A MODEL FOR THE FORMATION AND EMPLACEMENT OF 'POCKET' CLAY IN THE GRANITIC PEGMATITES OF SAN DIEGO COUNTY, CALIFORNIA

Foord and others (1986, Amer. Min. V71, p. 428) propose that the formation of 'pocket' clay (a Ca-Mg-Fe clay) occurred in the surrounding, dioritic country rock. However, they deal with emplacement only generally and descirbe it as 'hydrothermal circulation'. They also chacterize 'pocket clay' as supergene and hence relate it to near-surface weathering.

Assume that, at emplacement, these pegmatite magmas were nearly saturated with water (~5 wt.%), then ~4 wt.% water exsolved (pegmatite rock contains ~0.6 wt.% water)--the resultant fluid (when it escaped) represented a volume approximately 2/3 the volume of the solidified body. If, in addition, the time, temperatures and pressures determined by Taylor and others (1979, Cont. Min. Pet., V68, p. 187) are correct, then the proposed, resulting sequence of events is as follows: 1)emplacement of pegmatite magma; time (before present) ~100 myr; 2) exsolution and loss of fluid until the bodies were ~70 percent solidified (early-fluid loss; ~4/5 of total fluid); escape of early fluid from the pegmatite body was controlled by plasticity of the body and country-rock permeability; ~100 myr; 3) subsequent trapping of the last (~1/5) of the fluid to form cavities (P(H2O)=P(total) only for the fluid trapped within the pegmatite body, otherwise P(H2O) ~ 1/3P(total)); ~100 myr; 4) crystallization and alteration in cavities plus country-rock alteration; ~100-90 myr; 5) brittle collapse of the pegmatite cavity rock after sufficient cooling and after the trapped fluids decrease in volume (forming a clast-supported breccia); cavity fluid was lost; because of the pressure difference, escape may have been violent; ~100-90 myr; 6) continued cooling (supercritical state) resulted in a lower pressure in the interstices of the breccia and previously expelled fluid was drawn back along with altered material from the country rock. This last event produced filled fractures leading to the cavities as well as filled breccia in the former cavity volume; ~100-90 myr; 7) initial country-rock alteration products (inplace) were converted to clay (note--the long period of time allows very slow reation rates) and the pegmatite body was uncovered; ~90-0 myr.