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REGIONAL GEOLOGYMost 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. 12 and 13 ). Relationships between the five terranes of the GCT are not known everywhere as most of the GCT is buried under large volumes of Cenozoic volcanic rocks.TAHUE TERRANEThe Tahue Terrane underlies most of western Sinaloa and Nayarit States. The basement is represented by the El Fuerte metamorphic complex, an Ordovician island arc assemblage exposed in northern Sinaloa and accreted to northern Mexico prior to the Pennsylvanian. The El Fuerte Complex is unconformably overlain by deformed Pennsylvanian to Permian deep-marine turbidites of the San Jose de Gracia Formation. Marine volcanic and sedimentary strata of Berriasian age and younger overlap both the Tahue and Zihuatanejo Terranes.ZIHUATANEJO TERRANEThe Zihuatanejo Terrane extends from Jalisco through Michoacan and Guerrero States, along the southwest Pacific coast of Mexico. Its basement consists of a metamorphosed Upper Triassic to Lower Jurassic subduction-related accretionary prism. Rock types in the metamorphic core complex include quartz-rich meta-turbidites imbricated with blocks and slabs of pillow basalts, diabase, banded gabbros, chert and limestone. These rocks have received different names at different localities: Zacatecas Formation, Arteaga Complex, and Las Ollas Complex. The metamorphic grade ranges from greenshist to amphibolite, with blueschists reported at Las Ollas (Centeno-Garcia et al., 2008). The Copper King VMS deposit in Guerrero is hosted in the Las Ollas Complex (Miranda-Gasca, 1995).Dispersed exposures of Middle to Late Jurassic volcanic arc rocks occur near the Pacific Coast. These rocks consist of submarine rhyolitic lavas of the Cuale Volcanic Sequence (CVS) that are intercalated with black argillite, volcanogenic massive sulfide deposits and minor andesite. The CVS ranges from more than 3000 meters thick at Cuale to about 500 meters thick at El Rubi. Coveal granitoids (e.g. Tumbascatio, Michoacan) intrude the basement schists (Centeno-Garcia et al., 2008). Silver-rich VMS deposits, including La Prieta, La Coloradita, Naricero, San Rafael, Amaltea, San Pedro, La America, El Rubi/Fenix and La Valenciana occur near the contact of the rhyolitic volcanic rocks with overlying and intercalated black shales. Current owners of these VMS deposits are Zinco Mining Corporation, Minera Vagabundo S.A. de C.V., Industrias Peñoles, Ing. Arnoldo Casteñada and Ing. Antonio Guerra. The Jurassic arc rocks interdigitate with, and grade upwards into, the Valanginian to Hauterivian Alberca Formation, which consists of 1500 meters of black shale, shaly limestone, fine-grained sandstone and andesitic tuff overlain by andesitic-basaltic lavas flows interbedded with limestone and shale (Centeno-Garcia et al., 2008). In turn, the Alberca Formation grades conformably upwards into the Barremian to Aptian Tecalitlán Formation, a 1200 to 2400 meter thick sequence of andesite flows, rhyolite pyroclastics, volcanogenic massive sulfide deposits (e.g. La Minita, Michoacan), siltstone, sandstone and conglomerate (Zürcher et al., 2001). The upper parts of the Tecalitlán Formation are correlated to the Encinos Formation in Jalisco (Léxico Estratigrafico de México). The Tecalitlán Formation is overlain by the Albian to Cenomanian Tepalcatepec Formation. The base of the Tepalcatepec Formation consists of near-shore mudstone, thinly layered carbonaceous shale, siltstone, sandstone, evaporitic horizons and local reef limestone. This lower member hosts lead and silver-rich SEDEX deposits at Arroyo Seco in Michoacan (currently owned by Endeavour Silver Corp.; http://www.edrsilver.com) as well as giant volcanogenic iron deposits such as Las Truchas, Aquila, Pluton and El Jovero. In Jalisco, this lower member is correlated to the Vallecitos Formation (Léxico Estratigrafico de México). The middle section of the Tepalcatepec Formation consists of gray, medium-bedded micritic shallow marine limestone with thin intercalations of limy shale. This member is the main host rock of the Peña Colorada iron deposits in Colima State (owned by Consorcio Minera Benito Juárez). Laterally, these rocks interdigitate with reef limestones of the Madrid Formation. The upper part of the Tepalcatepec Formation consists of subaerial andesite flows. Overall, the three