Petrogenesis and Postmagmatic Geochemistry of the Italian Mountain Intrusive Complex, Eastern Elk Mountains, Colorado

Home , Elk Mountains (Colorado), Italian Mountain

Petrogenesis and postmagmatic geochemistry of the Italian Mountain Intrusive Complex, eastern Elk Mountains, Colorado

CHARLES G. CUNNINGHAM, JR. U.S. Geological Survey, Denver Federal Center, Denver, Colorado 80225

ABSTRACT I06°40'

(82) The Italian Mountain Intrusive Complex lies within the Colorado mineral belt. It consists of Oligocene plutons, dikes, and associated hydrothermal lead-silver deposits. The rocks range in composition from quartz diorite to quartz monzonite. SOPRIS The core of the youngest intrusive mass is STOCK porphyritic and contains a central facies SAWATCH characterized by a partly aphanitic ground- RANGE mass, which was formed by quenching SNOWMASS and represents a late-stage venting of the STOCK intrusive complex. Upon venting, the youngest plutonic rocks fractured, fluids in the core boiled and were introduced into fractured quartz phenocrysts, and quartz veins were formed. The evolution of the late magmatic and TREASURE postmagmatic fluids is inferred from fluid MOUNTAIN inclusions. Pressure constraints imposed by DOME '.WHITE/ 39? : ROCK measured fluid compositions and homog- 00 PLUTON- enization temperatures indicate that a pres- ; Figure 2 sure of 250 bars existed on the fluids at the time of venting. Depth of emplacement was between 950 and 2,700 m. Key words: pet- \ rogenesis, fluid inclusions, geobarometry, Crested i Taylor epizonal plutons, hydrothermal fluids. Butte Park £ -f INTRODUCTION Or

The Italian Mountain Intrusive Complex 10 15 MILES is in the central part of the Colorado min- j eral belt, 25 km south of Aspen (Fig. 1). The 10 15 KILOMETRES complex consists of Oligocene stocks and dikes and associated lead-silver ore deposits. Almont j The Italian Mountain Complex was in- DENVER truded into Precambrian metamorphic and MAP igneous rocks and overlying Paleozoic AREA sedimentary rocks (Fig. 2, A and B) in a COLORADO complex structural setting. The Precam- MINERAL brian rocks consist of plagioclase-quartz- BELT /Gunnison muscovite-biotite schist and gneiss similar to rocks to the north described by Bryant (1970) and are intruded by granitic plutons. The principal granitic rock is coarse- EXPLANATION grained porphyritic quartz monzonite that resembles the Boulder Creek Granite, which Tertiary intrusive rocks Precambrian rocks was dated by Peterman and others (1968) at Figure 1. Location and geologic setting of Italian Mountain Intrusive Complex. Modified from 1,700 m.y. The porphyritic quartz monzo- Burbank and others (1935), Obradovich and others (1969), Bryant (1970), and Tweto and Case nite is cut by bodies of medium-grained (1972).

Geological Society of America Bulletin, v. 86, p. 897-908, 8 figs., June 1976, Doc. no. 60610.

897

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/6/897/3429252/i0016-7606-87-6-897.pdf by guest on 28 September 2021 I06°45'

Figure 2. A. Geologic map and sections of Italian Mountain Intrusive 1/2 I MILE I J Complex, Gunnison County, Colorado. Base from U.S. Geological Survey ~l— I Pearl Pass and Italian Creek quadrangles (location shown on Fig. 1; ex- 1/2 I KILOMETRE planation of symbols in Fig. 2B caption).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/6/897/3429252/i0016-7606-87-6-897.pdf by guest on 28 September 2021 13,000'

M 2,000

14,000

13,000'

"J-12,000'

II, 000

10,000'

12,000' -

11,000'

IC^OOO' 10,000' z g- H I D 14,000' -i r 14,000' Italia

Tmv P

13,000' -13,000'

12,000' 12,000

11,000 11,000' Figure 2. B. Detailed cross sections (locations shown on Fig. 2A). Qs, Quaternary surficial deposits — includes alluvium, colluvium, talus and landslide deposits, rock glaciers, and morainal deposits; Tr, rhyodacite dikes; Td, dacite porphyry dikes; Tv, vent facies; Tpq, porphyritic quartz monzonite — upper part may be contemporaneous with Tv, lower part may be contemporaneous with Tqm; Tqm, quartz monzonite; Tm, melagranodiorite — contemporaneous with Tqm; Tgc, pink granodiorite — coarse-grained; Tgf, pink granodiorite — fine-grained; Tqd, quartz diorite; ms, metasedimentary rocks — contact metamorphosed Paleozoic siltstone and shale converted to hornfels, limestone converted to marble, abundant calc-silicate minerals include gros- sularite garnet, idocrase, and lazurite; IPgb, Pennsylvanian Gothic Formation of Langenheim (1952), Belden and Molas Formations; Pzr, older Paleozoic sedimentary rocks — includes Mississippian Leadville Limestone, Mississippian(?) and Devonian Dyer Dolomite, Devonian Parting Formation, Ordovician Manitou Dolomite, and Cambrian Peerless Formation and Sawatch Quartzite; pCr, includes vogesite dike, porphyritic quartz monzonite and muscovite- bearing granite, and plagioclase-quartz-muscovite-biotite schist and gneiss. Scale same as Figure 2A.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/6/897/3429252/i0016-7606-87-6-897.pdf by guest on 28 September 2021 900 C. G. CUNNINGHAM, JR.

