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 

Introduction ---------------------------------- 1 

Geologic setting ------------------------------ 1 

Geologic setting-Continued 

Poison Ridge volcanic center -------------- 3 

Rock alteration and mineralization ------------ 6 

Geochemical anomalies ----------------------- 7 

Middle Park Formation ------------------ 3 s,ummary ------------------------------------ 8 

Rabbit Ears Volcanics -------------------- 3 References ----------------------------------- 8 

ILLUSTRATIONS 

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

2. Preliminary geologic map of the Poison Ridge volcanic center, Colorado 

Page 

2 

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 

The Poison Ridge volcanic center, of probable middle 

Tertiary age, in the Rabbit Ears Range, Grand County, 

Colo., is surrounded by a zoned halo of intense hydrothermal alteration. An inner alteration zone of intensely fractured rock veined by quartz, hematite, and feldspar is surrounded by a zone characterized by quartz, 

sericite, and pyrite. Geochemical anomalies in the 

altered area indicate that the altered rock offers a 

potential target at depth for exploration for disseminated molybdenum and perhaps copper. 

INTRODUCTION 

In the course of regional mapping in the 

Rabbit Ears Range, northwestern Colorado, 

an extensive area of red-stained pyritic rocks. 

south of Poison Ridge was discovered by D. M. 

Kinney of the U.S. Geological Survey. Stream.-

sediment samples taken downstream from the 

altered area in the summer of 1967 by Kinney 

and D. L. Wheat were analyzed using semiquantative spectrographic methods and were 

found to contain anomalous amounts of several 

metals. Samples of hydrothermally altered 

rocks taken late in October 1967 by G. A. Izett 

and R. B. Taylor also had anomalous m.etal 

contents, but further work was halted by snow. 

The area of altered rocks and an area peripheral to· it were studied in more detail during 

July 1968 by Izett, Taylor, and R. U. King. This 

work showed that the altered and mineralized 

rocks are closely associated with a volcanic 

center of probable middle Tertiary age, and 

the pattern of alteration and distribution of 

metal concentrations suggests that potential 

targets for exploration may be present at 

depth. 

This igneous center, here designated the 

"Poison Ridge volcanic center," is on the south 

1 

side of the Continental Divide between North 

Park and Middle Park, about 20 mil_es northnortheast of Kremmling and 9 miles southsouthwest of Rand, Colo. (fig. 1). A quartz 

latite porphyry stock cutting clastic sedimentary rocks of the Middle Park Formation of 

Late Cretaceous ( ?) and Paleocene age (fig. 2) 

is surrounded by asymmetric zones of contact 

metamorphism and hydrothermal alteration. 

The northwestern part of the stock and adjacent wallrocks were closely fractured and 

veined by quartz, hematite, and feldspar. An 

alteration halo 1112 miles across ·Characterized 

by quartz, sericite, and pyrite surrounds this 

inner zone. 

Semiquantitative spectrographic analyses of 

samples of the hydrothermally altered rocks 

show that molybdenum content-Mo, 10-150 

ppm (parts per million) -is highest in the 

inner alteration zone and that copper and lead 

contents-Cu, 15-150 ppm; Pb, 10-300 ppmare highest in the outer zone. All surface rocks 

have been leached and the sulfides at least partially oxidized ; the analytical results reflect 

both the hypogene and supergene alteration. 

The Poison Ridge center is easily reached 

from Rand, Colo., by way of a county road 

along Willow Creek, and thence via Forest 

Service Road 2042 to the Continental Divide at 

the east end of Poison Ridge. 

GEOLOGIC SETTING 

In latest Cretaceous ( ?) and early Tertiary 

time, the Poison Ridge area was part of a large 

sedimentary basin lying between the Front 

Range to the east and the Park and Gore 

Ranges to the west. This intermontane basin 
40° 

15' 

0 5 MILES 

Corral x 

Peaks x 

• Area of 

report 

COLORADO 

X Elk Mountain 

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

2 
was formed early in the Laramide orogeny 

when tectonic movem.ents outlined some of the 

main elements of the Southern Rocky Mountains. Debris shed from these early boundary 

ranges and from andesitic volcanic centers in 

and along the west side of the Front Range 

ac·cumulated in the basin to form clastic 

deposits called the Middle Park Formation. 

The early upwarps were more closely folded 

and faulted during later Laramide orogenic 

episodes, and the Middle Park Formation was 

extensively deformed. Following the final Laramide tectonic activity in late Eocene tim.e and 

the aecompanying erosion, silicic volcanism began in Oligocene time. One volcanic field, which 

covered an upland area underlain by the Middle Park Formation, now makes up the Rabbit 

Ears Range. Quartz latite and latite pyroclastic rocks and flows (Rabbit Ears Volcanics) 

were extruded from several igneous centers. 