members of the Tepalcatepec Formation have a combined thickness of up to 3750 m (Léxico Estratigrafico de México). The Tepalcatepec Formation is unconformably overlain by the 2000 meter thick Cerro de la Vieja Formation of Cenomanian-Turonian age that consists of conglomerates, redbed sandstones and shales. GUANAJUATO TERRANEThe basement of the Guanajuato Terrane is the Tithonian-Hauterivian Arperos Formation, a thick turbidite succession of volcanic greywackes, quartzites, micritic limestone, radiolarian chert, black shale and rare conglomerate resting on basaltic pillow lavas. These rocks are overthrust by pillow basalts (La Luz basalts) capped by the Esperanza Formation, a section of rhyolitic lavas overlain by volcanogenic massive sulfide deposits (e.g. El Gordo), volcaniclastic turbidites, chert and limestone. The rhyolitic lavas yield a U-Pb zircon age of 146.1 Ma (Mortensen et al., 2008). The Esperanza Formation is overthrust by the ultramafic-mafic rocks of the Cerro Pelón tonalite and Tuna Mansa diorite (Centeno-Garcia et al., 2008).ARCELIA TERRANEThe Arcelia Terrane lacks a basement and is made up of Early Cretaceous deep-water pillow basalts, ultramafic intrusions, black shale, chert and volcanic turbidites, all intensely deformed and partly metamorphosed (Centeno-Garcia et al., 2008).TELOLOAPAN TERRANEThe basement of the Teloloapan Terrane might be represented by the Tejupilco metamorphic suite of Triassic to Early Jurassic age. It is in fault contact with mafic volcanic rocks overlain by intermediate to felsic volcanic and volcaniclastic units and volcanogenic massive sulfide deposits interlayered with fine-grained clastic rocks and radiolarian chert. The entire sequence is interpreted to have been deposited in a deep water, intra-arc basin setting. U-Pb age dates from felsic volcanics in the Campo Morado VMS camp range from 146.2 to 142.3 Ma (Mortensen et al., 2008). Campo Morado was owned by Farallon Mining Ltd., until it was taken over by Nyrstar early in 2011.PROPERTY GEOLOGYThe Company's Property overlaps part of a window of the Guerrero Terrane exposed southeast of Puerto Vallarta (Fig. 13, Point 1). In this window, basement schists are mainly exposed west of Cuale and Bramador (Fig. 14). These are unconformably overlain by the Cuale Volcanic Sequence (Bissig et al., 2008). Regionally, the CVS trends east-northeast and dips gently to the south. From the base upwards, it consists of: (i) quartz and feldspar porphyritic ignimbrite, (ii) quartz and feldspar porphyritic flow-domes, (iii) minor limestone reefs, (iv) black argillite and fine rhyolite tuff intercalated with volcanogenic massive sulfide deposits, and (v) quartz and feldspar porphyritic hypabyssal intrusions and cryptodomes. The CVS grades upwards into the Alberca Formation, which consists of several hundred meters of black argillite overlain by greywackes and basaltic andesitic volcanic rocks and subvolcanic intrusions. The supracrustal rocks are intruded by the Early Cretaceous Puerto Vallarta Batholith.BASEMENT METAMORPHIC COMPLEX (UNITS 50, 51)West of Bramador and north of Cuale, the basement is dominated by chlorite-rich greenschist or meta-basalt (Unit 51). Equivalent rocks in the Arteaga schist (300 kilometers to the southeast) have geochemical signatures typical of mid-ocean ridge basalts (Centeno-Garcia et al., 2008). Pelitic schist, intercalated with sericitic schist and meta-arkose is exposed west of Cuale (Unit 50). These rocks formed part of the Potosi Fan, a thick section of turbidites formed at a passive margin. Deformation is characterized by strong shearing, tight folding and block-in matrix structures formed by tectonically incorporating slivers of oceanic basalt into the passive margin sediments. West of Bramador, the main foliation is oriented east-west, and dips gently to the north. The metamorphic complex is separated from the overlying volcanic rocks by an angular unconformity and is locally intruded by hypabyssal rhyolite, mafic dikes and granodiorite.CUALE VOLCANIC SEQUENCEUNIT 10C_FX: QUARTZ-FELDSPAR PORPHYRITIC RHYOLITE IGNIMBRITEIgnimbrites are formed when the shallow parts of batholiths leak, blister and finally burst along ring fractures as their cupolas approach the surface (Elston, 1994; Fig. 15), 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 kilometers 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). Quartz-feldspar