Contrib Mineral Petrol (1992) 110: 370-386 Con t r i bu t i ons to Mineralogyand Petrology �9 Springer-Verlag 1992

Hydrothermal alteration and REE-Th mineralization at the Rodeo de Los Molles deposit, Las Chacras batholith, central Argentina Raul Lira 1 and Edward M. Ripley 2

1 Department of Geology, National University of Cordoba, CONICET, Velez, Sarsfield 299, (5,000) Cordoba, Argentina 2 Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA

Received March 4, 1991 / Accepted October 9, 1991

Abstract. REE (rare-earth-element) and Th mineraliza- tion at the Rodeo de Los Molles deposit occurs within an elliptical body of hydrothermally altered rocks (fen- ite) located in a biotite monzogranite of the Las Chacras batholith. Ore assemblages are found as isolated patches of intergrown britholite, allanite, apatite, bastnaesite, fluorite, sphene, quartz, and aegirine-augite, as well as nodules of uranothorite and late-stage veins of calcite, fluorite, and bastnaesite. Composition-volume computa- tions suggest that the fenite was produced by alteration of the biotite monzogranite by addition of K and Na, and loss of Ca and Sr. Petrographic evaluations indicate that microcline and plagioclase have been replaced by perthite, and biotite was converted to aggregates of clinochlore, anatase, kaolinite, and hematite. Relict bio- tite is characterized by lower Fe/(Fe + Mg) and Ti values with progressive alteration. Fluorine-rich phlogopite is present in mineralized areas, but textural evidence sug- gests that it was not produced via biotite alteration. Mass-balance constraints also show that Ca and Mg in ore zones may result from redistribution, rather than their being a result of external derivation. The ~180 values of quartz (8.6-11.1%o) and feldspar (7.8-10.6%o) suggest that feldspar continued to exchange oxygen isotopes with a fluid to lower temperatures than did quartz. Feldspars equilibrated with a fluid of 6 180 ~ 8%0 at a fluid/rock ratio less than 1. The 6 tsO values of quartz and aegirine-augite that crystallized during REE mineralization also suggest equilibration with a fluid of

180 ~ 8%0. The 6 D values of biotite ( - 8 3 to -120%o ) are relatively low for igneous rocks and are thought to have resulted from exsolution of a D-enriched magmatic vapor. The 6 D values of both mineralized and barren fenites are consistent with equilibration with fluid of magmatic origin. Meteoric water was involved in the production of calcite and clinochlore alteration, and late-stage calcite-fluorite-bastnaesite veins. The 613C values of calcite and bastnaesite ( -7 .8 to -13 .5%) sug- gest that carbon was derived by leaching of carbon from

Offprint requests to: E.M. Ripley

igneous and/or enclosing metamorphic rock types, and that a majority of carbon ultimately was derived from sedimentary organic material.

Introduction

The Rodeo de Los Molles rare-earth-element (REE) and thorium deposit is located in granitic rocks of the Las Chacras-Piedras Colorados (LCPC) batholith in the southern block of the Eastern Sierras Pampeanas, north of San Luis province, central Argentina (Fig. 1). Miner- alized outcrops were discovered in 1982 by the Argentina National Commission of Atomic Energy using airborne and ground radiometric techniques. More detailed ex- ploration work began in 1986, and has led to the discov- ery of several mineralized bodies, both at the surface and at shallow depths. Preliminary analyses indicate variable enrichment in light REE (LREE) ( C e - L a - N d - S m ) and Th, with ore grades ranging from ~ 500 ppm to several weight percent. Mineralization is hosted within a hydrothermally altered zone of alkali- feldspar granite and syenite that occurs within a biotite monzogranite. Fluid inclusion studies by Lira and Rip- ley (1990) have documented a complex history of open- system fluid migration in the deposit. This study consists of a petrologic investigation of major-element mass transfer accompanying conversion of monzogranite to alkali-feldspar granite, and a stable isotopic evaluation pertaining to the evolution of fluids responsible for alter- ation and ore deposition. The combined petrologic-sta- ble isotopic study in conjunction with data collected on fluid inclusions, permits an assessment of the processes that governed fluid-rock interaction in the Las Chacras batholith, as well as the diverse origins of involved fluids.

Regional and local geology

The LCPC is a composite batholith that consists of at least four circular stocks with an approximate surface area of 500 km 2 (Bro-

371

65 ~ 47' W,

AREA

~,NATION

] NEISSIC COMPLEX

~ MIGMATITES

E " ~ GRANITOIDS

• ALLUVIAL FANS

~ INTERMONTAIN DEPOSITS

' ~ ZONAL ULTRAMAFIC COMPLEXES Fig, 1. Location and regional geo-

logic setting of the Rodeo de los Molles deposit (from Yrigoyen 1981)

gioni 1987). A K/Ar age of 320-335 Ma has been obtained for the batholith, which represents the culmination of the granitoid cycle in the southern Sierra s Pampeanas in the Lower Carbonifer- ous (Brogioni 1987). Rapela et al. (1990) suggest that the Eastern Sierras Pampeanas represent part of an inner, back-arc zone of a continental, Paleozoic magmatic arc. Petrochemical and geo- chronological characteristics of the LCPC batholith, along with several other Argentinian granite intrusions, are similar to those defined by Rapela et al. (1990) as the alkali-calcic stage of granitoid activity, produced during late stages of arc evolution. Country rocks include Upper Precambrian to Lower Paleozoic metasedi- mentary rocks. Schists and tonalitic gneisses of the almandine- amphibolite facies are most prevalent with lesser amounts of phyl- lites and schists of the greenschist facies (Kilmurray and Villar 1981).

The Rodeo de Los Molles deposit is located at the northeastern edge of the batholith (Fig. 1). Gay and Lira (1984) have identified two major granitic facies in the mineralized area. Following the scheme of Streckeisen (1976) the predominant rock types are a biotite monzogranite with discontinuous muscovite-garnet-silli- manite zones along the northwestern contact zone (Fig. i), and a more restricted alkali-feldspar granite (fenite, see below). Both REE and Th mineralization are hosted in the fenite which occurs as an elongate body (2 km • 0.6 km) having transitional contacts with the monzogranite (Fig. 2). REE mineralization within the fen- ite occurs as nodules that range from a few millimeters to more than 2 m in their largest dimension. Enclosing rock types range in composition from quartz alkali-feldspar syenite to alkali-feldspar syenite, and both are thought to be of metasomatic origin.

Analytical methods

Mineral assemblages were identified by petrographic examination of polished thin sections. Mineral compositions were determined by wave-length dispersive electron-microprobe analysis, using the Bence-Albee correction scheme following procedures of Albee and Ray (1970). Modal analyses were made by counting between 800 and 1500 points per sample. Volume percentages of minerals were converted to molar abundances using molar volume data of Robie et al. (1978). Whole-rock chemical analyses were performed by ICP spectrophotometry after a Li-metaborate fusion dissolution. USGS standards G-2 and AGV-1 were employed. Specific gravities were determined by water immersion, with rock densities computed based on the density of water at room temperature. Water contents were measured using procedures similar to those employed for hydrogen isotope measurements. Water was liberated by melting samples in vacuum, and collected water was reduced to H2 by reaction with heated zinc in sealed tubes (e.g., Coleman et al. 1982; Kendall and Coplen 1985). Collected gas was measured either by a capacitance resistance manometer or by a calibrated ion gauge on the mass-spectrometer inlet. Samples for oxygen isotope analy- sis were prepared following standard methods using BrFs (Clayton and Mayeda 1963), Replicate analyses of N BS-28 quartz gave 6 l sO values of 9.6 + 0.2%o. Carbon dioxide was extracted from carbonate by reaction with 103% phosphoric acid at 50 ~ C (Rosenbaum and Sheppard 1986). Mineral separates for isotopic analyses were ob- tained by heavy liquid concentration followed by hand-picking. Purity was about 98% for micas and 99% for quartz and feldspars.

372

6 5 ~ 47' W

3 2 ~ 23' S

I V l V I ' ~ U ' I ~111 I L IV I [ - I f - ' , l V l ' , . J r ' t I -R l ' ~ . s i - q l t , . ) l , . ~ r~

V--] F E N I T E FLUORITE-QUARTZ VEINS

RARE EARTH-BEARING BODIES APLITIC-PEGMAT T C VEINS

Fig. 2. Local geoIogy and sample localities in the area of the Rodeo de Los Molles deposit. Sample lo- cations are indicated by small open circles. Larger circles refer to highly mineralized a r e a s : JL, La Juli; ER, E1 Rulo; AN, Ano- malia Norte

Mineralogy and mineral chemistry

Biotite monzogranite

Biotite monzogranite is a fine- to medium-grained (min- eral grains vary 0.15-2.5 mm) rock composed of quartz (30 32%), microcline (33-39%), plagioclase (29-31%), biotite and muscovite (total mica content ranges 1.5- 8%, with biotite/muscovite ratios variable, but generally > 3), and less than 1% magnetite-ilmenite, hematite, zir- con, apatite, rutile, sericite, and clay minerals. Micro- cline varies in composition from Ab6Or9~ to Ab~2Or88, whereas plagioclase ranges from AbsoOrlAn19 to Ab89Or~Anlo (Table 1). Unlike in fenites, perthites are rarely found in the monzogranite. Microcline tends to be clear and devoid of obvious signs of alteration. Some cores of plagioclase grains partially are replaced by seri- cite and clay minerals. Biotite occurs primarily as sub- hedral crystals, frequently in clusters with medium- grained muscovite. Both minerals appear to represent early crystallizing hydrous phases. Values of Fe/(Fe + Mg) in biotite fall within a relatively narrow range of 0.46 to 0.54 (Table 2). Values of AI+Si are less than

4.5 per 11 oxygens, and Ti averages 2.18 wt%. Apatite is a rare mineral, locally found as small inclusions within biotite. Magnetite and ilmenite may occur as separate grains, or as composite grains with exsolution lamellae. Both oxide minerals show variable degrees of replace- ment by hematite.