These centers, including the Poison Ridge center, are marked in the present terrain by sills, 

dikes, and small stocks cutting the underlying 

sedimentary rocks. Most of the volcanic activity probably took place in Oligocene time, but 

volcanism may have continued into Miocene 

time. Basalt plugs and flows formed after 

silidc volcanism. These rocks have been assigned tentatively to the Pliocene because some 

of the flows rest on well-dated Miocene sedimentary rocks northwest of Hot Sulphur 

Springs. 

MIDDLE PARK FORMATION 

The Middle Park Formation of Late Cretaceous ( ? ) and early Tertiary age is the only 

sedimentary unit exposed in the vicinity of the 

Poison Ridge center. The base of the formation is not exposed near the stock, but about 

8 miles southwest, the Middle Park beds rest 

unconformably on the Pierre Shale of Late 

Cretaceous age. As described by Tweto (1957), 

the formation typically is a sequence of interbedded arkosic ·conglomerate, sandstone, and 

mudstone at least 6,000 feet thick. Virtually 

unaltered outcrops of the formation near the 

Poison Ridge stock occur along north-flowing 

tributaries of Sheep Creek on the north flank 

of Sheep Mountain in the NWlit, sec. 16, T. 4 

N., R. 79 W. (unsurveyed). The conglomerates 

and sandstones at this locality are typical of 

the Middle Park Formation and are easily rec3 

ognized by the fragments of Precambrian rocks 

and Laramide porphyries that they contain. Interlayered mudstones contain abundant mica 

and are dark and massive. The Middle Park 

Formation is less easily identified around the 

Poison Ridge stock, where the rocks are baked 

and hydrothermally altered. 

RABBIT EARS VOLCANICS 

Extrusive fragmental volcanic rocks and 

flows of the Rabbit Ears Volcanics of Oligocene 

and Miocene ( ?) age (lzett, 1966) overlie the 

Middle Park Formation. The formation is as 

much as 1,500 feet thick and crops out in an 

arcuate pattern on the west side of the Poison 

Ridge stock (fig. 2). The formation is made 

up of interbedded tuff, tuff breccia, and volcanic 

breccia interlayered with a few lava flows. The 

breccia fragments are composed of several 

types of porphyritic intermediate volcanic 

rocks including biotite- and hornblende- and 

pyroxene-bearing latite, quartz latite, and 

rhyodacite porphyry. Sparse fragments of 

rhyolite welded tuff containing quartz phenocrysts als9 occur in the breccia. The flows are 

chiefly latite and quartz latite in composition, 

but trachybasalt flows have been found locally 

near the base of the formation. Numerous 

dikes, sills, and larger discordant masses of 

porphyry cut both the extrusive rocks of the 

Rabbit Ears Volcanics and the underlying sedimentary rocks and are clearly related to the 

middle Tertiary period of igneous activity. 

POISON RIDGE VOLCANIC CENTER 

The Poison Ridge volcanic center is marked 

by a composite quartz latite porphyry stock 

that is the focus of a radiating dike swarm 

and is surrounded by a zone of baked and 

hydrothermally altered rocks of the Middle 

Park Formation. 

The Poison Ridge stock is rudely ovoid in 

plan and measures about 2,600 feet along its 

northwest axis and about 1,500 feet along its 

northeast axis. It is best exposed in the north-~ 

west-facing cliffs along Sheep Cre~k; in most 

other places, it is covered by talus. and rock 

slides. Its·. contacts are generally covered except on the northwest side where relations are 

partially masked by rock alteration. The ·contact on this side seems to be steep, perhaps 

vertical. 
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 fr om U. S. Geological Survey, scale 1 : 24,000-Hyannis Peak and Parkview Mountain, 1956. Geology by D. M. Kinney with additions by G. A. Izett, R. U. King, and R. B. Taylor. 

4 
EXPLANATION 

Surficial deposits 

'l'rachyba.sal t intrusive rocks 

Rabbit Ears Volcanics and related 

intrusive rocks 

Light pattern denotes flows, tuffs, and volcanoclastic rocks. Dark pattern denotes latite 

and quartz latite porphyry intrusive rocks 

Middle 

~ Park Formation 

Contact 

Approx:i.rrately located 

Fault 

Approximately located. 

Dashed where inferred 

7 

-'-

Strike and dip of beds 

>-

Ad it 

FIGURE 2.-Continued. 

