyke excluded), is marked by the presence of the mineralized skarn itself affecting the Mesozoic carbonates, and of heavily recrystallized rocks, locally classified as "hornfels", affecting the more argillaceous sedimentary materials (San Felipe and Méndez formations). The size and form of the metasomatic and metamorphic aureoles are directly related to the arrangement of the main anisotropies affecting the Mesozoic sediments (estratificationjoints, type and intensity of fracturation, folding, etc.) as well as to their chemical composition. The hornfels present granoblastic textures with garnet, wollastonite and diopside as main minerals.

A pervasive retrograde alteration affects both the endoskarn and exoskarn. This alteration phase is characterized by a penetrative stockwork structure composed by thin mineralized veinlets with associated propylitic alteration (actinolite, tremolite, chlorite, epidote, sericite, calcite and quartz) usually with sulfide minerals (mainly Ag–rich galena, sphalerite, chalcopyrite, pyrite, pyrargyrite, andtetrahedrite; Figure 3). Most of the gold present either in the endoskarn or exoskarn is associated with the retrograde veinlets. Also, the Dolores skarn presents a significant higher gold grade (>0.5 gr) than the Cobriza and Membrillo skarns.

U–Pb dating

Twenty–five analyses performed on the Dolores granodiorite zircons provided ²⁰⁶Pb/²³⁸U ages dispersed between 36.0±0.6 Ma and 1,356.9±96 Ma. The weighted mean crystallizing age of these zircons was calculated according to Ludwig (2003) as 36.8±0.5 Ma (n=20; MSWD of 1.4; see Figure 4 and Table 1). We also found two other concordant ages that revealed the existence of two older magmatic events of respectively Paleocene (57±5 Ma, n=1) and Bathonian (171±6 Ma, n=l) ages. Three other discordant ages exhibit inherited Mesoproterozoic (²⁰⁶Pb/²⁰⁷Pb: ca. 1,000 Ma, upper intersect; n=2) and Paleoproterozoic ages (²⁰⁶Pb/²⁰⁷Pb: 1,890±57 Ma, n=1).

Twenty–one analyses performed in the Cobriza intrusive zircons indicated ²⁰⁶Pb/²³⁸U ages dispersed between 35.3±2.1 Ma and 37.5±2.8 Ma. The weighted mean crystallizing age for these zircons is calculated, according to Ludwig (2003), as 36.1±0.4 Ma (n=20; MSWD of 0.8; Figure 4 and Table 1). One concordant age suggested the existence of an older magmatic event of Maastrichtian age (72±5 Ma, n=1).

For the El Membrillo intrusive, twenty–five analyses provided ²⁰⁶Pb/²³⁸U ages comprised between 35.2±3.0 Ma and 731.6±25.5 Ma. The weighted mean crystallizing age of zircons is calculated according to Ludwig (2003) at 36.9±0.4 Ma. (n=22; MSWD of 1.3; Figure 4 and Table 1). Three other discordant ages suggest the existence of three older magmatic events of namely Jurassic (²⁰⁶Pb/²⁰⁷Pb: ca. 200, n=1), Neoproterozoic (²⁰⁶Pb/²⁰⁷Pb: ca. 700 Ma, n=1), and Mesoproterozoic (²⁰⁶Pb/²⁰⁷Pb: ca. 1,000 Ma, upper intercept, n=1) ages.

For the San Acacio dykes, twenty–two analyses provided ²⁰⁶Pb/²³⁸U ages dispersed between 33.0±3.2 Ma and 384±14 Ma. The weighted mean crystallizing age of zircons from the San Acacio granodiorite dykes, according to Ludwig (2003) is 35.0±0.5 Ma (n=20; MSWD of 1.5; Figure 4, Table 1). Two older magmatic events were also revealed with Permian (²⁰⁶Pb/²⁰⁷Pb: ca. 285 Ma, n=1) and Cambrian (²⁰⁶Pb/²⁰⁷Pb: ca. 537 Ma, n=1) ages.

