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VMS Project Geological Setting


Most of the known volcanogenic massive sulfide districts in Mexico are hosted in the Guerrero Composite Terrane (GCT), a complex assemblage of mostly submarine volcanic and volcaniclastic rocks of Jurassic to Cretaceous age built on older metamorphic core complexes. The GCT is the largest of all the Mexican Terranes, and probably the second largest of the North American Cordillera after Wrangellia (Centeno-Garcia et al., 2008). The GCT is composed of at least five terranes: Tahue, Zihuatanejo, Guanajuato, Arcelia and Teloloapan (Figs. 7 and 8). The Property is located in the northernmost portion of the Zihuatanejo Terrane, an area that includes a coastal belt between Puerto Vallarta and Tecpan de Galeana, an inland belt at Huetamo (Michoacan), and a belt in central Mexico (Zacatecas).

The basement of the Zihuatanejo Terrane consists of a metamorphosed Upper Triassic to Lower Jurassic subduction-related accretionary prism that has different names in different localities: (i) Arteaga Schist in Michoacan, (ii) Rio de Placeres Formation in northern Guerrero, (iii) La Ollas Complex in southern Guerrero, and (iv) Taray Formation in Zacatecas (Centeno-Garcia et al., 2008). Late Jurassic marine sedimentary and intrusive rocks of the Cuale Volcanic Sequence (Bissig et al., 2008) unconformably overlie and intrude the schists in western Jalisco (Fig. 7.3). In Huetamo, rocks of the >1200 m thick Angao Formation are coeval with the Cuale Volcanic Sequence based on the presence of Late Jurassic fossils (Martini et al., 2009). To the south (Zihuatanejo area), the Cuale Volcanic Sequence and Angao may be equivalent to the Lagunillas Formation, quartz-rich sandstone and shale sequence with minor conglomerate composed of quartzite and foliated quartz-muscovite schist (Martini et al., 2010).

Late Jurassic rocks grade upwards conformably into Early Cretaceous marine turbiditic rocks. In the Huatemo area, there are more than 3400 m of Early Cretaceous strata, including the San Lucas, Comburindio, El Cajon and El Paso Formations. In southern Guerrero and Michoacan, Early Cretaceous strata are about 1500 m thick, and include numerous formations at different localities (Martini et al., 2010; Centeno-Garcia et al., 2011). In western Jalisco, the Early Cretaceous is represented by three Formations: (i) Alberca Formation, marine turbidites partly derived from the Cuale Volcanic Sequence intercalated with thinly bedded calcareous black argillite and sandstone and minor andesitic flows that grades upwards into (ii) Tecalitlán Formation, 2400 m of andesite intercalated with black argillite and rhyolite, volcanogenic sandstone and tuff, that in turn grades upwards into (iii) Tepalcatepec Formation, up to 3800 m of sedimentary rocks, carbonate rocks and minor volcanic rocks.



The basement metamorphic complex comprises pelitic schists, intercalated with chloritic, biotitic and sericitic schists and meta-arkoses, and crops out to the west of the Cuale and Bramador mining districts (Fig 9). This package is folded and metamorphosed to subgreenschist facies assemblages. The base of the unit is not exposed, but outcrops continuously over elevations ranging from 350 m west of Bramador to more than 2150 m north of Cuale, over a vertical elevation range of more than 1800 m. It is separated from the overlying volcanic rocks by an angular unconformity and is locally intruded by hypabyssal rhyolite of the Cuale Volcanic Sequence. Valencia et al. (2013) dated detrital zircons from several geologic units in the Jalisco Block and determined that the age of the basement schist ranges from 240-160 Ma. This is the same age as other schists (e.g. Arteaga, Rio Placeres) derived from the Early Mesozoic Potosi Fan (Centeno-Garcia et al., 2008, 2011).


The Cuale Volcanic Sequence (CVS) is a rhyolitic volcano-sedimentary formation (Bissig et al., 2008) that has a lumpy distribution that probably reflects the location of individual volcano-plutonic edifices. At Cuale, the CVS outcrops over an area 14 km long and more than 4 km wide. At Bramador, the CVS outcrops over an area about 6 km long by 4 km wide. At Aranjuez, the CVS is exposed over an area about 6 km long and 2 km wide. At El Rubi, the CVS is quite thin, perhaps a few hundred meters at most.

