The magnetite-rich breccia masses at Iron Mountain, Silver Lake district, San Bernardino County, California

A survey of the vertical magnetic field of the earth was made with an Askania magnetometer at Iron Mountain, Silver Lake District, San Bernardino County, California. The readings obtained were used to draw the lines of equal anomalous intensities, or "isogams", on a geologic map of the mountain. The pattern of the isogams over the alluvium is in accord with the southeast elongation and dip of the orebodies exposed on the hills. The anomalies over the alluvium are probably due to sizable orebodies rather than alluvial material rich in magnetite derived from the exposed orebodies, for the anomalies are of large magnitude and their trend seems unrelated to the present drainage pattern. In some places, the anomalies confirm the fact that the exposed orebodies extend far beneath alluvial cover. Ore samples were analyzed for magnetic properties and composition. Their mean susceptibility is between 0.10 and 0.17 and their remanent magnetism is about 10 times larger than the magnetism induced by the magnetic field of the earth. Polished sections of the ore indicate that it consists mostly of hematite and magnetite in various ratios, the hematite predominating. Depths to the centers of magnetic concentrations in the alluvium were determined in two different ways; from application of half-peak value rules to isogams, and from the analysis of data obtained by measuring magnetic intensities atop a platform placed at the centers of anomalies. Both methods gave values of less than several hundred feet for the depths. Peak anomalies over vertically polarized, surface-contacting ellipsoids of revolution were computed and plotted as function of true susceptibility, and of the ratio of the principal axes of the ellipsoids. The curves obtained indicate that the exposed orebodies are rather flat. This agrees with the large magnitude of the negative anomalies surrounding the orebodies and with data obtained from holes drilled through three of the orebodies by the the U.S. Bureau of Mines. The ratios of the anomalies expected over vertically polarized ellipsoids of revolution at depth to the anomalies expected for the same ellipsoids at the surface were computed and plotted as a function of the ratio of the principal axes of the ellipsoids. The curves obtained were used to estimate minimum and maximum amounts of ore which could produce the anomalies observed over the alluvium. Iron Mountain probably consists of two layers of breccia alternating with two layers of conglomeratic sandstone. The ore is confined to the lower layer of breccia. The intensity of deformation and faulting increases westward toward the crystalline-complex. The breccias and conglomeratic sandstone appear to be arched by the uplift of the crystalline-complex. Four faults trend northeastward. One of the faults offsets a large orebody. The exposed ore represents erosion remnants capping small hills and ridges. Upon the basis of the magnetic data, the inferred ore beneath alluvial cover was calculated as equal to about 10,000,000 long tons. The source and structural history of the ores are presented upon the basis of combined geologic, magnetic, and drillhole data