USGS Professional Paper 1361 - State of Arizona Department of ...

30 GEOLOGY Ai'\'D GOLD MINERAliZATION Of THE GOLD BASIN·LOST BASIN MINING DISTRICTS, ARIZONAearliest stable assemblages in the gneiss. However, theparagenetic position ofsillimanite in garnet-biotite assemblage(5) is problematicaL Careful examination ofgarnetsillimanitetextural relations reveals the crystals ofgarnetto be corroded marginally and embayed locally by stoutcrystals of sillimanite (fig. 1M). Further, excellent texturalevidence in such rocks shows that garnet is veinedand partially replaced by assemblage (6). Nonetheless,many garnet crystals contain swarms of very fine grainedneedlelike crystals of sillimanite aligned parallel to thetrace of the flowlines in the enclosing matrix of gneiss.Such sillimanite elsewhere has been interpreted by others(Reinhardt, 1968) as an early relict phase initially stablewith its host garnet as a two-phase subsystem. However,the overall fabric of such needles of sillimanite in garnethere suggests to us that crystallization of these needlesoccurred penecontemporaneously with the crystallizationof the surrounding stout sillimanite, which in places embaysand thus somewhat postdates the parageneticallyearlier garnet. Nonetheless, because textural criteria arehighly interpretive as to compatibilities and because somebiotite seems to have replaced an early sillimanite (fig.138), we assign sillimanite tentatively as a queried phasein early assemblage (5).D '-l --"o,~ ..lLI.IMfTUlFIGURE 13.-Textural relations ofsillimanite. Plane-polarized light. A,Embayed IU1d highly corroded crysta.l ofgarnet (G) showing replacementand microveining by assemblage of biotite (B), sillimanite (5),quartz(Q), and opaque mineral(O). Sample GM-406. B, Greenish·brownbiotite (8) completely replacing pseudomorphieaUy earlier crystallizedsillimanite in l-em-Iong Iensoid domains within IU1 assemblage ofgarnet, biotite (red-brown, Z axis), cordierite, pIagjoclase(Anlly, andquartt, where biotite is partially replseed by serpentine. SampleGM·I25. C, Sillimanite (5) partially replaced by biotite (B). SampleGM·406. D, Rounded porphyroclasts of garnet (G) in a qUllrtr. (Q)­5illimanite (5) mylonitic schist. Sillimanite is apparently replacing Ixlthgarnet and biotite (B). Sample GM-83-1.