quartz monzonite that resembles the quartz diorite and pink granodiorites has l,400-m.y.-old Silver Plume Granite and by been repeatedly fractured by successive in- finer grained, leucocratic muscovite granite. trusions and reactivation of the main fault The Precambrian rocks are overlain by during emplacement of the complex. more than 430 m of quartzite and carbo- The youngest plutonic rocks of the com- nate rocks of the Mississippian Leadville plex form the northern intrusive center. Limestone, Mississippian(P) and Devonian This center, which constitutes the bulk of Dyer Dolomite, Devonian Parting Forma- Italian Mountain, is rudely concentric and tion, Ordovician Manitou Dolomite, and contains four distinct rock types. A thin Cambrian Peerless Formation and Sawatch outer border zone is melagranodiorite; it is Quartzite. These are followed by more than succeeded inward by a discontinuous zone 300 m of dominantly coarse clastic of quartz monzonite. The core of the pluton sedimentary rocks of the Pennsylvanian is leucocratic porphyritic quartz monzonite, Gothic Formation of Langenheim (1952) which is pierced at the center by a pipelike and the Belden and Molas Formations. body of partly aphanitic porphyritic quartz The major preintrusive structural feature monzonite that is referred to hereafter as in the vicinity of Italian Mountain is a re- the vent facies. cumbent syncline of Laramide age that is The melagranodiorite forms a border overturned to the west (Fig. 2, A and B). zone as much as 12 m wide around part of Local drag folds in the Belden Formation of the northern intrusive center (Fig. 2, A and the overturned limb trend N30°E, plunge 3° B). The rock is predominantly equigranular to 5° southwest, and are parallel to the axis with an average grain size of 1 to 2 mm. of the major fold. The fold opens and be- Locally, K-feldspar phenocrysts and as- comes upright to the north and south of sociated schlieren are aligned parallel to the Italian Mountain. Sedimentary rocks on the contacts with sedimentary rocks. Mafic east flank of the syncline at Mount Tilton (3 minerals are interspersed in a matrix of km north of Italian Mountain) and Ameri- plagioclase (An 55) and quartz, which can Flag Mountain (3 km southeast of causes a "salt-and-pepper" appearance. 0 I KILOMETRE Italian Mountain) are right side up. The mafic grains are dominantly equant, The major recumbent fold is cut by the Figure 3. Location sketch showing intrusive chiefly black biotite and hornblende with centers and distribution of igneous rocks. See north- and northwest-trending Castle Creek sparse, dark-green augite. The rock is gen- Figure 2 for explanation of symbols. fault zone, which borders the Sawatch erally unaltered. The melagranodiorite is Range. The Tertiary plutons invaded the partly altered to epidote, quartz, K-feld- present only where the quartz monzonite is fault zone and apparently were localized by spar, and magnetite-ilmenite. Modes are in contact with sedimentary rocks and is it. In the vicinity of Italian Mountain, indicated in Table 1 and Figure 4. The rock thickest adjacent to limestone. It has a high-angle reverse faults cut the folds, and is locally propylitized. sharp contact with the quartz monzonite. these faults are cut by rocks of the complex, The next oldest plutonic rock forms Spatial distribution, composition, and tex- suggesting that most of the folding and the southern intrusive center and is com- tures indicate that the melagranodiorite faulting took place during the Laramide posed principally of coarse-grained pink formed as a reaction zone between the orogeny. Both the east- and the north- granodiorite but contains a finer grained quartz monzonite and sedimentary rocks trending faults cut the plutons, and thus, facies near the western border. Both rocks rich in calcium. they were reactivated during late Cenozoic are pink owing to disseminated hematite in The contact between the melagranodior- time. Movement on the east-trending fault the interstitial orthoclase. The rocks consist ite and the quartz monzonite is generally system must have been dominantly vertical of euhedral andesine in a matrix of altered parallel to the contact with the sedimentary after the intrusive complex was emplaced, orthoclase, chloritized hornblende, and units. The lack of quenching at the contact because there is no detectable horizontal quartz and are hypidiomorphic-granular in of the quartz monzonite suggests that fresh offset of the steeply dipping intrusive texture. In the altered facies, plagioclase quartz monzonite magma was emplaced contact. cores are replaced by epidote. The opaque while the melagranodiorite was still hot. In mineral is generally magnetite that contains places along the quartz monzonite— ROCKS OF THE COMPLEX ilmenite lamellae. Miarolitic cavities are melagranodiorite contact the quartz mon- moderately common. The quartz in the zonite contains hornblende crystals The Italian Mountain Intrusive Complex includes two simple plutons and one com- TABLE 1. AVERAGE MODAL COMPOSITIONS OF PLUTONIC ROCKS FROM THE posite pluton (Fig. 3). ITALIAN MOUNTAIN INTRUSIVE COMPLEX The rocks range in composition from quartz diorite to quartz monzonite and are 1 2 3 4 5 6 7 8 9 10 11 12 subdivided into six varieties (Fig. 2, A and B). The youngest rock (porphyritic quartz Porphyritic quartz monzonite (Tpq) 31 29 34 3-4 •• tr. tr. 2 tr. tr. tr. monzonite) is dated at 33.0 ± 1.8 m.y. by Quartz monzonite (Tqm) 39 27 25 •• 5 •• tr. tr. 2 1 tr. tr. Melagranodiorite (Tm) 51 16 16 6 7 •• tr. 0-1 2-3 1 tr. the fission-track technique (Cunningham Coarse-grained granodiorite (Tgc) 47 16 26 0-2 •• 1-2 3-4 tr. 2-3 tr. tr. tr. and Naeser, 1975). Fine-grained granodiorite (Tgf) 52 18 16 •• •• 5 4 1 3 tr.tr. 1 The oldest intrusive rock is quartz dior- Quartz diorite (Tqd) 67 8 12 3-5 2-3 1-2 1 1 2-3 1 tr. tr. ite, which forms the central intrusive center. It is medium to dark gray, allotriomorphic Note: 1, Plagioclase; 2, K-feldspar; 3, quartz; 4, hornblende; 5, biotite; 6, epidote; 7, chlorite; 8, sphene; 9, opaques; 10, apatite; 11, zircon; 12, calcium carbonate. in texture, and consists of andesine- Average modal composition of constituents in percentage; tr. = trace; •• = not present. labradorite rimmed by albite, hornblende

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/6/897/3429252/i0016-7606-87-6-897.pdf by guest on 28 September 2021 ITALIAN MOUNTAIN INTRUSIVE COMPLEX, EASTERN ELK MOUNTAINS, COLORADO 901

oriented perpendicular to the contact. The cm in length. The groundmass consists pre- dike cuts the melagranodiorite and quartz crystals may have grown by reaction of dominantly of quartz and feldspar crystals monzonite of the northern pluton just volatile-rich portions of the quartz monzo- that average 2 mm in length. Biotite is the below the summit of Italian Mountain. A nite magma, concentrated in flow layers, chief mafic mineral. The porphyritic quartz rhyodacite dike about 1.5 km north- with the more calcium-rich melagranodior- monzonite contains more miarolitic cavities northeast of the summit cuts a dacite por- ite. Quartz monzonite is found only in the than any other plutonic phase in the intru- phyry dike. vicinity of the summit of Italian Mountain sive complex. The miarolitic cavities are as and forms a discontinuous zone peripheral much as 3 mm in diameter and make up VENTING OF THE COMPLEX to the porphyritic quartz monzonite. The about 2 percent of the volume of the rock. quartz monzonite is dominantly hypidio- The vent facies of the porphyritic quartz The central plug of partly aphanitic por- morphic granular and locally trachytoid. It monzonite forms a vertical, pipelike body phyritic quartz monzonite is interpreted as is light gray, has an average crystal size of 1 200 m in diameter, which is located in the vent filling. Features that indicate venting mm, and contains a few scattered alkali topographic saddle 0.4 km north of the (Neuerburg, 1958; Jahns and Tuttle, 1963; feldspar phenocrysts as much as 1 cm in summit of Italian Mountain. The vent facies Neuerburg and others, 1974) are (1) clastic length. Quartz occurs both interstitially and is similar in mineral content to the por- texture, (2) partly aphanitic groundmass, as phenocrysts as much as 0.8 cm in diame- phyritic quartz monzonite, but its matrix is (3) vertical pipelike form located at the ter. The rock is characterized by biotite distinctly finer grained; crystals in the center of the youngest plutonic rock, (4) crystals, which are splayed in a radiating groundmass average 0.1 mm in diameter. pervasive alteration and disseminated iron pattern on (001), reach a diameter of 1.5 The contact with the surrounding porphyri- oxides, (5) evidence of a high volatile con- cm, and near the outer contact define a foli- tic quartz monzonite is generally grada- tent as indicated by miarolitic cavities, and ation parallel to the contact. Miarolitic tional over an interval of 15 m. The rock is (6) fluid inclusion evidence, discussed be- cavities 1 to 2 mm in diameter make up ap- stained reddish brown by hydrous iron low. Venting evidently resulted from an in- proximately 1 percent of the volume of the oxides that are distributed throughout the crease in pressure brought about by the so- rock; they increase slightly in abundance groundmass. The rock is miarolitic, has a lidification of the northern intrusive center inward toward the porphyritic quartz mon- protoclastic texture, and is cut by small from the edges inward and the resulting zonite. quartz veins. It is also slightly altered. concentration of volatiles. When the vent- Porphyritic quartz monzonite has a Plagioclase is sericitized along fractures, ing occurred, the volatiles were released sharp, intrusive contact with the quartz and biotite is sericitized. rapidly, thus causing a drop in pressure and monzonite. The porphyritic quartz monzo- Dikes of dacite porphyry or rhyodacite quenching of the remaining magma. nite contains sanidine megacrysts as much cut the plutons in places but are more as 6 cm in length and quartz phenocrysts abundant in the bordering sedimentary COMPOSITIONAL TRENDS (with a suggestion of beta morphology) 1 rocks (Fig. 2, A and B). A dacite porphyry AND VARIATIONS QUARTZ The average modal proportions of quartz, plagioclase, and K-feldspar in the major rock units of the complex are plotted on Figure 4. The differentiation index, silica content, and proportion of alkali feldspar to plagioclase increase from the older to the younger rocks. This trend is similar to that in the central part of the Colorado mineral belt (Tweto and Case, 1972). Fourteen samples from the main plutons and three from associated dikes, which were selected to represent all parts and facies of the complex, were analyzed (Table 2). Sample locations are shown in Figure 2. Normative mineral assemblages are plotted in Figure 5. Oxides of unaltered rocks plotted as a function of age fall close to smooth curves that suggest systematic differentiation of a single parental magma. These trends, in conjunc- tion with field evidence, suggest that the rocks of the Italian Mountain Intrusive Complex are part of a single comagmatic

