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Mineralogy and Geochemical Evolution of the Little Three

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Mineralogy and Geochemical Evolution of the Little Three American Mineralogist, Volume 71, pages 406427, 1986 Mineralogy and geochemicalevolution of the Little Three pegmatite-aplite layered intrusive, Ramona,California L. A. SrnnN,t G. E. BnowN, Jn., D. K. Brno, R. H. J*rNs2 Department of Geology, Stanford University, Stanford, California 94305 E. E. Foono Branch of Central Mineral Resources,U.S. Geological Survey, Denver, Colorado 80225 J. E. Snrcr,nv ResearchDepartment, Gemological Institute of America, 1660 Stewart Street,Santa Monica, California 90404 L. B. Sp.c,uLDrNG" JR. P.O. Box 807, Ramona,California 92065 AssrRAcr Severallayered pegmatite-aplite intrusives exposedat the Little Three mine, Ramona, California, U.S.A., display closelyassociated fine-grained to giant-texturedmineral assem- blageswhich are believed to have co-evolved from a hydrous aluminosilicate residual melt with an exsolved supercriticalvapor phase.The asymmetrically zoned intrusive known as the Little Three main dike consists of a basal sodic aplite with overlying quartz-albite- perthite pegmatite and quartz-perthite graphic pegmatite. Muscovite, spessartine,and schorl are subordinate but stable phasesdistributed through both the aplitic footwall and peg- matitic hanging wall. Although the bulk composition of the intrusive lies near the haplo- granite minimum, centrally located pockets concentratethe rarer alkalis (Li, Rb, Cs) and metals (Mn, Nb, Ta, Bi, Ti) of the system, and commonly host a giant-textured suite of minerals including quartz, alkali feldspars, muscovite or F-rich lepidolite, moderately F-rich topaz, and Mn-rich elbaite. Less commonly, pockets contain apatite, microlite- uranmicrolite, and stibio-bismuto-columbite-tantalite.Several ofthe largerand more richly mineralized pockets of the intrusive, which yield particularly high concentrationsof F, B, and Li within the pocket-mineral assemblages,display a marked internal mineral segre- gation and major alkali partitioning which is curiously inconsistent with the overall alkali partitioning of the system. Calculationsof phaserelations betweenthe major pegmatite-aplitemineral assemblages and supercritical aqueous fluid were made assuming equilibrium and closed-systembe- havior as a first-order model, although these assumptions may not be wholly correct. Isobaric phase diagrams were generatedand are used to constrain the observed mineral assemblagesin the system KrO-NarO-AlrO3-SiOr-HrO as a function of temperature and cation-activity ratios of the coexisting fluid phase. Log activity-temperature plots of the stoichiometric assemblagemicrocline-albite-quartz-muscovite at 2 kbar show that log a**r", of the fluid phaseremains essentiallyconstant throughout crystallization, while log QN"*nr1 rises slightly with falling temperatures.Modeling muscovite nonstoichiometry by decreas- ing activity of the pure component from unity to 0.5 markedly shifts the upper thermal stability limit of the assemblagemicrocline-albite-quartz-muscovite from approximately 630'C upward to 710'C. In the pegmatite-aplite system exposedat the Little Three main dike, crystallization temperaturesare thought to have ranged from approximately 700t downward toward 540'C at the central pocket zone; muscovite nonstoichiometry caused by F, Li, and Mn substitution may have contributed to the stability of this assemblageat the outer zonesof the system,where stoichiometric muscovite is not predicted to be stable. INrnorucrroN Diego County, California, is famous for its past produc- The Litfle Three layered pegmatite-apliteintrusive, lo- tion of gem-quality tourmaline and topaz, as well as for cated in the Ramona pegmatite district of central San its yield of exceptionalpocket matrix specimens.From a presents 'Presentaddress: U.S. GeologicalSurvey, 345 Middlefield petrologicalviewpoint, this system an interesting Road,Mail Stop977, Menlo Park, California 94025. example of a crystallization sequenceof magmatic origin 2Deceased December 31, 1983. which is believed to have involved an initially water- 0003{04x/86/03044406$02.00 406 STERN ET AL.: LITTLE THREE PEGMATITE-APLITE 407 Little Three Mine, 1983 locofions F'.jl Pocket c ross seclfon ,\ j' loco fions 1 New Spoulding Fig. l. Presentworkings of the Little Three mine. saturatedgranitic residual fraction with exsolvingand co- et. Equilibrium-phase-relation calculations are also pre- existing supercritical aqueousfluid (Jahnsand Burnham, sentedhere, serving as a first-order model to place some 1969).The Little Three main dike, exposedat the Little constraintson the fluid-solid interactions that took place Three mine, exhibits a marked vertical contrastin texture, during the evolution of this system. These calculations composition, and zoning