CHAPTER 3

PRELIMINARY GEOLOGIC MAP AND ALTERATION MINERALOGY OF THE MAIN SCARP OF THE SLUMGULLION LANDSLIDE

by Sharon F. Diehl and Robert L. Schuster

Introduction

The main scarp of the Slumgullion landslide lies near the eastern edge of the Uncompahgre-San Juan caldera complex and at the eastern edge of a regional limonitic alteration of rhyolitic volcanic rocks (Lipman, 1976; Sharp and others, 1983; Bove and Hon, 1992). In the San Cristobal quadrangle, hydrothermal alteration products such as alunite and smectite have long been known to be associated with weakening and failure of volcanic rock, with the resulting potential for landslides (Cross, 1909; Larsen, 1913). Larsen (1913) identified hydrothermal alteration products such as alunite and opal in the Slumgullion earth flow.

We investigated the geology of the main scarp because the geological characteristics and mineralogy of the hydrothermal alteration are important factors in the weakness of the rock, and therefore, in the initiation and intensity of landslide movement. The main scarp is composed of "pancake" stratigraphy: a series of relatively unaltered basaltic flows and ash-flow tuff overlying highly altered andesitic to rhyolitic flows. These units are intruded by porphyritic rhyolites and breccia pipes. Landslides caused by weakening of the rock by hydrothermal alteration were recognized and mapped in the Lake City area of the San Juan Mountains 85 years ago by Howe (1909).

Preliminary investigations of the main scarp of the landslide indicate that faults (fig. 1) are important structural features that add to the instability of the main scarp. Faults and breccia pipes have been conduits for hydrothermal fluids, as evidenced by the association of alunite-altered rock with fault traces and breccia deposits.

Methods of Study

Because of the inaccessibility of units on the steep and unstable face of the main scarp, it was nearly impossible to collect in situ samples of all units. Thus, float material collected at the base of the scarp supplemented in situ samples that were collected from accessible outcrops along the sides of the scarp (fig. 1). The results of thin-section and bulk X-ray analyses of unaltered volcanic rock were compared with thin sections and X-ray traces of altered volcanic rock to determine the change in mineralogy caused by hydrothermal alteration (table 1). Powdered bulk samples were X-rayed using CuK radiation on a Philips-type 150-100-21 goniometer from 2° to 65° 2 at a scan speed of 2° 2 per minute. A Cambridge Mark 2 scanning electron microscope (SEM) with an attached energy-dispersive X-ray (EDX) was used to confirm the identity of the alteration mineralogy.

Geology

Stratigraphy

Based on stratigraphic descriptions of Lipman (1976), Tertiary volcanic units exposed in the main scarp of the Slumgullion landslide, from youngest to oldest, are the Hinsdale Formation, the Sunshine Peak Tuff, and volcanic units of Uncompahgre Peak (fig. 1). The unaltered Miocene Hinsdale Formation is composed of vesicular basaltic lava flows that contain 5-15 percent phenocrysts of plagioclase, biotite, clinopyroxene, and opaque minerals (probably magnetite and (or) ilmenite). The Miocene Sunshine Peak Tuff is a welded ash-flow tuff. It was identified by float material, and this unit may also be present as a breccia deposit in the western portion of the main scarp. The underlying Uncompahgre Peak volcanics of Oligocene age are composed of andesitic flows that contain 10-20 percent phenocrysts of plagioclase, biotite, hornblende, clinopyroxene, and quartz. This unit has undergone acid-sulfate alteration. There is an erosional unconformity between the Oligocene Uncompahgre Peak volcanics and the younger Miocene formations.

Intrusives

Steep, dangerous topography prevented close examination of many of the intrusives recognized on aerial photographs. The porphyry intrusives and associated breccia pipes occur in rough alignment along the trend of the main scarp and along fault traces (fig. 1).

A domal intrusive sampled at the intersection of two major faults in the main scarp (fig. 1) is a medium-gray porphyry composed of 30-35 percent phenocrysts of feldspar, quartz, and biotite, and is mineralized with pyrite, barite, and rare-earth minerals. The feldspar phenocrysts are altered to clay minerals, and pyrite cubes are highly etched.

Breccias

Breccias are abundant and are important because their porous and permeable texture allowed the influx and passage of mineralizing solutions (fig. 2). The stratigraphy exposed in the main scarp is complicated by the presence of flow breccias, fault breccias, and breccia pipes, which are difficult to differentiate from one another. Their distribution is associated with faults and intrusives (fig. 1). Furthermore, there are surfaces of ferricrete that resemble breccias. Breccia fragments are very angular (fig. 2) and are composed of alunite-altered, as well as fresh, volcanic rock fragments. Breccias are commonly cemented by (1) silica and alunite (fig. 3), (2) silica and jarosite, and (3) iron oxides.