The muscovite-rich facies located in the northwestern section of the batholith differs from monzogranite in having a lower biotite/muscovite ratio (1.0-0.7) and by the presence of minor amounts of sillimanite and garnet associated with muscovite. Muscovite occurs as well-de- veloped booklets generally in close proximity to biotite. A late generation of muscovite is found also as a frac- ture-filling, but volume abundance of this type of occur- rence is low. Plagioclase varies in composition from Ab91Oro.1Ans.5 to Ab98Orl.sAno.s, whereas microcline ranges from Ab4Or96 to Ab6Or94. Plagioclase cores show preferential replacement by sericite-muscovite and clay minerals. Some samples show a second microcline generation that corrodes and replaces early-formed mi- crocline and plagioclase. Clusters of zircon crystals are found included within quartz or biotite. Rutile needles frequently are found enclosed by quartz. Iron-oxide min-

373

Table 1. Representative feldspar analyses from the Rodeo de Los Molles deposit

Sample Plagioclase analyses

Mg Lg Mg-Fen Fen Min-Fen

22(2) 18(2) 19(3) 8(4) 15(2) 14(2) 2(2) 7(2) L J7-0(2) L J3(2)

SiO2 64.13 66.34 65.15 67.82 67.85 67.70 67.54 67.66 67.73 66.85 A1203 23.10 19.36 21.62 20.70 19.76 19.92 20.11 19.94 19.73 20.31 CaO 3.84 2.78 2.54 0.94 0.14 0.11 0.02 0.02 0.07 0.00 Na20 9.22 10.97 10.03 11.05 11.43 11.59 11.51 11.67 10.95 11.48 K20 0.18 0.16 0.21 0.13 0,07 0.11 0.15 0.10 0.11 0.12

Total 100.49 99.61 99.54

Number ofcations on the basis of 8 oxygens

100.63 99.25 99.42 99.33 99.38 98.59 98.75

Si 2.814 2.937 2.877 2.947 2.980 2.974 2.970 2.973 2.992 2.958 A1 1.195 1.010 1.t24 1.060 1.024 1.031 1.041 1.033 1.027 1.059 Ca 0.179 0.131 0.120 0.042 0.006 0.004 0.001 0.00l 0.002 0.000 Na 0.784 0.941 0.858 0.930 0.975 0.986 0.980 0.994 0.937 0.984 K 0.006 0.008 0.010 0.006 0.003 0.005 0.008 0.005 0.005 0.005

Mol%

Ab 80.7 87.1 86.8 95.1 99.1 99.2 99.2 99.5 99.3 99,6 An 18.4 12.2 12.1 4.3 0.6 0.4 0.1 0.1 0.2 0,0 Or 0.9 0.7 1.1 0.6 0,3 0.5 0.8 0.5 0.5 0.5

Table 1 (continued)

Sample Microcline analyses

Mg Lg Mg-Fen Fen Min-Fen

18(2) 19(2) 8(2) 15(2) 14(2) 2(2) 7(2) LJT-0(2) L J3(2)

SiO 2 66.24 63.84 64.95 64.79 64.47 64.00 64.18 64.32 63.72 A1203 16.00 18.20 18.24 18.16 18.53 18.63 18.21 18.50 19.09 CaO 0.01 0.02 0.00 0.00 0.00 0.03 0.00 0.22 0.01 Na20 1.15 0.67 0.58 0.64 0.63 0.47 0.35 0.49 0.48 K20 15.20 15.95 15.81 15.59 15.77 16.07 16.18 15.71 16.03

Total 98.60 98.67 99.57

Number of cations on the basis of 8 oxygens

99.18 99.39 99.19 98.91 99.23 99.32

Si 3.074 2.991 3.006 3.009 2.992 2.982 2.999 2,994 2.965 A1 0.892 1.005 0,994 0.993 1.013 1.023 1.003 1.014 1.047 Ca 0.000 0.001 0.000 0.000 0.000 0.001 0.000 0.001 0.000 Na 0.103 0.060 0.052 0.057 0.056 0.042 0.031 0.044 0.042 K 0.901 0.953 0.932 0.923 0.934 0.955 0.964 0.932 0.951

Mol%

Or 89.8 94.l 94.8 94.2 94.4 95.8 96.9 95.4 95.8 Ab 10.2 5.8 5.2 5.8 5.6 4.2 3.1 4.5 4.2 An 0.0 0.1 0.0 0.0 0.0 0.1 0.0 0.1 0.0

Mg, biotite monzogranite Lg, muscovite-rich, garnet, sillimanite-bearing monzogranite Mg-Fen, monzogranite partially fenitized, transitional zone

Fen, barren fenite Min-Fen, mineralized fenite Values in parentheses indicate number of analyses

erals include t i taniferous magnet i te and ilmenite, and they may be in te rgrown with exsolved hematite. Discrete hemat i te grains occur in some samples as a rep lacement of i lmenite and magnet i te .

Fenite

Alkal i-feldspar granites are textural ly and mineralogi- cally distinct. Evidence to be presented below strongly suggests tha t this rock type was p roduced by hydrother - real a l te ra t ion of monzogran i te , and the rock is more

374

Table 2. Representative biotite-phlogopite analyses from the Rodeo de Los Molles deposit

Sample Mg Lg Mg-Fen Min-Fen

18(3) 22a(2) 8(2) 14(4) 15(3) LJZB(4)M70(1)

SiOz 39.32 36.92 37.13 40.70 42.89 45.72 46.39 TiO2 2.29 2.08 2.31 2.17 1.38 0.53 0.24 A1203 16.97 15.86 15.44 13.15 12.33 8.96 7.20 Cr203 0.02 0.02 0.00 0 .01 0.00 0,01 0.00 FeO 17.88 18.87 17.82 15.28 12.84 9.07 6.89 MnO 0.48 0.53 0.45 1.24 1.62 1.38 0.63 MgO 10.32 9.27 9.26 12.73 14.91 18 .91 19.14 CaO 0.00 n.a. n.a. 0.00 0.00 0.00 n.a. Na20 0.01 0.04 0.07 0 .01 0.02 0.07 0.04 K20 9.72 10.04 9.92 10.01 9.72 9.96 10.43 C1 0.05 0.04 0.24 0.04 0.03 0.02 0.01 F 1.12 1.25 1.50 2.14 2.50 2.64 3.29

98.12 94.89 94.11 97.46 98.24 97.26 94.26 - O - F , C1 0.48 0.54 0.68 0 .91 1.06 1.11 1.39

Total 97.64 94.56 93.43 96.55 97.18 96.15 92.87

Number of cations on the basis of 22 (O, C1, F)

Si 5.736 5.660 5.711 5.948 6.120 6.462 6.689 A1 2.917 2.865 2.798 2.265 2.073 1 .493 1.222 Yi 0.246 0.240 0.267 0.239 0.149 0.056 0.026 Cr 0.002 0.002 0.000 0.001 0.000 0 .001 0.000 Fe 2+ 2.180 2.419 2.293 1.867 1.532 1 .075 0.831 Mn 0.061 0.069 0.058 0.154 0.196 0.165 0.077 Mg 2.244 2.120 2.121 2.773 3.171 3.982 4.114 Ca 0.000 0.000 0.000 0.000 0.000 0,000 0.000 Na 0.003 0.012 0.020 0.007 0,006 0,020 0,011 K 1.810 1.963 1.946 1.866 1.769 1 .769 1.919 C1 0.011 0.009 0.063 0.011 0.0006 0.004 0.002 F 0.514 0.606 0.727 0.990 1.128 1 .181 1.500

Mg, biotite monzogranite Lg, muscovite-rich, garnet, sillimanite-bearing monzogranite Mg-Fen, monzogranite partially fenitized, transitional zone Min-Fen, mineralized fenite Fe 2+, total iron reported as Fe z+ n.a., not analysed Values in parentheses indicate number of analyses

a p p r o p r i a t e l y t e rmed a fenite. G r a i n sizes are var iable , bu t in genera l qua r t z and per th i t e gra ins (up to 4 m m in length) occur in a f ine r -g ra ined g r o u n d m a s s r ich in albi te . Relicts o f b o t h p lag ioc lase and microc l ine are f o u n d enclosed in large per th i te grains. D i scon t inuous , a lb i te - r ich marg ins are a c o m m o n fea ture o f m a n y per th - ite grains. Coar se vein and pa tch- l ike textures are preva- lent in the per thi tes , wi th a lb i te con ten t s rang ing f rom 30 to m o r e than 50 vo lume percent . A l t h o u g h m o d e s va ry widely, r epresen ta t ive ranges are : 15 -25% quar tz , 4 5 - 6 5 % per th i te , 1 5 - 3 0 % free albi te , and var ious acces- sory and s econda ry minera l s inc luding biot i te , musco- vite, z ircon, apa t i te , sphene, hemat i te , f luori te , ana tase - leucoxene, and c lay minera l s ( i l l i te-smecti te) tha t to ta l less than 5%. Po t a s s ium fe ldspar in the per th i te ranges in c o m p o s i t i o n f rom Ab3Or97 to Ab6.5Or93.5, wi th a t r ic l inici ty index o f 0.87. A lb i t e is near ly pu re (Ab99) in bo th per th i tes and g r o u n d m a s s crystals . Biot i te gra ins pa r t i a l ly are r ep laced by aggrega tes o f hemat i te , ana tase , c l inochlore , a n d c lay minera ls . Ra t ios o f F e / M g in relict

b io t i te range f rom 0.31 to 0.43 (Table 2). Con ten t s o f Ti va ry f rom 1.31 to 2.45 w t % , and cor re la te wi th Fe/ Mg. Conse rva t i on o f Ti is s u p p o r t e d by the presence o f ana tase . Rela t ive to b io t i te f rom monzogran i t e , SiO 2 con ten t in rel ict b io t i tes f rom fenites is higher , r ang ing f rom 39.96 to 43.18 w t % .