porphyritic rhyolite ignimbrite forms the base of the Cuale Volcanic Sequence and consists of very thickly bedded volcanic breccias, moderately sorted, thickly bedded lapilli tuffs and quartz-feldspar crystal tuffs. Crystal tuffs form by separation of low density glassy shards into the eruption column from the higher density crystals which are sedimented downhill from the vent (McPhie et al., 1993). The hot turbulent suspensions mix with sea water and sediments on the sea floor to form steam-inflated density slurries. Figure 16 from Bramador shows chaotic rip-up fragments of black argillite supported in a matrix of crystal tuff. Bissig et al., 2008 report a U-Pb zircon age of 159.2 +/- 2.2 Ma for quartz and feldspar phyric crystal tuff exposed in the north wall of La Coloradita open pit. They interpret the age of Unit 10C_FX to be anomalously old compared to "stratigraphically underlying units". The author of this report contends that their U-Pb data is correct as published, and that other rhyolite facies that occur under these tuffs are either intrusive or structurally juxtaposed by faulting. Furthermore, calderas normally evolve by catastrophic eruption of ignimbrites and magmatic volatiles first, followed by effusive eruption and intrusion of volcanic flows and subvolcanic intrusions and cryptodomes (Elston, 1994). Quartz-feldspar porphyritic rhyolite ignimbrite is welded deep in the Cuale Volcanic Sequence, but up-section it is typically re-sedimented with contemporaneous black argillite horizons (Figs. 16 and 17 ). UNIT 10C_FL: QUARTZ-FELDSPAR PORPHYRITIC RHYOLITE FLOW-DOMES Quartz-feldspar porphyritic rhyolite flows, domes and cryptodomes a few tens to several hundred meters thick are the dominant rock type in the CVS. The rhyolite flows rarely are massive, but normally exhibit flow-banding, spherulitic devitrification, amygdules and lithophysae (Figs. 19 and 20 ). Quartz-feldspar porphyritic rhyolites are characterized by 1 to 7% 1 to 10 mm dipyramidal quartz phenocrysts and 1 to 10% feldspar phenocrysts 1-3 mm long. Dipyramidal phenocrysts are the high-temperature form of beta-quartz (750ºC; MacLellan and Trembath, 1991), and are preserved in magmas that were quickly frozen (have a short cooling history). The upper parts and flanks of flows and domes are commonly enveloped by large volumes of monomictic hyaloclastite flow breccias (Fig. 18) that are locally re-worked to immature conglomerates. A U-Pb zircon age of 157.2 +/- 0.5 Ma is reported for hyaloclastite breccia north of Cerro Caracol, Cuale District (Bissig et al., 2008). UNIT 2: LIMESTONE A single outcrop about 20 meters long by 10 meters wide of organic rich limestone occurs in the floor of the San Nicolas pit. This rock might have been a biogenic mound that grew around the hydrothermal vent. UNIT 7,1E: VOLCANOGENIC MASSIVE SULFIDE/EXHALITE Volcanogenic massive sulfides in the property mainly occur at or near the contact of Unit 10C_FL with overlying tuffs and argillites. Economic stringer and stockwork zones can penetrate more than 100 meters into the underlying rhyolite flow domes. Massive and semi massive sulfides occur: (i) as replacements of glass, spherulites, lithophysae and breccia fragments in the upper parts of Unit 10C_FL and (ii) as syngenetic, finely laminated massive sulfide intercalated with black argillite above Unit 10C_FL (Fig. 21). Locally, massive sulfides grade laterally into siliceous chert (exhalite). UNIT 10C_F/1A: RHYOLITE FINE TUFF/ARGILLITE Thinly bedded rhyolite fine tuff and black argillite occur above Unit 10C_FL, and can be intercalated with hyaloclastite breccias and re-sedimented ignimbrite. At Jesus Maria, Bissig et al. (2008), observed accretionary lapilli in Unit 10C_F. Rhyolite tuffs conformably grade upwards into the massive black argillites of the Alberca Formation. UNIT 10C_INT: QUARTZ-FELDSPAR PORPHYRITIC RHYOLITE DIKES, CRYPTODOMES AND INTRUSIONS Unit 10C_INT ranges from aphryic, rapidly quenched dikes and cryptodomes that intrude the soft-sediment pile to coarsely crystalline, deeper equivalents. Coarsely crystalline rocks are characterized by about 5% embayed quartz phenocrysts up to 3 mm across, up to 20% feldspar phenocrysts up to 10 mm long and minor hornblende phenocrysts less than 5 mm long (Figs. 22 to 24 ). In the near surface environment, flow-fronts of cryptodomes are commonly