TECTONIC, PETROLOGIC, AND HYDROTHERMAL DYNAMICS OF GOLD MINERALIZATION IN THE NUKAY DISTRICT OF GUERRERO, MEXICO

The Nukay district is hosted by upper Cretaceous carbonate platform rocks of the northern Sierra Madre del Sur of central Guerrero state. Mineralization is centered on two early Tertiary granodioritic stocks that are part of a NW-trending porphyry belt (~65-62 Ma) that extends across central Guerrero and is associated with variable but regionally persistent Au-Cu mineralization.

Mineralization at Nukay lies along the margins of two porphyritic granodiorite-tonalite intrusions, termed the ‘West’ and ‘East’ stocks. Field evidence and 40Ar/39Ar age dates indicate that the East stock was emplaced first a complex body into relatively cool wallrocks (~64.99-64.67 Ma). This stock was intruded as a paired diorite-granodiorite magma series, the diorite forming a quenched sill that became the passive host to the Los Filos deposit (~1.4 Moz). The West stock was subsequently emplaced as a relatively simple steep-sided pluton (~64.17-63.39 Ma). The nature and style of mineralization is distinctly different for the two stocks: the East is dominated by structurally controlled meso to epithermal mineralization and the West by epithermal retrograde of prograde skarn.

Skarn alteration is locally well developed along stock contacts as endoskarn and as calcic and magnesian exoskarn. Gold mineralization shows no temporal or genetic association with prograde skarn development, but is related to meso- to epithermal alteration dominated by orthoclase, silica, sericite, clay, calcite, and sulfides dominated by pyrite and arsenopyrite.

Mineralization is associated with granodiorite porphyries that contain greatly elevated concentrations of beta-quartz phenocrysts. Fluid inclusions in these phenocrysts are high-T (>600C), hypersaline (>50%), and contain up to five solid daughter products. These intrusions are inferred to have crystallized in the presence of a flux of degassing magmatic-hydrothermal fluids that resulted in mineralization.

Stock emplacement took place during a period of local, if not regional, extension. Low-angle normal faulting and tension-gash infill veining were the dominant structural controls on mineralization in the Los Filos deposit. The Guerrero porphyry belt may represent an extensional back-arc setting developed to the east of the accreted volcanic arc terranes of western Guerrero.