Sillimanite shows additional apparently metastable-relationswith biotite and muscovite in the gneiss and schist.Some sillimanite appears to be in the initial stages <strong>of</strong>replacement by biotite (fig. 13C). Most <strong>of</strong> this biotite thatpartly or wholly replaces sillimanite is various shades <strong>of</strong>greenish brown (Z axis) W1der plane light in contrast tothe more typical reddish-brown (Z axis) cdors<strong>of</strong>the otherbiotites. We have already briefly described the growth <strong>of</strong>sillimanite in muscovite, which previously had replacedkyanite (fig. 12D). Such textural relations among thesethree minerals may be interpreted most simply as reflectingthe following cyclic orpaired reactions wherein muscoviteoccurs as an intermediate phase (Carmichael, 1969):3 kyanite+3 quartz+2K ++3H~2 muscovite +2H +(10)2 musoovite+2H+-3 sillimanite+2K+. (11)A critical implication <strong>of</strong> reactions (10) and (11) is theapparent immobility <strong>of</strong> A13+ (Carmichael, 1969). However,as pointed out by Glen (1979), if overall volumetricchanges and half reactions in physically separate domains<strong>of</strong> the rocks during metamorphism also are considered insuch metapelites. then some elements (A13+, Si 4 +) mayindeed be relatively mobile.The metamorphic rocks thus contain textural evidencedocumenting repeated and possibly prolonged periods <strong>of</strong>sillimanite crystallization after the onset<strong>of</strong>crystallization<strong>of</strong> a biotite-gamet-quartz±potassium feldspar±plagioclase±opaque minerals ± hercynite assemblage. Thepelitic gneiss also may have included sillimanite as an earlyphase.Some high-grade-metamorphic rocks host an assemblage<strong>of</strong>quartz and sillimanite, (7), which together makeup the bulk <strong>of</strong> the blastomylonitic matrix <strong>of</strong> these rocks(fig. laD). These mylonitic rocks crop outdiscontinuouslyin the metamorphic terrane, and the white mica in themis present as poorly developed coronas that surrOW1dsillimanite. In these rocks, which also contain previouslycrystallized porphyroclasts <strong>of</strong> garnet and biotite, thephyllonitic structure is defined mostly by sillimanite,which crystallized preferentially along the mylonitic s sur·face. In addition, quartz shows a strong crystallographicand dimensional orientation wherein the orientations <strong>of</strong>its 0001 axes coincide closely with the s surface <strong>of</strong> themylonite. Although quartz in the matrix <strong>of</strong> the blastomyloniticrocks shows complex and highly sutured boundaries,the bulk <strong>of</strong> the quartz is strain free and t.riple jW1Ctions<strong>of</strong> 120 0dihedral angles are common. In somemylonitic rocks. white mica partly replaces sillimanite andmay reflect continued late-stage mylonitization underhydrous conditions. Thus, local mylonitization apparentlyoccurred initially during the metamorphic peak <strong>of</strong> theregion at upper·amphibolite-facies conditions, and perhapscontinued sporadically into the subsequent greenschistmetamorphism (see fig. 7).PETROCHEMIS1"RY OF CRYSrALLINE ROCKS 31The cordierite-bearing rocks most commonly include acordierite-biotite-garnet-potassium feldspar association,(8) (table 5), which does not contain white mica that isparagenetically the same age as the cordierite. The bulk<strong>of</strong> the white mica in the cordierite-bearing assemblage(table 5) is a very late mineral that partly replaces feklsparand (or) cordieritein the rocks. Thus, the progressive andconsistent decrease in the modal abundance <strong>of</strong>early whitemica in these rocks suggests that the peak <strong>of</strong> the progrademetamorphism occurred at physical conditions whereinthe white mica-quartz assemblage was not stable. Althoughcordierite isograds elsewhere commonly have beenmapped fairly concisely as halos around intrusive rocksemplaced into metapelitic terranes (Loomis, 1979; andmany others), we have established only a very approximateboundary for the distribution <strong>of</strong> con::l.ierite