The Poison Ridge volcanic center and related mineralization, Grand and Jackson Counties, Colorado

Robert U. King, and Richard B. Taylor

ILLUSTRATIONS

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

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.

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.

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.

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.

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.

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

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).

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 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.

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