Longo, Anthony A.¹,², Paez-Beltran³, Enrique, Smit Hans¹, Bourke, Fletcher M.¹, and Prefontaine, Marc¹, (1) Grayd Resources Corporation, Suite 420 - 475 Howe Street, Vancouver, B.C., Canada V6C 2B3, (2) Department of Geoscience, University of Nevada, Las Vegas, Las Vegas, NV 89154, (3) Departamento Geología, Edificio 3-C, U.R.C., UNISON, C.P. 83000, Hermosillo, Sonora, México. Apdo. Postal: 847
The La India gold deposit developed as a cluster of small gold ore bodies (Cerro de Oro, La Cruz, Viruela, La India, and Cieneguita) aligned N-S within a genetically related zone of hydrothermal alteration >20 square km in area. The deposit is located 20 km NW of the Mulatos high-sulfidation gold deposit in SE Sonora, Mexico. The total gold resource at a 0.5 g/t cutoff is currently 17.4 Mt grading 1.01 g/t Au for 559,000 oz (Indicated) and 9.4 Mt grading 1.05 g/t Au for 318,000 oz (Inferred). New 40Ar/39Ar ages are presented here on igneous rocks and alunite. Volcanism flared up in late Eocene and persisted for ~3 m.y. The magmas evolved from andesite to dacite and then rhyolite. Eruptions of andesitic tuffs and lavas (Lower Andesite, 36.91±0.19 Ma) were followed by the deposition of epiclastic rocks. Volcanism continued with eruptions of dacitic lavas (3-5% quartz) and crystal-poor (<3% quartz) tuffs (Upper Dacite), and culminated in explosive eruptions of rhyolitic ignimbrites (>10% quartz) in late Eocene (Nopal Rhyolite, 33.90±0.16 Ma). Conglomerates were then deposited atop Nopal Rhyolite during a ~6 m.y. period lacking volcanism. Volcanism again flared up in late Oligocene with effusive eruptions of andesitic lavas (Upper Andesite, 28.21±0.13 Ma). Acidic hydrothermal fluids entered the volcanic section along sub-vertical fracture zones and moved laterally in the epiclastic rocks along the Upper Dacite contact. Large tabular bodies of dense massive quartz alteration developed atop the Lower Andesite in the sediments and dacitic tuff. Quartz-dickite±alunite alteration was localized in the fracture zones and laterally along margins of the massive quartz, and defines the central alteration zone in areas containing gold ore. It is progressively enveloped by quartz-kaolinite±dickite and illite-smectite-chlorite±kaolinite, and overprints quartz-sericite±tourmaline alteration. Alunites collected from the Viruela and La Cruz ore bodies yielded a Miocene age of 12.5±1.6 Ma, much younger than all known volcanics. Their δ34S values are isotopically light (δ34Scdt (‰) = -2.7 to -4.6) indicating they were derived from pyrite and of supergene origin. Gold ore is confined to the late Eocene rocks within zones of intermediate and advanced argillic alteration originally containing sulfides, and subsequently oxidized by supergene processes. Gold occurs in a variety of locations: (1) Directly above (Viruela) and below (Cieneguita-Viruela) massive quartz with quartz-dickite-kaolinite-alunite ±barite (±illite-smectite), (2) Along fractures in the massive quartz where gold is late with specular and earthy hematite, and barite (Cieneguita-Viruela), (3) In late NE-striking crackle breccias cemented by alunite-dickite±kaolinite (La Cruz-Cerro de Oro), (4) In dacite porphyry with dickite-alunite-kaolinite-pyrite-enargite (La Cruz), or with magnetite and chalcopyrite in zones of illite-smectite-chlorite (La India-Viruela-La Cruz), (5) In coxcomb and drusy quartz veinlets-stockworks with Ag-Pb-Zn, silicification, late barite, and quartz-sericite-pyrite in Upper Dacite and Nopal Rhyolite (Cieneguita). Exploration activities and geologic studies continue at La India. The hydrothermal mineral assemblages define a complex history involving a diversity of deposit types that include: porphyry-style gold with magnetite-pyrite±chalcopyrite and illite-chlorite-smectite±kaolinite in dacitic porphyry rocks, high-sulfidation epithermal gold with dickite-kaolinite-alunite-quartz-pyrite±enargite and barite, and intermediate-sulfidation epithermal gold with quartz veins and quartz-sericite-barite.