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 de- posits. 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 de- posits. 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<n0 Mountain Tmv P<Jb 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 — contem- poraneous 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.