Alteration zones surround the breccia pipes. The alteration zones consist of vuggy silica, quartz-alunite veins, and argillite. The alteration mineralogy follows the classic hydrothermal alteration zonation (fig. 4) typified at Summitville, Colorado, and geothermal systems in general (Hayba and others, 1985, p. 153; Silberman and Berger, 1985). Rare xenoliths of pink granite occur in quartz-alunite cemented breccia.

The earliest period of brecciation and mineralization is assumed to be 23.1 Ma. This date for hydrothermal activity is based on dates for similar mineral and breccia deposits in the alunite deposit at Red Mountain, located about 8 km west of the Slumgullion landslide within the Lake City caldera (Bove and Hon, 1990).

Structure

Faults

The volcanic rock is weakened by the intersection of numerous, discontinuous, curvilinear faults (fig. 1), which are evident on aerial photographs. Faults are confirmed by breccias, alteration minerals, and slickensides. Breccias composed of breccia fragments of alunite with jarosite cement, and alunite-silica altered rock, are commonly associated with faults (table 1; fig. 1). Samples of these rocks commonly have slickensides that show several periods of movement. Faults were important as conduits for hydrothermal fluids in the region (Lee, 1986) and may have been a focus for hydrothermal fluid flow in the main-scarp area. A network of silica-filled veins occur at the extreme eastern and western portions of the main scarp (for example, samples SF 18 and SF 52), and these veins are associated with breccia pipes and fault traces shown on figure 1.

Active rock-avalanche chutes are located along fault traces and at fault intersections (figs. 1 and 5). These chutes funnel debris from the main scarp onto talus cones at its base (fig. 5). The talus cones are composed of unsorted, angular cobble- to boulder-size blocks of volcanic rock. The boulders are derived from failure of the main scarp due to vertical, through-going joints in the Hinsdale Formation.

Hydrothermal Mineralization

Hydrothermal explosion brecciation is suggested by the presence of siliceous veins and sulfide and rare-earth mineralization. The sulfide minerals have interacted with potassium-rich hydrothermal fluids and are sources for acid sulfate solutions. Pyrite occupies a microfracture network and is disseminated through portions of the volcanic sequence. Other diagenetic products of hydrothermal activity, identified by X-ray diffraction and scanning electron microscope, are chalcedonic quartz, potassium feldspar, sericite, smectite, kaolinite, jarosite, and alunite-group minerals. These assemblages of minerals are similar to the hydrothermal deposits at nearby Red Mountain (Bove and others, 1990). Chalcedonic quartz, sericite, and pyrite are early main alteration products in the lava flows. These minerals line void spaces and fill fractures. Chalcedonic quartz also acts as a cement in some breccia deposits.

Of interest is the distribution of the alunite and jarosite minerals, which are later alteration products (fig. 3). The presence of these potassium sulfate minerals suggests that hydrothermal fluids associated with the intrusives were potassium rich. Alunite occurs with silica (opal?) around the intrusives in breccia zones and along the fault system. Jarosite, the more iron-rich potassium sulfate mineral, is peripheral to the alunite and forms a cement between breccia fragments. Smectitic clay is found in the main scarp but is more common near and in the active head of the landslide (fig. 1).

Plagioclase phenocrysts are commonly altered to kaolinite in the main-scarp volcanic rocks. This alteration process releases calcium ions into solution. In addition, pyrite has oxidized to release sulfur into solution. The calcium and sulfur ions combine in an oxidizing environment to form gypsum--a soluble salt. The formation of these surficial alteration minerals suggests that the ground water contains a high concentration of total dissolved salts.

The effect of soluble salts on clay minerals, especially in the yellow, sulfate-rich landslide material, and on their material properties is a problem that is addressed by Chleborad and others (this volume). Flocculation of clay platelets is one problem that may be caused by the low pH environment and (or) the abundance of total dissolved salts in the ground water.

Discussion

Because altered rock is interbedded with unaltered flows within the Uncompahgre Peak volcanics, acidic solutions probably flowed laterally along horizontal platy jointing or flow lines in the andesitic and rhyolitic volcanic rocks. Perhaps because they are more porous and of rhyolitic composition, flow breccias of the Uncompahgre Peak volcanics were especially susceptible to acid sulfate alteration.

Alteration products in the volcanic rocks at the Slumgullion landslide are similar to those in the nearby Red Mountain alunite deposits near Lake City, Colo. Both are hydrothermal deposits in which an abundance of sulfur and water were introduced into the rock (Bove and Hon, 1990).

