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The mineralogy and geochemistry of about 20,000 feet of Belt rocks in the Mission Mountains Primitive Area, northwestern Montana, are compared with those of about A 40,000 feet of Belt rocks in the Fend Oreille area, northern Idaho and northwestern Montana. Semiquantitative spectrographic analyses of 254 samples from the Mission * Mountains and 230 samples from the Pend Oreille area form the basis for chemical comparison; quantitative X-ray mineralogic analyses of most samples provide the mineralogic data. Both analytical techniques are rapid and inexpensive. , All rocks were classified on the basis of grain size, texture, and carbonate content into one of four rock types: argillitic rocks (shale or mudstone equivalents), siltites (siltstone equivalents), quartzites (sandstone equivalents), and carbonate rocks. The semiquantitative spectrographic method has good precision for all rock types, but the *~ accuracy is only fair for rocks of moderate to high carbonate content. Comparison of clastic rocks from both areas shows the mineralogy to be very similar even to the ratio among quartz, potassium feldspar, and plagioclase in siltites. I.. Most rocks are graywacke or like graywacke. Mineralogy is, not surprisingly, directly related to grain size; for example, we find that from argillitic rocks to siltites to quartzites the amount of quartz increases, the amount of feldspar remains about the same, and the illite-sericite-chlorite content decreases. "** Several elements show distinct patterns of abundance related to the rock types. Elements directly related in abundance to amount of illite-sericite-chlorite in clastic rocks are B, Co, Cr, Ni, Sc, V, Fe, Mg, Ti, and possibly La and Y. Siltites and quartzites contain Cu in sulfide grains and flakes, and Cu is distinctly more abundant in those rocks or in fossil algal forms (usually calcareous or dolomitic siltite) in the Mission Mountains. Carbonate rocks or rocks containing moderately abundant carbonate minerals show increased amounts of Ca, Mg, Mn, Sr, and Pb. The higher B "** content of certain Pend Oreille area strata and the higher Co, Cu, and Ni contents of Mission Mountains strata seem most likely to reflect differences in source area of the original sediments. If so, the Cu abundance and distribution identified in this i study may aid in the search for stratabound copper deposits in Belt rocks. The excellent sorting of grain sizes in similar strata through tens of thousands of feet of rock in two widely separated areas, the remarkable uniformity of simple miner-

Mineralogy and geochemistry of some Belt rocks, Montana and Idaho