Areas o f R E E mineralization

R E E mine ra l i za t ion tha t occurs in nodules is loca ted wi th in smal l syeni te bodies enclosed in fenite. A l t h o u g h con tac t s wi th the fenite are diffuse, g ra in size o f the syenites ranges f rom much coarse r (aegir ine-augi te crys- tals up to 8 cm in length) to much finer than average fenite. Textures and c ompos i t i ons o f fe ldspars in syenit ic

Table 3. Representative clinopyroxene analyses from the Rodeo de Los Molles deposit

Sample LJ2B(3) Min-Fen PVI(4) ER(3)

SiOz 51.20 51.13 51.26 TiO2 0.02 0.06 0.02 A1203 0.58 0.68 0.55 FeO (T) 19.84 21.12 22.14 MnO 2.95 2.17 2.28 MgO 5.48 5.36 4.69 CaO 15.33 15.50 14.00 Na20 4.29 3.94 4.76

Total 99.69 99.91

Number of cations on the basis of 6 oxygens

99.69

T sites

Si 1.971 1.985 1.971 A1 IV 0.027 0.015 0.029 Fe 3 + 0.002 0.000 0.000

2.000 2.000 2.000

M1 sites

A1 VI 0.000 0.009 0.001 Fe 3 + 0.348 0.360 0.338 Ti 0.001 0.001 0.002 Mg 0.315 0.262 0.304 Fe z + 0.287 0.360 0.348 Mn 0.049 0.008 0.007

1.000 1.000 1.000

M2 sites

Fe z + 0.000 0.000 0.000 Mn 0.047 0.068 0.062 Ca 0.633 0.578 0.626 Na 0.320 0.354 0.312

1.000 1.000 1.000

Mol%

Ac 31.3 33.7 30.1 Di 30.9 24.8 29.4 Hd 37.7 41.4 40.4

Min-Fen, mineralized fenite Fe 3 +/Fe 2 +, calculated on the basis of 4 cations and 6 oxygens Values in parentheses indicate number of analyses FeO (T), total iron reported as FeO

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areas are similar to those of the surrounding fenite (Ta- ble 1). Turbid feldspar with abundant hematite and clay minerals is common in strongly altered areas. Original monzogranite biotite is replaced almost completely by hematite, anatase, clinochlore, and kaolinite. Minor amounts of phlogopite (Fe/Mg=0.15-0.28, Ti=0.47 wt%, Table 2) occur interstitially, and appear to have formed during hydrothermal alteration. Phlogopite F contents range from 2.42 to 3.29 wt%, and are higher than those of biotite. Fluorine concentration of micas is related linearly to Mg/(Mg + Fe) ratio, and the higher F content of phlogopite may reflect the higher capacity of Mg-rich micas to contain F (Munoz 1984). Unlike altered biotite in fenite, anatase is not associated with phlogopite. The most conspicuous feature of the syenites is the presence of massive aggregates of sub- to euhedral aegirine-augite crystals that range in length from a few mm to 8 cm. Both prism and pinaocoidal forms are also observed as linings within miarolitic cavities. Aegirine- augite is characterized by a relatively narrow range of Na/(Na + Ca) (0.29-0.44) and Fe 2 +/(Fe 2 + + Mg) (0.62- 0.81) values (Table 3). Epidote is found as a replacement of aegirine-augite along cleavage and fractures, and in more intensely altered areas both minerals are enclosed in massive aggregates of clinochlore, fine-grained spher- ulitic quartz, calcite, and hematite. Aegirine-augite is as- sociated strongly with the presence of REE mineraliza- tion. In addition to aegerine-augite, REE-minerals occur intergrown with apatite, sphene, phlogopite, calcite, he- matite, quartz (Q1 of Lira and Ripley 1990), and fluorite (F1 of Lira and Ripley 1990). REE-minerals are com- monly Ce-rich. Britholite (Na, Ce, Ca)s [F(SiO4PO~)3] and allanite may occur as massive to euhedral grains. In many samples allanite encloses britholite. Allanite also occurs along cleavages in feldspar grains. Both min- erals are partially to totally replaced by bastnaesite (Ce[F, CO3]) or thorbastnaesite.

Thorium mineralization

To date, thorium mineralization has not been found with REE-bearing assemblages. Uranothorite, along with M n - B a oxides, is distributed widely&hroughout the fenite. Mineralization most frequently occurs as thorite- rich nodules (~3 to 30 cm in diameter) dispersed through the fenite. Less commonly, uranothorite is found in veins or as euhedral tabular crystals up to 2 cm in length lining miarolitic cavities with quartz (Q2 of Lira and Ripley 1990) and fluorite (F2 of Lira and Rip- ley 1990).

Bastnaesite-bearing veins

Recent exploration work in the area has led to the dis- covery of two types of bastnaesite-bearing veins that occur in close spatial proximity within the fenite. In the first type, fluorite fragments are cemented by calcite and lesser amounts of bastnaesite. Calcite may also be re- placed by bastnaesite. Within the second type, fragments

of mineralized syenitic material including britholite, al- lanite, apatite, quartz, and aggregates of clinochlore+ quartz + calcite + hematite are cemented by bastnaesite.

Calcite deposition

At least four textural varieties of calcite can be recog- nized in the monzogranite, fenite, and syenites. One type pseudomorphously replaces sphene and also occurs as an alteration of plagioclase and pyroxene. A second vari- ety is found as a filling of triangular-shaped voids be- tween grains of aegirine-augite. Calcite that fills vugs in clinochlore represents a late stage of carbonate precip- itation. Veins containing fluorite, calcite, and bastnaesite define a final period of calcite deposition. A more thor- ough discussion of calcite paragenesis is given below in conjunction with C - O isotopic studies.

Hydrothermal alteration and ore deposition

Fenites

Several mineralogical and chemical indicators suggest that the alkali-feldspar granite, or fenite, was produced by hydrothermal alteration. Contact areas between mon- zogranite and fenite show gradational boundaries with intensity of alteration increasing toward the fenite. Prin- cipal mineralogical changes are related to the replace- ment of feldspars in the monzogranite, and include the breakdown of biotite and F e - T i oxides. Perthite is the most abundant phase in the fenite, and occurs at the expense of primary microcline and plagioclase. Relict cores of primary feldspar grains commonly are enclosed in large perthite grains (Fig. 3 a). The occurrence of in- terstitial albite crystals and the presence of albite at the margins of perthite grains (pods of albite that penetrate perthite grains are abundant) suggest the sequence: mon- zogranite feldspars -~ perthite --+ albite. Biotite was pro- gressively converted to hematite, anatase, and clay min- erals during fenitization (Fig. 3b). Decrease in biotite Fe/(Fe + Mg) ratios with Ti content (Fig. 4) from monzo- granite to fenite suggests re-equilibration (loss of Fe and Ti, relative gain of Mg) with fluids responsible for alter- ation, rather than an isochemical destruction of biotite. Phlogopite that is present in REE-bearing syenitic areas is unaltered and appears to have crystallized directly from a fluid phase. Magnetite and ilmenite are converted to hematite and anatase during fenitization.

In order to more thoroughly evaluate the proposed fenitization, composition-volume and mass-balance computations were applied to whole-rock and mineral compositional data. In order to successfully apply Gre- sens' (1967) method, the behavior of one element or volume changes during alteration must be known. In addition, compositional variations within the presumed protolith must not be greater than the computed compo- sitional changes. A simple comparison of bulk-rock chemical analyses of monzogranite and fenite (Table 4) suggests that compositional changes were minimal. Ra-

376

Fig. 3. A Photomicrograph showing relict plagioclase, pl, within perthite (albite fraction, ab; altered microcline fraction, mc). Field width is 1.8 man. Sample no. LJ3, crossed nicols. B Photomicro-

graph illustrating the replacement of biotite, bt, by hematite, hm, anatase, at, and a mixture of kaolinite and clinochlore, ke. Field width is 0.85 ram. Sample no. Mg-Fen 15, plane polarized light

3.5

3.0

o~ 2.5

~ 2 . 0 0 ~- 1.5

1.0

0.5

0 0.1

�9 Mg and Lg ~ Mg-Fen

Min-Fen o !

x x x

x

x

ols ols o7 FeO(T)/FeO(T)+MgO (wt. %)

Fig. 4. FeO/(FeO + MgO) vs TiO2 values of biotites from monzo- granite and fenites at the Rodeo de Los Molles deposit. Mg, biotite monzogranite; Lg, muscovite-rich monzogranite; Mg-Fen, monzo- granite-fenite transition; Fen, fenite; Min-Fen, mineralized fenite

tios of selected elements to aluminum were determined for several samples to test for element mobility (Fig. 5). Computed Si/A1 ratios of all rock types are in the same range. Minor, but significant differences in K/A1, Na/AI, and Ca/A1 exist between the biotite monzogranite and fenite. These differences are also apparent from Gresens' calculations. Because of the modest range in bulk-rock composition of both monzogranites and fenites, average bulk-rock compositions are used for Gresens' analysis (Table 4). Average densities are 2.63 g/cc for monzogran- ire and 2,58 g/cc for fenite. Figure 6a, b illustrates the results for average fenite and monzogranite. Assuming Si and A1 were immobile a volume factor < 1.02 is indi- cated, suggesting virtually no volume change during al- teration. Small gains in K and Na, along with a loss of Ca, are indicated for the change to an average fenite from biotite monzogranite. Such variations are in accord with element ratios in Fig. 5, and consistent with fluid compositions estimated from fluid inclusion studies by Lira and Ripley (1990).

