The Poison Ridge Volcanic Center And Related Mineralization Grand and Jackson Counties Colorado

The Poison Ridge Volcanic Center And Related Mineralization Grand and Jackson Counties Colorado

By Douglas M. Kinney, Glen A. lzett, Robert U. King, and Richard B. Taylor

G E 0 L 0 G I C A L S U R V E Y C I R C U L A R 594

Washington 1968 United State~ Department of the Interior STEWART L. UDALL, Secretary

Geological Survey William T. Pecora, Director

Free on application to the U.S. Geological Survey, Washington, D.C. 20242 CONTENTS

Page Page )Lbstract ------1 Geologic setting-Continued Poison Ridge volcanic center ------3 Introduction ------1 Rock alteration and mineralization ------6 Geologic setting ------1 Geochemical anomalies ------7 Middle Park Formation ------3 s,ummary ------8 Rabbit Ears Volcanics ------3 References ------8

ILLUSTRATIONS

Page FIGURE 1. Index map showing location of Poison Ridge volcanic center, Colorado 2 2. Preliminary geologic map of the Poison Ridge volcanic center, Colorado 4 3. Map showing distribution of rock alteration______6 4. Photograph of fractured and altered Middle, Park Formation and quartz latite porphyry 7

III

THE POISON RIDGE VOLCANIC CENTER AND RELATED MINERALIZATION, GRAND AND JACKSON COUNTIES, COLORADO

By DOUGLAS M. KINNEY, GLEN A. IZETT, ROBERT U. KING, and RICHARD B. TAYLOR

Abstract side of the Continental Divide between North The Poison Ridge volcanic center, of probable middle Tertiary age, in the Rabbit Ears Range, Grand County, Park and Middle Park, about 20 mil_es north­ Colo., is surrounded by a zoned halo of intense hydro­ northeast of Kremmling and 9 miles south­ thermal alteration. An inner alteration zone of intense­ southwest of Rand, Colo. (fig. 1). A quartz ly fractured rock veined by quartz, hematite, and feld­ latite porphyry stock cutting clastic sedimen­ spar is surrounded by a zone characterized by quartz, tary rocks of the Middle Park Formation of sericite, and pyrite. Geochemical anomalies in the altered area indicate that the altered rock offers a Late Cretaceous ( ?) and Paleocene age (fig. 2) potential target at depth for exploration for dissemi­ is surrounded by asymmetric zones of contact nated molybdenum and perhaps copper. metamorphism and hydrothermal alteration. The northwestern part of the stock and adja­ INTRODUCTION cent wallrocks were closely fractured and In the course of regional mapping in the veined by quartz, hematite, and feldspar. An Rabbit Ears Range, northwestern Colorado, alteration halo 1112 miles across ·Characterized an extensive area of red-stained pyritic rocks. by quartz, sericite, and pyrite surrounds this south of Poison Ridge was discovered by D. M. inner zone. Kinney of the U.S. Geological Survey. Stream.­ Semiquantitative spectrographic analyses of sediment samples taken downstream from the samples of the hydrothermally altered rocks altered area in the summer of 1967 by Kinney show that molybdenum content-Mo, 10-150 and D. L. Wheat were analyzed using semi­ ppm (parts per million) -is highest in the quantative spectrographic methods and were inner alteration zone and that copper and lead found to contain anomalous amounts of several contents-Cu, 15-150 ppm; Pb, 10-300 ppm­ metals. Samples of hydrothermally altered are highest in the outer zone. All surface rocks rocks taken late in October 1967 by G. A. Izett have been leached and the sulfides at least par­ and R. B. Taylor also had anomalous m.etal tially oxidized ; the analytical results reflect contents, but further work was halted by snow. both the hypogene and supergene alteration. The area of altered rocks and an area periph­ The Poison Ridge center is easily reached eral to· it were studied in more detail during from Rand, Colo., by way of a county road July 1968 by Izett, Taylor, and R. U. King. This along Willow Creek, and thence via Forest work showed that the altered and mineralized Service Road 2042 to the Continental Divide at rocks are closely associated with a volcanic the east end of Poison Ridge. center of probable middle Tertiary age, and the pattern of alteration and distribution of GEOLOGIC SETTING metal concentrations suggests that potential In latest Cretaceous ( ?) and early Tertiary targets for exploration may be present at time, the Poison Ridge area was part of a large depth. sedimentary basin lying between the Front This igneous center, here designated the Range to the east and the Park and Gore "Poison Ridge volcanic center," is on the south Ranges to the west. This intermontane basin