ASSIMILATION AND METASOMATISM

In addition to the systematic chemical variations accompanying the dominant trend of differentiation, petrography of the PLAGIOCLASE K-FELDSPAR hypabyssal rocks of the complex indicates Figure 4. Average modal proportions of quartz, plagioclase, and K-feldspar in plutonic rocks com- an overprint of chemical variations resulting prising Italian Mountain Intrusive Complex. from assimilation and metasomatism. Field

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/6/897/3429252/i0016-7606-87-6-897.pdf by guest on 28 September 2021 902 C. G. CUNNINGHAM, JR.

TABLE 2. CHEMICAL ANALYSES OF IGNEOUS ROCKS FROM THE ITALIAN MOUNTAIN INTRUSIVE COMPLEX

Oxides Rock type

Tqd Tqd Tgf Tgc Tgc Tm Tqm Tqm Tqm Tqm Tpq Tpq Tpq Tv Tdp Tdp Tdp

Field number

1-298 1-194 1-274 1-319 1-286 1-328 1-293 1-329 1-173 1-210 1-294 1-206 1-325 1-202 1-034 1-314 1-3 07A

Si02 64.0 63.2 61.2 64.3 64.8 60.1 68.7 67.2 69.5 67.8 71.5 70.4 72.4 67.3 67.3 68.1 65.4 AI2O3 16.5 16.6 16.7 16.4 16.4 17.1 16.0 16.4 15.6 16.2 15.0 15.2 14.5 15.0 15.4 15.8 15.4 Fe./), 2.5 2.4 3.3 2.4 2.7 3.9 1.6 1.8 1.8 1.7 1.1 1.2 1.2 2.3 2.4 1.6 1.0 FeO 2.4 2.2 2.8 2.3 2.1 3.0 1.4 1.4 1.3 1.3 0.80 0.80 0.68 0.40 0.80 0.40 3.4 MgO 2.0 2.1 2.0 1.4 1.3 2.1 0.83 1.1 0.76 1.0 0.44 0.64 0.32 0.38 1.4 1.2 1.6 CaO 4.3 4.9 5.1 3.8 3.5 5.9 2.6 2.8 2.3 2.6 1.8 2.0 1.8 3.2 3.3 2.2 3.3 Na20 5.3 5.3 3.8 4.9 3.4 3.2 3.2 3.2 3.3 3.3 3.3 3.1 3.2 3.0 3.1 3.4 3.2 K2O 0.67 0.83 0.92 1.3 3.4 1.9 4.0 3.9 4.0 3.9 4.4 4.2 4.2 4.3 3.6 3.8 3.4 + H2O 1.4 1.2 2.3 1.7 1.6 1.1 1.1 1.2 1.0 1.2 1.2 1.2 0.86 1.4 1.3 1.3 2.0 H2O- 0.26 0.55 0.73 0.17 0.24 0.68 0.53 0.83 0.49 0.86 0.29 1.1 0.24 0.74 1.0 1.5 0.71

Ti02 0.59 0.61 0.61 0.47 0.50 0.79 0.41 0.41 0.42 0.41 0.27 0.26 0.29 0.30 0.49 0.49 0.58 P2O5 0.41 0.44 0.45 0.34 0.33 0.53 0.24 0.24 0.22 0.24 0.16 0.15 0.13 0.18 0.24 0.26 0.33 MnO 0.12 0.09 0.12 0.09 0.09 0.20 0.05 0.09 0.09 0.09 0.01 0.05 0.09 0.12 0.05 0.01 0.09 co2 0.02 0.04 0.35 0.85 0.08 0.02 0.02 0.04 0.02 0.04 0.06 0.11 0.04 1.8 0.02 0.02 0.02 Total 100.5 100.5 100.4 100.4 100.4 100.5 100.7 100.6 100.8 100.6 100.3 100.4 99.9 100.4 100.4 100.1 100.4 Note: See Figure 2 for explanation of symbols. Data in weight percent. Method of Shapiro (1967). Analyst: Lowell Artis, U.S. Geological Survey.

relations (Fig. 2, A and B) show that lime- pink granodiorite and melagranodiorite) of similation include the formation of diop- stone beds of the Belden and Leadville For- the plutons. Assimilation is indicated chem- sidic augite and the abundance of modal

mations have been displaced or engulfed by ically by the low Si02 and high CaO con- hornblende in the melagranodiorite. the intrusions. The greatest effect of assimi- tent of these rocks (Tgf and Tm in Table 2). Potassium was lost from the older facies lation is on the contact rocks (fine-grained Pétrographie indications of limestone as- of the central and southern plutons, as indicated petrographically by greater alteration of K-feldspar than of plagioclase in the QUARTZ freshest samples. The loss is shown chemi-

cally by the low K20 content of altered coarse-grained pink granodiorite (sample 1-319, Table 2) as compared with that of a fresher sample (1-286, Table 2). The addition of sodium to the quartz diorite is ex- pressed petrographically by albitization and

by the anomalous Na20 values in Table 2. The vent rock contains significantly more

CaO, Fe203, C02, Si02, and FeO than the porphyritic quartz monzonite. Secondary calcite is present as fracture fillings in feldspar phenocrysts; it was evidently introduced during weathering of the fractured vent rocks. The iron oxide stain charac- teristic of the vent rocks, the higher total Fe content and Fe+3/Fe+2 ratio, and the petrographic indications of mafic mineral alteration indicate that some Fe was introduced into the vent area and some Fe was re- mobilized from the alteration of the mafic minerals.

