Geological Survey Bulletin 1359 Geology and Mineral Resources of the Northern Part of the North Cascades National Park, Washington

DESCRIPTION OF MINERALIZED AREAS
(continued)

LUNA CREEK AREA

Luna Creek originates in a steep-walled cirque carved in granitic rocks between Mount Challenger and Mount Fury. Sediment samples were collected along most of Luna Creek and many of the small tributaries that enter it, but samples containing the greatest amount of copper and molybdenum came from small tributaries that drain into the cirque at the head of the stream. Here, of 13 samples collected, all but three contained 30-50 ppm cold copper, and some of the samples also contained molybdenum. Many iron oxide-stained zones can be seen on the nearly vertical cirque walls. In the glacial till piled below the glaciers are boulders that contain disseminated pyrite and pyrrhotite. Fractures in a few boulders are also coated with chalcopyrite and molybdenite.

Small copper-bearing veins occur in two areas along the northwest wall of the cirque about 1-1/2 miles northeast of its head. One area is 2,900 feet south-southeast of the southern tip of Wiley Lake, where, east of a broad watercourse, a nearly vertical cliff of biotite quartz diorite is cut by mineralized joints. Epidote is the principal mineral, and in places it makes up the entire vein. Other minerals are quartz, pyrite, chalcopyrite, and chrysocolla. Copper minerals, as much as one-fourth inch thick, form a thin and generally erratic coating along parts of these joints. A grab sample (334, pl. 2) of vein material from one of the sulfide-rich parts of one joint contained 0.30 percent copper, 0.015 percent molybdenum, 0.20 ounce of silver per ton, and 0.002 ounce of gold per ton.

The second area of copper-bearing veins is on the west side of the same broad watercourse at an elevation of 5,850 feet, 4,200 feet south-southwest of the south tip of Wiley Lake and 7,100 feet north east of Mount Challenger. On this steep mountainside, five joints that cut biotite-quartz diorite are coated with as much as one-eighth inch of quartz, pyrite, chalcopyrite, and chrysocolla.

Five samples were taken from five small zones containing disseminated sulfides high on Luna Peak. Sample 340 (table 1), selected for its sulfide content, contained 0.05 percent total copper; the rest (335-337, 339, table 1) yielded 0.002-0.03 percent copper.

BIG BEAVER CREEK AREA

Three areas in and adjacent to Big Beaver Creek contain noteworthy concentrations of metals. The first of these is a northward-flowing tributary whose headwaters are just west of Sourdough Lake. A sediment sample and a panned sample were taken at the mouth of this tributary. The sediment sample did not contain anomalous amounts of cold copper or heavy metals, but a panned concentrate from the locality (391, pl. 2) contained 0.24 ounce of gold per ton and 0.15 ounce of silver per ton.

The second area is on both sides of Big Beaver Creek 2—3-1/2 miles above the mouth of McMillan Creek. Sediment samples collected from several tributaries in this area contained anomalous amounts of copper and molybdenum. Four samples (364-367, table 1; pl. 2) collected on a principal tributary on the west side contained 50-100 ppm cold copper and 5-15 ppm molybdenum. The source of the copper and molybdenum is about midway up the steep tributary, where joints in quartz diorite are coated with pyrite, chalcopyrite, and molybdenite. These joints, which can be traced about 300 feet vertically along the tributary, are fairly widely spaced, and many have few or no sulfides along them. A composite sample (362, pl. 2) of the rock from along several of the mineralized joints contained 1.5 ounce of silver per ton, >0.50 percent copper, and >0.20 percent molybdenum. On the east side of Big Beaver Creek, samples from four tributaries had anomalous amounts of copper. Near the mouths of these tributaries, samples (360, 361, 375, 378, pl. 2; table 1) yielded 7-70 ppm cold copper; farther upstream, samples (373, 374, 376, pl. 2; table 1) contained 15-100 ppm cold copper. The copper in most of these samples comes from zones of disseminated sulfides containing a little chalcopyrite. Samples from four of the zones (370-372, 377, table 1) had from 0.002-0.05 percent copper. The richest sample (371) came from near the ridge and was a selected sample of sulfide-rich material from a rather patchy zone about 500 feet across. Rock sampling was not carried out above the sediment sample (376, pl. 2) with the highest cold copper content (100 ppm), for we were unable to climb any farther. On the cliffs above this sample, however, green malachite coatings could be seen on some of the fracture surfaces.