in theGold Basin-Lost Basin districts. Con::l.ierite in the districtsseems generally to be confined to relatively fresh nonretrogradedmetamorphic rocks associated spatially withthe suite <strong>of</strong> Early Proterozoic igneous rocks that crop out.in the general area <strong>of</strong>Garnet Mountain east <strong>of</strong> the trace<strong>of</strong> the Grand Wash fault zone and northeast <strong>of</strong> the intersection<strong>of</strong> the inferred traces <strong>of</strong> the Grand Wash andHualapai Valley faults. In these rocks, cordierite does notreflect thermal recrystallization during a relatively drynondynamic contact event related primarily to fmalemplacement <strong>of</strong> nearby igneous rocks. Cordierite in many<strong>of</strong> these rocks forms an integral part <strong>of</strong> the gneissic fabric<strong>of</strong> the g'.lrnet-biotite assemblage(s). Many <strong>of</strong> the rocks containingcordierite are metamorphic schlieren and pendants<strong>of</strong> metapelites engulfed by more widespread igneousrocks. However, many pelitic rocks in these schlieren andpendants do not contain cordierite very close to their contactswith adjoining plutonic igneous rocks. Some peliticmigmatitic gneiss within 3 m <strong>of</strong> very large Early Proterozoicigneous bodies show quartz+biotite + plagioclase(oligoclase to even albite, in places)+microcline±whitemica ±garnet composite assemblages but also includecarbonate and clinozoisite-epidote alteration <strong>of</strong> earlierplagioclase. The only contact phenomena noted are theporphyroblastic growths<strong>of</strong>white mica and feldspar (bothalbite and microcline) and the alteration <strong>of</strong> plagioclase.Cordierite, as listed in reactions (8) and (9), is presentin at least two parageneses in nonretrograded peliticgneiss and migmatite. The more common, and probablyearly, association is with biotite and garnet (table 5), anassociation (8) that partly defines the schistose fabric <strong>of</strong>the host pelitic gneiss. Presumably early cordierite is alsopresent with biotite and garnet in the melanosome(Mehnert, 1968), or dark portion <strong>of</strong> migmatites, againstwhich the light micropegmatitic portion, or leuoosome, apparentlyhas advanced (fW- 14A). We herein use the termsmelanosome and leucosome in a purely descriptive sense.It is beyond the intended scope <strong>of</strong> our present study to
32 GEOLOGY AND GOLD MINERALIZATION Ot' THE GOLD BASIN·LOST BASlN MINING D1STRlCI'S, ARIZONAattempt a full documentation <strong>of</strong> the overall genesis <strong>of</strong>these extremely complex rocks. For example, we have notestablished whether the melanosomes reflect simply anin situ residue from the parent rock (paJeosome <strong>of</strong>Mehnert, 1968) or whether the melanosomes are chemicallytransformed even partially. However, modal analyses<strong>of</strong> adjoining layers <strong>of</strong> melanosomes and leucosomes inthese rocks show approximately equivalent abundances<strong>of</strong> the felsic minerals (quartz and feldspars). The majordifference between the two is the almost complete absence<strong>of</strong> biotite from the leucosome whereas the melanosomeA',-,:::;o~~Ltypically includes about 25 volume percent biotite.Further, in the melanosome some cordierite is concentratedin biotite-potassium feldspar (mostly microcIine)plagioclase (An2(}-zs) domains away from garnet-quartzdomains. Within these latter domains, xeooblastic garnetalso commonly shows highly irregular skeletal outlinesresulting from its growth along quartz crystal boundaries.In the melanosomes, garnet appears to be associatedstably with biotite. Biotite shows invariably an increasedabundance within about 0.2 mm <strong>of</strong>the sharply adjoiningleucosome, especially along fronts convex toward themelanosome. The leucosomes <strong>of</strong> these migmatitic rockscommonly contain a cordierite-sillimanite-hercyniteassemblage (9) (fig. 148). Contents <strong>of</strong> Al,o, and K,
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