FELDSPARS AS INDICATORS OF CHANGING CONDITIONS OF CRYSTALLIZATION

The feldspars of the Italian Mountain In- trusive Complex are sensitive indicators of the changing conditions of emplacement. Plagioclase and K-feldspar structural states and compositions were studied (Cun- Figure 5. Normative quartz, plagioclase, and K-feldspar from samples of Italian Mountain Intru- ningham, 1973) using optical and x-ray sive Complex. Tpq = porphyritic quartz monzonite; Tqm = quartz monzonite; Tm = melagranodior- methods described by Slemmons (1962) ite; Tgc = coarse-grained granodiorite; Tgf = fine-grained granodiorite; Tqd = quartz diorite. and Wright and Stewart (1968). The

K-feldspar data were reduced using a U.S. of sericite located about three-quarters of minology, and assumptions used in the Geological Survey least-squares regression the way out from the center. The sericite study of fluid inclusions have been dis- program with a fixed-index option. film probably represents a thin shell of cal- cussed by others (Roedder, 1967, 1972; A progressive change in the structural cic plagioclase replaced by sericite. Appa- Kelly and Turneaure, 1970; Nash, 1973). state of plagioclase correlates with the rently this set of crystals started to grow at Many of the ambiguities and problems as- sequence of intrusion. Plagioclase in the depth slowly enough to maintain equilib- sociated with the interpretation of relative older rocks has optical properties of rium with the silicate melt. Then, condi- ages of inclusions and the application of plutonic rocks (Slemmons, 1962); plagio- tions began to fluctuate, and the plagioclase data from synthetic systems to natural sys- clase in the younger rocks has optical prop- was unable to maintain equilibrium. This tems are discussed by Roedder (1967, erties of volcanic rocks. This change could have resulted from changes in total 1972). Recent studies by Nash and Theo- suggests that plagioclase in the older rocks pressure, fluid pressure, or, less probably, dore (1971), Roedder (1971), Kelly and crystallized more slowly than plagioclase in temperature (Vance, 1962), all consistent Turneaure (1970), Nash and Cunningham the younger rocks. Plagioclase twin laws with the interpretation that the magma was (1973, 1974), Moore and Nash (1974), and trend from complex to simple, paralleling being mobilized to higher elevations in an Cunningham (1975) have shown that fluid the transition in optical properties from episodic fashion. inclusions can provide valuable insight into plutonic to volcanic, and may also indicate Plagioclase crystals with the distinctive the chemical regime of fluids associated the changing conditions of crystallization. alteration band are more common in the with plutonic processes and, hence, into the In the coarse-grained pink granodiorite, quartz monzonite than in the porphyritic evolution of associated base-metal ore de- K-feldspar occurs interstitially, even filling quartz monzonite. This fact suggests that posits. cracks in broken plagioclase crystals, thus the fluctuations in the magma chamber at At the Italian Mountain Intrusive Com- indicating that K-feldspar continued to higher levels that affected the plagioclase plex, secondary fluid inclusions in quartz form after the crystallization of plagioclase crystallizing in the quartz monzonite phenocrysts and primary and secondary at the level of current exposure. magma did not exert a large effect on the fluid inclusions in quartz veins were used to K-feldspar in the older rocks (quartz dior- porphyritic quartz monzonite magma; evaluate the changes in temperature, pres- ite, fine- and coarse-grained pink granodior- therefore, the porphyritic quartz monzonite sure, and chemistry of the evolving aqueous ite) has a structural state similar to that of magma was probably at greater depths phase during and subsequent to solidifica- orthoclase (Wright and Stewart, 1968); the within the magma chamber and was af- tion of the magma. Fluid inclusions were pink granodiorites have the lowest struc- fected by lesser thermal and chemical gra- studied in thin sections and doubly polished tural state of any of the intrusive rocks. The dients. plates. A freezing stage similar to the one structural state is progressively higher and The K-feldspar megacrysts in the por- described by Roedder (1962) was used to closer to sanidine in the younger rocks of phyritic quartz monzonite have optically determine salinity by freezing-point depres- the northern intrusive center. The observed homogeneous cores sharply rimmed by sion. A heating stage was used to measure variation is interpreted as reflecting differ- feathery microperthite. The cores of the filling temperatures and the solution temp- ences in environment of crystallization — in megacrysts have a higher structural state erature of daughter minerals. A crushing particular, depth and time of formation. than that of the K-feldspar in the stage (Roedder, 1970) was used to examine The lowering of structural states by reheat- groundmass, which indicates that the cores the vapor pressure at room temperature. ing is not likely; if this were the case, the formed at higher temperatures than the The fluid-inclusion studies were based on quartz diorite should have the lowest struc- groundmass. At these higher temperatures, 58 samples. One or two thin sections of tural state, because it is close to a major, the growing megacrysts exsolved albite. each sample were examined, and more than younger, intrusive center. The microperthite rim suggests relatively 100 inclusions per thin section were ob- The structural states, compositions, and rapid formation under disequilibrium con- served. Fourteen samples were selected for zoning of the feldspars from rocks of the ditions during the last stage of crystalliza- detailed study, on the heating and freezing northern intrusive center suggest that the tion. stages, and doubly polished plates were melagranodiorite, quartz monzonite, and prepared; over 100 homogenization and sa- porphyritic quartz monzonite came from FLUID INCLUSIONS linity determinations were made. Figure 6 the same magma chamber, and they record illustrates the phase changes and terminol- the conditions of formation. Fluid inclusions in the intrusive rocks of ogy used in the present study. The data Plagioclase in the melagranodiorite is the Italian Mountain Complex were studied from the fluid-inclusion studies are sum- more calcic (An 55) than in any other intru- to investigate changes in the aqueous phases marized in Table 3. sive rock of the complex, and it is twinned associated with the silicate rocks. Samples Three types of fluid inclusions were ob- according to the largest variety of twin were chosen so that the variation in phase served (Fig. 7). Type I inclusions are low- laws. It has optical properties (Slemmons, assemblages and the homogenization be- salinity, two-phase inclusions that 1962) transitional to those of volcanic havior of the fluid inclusions could be homogenize to a liquid phase upon heating. affinities, which suggests rapid crystalliza- studied as a function of time of formation Type II inclusions are gas-rich, two-phase tion. K-feldspar from the melagranodiorite and of spatial distribution in the complex. inclusions, which commonly contain over has a structural state closer to sanidine than Quantitative information about the 60 percent gas, that homogenize to a vapor that of most of the K-feldspar in the north- evolution of fluids in a rock can be deter- phase upon heating. Type III inclusions are ern intrusive center. This structural state is mined from the measurement of phase high-salinity inclusions containing one or evidently a function of high-temperature changes in fluid inclusions. Semiquantita- more daughter minerals, described in Table formation and quenching when intruded tive information about fluid inclusions can 4. The reader is referred to Roedder (1972) against the sediments. be obtained by petrographic examination of for photographs of the various types of fluid Most of the plagioclase crystals from the standard thin sections. Observations on rel- inclusions. rocks of the northern intrusive center dis- ative abundance of fluid inclusion types, es- Type III inclusions contain as many as play oscillatory zoning throughout the en- timated homogenization temperatures, and seven daughter minerals within one indi- tire crystal. A distinctive set of larger daughter mineral identification, as well as vidual inclusion. Some daughter minerals plagioclase crystals, however, has identification of various gases including did not dissolve even when held at elevated homogeneous cores averaging An39 that are carbon dioxide and hydrogen sulfide, can temperatures for extended periods. Possible separated from more sodic, oscillatory- be made routinely. explanations for this are discussed by zoned rims by a well-defined, narrow film The basic equipment, methodology, ter- Roedder (1972, p. JJ-24). The identification

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/6/897/3429252/i0016-7606-87-6-897.pdf by guest on 28 September 2021 904 C. G. CUNNINGHAM, JR.