Mass-balance computations, in conjunction with variations in mineral proportions and composition, may also be used to evaluate mineral-fluid reactions. In the conversion of monzogranite to fenite two transforma-

tions are of particular significance. One is the replace- ment of plagioclase and microcline by perthite. Modal and compositional data indicate that addition of Na is required for the replacement, and because no other source of Na is available within the rock volume infiltra- tion of a Na-bearing fluid is indicated. The second trans- formation is the reaction of biotite to hematite, anatase, clinochlore, and kaolinite. Relict biotite grains often are enclosed within mixtures of the alteration assemblage, and where biotite is totally replaced, outlines of the origi- nal biotite grain may be recognized. For this reason a ratio of unreplaced biotite to alteration minerals may be computed, and is plotted against biotite Fe/(Fe + Mg) and Ti content in Fig. 7. It is clear that Fe and Ti content decrease, and in a relative sense Mg increases in residual biotite as alteration proceeds.

With these reactions in mind, an overall mineral-fluid reaction for conversion of biotite monzogranite to fenite was constructed following general procedures outlined by Ferry (1985). Primary minerals in unaltered biotite monzogranite were taken as biotite, quartz, microcline, plagioclase, magnetite, and ilmenite. Minerals in the fen- ite were taken as quartz, perthite, plagioclase (albite), clinochlore, anatase, hematite, and kaolinite. Amounts of quartz were determined using modal abundances and molar volumes of minerals. Because of the difficulty in accurately estimating feldspar mineral proportions, mass-balance techniques were used to calculate propor- tions of albite, anorthite, and orthoclase. Clinochlore abundance was computed based on approximate conser- vation of Mg. The elements Si, A1, Fe, and Ti were also treated as immobile. Bulk-rock composition was that of the average monzogranite using a 1,000 cm 3 ref- erence volume and a density of 2.63 g/cm ~.

Results of the evaluation, using an average measured molar ratio of kaolinite to clinochlore of 1:2, are given in Table 5. The overall reaction for the conversion of biotite monzogranite to fenite may be written as: 0.089 biotite + 0.028 ilmenite + 0.228 magnetite + 0.225 CAA12- Si205+1.354 quartz+0,272 Na++0.268 K++0.188

377

Table 4. Whole rock chemical analyses from the Rodeo de Los Molles deposit

Rock type Biotite monzogranite Average

Sample no. 1 3 4 6 10 18 19 22a 24 26 28 29 30

SiO 2 72.20 74.50 72.50 72.26 75.25 72.50 70.77 73 .03 75.07 73.61 74.03 75.32 74.57 74.14 TiOz 0.16 0.16 0.14 0.16 0.14 0.22 0.25 0.20 0.15 0.06 0.04 0.06 0.09 0.17 AlzO3 14.20 13 .02 14 .10 13.66 13 .42 13 .92 13 .57 14 .05 13 .69 12 .83 13 .67 13 .58 14.12 13.74 FezO3 (T) 1.16 1.41 1.34 1.64 1.25 1.94 1.79 1.79 1.16 1.90 0.63 0.58 0.45 1.52 MnO 0.08 0.02 0.03 0.03 0.02 0.03 0.04 0.04 0.03 0.01 0.03 0.02 0.03 0.03 MgO 0.25 0.29 0.26 0.38 0.33 0.39 0.61 0.35 0.31 0.19 0.34 0.19 0.11 0.34 CaO 0.40 0.22 1.09 0.99 0.58 1.08 1.37 1.18 0.69 0.76 0.26 0.83 0.69 0.88 Na20 4.43 4.22 3.88 4.22 3.87 4.06 3.56 3.70 3.86 4.28 4.58 3.85 4.02 3.78 K20 5.71 4.81 5.10 5.12 4.62 4.31 5.44 4.93 4.57 4.48 4.78 4.78 4.79 4.78 P205 0.14 0.16 0.10 0.11 0.10 0.15 0.15 0.11 0.06 0.03 0.10 0.10 0.09 0.11 LOI 0.71 1.02 0.55 0.40 0.62 0.53 1.00 0.52 0.68 1.04 0.49 0.61 0.77 0.69

Total 99.44 99.83 99.09 98.97 100.20 99.13 98.55 99.90 100.27 99.19 98.95 99.92 99.73 100,18

Sr (ppm) 577 384 344 390 424 697 474 381 324 727 635 380 286 403

Table 4 (continued)

Rock type Muscovite-rich Biotite monzogranite Fenite Average monzogranite partially fenitized fenite

Sample no. 8 9 27 13 14 15 A7 M1 2 7 LJ7 11 25

SiO2 73.19 75.08 7 3 . 8 1 72.94 71.84 72.42 74.70 72 .21 73.02 72.07 74.90 71 .71 73.52 72.99 TiO2 0.17 0.14 0.13 0.19 0.14 0.17 0.03 0.17 0.10 0.14 0.13 0.14 0.16 0.14 AlzO 3 13.66 13 .61 1 3 . 2 6 1 4 . 1 2 14.00 1 3 . 5 0 12 .90 14 .49 13 .43 14 .35 13 .20 13.62 13 .92 13.78 Fe203 (T) 1.35 1.24 1.04 1.33 0.99 1.50 1.13 1:66 1.27 0.97 1.18 1.55 1.52 1.28 MnO 0.01 0.02 0.02 0.03 0.03 0.04 0.05 0.02 0.02 0.02 0.01 0.05 0.04 0.02 MgO 0.33 0.24 0.38 0.26 0.34 0.32 0.03 0.24 0.17 0.16 0.19 0.18 0.37 0.24 CaO 0.39 0.42 0.99 0.50 0.69 0.76 0.43 0.39 0.75 0.57 0.52 1.64 0.48 0.59 NazO 3.88 3.48 3.82 4.06 4.68 3.99 5.36 4.39 4.49 4.64 3.89 4.59 4.40 4.36 K20 4.96 5.31 4.69 5.65 5.49 5.83 4.18 5.75 5.20 5.84 5.18 5.06 5.01 5.41 PzO5 0.08 0.08 0.09 0.10 0.13 0.11 0.03 0.12 0,05 0.08 0.09 0.19 0.10 0.10 LOI 0.89 0.76 0.82 0.99 0.84 0.80 0.45 0.98 0.76 0.88 1.13 1.56 0.77 1.01

Total 98.91 100.38 99.05 100.17 99.17 99.44 99.29 100.42 99.26 99.72 100.42 100.29 100.29 99.92

Sr (ppm) 244 265 323 729 336 501 138 220 172 152 96 158 340 163

FezO3 (T), total iron reported as F%Oa

H z O + 0.511 0 2 = 0.272 NaA1Si3Os + 0.437 KA1Si3Os + 0.049 ana t a se + 0.454 hemat i t e + 0.019 kao l in i t e + 0.039 c l i n o c h l o r e + 0 . 2 2 5 C a 2 + + 0 . 0 9 0 H +. The reac t ion is cons i s ten t b o t h wi th p e t r o g r a p h i c obse rva t ions and wi th results o f Gresens ' ca lcula t ions .

The Ca l ibe ra ted by the above reac t ion m a y have been ut i l ized in the f o r m a t i o n o f la te -s tage calci te or in R E E - b e a r i n g assemblages wi th such minera l s as apa - tite, br i thol i te , a l lani te , sphene, aegir ine-augi te , and f luori te . F igu re 6c, d i l lus t ra tes tha t the only s ignif icant difference be tween a weak ly minera l i zed syenit ic b o d y (fenite 11, Table 4) and unmine ra l i zed fenite is a gain o f Ca re la t ive to the p ro to l i th . The reference vo lume o f 1,000 cm 3 o f m o n z o g r a n i t e con ta ins 0.85 w t % CaO, and m a y l ibera te 0.225 moles o f Ca dur ing feni t iza t ion. W i t h i n minera l i zed syeni te a reas in the fenite, Ca con ten t grea t ly increases, ave rag ing ~ 3 . 5 w t % (or 1.57 moles / reference volume) . A t the p resen t t ime the k n o w n vol- ume o f minera l i zed a reas in the fenite is insuff ic ient to

accoun t for all o f the Ca lost du r ing feni t izat ion. I t is poss ib le tha t Ca in the minera l i zed bodies is o f local der iva t ion , and tha t an ex te rna l source is no t requi red . A l t h o u g h the extent o f ca lc i t e - f luor i t e -bas tnaes i t e veins is no t wel l -def ined, it is poss ib le tha t the con t a ined Ca represents only a r ed i s t r i bu t ion o f tha t re leased dur ing feni t iza t ion.

M a s s - b a l a n c e c o m p u t a t i o n s also ind ica te tha t de- s t ruc t ion o f b io t i t e l ibera tes sufficient M g to accoun t for tha t found in p h l o g o p i t e and aegi r ine-augi te o f min- era l ized areas. Hence, l ike Ca, it is poss ib le tha t M g has been red i s t r ibu ted dur ing ore depos i t ion , ra ther than its be ing o f ex terna l der iva t ion .

