Geology of the San Dieguito pyrophyllite area: San Diego County, California

Abstract

The San Dieguito area of west-central San Diego County, California, is one of the few localities in western North America where pyrophyllite has been produced in commercial quantities. The output from this area has been derived chiefly from one large deposit that was opened up for mining in 1945, and amounted to slightly more than 10,000 tons by mid-1949. Most of the pyrophyllite has been marketed as an insecticide carrier. The deposits occur in the Santiago Peak volcanic series, which comprises flows, breccias, agglomerates, and tuffs of probable Jurassic age. A group of intrusive rocks, in which tonalite is the most abundant type, is closely related in space to the volcanic series, and probably is of later Jurassic age. A second, more widespread group of intrusive rocks cuts all the other crystalline types, and forms a part of the southern California batholith, which is Cretaceous in age. Stock-like bodies of Escondido Creek leucogranodiorite and larger masses of Woodson Mountain granodiorite are the principal batholith representatives in and near the San Dieguito area. Both the volcanics and the older of the intrusive rocks have been affected by regional metamorphism of low rank. This metamorphism appears to antedate the rocks of the batholith. The crystalline rocks are unconformably overlain by poorly consolidated arkosic sands and interbedded silts that probably are a part of the La Jolla formation of Eocene age. These sediments are in turn unconformably overlain by coarse-grained Quaternary terrace gravels. The structure of the Santiago Peak volcanic series is essentially homoclinal. Shearing and schistosity are locally prominent, and commonly are subparallel with primary layering in the rocks. A low-angle discordance also is common. The intrusive rocks show little well developed planar structure. The Tertiary and Quaternary sediments have been very mildly deformed. The pyrophyllite-bearing rocks are discontinuously exposed within an area of about one square mile. They represent progressive stages in the alteration of the volcanic rocks at some time distinctly later than the regional metamorphism. The original rocks were mainly flows, breccias, and tuffs that ranged in composition from andesite to rhyolite. Pyrophyllitization appears to have been guided chiefly by pre-metamorphism fracturing and shearing, and to a lesser degree by variations in the original composition of the rocks. A marked silicification preceded the pyrophyllitization, but much quartz that is associated with pyrophyllite probably formed contemporaneously with it. The most thorough pyrophyllitization appears to have taken place in the less silicified rocks. The pyrophyllite-bearing rocks form elongate lenses that vary considerably in size and attitude and in general are conformable with the dominant planar structure of the host volcanics. Nearly all of the pyrophyllite mined to date has been obtained by open-pit methods from a single mass of high-grade schist at least 150 feet long and 15 feet in average thickness. Most of the other pyrophyllite bearing rock in the area is not of commercial grade. Petrographic data and chemical analyses indicate that the development of pyrophyllite was accompanied by the introduction of SiO2, Al2O3, and probably OH. A hypogene, rather than supergene, origin is indicated by (1) the localization of sulfide minerals in the pyrophyllite-bearing rocks. (2) alteration of an aggregate and nonselective, rather than a sequential type. (3) the occurrence of pyrophyllite and sulfides as consistently older minerals than those of distinctly supergene origin, and (4) by geochemical data that place the lower limit of pyriphyllite formation in the 250°-300° C. temperature range. The San Dieguito deposits are believed to have formed under conditions of intermediate temperatures and pressures, and from solutions related genetically to the older group of intrusive rocks.

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