CD Liquid filled ter minerals with low apparent bire- Vapor filled fringences are stacked upon each other, a LIQUID Liquid with vapor first-order red accessory plate was useful in PT path followed when sample is Vapor with liquid distinguishing them. heated and homogenizes to Liquid with vapor and doughter salt a liquid COMPOSITION AND DISTRIBUTION OF

iPOR POSTMAGMATIC FLUIDS

•PT path followed Every sample of intrusive rock from the when sample —homogenizes to Italian Mountain Complex contains secon- T, Tj Tj T4 a vapor dary fluid inclusions along healed fractures TEMPERATURE (T) • in quartz phenocrysts. Nearly every sample Figure 6. Diagram of behavior upon heating contains inclusions bearing halite cubes. of fluid inclusion containing NaCl daughter min- The only intrusive rocks lacking halite- eral and homogenizing to liquid phase. Modeled bearing inclusions are at the highest eleva- on P-T section at constant V in system NaCl- tions within the coarse-grained, pink H 0. Modified from Lemmlein and Klevtsov 2 TYPE III: HIGH SALINITY (1961) and Sourirajan and Kennedy (1962). T, = granodiorite; these contain abundant gas- rich (type II) inclusions. Figure 7. Types of fluid inclusions observed in room-temperature conditions; T2 = temperature quartz. at which salt daughter dissolves; T:l = homogeni- The evolution of the fluids associated zation temperature — everything within inclu- with the intrusive complex can be ascer- The wide range of fluid composition in the sion becomes one phase; T4 = temperature of tained in part from the data summarized in quartz diorite attests to the varied condi- trapping; T4 - T3 = pressure correction; CP = Table 3. The oldest fluids, which are de- tions to which this rock has been exposed; critical point. duced from spatial distribution within the the quartz phenocrysts have been fractured of daughter minerals is a problem, primar- intrusive complex, crosscutting relation- many times and new fluids were introduced ily because of their small size. To accentuate ships within the host material, and inclu- repeatedly. The highly saline fluid inclu- crystal morphology, inclusions were heated sion morphology and composition, contain sions in the quartz diorite are euhedral, until daughter minerals were partially dis- anhydrite; the youngest fluids contain syl- which suggests that the quartz was heating solved and then cooled, causing well- vite, thus indicating that the composition and partially recrystallized; these inclusions defined crystal edges to form. Where daugh- changed with time from Ca-rich to K-rich. contain the oldest fluids recognized. The

TABLE 3. ABUNDANCE, FILLING TEMPERATURES, AND SALINITIES OF FLUID INCLUSIONS IN QUARTZ

Rock unit* Number Percent abundance of Filling temperatures Salinities Comments of inclusion types (°C) localities If II§ III# I II III III

Quartz veins 35 40 25 385 385-450 223-455 10-20 38 Boiling 435°C, 38 wt percent NaCl. Fluids similar to those in porphyritic quartz monzonite Vent facies 3 20 5—(40) (40)-70 200-340 344-(400) •• 50 Complex fluids, boiling, KC1 common Porphyritic quartz 11 15-40 5-40 40-(80) 225e 400 (420)-543 •• 37-50 Excellent inclusions in large, monzonite quartz phenocrysts. KC1 common, boiling 435°C, 38 wt percent NaCl Quartz monzonite 12 (0)—25 (5)-60 40-(90) 175-260 425e 331-(474) 0.04 42-(46) Very few gas rich Melagranodiorite 6 (0)-20 0-(25) 70-80 230 313-464 0.12 38-54 KC1 relatively rare. Filling temperatures are random Coarse-grained 9 10-90 (5)-90 0-85 230-(358) 264-(342) 0.15 36-38 Abundant type II restricted to granodiorite high elevations, type III at low elevations. KC1 rare Fine-grained 4 (15)—70 5-20 10-80 325e 362 58 Highly variable phase pro- granodiorite portions. KCl very rare Quartz diorite 11 0-10 50-95 5 —(50) 280e 480 245-488 36-56 Type II inclusions more abundant at higher elevations and margins of the quartz diorite intrusive. Highly variable phase proportions. Anhydrite relatively common Note: ( ) = most prevalent abundance within a range of values; e = estimated; •• = not determined. * Rocks arranged in age sequence, oldest at the bottom to youngest at the top. t Two-phase inclusions; liquid phase predominant. § Two-phase inclusions; vapor phase predominant; salinities not determined. # High salinity; multiphase inclusions.