All the weighted mean crystallizing ages found are considered as representative of the emplacement ages of the intrusives. These newly found ages are in concordance with the data obtained by Tuta et al. (1988) (ca. 36 Ma, K–Ar in biotite from the Dolores intrusive).

At La Paz, the sampled granitoids present a small compositions variation in major and trace elements (see Table 2). As the samples were taken from both the graben and horst mining works and surface, these minor geochemical differences can be explained as representing different structural levels of the same intrusive. Samples of the La Paz Au–Cu skarn have variable loss on ignition (LOI; 0.52 to 5.13%; Table 2) reflecting variable H₂O contents, possibly due to different degrees of alteration. In general, the high–field–strength elements (HFSE) and rare earth elements (REE), are essentially immobile during intense hydrothermal alteration (Hawkesworth et al., 1997). The major elements contents (except Na₂O and K₂O) of La Paz intrusive show no obvious correlation with increasing LOI, indicating that their contents have probably not been changed by alteration. All rock types show mostly metaluminous (61% < SiO₂ < 68%), high–K calc–alkaline affinities (4% < K₂O < 7%); they belong either to a high–K calc–alkaline magmatic series (Le Maitre et al., 1989), or to the alkali–calcic metaluminous (Frost et al., 2001) series (Figure 5). Chondrite–normalized REE patterns (Figure 6) show that all samples are enriched in light REE (LREE) with respect to the heavy REE (HREE). They are moderately fractionated [11.64 < (La/Yb)_N < 15.2] with relatively low Yb_N contents (<10), small negative Eu anomalies (0.65 < Eu/Eu* < 0.9), and high REE contents (ΣREE up tol,800 ppm).

DISCUSSION

The scattered mean statistic ages found for the studied stocks and dykes can have a dual interpretation. On one hand, it can be due to the presence of several magmatic pulses from the Membrillo stock 36.9±0,4 Ma to the San Acacio dyke 35±0.5 Ma (Figure 4, Table 1); on the other hand, considering the short standard variation of the general statistic distribution, it can be an artefact caused by sampling at different vertical levels, obtaining slightly different samples within a narrow variability range. This last explanation have the authors preference.

The late Eocene age of La Paz intrusives (ca. 37 Ma) is contemporaneous with the Mapimi Cu–Zn skarn deposit (Durango; 36Ma, K–Arinplagioclase; Megaw et al., 1988) and the Ag–Pb–Zn skarn–vein system of Fresnillo (32 Ma, K–Arinplagioclase; Lang et al., 1988; Simmons, 1991). They represent the latests episodes of the syn/post–orogenic mag–matism and related mineralizations in the Sierra de Catorce district, that span from the Ag–Pb–Zn–Au veins at Real de Catorce (53±4 Ma, K–Arinplagioclase, Mújica–Mondragón and Jacobo–Albarrán, 1983) to the Zn–Cu skarn at Charcas (43±3 Ma, K–Ar in orthoclase; Mújica–Mondragón and Jacobo–Albarrán, 1963).

The general tendency, in terms of major elements, for the plutonic rocks associated to different types of skarn is towards calc–alkaline compositions (Fe, Au, Cu, Zn–Pb, W, Sn and Mo–bearing skarns; Figure 5; Meinert, 1995). All samples from La Paz granitoids correspond to high–K calc–alkaline rocks; the horst samples are tightly concentrated within the Cu and Zn rich skarns fields, whereas the graben granitoid samples and the San Acacio dyke plot scattered close to the Cu–rich skarns field. This scattering can reflect a hydrothermal influence on the original chemistry of the intrusives.

