The Coeur d'Alene district of this report is that area in northern Idaho between the Montana State line and longitude 116°17', and between latitudes 47°26'15" and 47°33'45".The country rock of the area consists of the Prichard, Burke, Revett, St. Regis, Wallace, and Striped Peak Formations of the Precambrian Belt Series. These formations are composed of fine-grained quartz-rich rocks that have been mildly regionally metamorphosed and which, in conse• quence now belong to the green schist facies. The ubiquitous sericite was oriented during the folding and metamorphism and the rocks now consist of quartzose slate, sericite phyllite, sericite quartzite, and in a few areas, schistose quartzite and sericite schist.Igneous rocks of the area include monzonite stocks and related monzonite dikes, which were probably intruded at the same time as the Idaho batholith, and biotite lamprophyre, diabase, and porphyritic hornblende-augite-olivine-dikes, the latter perhaps varieties of the diabase dikes. The two stocks are part of a line of northeastward-striking intrusives that apparently lie along a major fracture. This fracture is probably one of the controlling factors in the origin of the Coeur d'Alene district.Several sets of faults can be recognized; there is no certainty that any are older than the shear zones that contain ore deposits, but the great strike-slip faults-the Osburn and Placer Creek faults-have offset mineral belts.Idaho, within which the Coeur d'Alene district is the only important mining area, has ranked second in lead and in zinc production for many years. The first recorded production from the Coeur d'Alene district proper was
The ore bodies of the Highland-Surprise Mine are replacements along shear zones in quartzose slate, quartzite, and phyllite of the Prichard formation of the Belt series (Precambrian). Pyrite, arsenopyrite, pyrrhotite, sphalerite, chalcopyrite, and galena are the chief metallic minerals and were probably deposited in that order.Temperatures of formation were estimated for pyrrhotite-pyrite and sphalerite-pyrrhotite assemblages contained in the steeply dipping vein system. Measurements on sixty-two pyrrhotites indicate a range in temperature of crystallization between 370 ø and 492 ø C (no correction required for confining pressures <2,000 bars below 670 ø C), and measurements on fourteen sphalerites indicate crystallization temperatures between 375 ø and 460 ø C (uncorrected for confining pressure). No systematic temperature gradients were recognized.Minor elements in twenty-three pyrrhotite, eight pyrite, thirteen sphalerite, and two galena samples were determined. Partition ratios of nickel and cobalt in pyrrhotite-pyrite and sphalerite-pyrrhotite pairs are erratic. This suggests (a) a dis-equilibrium distribution of these elements between the phases of each mineral pair and/or (b) crystallization of the mineral pairs under different physical conditions.
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