presence of as many as five separate daugh- TABLE 4. DAUGHTER MINERALS those closer to and within the vent phase ter minerals, commonly including anhy- FROM FLUID INCLUSIONS OF THE have about equal proportions of gas-rich drite, indicates a multicomponent fluid con- ITALIAN MOUNTAIN INTRUSIVE (type II) and high-salinity (type III) inclu- + +2 + 2 COMPLEX taining Na , Ca , K , CI", S04" , and sions. Fe+3. The altered cores of plagioclase crys- The temperatures of dissolution for tals in the quartz diorite contain ilmenite, Daughter Physical and optical properties daughter minerals in these fluid inclusions minerals which is interpreted to indicate that have been used to estimate chemical compositions of the fluids. A partial ternary titanium was present in the fluids. Halite Cubic, isotropic, moderately high Halite daughter crystals in fluid inclu- optical relief diagram of the system NaCl-KCl-H20, modified from data compiled by Roedder sions from the fine-grained pink granodior- Sylvite Cubic, isotropic, moderate opti- (1971), was used for this purpose. Although ite generally dissolved after the filling temp- cal relief. When two isotropic erature had been reached, which indicates daughters are present (pre- there are many assumptions in this method, that a pressure differential existed between sumably NaCl and KC1), this for example, ignoring the effect of calcium filling and trapping temperatures. one dissolves at the lower on the aqueous solubilities of sodium and The distribution and composition of temperature potassium chlorides, the method furnishes some measure of the composition of hy- fluids in fluid inclusions from the coarse- Hematite Hexagonal platelets, opaque to grained pink granodiorite place limitations red. Does not dissolve in heat- drothermal fluids. Determinations of on the P-T conditions that existed at the ing tests weight percent NaCl, KC1, and HaO are compiled in Table 5. For purposes of com- time of the formation of the fluids. Rocks Anhydrite Prismatic, colorless, with high parison, the atomic ratios K/Na and Na/K from the center of the granodiorite body optical relief. Orthorhombic contain very few miarolitic cavities, and morphology. Length slow in are also listed. The fluids associated with high-salinity (type III) fluid inclusions are the orientation of minimum the porphyritic quartz monzonite are simi- predominent; miarolitic cavities that are birefringence. One dissolved at lar to the high-density fluids in the intrusive 200 m higher in the body constitute about 2 267°C; usually it does not rocks at Bingham (Roedder, 1971; Moore percent of the rock, and more than 90 per- dissolve. and Nash, 1974). cent of the fluid inclusions are gas-rich (type Calcite(?) Rhombohedral morphology, II). These contrasting features indicate that high birefringence, symmetri- BAROMETRY AND pressure differed appreciably in a vertical cal extinction. Changes optical DEPTH OF EMPLACEMENT interval of 200 m during the final crystalli- relief with rotation. Did not zation of the granodiorite: late magmatic appear to dissolve The igneous rock bodies of the Italian aqueous fluids in the lower part of the in- Dawsonite( ?) Birefringent, acicular mineral. Mountain Intrusive Complex have charac- trusive body were in the liquid field of the Moderate optical relief teristics typical of epizonal plutons (Bud- P-T diagram (Fig. 6), whereas those higher Bassanite(?) Hexagonal platelet, clear, mod- dington, 1959). As presently exposed, the in the intrusion were in the vapor field. If erate definition. No thermal northern intrusive body is believed to be near contemporaneity of trapping is as- data measured close to the top of the pluton. Remnants of sumed, the two-phase boundary existed Unknown A Opaque, morphology equant, a gently dipping roof of sedimentary rocks within that 200 m. The fluid-inclusion data sometimes appears cubic. Non- are still preserved on the flanks (Fig. 2, A also suggest, in this instance, that miarolitic magnetic. Did not dissolve and B), and a few roof pendants or cavities resulted from the separation of an xenoliths are present at the highest eleva- Unknown B Poor morphology, but generally immiscible vapor phase rather than from an equant and "cloudlike" in tions. The thickness of the sedimentary exsolved liquid. form. Moderately high defini- cover at the time of emplacement is not The rocks of the northern intrusive center tion, moderately high bire- known; stratigraphic reconstruction pre- have more and larger quartz phenocrysts fringence. Dissolves at approx- sents too many uncertainties to determine than do the older rocks; consequently, the imately 130°C depth of emplacement. fluid inclusions in plutonic rocks of the Unknown C Rectangular morphology, iso- Fluid-inclusion studies have been used to northern intrusive center record a more tropic, very low relief. Dis- determine limits on the depth of emplace- complete picture of the evolution of the solves at approximately 290°C ment of epizonal plutons (Nash and Theo- fluids in time and space than do the inclu- Unknown D Rectangular, occasionally ap- dore, 1971; Nash and Cunningham, 1973). sions in the older plutonic rocks. Fluids in pears hexagonal. High optical Estimates of pressure are elusive because the melagranodiorite are highly saline, mul- relief, moderate birefring- measured parameters are more sensitive to ticomponent fluids that show traces of in- ence, appears to have a yellow temperature change than to pressure termittent boiling. The NaCl cubes often tint. Dissolves at 300 to 353°C change, and vapor-pressure estimates in are larger than the vapor bubbles and dis- complex Na-K-Ca-Cl brines are lower than solve at temperatures higher than the filling higher homogenization temperatures than in the NaCl-H20 system. temperature (Fig. 6). Fluids associated with those in the interior of the quartz monzo- Refinement of pressure determinations in the quartz monzonite are generally highly nite body. ore deposits associated with subvolcanic saline but show considerable variation in Fluid inclusions in the porphyritic quartz settings has been based on P-T-X parame- phase proportions within an individual thin monzonite record further change of the ters measured in fluid inclusions (Kelly and section. Hematite is more common as a fluids with time. Many individual high- Turneaure, 1970; Nash and Theodore, daughter mineral than in any of the earlier salinity (type III) inclusions in the rock con- 1971; Roedder, 1971; Nash and Cunning- formed rocks. Some samples show an over- tain five or more daughter minerals; halite, ham, 1973; Moore and Nash, 1974). When print of fluids associated with the succeed- sylvite, and hematite are present in almost minerals form from boiling aqueous fluids, ing porphyritic quartz monzonite. Near the all. There is a roughly concentric zonation the pressure on the fluid equals the vapor contact between the two rocks, the fluids in of inclusion types away from the vent phase pressure, and, depending on the composi- the quartz monzonite are very similar to that forms the central part of the porphyri- tion of the fluids, the pressure may be de- those in the porphyritic quartz monzonite tic quartz monzonite core. Samples farthest termined using the data of Sourirajan and and different from those in most of the from the vent contain about 80 percent Kennedy (1962) and Haas (1971) on the high-salinity (type III) inclusions, whereas quartz monzonite. These inclusions have system NaCl-H20.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/6/897/3429252/i0016-7606-87-6-897.pdf by guest on 28 September 2021 906 C. G. CUNNINGHAM, JR.

TABLE 5. COMPOSITION AND CATION RATIOS OF FLUID INCLUSIONS THAT CONTAIN HALITE AND SYLVITE CRYSTALS; NORTHERN PLUTON

Locality Rock unit Solution T Filling KCl NaCl H2O Molai composition Atomic ratios KCl NaCl T (wt %) (wt %) (wt %) NaCl KCl K/Na Na/K (°C) (°C) (°C)

111 Vent facies 183 445 372 24 38 38 17.1 8.5 0.50 2.0 325a Porphyritic quartz monzonite 190 426 450 25 36 39 15.8 8.6 0.54 1.8 325b Porphyritic quartz monzonite 217 512 543 23 45 32 24.1 9.6 0.40 2.5 368 Porphyritic quartz monzonite 190 435 420 25 37 38 16.7 8.8 0.53 1.9 216 Quartz monzonite 186 400 474 26 33 41 13.8 8.5 0.62 1.6