The third area along Big Beaver Creek is on the ridge a little over 1 mile southeast of Luna Peak. The ridge is made up of alaskite of the Chilliwack batholith that is stained a light brown. Joints, 0.2-1 foot apart, cut across the ridge at a slight angle and form a zone that is 4-10 feet wide. Seams of chrysocolla occur in places along these joints over a distance of 225 feet. Small crystals of pyrite, commonly altered to limonite, were also found scattered along this zone. A 2-foot-long chip sample (381, pl. 2) taken across one of the most chrysocolla-rich parts of this zone contained 0.05 percent copper and 0.0005 percent molybdenum. In comparison, a chip sample (382, pl. 2) of the slightly iron-stained alaskite from a point on the ridge 150 feet southeast of the mineralized joint system yielded 0.015 percent copper and 0.0015 percent molybdenum.

WEST SIDE OF PICKET RANGE BETWEEN MOUNT CHALLENGER AND MOUNT FURY

The west side of the Picket Range consists of jagged peaks and near-vertical towers; below the upper pinnacles it consists of steep rock faces separated from each other by rockslides and snowfields. Veins were found in three places in the northern part of this area, and in several zones of disseminated sulfides in the southern part (pl. 2). The most northerly vein area lies along the crest of the Picket Range 1,250 feet north of Crooked Thumb Peak. It consists of three quartz veins in a zone 60 feet wide of iron oxide-stained biotite gneiss. Veins in the zone strike N. 36° E. and dip vertically. They are 1/2—4 inches thick and in addition to quartz contain pyrite and arsenopyrite. Two chip samples were taken in the area: one (600, pl. 2) across the entire iron oxide-stained zone and one (101) of a 4-inch vein plus 2 inches of gneiss on either side of it. The two samples, 600 and 101, contained 0.02 and 0.07 percent copper, 0.15 and 0.07 percent lead, 0.1 and 0.05 percent zinc, 0.04 and 0.58 ounce of silver per ton, and <0.003 and <0.03 ounce of gold per ton, respectively.

Two other veins were found about 2,500 feet southeast of the first set of veins on a small spur 1,500 feet southwest of Crooked Thumb Peak at an elevation of 6,650 feet. The two veins, which are about 20 feet apart at their westernmost exposure, cut biotite gneiss. The northerly vein strikes N. 74° E., dips 70° SE., and is 1-4 inches thick. The vein can be traced uphill to the northeast for 250 feet, and near its northeast end it splits into several smaller veins. The vein consists of quartz with abundant pyrite, arsenopyrite, galena, and tetrahedrite (?). Two 4-inch channel samples, 602 and 603 (pl. 2), that were taken across the vein near its southwest end contained >2 and >4 percent lead, 0.07 and 1 percent zinc, >0.5 and >1 percent copper, 5.8 and 2.9 ounces of silver per ton, and 0.006 and 0.12 ounce of gold per ton, respectively.

The southerly vein strikes about N. 68° E., dips 70° SE.-70° NW., and is 1/4—3 inches thick. The southern part of the vein appears to be mainly quartz with a little pyrite, chalcopyrite, and cerussite. Two 3-inch channel samples, 604 and 605 (pl. 2), taken across the southwest end of this vein contained 0.10 and 0.05 percent lead, 0.07 and 0.10 percent zinc, 0.05 and 0.04 percent copper, 0.58 and 0.09 ounce of silver per ton, and <0.003 ounce of gold per ton.

Another vein, approximately 2,200 feet southwest of Crooked Thumb Peak, at an elevation of 6,220 feet, strikes N. 78° E., dips 70° SE.-80° NW., and cuts biotite gneiss. The thickness ranges from 1/8 inch to 5 inches and averages about 1 inch. This vein, which was traced between snow patches for about 175 feet, is made up principally of quartz, with minor pyrite and arsenopyrite. A 5-inch channel sample (606, pl. 2) taken near its southwest end contained 0.07 percent lead, 0.10 percent zinc, 0.03 percent copper, 0.50 ounce of silver per ton, and 0.035 ounce of gold per ton.