Table 3 presents a comparison among the chemical data from La Paz granitoids and other Au–related and Cu–related intrusive bodies abroad (Meinert, 1995), as well as high–SiO₂ (HSA) adakite rocks (Martin and Moyen, 2003; Martin et al., 2005). The analyses of the major and trace elements reported by Meinert (1995) indicate that the plutonic rocks associated with Cu anomalies present a similar trend as the one shown by type I magmas. With the exception of the low Na₂O contents, the La Paz western intrusives (Dolores and Cobriza) display certain geochemical affinity with HSA major and trace element, following the criteria defined by Defant and Drummond (1990), Drummond and Defant(1990), Drummond et al. (1996) and Martin (1999). At La Paz, the higher Srvalues (467 to 653 ppm) shown by the granitoids, in contrast with the low Sr values found in the San Acacio dyke (248 ppm), are comparable with the values obtained for the giant porphyry copper deposits in the Andes (Reich et al, 2003).

Figure 7 shows the relationship between mobile vs. inmobile elements. The Rb/Sr ratio is very sensitive to magmatic differentiation, and usually the Sn–, Mo–, and W–rich skarn magmas are highly differentiated with respect to Fe–, Au– and Cu–rich skarn–related magmas. In our case, the La Paz analyses show the same general tendency displayed by the Cu–rich skarn deposits. Our samples present high Zr concentrations, ranging from 176 to 211 ppm, with a low magmatic differentiation grade. Notably, the San Acacio dyke is more differentiated and present a similar tendency as the Zn deposits. In the La/Yb vs. Yb diagram (Figure 8), the samples of La Paz intrusives are consistent with a partial melting trend, indicating that their compositional variation is mainly controlled by this process rather than by fractional crystallization. However, the presence of inherited zircon grains and the large Mg–number [100x Mg²7(Mg²⁺+Fe^total)] variation from 17 to 23 indicate that these magmas could be generated from metabasaltic magmas (Castillo et al., 1999).

It is generally believed that reaction between pure slab melts and surrounding peridotite in the sub–arc mantle wedge results in the high Mg–number and MgO contents typical of adakites (see Figure 9a reference). In the Figure 9a, the La Paz samples mimetize the fields of "thick lower crust–derived adakite rock" and "metabasalt and eclogite experimental melts".

Figure 9b shows the (La/Yb)_N vs. Yb_N relationships. The samples from the graben stocks clearly overlap the field of "island arc" compositions, whereas the samples from the horst stocks fall within the "subducted oceanic crust derived adakites" field and span towards the field of "delaminated lower crust derived adakitic rocks", with low to moderate (La/Yb)_N ratios. In La Paz samples, Y contents span from 12.3 to 14 ppm, with an average composition of 13 ppm, whereas Yb spans from 1.2 to 1.4, with an average content of 1.3 ppm. In contrast, well constrained adakites present compositions of Y < 18 ppm and Yb < 1.8 ppm, and typical calc–alkaline lavas have compositions of Yb>2.5 ppm and Y>25 ppm. Consequently, we can conclude that our intrusive probably represent a transition between these two compositional fields. The Sm/Yb ratios are used to calculate relative crustal thicknesses (Hildreth and Moorbath, 1988; Kay and Kay, 1991; Kay et al, 1999). The increase in the Sm/Yb ratio reflects the pressure–dependent changes that occur in the transition from clinopyroxene to amphibole and, then, to garnet in the refractary residue that is in equilibrium with an evolving magma (Kay and Kay, 1991). So, clinopyroxene is dominant at depths less than 35 km, amphibole is stable between 30 to 45 km, whereas garnet appears at depths greater than 45–50 km.

Major and trace elements display an evolution from low differentiated granodiorite magmas (horst) to the more differentiated shallowest magmatic bodies (San Acacio dyke). This continuum magmatic evolution supports the vertical geochemical variation already discussed above (U–Pb dating results and interpretation), rather than several magmatic pulses.

