FIELD TRIP DAY ONE Birch Creek, White Mountains, California
Total Page：16
File Type：pdf, Size：1020Kb
FIELD TRIP DAY ONE Birch Creek, White Mountains, California MARK D. BARTON† Center for Mineral Resources, Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Introduction Climbing toward the ridgeline the road passes down section THIS PAPER provides the log for the first full day of the field through folded and faulted exposures of Harkless (shale), Po- trip. Day 1 will be spent in the Birch Creek area, east of leta (mixed carbonate and shale), and Campito (sandstone) Bishop in the southeastern White Mountains. There are sev- Formations. These units are present in the distal part of the eral goals to the Birch Creek day: Birch Creek system where they locally host small quartz-base metal replacements and veins. A few of these deposits pro- 1. To see one of the best-exposed examples of a Cretaceous duced modest amounts of lead, zinc, silver and gold in the two-mica granite pluton in the Great Basin with its distinctive late eighteenth and early nineteenth centuries. style of emplacement and hydrothermal alteration; 2. To examine the relationships among magmatism, struc- STOP 1—Westgard Pass tural development, and emplacement; Purpose: overview of the Birch Creek district from south. 3. To examine the links between magmatism and hydro- Stop near the summit of Westgard Pass, 100 m southwest thermal activity; of the entrance to Cedar Flat group campground (11.9 miles 4. To ask the questions: Why are these Late Cretaceous from Big Pine). From here look north parallel to the road systems in the Great Basin not well mineralized? What differs along the axis of the White Mountains antiform. Low, boul- between these systems and better-mineralized Late Mesozoic der-dominated outcrops of the Birch Creek pluton form an examples elsewhere in the circum-Pacific? area of subdued relief below the high country. The ridge on 5. To consider how the characteristics of and processes in the far northesast skyline is Roberts Ridge which is in the these granite-related systems compare with other intrusion- middle of the quartz base-metal vein systems on the north related hydrothermal systems, specifically in the Yerington side of the intrusive center (Fig. 1). district and in the Humboldt mafic complex. At this point, we can get a sense of the overall structure of the southern White Mountains, with the major antiform See the accompanying paper (Barton, 2000) for an intro- plunging south towards this area (see fig. 2 in Barton, 2000). duction to the Birch Creek system as a whole and its broader The Birch Creek intrusion is on the steeper, eastern limb of context. Some local terminology used in this field guide is de- the fold where it intruded mainly uppermost Proterozoic to fined there. Lower Cambrian Reed Dolomite. The Reed and underlying It is important to get an early start as this is the most chal- Upper Proterozoic Wyman Formation wrap around the plu- lenging day of the field trip because of logistics and length ton on its northern and western margins. They change trend and because it is second only to the skarn traverse at Yering- by 90° and overturn as they approach the contact. In this part ton in the physical exertion required. If the upper Mollie Gib- of the aureole, folds that are broad and open to the north be- son road is impassable due to snow, ice, or obstacles, hiking in come isoclinal and generally concordant with the pluton con- and/or alternative stops can be considered. tact (Nelson and Sylvester, 1971). Begin Trip Drive up Mollie Gibson Canyon Leave Bishop and drive south on US 395 approximately From Stop 1 drive north on Highway 168 for about 3 miles 12.5 miles to the north edge of the town of Big Pine (14.0 descending into the upper part of Payson Canyon. Turn north miles from intersection of US 395 and East Line Street in (left) off the highway onto a very poor four-wheel drive road Bishop). Turn left (east) on California Highway 168 toward that goes up Mollie Gibson Canyon (at elevation 6,880 ft, just Westgard Pass and Fish Lake Valley. At the base of the range below the 7,000 ft elevation sign). Mileage is given from the (2.2 miles from Big Pine) stay on 168 as it bears to the left and turnoff into the Canyon. heads for Westgard Pass. The road to the right goes to Wau- The drive up Mollie Gibson Canyon is slow. Since its con- coba Wash, Saline and Death Valleys passing just north of the struction in the 1950s during tungsten exploration, the road Papoose Flat pluton. The latter shares many features with the has been severely damaged by several flash floods. Intermit- Birch Creek pluton (see preceding text