Geologic Setting, Mineralogy, and Geochemistry of the Early Tertiary Au-Rich Volcanic-Hosted Massive Sulfide Deposit of La Plata, Western Cordillera, Ecuador

Abstract

La Plata is, economically, the most important of several volcanic-hosted massive sulfide (VHMS) deposits of the Western Cordillera of Ecuador, with preliminary estimated reserves of 840,000 metric tons (t) at 4.8 g/t Au, 54 g/t Ag, 4.1 percent Cu, 0.7 percent Pb, and 4.2 percent Zn. Like the other VHMS deposits of the Western Cordillera, it is hosted by the Early Tertiary Macuchi unit, which represents an oceanic island-arc sequence, composed of submarine volcanic and volcaniclastic sequences of basaltic to andesitic compositions, accreted to the continent along the Chimbo-Toachi shear zone in the late Eocene.La Plata displays a typical VHMS vertical zonation, from bottom to top, of a pyrite stockwork, disseminated sulfides, massive sulfide lenses, and a thin red jasper bed with exhalative-sedimentary textures. The massive sulfide lenses are located within footwall andesitic volcanic and volcaniclastic rocks that are pervasively altered to a quartz-pyrite-sericite assemblage, and immediately below a hanging-wall sequence consisting of at least two basaltic to basaltic-andesitic lava flows, each capped by autoclastic breccia intervals. The hanging-wall rocks, especially the autoclastic breccias, are hematite altered up to 300 m above the ore level. Hanging-wall volcanic rocks have geochemical features typical of arc tholeiites. Dolerite dikes in the hanging wall have middle-ocean ridge basalt (MORB)-type geochemical features likely indicating deposit formation in an intra-arc extensional environment.The massive sulfide lenses are situated mainly on the eastern side of an antiform with footwall rocks at the core and hanging-wall rocks on both flanks. Lithologic contacts, volcanic strata, and axial planes of folds have a consistent north-south strike. Footwall rocks at the core of the antiform are locally affected by meter-thick, north-south-trending subvertical shear bands that result in an intense tectonic foliation. Deformation structures are absent in the hanging-wall rocks suggesting that the footwall rocks, due to a different rheology, were ductile during deformation. Centimeter-to decimeter-sized massive sulfide clasts are incorporated into a mylonitic matrix of altered footwall rocks within these shear bands that contain dextral movement indicators. The larger massive sulfide lenses (up to 100 m long) probably result also from tectonic dismembering, within the footwall rocks, of a former, larger, massive sulfide body. The deformation is probably related to the ChimboToachi dextral shear zone situated a few kilometers to the east of La Plata.The main ore minerals are pyrite, chalcopyrite, sphalerite, bornite, and galena, with minor tennantite and digenite. The gangue minerals are barite, quartz, and sericite. Mineralogical banding, defined by chalcopyriterich layers alternating with pyrite-and/or sphalerite-rich layers, is mostly the result of tectonic injection of ductile chalcopyrite, bornite, and galena within shear bands of cataclastic pyrite and sphalerite. Pyrite and sphalerite also show mild r...

Features of gold-bearing quartz veins in an artisanal mining-dominated terrain, Batouri gold district, Eastern region of Cameroon

“…Quartz from the Mama Shear zone show a stock work texture with hematite filling fractures. This is similar to mineralization reported by Kreuzer (2006) and Chiaradia et al (2008). Gold occurs as inclusions in hematite.…”

“…The absence of such features renders it difficult to identify the direction of shear which can be used to establish its kinematics. A number of gold mineralization associated with shear zones have been reported worldwide (Harraz, 1999;Khalil et al, 2003;Chiaradia et al, 2008;Zoheir, 2008;Zoheir et al, 2017). The most suitable veins for artisanal mining in this area should have the ductile features itemized above.…”

Gold- and Silver-Rich Massive Sulfides from the Semenov-2 Hydrothermal Field, 13°31.13′N, Mid-Atlantic Ridge: A Case of Magmatic Contribution?

“…Gold and silver are among the most valuable by-products of continental volcanic-hosted massive sulfide (hereafter, VHMS) deposits, especially in those of the Kuroko (or Baimak in the Urals) type associated with felsic volcanic rocks (e.g., Urabe and Sato, 1978;Prokin and Buslaev, 1998;Herrington et al, 2005;Dubé et al, 2007Dubé et al, , 2014Chiaradia et al, 2008;Glasby et al, 2008;Mercier-Langevin et al, 2014). In modern seafloor settings, high Au and Ag grades are typical of hydrothermal sulfide fields hosted by island-arc and back-arc structures (e.g., JADE field, Okinawa Trough, up to 24 ppm Au and 1.1 wt % Ag; Halbach et al, 1993; Brother volcano, Kermadec volcanic arc, up to 91 ppm Au; de Ronde et al, 2011; Vai Lili field, Lau back-arc basin, up to 28.7 ppm Au and 300 ppm Ag; Herzig et al, 1993; PACMANUS field, Manus back-arc basin, up to 57 ppm Au and 585 ppm Ag; Ihle et al, 2005) and by ultramafic rocks of the Mid-Atlantic Ridge (hereafter, MAR) (e.g., Logatchev-1 field, up to 56 ppm Au and 265 ppm Ag; Murphy and Meyer, 1998; Rainbow field, up to 51 ppm Au and 46.2 ppm Ag; Marques et al, 2007;Fouquet et al, 2010) (Table 1).…”

Inferences on the influences of age & porosity on oxidative weathering of seafloor massive sulfide deposits at the endeavour segment of the Juan de Fuca Ridge

“…Previous studies of massive sulfides have examined their formation processes (Ohmoto, 1996), internal structure (Ishizu et al 2019), and chemical composition (Corliss et al 1978;Firstova et al 2016) via methods like electrical resistivity tomography (ERT) (Ishizu et al 2019), X-ray Diffraction (XRD) (Firstova et al 2016), Atomic Absorption Spectroscopy (AAS) and X-Ray Fluorescence (XRF) (Corliss et al 1978). Geochemical properties of massive sulfides in various regions have been described (Relvas et al 2006;Chiaradia et al 2008;Sommerfield et al 2019;Conde & Torno, 2020). The fate of heavy metals during oxidative weathering (Hu et al 2022) and microbially-mediated massive sulfide degradation (Edwards, 2004) have been studied, however, the relationship between the physical and chemical characteristics of massive sulfide deposits, specifically as it influences oxidative weathering, has remained largely unresolved.…”