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FLOHR U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1502 Petrographic features, chemistry, and origin of protoliths, which included gel-like materials, of the manganese-rich carbonate, silicates, and oxides that form lenses in cherts of the Franciscan Complex UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1990 IV CONTENTS TABLES Page TABLE 1. Known and hypothetical manganese minerals and common analogues discussed in text..................................................... 6 2. Standards used for electron microprobe analysis of samples from the Buckeye deposit, California Coast Ranges................... 10 3. Constituents of graywackes from the Buckeye deposit, California Coast Ranges.............................................................. 13 4. Characterization of metasandstones, metacarbonates, and metavolcanic rocks from the Buckeye deposit, California Coast Ranges, by X-ray powder diffractometry ........................................................................................................... 14 5. Compositions of minerals from nonsiliceous host rock, in weight percent, from the Buckeye deposit, California Coast Ranges, determined by electron microprobe analysis ........................................................................................... 15 6. Chemistry and mineralogy of selected protoliths from the Buckeye deposit, California Coast Ranges.................................. 16 7. Characterization of siliceous samples from the Buckeye deposit, California Coast Ranges, by X-ray powder diffractometry.... 20 8. Compositions of gel-like materials, in weight percent, from the Buckeye deposit, California Coast Ranges, determined by electron microprobe analysis ............................................................................................................................ 35 9. Compositions of silicates from the Buckeye deposit, California Coast Ranges, determined by electron microprobe analysis .... 36 10. Compositions of vein minerals from the Buckeye deposit, California Coast Ranges, determined by electron microprobe analysis ........................................................................................................................................................ 37 11. Compositions of rhodochrosite from the Buckeye deposit, California Coast Ranges, determined by electron microprobe analysis ........................................................................................................................................................ 38 12. Compositions of the oxides braunite and hausmannite from the Buckeye deposit, California Coast Ranges, determined by electron microprobe analysis ............................................................................................................................ 39 13. Comparison of the Buckeye deposit, California, and some modern deep marine Mn-rich deposits........................................ 49 MICROBANDED MANGANESE FORMATIONS: PROTOLITHS IN THE FRANCISCAN COMPLEX, CALIFORNIA By J. STEPHEN HUEBNER and MARTA J.K. FLOHR ABSTRACT The processes that form recent marine hydrothermal mounds may be the same as processes that formed the Buckeye deposit. Features The Buckeye manganese deposit, 93 km southeast of San Francisco common to both include the presence of Mn-oxyhydroxide crusts in the California Coast Ranges, preserves a geologic history that (corresponding to the Buckeye orebody), a large Mn/Fe ratio, low provides clues to the origin of numerous lenses of manganese carbon­ abundances of most minor elements, and small size. The most impor­ ate, oxides, and silicates that occur with interbedded radiolarian chert tant differences are the absence of rhodochrosite and manganese and metashale of the Franciscan Complex. Compositionally and miner- silicates, interlayered with oxide, and the absence of adjacent chert in alogically laminated Mn-rich protoliths were deformed and dismem­ the contemporary deposits. These differences may be due to an absence bered, in a manner that mimics in smaller scale the deformation of the of the debris of siliceous pelagic organisms, which accumulated in the host complex, and then were incipiently metamorphosed at blueschist- Buckeye paleoenvironment. Periodic turbidity flows of chert-forming facies conditions. Eight phases occur as almost monomineralic proto­ radiolarian sand could provide the changes in the fluxes of silica and liths and mixtures: rhodochrosite, caryopilite, chlorite, gageite, taney- organic matter necessary to form manganese carbonate and silicates. amalite, braunite, hausmannite, and laminated chert (quartz). Turbidity flows of graywacke indicate proximity to an environment Braunite, gageite, and some chlorite and caryopilite layers were with high relief. A possible paleodepositional environment is an oceanic deposited as gel-like materials; rhodochrosite, most caryopilite, and at spreading center approaching a continental margin at which subduction occurred. least some hausmannite layers as lutites; and the chert as turbidites of radiolarian sand. Some gel-like materials are now preserved as trans­ parent, sensibly isotropic relics of materials that fractured or shattered INTRODUCTION when deformed, creating curved surfaces. In contrast, the micrites flowed between the fragments of gel-like materials. This report documents our initial effort to understand The orebody and most of its constituent minerals have unusually the origin and metamorphism of banded sedimentary Mn-rich compositions that are described by the system MnO-Si02- manganese formations. The term "manganese forma­ 02-C02-H20. High values of Mn/Fe and U/Th, and low concentra­ tions of Co, Cu, and Ni, distinguish the Buckeye deposit from many tion" (Huebner, 1976) was specifically chosen to empha­ high-temperature hydrothermal deposits and hydrogenous or diage- size the similarities in mineralogy and texture that are netic