Sequence of hydrothermal alteration and ore deposition

Pe t rog raph ic and chemica l analyses suggest t ha t hyd ro - t he rma l a l t e r a t ion a n d ore depos i t i on p roceeded in sev-

378

l o 8

8 6 t 6.

g4 LL 2 ~ 4 '

o 4.8

m Mg and Lg Mg-Fefl F~

o I ,,011 ,k, .... , , , 5.0 5.2 5.4 5.6 5.8 6.0 0.010 0.050 0.090 0.130

Si/AI Ca/AI

10 10

86 (D

ii

2

0

8 '

6,

ija. 2

O' 0.25 0.29 0.33 0.37 0.41 0.29 0.33 0.37 0.41 0.45

Na/AI K/AI

Fig. 5. Histograms showing distri- bution of K/A1, Na/A1, Ca/A1 and Si/A1 weight ratios for biotite monzogranites and fenites from the Rodeo de Los Molles deposit. The extreme right sample in the Ca/A1 histogram is a weakly min- eralized fenite (sample Fend 1)

~ 4

(I1

_~0 "o ~-2 e-

t a - 4 (.9

0.6

A

_. . . .#~-~ ~__.__r

0.8 10 1'2 14

- c ~ 4

~ 2 ~

ca

8o

~-2 ~ - 4

0.6 0.8 1 1.2 1.4 Fv

o.o5!, B

0.05 ',

-0.1 '.

0.15 i

0.6 o.8 i ~.'2 14

~ D

n ~ Mn ~ / ~ ,

01 ~ Sr

0.6 0.8 1 1.2 1.4 Fv

Fig. 6. A, B Composition-volume plots for average unmineralized fenite relative to average biotite monzogranite. C, D Composition- volume plots for weakly mineral- ized fenite relative to average bio- tite monzogranite

eral stages in the area of the Rodeo de Los Molles depos- it. Monzogranites are typically fresh, with only minor amounts of clay minerals and calcite produced during alteration of Ca-rich cores of plagioclase and locally mi- crocline. Alteration of monzogranite to quartz alkali- feldspar granite, or fenite, occurred by reaction with a K + Na-rich fluid, and is best defined by the conversion of plagioclase and microcline to albite and perthite, and

the destruction of biotite to produce hematite, anatase, clinochlore, and kaolinite. Local areas of Ca-enrichment (syenites) within the fenite contain Ca+REE-bear ing minerals such as britholite, allanite, and fluorite. Aegir- ine-augite and phlogopite also are found in these areas. The REE-bearing assemblages exhibit further signs of continued hydrothermal alteration, such as replacement of britholite by bastnaesite and conversion of aegirine-

3.0

O

,,oH Ia_

Y TiO 2

FeO(T)/FeO(T)+MgO

0 0.3 0.'5 0.'7 0.'9

AX Fig. 7. Plot of biotite alteration index, AX, ratio of biotite/(kaolin- ite + hematite + anatase + clinochlore + biotite) vs FeO/(FeO + MgO) and TiO2 content of biotite

Table 5. Moles of minerals in average biotite monzogranite and its altered equivalent (fenite). Reference volume is 1,000 cm 3 of biotite monzogranite

Mineral Biotite Fenite monzogranite

NaA1Si308 3.404 KA1Si3Os 2.573 CaAlzSi2Os 0.398 Plagioclase (3.416) Microcline (2.914) Quartz 12.839 Ilmenite 0.028 Magnetite 0.228 Biotite 0.089 Anatase Hematite Kaolinite Clinochlore

3.676 3.010 0.173

11.485

0.049 0.454 0.019 0.039

EARLY = LATE

I REE- Mineralization Th - Mineralization Bastnaesite-Bearing Veins MINERAL

Britholite

Apatite . . . .

Allanite . . . .

Aegirine-Augite

Phlogopite

Quartz Q1

Fluorite

Uranothorite

Calcite

Clinochlore

Bastnaesite

Q2

F2

Cc2 ~ L Cc.4

i_- r Fv

Ccv

By

Fig. 8. Generalized paragenetic diagram for late-forming minerals at the Rodeo de Los Molles deposit

augite to epidote and chlorite. Deposition of uranothor- ire in miarolitic cavities and with quartz in veins is thought to represent the next stage of ore mineral depo- sition. Calcite occurs as a late-stage mineral within vugs in clinochlore and in veins. Emplacement of calcite- fluorite-bastnaesite veins characterized a second period of REE deposition, Fragments of earlier-formed britho-

379

lite-apatite-quartz in bastnaesite veins suggests that REE may have been redistributed during the latest stages of hydrothermal activity. A generalized paragenesis of late- stage minerals at the Rodeo de Los Molles deposit has been presented by Lira and Ripley (1990). Figure 8 is a modified diagram based on recent data collected dur- ing on-going exploration.

Temperatures of alteration and ore deposition

The coexistence of Na-rich and K-rich feldspars in fen- ites permits an estimation of equilibration temperatures following methods outlined by Brown and Parsons (1981). Chemographic tests proposed by Brown and Par- sons (1981) were used to establish that the near end- member feldspars approached equilibrium. Pairs of feld- spar compositions (Table 1) yield temperature estimates of < 350 ~ C. Fluid inclusion studies by Lira and Ripley (1990) indicate that quartz (Q1 +Q2) in REE-bearing syenites formed at minimum temperatures of 450 ~ C (es- timated pressure 1-2 kbar), whereas fluorite (F1 and F2) crystallized between 271 and 340 ~ C. Later generations of calcite and vein fluorite formed at temperatures less than 250~ (Lira and Ripley, 1991). Homogenization temperatures of primary fluid inclusions in calcite, and secondary fluid inclusions in quartz and fluorite also indicate that low-temperature (< 200 ~ C) fluids were in- volved in late stages of alteration and carbonate deposi- tion.

S t a b l e i so tope s tudies

Results

Results of stable isotopic measurements are given in Ta- ble 6, with representative sample localities plotted on Fig. 2. Ranges of 6180 values for various minerals are as follows: quartz, 8.6-11.1%0; feldspar, 7.8-10.6%o; bio- tite, 5.7-7.9%0; muscovite, 6.1-7.2%o; aegirine-augite, 6.8 7.8%o; clinochlore, 2.3-3.8%0; and calcite, 5.2- 26.7%0, varying with paragenetic stage. Biotite and mus- covite 6D values generally fall between -83 and - 102%o, with some lighter values ( - 102 to - 135%o) restricted to the northwestern portion of the batholith. Whole-rock 6D values are commonly 20-40%0 lighter than biotite values. Carbon isotopic values of carbonates are relatively light, ranging from -7 .8 to -13.5%0.

Discussion

Oxygen isotopic values, c5 a 8 0 values of quartz in both biotite monzogranite and fenite fall in a range that is considered normal for felsic plutonic rocks. Although quartz feldspar A values (.-1.0 to 2.6, Fig. 9) are not far removed from those expected at magmatic tempera- tures, the number of samples with quartz-feldspar A values < 1 suggest disequilibrium. The scatter of values in Fig. 9 makes interpretation difficult. However, for

3 8 0

Table 6. Oxygen and hyd rogen stable i so tope da t a f rom the Rodeo de Los Molles deposi t

Sample Rock* ~ 1 s 0 S M O W (%o) no. type

6 D S M O W (%o)

Quar t z Feld- Biotite Musco- Aegirine- Clino- Biotite Musco- spar vite Augi te chlore vite

Whole- Rock

Clino- chlore

1 M g 10.72 9.44 7.90 - - - 92 - 3 M g . . . . . . . 96 - 4 M g 10.73 9.99 6.66 . . . . 90 - 6 M g 10.77 9.32 8.19 . . . . 89 - 10 M g 10.05 8.81 6.72 . . . . 91 - 18 M g 9.15 9.00 6.98 - - - 92 - 19 M g 10.71 8.55 6.08 . . . . 92 22a M g 9.83 9.17 6.13 . . . . 91 - 83 24 M g 10.68 10.54 6.14 - - - 1 0 2 - 84 26 M g 10.63 10.18 . . . . . 28 M g 10.74 9.07 6.79 - - - 1 0 2 - 29 M g 10.68 9.63 6.60 . . . . 87 - 30 M g 11.15 9.04 7.36 . . . . 96 8 Lg 8.98 8.92 6.15 6.06 - - - 1 2 2 - 1 3 6 9 Lg 9.64 9.29 6.83 7.25 - - - 102 - 100 27 Lg 9.81 - 5.72 . . . . 129 - 135 13 Mg-Fen 10.47 - 6.77 . . . . 14 M g - F e n 8.62 8.86 6.24 - - - 84 - 15 M g - F e n 10.70 9.34 5.69 - - - 97 A7 M g - F e n 8.67 9.00 . . . . A13 M g - F e n 11-14 9.43 . . . . M1 Fen 9.19 9.35 . . . . . 2 Fen 10.06 9.97 . . . . . 7 Fen 9.66 10.62 . . . . . L J7 Fen 10.47 10.00 . . . . . . 11 Fen 9.80 9.36 . . . . 17 Fen 9.20 8.62 . . . . . 25 Fen 11.03 9.53 . . . . . LJ7-O Min -Fen 10.26 9.43 - - - SWLJ Min -Fen 9.88 - - 6.83 - - PVI Min -Fen - - - 7.83 - - C C H L a Min -Fen 10.53 - - - 2.26 - - C C H L b Min -Fen . . . . 3.76 - - PG16 Min -Fen 10.27 . . . . L J3 Min -Fen 10.77 9.85 . . . . . . A G I Min -Fen . . . . . . . A G 1 0 Min -Fen . . . . . . A G 1 4 Min -Fen 10.73 . . . . . . . M70 Min -Fen . . . . . . . L J N D Min -Fen - 9.32 . . . . C M R a C a v - M i a r 10.08 . . . . C M R b C a v - M i a r 10.09 . . . . . C M R c C a v - M i a r 10.05 . . . . . . . C M R d C a v - M i a r 10.50 . . . . . . U T H N o d - U t h 10.58 . . . . 21 Tgn 10.59 10.15 6.76 - --111