All of the rhyolites of the CVS contain quartz and feldspar phenocrysts. Some QFP intrusions also contain minor amounts of hornblende and magnetite. There are some textural variations in the phenocryst population where extrusive rhyolites have dipyramidal quartz phenocrysts, and intrusive rhyolites have re-sorbed, embayed quartz phenocrysts. Dipyramidal phenocrysts are the high-temperature form of β-quartz (750ºC; MacLellan and Trembath, 1991), and are preserved in lavas that were quickly frozen due to interacting with cold water or ice. Rapidly frozen lavas also exhibit textures such as hyaloclastite breccias and peperites (McPhie et al., 1993). Devitrification of volcanic glass after cooling results in the formation of lithophysae and spherulites (McPhie et al., 1993). Collectively, the wide variety of textures can be complicated to interpret.


Ignimbrites are formed when the shallow parts of batholiths leak, blister and finally burst along ring fractures as their cupolas approach the surface (Elston, 1994), mainly in continental back-arc settings. As the roof of a magma chamber founders and walls collapse, great volumes of magma foam erupt catastrophically to form a cloud of hot gas, pumice, rock fragments and glass shards that can be several kilometres tall. Collapse of these eruption columns results in a hot, high concentration, ground-hugging, highly mobile gas-particle dispersion called an "ignimbrite flow" (McPhie et al., 1993). Submarine ignimbrite flow deposits are commonly zoned from lithic rich breccias at the base, to moderately sorted, thickly bedded lapilli tuffs in the center, to reverse-graded pumice-rich beds capped by fine tuff (with or without accretionary lapilli) deposits at the top (McPhie et al., 1993). Rhyolite ignimbrite forms the base of the Cuale Volcanic Sequence and consists of very thickly bedded volcanic breccias, moderately sorted, thickly bedded lapilli tuffs, quartz-feldspar crystal tuffs and fine tuffs with cherty, sulfide-rich layers intercalated with horizons of black argillite (Fig. 11). Bissig et al. (2008) observed accretionary lapilli in this unit in the footwall to the Jesus Maria deposit. On geological map F13A79 this unit is coded Knap-Rd-BvR. It is not identified as a separate unit on F13A89 or F13D71.

A sample of quartz and feldspar phyric crystal tuff from the hanging wall of the La Prieta fault taken in the Coloradita open pit yielded a 206Pb/238U age of 159.2 ± 2.2 Ma (Bissig et al., 2008). They interpret the age of this rock to be anomalously old compared to "stratigraphically underlying units". The author of this report contends that their data is correct, and that non-sedimentary rhyolite that occurs under these tuffs are intrusive (cryptodomes). Furthermore, calderas normally evolve by catastrophic eruption of ignimbrites and magmatic volatiles first, followed by effusive eruption of volcanic flows and intrusion of subvolcanic intrusions and cryptodomes (Elston, 1994).

The thickness of this unit ranges from a few 10's o meters to more than 400 m thick, and depends on the interaction of the paleo-topography and direction of flow at the time of eruption. At Cuale, the thickest known accumulation of rhyolite ignimbrite is exposed on the east flank of Cerro Canton with a vertical exposure of about 400 m between 1750 m elevation and 2150 m elevation. At Bramador, drill holes MJM6, 7 and 12 all intercepted more than 300 meters of rhyolitic ignimbrite and hole MJM8 intercepted 200 m of ignimbrite between 100 and 300 m depth, under Alberca (?) Formation argillite (Sanchez-Alvarado, 1987). At Aranjuez, the ignimbrites were intercepted in DDH MJM4 between 184 m and the end of the hole at 301 m depth, and MJM3 from surface to a depth of 220 m (Sanchez-Alvarado, 1987). At El Rubi, there are a few outcrops of ignimbrite a few 10's of meters thick.


A hypabyssal rhyolite cryptodome (a thick pile of rhyolite lava that is partially intrusive) centered in Cuale is exposed from 1000 m elevation at the bottom of the Arroyo Corazon Canyon, to the top of Cerro Caracol at 2300 m, over a vertical range of 1300 m. Unit 10A rhyolites have 1 to 7% 1 to 5 mm quartz phenocrysts, and feldspar phenocrysts were probably present when the rocks were new, but rarely observed now due to post-depositional hydrothermal alteration. The texture of the phenocrysts depends on the level of exposure (see comments in 7.2.2). On geological map F13A79 this unit is coded KnapPR. It is not identified as a separate unit on F13A89 or F13D71.

