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Description
The Sacatone Springs gem-bearing granite pegmatites of eastern San Diego County form part of the Jacumba dike swarm which is an extensive swarm of subparallel dikes that strike approximately N-S and extend for nearly 10 kilometers along strike. The swarm is hosted by the amphibolite facies metasedimentary and metaigneous wallrock of the Carrizo Gorge inlier, which separates the La Posta pluton into western and eastern parts. The lightcolored dikes of the Jacumba swarm contrast strongly with the dark metamorphic host rocks and hence are easily visible on aerial photographs. The garnetiferous 2-mica monzogranite Indian Hill pluton occurs to the NE of the main Jacumba swarm; the compositionally similarly Boulevard pluton 2-mica monzogranite occurs at the south edge of the swarm along Interstate-8. The Sacatone Springs pegmatites occur at the north end of the Jacumba dike swarm. Two main dikes, which are encompassed at their northern extent by the Pack Rat mining claim, were investigated in this study. The lowermost of the two dikes is famous for "Beebes Hole" at the northern tip of the dike where big kunzite crystals were discovered. The Sacatone Springs dikes exhibit strong internal textural variability and include aplite, pegmatite, and related textural variants. Pegmatite segregations typically exhibit a wall zone of graphic microcline ± biotite/muscovite and a core of megacrystic perthite and quartz ± muscovite ± garnet ± schorl. Pegmatitic segregations are bounded along sharp contacts by aplite and coarser granite. This granite "host" to the pegmatite ranges from a fine-grained biotite granite to a medium-grained 2-mica granite ± garnet. The source of the Jacumba dike swarm is unclear but the compositions of the dikes suggest it may have originated from one of the nearby Cretaceous plutons. This study had two main objectives: 1) to try and resolve the source of the Sacatone Springs pegmatite by comparison of whole rock X-ray fluorescence major and trace element analysis of the dikes with surrounding plutonic units, and 2) to investigate whether an individual dike shows any evidence for along strike compositional variation. Dike samples were taken from the aplitic parts of the dikes that are more representative of magma bulk compositions. The dike samples are monzogranites with SiO2 contents of~ 75 wt% and K/Na ratios of~ 1 and are similar in this respect to the Indian Hill and Boulevard plutons. The dikes on the other hand are compositionally very different from the La Posta pluton with the possible exception of one sample. Whole rock chemical analysis is therefore consistent with a monzogranite Indian Hills-type source for the Jacumba pegmatite swarm although none of the dikes have been traced back directly into this potential source. Internal along-strike variation within an individual dike was not detected at the sampling scale used.
