KUROKO-TYPE DEPOSITS IN THE MIDDLE CRETACEOUS MARGINAL BASIN OF CENTRAL PERU

Barite, massive sulfide, and siliceous stoekwork deposits of Kuroko type in the Lima region are associated with the Casma Group, a sequence of submarine volcanic rocks of Middle Cretaceous age. These deposits were formed in an ensialic marginal basin with predominantly basaltic to andesitic fill. Volcanic-hosted deposits occur in the entire region; sediment-hosted deposits are restricted to the eastern Casma volcanic facies, which intercalate with limestones and shales deposited on a shelf platform adjacent to the marginal basin. In most cases, mineralization is spatially associated with dacitic domes and tuff breccias with zones of quartz-sericite alteration; the latter have locally been dated at 116 to 106 m.y. by the K/Ar method. Strata-bound deposits of bedded barite, pyrite, sphalerite, and pyrrhotite overlie the feeder zones. The most important deposits of this kind are Leonila-Graciela and Juanita, with 4 million tons of produced barite and 2.5 million tons of production plus reserves of massive sulfide ore. They are located in a roof pendant of folded strata intruded by two plutons of the Coastal batholith. Contact metamorphism of hornblende-hornfels facies affects both ore deposits and wall rocks. K-Ar ages on hornblende-biotite pairs from the granitic rocks indicate that they were emplaced 82 and 65 m.y. ago. Whole-rock ages on postmetamor- phic dikes vary between 31 and 39 m.y. P-T conditions for contact metamorphism of hornblende-hornfels facies at Leonila-Graciela are estimated at 2.1 to 2.6 kb and 300° to 500°C on the basis of sphalerite geobaro- metry, stratigraphic reconstruction, metamorphic mineralogy, and interpretation of discordant K/Ar age patterns. Mole percent FeS in sphalerites increases in a prograde sense from the actinolite zone at Juanita to the biotite-muscovile zone at Graciela. In massive sulfide specimens it varies correspondingly from 15.4 ± 0.2 to 17.6 ± 0.7. Sphalerites from siliceous stockworks show the same trend with 14.7 ± 0.4 mole percent FeS and 17.6 ± 1.1 mole percent FeS. Metamorphic equilibration was reached only in the biotite-muscovite zone at Graciela. This is demonstrated by the homogeneity of high mole percent FeS values detected in sphalerite, which coexists in mutual contact with pyrite and hexagonal pyrrhotite. © 1987 Society of Economic Geologists, Inc.