Texturally, the flow-dome complex is complicated, and depends on local conditions. Hyaloclastite breccia occurs where flash-freezing of magma has occurred at the sediment-water interface, and ranges from in-situ, jigsaw-fit textures in deeper parts of the crypto-dome, to more disorganized, re-sedimented breccias at shallower (emergent) levels. A sample of hyaloclastite breccia from Unit 10A north of Cerro Caracol yielded a 206Pb/238U age of 157.2 ± 0.5 Ma (Bissig et al., 2008). Under San Nicolas and San Juan, parts of the flow-dome are characterized by dense spherulites and lithophysae up to several centimeters in diameter, often with sulfides in the center of these devitrification features (Fig.12). Several drill holes have bottomed in this unit, including ZIM 24 (76.5 m to 195.84 m) from San Nicolas and ZIM 34 from San Juan (71.41 m to 125.05 m).


The Ore Horizon occurs at the transition between active, volcanism of the older CVS to somewhat quieter sedimentation penecontemporaneous with intrusion of QFP rhyolite. Lithologies in the Ore Horizon include re-sedimented rhyolite lapilli tuffs and breccias, black argillite, chert and massive sulfide that form a package up to 240 m thick at Cuale. The basal contact of the Ore Horizon is gradational with the underlying and laterally equivalent ignimbrite. Fig. 14 shows thinly laminated massive sulfide intercalated with black argillite from Naricero. Later rhyolite cryptodomes and flows (Units QFP and 10U) burrow into these (wet) sediments and create steam explosions that form peperites (Fig. 13).


Deep exposures of QFP rhyolites are characterized by 5% embayed quartz phenocrysts up to 3 mm across, up to 20% feldspar phenocrysts up to 10 mm long and minor hornblende and magnetite phenocrysts less than 5 mm long (Fig. 15). Shallow, marginal and quenched QFP intrusions are characterized by dipyramidal quartz phenocrysts and spherulites. Feldspar, hornblende and magnetite may not be visually apparent in altered rocks. The QFP cryptodomes and dikes clearly cross-cut earlier rhyolitic and volcano-sedimentary rocks and are commonly mineralized and altered. A sample of an altered QFP dike cross cutting Unit 10A rhyolite in the footwall to San Nicolas yields a 206Pb/238U age of 155.9 ± 1.6 Ma (Bissig et al., 2008). At Cuale, the largest QFP flow-dome outcrops in the hangingwall to Naricero and has dimensions of 700 m long by 500 m wide by about 125 m thick. Based on geological map patterns at Cuale (Fig. 9), the QFP appears to have intruded and partially overlaps the Ore Horizon.


Rhyolite flows that occur above the Ore Horizon are weakly porphyritic and texturally range from massive with columnar joints (Cerro Canton) to flow-banded with flow-breccias (Jesus Maria). This upper rhyolite unit is up to 340 m thick and mainly outcrops between Cerro Canton and Cerro la Descubriadora in the Cuale District. A sample from an aphyric rhyolite flow in the Coloradita open pit yielded a 206Pb/238U age of 154.0 ± 0.9 Ma (Bissig et al. 2008). At Jesus Maria, flow-folded and flow brecciated rhyolite occurs in the hangingwall to the massive sulfides (Fig.16), and may have formed an impermeable cap that trapped mineralizing solutions in the receptive, carbon-rich sediments of the Ore Horizon.


The Alberca Formation is not formally recognized in the map area of Fig. 9 at this time. However, according to the stratigraphic lexicon of Mexico, the Alberca Formation is widely distributed in Jalisco State, is up to 1800 m thick, and is the only formally described black shale formation in the Mesozoic. While it is documented as Cretaceous, that information is based on fossil evidence from upper parts of the Formation. It is likely that the CVS grades upwards and laterally into the sediment-dominated Alberca Formation as volcanism ceased in the Latest Jurassic.

