Abstract

IZAGUIRRE, Aldo et al. Geochemical homogeneity of hydrothermal alteration in the orogenic gold belt of NW Sonora, Mexico: study of mass balance in host rocks in the mineralized areas. Bol. Soc. Geol. Mex [online]. 2012, vol.64, n.1, pp.119-153. ISSN 1405-3322.

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.