South of the previously mentioned veins and clustered on the west side of Mount Fury are five iron oxide-stained zones that contain disseminated sulfides. Two of the zones were of mineralized intrusive breccias. Samples 435 and 437 (pl. 2; table 1), made up mainly of the matrix of these breccias, which constitutes 20-30 percent of the rock and contains most of the sulfides, yielded 0.02 and 0.03 percent zinc, 0.0015 and 0.01 percent lead, 0.01 and 0.15 percent copper, and 0.0005 and 0.01 percent molybdenum, respectively. The other three zones were along shears in quartz diorite and gneiss. Chip samples 436, 438, and 439 (pl. 2; table 1), taken across thicknesses of about 1 foot each from these three zones, contained 0.05, <0.02, and 0.02 percent zinc; 0.07, 0.015, and 0.005 percent. lead; and 0.01, 0.03, and 0.020 percent copper, respectively.

PIONEER RIDGE

Pioneer Ridge rises abruptly more than 4,000 feet from adjacent valley bottoms. The ridge forms a massive buttress bounded by gray cliffs and is roughly triangular in shape having valleys on three sides. Picket Creek flows along the north side of Pioneer Ridge, and sediment samples (609-610, 612-614, pl. 2) from five tributaries of this creek, which descend the steep north face at Pioneer Ridge, contained 7-30 ppm heavy metals, 3-30 ppm cold copper, and 2-10 ppm molybdenum. The north face of the ridge was too steep to be climbed, and only the top of the ridge was examined. Small iron oxide-stained zones occur at several places along this ridge. On the ridgetop directly above the sediment—sample locality that contains the highest heavy metal and cold copper content is a fine-grained diorite dike that cuts volcanic rocks. Along the edge of this dike is a 2-inch-thick zone of iron oxide-stained rock. A sample (611, pl. 2) of the stained material yielded 0.10 percent zinc, 0.05 percent lead, 0.015 percent copper, and 2.92 ounces of silver per ton. This zone, even though relatively high in grade, is too small to produce the anomalous metal content in the sediment samples. Perhaps other similar zones occur on the steep mountain face.

About three-quarters of a mile south of this ridge, several iron oxide-stained zones occur near volcanic dikes in biotite gneiss. A chip sample (595, pl. 2) across a 1.5-foot-wide zone in one had 0.001 percent copper, <0.02 percent zinc, and 0.007 percent lead; a second chip sample (596) across a neighboring 5-foot zone yielded 0.01 percent copper, 0.03 percent zinc, and 0.02 percent. lead.

BACON PEAK AREA

One of the largest disseminated sulfide zones in the North Cascades National Park is along the cast flank of Bacon Peak (fig. 9.) This zone is 2.2 miles long, 0.3-0.9 mile wide, and is in part covered by the Bacon Peak glacier. The sulfide-bearing zone crops out on a high undulating upland surface, which to the east falls off into a series of nearly perpendicular cliffs to Bacon Creek more than 3,500 feet below. The disseminated zone is in greenschist, Chuckanut Formation, and a quartz diorite intrusive. It crosses and extends along several faults, which in part control its shape (fig. 9). Rocks in the disseminated zone have weathered on the surface to a dark brown, and most contain scattered grains of pyrrhotite and to a somewhat lesser extent pyrite. Chalcopyrite was noted only along some fracture surfaces, in a 5—12-foot-thick dike in the northern part of this zone. Sediment samples taken from tributaries that drain the western and southern parts of this zone are not anomalous in cold copper or molybdenum. Sediment samples from two tributaries of Bacon Creek that drain the northeast part of the zone contained 30 ppm cold copper (fig. 9), and one sediment sample taken at the mouth of Green Lake contained 100 ppm cold copper. Five chip samples (527-531, table 1; pl. 2) taken along the ridge east of Bacon Peak in the southern part of this zone of disseminated sulfides contained 0.007-0.02 percent copper, but no detectable molybdenum, lead, or zinc. Seven chip samples (519-521, 523-526, table 1; pl. 2) taken across the broad north end of the zone yielded <0.0003-0.03 percent copper and no detectable molybdenum. Sample 521, from the dike with the visible chalcopyrite, and 524, from the greenschist about 100 yards to the south, contained 0.15 and 0.03 percent zinc, respectively. Traces of gold and silver were found in several of these samples. Although this zone is large and the rocks in it contain at least several percent sulfides, most of these are iron sulfides. The copper and zinc content is low and erratic, and molybdenum, lead, gold, and silver, where detected, were in only negligible amounts.