patterns, such that massive and model the evolution of the pegmatite-apliteintrusive from layeredaplite definesthe footwall ofthe intrusive, overlain coexistingsilicate melt and supercriticalaqueous fluid and by increasinglycoarse-grained pegmatite and graphic peg- are assessedthrough a seriesof isobaric phase diagrams matite alongthe hangingwall. Centrally located"pockets" relating calculated activity ratios ofaqueous speciesand host a giant-texturedsuite ofgranitic mineral assemblages stable mineral assemblagesin the system KrO-NarO- as well as some unusual accessoryminerals. AlrO3-SiOr-HrO. This graphicalapproach models general This project was initiated over nine years ago when a trends among cation-activity ratios and observed stable large, richly mineralized pocket was discovered by L. B. mineral assemblages,efected by changing temperature Spaulding, Jr. A variety of museum-quality specimens and mineral stoichiometry. were excavated from this exceptional pocket, and the structure and contentsof the pocket were documented.A Loc.nrroN AND GENERALFEATURES continuing collaboration between Spaulding and the Stan- Numerous geologistshave documentedvarious features ford workers also permitted documentation of the con- of the Ramona pegmatitedistrict. The location of the San tents of other pocketswithin the main pegmatite dike, as Diego County pegmatite districts is shown by Jahns and well as of the zoning sequenceof this asymmetrically lay- Wright (1951), and the generalgeological features of the ered body. Ramona district are describedby Simpson (1965). Gen- The internal zonation and three-dimensionalnature of eral mineralogical and zonational features of San Diego the Little Three main dike are explored in the present County granitic pegmatitesare describedby Jahns(1955, study, in conjunction with a detailed mineralogic study 1982),Simpson (1962, 1965),Jahns and Tuttle (1963), of the pocket material recovered from the mine since Foord (1976, 1977),Shieley (1982), and Shigleyand Brown 1976,with emphasison the large"New Spaulding" pock- (1985).Analyses of bulk composition and of averagezone 408 STERNET AL.: LITTLE THREEPEGMATITE.APLITE qz- perthite grophic gronite qz - ol bi fe- pe rlhi t e pegm ot i I e + l0cm - loyered op/ite ("line rock" ) mossiveoplife counlry rock (mof ic tonolite ) .t A' I 9 cm j +ljcm- Fig.2a. Cross sectionA-A' acrossmain tunnel (darkly shadedarea), with vertical section showing units. Line rock locally splits and remergesdown dip. compositions of the Ramona district pegmatite-aplitein- pocket pegmatite-aplitebodies are documentedby Taylor trusives are reported by Simpson (1965) and compared et al. (1979). with layered pegmatite-aplite intrusives from other San The Little Three main dike is one of five principal peg- Diego County localities by Jahnsand Tuttle (1963).Fluid- matite dikes on the Little Three property. These dikes, inclusion and stable-isotopestudies of San Diego County including the Little Three main dike, the Hercules-Spes- B, f, scAG-, Fig. 2b. Cross section B-B' along main tunnel. Darkly shaded area indicates small adit of of main tunnel. STERNET AL.: LITTLE THREEPEGMATITE.APLITE 409 3m membersofthe CretaceousSouthern California batholith. A geologicmap and cross-sectionthrough the central por- tion of the Ramona pegmatitedistrict are given by Simp- son (1965,Figs. I and 2). The segment of the main dike exposed at the Little 2m Three mine averagesa N55"W strike, dipping 10'-65'SW, and transectsa mafic facies of the Green Valley tonalite. The Little Three main dike crops out along hillside ex- posureswhere it is folded on a dip-slope orientation and 1m runs perpendicularto it, presentingan irregularly rolling surfacein cross-sectionalview. A plan-view map of the present workings is shown in Figure l, and the cross- sectionalviews of the layered intrusive illustrated in Fig- ures 2a-2d show the rolling nature ofthe dike as well as the generalconcordancy ofthe internal zoning sequence. Locally the dike partially separatesinto two thinner, con- tiguouslayering sequences, remerging down dip to a single thick sequence(Fig. 2a). The contact betweenthe granitic pegmatite-apliteintrusive and the enclosinggabbroic-ton- Fig. 2c. Crosssection C{' acrossmain tunnel(darkly shaded alitic wall rock is sharply defined, and the host rocks gen' area). erally display little evidence of alteration except where proximal to large pockets. sartine dike, the Spauldingdike, the Sinkankasdike, and Despite similar bulk compositions and asymmetrical the axinite-bearing Hatfield Creek dike, apparently formed zoning featuresamong the dikes on the Little Three prop- aslocally anastomosingbut essentiallyseparate intrusives. erty, pocket-mineral assemblagesvary significantly; each The Little Three dikes occur with other Ramona district dike has its own characteristic pocket mineralogy,
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