Parts of the Formation consist of volcaniclastic turbidites derived from the CVS (Fig.18). Also present are rhyolite-derived conglomerate (Fig. 17) that appears to be re-worked peperite, and thinly laminated to thinly bedded black to grey argillite and lutite (Fig. 7.18) that is locally intercalated with basaltic andesite flows 10's of meters thick. Interbedded carbonaceous layers, sulfides, graphite and rhyolitic tuffs are common. On geological map F13A79 this unit is coded KnapA-Lu. It is not identified as a separate unit on F13A89 or F13D71.

Several drill holes have tested parts of the Formation for massive sulfides as the graphite-rich layers in this thick sedimentary package are good geophysical conductors. At Bramador, (i) drill hole MJM 8 intercepted about 85 m of argillite prior to intercepting rhyolite ignimbrite of the CVS below, (ii) drill hole MJM 9 intercepted 308 m of argillite and sandstone from surface, and (iii) MJM10 and 11 both intercepted about 320 m and 210 meters of argillite and sandstone, respectively, before bottoming in basaltic andesite (Sanchez-Alvarado, 1987). At Aranjuez, drill hole MJM1 intercepted 252 meters of intercalated argillite and sandstone from surface and MJM 4 intercepted 186 m of sandstone and argillite above rhyolitic ignimbrite of the CVS (Sanchez-Alvarado, 1987).

A sample of phyllitic schist from Mascota yielded a 206Pb/238U zircon age of 145.5 +/- 5.2 Ma, and a second sample of phyllitic schist from San Sebastian del Oeste yielded a 206Pb/238U zircon age of 138.9 +/- 3.72 Ma (Valencia et al., 2013). Valencia et al. (2013) describe these samples are described as belonging to marine turbiditic sedimentary rocks. It is the author's opinion that they probably correlate to this package.


The Tecalitlán Formation is not formally recognized in western Jalisco State at this time. However, the stratigraphic lexicon of Mexico mentions that Alberca Formation grades upwards into the 2400 m thick Tecalitlán Formation. According to Centeno-Garcia et al. (2003), the Formation consists of mainly andesitic and basaltic lava flows with minor rhyolite, pyroclastic and epiclastic deposits. East of Cuale and Bramador, andesitic volcanics that probably correlate to this Formation are abundant. A 206Pb/238U zircon age determination of amphibolite (metamorphosed basaltic andesite) from Ahuacatlán yields an age of 134.9 +/- 2.6 Ma (Valencia et al, 2013) that may correlate to the mafic volcanics (Units 11A, 11AF) overlying the sedimentary formations in Fig. 7.3a.

Andesite porphyry (Unit 11C) occurs as pillowed flows (Fig. 21) and agglomerates intercalated with black argillite and minor limestone. The rocks are characterized by feldspar megacrysts 1 to 3 cm long, hornblende phenocrysts up to 1 cm long, and up to 10% disseminated magnetite. Geophysically, these rocks have a strong and chaotic magnetic signature due to variable concentrations of magnetite. North of Aranjuez, there is a strong magnetic dipole 2.5 kilometres in diameter. Three dimensional inversions of the magnetic data imply that this dipole might represent an intrusive center at least 4 kilometres deep. At Bramador, the rocks are pillowed and clearly extrusive. Marginal to the pillowed flows, thick sections of volcanic breccias occur.

I-type granodiorite (Unit 21B) outcrops in a NNE trending belt between Desmoronado and Bramador. These rocks are characterized by abundant magnetite and xenoliths of mafic volcanic rocks. Roof pendants of andesite porphyry also occur. This area is co-incident with an area of markedly high magnetic susceptibility (Smith, 2006). Diorite mainly occurs in dikes and isolated apophyses that intrude the andesitic country rocks. In 2013, Valencia et al. determined a 206Pb/238U zircon age of 133.2 +/- 1.8 Ma for granodiorite from Ahuacatlán, western Jalisco State.


Outcrops to the west of Cuale are typically leucocratic alkali feldspar and biotite granites (Unit 21A). East of Cuale, two-mica granite with S-type characteristics occurs (Schaaf et al., 2003). Alternatively, the occurrence of muscovite might reflect metasomatism. 206Pb/238U zircon age dates yield an emplacement age of 83.8 to 80.1 Ma from granite samples west of Cuale (Valencia et al, 2013). Quartz monzonite (Unit 21G) is porphyritic with phenocrysts of quartz, K-feldspar, hornblende and biotite.


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