| Show ORIGIN OF THE MAGNETIC IRON ORES OF IRON COUNTY UTAH 3 by E P jennings of salt lake city utah the iron deposits of iron county utah which rival those of lake superior in extent are situated miles mile s south of salt both the magnetic and the hematite vary in quality from very pure ores high in iron and low in phosphorous and sulphur to sili Z ak al MAI ak 2 V rien aa AX 4 Z W X F tj V F Y 4 4 WIM A RK A 01 J FIGURE 1 GREAT WESTERN DIKE lake city and twenty five miles south of lund a station on the san pedro los angles salt lake railroad the range of hills containing the deposit has a general northeast and southwest strike and rise from a few hundred to 2000 feet above the escalante desert the ore being included in an area of five by twenty miles the basal rocks of these hills is diorite porphyry the flanks of the range are gov covered with limestone supposed to be of carboniferous boni ferous age in some localities by quartzite both formations are conformable to the porphyry and are remnants of beds that formerly covered the highest peaks but have been subsequently removed by erosion in the central and southern parts of the range beds of cretaceous maidstone sandstone said stone rest un conformably com on the erupt ives rhyolite flows occur to the south and southeast of the iron ore deposits and dikes and sheets of post tertiary lava are common with the exception of a few springs the range is destitute of water sage brush stunted cedars and the common desert brush b ush comp compose ps e the only vegetation large beds of coaloa coal of poor quantity occur in cretaceous sandstones fifteen ralles miles east pf af the iron ore re deposits clous cious ores of no commercial value the ores contain traces of copper but no tin banium the composition of the better grades of magnetite is as follows are from a large number made recently and represent the variations in the merchante merchantable mer chanta bi 0 ore the magnetite occurs in nearly vertical I 1 dikes in diorite porphyry and also as sheets sheet in overflows from these dikes very simile similar to the overflows of lava so common in southern utah in some instances the remnants of the sedimentary beds remaining in contact form one of the walls the presence ol of these thin beds of limestone or quartzite h has as led a few observers to conclude that th ore is in the form of a contact deposit but bilt as a matter of fact it is in diorite w walls alls below the sedimentary beds the dikes of hard black magnetite form bold outcrops that are prominent features 1 in the landscape projecting their black masses like great reat walls above the crest of the light brown porphyry hills some of the dikes are covered with an overflow sheet and their exact position under the sh sheet ee t is a matter of conjecture As a rule the original fissures are on the top ot of the hills and it may be assumed that the highest point of the overflow marks the position of the dike in some cases the sheet covers both sides of the hill and in these instances it is evident that the hill crest contains the original dike the typical dike form not complicated by accompanying sheet structure is shown in figures I 1 and 2 one of the largest of these dikes is the great western near the northeastern end of the range ten miles west of cedar city it is nearly feet long and thirty feet high the surface oi of the hill below the dike is thickly covered with angular blocks of ore which has been broken from the outcrop R f W 5 A P box IZZ tal V wl 41 NY N V AR n FIGURE 2 GREAT WESTERN DIKE fe 62 to 68 P to S to cu to per cent while the yield an analysis of fe 40 to 61 P 0 30 top to 04 S to 01 and cu to 0 09 per cent the above analyses the ore in all these outcrops is a meat m sive black magnetite without trace 01 crystallization except in bugs and joints 1 is strongly magnetite and natural magnets or are p not ot U uncommon 1 chief impurities are silica in the form of quartz and chalcedony and apatite the last named occurring in slender hexagonal crystals at t times imes six inches long and 05 inches in diameter in the dike the apatite is often I 1 ricilly arranged 1 in n bands parallel to the i walls the individual crystals that compose 1 these bands being perpendicular to the i 1 walls in the sheets or overflows from the dikes the apatite crystals are not so dis I 1 1 i but when observed are found to be i approximately normal to surface t 1 the ore in the sheets contain many small cavities lined with chalcedony resembling c in ift this respect surface flows of lava the ore of the dikes however is free from cavities 1 f where the sheets are intruded bet between I 1 the porphyry and limestone the magnetite li lias has as been converted into hematite which differs entire in physical character from the original magnetite the hematite is of the T soft earthy variety and is lower in iron and higher in lime content than the mag n petite my conception of the origin of these mag betic ores is that they are the result of ex basic differentiation from a basaltic magma magina in the deep and in a molten condition have been forced to the surface through fissures that were formed contemporaneously with the eruption of the magnetite in a manner similar to the formation of basaltic and other eruptive dikes when the quantity of material was more than sufficient to fill the fractures it overflowed in the form of sheets when the fractures extended into the overlying sedimentary beds they also have filled with ore erosion has destroyed most of the enclosing walls of this character but fragments of angular quartzite in the ore is sufficient proof that at least some of the upper parts of the dikes were contained in quartzite that these deposits were not simple seg legations in place is evident from the fact that they extend into the sedimentary beds the diorite porphyry is not sufficiently basic to have furnished iron for these enormous deposits by a process of segregation but the basic lavas that are extensively developed in southern utah may be another phase of the ma mama magma t ma that was the source of the magnetite more probably they have originated from the fusion of ancient ore beds in the deep professor kemp to whom thanks are due for his assistance has determined the wall rock to be diorite porphyry and has also examined thin sections of the ore for olivine and plagioclase with negative results the section examined by him showed considerable sid erable quartz in the form of small enfil te rations while these results are strongly against an eruptive origin tor these ores the t evidence in the field is strongly in favor ad of it lei t the utah iron fields will be of great im R importance ortance in the future andthil and this paper has been prepared in the hope that it will lead to a discussion of the method by which these great deposits have been formed atlantic city meeting february 1904 am dinst of mining engineers |
