Structural and stratigraphic setting of the Rey de Plata volcanogenic massive sulfide deposit in the Guerrero Composite Terrane, Mexico

Abstract

The Rey de Plata volcanogenic massive sulfide (VMS) deposit is located in southern Mexico and within the Teloloapan subterrane of the Guerrero Composite Terrane. It is hosted by the Middle to Late Cretaceous Villa Ayala Formation, which is itself subdivided into three informal members. The lower Ahuehuetla member consists of basaltic flows, sills, and volcaniclastic rocks. The middle Rey de Plata member, which hosts the Rey de Plata deposit, consists of rhyolitic to rhyodacitic flows, sills, and volcaniclastic rocks. The timing of felsic volcanism and the formation of the Rey de Plata deposit is constrained between 139.1 ± 0.4 Ma to 129.4 ± 0.7 Ma by U/Pb dating of zircons from an aphyric flow. The upper Villa Ayala member consists of basaltic amygdaloidal sills and volcaniclastic rocks. The slightly LREE enrichment, negative Nb and positive Zr anomalies of the mafic rocks suggest that they formed either in a primitive to slightly evolved arc to backarc setting with minor contamination by older arc (or continental) crust. The slight LREE enrichment, flat HREE pattern, and pronounced negative Nb anomaly of the felsic volcanic rocks are consistent with FII rhyolites, which indicate their formation and emplacement during rifting within an arc or nascent back-arc environment; an interpretation consistent with the geochemical signature of the associated basaltic rocks. The Rey de Plata and Villa Ayala members are unconformably overlain by argillite of the Acapetlahuaya and turbiditic sandstone of the Miahuatepec Formations, which are capped by Eocene conglomerates of the Balsas Group. Detrital zircons from the Acapetlahuaya Formation have U/Pb ages ranging from 115 ± 4 Ma to 152 ± 4 Ma, similar to the age of volcanic rocks of the Villa Ayala Formation. This suggests that the Acapetlahuaya sedimentary rocks were deposited in an intra-arc basin or in a back-arc basin, which separated the Villa Ayala arc from continental Mexico. This basin was closed during deposition of the younger Miahuatepec Formation sedimentary rocks as these strata yielded older, 800 Ma to 1200 Ma, zircon populations derived from the erosion of older terranes previously accreted to continental Mexico. D1 at Rey de Plata corresponds to the Upper Cretaceous to Lower Tertiary Siever- Laramide Orogeny. A shallowly SW-dipping S1 cleavage, L1 stretching lineation, and NE-verging F1 folds formed in altered volcanic rocks, argillite and turbiditic sandstone during NE-directed D1 thrusting. D2 extensional collapse of the orogen occurred either during or shortly after D1. It produced asymmetrical W-verging folds folding S1, and an S2 shear band cleavage (C’), indicating normal, west-directed slip down the dip of the S1 foliation. During D2, gold- and silver-rich epithermal veins were emplaced. These veins may have contributed to the high grade gold and silver zones of Rey de Plata deposit. Deformation of the Teloloapan subterrane resumed in the Eocene with the formation of D3 normal brittle faults ii linked by strike-slip brittle faults. W-verging F3 drag folds are associated with the normal faults. Extensional basins formed during D3 and were filled by fluvial conglomerate and ignimbrites of the Balsas Group. Late, NW-SE-directed, D4 compression produced upright to inclined, NE- and SW-verging F4 folds that overprint S1 and S2. Rey de Plata is a bimodal-felsic type VMS deposit. It formed during rifting of an arc or backarc and consists of stratigraphically stacked Zn-Cu-Pb massive sulfide ore lenses with high Ag-Au grades. The lenses range in thickness from 3m to 60m, in length from 300m to 1500m, and in width from 100m to 500m. They consist of massive, semi-massive, and disseminated pyrite ± sphalerite ± galena ± chalcopyrite ± silver sulphosalts ± gold, together with gangue minerals of quartz ± barite. Extensive quartz-sericite-pyrite alteration surrounds the ore lenses. Graphitic argillite units up to 40m in thickness typically occur at the top and bottom contacts of the ore lenses. The ore lenses underwent the same deformation history as their host rocks. During D1, they developed a strong S1 foliation, they were stretched parallel to L1, and their base metal zonation reflects clockwise rotation of the lenses (looking NW). Thereafter, during D2 and D3, the ore lenses were displaced southwestward by 200 meters along normal faults and shears and then openly folded by F4 during D4.