Fluids associated with rocks intruded trapped within a short span of time and Kennedy (1962) and Haas (1971). The prior to the porphyritic quartz monzonite under similar pressure-temperature condi- curves of Figure 8 follow the Haas model have highly variable salinities, and the tions. Boiling of fluids during crystallization with the data extrapolated to 450°C and 40 homogenization temperatures are minimum of the vent rocks may account for both the wt percent NaCl. Assumptions and limita- trapping temperatures. If boiling occurred, shattered crystals and the quenched tions were discussed in Haas (1971). Pres- it was intermittent and local. These fluids groundmass of this phase (Fournier, 1967; sure constraints imposed by measured fluid are characterized by halite daughter miner- Phillips, 1973). Small quartz veins cut the compositions and homogenization temper- als that dissolve at temperatures higher than rocks within the vent and the adjacent por- atures indicate a depth of emplacement of the temperature at which the vapor phase phyritic quartz monzonite. The quartz veins about 2,700 m, using a hydrostatic model. disappears. As discussed by Roedder were formed by late-stage fluids related to Pressure-temperature conditions close to (1972), it is most reasonable to suggest that the northern intrusive center. The fluids in hydrostatic are common at shallow depths the true solution temperature for the halite these veins are similar chemically and pet- as indicated by observed P-T profiles in ac- is above the filling temperature, and the dif- rographically to those in the vent rock and tive hot-spring systems. The effect of Ca ference between the temperatures corre- in the porphyritic quartz monzonite. and K would be to increase the density of sponds to a minimum pressure correction Ideally, proof of boiling requires evidence the brine and decrease the vapor pressure, that must be added to the filling tem- that inclusions formed at the same time and and the depth of formation would be cor- perature. temperature, some from the liquid phase respondingly shallower. If the system were High-salinity (type III) inclusions in the and others from the vapor phase. In actual- lithostatic, the density of the overlying quartz diorite and pink granodiorites ity, if fluids are trapped exactly on the material would be greater, and if 2.67 is as- homogenize to a liquid at temperatures up two-phase curve, it could result in trapping sumed, the depth of formation would be to 488°C (Table 3) and have salinities of 36 vapor bubbles of various sizes along with 950 m. to 58 wt percent NaCl equivalent. For a the liquid, which would result in a range of The figure of 2,700 m (8,800 ft) implies typical inclusion with homogenization homogenization temperatures. deep erosion in the Elk Mountains since temperature of 350°C and corresponding For the fluids at Italian Mountain, the middle Oligocene time, although Oligocene salinity of 40 wt percent NaCl equivalent, evidence of boiling is strongly suggestive sedimentary and volcanic rocks are widely the minimum pressure at which the liquid but not conclusive. The pervasive fracturing preserved in other parts of the state. How- could coexist with a gas phase is 110 bars of the quartz phenocrysts, spatial distribu- ever, many plutons much larger than the (Haas, 1971; extrapolated to higher salin- tion of fluid-inclusion types relative to the Italian Mountain Complex were intruded in ity). At any pressure less than this, the fluid vent, and presence of type II fluid inclu- the Elk Mountains in Oligocene time (Ob- would be in the vapor region of the NaCl- sions, together with type III fluid inclusions radovich and others, 1969), mainly into H20 system (Fig. 6) and thus would that have uniform homogenization temper- sedimentary rocks. They must have domed homogenize to a vapor phase rather than to atures, suggest that it was the venting pro- the sedimentary rocks to great height above the observed liquid. Measured homogeniza- cess that fractured the quartz, and the pres- those in the adjoining and unintruded tion temperatures as much as 488°C, in sure on the fluids was such that the fluids sedimentary basin to the west and added a fluids of 56 wt percent NaCl equivalent, are were close to, if not on, the boiling curve. pile of volcanic rocks to the top. Rapid ero- probably closer to the trapping temperature The depth of formation must provide sion of the volcanic and sedimentary rocks and require a minimum pressure of about enough pressure to prevent the fluids from (mainly Cretaceous shales) over the cluster 325 bars. being in the vapor region of P-T-X space at of plutons would have been inevitable. Typical high-salinity (type III) fluid inclu- the prevailing temperatures but to permit at sions in rocks from the northern intrusive least periodic boiling at times of higher HYDROTHERMAL ORE DEPOSITS center homogenize at 435°C to a liquid that temperature or reduced pressure. If boiling contains 38 wt percent NaCl equivalent. A is assumed, the fluids formed under P-T Hydrothermal ore deposits have been minimum pressure for the vapor phase that conditions in the two-phase region of the mined from breccia zones and replacement

could coexist with this NaCl-H20 fluid system H20-salt; consequently, a pressure deposits in the Leadville Limestone near the would be 250 bars. This vapor pressure correction is not required for the homog- Italian Mountain Intrusive Complex for would be equal to the fluid pressure if the enization temperature. Furthermore, with nearly a century. The deposits, which form fluids were boiling. reasonable assumptions, a unique solution a rough zonal pattern, are close to the Petrographic evidence indicates that the for pressure and depth of formation can be northern intrusive center. Sulfide ores are rocks in the vicinity of the vent were frac- determined. richest in zinc and copper near the intrusive tured and that the fluids were trapped as The calculated pressure of a vapor phase mass and in lead and silver farther away. secondary fluid inclusions in the fractured coexisting with boiling high-salinity fluids The zoning and the spatial association indi- quartz phenocrysts. The uniformity of the associated with the porphyritic quartz cate that the hydrothermal sulfides are re- fluid compositions and homogenization monzonite at time of venting was 250 bars lated to the northern intrusive center. temperatures indicates that the fluids were on the basis of the data of Sourirajan and The composition and evolution of the

