Lithologic controls on mineralization at the Lagunas Norte high-sulfidation epithermal gold deposit, northern Peru

The 13.1-Moz high-sulfidation epithermal gold deposit of Lagunas Norte, Alto Chicama District, northern Peru, is hosted in weakly metamorphosed quartzites of the Upper Jurassic to Lower Cretaceous ChimA(0) Formation and in overlying Miocene volcanic rocks of dacitic to rhyolitic composition. The Dafne and Josefa diatremes crosscut the quartzites and are interpreted to be sources of the pyroclastic volcanic rocks. Hydrothermal activity was centered on the diatremes and four hydrothermal stages have been defined, three of which introduced Au +/- Ag mineralization. The first hydrothermal stage is restricted to the quartzites of the Chimu Formation and is characterized by silice parda, a tan-colored aggregate of quartz-auriferous pyrite-rutile +/- digenite infilling fractures and faults, partially replacing silty beds and forming cement of small hydraulic breccia bodies. The delta S-34 values for pyrite (1.7-2.2 parts per thousand) and digenite (2.1 parts per thousand) indicate a magmatic source for the sulfur. The second hydrothermal stage resulted in the emplacement of diatremes and the related volcanic rocks. The Dafne diatreme features a relatively impermeable core dominated by milled slate from the Chicama Formation, whereas the Josefa diatreme only contains Chimu Formation quartzite clasts. The third hydrothermal stage introduced the bulk of the mineralization and affected the volcanic rocks, the diatremes, and the Chimu Formation. In the volcanic rocks, classic high-sulfidation epithermal alteration zonation exhibiting vuggy quartz surrounded by a quartz-alunite and a quartz-alunite-kaolinite zone is observed. Company data suggest that gold is present in solid solution or micro inclusions in pyrite. In the quartzite, the alteration is subtle and is manifested by the presence of pyrophyllite or kaolinite in the silty beds, the former resulting from relatively high silica activities in the fluid. In the quartzite, gold mineralization is hosted in a fracture network filled with coarse alunite, auriferous pyrite, and enargite. Alteration and mineralization in the breccias were controlled by permeability, which depends on the type and composition of the matrix, cement, and clast abundance. Coarse alunite from the main mineralization stage in textural equilibrium with pyrite and enargite has delta S-34 values of 24.8-29.4 parts per thousand and delta O-18(SO4) values of 6.8-13.9 parts per thousand, consistent with H2S as the dominant sulfur species in the mostly magmatic fluid and constraining the fluid composition to low pH (0-2) and logfO(2) of -28 to -30. Alunite-pyrite sulfur isotope thermometry records temperatures of 190-260 A degrees C; the highest temperatures corresponding to samples from near the diatremes. Alunite of the third hydrothermal stage has been dated by 40Ar/39Ar at 17.0 A +/- 0.22 Ma. The fourth hydrothermal stage introduced only modest amounts of gold and is characterized by the presence of massive alunite-pyrite in fractures, whereas barite, drusy quartz, and native sulfur were deposited in the volcanic rocks. The delta O-18(SO4) values of stage IV alunite vary between 11.5 and 11.7 parts per thousand and indicate that the fluid was magmatic, an interpretation also supported by the isotopic composition of barite (delta S-34 = 27.1 to 33.8 parts per thousand and delta O-18(SO4) = 8.1 to 12.7 parts per thousand). The Delta S-34(py-alu) isotope thermometry records temperatures of 210 to 280 A degrees C with the highest values concentrated around the Josefa diatreme. The Lagunas Norte deposit was oxidized to a depth of about 80 m below the current surface making exploitation by heap leach methods viable.