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

Field: Earth and Planetary Sciences (Physical Sciences)

The Rey de Plata volcanogenic massive sulfide (VMS) deposit is located in \nsouthern Mexico and within the Teloloapan subterrane of the Guerrero Composite \nTerrane. It is hosted by the Middle to Late Cretaceous Villa Ayala Formation, which \nis itself subdivided into three informal members. The lower Ahuehuetla member \nconsists of basaltic flows, sills, and volcaniclastic rocks. The middle Rey de Plata \nmember, which hosts the Rey de Plata deposit, consists of rhyolitic to rhyodacitic \nflows, sills, and volcaniclastic rocks. The timing of felsic volcanism and the \nformation of the Rey de Plata deposit is constrained between 139.1 ± 0.4 Ma to \n129.4 ± 0.7 Ma by U/Pb dating of zircons from an aphyric flow. The upper Villa \nAyala member consists of basaltic amygdaloidal sills and volcaniclastic rocks. The \nslightly LREE enrichment, negative Nb and positive Zr anomalies of the mafic \nrocks suggest that they formed either in a primitive to slightly evolved arc to backarc \nsetting with minor contamination by older arc (or continental) crust. The slight \nLREE enrichment, flat HREE pattern, and pronounced negative Nb anomaly of the \nfelsic volcanic rocks are consistent with FII rhyolites, which indicate their formation \nand emplacement during rifting within an arc or nascent back-arc environment; an \ninterpretation consistent with the geochemical signature of the associated basaltic \nrocks. \nThe Rey de Plata and Villa Ayala members are unconformably overlain by argillite \nof the Acapetlahuaya and turbiditic sandstone of the Miahuatepec Formations, \nwhich are capped by Eocene conglomerates of the Balsas Group. Detrital zircons \nfrom the Acapetlahuaya Formation have U/Pb ages ranging from 115 ± 4 Ma to \n152 ± 4 Ma, similar to the age of volcanic rocks of the Villa Ayala Formation. This \nsuggests that the Acapetlahuaya sedimentary rocks were deposited in an intra-arc \nbasin or in a back-arc basin, which separated the Villa Ayala arc from continental \nMexico. This basin was closed during deposition of the younger Miahuatepec \nFormation sedimentary rocks as these strata yielded older, 800 Ma to 1200 Ma, \nzircon populations derived from the erosion of older terranes previously accreted to \ncontinental Mexico. \nD1 at Rey de Plata corresponds to the Upper Cretaceous to Lower Tertiary Siever- \nLaramide Orogeny. A shallowly SW-dipping S1 cleavage, L1 stretching lineation, \nand NE-verging F1 folds formed in altered volcanic rocks, argillite and turbiditic \nsandstone during NE-directed D1 thrusting. D2 extensional collapse of the orogen \noccurred either during or shortly after D1. It produced asymmetrical W-verging folds \nfolding S1, and an S2 shear band cleavage (C’), indicating normal, west-directed \nslip down the dip of the S1 foliation. During D2, gold- and silver-rich epithermal \nveins were emplaced. These veins may have contributed to the high grade gold \nand silver zones of Rey de Plata deposit. Deformation of the Teloloapan \nsubterrane resumed in the Eocene with the formation of D3 normal brittle faults \nii \nlinked by strike-slip brittle faults. W-verging F3 drag folds are associated with the \nnormal faults. Extensional basins formed during D3 and were filled by fluvial \nconglomerate and ignimbrites of the Balsas Group. Late, NW-SE-directed, D4 \ncompression produced upright to inclined, NE- and SW-verging F4 folds that \noverprint S1 and S2. \nRey de Plata is a bimodal-felsic type VMS deposit. It formed during rifting of an arc \nor backarc and consists of stratigraphically stacked Zn-Cu-Pb massive sulfide ore \nlenses with high Ag-Au grades. The lenses range in thickness from 3m to 60m, in \nlength from 300m to 1500m, and in width from 100m to 500m. They consist of \nmassive, semi-massive, and disseminated pyrite ± sphalerite ± galena ± \nchalcopyrite ± silver sulphosalts ± gold, together with gangue minerals of quartz ± \nbarite. Extensive quartz-sericite-pyrite alteration surrounds the ore lenses. \nGraphitic argillite units up to 40m in thickness typically occur at the top and bottom \ncontacts of the ore lenses. The ore lenses underwent the same deformation history \nas their host rocks. During D1, they developed a strong S1 foliation, they were \nstretched parallel to L1, and their base metal zonation reflects clockwise rotation of \nthe lenses (looking NW). Thereafter, during D2 and D3, the ore lenses were \ndisplaced southwestward by 200 meters along normal faults and shears and then \nopenly folded by F4 during D4.