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

GEOCHEMICAL EXPLORATION

Geochemical exploration in this area consisted of systematic sampling and analysis of rocks, gossan, stream sediments, and panned concentrates. These materials were analyzed by using modern chemical or spectrographic techniques which are capable of detecting minute quantities of various elements. A large number of samples, collected systematically over wide areas, provides information on the distribution of potentially valuable metals. Samples that contain unusual concentrations of the metals aid in pinpointing localities most likely to contain potential deposits.

Three types of samples were collected: (1) heavy mineral concentrates panned from stream gravels, (2) stream-sediment samples, and (3) rock samples from areas that contain visible sulfide minerals, iron oxide-stained rocks or other types of altered rock.

Minerals which are resistant to abrasion and chemical weathering and which have a high specific gravity can be concentrated from stream gravels with a gold pan. Gold, monazite, cassiterite, wolframite, and scheelite are most readily detected in this manner. Their presence in stream gravels, as shown by analysis of panned concentrates, indicates the existence of rocks containing them in the area upstream from the point where the sample was collected. Subsequent panning farther upstream may locate the source area of the mineral. Certain sulfide minerals such as pyrite, chalcopyrite, or galena also may be found in panned concentrates, but these minerals are much more vulnerable to chemical and mechanical degradation and are rarely found far from their source.

Part of our geochemical studies in the northern part of the North Cascades National Park consisted of panning stream gravels for heavy minerals (pl. 2). Placer deposits are few and small, because of recent and intense glaciation. The great streams of ice that occupied the valleys less than 12,000 years ago effectively removed all weathered rock that was present, leaving fresh, bare rock surfaces from the valley floors to the crests of the ridges. Valleys were scoured out by the ice, and stream-washed gravels were removed, to be replaced by a jumble of glacial debris that consists mostly of poorly sorted fragments of fresh rock. Consequently panning in the streams of the North Cascades commonly produces only a very small amount of heavy mineral concentrates (generally less than 1 oz per pan). Very few of these concentrates contain gold, but even those that do are deceptive. A panned sample that contains gold in the ratio of 1 ounce per ton of heavy minerals is of no value if the heavy minerals themselves make up only a fraction of a percent of the stream gravels. It thus appears that none of the stream gravels we sampled contain significant reserves of placer gold.

Stream sediments of silt- and clay-sized particles carried by the stream or of material in contact with the stream water were sampled. A number of metals that commonly occur in bedrock as sulfides tend to weather rapidly under normal surface conditions and release metallic ions in solution. Copper, lead, zinc, cobalt, and molybdenum are among the metals that find their way into streams, where they are absorbed or adsorbed onto the silt- and clay-sized particles in the stream sediment. Analysis of the sample, using the citrate soluble heavy metals technique, can detect undifferentiated quantities of zinc, lead, copper, and cobalt, as small as 1 ppm (part per million). Another method, the "cold copper test," detects only the copper absorbed on the fine sedimentary particles. Both the cold copper and heavy metals tests are excellent guides to the presence of metalliferous ores in the drainage basin of the streams sampled. In general, sediment samples from the larger streams provide less useful information than those from smaller tributaries, for a concentration of metallic ions from small streams tends to become diluted when mixed with more water in larger streams.

We collected samples at intervals of 1 mile or less from the major streams and from most tributaries that contained running water at the time of our visit.

Minute amounts of copper and heavy metals are present in almost all stream sediment samples; we considered a sample to be anomalous if the heavy metals, cold copper, or molybdenum content of the sample was greater than 6 ppm. Anomalous amounts of potentially valuable elements in stream sediments indicate only that those elements occur in rocks somewhere upstream from the point of the sample.

Rock samples were collected for analysis wherever the appearance of the rock suggested the presence of potentially valuable ore minerals. These sampled localities included iron oxide-stained or otherwise altered rock, any rocks containing visible sulfides, and rocks from areas drained by streams that produced anomalous stream sediment samples. The rocks were analyzed spectrographically for 18 metals; where significant quantities of gold, silver, copper, lead, zinc, or molybdenum were suspected, chemical analyses also were made of the samples. The chemical analyses for gold, silver, and copper were done by atomic absorption; the molybdenum was analyzed colorimetrically. The atomic absorption and colorimetric determinations are quantitative and more accurate than the spectrographic determinations which are semiquantitative and which will contain quantitative results only about 30 percent of the time. Thus in table 1, semiquantitative spectrographic values are not given for those samples that have been analyzed by quantitative chemical methods.

The results of the analyses on 1,188 stream-sediment samples, 64 panned concentrates, and 450 samples of iron oxide-stained zones and veins are shown in table 1, and the sample localities are shown on plate 2. Because of the large number of sediment samples, only those that contain 6 ppm or more of heavy metals, cold copper, or molybdenum are given in table 1, but all the sampled points are shown on plate 2. The localities of the samples listed are shown, by sample number, on the plate. In the sections "Mineral Resources" and "Description of Mineralized Areas," where disseminated zones or veins are discussed, the analytical data on gold and silver have been converted to ounces per ton, and those data of other metals to percent. This conversion is solely for convenience of the reader, to enable him to quickly make comparisons with assay results determined by other methods and with the grade of deposits in other areas. The reader should be aware, however, of the limitations of those analyses determined by semiquantitative spectrographic analyses and should make his comparisons accordingly.