PEGMATITE DIKES OF THE JACUMBA PEGMATITE FIELD

Journal of Pegmatology VOLUME 1, NUMBER 1

Layering Diversity in the Jacumba Group Pegmatites, Jacumba, California

Cathleen D. Brown, Department of Mineral Sciences, National Museum of Natural History,
Smithsonian Institution, Washington, D.C., 20560, U.S.A.

Within a 5 km radius of the town of Jacumba, California lies the Jacumba pegmatite group. The pegmatites intrude Jurassic gneiss, schist, marble and quartzite; Cretateous granitoid bodies and dikes; and Tertiary andesitic pyroclastic tuffs, and agglomerates. The area is populated with mainly barren to weakly fractioned pegmatite dikes.

These chemically simple pegmatites typically contain microcline, plagioclase and quartz +/- biotite, muscovite, garnet and tourmaline. On rare occasions, the dikes may become sufficiently fractionated to produce beryl, columbite and/or spodumene as in the Pack Rat mine, the Charlie dog prospect and the School mine. With the exception of gemmy kunzite found at the Pack Rat pegmatite quarry, the occurrence of gem material is virtually absent. The pegmatite dikes are particularly well developed on the northeastern side of Mt. Tule.

Pegmatites may occur as discrete pegmatite dikes or as composite-layered dikes. Discrete pegmatite dikes typically display one or two zones.

Single zoned pegmatites generally contain graphic microcline ± biotite/muscovite. Pegmatites with two zones generally have a wall zone of graphic microcline ± biotite ± muscovite and a core of megacrystic perthite and quartz ± muscovite ± garnet ± schorl. Most of the pegmatite dikes of the Jacumba group are composite-layered dikes characterized by their unique variety of layering with aplite or granite.

The types of composite-layered dikes observed are as follows: 1) pegmatite margins with granite cores (PGP); 2) granite margins with pegmatite cores (GPG), 3) pegmatite on one margin with aplite or granite on the other (PA and PG respectfully); or 4) mulitple layering of granite and pegmatite (MGP).

One type of composite-layered dike may grade into a different type of composite layered dike or into a single aplite, pegmatite or granite dike. The granitic portions of the dike are most commonly fine-grained biotite granite, fine- to medium-grained two-mica granite ± garnet or fine- to medium grained muscovite aplite ± garnet. The fine-grained biotite granite is composed of 1-3 mm anhedral biotite flakes within a matrix of plagioclase, quartz and microcline. A weak foliation may be present. The two-mica granite is characterized by 1-3 mm anhedral biotite flakes, 1-3 mm euhedral to subhedral, psuedo-hexagonal muscovite books, plagioclase, quartz and microcline. Red to orange garnet may be present as euhedral 1 – 5mm crystals or as anhedral clots. The aplite contains plagioclase, microcline and quartz with euhedral to subhedral, pseudohexagonal books of muscovite ± red to orange euhedral garnet. Occasionally, the aplite will display distinct layering due to horizonally linear concentrations of 1mm sized garnet, schorl, muscovite and/or biotite.

Several granitoid bodies occurring as small plutons and/or dikes are intimately associated with the pegmatites and may be parental to the them. Field evidence indicates that the composite mulltiple pegmatite granite (MPG) dikes are related and/or cogenetic with the granite dikes in which they are occur. PGP, GPG, PA and PG dikes intrude all previously crystallized rocks and may be related to the granites that host them. Further field work and whole rock trace-element chemistry is necessary to identify the granite responsible for producing the Jacumba Group pegmatites.