Source of the La Paz intrusions

Under subduction settings, constrained by particular P–T–H₂O conditions (P>5 kbar, T°C > 750°C, >10 wt% H₂O), young (<25 Ma), mafic oceanic lithosphere melts before reaching dehydration, generating the "typical" adakitic magmas with a MORB–like isotopic signature, instead of the typical calc–alkaline arc andesite–dacite–rhyolite suites, originated by partial melting of a metasomatized mantle wedge (Drummond et al., 1996; Martin, 1999; Prouteau et al., 1999). However, adakite rocks have been found in various geological settings and their formation explained by several genetic models: 1) partial melting of a subducted oceanic crust slab (e.g., Defant and Drumond, 1990; Martin et al., 2005); 2) crustal assimilation and fractional crystallization processes (e.g., Castillo et al., 1999); 3) partial melting of a lower thickened crust (Kay et al., 1978, 1991; Petford and Atherton 1996; Kay and Mpodozis, 2001; Atherton and Petford, 1993; Xiong et al., 2003); 4) partial melting of a stalled slab in the mantle (e.g., Mungall, 2002); and 5) partial melting of a delaminated lower crust (e.g., Kay and Mahlburg–Kay, 1993; Wang et al., 2004). On the basis of the local and regional tectonic settings at La Paz and Mesa Central respectively, as well as of the geochemical characteristics and the zircon U–Pb ages, the last two models (4 and 5) are more plausible than the first three other models to explain the generation of the La Paz fertile granodiorites.

The Laramide Orogeny in western Mexico was the consequence of low–angle, high–speed (14 cm/year) convergence and subduction processes between the Farallón and the North America plates, which occurred from the Maastrichtian to the Paleocene (Dickinson and Snyder, 1978; Clark et al., 1982; Bird, 1988;Meschede et al., 1997; Bunge and Grand, 2000). The Oligocene trench is supposed to have been in a geographic position comparable to the present subduction trench, at around 500 km from La Paz district (Schmid et al., 2002). It is generally accepted that the Eocene–Oligocene magmatic activity in the Sierra Madre Occidental and the Mesa Central was still related to the sudbuction of the Farallón plate under North America (DickensonandSnyder, 1978; Clark et al, 1982;Bird, 1988; Meschede et al, 1997; Bunge and Grand, 2000). Internal deformation of the Farallón slab in the transition zone is proposed to explain the inland extension of contemporary magmatism activity (Schmid et al, 2002).

The orogen build–up was followed by two periods of post–orogenic extension illustrated by the appeareance of two volcanic events . The age of these volcanic sequences varies from 37 to 49 Ma and from 29 to 27 Ma (Labarthe–Hernández et al, 1989; Ferrari et al, 2005), respectively. The first event is interpreted as related to post–orogenic extension and the second one as a modification of the sub–duction direction (Ferrari et al, 2005). Recently, Orozco–Esquivel et al. (2002) distinguished within the second volcanic event two sub–units, one classically related to mantle–derived magmas, and a second related to lower crust (of supposed Grenvillian age) partial melting with low mantle contributions. This second volcanic sub–unit is comparable in age and chemistry with the La Paz mineralized intrusives. The same authors suggested that the well–documented early Oligocene crustal extension in the Mesa Central (Nieto–Samaniego et al, 1999) allowed basaltic melts to invade the crust, which subsequently acted as heat source for crustal melting. The granulite facies metamor–phism would be then associated with this heating event and a crustal melting process that generated the upper, younger sequence of rhyolites. Melting occurred at high rates, causing a rapid increase in pore fluid pressure that reduced rock strength and promoted rock fracturing. Then, such conditions enhanced rock permeability and rapid melt segregation at low degrees of melting before equilibrium was attained (Petford, 1995; Knesel and Davidson, 1999). The same effect of enhanced permeability would be produced by the dehydration and melting of granulites under water–undersaturated conditions (Rushmer, 1996). Another factor promoting rapid melt segregation and ascent is a low melt viscosity (McKenzie, 1984). High crustal extension rates, as occurred at this time in the Mesa Central, would also helped rapid magma ascent to the upper crustal emplacement level. The conditions of a short–lived event of melt generation in an extensional stress field associated to rapid heating of source rocks, high melting rates, and fast melt segregation, support the possibility of generating magmas with anomalous adakite–like affinities.

