PETROCHEMISTRY OF CRYSTALLINE ROCKS 35from 0.4 to 1.7 weight percent and is thus less than themean MgO composition <strong>of</strong> 61 graywackes (2.1 weight percent;table 4, analysis 10). Such low contents <strong>of</strong>MgO mayreflect the very low content <strong>of</strong>lithic volcanic detritus shedinto the protolith <strong>of</strong> the gneiss. Although these chemicalanalyses <strong>of</strong> quartz<strong>of</strong>eldspathic gneiss from the Gold BasinLost Basin mining districts show similarities generally tograywacke, all <strong>of</strong> these samples have been affectedmoderately to strongly by retrograde alteration phenomena.The rocks have been subjected to severed intense andprolonged metamorphic events and weathering phenomenathat contributed to the final chemical composition <strong>of</strong>the gneiss. 1'he chemical changes, if any, that accompaniedeach <strong>of</strong> the events cannot be ascertained.AMPHIBOLITEAmphibolite bodies <strong>of</strong> varying sizes are presentthroughout the Proterozoic metamorphic terrane in theGold Basin-Lost Basin mining districts. However, only afew <strong>of</strong> these amphibolite bodies are shown as separate'O',--;;;;:-_~~~ ~ _'0"z• u•••"I0"• '0•"z•"z0u~.."••'OooL-------,cC-;:------.,O-----,!2.0 4.0 6.0CaO CONTENT. IN WEIGHT PERCENTFiCURe 17.-Si~ versus Cao in four sample!:! (dotll) <strong>of</strong> quaJ1.z<strong>of</strong>eJdspathiegneiss (analyses 3-6. table 4); field (shaded area) establishedby Brown and others(l979) for 16 analyses <strong>of</strong>lTl4JtAsedimentary rocksfrom the Early Proteromie Vishnu Complex <strong>of</strong> Brown and others(1979).map units in the district-wide geologic map by Blacet(1975) because <strong>of</strong> the scale <strong>of</strong> the quadrangle map, theirdiscontinuous structure, and the overall highly irregularsize distribution <strong>of</strong> many individual outcrops. Most amphiboliteis present within the gneiss (fig. 2, unit Xgn),which includes mainly quartz<strong>of</strong>eldspathic gneiss asdescribed previously. Lesser amounts <strong>of</strong> amphibolite arepresent in the migmatitic units and also scattered aspendants in the various Early Proterozoic igneous rocks.In the gneiss unit, some outcrops consist <strong>of</strong> 30-m-thicklayers that are as much as 80 to 90 percent dark-greengrayto dark-greenish-black to black amphibolite (see fig.58). Such masses <strong>of</strong> amphibolite may grade across severalmeters into quartz<strong>of</strong>eldspathic gneiss by decreasednumbers <strong>of</strong>layers <strong>of</strong> amphibolite along strike <strong>of</strong> the foliation.Layers <strong>of</strong> amphibolite also may terminate abruptlyor be crosscut by coarse-grained, quartz-feldspar-richleucogranite which in tum then grades into the surroundingquartz<strong>of</strong>eldspathic gneiss. The overall abundance <strong>of</strong>amphibolite in the Proterozoic terrane here in the GoldBasin-Lost Basin mining districts is much greater thanin similar metamorphic terranes just to the north aroundLake Mead (P.M. Blacet, unpub. data, 1967-72). However,even within the districts themselves there is a widevariation in the proportion <strong>of</strong> amphibolite in the EarlyProterozoic gneiss map unit (fig. 2). Near the south end<strong>of</strong> the Lost Basin Range, amphibolite is derived largelyfrom igneous protoliths, an assessment based on the occurrence<strong>of</strong> widespread relict gabbroic and diabasic textures.Careful examination <strong>of</strong> outcrops there furtherreveals that rarely some amphibolite definitely crosscutsthe lithologic layering in the enclosing paraschist andparagneiss and clearly shows preserved chilled marginsagainst these rocks.Amphibolite in the Proterozoic terrane is eithermassive, showing no readily apparent macroscopic structure,or more commonly it contains a well-developedfabric. Generally, the fabric consists <strong>of</strong> at least one obviousfoliation and possibly a less well-developed lineation.The principal foliation (sd is defined by millimeter-sizelayers <strong>of</strong> different compositions and by differences ingrain size between adjoining domains <strong>of</strong> similar mineralogy(fig. 1M). Most commonly, the attitude <strong>of</strong> SI in theamphibolite is the sameas that in the surrounding gneiss,and 81 may be better developed near the margins <strong>of</strong> theamphibolite bodies. Lineation in the amphibolite locallyis defined by a strong preferred orientation <strong>of</strong>amphibolecrystals within 81. In addition, other types <strong>of</strong> lineation inamphibolite include minor fold axes and well-developedmullion structures consisting <strong>of</strong> aggregates <strong>of</strong> hornblendeshowing a strong preferred orientation <strong>of</strong> their [001] axesparallel to the mullions. The fold axes and mullion structurescommonly are coaxial. lsoclinally folded amphiboliteinterlayered with quartz<strong>of</strong>eldspathic gneiss is especially
36 GEOLOGY AND GOLD MINERALIZATION OF THE GOLD BASIN-LOS'f BASIN MINING DISTRICTS, ARIWNAabundant along the west flank <strong>of</strong> the Lost Basin Range.Lineation in amphibolite may also be defined by the intersection<strong>of</strong> a subsequent fracture cleavage (80 with 81.Locally, in amphibolite, ripplelike corrugations on 81plunge shallowly (about 10°) and reflect the intersection<strong>of</strong> 81 with the S2 fracture cleavage. Typically, such S2 frac·ture cleavages are more widely spaced than the 8} layering.In addition, chloritization occurred ooncurrently withthe development <strong>of</strong> 82, and thus the fracture cleavageFIGURE 18.-Stucture llIld textures obsen'oo in amphibolite. A, amphibole;Q, quartz:; P, plagioclase or retrogrnde aheration products<strong>of</strong> plagiodaae, including white mka, carbonate, and cla}'(s). A. Principalfoliation ('1) defined by tithokJgic Ia}'cring <strong>of</strong> varying compositionLSample GM-290, 5Eo,I, IiOC- 18, T. 29 N., R. 17 W. B, Fabric<strong>of</strong> hombleflde.clinop)'1'Oxene-pIagioclase (A~-biotite (B) amphiloIiteCf)1Itallizedat upper-~facieI;
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