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

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PETROCHEMISTRY OF CRYSTALLINE ROCKS 25the schist composes wispy stratiform zones containingsparse concentrations of other minerals that also haverelatively high specific gravities. The specific gravity oftounnaline is approximately 3.00 to 3.20 (Deer and others,1962a). These relations suggestthat the tourmaline schistor tourmalinite may reflect local syngenetic exhalativeemanations of boron-rich fluids onto the sea floor, wherethe protolith of the Proterozoic metamorphic rocks occurred.Such a syngenetic model for similar rocks elsewherehas been proposed recently (Ethier and Campbell,1977; Nicholson, 1980; Plimer, 1980; Slack, 1980). Theexploration implications of these rocks will be discussedin the section entitled "Suggestions for ExploratoryPrograms,"BFICHU IO.-Stn..:turaI and textural reIationsoftounnaline in metam0rphicroch. Plane-polarized light. A. Compollite small-scale folM intourma1illt lIChilit.~ eM·296. B, Disseminated euhedral ttyIlta!lIof fiIJeolT&ined garnet in a tourmaIine-biotite-museovite lldrist. T.tourmaline; Q. quartz; G. garnet; M,rnicroctine.SampIe GM·297LALMANOINE·BIOTITE±STAUROUTE SCHIST AND GNEISSPelitic schist and gneiss in the Proterozoic metamorphicterrane also include almandine-biotite-quartzpotassiumfeldspar-plagioclase assemblages. The potas·sium feldspar is mostly microcline, and the plagioclase ismostly oligoclase or andesine, although some of theserocks rarely contain albite. Minor aceessory minerals includeapatite, opaque mineral(s), and rutile. Some of theinitial crystallization stages ofalmandine are recorded byrocks still containing muscovite and biotite, as describedin the preceding section. In such a mineral association,almandine garnet is present aseuhedraJ O.l-mm-wXle porphyroblastsin a fabric wherein the schistosity and subsequentstrain-slip cleavage are defined by muscovite andbiotite. Textural relations suggesting specific reactionsthat controlled the crystallization and growth of almandineare very difficult to ascertain. Generally, however,increased modal abundances ofalmandine and microclineappear to have been compensated by (1) a decrease inoverall abundance and eventual disappearance of muscoviteand (2) a change in the color of biotite from darkbrown (Z axis) to reddish brown (Z axis). These changesare accompanied also by an apparent increase in grainsize. The first crystallization ofalmandine-rich garnet inthese rocks is not sharply defined areally and is notmarked by the disappearance ofa phyllosilicate phase. Instead,we infer from our petrographic observations thatthe initial crystallization of garnet reflects a reaction suchasbiotite! +m phengitic white mica....biotiten +a1mandine gamet+m potassium feldspar+ H20. (2)We suggest that early dark-brown (Z axis) biotite! is aniron-rich variety and that the ensuing red-brown (Z axis)biotiteu is a variety containing more magnesium thanbiotite!_ If reaction (2) is mostly correct for the onset ofthe biotiteu-almandine-potassium feldspar compatibility,then the magnesium content of biotiten may be derivedprincipally from the breakdown of phengitic white mica,which may have magnesium substituted for aluminum inits octahedral structural sites (Deer and others, 1962b).Further, m in reaction (2) may be greater than one, thusreflecting a preferred consumption of white mica relativeto biotite). In addition, reaction (2) provides a mechanismwhereby biotiteJl in the rocks becomes more magnesianand thus stable to higher temperatures than biotite).Staurolite (ideally F~A19S40d:OH» was found at onlyone locality (GM-922), near the south edge of the GoldBasin district, approximately 5 km south of the Cyclopicmine. Ribboned and spotted migmatitic gneisses there areretrograded partly to a chlorite-white mica greenschistassemblage. However, the melanosomes of these rocks
26 GEOLOGY AND GOLD MINERALIZATION Of' THE GOLD BASIN-LOST BASIN MINING DISTRlcrs. ARIZONAcontain a weU-developed, gamet-biotite-stauroli~andalusiU(r)assemblage with an absence of potassiumfeldspar and plagioclase. Andalusite is possibly presentin the rock in trace amounts as crystals apparently compatiblewith garnetduring the progradeevent, but subsequentlyhighly altered to white mica during the retrogradeevenl Equant to stubby-prismatic crystals of staurolitemeasure typically 0_5 to 1.0 rom wide and they form, inplaces, 120° dihedral angles against euhedral porphyroblastsof garnet (fig. IlA). Under the microscope, thestaurolite is strongly pleochroic from colorless (X axis) topale golden yellow (Z axis); some crystals are twinned.In the melanosomes of these staurolite-bearing rocks.chlorite and white mica partly replace mostly biotite asfine-grained porphyroblasts at high angles to the traceof the foliation defined by the biotite. Many of the staurolitecrystals are also rimmed by aggregates of very finegrained white mica. However, the crystalsofgarnet hereare remarkably free of replacement phenomena relatedto the retrograde evenlThe presena! ofstaurolite in the a1mandine-biotite schistand gneiss allows us to make some inferences concerningthe prograde metamorphism of the area during theEarly Proterozoic. The onset of the crystallization ofstaurolite is generally accepted to be at temperaturessomewhat higher than almandine garnet(Miyashiro, 1973)and to indicate thereby that regionally metamorphosedrocks have reached alleast a medium grade, or temperaturesin the 500 to 550°C range (Winkler, 1974).Although staurolite can apparently fonn across a broadspectrum of pressure environments (Miyashiro, 1973), itsmaximum stability determined experimentally in thepresence of quartz, muscovite, and biotite appears to beabout 675°C at 550 MPa and about 575 DC at 200 MPa,P H 2P = Plot and Mg/(Mg+Fe) - 0.4 (Hoschek, 1969).G. Hoschek further found that a relatively high FeJ(Mg+Fe) ratio will expand the stability field ofstaurolite.Thus, the very restricted presence of staurolite in contrastwith the very wide distribution of cordierite in theProterozoic terrane of the Gold Basin-Lost Basin districtsmay reflect such chemical controls. Although stauroliteis present here in an assemblage questionably containingandalusite, staurolite-bearing assemblages elsewhere arecF\cuRl: 11.-Textunli reIationa M'lOl'"C' minerals in gameWlearinggneiaG. garnet; S. staurolite; B. biotite; C. chkIrite; M. white mica; Q,~f'Iane.~ I:igttLA. Textural reIationr; in gamet·mwroote-biotitegneis& relict &om prograde evenL Odorite and ,,'hite mica a')"itallizedduring subeequent greenschist retrograde e\~nL SampleGM-922&. B. Rounded and higtlly a1tenxl crystals ofgarnet in retrogradt.odgarnetiIerotIlS Ieut'ogneia. Partially chloritiz.ed garneta arereplac:ed by aggregates oC very fine grained white mica. SampleGM-462. C. Mylonitic fabric COn9sting oflnotite-d1lorite-wrute mica·clinoww1e-()llartz _mblage superpo:l6ed during greenschi6t retrogradeewnt OIl an E'Jlrlier gamet-bearingassemblage. Sample GM·555.
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