There has been a growing interest in adakitic magma–tism and its relationship with copper and gold mineralization during the last decade. The physical association between adakites and ore deposits has been documented mainly in Philippines (Sillitoe and Gappe 1984; Malihan 1987; Imai et al, 1993); the Chilean Andes (Thieblemont et al., 1997) and inMexico (González–Partida et al., 2003). Porphyry copper and skarn deposits are generally derived from sulfur rich, highly oxidized magmatic system (Sillitoe, 1997; Oyarzun et al., 2001; Mungall, 2002; Richards, 2002; Rabbia et al., 2002). Mungall (2002) highlighted the importance of the JO₂ (JO₂ > FMQ buffer) of the magma as limiting condition for the transport of chalcophile elements. Thieblemont et al. (1997), Mungall (2002), and Defant et al. (2002) identified slab–derived adakite magmas as the most favorable for Cu–Au mineralization, because of their high oxidizing potencial. The tectonic scenarios considered favorable for the generation of Cu–Ag magmatic mineralization after slab melting are subduction of a very young lithosphere, flat subduction, oblique convergence, and the presence of stalled slabs (Sillitoe, 1997; Mungall, 2002; Wang et al, 2006).

Wang et al. (2006) proposed, besides fO₂ conditions, the generation of fertile adakite magmas by partial melting of a thick lower crust that interacted with the most important chalcophile reservoir, the mantle, in a geodynamic scenario without slab subduction. So, because the adakitic signatures are not exclusively generated by slab melting and can also been explained by crustal involvement, either as a magma contaminant or as a protholith after crustal thickening (Petford and Atherton, 1996; Kay and Mpodozis, 2001; Xu et al., 2002; Zang et al., 2005; Wang et al. 2006), a combination of geochemical and geodynamic evidences are needed to better constrain the adakite origin.

Paleotectonic reconstructions of the Mesa Central and geochemical evidences previously discussed from La Paz igneous rocks suggest that source magma could have been formed under garnet–amphibolite facies. It is noteworthy that the fluid release after breakdown of an amphibole–bearing mineralogy, passing to garnet–bearing residual assemblages during the melting process, has been considered of fundamental importance for the formation of the large central Andean ore deposits (Kay et al, 1999; Kay and Mpodozis 2001, Wang et al., 2006). As the chalcophile elements are mainly stored in mantle sulfides (Mungall, 2002), their transport from the mantle by magmas will only occur if the sulfide phases are completely consumed during partial melting under mantle oxidation conditions above the FMQ buffer (Mungall, 2002).

CONCLUSIONS

The intrusive rocks intimately related with the La Paz Au–Cu skarn deposits display a transitional geochemical signature between the adakite and calc–alkaline compositions. The La Paz stocks were emplaced during the late Eocene crustal extension (ca. 37 Ma) that occurred after the Laramide Orogeny. Deep seated fault systems, as the Taxco–San Miguel de Allende fault system, channelled up and transported substantial quantities of heat and metal–bearing fluids to the upper lithosphere, favoring the formation of ore deposits. The extensional setting played a crucial role in the generation of adakite–like magmas by a combined process of lower crust delamination and partial melting (Figure 11).

ACKNOWLEDGEMENTS

Our deep thankfulness to Lie. José Cerrillo Chowell, General Director of Negociación Minera Santa María de La Paz y Anexas, S.A. de C.V. for the support received for the fulfillment of the present study, and for allowing its publication. This study was financed by UNAM–PAPIIT projects IN114106 and IN100707, and CONACyT project 49234–F. We sincerely thank Professor Yang Xiaoyong, and Dr. Alaniz Alvarez for their constructive reviews.

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