1 • Area of report

COLORADO

40° 15'

Corral x Peaks x

X Elk Mountain

0 5 MILES

FIGURE 1.-Index map showing location of Poison Ridge volcanic center, Colorado.

2 was formed early in the Laramide orogeny ognized by the fragments of Precambrian rocks when tectonic movem.ents outlined some of the and Laramide porphyries that they contain. In­ main elements of the Southern Rocky Moun­ terlayered mudstones contain abundant mica tains. Debris shed from these early boundary and are dark and massive. The Middle Park ranges and from andesitic volcanic centers in Formation is less easily identified around the and along the west side of the Front Range Poison Ridge stock, where the rocks are baked ac·cumulated in the basin to form clastic and hydrothermally altered. deposits called the Middle Park Formation. The early upwarps were more closely folded RABBIT EARS VOLCANICS and faulted during later Laramide orogenic Extrusive fragmental volcanic rocks and episodes, and the Middle Park Formation was flows of the Rabbit Ears Volcanics of Oligocene extensively deformed. Following the final Lara­ and Miocene ( ?) age (lzett, 1966) overlie the mide tectonic activity in late Eocene tim.e and Middle Park Formation. The formation is as the aecompanying erosion, silicic volcanism be­ much as 1,500 feet thick and crops out in an gan in Oligocene time. One volcanic field, which arcuate pattern on the west side of the Poison covered an upland area underlain by the Mid­ Ridge stock (fig. 2). The formation is made dle Park Formation, now makes up the Rabbit up of interbedded tuff, tuff breccia, and volcanic Ears Range. Quartz latite and latite pyrocla­ breccia interlayered with a few lava flows. The stic rocks and flows (Rabbit Ears Volcanics) breccia fragments are composed of several were extruded from several igneous centers. types of porphyritic intermediate volcanic These centers, including the Poison Ridge cen­ rocks including biotite- and hornblende- and ter, are marked in the present terrain by sills, pyroxene-bearing latite, quartz latite, and dikes, and small stocks cutting the underlying rhyodacite porphyry. Sparse fragments of sedimentary rocks. Most of the volcanic activ­ rhyolite welded tuff containing quartz pheno­ ity probably took place in Oligocene time, but crysts als9 occur in the breccia. The flows are volcanism may have continued into Miocene chiefly latite and quartz latite in composition, time. Basalt plugs and flows formed after but trachybasalt flows have been found locally silidc volcanism. These rocks have been as­ near the base of the formation. Numerous signed tentatively to the Pliocene because some dikes, sills, and larger discordant masses of of the flows rest on well-dated Miocene sedi­ porphyry cut both the extrusive rocks of the mentary rocks northwest of Hot Sulphur Rabbit Ears Volcanics and the underlying sedi­ Springs. mentary rocks and are clearly related to the middle Tertiary period of igneous activity. MIDDLE PARK FORMATION The Middle Park Formation of Late Creta­ POISON RIDGE VOLCANIC CENTER ceous ( ?) and early Tertiary age is the only The Poison Ridge volcanic center is marked sedimentary unit exposed in the vicinity of the by a composite quartz latite porphyry stock Poison Ridge center. The base of the forma­ that is the focus of a radiating dike swarm tion is not exposed near the stock, but about and is surrounded by a zone of baked and 8 miles southwest, the Middle Park beds rest hydrothermally altered rocks of the Middle unconformably on the Pierre Shale of Late Park Formation. Cretaceous age. As described by Tweto (1957), The Poison Ridge stock is rudely ovoid in the formation typically is a sequence of inter­ plan and measures about 2,600 feet along its bedded arkosic ·conglomerate, sandstone, and northwest axis and about 1,500 feet along its mudstone at least 6,000 feet thick. Virtually northeast axis. It is best exposed in the north-~ unaltered outcrops of the formation near the west-facing cliffs along Sheep Cre~k; in most Poison Ridge stock occur along north-flowing other places, it is covered by talus. and rock tributaries of Sheep Creek on the north flank slides. Its·. contacts are generally covered ex­ of Sheep Mountain in the NWlit, sec. 16, T. 4 cept on the northwest side where relations are N., R. 79 W. (unsurveyed). The conglomerates partially masked by rock alteration. The ·con­ and sandstones at this locality are typical of tact on this side seems to be steep, perhaps the Middle Park Formation and are easily rec- vertical.