for brief discussion). tent repairs by hunters, field camp participants, and others Nelson provides a more detailed road log for the Bishop to has kept it marginally passable if driven slowly and with care. Mollie Gibson Canyon segment (in Nelson et al., 1991). In the lower part of Mollie Gibson Canyon, drive through The drive to Westgard Pass climbs about 1,300 m (4,000 ft) several tight bends in the narrow bottom where the canyon is and is mainly through exposures of Lower Cambrian sedimen- incised into the medium-bedded dark quartzites of the Campito tary rocks on the western limb of the White-Inyo antiform. Formation. Several dikes of dark green, sparsely porphyritic chlorite-actinolite-albite-epidote-bearing dikes cut through † E-mail, [email protected] this area (~0.5 miles). These dikes, although undated, have 27 28 MARK D. BARTON Roberts Ridge Schulman Grove Mexican mine outer lim it o Camp f h Birch Creek yd Hill ro (Stop 3) pluton th e rm a l f e a t u r e s Stop 2 • Mollie Gibson mine Mollie Gibson Canyon Payson Canyon Field trip routes 0 kilometers 5 driving Cedar Flat & Westgaard Pass N walking (Stop 1) 0 miles 3 FIG. 1. Aerial photograph of the Birch Creek area. The general route of the field trip is indicated. Compare with the sim- plified district geologic map (fig. 2 in Barton, 2000). been correlated with the Independence Dike swarm of east- dolomite, limestone) and Reed Dolomite make up the foot- ern California and may be Late Jurassic (147–172 Ma) or wall (east side) of the Mollie Gibson fault zone. The hanging Early Cretaceous (116–188 Ma) in age (Inan, 1996). These wall west of the fault consists primarily of Campito Formation dikes parallel and locally intrude faults with normal displace- in this area, although farther to the north older units (Deep ment, such as the Mollie Gibson fault system. Thus some or Spring, Reed and, ultimately, Wyman) lie in this position. The all of the normal movement on these structures could be fault dips at variable angles to the west and has a normal sense Early Cretaceous or older, consistent with evidence for ex- of movement, with stratigraphic offsets of >800 m where best tensional tectonism elsewhere in the western Great Basin in developed (Campito against Wyman, north of Stop 2). parts the Jurassic (cf. chapter on Humboldt mafic complex). Continuing north, isolated prospect pits occur along the As the valley opens up (>1.1 mi), drive past outcrops of western strand of Mollie Gibson fault (one is visible on the blue-gray Deep Spring Formation limestone on the east. In eastern canyon wall at 2.1 mi). Most of these veins contain this area, the Deep Spring Formation (mixed sandstone, quartz, which ranges from massive in veins to spider web silica 0361-0128/98/000/000-00 $6.00 28 DAY ONE: BIRCH CREEK, WHITE MOUNTAINS, CA 29 in carbonate host rocks. Also present are carbonates, typically relationships to each other and metasomatic features as well tan-weathering calcite, and minor sulfides (pyrite, galena, as some of the best evidence for timing and extent of syn- sphalerite, now mostly oxidized). Fluorite, muscovite and al- magmatic deformation. kali feldspar join these minerals in veins along the Mollie Gib- From Stop 2, continue driving north on the better pre- son fault system where it they are closest to the pluton on the served road toward Camp Hill. Just below Camp Hill (5.7 mi) northwest side. on the west side there is a very tight hairpin turn (5.5 mi). It The only development in the southwest sector is the Mollie is important to take this turn as far up (north) as practical to Gibson mine, which is located 300 m east of where the road avoid getting stuck. turns east and the wash narrows substantially (2.85 miles). The Mollie Gibson mine produced small amounts of silver STOP 3—Traverses in the Aureole and Pluton from sulfide-bearing ores along mafic dikes. Diggles and Beginning at Camp Hill (so called because it was the loca- others report a small resource averaging 0.1 oz/t (3 ppm) Au tion for camping during field work and field classes in the (Diggles et al., 1983). area), two traverses illustrate key features of the Birch Creek North of the Mollie Gibson turnoff, drive up a very rough, pluton and its aureole (Figs. 1, 3). A circuit of about 1 km bouldery section of road toward a saddle with an east-trending around the upper parts of Camp Hill illustrates many of the drainage. Drop down, cross this drainage, and climb toward structural, igneous and hydrothermal features of the aureole, the saddle to the north by way of a west-pointing switchback. including evidence for distribution and relative timing of de- formation, magmatism and metasomatism.