There appear to be several generations of brecciation and hydrothermal alteration as evidenced by (1) alunite-silica-altered rock fragments that occur with unaltered rock fragments in breccia deposits, (2) mineral-filled, in situ fractures in alunite-cemented breccia deposits, and (3) alunite-coated slickensides in breccia deposits.

The weathering of sulfides, which produces acid-sulfate solutions, is responsible for the precipitation of surficial alteration products such as limonite and gypsum. Jarosite may be a cold-water alteration product as well as a hydrothermal mineral. The precipitation of jarosite requires a low pH and a high-sulfate environment. These alteration minerals, as well as clay minerals, that are common in the main scarp also compose the bulk of the landslide material.

Conclusions

The instability of the main scarp is attributed to steep topography, intersection of faults, and to the high degree of alteration of the volcanic rocks. The main-scarp area of the Slumgullion landslide has been subjected to several generations of hydrothermal brecciation and alteration. The high degree of alteration in conjunction with the structural fabric is considered to be a prime factor in the original failure of the rock that formed the landslide. Failure of the main scarp has occurred in wide argillitic alteration zones surrounding the breccia pipes.

Fractures and void spaces in the altered volcanic rocks were flushed by meteoric waters resulting in infilling with smectitic clays, jarosite, and gypsum. Surficial cold-water alteration processes are important in the production of soluble salts, such as gypsum, in the landslide material.

Acknowledgments

Discussions with Dana Bove, Miles Silberman, and Karen Wenrich, U.S. Geological Survey, were very helpful in establishing the identity of the alteration mineralogy. Philip Powers assisted with the computer graphics.

References Cited

Bove, D.J., and Hon, Ken, 1990, Compositional changes induced by hydrothermal alteration at the Red Mountain alunite deposit, Lake City, Colorado: U.S. Geological Survey Bulletin 1936, 21 p.

______1992, Geologic and alteration map and drill-core logs of the Red Mountain area near Lake City, Hinsdale County, Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map I-2286, scale 1:12,000.

Bove, D.J., Rye, R.O., and Hon, Ken, 1990, Evolution of the Red Mountain alunite deposit, Lake City, Colorado: U.S. Geological Survey Open-File Report 90-0235, 30 p.

Cross, Whitman, 1909, The Slumgullion mud flow: Science, New Series, v. 30, no. 760, p. 126-127.

Hayba, D.O., Bethke, P.M., Heald, Pamela, and Foley, N.K., 1985, Geologic, mineralogic, and geochemical characteristics of volcanic-hosted epithermal precious-metal deposits in Berger, B.R., and Bethke, P.M., eds., Geology and Geochemistry of Epithermal Systems: Reviews in Economic Geology, v. 2, chap. 7, p. 129-167.

Howe, Ernest, 1909, Landslides in the San Juan Mountains, Colorado: U.S. Geological Survey Professional Paper 67, 58 p.

Larsen, E.E., 1913, Alunite in the San Cristobal quadrangle, Colorado: U.S. Geological Survey Bulletin 530-F, p. 179-183.

Lee, Keenan, 1986, Map showing areas of limonitic hydrothermal alteration in the Lake City caldera area, western San Juan Mountains, Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map MF-1868, scale 1:48,000.

Lipman, P.W., 1976, Geologic map of the Lake City caldera area, western San Juan Mountains, southwestern Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map I-962, scale 1:48,000.

Sharp, W.N., Martin, R.A., and Lane, M.E., 1983, Mineral resource potential and geologic map of the Powderhorn Wilderness Study Area and Cannibal Plateau Roadless Area, Gunnison and Hinsdale Counties, Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map MF-1483-A, scale 1:50,000.

Silberman, M.L., and Berger, B.R., 1985, Relationship of trace-element patterns to alteration and morphology in epithermal precious-metal deposits, in Berger, B.R., and Bethke, P.M., eds., Geology and Geochemistry of Epithermal Systems: Reviews in Economic Geology, v.. 2, chap. 9, p. 203-247.

Steven, T.A., and Ratté, J.C., 1960, Geology and ore deposits of the Summitville district, San Juan Mountains, Colorado: U.S. Geological Survey Professional Paper 343, 70 p.

[Data compiled from hand samples, thin sections, powder X-ray diffraction, and SEM/EDX. Blank boxes under Primary Minerals imply that alteration has completely obscured the original mineralogy of the sample. Sample locations are identified on figure 1]

 Table 1. -- Description of selected samples from the main scarp of Slumgullion landslide.