TEMPERATURE,IN °C gest that when the northern pluton vented, Cunningham, C. G., Jr., and Naeser, C. W., 300 400 500 the drop in pressure quenched unsolidified 1975, The Italian Mountain Intrusive groundmass in the core and expelled the Complex, west-central Colorado, in Cohee, volatiles (Jahns and Tuttle, 1963). At about G. V., and Wright, W. B., Changes in stratigraphic nomenclature by the U.S. 50 BARS the same time, the plutonic rocks surround- Two-phase coexisting Geological Survey, 1974: U.S. Geol. Survey (liquid and vapor) ing the vent were fractured, and boiling Bull. 1405-A, p. 27-28. 800 curves fluids were introduced into broken quartz Fournier, R. O., 1967, The porphyry copper de- 00 BARS phenocrysts, which subsequently healed posit exposed in the Liberty open-pit mine ,40 VAPOR and sealed off the fluids. A short time later, near Ely, Nevada — Pt. 1, Syngenetic for- 30 judging by the chemical similarity of the mation: Econ. Geology, v. 62, p. 57-81. i2°\V|50 BARS Haas, J. L., Jr., 1971, The effect of salinity on the 10 V fluids, quartz veins were deposited from boiling fluids. Causes of fracturing proba- maximum thermal gradient of a hydro- bly were the release of pressure by venting, thermal system at hydrostatic pressure: Econ. Geology, v. 66, p. 940-946. continued cooling and shrinkage, and re- 200 BARS Helgeson, H. C., 1970, A chemical and ther- surgent movements of the magma column. modynamic model of ore deposition in hy- Fluid inclusion data from the fractured drothermal systems: Mineralog. Soc. quartz phenocrysts and quartz veins, con- America Spec. Paper 3, p. 155—186. sidered in terms of the system NaCl-H20, Jahns, R. H., and Tuttle, O. F., 1963, Layered 2800 indicate a depth of emplacement between pegmatite-aplite intrusions, in Symposium 950 and 2,700 m. on layered intrusions: Mineralog. Soc. America Spec. Paper 1, p. 78-92. Figure 8. Boiling point curves for H20 liquid Fluids associated with the youngest por- (0 wt percent NaCI) and for brines of constant phyritic rock are quite similar in composi- Kelly, W. C., and Turneaure, F. S., 1970, Mineralogy, paragenesis and geothermome- compositions in weight percent NaCI. Data from tion to the fluids in inclusions in the try of the tin and tungsten deposits of the Italian Mountain Intrusive Complex added. mineralized quartz monzonite at Bingham, Modified from Haas (1971). eastern Andes, Bolivia: Econ. Geology, v. Utah. The inclusions in the Italian Moun- 65, p. 609-680. fluids associated with the intrusive rocks tain rocks indicate highly concentrated Langenheim, R. L., 1952, Pennsylvanian and provide insight into the genesis of the ore brines and intermittent boiling. Ore metals Permian stratigraphy in Crested Butte deposits. Almost every intrusive rock con- were probably partitioned into the brine quadrangle, Gunnison County, Colorado: tains halite-bearing fluid inclusions with from a granodiorite melt. The brine in- Am. Assoc. Petroleum Geologists Bull., v. dense, highly saline fluids — a feature teracted with meteoric water and deposited 36, p. 543-574. Lemmlein, G. G., and Klevtsov, P. V., 1961, Re- shared with 28 out of 30 porphyry copper the ores. lations among the principal thermodynamic deposits of the western United States inves- parameters in a part of the system H20- tigated by Nash (U.S. Geological Survey, ACKNOWLEDGMENTS NaCl: Geochemistry, no. 2, p. 148-158 1972, p. A12). Although the mechanism for [translated from Russian]. the transport of metallic sulfides in hydro- I thank Robert R. Compton, who di- Moore, W. J., and Nash, J. T., 1974, Alteration thermal solution is far from settled, many rected this work as part of a Stanford Uni- and fluid inclusion studies of the porphyry writers (see Helgeson, 1970; Anderson, versity Ph.D. thesis. I also thank R. B. copper ore body at Bingham, Utah: Econ. 1973) agree that complex chloride solutions Taylor, Bruce H. Bryant, J. Thomas Nash, Geology, v. 69, p. 631-645. may be the most important transporting William J. Moore, and Ogden L. Tweto of Nash, J. T., 1973, Geochemical studies in the agent. At Italian Mountain, spatial and the U.S. Geological Survey for their help. Park City district — 1, Ore fluids in the Mayflower mine: Econ. Geology, v. 68, no. geochemical conditions are consistent with 1, p. 34-51. the interpretation that high-salinity, high- REFERENCES CITED Nash, J. T., and Cunningham, C. G., Jr., 1973, density fluids derived from the porphyritic Fluid-inclusion studies of the fluorspar and quartz monzonite magma transported the Anderson, G. M., 1973, The hydrothermal gold deposits, Jamestown district, Col- metals as aqueous chloride complexes. De- transport and deposition of galena and orado: Econ. Geology, v. 68, p. 1247- position of the metals at the periphery of sphalerite near 100°C.: Econ. Geology, v. 1262. the intrusive mass may have been ac- 68, p. 480-492. 1974, Fluid-inclusion studies of the por- complished by mixing of the dense saline Bryant, Bruce, 1970, Geologic map of the phyry copper deposit at Bagdad, Arizona: fluids with lighter, cooler waters and by Hayden Peak quadrangle, Pitkin and Gun- U.S. Geol. Survey Jour. Research, v. 2, nison Counties, Colorado: U.S. Geol. Sur- their reaction with adjacent limestone. p. 31-34. vey Geol. Quad. Map GQ-863. Nash, J. T., and Theodore, T. G., 1971, Ore Buddington, A. F., 1959, Granite emplacement fluids in the porphyry copper deposit at CONCLUSIONS with special reference to North America: Copper Canyon, Nevada: Econ. Geology, v. Geol. Soc. America Bull., v. 70, p. 671 — 66, p. 385-399. The Italian Mountain Intrusive Complex 747. Neuerburg, G. J., 1958, Deuteric alteration of consists of three epizonal plutons and as- Burbank, W. S., Lovering, T. S., Goddard, E. N., some aplite-pegmatites of the Boulder sociated dikes that are emplaced along the and Eckel, E. B., 1935, Geologic map of batholith, Montana, and its possible sig- Castle Creek fault zone. Compositions of Colorado: U.S. Geol. Survey Map, scale nificance to ore deposition: Econ. Geology, the plutonic rocks range from quartz diorite 1:500,000 [repr. 1959]. v. 53, p. 287-299. (oldest) to quartz monzonite (youngest). Cunningham, C. G., Jr., 1973, Multiple intrusion Neuerburg, G. J., Botinnely, Theodore, and Wat- Chemical variations suggest that the rocks and venting of the Italian Mountain Intru- terson, J. R., 1974, Molybdenite in the of the complex constitute a comagmatic sive Complex, Gunnison County, Colorado Montezuma district of central Colorado: [Ph.D. thesis]: Stanford, Calif., Stanford series. U.S. Geol. Survey Circ. 704, 21 p. Univ., 168 p. Obradovich, J. D., Mutschler, F. E., and Bryant, Abundant miarolitic cavities in the por- 1975, The evolution of fluids associated Bruce, 1969, Potassium-argon ages bearing phyritic quartz monzonite suggest that an with an epizonal plutonic complex in the on the igneous and tectonic history of the appreciable fluid phase existed prior to final Colorado mineral belt: Geol. Soc. America Elk Mountains and vicinity, Colorado — A consolidation of the magma. The data sug- Abs. with Programs, v. 7, p. 601. preliminary report: Geol. Soc. America

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/6/897/3429252/i0016-7606-87-6-897.pdf by guest on 28 September 2021 908 C. G. CUNNINGHAM, JR.

Bull., v. 80, p. 1749-1756. Petrog. Mitt., v. 50, p. 41-58. Tweto, Ogden, and Case, J. E., 1972, Gravity Peterman, Z. E., Hedge, C. E., and Braddock, 1971, Fluid inclusion studies on the and magnetic features as related to geology W. A., 1968, Age of Precambrian events in porphyry-type ore deposits at Bingham, in the Leadville 30-minute quadrangle, the northeastern Front Range, Colorado: Utah, Butte, Montana, and Climax, Col- Colorado: U.S. Geol. Survey Prof. Paper Jour. Geophys. Research, v. 73, p. 2277- orado: Econ. Geology, v. 66, p. 98-120. 726-C, 31 p. [1973]. 2296. 1972, Composition of fluid inclusions, in U.S. Geological Survey, 1972, Geological Survey Phillips, W. J., 1973, Mechanical effects of ret- Fleischer, Michael, ed., Data of geochemis- research 1972: U.S. Geol. Survey Prof. rograde boiling and its probable importance try, sixth edition: U.S. Geol. Survey Prof. Paper 800-A, 320 p. in the formation of some porphyry ore de- Paper 440-JJ, 164 p. Vance, J. A., 1962, Zoning in igneous plagioclase posits: Inst. Mining and Metallurgy Trans., Shapiro, Leonard, 1967, Rapid analysis of rocks — Normal and oscillatory zoning: Am. sec. B, v. 82, no. 801, p. B90-B98. and minerals by a single-solution method, Jour. Sci., v. 260, p. 746-760. Roedder, Edwin, 1962, Studies of fluid inclusions in Geological Survey research 1967: U.S. Wright, T. L., and Stewart, D. B., 1968, X-ray — Pt. 1, Low temperature application of a Geol. Survey Prof. Paper 575-B, p. B187- and optical study of alkali feldspar — [Pt.] dual-purpose freezing and heating stage: B191. 1, Determination of composition and struc- Econ. Geology, v. 57, p. 1045-1061. Slemmons, D. B., 1962, Determination of vol- tural state from refined unit-cell parameters 1967, Fluid inclusions as samples of ore canic and plutonic plagioclases using a and 2V: Am. Mineralogist, v. 53, p. 38-87. fluids [Chap.] 12, in Barnes, H. L., ed., three- or four-axis universal stage — Revi- Geochemistry of hydrothermal ore de- sion of Turner method: Geol. Soc. America posits: New York, Toronto and London, Spec. Paper 69, 64 p. Holt, Rinehart and Winston, p. 515-574. Sourirajan, S., and Kennedy, G. C., 1962, The MANUSCRIPT RECEIVED BY THE SOCIETY 1970, Application of an improved crushing system H20-NaCl at elevated temperatures JANUARY 24, 1975 microscope stage to studies of the gases in and pressures: Am. Jour. Sci., v. 260, REVISED MANUSCRIPT RECEIVED JUNE 24,1975 fluid inclusions: Schweizer. Mineralog. u. p. 115-141. MANUSCRIPT ACCEPTED JULY 17, 1975

Printed in U.S.A.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/6/897/3429252/i0016-7606-87-6-897.pdf by guest on 28 September 2021