Abstract

Barite, massive sulfide, and siliceous stockwork 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-serieite 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 postmetamorphic 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 degrees to 500 degrees C on the basis of sphalerite geobarometry, 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-muscovite zone at Gracicla. In massive sulfide specimens it varies correspondingly from 15.4 + or - 0.2 to 17.6 + or - 0.7. Sphalerites from siliceous stockworks show the same trend with 14.7 + or - 0.4 mole percent FeS and 17.6 + or - 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.

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