3 40° 17' 20"

0 1000 2000 3000 4000 5000 FEET

CONTOUR INTERVAL 40 FEET

FIGURE 2.-Preliminary geologic map of the Poison Ridge volcanic center, Grand and Jackson Counties, Colo. Base map f r om U. S. Geological Survey, scale 1 : 24,000-Hyannis Peak and Parkview Mountain, 1956. Geol­ ogy by D. M. Kinney with additions by G. A. Izett, R. U. King, and R. B. Taylor.

4 The principal rock type of the Poison Ridge stock is a light-gray quartz latite porphry. Gray feldspar phenocrysts from 2 em ( centi­ meters) to less than 1 mm (millimeter) long and dark-greenish-gray hornblende crystals as EXPLANATION much as 5 mm long are set in a gray matrix of quartz and feldspar with lesser quantities of other minerals. Rare phenocrysts of clinopy­ roxene or bipyramidal quartz were observed in a few specimens. In thin section, the rock is Surficial deposits a seriate porphyry. The largest phenocrysts are orthoclase, with complex oscillatory zoning. The plagioclase phenocrysts are oligoclase, also with complex oscillatory zoning and complex twinning. The hornblende phenocrysts are tan 'l'rachyba.sal t intrusive rocks to greenish tan in transmitted light, are sieve­ like with many inclusions of other minerals, and have irregular outlines. The groundmass is a granular to subgraphic intergrowth of cloudy perthite and quartz. The perthite also Rabbit Ears Volcanics and related intrusive rocks thinly mantles both orthoclase and plagioclase phenocrysts. The modal composition of the Light pattern denotes flows, tuffs, and volca­ noclastic rocks. Dark pattern denotes latite principal rock type in the stock is quartz latite. and quartz latite porphyry intrusive rocks Modal analyses, in volume percent, of rocks from the Poison Ridge stock [Tr, trace] 1 2 ~ Phenocrysts: Middle Park Formation Oligoclase ------­ 25 28 Orthoclase ------­ 9 9 Hornblende ------­ 7 5 Clinopyroxene ------1 Groundmass: Perthite ------45 40 Quartz ------10 14 Contact Biotite ------1 1 Approx:i.rrately located Magnetite ------2 1 Zircon ------Tr Tr Apatite ------Tr Tr Sphene ------Tr Tr Fault Alteration minerals: Approximately located. Chlorite ------1 Tr Dashed where inferred Nontronite ------Tr 1 Sericite ------Tr Tr