intrusive into syn-volcanic black argillite sediments where they form chaotic deposits of blobular peperite (McPhie et al., 1993). The rhyolite cryptodomes and dikes cross-cut earlier syngenetic VMS deposits, and are themselves mineralized and altered, indicating that the mineralizing system was still active at the time of intrusion (Fig. 22). Three U-Pb zircon dates from aphyric to moderately porphyritic dikes and cryptodomes in the uppermost parts of the volcanic pile at Cuale yield ages between 152.5 +/- 1.5 Ma and 155.5 +/- 1.6 Ma (Bissig et al., 2008). No dates are available from coarsely crystalline, larger intrusions. Earlier mappers refer to coarsely-crystalline, intrusive rhyolite as "Footwall Dacite" (JICA-MMAJ Reports, 1985, 1986, 1987). While these rocks do commonly intrude the footwall to the VMS deposits, porphyritic cryptodomes and dikes can intrude the volcanic pile above the VMS horizons as well. Intrusive porhyritic rhyolite is the main ore host for the "Minas de Oro", including Grandeza, Las Talpas, El Carmen and Rosario. ALBERCA FORMATION (?)UNIT 1A: BLACK ARGILLITEMore than 300 meters of thinly laminated argillite and organic rich micrite with minor re-sedimented rhyolite tuffs have been intercepted in holes MJM 1 (Aranjuez) and Bramador (MJM 8, 9 and 10). This thick section of black argillite is the same age as, and possibly correlates to, the Alberca Formation. The base is conformable with the Cuale Volcanic Sequence, and the amount of intercalated rhyolite tuff decreases gradually up-section. Thinly laminated, pyritic, syngenetic sulfides occur throughout the argillite, with anomalous, but sub-economic metal values of Zn, Mo, V and Ni (Fig. 25). Locally, the rocks contain abundant graphite, particularly in the Bramador area. Geophysically, thick sections of black argillite stand out as strong resistivity lows (Section 10.2). Between Aranjuez and Bramador, the argillite strikes ENE and dips gently to moderately SSE. UNIT 11A: APHYRIC ANDESITE Aphyric andesite occurs mainly in the southeastern and northeastern parts of the Property. The rocks are aphyric to amygdaloidal, and are locally intercalated with felsic tuffaceous rocks, rhyolite flows and black argillite. Aphyric andesite is the main host rock to the La Fortuna silver vein prospect (Fig. 26). Andesitic agglomerate intercalated with fine tuffs and rhyolite flows occurs in the stratigraphic hanging wall to the San Rafael VMS deposit, Desmoronado, and was intercepted in the upper parts of Zimapan drill holes 100, 142, 143, 149, 151, 152, 153, 155, 156, 157, 159 and 160. UNIT 11C: ANDESITE PORPHYRY Andesite porphyry occurs as pillowed flows (Fig. 27) and agglomerates intercalated with black argillite, tuffs and minor limestone southeast of Bramador. North of Aranjuez, these rocks are massive, with no sedimentary textures. Everywhere, 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 kilometers in diameter. Three dimensional inversions of the magnetic data imply that this dipole might represent an intrusive center at least 4 kilometers deep. This interpretation is consistent with surface observations in the anomaly area that show that the bedrock consists of massive andesite (or diorite) porphyry with no sedimentary features. UNIT 1G: RED BED SANDSTONE, GREYWACKE Flat-lying red-bed sandstone occurs intercalated with aphyric andesitic volcanic rocks southeast of Aranjuez. The author of this Report has not mapped red beds elsewhere on the Property. PUERTO VALLARTA BATHOLITHUNIT 21B: GRANITEOutcrops to the west of Cuale are typically leucocratic alkali feldspar and biotite granites (Unit 21B). East of Cuale, two-mica granite with S-type characteristics occurs (Schaaf et al., 2003). Alternatively, the occurrence of muscovite might reflect metasomatism related to formation of porphyry Cu-Mo prospects as it does elsewhere on the Property. U-Pb zircon age dates yield an emplacement age of 103-91 Ma (Schaaf et al., 1995). A few isolated apophyses of granite occur with a few kilometers of Aranjuez. UNIT 32: MONZODIORITE I-type monzodiorite 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. Locally, monzodiorite is pervasively altered to muscovite greisen. One of the alteration zones, located north of Aranjuez, is co-incident with anomalous Au-Cu-Mo geochemistry in rocks and soil samples. |
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