- 1 1 4 - 1 1 7 - 1 1 6 - 1 1 1 - 1 0 9 - 1 3 3 - 1 2 1 - 1 2 1 - 1 3 1 - 1 2 9 - 129 - 1 1 4 - 1 0 9 - -130 - 96 --131

- 1 2 9 - -125 - - 1 4 1

- 1 3 6 - 1 1 9 - 1 3 0 - 122 - 1 1 6 - - 124 - -115 - 122

- -117 --131

--121

m

m

m

m

m

m

m

m

m

- 9 4 - 9 3

* Mg, biotite m o n z o g r a n i t e Lg, muscovi te- r ich , garnet , s i l l imani te-bear ing m o n z o g r a n i t e Mg-Fen , m o n z o g r a n i t e part ial ly fenitized, t rans i t ional zone Fen, bar ren fenite Min-Fen , mineral ized fenite

N o d - U t h , u r ano tho r i t e nodu le Cav-Mia r , miarol i t ic cavi ty Tgn , tonali t ic gneiss S M O W , s t anda rd m e a n ocean water

m o n z o g r a n i t e a n d l e u c o g r a n i t e s a m p l e s t w o t r e n d s a r e

r e c o g n i z a b l e . O n e t r e n d d e f i n e s a s t e e p l y d i p p i n g t o n e a r

v e r t i c a l a r r a y w i t h q u a r t z 6 1 8 0 v a l u e s n e a r 10.7%o. T h e

o t h e r is a n e a r l y h o r i z o n t a l a r r a y w i t h f e l d s p a r f i x s o

v a l u e s n e a r 9%o a n d q u a r t z fi 1 8 0 v a r y i n g f r o m ~ 9 t o

11.1%o. T h e t r e n d s a r e n o t r e l a t e d t o d i f f e r e n c e s i n m o d a l

m i n e r a l o g y , a s a l l s a m p l e s c o n t a i n s i m i l a r a m o u n t s o f

q u a r t z a n d f e l d s p a r . W e s u g g e s t t h a t t h e n e a r - v e r t i c a l

a r r a y r e s u l t s f r o m o p e n - s y s t e m e x c h a n g e b e t w e e n f e l d -

s p a r s a n d a f l u i d p h a s e . B e c a u s e t h e r a t e o f e x c h a n g e

b e t w e e n f e l d s p a r s a n d a f l u i d is e x p e c t e d t o b e m u c h

g r e a t e r t h a n t h a t b e t w e e n q u a r t z a n d f l u i d ( e .g . , G r e g o r y a n d C r i s s 1 9 8 6 ) , t h e h o r i z o n t a l a r r a y m a y r e f l e c t i n i t i a l

i s o t o p i c h e t e r o g e n e i t y o f q u a r t z i n t h e g r a n i t i c r o c k s .

M o s t s a m p l e s f r o m f e n i t e s p l o t e i t h e r o n o r b e t w e e n

t h e t r e n d l i n e s d e f i n e d b y t h e s a m p l e s o f m o n z o g r a n i t e .

381

12

~=0

~.~%11 ~ A=I

/4

O oo 7_~ 8 ~ o ~ Mg-Fen

" Fen * Min-Fen

7 8 lb 1~

d 18 O Quartz (~)

Fig. 9. Values of 6180quartzVS 8180feldspar. Mg, biotite monzogran- ite; Lg, muscovite-rich monzogranite; Mg-Fen, monzogranite-fen- ite transition; Fen, fenite; Min-Fen, mineralized fenite

"[ .0

rr

0.8

o~ E 0.6 0

0.4 r

>. 0.2 X �9

0 5.5 6.5 7.5 8.5 9.5 10.5

~180 go

Fig. 10. Calculated 5180 values of biotite and feldspar produced by isotopic exchange with a fluid of 51sO = 8%o at various water/ rock ratios (atom %). See Criss and Taylor (1986) for computation- al methods

These values are indicative of an initial quar tz - feldspar A near 1.8 to 2, and open-system exchange with a fluid at relatively low water/rock ratios. Fluid inclusion data, as well as the presence of mineralization, alteration and miarolitic cavities strongly indicate that fluids were pres- ent in the area of the Rodeo de Los Molles deposit. Figure 10 illustrates that observed 51sO feldspar values may be produced by isotopic exchange at temperatures near 400 ~ C between a magmatic fluid of 5 is O ~ 8%0 (lowest value determined from quartz and pyroxene, see below) and feldspar with an initial value of ~ 8.5%0 (low- est value in the vertical trend depicted in Fig. 9), at a water/rock ratio of ~ 1. Sample GM-27 shows an anom- alously low feldspar 51SO value, suggestive of interac- tion with an 1sO-depleted fluid. This sample will be dis- cussed below in conjunction with biotite 5180 and 5D values.

Quartz-biotite A values are only slightly lower than those reported from several granitic plutons (e.g., Turi and Taylor 1971; O'Neil et al. 1977; O'Neil and Chap- pell 1977). Larson and Taylor (1986) have shown that quartz-biotite A values from volcanic rocks are common- ly < 2-3, where those from more slowly cooled plutons are commonly > 4 due to continued exchange at lower temperatures. We suggest that biotite 51so values have been lowered by subsolidus exchange with a magmatic fluid as proposed for feldspar. Figure 10 illustrates that

biotite with a typical magmatic 81SO value of ~8%0, may undergo a decrease in 61SO by interaction with magmatic fluid of 6 tSO~ 8%o at temperatures of 500- 600 ~ C at water/rock ratios between 0.1 and 1. An alter- native is that biotite 8180 values near 7%o record prima- ry values inherited during magmatic crystallization at

700 ~ C, and exchange at lower temperatures has been limited. In this scenario the similarity between 5180 values of biotite and muscovite from the muscovite-rich facies suggests that both micas have resisted isotopic exchange with decreased temperature, and A values re- presentative of higher temperatures have been preserved.

Hydrogen isotopic values. Whole-rock, biotite, and mus- covite 5 D values are low compared to values considered typical for magmatic rocks (e.g., Friedman and O'Neil 1978). The relatively low 6 D values may indicate subso- lidus isotopic exchange with a low-D fluid, or crystalliza- tion from a D-depleted magma. Several features argue against interaction with a low-D fluid as a mechanism for producing all observed 6D values. First, oxygen isotopic values of feldspar and biotite do not suggest isotopic exchange with a low-180 fluid, except locally along the margin of the batholith (see below). Second, biotite and muscovite 6D values (except those from Lg 8, 9, and 27 located at the margin of the batholith) are considerably higher (20-40%0) than most whole-rock values. It is clear that a mineral with a much lower 5 D value than biotite must be present in the rocks. Although not isotopically analyzed, petrographic examination sug- gests that the low whole-rock 6 D values are related to the presence of fine-grained illite, smectite and other clay minerals that occur as alteration of feldspar. The differ- ences between whole-rock and biotite 6D values indi- cates a lack of hydrogen isotope homogeneity, and argues against widespread interaction with a low-D fluid at high temperature.

Taylor et al. (1983), Nabelek et al. (1983) and Brigh- am and O'Neil (1985) have shown that low 6D values in igneous rocks may result from loss of a D-enriched vapor phase rather than from low-temperature exchange with meteoric water. Experimental studies by Kuroda etal. (1982), Richet et al. (1986), and Dobson etal. (1989) indicate that fluids evolved from silicie magmas may be enriched in D by as much as 50%o relative to the coexisting melt, and such fluid liberation may pro- duce D-depleted magmas. Low-6D rocks which record vapor-phase exsolution generally exhibit a trend of de- creasing 6D with decreasing H 2 0 content (" O'Neil ef- fect"; Taylor 1988). For the Las Chacras batholith whole-rock monzogranite 6D values show no well-de- fined trend with water content. However, the range of whole- rock H 2 0 contents is restricted to values between 0.49 and 0.67 wt%. Correlation between 8D and water content observed in felsic plutons by several researchers (e.g., O'Neil and Chappell 1977; O'Neil et al. 1977; Na- belek et al. 1983) normally is detected over a much broader range (~0.2 to 1.0 wt% H20). The restricted range of H20 contents of rocks from Rodeo de Los Molles prevents a meaningful assessment of a possible H20-5 D trend. In addition, mass-balance computations

382

-20 Open System (700 ~ lOaln c~ V-M=+20~,( o~ v=1.02)

-40 103 In c~ BlOT-H20=-12 %o -60 ~ u s c - a 2 ~ =+5"4 ~

------___M~Me/t ~ "~~ v

-100

-120

-140

0.'2 0.'4 0.'6 0.'8 1.0

Fraction crystallized Fig. 11. The 5D trends of magma, vapor, biotite, and muscovite at 700 ~ C and a A fluid-melt value = 20%0 during Rayleigh distilla- tion of H20 vapor from granitic melt. 1000 In c ~ o 1 is from compo- sitional data and the equation of Suzuoki and Epstein (1976). 1000 In %~o is that for ideal muscovite

indicate that the bulk of the water in the monzogranites is not contained within biotite or primary muscovite, but rather is within secondary alteration minerals. For example, the mass fraction of water in sample Mgl con- tributed from illite and smectite is ~ 0.84, with a com- puted clay mineral 5 D value of -119%0. Other samples show a range in the mass fraction of water contributed from alteration minerals of 0.48 to 0.75 and 5D values of - 155 to -- 124%o. At temperatures between ~200 and 400 ~ C clay minerals may be from 30 to 50%0 heavier than coexisting water (e.g., Kyser 1987). Values of 5 DH2o near --100%o may be representative of small amounts of fluid remaining in pore spaces after extensive exsolu- tion of a vapor phase, or may indicate the involvement of later fluids that genetically are unrelated to crystalli- zation of the monzogranite.