7 1. 300 feet from northeastern contact of stock, about -'- 100 feet above Sheep Creek. Strike and dip of beds 2. 200 feet from northeastern contact of stock, about 100 feet above Sheep Creek. Mineralogic and textural variations in the >­ rocks found in talus covering the stock suggest Ad it that several different porphyry types are present. Scattered inclusions of dark horn­ FIGURE 2.-Continued. blende-rich porphyry in the outcrops of lighter colored quartz latite porphyry on Sheep Greek furnish the only clear evidence of relative age. The predominant coarse porphyry of the stock seems to be the last in an intrusive sequence of rock. The mudstone layers of the Middle Park generally finer grained qual'tz latite porphyries. Formation have been partially recrystallized A similar varied assemblage of porphyries as far as 2,000 feet from the stock. The mud~ n1akes up a group of dikes that radiate out­ stone has been baked to a black hard dense· ward from the Poison Ridge stock (fig. 2). hornfels composed of quartz, feldspar, chlorite, These rocks are principally quartz latite por­ and. sericite. Interlayered sandy mudstone, phyries in which an aphanitic groundmass en­ sandstone, and conglomerate beds are bleached closes abundant feldspar and hornblende and have been partially sericitized and locally phenocrysts. Many additional dikes were not silicified. mapped because of their small size, or because A highly asymmetric hydrothermal altera­ they are masked by hydrothermal alteration, tion pattern has been developed around the· or because of poor outcrop and talus cover. stock (fig. 3). Closely spaced and inte·rsecting The radial dike pattern, the complex central fractures and intensely altered rock are con­ stock, and the similarities of the porphyry centrated within the northwest part of the types to extrusive rocks in the Rabbit Ears stock and in the adjacent sedimentary rocks., Volcanics suggest that the Poison Ridge stock In this inner zone, the host rocks are alte:red represents the subvolcanic core of a volcano to quartz and fe1dspar and contain variable· that supplied pyroclastic ro·cks and flows to the amounts of pyrite, muscovite, and biotite Rabbit Ears Volcanics. (fig. 4) ; the innumerable minor fractures are filled by narrow quartz-hematite-feldsp·ar vein­ ROCK ALTERATION AND MINERALIZATION lets. This inner alteration zone grades outward The Poison Ridge stock is enveloped by into a peripheral zone of altered rock charac­ metamorphosed and hydrothermaHy altered terized by quartz, sericite, and pyrite.

EXPLANATION ~ Quartz-he~tite-feldspar Veinlets in silicified and sericitized rock

Quartz-sericite-pyrite Density of stipple shows relative intensity of alteration

0 1000 2000 3000 4000 5000 FEET

FIGURE a.-Distribution of rock altera.tion around the Poison Ridge volcanic center (compare with fig. 2). Bedrock units are shown in figure 2.

6 FIGURE 4.-Fractured and altered Middle Park Formation (left) and quartz latite porphyry (right) from inner altered zone. (Small-scale· divisions are millimeters.)

At the surface most of the intensely altered GEOCHEMICAL ANOMALIES rocks have been deeply weathered and leached. Semiquantitative spectrographic analyses of In particular, the pyrite-rich rocks on the 24 samples from the hydrothermally altered ridge north of the stock have been extensively area around the stock were made by Carl F:orn oxidized and now are stained or permeated by and D. J . Grimes. Analyses of rocks from the limonite. Sulfides are present only in the cores inner zone of quar·tz-hematite-feldspar altera­ of large blocks or in the least permeable silici­ tion (fig. 3) show the following ranges in fied rocks. The brightly colored red and yellow metal content: Mo, 10-150 ppm; Cu, 15-50 rocks of this ridge are perhaps the most strik­ ppm; Pb, 30- 70 ppm. Analyses of rocks from ing evidence of the large area of rock altera­ the outer quartz-sericite-pyrite alteration as­ tion. semblage show the following ranges in metal The general alteration pattern is modified content: Mo, < 3-10 ppm; Cu, 15-150 ppm; locally along the northwest-trending fault Pb, 10-300 ppm. Characteristically, the higher northwest of the central stock, where the inter­ values of molybdenum were obtained from the stices of fault breccia are generally filled with silicified rocks near the stock, and the higher limonite. In one place, at an altitude of about copper and lead values were obtained from 9,980 feet, waveilite spherulites partially fiJI sulfide-rich samples farther from the stock. openings and cement the brecciated Middle Values for individual samples are not given Park sandstone. In a second locality, on the here because they are highly dependent on the south side of Sheep Creek about 400 feet north­ local differences in hypogene alteration and east of the contact between the Rabbit Ears veining of the small grab samples and on the Vokanics and the Middle Park Formation variable superposed effects of supergene altera­ (fig. 2), several pits and two adits have been tion ~nd leaching. dug in altered Middle Park Formation. Work­ The effects of surficial leaching on most of ings are now caved and inaccessible. Ore thes~ samples are probably large, but are diffi­ samples collected from the dumps show abun­ cult to evaluate. The oxidation of pyrite and dant galena and resinous sphalerite in a matrix the concomitant formation of sulfuric acid of sooty manganese oxides and iron oxides. solutions in near-surface waters create an These rocks were apparently mined from frac­ ideal, environment for leaching of metals from ture fillings associated with porphyry dikes sulfide-rich rocks. Evidence for this supergene because the workings follow the surface trace leaching can be seen along Sheep Creek where of a major dike (fig. 2). springs fed from the area of pyri-tic alteration

