Boletín de la Sociedad Geológica Mexicana Volumen 64, núm. 1, 2012, p. 119-153 http://dx.doi.org/10.18268/BSGM2012v64n1a10

Homogeneidad geoquímica de la alteración hidrotermal del cinturón de oro orogénico del NW de Sonora, México: Estudio de balance de masas en rocas encajonantes de la mineralización

Aldo Izaguirre^1,*, Alexander Iriondo^{1, 2}, Juan Antonio Caballero-Martínez³, Francisco Moreira-Rivera³, Enrique Espinosa-Arámburu³

¹ Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Boulevard Juriquilla 3001, Juriquilla, 76230, Querétaro, México
² Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, 78712, Texas, EUA
³ Servicio Geológico Mexicano, Boulevard Felipe Ángeles km 93.50-4, Colonia Venta Prieta Pachuca, 42080, Hidalgo, México

^* This email address is being protected from spambots. You need JavaScript enabled to view it.

 Abstract

A geochemical mass balance study, combined with petrography, bivariate Au and Ag geochemical analysis, and inclusion study in pyrites and native gold, was performed on a suite of hydrothermally altered host rocks and contemporaneous quartz veins from the Caborca Orogenic Gold Belt in NW Sonora, Mexico (COOC; by its abbreviations in Spanish: Cinturón de Oro Orogénico Caborca). These combined studies helped evaluating the geochemical homogeneity of the mineralizing fluids that formed the alteration envelopes and the auriferous quartz veins in the entire area occupying the COOC (~400 km long and ~60-80 km wide belt).

The isocon statistical technique used in the mass balance study was applied on 20 pairs of mayor and trace element geochemical data of altered and fresh rock samples. The results show that the majority (>50 %) of the altered host rocks are enriched in K₂O, W, Cu, Pb, Mo, Tl, Be, Zn, V, Sb, Ag, Sn, SiO₂, Se, Ni and FeO, and are clearly depleted in Ca, Y, Na, Ho, Co, Sr, Yb, Mn, Dy, Er and Tb. The similar pattern of enrichment and depletion was shown by most of the studied samples suggesting a high degree of geochemical homogeneity of the mineralizing fluids.

Petrography on both the altered and fresh rocks indicates that the hydrothermal alteration primarily consisted of silicification, sericitization, chloritization, epidotization and pyritization. The bivariate Au and Ag geochemical studies, done in auriferous quartz veins, show a strong positive correlation between these two elements and the enriched elements shown by the mass balance study on the altered host rocks. Therefore, these elements could be utilized as a geochemical exploration guide to targeting orogenic gold deposits in the region. Multiple inclusions of native gold and silver, galena, covellite, sphalerite, molybdenite, scheelite and tourmaline were encapsulated in pyrite and native gold grains in the quartz veins. Presence of these mineral occurrences explains most of the elemental anomalies obtained geochemically on the quartz veins and altered host rocks.

In conclusion, the geochemical homogeneity of the mineralizing fluid, proposed in this study, implies that the mineralization (Au-rich quartz veins) and associated alteration assemblages were part of a large-scale event synchronous with the Laramide orogeny when compressional tectonism was active during Late Cretaceous to Early Tertiary time.

Keywords: Mass balance, homogeneity, hydrothermal alteration, mineralization, orogenic gold, Sonora, México, isocon method, quartz veins, electron microscopy, bivariate correlaction.