Figure 11 illustrates 5D values of melt and vapor produced during magmatic vapor-phase exsolution fol- lowing a Rayleigh process (open system) with A fluid- melt = 20%0 at 700 ~ C. The low 5 D values of biotite and coarse-grained, early-formed muscovite may have been produced after ~60-80% crystallization of a magma characterized by an initial 5 D of -75%o. Assuming con- tinuous vapor-phase exsolution, 60-80% of initial H/O would be lost during crystallization. Either initial or con- tinued formation of hydrous mafic silicates is still possi- ble after such vapor loss, as water fugacity in the melt may be buffered at relatively high values (e.g., Holloway 1976); Naney (1983) has shown that biotite may crystal- lize from melts having water contents as low as 0.5 wt% at 2 kbar.

Interaction with meteoric water. Three samples of the muscovite-rich facies located along the contact of the batholith with enclosing metasedimentary material (Fig. 2) show evidence for interaction with low-180 low-D meteoric H20. Figure 12 illustrates a trend of decreasing 5180 and 6D for biotite samples located in the contact area. Feldspar 5 t80 values also show a trend with biotite 5 D values (Fig. 13) that is characteristic of isotopic exchange with meteoric fluids. Muscovite 5D values are also the lowest observed. Whole rock-biotite

-60

-80

i~ -100, rn

,,~D .120 ,

-140

5

r

I

�9 Mg

I~gg. Fen

~180Biotite (~o) Fig. 12. Biotite 5D vs 5180. Abbreviations are the same as those of Fig. 9

-6O

-80

~,~-100 a "o-120

-140

�9 E i m l �9 �9 o

�9 Mg

* i~gg. Fe n

lb ~18 0 Feldspar (~)

Fig. 13. Biotite 5D vs feldspar 5180. Abbreviations are the same as those of Fig. 9

A values are reduced considerably, and they vary be- tween - 8 and + 6%o. Isotopic exchange between igneous rocks and meteoric water at the margin of batholiths is a commonly reported phenomenon (e.g., Taylor 1989). Temperatures of isotopic exchange between meteoric water and igneous rocks at the pluton margin are diffi- cult to ascertain. A maximum temperature of ~ 350 ~ C is estimated based on mineralogical and isotopic charac- teristics of feldspars located near the interior of the intru- sion that show no evidence of interaction with a low-lSO fluid. Utilizing the Suzuoki and Epstein (1976) relation- ship for muscovite-water fractionation, the 5 D value of the meteoric water is estimated at ~ - 100%0. Muscovite exchanged with the meteoric water more readily than did biotite, as evidenced by the reversal of A biotite- muscovite values.

Fenitization and mineralization. Field and petrochemical evidence suggests that the area of fenitization and REE deposition formed as a result of localized fluid infiltra- tion. Gradational transition zones between the fenite and surrounding monzogranite suggest that the reaction front moved outwards from the core of the fenitized area. Fluid inclusion studies (Lira and Ripley 1990) have documented a complex history of open-system, multiple fluid migration in the mineralized zone. Fluid composi- tions range from moderately saline in early quartz (Q1 ; 15-25 equilibrium wt% NaC1), CO2-bearing and lower salinity in paragenetically later fluorite (F1 : Xco2 = 0.07 to 0.13, 4.3-10.4 equivalent wt% NaC1), to lower tern-

383

perature and salinity in late-formed calcite (< 1 equiva- lent wt% NaC1).

The 6180 values of quartz that occurs with REE- minerals in nodules, and that found in miarolitic cavities within fenites suggest isotopic equilibration with fluids of magmatic origin. Using fluid-inclusion trapping tem- peratures of ~600~ (Lira and Ripley 1990) and the quartz-H20 fractionation factor of Matsuhisa et al. (1979), a c5180 value of H20 in equilibrium with quartz is ~ 10%% which is typical of a magmatically derived fluid. Utilizing temperatures between 400 and 550 ~ C, 61 s 0 values of aegirine-augite also suggest equilibration with a fluid of 6180 between 8 and 10%o, which is consis- tent with formation involving a magmatic-hydrothermal fluid.

Whole-rock 5 D values of both mineralized and un- mineralized fenites are similar to those of biotite monzo- granite, indicating that the production of clinochlore and kaolinite from biotite resulted in little isotopic frac- tionation of H - D , and that fluids involved in fenitiza- tion may have been isotopically similar to those that equilibrated with clay minerals in the biotite monzogran- ire. Because of uncertainties in fractionation factors, esti- mation of the hydrogen isotopic composition of water in equilibrium with clinochlore and kaolinite is difficult. However, fenite 6 D values are consistent with equilibra- tion with a D-depleted fluid produced during late stages of magmatic vapor exsolution. Such an interpretation is supported by 6180 values of quartz and aegirine-au- gite that crystallized in areas of mineralization.

A further stage of alteration is recognized by the con- version of aegirine-augite to clinochlore, and the deposi- tion of nearly massive clinochlore in mineralized areas. The bulk of water included within REE-mineralized rocks can be accounted for by that contained within late-stage clinochlore (5 D--~ - I00%o, 5 tsO = 2.26- 3.76%0, Table 6). Computed 5180 of water in equilibri- um with late clinochlore at 200 to 300 ~ C is estimated at 2.2 to 4.7%o (Taylor 1979). These c51sOH~o values are similar to those computed from calcite that occurs in vugs within massive clinochlore, and suggests the in- volvement of meteoric water in the late stages of alter- ation and carbonate genesis (see below).

Oxygen-carbon isotopic values - carbonate genesis. At least four textural varieties of calcite are recognized in the Rodeo de Los Molles area. However, the establish- ment of a paragenetic sequence is difficult. Only two of the textural varieties can be placed within well-defined time frames. Each variety of calcite is characterized by a restricted range of 5180 values (Fig. 14, Table 6). The 513C values are variable, with no clear distinctions be- tween textural types. One variety of calcite (Ccl) occurs as a filling of triangular void spaces between aegirine- augite grains. The 513C values for this calcite range from --8.6 to - 13.5%o, but 518 0 values fall in a narrow range of 5.2 to 5.9%0. A second type of calcite deposition (Cc2- Cc3) is recorded by a series of replacement textures. Calcite may pseudomorphically replace sphene, and oc- cur as an irregular replacement of plagioclase and aegir- ine-augite. The 513C values of this textural type vary

.g-8

~ -12 ( 'o

~='-16

-20 0

�9 § 2 4 7 �9 * , o

m m

�9 C c l + Cc2-Cc3 * Cc4 " CeV

4 8 12 16 20 24 28

61B0 Calcite (~)

Fig. 14. Values of 5180 vs 613C for various textural varieties of calcite from the Rodeo de Los Molles deposit

from -7 .8 to -10.1%o, and 6tsO values from 11.6 to 19.4%o. A third textural type (Cc4) represents a late stage of deposition with calcite as small crystals in vugs within nearly massive clinochlore. The clinochlore itself has been produced by alteration of aegirine-augite and pla- gioclase. Calcite Cc4 is characterized by the highest 51 s O values of all carbonate samples, 22.3-26.7%0; 613C values range from -9 .5 to -12.6%o. The latest calcite in the deposit (CcV) is that found in late-stage veins with bastnaesite and fluorite. Four samples of calcite from this type have 613C and 61so values between - 1 1 . 2 and -11.5%o, and 9.0 to 9.4%0, respectively.

Only calcites that occur as replacements of earlier- formed minerals show a possible 513C-5180 covariant relationship perhaps related to reservoir effect or CO2- degassing (e.g., Zheng 1990). However, we suggest that the range of observed 51 s 0 values is due primarily to differences in depositional temperatures, as well as ef- fects caused by fluid mixing. Because of the small size and scarcity of fluid inclusions in calcite that occurs as a replacement of sphene, aegirine-augite, or plagio- clase, an estimate of formation temperatures is difficult. However, based on temperature estimates for aegirine- augite formation and plagioclase alteration, a tempera- ture range of 200 to 350 ~ C appears reasonable. At these temperatures computed 5180~2 o values range from ~ 5 to 14%o, and may represent late-stage, deuteric fluids of magmatic origin. Fluid-inclusion homogenization temperatures for calcite that occurs in voids between aegirine-augite grains varies from 235 to 245 ~ C. Al- though possible pressure effects on homogenization tem- peratures are not considered, computed 6 lsOH2 o values are <2%0, and suggest the involvement of low-lSO me- teoric water. Late-stage calcite that occurs in veins with bastnaesite and fluorite also appears to be related to the influx of meteoric H20. Fluid-inclusion homogeniza- tion temperatures for calcite (160~ and fluorite (183 ~ C) lead to computed 6tsOr~2 o values of ~ - 2 to -3%o. Waters from fluid inclusions in two samples of vein calcite were collected by decrepitation methods, and are characterized by 5 D values of - 3 4 and -48%0. We suggest that these values represent meteoric water that underwent a modest 1sO shift due to exchange with metasedimentary and igneous rocks. Meteoric water of 5 D ~ -40%o, 5180 ~ -6%0 would be distinctly different

384

Table 7. Carbonate stable isotope values from the Rodeo de Los Molles deposit

Sample Rock* Calcite Bastnaesite no. type

c~18 O c~13C cS13 C SMOW PDB PDB (%) (%) (%)