The principal rock type of the Poison Ridge 

stock is a light-gray quartz latite porphry. 

Gray feldspar phenocrysts from 2 em ( centimeters) to less than 1 mm (millimeter) long 

and dark-greenish-gray hornblende crystals as 

much as 5 mm long are set in a gray matrix of 

quartz and feldspar with lesser quantities of 

other minerals. Rare phenocrysts of clinopyroxene or bipyramidal quartz were observed 

in a few specimens. In thin section, the rock is 

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 

to greenish tan in transmitted light, are sievelike 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 

thinly mantles both orthoclase and plagioclase 

phenocrysts. The modal composition of the 

principal rock type in the stock is quartz latite. 

Modal analyses, in volume percent, of rocks from the 

Phenocrysts: 

Poison Ridge stock 

[Tr, trace] 

Oligoclase --------------------- Orthoclase -------------------- Hornblende ------------------- Clinopyroxene -----------------

Groundmass: 

1 

25 

9 

7 

Perthite ---------------------- 45 

Quartz ----------------------- 10 

Biotite ------------------------ 1 

Magnetite -------------------- 2 

Zircon ------------------------ Tr 

Apatite ----------------------- Tr 

Sphene ----------------------- Tr 

Alteration minerals: 

Chlorite ---------------------- 1 

Nontronite -------------------- Tr 

Sericite ----------------------- Tr 

2 

28 

9 

5 

1 

40 

14 

1 

1 

Tr 

Tr 

Tr 

Tr 

1 

Tr 

1. 300 feet from northeastern contact of stock, about 

100 feet above Sheep Creek. 

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 

that several different porphyry types are 

present. Scattered inclusions of dark hornblende-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 

generally finer grained qual'tz latite porphyries. 

A similar varied assemblage of porphyries 

n1akes up a group of dikes that radiate outward from the Poison Ridge stock (fig. 2). 

These rocks are principally quartz latite porphyries in which an aphanitic groundmass encloses abundant feldspar and hornblende 

phenocrysts. Many additional dikes were not 

mapped because of their small size, or because 

they are masked by hydrothermal alteration, 

or because of poor outcrop and talus cover. 

The radial dike pattern, the complex central 

stock, and the similarities of the porphyry 

types to extrusive rocks in the Rabbit Ears 

Volcanics suggest that the Poison Ridge stock 

represents the subvolcanic core of a volcano 

that supplied pyroclastic ro·cks and flows to the 

Rabbit Ears Volcanics. 

ROCK ALTERATION AND MINERALIZATION 

The Poison Ridge stock is enveloped by 

metamorphosed and hydrothermaHy altered 

rock. The mudstone layers of the Middle Park 

Formation have been partially recrystallized 

as far as 2,000 feet from the stock. The mud~ 

stone has been baked to a black hard dense· 

hornfels composed of quartz, feldspar, chlorite, 

and. sericite. Interlayered sandy mudstone, 

sandstone, and conglomerate beds are bleached 

and have been partially sericitized and locally 

silicified. 

A highly asymmetric hydrothermal alteration pattern has been developed around the· 

stock (fig. 3). Closely spaced and inte·rsecting 

fractures and intensely altered rock are concentrated within the northwest part of the 

stock and in the adjacent sedimentary rocks., 

In this inner zone, the host rocks are alte:red 

to quartz and fe1dspar and contain variable· 

amounts of pyrite, muscovite, and biotite 

(fig. 4) ; the innumerable minor fractures are 

filled by narrow quartz-hematite-feldsp·ar veinlets. This inner alteration zone grades outward 

into a peripheral zone of altered rock characterized 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 

rocks have been deeply weathered and leached. 

In particular, the pyrite-rich rocks on the 

ridge north of the stock have been extensively 

oxidized and now are stained or permeated by 

limonite. Sulfides are present only in the cores 

of large blocks or in the least permeable silicified rocks. The brightly colored red and yellow 

rocks of this ridge are perhaps the most striking evidence of the large area of rock alteration. 

The general alteration pattern is modified 

locally along the northwest-trending fault 

northwest of the central stock, where the interstices of fault breccia are generally filled with 

limonite. In one place, at an altitude of about 

9,980 feet, waveilite spherulites partially fiJI 

openings and cement the brecciated Middle 

Park sandstone. In a second locality, on the 

south side of Sheep Creek about 400 feet northeast of the contact between the Rabbit Ears 

Vokanics and the Middle Park Formation 

(fig. 2), several pits and two adits have been 

dug in altered Middle Park Formation. Workings are now caved and inaccessible. Ore 

samples collected from the dumps show abundant galena and resinous sphalerite in a matrix 

of sooty manganese oxides and iron oxides. 