7 enter the main stream. In the oxygenated Molybdenum concentrations are highest in reaches of the stream, its bed is ·covered by a rocks of the inner zone (Mo, 10-150 ppm), and flocculent limonite precipitate which tends to copper and lead values are greatest in the outer entrap other metals. Semiquantitative spectro­ zone (Cu, 15-150 ppm; Pb, 10-300 ppm). The graphic analyses of iron-rich sediments from intense fracturing of rocks in the inner zone Sheep Creek and its minor tributaries in the and the dispersion of metals (sulfides) out­ area of hydrothermal alteration showed the fol­ ward from this zone are favorable indications lowing ranges in metal content: Mo, <3-70 of potential ore. The zonal arrangement of ppm; Cu, 30-700 ppm; Ph, 30~500 ppm.; Zn, potassic alteration minerals, with at least two <200-500 ppm. As much of the flocculent iron distinct mineral assemblages, is similar to that precipita,te is removed by torrential flow dur­ found at many other intrusive centers associ­ ing the spring runoff' in this area of high ated with disseminated (porphyry type) ore stream gradient, most of this material accumu­ deposits (Creasey, 1966). The shallow hypa­ lates during a single season. The Hmonitic byssal (subvolcanic) environment and the material indicates active and continuing leach­ hypogene hematite of the inner zone suggest ing of the pyritic rocks in this hydrothermally that ore would be most likely to occur at depths altered area. considerably below the present levels of exposure. SUMMARY The Poison Ridge volcanic center of probable REFERENCES middle Tertiary age is marked by a composite quartz latite porphyry stock at the focus of Oreasey, S. C., 1966, Hydrothermal alteration, in Titley, S. R., and Hicks, C. L., eds., Geology of the por­ a radiating dike swarm. It is surrounded by phyry copper deposits, southwestern North Ameri­ baked and hydrothermally altered rocks of the ca: Arizona Univ. Press, 287 p. Middle Park Formation. An inner alteration zone is ,characterized by closely spaced and in­ IzeU, G. A., 1966, Tertiary extrusive volcanic rocks in Middle Park, Grand County, Colorado, in tersecting fractures filled with quartz-hematite­ Geological Survey research 1966: U.S. Geol. Survey feldspar veinlets and by alteration of the rocks Prof. Paper 550-B, p. B42-B46. to pyrite, quartz, feldspar, muscovite, and biotite. An outer concentric zone of quartz­ Tweto, Ogden, 1957, Geologic sk~tch of southern Middle Park, Colorado, in Rocky Mountain Assoc. sericite-pyrite alteration extends as much as Geologists Guidebook 9th Ann. Field Conf., North three quarters of a mile from the stock and Middle Park Basins, Colorado, 1957: p. 18-31.

GPO 863.607

8