CCHL-I Min-Fen 26.68 - 12.60 - CCHL-2 Min-Fen 22.29 - 12.03 CCHL-3 Min-Fen 25.69 - 9.94 - CCHL-4 Min-Fen 25.85 - 9.97 - CCHL-5 Min-Fen 25.04 - 10.07 - CCHL-6 Min-Fen 25.38 - 9.47 - 11-1 Min-Fen 19.37 - 8.28 - 11-2 Min-Fen 19.25 - 7.79 - 11-3 Min-Fen 18.99 - 8.27 - 11-4 Min-Fen 18.44 - 8.03 LJ3-1 Min-Fen 11.60 - 1 0 . 1 1 -

LJ3-2 Min-Fen 12.19 - 10.08 - L J3 - 3 Min-Fen 11.58 - 10.08 PVI-1 Min-Fen 13.64 - 8.97 PVI-2 Min-Fen 12.19 - 8.94 - LJ7-O Min-Fen 14.00 - 9.79 - PG16-1 Min-Fen 12.64 - 8.70 - PG16-2 Min-Fen 12.77 - 8.68 N L J - 1 Min-Fen 5.23 - 13.49 - NLJ-2 Min-Fen 5.80 - 13.05 - NLJ-3 Min-Fen 5.91 -13.08 - AG25-1 Min-Fen 5.60 - 10.63 - AG25-2 Min-Fen 5.86 - 9.95 - AG25-3 Min-Fen 5.95 - 9.92 - SWLJ-1 Min-Fen 5.52 - 8.61 - SWLJ-2 Min-Fen 5.62 - 8.62 - LJND-I Min-Fen - - -10.86 LJND-2 Min-Fen - - 11.07 LJND-3 Min-Fen - - 7.22 M70-1 Min-Fen - - 9.77 M70-2 Min-Fen - - - 8.76 M80-3 Min-Fen - - - 7.79 M70-4 Min-Fen - - 7.99 M70-5 Min-Fen - - - 7.91 P47-B1 B-Vein - - - 9.83 P47-B2 B-Vein - - - 9.56 P47-C1 FCB-Vein 9.04 - 11.36 - P47-C2 FCB-Vein 9.35 - 1 1 . 1 7

P47-C3 FCB-Vein 9.03 - 11.36 P47-C4 FCB-Vein 9.41 - 11.46 -

* Min-Fen, mineralized fenites B-Vein, bastnaesite veins FCB-Vein, fluorite-calcite-bastnaesite veins PDB, Peedee Formation belemnite

f rom tha t p r o p o s e d to have in te rac ted wi th igneous rocks near in t rus ive con tac t s wi th coun t ry rocks. How- ever, the vein calci te is pa ragene t i ca l ly later , and H 2 0 i so top ic values a re m o r e s imilar to m o d e r n - d a y meteor ic values (surface wa te r ~ D ~ - 2 2 % o , c~ 180, -~-4%0) . I t is poss ib le tha t c l imat ic cond i t ions a n d / o r re la t ive geo- g raph ic loca t ion tha t con t ro l the i so top ic c o m p o s i t i o n o f me teor i c wa te r va r i ed be tween the a l t e r a t ion and vein- f o r ming event.

Values o f 813C for all s tages o f calcite, as well as bas tnaes i t e (Table 5), are i so top ica l ly l ighter t han those typ ica l ly ass igned to man t l e - or juven i l e -de r ived ca rbon .

Because o f the p r o p o s e d var iab le f luid sources involved in calci te depos i t ion , it is sugges ted tha t 613C values reflect leaching o f c a r b o n f rom igneous or m e t a m o r p h i c rocks tha t had i n c o r p o r a t e d o rgan ic c a r b o n dur ing for- ma t ion , or direct i nvo lvemen t o f i so top ica l ly l ight car- b o n in a f luid der ived f rom a m a g m a tha t m a y have fo rmed by pa r t i a l mel t ing o f o rgan ic c a r b o n - b e a r i n g sed- i m e n t a r y rocks.

Conclusions

Field, pe t rog raph i c , and chemica l da t a ind ica te tha t R E E mine ra l i za t i on o f the R o d e o de Los Mol les depos i t is local ized wi th in a fenite p r o d u c e d by h y d r o t h e r m a l a l t e r a t ion o f the hos t b io t i t e monzogran i t e . Gresens ' c ompu ta t i ons , a long with va r ia t ions in mine ra l p r o p o r - t ions and c ompos i t i ons suggest tha t fenites ga ined N a and K and lost Ca and Sr. Mic roc l ine and p lag ioc lase were r ep laced by per th i te , and b io t i t e was conver t ed to aggregates o f ana tase , hemat i t e , c l inochlore , and kaol in- ite. Relict b io t i te became progress ive ly M g rich du r ing feni t iza t ion. R E E mine ra l i za t i on m o r e s t r ic t ly is con- f ined to a reas o f qua r t z a lka l i - f e ldspa r syeni te and a lkal i - f e ldspa r syeni te f o u n d wi th in the fenite. These areas are ca lc ium rich, and represen t con t i nued h y d r o t h e r m a l al- t e ra t ion wi th a ga in o f Ca tha t is poss ib ly der ived f rom fe ldspar a l t e r a t ion in the large vo lume o f s u r r o u n d i n g fenite. P r i m a r y R E E minera l s are Ca rich (e.g., b r i tho- lite) and are assoc ia ted with apa t i t e , f luori te , aegir ine- augite, ph logop i t e , and quar tz . Ph logop i t e is a precipi - t a ted mine ra l and no t a rel ict o f a l te red m o n z o g r a n i t e biot i te . C o n t i n u e d f luid f lux into the local ized a rea p ro - duced a l t e ra t ion o f b r i tho l i t e to bas tnaes i te , and aegir- ine-augi te to ep ido te and chlori te . La te calci te is found in vugs wi th in mass ive c l inochlore and in veins.

Stable i so top ic d a t a indica te tha t fluids o f m a g m a t i c or igin were involved in the ear ly stages o f feni t iza t ion, wi th l o w - t e m p e r a t u r e in t e rac t ion with me teor ic wa te r res t r ic ted to the no r thwes t e rn p o r t i o n o f the ba tho l i th . Biot i te and muscov i te c~ D values are re la t ive ly low, and are t h o u g h t to have been p r o d u c e d as a resul t of exsolu- t ion o f a v a p o r phase f rom the p a r e n t m a g m a o f the monzogran i t e . Qua r t z - f e ld spa r A values ind ica te tha t al- t h o u g h qua r t z resis ted i so top ic exchange with l ibera ted m a g m a t i c - h y d r o t h e r m a l f luids, f e ldspa r con t inued to ex- change to m u c h lower t empera tu res . Fe ld spa r ~180 values are cons is ten t wi th i so top ic exchange with a mag- ma t i c f luid ( 6 1 8 0 ~ 8 % o ) a t t e mpe ra tu r e s near 5 0 0 ~ and w a t e r - roc k ra t ios less t han 1. M a g m a t i c fluids tha t depos i t ed qua r t z and aegi r ine-augi te are also charac te r - ized by 618 0 values near 8%0.

Bo th pe t rochemica l and i so top ic studies are consis- tent wi th the de r iva t i on o f fenite due to h y d r o t h e r m a l a l t e ra t ion o f the b io t i t e m o n z o g r a n i t e as f luids o f mag- mat ic or ig in were channe led to the t op o f the in t rus ive system. C o n t i n u e d f luid in f i l t ra t ion p r o d u c e d Ca- r ich areas o f R E E mine ra l i za t i on wi th in the fenites. Mass- ba lance re la t ionsh ips suggest t ha t b o t h Ca and M g in the minera l i zed areas m a y resul t f r om red i s t r ibu t ion o f Ca and M g ini t ia l ly f o u n d in the feni t ized rock , r a the r

385

than by a d d i t i o n f rom an ex te rna l source. L a t e r s tages o f calci te a l t e r a t i on a n d ca lc i t e - f luor i t e -bas tnaes i t e vein depos i t i on were p r o d u c e d by in f i l t r a t ion o f me teo r i c waters (6 180 ~ - 2 to - 3%0, b D shif t ing f rom - 100 to -40%0) tha t g r adua l l y were evolving t o w a r d i so top ic values s imi lar to p r e s e n t - d a y meteor ic waters (b 1 8 0 ~ - 4 % % 6 D ~ -22%0) . Smal l f r agmen t s o f p r i m a r y R E E - bea r ing mine ra l a ssemblages cemen ted by f luor i te and bas tnaes i t e in the la te -s tage veins suggest t ha t s econda ry R E E m i n e r a l i z a t i o n m a y have been der ived f rom pa r t i a l d i s so lu t ion o f p r i m a r y mine ra l i za t ion . L o w calci te b 13C values suggest t ha t the p a r e n t m o n z o g r a n i t e m a g m a con- t a ined a large p r o p o r t i o n o f c a r b o n der ived f rom sedi- m e n t a r y o rgan ic mate r i a l , and tha t c a r b o n a t e c a r b o n depos i t ed du r ing la te r s tages o f a l t e r a t ion was der ived by leaching o f c a r b o n f rom igneous a n d / o r enclos ing m e t a s e d i m e n t a r y rock types.

Acknowledgements. Raul Lira was supported by a fellowship given by the Consejo Nacional de Investigaciones Cientificas y T6cnicas de la Repfiblica Argentina (CONICET), to whom he is greatly indebted. Thanks also go to Michelotti e Hijos S.R.L. and N. Vifias for support during field work and for providing new samples. Appreciation is expressed to Mark Gilstrap (Dept. of Geological Sciences, Indiana University) for his assistance with bulk-rock chemical analyses, and to Mike Dorais (Indiana University) and Ian Steele (University of Chicago) for their help with electron mi- croprobe analyses.

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