These rocks were apparently mined from fracture fillings associated with porphyry dikes 

because the workings follow the surface trace 

of a major dike (fig. 2). 

7 

GEOCHEMICAL ANOMALIES 

Semiquantitative spectrographic analyses of 

24 samples from the hydrothermally altered 

area around the stock were made by Carl F:orn 

and D. J. Grimes. Analyses of rocks from the 

inner zone of quar·tz-hematite-feldspar alteration (fig. 3) show the following ranges in 

metal content: Mo, 10-150 ppm; Cu, 15-50 

ppm; Pb, 30- 70 ppm. Analyses of rocks from 

the outer quartz-sericite-pyrite alteration assemblage show the following ranges in metal 

content: Mo, < 3-10 ppm; Cu, 15-150 ppm; 

Pb, 10-300 ppm. Characteristically, the higher 

values of molybdenum were obtained from the 

silicified rocks near the stock, and the higher 

copper and lead values were obtained from 

sulfide-rich samples farther from the stock. 

Values for individual samples are not given 

here because they are highly dependent on the 

local differences in hypogene alteration and 

veining of the small grab samples and on the 

variable superposed effects of supergene alteration ~nd leaching. 

The effects of surficial leaching on most of 

thes~ samples are probably large, but are difficult to evaluate. The oxidation of pyrite and 

the concomitant formation of sulfuric acid 

solutions in near-surface waters create an 

ideal, environment for leaching of metals from 

sulfide-rich rocks. Evidence for this supergene 

leaching can be seen along Sheep Creek where 

springs fed from the area of pyri-tic alteration 
enter the main stream. In the oxygenated 

reaches of the stream, its bed is ·covered by a 

flocculent limonite precipitate which tends to 

entrap other metals. Semiquantitative spectrographic analyses of iron-rich sediments from 

Sheep Creek and its minor tributaries in the 

area of hydrothermal alteration showed the following ranges in metal content: Mo, <3-70 

ppm; Cu, 30-700 ppm; Ph, 30~500 ppm.; Zn, 

<200-500 ppm. As much of the flocculent iron 

precipita,te is removed by torrential flow during the spring runoff' in this area of high 

stream gradient, most of this material accumulates during a single season. The Hmonitic 

material indicates active and continuing leaching of the pyritic rocks in this hydrothermally 

altered area. 

SUMMARY 

The Poison Ridge volcanic center of probable 

middle Tertiary age is marked by a composite 

quartz latite porphyry stock at the focus of 

a radiating dike swarm. It is surrounded by 

baked and hydrothermally altered rocks of the 

Middle Park Formation. An inner alteration 

zone is ,characterized by closely spaced and intersecting fractures filled with quartz-hematitefeldspar veinlets and by alteration of the rocks 

to pyrite, quartz, feldspar, muscovite, and 

biotite. An outer concentric zone of quartzsericite-pyrite alteration extends as much as 

three quarters of a mile from the stock 

8 

Molybdenum concentrations are highest in 

rocks of the inner zone (Mo, 10-150 ppm), and 

copper and lead values are greatest in the outer 

zone (Cu, 15-150 ppm; Pb, 10-300 ppm). The 

intense fracturing of rocks in the inner zone 

and the dispersion of metals (sulfides) outward from this zone are favorable indications 

of potential ore. The zonal arrangement of 

potassic alteration minerals, with at least two 

distinct mineral assemblages, is similar to that 

found at many other intrusive centers associated with disseminated (porphyry type) ore 

deposits (Creasey, 1966). The shallow hypabyssal (subvolcanic) environment and the 

hypogene hematite of the inner zone suggest 

that ore would be most likely to occur at depths 

considerably below the present levels of 

exposure. 

REFERENCES 

Oreasey, S. C., 1966, Hydrothermal alteration, in Titley, 

S. R., and Hicks, C. L., eds., Geology of the porphyry copper deposits, southwestern North America: Arizona Univ. Press, 287 p. 

IzeU, G. A., 1966, Tertiary extrusive volcanic rocks 

in Middle Park, Grand County, Colorado, in 

Geological Survey research 1966: U.S. Geol. Survey 

Prof. Paper 550-B, p. B42-B46. 

Tweto, Ogden, 1957, Geologic sk~tch of southern 

Middle Park, Colorado, in Rocky Mountain Assoc. 

Geologists Guidebook 9th Ann. Field Conf., North 

and Middle Park Basins, Colorado, 1957: p. 18-31. 

GPO 863.607